Seismic stratigraphy and late Quaternary shelf history, south-central Monterey Bay, California

Marine Geology, 81 (1988) 137-157

137

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

SEISMIC STRATIGRAPHY AND LATE QUATERNARY SHELF HISTORY, SOUTH-CENTRAL MONTEREY BAY, CALIFORNIA J O H N L. CHIN 1, H. E D W A R D CLIFTON 1 and H E N R Y T. M U L L I N S 2 U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 (U.S.A.) 2Departrnent of Geology, Syracuse University, Syracuse, N Y 13210 (U.S.A.) (Received July 9, 1987; revised and accepted November 23, 1987)

Abstract Chin, J.L., Clifton, H.E. and Mullins, H.T., 1988. Seismic stratigraphy and late Quaternary shelf history, south-central Monterey Bay, California. Mar. Geol., 81: 137-157. The south-central Monterey Bay shelf is a high-energy, wave-dominated, tectonically active coastal region on the central California continental margin. A prominent feature of this shelf is a sediment lobe off the mouth of the Salinas River t h a t has surface expression. High-resolution seismic-reflection profiles reveal t h a t an angular unconformity (Quaternary?) underlies the entiro shelf and separates undeformed strata above it from deformed strata be]ow it. The Salinas River lobe is a convex bulge on the shelf covering an area of approximately 72 km 2 in water depths from 10 to 90 m. It reaches a maximum thickness of 35 m about 2.5 km seaward of the river mouth and thins in all directions away from this point. Adjacent shelf areas are characterized by only a thin (2 to 5 m thick) and uniform veneer of sediment. Acoustic stratigraphy of the lobe is complex and is characterized by at least three unconformity-bounded depositional sequences. Acoustically, these sequences are relatively well bedded. Acoustic foresets occur within the intermediate sequence and dip seaward at 0.7 ° to 2.0 ° . Comparison with sedimentary sequences in uplifted onshore Pleistocene marine-terrace deposits of the_ Monterey Bay area, which were presumably formed in a similar setting under similar processes, ~uggests t h a t ~ general interpretation can be formulated for seismic stratigraphic patterns. Depositional sequeno~s are irttorpreted to represent shallowing-upwards progradational sequences of marine to nonmarine coastal deposit~ form~d during interglacial highstands and/or during early stages of falling sea |~vel. Acoustic foresets within the intermediate sequence are evidence of seaward progradation. Acoustic unconformities t h a t separate depositional sequences are interpreted as having formed largely by shoreface planation and may be the only record of the intervening transgressions. The internal stratigraphy of the Salinas River lobe thus suggests t h a t at least several late Quaternary regressions and transgressions may be recorded under the present shelf. This record may represent the last major eustatic cycle of sea level, an interval not observed in uplifted onshore Pleistocene marine terraces.

Introduction

This paper addresses the depositional history of a lobate mass of sediment in the southern part of Monterey Bay, central California (Fig.l). Monterey Bay is the largest open embayment along the central California coast and is characterized by a narrow shelf split by the Monterey submarine canyon system. The 0025-3227/88/$03.50

central Califorpi~ coast is an oceanograph i, cally dynan~ic~ high-energy wave-dominated region; it also is geologically dynamic - - in part, a function of its location along an active transform continental margin. The variability in rate and style of wrench tectonics associated width transform continental margins commonly produces complex stratigraphic" packages in shelf and coastal deposits (Mullins et al., 1985).

© 1988 Elsevier Science Publishers B.V.

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Uplifted late Quaternary coastal terraces preserved along the central California margin attest to both tectonic and eustatic fluctuations of sea level in this area during the last one million years of earth history (Weber et al., 1979). At least six raised marine terraces have been documented in the Santa Cruz area, the youngest three of which have been recognized in the central and north-central Monterey Bay region (Dupr6, 1975). Marine and fluvial terraces have also been recognized in the Salinas Valley (Tinsley, 1975; Dupr6 and Tinsley, 1980) and on the Monterey Peninsula (Hart, 1966). Dupr~ (1975) and Tinsley (1975) recognized

other Pleistocene coastal deposits in subsiding regions of central Monterey Bay which provide further evidence of tectonic and eustatic fluctuations the region has undergone. Tectonic and eustatic sea level fluctuations have significantly influenced depositional and erosional processes over the entire Monterey Bay coastal region as well as the preservation of the resulting upper Neogene and Quaternary deposits. The late Quaternary depositional history of the central California shelf remains poorly understood. Moore and Shumway (1959), Moore (1960), and Shepard (1973), among

139 others, characterize the uppermost shelf subbottom (50-60 m) as consisting of a variable but typically thin unconsolidated sediment lens that unconformably overlies older sedimentary strata (usually deformed Tertiary bedrock). In some places this thin sediment lens is a piano-convex mound that is thickest over the central shelf and thins landward and seaward, and elsewhere is a thin tabular sheet (drape) that is fairly uniform across the shelf. The lens is thought to be undeformed and acoustically transparent to bedded on seismic reflection profiles. Moore (1960) suggests that because of the piano-convex geometry of the lens, relict sediment is exposed over the innermost and outer parts of the shelf. Bradley (1957, 1958), Greene (1970), Dupr~ (1975) and Bradley and Griggs (1976) characterize the northern Monterey Bay shelf as having only a thin veneer of sediment over Tertiary bedrock. Locally, the Monterey Bay shelf surface is thought to coincide with an erosional wave-cut platform developed during the Holocene transgression (Bradley and Griggs, 1976). The lobe of sediment off the Salinas River appears to represent an exception to the pattern postulated for that of the northern Monterey Bay shelf. The purposes of this paper are to: (1) delineate and characterize the geometry and internal stratigraphy of the Salinas River lobe and its relation to the adjacent uppermost shelf subbottom, (2) analyze the depositional-erosional history of the uppermost shelf subbottom (50-60 m) during the late Quaternary, and (3) relate the history of sedimentation on the inner shelf to that of the adjacent coastal plain and examine the effects of tectonics and fluctuating eustatic sea level on deposition, erosion and preservation.

Previous investigations The south-central Monterey Bay region has been investigated by a number of authors. Dorman (1968) and Yancey (1968) described the presence of a prominent sediment lobe, manifested on bathymetric charts by a seaward

deflection of isobaths off the moutl~ of the Salinas River. Yancey suggested the sediment lobe represented a subaerial fan deposited during a previous lowstand of sea level. Dorman, in contrast, argued that the lobe represents recent sedimentation since the last lowstand of sea level. Dorman's hypothesis was based on changes in the bathymetric configuration of the shelf. Greene (1970) who surveyed the Monterey Bay shelf using deep- and intermediate-penetration geophysical systems, postulated the presence of a Quaternary deltaic unit off the river mouth, 0-85 m thick (average thickness, 45-50m). Arnal (1971) reported the presence of a 25 m thick (average thickness, 8 m) delta off the Salinas River mouth. He postulated that the bulk of this sediment was deposited during the last 10,000 yrs B.P. The Monterey Bay shelf is cut by Monterey Canyon to within 0.5 km of the shoreline off Moss Landing (Fig.l). Dorman (1968) and Yancey (1968) noted a submarine re-entrant near the head of Monterey Canyon that cuts the south-central Monterey Bay shelf and trends southeastwardly, directly in line with the subaerial mouth of the Salinas River (Fig.l). Yancey (1968) suggested that this reentrant was cut subaerially by the Salinas River duri/lg a lowstand of sea level. In contrast, Dorman (1968) suggested that the reentrant is a very recent feature resulting from submarine erosion of the canyon head.

Methods Data used in this investigation were collected on three cruises of the U.S. Geological Survey aboard the R.V. David Johnston in 1981 and 1983. Navigation was provided by two different systems, a Motorola Mini Ranger III and a Del Norte 540 system. Navigation data were taped and later processed by computer. The positional accuracy of both systems was within several tens of meters. Surface sediment samples (Fig.2) were obtained with a Smith-McIntire grab sampler which yields an average volume of 0.0125 m 3 in

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Fig.3. Trackline map of one-quarter second sweep seismic reflection profiles. Water depths are in meters. The numbers on the tracklines are profiles used in subsequent figures.

addition, one-eighth second sweep profiles were collected along 75 km of trackline (Fig.4) using a double-plate EG&G Uniboom sound source with a power output of 400 joules. Incoming signal was filtered between 500-1500 Hz. Resolution of this system was about 1.0 m while vertical exaggeration averaged about 9 times. Subbottom penetration of both systems was about 50-60 m. An assumed seismic velocity of 1500 m s- 1 was used to derive both water depth and thickness of sediment above a regional angular unconformity. This seismic velocity is close to t h a t derived by Moore and Shumway (1959) for unconsolidated (?) shelf sediments on the Pigeon Point shelf to the north of the study area. Recognition of depositional sequences and unconformities is based on seismic stratigraphic analysis methods outlined by Mitchum

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et al. (1977), Vail et al. (1977) and Brown and Fisher (1980). Primary seismic reflections and reflector truncations are the key features utilized in seismic stratigraphic analysis (Mitchum et al., 1977; Payton, 1977; Brown and Fisher, 1980).

Geometry o f the Salinas River lobe

Bathymetric expression The seafloor in the study area (Fig.l) slopes continuously seaward from the shoreface to the shelf break. Off the Salinas River mouth the slope is about 0.25 ° while off Fort Ord it is about 0.31°. The slope over the outer shelf (greater than 90 m depth of water) off both areas is flatter, averaging 0.13-0.21 °. This flat outer shelf platform averages 3-5 km in width. The shelf as a whole varies in width from less than 1.0 km to 12 km. Shelf areas of northern

and southern Monterey Bay are separated by the Monterey Submarine Canyon system (Fig.l). The canyon-head re-entrant t h a t cuts across the south-central Monterey Bay shelf (Dorman, 1968; Yancey, 1968) is approximately 1.5 km in length and varies in width from 0.2 to 0.5 km (Fig.l). It occurs under present water depths of 37 to 130 m. A prominant feature of the seafloor is the pronounced seaward deflection of isobaths off the Salinas River mouth from about 10 m water depth out to about 80-90 m water depth (Fig, l). This seafloor feature, the "Salinas River lobe", contrasts markedly with the rest of the Monterey Bay shelf, where isobaths largely parallel the adjacent shoreline (Fig.l). The lobe covers an area of approximately 72 km 2.

Delineation of the lobe High-resolution shallow-penetration uniboom profiles reveal that the uppermost subbottom of the study area is grossly similar to that reported for other California shelf areas (Moore and Shumway, 1959; Moore, 1960; Moore and Silver, 1968; Field et al., 1980; Osborne et al., 1980; Mullins et al., 1985). In general, an angular unconformity of high acoustic amplitude in the uppermost 40-60 m of shelf subbottom separates acoustically transparent to bedded and undeformed strata above it from deformed strata below it (Fig.5). This angular unconformity was evident on all profiles within the study area. All reflectors above the angular unconformity dip continuously seaward and are parallel or subparallel to the seafloor. Since the angular unconformity separates undeformed and presumably unconsolidated strata above it from deformed and presumably semi-consolidated to consolidated strata below it, it is used as a convenient base for the uppermost shelf subbottom of south-central Monterey Bay. A contour map of the surface of the unconformity prepared from one-quarter second sweep profiles shows that the erosional surface is both irregular and has a pronounced break in slope 40-60m below present sea level

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(Fig.6). The erosional surface is flatter under the middle and outer shelf areas seaward of this break and steeper under the inner shelf shoreward of this break. The unconformity occurs at the landward limit of every profile and must extend under the surf zone and the b e a c h - c o a s t a l plain. This angular unconformity is similar to t ha t occurring under the shelf west of Santa Cruz and north of the study area, where the surface has a break in slope 50-60 m below present sea level (Mullins et al., 1985). The angular relationship of overlying undeformed strata, underlying deformed strata and the pronounced break in slope near the landward end are similar to the profiles of late Q u a t e r n a r y uplifted marine terraces in the Santa Cruz area studied by Bradley (1957, 1958) and Bradley and Griggs (1976), as pointed out by Mullins et al. (1985). Uniboom profiles reveal th at the unconformity beneath the Salinas River lobe is at a depth of 20-25 m below sea level within 1 km of the shoreline

and drops to - 1 0 0 m under the outer shelf. St rat a occurring below the unconformity are very near the seafloor under the outer shelf and may outcrop, as suggested by dredge hauls made by Greene (1977).

Stratigraphy Stratigraphy off Salinas River mouth The geometry of the uppermost subbottom varies across the study area both in an east -w est and n o r t h - s o u t h direction. West of the Salinas River mouth a thick lens of sediment overlies the angular unconformity. In plan view it is thickest 3 km from the shoreline and thins in all directions (Fig.7). In cross section, this body of sediment is lenticular as seen in a shore-normal uniboom profile (Fig.8). This thick lobe of sediment coincides with and causes the pronounced seaward deflection of isobaths west of the Salinas River mouth

143

Fig.6. Contour map of the angular unconformitybased on one-quarter second sweep profiles. Datum is depth in meters below present sea level. Contour interval is 5 m. Bathymetric contours are in meters.

Fig.7. Isopach map of all sediment occurring above the angular unconformity on one-quarter second sweep profiles. Contour interval is 4 m. Water depths are in meters.

(Fig.l). In general, the lobe is well-bedded although transparent zones do occur. In contrast to the lobe area, the inner shelf west of Fort Ord (Fig.l) is characterized by only a thin and uniform veneer of sediment, 2-5 m thick, which overlies the angular unconformity across the entire shelf (Fig.8). The same geometric relationships can be observed on profiles oriented parallel to the coast (Fig.9). The previously mentioned Monterey Canyon re-entrant (Fig.l) was investigated to determine its relationship to shelf stratigraphy. High-resolution profiles across the trend of this re-entrant (Figs.3, 4) reveal that it cuts through the lobe of sediment off the Salinas River (Chin, 1984). Reflectors within the lobe and the angular unconformity both appear to have originally continued uninterrupted beneath the re-entrant (Chin, 1984). Seismic reflection profiles reveal t h a t the

uppermost shelf subbottom off the river mouth can be subdivided into depositional sequences (Mitchum et al., 1977). The two different seismic systems used in this investigation yielded different internal configurations on the profiles studied. One-quarter second sweep profiles oriented normal to the coastline reveal two depositional sequences (upper and lower) within the lobe off the river mouth separated by an unconformity. A combination of different sweep rate, expanded vertical scale, power output and filtering on one-eighth second sweep profiles reveals t h a t the lobe consists of at least three depositional sequences (A, B and C) overlying the angular unconformity (Fig.10). Each depositional sequence is separated from the adjacent sequence by an unconformity, delineated by toplap and downlap. Comparison of the two sets of profiles (Figs.3 and 4) at crossing points of tracklines shows

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t h a t the lower sequence of o n e - q u a r t e r second sweep records c o r r e l a t e s with d e p o s i t i o n a l sequence A of one-eighth second sweep records. The u n c o n f o r m i t y b e t w e e n the u p p e r and l o w e r sequences on o n e - q u a r t e r second sweep records thus c o r r e l a t e s with u n c o n f o r m i t y A of oneeighth second sweep records. H o w e v e r , no i n t e r n a l u n c o n f o r m i t y w i t h i n the u p p e r depositional sequence was delineated w h i c h correlates with u n c o n f o r m i t y B (Fig.10), n o r were

acoustic foresets recognized within the u p p e r depositional sequence. Thus, depositional sequences B and C (Fig.10) c o r r e l a t e with the u p p e r depositional sequence. Since the oneeighth second sweep records reveal the g r e a t e s t a m o u n t of i n t e r n a l r e s o l u t i o n t h e y will be described in detail. However, the limited s u r v e y grid of one-eighth second sweep records m a d e it n e c e s s a r y to refer to the o n e - q u a r t e r second sweep s u r v e y for i s o p a c h maps.

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Depositional sequence A is a thin lens of acoustically transparent (reflection-free) sediment that overlies the angular unconformity under all but the innermost part of the lobe (Fig.ll). It pinches out landward against the angular unconformity about where the break in slope occurs (50 m below present sea level). This depositional sequence also thins seaward and probably pinches out under the outer shelf near 100 m water depth. The average thickness is 7 m and the maximum thickness of 18 m occurs about 5 km nortl~west of the river mouth. The top of sequence A is an unconformity (A) that is delineated by the downlap of

acoustic foresets in the overlying depositional sequence (B) (Fig.10). This unconformity is planar and merges landward with the angular unconformity at a depth of about 50 m below present sea level. Depositional sequence B is reflection-free to well-bedded and is recognized on profiles by its acoustic foresets ("progradational clinoforms" of Mitchum et al., 1977; Sangree and Widmier, 1977; Vail et al., 1977; Brown and Fisher, 1980, and Berg, 1982) that dip more steeply s e a w a r d than the unconformities (A and B) which bound the sequence (Fig.10). These foresets are monoclinal, dip seaward, and range in

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apparent dip from 0.7 ° to 2.0 °. They occur under the shelf in present water depths of 18 to 55 m. The average thickness of sequence B is 11 m and the maximum thickness is 18 m. The top of sequence B and the base of sequence C is defined by toplap (unconformity B) of the foresets in sequence B (Fig.10). Depositional sequence C is acoustically t r a n s p a r e n t to weakly parallel-bedded. Its base is defined by unconformity B while its top is defined as the present seafloor (Fig.10). The average thickness of sequence C is 8 m, and the maximum thickness of 14 m occurs about 3 km northwest of the river mouth. This sequence, like A and B, is lenticular and is thickest in water depths of 29-35 m. An isopach of the upper depositional sequence prepared from one-quarter second profiles reveals the approximate aggregate thickness of sequences B and C t hrough the study area (Fig.12).

Stratigraphy of Fort Ord and outer shelf areas

Fig.ll. Isopach map of depositional sequence A of oneeighth second sweep profiles, equivalent to lower depositional sequence of one-quarter second sweep profiles. Contour interval is 2 m. Water depths are in meters.

Examination of b a t h y m e t r y (Fig.l), Uniboom profiles (Figs.3 and 4) and the total sediment isopach map (Fig.7) reveals t hat the uppermost subbottom off Fort Ord and outer

147 Fort Ord and outer shelf areas and its complete absence locally is similar to conditions on the northern Monterey Bay shelf and the shelf off Santa Cruz and Davenport to the north of Monterey Bay, as described by Bradley (1957, 1958), Dupr~ (1975, 1984), Bradley and Griggs (1976) and Mullins et al. (1985). Bradley and Griggs (1976) and Dupr~ (1975, 1984) interpret the northern Monterey Bay shelf surface as a wave-cut platform eroded during the Holocene transgression and mantled by a thin veneer of sand, which in places gives way to reworked concretions and boulders (Greene, 1977). The inner shelf north of Santa Cruz (Fig.l) also consists, at least in part, of a modern wave-cut platform (Bradley, 1957, 1958; Moore and Shumway, 1959; Moore, 1960; Bradley and Griggs, 1976; Weber et al., 1979; Mullins et al., 1985).

Composition Fig.12. Isopach map of aggregate thickness of depositional sequences B and C, equivalent to the upper depositional sequence of the one-quartersecondsweepprofiles.Contour interval is 4 m. Water depths are in meters. shelf areas (water depths greater than 90100 m) differs considerably from t h a t off the Salinas River mouth. In contrast to the shelf off the river mouth, Fort Ord and outer shelf areas are characterized by only a thin 2-5 m thick veneer of sediment which overlies the angular unconformity (Fig.8). If any seismic stratigraphic units occur within this drape, they are masked by the seismic bubble pulse. In places, the veneer is so thin that sediments underlying the angular unconformity are exposed at or very near the seafloor. Surface grab samples and short gravity cores taken by Chin (1984) suggest that in water depths greater than 100 m the veneer is less t h a n 0.2-1.0 m thick. Dorman (1968) and Greene (1970, 1977) report that the shelf further to the south, off Monterey, has bedrock outcrops locally, suggesting that the veneer is thin to absent on that part of the shelf. The uniform thickness of the veneer over the

Surface sediment data Surface sediment patterns for the study area were determined through the use of 140 grab samples (Fig.2). Complete grain-size descriptions and statistical parameters are detailed in Chin (1984). Analyses show that 96% of all samples contain less than 1% gravel. Thus, sand and mud are the dominant sediment types on the south-central Monterey Bay shelf in water depths less than 100 m. A distribution map of percentage sand in surface sediments (Fig.13A) shows that 31% of all samples contain greater than 90% sand while 54% of all samples contain at least 50% sand. Sand dominates in water depths out to 60 m off the river mouth and out to 80-90 m water depth off Fort Ord (Fig.13A). Silt is the dominant component of the mud fraction and is secondary to sand in abundance in surface sediments. Silt dominates the shelf surface in water depths from 70m to about 100m (Fig.13B). Approximately 40% of all samples c o n t a i n greater t h a n 50% silt. No samples consisted of more than 16~/o clay, and 95% of all samples contained less than 5% clay. Most clay that is

148

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149

transported to the south-central Monterey Bay shelf probably bypasses the relatively narrow shelf in suspension and is deposited in deeper water (Arnal et al., 1973). Average grain-size variability in terms of mean diameter (Fig.13C) shows that the study area is dominated by fine to very fine sand out to about 60 m water depth. Seaward, mean grain-size decreases and silt dominates to about 90 m water depth. Mean grain-size then reverses over the outer shelf in water depths of 100 m and greater. The outer shelf platform (Fig.l) between the 100 m isobath and the canyon edge (west of the river mouth and Fort Ord areas) is mantled by sandy gravels and reworked concretions of sandstone and siltstone (Yancey, 1968; Malone, 1970; Greene, 1977; Chin, 1984; Powell and Chin, 1984). The same seaward-fining trend with a reversal over the mid-outer shelf characterizes the shelf west of Santa Cruz (Mullins et al., 1985) as well as other shelf areas off California (Moore, 1960; Klise, 1983).

Relict outer shelf sediments Several surface grab samples and short gravity cores (Chin, 1984) reveal that coarse gravelly sediments and reworked concretions occur on or within 0.2-1.0 m of the seafloor in water depths of 100 m or greater (Fig.l). The gravel fraction of outer shelf samples is composed of both lithic and biogenic clasts. The lithic clasts include subrounded to subangular granules to pebbles that are predominantly siliceous siltstones and sandstones. These rock fragments are commonly bored and encrusted with worm tubes, barnacles and bryozoans. The biogenic component of these outer shelf gravels is described by Chin (1984) and Powell and Chin (1984). Biogenic gravel-sized clasts consist primarily of unabraded molluscs dominated by a fauna indicative of late Pleistocene shallow-water environments (deep inner sublittoral, 18-46 m water depth, Powell and Chin, 1984). The late Pleistocene fauna also suggests cooler water conditions in Monterey Bay

during the late Pleistocene glacial interval. Several pectens (Chlamys rubida) obtained from the outer shelf grabs were radiocarbon dated at 17,000 yrs B.P. (Powell and Chin, 1984), which broadly coincides with the close of the late Wisconsin glacial maximum (Curray, 1965) and a lowstand of sea level over this part of the shelf. The occurrence of shallow-water mollusks, sandstone concretions and lithic gravels (which could not have been deposited by modern processes) and the reversal in seawardfining trend suggests that these gravels which mantle the outer shelf in water depths greater than 100 m are relict. Investigations by Yancey (1968), Malone (1970), and Greene (1977) on the outer shelf platform document that the entire southern Monterey Bay outer shelf is mantled by relict gravels. The age of sediments that comprise the lobe and adjacent shelf is not known at this time. However, a maximum age for the uppermost subbottom sediments can be derived from Greene's (1977) work on the outer shelf and canyon edge areas of this part of the bay. Greene dredged fossiliferous siltstones and sandstones from both the south wall of Monterey Canyon as well as from the outer shelf platform. These clasts were identified as from the Purisima Formation on the basis of lithology and fauna (Greene, 1977), and occur extensively along the upper south wall of Monterey Canyon in water depths as shallow as 18 m near Moss Landing. In general, Greene (1977) reports that they occur under a cover of unconsolidated sediment. The stratigraphic position, subsea elevation, and structure of the Purisima Formation in south-central Monterey Bay reported by Greene (1977) suggest that these rocks should occur immediately below the angular unconformity identified in this investigation under the outer shelf and at a greater depth below the angular unconformity under the inner shelf. The age of the Purisima Formation in central California is generally accepted to be Pliocene (Greene, 1977). Dupr~ (1975) reports that the upper part of the Purisima Formation in the Monterey

150

Bay region may be as young as early Pleistocene. Thus the unconsolidated(?) sediments overlying the regional angular unconformity under the south-central Monterey Bay shelf are probably younger than early Pleistocene and no older than late Pliocene. Discussion

The Monterey Bay coastal plain record of late Quaternary history The onshore record of late Quaternary history in central Monterey Bay is preserved primarily in coastal terrace deposits, fluvial terraces, valley fills of the Pajaro River, Elkhorn Slough and Salinas River, coastal dunes and the largely undifferentiated Pleistocene Aromas Sand. The Aromas Sand, together with its associated coastal terrace deposits, cumulatively records the most complete record of Quaternary sedimentation in the central California coastal region (Dupr~, 1975). Quaternary tectonism has played an integral role in the initial distribution of depositional systems as well as their ultimate preservation (Dupr~, 1975; Mullins et al., 1985). Coastal plain deposits in north-central Monterey Bay (near Watsonville, Fig.l) have been progressively folded, faulted and partly buried during the Quaternary (Dupr~, 1975). To a degree, the same is probably true of south-central Monterey Bay (Salinas River area), including the study area. Alexander (1953), Bradley (1957, 1958), Dupr~ (1975), and Bradley and Griggs (1976) have shown that the northernmost part of Monterey Bay (Santa Cruz vicinity) has been a region of Quaternary uplift and is characterized by uplifted coastal terraces and a cliffed shoreline. In contrast, much of central Monterey Bay (Watsonville to Fort Ord areas) has been a relatively stable and/or subsiding area during the Quaternary as suggested by extensive coastal lowlands and coastal deposits (Cooper, 1967; Dupr~, 1975; Tinsley, 1975). Late Quaternary marine terraces in the Monterey Bay region are believed to have been

formed in a two-stage process (Bradley, 1957, 1958). They consist of a wave-cut platform formed largely in response to rising sea level on which a regressive veneer of marine to nonmarine sediments were deposited during a subsequent fall of sea level (Bradley, 1957, 1958; Bradley and Griggs, 1976; Dupr~ and Clifton, 1979; Weber et al., 1979; Dupr~ et al., 1980). Detailed study of sedimentary sequences within these terrace deposits by Dupr~ (]975), Dupr~ and Clifton (1979) and Dupr~ et al. (1980) reveal that they can be characterized as unconformity-bounded progradational sequences preserved largely in shallowing-upward packages. These shallowing-upward packages are composed of marine to nonmarine deposits that record progradation of the shoreline during highstands and/or during early stages of falling sea level. The unconformities that bound these sequences represent the only record of the intervening transgressions and are characterized by an erosional surface of low relief often mantled by a thin lag deposit. This asymmetry of coastal deposits has also been documented in other investigations (Fischer, 1961; Ryer, 1977; Clifton, 1981; Hunter et al., 1984). Dupr~ (1975) developed a conceptual model of sedimentation for a tectonically active highenergy coastal region, such as Monterey Bay affected by eustatic fluctuations in sea level (Fig.14). Dupr~'s model (Fig.14) suggests that in a subsiding region (such as the Salinas River valley) one would expect the most complete initial record of sedimentation. Moreover, from a preservation standpoint, it is in subsiding regions that the model predicts that the most extensive sedimentary record would also be preserved within the coastal region. Dupr~'s (1975) conceptual model of coastal sedimentation provides a useful framework by which to interpret depositional-erosional events as recorded in seismic stratigraphic patterns observed in the south-central Monterey Bay shelf subbottom. Adjacent onshore coastal plain deposits suggest that the tectonic regime in south-central Monterey Bay has been one of continuous subsidence throughout the Quater-

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nary (Cooper, 1967; Dupr6, 1975; Tinsley, 1975). The south-central Monterey Bay region thus should be an area, according to the Dupr6 model, where a significant sedimentary record of late Quaternary depositional-erosional history is preserved both onshore and offshore. Furthermore, the sedimentary record preserved in the shelf subbottom (offshore) should be very much like that preserved in northcentral Monterey Bay uplifted marine terrace deposits (onshore), which formed in presumably the same environments and under the same processes.

The south-central Monterey Bay shelf record of late Quaternary history The coastal plain of central Monterey Bay, as seen from the previous summary, contains an extensive record of late Quaternary sedimentation. However, little is known to date as to what record of late Quaternary sedimenta-

tion exists under the continental shelf of this area. Bradley (1957, 1958), Dupr~ (1975, 1984), and Bradley and Griggs (1976) suggest t h a t the modern shelf surface, as characterized by the northern Monterey Bay shelf, is for the most part a wave-cut erosional platform developed during the last eustatic rise in sea level (Holocene). Much of the shelf, particularly in water depths greater than 20 m, consists of only a thin fine-grained veneer overlying Tertiary bedrock (Bradley and Griggs, 1976). It would thus appear from the foregoing studies that much of the northern Monterey Bay shelf (north of Monterey Canyon) has very little to no record of late Quaternary coastal sedimentation preserved in the uppermost shelf subbottom, as one would expect from the tectonic uplift that has characterized much of the northern part of the bay. Somewhat thicker late Quaternary deposits may occur on the shelf off the Pajaro River, where subsidence has been greater.

152 The south-central Monterey Bay uppermost shelf subbottom is characterized by two contrasting configurations, as mentioned earlier. The shelf subbottom off Fort Ord and over outer shelf areas appears to be similar to most of the northern Monterey Bay shelf described by Bradley (1957, 1958), Dupr~ (1975), and Bradley and Griggs (1976). In general, it consists of a thin uniform fine-grained veneer of sediment over an angular unconformity. In places, this thin veneer is mantled by relict sediment, just as on the northern Monterey Bay shelf (Greene, 1977). These shelf areas may, as suggested by Bradley (1957, 1958), Dupr~ (1975), and Bradley and Griggs (1976), be locally coincident with the wave-cut platform eroded by the Holocene transgression. The shelf subbottom off the Salinas River mouth, in contrast, consists of a thick piano-convex lens of sediment (up to 35 m) overlying the same angular unconformity which characterizes the rest of the study area. The shelf area off the Salinas River thus seems to contain the most significant record of late Quaternary sedimentation on the Monterey Bay shelf. The Salinas River lobe consists of at least three unconformity-bounded depositional sequences defined by seismic stratigraphic analysis. The pattern of seismic stratigraphic sequences and their occurrence in a shallow inner-shelf setting appears to be very similar to the depositional-erosional patterns reported in adjacent onshore marine terrace deposits in central and northern Monterey Bay (Dupr~, 1975, 1984; Dupr~ and Clifton, 1979; Dupr~ et al., 1980). Using the Dupr~ (1975) conceptual model (Fig.14) and sedimentary sequences within uplifted marine terraces as a framework, an interpretation of seismic stratigraphic patterns observed in the Salinas River lobe is possible. Depositional sequences comprising the Salinas River lobe are interpreted to represent shallowing-upwards progradational sequences of marine to non-marine coastal deposits formed during progradation of the shoreline at highstand and/or during early stages of falling sea level. Unconformities which separate depositional

sequences are interpreted as transgressive unconformities formed largely during rising sea level by erosional transgression as the shoreface translated landward across the shelf. The shoreward migration of the relatively steep shoreface and high wave energies resulted in the selective removal of most (if not all) of the transgressive record during erosional transgression (Dupr~, 1975). Moreover, the close proximity of the Monterey Canyon system probably resulted in the permanent removal of shelf sands to deep-sea areas. These unconformities may be compound erosional surfaces (Swift et al., 1970), representing some component of Subaerial erosion/nondeposition from the preceding lowstand of sea level as well as marine erosion during transgression. At this time one other interpretation of Salinas River lobe stratigraphy appears tenable. Toplap and downlap, which define the boundaries of the depositional sequences, may represent surfaces of minor erosion (or sedimentary bypassing) rather than surfaces of significant erosion associated with rising sea level. The three depositional sequences, according to this interpretation, would be analogous to facies of a marine delta. Unit A would correspond to bottomset beds, unit B to foreset beds, and unit C to topset beds. The Salinas River lobe, using this interpretation, would appear to represent a single progradation of the shoreline. The Dupr~ (1975) conceptual model, as applied to the Salinas River lobe and the southcentral Monterey Bay shelf record, suggests that at least several regressions and several intervening transgressions may be recorded within the sediments above the angular unconformity. Whether the regressions and transgressions represent discrete (major) lowstands and highstands (Curray, 1965) cannot be ascertained at this time with existing data. It is evident from Fig.10 and a MLML profile (Fig.15) that the stratigraphy of the shelf off the Salinas River may be very complex. Relative ages for the depositional sequences within the Salinas River lobe cannot be assigned due to the stratigraphic complexity

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154 and lack of drilling and radiometric age data. However, several lines of evidence suggest that the lobe may be largely Pleistocene in age. The angular unconformity that forms the base of the uppermost shelf subbottom throughout the study area is evident on all lines at their landward limit (i.e., just outside of the surf zone). At this location the angular unconformity is approximately 20-25 m below present sea level (Fig.6) and most likely continues under the present shoreface and beach/coastal plain. If projected onshore, from its landwardmost position on seismic profiles, the angular unconformity appears to coincide with the position and elevation of a marine terrace of Sangamon(?) age near Castroville delineated by Dupr6 and Tinsley (1980). This correlation suggests that the angular unconformity, delineated throughout south-central Monterey Bay, may be the offshore extension/equivalent of the Sangamon wave-cut platform delineated by Dupr~ (1975) throughout north-central Monterey Bay as well as by Dupr6 and Tinsley (1980) near Castroville. If the angular unconformity is Sangamon in age, the Salinas River lobe may record shelf history during the last major eustatic cycle of sea level, an interval not observed in onshore north-central Monterey Bay marine terrace deposits. A second, but perplexing, line of evidence on the relative age of the lobe is the relationship of the Monterey Canyon re-entrant (Fig.l) to the shelf subbottom. At its seaward end, the reentrant cuts through the lobe and down to the subsea elevation of the angular unconformity (Chin, 1984). Furthermore, at its seaward terminus its thalweg is approximately 130 m below present sea level, a subsea depth coincident with the late Wisconsin maximum lowstand of sea level (Curray, 1965). Seismic reflection profiles run across the trend of the re-entrant and the shelf area between its head and the subaerial river mouth suggest several possibilities. No buried channel was found which would connect the re-entrant to the subaerial river mouth (Chin, 1984). However, it is possible that one exists outside of the survey area (Figs.3 and 4) or that the orientation of

the tracklines did not allow for delineation, it is not clear from available evidence whether a genetic relationship exists between the reentrant and the late Wisconsin Salinas River. Tinsley (1975) and Chin (1984) estimate that the late Wisconsin Salinas River entrenched valley and subsequent early Holocene embayment may have been as deep as 60-90 m and may have extended up to 40 km inland from the present coastline. It thus may have been comparable in size to modern South San Francisco Bay. Considering the depth and extent of the early to mid-Holocene Salinas embayment and the sediment trapping capability of embayments (Dupr~, 1975; Atwater et al., 1977; Clifton and Davis, 1983), it is conceivable that very little sediment has reached the shelf since the late Wisconsin lowstand. This would support Dupr~'s (1975, 1984) supposition that the Monterey Bay shelf has received very little sediment during the Holocene. It does appear, though, that at least the surface veneer of the shelf off the Salinas River is the result of modern sedimentation, as suggested by the seaward fining trend out to 90-100 m water depth. Dorman (1968), Arnal (1971), and Arnal et al. (1973) also suggest that modern sedimentation has occurred off the Salinas River. In summary, it is possible that off the Salinas River a relatively thick sediment lobe may contain the record of the last major eustatic cycle of sea level as represented by at least three unconformity-bounded depositional sequences. However, the shelf off Fort Ord (and most of the northern shelf) appears to have had its late Quaternary record erased by shoreface planation associated with the Holocene transgression, as suggested by the presence of only a thin veneer of sediment remaining over the angular unconformity. Conclusions The late Quaternary history of the shallow shelf off California is poorly understood at this time. Where a sedimentary record is preserved on the shelf, it is likely to be stratigraphically complex as a function of being located on an

155 active transform continental margin that has experienced relatively rapid fluctuations of glacio-eustatic sea level. A reconnaissance geophysical survey by Moore and Shumway (1959) and Moore (1960) suggests that the uppermost subbottom of California inner shelves can be characterized in two broad categories. The first type is where an acoustically transparent (to weakly bedded), undeformed lens of sediment (presumably unconsolidated) overlies a Quaternary angular unconformity that truncates underlying deformed and older (early Quaternary and Tertiary) strata. The second broad category is where only a thin veneer of unconsolidated sediment overlies a Quaternary angular unconformity and deformed early Quaternary and Tertiary strata. The uppermost shelf subbottom of southcentral Monterey Bay can also be characterized by the aforementioned categories. The shelf west of the Salinas River mouth (Fig.l), and within the area delimited by the seaward deflection of isobaths (Fig.l), is characterized by a relatively thick (up to 35m) lens of undeformed, acoustically transparent to bedded sediment overlying an angular unconformity. This angular unconformity occurs throughout south-central Monterey Bay and truncates deformed strata below it. It is characterized by a concave-up profile and has a pronounced change in slope in present water depths of 40-60 m (Fig.6). This angular unconformity is similar in profile and subsea depth to others found in the uppermost shelf subbottom off California (Moore and Shumway, 1959; Moore, 1960; Field et al., 1980; Mullins et al., 1985). Mullins et al. (1985) suggest that the concave-up profile and break in slope of their angular unconformity represent a buried marine terrace. Shelf areas off Fort Ord (Fig.l) and in water depths greater than 90-100 m are characterized by only a thin (2-5 m or less) sediment veneer which overlies the same angular unconformity observed off the Salinas River mouth. The sediment veneer is masked within the seismic bubble pulse. Locally, particularly on the outer shelf in water depths greater than 90-100 m,

the veneer is entirely absent or is mantled by relict gravels. The thin veneer overlying an angular unconformity appears to be common for most of the northern Monterey Bay shelf as described by Dupr~ (1975, 1984) and Bradley and Griggs (1976). Dupr~ (1975) states that shelf areas, which are not off major sediment sources (such as the Salinas River), have not received much sediment since the Holocene transgression and are characterized by the aforementioned thin fine-grained veneer, or in places Tertiary bedrock may comprise the seafloor surface. This thin veneer seems to characterize areas of the Monterey Bay shelf which have received little sediment and/or have been uplifted and planed off by transgression. Surface sediment trends tend to corroborate Dupr~'s (1975) interpretation. The shelf area off the Salinas River, and within the area characterized by a seaward deflection of isobaths (Fig.l), shows a seaward-fining of surface sediments (Fig.13) from the 10-m isobath out to about the 90-m isobath. Seaward of 90 m the fining trend reverses and relict gravels and reworked concretions from previous lowstands of sea level mantle the seafloor. This reversal in seaward fining of shelf sediments has also been documented on the Santa Cruz shelf (Mullins et al., 1985) and on the Russian River shelf (Klise, 1983) off California. The seaward fining of surface sediments west of the Salinas River mouth suggests that some Holocene sedimentation has occurred on the shelf out to about 90-100 m water depth. In contrast, the shelf off Fort Ord (Fig.13) appears to be characterized by a patchy distribution of fine (sandy to silty) sediment. Shelf sediment dynamics and oceanography are poorly understood, adding to the perplexing character and evolution of this shelf area. Chin (1984) suggests that a partial explanation for the patchy sediment distribution and thin veneer relates to its late Quaternary evolution. The thin veneer may result from shoreface planation of pre-existing Pleistocene morphology during the Holocene transgression as well as from the shelf area being an area of very low sediment

156 i n p u t and slow uplift. The late P l e i s t o c e n e S u n s e t D u n e s (Dupr~, 1975) w h i c h c o v e r the p r e s e n t F o r t Ord c o a s t a l plain a r e a (Fig.l) most likely extended across the shelf d u r i n g the late W i s c o n s i n sea level lowstand. As only a t h i n v e n e e r of sediment is n o w p r e s e n t above the a n g u l a r u n c o n f o r m i t y , no evidence of this P l e i s t o c e n e m o r p h o l o g y was preserved on this part of the s o u t h e r n M o n t e r e y B a y shelf. The shelf surface off F o r t Ord m a y thus be a n a l o g o u s to a t r a n s g r e s s i v e sand sheet developed by p l a n a t i o n of the shelf d u r i n g the H o l o c e n e t r a n s g r e s s i o n (Chin, 1984). The r e c o r d of shelf s e d i m e n t a t i o n preserved w i t h i n the Salinas River lobe and the a d j a c e n t onshore south-central Monterey Bay coastal plain r e c o r d a p p e a r to s u p p o r t the Dupr~ (1975) c o n c e p t u a l model of s e d i m e n t a t i o n for subsiding regions. A d d i t i o n a l h i g h r e s o l u t i o n seismic reflection profiling and surface grab sampling on the s o u t h e r n m o s t M o n t e r e y B a y and n o r t h e r n M o n t e r e y B a y shelves are needed to fully apply and test the Dupr~ model for shelf areas. Moreover, extensive drilling and vibracoring, and age d a t i n g are needed to identify s e d i m e n t a r y facies, t h e i r r e l a t i o n s h i p to seismic s t r a t i g r a p h y , and to d o c u m e n t how seismic s t r a t i g r a p h i c p a t t e r n s w i t h i n the shallow shelf s u b b o t t o ~ relate to s e d i m e n t a t i o n p a t t e r n s observed in o n s h o r e uplifted m a r i n e terraces.

Acknowledgements This paper resulted from a thesis investigation by the senior a u t h o r at S a n J o s e S t a t e University. The s t u d y was suggested to the senior a u t h o r by G a r y Hill, to w h o m I would like to express my deepest g r a t i t u d e for his s u p p o r t a n d g u i d a n c e t h r o u g h the years. Special t h a n k s are due to R a l p h H u n t e r and Bill Dupr~ w h o reviewed the m a n u s c r i p t and whose c o m m e n t s are g r a t e f u l l y a c k n o w l e d g e d . Apprec i a t i o n is also due to J o - A n n Gibbs, G a r y Greene, C h u c k Powell, B r i a n H o and the crew of the R.V. David Johnston. P a r t i a l f u n d i n g for this i n v e s t i g a t i o n was provided by the U.S.G.S. G r a d u a t e I n t e r n s h i p P r o g r a m . The M L M L profile was a c q u i r e d by H.T. Mullins while

at Moss L a n d i n g M a r i n e L a b o r a t o r y u n d e r N.S.F. g r a n t CDP-7926669.

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