The presence of thick carbonate platforms and reefs beneath the

Marine Geology, 44 (1981)97--117
97
Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
MESOZOIC C A R B O N A T E P L A T F O R M S A N D B A N K S OF THE
EASTERN NORTH AMERICAN MARGIN
L.F. JANSA
Atlantic Geoscience Center, Geological Survey of Canada, P.O. Box 1006, Dartmouth,
N.S. (Canada)
(Accepted for publication March 12, 1981)
ABSTRACT
Jansa, L.F., 1981. Mesozoic carbonate platforms and banks of the eastern North American
margin. In: M.B. Cita and W.B.F. Ryan (Editors), Carbonate Platforms of the PassiveType Continental Margins, Present and Past. Mar. Geol., 44: 97--117.
The Jurassic--Lower Cretaceous carbonate platforms and banks form a discontinuous
belt extending over distance of 6000 km from the Grand Banks up to the Bahamas. Six
types of carbonate buildups are recognized and document the variability of depositionai,
paleo-oceanographic and tectonic processes on the eastern North American margin. The
composition of the carbonates closely resembles the Recent deposits of the western Great
Bahama Bank since oolitic shoals were present near to the shelf edge, and skeletal, peloid
wackestones and biomicrites were deposited in the inner part of the platform. Coraistromatoporoid and sponge bioherms were only rare constituents of the carbonate banks.
The thickness of carbonate buildups progressively increases southward along the margin,
attaining a thickness of more than 5 km on the Bahamas. The carbonate platforms also
become younger southwards, which is thought to reflect the northward movement of the
North American plate of less than 1.5 cm]yr. The carbonate platforms were seeded over
the continental basement following the taphrogenic period of plate tectonics.
Building of carbonate ramps which characterized the Early Jurassic, has began during
the transitional period between continental rifting and early drift. The second stage in
construction of the carbonate platforms and offshore banks proceeded mainly after
separation of the continental plates.
INTRODUCTION
The presence of thick carbonate platforms and reefs beneath the eastern
North American margin was first indicated by geophysical studies and dredging
in the mid 1960's by Heezen and Sheridan (1966), Sheridan et al. (1969),
Emery and Uchupi (1972), Sheridan (1974, 1976}. However, it was not until
extensive offshore exploration for hydrocarbons, that the thick carbonate
sequences beneath the shelf and upper slope were confirmed by drilling
(McIver, 1972; Jansa and Wade, 1975; Benson/Sheridan et al., 1978; Eliuk,
1978; Amato and Simonis, 1980).
Presently more than 150 wells have been drilled on the shelf and upper
slope off eastern Canada. In addition there are five deep stratigraphic tests on
0025--3227/81/0000--0000/$02.75 © 1981 Elsevier Scientific Publishing Company
98
the U.S. shelf and ten Deep Sea Drilling Project holes on the U.S. lower slope
and rise (Fig. 1). This drilling activity confirmed that a belt of discontinuous
carbonate banks and platforms extends from the Grand Banks southwards to
the Bahamas. This belt can be traced further around the northern periphery
of the Gulf of Mexico up to Campeche platform in Mexico (Meyerhoff, 1967;
Bryant et al., 1969; Paulus, 1972). The total length of this carbonate belt
exceeds 8500 kin. Except for local areas in Mexico, this belt has not been
tectonically disturbed, in contrast to similar Mesozoic carbonate platforms in
Europe, which were involved in an Alpine orogenesis. Thus, off the eastern
North American margin a continuous transition from the nearshore facies
across the shelf into the deep ocean basin can be studied, providing information
P
99
about the role of carbonates in the process of constructing the passive continental margin. The development of carbonates further provides an insight
into the early stages of the plate tectonic processes, particularly the late rifting
and early drifting periods.
In contrast to principally E--W orientation of the Mesozoic carbonate platforms surrounding the European Tethys, the carbonate platforms of the
eastern North American margin are mainly NE--SW oriented. This orientation
permits comparison of the latitudinal movements of the continental plates
with the path of suggested apparent polar wandering (Van der Voo and French,
1974; Irwing, 1977) through the changes in carbonate deposition.
METHODS
In general, the Mesozoic--Cenozoic of the eastern North American margin
is not exposed, and mostly is covered by ocean. Thus all interpretation is
based on subsurface studies such as the study of drill cuttings, mechanical
logs, and multichannel reflection seismic profiles. Except for the Bahamas,
only limited possibility exists for direct observation from submersibles of the
carbonate in some of the canyons which cut into the upper continental slope,
as has been pioneered by Neumann and Ball (1970) and Ryan et al. (1978).
Recent rapid growth in the understanding of the formative processes of
carbonate buildup has resulted in confusion regarding the terminology used to
describe such sedimentary bodies. The terminology used in this paper principally follows Wilson (1975) and is summarized in Table I.
Regional geology
The eastern North American margin has been built over the fractured
continental edge of the North American continental plate in response to the
Fig.1. Location map. Locations of offshore wells which encountered carbonate sequences
are marked by a black triangle. Continental offshore stratigraphic test wells (COST) are
marked by a white triangle and the Deep Sea Drilling Project wells (DSDP) are marked by
a square.
The full identification of abbreviated names of the wells are: K-36 = Amoco-Imp-SkeUy
Egret K-36; J-34 = Amoco-Imp-Skelly Carey J-34; G-72 = Amoco-Imp Heron G-72; C-56
= Elf et al. Emerillon C-56; D-35 = Mobil-Tetco Dauntless D-35; A-57 = Shell Sauk A-57;
J-56 = Shell Abenaki J-56; D-42 = Mobil-Tetco Cohasset D-42; G-32 = Shell Demascota
G-32; 0-25 = Shell Oneida 0-25; 1-100 = Shell Mohican 1-100; G-2 = COST G-2; G-I =
COST G-1 ; B-2 = COST B-2; B-3 = COST B-3; GE-1 ffi COST GE-1 ; 107 = DSDP Site 107;
108 = DSDP Site 108; 390 = DSDP Site 390; 39211= DSDP Site 392; 391 = DSDP Site 391;
98 = DSDP Site 98; 101 = DSDP Site 1 0 1 ; 5 = DS~)P Site 5; 100 = DSDP Site 100; 4 =
DSDP Site 4; 99 = DSDP Site 99; G.L = Great Isaac 1; W. = Williams 1;A.I. = Andros
Island 1 ; E. = Eleuthera; N.P. = New Province; S.S. = San Salvador; L.L = Long Island 1.
The figure also shows the location of the seismic lines shown in Figs.4, 5 and 8. The
dotted line shows position of the carbonate shelf trend as established by reflection surveys
data. The continuity or discontinuity of carbonate platforms along this trend has not yet
been established, for lack of drilling data and large separation of the seismic lines.
100
TABLE I
Terminology of carbonate masses
Carbonate ramp:
Carbonate buildup:
Carbonate platform (carbonate shelf):
Bioherm:
Reef:
Offshore banks:
huge carbonate bodies build away from positive areas and down a gentle paleoslope; no
significant break in slope exists
a body of carbonate sediment formed by
combined action of organic production and
chemical precipitation; no inference to origin
or size is included
huge carbonate bodies build up with a more
or less horizontal top and abrupt shelf
margin, where "high-energy" sediments occur
a buildup whose internal composition is
shown to be largely derived from in situ production of organisms, and which does not
exert any control on the surrounding
environment (usually smaller in size)
a buildup formed in part by a wave-resistant
framework constructed by organisms, and
exerting some control over the surrounding
environment
complex carbonate buildups of great size and
thickness located well offshore from the
coast
North Atlantic opening. Infilling over marginal basins and highs has created a
continental shelf ( < 2 0 0 m depth) 2--540 km wide. The shelf is widest off
Newfoundland and narrowest adjacent to southern Florida (Fig.l). Several
Mesozoic--Cenozoic sedimentary basins underlie the shelf (Sheridan, 1974,
1976) which are separated by shallower subsided platforms or basement
"highs" (Maher, 1971; Jansa and Wade, 1975; Klitgord and Behrendt, 1979)
(Fig.2). The basins are filled by up to 17 km of sediments which record the
complex tectonic history of the margin (Schlee and Jansa, 1981).
The sedimentary history of the recent continental margin started in the
Triassic. In the Scotian and Georges Banks basins, the Pateozoic basement is
overlain by Triassic red beds and evaporites, with evaporite deposition extending f r o m Rhaetian into Hettanginian--Sinemurian (Jansa and Wade, 1975;
Barss et al., 1979). The continental margin was transgressed by an open sea in
Sinemurian time, and shallow-water carbonates became to be deposited.
Carbonate deposition with minor interruptions continued into Early Cretaceous when, in response to a major regression, the deltaic clastics prograded
over the carbonate platforms and offshore banks, and terminated the carbonate sedimentation on the margin. Even though the carbonate deposition was
temporarily resumed on the margin during the Albian, Turonian, Santonian-Maastrichtian and Eocene, the carbonate platforms similar to those of the
Jurassic--Early Cretaceous were not constructed during these later periods.
Through most of the Mesozoic the margin was passively subsiding, with only
minor tectonic disturbances occurring near the Triassic--Jurassic boundary,
101
~TS
ture
GSC
Fig.2. Map showing the location of sedimentary basins, basement highs and platforms
separating the basins. (Compiled from Jansa and Wade, 1975; Klitgord and Behrendt,
1979.)
mid-Jurassic and Early Cretaceous (Jansa and Wade, 1975; Schlee and Jansa,
1981). The Early Cretaceous disturbance is locally manifested by profound
unconformity.
As will be discussed later, the carbonate platforms along the eastern North
American margin are diachronous, the platforms to the south being younger
than those occurring along the northern part of the margin. The carbonate
deposition in the southernmost part of the margin (Florida and the Bahamas)
still continues today. The origin of the Bahama Platform was reviewed by
Mullins and Lynts (1977) and in detail is discussed by Schlager and Ginsburg
(1981, this volume).
102
Detailed stratigraphic description of the Mesozoic-Cenozoic sedimentary
strata of the eastern North American margin can be found in Jansa and Wade
(1975), McIver (1972), Given (1977), Schlee and Jansa (1981), Scholle
(1977}, Amato and Simonis (1980), and schematically is presented in Fig.3.
EASTERN " - - ~
i
NORTH AMERICAN
SHELF
(.9
AGE
i ~
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L
5
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.
.
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.
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.
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HETT,
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.
NORIAN
--- = =-~___
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CARNIAN . . . . . .
.~ '~,. ':9:g,z
GY;
Fig.3. Generalized stratigraphic column of the Mesozoic--Cenozoic of the eastern North
American margin. Extensive carbonate development is characteristic of the Jurauic--Early
Cretaceous Period (Superunit 2). The designation of the seismic markers horizons on this
Figure differs from the local designation as used on Figs.4, 5 and 8. The seismic horizon
K w ffi Wyandot marker; Kp ffi Petrel; Jl ffi Abenaki; J4 ffi Salt. The key to the ltthologic
symbols is shown in Fig.6.
103
E X T E N T A N D F O U N D A T I O N O F T H E C A R B O N A T E BUILDUPS
The carbonate buildups on the eastern North American margin have a
complex history with stages of carbonate ramp, platform and offshore carbonate banks development. To simplify the discussion, the term carbonate
platform and/or carbonate buildup is used for all these facies during the
general description of the carbonate deposition.
The regional extent of the carbonate platforms can be established off
eastern Canada from drilling data. Because limited information exists off the
U.S. coast the position of the carbonate shelf edge can only be established by
the integrating multichannel reflection seismic data with sparse well control
(Schlee et al., 1979}.
This study shows that the modern and paleoshelf edge fortuitously coincide
only locally. In the central and northern parts of the Scotian Basin, the carbonate paleoshelf edge occurs beneath the present mid-shelf (Fig.4), but in the
southwestern portion of the same basin the carbonate paleoshelf edge is
beneath the present continental slope (Fig.5). The position of the paleoshelf
edge similarly varies off the U.S. margin. From Georges Bank southwards, the
carbonate paleoshelf edge occurs under the lower part of the continental slope
(1000--2000-m isobath) and probably under the upper continental rise off
the Long Island platform and off Cape Hatteras (Schlee et al., 1979). South
of Cape Hatteras the carbonate platform edge is under the upper part of the
Blake Escarpment where 2500 m water-depth are prevalent. Further southwards the carbonate banks are strikingly higher and are partially exposed on
the Bahamas and in Florida, where the paleoshelf edge is located approximately near the present shelf edge.
Seismic data and drilling information demonstrate that the carbonate buildups form a discontinuous belt along the entire margin, probably composed of
more or less discrete segments of carbonate banks and platforms up to 400 km
in length and variable in width. The shape and size of the carbonate platforms
cannot be presently established with any certainty because of the limited
control. Dimension of the carbonate platforms are dependent on the sedimentary processes active in the region and vary in both vertical and horizontal
directions. The resulting morphological shapes of carbonate platforms recognized from the combination of multichannel reflection seismic data and offshore boreholes on the eastern North American margin are schematically
summarized in Fig.7 and will be discussed further in the paper. In general, the
Lower Jurassic carbonate platforms are much broader than the Upper
Jurassic--Lower Cretaceous carbonate banks. On the Scotian Shelf the width
of the Upper Jurassic platforms varies between 20 and 80 km (Eliuk, 1978).
Similar widths can be assumed for the carbonate buildups off the U.S. from
the seismic reflection profiles north of Cape Hatteras (Schlee et al., 1979).
South of Cape Hatteras the width of the platforms rapidly increases. The
carbonate platform underlying the Blake Plateau is approximately 200 km
104
NNW
SSE
0 km
20
40
60
"V
I"11
r~
Z
°
o
LI,I
Z
--4
2.0"
bJ
r"-n
I-
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rn
4,0-
m
e4
6.0-
G$C
CARBONATESHELFEDGE
Fig.4. The reflection seismic line from the southern part of the Scotian Basin (for location,
see Fig.1 ) shows the carbonate platform and the paleoshelf edge located beneath the
present mid-shelf. The carbonate platform (A b) overlies the basement (B), A brief stage of
the building of the carbonate platform occurred again at the time of horizon O, but at this
time the basin was filled by sediments and the carbonate platform developed on the clastie
sediment substratum and seaward of the previous position of the platform. For legend
see Fig. 8.
NW
0 ~
SE
20
40
60
O
Z
8w
20
03
w
k-
40
60
Fig.& The multichannel seismic line from the Scotlan Basin (for location, see Fig.l) showing seaward progradation of the Jurassic carbonate platform (Pp). This platform corresponds to the morphological carbonate platform type A on Fig.7. This Figure also shows
the sharp seaward edge of the platform (A b) resulting either from faulting or erasion. The
carbonate platform paleoshelf edge at this locality was located during the Jurassic near to
the present shelf edge. For legend see Fig. 8.
105
wide and in the vicinity of Bahama Banks it is about 240 km wide. The thickness of the carbonate sequences in the belt also varies and increases southwards
from 343 m on the Grand Banks through 3300 m in the Georges Banks Basin
to more than 5000 m on the Bahama Banks (Fig.6).
The segmentation of the carbonate belt into separated platforms and banks
is due to several reasons. One of them is local development of prograding
clastic deltas, which breached the carbonate belt and prevented carbonates to
be deposited as in the Scotian Basin (Jansa and Wade, 1975). Another reason
is the variable subsidence of the margin. The rapidly subsiding continental
blocks developed into marginal basins and were filled by clastic deposits which
did not provide a suitable environment for sustained carbonate deposition.
More favourable conditions for development of carbonate platforms existed
over more slowly subsiding basement blocks. A seismic profile over one of the
basement highs (the LaHave platform; Fig.4) shows that the Jurassic carbonate platform overlies a Paleozoic basement high. However, this profile also
demonstrates that the shallow-water carbonate deposits extended into the
S
H 1230km ~
GRAND
BANKS
46O ~
SCOTIAN
BASIN
GEORGES
BANKS
Sandstone
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Limestone
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Peloid
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Dolomite
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GREAT
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SCHEMATIC STRATIGRAPHY OF THE
CARBONATE SEQUENCES OF
ENA MARGIN
Fig.6. Lithology and the stratigraphic position of strata enclosing the carbonate sequences
on the eastern North American margin. Note the N--S diachronism in the initiation and
termination of the shallow-water carbonate deposition.
106
sediment filled portion of the sedimentary basin during Early Cretaceous, so
that the substratum for the carbonate buildup in this area was the previously
deposited sediments. Thus carbonate platforms could be initiated over any
type of substratum -- continental, igneous, metamorphic basement or a previously filled sedimentary basin, as long as that the floor was shallow enough to
lie within photic zone.
From the interpretation of the East Coast Magnetic Anomaly as thickened
oceanic crust, Klitgord and Behrendt {1979), Grow et al. (1979), Schlee et al.
{1979) have suggested that the carbonate platforms off some parts of the U.S.
coast were built over oceanic crust. The newly formed oceanic crust in the
present-day oceans generally occurs 2.5 km below sea level and either forms
ridges (Sclater and Francheteau, 1970) or deep axial troughs such as the Red
Sea (Ross and Schlee, 1973). The present author thus finds it difficult to find
a mechanism which could build a carbonate platform comprised of a shallowwater facies directly on the oceanic crust. Exceptions, however, exist over
some of the structurally higher features such as oceanic ridges (J-anomaly
Ridge, Tucholke/Vogt et al., 1979) islands and seamounts {Pacific Ocean)
which can be the loci of shallow-water carbonate deposition due to proximity
to sea level. But these exceptions at least are only isolated occurrences of
carbonates and do not favour continuous features along the continental
margin. In addition they are mainly related to the intra-plate volcanism.
The reflection multichannel seismic data cannot resolve the character of
the substratum or the internal structure of the carbonate platforms specially
where these are thick. The mode of origin of carbonate platforms on the
deeper parts of the North American eastern margin still remains enigmatic.
M O R P H O L O G I C A L TYPES OF C A R B O N A T E P L A T F O R M S
The construction of carbonate buildups is achieved by a combination of
biochemical and mechanical systems {Wilson, 1974). Thus any variation in
these systems (subsidence, tectonism, eustatic sea-level change, climatic
changes, biological productivity) is reflected in a change of the morphology
of the deposited carbonate buildup. The shape of the carbonate platforms on
the eastern North American margin was further influenced by their initial
location {basement re-entrant or promontory) on the continental margin. Six
types of carbonate buildups have been recognized which vertically and laterally grade into one another in response to local geologic conditions. The
carbonate platform types are:
(1) Prograding carbonate platform (Fig.7, A). In this type the shallowwater carbonates prograde seaward over the deeper slope deposits. This type
of platform or bank will develop when the carbonate production exceeds the
combined rate of subsidence and sea-level fluctuation. The presence of such a
carbonate buildup is indicated by U.S.G.S. seismic line 2 in the Baltimore
Canyon Trough (Fig.7) (Schlee et al., 1979; Schlee and Jansa, 1981) and also
occurs in the Scotian Basin (Fig.5).
(2) Stationary carbonate platform (Fig.7, B). This type of carbonate platform is the result of an equilibrium between carbonate production and the
107
......
,
---I
,o
,
[7:.: ::---~
....
B ~~-~
~
I i iI J II ~
i
i
i
~ ~ ,
CI
~.l!i!l!l
I
c2
I I I
]
PROGRADING
(Corboneleproduction)
subsidence)
STATIONARY
(Cc]rbonoteproduction:
~ _ _ _
\ Seismicollytronsperent
{ reef or dic)geneticfront)
RETREATINGLANDWARD
1
~
(Corboneteproduction(
[I. I i ~ i ~ ~
subsidenceond eustasy}
i ] i
I
r"~7~=~--
~ ~ ......./..
'
RETREATINGSEAWARC
(Clastics input)
corbonoteproduction)
DESTRUCTIONAL
( Ero,sional)
D2~
~
{)ESTRUCTIONAL
~.-~ ~ 1
( Faulled)
G$C
Fig,9, Schematic presentation of the morphological types of carbonate platforms recognized f r o m the combination of reflection seismic profiles and offshore drilling results. The
integrated influence of carbonate production, subsidence and eustatic sea-leve] fluctuation
played a major role in shaping of the carbonate platforms (Types A--C). The seaward edge
of the carbonate platforms was later modified by tectonics and submarine erosion (Types
D 1 and D:).
combined effects of subsidence and eustatic sea-level change. It is characterized by the steep seaward edge of the carbonate platform with the shelf edge
being stationary and building upward. This type of platform is characteristic
of the southern part of the eastern North American margin with particular
examples being the Blake Plateau and Bahamas.
The seismic section across the seaward edge of the Blake Plateau shows the
seaward edge of the platform to be seismically transparent (Shipley et al.,
1978), which has been interpreted by Benson/Sheridan et al. (1978) as an
indication of a buried reef. According to the present author, the steep slope
along the Blake Plateau can be maintained by early submarine lithification of
carbonate sediment, as previously suggested for the margin of Bahamas by
Neumann {1974).
(3) Retreating carbonate platforms can be subdivided into two subtypes:
(a) landward, and (b) seaward retreating.
(a) The landward retreating carbonate platform (Fig.7, C, ) results from prolonged subsidence and seaward tilting of the carbonate platform, with carbonate production at the shelf edge finely balanced with the subsidence. During
108
transgressive periods, the carbonate shelf edge is drowned and carbonate
production is unable to re-establish the growth at the point of the previous
shelf edge, but a new favorable condition develops in a shallower, submerged
portion of the carbonate platform, where a new carbonate shelf is reestablished. This type of carbonate platform edge development probably
characterizes the late Middle Jurassic of the Scotian Basin.
(b) The seaward retreating carbonate platform (Fig.7, C2) was widespread
during the Late Jurassic--Early Cretaceous and is interpreted to occur in the
Scotian Basin, Georges Bank Basin and the Baltimore Canyon Trough. This
type of platform develops in response to the increased input of clastics into
the basin by rivers and deltas, which dilute the carbonate and form unfavorable conditions for the carbonate secreting organisms in the nearshore zone.
Thus the clastics slowly displace the carbonate platform in an offshore direction and in the final phase, particularly during a major regression, they
progress over the carbonate shelf edge and terminate carbonate deposition.
This development is characteristic for most of the Early Cretaceous of the
eastern North American margin.
(4) Destructional carbonate platforms can also be subdivided into two
types: (a) erosional and (b) faulted.
(a) The carbonate paleoshelf edge located under the present upper continental slope, was subjected in some places to extensive deep-sea erosion,
during pre-Early Miocene time (Figs.7, D1; 5, 8). This erosion was related to
the development of a contour current flow in the North Atlantic (Jansa et al.,
1979) which locally cut back the paleoshelf edge up to 30 km (Schlee et al.,
1979; Ryan, 1978).
(b) In some areas of the continental margin the carbonate shelf edge was
downfaulted, perhaps as a result of the growth fault (Fig.7, D2). Such a fault
is recognizable on a seismic section off the LaHave platform (Fig.8).
Both of these destructional mechanisms result in the superposition of
pelagic carbonates or deep-water shales over shallow-water carbonates. Some
of the Ammonitico Kosso lithofacies of the Mediterranean Jurassic may have
originated by a similar process.
LITHOLOGY AND COMPOSITION OF THE CARBONATE SEQUENCES
The composition of the carbonate sequences reflects the eustatic-tectonic
variations in the development of the margin. The carbonate sequences encountered by drilling along the margin were fully penetrated only north of Long
Island platform (Fig.6). Lithologic description of rocks given here is pertinent
only to that area. In the extreme south the platform was again fully penetrated on the Bahama Banks (Tator and Hatfield, 1975; Meyerhoff and
Hatten, 1974, fig.l).
At the northern part of the eastern North American margin two major
periods of carbonate deposition which constitute the second super unit of the
lithologic subdivision of the Mesozoic--Cenozoic, can be recognized (Fig.3).
The first period of carbonate deposition spans Sinemurian--pre-Bajocian and
109
SE
NW
f) km
40
20
60
BO
2.0
Z
oo
w 40
h-
e,l
6.0
8.0
S
SALT
U
UNCONFORMITY
r ' g ~ l r ~.)f~I,I
GSC
Fig.8. Downfaulted carbonate shelf edge of the carbonate platform (A b). The carbonate
platform as shown by this multichannel seismic section (for location, see Fig. 1) extends
beneath the present continental slope, with the seaward edge of the platform being faulted.
The carbonate platform at this locality corresponds to the destructional-type platform
(Fig.7, type D2).
the other Bathonian--Hauterivian (Fig.3). These two carbonate depositionaJ
periods are separated by a period of clastic deposition during Bajocian--Early
Bathonian, when continental clastics were deposited on the shelf as a result of
rejuvenation of the source area and a minor tectonic disturbance correlated to
the mid-Kimmerian tectonic phase by Jansa and Wade (1975). Another interruption of carbonate deposition occurred during the Callovian transgression,
when mainly marine shale was deposited (Jansa and Wade, 1§75; Given, 1977).
The third one, which is less pronounced than the previous two occurred in the
middle of the Kimmeridgian and is characterized by alternation of argillaceous
carbonates and shales, resulting from an eustatic sea-level rise at this time.
The oldest carbonates which are probably Sinemurian in age (Barss et al.,
1979) overly Lower Jurassic halite in the Scotian Basin. These carbonates
consist of primary and secondary dolomites with minor limestones. In some
of the dolomites, stromatolites and nodular anhydrite interbeds indicate tidal
deposition under hypersaline conditions. Later in the Lower Jurassic, oolitic,
peloid, dark-gray micritic and minor skeletal wackestones, which are intercalated with secondary dolomites, indicate normal salinity. In the Georges
Bank Basin, the bedded anhydrite is interbedded with shallow-water carbonates (Amato and Simonis, 1980) indicating more restricted depositional conditions than those in the Scotian Basin during late Early and early Middle
Jurassic. The early period of carbonate deposition was terminated by a
regression in the Bajocian when continental clastics were deposited over the
carbonate surface.
110
Carbonate sedimentation was renewed by a transgression in the Late
Bathonian when widespread blankets of high-energy, shallow-water oolitic
and oncolitic limestones in the Scotian and Georges Bank basins were
deposited. The oolitic shoals were drowned by a minor transgression in the
Callovian when a widespread shale was deposited over the carbonate (Fig.3).
Renewed oolitic, peloid and minor skeletal limestone deposition characterizes
the Oxfordian. During the Kimmeridgian the depositional environment
deepened and skeletal wackestone, packstone, biomicrites and shales were
deposited on the shelf. During the Late Kimmeridgian in local areas near to
the paleoshelf edge, conditions favorable for the growth of coral-stromatoporid bioherms and perhaps reefs were established. Pelagic sedimentation
continued during the Tithonian when near to shelf edge biomicrites with
Calpionellids were deposited (Jansa et al., 1980; Ryan et al., 1978).
At the Jurassic--Cretaceous boundary the sea retreated from most of the
paleoshelf region as the result of major regression which is indicated by an
increase in the amount of quartz sand mixed with skeletal carbonate. Clastics
eventually choked carbonate deposition at the northern part of the margin in
the Berriasian--Hauterivian. During the transitional period from dominantly
carbonate to dominantly clastics deposition, sponge biostromes were locally
developed near to the carbonate paleoshelf edge (Eliuk, 1978).
The cementation of carbonates in the platforms is pervasive. The cement is
mainly blocky, and both low and iron-rich sparry calcite. Minor porosity
exists in the Lower Jurassic dolomites and the Upper Jurassic secondary dolomites and averages about 6%; permeability is less than 0.1 md (Eliuk, 1978;
Amato and Simonis, 1980).
In addition to the vertical variability in composition, the carbonate
sequences also show significant lateral facies changes. These changes are
schematically illustrated on Fig.9, which demonstrates that the Lower Jurassic
nearshore sabkha and tidal-flat deposits grade in an offshore direction into
oolitic shoals and shallow subtidal environments characterized by pelleted
CARBONATE PLATFORM WITH LOCAL BUILDUP DEVELOPED FROM A RAMP
~
~
.~
!
~
'
^
I ~ ~ / ' / / ' ' ' `
I
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'~
1
''
,.........
rt
~-PlatformStage
. ....., .
"ff~,..':,.
"
'
Ar,',ericonMor,
I00km
~tage
.''" //
11,. . . .
. -
(example:
iol
Fig.9. Schematic cross-section of the Jurassic continental margin on the northern Eastern
North American margin. The section demonstrates the lateral facies variation across the
margin and the three principal stages of the Jurassic carbonate buildups: the Lower
Jurassic carbonate ramp, the Upper Jurassic carbonate platform and latest Upper Jurassic-early Lower Cretaceous carbonate bank.
111
carbonates and intercalated dark-grey micritic limestones. Because no pronounced shelf edge has been recognized in the Lower Jurassic carbonates it is
suggested here that they formed a ramp (Ahr, 1973), similar to that in the
present-day Persian Gulf. The carbonate platform was constructed later, during the second, Late Jurassic period of carbonate deposition, which started
with the development of extensive oolitic shoals comprised of radial oolites,
algal oncolites, minor quartz sand grains and skeletal debris. The cyclic alternation of oolitic grainstones, skeletal and peloid wackestones continued
throughout the Late Jurassic, with bioherms developed near the shelf edge
during periods of minor increases of sea level. Accentuated seaward progression of clastic deposits caused during the Late Jurassic the carbonate platform
to retreat seaward. During the final phase of carbonate deposition, the
morphological form of an offshore bank was attained (Fig.9). This bank had
an inclination of the seaward slope of 20--30 ° (Eliuk, 1978). The textural and
lithologic composition of the carbonates strongly suggest that the depositional
conditions and type of sediment generated during the Jurassic, is similar to
those presently occurring on the western side of the Great Bahama Bank.
The petrological details of the carbonates from the deeper slope of the
Jurassic carbonate platforms are not known. The evolution of the carbonate
platform, therefore cannot be followed into the deep oceanic basin, where
pelagic carbonates were deposited (Hollister/Ewing et al., 1972; Benson/
Sheridan et al., 1978). The oldest known deep-water sediments in the North
American Basin are Oxfordian--Lower Tithonian reddish pelagic limestones
which are developed in a microfacies similar to those in Tethys regions such
as nannofossil limestones with filaments, Saccocoma and ammonite aptychi
{Bernoulli, 1972; Jansa et al., 1979). These are overlain by Upper Tithonian-Barremian white pelagic limestones rich in radiolarians, calcispherulids and
rare calpionellids. Both of these limestone sequences, named the Cat Gap and
Blake Bahama Formations in the western North Atlantic (Jansa et al., 1979)
are synchronous with the second period of limestone deposition on the shelf.
Lack of terrigenous grains in the deep North American Basin limestones is a
further indication that the Jurassic outer-shelf carbonate banks were effective
barriers for trapping of clastics and were thus instrumental in building a broad
continental margin.
Diachronism and plate motion
The stratigraphic development of the carbonate platforms along the
eastern North American margin is far more complex than presented here. The
previous stratigraphic discussion is derived mainly from the Scotian Shelf and
Georges Bank area. If we examine the rather sketchy stratigraphic information
available for the margin south of Georges Bank, it will be noticed that the
vertical facies development remains similar to that described here (Fig.6). The
evaporites at the base of the sedimentary wedge are overlain by dolomites
interbedded with evaporites. The second carbonate cycle is characteristic by
presence of shallow-water limestones, mainly pelleted, oolitic and less skeletal
and biomicritic in composition, with reefs occurring more frequently toward
112
the southern end of the margin (Tator and Hatfield, 1975; Enos and Freeman,
1978). Despite inconclusive age determination for the base of the basal dolomitic sequence south of the Scotian Basin, available data indicate that the
carbonate sequences along the margin are diachronous and become progressively younger southward. At the southern end (Bahamas) the carbonate
deposition is continuing until today.
If we assume that cessation of carbonate deposition is due to a shift of the
region through the climatic zone, then the maximum northward movement of
the North American plate can be calculated (Fig.10). The obtained value is
1.5 cm/yr, which represents the maximum value for the northward movement
of the North American plate. This value is comparable to the change in paleolatitude calculated from the apparent polar wander path by geomagnetic
methods, which indicate an approximate 1 cm/yr northward movement for
the North American plate during Late Mesozoic-Cenozoic time (Irwing,
1979).
Another theory to consider is that the cessation of carbonate deposition
along the eastern North American margin resulted from changes in the width
of the climatic zones. During the Jurassic the climate was more uniform, so
that no more than three climatic zones can be recognized, the Polar zone
being lacking. Deterioration of the climate during the Cenozoic resulted in the
N
S
MY'
400
300
______J_
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:&LOG
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65
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i
,
i
,
i
G
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E
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i- CAMPANIAN
i
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l184~lual
136
l~r~
157
b
'
~
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ALBIAN
i
~
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I
~RREMIAN
..... +-
-
+
;
-I- OXFORDIAN
PALEOLATITUDESHIFT OF
THE NORTHAMERICANPLATE
(I.5 cm./yr. )
GSC
/
19r-J
Fig.10. Graphic presentatlon o f the cessation o f carbonate deposition on the margin. The
Figure demonstrates that the termination of the carbonate deposition along the margin is,
in general, aligned along the slightly curved line. The intersection of this line with the time
lines and latitudinal po6ition of the locality,has been used to calculate the northward
movement of the North American plate during the last 180 m.y. The average movement
was less than 1.5 cm per year. For the location of individual wells, see Fig.1.
113
development of four climatic zones and narrowing of the previous climatic
belts in an equatorial direction. This progressive shift of the boundaries of the
climatic zones southward could explain the progressive cessation of the carbonate deposition in the same direction. However, there did not appear to have
been any indication of climatic deterioration during the Late Jurassic-Cretaceous when latitudinal changes in the carbonate distribution had already
occurred. Thus in view of the presently available data, the hypothesis of
northward moving continental plate provides the most satisfactory overall
explanation for the distribution of carbonate deposits on the eastern North
American margin.
Carbonate platforms and plate tectonics
The carbonate platforms developed in low and mid paleolatitudes of the
eastern North American margin at a certain stage of the plate tectonic process.
Their initiation followed the late period of taphrogenesis and they are locally
unconformable on the underlying evaporite or red bed sequences (Jansa and
Wade, 1975; Schlee and Jansa, 1981). Schlee and Jansa have suggested that
this unconformity may be correlative with the breakup unconformity of
Falvey (1974}. The oldest oceanic crust drilled to the present day in the North
Atlantic is mid-Oxfordian in age (Hollister/Ewing et al., 1972; Jansa et al.,
1978), however, the reflection seismic profiles show that older sediments
overly the oceanic crust closer to the continental margin (Shipley et al., 1978).
Thus the separation of the continental plates and incipient continental drift
must have occurred between the Sinemurian and the Bajocian.
The initiation of the carbonate buildups on the eastern North American
margin post-dates a major change in the structural behaviour of the margin
{Schlee and Jansa, 1981), when the mostly vertical tectonics associated with
the formation of grabens and half-grabens was replaced by periods of regional,
gentle, downwarping of the continental crust. The downwarping contributed
to the larger uniformity of deposition allowing a broad transgression of the
shallow open sea over the margin. Such conditions were very favorable for
carbonate deposition, because the land relief was low, and the climate was
warm and dry as indicated by deposition of widespread continental red beds
associated with evaporites during the earliest Jurassic.
Extensive development of carbonates follows the taphrogenic period not
only on the eastern North American margin, but also on the eastern South
American margin where the carbonate platforms are Albian to Cenomanian
(Campos et al., 1975) and in the Gulf of Suez, where they are Cenomanian to
Early Eocene (Garfunkel and Bartov, 1977). In the Suez area, the development of the carbonate platforms predates the continental separation. On the
North and South American margins the initial stages of the carbonate buildups
span the transitional pre-rifting to post rifting period. Carbonate buildups
which were constructed on the eastern North American margin during this
transitional period developed into carbonate ramps because of rapid subsidence
and tilting of the margin during the Early Jurassic. Buildups which formed
114
during the later drifting-stage of continental plates, had developed into carbonate platforms and offshore banks as the marginal basins became filled up and
the subsidence of the shelf areas decreased, and the bathymetric difference
between the shelf and forming deep-sea basin increased.
CONCLUSIONS
(a) A discontinuous belt of Jurassic--Early Cretaceous shallow-water carbonate buildups up to 5 km thick extends for more than 6000 km along the
offshore of the eastern North American margin. Six types of carbonate buildups recognized document the variability of depositional, paleo-oceanographic
and tectonic processes on the margin.
(2) The position of the carbonate paleoshelf edge along the margin is highly
variable and is located under the present mid-shelf as well as under the lower
continental slope (near to 2000-m contour). Some of the carbonate platforms
extending seaward underneath the present continental slope were deeply
eroded by contour currents during pre-Early Miocene time.
(3) The carbonate buildups penetrated by drilling, were constructed over
the continental basement or overlie sedimentary strata which are underlain by
continental basement. The Lower Jurassic carbonates form a ramp which was
constructed during the transitional period between the late rifting and early
drifting stage of plates in the North Atlantic and is the result of rapid subsidence of the margin at this period.
(4) The Upper Jurassic carbonates deposited after the Bathonian transgression developed into carbonate platforms which evolved into off-shore carbonate banks during the early drifting stage of the continental plates in the North
Atlantic. Construction of the carbonate platforms at the southern end of the
margin was sustained into the late drifting stage (the present time). The
stromatoporid-coral and sponge bioherms are only a minor element of the
banks at the northern part of the margin. Their contribution to the construction of the carbonate buildups in the central part of the margin (Baltimore
Canyon Trough and Blake Plateau) is presently unknown, but the seismicreflection data indicate th~ presence of a reef structure near to the paleoshelf
edge in the Baltimore Canyon m~ea. The major components of the buildups
are oolitic grainstones, skeletal and peloid wackestones and packstones
suggesting that the buildups are similar in composition to those of the Recent
western Great Bahama Bank.
(5) The carbonate sequences along the margin are diachronous, with the
cessation of carbonate deposition being explained here as reflecting a northward movement of the North American plate through the climatic zones. Such
calculated movement was less than 1.5 cm/yr, and supports the paleomagnetically derived 1 cm/yr northward shift of the North Atlantic plate.
(6) The carbonate platforms were effective barriers for the dispersal of the
terrigenous sediment into the deep ocean basin, and thus helped to build wide
continental margins.
115
ACKNOWLEDGEMENTS
The research has been supported by the Geological Survey of Canada. The
author is indebted to J. Wade and A. Grant who helped with the interpretation
of the seismic data. J. Bujak and D. Umpleby reviewed the paper with utmost
patience and gave helpful suggestions to improve it.
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