THE ORIGIN, EVOLUTION, AND CONTEMPORARY MORPHOLOGY OF

SOQUEL SUBMARINE CANYON, MONTEREY BAY, CALIFORNIA

A Thesis

Presented to San jose State UniYersity

and

!\loss Landing l\!arine Laboratories

In Partial Fulfillment

of the Requirements for the Degree

!\laster of Science

By

Deidre Ellen Sullivan

December, 199-1

© 1994

Deidre Ellen Sullivan

ALL RIGHTS RESERVED

ABSTRACT

THE ORIGIN, EVOLUTION, AND CONTEMPORARY MORPHOLOGY OF SOQUEL SUBMARINE CANYON, MONTEREY BAY, CALIFORNIA

by Deidre Ellen Sullivan

Soquel Canyon is the product of eustatic, tectonic, biological and

erosional processes. Soquel Canyon formed as the San Lorenzo River cut

across the emergent continental shelf during Pleistocene low stands of sea

level. At least three erosional episodes have taken place. Faulting and gas

discharge may have influenced the position of the canyon head. Extensive

mass wasting, in the past, is indicated by the large area covered by slumps

(40% of the 17 km2 surveyed). Presently, erosion in Soquel Canyon is less

vigorous and visible signs of erosion include biological modification, stress

release fracturing, water currents, and mass wasting on a small-scale.

ACKNOWLEDGMENTS

I would like to acknowledge the help and support of many people in

completing this thesis: Gary Greene for assuming the role of thesis advisor and

providing much needed e:,:pertise on the geology of !v!onterey Bay, l\1ik.e

Ledbetter my former advisor for his excellent presentation of the field of

marine geology, and Greg Cailliet for his thoughtful review of this

manuscript. I wish to give special thanks to Mary Yoklavich for her guidance

and support that made this study possible.

This study involved many people and many modes of transportation. I

thank. the captains and crew of the R/V jolly Roger, R/V ca,·afier, R/V McGaw,

Delta Oceanographics, and R/V Point Sur. The U.S. Geological Survey, Branch

of Pacific 1\!arine Geology provided navigational plots for each cruise, merged

data sets to produce bathymetric maps of Soquel Canyon, and reproduced

sonographs. I would like to thank. Clint Steele, Carolyn Degnan, Harry Lucky,

Tom Chase and Mike Boyle of the Survey for their assistance. Side scan sonar

data was e:>.:pertly gathered by David Caswell of Pelagos, San Diego. A special

thanks to Lynn McMasters for her help with figures. The help of fellow

students judy Mariant, Bob Barminski and Karen Press with data logging

during long cold nights on the geophysical cruises was much appreciated. I'd

like to thank Shelia Baldridge for finding obscure geology literature and not

forgetting the Geology Lab, or should I say me. I greatly appreciated the

organization and professionalism of the Moss Landing Marine Laboratories

administrative staff. Most of all I would like to thank my parents for their

support and encouragement in pursuing a graduate degree.

Research was supported by National Undersea Research Program, West

Coast Center, Fairbanks, Alaska (#CA 92-02, #CA 93-04). The Packard

Foundation provided a travel stipend to present my results at AGU Ocean

Sciences 1994, San Diego, CA.

\'

TABLE OF CONTENTS

ACKNOWLEDGMENTS .......................................................................................... v

UST OF FIGURES AND TABLES .................. :......................................................... vii

ABSTRACT ............................................................................................................. \'iii

INTRODUCTION .................................................................................................... 1

Geologic Setting ........................................................................................ 3

Stratigraphy .............................................................................................. 5

METHODS.............................................................................................................. 11

Bathymetric Mapping .............................................................................. 11

Side Scan Sonar Data ............................................................................... 13

Seismic Reflection Data ........................................................................... 15

Submersible Operations .......................................................................... 15

RESULTS................................................................................................................ 20

Geophysical Mapping .............................................................................. 20

Submersible Observations ....................................................................... 3 5

DISCUSSION .......................................................................................................... 45

Origin and Evolution of Soquel Canyon ................................................ 45

Small-Scale Geology from Submersible Operations .............................. 50

CONCLUSIONS ....................................................................................................... 54

REFERENCES .......................................................................................................... 56

APPENDICES.......................................................................................................... 61

vi

LIST OF FIGURES

Fig. 1. Location map of the study area ............................................................ 4

Fig. 2. Geologic structure map of Monterey Bay region ................................ 6

Fig. 3. Stratigraphy of Monterey Bay ............................................................... 7

Fig. 4. Purisima Formation exposed ................................................................. l 0

Fig. 5. Ship's track for the 3.5 kHz bathymetric survey................................ 12

Fig. 6. Ship's track for the side scan survey.................................................... 14

Fig. 7. Ship's track for the seismic reflection survey...................................... 17

Fig. 8. Submersible tracks for the Delta .......................................................... 18

Fig. 9. The Delta Submersible ............................................................................ 19

Fig. 10. Bathymetric chart generated from 3.5 kHz profiles........................... 21

Fig. 11. Computer generated physiographic diagram ...................................... 22

Fig. 12. Map showing various segments of Soquel Canyon .............................. 23

Fig. 13. A 3.5 kHz profiles of Soquel Canyon.................................................... 24

Fig. 14. l\lap showing location of slump deposits ............................................. 26

Fig. 15. A 3.5 kHz profile of a slump.................................................................. 27

Fig. 16. Line drawn interpretation of side scan sonographs ........................... 28

Fig. 17. Side scan sonographs of rocky ex-posures ............................................ 29

Fig. 18. Side scan sonographs of mass wasting ................................................. 31

Fig. 19. Seismic reflection profile shoreward of canyon head ......................... 32

Fig. 20. Correlation of faults or channel walls ................................................... 33

Fig. 21. Gas anomaly in the water column ......................................................... 34

Fig. 22. Float rock collected by the submersible ............................................... 36

Fig. 23. Diagram of Dive #2978 .......................................................................... 38

Fig. 24. Photographs from the submersible ....................................................... 40

Fig. 25. Sea-level curve of the Neogene .............................................................. 46

vii

ABSTRACT

Seventeen square kilometers of seafloor in Soquel Canyon, one of the

tributaries to the Monterey Canyon system off central Califon1ia, was

surveyed using 1) a 3.5 kHz depth recorder 2) a side scan sonar 3) a seismic

reflection profiler and 4) the Delta submersible in 1992 and 1993. Surveys

revealed a canyon of steep to gentle relief with close to 40% of the area

covered with well-defined slump deposits. Over one hundred linear outcrops

paralleling the canyon axis were detected. All rocky outcrops consis l of the

Pliocene Purisima Formation, a nearshore marine deposit.

The origin, evolution, and present morphology of Soquel Canyon are

the result of complex interactions of eustatic, tectonic, biological, and

erosional processes. Soquel Canyon fonned during the Pleistocene as a river,

probably the San Lorenzo, cut across the emergent continental shelf during a

low stand of sea level. Faulting and gas discharge may ha,·e influenced the

positioning of the head of Soquel Canyon. Based on sequence stratigraphy, at

least three major erosional episodes took place during Pleistocene low stands

of sea le\·el as the head of Soquel Canyon intercepted littoral and fluvial

sediments. Mass wasting and turbidity currents during these periods were

most important volumetrically in excavating and shaping the canyon.

Soquel Canyon, during the present high stand of sea level, is cut off

from coastal sediment supplies, resulting in a much less active canyon.

Presently, potential erosional processes modifying Soquel Canyon include

bioerosion and bioturbation, stress release fracturing, various water

currents, and mass wasting on a small-scale.

viii

INTRODUCTION

Soquel Canyon is one of the smallest and least studied of six canyons in

the Ascension-Monterey Canyon System, which includes Ascension, Ano

Nuevo, Cabrillo, Monterey, Soquel, and ~armel Canyons. The origin of Soquel

Canyon is uncertain. Previous studies have suggested subaerial erosion by the

San Lorenzo River flowing from the north across the shelf during the last low

stand of sea level (Martin, 1964; Greene, 1977), but there has been no direct

evidence to support this. This inference is not une>;pected because the

inception of many submarine canyons is generally thought to occur under

similar conditions (Shepard, 1952; Shepard, 1981; May eta!., 1983 ).

Other processes that have been proposed to influence submarine

canyon formation and modification include faulting, turbidity currents, water

currents, artesian spring sapping, fluid flow, biological modification,

tsunamis, and mass wasting (Shepard and Dill, 1966; Inman eta!., 1976; Greene,

1977; Shepard and Marshall, 1973; Shepard, 1981; May et al., 1983; Nagel et al.,

1986; Paull et al., 1990; Orange et al., 1993 ). The importance of each of these

processes varies regionally and over time. The effects of turbidity currents

and mass wasting generally dominate, but during high stands of sea level

canyon heads may be removed from sediment sources and become dormant

(!\-lay et al., 1983). During high stands of sea level, modification by biological

activity and water currents may be significant agents of canyon erosion

(Dillion and Zimmerman, 1970; Rowe et a!., 197 4; Warme eta!., 197 8; Valentine

et al., 1980; Hecker, 1982; Twichell et al., 1985 ).

Submarine canyons form a unique interface between the shallow

continental shelf and deep continental slope marine environments and

contain a variety of substrata, such as rock walls, ledges, talus slopes, and large

slumps, in close association. Surveying benthic canyon environments is

difficult and expensive, because of the steep and rugged relief. There is an

increasing need, however, to describe canyon geology and faunal associations

as more activities, such as fishing, dumping, and marine transport, target

these locations.

A study to assess deep-water rockfish populations and their habitat

associations in Soquel Submarine Canyon, Monterey Bay, central California,

was conducted in 1992 and 1993. The geology of the canyon was mapped and

described in detail to identify and quantify suitable rockfish habitat. The area

of primary interest was from 90 to 400 m in the canyon. The study was carried

out using data from 3. 5 kHz depth recorders, side scan sonar, seismic reflection

and a manned submersible. Synthesis of these data resulted in a more detailed

description of present morphology than in previous studies of Soquel Canyon

(lv!artin, 1964; Greene, 1977).

The geological portion of this multi disciplined study had three

objectives. The first was to provide direct evidence of the origin of Soquel

Canyon. The second was to describe the processes that may have contributed to

present canyon morphology. The third was to describe the canyon's geology

on a small spatial scale from submersible observations, and speculate on

processes that presently influence canyon erosion.

2

Geologic Setting

Monterey Canyon system, comprising Monterey, Soquel, and Carmel

Canyons, is located offshore of central California, and is the largest submarine

canyon system along the conterminous United States. The vertical relief of

the l\lonterey Canyon is comparable to that of the Grand Canyon of the

Colorado River (l\lartin, 1964; Shepard and Dill, 1966). The head of Monterey

Canyon begins less than 100m offshore of Moss Landing in the central region

of l\!onterey Bay, approximately 115 km south of San Francisco (Fig. 1 ). Soquel

Canyon is the shallowest canyon to intersect the Monterey Canyon 18 km west

of l\Ionterey Canyon's head at a depth of 915 m (Greene eta!., 1989). Carmel

Canyon joins Monterey Canyon 30 km southwest from the head of l\lonterey

Canyon at a depth of 1900 m. To the north, the Ascension Canyon system joins

the Monterey Fan-Valley system near the base of the continental slope at 3290

m.

The Monterey Submarine Canyon system along with the sou them

Coastal Ranges and valleys of west-central Califomia is located on an

allochthonous crustal block referred to as the Salinian terrane (Page, 1970).

The San Andreas fault, and the offshore Palo Colorado- San Gregorio and

onshore Sur-Nacimiento fault zones form the northeast and southwest

boundaries, respectively, of the block, which extends from the Transverse

Ranges northward to Cape l'v!endocino (Page, 1970). It is not clear how far the

Salinian block has been displaced from its point of origin, however, most

minimum estimates are in the range of 350 km (Graham, 1976; Page and

Engebretson, 1984 ).

Faults in the Monterey Bay region lie primarily within two north\l·est­

trending converging fault zones, the Palo Colorado-San Gregorio and the

3

122' 55' 122' 50' 122' 45'

--------~

i i .. , o··,_

<;/.:, / 1': ·-, 36' 55' o,. ..

'/>--'•) . -- ..

36' 50'

36' 45'

36' 40'

S.F. =San Fransisco 0 1 2

Nautical Miles 0 5

Kilometers

Bathymetric base map taken !rom NOAA char! N365121W, N36122W

Depth contours in meters

Figure 1. Location map of the study area.

4

Monterey Bay (Greene, 1990) (Fig. 2). Soquel Canyon lies within the Monterey

Bay fault zone. Faults of the Monterey Bay zone have been identified from

high resolution seismic reflection profiles (Greene et al., 1973; Greene 1977;

Gardner-Taggart et al., 1993). This 10)5 km wide fault zone consists of

discontinuous en echelon faults trending N50°W. Gardner-Taggart (1991)

reports two types of faults in the southern part of the Monterey Bay fault zone,

normal or strike-slip and thrust. The primary NW-SE oriented faults appear as

right lateral-strike slip faults whereas the conjugate faults are thrust faults

and generally trend E-W (Gardner-Taggart, 1991). Most of these faults cut the

sea floor and displace Pliocene deposits.

Stratigraphy

The geology of the Monterey Bay region has been mapped extensively

onshore by Clark et al. (1974) and Clark (1981), and offshore by Ivlartin (1964)

and Greene (1977). The stratigraphy reflects a complicated late Cenozoic

tectonic history of uplift and depression that has produced a succession of

regressive and transgressive sedimentary units (Greene, 1977; Greene and

Clark, 1979). Greene (1977) correlated four distinct seismic stratigraphic

sequences with major onshore sequences: (1) Mesozoic basement rocks

overlain by sedimentary sequences of (2) middle Miocene, (3) upper Miocene

to Pliocene, and ( 4) upper Pliocene to Holocene ages (Fig. 3 ).

The Mesozoic basement consists of granodiorite and underlies all of

Monterey Bay. Greene (1977) refers to a basement called "Monterey High" that

extends northeastward from the city of Monterey to Aptos. This basement high

is bisected by the submarine Monterey Canyon. Greene (1977) suggests that

the Monterey High has undergone episodes of uplift from the early l\!iocene to

5

Figure 2. Geologic structure map of the Monterey Bay region showing locations of faults, folds and major physiographic features. Outline of submarine canyon shown in broad, gray lines; Monterey Canyon axis shown by broad, gray, dashed line. (from Greene, 1990).

6

w u z

AGE w 0 0 w •

> . " < 0 z ' " " w

" < 0 0 ' ';;~

:!:':: ~

" ~

1- ~

> j " < ~ . ' " w

" ~~ . f-0

0

i

' ' • ~ MESOZOIC

OR OLDER

AGE

CLOER

Figure 3.

NORTHERN MONTEREY BAY REGION

FORMAT !ON

Recent marine sedi..,ents

0 ' '"" 0 - Submarine 1 and - 0 !.~;~es and slump c 0 , 0 <>rill ~~

Canyon fl 11 / material

Aromas Sand

Deltaic material

Purlsi"'a F ormil t ion

" Santa Cruz Mudstone of Clark ( 1966b}

Sante Margarita(?)

~ Monterey Fon::Hlon

Granitic rocks (crystallin~ base:n~nt}

THICK-

LITHOLOGY NESS (mtltrl)

* 40

240

DESCRIPTION

Rec~ntl}' depo;i ted muine sand and mud.

Subrr.olrir.e landslide and slump ~1aterial

g 50 Gravel, sand, and mud; ~o,ue broHm <.:·"": consolidated materli!l.

.':·.:~;,; 300 \~ell sort~d. cross-bedded, ~uarnose sand~ '.::·_;~ nonmarine, eolian. fluvial 1oco11y at ba>~

§::2, \__'_'_f-'-'_"_'_'_"_'_"_"_'_'_"_''_'_'"_'_·_'_'_'_"_'_'_· _______ ~

~·== c~:~2 ~:.~':

670•

~:~·. 200+

~~---=--

X X X X X X X

X X X X X X X

X X X

370?

550

{;reenl sh-gray, semi -consol il!,n~~ to conoolid,J!ed sandston~. siltstone, and sMll'; marin~. generally fossiliferous.

Siliceous. organic mud;tone; m~rine

Bedded arkosic sandstone{?)

Li~ht olive-~r3y, rhythmically bedded, oroonic mudstone, diato~laceous df'ld ~ificeous st1ale MIC siltstone; r.:arine .

Probab 1 y rredo.Hi nantl y granod i ori te.

SOUTHERN MONTEREY BAY REGION

FORMATION

Monterey Formation

Granitic rocl:s (Crystalline basement)

DESCRIPTION

Light olive-gray, rhythmtu11y bedded, org<111ic mudstone, diatomac~OliS and siliceous shale and siltstone; marine

5a~e of unit locally contains coar~o-grained, 11hite, granitiC or ~rl.osic sandstone

Predominantly porphyritic biotite granodiorite

Stratigraphy of the Monterey Bay (from Greene, 1977).

7

Holocene based on progressiYely shallowing benthic micro-fossils in the

overlying sedimental}' units.

In northern Monterey Bay, the middle Miocene Santa Cruz Mudstone,

the late 1\liocene Santa Margarita Sandstone, and the Pliocene Purisima

Formation unconformably overlie the Monterey Formation and in some areas

the basement rocks. In southern Monterey Bay, the Purisima Formation

unconformably overlies the Monterey Formation. This change in

stratigraphy represents a time transgressive sequence with several facies

changes resulting from a general north to south transgression (Greene, 1977).

Rock exposures in the study area of Soquel Canyon are predominantly

from the Purisima Formation. The offshore thickness of the Purisima

Formation ranges from zero meters in the Monterey bight to 600 min the

northwestern part of the Bay (Greene, 1977). The Purisima Formation

mapped in the northern Santa Cruz Mountains consists of marine, fine-

grained sandstone, shale and conglomerate and has a maximum thickness of

1,722 m (Cummings eta!., 1962; Clark, 1981 ). The formation has been divided

into five major lithologic members (Cummings eta!., 1962). From dredge

samples collected in Soquel Canyon, Greene ( 1977) has identified the

presence of three of these Members: Tahana, Pomponio, and San Gregorio.

The Tahana Member forms the basal part of the Purisima Formation and

consists of medium to fine grained sandstone and siltstone with pebble

conglomerate beds at its base. The Pomponio Member consists of alternating

beds of silicified mudstone, siltstone and porcellanite. The San Gregorio

Member consists of fine to coarse grained massi\·e sandstone with

irregularly distributed pebbles of chert and volcanic rocks (Cummings et. a!,

1962). Megafossils are common throughout the Purisima.

8

The Purisima Formation is an important aquifer on land (Greene,

1970). Groundwater can be seen flowing from bedding contacts in the sea

cliffs at Capitola Beach, California (Fig. 4).

Holocene sediment unconformably m·erlies the Purisima Formation and

the Aromas Sands in northern Monterey Bay. These deposits extend as an

elongate body from the mouth of the Pajaro River westward for more than 25

km and attain a ma'>dmum thickness of nearly 40 m thick near the head of

Soquel Canyon (Greene, 1977).

9

Figure 4. Purisima Formation exposed along the sea cliffs in Capitola south of Santa Cruz. Groundwater can be seen flowing out from bedding contacts.

METHODS

Data for geological mapping in Soquel Canyon were collected using four

methodologies during four cruises. A bathymetric survey of Soquel Canyon

was conducted aboard the R/V Point Slfr. March 20-21, 1992. A side scan sonar

survey of Soquel Canyon was conducted aboard the 1\1/V jolly Roger, August

21-27, 1992. Ground truthing of sonographs also was conducted during this

cruise by in situ obsen·ations from the Delta submersible. During the third

cruise aboard the M/V McGaw seismic reflection data were collected in Soquel

Canyon, June 7-14, 1993. During the forth cruise, we continued detailed

observations of Soquel Canyon morphology aboard the M/V Cavalier using the

Delta submersible, October 5-15, 1993.

Bathymetric Mapping

High resolution bathymetric mapping of Soquel Canyon was

accomplished using a 3.5 kHz precision depth recorder to produce profiles of

the sea floor. A Trimble lOX Loran-GPS (Global Positioning System) with± 15m

accuracy was used to estimate position. Ship track lines were run at 80 m

intervals (Fig. 5; Appendix 1). The precision depth recorder penetrated 20-

50+ m beneath the ocean floor, and was used to distinguish hard from soft

bottom. All bathymetric profiles were digitized at the U.S. Geological Sun,ey

(USGS}, Branch of Pacific Marine Geology, and these data were merged with

navigational data to construct a detailed bathymetric chart of Soquel Canyon at

20m contour inten,als. Erroneous data points were identified visually then

removed if obviously incompatible with original bathymetric profiles.

Physiographical representations of the bathymetric data also were produced at

the USGS to aid in visualizing the canyon morphology.

11

122"01' 121 '57'

Figure 5. Ship's track during the 3.5 kHz bathymetric survey in Soquel Canyon.

12

Side Scan Sonar Data

The side scan sonar survey was conducted with EG&G sonar equipment

and technical personnel contracted from Pelagos Corporation, San Diego,

California. Navigation data were collected with a differential GPS (±. 1 m

accuracy) on loan from 1\lonterey Bay Aquarium Research Institute (MBARI),

using the differential-correction signal broadcast from Mt. Taro.

13

Side scan sonar produces images similar to low-angle, oblique, aerial

photography, except that the images are based on differences in the intensity

of reflected acoustical sound rather than reflected light (Belderson et al.,

1972). The images produced on the side scan sonographs largely are

determined by two factors, topography and fine scale texture of the ocean

bottom (Able eta!., 1987). Topographic features such as ledges, vertical walls,

and boulders produce dark and light images on the records depending upon

the orientation of the relief. A strong signal (dark) is received from the side

of the feature facing the transducer while a weak signal or shadow (light) is

received from the side sloping away from the transducer. Sediment size also

impacts the acoustic signal. For example, gravel produces a much stronger

return signal than mud because the many small facets facing the tranducers

appear as a darker gray while mud appears much lighter. These textural

differences can be distinguished from topographic differences because there

is no shadow associated with them.

A 1. 5 m long towfish that emitted a 100 kHz acoustic signal was towed

behind the ship at 3.5 knots. The acoustic signal produced a swath width of 600

m, 300m per side. One hundred and ten kilometers of track lines were

recorded (Fig 6; Appendix II). Because of steep relief, two hundred percent

overlap was necessary to produce a complete mosaic of the canyon. Records

36' 50'

122°01' 122'00' 121 °58'

Figure 6. Ship's track during the side scan survey in Soquel Canyon.

14

were microfilmed and photographically reproduced at the USGS to construct a

mosaic of Soquel Canyon.

Seismic Reflection Data

Seventy kilometers of seismic reflection data were collected on the

continental shelf north of Soquel Canyon (Fig. 7; Appendix III). Single

channel seismic reflection data were collected using a dual plate Geopulse

58 lOA sound source fired by a Geopulse 5-+20A power supply. A sweep of 112

second was used with a maximum power output of 900 joules. Recording

frequencies were from 500Hz to 1500Hz. A filter was attached to the

triggering device on the recorder to eliminate noise or erroneous signals. .L\n

EPC -+BOO recorder produced analog records and data were stored

simultaneously on a 16 bit digital optical disk at 8kHz. Stratigraphic resolution

of this system is about 2 meters. Sub-bottom penetration averaged about 80

meters.

Seismic reflection involves measuring the time required for a seismic

wave (or pulse) generated by a mechanical impact to return to the surface

after reflection from subsurface interfaces. An interface between two layers

may be detected if the acoustic impedance (product of acoutic velocity and

density) of the layers is different (Ewing and Ewing, 1970).

Submersible Operations

TVI·enty-nine submersible dives were made from August 21-27, 1992, and

22 dives were made from October 5-15, 1993, in Soquel Canyon using the Della

submersible (Fig 8). The Delta is a 4.7 m long two-person submersible that has

an operational depth to 380m (Fig. 9). A fusion track point system coupled

with the ship's navigation allowed for precision navigation of the

submersible. Of the 51 submersible dives, 8 dives specifically were devoted to

15

describing canyon geology on a small spatial scale (6 of the 8 are described in

the results). The primary objective of the other 43 dives was to survey fish

assemblages and associated habitat. All the dives were recorded

simultaneously by externally and internally mounted video (Hi-8) and still (35

mm) cameras. The video tapes include a voice overlay. The video tapes of the

fish surveys were reviewed to verify interpretations of remotely-sensed

images from 3.5 kHz bathymetric and side scan surveys. Rock samples were

taken with a manually operated remote arm from the submersible.

16

36' 59,.--------,\---------,------------------~-----------.

Santa Cruz

36' 55'

36' 50'

-122'

18

-121'55'

3

155 ls4 As

·121'50'

Moss Landing

Figure 7. Ship's track during the seismic reflection surYey in Monterey Bay, northeast of the head of Soquel Canyon.

17

36° 51'

36' 50'

36' 49'

122'01' 122'00' 121 '59' 121 '58'

Figure 8. Track lines of the Delta submersible in Soquel Canyon. Sites of dives dedicated to geologic characterization are indicated by the bold lines with corresponding dive number. The gray lines are fish suf\'ey dives.

18

19

f'igure 9. The Delta Submersible of Delta Oceanographics.

RESULTS

Geophysical Mapping

From the bathymetric chart constructed from 3.5 kHz bathymetric

profiles, Soquel Canyon is steeper and more rugged than previously

interpreted from NOAA Sea Beam bathymetric data (Fig. 10; Map 1). The steep

relief and gentle slopes of the canyon were vizualized easily from the

physiographic diagram (Fig. 11 ), to give an overall impression of canyon

morphology. Bathymetric data indicate that the head of Soquel Canyon begins

9 km south of Santa Cruz at a water depth of 90 m. From the canyon head the

axis trends 205° (S25°W) for 4.5 km then abruptly bends twice, first west then

south to continue along a new trend of 195'' (S15'W). The canyon continues

along this trend for 5 km at which point it joins the Monterey Canyon at a

20

depth just over 900 m. This study co\·ered the first 6.7 km of the canyon

(approximately 2/3 of the canyon) in the 90- 400 m depth range. At this point,

i.e., 6.7 km from the head, the canyon is 3.5 km wide and 650 m deep at the axis.

Seventeen square kilometers were surveyed. For reference, the canyon is

broken into four segments (with respective lengths): head (1.4 km), central

(1.8 km), bends (1.3 km), and lower segment (2.2 km) (Fig. 12). Each segment

has a west and east wall. The southen1most 4 km of the canyon were not

examined in this study.

The head of Soquel Canyon is U-shaped in profile with the exception of

the thal\\·eg, the deepest part of channel (Fig. 13a). The central segment

exhibits a steep V-shaped profile (Fig. 13b). Farther to the south the lower

segment of the canyon has aU-shaped profile and unconsolidated material has

accumulated in the axis (Fig. 13c). A thalweg breaches the eastern side of the

36°~----------------------------------------------~ 52'

36° 50'

36° 49'

121 '58

Figure 10. Bathymetric chart generated from 3.5 kHz bottom profile survey. Contours are at 20m intervals.

21

i

\

\ I

\

\ \

\ \ '

\

22

36' 52'

36' 51'

36'' 50'

36' 49'

'"""'

t..':l::J ~ /

/

/

c -a " "' E " Cl I <ll

rJl

iii c <ll

c E <ll Cl () <ll

rJl

c "' <ll -a E c <ll Cl

CD " rJl

:;; c <ll

:: E 0 Cl -' " rJl

122'01' 122'00' 121 '59' 121 '58' 121 '57'

Figure 12. Various segments of Soquel Canyon studied in this investigation. The lines and numbers correspond to figure numbers and locations.

23

24

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(b)

... , __ .._ ___ ,, __ _ .. ,.,...,._, ... _

""' """' "~

Lme wG

""'"- --­'"'""-

f . ' ,. ... ·.,

Figure 13. (a) A 3.5 kHz profile across the head of Soquel Canyon showing U­shaped profile and thalweg. (b) A 3.5 kHz profile across the central region of Soquel Canyon showing V-shaped profile. (c) A 3.5 kHz profile across the lower region of Soquel Canyon showing U-shaped profile and recently eroded sediment pond on the right side of the canyon axis. (d) A 3.5 kHz profile showing extreme relief along a portion of the west wall of Soquel Canyon representative of the many small tributaries that cut canyon walls. See Fig. 12 for exact locations.

200 g

J50

ponded sediment in the lower segment profile. In some areas, such as the west

canyon wall in the central region, relief can vary greatly and the walls are

cut by small channels and gullies up to a 100m in depth (Fig. 13d ).

Close to 40% of the study area.was covered with 20m or more of slumped

material as indicated from the interpretations of 3.5 kHz profiles. These slump

deposits covered large parts of the canyon walls and portions of the canyon

axis (Fig. 14). In the bends segment of the canyon two extensive rock

exposures were identified on each canyon wall from the 3.5 kHz profiles; on

the west wall the rocky exposure is close to 1 km2 and on the east wall the

rocky area is half the size. Large slump deposits were found along both walls

of the central and lower segments of the canyon. Many of these deposits and

slump scarps exhibit youthful geomorphology indicated by sharp relief, little

or no sediment cover, the hummocky nature of the surface, and informal

stratigraphy of the deposits (Fig. 15).

The side scan sonographs indicate that Soquel Canyon has a series of

reflectors along the canyon walls that parallel the axis of the canyon (Fig. 16).

These reflectors were verified from submersible observations as exposures or

outcrops of the Purisima Formation composed of sandstone, mudstone and

coquina that form ledges running along the canyon walls. In the area of the

two large rocky exposures at the bends segment of the canyon, the side scan

sonographs revealed tens of closely spaced reflectors, or ledges, running

parallel to the canyon axis (Fig. 17a). Outside of the rocky exposures more

than fifty small groups and isolated individual reflectors (outcrops) were

identified from side scan sonograph throughout the canyon surrounded by

mud (Fig 17b). Some of these isolated reflectors were not detected in the 3.5

kHz profiles. Signs of past mass movement, such as block glides, and slump

36° 52'

36' 51'

/ I

/ /

/

..... -, /

122'01' 122'00' 121 '59' 121 °58' 12P57'

- - - - Inferred Scarps millJ Slump Deposits> 20m thick

Figure 14. Map showing location of slump deposits based on 3.5kHz profiles.

26

N

.,.,

Line wk

'•"'··""·""'· Lll!=-.,... .... JU:l..'JJ.AIIIIli..!J....lU llWrPI~

"'-~­IUU 5e !lll.l..LntiP l'ftll--.--!!~~ fllllll IIPI_(b_ llH!OWlfU"_

-"-­ltalf!l

--~----~

s

100

150 0 CD

"!l. :r

2003 ~ CD til

250 e­m

~ (/)

300 ill ~ ~

350

400

450

Figure 15. A 3.5 kHz profile of a slump deposit along the west wall of the lower segment in Soquel Canyon showing its hummocky nature. See Fig. 12 for exact location.

36° 52'

36° 51'

36' 50'

36° 49'

Figure 16.

c: "0 "' "' E "' "' I Ql

Ul

-C: !!' "' C: E Ql "' () "' Ul

"' c:

"0 " c: E " "' rn " Ul

'" c: " ;:: E

0 "' _J " Ul

Une drawn interpretation of side scan sonographs of Soquel Canyon; the dark lines are reflectors (outcrops).

28

120m Line 21

(a)

120m Line 24

',_', :

Figure 17. (a) Individual sonograph of the rocky exposure on the west wall of Soquel Canyon. (b) Individual sonograph of isolated outcrops found in the headward segment of Soquel Canyon. See Fig. 12 for exact locations.

29

scarps, were inferred from the side scan sonographs and verified by

submersible observations in many parts of the canyon (Fig. 18).

Horizontal bedding of the Purisima Formation that is overlain by recent

unconsolidated sediment from the shelf was determined from seismic

reflection profiles. Gas charged sediments (bright spots) along buried eroded

channel wails or faults and within folds were identified from two closely

spaced profiles running perpendicular to the axis just shoreward of the head

of Soquel Canyon. In addition, at least four buried channels can be identified

within these folds in the profiles (Fig. 19). Correlation of the buried channel

walls or faults from track line to track line indicate a trend of approximately

02S (or N25'E or S25"W; Fig. 20). The depth to which the buried channel walls

or faults extend could not be determined because of the loss of signal, however,

these features are truncated by an inferred erosional surface that is covered

by approximately 15 m of undisturbed flat lying sediment. Water column

anomalies on the 3.5 kHz profiles also suggest gas discharge along these faults

or channel walls (Fig. 21 ). This water column anomaly could also indicate an

aggregation of pelagic fishes, but the close alignment to the buried channel

walls or faults and the configuration of the plume in the overlying water

column suggest gas bubbles.

30

120m Line 26

+ + + + +

+

(a) ..

300m

Figure 18. (a) Side scan sonograph of a block glide found near the head of Soquel Canyon. (b) Side scan mosaic of slump scarps on the west wall of the central segment of Soquel Canyon. See Fig. 12 for exact locations.

31

(a)

(b)

Line 64

> :..:::: •• - ••

~0

\\ \~10 4030 20

Figure 19. Geopulse seismic reflection profile (line (J4) just shoreward of the head of Soquel Canyon. (a) uninterpreted seismic reflection profile (b) interpreted profile showing three faults or eroded channel walls and four buried channels. The bold lines show angular unconformities that are inferred erosional surfaces. The bright spots (light areas) are inferred gas charged sediments. See Fig. 2.0 for exact location.

0

32.

0 <P

"!/. 58 ::r

3 ~ <P iil

77 Ci" <P 0

" "' 95 2l ~ ~

113 ~ :J 0 0

132 ~ <n Eo

36' 59',---------,---------.------------------,------------,

36' 55'

36' 50'

Santa Cruz

-122'

Figure 20.

68

-121'55'

64

Moss Landing

Correlation of faults or channels walls from track line 64 to track line 68 showing trend of features. * Denotes the location of the gas anomaly in Figure 21.

33

34

Figure 21. Anomaly in the water column interpreted as discharge from a gas seep. See Fig. 20 for exact location.

Submersible Observations

Observations from the submersible were critical to interpretations of

remotely-collected data. The location, size, and substratum type of reflectors

(outcrops) that were identified on the side-scan songraphs were verified by in

situ obsen·ations from the submersible. Using differential GPS, the fusion

track point system, and depth of submersible, we also improved the

bathymetric data on a fine scale. We verified the lithology of the reflectors

with rock samples collected from the submersible.

The three samples were float rock composed of sandstone from the

Purisima Formation, as would be expected from previous investigations of the

area (Greene, 1977). The samples appeared to be derived from the outcrops

close to point of collection. Samples were not collected directly from the

outcrops because the submersible's mechanical arm was not appropriate. The

samples appeared to ha,·e been in place for some time because encrusting

organisms (e.g., solitary cup corals) were found on the top side of all the rocks.

Borings by invertebrates also were present in all the samples, however the

organisms responsible for the boring were not obsen•ed (Fig 22).

Six dives were dedicated to geologic obsen•ations and verification of

remotely collected data from Soquel Canyon. Relief, substratum type, size and

depth range of features, and signs of small-scale canyon modification were

described. See Figure 8 for dive locations.

On some dives bottom currents carrying suspended material decreased

visibility and made it difficult to maneuver the submersible. Current speeds

estimated from the submersible cruising speed and distance covered were close

to one knot.

35

Figure 22. Float rock collected by the submersible. Note the borings, solitary coral and the megafossils characteristic of the Purisima Formation.

36

37

Dive 7078 August 7S. 1002

Dive 297 8 traversed up the rocky exposure on the west wall of the bends

segment (Fig. 23 ). From bathymetric and side scan surveys, this segment was

characterized by steep relief and 30- 40 linear reflectors (interpreted as

outcrops paralleling the canyon axis). The dive commenced in 242 m on a 30'

mud slope scattered with boulders and cobbles. From 235m to 220m, the slope

alternated from massive friable mudstone with a 50- 90' slope to mud

interspersed with varying amounts of rubble composed of cobbles and

boulders on a slope of 20- 40'. The outcrops were heavily encrusted with

invertebrates. White sponges were a dominant taxa encrusting vertical

surfaces, and basket stars were common on many of the rocks. From 200m to

165 m, the slope steepened to 80 - 90' and outcrops began to show weak

horizontal bedding cut by fractures. The topography was convoluted and

rugged in three dimensions with small box gullies, up to 10 m across, cut into

the slope associated with fracture planes. At 165 m, distinct bedding planes

formed ledges, overhangs and caven1s created by alternating resistant and

friable sandstone, mudstone and coquina. Large rockfish (Sebastes spp.), some

close to a meter in length were found in and near these caverns (Fig. 24a).

Continuing upward the angle of slope decreased to 20' and the bottom was

covered with mud and cobbles. From 120m to 108 m crinoids covered the tops

of a few small ledges (0.2- 0.5 m). Dive 2978 terminated on a nearly horizontal

bottom at 108m, covered 0.3 km and had a bottom time of 58 minutes.

Dive 2984 August 7(), 1907

Dive 2984 traversed the northern side of the the west wall rocky

eJ<.'POsure of the bends segment. The submersible landed at 253m on a gently

lOOm

200m

300m

Ledges and caverns

Small ledges interspersed with mud and tal us slopes

Joints and fractures cutting across bedding planes

Distinct bedding planes of sandstone and mudstone

Massive mudstone, sandstone and coquina interspersed with unconsolidated muddy substrate

Boulders and loose debris on unconsolidated muddy substrate

KEY

(:? Boulders

Cobble

Indistinct Bedding

Distinct Bedding

~Ledges and Caverns

II Fractures and Joints

I Massive Rocky Outcrops

~ Basket Stars

q White Sponges

':f Crinoids

Figure 23. Diagram of the substrate and canyon wall stratigraphy encountered along submersible Dive #297 8 on the west rocky exposure in the bends segment of Soquel Can von.

Figure 2-1. Next page.

(a) Photograph of rockfish Sebasles chlorostictus occupying a cave.

(b) Photograph of a large rockfish disturbing soft sediment. (c) Photograph of crinoids on top of cobbles. (d) Photograph of a slump hea\·i!y bioturbated with a small

rockfish Sebastes diploproa near an indentation in the slope. Brittle star arms are protruding from the soft sediment.

39

40

sloping (20') mud bottom. From 227m to 156m, the substrata altemated

between massive friable mudstone and mud slope on a 30 - 50' slope. In a

number of areas, holes and indentations appeared to have been carved in the

soft friable rock. Rockfish, up to 0.5 m in length, occupied some of these

indentations. The fish at times were disturbed by the submersible and their

actions disturbed the bottom (Fig. 24b). Fish could modify the soft mudstone

by nestling, creating indentations in the slope; this could significantly alter

the appearance of the substrata. From 150m to 125 m, outcrops of varying

resistance formed ledges and overhangs interspersed with boulders, cobble,

and mud slopes. Invertebrate life was dense on the rock surfaces and fish

were found under a number of the ledges. Scours around some of the boulders

were occupied by large fish that presumably enhanced or created these

scours. The interspersed muddy slopes were covered with signs of faunal and

infaunal activity. From 124m to 115 m angular cobbles were scattered along

the mud slope. Crinoids were attached to a some of the cobbles and small

rockfish were associated with the cobbles and crinoids (Fig. 24c). Dive 2984

terminated at 115m, covered 0.4 km and had a bottom time of 47 minutes.

Dive 7989 August 7(), 1992

Dive 2989 traversed up a rocky exposure on the east wall of the bends

segment. From the bathymetric and side scan surveys, this segment was

characterized by steep relief and a series of linear reflectors (outcrops)

paralleling the axis, but not as extensive as those on the west wall. The

traverse over this section began at 288m on a 20' soft muddy slope. At 275m

the slope steepened to 30 - 40' and 50% of the area was covered with large

boulders, many of these were in the form of sharp-edged slabs on a mud

bottom. Continuing up slope, vertical faces of uniform rocky outcrops were

41

comprised of sandstone/mudstone. At 260m no distinct bedding was present

but vertical fracturing was moderate and open space between fractures was

minimal. Few fish were observed on these outcrops and white sponges were

42

the dominant invertebrate covering rock surfaces. From 250m to 200m the

bottom alternated amoung bedded vertical walls 1- 6 m in height, terraces

covered with mud, and talus slopes. All the bedding planes were nearly

horizontal. At 200m, ledges with caverns were prevalent and rockfish were

more abundant. From 178m to 165m the wrtical rock faces were highly

fractured. Continuing up slope from 145 m to 130 m, the height of the ledges

decreased to 0.3 m- 1 m and were a series of steps with broad terraces. Crinoids

and small rockfish were common on the tops of the ledges. Dive 2989 ended in

130m of water, covered 0.3 km and had a bottom time of 47 minutes.

Dive 79CJ1 August 77, 1997

The objective of di\·e 2991 was to examine the canyon axis and large

slump (0.3 km2) on the east wall of the central segment, and to look for signs

of recent instability such as glide planes and fresh scarp faces. These slumps

appeared fresh from 3.5 kHz bath) metric profile interpretations. This dive

commenced in a 11·ater depth of 245m and terminated at a depth of 112 m. The

entire dive was in mud with the exception of one rock with 4 Metridium spp.

The slope varied from <1 0- 20° and gently undulated with indentations and

mounds from recent fauna and infauna activity. Small rockfish were observed

in some of the indentations (Fig. 24d). No signs of scarps or glide planes were

present. It appeared that slumps that can look fresh from high resolution

bathymetric profiles could be covered with sediment cover whose thickness is

below the resolution of the bathymetric profiles. Dive 2991 covered 0.9 km and

had a bottom time of 98 minutes.

Dive 3104 October 6. l <J93

The purpose of dive 3104 was to examine the largest and most pronouced

reflector detected by the side scan survey in Soquel Canyon. This large black

reflector was interpreted as a massive vertical wall on the deepest portion of

the rocky exposure on the west wall of the bends segment. The submersible

reached a maximum depth of 376m at the base of a vertical wall. The

submersible traversed up this wall from south to north and reached the top of

the wall at 267m. The wall, over 100m in height, consisted of uniform massive

sandstone/mudstone with no bedding. !vlinor fracturing was present near the

top of the wall. o,·erall, invertebrate cover was low compared to other vertical

walls in the canyon and increased near the top. There were similar trends in

fish abundance. A blanket of detrital material covered available surfaces.

From the top of the wall to 230 m the slope varied from 30 to 90' and comprised

massive mudstone, taius blocks and mud. Fish and invertebrate abundances

were higher here than on the vertical wall below. Dive 3104 terminated at 230

m, covered 0.6 km and had a bottom time of 45 minutes.

Dive 3111 October 7 1 9<J3

The objectives of dive 3111 were to investigate the shallow portion of a

large slump (1 km2) and adjacent steep gully cut into the southern portion of

the lower segment's west wall. Side scan sonar only surveyed the shallow

portion of the canyon wall in this location, but a reflector over 0.5 km in

length ran along the shelf-edge of the canyon at 130m-110m. At 262m the

submersible landed on a mud bottom surrounded by a school of hake that

reacted to the submersible by di\'ing into the mud. The mud slope was close to

30' and the submersible sent small turbidity flm,·s down the slope upon

.:..1-3

landing. Benthic fauna and the traces of infauna mostly were absent.

Traversing to the south from 250 m to 160m rock outcrops a.ssociated with the

steep gully consisted of massive sand tone and mudstone. The slope had an

overall lumpy appearance and varied from 60- 90' in the rocky areas to 35+'

for the mud/sand areas. Chutes formed by fractures in the rock surface

transported sediment down the canyon wall. These chutes ranged from 1- 10

em across. A few sediment cones were associated with the chutes. Burrows

were abundant and gallethid crabs and hagfish occuppied some of the holes.

From 160m to 104m the slope altemated amoung ledges 0.5- 3m in height, to

talus slopes of cobble to boulder size slabs, to mud on a 20" slope. Dive 3111

terminated at 104m at the base of a 4 m vertical wall, covered 0.-l km and had a

bottom time of -16 minutes.

44

DISCUSSION

Origin and Evolution of Soquel Canyon

Seismic reflection profiles just north of the head of Soquel Canyon

suggest that the formation of the canyon. was controlled by eustatic processes.

Four shallow small buried channels were found between larger buried eroded

channel walls or faults, just shoreward of the head of Soquel Canyon. These

four buried channels are probably remnants of repeated episodes of subaerial

erosion during the Pleistocene. A stream or river, possibly the San Lorenzo

(lvlartin, 1964; Greene, 1977), eroded the continental shelf as it made its wav to

sea during low stands of sea level. The fill within these channels may be shelf

deposits from subsequent high stands. At least three transgressi\·e-regressive

sequences may be bounded by the unconformities in the seismic reflection

profiles. The most recent erosional surface truncates the buried channel walls

or faults and two of the smaller buried channels, and is covered by 15 m of

recent undisturbed sediment. This erosional surface may be the result of the

coastline transgressing across the continental shelf at the end of the last low

stand 18,000 years ago (Fig. 25; Vail and Hardenbol, 1979). The channels must

have been filled prior to this time because they are truncated by the erosional

surface. This may suggest that the upper two channels were created and filled

prior to the last low stand of sea le\·el.

Soquel Canyon lies within a tectonically active region bounded by two

large fault zones, the San Andreas to the east and the San Gregorio to the west

and within a smaller zone, the Monterey Bay fault zone (Greene, 1977).

Tectonic activity may have influenced the positioning of the headward region

of Soquel Canyon. The trend (N25oE) of the head of the canyon for four km

45

46

0 I z~w 0 0 w"' RELATIVE CHANGES OF SEA LEVEL a: 0 ~::;: 0~(!) . w a_ 1- Oa:<t a_ w zo

-10p m -50 m 0 50m 100m 150m ::::>u_ ' HOLOCENE 1- .018 >- w ,.-L a: I- .75 1-<( z

' z w -a: 0 M w 0 ~ 1- 1.75 - - - - .........

1- 1- -<( (f)

::::> w E -~ 0 ....J

a_ I- 2.8 - - - - - - -- \ w

z w \ 0 -0

\ ::i a_

5 -

- ~ t-- 6.6 - - - - - 1-w >- z -a: w <( 0 i= Q a: ::;: -w 1- w

\ 1-<( -....J

I- 9.8 -10-

w -w::Jz l-Ow

\ <COO ....J-0 -::;:-::;:

Figure 25. Sea-level curYe of the Neogene (after Vail and Hardenbol, 1979).

aligns with the buried eroded channel walls or faults mapped just shoreward

of the head. In the seismic profiles these boundaries appear to be faults, but

this is not conclusive. juxtaposition of differing lithologies or faulting and

gas discharge may have controlled the location and trend of the head and

central segments of Soquel Canyon. This is a reasonable inference as many

canyons originated as fault valleys that were greatly modified by marine

erosion (Shepard, 1981). If the gas-charged features bounding the buried

channels are fau! ts, their trend may indicate a conjugate relationship to the

northwest-southeast oriented faults of the l\lonterey Bay fault zone described

by Gardner-Taggart et a!. (1993). If faulting controlled canyon development,

then the faults ha\·e not been active since the last low stand because no offset

was detected in the top 15 m of recent sediment.

The inferred gas charged sediment zones and the presence of anomalies

in the water column along trends of the buried eroded channel walls or faults

suggest that gas is being released from the sedimentary channel fill, or

perhaps from greater depths into the overlying water column. Even though

gaseous hydrocarbons were not collected in this study, they have been

sampled 10 km to the west (Mullins and Nagel, 1982). Mullins and Nagel ( 1982)

found these hydrocarbons to be thermogenic in origin, but suggested that

some of the shelf edge anomalies may be biogenically produced within the

Quatemary sediment. Because the depth to which these faults or channel

walls extend is not clear from the seismic reflection data, it is not possible to

deduce whether these gases are thermogenic and from great depths or

biogenically produced within channel fill deposits. Therefore, it is not clear

whether these gases were present before the initial formation of Soquel

Canyon and may have facilitated early canyon formation. However, the

47

presence of gas charged sediments would have facilitated exhumation of later

buried channels.

The stability of most of the slumps and the absence of fresh scarps

suggest that Soquel Canyon presently is less active then it has been in the past.

The mass wasting, which dominates the canyon's morphology, probably took

place during the Pleistocene when sea level was lower. During low stands of

sea level, the head of Soquel Canyon intercepted the coastline and the canyon

became an active conduit transporting littoral and fluvial sediments to the

deep sea (Greene, 1977). Once coastal sediments were intercepted by the

canyon, they were deposited temporarily in the canyon axis near the head

until destabilized by some episodic event (e.g., storms and earthquakes) or

chronic activity such as tides (Inman eta!., 1976). Turbidity currents,

generated by destabilized sediments, would move the axis sediments farther

down canyon (Moore, 1965). Examination of modem canyon heads that are

accessible by scuba indicates that discreet flushing events occur on a yearly

basis and many times are associated with the first fall storm (Shepard and Dill,

1966; Shepard and l\larshall, 1973; Shepard, 1975; Okey, 1993).

A consequence of turbidity flows in submarine canyons is the

undercutting and oversteepening of canyon walls (Andrew and Hurley, 1978).

This makes the walls susceptible to mass wasting and the process begins again

(Martin, 196-1; Martin and Emery, 1967). The large amount of area covered by

slumps, close to -10% of Soquel Canyon, suggest that mass wasting was a

significant erosional process in the past.

The cross axis profile of Soquel Canyon changes from the head to the

lower segment. The U-shaped profile near the head of the canyon suggests

that little or no activity is presently occurring and that past mass wasting has

-18

piled up slump debris in the canyon axis. Farther to the south, the V-shaped

profile in the central and bends segments suggests that this region of the

canyon may have been cleared recently of accummulated sediment or has not

been subjected to extensive mass wasting. Continuing farther to the south the

profile in the lower segment is l.J-shaped with sediment in the axis, and

appears to have been breached recently because a thalweg cuts into the

sediment on the narrow flat canyon floor. This sediment may be a result of a

slump, as obserced on the lower west wall of the canyon, spilling into the

canyon axis or by sediment blocked by a large landslide that dammed the

canyon near it's mouth (H. G. Greene, 1994, pers. comm.).

49

50 Small-Scale Geology from Submersible Observations

General obserYations from the submersible verified that a large area of

Soquel Canyon is covered with mud. Even in the bends segment, where the

walls appeared to be dominated by rocky exposures from 3.5 kHz profiles and .

the side-scan sonographs, were interspersed with up to 50% mud. In many of

t: • ..:se areas within the rocky e:-;posures, the mud slopes were steeper than the

angle of repose (22- 2T for subaqueous grain flows, Lowe, 1976) suggesting

that hard substrate was not far below the surface.

Contemporarv Submarine Erosion

Evidence of mass wasting on a small scale was observed from the

submersible, including sediment mm·ing down slope via sediment chutes

located in fractures and turbidity-like flows initiated by the submersible

coming in contact with the slope on the west wall of the lower segment of the

canyon. Benthic fauna and signs of infaunal activity were much less here

than on other slopes. Lower density and diversity of fauna have been

documented in areas that are subject to disturbance (Trumble and McCamis,

1967; Stanley, 1971; VanBlaricom, 1978; Okey, 1993).

Gravity-induced rock fails along fractures are an inferred

contemporary erosional process that also was important during the

Pleistocene. Although rock falls were not observed directly, accumulation of

slabs and blocks of rock at the base of cliffs where fractured rock is exposed

suggests their occurrence. 1\lcHugh et al. ( 1993) suggest that stress release

fracturing can contribute to canyon growth as the unloading of rock from the

canyon wails leads to loss of support. The tectonically active region of

Monterey Bay also may contribute to fracturing in Soquel Canyon.

Biological activity has been identified as an important erosional

influence on canyon walls (Dillion and Zimmerman, 1970; Rowe et al., 197 4;

Warme et a!., 1978; Valentine et al., 1980; Hecker, 1982; Twichell et a!., 1985 ),

and this appears to be true in Soquel Canyon. Biologically induced erosion

includes bioturbation (i.e., the reworking and modification of unconsolidated

sediment by biological activity) and bioerosion (i.e., the breakdown of rock

outcrops by marine borers that range from small invertebrates to large fish;

Neumann, 1966; Warme et al., 1978; Hecker, 1982).

Most of the mud deposits in Soquel Canyon were covered with benthic

fauna and traces of infauna activity such as mounds, pits, and burrows. Both

fish and invertebrates appeared to be actively moving the sediment. On steep

slopes this would result in a net transport of unconsolidated sediment down

slope over time (Hecker, 1982).

The importance of biological activity on canyon erosion can vary

greatly with the type of substrate (Valentine eta!., 1980). In Soquel Canyon

this appears to be a function of the rock's resistance to erosion and degree of

surface complexity. Fish may be modif)ing semiconsolidated rock by creating

indentations in the friable mudstone for refuge. Bioerosion by tilefish

impacted semi-lithified silty clay of Hudson Canyon on the East Coast of the

United States (Grimes et al., 1986). Twichell eta!. (1985) has estimated the rate

of erosion by tilefish to be 13 em per 100 y as each generation of fish creates a

new burrow.

The surface complexity of the substrata influenced the size and

composition of associated fauna. At least two distinct assemblages of rockfishes

were present in Soquel Canyon. Large rockfish species (up to 1.0 min total

lenght) associated with rock ledges, cavems, and boulders of high relief, and

51

small rockfish species associated with mud and cobble of low relief (Yoklavich

eta!., 1992; 1993 ). The actions of fish enlarge the caverns they inhabit, or at

least prevent sedimentation (Palmer, 1976). Changes in fauna with depth also

could influence the degree of bioerosion on outcrops.

Boring by invertebrates was evident in all the rock samples and is

undoubtedly an important process in breaking down outcrops in the marine

environment (Warme and Marshall, 1969; Dillion and Zimmerman, 1970;

52

Warme eta!., 1971; Warme et al., 1978). Pholads can bore a foot or more into

rocks and occur at depths up to 1-lO m (Palmer, 1976). The degree of boring has

been associated with the grain size of the country rock, with the number of

borers decreasing with increasing grain size (Warme and Marshall, 1969). It

should be noted that some of these bored rock samples may be remnants of

Pleistocene activities when sea level was lower.

Bioerosion by macro borers, such as sponges, dominates all samples in

recent and fossil coral material from deep sea environments (Bromley and

D'Alessandro, 1990). The high frequency of occurrence of sponges on many of

the rock faces suggests that they are agents of bioerosion and should be

examined more closely in Soquel Canyon. little is known of the ecology of

Hexactinellida, the dominant sponge taxa in the canyon, but the erosive

capabilities of tropical sponges have not been described for this group

(]. Nybakken, 199-l, personal com.). Further investigations are necessary to

adequately address the importance of bioerosion to contemporary canyon

morphology.

Bottom currents work synergistically with biological acth·ities to

increase their effects on erosion (Rowe et al., 197-l; Stanley and Freeland,

1978). Canyon currents are generated by many forces including wind, surface

waves, intemal waves, tides, and spin-off eddies from large scale geostrophic

current systems (Inman et a!., 1976 ). Maximum bottom currents in Soquel

Canyon were estimated to be close to one knot (50 em s-1 ). Bottom current

speed up to 50 em s-1 also has ben estimated in one study in the head of

Monterey Canyon (Rosenfeld, 199-1 ). At times, especially in the shallow end of

the axis during falling tides, large amounts of suspended material indicated

these currents had the capability of removing sediment.

Submarine spring sapping can contribute substantially to canyon

erosion (Johnson 1939; Paull et a!. 1990). The Purisima Formation is an

important aquifer on land (Greene, 1970; Greene eta!., 1993), and groundwater

can be seen flowing from the cliffs composed of the Purisima Formation (Fig.

-1). Cold seep communities of chemos.ynthetic clams are associated with faults

and fluid flow within the Purisima Formation in Jvlonterey Bay (Greene et a!.,

1993; Barry et al., 1993). Groundwater discharge could directly cause grain by

grain erosion of strata and result in slumping of weaken substrata (Robb,

198-1). No signs of fluid flow, such as chemical stains or bacterial matts, were

seen during this study but spring sapping potentially can contribute to

growth of Soquel Canyon.

53

CONCLUSIONS

Eustatic, tectonic, biological and erosional processes contribute to the

origin, evolution and contemporary morphology of Soquel Canyon. During

the Pleistocene, when sea level was lower, the San Lorenzo Rh·er subaerially

cut a channel across the continental shelf south of Santa Cruz, CA. The river

cut through deposits of the Purisima Formation, a nearshore marine deposit of

Pliocene age. The placement of the newly created proto-Soquel Canyon may

have been fault controlled following a zone of weakness created by conjugate

faults of the Monterey Bay fault zone. Gas discharge along these faults also

may have contributed to canyon formation, hm,·ever, this remains unclear.

The coastline of Jvlonterey Bay fluctuated during the Pleistocene from

close to its present day position during high stands of sea level to over 100m

lower and near the shelf break during low stands. Once canyon formation

commenced on the emergent continental shelf several periods of erosion

followed during successive low stands of sea level. At least three erosional and

depositional events may have taken place to shape Soquel Canyon during the

Pleistocene. The periods of low stand were most important in excavating and

shaping the canyon. The head of Soquel Canyon was very near the ancient

low stand shoreline and acted as an active conduit transporting littoral and

fluvial sediment offshore. Mass wasting accompanied this transport of coastal

sediments as turbidity currents flushed sediment down the axis of the canyon.

Undercutting of canyon walls and down-cutting of the canyon axis are

consequences of turbidity currents that led to rapid periods of mass wasting

and canyon growth. Later transgressions planed the continental shelf and

subsequent high stands partially buried the shallow channels with shelf

54

deposits. Exhumation of buried channels resumed with low stands of sea level,

and was facilitated by gas discharge from the sediment.

Erosional processes during the present high stand of sea level result in

a much less active canyon, which is cut off from coastal sediment supplies.

Processes that currently modify Soquel Canyon include bioerosion, stress

release fracturing, water currents, and mass wasting on a small-scale.

55

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APPENDICES

The following appendices contain the nayigational starting and ending points for the track lines run on the geophysical cruises.

61

APPENDIX I 62

Ship track lines for the 3.5 kHz presicsion depth recorder survey. Lines are spaced at 80 m.

Line Start: Lat Start: Long End: Lat End: Long

eA 36 50.77 171 57.74 36 48.75 121 59.08

eB 36 50.77 121 57.68 36 48.66 12159.08

eC 36 50.77 121 57.62 36 48.57 121 59.08

eO 36 51.13 121 57.32 36 48.48 121 59.08

eE 36 51.13 121 57.26 36 48.39 121 59.08

eF 36 51.13 121 57.20 36 48.30 121 59.08 wG 36 48.63 122 01.06 36 51.64 121 57.09

eG 3651.13 121 57.14 36 48.21 121 59.08

wi-1 36 48.60 122 01.02 36 51.61 121 57.05

el-l 36 51.13 121 57.08 36 48.12 121 59.08

wi 36 48.57 172 00.98 36 51.58 121 57.01

e I 3651.13 171 5t.02 36 48.03 121 59.08

w.J 36 48.54 122 00.94 36 51.55 171 56.97

ej 36 51.13 171 56.96 36 47.94 121 59.08

wK 36 48.51 172 00.90 36 51.52 171 56.93

eK 36 51.13 121 56.90 36 47.90 171 59.03

wL 36 48.48 172 00.86 36 51.49 121 56.89

wM 36 48.45 177 00.87 36 51.46 171 56.85

wN 36 48.42 122 00.78 36 51.43 121 56.81

wO 36 48.39 122 00.74 3651.40 121 56.77

11 36 49.47 122 00.02 3651.06 121 58.46

12 3651.06 121 58.46 36 50.05 121 57.14

13 36 50.05 171 57.14 36 49.40 121 57.55

14 36 49.40 121 57.55 36 50.33 121 58.79

15 36 50.33 171 58.79 36 50.05 171 59.33

16 36 50.05 121 59.33 36 48.98 121 57.97

17 36 48.98 121 57.97 36 50.72 171 57.03

18 36 50.72 121 57.03 3651.98 121 58.77

19 36 52.00 121 57.79 36 51.97 121 54.99

20 3651.44 171 55.38 36 52.01 121 56.95

21 36 52.00 121 57.13 36 49.83 121 58.38 27 36 49.83 121 58.38 36 49.32 ]71 59.44

63 APPENDIX II

Ship track lines for the side scan sonar survey.

LINE North pt North pt South pt South pt

latitude longitude latitude longitude

16 36 52' .63 121 58' ,25 36 48' .00 121 59' .71

17 36 52' .63 121 58' .15 36 48' .00 121 59' .61

18 36 51' .63 121 58' .05 36 48' .00 121 59' .51

19 36 52' .63 121 57' .95 36 48' .00 121 59' .41

20 36 52' .63 121 57' .85 36 48' .00 121 59' .31

21 36 52' .63 121 57' .75 36 48' .00 121 59' .21 22 36 52' .63 121 57' .65 36 48' .00 12 I 59' .11

23 36 52' .63 121 57' .55 36 48' .00 121 59' .01

24 36 52' .63 121 57' .45 36 48' .00 121 58' .91 25 36 52' .63 121 57' .35 36 48' .87 121 58' .58

26 36 52' .63 121 57' .25 36 48' .87 121 58' .46

27 36 52' .63 121 57' .15 36 48' .87 121 58' .34 28 36 52' .63 121 57' .05 36 48' .87 121 58' .22

D 36 49.96 Pl 57.71 36 47.57 122 00.18

64

APPENDIX III

Ship track lines for seismic reflection surveys.

LINE Start: Lat Start: Long End: Lat End: Long

18 36 54.010 Pl 54.993 36 57.376 121 55.182 19 36 57.562 121 55.i66 36 51.316 121 58.881 20 3651.153 121 58.981 36 49.482 127 08.457 so 36 56.309 ]7] 58.378 36 57.704 121 55.578 51 36 57.736 121 55.516 36 55.017 121 56.543 52 36 54.649 171 56.799 36 57.146 171 54.577 53 36 57.364 121 54.333 36 55.549 121 54.169 54 36 55.336 121 54.171 36 56.363 121 53.523 55 36 56.575 ]71 53.372 36 55.370 121 53.254 56 36 54.882 121 53.233 3655.140 171 52.477 64 3651.426 121 54.775 36 52.641 121 58.444 68 36 57.475 171 58.773 36 51.156 ]71 55.051