Depositional Environments in the Monterey Formation, Cuyama Basin, California

Depositional environments in the Monterey Formation, Cuyama Basin, California

MARTIN B. LAGOE* ARCO Exploration Co., Geological Services, P.O. Box 5540, Denver, Colorado 80217

ABSTRACT borderland province, an area of discrete depositional basins that are inter- connected to various degrees, similar to the modern continental border- The Monterey Formation of the Cuyama Basin accumulated in an land off southern California. Such a borderland (Fig. 1) existed during the inboard basin within a Miocene borderland analogous to the Recent Miocene in California (Ingle, 1980). The diverse basin topography asso- continental borderland off southern California. Analysis of benthic ciated with such a borderland, and its interaction with a variety of océano- foraminiferal biolfacies, lithology, and sedimentary structures permits graphie and depositional processes, produces basin-filling sediments recognition of several depositional environments within the Monterey characterized by internal stratigraphic complexities and significant regional Formation in this; basin. The lower member of the Monterey Forma- variations. tion, the Saltos Shale (early Miocene), contains basin-plain, slope, and The Miocene Monterey Formation exhibits many of these stratigra- various submarine-fan subenvironments. These predominantly ter- phic complexities which have attracted the attention of California ¡itratig- rigenous rocks are interbedded with impure carbonates composed raphers for decades (for example, Reed, 1933; Bramlette, 1946; Kleinpell, primarily of foraminifera and calcareous nannofossils. The upper 1938; Woodring and Bramlette, 1950; Isaacs, 1983). Several recent studies member, the Whiterock Bluff Shale (middle Miocene), contains highly biogenous deposits representing slope and basin-plain environments. Both siliceous and calcareous biogenous rocks are prominent in this unit. The complex: array of lithofacies within the Monterey Formation is the product of the interplay between terrigenous and biogenous sedimentation. Various factors control this interplay, including global sea level, climate, paleo-oceanographic changes, local tectonics, and paleogeography. The Monterey Formation in the Cuyama Basin is more terrigenous than coeval sections in more outboard basins of the Miocene borderland (as represented by the Santa Barbara Basin). These latter areas contain many lithofacies indicative of low terrigenous sedimentation rates: highly siliceous and calcareous rocks, phosphatic rocks, and highly organic rocks. The inboard Cuyama Basin acted as a sediment trap during much of the Miocene and is partly responsible for these lithofacies patterns. Both inboard and outboard basins contain variations in bulk-accumulation rates that correlate to variations in relative sea level. This study demonstrates the importance of depositional patterns, psJeogeography, and multidisciplinary analysis for understanding the complex stratigraphic units found along active continental margins.

INTRODUCTION

Statement of Purpose Ü3GRANITIC EH FRANCISCAN ROCKS COMPLEX This paper discusses the deposition of the Monterey Formation within a Miocene basin developed along the tectonically active continental Figure 1. Major depositional basins in central California. The margin of California. Basin development along such margins is complex basins west of the San Andreas fault formed a borderland during the (for example, Blake and others, 1978) and commonly incorporates a Miocene. Major faults: SAF, San Andreas fault; GF, Garlock fault; RNF, Rinconada-Nacimiento fault; BPF, Big Pine fault; SGF, San •Present address: Department of Geological Sciences, University of Texas at Gabriel fault; SYF, Santa Ynez fault. Cities: B, Bakersfield; SLO, San Austin, Austin, Texas 78712. Luis Obispo; SB, Santa Barbara; LA, Los Angeles.

Geological Society of America Bulletin, v. 96, p. 1296-1312, 19 figs., 3 tables, October 1985.

1296

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in the Santa Barbara and Santa Maria Basins of coastal California have of the Monterey Formation will provide the basis for understanding how greatly increased our knowledge of the lithologic variations, diagenetic regional controls interact with local basin settings to produce a variety of aspects, and depositional components of the Monterey Formation (for depositional responses across the Miocene borderland. example, Pisciotto, 1978, 1981a, 1981b; Isaacs, 1981a, 1981b, 1981c, 1981 d, 1983; Surdam and Stanley, 1981a, 1981b). It is unclear from these Objectives studies, however, whether these local relationships are typical for the Miocene borderland as a whole. In order to understand the depositional Previous studies (Lagoe, 1981, 1982) demonstrated that the Monte- history of the Miocene borderland from a regional context, it is necessary rey Formation in the Cuyama Basin is a bathyal marine deposit with to examine coeval sediments in a variety of tectonic, geographic, and several subenvironments. The present study describes these subenviron- depositional settings. This regional context needs to include (1) the geo- ments in detail and assesses their importance in understanding the deposi- graphic and local basin setting within the Miocene borderland and its tional history of the Miocene borderland in California. Specifically, these development through time; (2) the types of lithofacies present, their distri- objectives are: (1) to define lithofacies within the Monterey Formation of bution, and their depositional significance; and (3) the major controls on the Cuyama Basin on the basis of lithology and sedimentary structures of this deposition and their variation through time. the rocks; (2) to review the paleobathymetry of these lithofacies as based The purpose of this paper is to discuss the depositional environments on biofacies patterns of benthic foraminifera; (3) to infer depositional in the Monterey Formation within the Cuyama Basin of central California environments on the basis of lithofacies associations and biofacies patterns; (Fig. 2). This information, when compared with similar information from (4) to document distribution of these depositional environments within a adjacent basins, will contribute to the formation of the regional context part of the Cuyama Basin; (5) to develop a preliminary depositional model mentioned above. The subsidence history and general stratigraphic rela- for the Monterey Formation in the Cuyama Basin; and (6) to briefly tionships of this basin were described previously (Lagoe, 1984). This study compare the depositional character of the Monterey Formation in the will show that the Monterey Formation contained a variety of deep- Cuyama Basin with the outboard Santa Barbara Basin. marine basinal environments which varied greatly, both temporally and geographically. The nature of lithofacies within these environments was Methods predominantly terrigenous with significant biogenous inputs ranging from calcareous during the early Miocene, to mixed calcareous/siliceous during Lithofacies are based on the description of well and outcrop samples. the middle Miocene. These basinal rocks graded laterally into shallow- Samples were described with respect to lithology and sedimentary struc- marine and nonmarine rocks, which dominated the late middle and late tures. These descriptions were plotted on graphic logs (see Lagoe, 1982, Miocene history of the basin. for examples) for key well and outcrop sections and provide the basis for The nature and distribution of basinal sediments are related to a defining lithofacies. Additional information is provided by sample descrip- variety of controls, including local basin topography, global eustatic sea tions from oil company well files and correlation of lithofacies to electric level, regional variations in biotic productivity, and access to sources of log response. terrigenous sediment. Mutti and Ricci Lucchi (1972) presented a lithofacies classification Comparison of the Cuyama Basin with the more "outboard" (in the for deep-marine clastic rocks. This classification is useful for describing the sense of being more seaward within the Miocene borderland) Santa Bar- Monterey Formation in the Cuyama Basin. The lithofacies used in this bara Basin shows that Cuyama contained, on average, more terrigenous study are a modification of the Mutti and Ricci Lucchi scheme. The lithofacies. This was due to its position adjacent to the primary Miocene modifications used adapt their classification to subsurface information (see strandline; thus, paleogeographic position within the Miocene borderland Lagoe, 1982; Fig. 3). was also an important control on Monterey deposition. Such comparisons The biostratigraphy of Miocene deep-marine rocks in California is

Figure 2. Index map of the Cuyama Basin. Major faults: SAF, San Andreas fault; BSF, Big Spring fault; SJCF, San Juan-Chi- mineas fault; MF, Morales fault; WBF, Whiterock Bluff fault; RF, Russell fault; CF, Cox fault zone; OF, Ozena fault; SCF, South Cuyama fault; LPF, La Panza fault; RNF, Rinconada-Nacimiento fault.

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MODIFIED MUTTI AND RICCI LUCCHI LITHOFACIES

LITHO- LITHOLOGY BEDDING SEDIMENTARY E-LOG FACIES CHARACTERISTICS STRUCTURES

SP 1 MOHAWK LUNDSTR UM 48-2JRES DOMINANTLY SANDSTONE, FINE TO VERY THICK TO MASSIVE BEDS, OFTEN CORE SAMPLES ARE NORMALLY MASSIVE COARSE GRAINED, PEBBLY SANDSTONE, FORMING VERY THICK AMALGAMATED OR CONTAIN INDISTINCT CURRENT CONGLOMERATE AND VERY RARE UNITS, POOR LATERAL CONTINUITY LAMINAE. WHOLE CORE CONTAINS A/B INTERBEDDED MUDSTONE OCCASIONAL GRADING AND OCCASIONAL TO ABUNDANT MUD CLASTS

^' a • C R DM i4(-2| SANDSTONE, NORMALLY VERY FINE THINNER, MORE DISCRETE BEDS THAN COMPLETE BOUMA SEQUENCES, GRADED 3 TO MECIUM GRAINED WITH SOME FACIES A/B. BETTER LATERAL BEDDING, PARALLEL AND CROSS INTERBIEDDED MUDSTONE CONTINUITY THAN FACIES A/B, LAMINATION, CONVOLUTE BEDDING, SOME

E-LOG MARKERS MORE PERSISTENT MUD CLASTS, OCCASIONAL BROAD, § C LOW RELIEF CHANNELS; BOUMA 990 0 SEQUENCES EVIDENT ONLY IF 1 l WORKING WITH FULL CORES 10,00 0 JUIU i

PFER #1 I o t SANDS""ONE, VERY FINE TO FINE GRAINED, THIN BEDDED WITH EXCELLENT LATERAL THIN CURRENT LAMINAE PREDOMINANT — SILTY, INTERBEDDED MUDSTONE AND CONTINUITY, MINOR E-LOG MARKERS MAY BE PARALLEL, WAVY, CONVOLUTE SOME IMPURE CARBONATE PERSISTENT OVER LONG DISTANCES OR CROSS LAMINATED, BOUMA SEQUENCES MISSING THE A DIVISION ARE COMMON DI E SANDS TONES OFTEN NOT APPARENT ON E-LOG DUE TO THIN-BEDDING AND 680 0 SILTINESS 690 0

VARIABLE, CAN BE SANDSTONE, CHAOTIC FEATURES INDICATIVE OF MASS SILTSTONE, MUDSTONE AND/ OR MOVEMENT — SLUMPING, ETC F CONGLOMERATE VARIABLE

1 ROGO RUS s ELL A28-5 I MUDSTONE, IMPURE CARBONATE, MASSIVE TO VERY THINLY BEDDED, SOME LAMINATION, ELONGATE GRAINS SILICEOUS MUDSTONE AND BIOGENIC SOMETIMES LAMINATED ORIENTED PARALLEL TO BEDDING, S o \ O . OOZE MASSIVE IF BIOTURBATED, OCCASIONAL I CT) CROSS LAMINATION DUE TO BOTTOM G AND CONTOUR CURRENTS

( O ¿ O - 1 O

Figure 3. Modified Mutti and Ricci Lucchi lithofacies. Electric log examples are from the Cuyama Basin. See Lagoe (1982) for details of how this classification was compiled.

based primarily en benthic foraminifera (Kleinpell, 1938,1980). Several vert, 1964), and other relevant areas (for example, studies by Rupke and benthic foraminileral stages are used to biostratigraphically characterize Stanley, 1974; Stanley and Maldanado, 1981, in the Mediterranean Sea). Miocene rocks (Fig. 4). The biostratigraphic framework used here cali- As soon as the depositional environments are defined, they can be corre- brates these beni:hic stages to planktic zonations and commonly used lated to electric log responses in some cases (compare Selley, 1978;, 1979; megafossil stages (Fig. 4). Webb, 1981). This permits the mapping of environmental trends in sub- Biofacies of benthic foraminifera were studied from published species surface areas of the basin where lithologic sampling is limited to w ells. lists (for example, Phillips, 1976; Poore and others, 1981), raw species checklists, and picked slides from oil company well files and samples Geologic/Stratigraphic Setting collected specifically for this study. Distinctive biofacies are used to infer paleobathymetry. The biofacies classification of Ingle (1980) is used with The Cuyama Basin is located in the southernmost Coast Ranges of minor modification (see Lagoe, 1984). This biofacies classification allows California, in an area now occupied by the Caliente, La Panza, and Sierra samples to be ass igned a paleodepth based on the deepest-dwelling species Madre Ranges, Cuyama Valley, and Carrizo Plain (Fig. 2). This area also present in a sample. Shallower species are commonly present in samples represents the southernmost Salinian Block (Reed, 1933; Page, 1981), a due to downslope transport. geologic terrain underlain by granitic and gneissic basement and bordered The combination of lithofacies and biofacies information is used to by major faults—the San Andreas to the northeast, the Big Pirn; to the infer depositiona.1 environments. Biofacies contribute paleobathymetric south, and the Rinconada-Nacimiento to the southwest. data; lithofacies provide information on specific depositional processes. The Cuyama Basin is part of a Miocene borderland province in Interpretations also utilize the many studies of depositional processes and which the regionally widespread and stratigraphically important M onterey products in the modern borderland off southern California (Emery, 1960; Formation was deposited (Bramlette, 1946; Ingle, 1980; Piscictto and Emery and Hiilsemann, 1962; Moore, 1969; Gorsline, 1978, 1980, 1981; Garrison, 1981). The well-documented Miocene strandline and extensive Soutar and others, 1981), the Gulf of California (van Andel, 1964; Cal- nonmarine rocks indicate that the Cuyama Basin was an inboard basin

EPOCH STAGE STRATIGRAPHIC UNITS PLIO- Morales Fm CENE Santa 12H Mohnian Margarita Caliente Fm Fm M White- Luisian rock Branch 13- LU Bluff « Shale Canyon Z Relizian Sandstone Saltos ILI c o Shale 14- O 5 o E Saucesian u. Painted Rock Sdst m o « CO 15- O 3 Soda Lake Shale IT CO > OLIGO- Quail Canyon Sdst 16- Zemorrian CENE Simmler Fm

17- EOCENE Paleogene Sedimentary Rocks

Figure 5. Generalized stratigraphy of the middle Cenozoic se- 18- .2 quence in the Cuyama Basin. Modified from Vedder (1973) and Lagoe (1982). Stages are benthic foraminiferal stages after Kleinpell (1938).

19- T. 11 N., R. 27 W., San Bernardino Base and Meridian). It was originally described as consisting of claystone, siltstone, and minor impure limestones and siliceous shale. In its type area, the unit ranges in age from Saucesian 2 0 — at its base to Relizian (see Fig. 5 for calibration of California stages to other biostratigraphic units) at its top (Hill and others, 1958). The Saltos Figure 4. Biostratigraphic framework for the Cuyama Basin. Shale Member thus is lower Miocene, using the biochronology of Keller Compiled from Kleinpell (1938,1980), Vedder (1973), Phillips (1976), and Barron (1981) and Poore and others (1981). Addicott and others (1978), Keller and Barron (1981), Poore and The Whiterock Bluff Shale member overlies the Saltos Shale member others (1981), Baldauf and Barron (1982), Lagoe (1982), and H. E. and is more limited in extent. Originally named by English (1916), it was Harper (1980, personal commun.)- Figure from Lagoe (1984). described by Hill and others (1958), its type section being at Whiterock Bluff (sec. 25, T. 11 N., R. 28 W„ San Bernardino Base and Meridian). (that is, adjacent to primary strandline) within this borderland province, This type description of the unit mentions siliceous shale, fissile shale, and close to sources of terrigenous sediment. In this sense, it would be similar diatomaceous shale as the major lithologies. The age of the Whiterock to the modern Santa Monica and San Pedro Basins off southern California Bluff Shale at its type area is uppermost Relizian at the base but Luisian which are dominated more by terrigenous sedimentation than are more throughout most of the unit (Hill and others, 1958; Phillips, 1976), making outboard basins (Gorsline and Emery, 1959; Gorsline, 1978,1980,1981). the member largely middle Miocene. A thick sequence (up to 10,000 ft; Dibblee, 1976) of sediments was deposited within this area during the middle Cenozoic (Fig. 5). This se- THE SALTOS SHALE quence contains both marine and nonmarine rocks, the latter becoming more predominant in the eastern part of the basin. This area contains one Sources of Data of the best exposed and best documented marine to nonmarine transitions in California (Clifton, 1968,1981; Repenning and Vedder, 1961; Vedder, The Saltos Shale is not well exposed in surface outcrops of the 1973). The Monterey Formation is an important part of this unconfor- Cuyama Basin. Fortunately, there are abundant wells which penetrate this mity-bounded, middle Cenozoic sedimentary sequence within the Cuyama unit, and these yield samples for study. In particular, 180 ft (55 m) of Basin (Fig. 5). It represents a deep-marine phase of deposition within this continuous core from the ARCO Federal Caliente Unit number 1 permits sequence. comprehensive sampling in a portion of this unit. The ARCO Federal This formation was originally subdivided into two members in the C.U. number 1 was drilled on the southeast flank of the central Caliente Cuyama Basin (English, 1916; Hill and others, 1958). The lower member Range (sec. 35, T. 11 N., R. 27 W., San Bernardino Base and Meridian; is the Saltos Shale and is the more widespread unit, occurring in both the Fig. 6). It penetrated a section of Saltos Shale with a subdued electric log Caliente and Sierra Madre Ranges and beneath Cuyama Valley. The type response typical of the unit over large areas of the Cuyama Basin (Fig. 7). locality is on the southeastern flank of Caliente Mountain (sees. 28 and 19, The section penetrated at the base of the well is beneath the range-front

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CUYAMA BASIN Figure 6. Map of the Cuyama Basin showing data 0 Ml 6 1 1 1 localities. Well sections: 2, Richfield Wegis-Reyes B-l; 0 KM 10 3, Richfield Lundstrom Becher 1; 4, Mohawk Humble 9 WELL OR * SURFACE Lundstrom 48-2; 5, Richfield James 1; 6, Richfield Per- SECTION kins 33-35; 7, Richfield Perkins 1; 8, Richfield Perkins OUTCROPS OF MIDDLE 33-26; 9, Richfield Schaeffer 1; 10, Seaboard Kirsch- CENOZOIC enmann 1; 11, Ohio Kirschenmann 1; 15, ARCO ROCKS REVERSE Federal Caliente Unit number 1. Outcrop sections: 12, FAULT Whiterock Bluff section; 13, Post Canyon section; 14, Head of Morales Canyon section. Also shown is location of facies cross section A-A' which is illustrated in Figure 14. Vertically ruled pattern represents outcrops of middle Cenozoic rocks. -MAP . AREA x tra

RNF

thrust faults of the Caliente Range and is depositionally continuous with 1976; Hill and others, 1958). These biofacies indicate that the Saltos Shale similar sections beneath Cuyama Valley. is a deep-marine, basinal deposit.

Biofacies Lithofacies

Benthic foraminiferal biofacies in the ARCO Federal C.U. number 1 In the discussions below, the data from the ARCO Federal C.U. indicate middle bathyal water depths for the cored interval (Fig. 7). Diag- number 1 will be mentioned first, then information from more widely nostic bathyal s]Decies include "Siphogenerina" transversa, Uvigerinella scattered samples in other Saltos Shale sections similar to the ARCO obesa impolita, Bulimina inflata alligata, Stilostomella advena, Bolivina Federal C.U. number 1 will be covered. Finally, data from Saltos Shale floridana, and Plectofrodicurlaria sp. Similar middle to upper bathyal sections representing different lithofacies will be summarized. biofacies are widespread in the Saltos Shale (Lagoe, 1981, 1982; Phillips, Lithology. The ARCO Federal C.U. number 1 cores are composed mainly of interbedded sandstone, mudstone, and impure carbonat e rocks. The interbedding is of very fine scale, often approaching interlamination. This thin interbedding produces a distinctively subdued spontaneous potential and resistivity signature on electric logs (Fig. 7). PALEOBATHYMETRY CORED INTERVAL S

Figure 8. Mudstones from the Sal- tos Shale in the ARCO Federal C.U. number 1. A, Interlaminated brown, foraminiferal mudstone and gray, non- calcareous mudstone; B, Fractured dolostone with calcite vein fillings; C, Thin interbedded sandstones and brown, foraminiferal mudstone. Note sharp and often irregular bases of sandstone beds. D, Oasts of brown, foraminiferal mudstone in sandstone. Scale bars on all photos are 5 cm long (2 in.).

The carbonate fraction in some examples of the gray-brown forami- "base-missing Bouma sequences" (Mutti and Ricci Lucchi, 1972). Clearly, niferal mudstone is predominant, and these rocks are actually impure most of the sandstones examined are the product of turbidity currents. carbonates. In some samples, the carbonate component has been dolomit- The mudstones contain fewer types of sedimentary structures which ized, and the dolomitized rock was pure enough to fracture brittlely are also less easily seen in hand samples. As mentioned above, foraminifera (Fig. 8). Rocks assigned to this foraminiferal mud type obviously include are often oriented parallel to bedding, giving the rock an indistinct, lami- both true mudstones and some limestones or dolostones which intergrade nated fabric. In addition, some gray mudstone layers and laminae are with each other. These gradations are often difficult to distinguish in hand graded, their coarser silt grains concentrated at the base of layers. The samples and, for the purposes of this study, are not differentiated. different mudstone types are commonly thinly interbedded to interlami- Sedimentary Structures. Both sandstones and mudstones contain a nated with sharp contacts (Fig. 8). Where mudstone and sandstone are variety of sedimentary structures which can be used to infer depositional interbedded, some of the mudstone has been deformed by the overlying processes. sandstone beds to form flame structures. This deformation suggests rapid The sandstones show the widest variety of sedimentary structures. deposition of the sandstone and resulting soft-sediment deformation due to The most common are various types of current lamination. These include a density inversion. Mudstone also occurs in the interbedded sandstones as parallel lamination, ripple cross-lamination, wavy lamination, wispy lami- discrete clasts (Fig. 8), illustrating the erosive action of the sands on a nation, and convolute lamination (Fig. 9). The sandstones often have semiconsolidated to consolidated muddy substratum. Again, there is little irregular (scoured or loaded?) bases. Less common structures include evidence of extensive bioturbation in the mudstone, although some thin graded bedding, massive beds, and sandstone dikes. There is little or no intervals of brown mudstone do not show an oriented fabric. evidence of bioturbation in the sandstones. The lithology and sedimentary structures in the ARCO Federal C.U. Most of these structures indicate that the sandstones are the product number 1 cores were plotted on detailed graphic logs (see Lagoe, 1982). of rapid deposition by a variety of processes including turbulent flow, Contact relationships between the various lithologies were also recorded. traction, and settling from suspension. The benthic foraminiferal biofacies Graphic logs of this continuously cored interval permit certain bedding in the interbedded mudstones point to a bathyal depositional site, hence characteristics to be calculated as well. some sort of gravity process is responsible for the deposition of these rocks. The thickness of sandstone beds is an important attribute for defining Some sequences of sedimentary structures in the cores fit the Bouma lithofacies (Fig. 10). All sandstone beds thicker than 1.25 cm (0.5 in.) were sequence (Bouma, 1962) of classic turbidites. Most, however, lack the counted, 186 in all. The resulting data show that most of the sandstones in basal A division, massive or graded sandstone, and can be referred to as the ARCO Federal C.U. number 1 cores are thin to very thinly bedded.

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Figure 9. Sandstones from the Saltos Shale in the ARCO Federal C.U. number 1. A, Wavy to ripple cross-laminated sandstone; B, Par- allel laminated sandstone. Prominent black laminations contain abundant black, carbonaceous grains; C, Flame structures at the base of a parallel laminated sandstone bed; D, Graded bedding; E, Mi ssive sandstone with sharp irregular base overlain by wavy laminated ¡sandstone (includes Bouma divisions a, b, and c); F, Convolute bedding. Scale bars on all photos are 5 cm long (2 in.).

SAND % o 20 40 60 DEPTH SAND %/CORE DRILLED (FT) T CORE 3 41% 11,400-

CORE 4 11,420—

CORE SECTION AVERAGE

CORE 5

SAND % OF ENTIRE INTERVAL

4 3 %

CORE 6

CORE 7

CORE 8

Figure 10. Histogram of sandstone bed thickness for 1 cored interval of the Saltos Shale in the ARCO Federal C.U. Figure 11. Graph of percentage of sandstone beds in the cored number 1. All sandstone beds interval of the Saltos Shale in the ARCO Federal C.U. number 1. Dots thicker than 1.25 cm (0.5 in.) represent values for approximately 1 m (3 ft) of section. Verticiil bars were counted. are average for individual cores.

The vast majority of these beds are < 15 cm (6 in.) thick. The percentage of sandstone beds in each core can be compiled from the bed thickness data (Fig. 11). Percentages vary from 12% to 64% sandstone in the cores, with an average of 43% for the entire cored interval. The sand-to-shale ratio thus is near 1:1 for this part of the Saltos Shale. Lithofacies. The rocks in the ARCO Federal C.U. number cores are thinly interbedded, very fine to fine-grained sandstone and mudstone which were deposited at bathyal depths. As mentioned above, Mutti and Ricci Lucchi (1972) presented a lithofacies classification applicable to BED THICKNESI5 (INCHES) these rocks. According to this classification, deep-water sandstones and

1X - Q. «z PALEO- HI _ to z C < C/> Q t- X UJ BATHYMETRY <«E>- w OÜ o <3 ENVIRON- HOE z oc o < Ex m MENTS «V/O zee DC X O a. 1- ZU) s < ca OC CO UJ UJ s a> o N oc © — UJ UJ ö o -J 1- a> z < ra Ul BASIN o X c s OC s PLAIN BASALT 05 o cn 5 O Iriterbedded MUDSTONE, brown, 1- _i foraminiferal; SILTSTONE, grey, < sandy; and SANDSTONE, light brown, CO > medium to very fine-fine grained, sandstone beds have sharp bases, oc z I some sole marks, some graded beds.

< « JCESI A N UJ CO s DC < (ß u. CD o UJ SHELF S ï E o CO 3 U. 3 o OC oc a < O n UJ UJ co 3 1- o z >< I Figure 12. Summary of lithofacies and benthic foraminiferal biofacies in the Richfield Schaefter number 1. Abbreviations for Figure 13. Stratigraphie and lithofacies summary for the Head of paleobathymetry column: NM, Nonmarine; SH, Shelf; UB, Upper Morales Canyon section. Lithologie descriptions and lithofacies de- Bathyal; MB, Middle Bathyal. Note subdued to serrate electric log terminations are from this study. General lithologie column modified response for majority of Saltos Shale. See Lagoe (1981) for additional from Phillips (1976). Location of section shown in Figure 6. Addi- information on this well. See Figure 6 for location of well. tional detailed information on this section available in Lagoe (1982).

mudstones are subdivided into seven lithofacies. These lithofacies are Lithofacies variations within the Saltos Shale consist primarily of the based on lithology, sedimentary structures, and bedding characteristics. bedding characteristics (thickness, sedimentary structures) and the amount Their facies D includes very fine to fine-grained sandstone interbedded of the interbedded sandstone. The variations are most prominent in the with mudstone. The sandstones typically contain thin current laminae of eastern part of the basin, beneath Cuyama Valley. The Saltos Shale be- various sorts as the dominant sedimentary structure. "Base-missing Bouma comes sandier to the east, possessing thicker units of sandstone interbedded sequences" (beginning with the B division) are commonly found, and the with decreasing amount of mudstone (Fig. 14). Farther to the east, the sandstones tend to be very thinly bedded. Sand-to-shale ratios are low, sands become amalgamated into thick units (Fig. 14), although the mud- commonly near one. The rocks examined in the ARCO Federal C.U. stone interbeds continue to contain bathyal foraminifera. The lithofacies number 1 cores closely fit this description and belong to facies D/E of the variations thus are not a function of significant shallowing to the east. modified classification used here (Fig. 3). The east-west cross section beneath Cuyama Valley (Fig. 14) can be Similar lithofacies D/E rocks are found in samples from other described using the modified Mutti and Ricci Lucchi classification. Facies Cuyama Valley wells where samples are more scattered than in the ARCO D/E is predominant in the west and interfingers with facies C to the east. Federal C.U. number 1 (Fig. 12; see also Lagoe, 1981). Even though the Facies C, in turn, grades into sections with facies A/B. The eastward sampling is not continuous, the same lithologies and sedimentary struc- traverse eventually encounters equivalent sections of entirely shallow- tures are found. Electric log responses in these wells are also similar— marine to nonmarine rocks. a subdued spontaneous potential due to the mudstone and the silty, thin- bedded sandstones (Figs. 7 and 12). A review of wells in the Cuyama Depositional Environments Basin shows that this lithofacies is widespread. Poorly exposed Saltos Shale outcrops examined in the area west of Mutti and Ricci Lucchi (1972) and Walker (1978) demonstrated Caliente Mountain (Fig. 13) can be classified as largely facies D/E. In how lithofacies form associations that can be interpreted with respect to a some outcrops, turbidite sandstone beds are quite prominent. submarine-fan model. The more sand-rich lithofacies of the Saltos Shale

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can be interpreted in this way when combined with paleobathymetric the Cox fault zone suggest some topographic relief on this fault during information from benthic foraminiferal biofacies. Facies A/B represents Saltos Shale deposition (Fig. 15). This would explain the limitation of the inner to upper mid fan where high-density turbidity currents and grain sand-rich facies to the east of the fault. flows deposit predominantly sandy sediments. Facies C and D/E are The two types of mud recognized within facies D/E also reflect products of the middle to outer fan. In addition, the paleobathymetry variations in terrigenous sedimentation and depositional processes. Work inferred from benthic foraminiferal biofacies supports this deep-water on the deposition of Holocene, deep-water mud (for example, Rupke and submarine-fan interpretation. Similar facies patterns (Fig. 15) are recog- Stanley, 1974; Stanley and Maldanado, 1981; Stow, 1981; Stow and nized in the Holocene deposits of the modern California borderland (Gors- Shanmugam, 1980) often recognizes two major mud types analogous to line and Emery, 1959; Gorsline, 1978, 1980, 1981). those found in this study. Rupke and Stanley (1974) defined type A mud Facies A/B a nd C are formed in environments in which terrigenous as being fairly well sorted, composed primarily of terrigenous silt and clay, sedimentation waii dominant. Facies D/E represents a mixture of coarse and often graded. It is also normally carbonate-poor. They considered this terrigenous ("distal turbidites") and fine terrigenous/biogenous ("hemipe- sediment as being the product of dilute turbidity currents—either the tail of lagic") sedimentation. This latter facies is present not only in outer fan a normal density current or a low-density, nepheloid-type current (com- environments but also in basin plains unassociated with submarine fans in pare Moore, 1969). The gray, barren mudstone of this study is also pre- the strict sense. This accounts for the more widespread extent of this facies dominantly terrigenous, fairly well sorted, and sometimes graded, making which is dominanl. in the type section of the Saltos Shale. it analogous to Rupke and Stanley's type A mud. The more sand-rich, submarine-fan facies in the Saltos Shale are Type B mud was defined by Rupke and Stanley (1974) as being restricted mainly to the southeastern end of the Cuyama Basin, now buried more poorly sorted, carbonate-rich, and abundantly microfossilrerous. beneath the eastern end of Cuyama Valley. This was an area of major They interpreted this mud as being the product of hemipelagic processes— terrigenous sedime nt input during early Miocene and strongly indicates the pelagic and benthic biogenous sedimentation mixed with some terrigenous existence of a river system or confluence of longshore sand transport mud from dilute turbidity currents. Type B mud is analogous to the brown, associated with a submarine canyon to supply the massive amounts of foraminiferal mudstone of this study. sand contained in these sand-rich facies. The rapid facies changes across The different mud types reflect the interplay of biogenous and terrig-

Figure 14. East-west-oriented lithofacies cross section A-A' beneath Cuyama Valley. See Figure 6 for location of section. Numbers above wells correspond to identification numbers in Figure 6. Also shown is the interpretation of depositional environments with respect to a submarine-fan model: 1, Basin plain; 2, Fan fringe; 3, Outer to middle fan; 4, Inner fan; 5, Shallow-marine environments. Lithofacies E>/E, C, and A/B all contain bathyal foraminifera in mudstone interbeds (see Lagoe, 1982, for details).

enous sedimentation within the Cuyama Basin. Nearly all the biogenous comes from outcrop sections in the Caliente Range (Fig. 6). Key sections sedimentation in the Saltos Shale is calcareous (planktic and benthic fo- include the type section at Whiterock Bluff and the section at the head of raminifera plus calcareous nannoplankton). Within the overlying White- Morales Canyon. rock Bluff Shale, this biogenic component changes in nature and becomes more important. Biofacies

THE WHITEROCK BLUFF SHALE Phillips (1976) studied the foraminiferal fauna in the Whiterock Bluff Shale in detail. He interpreted the fauna as being outer neritic in depth In contrast to the preceding section, most of the information studied (< 180 m; 600 ft). An analysis of his data and samples collected for this from the Whiterock Bluff Shale Member of the Monterey Formation study suggest that an upper bathyal depth is more appropriate for these faunas. Specifically, the presence of Epistominella subperuviana, Bolivina imbricata, Bolivina túmida, Bolivina salinasensis, and, in the lowermost part of the member, Bolivina floridana indicates depths in excess of 150-180 m (500-600 ft). The benthic foraminifera from this unit are Relizian in the lowermost part and Luisian (see Fig. 16) in most of the member (Phillips, 1976; Hill and others, 1958). Diatom data from the upper part of the member (Fig. 16) correlate with assemblages from high in the type section of the Luisian stage (H. E. Harper, 1981, personal commun.; Poore and others, 1981).

Lithofacies

Lithology. The type section of the Whiterock Bluff Shale was described by Hill and others (1958) as mainly siliceous shale and diatomaceous shale. An analysis of the rocks in this section shows this to be only partly correct. The section is composed of thinly bedded, white to Figure 15 A. Distribution of sandstone and mudstone across a light buff-colored shale. Most of the samples collected from this section submarine canyon-fan system in the modern California borderland contain abundant foraminifera and are very calcareous. Diatoms are con- (after Gorsline and Emery, 1959). spicuous in hand samples only in the upper part of the section. A suite of

Figure 15B. Analogous cross section representing submarine-fan deposits in the Saltos Shale of Cuyama Valley, showing depositional trends and typical electric log responses in various facies of the fan. Examples of electric log responses come from (A) Mohawk Humble Lundstrom 48-2, (B) ROCO Perkins 1, (C) ROCO James 1, and (D) ROCO Schaeffer 1.

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zUi O UJ PALEO- Oco N O < 111 < BATHYMETRY SPLS. 00 AAA/V — OC O 3 KEY SPECIES Ej: t-: N CO CO c « 1 o a I I — L. -80 - 79 O O \ O a. co O < t/5 AAAV, H < O — 77 => O X z O < a. TUFFACEOUS OR ASHY BEDS-» z oc co co o z < CO S 111 LU o — 70 (O O OC Q. UJ -67 o O < DIATOMACEOUS UJ => oc co a X 3 AND CALCAR- a. _l - 63 o O O I- >- EO JS MUDST0NE z o UJ o -59 o z I- o< '56 55 \7 e I LU < < < Z o 3 < < X VERY CALCAR- Ui a. co co co co EOUS MUDSTONE; 3 co O < ~ COMMON FORAM- co -J a. < oc S INIFERAL MICRO- — 44 o < _i oc co COQUINA 3 o 00 O * >- CL co < o UJ o -34 oc Ui I-

5 -24 DOLOM ITI C BEDS-

-16 m

100 -I

-6 FT. N _l UJ - 1 OC 0 -1 XAFTER BARRON(198Û) xx AFTER K LE IN PELL (l338)

Figure 16. Stratigraphie and paléontologie summary of the type section of the Whiterock Bluff Shale Member, Monterey Formation. Lithologie column and descriptions, foraminiferal data from this study. Diatom data provided by H. E. Harper, ARCO Exploration. See Figure 6 for Ideation of section.

samples analyzed for diatoms by H. E. Harper, ARCO Exploration, con- nannoplankton) increases toward some slopes removed from sources of firms this field observation (Fig. 16). Diatoms are present only in the upper terrigenous sedimentation. Basin-plain deposits also show an increase in third of the section and quickly become less abundant and more poorly carbonate content away from the submarine fans which introduce ter- preserved downsection before disappearing entirely. The oldest sample rigenous sediment into the basin (Gorsline, 1980, 1981). The depth of containing diatoms belongs to the Denticulenopsis lauta Zone, Subzone A deposition and presence of slump folding in the type area also point to a (~15 m.y. B.P., according to Keller and Barron, 1981). slope environment in this area. A basin-plain environment is likely for this The lower part of the Whiterock Bluff section consists primarily of unit in other parts of the basin. In this context, the microcoquina layers calcareous mudstone and shale with minor interbedded, hard, buff, dolo- may reflect winnowing by deep tidal or storm currents or possibly by mitic layers. Foraminifera are extremely abundant and are often concen- contour currents (Stow and Lovell, 1979). trated in thin layers constituting a microcoquina. These microcoquinas suggest some sort of winnowing process to concentrate the microfossils. In DEPOSITIONAL RELATIONSHIPS the upper part of the section, the calcareous shale is gradually replaced by diatomaceous shale (Fig. 16). The diatoms are preserved best and were The Monterey Formation in the Cuyama Basin is the product of most abundant in the uppermost part of the section. Interbedded with several depositional processes. These processes were controlled by both these siliceous rocks, there are several, generally thin (<1 ft), ashy and local and regional-to-global factors. The nature of the sedimentary deposits tuffaceous layers. Although not present in the type section, some minor at any one place is dependent on the interplay of biogenous and terrige- lithologies occurring in this member include rare, thin sandstones and rare nous sedimentation. These processes varied both spatially (for example, pelletal phosphate. In some areas of the Cuyama Basin, the siliceous rocks proximity to a submarine fan) and temporally (for example, the change in have undergone diagenesis and have converted to siliceous shale and the biogenous component from calcareous to siliceous in the Whiterock porcelanite. Bluff Shale). Most of the rocks in the Whiterock Bluff section have a significant The clastic facies recognized by Clifton (1981) in nearshore rocks biogenous component. These lithologies are typical of the other Whiterock equivalent to the Monterey Formation complements the offshore facies Bluff Shale sections and outcrops studied, including the Head of Morales described above and completes the general picture of deposition in the Canyon section (Fig. 13). A conspicuous aspect of this member is the Cuyama Basin. A generalized model summarizing the manner in which nature of the biogenous component which changes from calcareous in the the terrigenous and biogenous processes interact suggests how the diversity lower part and becomes increasingly siliceous upsection. of lithofacies was produced (Fig. 17). Sedimentary Structures. Rare sandstones in the Whiterock Bluff The controlling factors can be either local or regional-to-global in Shale contain sedimentary structures like those found in facies D/E of the scope. Controls on the magnitude and distribution of terrigenous sedimen- Saltos Shale. These thin sandstones exhibit parallel, convolute, and ripple tation can include basin geometry, nature of sediment source areas, cli- cross-lamination and can be classed as base-missing Bouma sequences mate, and relative sea level. Basin geometry and paleogeography are the (Mutti and Ricci Lucchi, 1972). products of local tectonics, although the local tectonics may be part of a The fine-grained rocks within this unit contain more subtle structures. more regional pattern. The formation of the Cuyama Basin resulted from The most apparent feature of the fine-grained rocks is the even, thin subsidence associated with the Oligocene impingement of the Pacific and bedding. The rocks are not distinctly laminated, as are many Monterey North American plates and the subsequent development of a translational Formation intervals in other basins, but such laminated and bedded fabric margin (Atwater and Molnar, 1973; Blake and others, 1978; Graham, as is observed is not disturbed by burrowing. The most distinctive lamina- 1978; Ingle, 1980). Patterns of subsidence and locations of submarine tions present are the thin, microcoquina layers of benthic foraminifera. canyons or major rivers (Lagoe, 1982, 1984) were affected by more local These layers commonly occur in the lower, calcareous part of the member factors. Other local controls include the inboard position of the basin and are increasingly rare upsection. The layers consist of the same species within the Miocene borderland and the presence and nature of uplifted as are found in the enclosing sediments, suggesting a winnowing process source terranes immediately to the east (Clifton, 1968, 1981). for their formation rather than a downslope sediment-flow process. This The inboard position of the basin puts it adjacent to major inputs of latter process would introduce mainly shallower-dwelling species into the terrigenous sediment, and uplifted source terranes insure an abundant layers. Another distinctive sedimentary structure found in the Whiterock supply of coarse clastics. Biogenous and terrigenous sedimentation reflects Bluff Shale is rare slump folding. This reflects soft-sediment deformation of global controls such as climate, paleo-oceanography, and sea level. Cli- mud deposited on a slope. matic factors such as aridity and seasonality of rainfall affect the amount Lithofacies. The Whiterock Bluff Shale Member is composed pri- and time of terrigenous sediment input into the basin. The seasonality of marily of very calcareous to diatomaceous shale and mudstone. The high rainfall is evident in the modern borderland off southern California and the biogenic component in the Whiterock Bluff Shale suggests that these de- Gulf of California where the distinct rainy season plays a major role in the posits primarily belong in facies G of Mutti and Ricci Lucchi formation of varved, slope and basin-plain sediments (Calvert, 1964, (1972)—hemipelagic shales and marls (see Fig. 3). These deposits are a 1966; Donegan and Schrader, 1981; Soutar and others, 1981). These mixture of biogenous sediment and terrigenous mud, the latter being de- varves consist of dark, terrigenous-rich laminae deposited during the rainy posited by dilute turbidity flows. Some interbedding of facies D/E season, and lighter, biogenous-rich laminae, deposited during the dry sea- accounts for the more terrigenous shales and rare sandstones found within son. Such varved laminations are commonly found in the Monterey For- the member. mation of the outboard Santa Barbara and Santa Maria areas. The large amount of terrigenous sediment in the inboard Cuyama Basin tends to Depositional Environments obscure such varves. Sea level is a global factor of major importance. The Cuyama Basin The lithofacies and paleobathymetry within the Whiterock Bluff was actively accumulating sediment during a major change in sea level Shale point to either a slope or a basin-plain environment. Studies of during the Miocene (Fig. 18). Relatively low sea level existed during the Holocene sediments in the modern borderland off southern California early Miocene (Vail and others, 1977; Vail and Hardenbol, 1979). Rela- show that the amount of carbonate (largely foraminifera and calcareous tively low sea level allowed easier access of coarse terrigenous sediment to

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DEPOSITIONAL MODEL CUYAMA BASIN

Fields are qualitative Bc= Rate of calcareous biogenous sedimentation = Bs Rate of siliceous biogenous sedimentation

Tc = Rate of coarse terrigenous sedimentation

Tf = Rate of fine terrigenous sedimentation

SALTOS SHALE WHITEROCK BLUFF SHALE

BIOGENOUS SEDIMENTATION CONTROLS: PALE00CEAN0GRAPHY- UPWELLING, CURRENT PATTERNS, WATERMASSES CLIMATE PALEOGEOGRAPHY

Settling. deep currents

BASIN PLAIN SUBMARINE FAN

Figure 17. Depositional model for the Monterey Formation, Cuyama Basin, California. Ternary diagrams are meant to show general relationships, not quantitative analyses.

basinal areas and coincident dilution of biogenous sedimentation. This at high nutrient levels, to calcareous nannoplankton (Garrison, 1981), this increase in terrigenous sedimentation helps to explain the development of change in biogenous sedimentation probably reflects an intensification of a submarine fan in the eastern Cuyama Basin during the Saucesian and upwelling at this location in the Miocene borderland. The magnitude of Relizian. Sea-level rise to a maximum during the middle Miocene tended biogenous silica accumulation in the borderland is not evenly distributed to preferentially accumulate terrigenous sediment in nonmarine and in time. Isaacs (1983) demonstrated an increase in silica accumulation in shallow-marine environments of nearshore basins (for example, Swift, the more outboard Santa Barbara Basin at — 11 m.y. B.P., much later than 1974), thus starving basinal areas and outboard basins of terrigenous de- in Cuyama. There is, however, a progressive shift from calcareous sedi- bris. This explains the general lack of submarine-fan facies in the White- ments in the lower Monterey Formation to siliceous sediments in the rock Bluff Shale and the thick accumulation of middle Miocene shelf and upper part of the formation (Isaacs, 1983; Pisciotto and Garrison, 1981). nonmarine rocks in the Cuyama Basin (Clifton, 1981; Vedder, 1973; These relationships most likely mean some regional or global change in the Repenning and Vedder, 1961). The inboard location of the Cuyama Basin, nature of primary productivity (see, for instance, Keller and Barron, 1983), with its access tc major sources of terrigenous sediment, makes it an upon which, local variations in the distribution of upwelling and amount excellent recorder of sea-level fluctuations. This basin was a major sedi- of primary productivity were superimposed. This latter relationship would ment trap for terrigenous debris during both low stands (as submarine-fan be analogous to conditions in the modern borderland off southern Califor- deposits) and high stands (as nonmarine and shallow-marine deposits) of nia (Douglas, 1981; Gorsline, 1981). sea level. The changing nature of biogenous sediment in the Monterey Forma- MIOCENE DEPOSITIONAL HISTORY tion during the Luisian reflects a change in the type of primary productiv- OF THE CUYAMA BASIN ity dominating ths Miocene borderland. The change from calcareous to siliceous sedimentation in the Cuyama Basin occurred at ~ 15 m.y. B.P. The The depositional history of the Cuyama Basin during the early and dominant primary producer changed from calcareous nannoplankton to middle Miocene can be reconstructed from the discussion above and from siliceous diatoms at this time. Because diatoms are competitively superior, regional stratigraphic relationships (Lagoe, 1981, 1982, 1984). This his-

Pacific Sea Level Paleo- Ocean [meters! SE ,8 8 0 Cuyama bathymetry Q. ° o ° 2 >00- o o O O Ei .o Stages Basin E Caliente Figure 18. Cuyama Basin stratigraphy as it Fm. 8H LU relates to relative global sea level and oxygen- H < Santa isotope record. Isotope curve from Woodruff 10 Mohnian and others (1981); sea level from Vail and others Margarita UJ -- (1977) and Vail and Hardenbol (1979); Califor- 12 Fm. nia benthic foraminiferal stages from Kleinpell Z LU P P PP (1938). UJ O 14 a O Luisian 16 O Relizian 18 S >- -I CC Saucesian 20 < Vaqueros UJ Fm.

PPP phosphatic rocks

TABLE 1. DEPOSITIONAL HISTORY OF THE CUYAMA BASIN

Stage* Subsidence Sea level* Biogenous Local tectonics Depositional environment sedimentation

Early Mohnian Slow Relatively Calcareous and Cessation of extension. Movement Mainly shallow marine in Cuyama Basin; (Il to 14 m.y. B.P.) high siliceous of San Andreas fault system some phosphatic deposition in shelf-edge areas. accelerates.

Luisian Slow Relatively Calcareous and Slowing or cessation of extension. Slope and basin-plain environments predominant; (14 to 16 m.y. B.P.) high siliceous very biogenic sediments. Change from calcareous to siliceous at -15 m.y. B.P. Submarine fans starved.

Relizian Rapid Relatively Calcareous Continued extension. Maximum development of submarine fan in (16 to 18 m.y. B.P.) low eastern part of basin; basin plain also prominent.

Late Saucesian Rapid Relatively Calcareous Extension, initiation of movement Basin plain predominates; incipient development low on some local faults and anticlines, of submarine fan in eastern part of basin.

•Of Kleinpell (1938,1980). tFrom Vail and others ( 1977).

tory (Table 1) reflects the interplay of local and regional/global controls will be illustrated by comparing the Cuyama Basin with the Santa Barbara just summarized. Patterns of subsidence (Lagoe, 1984) and relative sea Basin (Fig. 19). This comparison is made with reference to three time level play a major role in the distribution of lithofacies. Local variations in intervals—Relizian (early Miocene), late Luisian (middle Miocene), and depositional patterns can be due to such local tectonic controls as contem- late Mohnian (late Miocene). A more detailed comparison of inboard poraneous growth on anticlines (Lagoe, 1981) and topographic relief on versus outboard basins, involving the Cuyama, Santa Maria, and Santa fault scarps (see earlier discussion of Saltos Shale submarine-fan facies). Barbara Basins, is presented elsewhere (Lagoe, in press). The Santa Bar- This depositional history can now be compared to contemporaneous ba- bara Basin provides excellent opportunities for comparison because of the sins in the Miocene borderland. detailed studies on the Monterey Formation by Isaacs (1980, 1981a, 1981b, 1981c, 1981d, 1983). These works present a detailed view of COMPARISON OF INBOARD AND OUTBOARD BASINS Monterey Formation deposition in an outboard basin. The Monterey Formation is well exposed along the coast near Santa The Cuyama Basin was only one locus of deposition for the Monte- Barbara. The formation in this area is subdivided into five informal rey Formation within the Miocene borderland of California. This paleo- members (Fig. 19). The following brief descriptions are abstracted from geographic regime provided numerous depositional basins in which this Isaacs (1983). The lower calcareous-siliceous member (late Saucesian- formation accumulated. Some, like the Cuyama Basin, were adjacent to early Luisian) is composed of thick-bedded to massive, calcareous mud- the primary Miocene strandline. Others, such as the Santa Maria and stone and siliceous rocks. The overlying carbonaceous marl member (late Santa Barbara areas, were seaward of these inboard areas and were Luisian-early Mohnian) consists of irregularly laminated, organic-rich blocked from receiving as much terrigenous sediment by intervening bath- mudstone which is commonly phosphatic. This member is overlain by the ymétrie highs and/or basins. The position of a basin within the border- transitional marl-siliceous member and upper calcareous-siliceous member land affected the nature of its basinal sediments. A brief comparison of two (early late Mohnian). These members contain a heterogeneous mix of contrasting basins will highlight these effects. calcareous, siliceous, and organic-phosphatic mudstones which are com- The stratigraphie differences between inboard and outboard basins monly laminated. The uppermost unit of the Monterey Formation is the

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clayey-siliceous member (latest Mohnian-early Delmontian). This mem- Epoch Stage Santa Barbara Cuyama ber consists of laminated to massive siliceous mudstone, porcelanite, chert, Clayey-Siliceous diatomaceous mudstone, and diatomite. UJ Mbr. Santa Isaacs (1983) provided detailed information on bulk-accumulation t— Upper Calc.-Sil.Mbr. rates for these uniis and their various lithologic components (carbonate Mohnian Margarita Trans. Marl - Si Mbr. minerals, detrital minerals, silica, organic matter). These values for the 1. Fm. three time interval:! of interest are given in Table 2. These data illustrate important variations in the composition of deposition within the Monterey Carbonaceous Formation of the Santa Barbara Basin. These variations can be compared UJ LL_ Whiterock j with coeval sediments in the Cuyama Basin. Most of these comparisons Marl Mbr. Bluff a Luisian will be qualitative, due to the lack of quantitative lithologic analyses in the E OD E Shale Cuyama Basin. LU Od Ll_ Mbr. Relizian (Late Early Miocene). The lower calcareous-siliceous C Lower O member is dominated by highly biogenous sediments. Bulk-accumulation O Calcareous- o> 03 rates indicate subequal amounts of biogenous silica and carbonate (Table Siliceous Saltos >- Relizian 2). This member contains benthic foraminifera, indicating middle bathyal J — Mbr. Q Shale water depths (Isaacs, 1980; Lagoe, unpub. data) and representing hemipe- as < Mbr. lagic deposition (Isaacs, 1983). There are no significant accumulations of UJ coarse clastic sediments in the coastal outcrops of this member. Saucesian The Relizian part of the Monterey Formation in the Cuyama Basin Rincón Fm. ^J/a^ueros^ Fm. represents two major depositional settings—basin-plain environments Isaacs(1983) Lagoe (1984) (primarily facies D/E and G) and submarine-fan environments (combina- tions of facies D/E, C, and A/B). The basin-plain rocks are similar to Figure 19. Comparative stratigraphy of the Santa Barbara and some of the lithologies in the lower calcareous-siliceous member in the Cuyama Basins. Santa Barbara stratigraphy from Isaacs (1983). Cross Santa Barbara area and have similar bulk-accumulation rates (Table 3). hatching represents time intervals compared between the two bsisins. The deep-marine coarse clastics found within the Relizian part of the Monterey Formation in the Cuyama Basin are not found in the Santa Barbara coastal area. Bulk-accumulation rates for these Cuyama phosphatic blebs and laminations in many of these mudstones is particu- submarine-fan lithofacies are also higher than those recorded for the Reliz- larly informative about the depositional environment. ian in Santa Barbara (Tables 2 and 3). These submarine-fan deposits occur Studies of modern marine environments in which phosphatic miner- in the Cuyama Basin because of that basin's inboard position and access to als are accumulating indicate that low sedimentation rates and nutrient- coarse clastic sediments during low stands of sea level. The Santa Barbara rich waters are prerequisites for this type of authigenic sedimentation area, being outboard of the Cuyama Basin, was isolated from coarse clastic (Burnett, 1977; Pisciotto, 1978; Burnett and others, 1980). This modern sedimentation. Hig;h bulk-accumulation rates (Table 2) do indicate that relationship suggests that the phosphatic, organic-rich mudstones in the outboard areas did receive larger amounts of fine-grained sediment during Santa Barbara Basin are the products of slow sedimentation in association low stands of sea level. with high organic productivity. This interpretation is supported by the Late Luisian (Middle Miocene). The late Luisian in the Santa Bar- lower bulk-accumulation rates in this unit. Less common lithologies within bara coastal area includes the lower part of the carbonaceous marl the carbonaceous marl member (for example, impure limestone, calcare- member of the Monterey Formation (Fig. 19; Isaacs, 1983). This distinc- ous porcelanite, dolostone) also indicate low terrigenous input. tive unit is composed of organic-rich, calcareous mudstone which is phos- Luisian foraminifera from the carbonaceous marl member indicate phatic in many places. These rocks tend to be relatively low in detrital middle bathyal depths (500-2,000 m; 1,650-6,600 ft). Characteristic spe- minerals and high in biogenous, organic, and authigenic components cies include Anomalina salinasensis, "Siphogenerina" collomi, Piillenia (Table 2). Bulk-accumulation rates are much lower than during the Reliz- miocenica, and Stilostomella advena. ian, reflecting relatively high sea level and sediment-starved conditions The late Luisian in the Cuyama Basin is represented by the upper part within outboard basins of the Miocene borderland. The abundance of of the Whiterock Bluff Shale Member of the Monterey Formation. As shown above, this unit consists largely of calcareous mudstone and diato-

TABLE 2. BULK-ACCUMULATION RATES FROM THE SANTA BARBARA BASIN

TABLE 3. PRELIMINARY BULK-ACCUMULATION RATES FOR THE CUYAMA BASIN Relizian Late Luisian Late Mohnian

Total bulk-accumulation Time interval Facies Bulk- rate (m/m.y.) 180-370 110-140 96-500 accumulation rate (m/m.y.) Detrital accumulation 3 rate (g/cmVlO yr) 0.9-1.7 0.6-0.7 0.9-4.4 Luisian Slope/Basin plain 100 to 130 Relizian Basin plain 125 to >150 Carbonate accumulation Relizian Submarine fan >400 rale (g/cm2/103 yr) 1.5-2.8 1.2-1.5 0.0-0.1

Silica accumulation Note: accumulation rates based on a limited number of sections. Luisian rates based on type Luisian section (aoore and rate (g/cmVlO3 yr) 1.8-3.4 0.4-0.5 1.3-6.3 others, 1981), Whiterock Bluff section (Lagoe, 1982), and Head of Morales Canyon section (Lagoe, 1982). Relizian rates based on the Richfield Schaeffer number 01, Richfield James number #1, Richfield Perkins numb;r #33-35 Organic Matter (see Fig. 6 and Lagoe, 1981, for locations), and the Head of Morales Canyon section (Lagoe, 1982). Relizian rates tend to be accumulation rate underestimated because Monterey deposition did not encompass the entire Relizian in many places, and the present (g/cm^ltPyr) 0.3-0.6 0.3-0.4 0.1-0.7 biostratigraphy does not permit further refinement of ages. Estimates should be regarded as minimum rates.

maceous mudstone with minor dolostone, volcanic tuff, and diatomite. nant biogenous component changes, during the middle Miocene, from Notably absent are any phosphatic mudstones such as those in the out- calcareous to siliceous at ~ 15 m.y. B.P., probably reflecting an increase in board area described above. The only phosphatic rocks found in the upwelling. Monterey Formation of the Cuyama Basin are very rare occurrences of 3. Deep-marine deposition ceased in the Cuyama Basin at the end of phosphatic pellets in the upper Whiterock Bluff Shale Member. the middle Miocene (early Mohnian). The lack of phosphatic mudstones suggests that rates of terrigenous A comparison of depositional environments in the Cuyama Basin to sedimentation were higher, or availability of nutrient-rich water, lower, in the more outboard Santa Barbara Basin yields the following conclusions the Cuyama Basin than in the Santa Barbara Basin. Limited data on regarding regional variations in the Miocene borderland. bulk-accumulation rates for this interval in the Cuyama Basin are similar 1. Deposition in the Cuyama Basin differed significantly from that in to those computed for the Santa Barbara area (Tables 2 and 3). The lack of the Santa Barbara Basin. phosphatic mudstones may be due to higher terrigenous sedimentation 2. During the early Miocene, the Santa Barbara coastal area accumu- rates in the Cuyama Basin, even though total bulk accumulation for the lated biogenous hemipelagic deposits but not coarse submarine-fan depos- two areas was comparable. Quantitative lithologic work will be needed to its. The latter were trapped in the Cuyama Basin. substantiate this possibility. 3. Sedimentation, during the middle Miocene, in the Santa Barbara Both inboard and outboard areas of the Miocene borderland expe- Basin consisted of organic-rich, often phosphatic, sediments indicative of rienced marked changes in deposition with the onset of middle Miocene low sedimentation rates. Coeval deposits in Cuyama are also biogenous high sea level. In the Cuyama Basin, coarse clastic submarine-fan deposi- and suggest lowered sedimentation rates but do not contain any significant tion was shut off and succeeded by fine-grained biogenous and terrigenous deep-marine, phosphatic sediments. sedimentation. In the Santa Barbara area, the calcareous/siliceous hemipe- 4. During the late Miocene, the Santa Barbara Basin continued to lagic sedimentation changed to organic-rich and, at times, phosphatic accumulate bathyal, biogenous sediments while the Cuyama Basin was deposition indicative of very low sedimentation rates. Both areas experi- rapidly filling with coarse clastic shallow-marine and nonmarine deposits. enced substantial declines in bulk-accumulation rates (Tables 2 and 3). 5. Despite the differences in lithofacies between the two basins, both Late Mohnian (Late Miocene). Lithofacies were radically different areas show relatively high accumulation rates for deep-marine deposits in in the two basins during the late Mohnian. In the Santa Barbara Basin, this the early Miocene, lower rates in the middle Miocene, and higher rates time interval includes deposition of the clayey-siliceous member of the again in the late Miocene. These rates correlate with variations in global Monterey Formation (Isaacs, 1983). This unit is composed of various sea level, higher rates occurring during low stands and lower rates, during types of siliceous rocks (for example, porcelanite, chert, siliceous mud- high stands. stone, diatomaceous rocks), still indicative of bathyal conditions. Bulk- 6. The differences in lithofacies and depositional environments be- accumulation rates were much higher than during the middle Miocene tween the two areas can be explained partially by the paleogeographic (Table 2), pointing to relatively low sea level during the late Miocene. position of the two basins. Throughout the Miocene, the inboard Cuyama The coeval rocks in the Cuyama Basin are the Santa Margarita Basin acted as a sediment trap, relative to the outboard Santa Barbara Formation (shallow-marine sandstone) and nonmarine rocks of the upper Basin, shielding it, to various degrees, from the diluting effects of terrige- Caliente Formation (Fig. 19). Bulk-accumulation rates are not calculated nous sedimentation. because of the lack of refined age control. The change in deposition from The dramatic variation of depositional environments across the Mio- the middle Miocene is even more dramatic than that occurring in the Santa cene borderland is obviously the result of a variety of controls. Individual Barbara Basin. While that outboard basin was still dominated by fine- lithofacies are products of the interplay between terrigenous and biogenous grained, bathyal deposition, the Cuyama Basin was in the final stages of sedimentation. Terrigenous sedimentation at any one place is controlled by basin-filling. The coarse, shallow-marine and nonmarine clastic sediments local tectonics, basin geometry and paleogeographic position, nature of graphically illustrate the Cuyama Basin's role as a sediment trap, with source areas, and relative sea level. Biogenous sedimentation reflects cli- respect to more outboard basins. Analogous relationships exist in the matic and paleo-oceanographic factors plus local variations in primary modern California borderland where the Los Angeles and Ventura Basins productivity of sediment-forming micro-organisms. The diverse and com- became sediment-filled in the Pleistocene, and the primary strandline in plex stratigraphy of these active margin basins becomes more understand- the modern borderland migrated to the west. able in light of these various controls on regional deposition.

CONCLUDING REMARKS ACKNOWLEDGMENTS

Lithofacies and depositional environments varied dramatically within Many people aided this project. I particularly wish to thank my thesis the Miocene borderland of California. The analysis of depositional envi- advisor, James C. Ingle of Stanford University, and William J. M. Bazeley ronments in the Cuyama Basin reveals the following, concerning of ARCO Exploration Company for their support and guidance. I also intrabasinal variations. thank the following for their aid with various aspects of this work: Steven 1. Early Miocene (late Saucesian and Relizian), deep-marine deposi- A. Graham, Tjeerd van Andel, William Evitt, Cecelia McCloy, Javier tion is predominantly terrigenous and calcareous. Widespread hemipe- Helenes, and Mark Boehm of Stanford University; Paul Lillis, Laura Mer- lagic, basin-plain deposits grade into a variety of coarser clastic rill, Thom Davis, Bob Lewy, and Heather Murphy of ARCO Exploration submarine-fan environments. The distribution of these environments is at Company; and Paula Jefferis and Kris McDougall of the U.S. Geological least partially controlled by basin topography. The formation of these fans Survey. Caroline Isaacs and John Armentrout reviewed the manuscript is due to relatively low sea level and the Cuyama Basin's inboard position, and made numerous suggestions for its improvement. Atlantic Richfield adjacent to the primary Miocene strandline. Company provided samples and financial support for field work. Addi- 2. Middle Miocene (Luisian), deep-marine deposition is dominated tional funding was obtained from the Geological Society of America, the by biogenous, hemipelagic processes. No significant submarine-fan depos- McGee Fund of Stanford University, and National Science Foundation its are found, due to relatively high middle Miocene sea level. The domi- Grant 468. My thanks to all.

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