SEDIMENT DYNAMICS
IN
MONTEREY CANYON
CENTRAL CALIFORNIA
A Thesis
Presented to
The Faculty of the Department of Geology
San Jose State University
In Partial Fulfillment
of the Requirements for the Degree
Master of Science
By
Susan Jean Chinburg
December 1985 ACKNOWLEDGEMENTS
I would like to acknowledge the help and support of people, without whom this thesis would have never been com- pleted .. Many thanks go to the students and staff of Moss
Landing Marine Laboratories who graciously lent in the pursuit of scientific knowledge.
The captains and crews of the RVs Oconostota, and Ed.. Ricketts also provided valuable help the data collection. Their experience and knowledge allowed the ld work phase of this study to progress smoothly.
Dr. H.G .. Greene of the Marine Geology Branch of the U
Geological Survey provided the initial impetus to begin is study and much of the data (the 3 .. 5 and uniboom sei reflection data and some of the gravity cores) .. He also provided much technical help during the study.
Financial support for this project was provided by Moss Landing Marine Laboratories, Sigma Xi the Research
Society, and the Central Region Coastal Commiss of the
California Coastal Commission.
During the arduous writing phase of s ject many people provided much needed support .. Drs R.E .. Arnal H G
Greene, R. Gram, C.H. Stevens, and S. Creely cr 1 re- viewed this manuscript. Dr. Ralph Gram deserves spec mention, without his support and advice over s iii project would have never been completed. My heart fe s go to him.
I would also like to thank Conoco, Inc. for use of office equipment and supplies to finish the final draft of the manuscript.
Last but not least, much appreciation and many to my husband, Steven Locy, for his unfailing support. TABLE OF CONTENTS
page
ABSTRACT e e e e e e e e o e e e o o e e e e o e o e • e o e e e e o e e e o o o e e e e o e e e e e e X
INTRODUCTION . .•....•....•...... •••.•...... •.. e • • • • 1
Purpose of Study...... • ...... 1
Previous Work...... 3
REGIONAL SETTING AND GEOLOGY ......
DESCRIPTION AND INTERPRETATION OF BATHYMETRIC MAP...... 1
Methods of Data Collection...... 1
Interpretation...... 17
DESCRIPTION AND INTERPRETATION OF SEISMIC REFLECTION
PROFILES ...... ee•e• 21
Methods of Data Collection and Analysise ...... 21
Seismic Stratigraphic Units...... 21
Quaternary Aromas Formation (Qar) ...... 23
Undifferentiated Quaternary Sediment ( 23
Quaternary Submarine Landslides ( s) .. 23
Quaternary Canyon Fill (Qcf) ...... 7
Quaternary Deltaic Sediment (Qd). e •e•••·· 27
Geology and Structural Features ...... 27
Area 1: The Head of Monterey Canyon. 31
Area 2: The First Bend in the Canyon Axis 34
Area 3: The Northern Shelf ...... •... 34
Area 4: The Most Seaward Area 34
v Submarine Landslides ...... 3
Area ...... 40
Thickness .. 0
Volume ...... 2
DESCRIPTION AND INTERPRETATION OF BOTTOM SEDIMENT SAMPLES .. 43
Methods of Data Collection and Analysis...... 3
Types of Sediments ...... 5
Fine-grained Turbidites .. 5
Sample C-1-1 .. 50
Sample C-2-1 .. 53
Samples G-8, 9, and 10. 53
Sample C-2-4 .. 55
Slump Deposits .... 55
Sample G-5. 55
Sample G-7 ... 58
Sample G-11 ... 60
Sample C-2-2 .. 60
Samples C-2-3 and C-2-5. 60
Sample C-3-1 .. 61
Sample C-3-2 .. 61
Sample C-4-1 .. 63
Sample G-2 .... 63
Samples G-3 and 4 .. 66
Modern Marine Sediment .. 67
Possible Tertiary Purisima Formation. 68 vi lagic Sediment...... • . 69
DISCUSSION e e e e e e e e e e o e e e o e o e e o e e e e e e e e o e e o e e e e e e e o G e 7 Q
Bathymetry Features ..• 0 e ••• G e •• 0 tD •••••••• e ••• e e •• e •• 0 70
Slump Features•••••••••••••••••••••••e•••••• 70
Canyon Floor ••••••••.••.••.•••••••••••••• e • • • • • • 7 0
Canyon Gradient ....•...... •...... 71
Geology and Structural Features...... 72
Area 1: The Head of the Canyon ...... e • • • • • • • • • 7 2
Area 2: The First Bend in the Canyon Axis ..... 7
Area 3: The Northern Shelf ...... 74
Area 4: The Most Seaward Area...... 7 5
Submarine Landslides (Qls) .....••...... 75
Sedimentary Settings ...... e••••••••••e•• 76
Fine-Grained Turbidites ...... 77
S 1 urn p Deposits ...... 7 8
Hemipelagic Sediment ...... 79
CONCLUSIONS AND SUMMARY...... 8
REFERENCES CITED ...... e ...... , ...... • • • .. • • .. • .. .. • .. • .. 81
vii LIST OF ILLUSTRATIONS
Figure page
1 . Index Map...... 2
2. Drainage basins and mountain ranges ...... 11
3. Seismic stratigraphic sequences ...... e. 13
4. Location of bathymetric profiles ...... 15
5. Cross-sections of canyon...... 20
6. Location of seismic and sediment data ...... 22
7. Seismic reflection profile 79-3 ••••.•••....•.. 2
8. Seismic reflection profile 79-2 •.•••.....•••. 25
9. Seismic reflection profile 78-2...... •.. 26
10. Seismic reflection profile 79-0...... 28
11. Seismic reflection profile 79-5...... 29
12. Seismic reflection profile 79-6 ..•.•...... • 30
13. Seismic reflection profile 79-4 .••..•••...... 33
14. Seismic reflection profile 78-12 .•.••••..•.•• 35
15. Seismic reflection profile 78-9...... • .. 36
16. Seismic reflection pro le 78-14. •• 37
17. Seismic reflection profile 78-4...... 38
18e Area of Submarine Landslides .... 41
19. Sample transect 1 •.•...... •••.•.. 49
20. Sample transect 2 ...... 52
21. Sample transect 3 ...... 56
22. Sample transect 4 ...... 57
viii 23 .. Sample transect s ...... oeoeoee tteeeeeeee 62
24 .. Sample transect 6 •••••••••••.•• eeooeoeeeeeeeo 63
25 .. Sample transect ? •••••••••.•.••••••••••••••••• 65
Tables
1. Foraminifera from selected samples ...... 46
2. Characteristics of sediment samples...... 48
Plate
1 .. Geology of Monterey Canyon ...... in pocket
ix ABSTRACT
High resolution seismic reflection data and bottom ment samples collected throughout the upper 25 km of
Canyon, central California, were used to investigate sediment transport in the headward regions of the canyon.
The most active method of sediment transport is s ing. A continuous unit of Quaternary landslides and s covering both canyon walls of the upper 14 km of the was mapped. Over 150 million m3 of sediment are involved slumping.
Only in the sinuous, very narrow thalweg were sands being transported by sediment gravity flow mechanisms A striking feature of the sediment throughout the canyon is the lack of sand. Canyon sediment is almost silt-clay except for medium to fine sand in the canyon thal weg ..
In at least two places the canyon axis is blocked by a
slump. Sediment carried downcanyon is ponding se
slumps.
In the subsurface the only formation ised the
canyon in the area study is Quaternary Aromas The walls of the canyon head are composed of Aromas th a cover
of Quaternary submarine slumps and landslides. On shelf
the Aromas is covered by Quaternary marine
X 18 km of a sample of Tertiary sima was collected indicating that the canyon probably cuts older farther offshore.
Two major reentrants into the northern canyon wall rep resent the reexcavation of buried erosional channels cut
Monterey Bay shelf during the late Pleistocene.
xi INTRODUCTION
Monterey Canyon is one of the world • s largest ac submarine canyons. Located in Monterey Bay, central Calif ornia, it heads at the shoreline of the fishing village of
Moss Landing, about 115 km (71 mi) south of San Francisco
(fig" 1) . Throughout Holocene time, most submarine canyons a the United States Pacific Coast received little coarse grained sediment due to the existing high stand of sea level
(Nelson and Klum, 1973) . Modern Pacific coast canyon deposits are characterized by thin-bedded turbidites made of silt and clay (Bouma, 1965; Stanley, 1967; Carlson and
Nelson, 1969; Felix and Gorsline, 1971; Stanley and Freland
1978; Nelson and others, 1978).. Following this trend Mon terey Canyon is also characterized by silt and c sized sediment. Previous studies (Galliher, 1932; Shepard, 1948
Martin, 1964; Martin and Emery, 1967; Yancey, 1968 Wolf
1970; Oliver and Slattery, 1973; Arnal and s, 1973
Greene, 19 7 0 and 19 7 7) have reported finding mo mar muds in the canyon .. However, Holocene sand depos have been found on the Monterey Deep-Sea Fan (Hess and
1976) indicating that sand is being transported downcanyon
The purpose of this study was to qualitat descr 2
Figure 1 .. Index map of the study area 3 sediment transport the headward regions of Monterey Can yon. I will try to answer three questions: 1) is sediment entering the canyon being transported to deeper regions? 2) what if any, mode of transportation is active? and 3) where is deposition occurring?
High-resolution bathymetric ( 12 and 28 kHz) data were used to construct a detailed bathymetric map to examine metric features. Other seismic data were used to seismic stratigraphic and structural features in addition to adding more bathymetric data. Bottom sediment samples (grav ity cores and bottom grabs) were used to determine types. The study area was limited to the headward regions of
Monterey Canyon from Moss Landing, California seaward to the intersection of Soquel Canyon and Monterey Canyon,
imately the first 24 km of the canyon course (fig. 1) e
Previous Work
Quaternary sediment of Monterey Bay has been s
Galliher (1932), Sayles (1966), Dorman (1968), Yancey (1968
\volf (1970), Dittmer (1972), Arnal and others (1973
Combellick and Osborne (1977), Porter and others 1979 , and
Clark and Osborne (1982). Although these s s dealt marily with beach or very nearshore sediment f tive discussions of sediment transport into were presented. Several of the above-ment s reached conflicting conclusions about Monterey Canyon 4 Galliher (1932), conducting the t general of
the offshore sediment in Monterey Bay, found the canyon to
be covered by silts and clays and concluded that the canyon
provided an effective barrier to mixing of sediment from
northern and southern sources. Sayles (1966) and
(1968), while studying beach sands of Monterey Bay, were able
to identify mineral suites distinctive of Salinas River ment south of the canyon, and distinctive mineral suites from
the Pajaro River, north of the canyon (fig.1). The 1
suites of the Salinas River were not found north of the can-
yon and the Paj aro River sediment were not found south of
the canyon. From this, the inference was made that there is
no cross canyon transport of sediment and sediment c
by longshore currents was funneled into the canyon.
Wolf (1970), who conducted a study of coastal currents
and mass transport of bottom sediment in Monterey Bay stat-
ed:
"During winter months there is a predominate down-canyon mass transport of water bringing sedi ment into the canyon .... This supply of sediment into the canyon head results in infilling and oversteepening of the canyon heads with subsequent mass movement of sediment seaward followed by deposition in channels and on the broad deep sea fan."
More recent work such as that of Broenkow and (1978) and Shepard and others (1979 inve currents
in Monterey Bay, have indicated that transport of water
the canyon is more complex than that measured Wolf .. 5 Down-canyon transport may not dominate at any of the year.
Dorman (1968), in a study of the southern Monterey littoral cell and sediment budget, calculated a fa sediment budget deficit. To account for the defic Dorman speculated that the lost sediment moved downcanyon, making the canyon an avenue of major sediment loss from the
Bay sediment budget.
Dittmer (1972) estimated the amount of sediment into the canyon by dumping tagged sand just north of the canyon head. A few days later, only a few grains of the tagged sand were found south of the canyon. The conclus was that most of the sand went downcanyon, therefore, inferring that large amounts of sediment are transported the canyon.
A rna 1 and others ( 19 7 3) , however, cone 1 uded that canyon was not an avenue of major sediment loss to the
This conclusion was based on the presence of muds the canyon head and the fact that large amounts of dredge spoils dumped into the canyon from Moss Landing Harbor were until winter storm waves broke up the depos many s later ..
Combellick and Osborne (1977), Porter and 1979 and Clark and Osborne (1982), while studying the and sources of beach sand of southern Monterey he define the sediment budget, also concluded that was 6
1 le cross-canyon transport of sediment. Using parameters such as grain shape, litho logic compos and grain size analysis, distinctive characteris s of var sediment sources for Monterey beach sands were identif
It appeared that very little mixing of sediment from north ern and southern bay sources occurs. The inference is that longshore transport moves sediment into the canyon.
The short-term instability within the head of
Canyon have been discussed in other reports. Shepard (1948 reported periodic mass movements of sediment occur the three branches of the canyon head. Bathymetric and sed surveys were performed in the canyon head that indicated that the shape of the canyon head had not changed signi since the earliest United States Coast and Geodetic charts were made. Shepard inferred that there are h sedimentation rates in the canyon based on thick algal mats overlain and intermixed with muds and sands. He cone 1 uded that this type of sedimentation would soon fill in the canyon heads unless periodic mass movements of sediment the canyon heads clear.
Oliver and Slattery (1973), during several SCUBA s into and around the canyon head, observed many small s s and slides leading into the canyon head branches. The most dramatic example was the excavation of a transect 1 that was permanently installed. A large fence anchor used to moor the transect line was undercut, allowing to over 7
the canyon. Arnal and others (1973) calculated that 610 m3
(880 cu yd) of sediment slumped into one of the canyon heads during that episode.
The above discussion indicates that most researchers
agree that the canyon provides an effective barrier to of sediment from northern and southern sources. There is however, controversy over whether Monterey Canyon is an ave nue of major sediment loss to the sediment cell of Monterey
Bay. Arnal and others (1973) calculated that 300,000-450 000 m3 ( 400,000-600,000 cu yd) per year of sand is carr
longshore transport southward towards the canyon. cal
culated that the amount of sediment entering the canyon is
only 10% of that carried by longshore transport Where the
remainder of the sediment goes has been a matter of specu
lation.
Martin (1964), Martin and Emery (1967), and Greene 197
and 1977) conducted geological and geophysical investigations
in the Monterey Bay region .. They studied Monterey
its genesis, and its relationship to surrounding 1
geology. Martin (1964) stated that very little coarse sed
ment (gravel and sand) was entering or moving down
Canyon. This conclusion was reached because coar
sediment was not found in bottom samples from the canyon and floor ..
Active slumping of the canyon walls at head of the canyon was first suggested by Greene (1970) and later mapped 8 in detail (Greene, 1977). High-resolution seismic re profiles showed several slumps cutting into Quaternary val fill, deltaic deposits, eolian sands and Tertiary non-mar alluvium, marine mudstone, sandstone, silt and clay. These materials for the most part are unconsolidated to poorly consolidated and many were, and some still are, fresh-water bearing. These factors can favor instability.
The large number of slumps found by Greene indicate that erosion, probably due to oversteepening and undercutting bottom currents, is presently active in the canyon. Dur a submersible dive, Greene saw indications that at least one slump had blocked the canyon axis and may be impeding down canyon transport of sediment. A shallow core taken in s area lacked sand and did not show bedforms indicative of transport.
Although sediment is being brought into the canyon slumping and possibly from the littoral zone, s downcanyon transport of sediment from the upper reaches of the canyon to the Monterey Deep-Sea Fan may not be at the present. Previous studies of the Quaternary sediment and geology of the bay have not yielded a de ive des tion of the sediment transport within Monterey REGIONAL SETTING AND GEOLOGY
The cresent shape of Monterey Bay, 37 km (22 mi) wide at the mouth, indents the fairly straight California coastl about 115 km (70 mi) south of San Francisco (fig. 1) Santa
Cruz lies on the north side of the bay, and Monterey on the south. Moss Landing, at about 36°45'N and 121°47 1 W, is at the center of the crescent.
While most of the California shoreline is dominated steep bluffs with flat-topped terraces and small, sandy et beaches, the central Monterey Bay shoreline commonly has broad sandy beaches backed by large eolian sand dunes On shore topography is dominated by low- to high-re f mountain ranges and broad, flat-floored river valleys.
The dominant submarine geomorphic feature is
Canyon. Monterey Bay's flat shelf is bisected by the canyon
For over 90 km (55 mi) the canyon extends from the shorel seaward to the Monterey subsea fan.. Monterey Canyon has three tributaries, Soquel, Carmel, and Ascension Canyons
With respect to width, depth, and length, the canyon 's overall dimensions are comparable to those of the Grand Can yon of the Colorado River (Martin, 1964; and Dill
1966). The canyon's axis meanders with two major 90° bends Greene (1977) suggested that these bends are related to the presence of resistant rock types and the offshore Palo Colo rado-San Gregorio and Monterey Bay fault zones .. 10
Two major rivers and three smaller rivers and streams supply water and sediment to Monterey Bay (fig. 2 . About 8 km (5 mi) south of Moss Landing, the Salinas River a drainage basin encompassing the Santa Lucia Mountains south of Monterey Bay and Gablian Range east of the bay breaks the coastal dune field.
The Pajaro River, the second largest river draining
Monterey Bay, drains the Watsonville flatlands from the lower
Santa Clara Valley and Diablo Range northeast of
Bay. It enters the bay about 6 km north of Moss Entering northern Monterey Bay from very small drainage ba sins in the Santa Cruz Mountains are San Lorenzo River and
Soquel Creek. Elkhorn Slough extends inland from Moss Land ing about 10 km and has a very small drainage basin from the immediate area.
The Monterey Bay area is geologically complex and tee- tonically active. It is located on the Salinian a major structural element of California (Compton, 1966;
1966) . The basement rocks of the offshore area are Creta- ceous biotite granodiorite porphyry similar to that of the
Monterey Peninsula (Ross and Brabb, 1973; Greene, 1977
Overlying the crystalline basement on an undu erosi surface are three sedimentary sequences identi seismic reflection profiles (Greene, 1977; Greene and Clark, 1979 first, a middle Miocene sequence unconformably over the crystalline basement; second, an upper Miocene-to-Pliocene 11
N
0
Figure 2. Drainage basins ranges adjacent to Monterey 12 sequence overlying the middle Miocene; and third, an upper
Pliocene-and-Pleistocence to Holocene sequence on
3) • 13
ll E IE QUENCE MA TION E Rl N
recently surficial d d ... -·-. -·-· -·-· marine dep i t i w z UJ w deposits > u· UJ submarine Ian I id 0 u ... and tl u mp materia I 0 Ols ::1: Q UJ ::t deltaic sand and mud Salin a 41 w I z marine w .... u II Vt quartzos s w II.. _, G) n nmarine; a.. L a.. ~
unconform w greenish g r e n- PUR- z I solidated t 0 con 1 o i- w w u Q z I S I A dated lands ton •, I i t- 0 ~ w 0 u and s h a I •; _, ... 0 L - fossilifer 11:'&. w ....a z ~ > A w a: ;::, u < I- a: w z di tom ( w w w . I- u NT z 0 ilid U I sh w EY u ~ and an n 0 I ..... c ~ ~
unconform granitic r o dt 1 E S z c or o I de r Figure 3. Seismic stratigraphic sequences central Monterey (after Greene, 1977 . DESCRIPTION AND INTERPRETATION OF BATHYMETRIC MAP
This study focuses on small-scale sedimentary features
( i .. e .. , slumps and other sediment gravity flows) thin the canyon., A detailed bathymetric map of Monterey Canyon was constructed to aid in this work. Greene (1977) had previous ly published a detailed bathymetric map based on a 1 .. 9-km
(1-nautical-mile) space data collection grid. While Greene's map is not of the detail needed for this study, comparison of the two maps allowed detection of possible large-scale changes that might have occurred since the data for Greene s map were collected.
Methods of Data Collection and Analysis
The bathymetric base map of the headward of
Monterey Canyon was constructed using a grid spaced at 0 9 km
(0.5 nautical mile) (fig. 4). Track lines were laid out both normal (north-south) and transverse (east-west) to canyon axis. Bathymetric data were collected using a 28-kHz i- sion depth recorder (PDR) aboard the RV Oconostota the fall of 1978.. Navigational control was Loran-e.. 5 minutes a Loran-e time delay reading was taken s taneous as an event mark was recorded on the PDR trace.. le an attempt was made to maintain a constant boat of 16 km/hr ( 9 knots) , current and wind conditions, adjustments in speed for passing shing boats and at 15
Figure 4.. Locat-ion of bathymetric ( 28 kHz) profiles used to construct bathymetric map. the end of track 1 s, caused variation speed to the actual distances between Loran-e readings to vary from
0.7 to 1.1 km (0.4 to 0.75 nautical miles).
Loran-e has an accuracy of approximately ±450 m
(1500 ft) (Bodwitch, 1977).. Precision of the Loran-e within Monterey Bay and the instrumentation abroad the RV
Oconostota throughout the study period was very good. s of cross-over points between intersecting pro les consistently agreed within 3 m.. In addition, the Loran-e readings for the boat dock and the Moss Landing A bouy were checked as a control at the beginning and the end of each cruise.
The bathymetric map, constructed to a scale of 1 =
0.9 km (0.5 nautical miles), or approximately 1:36,500, used
Loran-e time delay lines as a base. The time delay 1 s were interpolated from the National Ocean Survey chart
#18680 ..
A latitude and longitude overlay for the Loran-e map was
constructed after contouring using two separate s
First, 25 uniformly distributed Loran-e readings were
ematically converted to latitude and long us
computer facilities at the United States cal
(USGS) in Menlo Park, California. Second, and lon
gitude for the RV Oconostota dock and the Moss A
were obtained independently of the ca
location. The readings for the dock were calcul surveying the distance from a nearby USGS benchmarker for which very precise readings are known. Latitude and tude readings for the bouy were obtained from the National
Ocean Survey chart #18685. Using the latitude and longitude for the boat dock and the bouy as control points, the cal culated values were overlaid upon the Loran-e map.
To construct the map, water depths were plotted at each
5-minute Loran-e reading and at every change of slope.. A depth correction of 3.1 m was added to the depth read off
PDR trace to compensate for the depth of the PDR transducer below the water line. No correction in water depth because of tidal variations was made, because the 3-m tidal excurs common in Monterey Bay is usually masked by wave height deep water. Tidal excursion is important at the shorel and the shoreline for this map was interpolated from National
Ocean Survey chart #18685. Depth contours were drawn every
10 m to 100 m depth and every 20 m below 100 m
Interpretation
Analysis of the bathymetric data revealed that s of the canyon walls is prevalent throughout the head of the canyon (Plate 1). Many previously unmapped slumps were tified from the bathymetric profiles. A discrete of slumping along both walls of the canyon was from the bathymetric profiles. This band consists of many vidual localized slumps, usually not more than 200 m wide (Plate 1). Submarine slumps and landslides are recognized bathymetric profiles by the hummocky surface of the toe of
the slump and steep scarp at the head. After the contour map was made, the crescent-shaped scarp at the head of many of
the slumps also could be identified.
The bathymetric profiles also showed that in the canyon
axis there is a very narrow thalweg that meanders back and
forth across the canyon floor.. A small but identi le
thalweg was seen throughout the canyon except for an area
21-24 km offshore.
About 21 km offshore (36°47.6'N and 121°55.3' the
canyon floor flattens for about 4 km. From the shorel to
12 km offshore (121°51.6'W) the gradient of the canyon floor
averages 0.02, and to 21 km (121°54.6'W); the average
ent is 0.01. For the next 4 km the canyon floor is
essentially flat. Down canyon from this area the gradient of
the canyon floor is 0 .. 03-0.04.. The flat area is d
below a large ( 2 .. 8 km wide) slump scarp that ises the
north canyon wall. The leveling of the canyon floor is
result of displaced material from the above ment scarp
filling the canyon axis. Also downcanyon transport of
sediment is impeded at the area because the canyon axis bends
to the south (Plate 1).
Another feature found first in the bathymetric data and
then later confirmed by the seismic reflect data is a
mid-channel mound in the very nearshore area of the canyon 19 axis (Plate 1, fig. 5). This feature is about 1.8 km
100 m wide and 10-20 m high. On each side of the mid-channel mound the canyon floor appears to have been eroded between
15-20 m creating the mid-channel mound.
The channel on the south side of the mound bifurcates at the very head of the canyon creating three branches of the canyon head. The two sub-channels of the south channel merge into one further downcanyon (cross section B, fig. 5). Less than 0.2 km from cross section B the mid-channel mound is not seen, indicating all of the branches of canyon have together by this point (cross section C, fig. 5). 20
n
5
50
76
6 km 1
h north
m
0 .. 5 km
1
125 south 2
75 \ \ \ 0 \ \ 7 11) 125
Figure 5.. Cross-sections of canyon showing mid-channel mound. DESCRIPTION AND INTERPRETATION OF SEISMIC REFLECTION PROFILES
High resolution 3.5 kHz uniboom seismic reflection data collected for this study were used to enhance and expand on the geology of Monterey Canyon previously reported by
(1964) and Greene (1970, 1977) ..
Methods of Data Collection and Analysis
The seismic reflection data, and assistance in preting the data in this study, were supplied by Dr H G ..
Greene of the USGS Pacific-Arctic branch of Mar
These data were collected as a part of an ongoing study of
Monterey Bay and Monterey Canyon during the 1978 and 1979 cruises in the RV Sea Sounder.
Navigational systems used while collecting the seismic reflection profiles were a combination of Loran-e, radar and satellite navigation.. The profiles were ally located upon the Loran-e derived base map using the prel gation from the USGS and reconciled to the bathymetry of the base map (fig. 6).
Of the three seismic stratigraphic sequences des by Greene (1977) sequence three, the upper Pl
Pleistocene to Holocene sequence (fig. 3), was on 22
Figure 6. Locat of seismic reflect sediment samples. Outlined areas were u 23
sequence seen in s study. S seismic s c un s of sequence three were defined.
Quaternary Aromas Formation (Qar)
The seismic data show the Aromas Formation at the same
locations previously mapped, along the canyon walls and at the shelf break (Plate 1) where submarine landslides and
slumps have stripped away most of the overlying and more recent cover. Large-scale, low-angle 11 cross beds 11 and lled buried erosional channels common to the Aromas (fig. 7) are indicative of eolian stratification similar to that seen the Aromas onshore (Dupre, 1975; Greene, 1977).
Undifferentiated Quaternary Sediment (Q)
Undifferentiated Quaternary sediment (Q) shows flat
lying, parallel seismic reflectors that are regular dis
continuous (fig. 8). This unit was found on the north shelf and canyon walls very near shore.
Quaternary Submarine Landslides and Slumps (Qls)
Quaternary submarine landslides and slumps s) are
very common along the canyon walls (Plate 1) s t
consisting of many individual slumps and slides is found a
continuous band along each wall.. An of s
(profile 78-2, fig. 9) shows the cal sei feature
rough hummocky disturbed reflectors and the lure surface
along which the slump moved. Another dramat 24
I lf) N l()
Figure 7 .. Seismic reflection pro le 79-3 and line drawing interpretation .. (
Figure 8. Seismic reflection pro le 79-2 line drawing interpretation .. 26
0-
-NORTH ~~:' - j/~}[f;, ;_yOor-, Q/Qar~-~ - 7 Ifau It · - ·\ (?) \ \ \
....\ \
15 f.2 Km
Figure 9. Seismic reflection profile 78-2 line drawing interpretation. 27 of submarine landslides and slumps is seen pro le 79-
( fig.. 10) .. Large areas of the south wall and some of the north wall have slumped, contributing material to the canyon floor.
Quaternary Canyon Fill (Qcf)
The Quaternary canyon fill sequence was found only on the canyon floor (Plate 1). Canyon fill was distinguished seismic characteristics of weak horizontal reflectors
(fig. 7) .. The sediment in this unit was deposited land- slides, slumps, and turbidity currents.
Quaternary Deltaic Sediment (Qd)
The Quaternary deltaic unit was found on the south shelf of Monterey Bay (Plate 1).. Greene (1977) mapped a de c sequence extending from the Salinas River mouth to the canyon edge. A wedge-shape sequence of sediment is seen along the very southern edge of the study area (figs. 7, 11 and 12)
Uniboom and 3.5 kHz seismic reflection pro les are grouped and discussed in four geographic areas: 1) the head of Monterey Canyon, between 36° 46' N and 121° 49 W and the shoreline; 2) the area around the first bend the s of the canyon, between 36° 47' N and 36° 47.3'N and 121 51 5 W
3) the north shelf near the head of the between 36°47.3'N to 36°49'N and 121°57'W and the shorel 28
-----r-----I
.... • .. h) •• t .,.
2.0 km
Figure 10 .. Seismic reflection le 79-0 and line interpretation .. 29
···--·....,·--·-~··-- ... _.... _ ... ··-- ... __ ...... ---- ... -~ ...... ·-·· •'l'fl'1;1fr·•t-· '• '' ·~!· .. ~.··.:\: I ··.~~~~·- ... ·~·•••••,•··~·' -111'" .... ~"''ii·f'OII•of" ... ti•~IL~II!IIII.A;,'; 4,'f .. •fl'"""""•\.0o J-'!!!t:....~~... :!::::·-. :¥::·::·.:.:_·_·_~.• ... --~ ~ . ·.:: :·... ,·,'. ~:~: :· .. ·... ~; =~ .. -· :·,. ·:. ~ :·;·~ ~ ·..:· ..~:. :.. i.J~iur~·.:~:.::~~-.:.~ .. "·.: ...... _ ,..... __ ...... ::t~fr7·"..:.·:~~ :-:-~-:::~·-·· --····-·-. ·----· .. ··· ...... ----··"' ;~=..:~::_:··:<::· :--:--·;-~ ·:·---~~- :. :~.--~ ·::-·· .. ~--···· . "' ...... ·--···. ?:-'.~;t·:~~::~~-~·.:·- ...... :: .... . ::J.'::£;:g:;:..':':j: :.::.:;::;;:.~... : ~-. -:::.:;:..:;:::::.:,:;;..:: ~ .::. ~~~:::',::.:-:::: :~ :.~· ~~,;: ·~ ...... _.....,.. ____._..,,or..,. ,.., •••••~••- ... ., ooo.••wo~.o..- ..... _..,, , ... - ... • l'~ ... ,.~ ... -· ...... ,.._,...... , ...... ··--· ~~~~;::~!r-: . -· ·-··· T t.,...~••·•· })' ~·.o:t.~· ·._,-:Y· • ·.. · -~it -~i.;.;;,_~ ...... : ...... ~ ..... -·· ...... _ .. :- .~~.\~~~~--::~:.... ··~ ...J ...J .. ~· ~;~~:~~-~~~·-:·:·~·: ... ,...... ,. . .,. ., \l~·l\,1' .a.r ft ~·· ~- •· ,,., ... ~· ~ ~~~~·.;~:-.:·:",:'.:"-··~--:. ··········"··"--· I : .... :!.!:;~.··~- 1- ~ !.: ~r::-~:c .. 0 (J)
·~\i,.O ~,. ------
...J ...J
...... _...... ,. --.·· • f, ...... ,.,.. ., ...... _ •• .....,., ... , .,o.~::.·-- ... ~- •••• -·· •• , ...... , •.•.
-. .. , ... --;-...;.
~~ ,7.:;·.,:::·:·.•·:_,-:-,~~~~~.~--~~~~~------~·~~~ .. -~····!··; ;.,,,. J-,"1 I ,,'./.:f, ~~~~l~~~~~-tlil~~i~~
Figure 11. Seismic reflection and line drawing 30
~~-~~.. -=--:.-:-.· ~-~-~:;. ..:~_-. -:------··--:·-·::--·-:::· _:_. .... :····-·-·-·--·
......
-~:.:~_-:;:;~·.-::_~_~:~.:.::;":"";~~~~~~~.:_~_~;;::::~.;~E.;.::::~;:_:;;·~~ .. ------· ·-:.- .. :..:._;;qf~:- ~ - ..;.. ; --- ~-~- .. ==- -~~ •iii~~~:~~~\.;~~ ~
Figure 12. Seismic reflection le 79-6 and line drawing interpretation. 31 and 4) the most seaward bend in the canyon s that is covered by this study, between 36°45' N to 36°49 N and
121°57' W (fig. 6 and Plate 1).
Area 1: The Head of Monterey Canyon
The shelf south of the canyon is made of two units The most southerly unit is the Quaternary deltaic sediment from the Salinas River, first mapped by Greene (1970; 1977 and mapped in greater detail in this study. This s package has the typical deltaic wedge cross-sectional thicker towards the Salinas River mouth and thinner towards the canyon.
Greene (1977) mapped this unit as extending to the can yon wall, but most of the seismic reflection data collected in this study show this unit pinching out about 0.25 km to 1 km from the canyon edge (Plate 1). However, in profile 79-6
(fig.. 12) Quaternary deltaic sediment can be identified to reach the canyon edge.. This profile is parallel to an ero sional channel in the south canyon wall at 36°47 N and
121°50'W (Plate 1). A very thin layer of deltaic mater 1 thinner than the resolution of the data collected for s study could extend to the canyon edge.
The northern shelf nearshore is interpreted to be f- ferentiated Quaternary (Q) sediment. well data examined for an unpublished report to Cali a Coastal
Commission and other well data reported Greene ( 1977 32 indicates the Aromas Formation onshore is 180 m (600 ft) thick at the surface. Offshore, the Aromas could be at least this thick, although there was only 25 m or less seen the seismic profiles in this study.
Further offshore on the northern shelf, Aromas is cov ered with a thin veneer of Quaternary sediment (Q/Qar). The seismic reflection data show both canyon walls in Area 1 except where the walls are covered by a landslide or slump to be the Aromas Formation (Qar) .
Above the canyon floor along the southern wall there is a large and continuous slump. This feature appears all profiles (fig. 7, 8, and 9) from Area 1 except 79-4 and 79-5
(fig. 11 and 13). Just below the shelf break in the area of profiles 79-4 and 79-5, the upper slope of the southern wall is more gentle than elsewhere with an abrupt steepening far ther downslope. The change in slope of the southern wall probably is due to a large slump moving off the wall It appears that the entire southern wall Area 1 has potential of being affected by this large and slump.
Another feature of Area 1 is the presence of a fault on the northern shelf near the shelf break ( 7 and
9 and Plate 1) . This fault was seen two pro les 79 3 and 78-2, figs.7 and 9) but does not cut over
Holocene sediment or the sea floor. The trend of s is sub-parallel to the Monterey Canyon Fault mapped Greene I I
tt't' I ,I I
·.!!! ~·-...... _0 '~~------
Figure 13. Seismic reflection le 79- and 1 drawing interpretation. 34 and others (1975), trending east-west and is about 0 .. 7 km
(0.4 nautical miles) long.
Area 2: The First Bend in the Canyon Axis
The canyon floor in this area is relatively broad slump and submarine deposits covering large areas 11 and 12). Profile 79-6 (fig. 12) shows the canyon this area, a broad floor covered by landslide depos a very narrow thalweg against the south wall. The narrow thal weg in the canyon floor in this area runs along the southern side of the canyon and then turns to the northern s of the canyon probably around a slump (Plate 1).
The large continuous slump on the southern canyon wa 1 first described in Area 1 is also seen in area 2 pro le
78-12, fig. 14). Along the southern wall there is a basin-shape feature created by a slump that has moved obliquely down the canyon wall (profile 78-9, fig. 15 . The canyon has widened from 1.5 km to 2.5-3 km due to s
Area 3: The Northern Shelf
Buried erosional channels were ident sei reflection profiles north of Monterey Canyon 16 and
1 7) . These channels lead into two large slump scarps that have incised the northern canyon wall at 36°48 4N and
121°52'W (Plate 1) ..
The northern shelf is comprised of Aromas
Formation with a cover of Quaternary marine sediment 35
..... : -... .,_. ~ :· ·~...
j : ·_ (~ . :~·:.. ·.. ~- -
] ......
Figure 14e Seismic reflection file 78-12 and line drawing interpretation. 36
e N 225 ... \,...... _
\ I I I ~· 300 Is ~I
Figure 15. Seismic reflection profile 78-9 and line drawing interpretation. 37
Figure 16. Seismic reflection profile 78-14 and line drawing interpretation. ~ ,: I ·I I, - 1- If I 0:: I J 0 II t '- z (}010 I~\,· 0' '''·"il:•.)
!illl,. I
if\11\\,11 I
~~~ W'll '- 0 \\ \ ,'J 0 I~ \I I 0' \~\"' ~ 1\ ~ ,/• § ~~~.c~·, u ::l; 1111 'I\)~ I \'•' \~ \II \ .... _ ~i lo J( \( 1 qA~ \ I I,
Figure 17. Seismic reflection 78-4 and line drawing interpretation. 39 Along the shoreline is a thick band of undiffe
Quaternary sediment Previous workers found this sediment package to be up to 25 m thick (Greene, 1977). The proximity of this sediment to the Pajaro River indicates the source probably was the Pajaro River (fig.2 and Plate 1
Structurally, this area is relatively simple. There is a small fault located in the very northern part of Area 3.
This fault appears in only one profile, 78-12 (fig. 14) and does not break the overlying Holocene sediment or the sea floor. Because this fault was not seen in any other data collected, it is not possible to determine the trend of the fault; therefore, it is not shown on Plate 1.
Area 4: The Seaward-Most Area
Only one 3.5 kHz seismic reflection profile, 79-0 (f
10) was taken in this area. This profile showed amounts of slump material covering the canyon floor An outline of the canyon floor prior to slumping was able. The gradient of the floor was s pr to infilling because the canyon floor was at least 75 m deeper pubmarine Landslides
Upon examination of the seismic reflection and metric data, it was apparent that slumping was a mass-movement mechanism active in the canyon Further s tigation revealed that the seismic reflection data could be 40 used to estimate the parameters of the sediment mass in slumping ..
Area
Using the seismic reflection data in the headward areas of the canyon, slumps and landslides (Qls) were areally map ped (fig. 18). Regions A and B represent the band of land slides and slumps (Qls) mapped along each wall of the
Region C encompasses the two large slumps that incise the north wall of the canyon at approximately 36°48.5'N and
121° 52' W.. A standard planimeter was used to determine the area of each region.
Thickness
The thickness of the Qls material was measured in each appropriate profile.. Seven profiles cross region A, e cross region B but only one profile 78-12 (fig. 12) cove region C is of sufficient quality to be used for th ss measurements. Using a standard 1.5 km/sec time-depth conver sion factor the time measurements from the seismic ref profiles were converted to meters. Using a model of the s material forming a blanket across each region, an thickness of Qls was determined from each pro le. In
A the individual measurements ranged from 4 to 31 m an average thickness of 10 m .. In region B the is from 5 to 10 m with an average of 8 m. 41
;.., 'It
oc:
0 II)
;
Figure 18. areas used to ca of sediments in Quaternary landslide (Qls) • Volume .. Us the formula Volume = Area x Th ss the volume of sediment was calculated .. The results are as follows:
Headward parts of canyon 6 Region A 10.9 km 2 x 10m = 109xl0 m3
(south wall) (143,000,000 cu 6 Region B 7 .. 5 km 2 X 8m = 60xl0 m3
(north wall) (78,000,000 cu 6 Total 18 .. 4 km 2 169xl0 m3
(221,000,000 cu
Large Slumps in North wall 6 Region C 21.0 km 2 x 35m* = 735xl0 m3
(961,000,000 cu
*based on one thickness measurement DESCRIPTION AND INTERPRETATION OF BOTTOM SEDIMENT SAMPLES
Along seven sample transects twenty-two core samples were collected (fig. 6). The sample sites were se lected from the bathymetric and seismic surveys to investi gate areas that would provide information concerning tation within the canyon.
Methods of Data Collection and Analysis
Samples G-1 to 11 were collected aboard the USGS RV Sea
Sounder on cruise S-79 with a 7.6 em (3 in) diameter dart core.. Samples C-1 to 4 were collected aboard Moss
Marine Laboratory's boat the RV Cayuse during mul cruises using a Benthos 7 .. 3 em ( 2 7/8 in) diameter core. All of the samples retrieved were 3 m or ss length.
Samples (G-1 to 11) were split immediate and cold storage aboard the vessel. Later, on land, se sam ples were photographed in black and white and color x-ray radiographed, visually described and sub-sampled at the USGS
Marine Geology cold storage facilities in Palo Alto Cal fornia.. Samples C-1 to 4 were transported un it to the
USGS cold storage facilities several of e cruise .. Later, these cores were split, sual de
Photographed in black and white and color x-ray graphed, and sub-sampled. One half of sample was 44 retained as archive splits and placed USGS Mar Geology cold storage facilities.
Small sub-samples of 30-100 gms were taken from each core in areas of apparent textural change, above and below visible contacts, and at intervals that would provide infor mation important to the characteristics of the samples i ee grain size change, color change, bioturbation change or sed- imentary structure change) . Color descriptions were on the Geological Society of America Rock-Color Chart us the
Munsel color system. Original color before oxidat was noted whenever possible, along with oxidized color.
Textural properties of the subsamples were ascerta using standard sedimentological techniques.. Silt and c fractions, the fraction that washed through a 62 sieve, were analyzed by the settling tube method
1938) . Statistical parameters of mean and median grain size sorting, skewness, and percent sand, silt, and clay presented in Appendix B were calculated according to Folk (1974)
Foraminiferal analysis was accomplished separat the foraminifera from the sand fraction of each sample. The oven-dried sand fraction was put into carbon-tetrach where heavier minerals sank to the bottom of the conta r and the less dense foraminifera floated .. The were identified by Dr. R.E. Arnal of the USGS Branch of 1 and Gas ..
The above procedure was used for those samples that were 45 not the obvious product of resedimentation and for sub-sam- ples with sand fractions. Samples G-7, 8, and 11 the only foraminifera for identification e The results are presented in Table 1.
Types of Sediment
Analysis of the bottom sediment samples and seismic reflection profiles revealed that there are two primary meth ods of sediment transport active in the canyon: the movement of sediment by turbidity currents and by slumping. Bes s these, two other distinct sediment types were sampled hemi- pelagic mud and modern marine shelf sand .. A sample that could be Tertiary Purisima Formation was also recovered Ta- ble 2) .
Fine-Grained Turbidites
This sediment group is represented by six samples
These samples are from features that did not appear to be slumps in the seismic reflection or bathymetric data and are interpreted to be in place features.
Bouma (1962) proposed a classification for sandy ites that lumped all fine-grained material uppermost divisions (D and E) .. Internal structures of silts and clays are not readily preserved anc mudstones the study of fine-grained turbidites was hampered until sea sediment became better known (Rupke and , 1974; Hess Table 1. Foraminifera from selected samples Monterey
Sample Interval Foraminifera Comments
G-7 22cm Nonionella grateloupi Arenaceous lagoonal Textularia earlandi bay; Neogene Reophax nana Quaternary upon Buliminella elegantissma preservation
45cm Ammonia beccarii Neogene, very shallow water
63cm Bulimina ovula Abundant mica, looks like Holocene
93cm Nonionella grateloupi Neogene, probably Elphidium incertum Holocene, very Bulimina ovula shallow water, less than 50m
130cm Nonionella grateloupi Neogene Epistominella pacifica Bulimina ovula
155cm Elphidium incertum one helix type of gastropod
G-8 210cm Nonionella grateloupi Neogene, very shallow Ammonia beccarii repida water
220cm Globigerina bulloides Neogene, probably Elphidium incertum Quaternary Nonionella grateloupi Ammonia beccarii
230cm Elphidium incertum Miocene to recent Ammonia beccarii very low water Nonionella grateloupi lagoonal, Trachammina pacifica and anktonic Bulimina ovula
270cm Nonionella 47 Table 1. Foraminifera from selected samples, Monterey (cont).
Sample Interval Foraminifera Comments
G-11 12cm Nonionella grateloupi Quaternary, very Buccella frigida nearshore, fresh Ammonia beccarii or brackish water Bulimina ovula in place or close Lagena costata Proteonina sp .. Wood fragments
30cm Nonionella grateloupi Neogene, poss Elphidium incertum Quaternary Plant fragments
55cm Nonionella grateloupi Neogene, poss Elphium incertum Quaternary Plant fragments
133cm Nonionella grateloupi Neogene, poss vary Polystomella sp .. late Pliocene to Pleistocene 48
2 • sties of canyon sediment s ..
.... u w z SAMPLE c:c ....Cll:
FINE-GRAIN TURBIDITES A A R C R A R c 79 102 R A C 2 c -2-1 c c R C R C 47 80 R A A 3 G- 8 c c c R C c C 305 225 C A C 3 G- 9 35 245 A R 3 G -10 c c R R R C R c 105240 c c
4 C -2-4 en tire sa rnpl d stu bed 19 290 A R R
SLUMP DEPOSITS 2 G - 5 A R c c c 174 100 c c 3 G - '1 C R R C C R C C R c 233 125 c c 3 G - 11 c c 13.5 64 c c 4 C-2-2 R R R 131 138 A A 4 C-2-3 C c 143 164 A A 4 C-2-5 R A c 52 188 A A 5 C-3-1 R c c 48 328 A A 5 C -3 -2 R 133 175 A C
6 C-4-1 R R' 18 367 A A 7 G- 2 A C C R C C c 248 145 A A '1 G - 3 A R R C 210 553 A 7 G- 4 c c c 47 533 R C A
MODERN MARINE SHELf DEPOSITS 7 G-1 C R R 36 94 A C
POSSIBlY TERTIARY PUR IS I FOR MAT ION 1 5 77 G) RARE c] COMMON A ABUNDANT L-- ABSENT ---- 49 1975; Mutti, 1977; Piper, 1978; Stow, 1979; Stow and Shan- mugam, 1980) ..
Stow and Shanmugam (1980) proposed a structural for fine-grained turbidites that is analogous to Bouma s
( 19 6 2) structural sequence for sandy turbidites and is ap- proximately equivalent to Bouma's (1962) (C) DE divisions
The complete fine grained sequence has nine subdivis termed TO - T8 ..
"The lower subdivision (TO) comprises a silt lam inae which has a sharp, scoured and load cast base, internal parallel-lamination, cross-lamination, and a sharp current-lineated or wavy surface with 'fad ing ripples'.. A convolute-laminated sub-division (T1) is overlain by low-amplitude climbing ripples (T2) , thin regular laminae (T3) , thin indistinct laminae (T4), and thin wispy or convolute laminae (TS). The topmost three divisions, graded mud (T6), ungraded mud (T7) and bioturbated mud (T8) do not have silt laminae but rare patchy silt lens es and silt pseudonodules and a thin zone of micro -burrowing near the upper surface."
However, Stow and Shanmugam (1980) reported that the en- tire sequence is rarely seen and that it is the recognit of partial sequences that will aid in paleoenvironmental interpretation.
Sample C-1-1 .. Sample C-1-1, from 102 m of water was taken along transect 1 (fig. 6 and 19). It consists of dark gray to black sandy silt and clay several sequences with distinct and sometimes eros 1 contacts
(Table 2) .. Erosional contacts are those that show scour of the underlying sediment. The most common 2
7
graded· bedding bioturbation
mud (clay and/or parallel soupy silt) size grains laminations
shells and shell sand size cross bedding fragment.s
Figure 19 .. Sample transect 1 G cross-sect from profi 78-3. 51 structures of the sample are textural and color s
(Table 2) with one interval of faint cross laminations. The cross laminations are similar to fading ripples described
Stow and Shanmugam ( 1980) as common features of basal parts (sequence TO) of fine-grained turbidites.
A sequence consisting of very organic-rich, black, clayey sandy silt had several partially decomposed ant fibers present. The parallel laminations appear somewhat tilted in the lower parts of the sample. While this ti could be the result of deformation during the operations, it could also be related to depos processes. Stow and Shanmugan (1980) reported that of laminations is common for fine-grained ites
(sequence TO to Tl).
Sample C-1-1 represents several pulses of downcanyon transported, fine-grained sediment and organic-rich accumu lations from the head of the canyon. The graded sequences erosional contacts, cross bedding, and tilted are all indicative of fine-grained turbidites .. The layer of black, organic, clayey-sandy silt represents an accumulation of organics, possibly algae, from the head of the canyon
Shepard (1948), Oliver and Slattery (1973) and Arnal and others (1973) have reported the presence of a 1 mats perched along the canyon wall and lf break These a 1 mats could easily be transported downcanyon. 1948 believed that these organic mats provide for 52 slippage of sediment.
Sample C-2-1 from 80 m water was suc cessfully recovered from the midchannel erosional remnant present in the very nearshore part of the canyon . 6 and
19). The upper 13 em of this sample are soupy and disturbed
Texturally this sample is very fine sand to coarse silt (Ta ble 2) .. A distinct contact at 20 em is marked by a color change from dark greenish gray to grayish black.. The on other structures present are plane parallel laminations. The lower part of this sample consists of silt and very sand. A bioturbated interval at about 40 em (fig. 19 cates that there was rapid deposition of the upper over a surface that had been established long enough to allow biological activity. Sample C-2-1 represents TO, T6, and T7 sequences .. Several attempts were made to recover a sample from south channel along transect 1 (fig. 19). During one some coarse sand-sized material and shell fragments were brought up around the gravity-core catcher; this a the inability to sample this site, indicates that there may be fairly well-sorted coarse sand in the south channel. The gravity core used in s study would not have been able to penetrate well-sorted coarse sands and any sand could have easily washed out ing recovery 53
These s s were recovered from the canyon floor transect 3 (f . 6 and 20)
Sample G-8 is from 225 m water, G-9 from 245 m, and G-10 from
240 m. The top centimeters of sample G-8 are soupy, sh
black silts over a distinct contact at 40 em a This sample is mostly silty clay or clayey silt with several very to coarse silt laminae with very distinct upper and lower con tacts (sequence TO). A few mollusk shell fragments are seen in the radiographs deeper in the sample, but most of bioturbation (sequence T8) is confined to the top 40 em
Sample G-9, one of the shortest and sandiest samples is mostly dark greenish gray, micaceous sand that is skewed and moderately well sorted (Table 2) . Shell s and plant debris are also visible but no other structures are seen except for a few faint parallel laminations seen in x-ray radiographs.
Sample G-10 also shows the upper soupy disturbed layer
Below the soupy layer are grayish black, si c intercalated with sandy silt laminae. Plane parallel tions that become more distinct with oxidation and are the dominant structures. Below the erosional contact at
87 em deep in the sample is a normally graded 1 th plane parallel laminations and faint cross at bottom of the sequence.
Sample G-9, recovered from the tha repre sents the type of material carried by a the 54
.. "'". ., #
.. '
26
2
. -
Figure 20. Sample transect 3. Canyon cross- section from profile 79-6. See fig. 19 55 canyon today. The characteri cs and structures are indicative of turbidites. Both seismic pro- file 79-6 (fig. 12) taken near this station and the on-sta tion records indicated that G-8 and 10 were taken from a levee-type deposit that built over and around a slump. Sam ples G-8 and 10 represent overwash deposits of a current ..
Sample C~2-4 from 290 m of water con sists entirely of well sorted, fine skewed, gray green ceous sand., The entire core was disturbed during recovery operations so there were no original structures preserved
The 12 kHz records of transect 4 (fig. 21) indicate that s sample is from the canyon thalweg. Although no 1 structures are available for observation the simi s of grain size and location to sample G-9 indicate that C-2-4 could also be the product of turbidite deposition.
Slump Depos s
The predominant feature relating to sediment in the canyon is slumping. Seismic reflection and metric data suggest that the samples described below are from slump deposits ..
This sample from 100 m water con- sists of a series of reverse graded sequences . 22 ing from clayey silt to very fine sand (Table 2). Contacts ~ I 1 \ \ \ \ I \ U B ~ a U I U • . I
Figure 21. Sample transect 4. Canyon cross from profi C-2-79. See fig. 19 for 57
E -0
Figure 22. Sample transect 2. Canyon cross from profile 79-4. See f . 19 for between sandy layers and overlying clayey si s are t and erosional. The lower part of this core shows deforma tional features such as disturbed laminations.
Seismic reflection profile 79-5 (fig. 11) and on-station records indicate that this sample is from a slump that moved down the south wall of the canyon. Since the or position of this slump was higher up on the canyon wall, the sediment was deposited in a shallow water environment
Reverse graded sequences of mud to sand, erosional contacts between laminae, and disturbed laminae have been from river delta fronts (Coleman and Prior, 1980) .. Thus sample G-5 with reverse graded sequences, erosional contacts and disturbed laminae probably represents late Pleistocence shallow, fresh-to brackish-water, deltaic sediment depos on the marine shelf.
Foraminiferal data for sample G-11 (Table 1) taken from a similar slump farther downcanyon indicates that G-11 was deposited in a shallow fresh-to-brackish water
This could also be true for sample G-5 for which foraminif eral data are not available due to the lack of sand
Seismic profiles 79-6 and 78-9 ( . 12 and 15) and on-station records indicate sample G-7 is from a slump off the north canyon wall at 125 m water
(fig. 6 and 20). The upper part of this sample was soupy
This sequence rests over a distinct contact under are 59 clayey silts that show plane parallel laminations and bation. Besides plane parallel laminations there are sequences of cross laminations and several interbedded layers of fine sand.
Foraminifera identified are Neogene, possibly Holocene lagoonal, shallow-water species (Table 1) .. The remains of these animals along with intact arenaceous foraminifera cate that this sample was deposited with little turbulence
Since arenaceous foraminifera break up with little tran samples with intact arenaceous foraminifera are cons to be in place or very close to being in place.
This sediment probably was deposited during a low stand of sea level during the late Pleistocene when Monterey would have been drained of sea water. The shelf areas then have been dry or covered by fresh-to-brackish water lagoons. Thus, this sample also represents a displaced block of shelf sediment slumped into the canyon.
Sample G-11. Sample G-11 from 64 m water
6 and 20) is one of the coarser grain-sized samples during this study. It consists of dark greenish gray cl
-silty to silty, fine to very fine sand and silt Table 2
X-ray radiographs show plane parallel laminations. The
10 centimeters are soupy and highly b worms along with a few mollusks.
The foraminiferal assemblage found s s (Table 60
1) is indicative of a shallow fresh-to bracki deposi
tional environment. The sediment of sample G-11 was depos
ited during a low stand of sea level in a lagoon or fresh water estuary near the mouth of a river such as the Sal
River during the Pleistocene, as were samples G-5 and 7.
Sample C-2-2. Sample C-2-2 (fig. 6 and 21) consisted of an upper interval of mostly massive, dark gray, silty c to clayey silt. Below a distinct angular contact were sever al coarser silt laminae in finer silts and clays (Table 2
X-ray radiographs showed faint wavy laminae throughout the core.. Sample C-2-2 was taken from a slump off the canyon wall and was recovered from a water depth of 138 m
The lack of internal structures and very fine grained nature of the upper part of the sample suggests that this consisted of hemipelagic sediment. The lower part of the sample was from the slump as suggested by the wavy dis nature of the laminations.
Sample C-2-3 from 164 m of water (fig. 6 and 21) is primarily medium dark silt
several coarser laminae. These coarser laminae have s
and sometimes erosional contacts (Table 2) . There were discrete intervals of bioturbation among the parallel
laminations. These structures along with
sequence represents separate pulses of Each
pulse was followed by a period of quiescence 61 intense ion ..
The upper 10 em of sample C-2-5 from 188 m of water
(fig. 6 and 21) are soupy and disturbed.. The rest of the sample is medium gray to dark medium gray, bioturbated, c ey-silt to silty-clay. There are faint indications of parallel laminations. Sample C-2-3 and 5 are from s on the canyon wall ..
Extensive bioturbation in these samples is indi of periods of lack of sedimentation at the sample area al the animals to establish themselves and disturb the
Later pulses of sedimentation covered the area caus the biological activity to cease.. The highly bioturbated and extremely fine-grained nature of this sediment indicates that hemipelagic sedimentation has been very active in this area
Sample C-3-1 .. Sample C-3-1 recovered from 328 m of water was very dewatered and cohesive. This sample consisted of a series of upward-fining sequences 1 to 5 em
These sequences graded from clayey-silt at eros
basal contact to silty clay e Two worm tubes were the indications of bioturbation. Sample C-3-1 was taken from a slump on the canyon floor (fig. 6 and 23 . Greene 1977 and pers.. comm.) described submarine landslides s area which he observed from a submers le as be and made of very cohesive sediment .. Sample C-3-1 may have come from such a feature .. 62
Figure 23. Sample transect 5. Canyon cross from pro le C-3-79. See f • 19 for 63
Sample C-3-2 consists of a re homogeneous, dark gray, clayey silt with some ane parallel laminations (fig. 6 and 23).. The lower 18 em of this sample show disturbed laminations and were also slight coarser. Core C-3-2 is from the central axis of an old s scarp on the south canyon wall at 175 m of water depth. The lower 20 em of the core shows disturbance that could repre sent the material moved by the slump while the remainder of the sample probably was deposited after movement of the slump.
Sample C-4-1. The upper 45 em of this sample are dark gray silty clay-clayey silt that show plane lel laminations .. Below a distinct contact in the medium gray silt clay are a few worm holes. The lower 18 em of s sample show wavy internal laminations. This sample from 367 m of water is part of a slump that moved off the south wall
(fig. 6 and 24). The wavy nature of the internal laminat s in the lower part of the sample was caused by slumping Since the deformation was not consistent between laminations is an original sedimentary feature and not caused by the cor operations. The contact at 45 em (fig .. 24) represents the top of the slump feature at the time of failure.. The over lying fine sediment was deposited later
Sample G-2 from 145 m water con- sisted of extensively bioturbated, greenish black 64
150 20
40
200
10
100 250
E -N
300
350
Figure 24. Sample transect 6. Canyon cross- from profile C-4-800 See . 19 for 65 clayey-silt fig. 6 and 25, Table 2). Plane parallel tions are the main internal structures evident terval of cross laminations just below a distinct erosional contact. The lower 30 em of the sample shows evidence of disturbance. A distinct contact in the lower part of sample is typical of a slump deposit along with deformed wavy internal structures. Among the laminations are small elon- gate finer clasts in coarser sediment oriented sub-normal to the parallel laminations.
Sample G-2 represents silt and clay deposited el and then remobilized. Many of the structures, such as parallel and cross laminations, distinct contacts and r clasts have been reported by Stow and Shanmugam ( 1980) for fine-grained turbidites. The disturbance of the contacts and laminations was due to slumping and not deformation coring operations as shown by the lack of consistent deforma tion throughout the sample.
Samples G-3 and G-4 are from the same feature , a large slump deposit that lls the can yon axis at 553 m of water (fig. 6 and 25). The soupy upper interval of sample G-3 graded into a disturbed, grayish black, micaceous silty c The on structures evident are plane parallel a few shell fragments. There are thin (less than 2 em laminae of coarser material the silty clay. S G-3 66
-4
s-
30 -
l( 37.5-
Figure 25. Sample transect 7. Canyon cross- from pro le 79-0. See • 19 for 67 represents the gradational change to hemipelagic tion common in the deep marine environment.
Sample G-4 bottomed in sandier sediment than G-3 several lenses of sandier material in the silt s
These very fine sand lenses have very distinct contacts. The upper soupy layer is absent in this sample indicating that the area of this sample current activity and/or movement was strong enough to keep hemipelagic sediment from settling. In this area, the ocean bottom is very hummocky and uneven due to effects of slumping (fig. 25). Sample G represents sediment that was moved by slumping or some of gravity flow. This sediment may have been deposited after the initial slump movement.
Modern Marine Sediment
One sample (G-1) is from the shelf above the canyon shelf break at 94 m deep (fig. 6 and 25). Sample G-1 con- sists of very poorly sorted, coarse-skewed, to very fine, greenish black to dark greenish, gray micaceous sand
The upper few centimeters are finer than the lower s of the sample. X-ray radiographs show ane parallel laminations with some broken shell fragments lower core.
This sample is similar to shelf samples de
Galliher (1932), Yancey (1968), and Wolf (1970 for the same area .. The very sand in the top few cent 68 represents the type of sediment that is present on the shelf
Sample G-1 is slightly coarser than samples reported Wolf (1970). He reported coarse silt. However, Galliher 1932 and Yancey ( 19 6 8) reported finding fine sand in the same area ..
Since the difference between very fine sand and coarse silt is less than 20 microns ,sampling and is techniques could cause the difference in the reports. Size classification is based on relative percentages of fferent sizes of clasts. A shift of a few percentage points of any size group could also cause the sample to be in another size classification.
Possible Tertiary Purisima Formation
Sample C-4-2 from 77 m water depth (fig. 6 and 24) is so cohesive that it encompassed the entire core cutter. It had to be scraped from the cutter in the laboratory as it not come off the instrument while aboard the boat. There is a layer of gray, clayey silt over another layer of sh black, clayey silt with a distinct contact between them In the upper layer there is one worm hole. The lower layer is hard, almost brittle, showing a fracture on the base of the sample.
Sample C-2-4 could be from an outcrop of Pur- isima Formation. The very fine texture, hard nature of the sediment, the dark grayish color and the worm 69 hole are all features of the Purisima Format
(1964) and Greene (1970 and 1977) reported sima s area.
Hemipelagic Sediment
Several samples (fig .. 19 1 20 1 21, 23, and 25; Table 2 had an upper layer of very soupy material. While s
could have been disturbed during coring operations 1 it was usually 20 em or more thick and quite often there was a sharp contact with the layer below.
This soupy layer 1 widespread in the canyon is interpreted to be the result of hemipelagic
The major source of suspended sediment is the s that drain into the bay (fig. 1 and 2) with the most s sources being Elkhorn Slough and the Salinas River lf
1970; Arnal and others, 1973).
The absence of hemipelagic sediment indicates that some areas either bottom currents were strong enough to sediment in suspension or gravity flow mechanisms removed them .. DISCUSSION
Bathymetric Features
The major features of the canyon identified on the basis of the bathymetric data are slumping along the canyon walls a meandering canyon thalweg and gradient changes of the can yon floor ..
Slump Features
In the upper reaches of the canyon (0-20 km) s along the canyon walls is the most dominant sediment trans port feature. Small slumps are so numerous that a unit of Qls could be mapped along each canyon wall (Plate 1
Individually a slump could be meters to hundreds of meters length. Small-scale slumping such as this has been shown to be a major force in moving sediment downcanyon (F ld and
Clarke, 1979; May and others, 1983). Slumping can be caused by undercutting bottom currents and oversteepening of depos- ' its on canyon walls due to high sedimentation rates common submarine canyons (Shepard and Dill, 1966; Nelson and s
1979) ..
Canyon Floor
The sinuosity of the axis is an un feature of the canyon floor and thalweg and appears to be related to s ing. The bathymetr data show that s a 71 very narrow thalweg is deflected back and forth across the canyon floor.. This meandering is in response, to the many slumps that have come off the canyon walls. Slump features large enough to block the canyon s cause sediment carried downcanyon to either be deflected around the slump or pond behind the slump until the blockage is breached ..
Farther downcanyon (20-25 km), the canyon floor has been filled with slump material (fig. 10) .. A small thalweg was noticed on the north side of the canyon where the s could be carried. The canyon floor is essential flat up canyon from this slump indicating that material the thalweg was being deposited.
Canyon Gradient
The upper 20 km of the canyon floor slopes seaward at a gradient of 0.01-0.02. From 21 to 24 km offshore the canyon floor is flat. Immediately downcanyon the slope of the f is 0.03-0.04. The relative flatness of the upper reaches of the canyon floor can be related to the pending of s behind blockages of the canyon floor such as those descr earlier ..
During this study no major slope reversals floor were observed Greene (1977) reported ously unmapped depression in the canyon thalweg 4 km of the head of the canyon. The canyon floor was also blocked 72 by a slump. My inability to find the depression mentioned
Greene can be explained by examining the scale of inves- tigation of the two studies. Greene (1977) identified the slump that blocked the canyon axis during a dive in a small submersible while this study used seismic reflection and bathymetric data collected on 0. 9 km ( 0 .. 5 nautical mile grid.. The blockage described by Greene was probably smaller than the resolution of the data for this study.. Localized areas of channel fill and later erosion are also common histories of submarine canyons (Shepard and Dill 1966
Shepard, 1981).
Geology and Structural Features
Area 1: The Head of the Canyon
While deposition appears to have happened in the deeper regions of the study area, erosion has occurred very near the head of the canyon. The mid-channel mound near the head of the canyon is an erosional remnant of an earlier episode of canyon infill (fig. 5, 7, and 9). The earlier canyon floor was interpreted to be as least as shallow as the top of the mound indicating a water depth of at least 10-20 m less than present depths.. The mound was deposited flow mechanisms ..
After the channel was filled, funnelling of ment into the different branches of the canyon head eroded the two channels leaving the mid-channel mound Silt and 73
sand turbidite deposits found on the canyon floor
sediment was continuing to move downcanyon and would have the potential to further erode the canyon floor.
On the southern shelf in area 1 the surface formation appeared to be Qd. This unit, first mapped by Greene (1977
and later by Powell and Chin (1984), is a fluvial delta unit deposited during several sea-level regressions. pinches out at or very near the canyon edge.. This
thickens near the Salinas River indicating that the Sal s
River was the source.
The Qd unit unconformably rests upon Q/Qar immediate
south of the canyon. Powell and Chin (1984) described s of the southern Monterey Bay shelf as Qd resting upon
ary Purisima Formation. Qar either was not depos or was eroded away before Qd deposition on the central part of
southern Monterey Bay shelf.
Along strike with the canyon in area 1 is a
channel (Plate 1, fig .. 11) that terminates into a reentrant
in the southern canyon wall (Plate 1) Buried channels are
common in Qar, the major subsurface seismic c
sequence seen in area 1.
Portions of the northern shelf were mapped as ffer-
entiated Q but they very likely are Qar.. Immediate onshore
the Aromas Formation was found at or very near surface
and at least 180 m thick .. Farther off shore the northern
shelf was identified in this study as Qar. Re conduct- 74 ed by Science Associates (ESA, 1980) for Paci Gas and Electric Company described the same area as Aromas Forma tion. Two units of Aromas were differentiated in the ESA study using a mini-sparker seismic reflection system that achieved much greater penetration than the system used in this study ..
The shelf around the canyon head in area 1 is struc turally simple. Only one small buried fault disturbed the northern shelf. The southern canyon wall has been disturbed by a large continuous slump, parts of which have moved off the wall entirely and other parts have moved only sli
Area 2: The First Bend in the Canyon Axis
The large continuous slump on the southern canyon wall in Area 1 continues into Area 2. Evidence of extens slumping is seen on the canyon floor where the canyon tha is deflected around slumps that had moved onto the canyon floor. Active sedimentation is occurring around one slump as indicated by the turbidite deposits found the thalweg next to a levee that had built next to and around a slump. At least in this area of the canyon, s are actively shaping the canyon.
Area 3: The Northern Shelf Area
Buried channels that lead into large slump northern canyon wall were identified in the sei reflec tion data.. These buried channels, excavated dur a low 75 stand of sea level, were s of the As sea level rose the channels were filled. less consol nature of this sediment compared to the surrounding material would make the channels more prone to slumping. It is the location of these earlier erosional features that control location of the major slumping found along the canyon walls
Area 4: The Most Seaward Area
The canyon floor in area 4 was covered by slump mate rial. Other data indicated that this slump material blocked the canyon causing sediment carried downcanyon to be ponded behind the slump and fill the canyon floor
Submarine Landslides (Qls)
Slumping as a mechanism of sediment transport utes more sediment to the canyon than downcanyon transport of sediment from the canyon head. Over 150 million m3 of ment are in the small-scale slumping along just the 2 km of the canyon. This sediment could be added s to the downcanyon transport system.
Arnal and others (1973) estimated that 30,400 m3 40,000 cu yd)/year are added to the canyon from the 1 zone at the present time. At this rate the volume of entering through the head of the canyon would 1 the vol ume of Qls in 30,000 yrs.
In the past, more sediment entered c from the littoral zone. Prior to 1908 the Salinas entered
Monterey Bay just north of the canyon head where would have delivered considerable amounts of sediment to the canyon
(Arnal and others, 1973). When the river mouth shifted south of the canyon this source of sediment was removed. Also prior to the 19so•s, when cultural influences, such as dam building, decreased the flow in the Salinas River, more ment would have been delivered to the ocean than is be delivered today. During the Pleistocene low stands of sea level, more sediment would have been added to the canyon
Sedimentary Settings
The sediment in the headward regions of Monterey consists of thin-bedded clays and silts with deposits of thicker bedded fine-grained sands within the narrow tha
The finding of these deposits confirmed earlier reports such as Wolf (1979) and Arnal and others (1973) of very
-grained sediment in the canyon. The predominance of silt and clay at first indicated that very little sediment was being actively transported in the canyon today However interpretation of the depositional setting of the sed collected during this study indicated that there are areas of active sedimentation, characterized ined ites, slump deposits and the lack of a surface of soupy sediment. 77 Fine-Grained Turbidites
Several fine-grained samples are interpreted to be the product of turbidite deposition in the canyon. Samples taken
from the thalweg were mostly sand indicating that sand was being transported.. The source for sand in the canyon axis nearshore could be sediment entering through the head of the
canyon. However, the depression in the canyon axis and the
slump blocking the canyon axis reported by Greene ( 1977 presumably would block any downcanyon transport of sand brought into the canyon head. Sand found downcanyon from the blockage would be either relict deposits from before the canyon was blocked or could be sediment fed into the canyon by slumping ..
It is unlikely that the sand found downcanyon from blockage represents a relict deposit because a top layer of hemipelagic sediment was not present in samples col in the thalweg. Deposits stable for any appreciable amount of time would be expected to have a surface layer of hemipe
sediment. Samples taken from the canyon thalweg did not have the surface soupy layer while samples from the levee adjacent
to the thalweg showed the hemipelagic surface layer ..
The canyon incised the very sandy Quaternary Aromas For- mation, Qar, which could be the source of sand. S of
Qar would activate turbulent suspensions the well
sorted sandy deposits such as those the
Other fine-grained turbidite deposits consi of 78 mostly silt and clay were found. These depos s could be result of turbulent suspensions created by off the canyon wallse
Slump Deposits
Samples taken from slump features identified by sei reflection and bathymetric data were described as being ei ther intact or disturbed.
Those samples interpreted to be intact showed depositional features and little or no disturbance .. These features moved off the canyon walls without internal deforma- tion .. Foraminifera and sedimentary structures were those of shallow lagoonal, brackish-fresh water environment, not the deep water canyon floor where the samples were
Many samples showed internal disturbance or de
Usually a wavy or deformed contact was present with consis tent deformation below the contact while above the contact the internal structures would be undeformed. This teristic differentiated these samples from those were disturbed during coring operations.
Other samples taken from slump features showed charac teristics of turbidite or other gravity flow depos ional processes. It was unclear whether the flow features were primary depositional or reworked slump either moved intact off the wall or ited on top of the slump after 1 movement. 79 Hemipelagic Sediment
Another source of sedimentation in the canyon was sus pended sediment brought into the bay. While most suspended sediment is carried out of the bay by surface water c lation (Arnal and others, 1973; Griggs and Rein, 1980 a certain amount settles out and remains in areas where bottom current activity is minimal.
Bottom currents within the canyon are complex, due to the complex topography of the canyon. Currents in the have been reported fast enough to keep silts and c suspension and erode fine-grained soupy material (Gatje and
Pizinger, 1967; Hollister, 1970; Shepard and others, 1979
The absence of the hemipelagic layer in some areas was due to bottom current activity, where as in other areas, such as the canyon axis, the absence probably was due to a com bination of bottom current activity and sediment tran removing the hemipelagic layer. SUMMARY
In the upper 25 km of Monterey Canyon sediment is be transported and deposited. The major source of sed input to the canyon is from slumping of the canyon walls
Over 150 million m3 of sediment are involved in s
Smaller amounts of sediment enter the canyon from hemipe sedimentation and transport of sediment through the canyon head.,
Sands that enter the canyon head are limited to the narrow thalweg in the upper 4 km.. Farther offshore the source of sands found in the canyon thalweg is very 1 from the slumping of the canyon walls.
At about 4 km and 24 km offshore the canyon is b by slump material filling the canyon floor. Sediment c along the canyon floor are being trapped behind se deposits ..
The major reentrants the northern canyon walls represent the re-excavation of previous filled eros channels in the Quaternary Aromas Formation It appears that the location of the major slumping of the walls is controlled by the location of these earl features REFERENCES CITED
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