RECENT MARINE SEDIMENTS OF THE CENTRAL CALIFORNIA CONTINENTAL SHELF BETWEEN POINT LOBOS AND POINT SUR Herbert Palfrey Colomb Library Naval Postgraduate School Monterey, California 93940 M A ) \ ft r -":< ■ '• ^ t n n a n 1 1 a t r NAVhl i tfoibiitAuUAIt Monterey, California >f8S in Ju T RECENT MARINE SEDIMENTS OF THE CENTRAL CALIFORNIA CONTINENTAL SHELF BETWEEN POINT LOBOS AND POINT SUR by HERBERT PALFREY COLO MB , JR. Advisor Robert S. Andrews March 19 73 Apptiovzd §on pubtic KzZzaAe.; d^tnJJbatior. unLimitcd. T15337 Recent Marine Sediments of the Central California Continental Shelf betwsen Point Lobos and Point Sur by Herbert Palfrey Colomb , Jr. Lieutenant, United States Navy B.S., United States Naval Academy, 1967 Submitted in partial fulfillment of the requirements for the degree of MASTER OF .SCIENCE IN OCEANOGRAPHY from the NAVAL POSTGRADUATE SCHOOL March 19 7 3 Library Naval Postgraduate School Monterey, California 9 .. ABSTRACT Seventy sediment samples were collected from the continental shelf between Pt. Lobos and Pt . Sur for textural analysis to determine their statistical properties. Based upon the parameters of mean grain-size, standard deviation, and skewness , three distinct belts of sediment were found. Along the coast the sediments were composed of medium sand- sized, moderately to well sorted particles. Down the center of the shelf a discontinuous band of fine sand occurs in depths of 35 to 65 fm. The outer band appears to follow the break in the continental shelf and is composed of relatively coarse sediment. Two bathymetric and 3.5 kHz seismic profiling cruises were made through the area. The results of these cruises indicate that the offshore extension of the Palo Colorado Fault follows a branch of .the Carmel Submarine Canyon. The primary source of sediments appears to be weathering of coastal rock formations, with sediment distribution due to wave action. TABLE OF CONTENTS I. INTRODUCTION 8 A. OBJECTIVE 8 B. AREA DESCRIPTION 8 C. BATHYMETRY 10 D. REGIONAL GEOLOGY 10 E. PREVIOUS WORK 14 II. DATA COLLECTION 15 A. SAMPLE COLLECTION 15 B. PRECISION DEPTH RECORDING 15 III. DATA ANALYSIS 18 A. MECHANICAL GRAIN-SIZE ANALYSIS PROCEDURE 18 B. COMPUTER ANALYSIS OF RAW DATA 19 C. BATHYMETRIC PROFILES 19 IV. PRESENTATION OF DATA 20 A. TEXTURE OF SEDIMENTS : 20 1. Mean Grain-Size 20 2. Standard Deviation 27 3 . Skewness 2 7 4. Bimodal Distributions 27 B. PEBBLE COMPOSITION 32 C. SHELL CONTENT 32 D. BATHYMETRY 32 V. SUMMARY AND CONCLUSIONS 36 VI. SUGGESTIONS FOR FURTHER STUDY 38 BIBLIOGRAPHY 39 INITIAL DISTRIBUTION LIST 41 FORM DD 1473 43 n LIST OF FIGURES 1. Bathymetry from Monterey Bay to Point Sur 9 2. Regional Geology 1 3. Location of Sample Stations 16 4. Mean Grain-Size Distribution 24 5. Tertiary Diagram: Gravel-Sand-Silt Relationships 25 6. Mean Grain-Size vs Depth 26 7. Standard Deviation Distribution 28 8. Mean Grain-Size vs Standard Deviation 29 9. Skewness Distribution 30 10. Mean Grain-Size vs Skewness 31 11. Bimodal Sample Distribution -33 12. Offshore Extension of Palo Colorado Fault and Location of Continental Shelf Break 34 13. Precision Depth Recorder Profile Showing Fault Scarp and Shelf Break 35 LIST OF TABLES I. Explanation of Geologic Abbreviations 13 II. Sample Locations, Gravel-Sand-Silt Relationships, and Size Statistics 21 ACKNOWLEDGEMENTS The author is deeplv grateful for the assistance of Dr. Robert S. Andrews of the Department of Oceanography, Naval Postgraduate School, Monterey, California. Sample collection was facilitated b* the ar. si stance of the Master and crew of the Research Vessel ACANIA. Acknowledgement must also be made to my wife, Jill Colomb, for her assistance and cooperation. I. . INTRODi;CT"CiT A. OBJECTIVE The objective of this study was to describe and explain the distribution of recent marine sediments along the Central California continental shelf between Point Lobos and Point Sur. In order to accomplish this objective a total of 70 sediment stations were sampled from the Naval Postgraduate School Research Vessel (R/V) ACANIA. B. AREA DESCRIPTION The area of investigation is located along the Central California coast 125 km south of San Francisco. Carmel Bay represents the northern limit of the area, and the southern boundary is the continental shelf seaward of the Pt . Sur tombolo (Fig. 1) . The Carmel and Monterey Submarine Canyons to the north and the Sur Submarine Canyon to the south of Pt. Sur isolate this shelf section from most littoral sediment sources. The eastern boundary is the shoreline and the western edge of the area is the 100 fm curve. The topography of the onshore area is rugged and mountain- ous. Rising abruptly from Carmel Bay and running southward along the coast/ the Santa Lucia Range forms an effective drainage divide. A maximum separation between the coastline and the Santa Lucia Range exists at Pt. Sur, where the divide is found 16 km inland. Several youthful, perennial streams drain into the area, with the largest being the Sur and 8 Fig. 1 Bathymetry from Monterey Bay to Pt. Sur Little Sur Rivers. Along the coast a series of terraces can be seen extending to a height of 175 m between the promon- tories. Alluvial deposits resting on wave-cut terraces are observed along the coast (Trask , 1926). C. BATHYMETRY Dohrenwend (19 71) studied the bathymetry of the area and found that 90% of the area could be described as relatively smooth and featureless with a slope of about 1.5° out to the shelf break. The major features of the area are two branches of the Monterey Canyon, a branch of the Carmel Canyon, and the relatively broad expanse of the continental shelf off of Pt. Sur. The canyon branches mentioned are labeled M-l, M-2 , and CC , respectively, on Fig. 1. Approximately 1.5 km west of Point Lobos the branch (CC) of the Carmel Canyon intersects the shelf, draining northward into the Carmel Canyon. The first branch (M-l) of the Monterey Canyon crosses the shelf 3.5 km west of Yankee Point and runs northwest for 3 km before turning westward to join the Monterey Canyon. A second branch (M-2) of the Monterey Canyon extends almost due west from the center of the area. Finally, the wide shelf off Pt. Sur can be assumed to be the result of the northwestward movement of the block south of the Sur Fault described by Trask (1926) . D. REGIONAL GEOLOGY The geology of the quadrangle inland from the area of investigation was described by Trask (1926) . Cretaceous Santa 10 Lucia Formation granitic rocks predominate in the northern sector down to the Palo Colorado Fault (Fig. 2) . In the extreme northern part of the area the Santa Lucia occurs as a porphyritic granodiorite with orthoclase feldspar phenocrysts ranging from 3 to 10 cm, grading rapidly southward into the main quartz diorite of the region. Farther south the mass terminates in a series of broad tongues extending into schist. South of the Palo Colorado Fault the pre-Cretaceous Sur series contains the oldest rocks in the region in the form of a metamorphic series of schists, quartzites , gneisses, and crystalline limestones of sedimentary origin. Extending southward from the mouth of the Little Sur River is a 3-km wide outcropping of pre-Cretaceous Franciscan rocks composed primarily of sandstones, radiolarian chert, dark grey shales, schist, and greenstones and serpentines. The structural geology of the quadrangle is characteri sd by a series of nearly parallel faults which extend offshor at small angles from the coastline. All of the faults ha\ ; a northwest trend and all, except the Sur Fault, dip 40° to 70' northeast. Near the coastline the offshore geology conforms with that found onshore. In the northeast, the Santa Lucia quartz diorite terminates at what appears to be an extension of the Palo Colorado Fault. South and west of the Santa Lucia Formation, an area of faulted Tertiary and Quaternary marine sediments are found. 11 122°00 W 58 56 54' _52 50 PT LOBOS ^ . 1 1 > E" \ \ \ \ \ L 9r \ \ \ \ YANKEE r- \ pT 4r> ) Om N \ \ 9r \ \ x * \ v i \ '"1 ^ \ \ \ . L M J i , \ ) \ TOu %>o f \ \v --" :rr-soo — -y . \ \^" "^ «.-' * ^° ' \ ? m r\ 91" Mm \ ■ P \ ^. \ *• "V s Mm * ^ \ N \\ o - \ I \ N * s S jL \ v *X \ sQmy m +■ * \ X f Ka /*fc\uf - 1 ■ •f I / Vos \ PT SUR \Z^ — \\ AFTER DOHRENWEND (1971 ) Fig. 2 Regional Geology (geologic abbreviations listed in Table I) 12 TABLE I EXPLANATION OF GEOLOGIC ABBREVIATIONS TQu Tertiary - Quaternary undifferentiated Mm Miocene marine (Monterey Formation) Qm Pleistocene marine Qs Quaternary dune sand Ep Paleocene marine (Carmelo Formation) Ku Upper Cretaceous marine KJf Franciscan Assemblage gr Cretaceous granitic rocks (Santa Lucia granodiorite and quartz diorite) m Pre-Cretaceous metamorphic rocks (Sur Series) Sedimentary rock isopach contour line (meters) 40 0 13 The northwest-southeast trend found in the Santa Lucia Range continues in the offshore formations. There is evidence of much faulting throughout the area which appears to be a continuation of onshore faults (Dohrenwend, 1971). E. PREVIOUS WORK The geology of the onshore area of the study has been well described by Trask (1926). Very little bathymetric or geologic work has been completed in the offshore area of investigation. A bathymetric chart has been compiled by the Coast and Geodetic Survey (C.&G.S. 5476) and a recent seismic survey was conducted by Dohrenwend (1971). Studies of oceano- graphic features are almost non-existent. 14 II. DATA COLLECTION A. SAMPLE COLLECTION Seventy samples were collected between ?t. Lobos and Pt. Sur by ship , using a Shipek grab sampler. In Jai.uarv 1972 a reconnaissance was conducted during which 15 samples were taken throughout the area and bathymetry v. as ch.-;.->ed against the existing navigational chart (C.&G.S. 5476). Navigation was performed by the master of the research vessel and sample depth was obtained by fathometer. Based upoi; the results of the reconnaissance, 55 additional stations were sampled (Fig. 3) . Samples were not recovered from each station, but rock dwelling organisms were present in the sampler bucket when no sediment was collected. The R/V ACANIA, used for sample collection, is small enough for easy maneuvering, yet has the stability for good navigation and safe platform work. Grab samples were tagged, placed in double plastic bags, and refrigerated until processed. Any rock fragments collected were saved for future analysis. B. PRECISION DEPTH RECORDING In conjunction with 3.5 kHz seismic reflection profile and bathymetric studies being conducted in the vicinity of the study area, two traverses of the study area were made by oceanographic research vessels, USNS BARTLETT and USNS DE STEIGUER. The equipment onboard the two vessels was almost identical. 15 30 28 _Q£ 122^00 W 58_ 77^79 80 ) 14 0 71 72 73 74 76 o o o o o o ^O 75 62 64 66 68 o o o o o o 63 65 12 11 a o y 12A Naufica! Miles 0 1 2 26 36° 24 N 22 54 ca °o o°o5860 Ai 55 57°5$ $ 45 47 48 50 52 OOO o ooooo 46 49 51 53 3' ^ 3o? ¥ 309 40 „ 42 Yord$_ 0 1000 2000 3000 ^o \<* 6A o 20 26 27 28 OOO 1S 29 o 30 31 32 o 33 o Fig. 3 Location of Sample Stations 16 The primary mission of the bathymetry transects was to delineate the break in the continental shelf and to plot the location of any possible fault scarps. Within the study area, the cruise of the USNS BARTLETT confirmed the fathometer depths recorded during grab sampling and gave a broad over- view of the area. Several tracks from the cruise of the USNS DE STEIGUER were helpful in tracking the Palo Colorado Fault northward. 17 III. DATA ANALYSIS A. MECHANICAL GRAIN-SIZE ANALYSIS PROCEDURE Kruinbein and Pettijohn (1938) outlined the procedures followed in the grain-size analysis: onduc'.t d for this in- vestigation. Large subsamples of 2.r ' •: c 3ou g were taken from each grab sample to insure that sufficient 3- and 4-0 fractions would be obtained for later heavy mineral analysis. Any sample with a total weight of less than "00 c or made up predominately of pebbles was analyzed in its entirety. Each subsample was shaken with distilled water in a 500-ml jar to desalt. After being allowed to settle, the excess water was decanted by siphon. Foilr :ing a repetition of this process, the subsample was wet-seived through a 4 0 screen thereby removing the silt- and clay-size fractions from the sand- gravel-size fractions remaining in the screen. After passing through the screen, the fine fraction was transferred to a 1000-ml sedimentation cylinder and the coarse fraction was dried in a 500-ml beaker. Upon removal from the oven and being allowed to cool, the dried coarse fraction was size graded with screens according to the Wentworth scale using phi notation (Folk, 1968). Sieves of 8-inch diameter and 0.5-0 increments were employed for the separation process. The fractions coarser than 1 0 were hand brushed through the appropriate sieves; the remaining sieves were then shaken on a Ro-tap automatic shaking machine for 10 min. The fraction removed from each 18 sieve was weighed to the nearest 0.1 mg , with the 3.0- f 3.5-, and 4.0-0 fractions saved for later analysis. Pan fractions obtained during the shaking of the coarse fractions were placed in the 1000-ml cylinder containing the respective fine fraction for that subsample. A peptizing agent was added to each cylinder and a O.S-O' interval separation was performed by pipette. Wadell's correction of Stokes law at 20 C was used to determine the settling veloc- ities of the particles involved. Each 20-ml pipette fraction was dried in a 50-ml beaker and weighed to the nearest 0.1 mg, subtracting the weight of the peptizing agent (Calgon) . When cumulative fraction weights reaching 9 5% of the total weight were attained, the remaining sediments were combined as a final fraction. B. COMPUTER ANALYSIS OF RAW DATA Carter (1971) has prepared a size analysis program for use on the IBM 36 0 computer which will give an output using the statistical parameters of Folk and Ward (1957) . In addition gravel-sand-silt-clay relationships, along with Trask values and Inman values are supplied. C. BATHYMETRIC PROFILES The bathymetric records and 3.5 kHz profiles were studied from the viewpoint of linking sediment size distributions and discontinuities with bottom features. Thus, concentration was placed upon looking for irregularities in the profiles while temporarily disregarding /the generally smooth bottom. 19 IV. PRESENTATION OF DATA A. TEXTURE OF SEDIMENTS Statistical results of the computer output are presented in tabular form giving the gravel-sand-silt percentages and Folk and Ward parameters. The term gravel is used in this work in its commonly applied sense, i.e., particles between -1.00 and -6.00 tf (Folk, 1968). Those stations where no sediment sample was obtained are marked with an asterisk in Table II. 1 . Mean Grain-Size The mean grain-size of the sediments on the continental shelf ranged from -1.69 (Sample 66) to 3.53 0 (Sample 37) (Fig. 4) . The gravel-sand-silt relationships are plotted (Fig. 5) on one face of the tetrahedron used by Krumbein and Sloss (1963). By percentages, there was little veriation in text re with 1 sample classified as silty-sand, 2 as s an dy- grave 1 , il as pebbly-sand and 52 as sand. The finer sediments seem to be concentrated along the center of the shelf at depths of 40 to 60 fm in both the north and south. There is a marked increase in grain-size at the shelf break. Above the lower branch (M-2) of the Monterey Canyon an area of very uniform material varying from 0.76 0 in 55 fm to 0.49 0 in 30 fm, bisects the shelf. 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Contours in 0 24 Q. E o 4/> o. E o V) o CM a •H c o •H +J rd H Q) pci •H I C rd en i H CD > rd e u Cn rd •H Q >i U fd ■H -p u 1.5 0 36° 24 N 22 20 18V Naul icol Mtje* 0 1 2 3000 Fig. 7 Standard Deviation Distribution 28 I 1 V Q o e o wo O o o o o O o ° O o o CO OD d& O o 8 o I 1 -1 0 +1 *2 Mean Grain-Size, 0 ♦3 Fig. 8 Mean Grain-Size vs Standard Deviation 29 122^00 W 58 30 28 26 36° 24 N 22 20' IS N a u I ical Miles 0 Yards / Negative Skewness Positive Skewness Fig. 9 Skewness Distribution 30 Fig. 10 Mean Grain-Size vs Skewness 31 bimodal (Fig. 11) . All of these samples fell within the depth range of 55 to 115 fm; however, there were many other samples in this depth range not classified as bimodal. B. PEBBLE COMPOSITION In studying the pebble composition/ samples 13, 31, 46, 47, and 62 were found to contain quartz diorite pebbles from the Santa Lucia Formation. Andesite pebbles from the Carmelo Formation were found in samples 46, 62, and 66; additional felsite pebbles were found in sample 31. Mudstones were discovered in samples IIS and 66 , with the latter also con- taining sandstone and Miocene Monterey cherty shale, while the former contained shale. Gneiss and chert pebbles were found in sample 31. A granule conglomerate composed of angular fragments with a calcareous cement was found in sample 46. C. SHELL CONTENT No analytical measurement was made of the shell content in each of the samples. However, notice was taken of several samples which seemed, by visual estimate, to be predominately composed of calcareous biological detritis. Samples 58, 68, and 79 appear to be entirely biological remains. D. BATHYMETRY Of all the bathymetric features studied, only two displayed linear continuity. These were the shelf break occurring at about 70 fm and an exposed fault scarp having, in general, 20-m relief (Fig. 12 and 13) . 32 i2rogw 50 30 28 26 36°24' N 22 20 Ifl Koutical Miles 0 1 2 Yords O 3000 Fig. 11 Bimodal Sample Distribution 33 Fig. 12 Offshore Extension of Palo Colorado Fault and Location of Continental Shelf Break 34 Q) u CQ IM 0) CO T3 a, u fd r» o m to ^ -p ■H P fO O Cm in J2 tn C •H £ in o m ^ ^ « CO J (L) >- iH •H rr IN in 0 T— u Pn u ro a f mean grain-size, standard deviation, and skewness , three listinct belts of sediment were found. Along tharticles. Down the center of the shelf a discontinuous band of :ine sand occurs in depths of 35 to 65 fm. The outer band appears K follow the break in the continental shelf and is composed of relatively coarse sediment. Two bathymetric and 3.5 kHz seismic profiling cruises were nade through the area. The results of these cruiises indicate that -he offshore extension of the Palo Colorado Fault follows a branch If the Carmel Submarine Canyon. The primary source of sediments appears to bxe weathering of coastal rock formations , with sediment distribution due to wave action . p ,F°„";..1473 lPAGE " 5. 0101-807-6811 UNCLASSIFIED 43 "Security Clarification A-S1408 UNCLASSIFIED CALIFORNIA CONTINENTAL SHELF MARINE SEDIMENTS SEDIMENT ANALYSIS ' *++ uimm DD ,Frf..1473 'sack. UNCLASSIFIED S/N 0101-807-6821 44 Security CUnificetion A- 3M09 J 29 N S 1 0 i 3 ** Thesis C645 c.l Colomb H2019 Recent marine sedi- ments of the Central California continental shelf between Point Lobos and Point Sur. 7a SUii1* Thesis C645 c.l 1^2013 Colomb Recent marine sedi- ments of the Central California continental shelf between Point Lobos and Point Sur. thesC645 Recent marine sediments of the Central C II Mill II 3 2768 002 08394 1 DUDLEY KNOX LIBRARY