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Marine Geology, 102 (1991) 63-130
63
Elsevier Science Publishers B.V., Amsterdam
Rise and demise of the Bahama-Grand Banks gigaplatform,
northern margin of the Jurassic proto-Atlantic seaway
C. Wylie Poag
U.S. Geological Survey, Woods Hole, MA 02543, USA
(Received May 29, 1990; revision accepted February 10, 1991)
ABSTRACT
Poag, C.W., 1991. Rise and demise of the Bahama-Grand Banks gigaplatform, northern margin of the Jurassic proto-Atlantic
seaway. In: A.W. Meyer, T.A. Davies and S.W. Wise (Editors), Evolution of Mesozoic and Cenozoic Continental Margins.
Mar. Geol., 102: 63-130.
An extinct, > 5000-km-long Jurassic carbonate platform and barrier reef system lies buried beneath the Atlantic continental
shelf and slope of the United States. A revised stratigraphic framework, a series of regional isopach maps, and paleogeographic
reconstructions are used to illustrate the 42-m.y. history of this Bahama-Grand Banks gigaptatform from its inception in
Aalenian(?) (early Middle Jurassic) time to its demise and burial in Berriasian-Valanginian time (earty Early Cretaceous).
Aggradation-progradation rates for the gigaplahbrm are comparable to those of the familiar Capitan shelf margin (Permian),
and are closely correlated with volumetric rates of siliciclastic sediment accumulation and depocenter migration. Siliciclastic
encroachment behind the carbonate tracts appears to have been animportant impetus for shelf-edge progradation. During the
Early Cretaceous, sea-level changes combined with eutrophication (due to landward soil development and seaward upwelling)
and the presence of cooler upwelled waters along the outer shelf appear to have decimated the carbonate producers from the
Carolina Trough to the Grand Banks. This allowed advancing siliciclastic deltas to overrun the shelf edge despite a notable
reduction in siliciclastic accumulation rates. However, upwelling did not extend southward to the Blake-Bahama megabank,
so platform carbonate production proceeded there well into the Cretaceous. Subsequent stepwise carbonate abatement characterized the Blake plateau Basin, whereas the Bahamas have maintained production to the present. The demise of carbonate
production on the northern segments of the gigaplatform helped to escalate deep-water carbonate deposition in the Early
Cretaceous, but the sudden augmentation of deep-water carbonate reservoirs in the Late Jurassic was triggered by other agents,
such as global expansion of nannoplankton communities.
Introduction
In the early part of the Middle Jurassic (ca. 186
Ma) scattered clusters of small carbonate banks
began to form along the North American margin
of the proto-Atlantic seaway. The banks flourished
and expanded for 42 m.y., culminating in a Late
Jurassic carbonate gigaplatform (from the Greek
"gigas", meaning "giant") that stretched > 5000
km from the Bahamas to the Grand Banks of
Newfoundland (Fig. 1, Jansa, 1981, 1984) ("Great
Mesozoic Carbonate Bank" of Meyer, 1989). Sheridan (1974), who was among the first to suspect
its magnitude, used seismostratigraphy (e.g., Press
and Beckman, 1954; Ewing et al., 1966; Sheridan
et al., 1966, 1970; Sheridan and Drake, 1968;
0025-3227/91/$03.50
Emery and Uchupi, 1972), seafloor dredgings
(Sheridan et al., 1969) and extrapolation of early
drilling results from the Canadian margin
(Laughton et al., 19'72; McIver, 1972) to construct
representative dip sections across the gigaplatform.
By 1976,. ~.heU.S. Geological Survey had collected
enough 24- and 48-channel, common-depth-point
(CDP) seismic reflection profiles to show that a
prominent buried "'outer ridge" in the Georges
Bank Basin, Baltimore Canyon Trough, and Carolina Trough (Fig.2) probably represented a [.ate
Jurassic-Early Cretaceous reef trend at the seaward edge of a prograded carbonate bank (Schlee
et al., 1976; Grow et al., 1979). That same year,
exploratory drilling in the Baltimore Canyon
Trough and Georges Bank Basin confirmed the
© 1991 - - Elsevier Science Publishers B.V. All rights reserved
64
C.W. POAG
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1981, 1984; Poag, 1985). It was not until 1983,
however, that the first of three boreholes penetrated the Upper Jurassic shelf edge in the Baltimore Canyon Trough (Fig. 3; Shell 372-1, 586-1,
and 587-1), providing incontrovertible proof of a
buried barrier reef in that basin (Edson, 1986a, b,
1987, 1988; Eliuk et al., 1986; Karlo, 1986; Meyer,
1986, 1989; Ringer and Patten, 1986; Erlich et al.,
1988, 1990; Prather, 1988; Lawrence et al., 1990;
Prather, 1991). A grid of seismic profiles across
the continental shelf (Fig. 3) verifies the extent of
the gigaplatform far beyond the Baltimore Canyon
Trough (e.g., Emery and Uchupi, 1984; Uchupi
and Shor, 1984; Schlee et al., 1988).
To the south, the platform rims the Carolina
Trough and the Blake Plateau Basin (Sheridan et
al., 1979; Dillon and Popenoe, 1988), then spreads
across the Bahama Basin (Fig. 2) and into Cuba
(Tator and Hatfield, 1975) and the eastern Gulf of
Mexico (West Florida Platform, Salvador, 1987;
Ball et al., 1988; Buffler, in press). Direct evidence
of the ancient platform is also sparse in this
southern area, but several exploratory wells and
seafloor samplings have corroborated seismostratigraphic interpretations (Meyerhoff and Hatten,
1974; Tator and Hatfield, 1975; Benson et al.,
1978a, b, c; Dillon et al., 1979, 1988; Freeman-
f
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Fig. I. Comparison of Late Jurassic configuration of BahamaGrand Banks gigaplatform with modern Great Barrier Reef of
Australia. Great Barrier Reef rotated by 180° so equator is at
bottom of page relative to both platforms. Carbonate tracts
stippled.
presence of Jurassic shallow-water carbonate rocks
(Smith et al., 1976; Scholle, 1977, 1980; Scholle
and Wenkam, 1982; Amato and Simonis, 1979,
1980; Amato and Bebout, 1980; Libby-French,
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Fig. 2. Physiographic, structural, and depositional features of proto-Atlantic seaway as reconstructed at end of Jurassic. Landmass
borders stippled. Northern paleolatitudes shown at left. AD=Adirondack Mountains; GBB=Georges Bank Basin; S. Fla. = South
Forida. Reconstruction adapted from Jansa (1986) and Tucholke and McCoy (1986). For simplicity, structurally high platforms
separating ~ome basins are not shown.
THE BAHAMA-GRAND BANKS GIGAPLATFORM AND THE JURASSIC PROTO-ATLANTIC SEAWAY
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Lynde et al., 1981; Austin et al., 1988a, b; Schlager
et al., 1988).
To the north, the gigaplatform extends across
the outer edge of the Georges Bank Basin onto
the Canadian margin (continental shelf off Nova
Scotia and Newfoundland), where another extensive seismic grid (tied to wells) records its presence
(Mclver, 1972; Jansa and Wade, 1975; Given, 1977;
Eliuk, 1978, 1981, 1988; Jansa, 1981, 1984; Jansa
and Wiedmann, 1982; Jansa et al., 1982, 1988b;
Eliuk et al., 1986). Only two wells, however, have
penetrated the shallow-water Jurassic reef facies
on the Canadian margin (Shell Deroasco~.a G-32
and Mobil-Texaco-PetroCanada Cohasset L-79;
Pratt and Jansa, 1988). Two additional wells
penetrated peri-reef debris (Shell-PetroCanada
Penobscot L-30 and Chevron-PEX-Sheli Acadia
K-62). Recent drilling by the Ocean Drilling Program on the margin of Galicia Bank (off Spain,
Boiilot et ai., 1988) has indicated that the gigaplatform may have extended even farther around
the eastern end of the proto-Atlantic (Jansa et al.,
1988). Coeval carbonate platforms stretched from
the Grand Banks across the Galicia margin to the
Lusitanian and Algarve basins of Portugal.
The purpose of this paper is to describe, illustrate, and interpret the geologic and seismic
data concerning this carbonate gigaplatform as
known along the U.S. segment of the protoAtlantic seaway. I will attempt to discern its initiation, growth, and disappearance in the context of
the regional depositonal history of the margin; to
examine its relationships to tectonic events, sealevel changes, paleoclimates and paleoceanographic regimes; and briefly to compare its development with coeval deposits on the Canadian and
Gulf Coast margins.
Stratigraphy and correlation: strategy and
problems
My general approach to correlation over this
vast region has been to choose key wells (COST
wells and selected exploratory wells) as iithostratigraphic and biostratigraphic standards by which
to establish stratigraphic reference sections along
dip-oriented seismic reflection profiles (Fig. 3;
Poag, 1982b, 1985, 1987). Starting with reference
C.W. POAG
sections in the Georges Bank Basin and Baltimore
Canyon Trough, I have used seismostratigraphy
(to identify depositional sequences; Mitchum et
al,, 1977a) and seismic facies analysis (to interpret
lithofacies; Mitchum et al., 1977b) to extrapolate
the stratigraphic and depositional framework
through a margin-wide network of intersecting
multichannel seismic reflection profiles (> 30,000
line-km) that covers the areas not yet (or only
sparsely) drilled (e.g., Carolina Trough, Blake Plateau Basin, Bahama Basin; Fig. 3). The southern
end of the seismic grid was tied to the stratigraphy
of the Great Isaacs No. 1 well in the Bahama
Basin (Fig. 3) and to Deep Sea Drilling Project
Sites 390 (Blake Spur) and 391 (Blake Basin;
Fig. 3). Seafloor samples from the Blake Escarpment (Dillon et al., 1988) provided further age
constraints on the southern profiles. From these
analyses, 1 defined 23 margin-wide depositional
sequences (Poag and Sevon, 1989) which can be
correlated and mapped from the Bahama Basin to
the Georges Bank Basin. The stratigraphic correlations are highly interpretive in some areas however,
especially in the deeper undrilled sections (older
than Bajocian). Thus, other authors could be
expected to define and correlate sequences somewhat differently (e.g., Lawrence et al., 1990). Indeed, as discussed later on, I have considerably
revised some of my own previously published
interpretations.
A host of authors (cited throughout the text)
have analyzed various aspects of U.S. Atlantic
margin stratigraphy, having relied mainly on
seismic profiles. To date, however, only four principal groups of authors have offered detailed, stagelevel, chronostratigraphic interpretations of the
shelf-edge stratigraphy:
(l) U.S. Geological Survey (Poag, 1982a, b,
1985, 1987; Valentine, 1980; Poag and Valentine,
1988).
(2) Minerals Management Service (Adinolfi,
1986; Amato, 1987; Amato and Simonis, 1979;
Edson, 1986, 1987, 1988).
(3) Shell Offshore Inc. (Karlo, 1986; Eliuk et al.,
1986; Meyer, 1986, 1989; Ringer and Patten, 1986;
Prather, 1988, 1991; Lawrence et al., 1990).
(4) Amoco Production Co. (Erlich et al., 1988,
1990).
THE B A H A M A - G R A N D BANKS G I G A P L A T F O R M AND THE JURASSIC PROTO-ATLANTIC SEAWAY
Figure 4 shows the contrasting correlations of
these four groups of authors for two Shell wells
that drilled and cored the shelf-edge carbonate
section in the Baltimore Canyon Trough. The
coarse lithostratigraphic and biostratigraphic correlations are similar, but in detail there are some
significant contradictions, especially among the
Minerals Management Service (MMS) correlations. It is especially notable that the MMS correlations (based on palynological interpretations of
Cousminer, in Edson, 1986, 1987) show that the
lower section of each well is older by 15-20 m.y.
than any other group of researchers has indicated.
I interpret this discrepancy to mean that older
Jurassic palynomorphs have been reworked into
the Upper Jurassic sections in this area.
The most contentious correlation problem encountered in the study area involves the Jurassic
section in the Georges Bank Basin. Most authors
who have studied this section (Amato and Simonis,
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Fig. 4. Comparison of four different stratigraphic interpretations of two key wells drilled by Shell Offshore Inc. into
Bahama-Grand Banks gigaplatform on the seaward margin of
Baltimore Canyon Trough.
67
1980; Ascoli, 1982, 1983; Jansa and Wiedmann,
1982; Poag, 1982a, b; Schlee and Fritsch, 1982;
Manspeizer, 1985; Schlee et al., 1985; Klitgord et
al., 1988; Poag and Valentine, 1988; Schlee and
Klitgord, 1988; several unpublished oil company
reports) have interpreted the oldest non-evaporitic
marine sediments above the basement to be of
Early to Middle Jurassic age (e.g., COST G-l well,
~2200 to ~4600 m; COST G-2 well -,,3000 to
~6500 m; Fig. 5). In stark contrast is the interpretation made by Cousminer and his coauthors
(Cousminer et al., 1984; Manspeizer and Cousminer, 1988; Cousminer and Steinkraus, 1988) and
accepted in Manspeizer's (1988) and Schlee et al.'s
(1988) latest interpretations. The Cousminer group
claimed that the lower 2200 m of the G-2 well
(and thus equivalent sections throughout the other
east coast basins) are of Triassic (Carnian-Norian)
age. This disparate interpretation is based on Cousminer's identification of rare, poorly preserved,
Triassic palynomorphs in a single core (core 5 at
4434 m) in the COST G-2 well (Fig. 5). Another
core (core 4), 400 m higher in the G-2 well, also
contains Triassic palynomorphs (in greater abundance), but Cousminer contends that these have
been reworked into Bajocian rocks (according to
Davies (1985), however, these "Bajocian" rocks
could be as young as Bathonian, which would
agree with my correlations, as based on Ascoli's
(1982) biostratigraphy). Cousminer and Steinkraus
(1988) also rejected, as "contamination", the Middle Jurassic palynomorphs found by Fairchild and
Gartner (1977) in a core at 5612 m in the COST
G-2 well.
Placing the top of the Triassic at such a high
stratigraphic position has untenable implications
for the tectonic and depositional history of the
Atlantic margin basins. Cousminer and colleagues
used their interpretation of the biochronologic age
of the rocks to argue that a "'postrift unconformity" is present in the COST G-2 well either at
4051 m (Cousminer and Steinkraus, 1988) or at
4153 m (Manspeizer, 1988). They claimed that this
unconformity is associated with an attenuated
Liassic section, although they found no evidence
of Liassic mic~cofossils in the well (they cited only
a personal communication to support the Liassic
age). The stratigraphic position of the postrif!
68
c.w. POAG
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Fig. 5. Comparison of four different stratigraphic interpretations of key COST (Continental Offshore Stratigraphic Test) wells drilled
in Georges Bank Basin. C - N = Carnian-Norian.
unconformity, however, is not dependent upon the
age of the rocks above and below it. As generally
defined (e.g., Falvey, 1974), the postrift unconformity is the surface that separates deposits formed
during continental rifting (synrift deposits such as
rift-basin fill) from deposits formed after seafloor
spreading (drifting) had begun (Oostrift sediments).
As such, the unconformity marks a profound
change in structural and depositional style, which
is manifest in two principal ways: (1) On seismic
reflection profiles, synrift sediments typically show
non-parallel (convergen0, fan-like (wedge-shaped)
reflection geometries (e.g., Montadert et al., 1979;
Ladd and Sheridan, 1987; Hutchinson and Klitgord, 1988; Klitgord et al., 1988). These geometries
result from the fact that the fault-bound margins
of the rift basins subsided faster (collecting thicker
sediment columns) than the non-faulted margins.
This structural and depositional style contrasts
markedly with that of the postrift deposits, which
display principally horizontal (or gently warped),
parallel reflection geometries, having accumulated
on a more uniformly (thermotectonicaUy) subsiding basin floor. The surface separating these two
THE BAHAMA-GRAND BANKS GIGAPLATFORM AND THE JURASSIC PROTO-ATLANTIC SEAWAY
co1~trasting structural and depositional regimes is
easily discernible on many seismic sections as an
angular unconformity (the postrift or "breakup
unconformity"; Falvey, 1974). In addition, synrift
and postrift deposits display distinctly different
patterns of areal distribution. Synrift deposits typically occupy narrow, elongate, isolated basins
(long axes parallel to subsequent spreading axis),
and accumulate almost e_n.ti_relyupon continental
or "transitional" crust. Postrift deposits, on the
other hand, are more widespread (broader basins
due to spreading seafloor and regional thermotectonic subsidence) and are well represented as
deeper water deposits on the oceanic crust.
Along the U.S. Atlantic margin, synrift deposits
can be clearly distinguished from postrift deposits
by both their cross-sectional geometry and their
areal distribution. Seismic sections (Figs. 6-10)
and isopach maps (e.g., Figs. 11-13) show that
most of the section in the COST G-2 well designated by Cousminer and Steinkraus (1988) and
Manspeizer (I 988) as "Liassic-Triassic'" comprises
unequivocal postrift deposits. It follows then, that
if the "Liassic-Triassic" interval in the COST G-2
well is a postrift deposit, it can not also be Triassic
in age, because all avdilable onshore and offshore
data indicate that seafloor spreading did not start
until well into Jurassic time (see discussion below
and references therein). Manspeizer himself (! 985,
1988) has corroborated this, which is, of course,
the reason that he, Cousminer, and Steinkraus
placed the postrift unconformity so high in the
COST G-2 well.
The Cousminer-Manspeizer-Steinkraus interpretation would place the postrift unconformity as
much as 2600 m above where seismic sections and
maps show it clearly to be in the Georges Bank
Basin (and where it was drilled in both the G-I
and G-2 wells; e.g., Grow et al., 1988; Klitgord et
al., 1988; Figs. 5 and 6). Correlation with equivalent rocks in the Baltimore Canyon Trough would
place the Cousminer-Manspeizer-Steinkraus
"postrift unconformity" 5500 m above it~ seismically determined position in the central part of the
Baltimore Canyon Trough (e.g., Benson and
Doyle, 1988; Klitgord et al., 1988; Lawrence et al.,
1990; Fig. 7).
Seismic correlations along intersecting profiles
69
from the COST G-I to the COST G-2 wells in the
Georges Bank Basin are straightforward and uninterrupted by structural complexities (essentially
layer-cake geology, with thickening in the older
section in a downd;~p direction, towards the G-2
well; Fig. 5). The stratigraphic interval that Ascoli
(1982) interprets as Bajocian (on the basis of
ostracodes) in the G-I well (4407-4557 m) correlates with the bottom of the sequence I correlate
as Bajocian-lower Bathonian. The top of my
Bajocian-lower Bathonian sequence is, in turn,
the top of the Triassic section according to Cousminer and Steinkraus (1988) and Manspeizer
(1988). This age disparity (a difference of 35-4',)
m.y.) is reminiscent of the problem with CousmiT~er's "'old" ages in the Baltimore Canyon Troagh
wells (Fig. 4, in column labeled Edson, 1986, 1987).
After considering all the available data, it seems
clear to me that reworking of older Triassic and
Jurassic palynomorphs into younger Jurassic strata
is much more pervasive in the east coast offshore
basins than Cousminer and his colleagues have
recognized. I therefore reject the Triassic interpretation proposed by Cousminer et al., and reaffirm my (and others') earlier conclusion that all
the marine rocks below -,-4000 m in the COST
G-2 well (and equivalent sections in the other
basins of the east coast) are most likely Early to
Middle Jurassic deposits (but are probably limited
to Aalenian and Bajocian-early Bathonian age;
Poag, 1982a, b; Schlee et al., 1985; Manspeizer,
1985; Poag and Valentine, 1988; Klitgord et al.,
198S).
In addition to these principal correlation problems, there are a number of minor correlation
discrepancies of the type to be expected in a
frontier exploration region where well data are
sparse. The use of different fossil groups by
different analysts, for example, causes variable
placement of biozonal and chronostratigraphic
boundaries; many of the benthic fossils used for
correlation are facies-dependent, and may have
different stratigraphic ranges in different paleoenvironmental settings. Other problems include the
dependence on rotary cuttings for most samples
(none of the sequence boundaries wen: encountered in ceres), intense diagenesis of some carbonate sections, difficulty in obtaining correct seismic
C.W. POAG
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THE BAHAMA-GRAND BANKS GIGAPLATFORM
A N D T H E J U R A S S I C P R O T O - A T L A N T I C SEAWAY
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BAJOCIANLOWER BATHONIAN
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25 °
Fig.13. Bajocian-lower Bathonian isopachs. For full caption, see p. 78.
78
C.W. POAG
velocities for depth conversion of seismic profiles,
differences in seismic processing between different
companies and institutions, poer resolution of thin
stratigraphic intervals on multichannel seismic profiles, widespread stripping and reworking of sediments during sea-level falls, assumption of uniform
depositional rates to extrapolate stratigraphic age,
and great variablility of depositional rates from
place to place within a given basin.
These discrepancies can not be resolved using
the data at hand. A much denser array of deeper
wells in appropriate paleodepositionai settings and
with more complete coring is required to appreciably improve the correlations. Thus 1 rely heavily
on modified versions (revisions herein) of the two
standard reference sections i proposed for the
Baltimore Canyon Trough (Poag, 1985, 1987) and
the Georges Bank Basin (Poag, 1982b; Schlee et
al., 1985) as the basis for the following stratigraphic interpretations.
Sedimentary evolution of the gigaplatform
Poag and Sevon (1989) have published a series
of simplified isopach maps which show the postrift
stratigraphic and depositiona! evolution of the
Baltimore Canyon Trough region. These maps are
used here in more complete form (although still
simplified), along with additional ones for the
Georges Bank Basin, Carolina Trough and Blake
Plateau Basin in order to interpret the initiation,
growth, acme and demise of the Bahama-Grand
Banks gigaplatform (United States segment). A
depth section across each trough and basin provides additional data for this discussion (Figs.
6-10). Outside the Georges Bank Basin, the oldest
depositional sequences (pre-Bathonian) are interpreted entirely on the basis of seismostratigraphy
and seismic facies analysis, because only in the
Georges Bank Basin have wells penetrated deep
enough to sample these sequences directly (e.g,,
COST G-2 well). As noted above, the early postrift
stratigraphic succession described herein differs
significantly from that presented in my earlier
papers (Poag, 1982a, b, 1985, 1987), mainly because of constraints on the earliest reasonable age
for initial postrift sediments, the release of industry
data, and the availability of Ascoli's (1982, 1983)
biostratigraphic revisions. The maximum possible
age of the initial postrift sequence is still controversial, but most workers now agree that seafloor
spreading (and postrift deposition) began at approximately 165-190 Ma (see Klitgord and
Schouten, 1986). My current seismostratigraphic
analysis, along with Ascoli's (1982, 1983) revised
borehole data from the Georges Bank Basin, together with unpublished industry biostratigraphy,
saggests that a 190-187 Ma date (late Toarcianearly Aalenian) is probably most realistic, a conclusion also reached by Jansa (1986) and Benson and
Doyle (1988) (see also Lawrence et al., 1990).
Furthermore, the Toarcian-Aalenian age agrees
closely with a radiometric date of 183 + 12 Ma
(Jansa, 1986) for evaporites presumed to have
precipitated in the Baltimore Canyon Trough during the transition from rifling to seafloor spreading. Nevertheless, because of continued uncertainty
regarding the age of the stratigraphic unit underlying the dated Bajocian unit, I shall use a question
mark when referring to this unit [ = Aalenian(?)].
Fig. !i. lsopach map of evaporite beds [Toarcian(?)-Aalenian(?)]deposited during rift-drift transition along U.S. Atlantic margin.
Contours represent equal two-waytravel time at 0.1 s intervals: unnumbered,they show distribution and relative thickness only.
Small dots indicate positions of diapiric structures. ECMA East Coast Magnetic Anomaly, which marks approximale edge of
African plate during this interval. Dashed line around profile 32 indicatesextent of evaporites in Carolina Trough accordingt,j
Dillon et al. (1982). Positionsof fivedepth sections(Figs. 6-10 herein)indicated.
=
Fig. 12. Isopach map of Aalenian(?)sequence,the first postrift sequencedeposited along U.S. Atlantic margin. Contours represent
two-waytravel time at 0. I s intervals. Shelfand slopecarbonate tracts stippled. Arrows indicatemain routes of siliciclasticsediment
supply. Positionsof fivedepth sections(Figs. 6-10 herein)indicated. BlakePlateau Basin still in rifting stage.
Fig. 13. isopach map of Bajocian-lowerBathonian sequencealong U.S. Atlantic margin. Contours represent two-waytravel time
at 0,1 s intervals. Shelfand slope carbonate tracts stippled. Arrows indicatemain routes of siliciclasticsediment supply. Position
of fivedepth sections(Figs. 6-10 herein) indicated.
THE BAHAMA-GRAND BAI,K S GIGAPLATFORM AND THE JURASSIC PROTO-ATLAN'IIC SEAWAY
Transition from synrift to postrift depostion
Shallow-water carbonates were probably only a
minor part of the initial postrift depositional sequence in the study area, if they were present at
all. The transition from continental rifting to drifting along the U.S. Atlantic margin was initiated
by a period of intense erosion [late Toarcian(?)early Aalenian(?)], which smoothed the irregular
topography presumed to have developed around
the Triassic and Early Jurassic rift basins. This
erosion formed a distinctive angular unconformity
across the rift basins (e.g., Hutchinson and Klitgord, 1988; Figs. 6-10). The transition was completed in the early Aalenian(?) when horizontally
bedded evaporites (halite and anhydrite) accumulated in isolated, elongate depressions (Fig. !1).
The early evaporite-filled depressions of the
Georges Bank Basin formed five small depocenters
or subbasins ( ~ 35-40 km maximum lateral dimension), which held 0.8-1.0 km (~0.2-0.3 s) of
bedded evaporitic strata. Diapiric structures are
rare in this basin, but one has been located in the
northernmost subbasin, where it pierces ,-.,6 km
(3.3 s) of the overlying sedimentary section. In
addition, several thickened salt swells appear to
be present just above the postrift unconformity
(Schlee and Fritsch, 1982; High, 1985).
The structurally high Long Island Platform separated the Georges Bank evaporite basin from the
Baltimore Canyon Trough evaporite basin by a
distance of ~ 330 km (Fig. ! 1). The evaporite basin
in the Baltimore Canyon Trough covers ~ 15000
km 2 (,-,300 x 50 km), contains at least 2-3 km
(0.5-0.7 s) of bedded evaporites, and rims the
landward edge of the East Coast Magnetic Anomaly (ECMA), which approximates the continentocean boundary (Klitgord et al, 1988; Fig. 7). The
thickest depocenters surround the Schlee Dome
(previously cited informally as the "Great Stone
Dome"), an igneous intrusion near the northwestern edge of the evaporite depression (Fig. 2; Grow,
1980; Lippert, 1983; Poag, 1987; Grow et al., 1988;
Jansa and Pe-Piper, 1988). A few prominent diapirs
rise as much as 5-6 km into the overlying section,
and halite was drilled on the flank of Schlee Dome
(Houston Oil and Minerals 676 No.! well; Fig. 3).
The Carolina Trough evaporite basin is sepa-
79
rated from the Baltimore Canyon evaporites by a
distance of ~ 300 km across the intervening Carolina Platform (Fig. 1I). The Carolina Trough evap~
orite basin is similar in length and position (relative
to the ECMA) to the Baltimore Canyon Trough
evaporite basin, but it is much narrower (,-, 20 km
wide). The evaporite section of the Carolina
Trough is also thinner (maximum ~ 1.6 km, 0.4
s), and forms many more diapiric structures. At
least 26 diapirs, identified in the Carolina Trough
from seismic profiles and sidescan sonographs of
the seafloor have been interpreted as evaporite
structures (Dillon et al., 1982). One ;additional
evaporite sequence ( ~ 20 x 20 km) appears to have
filled a small graben on the landward margin of
the Carolina Trough off Cape Hatteras.
Equivalent evaporites are presumably present
also in the Blake Plateau Basin (Dillon et al.,
1982), but according to Klitgord and Schouten
(1986) and Klitgord et al. (1988), rifting was still
going on in the southern basins, so any Toarcian(?)-Aalenian(?) evaporites in this region would
have been part of the synrift section. Early to
Middle Jurassic(?) evaporites have been drilled in
the Bahama Basin (Tator and Hatfield, 1975), and
a few Late Jurassic(?) domed features seen on
seismic reflection profiles have been interpreted as
salt structures (Ball et al., 1968; Sheridan et al.,
1981). In general, however, evaporites in this region are difficult to identify on seismic profiles
because of the thick (2-4 km) shallow-water carboxate section overlying them.
Initiation of shallow-water carbonate deposition:
Aalenian(?) sequence ( ~ 186-183 Ma)
The first major postrift sedimentary sequence
deposited on the U.S. Atlantic margin probably
accumulated during Aalenian time. The principal
east coast basin (the largest, and with the thickest
sediment column) was the Baltimore Canyon
Trough, which at this time formed a broad, 45,000
km 2 depression (at 100x450 km, approximately
three times larger than the preceding evaporite
basin) off the coast of what now is New Jersey to
North Carolina (Figs. 7, 8 and 12)~ Deposition
rates in the Baltimore Canyon Trough (calculated
from sediment volumes and using the Decade of
80
North America (DNAG) time scale; Palmer, 1983;
Kent and Gradstein, 1986) were higher (~25,000
km3/m.y.; Poag and Sevon, 1989) during this
interval than at any other time prior to the Neogene. As a result, very thick depocenters developed
on the middle and outer part~ of a di,stinct, 100kin wide, continental shelf. The greatest thicknesses, however, reached ,,. 5 km (I.6-2.2 s) along
the continental slope off New Jersey. These depocenters appear to have been fed mainly by ancient
equivalents of the Schuylkill and Delaware rivers
bringing terrigenous sediments from the Central
Appalachian Highlands (Poag and Sevon, 1989).
Several thick siliciclastic lobes spread out fiver the
incipient continental rise forming in the SohmHatteras Trough (on oceanic crust) and lapped
onto the western flank of the young mid-Atlantic
spreading center. Seismic facies analysis suggests
that the bulk of the Aalenian(?) sediments are
siliciclastics. The presence of some high-amplitude
reflections in the middle of the sequence, however,
is possible evidence that a linear trend of four
narrow (20 kmx70-80 km) carbonate tracts
(oolite shoals?) formed on the middle-to-outer shelf
during this time (Fig. 12) but did not reach the
shelf edge. Lawrence et ai. (1990) suggested that
halite may be interbedded with the carbon~,.te in
these tracts. In the late deposit~onai history c.f this
sequence, carbonate may have formed a narrow,
segmented platform (5-10 km wide) at the shelf
edge in some locations (e.g., line 25), but it is
difficult to substantiate the configuration and lithofacies of this deeply buried feature.
To the north, separated from the Baltimore
Canyon Trough by the .,,200-km-wide Long Island Platform, the Georges Bank Basin also received a sequence of probable Aalenian deposits
as the first postrift accumulation (Fig. 12). The
Aalenian(?) Georges Bank Basin was much smaller
(100 x 180 km), however, than the coeval Baltimore
Canyon Troughs and the sediment column was
considerably thinner (0.8-1.0 km, 0.2-0.3 s). Carbonate deposits (known from drilling and seismic
facies) appear to have composed about half the
basin-fill, and were concentrated on the middle to
outer shelf in the northern two-thirds of the basin.
The carbonates constructed a broad (65 km wide),
shallow-water, shelf platform (dolomites, lime-
c.w. POAO
stones, evaporites and calcareous shales in the
COST G-2 well; Scholle and Wenkam, 1982; Figs.
3 and 5), with no obvious mounded or reefqike
bu!ldups. Two primary siliciclastic sources appear
to have built broad delta-like depocenters landward of the carbonate tract. Very little Aalenian(?)
sediment was deposited beyond the shelf edge in
the Georges Bank Basin, although a siliciclastic
slope apron ( -,, 20 x 40 kin) appears to have formed
near the southern end of the basin, and a carbonate-dominated slope apron ( ~ 15 x 60 kin) developed seaward of the carbonate platform (Fig. 12).
Aalenian(?) deposition was continuous southward from the Baltimore Canyon Trough into the
,ong, narrow (,-,20-40 km x 380 kin) Carolina
1rough down to about Cape Fear, North Carolina
(Fig. 12). No large siliciclastic depoceaters occupied the Carolina Trough, but one major elongate
depocenter (-,, !.2 kin, 0.9 s thick) developed on
the upper continental rise in the Sohm-Hatteras
Trough. Siliciclastic deposition appears to have
prevailed over most of the Carolina Trough
(seismic facies) and adjacent parts of the SohmHatteras Trough, but high-amplitude reflections
landward of the shelf edge (off Cape Fear) suggest
that a narrow ( ~ 20 x 70 kin) carbonate platform
existed in that area during Aalenian(?) time.
The presence of Aalenian(?) deposits in the Blake
Plateau Basin is difficult to ascertain because the
sequence can not be traced from the Carolina
Trough. However, near the southern end of the
basin, along the southeastern l~art of seismic profiles TD-3 and FC-1 (Fig. 3), there appears to be
a sedimentary section between basement and the
inferred Bajocian-lower Bathonian sequence,
which could represent the Aalenian(?) sequence.
This "Aalenian(?)" sequence is confined to a narrow depression (~ 150 x 50 kin) in the basement
in which two small depocenters formed ( ~ 500 m
thick, 0.5-0.6 s). Some high-amplitude, continuous
reflections in the middle of the sequence suggest
that either carbonates or evaporites were deposited
during part of Aalenian(?) time. Indeed, Dillon et
ai. (1982) interpreted the entire interval as a salt
layer, but the predominance of discontinuous, lowamplitude reflections suggests to me that the bulk
of the deposit is siliciclastic. The Blake Plateau
Basin had not yet entered the drifting stage, so the
THE BAHAMA.GRAND BANKS GIGAPLATFORM AND THE JUPASSIC PROTO-ATLANTIC SEAWAY
Aalenian(?) sequence here was still part of the
synrift deposition (Klitgord and Schouten, 1986;
Dillon and Popenoe, 1988; Klitgord et al,, 1988).
Development of southern "megabank": Bajocianlower Bathonian sequence ( ~ 183--172 Ma)
By the end of Bajocian-early Bathonian time,
deposition in the U.S. Atlantic margin basins was
nearly continuous for ~ 3000 km, from Georges
Bank to the Bahamas (Fig. 13). The only depositional gap was a 50-60 km strip along the Long
Island Platform. Carbonates were widespread in
the Georges Bank and Blake Plateau basins, but
were sparse in between, having been suppressed
there by greater supplies of siliciclastic detritus.
The Georges Bank Basin had nearly doubled ~n
area (~40-160x350 km), and most deposition
still took place on the ci~ntinental shelf, although
a narrow continental slope and rise (18-20 km
wide) had begun to develop. The seaward half of
the shelf was occupied by a moderately broad
(50-75 km) carbonate platform with a maximum
thickness of ~ 1.5 km (0.7 s). The platform was
bifurcated, however, by a siliciclastic wedge (fed
by source terrains in the New England Appalachian highlands of Maine) that extended across
the central part of the shelf and spread across the
incipient continental rise (Fig. 13). The northern
segment of the Bajocian-early Bathonian platform
on Georges Bank is characterized by a series of
isolated mounds, patch reefs?, and oolite shoals;
six have been identified on seismic profiles, and
five have been drilled (High, 1985; Mattick and
Libby-French, 1988). One high-amplitude lenticular feature near the center of the basin turned out
to be a bed of halite (drilled in the Exxon 955-1
well; Figs. 3 and 13). Along the outer edge of this
northern platform is a low, raised rim, which has
the configuration of a barrier reef ( ~ 110 km long;
Figs. 7 and 13). Narrow, carbonate-rich, slope
aprons (500-800 m (0.4-0.7 s) thick) developed
just seaward of the platform edge. Siliciclastic
debris that reached the Sohm-Hatteras Trough
seaward of Georges Bank spread out as a bilobate
submarine fan complex and filled in a series of
basement troughs, whose long axes parallel that
of the mid-Atlantic spreading center.
81
In the Baltimore Canyon Trough, the continental shelf was comparable in area (-,- 100 x 450 kin)
to that of the Georges Bank Basin, but the SohmHatteras Trough in this area had expanded to
about 5-6 times (,,-150 km) the width of its
Georges Bank counterpart (Fig. 13). Bajocianlower Bathonian deposits in the Baltimore Canyon
Trough appear to have been domiuantly siliciclastics (seismic facies) derived from the central Appalachian Highlands and Adirondacks which built
thick depocenters (1.5-2 kin, 0.5-0.7 s) on the
middle part of the continental shelf (Fig. 13). Siliciclastic detritus reached the shelf edge at many
points along the outer margin of the trough and
formed slope aprons as *.hick as 2 km (0.8 s).
The presence of high-amplitude, continuous
seismic reflections along several profiles in the
Baltimore Canyon Trough suggests that narrow
carbonate platforms (100-115 km long, 15-25 km
wide) developed at several places along the outer
continental shelf and at the shelf edge (Fig. 13)
(the thickness of the carbonate platforms was
~ 1.5-2 km (0.7-1.2 s); see also Lawrence et al.,
1990). At three sites, small, mounded, carbonate
buildups, or reefs, were constructed landward of
the shelf edge during the latter part of Bajocianearly Bathonian time.
The Bajocian-early Bathonian continental shelf
narrowed southward from ~ 100-130 km in the
Baltimore Canyon Trough to ~ 50-75 km in the
Carolina Trough, where two siliciclastic depocenters dominated shelf deposition (Fig. 13)--one seaward of what is now Cape Lookout ( ~ 2 km (1.0
s) thick at the shelf edge), and the other south of
Cape Fear ( ~ 1.8 kin (0.7 s) thick). Elongate slope
aprons fringed these shelf depocenters, and a narrow abyssal plain (10-40 x 300 km) developed in
the Sohm-Hatteras Trough from Cape Fear to
Cape Lookout.
Small carbonate platforms (10-20 x 60 km) appear to have developed at two places in the Carolina Trough during Bajocian-early Bathonian
time, one off Cape Lookout and one off Cape
Fear (Fig. 13). The maximum thickness o.? the
platforms is ~ 1.8-2 km (0.6-1.1 s).
Farther south, in the Blake Plateau Basin, the
physiography and lithofacies distribution appear
to have been considerably different from that in
82
the northern basins. In particular, the continental
shelf widened markedly to 350-450 kin, and most
of this huge shelf area ( ~ 160,000 km 2) appears to
have received mainly carbonate deposition, forming a "megabank" (Fig. 13). Thus, the northern
margin of the Blake Plateau Basin marked a
significant physiographic, depositional and perhaps climatic boundary between carbonate-dominated and siliciclastic-dominated shelf regimes,
which lasted for ~65 m.y. into Aptian-Albian
time (Benson et al., 1978a; Ladd and Sheridan,
1987; Austin et al., 1988a), In the southern part
of the Blake Plateau Basin, a broad, relatively flattopped, carbonate platform stretched essentially
from the shoreBne to the shelf edge. Subtle variations in thickness of the platform sediments
(average 3-3.5 kin, 0.8-0.9 s) appear ~o have been
caused mainly by topographic relief on the underlying basement surface. A few small (2-5 km diameter) mounded buildups, or reefs, are scattered
around the platform (twelve have been identified
on seismic profiles; Fig. 13), and two larger (I 5 x 50
km and 3 x 20 kin) shelf-edge reefs reached thicknesses of ~ 4 km (!.3 s). A large (90 x 150 kin),
elevated (karstic?) bank appears to have developed
on the inner edge of the Blake Plateau Basin along
the central part of the Florida peninsula (Shipley
et ai., 1978~ Popen6c et al., 1984).
A single elongate swath (200 x 75 kin) of siliciclastic sediments appears to have entered the
northern part of the Blake Plateau Basin (Fig. 13)
from a source in the southern Appalachian Highlands and the surrounding coastal plain. The maximum thickness of this siliciclastic wedge is near its
termination on the middle of the continental shelf
(~ 3.5 kin, i.1 s).
~.W. POAG
The original Bajocian-early Bathonian shelf
edge appears to have formed a carbonate escarpment, which extended ~ 1000 km from the Blake
Spur to the southern termination of the Bahama
Platform (approximately at modern 25°N). Subsequently, however, the escarpment has retreated as
much as 10-30 km from its original position
between seismic profiles TD-5 and TD-2 (Figs. 10
and 14; Paull and Dillon, 1980; Sheridan, 1981;
Freeman-Lynde et al., 1981; Dillon et al., 1985;
Freeman-Lynde and Ryan, 1985). A thinned, narrow remnant of this shelf-edge retreat presently
forms a flat-topped terrace, which is buried by
Berriasian-Valanginian and younger abyssal deposits. If this stratigraphic relationship is correct,
the terrace appears to have been formed during
Jurassic time, rather than in the Tertiary, as asserted by Paull and Dillon (1980), or the Cretaceous, as interpreted by Sheridan (1981) and
Sheridan et al. (1988). The Bajocian-lower Bathonian section is exposed along the face of the
Blake Escarpment along seismic profile FC-1 and
its vicinity (Fig. 14). Ladd and Sheridan (1987)
have suggested that Middle Jurassic strata (perhaps including a Bajocian-lower Bathonian section) may be present farther south in the Bahama
Basin beneath Andros Island and the southeastern
side of Great Bahama Bank. Klitgord et al. (1988),
however, believed that the oldest postrift sediments
in the Bahama Basin are probably Oxfordian.
Platform expansion in the northern basins: upper
Bathonian-Callovian sequence ( ,,~172-163 Ma)
and Oxfordian sequence (163-156 Ma)
The h~story of carbonate platform development
along the U.S. Atlantic margin following Bajo-
Fig. 14. Shelf-edge stratigraphy and morphology along four seismic profiles across Blake Escarpment (Blake Plateau Basin). Seismic
reflections traced for inferred Kimmeridgian-Tithonian and Berriasian-Valanginian sequences; Kimmeridgian-Tithonian sequence
stippled. Diagonal shading indicates Cretaceous and Cenozoic sedimentary rocks lying above studied section. Random dash pattern
at bottom of seismic sections represents continental basement; "V" pattern represents oceanic basement. VE= vertical exaggeration.
Note extensive faulting on TD-3 and TD-2 and eroded Bajocian-lower Bathonian terrace on TD-5 and TD-3. Note also different
scale for line FC-1. Dots along escarpment of line TD-3 indicate dive samples reported on by Dillon et al. (1988). Lowest sample
(at ~ 3.3 s) is the only one whose nannofossil age (Hauterivian-Valanginian) does not agree with seismostratigraphic interpretation
shown here. However, because Dillon et al. reported two displaced samples (ages too young and are not shown on figure) from
higher in the inferred Kimmeridgian-Tithonian section, and because of the faulted nature of the platform edge on this line, I infer
that the lowest dive sample has also been displaced from its original stratigraphic position. A - A = Aptian-AIbian; B-B= Bajocianlower Bathonian; B - C r u p p e r Bathonian-Callovian; B-V=Berriasian-Valanginian; K-T=Kimmeridgian-Tithonian; OX=
Oxfordian.
THE BAHAMA-GRAND BANKS GIGAPLATFORM AND THE JURASSIC PROTO-ATLANTIC SEAWAY
83
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C.W. POAG
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THE BAHAMA-GRAND
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86
clan-lower Bathonian deposition is largely one of:
(1) progressive seaward progradation of the shelf
edge (interspersed with intervals of aggradation
and regressive hiatuses), (2) expanded areal distribution of carbonates on the shelf, slope and upper
continental rise, and (3) heightened shelf-edge relief. These changes are most notable in the northern
basins, as typified by the Baltimore Canyon
Trough, so for the sake of brevity, I shall confine
description of the late Bathonian to Oxfordian
sedimentary history to the record there.
Carbonate deposition in the Baltimore Canyon
Trough accelerated during late Bathonian-Callovian time. The shelf-edge carbonate tract was now
nearly continuous from the Bahamas to the Grand
Banks, forming a > 5000-km-long gigaplatform.
Continuous, high-amplitude reflections are present
in an aggrading succession that comprises the
lower half of this sequence along the outer part of
the paleoshelf off New Jersey (Fig. 15). During
deposition of the upper half of the sequence, the
shelf edge prograded dramatically ( ~ 25 kin) seaward of its early late Bathonian position (Fig. 7).
The presence of numerous mounded seismic reflections suggests that an elevated reef system rimmed
the shelf edge during the later part of the late
Bathonian-Callovian interval [=Oxfordian(?) of
Lawrence et al., 1990]. The upper carbonate strata
of this sequence (="Wilmington Platform" of
Prather, 1988, and Schlager, 1989) have been
drilled by some of the deeper wells in the Baltimore
Canyon Trough (e.g., Tenneco 642-2 well; Bielak,
1986; Fig. 3). The maximum thickness of the gigaplatform near the shelf edge in this depositional
sequence is --,4 km (I.8 s).
Landward of the gigaplatform, a series of four
elliptical to subcrescentic, siliciclastic depocenters
was present. These depocenters appear to have
represented outer-shelf deltas, whose seaward advance was driven by a significant acceleration in
silictclastic accumulation rate (increase from 9000
to [4,000 kma/m.y.; Poag and Sevon, 1989). Siliciclastic sediment from these deltas appears to have
C.W. POAG
transited the shelf edge mainly through four large
submarine canyons (Fig. 15), and formed sediment
pondo on the widening continental rise. Their distal
margins onlapped the western flanks of the midAtlantic spreading center where they filled narrow
troughs in the furrowed basement surface.
During Oxfordian time siliciclastic deposition
decreased somewhat (from 14,000 to I 1,000 kin3/
m.y.) in the Baltimore Canyon Trough, and
encroached less rapidly on the benthic carbonate
producers at the shelf edge. This slowed deposition
was in part a consequence of diminished thermotectonic subsidence of the basin as sediment loading began to dominate subsidence on this passive
continental margin (Watts and Steckler, 1979; Poag
and Sevon, 1989). In addition, increased climatic
aridity (Hallam, 1984; 1986) may have slowed
delivery of terrigenous detritus to the basin.
Carbonate production appears to have developed a series of narrow, elongate depocenters that
stretched nearly 400 km along the Oxfordian shelf
edge from what now is Long Island to Virginia
(Fig. 16). The principal carbonate depocenters
were ~ 1.5 km (0.8-0.9 s) thick, and the gigaplatform reached widths of 20-25 kin. The carbonate
tract reached a total seaward progradation of ,,, 50
km off New Jersey (Fig. 7). Landward of the
gigaplatform a large mid-shelf delta complex built
up off New Jer~-,y, fed by the ancient Hudson,
Delaware and Schuylkill rivers which drained the
Adirondack and central Appalachian highlands
(Fig. 16; Poag and Sevon, 1989). This delta was
subsequently dissected into four small depocenters,
from which terrigenous detritus moved over the
shelf edge through pre-existing submarine canyons,
eventually filling the canyons. Thick (,,, ! km, 0.7
s) slope aprons were again prominent, and several
moderately large (50-75 km wide, 100-150 km
long) submarine fan complexes spread into the
Sohm-Hatteras Trough. The oldest clearly defined, very large [100 km wide, 0.4-1 km (0.3-0.7
s) thick] submarine fan complex yet discovered in
the trough was constructed seaward of the south-
Fig. 17. lsopach map of Kimmeridgian-Tithonian sequence along U.S. Atlantic margin. Contours represent two-way travel time
aLO~I s intervals. Shelf and slope carbonate tracts stippled. Arrows indicate main route of siliciclastic sediment supply. Diagonal
shading indi~:ates sequence missing or too thin to resolve on seismic lines. Position of five depth sections (Figs. 6-10 herein)
indicated.
THE BAHAMA-GRAND BANKS GIGAPLATFORM AND THE JURASSIC P R O T O - A T L A N T I C SEAWAY
87
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45 °
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IMENT
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POND
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Present Sea'Floor =1"2 Sec.~~a!~
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VE=~4:I
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:;::~ .........
~v~; rHONIAN'CALLOVIAN
Fig. 18. Shelf-edge stratigraphy and morphology along four seismic lines across Baltimore Canyon Trough segment of gigaplatform
showing pinnacled barrier reef and overlying Berriasian-Valanginian siliciclastics. Seismic reflections traced for KimmeridgianTithonian and Berriasian-Valanginian sequences: Kimmeridgian-Tithonian sequence stippled. Diagonal shading indicates Cretaceous
and Cenozoic sedimentary rocks overlying studied section. Shell wells projected to lines 6 and 220. VE= vertical exaggeration.
Uninterpreted version of line 6 can be seen in Grow et al. (1979).
89
' "W
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II
- - ~ ' P ' ~ A ' s ~ P ~ r ¢.~ ~". /
~.,
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5
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SeaRoot
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~,v / ~
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. . . . . .
7
~RRIASIA~-"
s~l
--,.
I
::,:;,,~-~:.:~:::::;r:~¢~rs;..-,:;;.~::~:::.:.-;:.
-.,:
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.......,~::,~..,:~:i~i-. ......... ~%.'~',~ ,
IA
'::'::.:::~>: ~.:,:':' :~ ':-,:~xa
-4" ~-.~ U~,r~e e,~r,,~,'~""~
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-LINE
4
,
>...,
_ o~O~o~. '~:~'-,::;"~'~
~:'u "-''~-'2Z
Fig, 19. Shelf-edge of New England segment of gigaplatform, For full caption, see p.90.
-'1
/
,
/
90
ernmost submarine canyon, and stretched at least
350 km toward the mid-Atlantic spreading center
(wbich was outside the mapped area by this time;
Fig. 16). Abyssal sediments of the Oxfordian sequence have been sampled at DSDP Site 105 (100
km south of the southeastern corner of Fig. 16;
HoUister et al., 1972), where they consist of mixed
variegated calcareous claystones, marls, and clayey
limestones. These characteristics are evidence of
the long-distance transport of terrigenous sediment
within the Sohm-Hatteras Trough.
Maximum development of the gigaplatform:
Kimmeridgian- Tithonian sequence ( .,. 156-144
Ma)
The acme of gigaplatform development and reef
growth on the U.S. Atlantic margin spanned the
late part of the Jurassic (Kimmeridgian and Tithonian time). Details of this acme in the Baltimore
Canyon Trough region have been presented by
Meyer (1989), who emphasized the stratigraphy,
lithofacies and different stages of platform construction (see also Edson, 1986, 1987, 1988; Erlich
et al., 1988, 1990; Prather, 1988, 1991; Lawrence
et al., 1990). For the Georges Bank Basin more
generalized treatments of Kimmeridgian and Tithonian stratigraphy and deposition have been
presented by Ascoli (1982), Poag (1982a, b) and
Schlee and Fritsch (1982).
The isopach map, seismic sections, and boreholes of the Baltimore Canyon Trough (Figs. 7
and i 7) show that the lower part of the Kimmeridgian-Tithonian gigaplatform consis,ts of shallowwater carbonates that continued to extend the shelf
edge seaward along a -,-1000-kin-long segment
from Massachusetts to Virginia. The carbonate
rim had now prograded -,-55 km onto the oceanic
crust off New Jersey (maximum progradation:
c.w. POAG
Fig. 7). The upper half of the KimmeridgianTithonian sequence also consists of shallow-water
carbonates at the shelf edge, but aggradational
sedimentation dominated to produce a thick, elevated rim topped by a discontinuous, pinnacled,
barrier reef system. The reef crest towered as much
as 3 km above the floor of the Sohm-Hatteras
Trough in some places, and as much as 150 m
above the adjacent backreef shelf (Fig. 18). However, the gigaplatform was relatively narrow in
this region, ranging from ~10 to --30 km in
width.
Landward of the gigaplatform were a series of
siliciclastic deltas (Fig. 17). One large system of
coalescing deltas, for example, extended 250 km
from northern New Jersey to Maryland, and was
more than 100 km across (dip direction). Separate
delta lobes were fed by the ancient Hudson, Delaware, Susquehanna and Potomac rivers, carrying
detritus from the northern part of the central
Appalachian Highlands (Poag and Sevon, 1989).
Siliciclastic accumulation rates had dropped from
II,000 to 6000 kma/m.y, by this time, however
(Poag and Sevon, 1989), which slowed encroachment on the shelf-edge carbonate communities and
allowed the platform to aggrade.
Along the Kimmeridgian-Tithonian continental
slope, carbonate-rich aprons received detritus from
the reef tract, and several small carbonate-enriched
submarine fan complexes (25-50 x 100 km) formed
on the upper continental rise (Fig. 17). The most
prominent depositional features in the Sohm-Hatteras Trough, however, were four large siliciclastic
submarine fan complexes, which derived their constituents mainly from the shelf delta systems. Distal
sediments from these four fan complexes reached
the lower continental rise 300-400 km from the
shelf edge, having bypassed the shelf edge through
prominent gaps in the barrier reef. Some of the
Fig. 19. Shelf-edge stratigraphy and morphology along four seismic lines across New England segment of Bahama-Grand Banks
gigaplatform. Seismic reflections traced for Kimmeridgian-Tithonian and Berriasian-Valanginian sequences: KimmeridgianTithonian sequence stippled. Diagonal shading indicates Cretaceous and Cenozoic sedimentary rocks lying above studied section.
Pattern below Bajocian-lower Bathonian sequence represents basement. VE=vertical exaggeration. Lines 23 and 5 cross Long
Island Platform and show absence of Berriasian-Valanginian siliciclastic sediments on foreslope of Kimmeridgian-Tithonian reef
( = drowning unconformity). Lines 213 and 4 cross Georges Bank Basin and show Berriasian-Valanginian siliciclastics burying entire
surface of underlying gigaplatform and reef. Uninterpreted versions of lines 5 and 213 can be seen in Grow et al. (1979) and Schlee
et al. (1988) respectively.
91
COST GE-I
SERIES OR
STAGE
(POAG 8 HALL,
1979),
SERIES OR
DEPTH
STAGE
(VALENTINE
1979
0
SEA FLOOR
PLEISTOCENE
UP PLIOCENI NEOGENE
MID. MIOCENE
MID. OLIGOCENE
LOWER
OLIGOCENE
OLIGOCENE
UPPER
EOCENE
MIDDLE
EOCENE
LOWER
EOCENE
LITHOLOGY
-0
NOT SAMPLED
SHELL FRAGMENTS
AND TERRIGENOUS
3LASTIC SEDIMENT:
UPPER
/ OCE~','E
POAG
(Herein)
ARGILLACEOUS
LIMESTONE
WITH CHERT
.....
MIDDLE
EOCENE
SHALE AND
LIMESTONE
UP PALEOCENE LO EOCENE
gAESTRICHTIAN PALEOCEN,E
I.=J
.._F- --T_
CAMPANIAN
CAMPANIAN _
! SANTONIA~
__
_L,.
.L_
.-L
CONIACIAN
L
_F
TURONIAN
--]_.~
T----
z
,¢Z
"1-
.J_
SANTONIAN
CONIA~IAN
LOWER
TURONIAN
REVISED
STRATIGRAPHY
ARGILLACEOUS
CHALK AND
CALCAREOUS
SHALE AND
CLAYSTONE
0
Z
>-
--r
._j
CALC. SANDSTONE
SILTSTONE,SHALE
ALBIAN
~l'lL
e
g
,,,~
--z
]~
I / I/1
~ l , .i ,
0
i
-7
|
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tT'l
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,7" ~
laJ
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__:::--:
~
,-
/v~rL~/vv~A~-~vvvv~
I_)
- - ~ _ T-------_
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APTIAN
LIMESTONE,SHALE,
ANHYDRITE
S
-- 0
_
. ~
X
~.~
-:-_.-o"
RED AND GRAY
SANDSTONE,SHALE
MINOR LIMESTONE
ANHYDRITE
COAL, DOLOMITE,
PYRITE
.
I ::'
BARREMIAN
(EXTRAPOLATED
: .I
Z--/-7-7
.....
8ERRIASIANVALANGINIAN
KIMMERIDGIANTITHONIAN
I A
, -i(i
HAUTERIViAN
[EXTRAPOLATED)
VALANGINIAN
(EXTRAPOLATED
UPPER
DEVONIAN
CONGLOMERATE
-II
-,2 f
-13
ARGILLITE
WITH TRACHYTE
BASEMENT
4
Fig. 20. Revised stratigraphy of lower section drilled in COST GE-I well, in shoreward part of Blake Plateau Basin (= Southeast
Georgia Embayment). Modified from Dillon and Popenoe (1988). Reinterpretation based on seismic extrapolation from Georges
Bank Basin and Baltimore Canyon Trough wells, Great lsaacs well in Bahama Basin, and dive samples on outer end of line TD-3.
Lowest definitive fossils identified in GE-I well [Aptian palynomorphs (R. Christopher and J. Bebout, in Poag and Hall, 1979)]
came from cuttings sample at 2259 m.
c.w. POAO
92
.,
?
-~¢
t
/
/
45*
500
o
KM
\
.
.
t
k
:.~ CAPE
COD
DELTA
%°%~4,+~ •
:.~.'
FAN
COMPLEX
40°
.
"6
AN CITY
COMPLEX
OSDP603
HAT'rE~
•k" ~oP ,o,
35°
"FAN COMPLEX
p :SLOPE APRON
30°
'0,~
SILICICLASTIC CHANNEL-FILL
REEF'?'~.-
-BLAKE
SPUR
(DSDP SITES 390, 39~)
TD'4
"\
"-"POS
ISOP 534
!
\
...
IISAACS
I We:LL
25"
CONTOURITES
°J
l
/"
%
BERRIASIANVALANGINIAN
~.~
t~ANOROS
*E, L - -
~
"L
OSOPI00
THE BAHAMA-GRAND BANKS GIGAPLATFORM AND THE JURASSIC PROTO-ATLANTIC SEAWAY
deepest parts of the continental rise in this area,
however, were still accumulating mainly carbonates, as shown by the presence of fine-grained
pelagic limestones in the Kimmeridgian-Tithonian
section at DSDP Site 105 (Hollister et al., 1972).
To the north of the Baltimore Canyon Trough,
along the outer edge of the Long Island Platform
and Georges Bank Basin, the shelf-edge reef system
can also be identified on seismic profiles (Fig. 19),
but it has not been directly sampled by drilling.
The gigaplatform was significantly wider here
( > 40 km), but the associated reef system was more
segmented and discontinuous than off New Jersey;
its general morphology, architecture and lithofaties, however, are similar (Fig. 17). Slope aprons
(presumably carbonate-rich~, were associated with
the reef fronts in this area a~ well. Two broad gaps
in the gigaplatform separated the Long Island and
Georges Bank segments and appear to have allowed siliciclastic sediments to pass from outershelf delta systems onto the continental slope and
rise. The resultant submarine fan complexes are
not nearly as large or distinctive as those in the
New Jersey region. Another siliciclastic shelf-delta
system developed on the northern edge of the
Georges Bank Basin during KimmeridgianTithonian time, but its sediments appear not to
have reached the Sohm-Hatteras Trough (Fig. 17).
Extension of the gigaplatform and shelf-edge
reef system into the Scotian Basin has been described by several authors (Jansa and Wade, 1975;
Eliuk, 1978, 1981, 1988; Jansa, 1981; Eliuk eta!.,
1986; Jan~a et al., 1988b). The general structural,
stratigraphic and lithofacies relationships are similar to those along the U.S. margin. Jansa et al.
(1988a) have suggested that the gigaplatform did
not end at the Grand Banks. A segment of Kimmeridgian-Tithonian platform cored at ODP Site
639 on the Galicia Bank margin (Boillot et al,
1988) was interpreted by Jansa et al. (1988a) as a
connector between the Grand Banks platform and
that of the Lusitanian and Algarve basins, which
93
rimmed the eastern end of the proto-Atlantic seaway during the Late Jurassic.
South of the Baltimore Canyon Trough a distinctly different stratigraphic and depositional history is recorded by the Kimmeridgian-Tithonian
sequence. As in the Bajocian-early Bathonian
interval, and in a,i subsequent sequences, the most
obvious difference is the greater expanse of the
carbonate platform, which widened progressively
southward until it occupied the entire continental
shelf of the Blake Plateau and Bahama basins
(Fig. 17). In fact, in Kimmeridgian-Tithonian
time, an enormous carbonate megabank formed
the southern part of the gigaplatform, extending
,,-900 km from the the Bahama Basin to the South
Florida Basin (Gulf of Mexico) (Tator and Hatfield, 1975; Sheridan et al., 1981; Applegate et al.,
1982; Ball et ai., 1988). No accompanying barrier
reef system has been documented south of Virginia,
but two large individual reefs appear to have
occupied the Blake Nose, and two additional isolated reef complexes appear to have becn built on
the gigaplatform off Florida (Fig. i 7). The seaward
edge of the gigaplatform may have originally contained a more extensive reef tract, but its seaward
edge has retreated as much as 10-30 km in the
southern part of the Blake Plateau and Bahama
basins (Fig. 14; see mention in the discussion of
Bajocian-lower Bathonian deposits; Paull and Dillon, 1980; Freeman-Lynde et al., 1981; FreemanLynde and Ryan, 1985). This erosion appears to
have exposed the Kimmeridgian-Tithonian section
(and older Jurassic sections) at the seafloor along
the face of the Blake and Bahama escarpments.
The remaining major carbonate depocenter in this
region is near the middle of the megabank, rather
than at its edge.
Carbonate accumulation on the megabank was
interrupted, however, by a large siliciclastic depocenter that cxtended from southern Georgia to
within 50-100 km of the shelf edge (Fig. 17). This
depocenter appears to have formed from coalesc-
Fig. 21. lsopach map of Berriasian-Valanginiansequencealong U.S. Atlantic margin. Contours represent two-way travel time at
0.1 s intervals. Shelf and slope carbonate tracts stippled. Arrows indicate main routes of siliciclasticsediment supply. Diagonal
shading indicates sequence missing or too thin to resolve on seismic lines. Position of fi'~e depth sections (Figs. 6-10 herein)
indicated. Long Island and Cay Sal wellsand DSDP Site 99 (mentionedin text, but not shown here) are south of mapped area.
94
SE'
'
=.,--.--...,,---. ~ . - . ~ : .
...:~!!~!~:::~:!..~ - < ~
.
F MW l
o
/o
.
PresentSeo
r
ot 2 Se/(~
........>_~ .....................i~i::K .................
~
L--.
I,
I
I~W / , r
=3
,/
-- ~-
::"
I
/
,/
_~_
~
I
-
BERR~ASIAN/
~
,::~
/
/"
'~.=P:esem S~ Floor~t I 5 Se'¢
/
//
-
,/
/
//
/
_~----~....~
.
:~
" ~
i
~,~,~.'.~,~
' ......."" :::::~~ " ~ ~ ~"~':"""'~
~
/"
/
./
~
/
-~,.~-- . ~
--~
~i!!":ii~
O~FOI~oI4
~ - ~ L I N E
"E
v
2..._..
=~
I N W ~
~
~--""~ "~
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~
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4"1
•
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BATHONI~N-
upp( ~
~
,5 K
M
~
:::i::::i:i!~i:::~:!:!
:~
:~!!i!i!iii::ii:i:::i:iiii~::ii!~i:::i
...........
= CALLOVIAN
"
~ . ~ : . ~ ~
"
, ~ BERRI~SIAN /
/,~,,,=Presept Seo Floor of 2.6 Sec./
- , . . : : ~ : ~ ~.'~:-~
~::::::~::~.. I r
,,,
•~.:
CALLOVIAN
o
;~:,~..::
,'I
~"~.
~ - - - ~
:::;.,~:: :.: :::::::::::::::::::::::::::===========================....::~.,
:.:. -..,...::: .....
~
":':~:~..:
: ~ : : : ~ . : ~ ~ !
..
.............. :~ ~ ............ •...............................................:-:....~........
~UIAN
-~ •
~.~
..~.~:~:~ ......
~lC' I
/
/
•
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~ : . ~ ~ ~ : ~
. I...~
>.
"S-'~
/
/
~
:' :';:::i: i : : : ~ i :
: . ~ - ~
fv - " ..... :-,':~:i:i:i: i ~ : ! :
::i~ ' :::::: ..
..~
..........:.:...........................
~ ...... T I F f
~:"
....................: ......... : : . . . .
~
0
SE
,
~.....~
g4:. ~ ..............
:~::::"!
i : " :':::i~ : : :: ::::::::::::::::::::::::::::::::::::::::::::
'~
::::: K a*
: ~ ~
~
~ n . , ...................
/
"":~:-!~!i~!~!~i~
'::::~:?i:i:;;:.:!.ii:::::!::.ii:.
VALANGINIAN
,~::::.~:!:!:~.;
SHELL ~
....................
I
,/
6/
........." "
J
.................. ~ : ~
~"~:~
i : ~ : i : ! ~ : : : : ~ ..
!"~:::~i~i~!~i!~!i~:iii~
Fig:. 22. Shell-edge stratigraphy and morphology along four seismic lines across Baltimore Canyon Trbugh segment of gigaplatform
showing Berriasian-Valanginian siliciclastic deltas burying gigaplatform. Seismic reflections traced for Kimmeridgian-Tithonian
and Berriasian-Valanginian sequences; Kimmeridgian-Tithonian sequence stippled. Diagonal shading indicates Cretaceous and
Cenozoic sedimentary rocks overlying studied section. VE=vertical exaggeration. Uninterpreted versions of lines 2 and 10 can be
seen in Grow et al. (1979).
_
/
•
•
/
/
~.e~se,.se'oFIoor~t2Se~ /
W
/'~melveuc~H"
•
/
/
:..~~~¢/"
'....~
i ~ .....
/
/
/
/
.~~ ~ i ~ ~ . . ' , k
/
/
/
/
SE
/
LINE 9
-'-,<.j
95
-
.
/~,,,,~,,,./
$[
II
~
~
~
I
M
~
I" ~
'
~'~
~
M
E
R
~
%
I
D
~
G
I
A
/
N
- /
T!THONIAN/
/
/
/
/
/
v~:-,:,
~
i ~
/
LINE 16
~lilRl~o
--~.,..
~
/
....
/
/I
,,I
,K~ I
.~:~....,
.....
2.25 Sec.
%
/
x
~
j
-
,
~
" ~
LINt 2;
/ /
/
j/ /_j
"::~.:.
•"
.,.
Fig. 23. Shelf-edge stratigraphy and morphology along four seismic lines across Long Island Platform segment of gigaplatform
showing "'empty bucket" lagoon behind reef and variable degrees of burial or exposure (drowning unconformity) of forereef slope.
Seismic reflections traced for Kimmeridgian-Tithonian and Berriasian-Valanginian sequences; Kimmeridgian-Tithonian sequence
stippled. Diagonal shading indicates Cretaceous and Cenozoic sedimentary rocks overlying studied section. Pattern at bottom of
lines 23 and 16 represents basement. VE--vertical exaggeration. Uninterpreted version of line 9 can be seen in Grow et al. (1979).
96
ing delta complexes fed by three river systems,
which derived terrigenous detritus from the southern Appalachian Highlands. The updip margin of
this delta system has been drilled by exploratory
wells and by the COST GE-1 stratigraphic test on
the inner part of the continental shelf (Fig. 3;
Scholle, 1979). The inferred' Kimmeridgian-Tithonian sediments sampled there (age estimated
herein by seismostratigraphic extrapolation) are
coarse, unfossiliferous (presumably non-marine)
sands and associated terrigenous facies (Fig. 20).
The fabric of the isopach patterns on the continental slope and rise is also different seaward of
the Blake Plateau and Bahama basins compared
to along the northern basins. Elongate pods of
sediment rim the southern shelf edge and the Blake
Escarpment, and probably consist primarily of
debris eroded from the gigaplatform. No distinctive fan-like bodies of sediment appear to extend
across the continental rise in this southern region,
but one thick pod on the upper part of the rise
north of the Blake Spur (Fig. 17) [700 km (0.7 s)
thick] and another south of the Blake Spur [500
m (0.5 s) thick] appear to have intermittently
received siliciclastic deposits from the large shelfdelta complex. At DSDP Site 534 (Fig. 3) in the
Blake-Bahama Basin for example, the lower part
of a Kimmeridgian-Tithonian section comprises
90 m of red calcareous claystones and marly
limestones (50-80% siliciclastics) interspersed with
greenish-gray claystone and marl, black carbonaceous clay, and turbidites of light gray and pink
micritic, bioclastic and oolitic limestone (Ogg et
al., 1983). Similar lithoiogies were encountered
,,-28 km to the west, at DSDP Site 391, which is
intersected by the seaward end of seismic profile
TD-3 (Figs. 3 and 14; -qenson et al., 1978b).
Demise of the gigaplatform: BerriasianValanginian sequence (,,, 144-131 Ma)
The widespread deposition of shallow-water carbonate sediment along the outer margin of the
Carolina Trough, Carolina Platform, Baltimore
Canyon Trough, Long Island Platform and
Georges Bank Basin essentially ended at the close
of Jurassic time. In isolated patches, shallow-water
carbonate production continued into the Early
c.w. POAG
Cretaceous (e.g., Ryan et al., 1978; Erlich et al.,
1988, 1990; Meyer, 1989), but most of the earliest
Cretaceous carbonate appears to have been of
deep-water pelagic origin (Meyer, 1989; Erlich et
al., 1990). The same was true along the Scotian
Shelf (Eliuk, 1988; Eliuk and Levesque, 1988;
Jansa et al., 1988b), but to the south, in the Blake
Plateau Basin, shallow-water carbonates persisted
across the broad megabank into at least Barremian
time, and in much of the Bahama Basin they have
persisted to the present (Benson et ai., 1978b, c;
Schlager and Ginsburg, 1981; Ladd and Sheridan,
1987; Dillon and Popenoe, 1988; Dillon et al.,
1988).
Poag et al. (1990) have described the BerriasianValanginian depositional regime of the Baltimore
Canyon Trough area, where three large shelfedge delta systems built out from sources in the
central Appalachian Highlands and Adirondacks
(Fig. 21). Relict (and/or still growing) elevated reef
gtructures prevented access to the Sohm-Hatteras
'Trough by these shelf-edge delta systems, except
where deposition was rapid enough to bury them
(Fig. 22). At these points, large volumes of siliciclastic sediment reached the continental slope and
rise in the form of extensive, thick, slope aprons
and submarine fan complexes. The most notable
submarine fan complex of the Berriasian-Valanginian sequence is the Ocean City Fan, which
extends .,-500 km from the shelf edge to DSDP
Site 603 (Fig. 3), where terrigenous turbidites constitute the distal elements of the system (Sarti and
Von Rad, 1987; Holmes et al., 1987).
Along the seaward edge of the Long Island
Platform," Berriasian-Valanginian sediments fill
elongate depressions (scour channels?, "empty
bucket" lagoons?) parallel to and landward of the
Kimmeridgian-Tithonian reefs (Fig. 23). These
Berriasian-Valanginian strata lap onto the crests
of the reefs, but generally are missing on the
forereef slopes (=drowning unconformity of
Schlager, 1989). The upper part of this broad
seafloor exposure of the Kimmeridgian-Tithonian
gigaplatform may never have been covered with
Berriasian-Valanginian sediment because of its
steep slope. Lower down, however, subsequent
erosion may have removed some of the siliciclastic
cover from the escarpment face, On the outer shelf
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Fig. 24. Shelf-edge stratigraphy and morphology along four seismic lines across Georges Bank Basin segment of gigaplatform
showing complete budal of gigaplatform by Berriasian-Valanginian siliciclastic sediments. Seismic reflections traced for Kimmeridgian-Tithonian and Berriasian-Valanginian sequences; Kimmeridgian-Tithonian sequence stippled. Diagonal shading indicates
Cretaceous and Cenozoic sedimentary rocks overlying studied section. Pattern at bottom of lines 8 and 21 i represents basemenl.
VE= vertical exaggeration.
98
seaward of Cape Cod, an area where the Kimmeridgian-Tithonian reef ~ystem was never (or was
only weakly) developed (Fig. 24), the BerriasianValanginian siliciclastics formed a broad delta
complex (Fig. 21), which supplied terrigenous detritus to the Sohm-Hatteras Trough. At least two
primary and one secondary distributary routes are
indicated by the isopach contour pattern. A
multilobed submarine fan complex formed in this
region, but ,~xtended only a relatively short distance (50-i00 km) across the upper continental
rise. Along the northern margin of the Georges
Bank Basin, the Berriasian-Valanginian siliciclastics also failed to cover the upper parts of the
gigaplatform escarpment. The presence of several
small, coeval, submarine fan complexes attests,
however, to sporadic spilling of siliciclastic detritus
into the Sohm-Hatteras Trough in this area. Similar relationships have been described for the Scotian Basin (Jansa and Wade, 1975; Given, 1977;
Eliuk, 1978, 1988; Eliuk and Levesque, 1988).
South of the Baltimore Canyon Trough, in the
vicinity of what now is Cape Hatteras, another
major Berriasian-Valanginian siliciclastic depocenter was formed by a shelf-edge delta complex
(Fig. 21). Sediments from this complex also buried
the Kimmeridgian-Tithonian gigaplatform, and
spilled into the Sohm- Hatteras Trough (Fig. 25).
The thickest part of the siliciclastic prism (800 kin,
0.7 s) occupied the continental slope and rise, and
from it, a broad (100-150 km wide) submarine
fan complex distributed siiiclastic sediments at
least 500 km basinward. This fan is the most
southerly Berriasian-Valanginian fan complex
presently known i:~ ~he Sohm-Hatteras Trough.
Moving southward, there is a 500-kin stretch
from Cape Hatteras to southern Georgia in which
the prograding Berriasian-Valanginian siliciclastic
blanket did not cover the steep upper slope of the
Kimmeridgian-Tithonian gigaplatform. In fact,
strata even older than Kimmeridgian-Tithonian
appear to have been exposed along this escarpment
at that time (Fig. 26). I infer, however, from truncated reflections at the seaward edge of the Berriasian-Valanginian sequence, that its sediments
originally covered at least part of this escarpment,
and have subsequently been eroded. About 150
km north of the Blake Spur, a Berriasian-Valan-
c.w. POAG
ginian carbonate tract developed on the outer shelf
(profile FC-7, Fig. 27), and a series of mountled,
somewhat chaotic reflectors suggests that a sm~dl
reel ~'eveloped in the upper part of the sequence.
Further landward, a mid-shelf carbonate tract (as
determined from seismic facies analysis) is separated from the shelf-edge tract by a 30-kin-wide
swath of siliciclastic sediments. This swath joins a
major siliciclastic shelf-edge depocenter, which
abuts the northern margin of the Blake Spur
(Fig. 21). Sediments of this depocenter buried the
Kimmeridgian-Tithonian gigaplatform escarpment under at least l km of terrigenous detritus.
Seismic profile FC-8, which crosses this terrigenous
deposit (Fig. 27), probably shows the original (preerosion) configuration of the siliciclastic prism as
it draped the gigaplatform. This depocenter appears to have formed at the confluence of two
major distributary systems, which originated in the
southern Appalachian Highlands. Siliciclastics
from this depocenter entered the Sohm-Hatteras
Trough and filled a basement depression along the
northern flank of the Blake Spur. From there, the
sediments entered a narrow channel between the
Blake Escarpment and a basement high about 30
km to the east (Fig. 21).
A series of elongate slope aprons skirted the
gigaplatform from Cape Hatteras to the Blake
Spur, from the thickest of which ( ~ l km, > l. ! s)
a sediment lobe extended southward, wrapped
around the seaward margin of an elongate basement high, and then turned westward again to
parallel the Blake Escarpment. This contourfollowing isopach pattern suggests that abyssal
bottom currents redistributed sediments into contourite drifts along the base of the Blake Escarpment in Berriasian-Valanginian time. Such an
interpretation was broached by Tucholke and
Mountain (1979), but was refuted by Freeman and
Enos (1978) and Robertson and Bliefnick (1983)
after analyzing the sedimentologic properties of
cores taken in the Blake Basin.
Farther southward, on the Blake Spur, the shelf
carbonate tract was ,,-180 km wide (Fig. 21).
Seismic line TD-5 indicates that strata prograded
across the spur from its landward edge, where a
prominent, elongate, mounded reef structure forms
part of what I correlate as the Berriasian-Valangin-
NW
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Fig, 25, Shell-edge stratigraphy and morphology along three seismic lines across southern Baltimore Canyon Trough segment of
gigaplatform showing lack of prominent reefs and burial of gigaplaar~rm under Berriasian-Valanginian siliciclastics. Seismic
reflections traced for Kimmeridgian-Tithonian and Berriasian-Valanginia, sequences: Kimmeridgian-Tithouian sequence stippled.
Diagonal shading indicates Cretaceous and Cenozoic sedimentary rocks overlying studied section. V E = vertical exaggeration.
| O0
C.W. POAG
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THE BAHAMA-GRAND BANKS GIGAPLATFORM AND THE JURASSIC PROTO-ATLANTIC SEAWAY
ian sequence (Fig. 28). Benson et al. (1978c) interpreted this Blake Spur reef to be of Barremian
age, based on correlations with their interpretation
of the stratigraphy at DSDP Sites 390 and 392.
Dillon et al. (1985, 1988), on the other hand, used
the same DSDP data together with outcrop samples from the face of the Blake Ecarpment to infer
an Aptian-Albian age for a coeval reef crossed by
line TD-4. I infer that the main reefs formed during
Berriasian-Valanginian time, but that their crests
were reoccupied by reef organisms in AptianAlbian time. From this reef, the sedimentary section thins gradually along line TD-5 to the nose
of the Blake Spur, where it thickens again slightly.
This thickening suggests that another BerriasianValanginian reef originally occupied the nose, but
subsequently has been planed off, perhaps during
a ,~ ',vel fall at the end of Valanginian time. The
Berriasian-Valanginian section is now exposed at
the seafloor along the Blake Escarpment, and a
sample of Valanginian-Hauterivian age was collected from a depth of 3390 m below sea level,
where seismic profile TD-5 crosses the nose of the
escarpment (Fig. 28; Dillon et al., 1988). In addition to the escarpment sample, the DSDP drilled
Sites 390 and 392 on Blake Nose (Benson et al.,
1978a, c). Hole 390, which is Ioc~',ed on seismic
profile TD-5, penetrated a w:ll-dated BarremianAptian section (age based on foraminifera and
nannofossils), but in the underlying 35-m section
only disaggregated fragments of shallow-water
limestone were recovered, the fossil constituents
of which yielded no age-diagnostic forms. This 35m section corresponds to the upper two-thirds of
the Berriasian-Valanginian section as I have correlated it on profile TD-5 (Figs. 14 and 28).
At Site 392, about 25 km due south of Site 390
(still on the Blake Nose), a Barremian section of
nannofossil ooze and reddish brown, ooidal limestone was penetrated, which unconformably overlies a 260-m section of shallow-water limestones
101
(fenestral, oolitic, and skelmoldic) containing no
dateable fossils (Fig. 28). Freeman and Enos (1978)
interpreted this succession of limestones to represent a single regressive depositional sequence, but
Benson et al. (1978c) suggested, on the basis of
evidence for freshwater diagenesis, that one or
more emergent intervals could be represented in
the succession. Poor core recovery (,-, 3% of the
cored interval) in the limestone section precludes
direct identification of possible sequence boundaries within the succession. However, judging from
its much greater thickness and lithologic variability
relative to the equivalent section at Site 390, I
suggest that only the upper ~ ! 13 m of fenestral
limestone correlates with the section I assign to
the Berriasian-Valanginian. The lower ,-, 176-m
succession of skelmoldic to oolitic limestone
may well represent the top of the KimmeridgianTithonian gigaplatform.
Beginning at the Blake Spur, the seaward edge
of the Berriasian-Valanginian megabank (along
with older strata) has been eroded landward by
about 2-5 km from its estimated original position
(Figs. 10, 14 and 29; Paull and Dillon, 1980;
Freeman-Lynde and Ryan, 1985). Reefs may have
occupied this missing segment of the platform
during Berriasian-Valanginian time, as Benson et
al. (1978c) suggested, on the basis of sediments
drilled at DSDP Site 392. If so, hewever, it is
likely that they were eroded during the interval of
reef setback.
From the Blake Spur southward, a continuous
shallow-water carbonate tract covered the outer
third of the continental shelf in Berriasian-Valanginian time (Fig. 21). The next large BerriasianValanginian reef is crossed by seismic profile TD4 (Fig. 3), ,--20 km from the present shelf edge
(Figs. 10 and 21). Berria.sian-Valanginian strata
prograded slowly across the remaining segment of
the megabank and appear to have been sharply
truncated at the seafloor along the upper part of
Fig. 26. Shelf-edge stratigraphy and morphology along four seismic lines across Carolina Trough and Carolina Platform segments
of gigaplatform showing lack of, or moderate development of, reefs and exposure of steep platform edge (drowning unconformity)
at end of Berriasian-Valanginian deposition. Seismic reflections traced for Kimmeridgian-Tithonian and Berriasian-Valanginian
sequences; Kimmeridgian-Tithonian sequence stippled. Diagonal shading indicates Cretaceous and Cenozoic sedimentary rocks
overlying studied section. Pattern at bottom of lines 6 and 30 represents basement. VE=vertical exaggeration. Uninterpreted
versions of lines TD-6 and USGS-1POD can be seen in Dillon et al. (1982) and Grow et al. (1979) respectively.
102
C.W.POAG
SE
BERRIAE
~
~
&~,Present Seo Floor
~
LINE FC-8
~'-
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KIMMERIDGIAN-
L.4~
TITHONIAN
.... ~
V.E.="7: I
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I0 KM
5,
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BAJOCtAN . LOWER 8
,
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SE
BERRIASIAN-/
VALANGINIAN
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................................................
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A,,=Present Seo Floor
KIMMERIDGIAN- /
TITHONIAN../,/"
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/ / / /
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LINE
Upp£+
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co.,e+
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Fig. 27. Shelf-edge stratigraphy and morphology along two seismic lines across southern Carolina Trough (FC-8) and northern
Blake Plateau Basin (FC-7) segments of gigaplatform. Seismic reflections traced for Kimmeridgian-Tithonian and BerriasianValanginian sequences: Kimmeridgian-Tithoniar, sequence stippled. Diagonal shading indicates Cretaceous and Cenozoic sedimentary
rocks overlying studied section. VE=vertical exaggeration. Note striking differences in reef development, height of forereef
escarpment, and degree of platform burial. Vertical exaggeration nearly twice that of the previous shelf-edge sections.
103
THE BAHAMA.GRAND BANKS GIGAPLATFORM AND THE JURASSIC PROI'O-ATLANTIC SEAWAY
('33S) ":1~1./. "I3AWlJ. ArM- OM.L
Od
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104
C.W, POAG
....
TD'5
TD-4
. BAJOCIANLOWER BATHONIAN
SHELF EDGE
BERRIASIAN'_--~
VALANGINIAN
SHELF EDGE
"
TD-3
FC-I
TD-2
0
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,
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Km
Fig. 29. Tracing of shelf edge position (map view) at end of
early Bathonian time versus position at end of Valanginian
time showing amount of platform retreat (approximating the
width oferoded Bajocian-lower Bathonian terrace) along Blake
Escarpment segment of gigaplatform. Crossing seismic profiles
indicated by numbered lines.
the Blake Escarpment. Here again, a segment of
the Berriasian-Valanginian platform edge, ~ 2.5
km wide, appears to have been removed by erosion.
Two sediment samples taken from the face of
the Blake Escarpment along line TD-4 yielded
"approximate" Barremian? nannofossil dates (Dillon et al., 1988) for the section I have correlated
as Berriasian-Valanginian (Fig. 10). Landward of
the outer-shelf carbonate province crossed by
seismic profile TD-4, a broad (,-, 200 km) tongue
of siliciclastic sediments divided the megabank into
an inner-shelf and outer-shelf segment (Fig. 21).
Seismic profiles along this part of the inner shelf
show an abrupt, buried, seaward-facing escarpment, which has been interpreted by Shipley et al.
(1978) and Popenoe et al. (1984) to be a carbonate
reef or bank. Its chaotic seismic signature is much
like the sections in the Bahama Basin that Ladd
and Sheridan (1987) interpreted as diagenetically
altered, karstic, limestone terrain. The upper surface of this reefal? mass is onlapped by the edge
of the broad siliciclastic tongue that originates
from distributaries in southern Georgia to form a
mid-shelf depocenter approximately 700 m (0.6 s)
thick (Fig. 21). Several deep boreholes, including
the COST GE-I stratigraphic test, have penetrated
this segment of the siliciclastic wedge (Fig. 20;
Scholle, 1979; Dillon and Popenoe, 1988). The
wedge is, however, unfossiliferous in this updip
position, so its Berriasian-Valanginian age can not
be confirmed by paleontology. The beds in the
COST GE-I well, which immediately overlie the
section I correlate as Berriasian-Valanginan, contain Aptian palynomorphs (Poag and Hall, 1979).
Along the base of the Blake Escarpment, Berriasian-Valanginian strata (in this region and southward) fill an elongate basement depression and lap
seaward onto the flank of a broad basement plateau (where the section thins significantly). Presumably, these strata include primarily carbonate
sediments eroded from the escarpment and swept
off the outer edge of the megabank.
Seismic line TD-3 crosses a series of prominent
down-to-the-basin normal faults near the Blake
Escarpment (Fig. 14). These faults displace the
surface of the ancient shelf several hundred meters
downward toward the Sohm-Hatteras Trough.
The main fault trace appears to intersect the
seafloor and the upthrown block forms the seaward
face of a prominent cliff at the present shelf break.
Three sediment samples taken from the escarpment
along seismic profile TD-3 have yielded nannofossils of Hauterivian-Valanginian age (Dillon et al.,
1988) in the section I correlate as BerriasianValanginian (3300-3500 m below sea level).
DSDP Hole 391C was drilled on seismic profile
TD-3 in the Sohm-Hatteras Trough (Fig. 3;
Blake-Bahama Basin of some authors; Benson et
aL, 1978b). Cores recovered from the section I
correlate as Berriasian-Valanginian contain lower
Berriasian-upper Valanginian shale and calcilutite
( ~ !100-.1260 m below the seafloor). These sediments were interpreted by Benson et al. (1978b)
to represent a mixture of distal siliciclastic turbidites (cross-laminated mudstones), coarse oolitic
calcareous turbidites derived from the nearby shelf
(Blake Plateau), and laminated nannofossil (pe-
THE BAHAMA-GRAND BANKS GIGAPLATFORM AND THE JURASSIC PROTO-ATLANTIC SEAWAY
lagic) limestones. At nearby Site 543 (26 km northeast) the Berriasian-Valanginian section comprises
mainly lime mudstones, marly limestones, black
claystones and calcilutites (Sheridan et al., 1983).
lnterbedded sandy turbidites contain ooids and
neritic fossils derived from the Blake Plateau.
Seismic profile FC-I crosses another BerriasianValanginian reef at the shelf break (Fig. 14). The
top of the reef has been eroded to an unusually
fiat, level surface about 10 km across. The escarpment edge is not significantly faulted at this
crossing.
The southernmost seismic profile used for this
study is profile TD-2 (Fig. 14). Where profile TD2 crosses the Blake Escarpment, the Berriasian--'
Valanginian section has been truncated and
pinches out on (onlaps) the underlying fiat, eroded
surface of the Kimmeridgian-Tithonian gigaplatform ,,-4 km from the pre~ent shelf break (not
shown on Fig. 14). Here, as at the profile TD-3
crossing, the outer part of the megabank has been
faulted d o w n w a r d into the Sohm-Hatteras
Trough, but here the faulting is much more pervasive, as shown by a series of en echelon blocks,
which have dropped down to the southeast. Total
vertical displacement is of the order of 1400-3000
m. A small prism of Berriasian-Valanginian strata
appears to partly fill a narrow depression b-*.wcen
the base of the escarpment and a broad basement
high (Blake Spur Ridge; Fig. 21). Berriasian-Valanginian strata onlap the basement high, but
along profile TD-2 do not cross it. South of Little
Bahama Bank the Berriasian-Valanginian section
has been drilled in the Great Isaacs well (Tator
and Hatfield, 1975; Sheridan et al., 1981; Schlager
et al., 1988), in several wells in southern Florida
(Applegate et ai., 1982), and in wells on the West
Florida Platform (Ball et al., 1988). All these wells
penetrated shallow-water carbonate and evaporite
lithologies in the Berriasian-Valanginian interval.
Bryant et al. (1969), Sheridan et al. (1981) and
Ladd and Sheridan (1987) have shown, on the
basis of seismic reflection profiles, boreholes and
seafloor dredgings, that the Berriasian-ValanginJan megabank extended westward across the Bahama Basin to Florida (and perhaps northern
Cuba) and into the Gulf of Mexico as part of a
vast, tropical carbonate province.
105
Gigaplatform growth patterns and shelf-edge
configuration
Figures 14, 18, 19, and 22-27 show that the
growth patterns, cross-sectional geometry and surficial morphology of the Bahama-Grand Banks
gigaplatform have varied considerably within a
broad continuum of rates, facies, styles, stages and
shapes, as Jansa (1981, 1984), Eliuk (1978, 1981),
Emery and Uchupi (1984), Schlee et al. (1988) and
Meyer (1989) have expressly pointed out.
In the most recent comprehensive analysis,
Meyer (1989) recognized a "genetically" oriented
system of platform-margin categorization based
on three successive depositional "stages", which
he applied to the Kimmeridgian-Berriasian section
of the Baltimore Canyon Trough:
Stage I (Progradation): A period of rapid seaward platform growth. Carbonate-production potential is presumed to have been sufficient only for
platform aggradation, but the carbonate tract prograded because rapid siliciclastic deposition helped
to fill the adjacent accommodation space in the
Sohm-Hatteras Trough. Stage I ended with a
relative sea-level fall and subaerial exposure of the
gigaplatform.
Stage H (Aggradation): The supply of siliciclastic detritus to the Sohm-Hatteras Trough dwindled. Accelerated relative sea-level rise was
matched by accelerated carbonate production to
produce a relatively stationary barrier reef, topped
by a series of pinnacles. Stage II ended with
another fall in relative sea level and subaerial
exposure of the gigaplatform.
Stage III (Drowning): Rapid and large relative
sea-level rise drowned the neritic carbonate producers, and deposited a thin veneer of largely
pelagic carbonates. Sediments of Stage III are
normally too thin to be distinguished on seismic
profiles.
In an exemplary study of the Permian Capitan
shelf margin (Delaware Basin) Garber et al. (1989)
recognized a progradation-aggradation succession
similar to Meyer's Stages I and II. They also
concluded that siliciclastic basin-fill, along with
sea-level change and emplacement of allochthonous carbonate debris ot~to the basin margin, was
a major mechanism for shelf-edge progradation.
106
I agree in general with Meyer's interpretation
of a tripartite Late Jurassic-Early Cretaceous platform evolution in the Baltimore Canyon Trough,
a!though we disagree on the details of timing, the
lateral continuity of carbonate deposition, and the
relative importance of forcing mechanisms for
progradation. I, for example, would place most of
the prograding Kimmeridgian strata (Stage I;
shown in Meyer's fig. 3A, B) in the Oxfordian
sequence, more like what he shows for the Scodan
Basin (Meyer's fig. 14). Furthermore, the depositional variability along strike may have been
greater and more persistent through Stage II (aggradation) than implied by Meyer (e.g., his fig. 13).
As a case in point, Fig. 17 shows four large passages through the carbonate tract (20-30 km wide),
where siliciclastic deposition appears to have stifled
carbonate production throughout most of Kimmeridgian-Tithonian time.
The lateral variation of gigaplatform growth is
even more striking when viewed on a margin-wide
scale (Bahamas to Georges Bank). For example,
progradation was most pronounced in the central
part of the Baltimore Canyon Trough and tapered
off toward the bounding structural platforms
(Fig. 30; see also Emery and Uchupi, 1984). A
similar, but more irregular progradation took place
in the Georges Bank Basin. Selected depth-converted cross sections (Figs. 6-10) indicate that
each basin or trough had its own individual growth
history. From Aalenian(?) to early Bathonian time,
the shelf edge prograded slowly (70~-900 m/m.y.)
in the Baltimore Canyon Trough (Fig. 31), At the
same time, the shelf edge retreated slightly in the
Georges Bank Basin ( - 700 m/m.y.) and the Carolina Trough ( - 180 m/m.y.), but aggraded in the
Blake Plateau Basin. In the late Bathonian-Callovian interval, rapid progradation (800-2800 m/m.y.)
was the rule for the entire margin north of the
Blake Plateau Basin (where a retreat of -800 m/
m.y. took place). This seaward thrust continued
~hrough the Oxfordian (200-3000 m/m.y.), except
in the southern Baltimore Canyon Trough (line
28), where a slight retreat ( - 3 0 0 m/m.y.) appears
to have begun (and aggradation again dominated
the Blake Plateau Basin). Through the Kimmeridgian and late Tithonian slight progradation (400
m/m.y.) took place in the central part of the
c.w. POAG
~'-~,,,,.
Xx
KIMMERIDGIANTITHONIAN
25
BeT
0
I00
2100
Km
Fig. 30. Tracing of platform edge positions (map view) at end
of Aalenian(?) and Kimmeridgian-Tithonian time show;~ng
variable maximum progradation of platform in Baltimore
Canyon Trough (BCT) and Georges Bank Basin (GBB). Note
Aalenian(?) sequence missing from Long Island Platform. Three
depth sections (Figs. 6-8 herein) indicated.
Baltimore Canyon Trough, but in the other northern basins the shelf edge retreated ( - 200-300 m/
m.y.).
It is instructive to compare the growth rates of
the Bahama-Grand Banks gigaplatform with
those of the Capitan shelf margin calculated by
Garber et al. (1989). Table 1 shows that the gigaplatform and its pre-Caiiovian predecessors aggraded at rates of ~50-400 m/m.y., which are
comparable to a range of 95-430 m/m.y, for the
entire Guadalupian sequence of the Delaware
Basin. Maximum Capitan rates of 335-430 m/m.y.
at the shelf margin match iq~ajocian-early Bathonian rates in the Blake Plateau Basin and late
Bathonian-Callovian rates in both the Blake Plateau Basin and central Baltimore Canyon Trough.
Progradation rates of the gigaplatform range
from 200 to 3000 m/m.y., which are similar to the
minimum rates (1900-3200 m/m.y.) on the distal
extremities of the Capitan margin. At its maximum
along the northern margin, however, the Capitan
THE BAHAMA-GRANDBANKSGIGAPLATFORMAND THE JURASSICPROTO-ATLANTICSEAWAY
107
TABLE 1
Aggradation, progradation, and retreat rates and growth-retreat d;fferential for five seismic sections (Figs.6-10
herein) across Bahama-Grand Banks gigaplatform
Seismic line
19
25
28
32
81
11 I
166
125
200
164
378
292
158
63
45
133
183
67
50
191
222
250
104
- 727
889
200
- 333
909
2778
2000
417
727
2222
-333
- 208
-182
88~
~000
- 333
TD-4
Aggradation (m/m.y.)
Aalenian(?)
Bajocian-early Bathonian
late Bathonian-Callovian
Oxfordian
Kimmeridgian-Tithonian
Berriasian-Valanginian
364
333
133
146
85
Progradation and retreat (m/m.y.)
Bajoeian-early Bathonian
late Bathonian-Callovian
Oxfordian
Kimmeridgian-Tithonian
Berriasian-Valanginian
- 833
-
192
Growth-retreat differential
Bajocian-early Bathonian
late Bathonian-Callovian
Oxfordian
Kimmeridgian-Tithonian
Berriasian-Valanginian
-9:i
8:1
!:1
-3:1
margin prograded 2.5 times as fast as the maximum
gigaplatform rate (7600 vs 3000 m/m.y.). Retreat
of the gigaplatform varied from - 182 to - 833
m/m.y.
Growth differential (horizontal versus vertical)
for the Bahama-Grand Banks gigaplatform
ranged from 1:1 to 17:1 (Table 1), which is considerably less than that of the Capitan margin
(25:1-30:1). It is curious that the growth
differential of 3:1 in the central Baltimore Canyon
Trough during Kimmeridgian-Tithonian time was
matched by an identical retreat differential in the
other northern basins.
When Meyer (1989) compared the Baltimore
Canyon Trough's Late Jurassic evolution with that
of the coeval Scotian Basin, he found that these
two segments of the gigaplatform also had distinctly divergent growth histories. Whereas rapid
platform progradation took place throughout
much of the Baltimore Canyon Trough, aggradation dominated most of the Scotian Basin. Meyer
ascribed this divergent growth history to the
13:1
14:1
7:1
3:1
16:1
17:1
-2:1
-3:1
-I:1
4:1
12:1
-3:1
-2:1
-2:1
greater rate of siliciclastic basin filling that took
place seaward of the Baltimore Canyon Trough.
My data suggest, however, that siliciclastic
encroachment from the landward side of the platform may have been equally as important (or
sometimes more important) than siliciclastic basin
filling on its seaward side (see also Erlich et al.,
1990). This conclusion is supported by Fig. 30,
which shows that the locus of maximum progradation was along line 25, in alignment with the locus
of maximum siliciclastic encroachment (Figs. 7
and 17).
The record of volumetric siliciclastic accumulation rates for the Baltimore Canyon Trough
(Fig. 3 IA; Poag and Sevon, 1989) provides further
evidence that Meyer's and Garber et al.'s basinfill hypothesis needs modification. During Aalenian(?) time, unusually high accumulation rates
(25,000 km3/m.y.), associated with rapid thermotectonic subsidence (Watts and Steckler, 1979),
restricted carbonate production to a few scattered
tracts, and shelf-edge progradation was minimal
108
c.w. POAG
AI
Mo ILl
-140
STAGE
BERRIASIAN
MIGRATION OF PLATFORM EDGE
- 0+
I0
20
30
40
,
,
,
,
,
I0
~:
\
~
\
I~
/"5,.... j
i
50 Km
,.~
..... .r:
TITHONIAN
KIMMERIDGIAN
160
2:)
~NVVVVVV~A~NV~
,',,
OXFORDIAN
i
3
.~
.,,,.':"
\
/
\f"
',
CALLOVIAN
UP. BATHONIAN
'~APaVVVV~e~AA~
LO. BATHONIAN
-180
\
~',..'//
:u;//"
BAJOCIAN
ACCUMULATIONRATE
AALENIAN(?)
0
I0
20
SIL. ACCUM. RATE (103Km 3/my.)
B"
BATHYMETRIC POSITION
PERIOD,
EPOCH,
AGE
OF
MAIN DEPOCENTERS
SHELF
|
SLOPE
QUATERNARY
* •
I
RISE
e
/
e
PLIOCENE
LATE MIOCENE
MI_DOLE MINCENE
EAR:Y MIOCENE
o,---~ ......e ......... ~
__.___o.--.---o
LATE OL~gCENE
LA~'E EOCENEEARLY O!.IGOCEN[
MIDDLE EOCENE
EARLY EOCENE
PALEOCENE
MAESTRICHTIAN
CAMPANIAN
•
.
..... • ....
•
;::e
o'".•
.
..... e,
~21;IL,]~[M;
~
g
•
.:.'J
- e ~ e
~p-9
CENOMANIANTURONIAN
APTIANALBIAN
8ARREMIAN
HAUTERIVIAN
8ERRiASIANVALANGINIAN
KIMMERIDGIANTITHONIAN
OXFOROIAN
L1IaATHONIAN-' ....
CALLOVIAN
OAJOCIANE, BATHONIAN
AALENIAN
• .....
•
/ O f
O / O
•
THE BAHAMA-GRAND BANKS GIGAPLATFORM AND THE JURASSIC PROTO-ATLANTIC SEAWAY
(Fig. 12). A significant reduction in Bajocian-early
Bathonian siliciclastic deposition (9000 kma/m.y.)
allowed carbonate-producing communities to
flourish widely, which significantly expanded the
shelf-edge platforms. An increase to moderate
siliciclastic accumulation rates in the late Bathonian-Callovian and Oxfordian (11,000-14,000 km3/
m.y.) caused the carbonate communities to migrate
rapidly seaward, consequently accelerating shelfedge progradation. Siliciclastic accumulation rates
fell to 6000 km3/m.y, in the KimmeridgianTithonian, reducing the rate of encroachment
toward the shelf-edge carbonate tract, and the
gigaplatform either aggraded or retreated. Note
also that the gigaplatform was significantly wider
(expanded landward) in the Ge,arges Bank and
Blake Plateau basins (Figs. 6-10), where the supply
of siliciclastic shelf sediments (and encroachment
on the carbonate tract) was least. Meyer (1989),
in contrast, believed that siliciclastic accumulation
rates increased during this interval, having used
the common "dip-stick" method of rate calculation
based on the thickness of the sediment column in
selected wells. But as Poag and Sevon (1989)
showed (and the data herein confirm), a few local
dip-stick measurements are by no means adequate
for measuring regional sedimentation rates in the
highly variable 1,000,000 km 2 covered by the
Baltimore Canyon Trough-Hatteras Basin depositional regime.
The bathymetric migration of principal Jurassic
siliciclastic depocenters in the Baltimore Canyon
Trough region (Fig. 31B; Poag and Sevon, 1989)
also bears on the growth history and processes of
this segment of the gigaplatform. During Aalenian(?), Bajocian-early Bathonian, and late Bathonian-Callovian time, for example, principal
siliciclastic depocenters were located on the conti-
109
nental shelf (Fig. 31B), where maximum encroachment rates could be maintained on shelf-edge
carbonate tracts. During the Oxfordian to Tithonian interval, however, large volumes of siliciclastics
began to bypass the shelf to form bathyal and
abyssal depocenters (Meyer reported a succession
exactly the reverse of this). This bypassing may
have further reduced siliciclastic encroachment on
shelf-edge carbonate communities.
Thus Meyer's (1989) and Garber et al.'s (1989)
hypothesis that siliciclastic basin filling can enhance the progradation of carbonate plattbrms is
only weakly supported by the diverse relationships
highlighted herein. Along the U.S. margin it appears, rather, that siliciclastic encroachment was
often equally or more important. The divergent
basin histories described above show, however
[and so do many modern and additional fossil
examples (e.g., Doyle and Roberts, 1988)], that
contiguous siliciclastic deposition was but one of
several interactive bio-physico-chemical agents
that created, maintained, and terminated the gigaplatform.
Paleogeography, paleoceanography and
paleoclimate
In order to fully appreciate the development of
the Bahama-Grand Banks gigaplatform it is important to understand its relationships with regional
paleogeography,
paleoceanographic
conditions, and prevailing paleoclimate. A number
of authors, including Manspeizer (1981, 1982),
Hay et al. (1982), Parrish and Curtis (1982), Parrish
et al. (1982), Emery and Uchupi (1984), Hallam
(1984, 1986), Jansa (1986), Roth (1986, 1989),
Tucholke and McCoy (1986), Barron (1987) and
Schlee et al. (1988), have analyzed and recon-
Fig. 31. (A) Chart of migration (progradation or retreat in kilometers)of gigaplatform edge through last ,-, 183 m.y. (upper scale)
as measured on five depth sections (Figs. 6-10 herein) beginningat terminal Aalenian(?) position. Lowerscale shows volumetric
siliciclastic accumulationrates calculated for BaltimoreCanyon Trough region by Poag and Sevon (1989). BPB= Blake Plateau
Basin; GBB= Georges Bank Basin; CT= Carolina Trough; BCT S= southern part of Baltimore Canyon Trough; BCT C= central
part of Baltimore Canyon Trough; LAWRENCE=data from Lawrenceet al. (1990). Age scale is that adopted by Geological
Societyof Americafor Decadeof North AmericanGeology(Palmer, 1983).(B) Chart showingbathymetricmigrationof depocenters
in BaltimoreCanyon Trough region through last ~ 187 m.y. (from Poag and Sevon, 1989). Large dots indicate main depocenters
(thickest sedimentcolumns),small dots indicate secondarydepocenters.Solid line tracks bathymetricshifts of main deepest-water
depocenters through time, dashed line tracks migration of main shallowest-waterdepocenters. Note progressive shift of main
depocentersonto continentalslope and rise from Oxfordian to Valanginiantime.
I I0
c.w. POAG
also have accumulated farther south beyond the
seaway, however, in the Blake Plateau rift basin.
According to most modern plate reconstructions
this long, narrow seaway (50-200 x 4300 km) lay
in a low-latitude (10-20°N) position (Sclater et al.,
1977; Smith and Briden, 1977; Firstbrook et al.,
1979, Scotesc et al., 1981; Emery and Uchupi,
1984; Zeiglcr et al., 1984; Manspeizer, 1985; Klib
gord and Schouten. 1986; Jansa, 1986). Its sinuous
long axis was oriented approximately east-west
from a Tethys gateway (Biscay Seaway of Jansa,
1986) to the Long Island Platform, at which point
the axis turned gently to the southwest (Fig. 32,
Long Island Bend). A second gateway from Tethys
(Gibraltar-Rif Seaway of Jansa, 1986) channeled
surface currents northwestward to the Grand
Banks.
Parrish et al. (1982) used the global distribution
of coal and evaporitcs, which reflects the balance
between precipitation and evaporation, to predict
the relative amount of rainfall on Pangea. For the
proto-Atlantic seaway, the model predicts low
rainfall in the Triassic and moderately low rainfall
structed the Jurassic and Early Cretaceous paleogeography, paleoceanography and paleoclimate of
the proto-Atlantic seaway. In the most recent
comprehensive interpretations (Jansa, 1986;
Tucholke and McCoy, 1986), shallow-marine
waters from the Tethys Sea entered the developing
Pangean rift zone between North America and
Africa from two northeastern gateways during
Late Triassic time (Norian). Evaporites, the chief
sedimentary deposits of the Triassic and Early
Jurassic, precipitated in what was essentially a
single intracratonic basin, which extended as far
southwest as the southern end of the Baltimore
Canyon Trough. My interpretation (Fig. 32) is
more like that of hoiser et al. (i988) in that: (1)
along the U.S. margin, in late Toarcian(?)-early
Aalenian(?) time, three discrete evaporite basins
were separated by structurally high platforms upon
which no seismically detectable sediment accumulated, and (2) evaporite deposition reached the
Carolina Trough, which at that time appears to
have been the most southwestern depocenter of
the proto-Atlantic seaway. Coeval evaporites may
TOARCIAN(?)- AALENIAN(?)
¢•
20 ° -
ISLAND.,--"-"i
BEND
e
•
--JI,
",
,/~
"
LONG
• -~
""
"~ ") t ~
". ~
• ..".': ......
::. ~-..
't /l~
~
.
" '~
\
".
.
C
i
.
.
,
~
4
.
.::
Y
: . ~'EAWAY.
',',,."
".:
%%
t
• s * " " " '-' "
I0 o-
o I
~,~
~
I,..
o_
,~
,
II
%
% I
%I
'"" " ' . . , '" :'." :
;;..~:~:;E;~:~::~:_-:[~ s..,'AwAr
. ' . ~ ~ , . . . ~ . TETHYS
t
I
,/
%
:':;"
~':I%
•
~L
/iI
0
I000
2000
•
I
I
0
% s"'',,~%1
KM
EVAPORITES ~ !
Fig. 32. Paleogeographic reconstruction of North American-African plate junction during transition from rifting to seafloor
spreading [late Toarcian(?)-early Aalenian(?)] showing evaporite basins. Dashed lines indicate present outline of continents. Arrows
indicate dominant flow direction of surface currents. Scale approximate. Adapted from Jansa (1986) and Tucholke and McCoy
(1986).
THE
BAHAMA-GRAND BANKS GIGAPLATFORM AND THE JURASSIC PROTO-ATLANTIC SEAWAY
throughout the Jurassic and Early Cretaceous. The
Blake Plateau and Bahama basins would have
become increasingly wetter (moderately high rainfall), however, near the Jurassic-Cretaceous transition.
By the end of Aalenian(?) time, seafloor spreading had split the nascent proto-Atlantic seaway
into a northern (Sohm-Hatteras) and southern
(Madeira-Gambia) deep-water trough, each
bounded landward by shelf basins and separated
from each other by a mid-ocean (mid-seaway)
ridge, or spreading center (Fig. 33; Jansa, 1986;
Klitgord and Schouten, 1986; Tucholke and
McCoy, 1986). The cessation (or diminution) of
evaporite deposition and the beginning of benthic
carbonate deposition on the continental shelves
during the Aalenian(?) attest to improved circulation at shallow and mid-water depths as far west
as the Georges Bank Basin. Carbonate production
was still sparse farther down the seaway, however,
and restricted surface-water circulation may still
have characterized the margin from Baltimore
Canyon Trough to the Carolina Trough, which
was closed to communication with the Pacific or
the yet to be formed Gulf of Mexico, although
]lI
some shallow Aalenian(?) marine waters may have
invaded the still-active rifts of the Blake Plateau
Basin (Phair and Bufflcr, 1983; Salvador, 1987;
Schlager et al., 1984; Buffler, in press).
No Aalenian deep-sea sediments have been recovered from the Sohm-Hatteras Trough, but DSDP
Site 547 (Fig. 33), near the Tethys end of the
Madeira-Gambia Trough, appears to provide
some evidence of Aalenian bathyal circulation.
Jansa et al. (1984) described a poorly recovered
succession of mainly pale yellowish-brown and
pale red micrite nodules encompassed in a pale
yellowish-brown, burrowed, calcareous claystone
matrix (cores 547-14, 15 and 16). Most of the
nodular clasts are displaced biomicrites composed
mainly of radiolarian casts, together with ostracodes, foraminifera and Globochaete. Theses characteristics prompted these authors to infer
deposition in oxygenated bottom waters at outer
neritic or upper bathyal depths. One prominent
black shale interval was interpreted as evidence of
occasional periods of relative stagnation, probably
associated with an oxygen-minimum layer. The
distribution of Aalenian(?) deposits in the Gcorges
Bank region is evidence that maximum water
AALEN IAN (?)
20 o -
CARBONATE~
S BEND
t "-
SOHM" HATTERAS~
/
~
..........
~. ~ : ~
""
~
-~ ~. ,.,~
•
I0 °-
,.,.,~_: ..:
%
. ~ ~..=r."
,'
I'
~
I . ~ O G ,
l' i " r / ~
Y
:::I
~U NI4IL.'~
- "~E:THYS
." .:..~.
_
|
. j'
'~
%
°'1
~,
I
", I
• • . , j r -- %,
0
~t
I
~l
tt
|t
I000
I
m
2000
i
I
KM
SILICICLASTICS[ ]
Fig. 33. Paleogeographic reconstruction of proto-Atlantic seaway at end of Aalenian(?) deposition, after seafloor spreading had
begun. Spreading axis indicated by segmented line. Carbonate tracts shown. Arrows indicate dominant flow directior, of surface
currents. Dashed lines indicate present outline of continents. Scale approximate. Adapted from Jansa (1986) and Tuchoike and
McCoy (1986).
112
depths there were probably bathyal, as only a
restricted (,-, 20 km wide) continental slope existed,
and apparently no continental rise had yet formed
(Fig. 12). Beyond this narrow passageway, however, just southwest of the Long Island Bend, an
incipient continental rise and abyssal plain appear
to have been present where the Sohm-Hatteras
Trough widened to ~ 80-170 km in front of the
Baltimore Canyon Trough. The abyssal trough
narrowed again to ~ 10-I 5 km along the Carolina
Platform, only to widen again (40-90 km) seaward
of the Carolina Trough (Fig. 12). The SohmHatteras Trough terminated in a cul-de-sac, approximately seaward of what is now Cape Fear,
North Carolina. Here the trough was closed on
three sides by elevated seafloor topography (Carolina Trough, Blake Plateau Basin, mid-seaway
spreading center), which may have prevented deepwater connections with the Madeira-Gambia
Trough. Under such conditions, as Jansa (1986)
suggested, a Mediterranean type of stratified circulation probably developed in the Sohm-Hatteras
Trough. Warm, clear, wind-driven Tethyan surface
water would have entered the eastern end of the
trough, flowed ,,-3000 km through the narrow
seaway within a relatively arid climatic regime,
and descended as it became more saline and denser
near the cul-de-sac. The return flow must have
~ e n relatively stagnant and sluggish as it passed
through narrow, circuitous passageways back to
Tethys.
Bajocian-early Bathonian time saw significant
changes in the paleogeography and paleoceanographic conditions along the U.S. margin of the
proto-Atlantic. Seafloor spreading had widened
the seaway for ,~ 15 m.y., and by the middle of
Bathonian time the Sohm-Hatteras Trough (from
shoreline to sedimented flank of the spreading
center) was on average ~300-400 km wide
(Fig. 34; the seaway would have been more than
t~.t4ce as wide). Increased benthic carbonate production created wider, more extensive outer-shelf
platforms in the Georges Bank Basin. The Blake
Plateau and Bahama basins in particular underwent a vast expansion of carbonate production,
which resulted in construction of a 450 x 600-km
megabank (Fig. 13). These developments indicate
that warm, clear, well-circulated surface waters
c.w. POAG
were reaching the southern terminous of the seaway. Carbonate production was more limited in
the Baltimore Canyon and Carolina troughs, because large volumes of siliciclastic sediments
(Fig. 31A; Poag and Sevon, 1989) still suppressed
benthic carbonate communities.
The best evidence of deep-water circulation for
Bajocian-early Bathonian time comes again from
DSDP Site 547 (Fig. 34). There, an upward change
to reddish brown, sandy, limestone breccias and
conglomerates (lithologic subunit VIA3 of Jansa
et al., 1984), topped by a series of bioturbated,
fossiliferous, pelagic wackestones, indicates good
abyssal ventilation and bottom circulation near
the widened Tethys gateway. Bottom-water circulation in the Bahama cul-de-sac, however, was probably much poorer than at Site 547, because of the
great distance from Tethys and because of the still
quite narrow dimensions (no more than 50-100
km wide in places seaward of the Carolina Trough
and Blake Plateau Basin) and circuitous axis of
the Sohm-Hatteras Trough (Fig. 13). The trough
may also have been shalio,ver here than at Site
547; the present relief between the Bajocian-lower
Bathonian sequence seaward of the Blake Escarpment and its neritic equivalent on the escarpment
is 1500 m (Fig. 10), which indicates a similar
paleodepth in Bajocian-early Bathonian time.
The slow northward migration and counterclockwise rotation of the North American plate
had not appreciably changed the low-latitude position (,-,5-22°N) of the U.S. marginal basins by
the end of Bajocian-early Bathonian time, as
reflected in the continued presence of thick carbonate deposits. No significant alterations in the paleoclimate during this time interval are indicated by
the global geologic record or by theoretical models.
By the end of late Bathonian-Callovian deposition the proto-Atlantic seaway had widened to as
much as 1000-1200 km, and was probably connected with the Gulf of Mexico (Fig. 35; Scott,
1984; Taylor et al., 1984; Westermann, 1984) and
to the Pacific Ocean ("Hispanic Corridor" of
Smith, 1983; "Americas Seaway" of Jansa, !986;
Tucholke and McCoy, 1986; see Salvador, 1987,
however, for a countervailing opinion). Benthic
carbonate production continued te pr.~liferate, and
lateral expansion of the shelf-edge tracts created
THE BAHAMA-GRANDBANKSGIGAPLATFORMAND THE JURASSICPROTO-ATLANTICSEAWAY
113
BAJOCIAN- LOWER BATHONIAN
,;.,.:.~."
20 ° -
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,- ~
:..-~. ; .
.
.
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2000
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CARBONATES B
SILICICLASTICS D
Fig. 34. Paleogeographic reconstruction of proto-Atlantic seaway at end of Bajocian-early Bathonian deposition. Spreading axis
indicated by segmented line. Carbonate tracts stippled. Arrows indicate dominant flow direction of surface currents. Dashed lines
indicate present outline of continents. Scale approximate. Adapted from Jansa (1986) and Tucholke and McCoy (1986).
UPPER BATHONIAN - CALLOVIAN
. ° - - ; ' ~ .- ~ 3 : , : . : : . . . ~ ~ ' . . , ./.:3. ~
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CARBONATES ~
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.~, ~//,',-,-,-,-,-,-,-j-~,g~T~r/.~-%.-.:
...,.
." . .....:
%'-'J
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EVAPORITES I ~
SILICICLASTICS D
Fig. 35. Paleogeographic reconstruction of proto-Atlantic seaway at end of late Bathonian-Callovia,~ deposition. Spreading axis
indicated by segmented line (axis has jumped to east and extended south and west into Gulf of Mexico (vast evaporite beds there
in Callovian)). Carbonate tracts stippled. Plain arrows indicate dominant flow direction of surface currents; broken arrow indicates
possible deep-water outflow to Tethys Sea. Dashed lines indicate present outline of continents. Scale approximate• Adapted from
Jansa (1986) and Tucholke and McCoy (1986).
114
the Bahama-Grand Banks gigaplatform. Moreover, there is evidence that the Blake-Bahama
megabank had expanded into the southeastern
Gulf of Mexico (Schlager et al., 1984; Buifler, in
press). In addition, sandy, shaley limestones of the
Tepexic Formation in Mexico (Cantu Chapa, 1971;
Scott, 1984; Taylor et al., 1984) provide evidence
of a Pacific connection (Jansa, 1986). Callovian
sediments are the oldest to have been recovered
from the abyssal Sohm-Hatteras Trough (BlakeBahama Basin). Sheridan et al. (1983), Ogg et al.
(1983), and Robertson and Ogg (1986) reported a
succession of dusky red, calcareous, silty marls
overlain by alternations of non-calcareous claystones, marly limestones, black shales, thin radiolarites, silts and sands near the base of the Blake
Escarpment at DSDP Site .534 (Figs. 3, 17 and
35). These authors estimated that the paleodepth
at the time of deposition was approximately 3000
m; this estimate is corroborated by the present
~2800 m of vertical relief between the abyssal
Callovian section and its inferred neritic equivalent
exposed along the Blake Escarpment (Fig. 10).
Seismic facies and sedimentologic analyses suggested to Ogg et al. (1983) and Robertson and
Ogg (1986) that the basal Callovian claystones
may represent contourites, formed by sluggish
bottom currents. Tyson (1984), on the other hand,
favored redeposition by slumps, debris flows and
turbidity currents. The dark colors, extensive pyritization, relative abundance of marine and terrestrial organic matter, and the presence of
phosphate concretions and glauconite in the claystones and shales, suggest periodic oxygen deficiency at, or just below, the seafloor, possibly in
local hollows. This conclusion is supported by the
lack of nearly all fossils except radiolarians and
fish skeletal debris, although bioturbation (Chondrites) is evident in some gray to green claystones.
Green-gray limestones, which represent calciturbidites, become numerous in the upper part of the
Callovian succession. Ogg et al. (1983) and Robertson and Ogg (1986) concluded that abyssal circulation near Site 543 was mainly an intra-seaway
system, and that exchange of bottom waters with
Tethys and the Pacific was minimal because of
shallow sill depths. Although the southern abyssal
portion of the narrow (50-100 km) proto-Atlantic
C.W. P O A G
seaway appears still to have been a virtual cul-desac, surface circulation was evidently sufficient to
allow nektonic Tethyan ammonite populations to
reach Cuba, the Gulf Coast and Mexico (Scott,
1984; Taylor et al., 1984; Westermann, 1984; Jansa,
1986).
The presence of broad, carbonate-rich shelves
in the Blake Plateau and Bahama basins indicates
continued vigorous circulation of warm, clear,
surface waters in the southern reaches of the protoAtlanti'c. In contrast, partly coeval precipitation
of vast evaporite beds in the Gulf of Mexico
(Louann Salt) indicates restricted marine circulation there. The eroded terrace (cut into the inferred
Bajocian-lower Bathonian sequence; Figs. 10 and
14) along the buried base of the Blake Escarpment
may be an indication, however, of important
changes in shallow-water circulation of the southwestern Sohm-Hatteras Trough during the late
Bathonian-Callovian interval. This is the time
when the mid-Atlantic spreading center jumped
eastward and propagated southward, skirted the
Blake-Bahama megabank, and connected through
fracture zones to a nascent Gulf of Mexico and/
or Caribbean spreading center (Fig. 35; Anderson
and Schmidt, 1983; Klitgord and Schouten, 1986).
This rearrangement created a narrow passageway
between the North American and African plates
along the Blake-Bahama megabank. Presumably,
surface currents accelerated by this bottleneck
could have eroded the outer raargin of the megabank and pushed the carbonate production zone
as much as 30 km landward, producing the terrace
(Figs. 14 and 29). This situation may have been
sustained "until the seaway became appreciably
wider in Berriasian-Valanginian time. Meanwhile,
an upper Bathonian to Tithonian succession could
still accumulate in deeper parts of the BlakeBahama Basin (e.g., DSDP Site 534). By the early
Berriasian the passageway had widened enough to
reduce current flow significantly, and deposition
resumed on the Bajocian-early Bathonian terrace.
However, the bounding escarpment was now so
steep and high t~~."
" km) that the .........
..... ~;-~.~ basinal
accommodation space could not be filled, thus
preventing shelf-edge progradation.
Jansa (1986) cited an explosive expansion of
radiolarian populations during Bathonian-Callov-
THE BAHAMA-GRAND BANKS GIGAPLATFORM AND THE JURASSIC PROTO-ATLANTIC SEAWAY
ian time as an indication of Unhindered circulation
between Tethys, the proto-Atlantic and the Pacific,
but this radiolarian distribution reflects only circulation of surface to mid-depth water masses, where
most of these planktonic organisms lived. Jansa
(1986) also suspected that the radiolarian blooms
were triggered by changing circulation patterns,
which created coastal upwelling zones, including
stretches along the seaward margins of the Carolina Trough and Blake Plateau Basin. It is unlikely,
however, that significant upwelling took place at
these locations, for the heightened nutrification
would probably have depleted the carbonate-producing communities that built the gigaplatform
(Hallock and Schlager, 1986; Hallock, 1988; Hallock et al., 1988).
Oxfordian time brought further improvement in
shallow-water circulation in the proto-Atlantic seaway, manifest by sustained aggradation of the
gigaplatform in the Blake Plateau and Bahama
basins, and by continued progradation in the
Georges Bank Basin, Carolina Trough, and parts
of the Baltimore Canyon Trough (Figs. 6-10 and
31A). A dramatically improved Gulf of Mexico
connection led to extensive platform and reef
building there as well (Smackover Formation;
Baria et al., 1982; Scott, 1984; Jansa, 1986; Salvador, 1987; Buffler, in press).
Jansa (1986) envisaged a northern (clockwise)
and southern (counterclockwise) gyre in the surface-current systems of the Oxfordian proto-Atlantic, although a dominant southwesterly flow was
still maintained from Tethys to the Gulf of Mexico
and Pacific. Abyssal circulation in the southern
part of the seaway was still rather restricted however. Abyssal depths of ~ 3000 m were maintained
along the Blake-Bahama Escarpment (Fig. 10)
near DSDP Site 534 (Fig. 3), where in situ Oxfordian sediments are black, horizontally bedded,
non-bioturbated, sparsely fossiliferous claystones.
These characteristics suggest that bottom waters
were poorly oxygenated and sluggish (Ogg et al.,
1983). Interbeds include redeposited reddish and
greenish claystones, representing distal turbidites
(probably from the nearby slope of the Carolina
Trough), and greenish-gray, marly, pelmicritic
limestone turbidites derived from the adjacent
Blake Plateau platform. Chondrites burrows in the
l 15
upper portions of some claystone turbidites, however, indicate intermittent oxygenation of this site.
A similar environment, ,,, 1000 km farther northeast at DSDP Site ! 05 (Fig. 3), is indicated by the
presence of greenish-gray marly limestone and
marl containing a variety of bioclasts and peloid
grains rel- oenting carbonate turbidites (Hollister
et al., 1972; Ogg et al., 1983).
Five hundred kilometers to the southeast, however, at DSDP Site 100 (Fig. 3), C,xfordian abyssal
sediments are mainly greenish-gray pelagic limestones containing numerous burrows and some
current bedding, perhaps reflecting a greater distance from terrigenous sources and more oxygenated bottom waters.
By the end of Jurassic time the proto-Atlantic
seaway had widened to ,,-1500-2000 km, and
developed broad, relatively deep connections with
Tethys, the Gulf of Mexico and the Pacific
(Fig. 36). A relatively wide abyssal plain was present in both the Sohm-Hatteras and MadeiraGambia troughs, and submarine fan complexes
were actively filling them (Fig. 17; Poag and Sevon,
1989). Unencumbered connections with Tethys and
the Pacific further improved general circulation
systems at all depths. On the shelves, reduced
siliciclastic input (Fig. 31A; Poag and Sevon, 1989)
and the acme of benthic carbonate production
broadened and thickened the gigaplatform and
built a spectacular pinnacled barrier along the
Baltimore Canyon Trough (Figs. 7 and 18). Although the North Axnerican plate continued its
northward trek and counterclockwise rotation, the
Bahama-Grand Banks gigaplatform still lay between ~ 5 and 28°N, and the depositional record
reflects optimal environmental conditions for the
benthic carbonate producers.
Abyssal environments bordering the gig~platform also underwent a significant upgrading and
progressive carbonate enrichment. At DSDP Site
534 (Figs. 3, 17 and 36), for example, the Kimmeridgian-Tithonian succession began with grayishred, calcareous (nannofossiliferous) claystones,
l y l l a n n o l o~s S l l "'
graded upward into -~'~
V"" red l-l t a l-""
limestones, and culminated with white, nearly pure,
nannofossil limestones. Similar KimmeridgianTithonian successions have been cored at DSDP
Sites 99, 100, 105 and 391 (Figs. 3, 17 and 36;
116
C.W. POAG
KIMMERIDGIAN - TITHONIAN
"
o .
i
TETHYS
20 ° -
ti~',;,.,_£2j
•". ~
I 0 o•
0
I /
I
I000
,,,
=
.:..
2000
I
KM
OO
CARBONATES
SILICICLASTICS
Fig. 36. Paleogeo3raphic reconstruction of proto-Atlantic seaway at end of Kimmeridgian-Tithonian deposition, at acme of BahamaGrand Banks gigaplatform development. Spreading axis indicated by segmented line. Carbonate tracts stippled. Plain arrows indicate
dominant flow direction of surface currents; broken arrows indicate possible flow of deep-water circulation. Dashed lines indicate
present outline of continents. Scale approximate. Numbered dots represent DSDP core sites. Adapted from Jansa (1986) and
Tucholke and McCoy (1986).
Hollister et al., 1972; Jansa et ai., 1979; Ogg et al.,
1983; Jansa, 1986). Such lithofacies reflect welloxygenated bottom waters, a depressed carbonate
compensation depth, a significant increase in calcareous plankton production (Hallam, 1975; Roth
1986, 1989), and reduced siliciclastic input. There
is little sedimentary evidence of bottom currents,
however, and benthic organisms other than foraminifera and ostracodes are rare in these sediments.
Radiolarians, dinoflagellates, calpionellids, Saccocoma (a planktonic crinoid) and small ~pelagic
mollusks are common, but nannoplankton are
dominant among the planktonic fossils, and
moderately abundant ammonites represent the
nekton.
Ogg et al. (1983) and Jansa (1986) made special
note of this abyssal carbonate injection, which
lasted through much of the Early Cretaceous.
that such a carbonate spike might result from a
shift of carbonate production from the continental
shelves to the pelagic realm during the Late Jurassic (the "Keunen Event" of Roth, 1986). Jansa
described a gradual displacement of North Atlantic
shelf carbonates by siliciclastic encroachment beginning in the Late Jurassic to Early Cretaceous.
Jansa's timing of the neritic-to-pelagic carbonate
shift, however, does not fit the sequence of events
along the U.S. margin. On this margin, coincident
with the abyssal carbonate injection, carbonate
platforms and shelf-edge reefs reached maximum
development and siliciclastic encroachment declined (Figs. 6-10 and 31A). The shelf carbonate
production was not shut down until BerriasianValanginian time. Thus, repositioning the carbonate "factories" in deeper waters may have reinforced abyssal carbonate saturation during the
Early Cretaceous, but some other mechanism must
have initiated it in the Late Jurassic. One possible
mechanism is the reduction of sili6clastic sediment
supply during Kimmeridgian-Tithonian time
(Fig. 3!A), possibly associated with increased continental aridity (Hallam, 1986), which would have
reduced the noncarbonate component of abyssal
sediments. A second, probably more important
mechanism is the "'Keunen Event", a major pulse
THE BAHAMA-GRAND BANKS GIGAPLATFORM AND THE JURASSIC PROTO-ATLANTIC SEAWAY
of coccolith carbonate production that took place
in the Late Jurassic (Roth, 1986, 1989).
By the end of the first major Early Cretaceous
depositional episode (Berriasian-Valanginian)
benthic carbonate production along the BahamaGrand Banks gigaplatform had essentially ceased
as far southwest as the Carolina Trough (Figs.
6-10, 21 and 37). Meanwhile, light-colored pelagic
carbonates were accumulating at numerous sites
(DSDP Sites 99, 100, 105, 391 and 543) in the
Sohm-Hatteras Trough, except near major sources
of terrigenous detritus (e.g., DSDP Site 603). Marine connections with Tethys, the Gulf of Mexico
and the Pacific continued to broaden (Jansa, 1986),
and the Tethys gateway may have deepened to
3000 m (Tucholke and McCoy, 1986), and comparable depths characterized the southern part of the
Sohm-Hatteras Trough (Fig. 10). The Parrish and
Curtis (1982) atmospheric circulation model indicates that westward equatorial flow was maintained over the proto-Atlantic seaway, and Jansa
(1986) and Tucholke and McCoy (1986) suggested
300
I 17
that a clockwise North Atlantic surface-water gyre
may have developed, accompanied by the initiation
of proto-Gulf Stream circulation along the eastern
U.S. margin.
Tucholke and Mountain (1979) and Tucholke
and McCoy (1986) suggested that the lensoid shape
of the Early Cretaceous deposits in the BlakeBahama Basin (Fig. 21) might be due to intensified
bottom currents that impinged upon the BlakeBahama Escarpment. Freeman and Enos (1978)
and Robertson and Bliefnick (1983) argued, in
contrast, that ubiquitous fine laminations seen in
marly Berriasian-Valanginian nannofossil chalks
at DSDP Sites 543 and 391 (Figs. 3, 21 and 37)
are not current-derived, lacking ripples, crossbedding, scours, and other current-associated sedimentary structures. Neither are the seismic reflections in this interval hummocky, as in typical
contourite drifts.
An Early Cretaceous decline in neritic carbonate
production was felt widely in the proto-Atlantic
and Tethys region, extending from the Carolina
BERRIASIAN-VALANG!NIAN
|~-
•,,:...
LLING ~'-
~,os.P
603
TETHYS
eOSDP 105
20 °-
C
I0"-
' "~::::i:i:i:i:::?.i:i:i:.::i:i:i:i:::
•
"
• ~ ~,
st
|
CARBONATES
N
.
~..
" ---'' ~'rx
~
~"~"
,I
"#
0
',~
,,' ,, ~ , ,
I000
,
2000
]
KM
SILIClCLASTICS
Fig. 37. Paleogeographic reconstruction of proto-Atlantic seaway at end of Berriasian-Valanginian deposition following demise of
Bahama-Grand Banks gigaplatform. Spreading axis indicated by segmented line. Carbonate tracts stippled. Plain arrows indicate
dominant flow direction of surface currents; broken arrows indicate possible flow direction of deep-water circulation. Dashed lines
indicate present outline of continents. Scale approximate. Numbered dots represent DSDP core sites. Note upwelling along eastwest trending segment of North American shelf edge (Carolina Trough to Grand Banks), but not along north-south trending
segment (Blake Plateau and Bahama Basins). Adapted from Parrish and Curtis (1982), Jansa (1986) and Tucholke and McCoy
(1986).
118
Trough to North Africa and into the southern
USSR and the Arabian peninsula (Jansa and Wiedmann, 1982; Hallam, 1984). Jansa and Wiedmann
attributed this decline to siliciclastic replacement
due to Cimmerian tectonism (uplift of siliciclastic
source terrains). Hallam, on the other hand, suggested that it was caused by a dramatic increase
in continental runoff as a more humid Early Cretaceous climate replaced the generally arid Late
Jurassic climate. HaUam cited as evidence the
widespread disappearance of evaporites, reintroduction of kaolinite after a period of absence, and
abundance of siderite in "Wealden" clays.
Causes of demise of Jurassic shallow-water
platforms
Why, after surviving for 42 m.y. the vicissitudes
of environmental stress associated with the birth
of a new ocean basin (shifting sediment supply,
sea-level rise and fall, altered current patterns,
changing subsidence rates, plate migration) did the
gigaplatform languish at the end of the Jurassic?
Sheridan (1976), writing before the stratigraphy of
U.S. Atlantic shelf basins was constrained by
drilling, was the first to suggest an answer to this
question. He projected poorly dated stratigraphy
from a few coastal wells in New Jersey -,,20-50
km downdip to a series of offshore seismic reflection lines. From these jump correlations he deduced that a massive bank had built up on the
New Jersey continental shelf during BarremianAptian time. This bank-building episode was followed by construction of a smaller reef-bank complex, which endured from Albian to SantonianCampanian time. Believing that a similar stratigraphic succession was represented in the Blake
Plateau Basin, Sheridan concluded that the biphase
bank-to-reef developraent and its subsequent termination were coeval in both basins. Unfortunately, none of Sheridan's offshore profiles actually
crossed any wells, and as a result, his reef stratigraphy was ,,-64 m.y. too young.
To explain the te~ination of the "P,arremianAptian" bank, Sheridan called on a regression
brought about by a major change in seafloor
spreading and a tilting of the basins to the west.
He suggested, furthermore, that significant envi-
C.W, POAG
ronmental changes related to deep-sea anoxia
might have impeded recovery of the benthic carbonate producers. He envisaged a similar scenario
for the demise of the "Albian-Santonian" reefbank (reduction of seafloor spreading, opening of
the Labrador Sea, increased cold bottom currents,
end of abyssal anoxia). Of course, with the inaccurate stratigraphy, these specific events are irrelevant to the actual history of the gigaplatform, but
the inferred "one-two punch" of relative sea-level
change accompanied by prolonged environmental
disturbance was probably correct.
More recently, Jansa (1981, and repeated in
1984) presented a synthesis of the Mesozoic carbonate platforms of eastern North America as
known before the shelf-edger~:ef was drilled in the
Baltimore Canyon Trough. Jansa reported an apparent north-to-south stratigraphic progression
(Grand Banks to the Bahamas) in the termination
of carbonate production. This progression, he inferred, was due to northward movement of the
North American plate into climatic zones increasingly hostile to benthic caloonate producers
(Fig. 38). From this interpretation he derived a
maximum rate of plate migration ( ~ 1.5 cm/yr),
which was similar to rates derived from paths of
polar wander (Irwing, 1979), although he used the
present latitudes of the basins rather than their
paleolatitudes. Almost simultaneously, Schlager
(1981) reached the same conclusion regarding
North American plate migration, although he recognized an overprint of amphi-Atlantic platformdrowning events near the end of the Jurassic.
Several subsequent authors (e.g., Schlee et al.,
1988) have perpetuated Jansa's idea.
It now seems certain, however, that the global
carbonate-production zone remained between the
latitudes of 5 and 35° throughout Mesozoic and
Cenozoic time (Zeigler et al., 1984), and that the
Bahama-Grand Banks gigaplatform did not move
out of this zone until the Late Cretaceous. Zeigler
et al. (1984) noted furthermore, that this carbonate-production zone did not expand appreciably
into higher latitudes during the warmest parts of
the Cretaceous as would be expected in a climatecontrolled (temperature-sensitive) system. These
authors suggested that instead, reduced light refraction (in the upper water column) at higher
THE BAHAMA-GRAND
B A N K S G I G A P L A T F O R M A N D T H E J U R A S S I C P R O T O - A T L A N T I C SEAWAY
LATITUDE
MO ~ SERIES P"
•o
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n.h
men
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PALEOLATITUDE
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z
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MIDDLE
Fig. 38. Chart showing chronological cessation of shallowwater carbonate production along Bahama-Grand Banks gigaplatform. Scales at top show present latitudes (above) and
end-of-Jurassic paleolatitudes (below). Dotted line represents
early concept (Jansa, 1981) of carbonate cessation progressing
diachronously from northern Grand Banks to Bahamas. Solid
line shows revised cessation curve based on current information.
Early part of revised curve is identical to Jansa's curve, but
later part shows nearly simultaneous Late Jurassic-Early Cretaceous cessation along U.S. margin as far south as Carolina
Trough. Note stepwise cessation in Blake Plateau Basin (Blake
Spur) and northern Bahama Basin. Shallow-water carbonate
tract reoccupied shelf in northern Bahamas during late Paleogene (Oligocene). Deposition in southern Bahama Basin was
never seriously interrupted.
latitudes (which would have been relatively constant through post-Paleozoic time), must have
limited photosynthesis among the carbonate producing community. In contrast, Hay et al. (1988),
using a different paleogeographic reconstruction,
concluded that carbonate production did expand
appreciably northward in the middle part of the
Cretaceous in response to a more equable climate.
Regardless of the paleogeographic contradictions, Jansa's (198 l) curve for cessation of carbonate production (Fig. 38), is stratigraphically out of
] 19
date. As Eliuk and Levesque (1988) have pointed
out, we now are confident that the Late JurassicEarly Cretaceous termination of benthic carbonate
production was nearly synchronous from the
Grand Banks to the southern Carolina Trough
(through 15° of present latitude, but only 9° of
Late Jurassic paleolatitude). By contrast, the termination of benthic carbonate production in the
Blake Plateau and Bahama basins has been steplike. At the edge of the Blake Escarpment, for
example, a temporary step-back in reef construction took place in the Berriasian-Valanginian interval (line TD-5, Fig. 28), and a permanent
drowning event took place at the end of Barremian
time (e.g., DSDP Sites 390 and 392). In the northern part of the Bahama Basin (Great Isaacs well
and ODP Site 627; Figs. 3 and 21), on the other
hand, the shallow-water platform was not drowned
until Cenomanian time; moreover, benthic production of carbonate resumed there in the Oligocene, and continued to the present. In still other
Bahaman areas (e.g., Cay Sal, Andros and Long
Island wells; Meyerhoffand Hatten, 1974; Schlager
et al., 1988; Figs. 3 and 21) benthic carbonates
constitute the entire Mesozoic-Cenozoic section.
Thus a revised carbonate-cessation curve for the
North American margin (Fig. 38) would refute
Jansa's correlation with plate motion.
Furthermore, if the paleogeographic reconstruction used here (Fig. 37) is correct, we see that
Berriasian-Valanginian shallow-water carbonates
accumulated in the Gulf of Mexico at latitudes
higher than the Ba!timore Canyon Trough to
Grand Banks region. Also, we know that shallowwater carbonate production returned to the northern Atlantic basins periodically (although on a
smaller scale) during the Cretaceous (Scholle and
Wenkam, 1982; Eliuk and Levesque, 1988) and
CenozGic (Ward and Strickland, 1985).
Recent investigators of the Bahama-Grand
Banks gigaplatform (Eliuk and Levesque, 1988;
Meyer, 1989; Poag and Sevon, 1989; Erlich et al.,
1990) agree that the most likely explanation for
the sudden abatement of benthic carbonatc production is platform drowning. Schlager (1981)
eloquently argued that long-term eustatic rise alone
is not sufficient to drown a healthy carbonate
platform. But a rapid eustatic or tectonic pulse
120
probably could do so, especially if accompanied
by other environmental disturbances, such as "inimical bank water".
Several authors have amplified such a reefdrowning scenario (Hallock and Schlager, 1986;
Scott, 1988; Macintyre, i989), which begins with
a sea-level fall that exposes the continental shelf
to subaerial weathering and soil formation. Carbonate-producing benthic communities that occupied the shelf edge migrate downward to
compatible habitats on the upper slope. This activity is followed by a rapid sea-level rise, which
floods the shelf and erodes the nutrient-rich soil
to form a turbid, hypernutrient water column. The
turbidity and excessive nutrients prevent the displaced reef organisms from repopulating the shelf
edge, until they are submerged below the critical
depth from which they can catch up with rising
sea level. Stratigraphic and petrologic evidence
indicates that such events could have repressed
Late Jurassic carbonate production on the Bahama-Grand Banks gigaplatform. Seismostratigraphic analysis shows that a widespread erosional
unconformity separates the Kimmeridgian-Tithonian carbonates from the Berriasian-Valanginian siliciclastics in the northern basins. Evidence
of freshwater diagenesis in limestone cores from
the upper part of the Baltimore Canyon reef tract
(Eliuk et al.. 1986; Meyer, 1986, 1989; G. Edson,
pers. commun., 1988) indicates that the reef was
emergent, or nearly so. (This is less well recorded
on the Canadian margin, however; Eliuk et ai.,
1986; Pratt and Jansa, 1988.) Furthermore, the
reef communities (sampled in cores) were dominated by corals, algae and stromatoporoids (Eliuk
et al., 1986; Karlo, 1986; Pratt and Jansa, 1988:
Meyer, 1989; G. Edson, pets. commun., 1988), all
of which are zooxanthellate organisms that rely
on photosynthesis for their life processes. These
organisms are particularly susceptible to the lethal
effects of turbid, hypernutrient waters. Meyer
(1989) and Erlich et al. (1990) argued convincingly
that a marine flooding event, characterized by
markedly reduced deposition, deep-water shelf faunas, and "empty-bucket" morphology (raised rim
and deep lagoon typical of platforms forced to
keep up with a rapid elevation of sea level), finally
ended the ~42-m.y. history of Jurassic platform
c.w. POAG
building in the Baltimore Canyon Trough region.
But surely, marine flooding must have inundated
the gigaplatform repeatedly during those 42 million
years (e.g., Haq et al., 1987). What was special
about the terminal Jurassic event?
The coincidence of widespread drowning (including both marine flooding and siliciclastic
smothering) of Atlantic carbonate platforms (on
both North American and African margins) and
a marked turnover of terrestrial plant taxa (Delevyoras, 1970) at or near the Jurassic-Cretaceous
transition is evidence that these events were controlled by globally effective agents. A eustatic fall
in the early Berriasian may have started the sequence of platform exposure and drowning, but
this would hardly have affected the terrest.rial
ecosystems so significantly. Jansa and Wiedmann
(1982) and Hiscott et al. (1990) cited widespread
tectonism (uplift, intraplate stress) in the circumTethys region, although according to Hallam
(1984) the carbonate to siliciclastic switch occurred
in many sections devoid of independent evidence
of tectonism. Hallam (1984) suggested that the
continental botanic changes are partial evidence
of a significant paleoclimatic shift (increased humidity) in the Early Cretaceous. He further suggested (and Zeigler et al., 1987, concurred) that
the humidity increase may have been tied to a
critical threshold of minimum oceanic size, which
the proto-Atlantic seaway may have crossed in
Berriasian-Valanginian time.
The question remains, however, why such events
did not terminate the Blake Plateau and Bahama
segments of the gigaplatform. The qualitative models of paleoatmospheric circulation proposed by
Parrish and Curtis (1982) may provide part of the
answer. Their model predicts that coastal upwelling [which can elevate trophic resources (nutrients
and organic matter) in surface waters, and thereby
suppress reef and platform growth; Zeigler et al.,
1984; Hallock and Schlager, 1986; Hallock et al.,
1988] was initiated from the Grand Banks to the
Carolina Trough (but not farther southward) at
the end of the Jurassic. Certainly, significant paleoceanographic changes of some kind are indicated
by the burst of nannoplankton abundance and
diversity that marked the late Tithonian invasion
by these organisms of the oceanic realm (Roth,
THE BAHAMA-GRAND BANKS GIGAPLATFORM AND THE JURASSIC PROTO-ATLANTIC SEAWAY
1986). UpweUing could well have provided the
nutrient sources necessary to escalate nannoplankton production, while simultaneously decimating
the platform builders. At the same time, upwelling
may have lowered the surface-water temperature
along the platform margin to levels unacceptable
for extensive carbonate production. Cold polar
temperatures in the Early Cretaceous, for which
evidence is growing (Rich et al., 1988; Ginsburg
and Beaudoin, 1990; M.A. Arthur, pers. commun.,
1990), may have produced a cold, deep-Atlantic
water mass, which could be tapped by upwelling.
Another piece of the puzzle may involve the
proximity of siliciclastic sediment sources. Even
though the delivery of siliciclastics to the Baltimore
Canyon Trough actually declined in the Early
Cretaceous (Fig. 31A; Poag and Sevon, 1989), the
narrower northern shelves still provided a shorter
route to the shelf-edge carbonate tracts than the
broad shelves of the Blake Plateau and Bahama
basins. So perhaps the combination of rapid shelf
flooding, and intensified surface-water nutrification
and cooling (brought about by relative sea-level
rise and upweiling), accompanied by an injection
of terrigenous organic matter (associated with siliciclastic encroachment), provided the particularly
detrimental environment necessary to selecti-ely
decimate the northern carbonate tracts.
Conclusions
The principal results derived from this study are
enumerated below and in Table 2.
(I) A revised Jurassic-Lower Cretaceous stratigraphic and depositional framework demonstrates
that six thick postrift depositional sequences
[Aalenian(?), Bajocian-lower Bathonian, upper
Bathonian-Callovian, Oxfordian, KimmeridgianTithonian, and Berriasian-Valanginian] can be
mapped from the Georges Bank Basin to the Blake
Plateau Basin. All six sequences have been drilled
in the Georges Bank Basin, but most are extrapolated by seismostratigraphy into undrilled portions
of the southern basins. There is no convincing
evidence of thick Triassic deposits in the sequences
drilled on Georges Bank. Postrift deposition in
this northern half of the proto-Atlantic seaway
(Sohm-Hatteras Trough) and its marginal basins
121
began prior to Bajocian time, probably in the early
Aalenian ( ~ 187 Ma).
(2) Siliciclastic deposition dominated throughout
this region during the early stages of seafloor
spreading [Aalenian(?) and Bajocian-early Bathonian time], but small carbonate tracts developed
early on the continental shelves and persistently
expanded throughout the Jurassic. Alhough each
basin experienced a somewhat different history of
carbonate production and platform development,
a rapid seaward progradation of the carbonate
tracts generally characterized late Bat.honian-CalIovian or Oxfordian deposition. The impetus for
such rapid progradation has been attributed to a
combination of high carbonate-production potential (which produced large amounts of reef-slope
debris) and rapid filling of the contiguous basin
margin with siliciclastic sediments (which, together,
reduced accommodation space). Both these mechanisms were undoubtedly important along the U.S.
Atlantic margin, but the temporal migration of
siliciclastic depocenters in the Baltimore Canyon
Trough was out of phase with that predicted by
the basin-filling hypothesis. This suggests to me
that siliciclastic encroachment from the landward
side, which forced the carbonate tract to grow
seaward, may have been equally important as a
cause of rapid progradation (or more important
in some cases) as siliciclastic basin filling.
(3) The period of rapid progradation was followed by an interval of prolonged aggradation
(Kimmeridgian-Tithonian), during which peak
carbonate production created a >5000-km-long
gigaplatform and pinnacled barrier reef system.
Maximum aggradation (~300-400 m/m.y.) and
progradation (,,-2000-3000 m/m.y.) rates for the
Bahama-Grand Banks gigaplatform are comparable to those of the well-studied Permian Capitan
shelf margin, but maximum growth differential
(horizontal versus vertical growth = 17:1) of the
gigaplatform was approximately half that of the
Capitan margin.
(4) North of the Blake Plateau Basin, carbonateproducing neritic communities of the gigaplatform
were decimated during Early Cretaceous time (Berriasian-Valanginian) by a successive fall and rise
of sea level, presumably accompanied by eutrophication due to soil-derived nutrients and organic
C.W. POAG
122
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~
m~
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THE BAHAMA-GRAND BANKS GIGAPLATFORM AND THE JURASSIC PROTO-ATLANTIC SEAWAY
matter entrained in shallow waters shoreward of
the reef tract, combined with upwelling of cool,
nutrient-rich waters seaward of the tract. Although
siliciclastic accumulation rates were low at this
time, the northward termination of carbonate production allowed the dying shelf-edge platform in
that region to be extensively buried by terrigenous
sediments. A pronounced drowning unconformity
was developed, however, where the forereef slope
was too steep to retain a siliciclastic cover.
(5) A switch from dominantly carbonate to
siliciclastic deposition on continental margins at
the end of the Jurassic was widespread around the
proto-Atlantic seaway and Tethys region, and has
been attributed mainly to Cimmerian tectonism
(greater source-to-basin relief) and increased climatic humidity and rainfall (accelerated delivery
of terrigenous detritus to offshore basins). Neither
of these mechanisms, however, appears to have
applied to the U.S. margin, where siliciclastic
accumulation rates remained low during the Late
Jurassic carbonate abatement. The cessation of
carbonate production appears to have been sufficient in itself to a!!ow siliciclastics to begin filling
the vast backreef and forereef accommodation
space.
(6) The terminal Jurassic cessation of shallowwater platform growth has been attributed by
some authors to northward migration of the North
American plate margin into a climatic regime
unsuitable for voluminous carbonate production.
A revised cessation curve shows, however, that
carbonate production ceased almost simultaneously from Georges Bank to the Carolina
Trough. Furthermore, the paleolatitude of the
northern proto-Atlantic seaway never exceeded the
limits of the tropical carbonate-productivity zone.
Early Cretaceous strata deposited at equivalent
paleolatitudes in the Gulf of Mexico, for example,
were dominantly shallow-water carbonates.
(7) Contrasting opinions explain the formation
of a > 400-km-long terrace that rims the gigaplatform along the base of the Blake Escarpment. I
offer an additional alternative by suggesting that
the terrace was created in late Bathonian-Callovian time, when the mid-seaway spreading axis
jumped eastward to open a narrow strait between
the Blake Plateau segment of the gigaplatform and
123
the African margin. I infer that this bottleneck
sufficiently accelerated surface currents to push the
carbonate tract 10-30 km to the west, while at the
same time eroding the top of the Bajocian-early
Bathonian platform.
(8) A significant Late Jurassic shift in carbonate
production from the surrounding shelf margins
into deep waters of the proto-Atlantic seaway (as
well as the Tethys and Pacific oceans) has been
widely recognized for many years. Some authors
have suggested that this shift may have resulted
from the demise of shelf-dwelling carbonate communities. The record of carbonate production in
the U.S. marginal basins, however, disputes th~s
correlation, because shallow-water carbonate production peaked in concert with the deep-water
carbonate injection. Presumably, the abatement of
shelf carbonate production did eventually enrich
the deep oceanic carbonate reservoir, but the initial
basinal injection seems to be more reasonably
attributable to an expansion of calcareous nannoplankton populations combined with a general
reduction in siliciclastic sediment supply (perhaps
related to increased climatic aridity).
Acknowledgements
I thank William P. Dillon, John S. Schlee and
three anonymous reviewers for their careful scrutiny of an earlier version of this article, and Kim
D Klitgord for discussing several aspects of geohistory. I am also grateful to Audrey Meyer,
Thomas Davies and Sherwood W. Wise for inviting me to participate in this publishing effort and
in the symposium from which it arose.
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