Submarine geology of Cortes and Tanner Banks

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SUBMARINE GEOLOGY OF CORTES AND TANNER BANKS

A Thesis Presented to the Faculty of the Department of Geology The University of Southern California

In Partial Fulfillment of the requirements for the Degree Master of Science in Geology

by Johnston Earl Holzman August 1950

UMI Number: EP58433

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This thesis, w ritten by

JOHNSTON EARL HOLZMAN under the guidance of h%.8L„ F a c u lty Com m ittee, and approved by a ll its members, has been presented to and accepted by the C ouncil on G raduate S tudy and Research in p a r tia l f u l f i l l ­ ment of the requirements f o r the degree of

Master of Science

Faculty Committee

w rm an

V "

TABLE OF CONTENTS PAGE A B S T R A C T ................

1

INTRODUCTION ......................................

2

GEOLOGIC ENVIRONMENT

.....................

5

OCEANOGRAPHY......................................

9

P H Y S I O G R A P H Y ......................................

11

LITHOLOGY

...............................

12

RECENT S E D I M E N T S .................................

22

. . .

Grain Size D i s t r i b u t i o n ...................

22

Organic Carbon Content

29

...................

Calcareous and Siliceous Material of Organic Origin ..........................

33

Insoluble Residue.... ......................

36

.....................

40

Shape and SurfaceT e x t u r e ..................

40

Mineralogy

42

Sediment Color

CONCLUSION

.

.

...........................

......................................

B I B L I O G R A P H Y ..........

47 4£

LIST OF PLATES PLATE 1.

PAGE

Chart of Cortes and Tanner Banks with Inset Showing Relation to California Coast . .

2.

Location of Samples and Bottom Photographs

3.

Bottom Photograph of Cortes Bank at

..

4

..

8

Location nB " , Depth of 50 F a t h o m s ........ 4.

17

Bottom Photograph of Cortes Bank at Location "C", Depth of 25 F a t h o m s ........

18

5.

Areal Distribution of Lithology ...............

21

6.

Isopleth Chart of Grain Size Distribution . . .

27

7.

Isopleth Chart of Organic Carbon Content of Recent Sediments . . . . . . .

8.

. .

32

Chart Illustrating the Color of the Sediment.,..

41

LIST OF TABLES TABLE ‘ I.

PAGE General Description of Samples .............

6

I I . L i t h o l o g y ..................................

14

Parameters of the Original S e d i m e n t ........

24

III, IV,

Organic Carbon Content of Recent S e d i m e n t s ...............................

V.

31

Calcareous and Siliceous Material of Organic O r i g i n ......................

VI.

3

Parameters of the Insoluble Residueof the S e d i m e n t ...............................

37

LIST OF FIGURES FIGURE 1.

PAGE

An Illustration of the Frequency and Nature Of Occurrence of Each Lithologic Type . . . »

B.

BO

A Cumulative Curve of the Grain Size Distribution of the Average Sediment from the A r e a ....................................

3.

S6

An Illustration of the Relationship of the Precentages of Shell Detritus and Foraminifera Tests, to the Depth of Water . .

4.

An Illustration of the Heavy Mineral Composition of 14 Sediment S a m p l e s ....... .

5.

44

An Illustration of the Light Mineral Composition of 14 Sediment S a m p l e s ........

6.

35

45

Bar Graph Showing the Average Percentage of Detrital Heavy Mineral Grains, Derived from Each Petrologic Source . . . . . . . . .

46

ABSTRACT Cortes and Tanner Banks are 90 miles west of San Diego, California in a marine basin-range province, desig­ nated the "Continental Borderland."

The two banks and the

shallow saddle-like depression which separates them have an area of 230 square miles.

The region was recontoured at a

10 fathom countour interval.

This revealed an extensive

terrace at a depth of 40 to 60 fathoms on both banks. Sixty-six bottom samples were obtained from the region and detailed studies of the rock and recent sediments were made.

Only sedimentary and igneous rocks of Miocene

age were recovered from the area.

The characteristics of

the recent sediments are controlled by the topography, current activity, and depth of water.

An area of non­

deposition or unconformity exists at the top of the banks.

Z INTRODUCTION Geologic studies of continental shelf areas are of two general types.

The first is the investigation of

physiography, structure, and lithology in an effort to relate offshore geology to that observed on shore.

The

second is the study of marine environments and recent sedi­ mentation patterns; thus affording a key to the interpretation of ancient sediments.

This report is concerned with both of

these lines of endeavor. .previous studies which offer regional descriptions of continental shelf areas (Sverdrup, et al, 1938; Revelle and Shepard, 1939; Shepard, 1939; Shepard and Emery, 1941; Emery and Shepard, 1945), include general discussions of bank areas and their environments, but few investigations have been devoted specifically to banks.

A report by Cayeaux,

(1934) deals specifically with a bank area, but is primarily concerned with the origin of authigenic materials that are characteristic of marine areas of non-deposition.

Several

banks have been described from the area surrounding Japan (Niino, 1948), but these are very near shore, and represent a direct extension of shoreline geology.

In his discussion

of the submarine geology of Ranger Bank, Mexico, Emery (1948), offers one of the few reports which deals with a marine bank­ as a unit.

3 Two banks, Cortes and Tanner, located off southern California were selected for study.

Bank areas‘are of

particular interest in that they may represent the early stage in the development of "buried Hill" structures.

By

establishing the character and environment of present day sedimentation, ancient sediments of such structures may be more readily interpreted.

Furthermore, Cortes and Tanner

Banks are 50 miles from any surface geology (San Clemente Island); any geologic data obtained from this area may offer a control point to which the known geology may be extended. Cortes and Tanner Banks are 90 nautical miles west of San Diego, California (Plate 1, inset), in the region included by Latitude 32-BO N . , and Latitude 32-50 N., and Longitude 119-30 W., and Longitude 118-50 W.

By arbitrarily setting

the lower limit of the area at the 150 fathom contour, the two banks, and the intervening saddle-like depression, have an area of 230 square miles. Two sampling expeditions were conducted to the region. The first was in May of 1949, aboard the Hancock Foundation oceanographic vessel, the "Velero IV."

A subsequent trip

aboard the same vessel was made in August of the same year. . These trips yielded 50 bottom samples.

Eight samples were

secured from the Hancock collection; and these were later augmented by eight others from the Navy Electronic Laboratory

30' 5d

20 '

119*00'

CORTES & TANNER BANKS 10 FATHOM

CONTOUR

INTERVAL

FROM

O TO 100

50 FATHOM CONTOUR INTERVAL DEEPER TH AN COMPILED FROM HYDROGRAPHIC SURVEY NO. 6 2 0 6 NAUTICAL

FATHOMS

IOO FATHOMS SOUNDING

DATA

MILES

STATUTE MILES JUNE

40'

1950

40’

3? 30'

OFFSHORE AREA OF SOUTHERN

CA LIFO R NIA

500

20' 30'

20'

II9-* 00'

20' 118*50'

5 of San Diego, California.

Table I gives sample locations

and general information, while Plate 8, shows the position of sampling stations, and the nature of the devices used. The writer is indebted to Dr . K. 0. Emery who suggested this study, and offered invaluable criticism and encouragement; and to the Allan Hancock Foundation of the University of Southern California f o r ‘the use of the laboratory, and ship-board facilities.

Acknowledgements

are also tendered to the Navy Electronics Laboratory of San Diego, California for a series of bottom samples and photographs. GEOLOGIC ENVIRONMENT Cortes and Tanner Banks are located in a basin-range province between the narrow continental shelf and the contin­ ental slope of southern California.

Shepard and Emery (1941)

suggested' that this province be referred to as the ’’Continental Borderland.” The; physiography of the ’’Borderland” is fault con­ trolled and exhibits a northwest southeast trend, paralleling the structural trend of southern California.

In the northern

portion of the ’’Borderland” there is a counter east-west trend whose prime expression is the Channel Islands.

This trans­

verse element is an extension of the transverse ranges found on shore (Reed, 1933).

TABLE I GENERAL DESCRIPTION OF SAMPLES

sta # 1 2 3

4 5 6 7

a 9 10 11 12 13 14 15 16 17 IS 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33

Field #

Latitude North

1340-41 32 -4 1 .0 43.0 1348-41 2 0 .3 1330-41, 29.7 25.8 1335.41 25.5 3 0 .8 1341-41 30.5 1342-41 33.3 35.0 1343-41 35.1346-41 36.5 2 3 .0 N-643 N-644 23.4 N-756 28.5 42 *0 N-67 I N-672 4 2 .3 4 2 .2 N-753 43.0 N-754 N-755 42-5 27.6 1822-49 1823-49 25.3 1824-49 23.3 1825-49 24.5 2 6 .2 1826-49 1827-49 26.7 1828-49 27.7 3 0 .8 1829-49 26.3 1830-49 1831-49 29.9 1832-49 31.4 33.5 1833-49 1834-49 35.4 3 6 .1 1835-49 32-37.2 1836-49 1837-49 38.5 1838-49 40*9

Longitude West 119 -0 6 .0 11.7 0 5 .0 04.5 07.5 08.0 09.5 09.5 15.3 1 1 .8 11.9 2 0 .0 118 - 5 8 .0 119 -0 0 .0 0 9 .0 08.0 06.8 11.8 10.0 10.0 02.0 118-58.7 119-02.6 05.2 03.5 03.7 04.5 ‘ O6 .5 12.0 15.2 14.5 12.3 09.7 08.8 119-07.3 05.9 02.8

Depth Fathoms

Device

Bottom Notation

26 46 100

Drg Drg Drg

Sand-Rock Rock Sand

52

Drg

Sand

127

Drg

Sand-Rock

40. 105

Drg Drg

Sand-Rock Sand-Rock

75 56 60 45 29 42 52 35 32 100 99 98 63 46 53 50 95 75 82 48 56 136 86 150 51 108

Drg, Snp Drg Snp Snp Snp Snp Snp Snp Snp Snp Snp Snp Snp Snp Snp Snp Snp Snp Snp Snp Snp Snp Snp Snp Snp

Rock Sand Sand-Rock Sand Sand Rock Sand Rock Sand Sand Sand Sand Sand Sand Sand Sand Sand Sand-Rock Sand Sand-Rock Sand Sand Sand-Rock Sand Sand Sand

TABLE I (Con’t.)

Sta §

Field #

34 35 36 37 3# 39 40 41 42 43 44 45 46 47 4^ 49 50

1839-49 1841-49 1842-49 1843-49 1844—49 1845-49 1872-49 1873-49 1874-49 1875-49 1876-49 1877-49 1878-49 1879-49 1 8 8 0 -4 9 1881-49 1882-49

51 52 53

1883-49 1884-49 * * 'i 1335-40

54

1336-49

55 56 57

1337-49 1333-49 1339-49

53

1390-49

59 60 61

1391-49 1392-49 1393-49

62 63 64

1394-49 1395-49 1396-49

65 66

1397-49 1393-49

f

V

•'

Latitude North

Longitude West

40 •3 0 5 .6 07.5 41.1 07.3 41.1 4 1 .2 10.3 42.5 03.7 0 7 .2 4 3 .3 1 4 .d 42.3 4 O .4 15.7 16.3 23.3 36.9 1 7 .$ 35.1 13.9 19.9 33.3 31.6 20.7 13.5 30.5 32.2 16.9 15.6 34.1 33.3 15.3 15.2 33.4 30.3 13.4 29.2 12.6 23.6 12.3 29.2 12.6 23.6 12.3 2 9 .2 12.6 23.6 12.3 23.1 10.7 2 7 .6 09.3 03.1 27.1 26.3 07.3 26.3 07.3 26.5 07.7 11.3 32.4 34.3 10.9 119-10.6 32-35.3 10.4 35.7 40.3 07.3 0 7 .6 41.5 4 2 .2 0 7 .6 4 2 .6 07.5 42.3 0 7 .6 43.6 . 07.3 43.6 07.3

Depth Fathoms

Device

Bottom Notation

23 42 42 55 36 115 300 30 130 90 5& 66 39 35 45 45 42

Snp Snp Snp Snp Snp Snp Snp Snp Snp Snp Snp Snp Snp Snp Snp Snp Drg

Sand Sand Sand Sand Sand Sand Sand-Rock Sand-Rock Sand Sand Sand-Rock Sand Sand Rock Sand Rock Rock

47 26

Snp Drg

Sand Rock

24

Drg

Rock

24

Drg

Rock

3^ 31 20

Snp Snp Drg

Sand Rock Rock

13

Drg

Rock

60 35 123

Snp Snp Drg

Sand Sand Sand

42 22 30

Snp Snp Drg

Sand-Rock Sand Rock

39 . 40

Snp Drg

Sand-Rock Rock

30

50

10* 00'

20

CORTES & TANNER. BANKS SNAPPER DREDGE BOTTOM

STATUTE

SAMPLE SAMPLE PHOTOGRAPH

MtLES

39

,38

40

O 65 •A

37

63

>34

41 o 40

40

42 O

43 30 440 50

28< 59

48

'00 24

47

32'

32* 30'

26 54 55 23

25

30

19*00

The area is characterized by a series of deep basins and intervening ridges.

The basins have a maximum depth of

1,078 fathoms and are present-day areas of sediment accumu­ lation,

The ridges, which are often expressed as islands or

banks, rise from the basin areas with exceptionally steep slopes.

These topographic highs are areas of sediment

bypassing (Revelle and Shepard, 1939). Miocene sedimentary and volcanic rock are the pre­ dominant lithologic types cropping out in the "Borderland’1 (Emery and Shepard, 1945).

Some metamorphig rock has been

reported and is considered to be Franciscan (Jurassic ?). No Pliocene or Pleistocene rock is known to crop out in the region.

This lack of lithologic material younger than

Miocene is interpreted by Emery and Shepard (1945) to date the faulting as late Miocene. OCEANOGRAPHY Because no oceanographic data was collected during the course of this investigation, a report by Sverdrup and Fleming (1941) is used as a basis for the following dis­ cussion of the oceanographic conditions existing in the Cortes and Tanner Banks area. currents in the region.

They report two surface

A southeast current with a velocity

of 20 to 15 centimeters per second flowed on the west of Santa Rosa-Cortes Ridge, whereas the Southern California

10 Counter Current with a velocity of 10 to

centimeters

per second was active on the east side of the ridge.

Much

turbulence occurred at the interface of these two opposing currents.

This is shown by the vertical distribution of

temperature, salinity, and oxygen content, which reveals a thickening of the convection layer from 5 fathoms near shore to 75 fathoms in the Cortes and Tanner Banks area. The lower limit of current activity in the bank area is set at a depth of 108 fathoms (200 meters) by the topo­ graphic control of Santa Rosa-Cortes Ridge.

At this depth,

current activity consists of a north-northwest Coastal Deep Current with a velocity of 10 centimeters per second, which exists from the shore to the outer edge of the Santa RosaCortes Ridge.

Seaward from the ridge, the deep southeast

California Current is encountered. Throughout the region, surface waters are saturated with oxygen, but below the zone of photosynthetic activity the;

oxygen content rapidly decreases. In summary, the banks are in a region of shifting and

conflicting currents that give rise to much turbulence and eddy motion.

This turbulence may be significant in main­

taining and placing fine materials in suspension, thereby rendering them susceptible to transport by weak currents. However, in an area of shifting currents as found in the area of Cortes and Tanner Banks, the net sediment transport

11 is radially downslope,

(Sverdrup, et al, 1948), so that

little reflection of current directions may be expected in the sedimentation patterns.

PHYSIOGRAPHY Cortes and Tanner Banks are at the southeast end of the northwest-southeast trending, Santa Rosa-Cortes Ridge. The sides of the banks are steep, with an average slope of 6 degrees.

These steep slopes rise 850 to 1,080 fathoms above

the floor of the basins that flank the banks on three sides. Tanner Bank, the northernmost of the two, has an area of about 58 square miles, and a ridnimum depth of 12 fathoms. Gortes Bank is 120 square miles in area, with a minimum depth of 2 fathoms.

The saddle area, separating the banks,

has an area of 50 square miles, and an average depth of 120 fathoms. The most striking physiographic feature of the area is the extensive terrace on each bank at a depth of 40 to 60 fathoms (Plate 1).

Rising above this terraced surface

are three major topographic highs, one on Tanner Bank and two on Gortes.

Sampling shows that these are composed of

basaltic rock, whereas the terrace surface consists of sedimentary rock.

Thus, it seems that the terraced areas

are the result of wave erosion at a time when sea level was at a relatively lower stand, and the highs of volcanic

material are merely a manifestation of their greater resistance to wave erosion.

The depth of the terrace

surface corresponds with the values generally accepted fa* the lowering of sea level duii ng the Pleistocene epoch. Two major canyons cut the saddle region immediately to the south of Tanner Bank (Plate 1).

The axes of these

canyons are in a straight line paralleling the general trend of the ridge.

One deepends to the northwest and

presents an asymmetrical cross section; the other is symmetrical, and deepens to the southeast.

The alignment

of the axes of these canyons, and their parallelism to the structural trend of the regions suggests that they are of fault origin. LITHOLOGY Rock was obtained from 27 stations.

The majority of

these were dredge samples, but occasionally small rocks were recovered by the bottom snapper.

A total of 765 pieces of

rock were secured in the course of the sampling program. Of these, 125 fragments are considered representative of the rock in situ.

The criteria for distinguishing between rock

representative of the out-cropping material from that which is transported are given by Emery and Shepard (1945). Preliminary studies of the lithologic materials were conducted on b'oard ship immediately after the sample was

13 recovered.

The size, shape, rock type, and evidence of

being representative of rock in situ, was recorded for each rock specimen.

Representative samples of each lithologic

type were selected for laboratory study, after which the bulk of the sample was discarded.

This procedure eliminated

the return of large volumes of rock to the laboratory. The detailed description and classification of the rock specimens were conducted in the laboratory.

(Table II).

Thin-sections were prepared of the igneous rock, and their classification is based on analyses conducted with a pettographic microscope.

The sedimentary rocks were examined

with the aid of a binocular microscope and hand-lens.

In

many cases, where the sedimentary material appeared highly calcareous, it was necessary to determine the percentage of calcium carbonate in order to correctly classify the rock. This was done by digesting the rock in dilute hydrochloric acid and calculating the per cent calcium carbonate from the weight loss. Only igneous and sedimentary rocks were found in the bank area.

Of the rock specimens, 67.1% are igneous rock,

and the vast majority of these are of volcanic origin. Sedimentary rock constitutes the remaining 32.9%; authigenic phosphorite is the major rock type of the sedimentary suite. The majority of the rock specimens recovered from the area are well-rounded to subrounded clasts which ranged from

TABLE II LITHOLOGY

Sta #

Rock Type

Number of Pieces

Roundness

1

Phosphorite Pyroxene Basalt Porphyry

1 2

SR A - SA

2

Rudaceous Limestone Calcareous Medium Sandstone

1 3

R - SR R - SR

5

Arenaceous Limestone Calcareous Fine Sandstone

1 1

SR R - SR

6

Biotite Granodiorite Pyroxene Basalt Porphyry Hornblende Biotite Dacite Porphyry

1 3 1

R SA - SR SR

S

Pyroxene Basalt Porphyry

7

SA - SR

1 6 1

SR SR A - SA

10

Black Argillaceous Limestone Phosphorite -'Foraminiferal Limestone

13

Pyroxene Basalt Porphyry

26

A - SR

15

Pyroxene Basalt Porphyry

2

A - SA

25

Phosphorite

1

SR

27

Hornblende Diorite Quartzite Andesite Porphyry Pyroxene Basalt Porphyry

2 1 19 3

30

Pyroxene Basalt Porphyry Phosphorite

1

SR SR

40

Pyroxene Basalt Porphyry Phosphorite Argillaceous Limestone

2 1 2

SA - SR SR SA - SR

41

Phosphorite

1

SR

SR - R R SA - SR SR

TABLE II (Con'tJ

Rock Type

Sta #

Number of Pieces

Round­ ness

44

Biotite Granodiorite Pyrpxene Basalt Porphyry

3

B

R SA - SR

47

Argillaceous Limestone

1

SR - R

50

Argillaceous Limestone Arenaceous Limestone Calcareous Fine Sandstone Calcareous Medium Sandstone Ferruginous Coarse Sandstone Phosphorite Pyroxene Basalt Porphyry Vesicular Basalt Vitrophyre Hornblende Diorite

1

SA A - SA SA - SR SA - SR SA SR A - SR A - SA R

27 110 14 16 1 b 283

B

52

Pyroxene Basalt Porphyry

3

SA

53

Pyroxene Basalt Porphyry

2

A - SA

54

Vesicular Andesite Porphyry Pyroxene Basalt Porphyry

1 19

A - SA SA

5^

Pyroxene Basalt Porphyry

3

A - SA

62

Calcareous Medium Sandstone

1

A

64

Pyroxene Basalt Porphyry

1

A - SA

65

Phosphorite

1

SR

66

Calcareous Medium Sandstone Pyroxene Basalt Porphyry Phosphorite

*

14 62 21

SA - SR A - SA SA - SR

16 granule to boulder size.

Most of these rounded clasts are

unweathered, and often they exhibited a growth of encrusting organisms oh one side.

This coasting of organisms in indica

tive of long periods of stability, during which the clast continuously exposed the same side to the sea; furthermore, it indicates that sediment accumulation is not sufficient to cover the cobbles (Plate 3). Pieces of unweathered basalt with freshly fractured surfaces were recovered from dredge hauls that are character' ized by sharp shocks and by great changes of tension on the dredge cable.

These are considered to be indicative of rock

fragments broken from bottom outcrops (Plate 4).

A few

specimens of highly weathered basalt porphyry were found. The occurrence of outcrops of rock relatively free of sedi­ ment, cobbles with encrusting organisms on one side, and highly weathered material are indicative of an area of non­ deposition or unconformity at the top of the banks. The basalt porphyry, the most abundant rock in the s

area, consits of phenocrysts of labradorite (An^) in a matrix of microlites of labradorite (Ang^), and augite granules.

The basalt showed several textural variations;

the majority of the specimens have large plagioclase pheno­ crysts, in many cases Z inches in length, in a very fine grained holocrystalline matrix (Clements, 1945). samples showed a diabasic or seriate texture.

Other

The minor

y¥:?4r‘ PLATE 3 BOTTOM PHOTOGRAPH OE CORTES BANK AT LOCATION rTB % DEPTH OF 50 FATHOMS *

PLATE 4 BOTTOM PHOTOGRAPH OF CORTES BANK AT LOCATION "C" , DEPTH OF 25 FATHOMS

19 accessory minerals are magnetite and hypersthene.

Some

secondary chlorite is present, which is believed to be an alteration product of the abundant pyroxenes. The second most abundant rock in the area is an arenaceous limestone.

It is characterized by a light buff

color, a massive appearance, and consists of fine, subangular to subrounded, well sorted sand grains in a matrix of calcium carbonate.

The calcium carbonate shows no organic structures

and is 59.2 $ by weight of the rock. The other ro*ck types and their relative frequency of occurrence are shown in Figure 1.

Varieties of limestone

are shown to be in greater abundance than is generally found on shore.

Emery and Shepard (1945), also reported a relative

abundance of limestones, in contrast with the predominently clastic nature of the sediments on shore. Plate 5 shows the areal distribution of the rock types and denotes those deemed representative of rock in situ.

The outcrops of basalt porphyry coincide with the

topographic highs, whereas the sedimentary rocks, crop out in the terraced areas.

This distribution of rock types

corroborates an earlier statement which attributes the existance of the topographic highs above the terrace O w XE ha N • »E W

B S (A' L W rA \ v/ Im T I

1

ARENACEOUS

10

20

30

40

50

60

P R> P I O V | | H YR Y

L IM ESTO N E \

PH O S P H O R ITE CALCAREOUS

55883' M EDIU M

ARGILLACEOUS CALCAREOUS

SANDSTONE

LIMESTO NE FINE

9

SANDSTONE

BASALT

VITROPHYRE

B IO TITE

G R A N O D IO R ITE

3

,

Q U A R TZIT E PYROXENE

ANDESITE

HORNBLENDE

PORPHYRY

ES

' OUTCROP

mm

‘ TRANSPORTED

\

DIORITE

RUDACEOUS

LiM ESTONE

FERRUGIN OUS

C OA R S E

BIOTITE

a

DACITE

FORAM1N! F E R A L

SANDSTONE

PORPHYRY LIM ESTONE

FIGURE 1 FREQUENCY AND NATURE OF OCCURRENCE OF EACH LITHOLOGIC TYPE

• AUTH!GENIC

70

11000

W

118 5(J

CORTES & TANNER BANKS LITHOLOGY oo 00

77?/4 A/S P O P TED

O UTCROP

BASALT PORPHYRY A N D E S IT E PO RPHYRY BASALT V 1TR O PH YR E D A G IT E PO RPHYRY G R A N O D IO R IT E D IO R IT E Q R Z Q U A R T Z IT E L S • L IM E S T O N E SS PH

SANDSTONE P H O S P H O R IT E 40 I

AP Q R Z GD

P

119 00

118 50' ’

EE

limestone which contains abundant Siphogenerina reedi, Cushman, and Siphogenerina branneri (Bagg) indicative of the middle Miocene was dredged from the southeast end of Cortes Bank.

This is the only fossiliferous material

recovered in the present investigation.

Further indication

of the age of the sedimentary materials of the area is the ' report by Emery and Shepard (1945) of a Foraminifera-bearing mudstone of middle Miocene age from the Tanner Canyon area. Tentatively therefore, the age of the sedimentary rock of the Cortes and Tanner Banks area is considered middle Miocene. The age of the igneous material of the area is difficult to establish.

Submarine geologic techniques, as

yet, give no indication of the structural relationship of the volcanics and the Miocene sediments.

A Miocene age,

however, is given to the volcanics (Emery and .Shepard, 1945), on the basis of their lithologic similarity to the abundant Miocene extrusives of the islands of the ?TBorderland" and the adjacent mainland. RECENT SEDIMENT’S Grain-Size Distribution Mechanical analyses of the recent sediments were made by sieving, using a set of Tyler standard screen sieves having openings corresponding to Wentworth grade limits.

23 Cumulative curves were plotted on probability paper, and the median diameter, sorting coefficient, and phi skewness of each sample was determined.

The results of these analyses

are listed in Table III, Median diameters ranged from 1.17 millimeters to 0.10 millimeters and averaged 0.31 millimeters.

The average

median diameter was obtained from a curve (Figure 2) which is representative of the grain-size distribution of the average sediment of Cortes and Tanner Banks.

This curve

was constructed from the average percentage of each size grade.

Because this curve represents a composite of all

the mechanical analysis data, any parameters obtained from the curve represent the average value for the sediment of the bank area. An isopleth map was prepared to illustrate the areal distribution of the median diameters (Plate 6).

This

revealed a good grouping of median values, and led to the establishment of zones of grain-size distribution based on Wentworth’s sand grade limits.

The relationship of median

grain diameters to the topography is readily seen.

The tops

of the banks are characterized by very coarse sand; downslope the median values decrease to very fine sand grades. The grain-size distribution in the Tanner and Cortes Bank area is controlled by two factors: the size of the material available, and the subsequent working of the sediments by current activity.

TABLE III PARAMETERS OF THE ORIGINAL SEDIMENT

u

cd p co

1 3 4 5 6 7 9 10 11 12 14 16 17 IS 19 20 21 22 23 24 2$ 26 27 2S 29 30 31 32 33 34 33

•H

g

x) o o CO o

1.43 1 *44 1 .2 0 1 .6 4 2 .3 0 2.53 1 .4 1 1 .5 0 2.30 1 .4 0 1 .7 3 1 .1 7 1 .7 9 1 .3 4 4 .6 0 1 .3 0 1.3 5 2 .2 9 1.55

0 -0 .0 2 5 -0 .0 2 5 -0 .1 3 - 0 .3 2 0 .0 3 -0 .0 5 5 -0 .0 0 5 -0 .5 3 -0 .0 0 5 -0 .0 3 -0 .0 0 5 -O .36 0 -0 .3 1 - 0.01 -0 .1 2 5 0.6 2 -0 .0 2 5

mineral gray mineral gray light mineral gray light olive deep olive deep olive deep olive olive light olive deep olive olive light greenish mineral gray mineral gray light mineral gray deep olive deep olive mineral gray mineral gray mineral gray

99 5

9 8 .0

9 0 .0

8 0 .0 p

O R / G/NA L

S E D /M E N T

M D • 0.31 \

7 0 .0

qc

6 0 .0 5 0 .0

3 0 .0 D

INSOLUBLE

R ESID U E

" M D • 0.22 SO • 1.75 SK •-0.025

20.0

10.0

2.0 2 .0

1.0 0 .5 D IA M E T E R

IN

0 .2 5 0 .1 2 5 M IL L IM E T E R S

0 .0 6 2

FIGURE 2 A CUMULATIVE CURVE OF THE GRAIN SIZE DISTRIBUTION

119 00

CORTES & TANNER BANKS GRAIN SIZE DISTRIBUTIO N 2 .0 * l.o M M

|TTi I.O* 0 . 5 0

MM

VE R Y

C O A R SE

COARSE

0 .5 0 * 0 . 2 5 M M

M E D IU M

SAND

SAND SAND

0 .2 5 * 0 .1 2 5 M M

F IN E

SAND

0 .1 2 5 - 0 .0 6 2 MM

VE R Y

F IN E

SA N D

%

119 00

118 50

28 In shoal areas which lie within the zone of photosynthetic activity, 43 fathoms (Sverdrup, et al, 1942) ample food is' available to support large populations of benthonic organisms.

Most of these benthonic forms produce

calcareous tests, and on the death of the organism, the test is contributed to the sediment.

This coarse calcareous

debris,is the main constituent of the coarse sand on the top of the banks.

Below the euphotic zone, where there is less

food, these large forms can not exist and the prime materials contributed to the sediment are the minute test of pelagic Foraminifers. The great turbulence and alternating direction of current flow in the region have been discussed.

This

activity results in a winnowing of the fine materials from the top of the banks, yielding a lag concentration of coarse grained elements, and a net downslope transport and accumu­ lation of fine sediment. The geometric sorting coefficients range from 1.17 to 5.00 and have an average value of 1.85, which was computed from the quartiles and median of the curve repre­ sentative of the average sediment (Figure 2).

Of the samples

95 fo have sorting coefficients of less than 2.50; thus, they are considered well sorted by the criteria establishedo. by Trask.

The sorting coefficients showed no relationship

to topography or median diameters.

Values of phi skewness varied from -1.55 to 0.62.

The

phi skewness of the curve of the average sediment was -0.09 (Figure 2).

A total of 68 fo of the samples were skewed

toward the coarse grain sizes; 10 22

°/o

°/o

showed no skewness;

were skewed toward the fine grain size.

This indicated

that in the majority of the samples, the fine materials have been winnowed out, resulting in a curve skewed toward the coarse grain sizes.

The geologic interpretations which may

be derived from skewnessvvalues are rather obscure.

Krumbein

and Pettijohn (1938), state that skewness in many cases is merely the manifestation of the sampling error. Organic Carbon Content The organic carbon content of 37 sediment samples was determined by a chromic acid reduction method as described by Allison (1935).

This method consists of oxidizing 0.5

gram of sediment in concentrated sulphuric acid containing an excess of potassium dichromate.

The mixture is then

heated to ji750 C. in 90 seconds, cooled and the unused chromic acid is titrated with a standard solution 0.2 N ferrous ammonium sulphate, using diphenylamine as an indicator.

Allison has shown that the organic carbon

content determined by this method compares favorably with that obtained by other techniques.

A conversion factor of

0.7 as recommended by Trask (1932) may be used to estimate

30 the total amount of organic matter from the organic carbon t

content. The organic carbon content of the sediments ranged from 0.07 % to 1.57 % with an average value of 0.64 % IV).

(Table

An areal distribution chart was prepared and the

organic carbon values were contoured at 0.50 % interval (Plate 7).

Values less than 0.50 °/o are found in association

with the coarse grained materials of the bank tops.

Down-

slope the organic carbon content increases with a decrease in sediment grain size. The oxygen content of the overlying water is one of the prime controlling factors of the organic carbon content of the sediment.

The tops of the banks lie within the

photosynthetic zone and therefore are characterized by overlying water which is saturated with oxygen.

Organic

material are readily destroyed in this environment.

Below

the zone of photosynthetic activity the oxygen content rapidly decreases and the destruction of organic matter is retarded. The sedimentary processes of the area are also importantt in controlling the organic carbon content of the sediments.

Organic materials are generally minute particles

of low specific gravity.

The turbulence and shifting current

activity of the upper bank region causes the winnowing of the fine materials from the tops of the banks and their subsequent

TABLE IV

ORGANIC CARBON CONTENT OF RECENT SEDIMENTS

Sta. #

1 3 4 5 6 7 9 10 11 16 17 18 19 20 21 24 26 27

% Organic Carbon

Sta* #

% Organic Carbon

0.. 448 0.3 56 0 .1 5 6 0.204 0 .2 5 7 1 .1 6 0 0 .6 3 7 0.33S 0.1 7 9 0.775 1 .1 5 0 O.786 0.5 5 6 0.415 0.250 1.0 0 0 1 .1 9 0 O .4S9

28 29 30 31 32 33 37 38 39 40 41 43 44 45 46 48 55 61 62

O.366 1 .0 5 0 O.762 0 .372 0.263 0 .4 4 4 0.2 06 010724 0.872 1 .1 7 1 .1 0 1 .4 8 0 .6 9 0 I .46 0 .7 7 0 0 .5 30 0 .2 4 1 .5 7 1.55

118*50

110*00'

CORTES & TANNER BANKS ORGANIC

CARBON

C O NTENT

GREATER T H A N 1.00 P E R C E N T

LE S S

THAN

0 .5 0

PERCENT

33 deposition downslope.

This results in a depletion of organic

materials at the tops of the banks and their concentration at depth with the fine sediments. Calcareous and Siliceous Material of Organic Origin Determination of the percentages of shell detritus, foraminiferal tests, echinoid spines, sponge spicules, and bryozoa, composing the inert material of organic origin were made.

The samples were examined under a binocular microscope,

and the percentages were estimated to the nearest 5.0 %, based on mass relationship rather than on the numerical considera­ tion of the particles of each type.

The results of this

study are shown in Table V. The tops of the banks are characterized by an abundance of shell debris which continues as the dominant form of calcar­ eous material to a depth of 50 fathoms (Figure 3).

This depth

is significant because it closely approaches the lower limit of photosynthetic activity (Sverdrup, et al, 1943), thus indicating that the growth of the majority of the large benthonic organisms is restricted to regions in which the bottom lies within the euphotic zone.

They supply of test

of Foraminifera above 50 fathoms is probably larger than that indicated by the low percentages, but many of these tests, because of their small mass, are winnowed out, and deposited downslope.

Others are probably destroyed by

TABLE V CALCAREOUS AND SILICEOUS MATERIAL OF ORGANIC ORIGIN

Percentage

§Jkfk*~JL i 3 4 5 6 7 9 10 11 16 17 id 19 20 21 24 26 27 2d 29 30 31 32 33 37 3d 39 40 41 • 43 44 45 46 4d 55 61 62

Eehinoid

Sponge

Forams

Spines

Spicules

5 5 5 45 35 do 40 20 5 5 75 do 75 70 20 65 65 50 25 75 S3 20 40 55 22 5 65 do 77 70 50 70 75 do 5 90 5

10 20 15 10 15 5 15 15 5 10 5 5 5 0 10 5 5 5 5 5 2 5 10 10 3 10 3 2 10 15 10 0 3 3 5 0 10

5 0 0 5 0 10 0 5 0 0 10 3 10 10 10 10 20 10 5 0 0 5 0 5 5 0 5 3 3 0 0 5 2 12 5 5 0

Shell Detritus 60. 50 65 25 40 5 40 10 75 45 10 10 10 15 60 20 10 30 60 20 15 70 35 30 50 70 25 15 10 15 35 * 25 20 5 75 5 70

Bryozoa 20 25 15 15 10 0 5 50 10 40 0 2 0 5 0 0 0 5 5 0 0 0 15 0 20 15 2 Q 0 0 5 0 0 0 10 0 15

80

70

PERCENTAGE

60

50

• SHELL

D E TR ITU S

&F0RAM/NIFERA

40

30

20

10

20

30

40

50

60

70

80

90

100

no

120

FA THOMS

FIGURE 3 RELATIONSHIP OF THE PERCENTAGES OF SHELL DETRITUS AND FORAMINIFERA TESTS, TO THE DEPTH OF WATER

130

140

36 by solution in the acidic environment at the top of the, banks, where pH values ranging from 6.6 to 6.9 were recorded from the sediment.

Much of the shell debris also shows the

affect of the solvent action incurred by the acidic environ­ ment . Below 50 fathoms test of Foraminifera are the pre­ dominant form of calcareous material.

Many of these tests

have probably been swept downslope by current activity.

The

majority, however, represent the normal production of ben­ thonic and pelagic Foraminifera in the region. The percentages of echinoid spines, bryozoa, and sponge spicules showed no relationship to depth of water or to the sedimentation pattern of the area. Insoluble Residue The calcareous material of the sediment was removed by digestion in dilute hydrochloric acid, and the percentage of insoluble residue was determined by weight loss (Table VI). The percentage of insoluble residue shows a general relationship to the topography.

The top of the banks have

low values, whereas, an increase in the percentage of in­ soluble material is noted downslope.

A minimum value of

4.4 fo is recorded from Tanner Bank at a depth of 42 fathoms, whereas a maximum, value of 74.# % i§ found in the Tanner Canyon area at a depth of 150 fathoms.

The average per-

TABLE VI PARAMETERS OF THE INSOLUBLE RESIDUE OF SEDIMENT Continental Shelf Sediments: Symposium on Recent Marine Sediments. American Association of Petroleum Geologists. pp. 219-229. SHEPARD, F. P., REVELLE, R. R., and DIETZ, R. S., 1939. Ocean Bottom Currents off the California Coast: Science, vol. 39, pp* 433-439. SHEPARD, F. P.. 1941. Nondepositional Environments off the California Coast: Geological Society of America. Bulletin, vol. 52, pp. Io90-l396. SHEPARD, F. P., and EMERY, K. 0., 1941. Submarine Topography off the California Coast, Canyons and Tectonic Interpretations: Geological Society of America. Special Paper, no. 31• SHEPARD, F. P., 1943. Submarine Geology: Harper and Brothers. ^ New York. SVERDRUP,.H. U., and FLEMING, R. H., 1936-1941. The Waters off the Coast of Southern California, March and July,^ 1937: California University. Scripps Institute of Oceanography. Bulletin. Technical Series, vol. 4* SVERDRUP, H. U., JOHNSON, M. W . , and FLEMING, R. H., 1942. The Ocean, Their Physics, Chemistry and General ^ Biology: Prentice Hall Company. New York. TRASK, P. D., 1932. Origin and Environment of Source Sediments of Petroleum: American Petroleum Institute. TRASK, P. D., 1939. Organic Content of Recent Marine Sediments: Symposium on Recent Marine Sediments. Ammerican Association of Petroleum Geologists, pp. 428-452. WOODFORD, A. 0., 1924. The San Onofre Breccia: University of California. Publications in Geology, vol. 15, pp. 159-230.

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