EVOLUTION OF THE ASCENSION SUBMARINE CANYON

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EVOLUTION OF THE ASCENSION SUBMARINE CANYON
EVOLUTION OF THE ASCENSION SUBMARINE CANYON:
CENTRAL CALIFORNIA CONTINENTAL MARGIN
A Thesis
Presented to
The Faculty of the Department of Geology
San Jose State University
In Pa rti a1 Fu lfi 11 ment
of the Requirements for the Degree
Master of Science
By
David Kenneth Nagel
May 1983
ACKNOWLEDGMENTS
Long before I started my thesis research on the evolution of
Ascension Submarine Canyon, I had a great desire to get involved in
marine research and to learn the techniques of marine geologists.
I am
grateful to Moss Landing Marine Laboratories and to a large supporting
cast for helping me fulfill my longtime desires and for the wealth of
knowledge I gained.
I am deeply grateful to Henry T. Mullins, my thesis supervisor, for
his guidance, confidence, and friendship.
It was an honor and a
pleasure working with Hank, and I will be forever indebted to him for
all he has done for me.
I would like to thank Calvin Stevens for serving on my thesis
committee and providing critical review of my thesis.
A special thanks is owed H. Gary Greene, who originally directed me
toward Ascension Submarine Canyon.
He provided valuable discussions and
data pertaining to the Monterey Bay region which aided greatly in the
completion of my thesis.
Kevin Heston and Greg Lewis assisted in
obtaining access to valuable data.
I am indebted to Joseph Clark for his assistance in the
petrographic descriptions and correlations of rock formation
equivalences for the rock dredge samples.
He also shared his knowledge
of California tectonics that proved to be invaluable.
Valuable
discussions concerning tectonics were also generously supplied by Eli
Silver to whom I would like to extend my appreciation.
Many productive discussions with David McCulloch contributed
significantly to this thesis.
His sharing of knowledge and data
iii
concerning the structure of the Outer Santa Cruz Basin is greatly
appreciated.
Biostratigraphic analyses on selected dredge samples were done by a
group of individuals whom I would like to thank.
The foraminifera were
identified by Robert Arnal, Michael Mullen, Kristin McDougall, and
Joseph Clark.
The diatom biostratigraphy was done by John Barron.
The
megafossil identifications were provided by Ellen Moore, Daniel Fleming,
and Robert Stuart.
selected samples.
George Curtis ran X-ray diffraction analyses on
Their valuable input and cooperation is greatly
appreciated.
Harry Hill and Ken Delopst provided essential assistance in getting
the seismic reflection equipment operating and Bill Raub introduced me
to grinding thin sections and the use of the camera lucida.
Thank you
for your help.
A special acknowledgment and thanks are extended to Richard Rasch
and Madelain Urrere.
Their sharing, understanding, and support has
brought a great deal of strength and happiness to my life.
Finally, I wish to thank Moss Landing Marine Laboratories for
providing shiptime and the David and Lucille Packard Foundation for
providing partial financial support for my research.
My appreciation
goes out to Lew Skelton and crew of the CAYUSE, Steve Royce, and the
many scientific crew members who helped gather my data.
TABLE OF CONTENTS
ABSTRACT
x
INTRODUCTION
1
REGIONAL GEOLOGIC SETTING
7
Outer Santa Cruz Basin
7
Fault Zones
10
Offshore Geology East of the San Gregorio Fault Zone
12
METHODS
13
Rock Dredges
13
Seismic Reflection Profiles
13
Seismic Stratigraphy
16
RESULTS
19
Rock Dredge Results
19
Jurassic-Cretaceous Rocks
19
Latest Middle Miocene Rocks
23
Upper Miocene-Pliocene Rocks
23
Quaternary Unconsolidated Sediments
25
Rocks of Uncertain Association
26
Seismic Reflection Profiles
26
Seismic Stratigraphic Sequences, West of San
Gregorio Fault Zone
26
Evidence for Canyon Excavation
36
San Gregorio Fault Zone
41
Structural Basement Highs
44
v
DISCUSSION
48
Origination Point and Total Offset of the Study Area
48
Paleogeographic Reconstruction Parameters
49
Evolution of Ascension Submarine Canyon
52
CONCLUSIONS
62
REFERENCES CITED
64
APPENDIX I
69
vi
LIST OF ILLUSTRATIONS
Figure
1.
Generalized bathymetry of the central California continental
margin.
3
2.
Bathymetric map of the study area showing northwestern and
southeastern headward regions of Ascension Submarine Canyon
and the adjacent continental shelf as well as onland creeks
and rivers.
4
Drawing depicting the hypothesis that suggests that the
multiple heads of Ascension Submarine Canyon were originally
cut as the distal channel of the Monterey Submarine Canyon
and were progressively offset northward along the San
Gregorio fault zone.
6
4.
Locations of Neogene sedimentary basins along the central and
northern California continental margin.
8
5.
Generalized structural map depicting the Outer Santa Cruz
Basin and its bounding structural highs. San Gregorio and
Monterey Bay fault zones are shown.
9
3.
6.
Faults and epicenters (1911-1976) in the Monterey Bay area of
central California.
11
7.
Rock dredge sample location map for Ascension Submarine
Canyon.
14
8.
Moss Landing Marine Laboratories (MLML) seismic reflection
profile tract line map.
15
9.
U.S. Geological Survey (USGS) seismic reflection profile
tract line map.
17
10.
Stratigraphic column for Ascension Submarine Canyon with a
correlation to the seismic stratigraphic units (AC-1 through
AC-5), west of the San Gregorio fault zone.
24
Moss Landing Marine Laboratories one-kilojoule sparker
profile D'-E showing folded Monterey Formation (AC-1)
adjacent to the San Gregorio fault zone.
28
12.
Geologic map for Ascension Submarine Canyon and adjacent
continental shelf.
29
13.
U.S. Geological Survey BOO-joule uniboom profiles 18'-18"
depicting the truncation relationship between seismic
stratigraphic sequences AC-1 and AC-2 and between sequences
AC-2 and AC-3.
30
11.
vii
14.
15.
16.
Moss Landing Marine Laboratories one-kilojoule sparker
profile F-G' showing the complete seismic stratigraphic
sequence, AC-1 through AC-5, west of the San Gregorio fault
zone.
31
Relative changes of sea level correlated with the seismic
stratigraphic sequences (AC-1 through AC-5) for Ascension
Submarine Canyon, west of the San Gregorio fault zone.
33
Moss Landing Marine Laboratories one-kilojoule sparker
profile B'-C illustrating the shelf edge progradational
sequences (AC-3 through AC-5).
34
17.
Moss Landing Marine Laboratories one-kilojoule sparker
profile Q'-R showing two post-middle Pleistocene exhumations
of the northwesternmost head of Ascension Submarine Canyon, a
late-Miocene-Pliocene buried canyon, and a large anticline
associated with the uplifted Ascension-Monterey High.
37
18.
Line drawing of U.S. Geological Survey 160 kilojoule sparker
profile 4'-4'' showing the Santa Cruz High, the Outer Santa
Cruz Basin, the Ascension-Monterey High and the San Gregorio
fault zone.
38
19.
Structural map for the Ascension Submarine Canyon and
adjacent continental shelf.
39
20.
Portion from Moss Landing Marine Laboratories one-kilojoule
sparker profile Q'-R (fig. 17) showing late Miocene-Pliocene
buried canyon.
40
Portion from Moss Landing Marine Laboratories one-kilojoule
sparker profile Q'-R (fig. 17) showing two episodes of
post-middle Pleistocene canyon cutting of the
northwesternmost head of Ascension Submarine Canyon.
42
Moss Landing Marine Laboratories one-kilojoule sparker
profile A'-B showing folding adjacent to San Gregorio fault
zone.
43
Line drawing of U.S. Geological Survey 100-kilojoule sparker
profile 16'-16" illustrating the multi-ridge Pigeon Point
High, offset Nacimiento Fault, and the Outer Santa Cruz
Basin.
45
Generalized map depicting the Outer Santa Cruz Basin and its
associated bounding structural highs with the late
Pleistocene uplifted Ascension-Monterey High at the
southeastern end of the basin.
46
21.
22.
23.
24.
25.
Sequential development of the region surrounding the
Ascension Submarine Canyon including development of the Outer
Santa Cruz Basin.
50
viii
26.
27.
Fault wedge basin model depiciting uplift at the convergent
southern end of the Outer Santa Cruz Basin and a northwest
plunging basin.
59
Channel gradient comparison of Ascension and Monterey
Submarine Canyons, west of the San Gregorio fault zone.
61
Summary of Rock Dredges
20
Table
1.
ABSTRACT
Ascension Submarine Canyon, located along the central California
continental margin, is a shelf edge canyon system comprised of eight
canyon heads that coalesce into one major canyon near the 2700 m
bathymetric contour.
These multiple heads can be divided into a
northwestern region containing two discrete, well-defined heads, and a
southeastern region consisting of six diffuse heads.
During
approximately the past 12 m.y., this portion of the continental margin
has been displaced northward 105 km along the San Gregorio fault zone
which concurred with the formation of the Outer Santa Cruz Basin.
During this time, multiple episodes of canyon excavation which appear to
be related to both tectonic and eustatic processes occurred in the area
of Ascension Canyon.
Greene (1977) and Howell and others (1980) previously suggested
that the multiple heads of Ascension Canyon were originally cut one at a
time as the distal channels of the Monterey Submarine Canyon.
After a
channel was cut it was offset northward along the San Gregorio fault
zone, and a new channel was cut by the Monterey Canyon.
A buried canyon
landward of the second northwesternmost canyon head indicates that
Ascension Canyon may have been cut initially during late MiocenePliocene time.
This buried canyon may have been cut by the Monterey
Canyon as its distal channel or by headward erosion from a slope canyon
generated by tectonic uplift along the seaward margin of the Outer Santa
Cruz Basin.
Two southeastern heads of Ascension Canyon may have been
cut when they were juxtaposed against the Monterey Canyon during low
X
stands of sea level at 2.8 m.y.B.P. and 1.75 m.y.B.P.
By the late
Pleistocene, Ascension Canyon would have been displaced northwestward
beyond the channel of Monterey Canyon.
The northwestern region,
however, shows two episodes of late Pleistocene canyon exhumation which
may be due to repeated periods of erosion and deposition controlled by
eustatic sea level fluctuations.
The excavations have been tentatively
correlated with the 0.75 m.y.B.P. and 18,000 yr. B.P. low stands, but
the erosional processes responsible for them are not known.
Within the past 750,000 years, uplift of the southeastern
convergent end of'a fault wedge basin (i.e., Outer Santa Cruz Basin) may
have resulted in the oversteepening of the slope.
The oversteepened
slope may have generated submarine mass movements and sediment gravity
flows which resulted in canyon excavation.
The history of Ascension
Canyon depicts the close relationship between tectonics and the
evolution of a submarine canyon.
xi
1
INTRODUCTION
The origin of submarine canyons has been a controversial topic for
decades with a variety of possible. mechanisms for canyon cutting having
been proposed (Shepard and Dill, 1966; Shepard, 1972, 1981).
Early
hypotheses attempted to explain the origin of submarine canyons by means
of a single dominant process such as:
(1) faulting (Lawson, 1893;
Wegener, 1924); (2) turbidity currents (Daly, 1936); ( 3) drowned river
valleys (Shepard, 1936); (4) artesian spring sapping (Johnson, 1939);
and, (5) tsunamis (Bucher, 1940).
Many of these previous hypotheses
were partially correct, but failed to recognize the integration of
multiple processes which might be responsible for submarine canyon
development.
Today, the prevailing idea is one of a polygenetic origin
for submarine canyons, including multiple controls on canyon formation.
Shepard (1981) has summarized these controls as:
(1) submarine
fault valleys or the location of canyon axes along fault trends, (2)
submarine erosion including sliding, slumping, turbidity currents and
other sediment gravity flows, (3) subaerial erosion, such as stream and
river channels subsequently submerged due to eustatic sea level changes
and/or subsidence, and (4) upbuilding of canyon walls.
The various
combinations of geological processes responsible for the excavation of a
particular submarine canyon are largely dependent upon geographic and
geologic setting of the canyon.
Many submarine canyons show a close relationship to land rivers
(Shepard and Dill, 1966) which points to the importance of subaerial
2
erosion in the excavation of the shallow water heads of submarine
canyons during low stands of sea level prior to submergence.
However,
the relationship of the heads of submarine canyons to the adjacent
coastline is variable.
Many canyon heads are found near the littoral
zone off major river valleys (Shepard, 1972), such as the Congo Canyon,
while there are other submarine canyons whose heads are located at or
near the continental shelf/slope break which may or may not have an
obvious connection to adjacent rivers.
These include submarine canyons
off the east coast of the United States, such as the Baltimore and
Wilmington Canyons, and along the central California continental margin
such as Pioneer, Sur, and Ascension Submarine Canyons (fig. 1).
Ascension Submarine Canyon has eight heads that notch the
continental shelf above the 150 m bathymetric contour (figs. 1 and 2).
Seaward, these multiple segments of Ascension Canyon coalesce into a
single major canyon near the 2700 m bathymetric contour.
A distinct
geomorphic difference in the headward region of this canyon system
allows division into northwestern and southeastern sectors (fig. 2).
The northwestern region is characterized by two separate, distinct
canyons, whereas the southeastern region is characterized by six diffuse
canyon heads.
Greene (1977) and Howell and others (1980) have previously
suggested that the shelf edge submarine canyons between San Francisco
and Santa Cruz (i.e., Ascension Canyon, an unnamed canyon, and Pioneer
Canyon; fig. 1) originally developed as the seaward channel of the
Monterey Submarine Canyon and were subsequently displaced northward by
....
3
38"
NORTH
CONTOURS IN METERS
25
KMS
37"
3500
Figure 1. Generalized bathymetry of the central California continental
margin. Based upon NOAA bathymetric sheet NOS-1307 N-11B.
Drafted by H. T. Mullins.
--
4
3~;~~·;.3;:.0'----.,.\------,,...,..----~--.,-------------_.;.'2:;2:,·~~·10'
""'"'"'""'"
BATHYMETRIC
ASCENSION CANYON
ANa
\
MAP
o
s
KM
N~
I
NUEVO
I
I
/
f
I
"\..NoRTHWESTERN
""'HEAOWARD
"'-. REGION
............
.........../
/
I
I
I
I
I
I
I
I
',
so
'
'
'y
I
'
I
36°45'
122°30'
36"45'
122°00'
Figure 2. Bathymetric map of the study area showing northwestern and
southeastern headward regions of Ascension Submarine Canyon
and the adjacent continental shelf as well as onland creeks
and rivers. Bathymetry based upon NOAA bathymetric sheet
NOS-1307-11B and onland creeks and rivers based upon Jennings
and Strand (1958) and Jennings and Burnett (1961).
""
5
right lateral faulting to their present locations.
They have further
suggested that the multiple heads of Ascension Canyon were cut one at a
time and progressively offset along the San Gregorio fault zone
beginning approximately 7 m.y.B.P .. (fig. 3).
The primary objective of this study is to determine the evolution
of Ascension Submarine Canyon.
The development of the evolutionary
picture for Ascension Canyon will be approached by attempting to:
(1)
determine what geological controls were involved in the excavation of
the canyon, (2) determine where the canyon system was located
geographically during excavation with respect to fault displacement, (3)
determine the timing of excavation, and (4) determine whether or not
multiple excavations have occurred.
This approach will be based upon:
(1) an understanding of the stratigraphy and structure of the headward
region of Ascension Canyon and the adjacent continental shelf and (2) a
working knowledge of the regional tectonic setting and sea level
fluctuations.
A study area was selected that encompasses the headward
regions of Ascension Canyon and the adjacent continental shelf.
The
area extends from approximately 5 km north of Point Ano Nuevo to Point
Santa Cruz and approximately 20 km offshore (fig. 2).
-
6
--""'-~~
.-=----=--~-:
·---
------.... ..
,..,,..,., ··~'"' ·~we ''"'' ,..... ~
,.e ,~,,... ,,,... ~c ''""' ., ,,,..,
~
,
c...,_,,,,.""'''""''''""""''"''""
.. ..........
................ .
'"'"
..... - ,,_, ·""''"• " ......
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......
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......, •••
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..,,.,., aa, c,.,,,,...
,, .. c"""
"'~"''to'""""
"C "'"'• ,,._.,..,..., , , . ,..
_,_,_
·-·. ··
_.,.......
.......
... .........
........_
-,_
.....
.,
•'"''"'''""""'
...er ,. ..............
-- -- a.... ,. "'"'"""'
'
""""~
,,.,, ........ ,.,
h
........
ih::-···· ...
c......
..,. ''"'"'
..c .......... ;,.,,.
,~-
w•·•~•4
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••• ,, •
......... .
"'<O· " " ' .;,,,.,,,
._, •
-· ........... ..,... ,, .
.
..,c·-
'> ••
-
Figure 3. Drawing from Howell and others (1980) depicting the hypothesis
of Greene (1977) that the multiple heads of Ascension
Submarine Canyon were originally cut as the distal channel of
the Monterey Submarine Canyon and were progressively offset
northward along the San Gregorio fault zone.
7
REGIONAL GEOLOGIC SETTING
Outer Santa Cruz Basin
The study area lies within
Santa Cruz Basin (fig. 4).
th~
southeastern portion of the Outer
Right lateral shear between the Pacific and
North American plates has resulted in late middle Miocene extension and
the formation of the Neogene sedimentary basins along the northern and
central California continental margin, including the Outer Santa Cruz
Basin (Hoskins and Griffiths, 1971; Blake and others, 1978; Silver,
1978; Howell and others, 1980; fig. 4).
Hoskins and Griffiths (1971) mapped the general structural features
and compiled a stratigraphic column and cross sections depicting the
general stratigraphic relationships within this basin, using seismic
profiles and well data.
They described Cretaceous and early Teritary
(Oligocene?) marine sandstone units overlying crystalline basement.
Deposition of each of these units was followed by a period of
deformation and erosion, resulting in basinwide unconformities.
The
overlying Neogene section is divided by basin edge and basinwide
unconformities that reflect repeated episodes of erosion, followed by
marine transgressive deposition.
The northwest-plunging Outer Santa Cruz Basin is bounded by two
northwest-plunging structural highs, the Pigeon Point High on the
northeastern margin and the Santa Cruz High on the southwestern margin
(Hoskins and Griffiths, 1971; McCulloch and others, 1977, 1980; fig. 5).
The Pigeon Point High appears to be a southern extension of the
8
42·~------~--~~~----,-----~~-----r------r;42°
I
..J
(
EEL RIVERI
BASIN I
OFFSHORE
NEOGENE
8 AS INS
\
I
CENT./ NORTH. CALIFORNIA
(-
40°
\
PT. ARENA \ \
BASIN
40°
NORTH
\
i
\
38°
38°
BAY
36°
0
40
36°
.._
\
km
\
\
SANTA MARIA \
BASIN
\
\
(__) = SEDIMENTARY
0 "' EXPLORATORY
BASIN
WELL
\
'\
PT. CONCEPTION
Figure 4. Locations of Neogene sedimentary basins along the central and
northern California continental margin. Study area shown at
southeastern end of the Outer Santa Cruz Basin. From Mullins
and Nagel (1982).
9
>
1
N
0
10
20
30
40
'"
'
''
~
I
'
&\'\
GUIDE
/
SEAMOUNT
37"00'
\
\
'
HlGH
CAN YON
Figure 5. Generalized structural map depicting the Outer Santa Cruz
Basin and its bounding structural highs. San Gregorio and
Monterey Bay fault zones are shown. Map modified after
Hoskins and Griffiths (1971) and McCulloch and others (1977).
p
10
cretaceous quartz diorite Farallon Ridge (Silver, 1974; McCulloch and
others, 1977) and is bounded by the Nacimiento fault on its seaward
margin, which has been tectonically truncated and offset northwestward
along the San Gregorio fault zone !Silver and others, 1971; Silver,·
1974, 1978; Graham, 1978; Mullins and Nagel, 1981; fig. 5).
The
northwest-plunging Santa Cruz High is known to be composed of the
Franciscan Assemblage (Mullins and Nagel, 1981) and to be fault bounded
on its seaward margin by a high angle reverse fault (Hoskins and
Griffiths, 1971; McCulloch and others, 1977).
The high is also deeply
dissected by Ascension Canyon (Hoskins and Griffiths, 1971; fig. 5).
The irregular basement beneath the Outer Santa Cruz Basin has been
interpreted to be Franciscan Assemblage (McCulloch and others, 1977;
Mullins and Nagel, 1981) extending from the Santa Cruz High to the
Nacimiento fault along the seaward margin of the Farallon-Pigeon Point
High.
Fault Zones
Two parallel, nearly vertical, northwest-trending segments of the
San Gregorio fault zone cut diagonally across the continental shelf
through the study area (Hoskins and Griffiths, 1971; fig·. 6), separating
significantly different stratigraphic sequences (Clark and Brabb, 1978).
The fault zone is recognized as a series
of~
echelon right-lateral
strike-slip faults (Hill and Dibblee, 1953; Greene, 1977; Weber and
LaJoie, 1977, 1979; and Graham and Dickinson, 1978).
u
122"30'
121" 30'
FAULTS 8 EARTHQUAKES
MONTEREY BAY,CALIF.
'
'""-"'---. ..._
\
\
\
\
0
&
0
( B.ASED ON GREENE,I977)
.....
"
0
~
10
KM
37"00'
''·
)MOSS LANDING..__--!
0
NORTH
r
0
'' \
\
36"30'
\
FAULTS
\
--KNOWN
----INFERRED
-'-""
0
\
\
'\
\
EARTHQUAKE MAGNITUDES',
0
0
0
<I. 5
1.5 - 5.5
>5.5
(1911-1976)
Figure 6. Faults and epicenters (1911-1976) in the Monterey Bay area of
central California. Epicenters along the San Andreas fault
are not shown. Approximate boundaries of the study area are
shown by the box. From Mullins and Nagel (1982).
12
Striking obliquely into the San Gregorio fault zone is the
northwest-trending (N40°W), right lateral strike-slip Monterey Bay fault
zone (Greene and others, 1973; Greene, 1977; fig. 5).
It consists of a
diffuse zone, 10-15 km wide, of neaTly vertical en echelon faults and
folded Miocene and younger rocks.
Earthquake epicenters and Quaternary
seafloor and onland offsets are well documented along these two fault
zones (Griggs, 1973; Greene and others, 1973; Greene, 1977; Gawthrop,
1978), indicating that the San Gregorio and Monterey Bay fault zones are
seismically active (fig. 6).
Offshore Geology East of the San Gregorio Fault Zone
Greene (1977) and Greene and Clark (1979) have mapped two bedrock
units offshore, east of the San Gregorio fault zone, which lie within
the study area (i.e., the upper Miocene Santa Cruz Mudstone and the
upper Miocene-Pliocene Purisima Formation).
The Santa Cruz Mudstone
crops out along the nearshore shelf between Point Ano Nuevo and Point
Santa Cruz, and the unconformably overlying Purisima Formation crops out
on the outer shelf in the northern Monterey Bay region (Greene, 1977).
lA
13
METHODS
Rock Dredge
Eighteen rock dredge hauls
w~re
recovered by pipe dredging the
walls of Ascension Submarine Canyon during June through October, 1980
(fig. 7).
Samples were described from hand samples and thin sections.
Diatom and foraminifera (benthonic and planktonic) biostratigraphic
analyses and X-ray diffraction analyses were conducted on selected
samples.
The biostratigraphic ages were based upon foraminifera species
ranges (Kleinpell, 1938, 1980; Haller, 1980) and upon the northeastern
Pacific diatom zonation (Barron, 1980).
Correlations with rock
formations were based upon (1) biostratigraphy and (2) similar
lithologies and associated faunas after Clark (1966, 1981).
Seismic Reflection Profiles
A network of seismic reflection profiles were collected aboard Moss
Landing Marine Laboratories' R/V Cayuse and R/V Ed Ricketts during July
1980 through June 1981 (fig. 8).
Loran C was used to locate the
position of the ships while collecting geophysical and geological data.
Positions were supplemented by satellite fixes, radar, and bathymetry.
Two hundred and fifty kilometers of 300 joule (J) uniboom seismic
reflection profiles were collected 'using an EG&G "uniboom" for analysis
of seafloor and shallow sub-bottom features (0-70 m).
In order to
identify intermediate depth features (0-150 m), 320 km of 1 kilojoule
(KJ) sparker seismic reflection profiles were collected using a
14
::;
0
"'
::;
0
0
·.... ··:; ;...
..-:::·~·: '
"'
Figure 7. Rock dredge sample location map for Ascension Submarine
Canyon. Squares represent where dredging began and hexagons
represent where dredging stopped. SC-1 from Greene (1977).
15
z
0
>-
z
<{
u
Figure 8. Moss Landing Marine Laboratories (MLML) seismic reflection
profile tract line map. Lines with the following symbols
represent seismic systems used: (1) circles= 1 kilojoule
sparker; (2) crosses = 300 joule uniboom.
16
nine-electrode EG&G sparker as the sound source.
Both types of seismic
data were received by an eight-element EG&G hydrophone with a
pre-amplifier, filtered by a Krohn-Hite continuous, high-low pass filter
and graphically recorded on an EPC:3200 graphic recorder.
The uniboom
data were filtered at 15,000-400 Hz and the sparker data were filtered
at 1,500-200 Hz.
This seismic reflection profile data base was supplemented with
intermediate penetration (0-150 m) 800J uniboom and deeper penetration
(0-3000 m) 33 KJ, 100 KJ, and 160 KJ sparker as well as twin 40 cu. in.
air gun seismic reflection profiles.
These supplementary profiles were
collected by the U.S. Geological Survey in Menlo Park, California
(Greene, 1977; McCulloch and others, 1980; fig. 9).
Seismic Stratigraphy
Seismic sequence analysis, as proposed by Mitchum and others
(1977b), was used to subdivide the seismic section into individual
seismic stratigraphic sequences.
The seismic stratigraphic sequence
boundaries were determined by using the objective criteria of reflector
termination (i.e., erosional truncation, toplap, onlap, and downlap;
Mitchum and others, 1977a).
These seismic stratigraphic sequences were
interpreted to be depositional sequences.
They overlie an acoustic
basement which was identified by a definite surface reflector beneath
which no coherent reflectors were generated.
Rock formations and
lithologies have been correlated to the seismic sequences and the
acoustic basement based upon:
(1) offshore rock dredge samples, (2)
17
Figure 9. U.S. Geological Survey (USGS) seismic reflection profile tract
line map. Lines with the following symbols represent seismic
systems used: (1) squares = 160 kilojoule sparker, (2)
triangles = 800 joule uniboom and twin 40 cu. in. air gun, (3)
circles = 33 kilojoule sparker, and (4) crosses = 100
kilojoule sparker and 800 joule uniboom.
18
offshore well-hole data (Hoskins and Griffiths, 1971), and (3) known
onland stratigraphic units (Clark and Brabb, 1978; Clark, 1981).
The
relative ages of the bounding unconformities of seismic sequences were
determined by correlating the
sequ~nces
with Tertiary sea level curves
(Shackleton and Opdyke, 1973; Vail and Hardenbol, 1979; Beard and
others, 1982).
19
RESULTS
Rock Dredge Results
The rock dredge samples are organized and described according to
age.
The locations of the dredge hauls are shown in figure 7.
Lithologic, petrographic, and paleontologic descriptions of the samples
and coordinate positions are included in Appendix 1.
A summary of the
dredge haul results is given in Table 1.
Jurassic-Cretaceous(?) Rocks
Because Ascension Canyon dissects the Santa Cruz High, several
rocks recovered from the two northwesternmost canyon heads and from
their adjacent southeastern head may be representative of this
structural high.
Dredge A1 was filled with large, fresh, angular
pieces of serpentinite. X-ray diffraction analysis confirmed the
presence of lizardite, a common serpentine-group mineral (Mullins and
Nagel, 1981).
Dredge A recovered fresh, angular pieces of spilitic basalt
4
while dredges A1 and A23 contained vesicular basalts. In coastal
central California, serpentinites and mafic volcanics are most commonly
associated with the Franciscan Assemblage of Jurassic to early Tertiary
age.
Two exploratory wells in the Outer Santa Cruz Basin bottomed in
Upper Cretaceous marine sediments (Hoskins and Griffiths, 1971),
indicating this Franciscan-like basement must be older than Upper
,
::~;~~<
Table 1.
Dredge
Haul
Start
Dredging
Stop
Dredging
Depth (m)
Summary of Rock Dredges
Lithology
Age Range-Formation
Serpentinite
Jurassic to Cretaceous(?)-Franciscan
Assemblage(?)
Diatomaceous siltstone Late Pliocene-Purisima Formation
Spilitic basalt
Jurassic-Cretaceous(?)-Franciscan
Assemblage
A1
36°55'00"
122°28'05"
36°55'30"
122°28'30"
1080-845
A2
36°57' 30"
122°26' 10"
36°58'15"
122°25'05"
725-330
Lithic siltstone
Late Pliocene(?)-Purisima Formation(?)
A3
36°59'35"
122°25'15"
36°59'30"
122°26'00"
356-145
Dolomitic siltstone
Late Pliocene to Quaternary-Purisima
Formation
Late Miocene(?)-Quaternary-Purisima
Formation
Silty very fine
grained sandstone
A4
36°54'05"
122°25'00"
36°54'35"
122°25'10"
1055-875
A5
36°55'45"
122°23'25"
36°56'00"
122°22'30"
A6
36°57'55"
122°22'05"
A?
36°50'05"
122°18'20"
"
Spil iti c bas a1t
Jurassic-Cretaceous(?)-Franciscan
Assemblage
700-452
Diatomaceous siltstone
Late Pliocene-Purisima Formation
36°57'30"
122°22' 15"
375-233
Mud
Quaternary(?)
36°51'15"
122°20'00"
1095-840
Mud
Quaternary(?)
N
0
/,
C>i,
,','\
Dredge
Haul
Start
Dredging
Stop
Dredging
As
36°54'DO"
122°17'5D"
36°54'20"
122°18'0D"
A17
36°48 '55"
122°13'35"
36°48'20" 870-675
122°13'45"
Tuffaceous and diatoma- Pre-Quaternary probably Plioceneceo us siltstone
Purisima Formation
Diatomaceous opaline
Late middle Miocene(?)-Monterey
mudstone
Formation(?)
A18
36°48'40"
122°17'45"
36°47'50" 1440-730
122°15'35"
Siltstone
Pliocene(?)-Purisima Formation(?)
A20
36°51' 30"
122°12'45"
36°52'00"
122°12'05"
Diatomaceous siltstone
Pliocene to Quaternary-Pu ri sima
Formation
Depth (m)
635-425
700-325
Lithology
Diatomaceous and
micaceous siltstone
Age Range-Formation
Pliocene(?)-Purisima Formation(?)
Fine-medium grained
arkosic sandstones
Diatomaceous opaline
mudstone
Latest middle Miocene-Monterey
Formation
A2l
36°52'30"
122°17'45"
36°53'45" 1015-420
122°19'20"
Si1 tstone
Pliocene(?)-Purisima Formation(?)
A22
36°54'45"
122°24'30"
36°56'10" 1080-675
122°23'40"
Diatomaceous and
tuffaceous mudstone
Late Pliocene-Purisima Formation
A23
36°51'15"
122°21'00"
36°53'00"
122° 19' 00"
Calcareous siltstone
Late Pliocene-Purisima Formation
Vesicular basalt
Jurassic-Cretaceous(?)-Franciscan
Assemblage
765-720
N
~
r
0,
'
Dredge
Haul
A33
Start
Dredging
36°52'45"
122°22'15"
Stop
Dredging
Depth (m)
36°52'3D" 855-630
122°21'10"
Lithology
Massive mudstone
Opaline siltstone
A34
36°48'55"
122°12'05"
A35
36°48'45" 36°48'40"
122° 11' 00" 122°10'30"
A36
36°50'25" 810-740
122°10'15"
36°52'05" 36°52'15"
122°13'15" 122°12'45"
450-340
550-385
Age Range-Formation
Latest Miocene to PliocenePurisima Foundation
Late Pliocene-Purisima Formation
Siliceous mudstone
Latest middle Miocene(?)Monterey Formation(?)
Dolomitic concretion
Pliocene(?)-Purisima Formation(?)
Calcareous siltstone
Pliocene(?)-Purisima Formation
Mo 11 usc-bearing,
Pliocene-Purisima Formation
glauconitic siltstone
Silty, very fine
Pliocene(?)-Purisima Formation(?)
grained sandstone
Sandy siltstone
Pliocene(?)-Purisima Formation(?)
N
N
23
cretaceous.
Therefore, a probable Jurassic-Cretaceous age has been
assigned to these serpentinites and basalts (fig. 10).
Latest Middle Miocene Rocks
The oldest Tertiary sedimentary rocks were dredged from the
southeastern portion of Ascension Submarine Canyon in dredge hauls A20
and possibly A17 and A34 . Dredge haul A20 contained white to
light brown, thinly-laminated, organic, siliceous mudstones.
These
samples were dated as latest middle Miocene within the diatom subzone C
of the Denticulopsis hustedii - Denticulopsis lauta zone (J. Barron,
1981, written communication) which is age equivalent to the Mohnian
Foraminiferal Stage (Poore and others, 1981).
These mudstones and
lithologically similar samples collected in close proximity (dredge haul
A17 and A34 ) are correlated with the middle-lower upper Miocene
Monterey Formation (fig. 10). This correlation is based upon similar
lithology and age plus the close proximity of
~1onterey
Formation
outcrops along the sea cliffs north and east of Point Ana Nuevo and on
Ana Nuevo Island southwest of the San Gregorio fault zone (Clark and
Brabb, 1978; Clark, 1981).
In addition, dredge sample SC-1 was
collected in close proximity to dredge haul A20 and was reported to be
Monterey-like rock of probably middle Miocene age (Greene, 1977).
Upper Miocene-Pliocene Rocks
The vast majority of samples collected throughout Ascension
Submarine Canyon (i.e., in 14 out of 18 dredge hauls) was grayish-green
to olive green, highly bioturbed, moderately well indurated, massive to
very finely laminated, diatomaceous and tuffaceous siltstones.
In
24
(f)
2.
w
w
f-
w
(f)
fii
~ ~ f--=U'+A_C_·_St- UNCONFORMITY
~
W
1-<(
r
::>
0
Ll TH-
FORMAT! ON
0::
(f)
g
M
·;.;::;;;:';..:,;.';:.;.:.;;;,;:'~:
/d.·:,.;..;:~.::~:-~
t:; 1--t---t- UNCONFORMITY
w l AC-3
...J
1.75
w 0
u0
2.8 +------'----1
.....1
AC-2
DIATOMACEOUS
FORMATION
::2: 0::
a..
1-·-·t--·-·-
0:: w
w
a..
1--·--·-
'·· -·--·
:t §; 1--1- UNCONFORMITY
:::>
- .·--· - ·
-=-- -::
6.6-+---------
---=--F=--=--
r--r---_-
w
w
z
:---
u
0
SILTSTONE
t:::...-.=-·-. __:_
i= f----1
1-
TUFFACEOUS 6
PURISIMA
<(
w
SEDIMENTS
z
w w
z u
Cl:
UNCONSOLIDATED
w
0
f-
Cl:
.75-
AC·4
::.__f--"""-'-.l.-.-j--t- UN C 0 N F 0 R MIT Y
>-
DESCRIPTION
O LOGY
:--....='-
r-----
AC-1
::::!:
w
.....1
0
0
MONTEREY
FORMATION
::::!:
0::
w
a..
a..
I-
-
1---1-----1------1·-
THINLY LAMINATED
ORGANIC, SILICEOUS
MUDSTONE
:::>
!---~----~-+
UNCONFORMITY
CRETACEOUS
A" A
FRANCISCAN
TO
ASSEMBLAGE
JURASSIC
Figure 10.
AAAAI\1\/\AA
1\/\1\AAAAJ\1\
,1.
?-1---------1
AAAAAA
Al'oAf',,.J\AAAA
AAAA/\A/\ .... A
AAI\AI\ .... 1\AI\
AAA All /11'11\ .... A
................................ ""
SERPENTINITE 6
MAFIC VOLCANICS
""'" """"" .......
At'lr\AAAA ... AA
AA/\AA J\1\1\1\A
Stratigraphic column for Ascension Submarine Canyon \'lith a
correlation to the seismic stratigraphic units (AC-1 through
AC-5), \'lest of the San Gregorio fault zone.
25
addition, the dredge hauls contained silty, very fine-grained sandstone
(A ) and mudstone (A 22 and A33 ) members. Dredge hauls A23 and
3
A contained calcareous and dolomitic siltstones, while A35
35
included a dolomitic concretion ..
Five dredge hauls (A 1, A5, A22 , A23 , and A33 ) contained
samples that were determined by diatom biostratigraphy to belong to the
late Pliocene Denticulopsis seminar var. fossilis zone (J. Barron, 19B1,
written communication).
These samples and other probable Pliocene
samples have similar ages, lithologies, and faunal associations to the
upper Miocene-Pliocene Purisima Formation.
This primarily siltstone
lithology is correlated with the Purisima Formation (fig. 10) which
occurs onland southwest of the San Gregorio fault zone at Point Ano
Nuevo (Clark and Brabb, 1978; Clark, 1981).
Quaternary Unconsolidated Sediments
Unconsolidated sediments, generally gray-green cohesive mud with
some sand and gravel, were collected in 15 of the 18 dredge hauls
throughout the canyon.
(fig. 10).
These sediments are interpreted to be Quaternary
This interpretation is based upon:
(1) the extensive
recovery of unconsolidated sediments, {2) the lack of recovery of
Quaternary rocks, and {3) the Quaternary section of marine shallow-water
gravels, sands, and clays shown to exist in the Outer Santa Cruz Basin
by Hoskins and Griffiths (1971).
26
Rocks of Uncertain Association
Dredge haul A20 recovered numerous massive, fine-medium grained
arkosic sandstones that were fresh and angular in appearance and barren
of age diagnostic fossils.
Some
Qf
the sandstones were oil saturated
(Mullins and Nagel, 1982), and one sample was in sharp contact with a
siliceous mudstone, possibly belonging to the Monterey Formation.
These sandstones may represent one or more stratigraphic units.
They may correlate with the basal sandstone of the Monterey Formation
(Clark, 1g66, 1981; Greene and Clark, 1979) and with the basal sandstone
of the middle-upper Miocene sequence located in the Outer Santa Cruz
Basin (Hoskins and Griffiths, 1971).
An alternative correlative unit may be the lower upper Miocene
Santa Margarita Sandstone (Clark, 1966, 1981; Clark and Brabb, 1978).
This correlation is strengthened by the fact that the sandstones were
collected in the same dredge haul (A 20 ) with Monterey Formation and
Purisima Formation rocks, which stratigraphically underlie and overlie
the Santa Margarita Sandstone, respectively.
Because the age of these
samples was indeterminable, they were not assigned a stratigraphic
position.
Seismic Reflection Profiles
Seismic Stratigraphic Sequences West of the San Gregorio Fault Zone
Five seismic stratigraphic sequences,which are comprised of two
bedrock units (AC-1 and AC-2) and three unconsolidated sediment units
(AC-3 to AC-5),are recognized, west of the San Gregorio fault zone.
27
Middle Miocene-Early Late Miocene Sequence (AC-1).
The oldest
seismic stratigraphic sequence is defined by strong, continuous, high
amplitude, high-reflection frequency reflectors (fig. 11).
This
sequence is truncated by, and trends along, the San Gregorio fault zone
from north of Point Ano Nuevo to the southeastern headward region of
Ascension Canyon (fig. 12).
Near Point Ano Nuevo, this unit
unconformably onlaps and drapes the Pigeon Point High where it is
faulted and tightly folded.
The sequence extends across the Outer Santa
Cruz Basin conforming to the irregular basement topography and onlaps
the Santa Cruz High where it is tectonically folded.
The discordance
between this sequence and the overlying unit is not always easily
discernible.
However, erosional truncation of this sequence and onlap
by the overlying sequence defines the unconformity separating the two
sequences (fig. 13 and 14).
This sequence (AC-1) is correlated with the Miocene Monterey
Formation (fig. 10 and 15).
This correlation is based upon the
distribution of the AC-1 sequence that can be traced to known outcrops
of Monterey Formation as evident from:
(1} the late middle Miocene rock
dredge samples from the southeastern portion of Ascension Canyon, and
(2} the outcrops southwest of the San Gregorio fault zone in the
seacliffs north and east of Point Ano Nuevo and on Ano Nuevo Island
(Clark and Brabb, 1978; Clark, 1981}.
Late Miocene-Pliocene Sequence (AC-2).
This is a thick sequence
that unconformably overlies sequence AC-1 and is characterized by
continuous, variable amplitude and variable reflection frequency
reflectors (fig. 14).
A lower reflection frequency, more gentle folding
and rarity of faulting distinguishes this unit and overlying units from
::E
osw
)>
~~---
-l
-0.0
::E
0
::E
~ROFI E D'-
)>
-<
-l
IT1
:::0
:::0
0
-l
0.1
7 5.
IT1
""0
)>
<
IT1
r
-l
-l
-$::
I
~
$::
fll
~
~
(J)
0.2 IT1
150.
0
~
Figure 11.
Moss Landing Narine Laboratories one~kilojoule sparker
profile D'~E showing folded Monterey Formation (AC-1)
adjacent to the San Gregorio fault zone. Ml = first multiple
reflection.
N
CD
Figure 12.
Geologic map for Ascension Submarine Canyon and adjacent
continental shelf. See text for explanation.
IW
36.6-
.
.~o.o5
' '
I
:-O.IO
109.8
V. E.= 15X
219.6-
Figure 13.
-0.30
U.S. Geological Survey BOO-joule uniboom profiles 18'-18"
depicting the truncation relationship between seismic
stratigraphic sequences AC-1 and AC-2 and between sequences
AC-2 and AC-3.
31
TWO-WAY TRAVEL TIME (SEC)
....
"'
"'
0
N
N
"'....
(Vol) H.Ld30 1:13.LI;IM
Figure 14.
Moss Landing Marine Laboratories one-kilojoule sparker
profile F-G' showing the complete seismic stratigraphic
sequence, AC-1 through AC-5, west of the San Gregorio fault
zone. M1 = first multiple reflection.
32
the underlying AC-1 sequence.
The sequence (AC-2) overlies sequence
AC-1 along the continental shelf and crops out in Ascension Canyon along
the shelf break below the 150-250 m bathymetric contours (fig. 12).
The
upper unconformity of this AC-2 uoit is defined by erosional truncation
AC-2 reflectors and by downlap of an overlying progradational sequence
near the shelf break (figs. 14 and 16).
This AC-2 sequence is correlated with the upper Miocene-Pliocene
Purisima Formation (fig. 10) on the basis of: (1) the similar
stratigraphic relationships observed in outcrops of Purisima Formation
that unconformably overlie the Monterey Formation, e.g., southwest of
the San Gregorio fault zone at Point Ano Nuevo (Clark and Brabb, 1978;
Clark, 1981), and (2) rock dredge samples of Purisima Formation
recovered extensively throughout Ascension Canyon where the AC-2 seismic
reflectors crop out.
The upper and lower unconformities of this late Miocene to Pliocene
sequence are correlated with the low stands of sea level at 6.6 and 2.8
m.y.B.P., respectively (fig. 15).
These two low stands correspond to
the recognized late Miocene to late Pliocene age boundaries for the
Purisima Formation.
Early Pleistocene Sequence (AC-3).
This sequence is the basal unit
of a distinct group of superposed progradational shelf edge sequences
(fig. 16).
This basal sequence is generally characterized by strong,
continuous, variable amplitude, and moderate reflection frequency
reflectors (figs. 14 and 16).
In some profiles, the base of this unit
is composed of extremely low amplitude reflectors.
Reflectors are
0
0
:I:
0::
a.
>-
z
0::
w
1<!
::l
0
~>- u:::i!"'
t-:::i! za:-o:
-z
W:::J
w
Q_
0::
<!
:::i!ien-
0
w
•>a; Ziw<D
o-w
~en
u
en
:::>0
w
z
w
u
0
1-
L
AC-5
M
AC-4 1-
en
w
...J
E
AC-3
..;.
50M
AC-2
w
0::
<!
z
1-
u
0::
:a
w
0
w
<t
...J
1-
-50M
0
-lOOM
-150M
t-1.75-
------
~
~
~
--'-...
--....._
"-
- r-2.8- 1-------------
----\
,__ 6.6-
1-------------- ---
1-9.8
r---
-
t-
-
t-
-
t-1 0-
I
-
~
-"'.
\
-
1-
w
'Wz
w ...JW
AC-1
1- ou
<t ~0
...J
r
:a~
Figure 15.
-- ---
t- 51-
>-
t- .75-
- -
-...J0
a.
-
- -
a.
u
1-
.
lOOM
u..
HOLOC H
w
z
w
w
RELATIVE CHANGES OF SEA LEVEL
~
\
I-PRESENT SEA LEVEL
\
Relative changes of sea level correlated with the seismic
stratigraphic sequences (AC-1 through AC-5) for Ascension
Submarine Canyon, west of the San Gregorio fault zone. Sea
level curve after Vail and Hardenbol (1979).
r
75
I
-1
PROFILE 8'-C
~:
~·
0
I
I
~
'KM-{
)>
-<
I
-1
:::u
1-
a..
0.2
w 150-
)>
0
<
a::
r
ITI
w
-1
1-
s::
<(
3:
-
fT1
.U>
fTI
.0
Figure 16.
Moss Landing Marine Laboratories one-kilojoule sparker
profile B'-C illustrating the shelf edge progradational
sequences (AC-3 through AC-5). .
35
terminated by erosional truncation at the seafloor along the shelf and
by downlap near the shelf break (fig. 14 and 16).
This unit and the
overlying units extend along the shelf edge from north of Point Ano
Nuevo to where they pinch out between the northwestern and southeastern
headward regions of Ascension Canyon (fig. 12).
The upper unconformity of this sequence is defined by erosional
truncation of underlying AC-3 reflectors and by downlap of the overlying
AC-4 reflectors.
This discordance is tentatively correlated with the
low stand of sea level at 1.75 m.y.B.P. because this was the next major
sea level low stand to the 2.8 m.y.B.P. fall (fig. 15).
early Pleistocene age is assigned to this sequence.
Therefore, an
This unit and the
overlying units are inferred to be comprised of unconsolidated sediments
based upon:
(1) the unconsolidated sediments dredged from Ascension
Canyon, and (2) the section of Quaternary unconsolidated sediments in
the Outer Santa Cruz Basin (Hoskins and Griffiths, 1971; fig. 10).
Middle Pleistocene Sequence (AC-4).
This sequence is nearly
identical to the underlying sequence AC-3 in its acoustic character
except for the lack of low amplitude basal reflectors.
It downlaps onto
AC-3 sequence and is downlapped by the overlying unit (figs. 14 and 16).
The upper unconformity is defined ·by truncation of sequence AC-4 through
AC-2 reflectors near the shelf/slope break and by downlap of the
overlying unit (figs. 14 and 16).
This discordance is attributed to the
low stand of sea level at 0.75 m.y.B.P.; thus, this sequence is presumed
to be middle Pleistocene in age (fig. 15).
36
Late Pleistocene Sequence (AC-5).
This sequence is typified by
variable amplitude and variable reflection frequency reflectors (figs.
14 and 16).
Local, discontinuous unconformities are seen within this
unit, but none are correlated as a single entity due to problems in
resolution through the ''bubble pulse.''
These discontinuous
unconformities may be due to one or more of the at least five low stands
of sea level during the past 0.75 m.y.B.P. (Beard, 1982), particularly
the low stand prior to the Holocene transgression (fig. 15).
Because
this unit overlies an unconformity assumed to be 0.75 m.y.B.P., it is
assigned a late Pleistocene age (fig. 15).
Seafloor exposures of all five of the above seismic stratigraphic
sequences (AC-5 through AC-1) are probably almost everywhere covered by
a thin veneer of sediment due to the Holocene transgression beginning
approximately 18,000 yr. B.P.
Evidence for Canyon Excavation
Multiple episodes of canyon cutting on the shelf are observed on
intermediate and deeper penetration seismic reflection profiles
extending landward from the two canyon heads comprising the northwestern
portion of Ascension Canyon (figs. 17 and 18).
A buried canyon landward
of the second northwesternmost canyon head probably represents the
oldest canyon excavation (fig. 19).
Late Miocene-Pliocene (AC-2)
sequence reflectors are truncated and underlie the upper bounding 2.8
m.y.B.P. unconformity (fig. 20).
In addition, a zone of high amplitude,
chaotic reflectors of questionable origin is visible within the AC-2
sequence.
PROFILE' Q'-R
-0.10
1125-
150.0-
187.5-
-0.30
225.0-
Figure 17.
Moss Landing Marine Laboratories one-kilojoule sparker
profile Q'-R showing two post-middle Pleistocene exhumations
of the north~testernmos t head of Ascension Submarine Canyon,
a late-Miocene-Pliocene buried canyon, and a large anticline
associated with the uplifted Ascension-Monterey High.
PROFILE
4'-4"
KM
10
_,
::;:
0
::;:
);>
-<
~_,
rn:v
OJ;>
-<
2.0
OUTER SANTA CRUZ
"'r_,
s:
"'
Figure 18.
line drawing of U.S. Geological Survey 160 kilojoule sparker
profile 4'-4" showing the Santa Cruz High, the Outer Santa
Cruz Basin, the Ascension-Monterey High and the San Gregorio
fault zone.
w
00
-KM
0
....
5
--?__3-Q--0
~----
------- ----Ll------ ---<-1-L. '>
EA.U..I.Jli' AGES INDICATED AS FOLLOWS:
SOLID WHERE WELL-DEFINED
DASHED WHERE APPROXIMATELY
LOCATED OR INFERRED
QUE fliED WilER£ EXISTENCE
UtlCEfiTAitl
Figure 19.
~ ~~~~~~~ii!~~~~~t;~~o'!:~ASTEANARY
DISPLACES QUATERNARY SEDIMENTS
AND OLDER ROCKS
..£..DISPLACES PLIOCENE AND OLDER RO~KS
0
L::c,
DISPLACES MIOCENE AND OLDER ROCKS
~
TANTICUNE
-t-
SYNCLINE
n
SURFACE CANYONBCHANNELi
Ed QUATERNARY SEDIMENT FILLED
fg
BURIED CANYON IN UPPER
~TERTIARY STRATA
__.<
STRIKE
a
DIP
F-'JZONE OF CHAOTIC REFLECTORS i
~GAS
OIARGED CANYON FILLI?l
Structural map for the Ascension Submarine Canyon and
adjacent continental shelf. See text for explanation.
w
<D
'IU
-0.10
'
'
'
';
15
150
::r:
1CI..
~ 225.u--::. ..
5
1-
75.0- SE
~
112.5
150.0
187.5
225.0-
Figure 20.
0.25
V.E:"24X
BURl ED CANYON
-0.30
Portion from Moss Landing Marine Laboratories one-kilojoule
sparker profile Q'-R (fig. 17) showing late Miocene-Pliocene
buried canyon.
41
This zone extends landward from the clearly defined buried canyon (fig.
19).
The proximity and similarity in age of this chaotic zone to the
buried canyon suggests that the chaotic deposits may have formed by
slumping near the headward terminu? of the buried canyon.
Two episodes of canyon cutting are represented by a
northeast-trending, infilled and partially exhumed canyon extending from
the northwesternmost canyon head (fig. 19).
The first excavation is
shown by the truncation of middle Pleistocene (AC-4) through late
Miocene-Plioocene (AC-2) reflectors (fig. 21).
Subsequent canyon
infilling occurred shown by the stratified reflectors juxtaposing the
truncated AC-4 and older reflectors.
The second episode of canyon
cutting is displayed by the partial exhumation of the canyon fill.
This
is evident where canyon fill reflectors are truncated along the walls of
the modern canyon (fig. 21).
San Gregorio Fault Zone
Two parallel, nearly vertical, northwest-trending (N20-25°W)
faults are part of a series of faults comprising the right-slip San
Gregorio fault zone (fig. 11, 19, and 22).
The Monterey Formation
(sequence AC-1) on the southwest side of the fault zone is juxtaposed
with the Santa Cruz Mudstone and Purisima Formation on the northeast
side (fig. 12).
A series of northwest-southeast-trending (N50-60°W)
~echelon
folds occur adjacent to the fault zone (fig. 11, 19, and 22).
The
angles between the fold axes and the strike of the fault zone are
TWO-WAY TRAVEL TIME(SEC)
0
~
ci
ci
~L
0
"!
o
~
"!
o
0
1'1?
~
0
oa
0
C\J
o
0
('()
ci
I
I
X
'<I"
3
"'•
z
u.i
>
z
0
z>-
1-
<(
0
u
en
~
z
J
0
LLJ
-1
LLJo-1
1-<(li:
a:
::.::
C\J
[:3wz
1
;::::t:J:
>-
1-
-1
z
1-
0
.
-: ..
...J
a:
<(
a:
<(
a..
....
..
. c.
w
.
(/)
r
q
~
I'-
IQ
~
0
ci
~
~
r..:
(I)
I
0
0
C\J
C\J
I'-
tO
~
(l/ll) H.Ld30 ~3.L'I1M
Figure 21.
I
~
0
C\J
~
I!)
C\J
C\J
Portion from Moss Landing Marine Laboratories one-kilojoule
sparker profile Q'-R (fig. 17) showing two episodes of
post-middle Pleistocene canyon cutting of the
northwesternmost head of Ascension Submarine Canyon. M1 =
first multiple reflection.
43
0. 0
7 5,
~
(.)
w
(/)
~
0.2 w
:::;E
1-
::c
1a..
w
....J
w
>
<{
a::
Cl
1-
0::
w
>-
~
0
~
1-
I
3
0.1
1-
0.2
If. E.• 25X
Figure 22.
Moss
Land"
profile
A; ngB
Lab orator·
zo
- Marine
sh ~Wlng
· to
ne. Arrow
foldin
les one-kilo.
polnts
Pl elstocene
. g adjacent
t o Joule
San Grespark
_er
unconformit
y. gon o fault
44
approximately 30° in a counterclockwise direction (fig. 19).
The trends
of the folds are consistent with wrench tectonic models for
right-lateral strike-slip faulting (Tchalenko, 1970; Wilcox and others,
1973).
Structural Basement Highs
West of the San Gregorio fault zone, the dominant structural
feature is a northwest-striking acoustic basement ridge (i.e., Pigeon
Point High; figs. 5 and 19), which consists of two separate ridges
separated by a small intervening basin or syncline (figs. 19 and 23).
This granitic basement ridge terminates along the San Gregorio fault
zone where its seaward bounding fault (Nacimiento fault; figs. 5, 19,
and 23) intersects the San Gregorio fault zone at a point 16 km south of
Point Ano Nuevo (fig. 19).
Immediately seaward of the high, a
concentration of late middle-late Miocene compressional features trend
parallel to the high (i.e., generally, high angle reverse faults with
westward dips and relatively tight folds; figs. 19 and 23).
The southeastern boundary of the Outer Santa Cruz Basin is defined
by a structural high.
The high lies beneath the shelf and slope along
the entire southeastern headward region of Ascension Canyon and is
interpreted to be continuous with the Ascension-Monterey High,
previously described by Greene (1977) as an acoustic basement dome
supporting the submarine upland betweeen Ascension and Monterey Canyons
(fig. 24).
This high has undergone uplift along a newly identified
northeast-southwest-trending (N30°E) high angle reverse(?) fault
PROFILE 16~16"
w
E
--<
"'
"'
-<
0
NACIMIENTO
);>
FAULT
R SANTA CRUZ
BASIN
"-MULTI-RIDGE
/
0
;;;__,
rn:u
0);>
-<
rn
r
PIGEON POINT HlqH
--<
s:
"'
D U
2 KM
\I.E.= 6X
•I
2.0
Figure 23.
Line drawing of U.S. Geological Survey 100-kilojoule sparker
profile 16'-16" illustrating the multi-ridge Pigeon Point
High, offset Nacimiento Fault, and the Outer Santa Cruz
Basin.
46
12 2°00'
3Beoa
38'"00'
1
N
10
20
30
40
KM
@\
37"00'
/
.
' .A(
GUIDE
SEAMOUNT
\
\
\
37"00'
SANTA
HIGH
OUTER
FAULT
CANYON
Figure 24.
Generalized map depicting the Outer Santa Cruz Basin and its
associated bounding structural highs with the late
Pleistocene uplifted Ascension-Monterey High at the
southeastern end of the basin. Map modified after Hoskins
and Griffiths (1971) and McCulloch and others (1977). S.F.
= San Francisco.
47
located between the northwestern and southeastern headward regions of
Ascension Canyon (figs. 18 and 19).
Upturned middle Pleistocene (AC-4)
and older reflectors are located along the northwestern side of this
high indicating uplift (fig. 17). ·Uplift, folding, and subsequent
erosion has exposed the anticlinally folded sequence AC-1 and the
upturned and truncated sequences AC-2 through AC-4 at the seafloor (fig.
17).
Deep penetration reflection data also show a basement high
(Ascension-Monterey High), which has upturned and anticlinally folded
reflectors in this area (fig. 18).
Because middle Pleistocene (AC-4)
reflectors are folded and truncated, uplift and erosion apparently have
occurred within the past 750,000 years (fig. 15).
48
DISCUSSION
Origination Point and Total Offset of the Study Area
A place of origin for the study area must be established in order
to trace the structural evolution and the right-lateral strike-slip
displacement of Ascension Submarine Canyon along the San Gregorio fault
zone.
First, cross fault ties must be identified along the San Gregorio
fault zone, then the progressive northward right-slip offset along the
fault zone can be traced through time.
The southwestern boundary of the Pigeon Point High is defined by
the truncated Nacimiento fault (fig. 5), which separates Franciscan
rocks to the southwest and granitic rocks to the northeast (Mullins and
Nagel, 1981).
A Franciscan-granitic fault contact at Point Sur was
previously discussed as a possible cross fault tie along the San
Gregorio fault zone with 90 km of offset since mid-late Miocene (Silver,
1974; Mullins and Nagel, 1981).
More detailed mapping has extended the
Pigeon Point High approximately 16 km south of Point Ana Nuevo to where
the Nacimiento fault intersects the San Gregorio fault zone (figs. 19
and 24), thus suggesting 74 km of offset.
However, W. R. Dickinson
(written communication, 1982) has suggested that a more realistic
Franciscan-granitic cross fault tie, as opposed to Point Sur, would be
where Graham and Dickinson (1978) have interpreted the Nacimiento fault
to be truncated by the Sur fault (i.e., a southerly extension of the San
Gregorio fault), located approximately 25 km south of Point Sur.
cross fault tie is accepted because:
This
(1) a Franciscan-granitic contact
49
as well as the truncated Nacimiento fault can be matched and {2) the
Point Sur Franciscan block is located west of the San Gregorio fault
(Graham and Dickinson, 1978) and could not possibly provide a cross
fault tie.
Thus, the origination point for the study area is located
south of Point Sur and north of Point Lopez where the Nacimiento and Sur
faults intersect (fig. 25a).
Consequently, the study area must be
displaced south a1ong the San Gregorio fault zone approximately 105 km,
i.e., the measured distance between this origination point and truncated
Nacimiento fault south of Point Ana Nuevo (fig. 25h), in order to begin
the evolutionary history for Ascension Canyon.
Paleogeographic Reconstruction Parameters
Using the Franciscan-granitic cross fault ties described in the
previous section as points of reference, a paleogeographic
reconstruction model is developed that shows the northward migration of
the study area along the San Gregorio fault zone (fig. 25).
The model
will provide assumed paleogeographic locations for Ascension Canyon
during its evolution.
Greene (1977) and Howell and others (1980) have suggested that
movement on the San Gregorio fault zone began 14 to 10 m.y.B.P. which is
comparable to the post middle Miocene initiation time proposed by Graham
and Dickinson (1978).
For purposes of paleogeographic reconstruction,
the initial movement on the San Gregorio fault zone is assumed here to
be 12 m.y.B.P., i.e., the median value between 14 and 10 m.y.B.P. (fig.
25a).
50
A.
:.,
LATE MIDDLE MIOCENE
8.
EARLY LATE MIOCENE
INITIATION OF SAN GREGORIO
'•
DEVELOPMENT OF OUTER
SANTA CRUZ BASIN
PT. ANa\.
FAULT ZONE
NUEVO ' \ \ ,
\
SEA LEVEL. LOW STANO
\
·--.:~.~-~~--C:~uz
\ ....
MONTEREY \
CANYON
\
SAN GREGORIO
-._f~:.!::..:-:.:..:~Y::ti2.Q M.Y.B.P.
FAULT ZONE
,---
·· ....-._.-•\\
. __:y-:-~.-::.:==:;f
,
·---
,.,
:::.~~'TEREY
'
.
~\',l,
PIGEON
z9.8 M.YB.P
,~.
\
PO IN~['
HIGH
\
·-~:
'%'
0
10
20
30
o!Q
~
50
\_,
/.??'.:'('t:..:,..-~- ~
'"
SANTA CRUZ
HIGH
',
~/'<"'..::.-..,
"'<::;;.:
.,:.<;:..
OUTER SANTA CRUZ
FAULT
c.
oATE MIOCENE
·SEA LEVEL
'-,
'·
Low
sTAND
OUTER SANTA CRUZ
BASIN
',
.._
,',~
\"""'-
<.c. '>·-~
\', ,
D.
PLIOCENE
·-.
SEA LEVEL LOW STAND
.... _,,_ ......
:.-....r·::-.-..~-..:.-..""!i ~ 6.6 M.Y.B.P
Figure 25.
Sequential development of the region surrounding the
Ascension Submarine Canyon including development of the
Outer Santa Cruz Basin. See text for explanation. Dashed
coastline in sequence A-G is for reference purposes and does
not represent a paleocoastline configuration.
51
F
E.
LATE PLIOCENE
:,~
'.
~
MIDDLE PLEISTOCENE
SEA LEVEL LOW STANO
SEA LEVEL LOW STANO
CANYON CUTTING IN
MONTEREY CANYON
'··.,,_5~~~ ·~~RENZD RIVER
~2.8
~
M.Y.B.P.
1.75 M.Y.B.P.
',
G.
LATE PLEISTOCENE
INITIAL UPLIFT OF ASCENSION•
MONTEREY HIGH
SEA LEVEL-LOW STAND
H.
HOLOCENE
105KM TOTAL LATE CENOZOIC
OFFSET ALONG SAN GREGORIO
FAULT ZONE
_\_.-..
~.
7 5 M.Y. 8.~
TODAY
HIGH
-T-36"00'
52
Using offset pairs of upper Miocene Santa Cruz Mudstone, J. C.
Clark has suggested that there has been approximately 70 km of offset
along the San Gregorio fault zone since the late Miocene (personal
communication, 1983).
.
For paleoreconstruction purposes, this offset is
assumed to have occurred since the formation of the upper bounding
unconformity to the Santa Cruz Mudstone, i.e., approximately 6.6
m.y.B.P.
Considering there is a total offset of 105 km, the remaining
35 km of offset must be attributed to the period between 12 and 6.6
m.y.B.P.
The calculated slip rates are 10.6 mm/yr for the period since
6.6 m.y.B.P. and 6.5 mm/yr for the period from 12 to 6.6 m.y.B.P.
These
slip rates are used to provide approximate paleogeographic locations for
Ascension Canyon and are not meant to advocate constant slip for the San
Gregorio fault zone during the above time periods.
The present-day channel configuration of the Monterey Submarine
Canyon, east of the San Gregorio fault zone, is used in the
paleoreconstruction model (fig. 25 a-h).
Consideration was given to the
possibility that the channel of Monterey Canyon has followed different
courses throughout time.
However, present available data do not provide
evidence for alternative post-middle Miocene channel courses for the
Monterey Canyon.
Evolution of Ascension Submarine Canyon
The buried canyon landward of the second northwesternmost canyon
that truncates reflectors within the late Miocene-Pliocene (AC-2)
sequence provides the oldest evidence for cutting of Ascension Canyon
53
(figs. 19 and 20).
This buried canyon may represent headward erosion
from a pre-late Miocene slope canyon that has extended onto the shelf
during late Miocene-Pliocene times.
No evidence exists to substantiate
or refute the existence of a previously carved slope canyon.
Therefore,
this buried canyon is interpreted to mark the initiation of Ascension
Canyon.
Greene (1977) and Howell and others (1980) have suggested that
Ascension Canyon was cut as the distal channel of the Monterey Canyon
approximately 7 m.y.B.P.
This cutting can be correlated with the 6.6
m.y.B.P. low stand of sea level (fig. 15).
This suggestion is
improbable because the late Miocene-Pliocene buried canyon overlies the
6.6 m.y.B.P. unconformity.
This canyon excavation must have occurred
after 6.6 m.y.B.P. and prior to the 2.8 m.y.B.P. bounding unconformity
of the sequence AC-2.
The exact timing for exhumation is indeterminable
within these time boundaries.
Along the northwestern wall of the buried canyon, there is a
complex interfingering relationship between the erosional truncated
reflectors and the AC-2 sequence reflectors that appear to extend into
and infill the canyon (fig. 20).
There appear to be at least three
episodes of erosional truncation followed by canyon infilling.
Such a
relationship may suggest continuous marine sedimentation that was
punctuated by submarine canyon erosion, possibly via turbidity currents
and/or other sediment gravity flows.
The study area would have migrated along the San Gregorio fault
zone from its approximate location shown in figure 25b to its
,,,,
----.
54
1
approximate location shown in figure 25e during the late Miocene to the
late Pliocene.
During this time, the buried canyon could have passed
the channel of the Monterey Canyon and might have been juxtaposed to the
channel during mid-Pliocene time (i.e., approximately 3.8 m.y.B.P.; fig.
'25d).
There was a relatively moderate sea level fall at 3.8 m.y.B.P.
{Vail and Hardenbol, 1979; fig. 15), so this presents the possibility
that Monterey Canyon experienced a partial exhumation, and Ascension
Canyon was cut as its distal channel.
During low stands of sea level,
the frequency and magnitude of turbidity currents are greatly increased
on the outer shelf down to the deep sea, apparently because rivers
and/or littoral drift systems can transport sediments directly into
submarine canyon heads (Hess and Normark, 1976; Shanmugam and
1982).
t~oiola,
Increased turbidity current activity related to the 3.8 m.y.B.P.
sea level fall may have exhumed the Monterey Canyon while periodical
turbidity currents interrupted the continuous sedimentation in the
proximity of Ascension Canyon as indicated by the complex interfingering
relationship along the northwestern canyon wall (fig. 20).
possibility should be considered speculative because:
The
(1) there is no
evidence of a truncated ancestral channel continuously extending across
the shelf from the San Gregorio fault zone to Ascension Canyon, and (2)
there is no evidence for activity in the Monterey Canyon from the late
t1iocene (6.6 m.y.B.P.) to the late Pliocene (Martin, 1964; Greene, 1977;
Greene and Clark, 1979).
An alternative possibility is offered to explain this late
Miocene-Pliocene excavation.
The buried canyon is hypothesized to have
,---
55
1
resulted from headward erosion from a slope canyon.
Uplift of the Santa
Cruz High may have oversteepened slopes that generated slumping,
sliding, and/or sediment gravity flows.
These possible mass movements
and/or sediment gravity flows could have cut a slope canyon whose
headward erosion eventually notched the shelf.
Uplift along the Santa
Cruz High was active until late Pliocene-Quaternary times (Blake and
others, 1978), which coincides closely with the termination of erosion
of the buried canyon, i.e., prior to the late Pliocene-early Pleistocene
(2.8 m.y.B.P.) unconformity.
Also, the termination of erosion may be
attributed to the migration of the buried canyon away from the Monterey
Canyon channel by the late Pliocene (fig. 25e).
Thus, both of the above
mentioned possibilities seem to hold some merit for explaining the
evolution of the late Miocene-Pliocene buried canyon.
The formation of the modern Monterey Canyon probably began during
late Pliocene-early Pleistocene times, approximately 3 m.y.B.P. (Martin,
1964; Greene, 1977; Greene and Clark, 1979; Howell and others, 1980).
This timing can be correlated with the major sea level low stand at 2.8
m.y.B.P. (Vail and Hardenbol, 1979, fig. 15).
The most northwesterly
portion of the southeastern headward region of Ascension Canyon could
have been juxtaposed to the Monterey Canyon at this time (fig. 25e).
There is no evidence for cutting on the shelf landward of the
southeastern headward region.
The late Pleistocene uplift of the
Ascension-Monterey High and subsequent erosion could account for the
removal of all such evidence (fig. 17).
Therefore, based on the close
geographic proximity of Ascension Canyon to the Monterey Canyon during
56
its major excavation, it seems likely this portion of Ascension Canyon
may have been cut as the distal channel of the Monterey Canyon.
The most southeasterly extent of Ascension Canyon possibly was
juxtaposed against the Monterey Canyon during the 1.75 m.y.B.P. low
stand of sea level (fig. 25f).
Martin (1964) has indicated exhumation
in the Monterey Canyon at approximately 1.5 m.y.B.P.
If these events
correlate, then this southeasterly portion of Ascension Canyon may have
been cut as the distal channel of Monterey Canyon.
While a portion of
Ascension Canyon possibly was being cut by the Monterey Canyon, the
other canyon heads may have had the opportunity to be excavated by
streams and rivers that crossed the shelf during sea level low stands.
Martin (1964), Greene (1977), and Greene and Clark (1979) suggested
rivers and streams ran out to the shelf break of the Monterey Bay during
the late Pliocene low stand and Greene (1977) showed Pleistocene buried
stream channels on the northern Monterey Bay shelf.
Ascension Canyon would have been offset northwestward beyond the
channel of Monterey Canyon by the late Pleistocene (fig. 25g).
Two post
middle Pleistocene episodes of canyon cutting are shown by the
northwesternmost canyon head (fig. 21).
The first episode of erosion
truncated middle Pleistocene (AC-4) and older reflectors and is
speculatively correlated to the late Pleistocene low stand at 0.75
m.y.B.P. (fig. 15).
It is indeterminable if this was the initial
cutting because any evidence for an earlier cutting would have been
removed.
Stratified canyon fill may have been deposited during the
ensuing transgression (figs. 15 and 21).
The second episode of cutting
57
truncates the canyon fill reflectors that are exposed along the walls of
the modern canyon (fig. 21).
This partial exhumation may be correlated
with the last major low stand of sea level before the Holocene
transgression, i.e., at approximately 18,000 yr. B.P. (fig. 15).
The erosional processes responsible for the two late Pleistocene
exhumations are open to speculation.
processes were involved.
One possibility is that multiple
A creek from the Santa Cruz Mountains may have
initially carved a channel across the shelf during the 0.75 m.y.B.P. low
stand, although comparing the widths of this partially infilled canyon
and Santa Cruz Mountain creeks, it seems unlikely any Santa Cruz
Mountain creek could generate such a wide canyon.
This suggests that
submarine erosion may have enlarged this possible subaerial excavation
during the succeeding transgression prior to infilling.
The assumed
18,000 yr. B.P. excavation may have followed a similar scenario.
The
number and spacing of the Ascension Canyon heads are similar to the
number and spacing of the onland creeks and river adjacent to the study
area (fig. 2), suggesting that these creeks crossed the shelf and
initiated the cutting of canyon heads.
No buried stream channels were
found crossing the shelf in the study area, however.
Alternatively,
the excavations may have resulted entirely from submarine processes.
The first cutting may represent headward erosion from a slope canyon
or an excavation of the shelf by littoral zone erosion.
Then,
the partial exhumation of the canyon fill may have been the
58
consequence of scouring turbidity currents and headward erosion during
the low stand of sea level at 18,000 yrs. B.P.
The southeastern headward region of Ascension Canyon may have had
portions cut prior to the late Pletstocene as previously discussed.
However, the late Pleistocene to
Recen~
canyon morphology is
hypothesized to be predominantly influenced by the uplift of the
Ascension-Monterey High that has presumably occurred within the past
750,000 years (figs. 18, 24, and 25g).
This uplift of the southeastern
end of the Outer Santa Cruz Basin can be accounted for if the basin is
structurally viewed as a fault wedge basin (fig. 26).
The fault wedge
basin model (Crowell, 1974) demonstrates uplift and compression at the
end of the basin where two major right-lateral strike-slip faults
converge.
At this southeastern portion of the basin, the high angle
reverse fault along the seaward margin of the Santa Cruz High (referred
to as the Outer Santa Cruz fault) is inferred to be a convergent
right-lateral strike-slip fault and to demonstrate convergence with the
San Gregorio fault zone (fig. 14 and 26).
Thus, convergent wrenching
along the San Gregorio fault zone and the Outer Santa Cruz fault is
inferred to be responsible for the uplifted Ascension-Monterey High.
As
the uplift occurred, the slope probably was continuously oversteepened.
In response to the oversteepened slope, periodic slumping, sliding,
turbidity currents, and other gravity flows could have resulted in
canyon cutting on the slope and headward erosion onto the shelf.
oversteepening of the slope may be reflected in the vastly steeper
Ascension Canyon channel gradient compared to the Monterey Canyon
This
59
FAULT WEDGE BASIN MODEL
OUTER
SANTA CRUZ
BASIN
N
~
1
00'
N GREGORIO
FAULT ZONE
OUTER
SANTA CRUZ
FAULT
\
ASCENSIONMONTEREY
HIGH
~CONVERGENCE
OEPTH COHTOUR9 IN METERS
L
--=----------------------"--'"'"'---::-::-:-:'!
121.,45'
36.,30' 123"0
Figure 26.
36"30'
Fault wedge basin model depiciting uplift at the convergent
southern end of the Outer Santa Cruz Basin and a northwest
plunging basin. Crowell (1974) developed the general model
for fault wedge basins.
60
channel gradient, west of the San Gregorio fault zone, i.e., a place
where this southeastern portion of Ascension Canyon may have been cut
(figs. 25e, 25f, and 27).
The steeper gradient may attest to the
changing morphology due to the late Pleistocene uplift that may still be
occurring.
In addition, this southeastern headward region may owe its
diffuse morphology to this uplift and subsequent erosion.
After the low stand of sea level at 18,000 yrs. B.P., headward
erosion could not keep pace with the rising Holocene transgression, so
this shelf edge canyon system was beheaded.
Today, the activity in the
Ascension Submarine Canyon may be best typified by a description of the
activity on the Ascension fan valley.
Hess and Normark (1976) described
the sedimentation on the fan valley as only an occasional sand or silt
interval, reflecting a rare turbidity current event, interrupting the
predominantly muddy Holocene sedimentation.
HORIZONTAL DISTANCE FOLLOWING
AXES OF
0
20
10
0~--------~----------J_
''
500
1000
''
''
CANYONS IN KILOMETERS
40
50
60
_________ L_ _ _ _ _ _ _ _ _ _L __ _ _ _ _ _ _ __ L_ _ _ _ _ _ _ __ J_ _ _ _ _ _
70
60
'
''
ASCENSION AND MONTEREY CANYONS
''
''
''
1-
w
"'~
''
''
''
1500
1-
'
''\
\
"-
\
w
\
D
'
a:
w
EAST'.
----...:.....- LOCATION OF SAN GREGORIO FAULT
1-
WEST
<{
3:: 2. 000
1
'
WEST OF SAN GREGORIO FAULT
V.E.=30X
'
'-....,
MONTEREY CANYON'S AXIAL CHANNEL GRADIENT
'<--
AXIAL CHANNEL GRADIENT
2 500
' , CROSSING MONTEREY CANYON
',,
ASCENSION CANYON'S
' ',
WEST OF SAN GREGORIO FAULT
''
''
'
-- ---1.................... _
-- ----
3000
Figure 27.
90
~--L----------L--------~
CHANNEL GRADIENT FOR
''
"'a:w
r
30
Channel gradient comparison of Ascension and Monterey
Submarine Canyons, west of the San Gregorio fault zone.
r
62
CONCLUSIONS
1.
Both tectonic and eustatic processes appear to have been
responsible for the formation of Ascension Canyon.
2.
The initial cutting of Ascension .Canyon probably occurred between
6.6 and 2.8 m.y.B.P.
Multiple erosional events are recognized, but
the exact erosional processes involved are not known.
3.
No direct evidence was found to show that Ascension Canyon owes its
multiplicity of heads to the progressive offset of the distal
channel of the Monterey Canyon.
However, portions of Ascension
Canyon may have been juxtaposed against the Monterey Canyon when it
probably was being exhumed, thus providing indirect support to the
basic idea of Greene (1977) that the displacement of the distal
channel of Monterey Canyon resulted in portions of Ascension
Canyon.
4.
The northwesternmost head of Ascension Canyon shows multiple
episodes of late Pleistocene canyon cutting and infilling which
appears to be due to repeated periods of erosion and deposition
controlled by eustatic sea level fluctuations.
The excavations
have been tentatively correlated with the 0.75 m.y.B.P. and 18,000
yr. B.P. low stands.
The exact erosional processes responsible for
the excavations are not known.
5.
The late Pleistocene to Recent evolution of the southeastern
headward of Ascension Canyon may have been predominantly controlled
by the uplift of the southeastern convergent end of a fault wedge
r
63
'.
basin, i.e., Outer Santa Cruz Basin.
The uplift of
Ascension-Monterey High may have continuously oversteepened the
slope periodically giving rise to submarine mass movements and
sediment gravity flows that carved the slope; so, as the
Ascension-Monterey High was ascending, the Ascension Canyon was
evolving.
6.
Total late Cenozoic right-lateral strike-slip offset along the San
Gregorio fault zone is approximately 105 km, based upon
Franciscan-granitic offset pairs located 16 km south of Point Ano
Nuevo and 25 km south of Point Sur.
64
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65
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~~··
69
APPENDIX I:
ROCK DREDGE SAMPLES
Dredge Haul A1
Start:
Finish:
Depth:
36"55'00"
122"28'05''
36"55'30"
122"28'30''
1080 to 845 m
Sample Ala (99%)
Lithology: Serpentine - blue-green, angular, brittle, fibrous
serpentinite. Most of sample has fresh surfaces and are cobble size
fragments ( 10 x 11 x 9 em) with one boulder (30 x 40 x 22 em).
X-ray: Lizardite
Age: Jurassic to Cretaceous-Franciscan Assemblage(?)
Sample Alb (<1%)
Lithology: Siltstone - grayish green, subangular, massive,
moderately well indurated, highly bored, diatomaceous siltstone with few
siliceous sponge spicules. One piece (5 x 5 x 2 em).
Paleontology: Diatoms include:
Denticulopsis seminae var. fossilis
Melosira granulata (fw)
Rhizosolenia barboi
Stethanodiscus sp. (fw)
Tha assiosira antigua
l· oestrupii
Age: Late Pliocene- Denticulopsis seminae var. fossilis zonePurisima Formation.
Sample Ale (<1%)
Lith: Basalt - brownish black, highly angular, vescicular basalt.
Two fragments (5 x 4 x 2 em and 8 x 5 x 4 em).
Age: Unknown - Mafic volcanics in central California are commonly
associated with the Franciscan Assemblage.
Remarks: Serpentinite samples showed fresh surfaces and angularity
indicating samples were collected in situ. Dredge contained minor
amounts of gray green cohesive mud.
70
Dredge Haul A2
Start:
Finish:
Depth:
36°57'30"
122°26'10"
36°58'15"
122°25'05"
725 to 330 m
Samp 1e A2 (100%)
Lithology: Siltstone -brownish green (weathered surface) to
greenish gray (fresh surface), angular'to subangular, massive,
moderately well indurated, highly bored (polychaetes and pholads), very
fine grained, sandy, lithic siltstone with diatom fragments and
siliceous sponge spicules. Lithic fragments include rhyolitic glass
shards, pumice fragments, quartz, zoned plagioclase, and basaltic
hornblende fragments. Most of sample is cobble size (15 x 12 x 8 em)
with two boulders (25 x 20 x 18 em and 25 x 12 x 12 em).
Paleontology: Diatom fragments nondiagnostic and barren of
foraminifers.
Age: Pliocene(?) - Purisima Formation (?)
Remarks: All one rock type suggests samples were collected in
situ. Dredge was partially filled with minor amounts of gray green
cohesive mud.
71
Dredge Haul A3
Start:
Finish:
Depth:
36"59'35''
122"25'15''
36"59'30''
122"26'00''
356-145 m
Sample A3a (95%)
Lithology: Siltstone - grayish olive green (weathered surface to
buff gray (fresh surface), angular, massive, very well indurated, highly
bored (polychaetes and holads) dolomitic siltstone with siliceous sponge
spicules and glauconite. Four large slabs (37 x 25 x 10 em to 15 x 19 x
16 em) and boulders (20 x 18 x 14 em).
Paleontology: Diatoms include:
Stephanopyxis dimorpha
Age: Late Pliocene to Quaternary - Purisima Formation.
Sample A3b (5%)
Lithology: Sandstone - grayish olive green (weathered surface) to
buff gray (fresh surface), angular, massive, moderately well indurated,
highly bored (pholads and polychaetes), silty, very fine-grained
sandstone with sponge spicules, wood fragments, and glauconite. Cobble
size samples (1D x 6 x 3 em).
Paleontology: Foraminifera include:
Bulimina denudata (Cushman and Parker)
Epistominella pacifica (R. E. and K. C. Stewart)
Virtulina sp.
Age: Late Miocene ?) - Quaternary-Purisima Formation (?)
Remarks: Fresh surfaces on boulders and slabs suggest the samples
were collected in situ. Dredge was half filled with dark green very
cohesive clayey mud.
72
Dredge Haul A4
Start:
Finish:
Depth:
36°54'05"
122°25'00"
36°54'35"
122°25'10"
1055-875 m
Samp 1e A4 ( 100%)
Lithology: Basalt- dark gray, fine granular textured, very
angular, highly fractured, very friable metavolcanic with pyrite and
thick phosphatic coating (1! em). Majority of sample is small fragments
(5 x 5 x 3 em) with largest size (12 x 8 x 6 em).
X-ray: Spilitic basalt.
Age: Unknown - spilitic basalts are commonly associated Franciscan
Assemblage.
Remarks: Fresh surfaces indicate samples were collected in situ.
No mud in dredge.
73
Dredge Haul A5
Start:
Finish:
Depth:
36°55'45"
122°23'25"
36°56'00"
122°22'30"
700-452 m
Sample A5 (100%)
Lithology: Siltstone -greenish gray (fresh surface) to brownish
gray (weathered surface), angular-subangular, very finely laminated weak
to fairly well indurated, heavily bioturbed (polychaetes and pholads),
diatomaceous siltstone with siliceous sponge spicules and some quartz
silt. Majority of sample is cobble size (10 x 8 x 4 em) with largest
size (20 x 15 x 7 em).
Paleontology: Diatoms include:
Denticulopsis seminae var. fossilis
Epithemi a sp. ( fw)
Melosira granulata (fw)
M. sul cata
Rhaphoneis sp. (marine benthic)
Stephanopyxis dimortha
Ste~hanodiscus sp.
fw)
Tha assionema nitzschioides
Thalassiosira antigua
Thalassiosira eccentrica
Silicoflagellates include:
Distephanus polyactis
Age: Late Pliocene- D. seminae var. fossilis zone- Purisima
Formation.
Remarks: All one rock type and angularity of samples suggest in
situ collection. Dredge was partially filled with three distinctly
different muds. Mud types were: (1) dark green cohesive mud; (2) dark
green silty mud with siltstone chips; and (3) greenish very coarse grain
sandy (pelletal) silty mud.
74
Dredge Haul A6
Start:
Finish:
Depth:
36°57'55 11
122°22'05"
36°57'30 11
122°22'15"
375-233 m
Sample A
6
Lithology: Mud- dark grayish green cohesive mud with few shell
fragments.
Paleontology: None
Age: Unknown
75
Dredge Haul A7
Start:
Finish:
Depth:
36°50'05"
122°18'20"
35°51'15"
122°20'00"
1095-840 m
Sample A7
Litholog{: Mud- grayish green
Paleonto ogy: None
Age: Unknown
c~hesive
homogenous mud.
76
Dredge Haul As
Start:
Finish:
Depth:
36°54'00"
122°17'50"
36°54'20"
122°1S'OO"
635-425 m
Sample As (100%)
Lithology: Siltstone -brownish gray (weathered surface) to
greenish gray (fresh surface, angular-subangular, massive, moderately
well indurated; highly bioturbed (pholads), diatomaceous and micaceous,
lithic (?) siltstone with siliceous sponge spicules, some glauconite,
quartz, plagioclase, green hornblende and basaltic hornblende fragments.
Mostly cobble size fragments with two boulder size fragments (31 x 16 x
12 em).
Paleontology: Sample only yields nondiagnostic diatom fragments
and barren of forams. Molluscs include:
Yoldia sp., nuculanid bivalve.
Age: Pliocene(?) - Purisima Formation (?).
Remarks: All one rock type and no mud in dredge.
77
Dredge Haul A17
Start:
Finish:
Depth:
36"48'55''
122"13'35"
36"48'20''
122"13'45''
870-675 m
Sample A17 a (95%)
Lithology: Siltstone - greenish gray, very angular to subangular
massive, friable, moderately well indurated, highly bioturbed
(polychaetes), lithic, tuffaceous and diatomaceous siltstone with
Monterey chert fragments, basaltic hornblende, quartz and plagioclase
silt and obsidian shard fragments. Radiolarian also present. Mostly
cobble size fragments (19 x 14 x 7 em).
Paleontology: Diatoms include:
Coscinodiscus marJinatus
Epithemia sp. (fw
Nitzschia granulata (mb)
Melosira granulata (fw)
Rhizosolenia barboi
Stephanodiscus sp. (fw)
Thalassionema nitzschioides
Foraminifers include: barren
Age: Pre-Quaternary probably Pliocene - Purisima Formation
Sample A17 b (5%)
Lithology: Mudstone - light brown, angular, very thinly
laminated, very well indurated, diatomaceous opaline mudstone with few
fish fragments, sponge spicules, and glass shards. Sample consists of
blocky fragments up to 7 x 4 x 3 em in size.
Paleontology: Sample barren of diatoms and foraminifers.
Age: Late middle Miocene(?) - Monterey Formation (?)
Remarks: Lithology of A17 b very similar to samples known to be
Monterey Formation that were dredged at similar depths and close
proximity. Dark green cohesive mud and silt with minor amounts of dark
green medium grained sand surrounded rock samples.
7B
Dredge Haul AlB
Start:
Finish:
Depth:
36°4B'40"
122°17'45"
36°47'50"
122°15'35"
1440-730 m
Sample AlB (100%)
Lithologt: Siltstone - dark greenish gray subangular, massive,
moderately we 1 indurated, highly bioturbed siltstone. Sample consists
of small fragment (3 x 2 x 2 em).
Paleontology: None.
Age: Pliocene(?) - Purisima Formation(?)
Remark: Questionable if sample is representative of dredged area.
Sample's lithology similar to known Pliocene Purisima Formation.
79
Dredge Haul A20
Start:
Finish:
Depth:
36°51'30"
122°12'45"
36°52'00"
122°12'45"
700-325 m
Sample A20 a (45%)
Lithology: Siltstone - greenish gray, angular to subrounded,
massive, moderately well indurated, hi.ghly bioturbed (pholads and
polychaetes), diatomaceous siltstone with radiolarians, sponge spicules,
one glaucophane grain (?). Majority of sample is cobble size with many
boulders (23 x 16 x 7 em).
Paleontology: Diatoms include:
Coscinodiscus (fragments)
Melosira granulata (fw)
M. sulcata
Stephanodiscus sp. (fw)
Thalassiosira oestrupii
Thalassiothrix longissima
Forams include:
Epistominella sp.
Nonionella grateloupi (?)
Virgulina pyritized
Age: Pliocene to Quaternary - Purisima Formation
Sample A20 b (20%)
Lithologl: Sandstone - black, subrounded, massive, moderately well
indurated, we l sorted, slightly bored, fine-medium grained, oil
saturated arkosic sandstone. Sample mostly cobble size fragments (15 x
11 x 8 em) and smaller.
Paleontology: Sample barren of diatoms and forams.
Age: Early late Miocene(?) Santa Margarita Sandstone(?).
Sample A20 c (1%)
Lithology: Oil saturated, fine-medium grained arkosic sandstone in
contact with thinly laminated siliceous mudstone. Patches of solid
bitumen on surface of mudstone. Nature of contact between lithologies
is a sharp linear contact. Sample size is (10 x 7 x 7 em).
Paleontology: Sample yields non-diagnostic arenaceous forams from
mudstone, and sandstone was barren.
~:
Late middle Miocene(?) - Monterey Formation - mudstone.
Early late Miocene(?) - Santa Margarita Sandstone(?) - sandstone.
Sample A20 d (15%)
Lithololy: Mudstone -whitish, angular-subangular, thin
irregularlyaminated, well indurated, dolomitic, diatomaceous opaline
mudstone. Tar impregnation within thin laminae. Sample is notably of
low density and of cobble size fragments (8 x 5 x 4 em).
r
80
Paleontology:
Diatoms include:
Actinocyclus ingens
Actinoptychus undulatus
Auliscus sp.
Cocconeis sp.
Coscinodiscus marginatus
Coscinodiscus plicatus
Coscinodiscus tabularis
Denticulopsis hustedii
Diploneis sp.
Rhizosolenia barboi
Stephanopyxis schenckii
Silicoflagellates include:
Mesocena hexalitha
Foraminifers: non-diagnostic
Age: latest middle Miocene - subzone C of the Denticulopsis
hustedtii - D. lauta zone - Monterey Formation.
Remarks: Subzone C is age equivalent to the Mohnian benthic
foraminifer stage. Sample's dolomitic nature and well-preserved diatoms
may be indicative of an intraformational concretion (R. Garrison, oral
comm. ).
Sample A20 e (10%)
Lithology: Mudstone -whitish, angular to subrounded, thinly
laminated, extremely well indurated opaline mudstone with diatoms, fish
fragments, quartz (angular) and feldspar silt. Many cobble size (9 x 8
x 7 em) fragment, but mostly sample is found as angular gravel size
fragments. Also sample is of notable low density.
Paleontology: Foraminifera include: barren
Diatoms include: non-diagnostic.
Age: Latest middle Miocene{?) -Monterey Formation{?)
Sample A20 f (7%)
Lithology: Sandstone - buff, subrounded, massive, very friable,
moderate -weakly indurated, moderately well sorted, medium grained
arkosic sandstone with siliceous volcanic grains and sparry calcite
cement. Sample consisted of two boulders (22 x 14 x 12 em + 14 x 18 x
11 em) and three cobble size fragments (7 x 7 x 5 em).
Paleontolog~:
Barren of foraminifers and diatoms.
Age: Late iocene(?) Santa Margarita Sandstone(?)
Sample A20 g (<1%)
Lithology: Sandstone - gray, subrounded, massive, moderately well
indurated, moderately well sorted, fine grained arkosic sandstone.
Sample consists of one fragment (8 x 8 x 4 em). Same as A20 f only
finer grained.
Paleontology: None
Age: Same as A?Of
Remarks: Dredg~ partially filled with gray green cohesive mud.
81
Dredge Haul A21
Start:
Finish:
Depth:
36°52'30"
122°17'45"
36°53'45"
122°19'20"
1015-420 m
Sample A21
Lithologf: Siltstone - brownish gray, subangular, massive,
moderately w~l indurated, highly bioturbed (polychaetes) siltstone.
Sample consists of 6 rock fragments (6 x 4 x 1 em to 3 x 2 x 1 em).
Paleontology: None
Age: Pliocene(?) - Purisima Formation
Remarks: Dredge was filled with dark green cohesive mud. Rock
fragments' lithology similar to known Pliocene Purisima Formation.
82
Dredge Haul A
22
Start:
Finish:
Depth:
36°54'45"
122°24'30"
36°56'10"
122°23'40"
1080-675 m
Sample A22 (100%)
Lithology: Mudstone - greenish gray, angular-subangular, massive,
moderately well indurated, highly bored (polychaetes and holads),
diatomaceous and tuffaceous opaline mudstone with glauconite (rare).
Sample consists of many boulders (16 x 16 x 10 em) and many cobble size
rocks (8 x 5 x 5 em).
Paleontology: Diatoms include:
Actinoptychus undulatus
Coscinodiscus mar~1natus
Denticulopsis sem1nae var. fossilis
Melosira sulcata
Stephanopyxis turris
Rhizosolenia barboi
Thalassionema nitzschioides
Foraminifera include:
Elphidium clavatum var. (pyrite mold) (Cushman)
Epistominella pacifica (R. E. and K. C. Stewart)
Age: Late Pliocene D. seminae var. fossilis zone- Purisima
Formation.
Remarks: Rock fragments interspersed in green cohesive mud.
83
Dredge Haul A23
Start:
Finish:
Depth:
36°51'15"
122°21'00"
36°53'00"
122°19'00"
765-720 m
Sample A23 a (88%)
Lithology: Siltstone - greenish gray, subangular-subrounded,
massive, moderately well indurated, slightly bioturbed (polychaetes and
pholads), calcareous (micritic) siltstone with diatoms, sponge spicules,
and glauconite. Sample consists of numerous cobble size rocks (13 x 13
x 5 em).
Pa 1eon to 1ogy: Diatoms include:
Denticulopsis seminae var. fossilis
Melosira granulata (fw)
Rhizosolenia barboi
Steyhanodiscus sp. (fw)
Tha assiosira antigua
Thalassiosira oestrupii
Age: Late Pliocene D. seminae var. fossil is zone - Purisima
Formation.
Sample A23 b (10%)
Lithology: Siltstone - brownish gray, subrounded, irregularly
laminated, extremely well indurated, vaguely dolomitic siltstone with
glauconite. Sample consists of two cobbles (12 x 12 x 6 em).
Paleontology: Diatoms include:
Biddulphia aurita
Coscinodiscus mar~inatus
Dent1culopsis sem1nae var. fossilis
Epithemia sp. (fw)
Melosira granulata (fw)
Rhaponeis sp. (mb)
Thalassionema nitzschiodes
Thalassiosira antigua
Foraminifers include:
Bulimina sp.
Globobulimina pacifica(?) (Cushman)
Age: Late Pliocene D. seminae var. fossilis zone - Purisima
Formation.
Sample A23 c (2%)
Litholog1: Basalt - black, angular, light wei9ht, vescicular
basalt. Samp e consists of two rocks (6 x 6 x 2 em).
X-ray: None
Age: Unknown
Remarks: Dark green cohesive mud concentrated at top of dredge.
84
Dredge Haul A33
Start:
Finish:
Depth:
36°52'45"
122°22'15"
36°52'30"
122°21' 10"
855 to 630 m
Sample A33 a (50%)
Lithology: Mudstone - dark brown, angular, massive to faintly
laminated, well indurated, brittle (conchoidal fracturing) mudstone.
Sample consists of cobble size rocks (14 x 10 x 6 em) that readily
fractures conchoidally into gravel.
Paleontology: Foraminifers include:
Ammobaculites sp.
Bathysiphon sp.
Buliminella elegantissima (d'Orbigny)
Clavulina sp.
Cyclammina sp.
Eggere 11 a sp.
Gaudryina sp.
Gl omos ~ira sp.
Haplop ragmoides sp.
Reophax sp.
lextalaria sp.
lrochammina sp.
D1atoms include: barren
Age: Latest Miocene - Pliocene - Purisima Formation
Sample A33 b (50%)
Lithology: Siltstone - greenish gray, subangular to subrounded,
massive, moderately well indurated, bioturbed (polychaetes and pholads),
opaline siltstone with sponge spicules and glauconite. Sample consisted
of cobble size rocks (15 x 14 x 8 em).
Paleontology: Diatoms include:
Denticulopsis seminae var. fossilis
Rhizosolenia barboi
Stephanopyxis d1morpha
Thalassiosira antigua
Thalassiosira oestrupii
Foraminifers include: Rare modern forams contamination.
Age: Late Pliocene D. seminae var. fossilis zone- Purisima
Formation.
Remarks: Dredge filled with green mud.
85
Dredge Haul A34
Start:
Finish:
Depth:
36°48'55"
122°12'05"
36°50'25"
122°10'15"
810-740 m
Sample A34 (100%)
Lithology: Mudstone -very dark brown, very angular, very thinly
laminated, well indurated siliceous mudstone. Sample consisted of 3
small rock fragments (2.5 x 2.5 x 1.5 em) with freshly broken surfaces.
Paleontology: None
Age: Latest middle Miocene(?) - Monterey Formation(?)
Remarks: Age determination is based only upon Monterey-like
lithology and the fact that sample was collected in close proximity and
at greater depths than known Monterey Formation. Dredge was nearly
empty containing few rock fragments and a few pelletal gray green mud
ba 11 s.
86
Dredge Haul A35
Start:
Finish:
Depth:
36"48'45''
122"11'00"
36"48'40"
122"10'30"
450-340 m
Sample A35 a (20%)
Lithology: Concretion - greenish gray, subrounded-rounded,
massive, moderately well indurated, highly bioturbed, silty dolomitic
concretion with calcareous benthic forams and glauconite. Sample
consists of cobble size rocks (10 x 6 x 5 em).
Paleontology: None
Age: Pliocene(?) - Purisima Formation(?)
Sample A35 b (80%)
Lithology: Siltstone - greenish gray, subangular, massive, fairly
well indurated, slightly bioturbed, calcareous siltstone with sponge
spicules and glauconite. Sample consisted of many cobble size rocks (6
x 4 x 3 em).
Pa 1eo: None
Age: Pliocene(?) - Purisima Formation
Remarks: Siltstone and concretion age determinations based on
similar lithology to Purisima Formation and known carbonate concretion
within Purisima Formation. Dredge largely filled with weakly indurated
green mud and very coarse grained sand. A few rounded pebbles scattered
throughout the sand.
87
Dredge Haul A36
Start:
Finish:
Depth:
36°52'05"
122°13'15"
36°52'15"
122°12'45"
550-385 m
Sample A36 a (90%)
Lithology: Siltstone -dark brown to olive green,
angular-subangular, conglomeratic, extremely well to well indurated,
mollusc-bearing, glauconitic siltstone with benthic and planktonic
forams. Texture of samples highly variable ranging from predominantly
siltstone to extremely coarse-granular grained sandstone with silty
matrix.
Paleontology: Diatoms include:
Coscinodiscus marginatus
Cymbella sp. (fw)
Stephanodiscus sp. (fw)
Thalassiosira antigua
Foraminifers include:
Buccella friTida (Cushman)
Cassidulinaimbata (Cushman and Hughes)
Cassidulina translucens (Cushman and Hughes)
Cibicides fletcheri (Galloway and Wissler)
Elphidium articulatum (d'Orbigny)
Elphidium gunteri (Cole)
Nonionella stella (Cushman and Hughes)
Gastropods include:
Acteocina sp.
Am hissa(?) sp. cf. A. columbiana (Dall)
Bitt1um ?) sp. aff. B. eschrichtii (Middendorf)
Bittium sp. aff. ~· rugatum (Carpenter)
Calliostoma(?) sp.
Crepitatella n. sp. (?) (Verticumbo)
Mitre la(?) sp.
Puncturella sp. cf. P. galeata (Gould)
Pelecypods include: Astarte(?) sp.
C clocardia ventricosa (Gould)
Hiatella ? sp.
Macoma cf. M. calcaria (Gmelin)
Panomta(?) sp.
Sacce la(?) sp. cf. ~- taphria (Dall)
Tellina carpenteri (Dall)
~:
Diatom age determination was latest Miocene to Pliocene
probaoly Pliocene. Foraminifer and mollusc age determination was
Pliocene-Pleistocene probably Pliocene. Therefore, Pliocene will be the
determined age - Purisima Formation.
88
Sample A36 b (5%)
Lithology: Sandstone - buff green (weathered surface) to gray
(fresh surface) subangular, massive, extremely well indurated, silty
very fine-grained sandstone with sponge spicules, calcareous forams and
glauconite.
Paleontology: None
Age: Pliocene(?) - Purisima Formation(?)
Sample A36 c (5%)
Lithology: Siltstone- medium to.dark brown, subrounded, massive,
extremely poorly indurated, extremely friable, highly bioturbed,
slightly sandy siltstone. Sample consists of cobble size rocks (12 x 7
x 7 em).
Paleontology: None
Age: Pliocene(?) - Purisima Formation(?)
Remarks: Age determination and formation for A3hb and A
are
contaiMg& only
questionably based upon association with A36 . Dredge
minor amounts of gray green cohesive mud. a

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