Seafloor geology of the Monterey Bay area continental shelf

Marine Geology 181 (2002) 3^34 www.elsevier.com/locate/margeo

Sea£oor geology of the Monterey Bay area continental shelf Stephen L. Eittreim *, Roberto J. Anima, Andrew J. Stevenson United States Geological Survey, 345 Middle¢eld Rd., Menlo Park, CA 94025, USA Received 16 March 2000; accepted 16 July 2001

Abstract Acoustic swath-mapping of the greater Monterey Bay area continental shelf from Point An‹o Nuevo to Point Sur reveals complex patterns of rock outcrops on the shelf, and coarse-sand bodies that occur in distinct depressions on the inner and mid-shelves. Most of the rock outcrops are erosional cuestas of dipping Tertiary rocks that make up the bedrock of the surrounding lands. A mid-shelf mud belt of Holocene sediment buries the Tertiary rocks in a continuous, 6-km-wide zone on the northern Monterey Bay shelf. Rock exposures occur on the inner shelf, near tectonically uplifting highlands, and on the outer shelf, beyond the reach of the mud depositing on the mid-shelf since the Holocene sea-level rise. The sediment-starved shelf off the Monterey Peninsula and south to Point Sur has a very thin cover of Holocene sediment, and bedrock outcrops occur across the whole shelf, with Salinian granite outcrops surrounding the Monterey Peninsula. Coarse-sand deposits occur both bounded within low-relief rippled scour depressions, and in broad sheets in areas like the Sur Platform where fine sediment sources are limited. The greatest concentrations of coarse-sand deposits occur on the southern Monterey Bay shelf and the Sur shelf. ß 2002 Elsevier Science B.V. All rights reserved. Keywords: Side-scan sonar; Sea£oor geology; Acoustic backscatter; Continental shelf; Monterey Bay National Marine Sanctuary

1. Introduction The Monterey Bay National Marine Sanctuary (MBNMS) occupies a large area of coastal California, comprising more than 2‡ of latitude along the coastline. Occupying the center of the Sanctuary is the Monterey Bay continental shelf, a 35km-wide embayment on an otherwise straight and relatively narrow continental margin (Fig. 1). Monterey Bay is bounded by the tectonic highlands of the Santa Cruz Mountains on the north and the Monterey Peninsula granitic highlands on

* Corresponding author. E-mail address: [email protected] (S.L. Eittreim).

the south. The head of the bay is formed where the tectonic lowlands of the Salinas Valley emerge on the coast. At the center of this lowlands region lies the head of Monterey Canyon, located at the terminus of watersheds that have drained large areas of coastal and perhaps central California in the past (Greene, 1990). The incision of the Monterey Shelf by the canyon morphologically divides the shelf into north and south sectors. In 1992 the MBNMS was declared with the aim of protecting the region’s marine resources for present and future generations (Anonymous, 1992). Fundamental to e¡ective stewardship of the sanctuary is an understanding of the region’s ecosystems and how they function. The need for a good description of the sea£oor, its composition

0025-3227 / 02 / $ ^ see front matter ß 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 5 - 3 2 2 7 ( 0 1 ) 0 0 2 5 9 - 6

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Fig. 1. Track lines along which acoustic swath-mapping was conducted. The data north of Moss Landing consisted of Klein 100 kHz and EGpG 59 kHz side-scan sonar in conjunction with Geopulse 1-kHz vertical-re£ection pro¢les. The data south of Moss Landing consists of Simrad EM1000 and EM300 multibeam bathymetry and acoustic backscatter. See Table 1 for speci¢cations of each survey block.

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and morphology, is important for building such an understanding, particularly of the region’s benthic ecosystem dynamics. For example, rocky outcrops provide a kind of oases on the sea£oor for certain types of rock¢sh ecosystems. The distribution of these outcrops and their degree of connectivity may determine the survival of larvae that drift with prevailing currents across the vast ¢elds of mud that sometimes separate outcrops. Detailed sea£oor maps can also be used to monitor changes that occur in the sanctuary in the future, changes that may be both man-induced and natural. Full coverage of the sea£oor with acoustic imagery opens up new possibilities of understanding benthic processes at small scales. Digitally processed side-scan sonar and multibeam bathymetric survey systems developed in recent years allow for the building of mosaics of sea£oor images with detail not possible in previous years. In general the world’s continental shelves are not well covered with swath-map acoustic data due to the high cost and ine⁄ciency of surveys in shallow waters where swath widths are limited by water depth. To provide base maps for better understanding of benthic habitats of the Monterey Bay area we have surveyed the continental shelf from Point An‹o Nuevo on the north, to Point Sur on the south using various techniques (Fig. 1). The large area of the northern Sanctuary shelf from Point An‹o Nuevo northward to San Francisco has yet to be mapped with 100% acoustic coverage and, due to its large areal extent and exposure to the weather, will require a signi¢cant e¡ort to accomplish. The southern Sanctuary shelf south of Point Sur on the other hand is very narrow and could be surveyed at relatively low cost. The identi¢cation of sea£oor materials by direct correlation of rock or sediment type to intensity of acoustic backscatter is not yet possible, although attempts have been made (e.g. Lee et al., 1996). Using backscatter intensity and pattern recognition in multibeam bathymetric trends, some success has been achieved in automated sea£oor characterization over limited areas where bottom characteristics are distinctive (e.g. Mitchell and Hughs Clarke, 1994). Although backscat-

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ter values (if known in absolute sense) are not usually directly convertible to bottom type, patterns of variations in acoustic backscatter intensities can be used to infer bottom composition, especially if samples have been collected that can be correlated to a particular acoustic bottom type. Edwards (2002) discusses the suite of samples that have been collected here on the continental shelf that provide some of this groundtruth information for the soft sediments. Sea£oor photographs and video provide another kind of ground-truth information. Anima et al. (2002) use video information, samples, and correlations to coastal cli¡ rock types to identify the acoustic bottom types in terms of known rock units. Also, vertical-re£ection pro¢les give an image of the stratigraphic and structural underpinnings of what lies on the sea£oor, information also helpful in making a geologic interpretation. In this paper we will discuss some of the features that are seen in the resulting sea£oor imagery and show how we derived the sea£oor geologic interpretation from this imagery. This imagery and interpreted map includes a wealth of new information, too extensive to discuss completely here. To access the imagery and derived geologic interpretation in its entirety use the CD-ROM that accompanies this volume (Wong and Eittreim, 2002) or, for the imagery alone, the website, http://TerraWeb.wr.usgs.gov/TRS/ projects/MontereySonar/.

2. Methods We mapped the northern Monterey Bay continental shelf o¡ Santa Cruz with towed-¢sh sidescan sonar systems, and the southern shelf with hull-mounted multibeam swath-bathymetry/backscatter systems (Fig. 1). On the northern shelf, in conjunction with the side-scan sonar data, we collected high-resolution seismic pro¢les using a surface-towed boomer sound source, and a 10-m streamer for receiver. Lines were spaced at 150 m on the inner Santa Cruz shelf to about 5-km o¡shore, and spaced 400 m apart farther o¡shore. Di¡erential GPS navigation was used on all acoustic survey lines, providing boat-position ac-

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curacy better than 10 m. Table 1 gives the details of systems used in the di¡erent survey blocks. On the northern shelf from the sheltered area around Santa Cruz northwest to Point An‹o Nuevo, the inner-most survey lines were run into water depths as shallow as 5 m. In most other areas,

inner-most lines were at about the 20-m isobath, leaving a signi¢cant data gap to the shoreline. The greatest nearshore data-gap width is around the Monterey Peninsula and south where the innermost data was collected along approximately the 50-m isobath.

Table 1 List of sediment sources and sinks to the Santa Cruz shelf and estimates of transport rates, reservoir volumes and residence times in reservoirs Sediment sources San Lorenzo River Pajaro River Salinas River Stream and gully erosion Seacli¡ erosion Erosion of surf-zone bedrock Littoral sand from the north

Sediment sinks Mid-shelf mud belt Canyon heads: Monterey Canyon Soquel Canyon Ascension group canyons Cross-shelf break to continental slope Northern Monterey Bay shelf sediment budget components (m3 /yr)

Sediment inputs San Lorenzo River

Total 212 000

Littoral sand Mud (silt+clay) 56 875 155 625

Pajaro River

297 500

59 500

238 000

Salinas River

1 955 625

647 500

1 308 125

Seacli¡ erosion

26 695

8 996

17 699

Gully erosion

7 830

1 169

6 661

Stream erosion

49 368

6 076

43 292

Littoral sand from north Erosion of surf-zone bedrock Totals Totals without Salinas River Sediment sinks Monterey Canyon head Soquel Canyon head Shelf break Mid-shelf mud belt (area 421 km2 ; volume 3.8 km3 ) Transport rates Littoral sand belt transport rate

? ? 2 549 018 593 393

? ? 780 116 132 616

? 1 769 402 461 277

? ? ? 1 100 000

? ? ? nil

3 000 minimum ? ? 1 100 000

Mud belt suspended sediment transport rate Sediment reservoirs

200 000 220 000

Mid-shelf mud belt

Volume (m3 ) Years in reservoir 3 600 000 000 3273

Littoral sand belt

16 000 000

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References Best and Griggs (1991) and Griggs (personal communication, 2000) Griggs and Hein (1980) and Griggs (personal communication, 2000) Griggs and Hein (1980) and Griggs (personal communication, 2000) from average of Best and Griggs (1991) ranges from average of Best and Griggs (1991) ranges from average of Best and Griggs (1991) ranges

estimate from Oakey (1997) observations

accumulation rate from Lewis et al. (2002)Uarea from S.C. harbor dredging (Best and Griggs, 1991) mooring-transect measurements (Xu et al., 2002)

observed sur¢cial accumulation rate/volume total reservoir volume/littoral transport rate

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pp. 7-14

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Fig. 5

Fig 13

Fig 6

Fig 8

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Fig. 2. Geologic interpretation of sea£oor acoustic imagery. Rock types and other sea£oor characterizations were interpreted based on backscatter signature, morphology (where known) and samples taken on the shelf or adjacent upper continental slope by Edwards (2002), McCulloch et al. (1985), Nagel et al. (1986), Stakes et al. (1999a,b), unpublished sample descriptions of Greene and Stakes (1998, personal communication), in addition to regional interpretations of McCulloch and Greene (1989) o¡shore, and Clark (1981), Brabb (1989), Dibblee (1999), and Hall (1991) onshore. Boxes show locations of areas covered in succeeding ¢gures. Small numbers, 3a^3f refer to reference locations represented in Fig. 3. Refer to CD-ROM in back of volume (Wong and Eittreim, 2002) for a higher resolution version of this map and supporting data.

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The side-scan sonar mosaics on the northern shelf have a pixel resolution of 0.4 m for the inner shelf and 0.8 m for the outer shelf. Although this pixel resolution allows us to resolve relatively small features on the sea£oor, the accuracy of pixel location is considerably poorer than 1 m for a number of reasons. The theoretical resolution of the raw side-scan acoustic information from the sea£oor, determined across-track by the frequency, pulse-width and sampling rate used (100 kHz, 10 ms and 22 kHz, respectively, e.g. for inner shelf in the north), and along-track by the boat-speed and ping rate, is widened in practice beyond its theoretical size (3 cmU12 cm) because of raypath distortions and uncorrected source/receiver ¢sh motions. Also, the digitized backscatter information recorded from the sea£oor represents wider sea£oor footprints with greater distance from the ship, and the error in location also increases with distance from the ship. Thus, although the ability to distinguish the outlines of features and their relative location, or relative precision, is 0.4^0.8 m, the accuracy of the location of these features is much poorer than this, due to distortions in the acoustic system. Uncertainties in tow¢sh position behind the ship, discussed below, are an additional accuracy detriment. A good general discussion of such errors, uncertainties and their variations among di¡erent types of systems can be found in Hughes Clarke (1996). The 1-kHz high-resolution seismic system used on the Santa Cruz shelf can resolve bedding thicknesses as small as about 1 m and was used to image the sediment to depths of about 50 m below the sea£oor. Because the source and receiver were towed within 10 m of the vessel, the accuracy in position of this data is almost as good as the di¡erential GPS ship location. Two surveys of the southern Monterey Bay and Sur shelf were carried out with hull-mounted multibeam bathymetric systems that recorded both water depth and acoustic backscatter from the sea£oor. Both surveys were done under contract with CpC Technologies of Lafayette, LA, USA and the Ocean Mapping Group, University of New Brunswick, Canada. The ¢rst survey, in 1995, employed a Simrad EM1000 system (Godin

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et al., 1992), and although referred to above as ‘hull-mounted’, actually had transducers mounted on a sti¡ pole welded to the bow stem of the 125ft vessel used. The second survey on the southern Monterey Shelf, in 1998, employed a Simrad EM300 system with transducers mounted on the hull amidships of the 235-ft vessel used. The multibeam systems (60 and 96 beams for the EM1000 and EM300 systems, respectively) recorded backscatter intensities (‘side-scan’) in addition to bathymetry and were processed at pixel resolutions of 2.5 m in backscatter and 5 m in bathymetry. The higher resolution for backscatter is possible because backscatter is recorded as a function of range and can be binned at a smaller size than that for bathymetry which is determined by the intersection on the sea£oor of the discrete 2.5‡ or 1.5‡ beams. For the tow-¢sh mounted side-scan sonar data, processing was done with the USGS MIPS system (Chavez, 1984; Chavez et al., 2002), which involved various pre-processing steps including navigation corrections, noise ¢ltering and slant-range corrections based on the £at-bottom assumption. Then the individual line swaths were joined into mosaics in latitude and longitude space with adjustments made on each swath for the distance of the tow¢sh behind the vessel. Although ship navigation accuracy was better than 10 m, our side-scan source/receiver tow¢sh used for the northern shelf work was not equipped with internal navigational equipment or a motion sensor. The ¢sh was towed at distances behind the vessel of up to 100 m on the outer shelf. Therefore a ‘layback’ error is introduced by the necessary estimate of ¢sh position relative to the vessel. Where sea£oor geology contains easily recognized patterns, these patterns can be used to adjust for layback error and make the shift of the adjacent swaths relative to one another. These shifts were as much as 100 m. In addition, since the images are projected from 100^300 m laterally port and starboard from the ship position, errors propagate and worsen for the pixels on the outer edges of swaths. Due to these layback errors and outward propagated errors, ¢sh navigational uncertainties are signi¢cantly larger than the uncertainties in ship navigation.

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We conducted an experiment to determine the magnitude of these errors by locating distinctive sea£oor features or ‘signposts’ that appear in both side-scan sonar data, and in the vertical pro¢ling data. Because the vertical pro¢ling data were obtained with sensors towed at the sea surface, very close to the vessel, these locations can be assumed to be as good as the di¡erential GPS ship navigation, after correcting for the 10-m o¡set to the navigation satellite antenna. This locational experiment, using ¢ve identi¢ed ‘signposts,’ gave 3, 19, 38, 70, and 82 m discrepancies, respectively, for an average error of 42 m, with the largest errors occurring on the outer shelf where ¢sh layback was greatest. Thus the position accuracy of any given random pixel in our mosaic is likely to be within 50 m of its actual GPS position for data on the northern shelf, although there may be occasions with worse errors than this, especially for imagery on the outer shelf. The navigational accuracy for the southern shelf multibeam data that was obtained with hullmounted transducers was considerably better than this, with errors in the range of 1^10 m. Processing of the multibeam data on the southern shelf was accomplished with the University of New Brunswick Ocean Mapping Group’s suite of editing and gridding programs. Editing of data was done line-by-line shortly after its collection by: (1) deletion of bad navigation points; (2) deletion of bad acoustic data points, both across track and along track, and ¢ltering out acoustic arrivals from improbable angles; (3) gridding of bathymetric data into a digital terrain model and adding the backscatter data from each line to build a backscatter mosaic. These editing programs allowed the viewing independently of both bathymetry and backscatter ¢elds to determine that there is agreement between the two data sets. If there is disagreement, the program has various tools for examining the data in order to judge its validity before acceptance. A noise source in the EM1000 data (block 4 in Fig. 1) involves a so-called ‘twist’ error, because the motion sensor, which is intended to monitor the exact pitch and roll of the transducer array, was located too far from the bow-mounted transducer array, given the £exibility of the narrow

1925 vintage ship used. When the ship experienced heightened wave conditions, the calculation of beam directions became inaccurate, resulting in a miscalculation of locations for the outer-most pixels. This produced a mismatch between outer edges of adjacent swaths at overlap points. The artifacts appear as a ‘railroad-track’ pattern of background noise orthogonal to the ship track in the imagery, with a wavelength corresponding to the distance between rolls of the vessel, and with maximum noise at the line of data overlap between adjacent tracks. This noise a¡ected both bathymetry and backscatter although sea£oor features can easily be seen through this noise. On the basis of the acoustic imagery (normally backscatter, supplemented with shaded-relief imagery for data south of Moss Landing) borders were drawn around areas interpreted as di¡erent bottom types. Interpretation was done at the scale of 2.4 m per pixel. At an image resolution of 72 dpi, common to most video screens, this represents a scale of 1:6 803. We emphasize that the distributions of bottom types shown in Fig. 2 and the references to bottom types in succeeding ¢gures are based largely on the acoustic imagery and not on direct samples. However numerous sample identi¢cations, mostly from rocks dredged from the upper continental slope, and some from ROV sample identi¢cations, support the interpretations, as well as do, most importantly, the landties of the inner-shelf outcrops.

3. Results Four basic categories of sea£oor geologic materials are interpreted from the image data: (a) rock outcrops ; (b) coarse sand; (c) ‘mud’, consisting of silt, clay, and ¢ne sand and (d) ‘relict gravels’, probably Pleistocene sand, gravel or carbonate-cemented sediment. Fig. 3 gives examples of imagery that represent some of the di¡erent bottom types. For each of the categories the source of samples or other information on which the interpretation is based will be explicitly stated in the discussions of each category below. At the outset it should be reiterated that absolute backscatter intensities cannot be directly related to sea-

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Fig. 3. Examples of acoustic imagery over di¡erent bottom types. Scale bar at bottom of each window is 100 m long. Low backscatter rendered as dark in this and later ¢gures. Location of these windows shown in Fig. 2.

£oor materials with reliability. Rather, it is the distinct patterns in the backscatter and bathymetric trends that are used to interpret bottom type.

Rock outcrops generally produce high backscatter but the backscatter patterns are highly variable due to the microtopography caused by fracture patterns, bedding planes and di¡erential

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erosional morphology. Also, patchy sediment cover in small crevices and depressions in outcrops can cause variations in backscatter patterns. ‘Coarse sand’ has high and uniform backscatter. Where high enough resolution data is available, the coarse-sand deposits usually display symmetrical ripples, 1^2 m in wavelength, typically aligned parallel or semi-parallel to the coastline. The ‘mud’ category has low and uniform backscatter. Given the changes in signal to noise ratios caused by variable weather conditions and other changes to system gains from survey to survey, the uniform mud cover over relatively broad areas often highlights the intra-swath gain changes or irregularities in processing, so that swath boundaries stand out more readily. It should be kept in mind that both of the terms ‘coarse sand’ and ‘mud’ are labels for acoustic bottom types intended as good descriptors, but these labels do not imply a rigorous knowledge of sedimentologic characteristics, since samples are limited (see Edwards (2002) for distribution of soft-sediment characteristics). The ‘relict gravels’ category was the most di⁄cult category to interpret (Fig. 4).

Fig. 4. Backscatter imagery of EM300 data in area of outershelf relict gravel/sand at 100^150 m water-depth o¡ the Salinas River. See Fig. 2 for location.

This bottom type occurs in some outer shelf areas and generally shows medium but variable backscatter and gradational changes, i.e. a lack of sharp boundaries. Where bathymetry is available, this category shows no observable relief. An additional geologic characteristic that we mapped is ‘active fault lineation’, interpreted as sur¢cial fault-break lineations in Holocene sediment. Also included in the map of Fig. 2 are the three pipelines that extend across the coastline out onto the nearshore shelf to depths of 20^35 m. 3.1. Rock outcrops The rock types that crop out on the sea£oor can be correlated with the rocks exposed on land around Monterey Bay. These are the Monterey Formation, Santa Cruz Mudstone, Purisima Formation, Vaqueros(?) Formation, Salinian granitic rocks, and unidenti¢ed Tertiary-sedimentary and Franciscan-complex rocks of the Sur Platform. The di¡erent rock types have distinctive acoustic signatures associated with them in most areas. On the inner shelf near and northwest of Santa Cruz and around the Monterey Peninsula, sea£oor rock outcrops are continuous with coastal cli¡ outcrops so that onshore^o¡shore correlations are straightforward (Anima and Tait, 1994; Anima et al., 2002). Clark (1981) and Brabb (1989) mapped the area around Santa Cruz and northwest to Point An‹o Nuevo and de¢ned the rock units in that vicinity. Also Wagner et al. (in press) has digitally compiled data from Clark (1981), Brabb (1989) and Dupre¤ (1990) for the area around Monterey Bay from east of Santa Cruz to the Monterey Peninsula. Hall (1991) and Dibblee (1999) have mapped the onshore geology of the Monterey Peninsula and south. McCulloch and Greene (1989) and Greene (1990) have extended these rock units o¡shore based on interpretation of deep seismic re£ection data and samples of outcrops with standard dredge. ROV and submersible observations were used, mostly from the slopes of upper Monterey and Soquel Canyon walls (Greene and Stakes, personal communication; Stakes et al., 1999a,b). McCulloch et al. (1985) and Nagel et al. (1986) reported lithologies and ages of 69 samples from

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the Monterey Bay area and these provide further evidence of the rock types in our imagery. Most of the latter samples are from the upper continental slope or canyon walls, but can be correlated with some con¢dence to nearby rocks that crop out on the shelf sea£oor. A complete description of these samples, their locations and their interpreted onshore rock correlatives, after McCulloch et al. (1985), and Nagel et al. (1986) are found in the CD-ROM at the back of this volume (Wong and Eittreim, 2002). 3.1.1. The Monterey Formation The Monterey Formation of Miocene age is exposed on the northern Monterey Bay shelf

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south of Point An‹o Nuevo, the southern Monterey Bay shelf north of the city of Monterey, and northwest and perhaps southwest of the Monterey Peninsula. The Monterey Formation is predominantly middle Miocene, and is widely found in the Coast Ranges of central California. It consists of thin to medium-bedded, laminated semi-siliceous mudstone and sandy siltstone with dolomite interbeds (Clark, 1981; Brabb, 1989). In sea£oor imagery (Fig. 5), as along the coast at Point An‹o Nuevo (White, 1990) the Monterey Formation appears as a ¢nely laminated, uniformly layered stratigraphy. Siliceous layers and dolomite interbeds likely provide di¡erential resistance to erosion and produce the layering observed on the

Fig. 5. Perspective plot of side-scan sonar imagery from area of Monterey Formation outcrop south of Point An‹o Nuevo combined with vertical-re£ection pro¢le taken along one line showing sub-sea£oor stratigraphy. Vertical pro¢le shows high-re£ectivity as dark pixels. Note the ¢nely-laminated bedding shown in side-scan sonar imagery, and the association of this sea£oor type with the terrace-like elevated terrain caused by the outcrop of southeast-dipping bedrock. See Fig. 2 for location.

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sea£oor. Both locations of Monterey Formation outcrop, north and south of the canyon, are near active fault zones: the San Gregorio fault near Point An‹o Nuevo and the Navy or Chuppines fault north of the Monterey Peninsula (Greene, 1990). In both instances, the fault-bound blocks are relatively uplifted on the west, exposing the older, generally more deeply buried rocks of the Monterey Formation. 3.1.2. The Vaqueros(?) Formation The Vaqueros(?) Formation is interpreted to crop out in a small area along the axis of an anticline observed on the sea£oor south of Point An‹o Nuevo. In the Santa Cruz Mountains, the Vaqueros(?) Formation is a thick-bedded to massive ¢ne-grained calcareous arkosic sandstone with interbeds of shale and mudstone and is of early Miocene and late Oligocene age (Brabb, 1989). Clark (1981) included the question mark after its name because this unit, discovered and dated in the cli¡ at Point An‹o Nuevo, has signi¢cant di¡erences to the type section east of the San Gregorio Fault. Its acoustic signature on the sea£oor is generally thick-bedded with highly variable backscatter intensity from its di¡erent beds. This unit is quite distinct in bedding style from the superjacent Monterey Formation. Because this unit forms an anticline and occupies a topographic low, we interpret it as equivalent to the Vaqueros(?) unit mapped by Clark (1981) in a similar structural setting in the small section of the coastal cli¡s immediately onshore. The Vaqueros(?) sandstone has also been identi¢ed at Pescadero Point, north of Point An‹o Nuevo, and on the Monterey Canyon wall at about 800 m depth near the Soquel Canyon intersection by ROV sampling (Stakes et al., 1999a,b). 3.1.3. The Santa Cruz Mudstone The Santa Cruz Mudstone occurs on the northern Monterey Bay inner shelf from the area south of Point An‹o Nuevo to Santa Cruz. Clark (1981) and Brabb (1989) describe this unit as a late Miocene, medium to thick-bedded, blocky weathering, siliceous mudstone with spheroidal concretions in the upper part. The unit is identi¢ed in sea£oor

Fig. 6. Side-scan sonar imagery from west edge of the outcrop of Santa Cruz Mudstone, northwest of the town of Davenport. See Fig. 2 for location.

acoustic imagery as layered, but with coarser layers (from meters to tens of meters in layer thickness) and having less distinctive layering than the Monterey Formation. In a band along the southwestern edge of the Santa Cruz Mudstone outcrop (Fig. 6), these rocks dip southwestward, and the erosional edges of beds are aligned parallel to the San Gregorio fault zone, which is adjacent on the west. 3.1.4. The Purisima Formation The Purisima Formation crops out on the inner-shelf sea£oor east of the o¡shore pipeline o¡ the city of Santa Cruz, south of Point An‹o Nuevo, and in most areas on the outer shelf in water depths greater than about 85 m. Clark (1981) and Brabb (1989) describe the Purisima Formation as late Miocene and Pliocene in age and consisting of very thick-bedded siltstone containing thick interbeds of ¢ne-grained sandstone with carbonate concretions. It is identi¢ed in sea£oor acoustic images as generally non-layered or weakly layered (Figs. 7 and 8). Diving and bottom-camera reconnaissance have shown it to consist, on the sea£oor around Santa Cruz, of an assortment of clasts, whose common concretions are scattered into a

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Fig. 7. Side-scan sonar imagery o¡ the city of Santa Cruz. The northwest^southeast bands of high backscatter are zones of coarse rippled sands. See Fig. 2 for location.

loose rubble on the sea£oor (Anima et al., 2002). McCulloch and Greene (1989) conclude this unit to comprise the principal pre-Holocene rock types that occur on the outer shelf and upper slope in this region. We suspect that the loosely rubbled surface that is observed on the sea£oor masks any layering that might otherwise be seen in this rock type (e.g. Fig. 7). 3.1.5. Aromas sand In northeastern Monterey Bay on the shelf between Aptos and Capitola, the Pleistocene-age

Aromas Sand Formation overlies the Purisima Formation and forms the pre-Holocene surface (McCulloch and Greene, 1989). We believe this eolian sand unit is covered on the shelf by a thin veneer of Holocene sediment but is exposed on the upper continental slope on the north side of Monterey Canyon where it forms zones of high backscatter. Because the Aromas is a relatively ¢ne-grained sandstone, it may be di⁄cult to distinguish it acoustically from the modern muds of the shelf and may be uncovered on the sea£oor in some locales in this northeastern corner of Mon-

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Fig. 8. Perspective plot of side-scan sonar imagery combined with vertical-re£ection pro¢les along two lines showing sub-sea£oor stratigraphy associated with a bedrock outcrop on the mid to outer-shelf. Note the westward-dipping layers that produce erosional cuestas of bedrock on the sea£oor. See Fig. 2 for location.

terey Bay, though our preferred interpretation (Fig. 2) is for total coverage with the ‘mud’ category. 3.1.6. Salinian granitic rocks Salinian granitic rocks of the Monterey Peninsula crop out on the shelf sea£oor surrounding the Peninsula, on the upper continental slope, and on the narrow shelf and upper slope south of the Monterey Peninsula from Pt. Lobos to Rocky Pt. These granitic rocks form the core of the Monterey Peninsula (e.g. Greene, 1990). Acoustically, these rocks have a distinctive knobby joint and fracture pattern that is unlike any of the sedimentary rocks that crop out on the sea£oor (Fig. 3a). 3.1.7. Tertiary and Franciscan complex rocks of the Sur Platform Layered sedimentary rocks have been uplifted in a welt along the southern San Gregorio Fault west of Pt. Lobos (Fig. 9). The layered strata are bent into drag folds and truncated at the fault

lineament. McCulloch et al. (1985) reported siltstones, pebble conglomerates and limestones of Miocene age in ¢ve dredge hauls from the top and sides of this bedrock welt. West of Point Sur, rocks that form the bedrock of the Sur Platform, southwest of the San Gregorio Fault, consist of elliptical outcrops that appear to be northwest-plunging faulted-fold belts (Figs. 2 and 10). Based on dip directions, the oldest, or underlying, rocks are those southwest of Pt Sur. This outcrop is apparently the most resistant to erosion, forming a bathymetrically complex area with pinnacles that reach nearly 20 m above the surrounding sea£oor. The proximity to the rock units mapped onshore and inboard of Point Sur by Hall (1991) suggests that these rocks southwest of the San Gregorio Fault and forming the Sur Platform are correlatives with Tertiary (Monterey Formation?) units and rocks of the Franciscan complex in this area. 3.2. Coarse sand On the inner shelf around Monterey Bay,

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Fig. 9. Shaded-relief imagery, with sun angle from the east, showing area of bedrock welt that has been uplifted along lineament of San Gregorio Fault. Drag folds record right-lateral motion shown by large arrows. 500 m to the west of the main fault a lineament in the smooth, uniform-backscatter muds of the shelf shows the most recent trace of this fault. See Fig. 2 for location.

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Fig. 10. Shaded-relief imagery, EM300 data, with sun angle from the west, showing area of southern-most plunging fold belt, believed to be Cretaceous^Jurassic metasedimentary rocks of the Franciscan complex. North of the outcrop note the dunes of ¢ner sediment (‘mud’ category) overlying ‘coarse-sand’ substrate and the headwall scarp that separates the two sediment types. See Fig. 2 for location.

coarse sand occurs on the £oors of distinct, irregularly shaped, rippled depressions approximately 1 m deep. On the northern inner shelf Fig. 11 shows coarse-sand depressions in a valley o¡shore Santa Cruz and the San Lorenzo River mouth. The ripples are spaced at about 1^2-m intervals and are quasi-parallel to the coastline, or approximately parallel to the alignment of the approaching swell. On careful examination of the record of vertical-incidence re£ected sound arrivals, a step of about 1 m in relief can be seen at the trough edges.

The most distinctive and best-studied examples of coarse-sand rippled depressions occur in southeastern Monterey Bay (Fig. 2) where we have multibeam-survey coverage. In this area these depressions have been surveyed and sampled by Hunter et al. (1988) and Mariant (1993). The coincident bathymetric and backscatter data shows that the 1-m deep troughs consistently exhibit high backscatter £oors (Fig. 12). Rippled sand depressions similar to these have been studied previously on the northern California shelf (Cacchione et al., 1984). In both towed-¢sh side-scan

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Fig. 11. Examples of two rippled scour depressions, south of the city of Santa Cruz in 15 m water-depth. Ripples have 1^2-m wavelength. See Fig. 7 for location.

sonar data and multibeam backscatter data these ripples impart a high backscatter strength that is highly dependent on relative look-azimuth, with maximum amplitude backscatter along tracks parallel to ripples. This suggests that the ripple microtopography rather than the coarse-sand size cause the high backscatter of this bottom type. Fig. 12 shows shaded-relief imagery of the deepest set of Monterey Bay rippled sand depressions. The outer limiting water depth of the tip of the deepest depression is 59 m, about the same as for those observed o¡ the Mendocino coast (Cacchione et al., 1984). In the 10^30 m water-depth range similar rippled sand depressions occur up and down the southeast Monterey Bay coast north of the town of Seaside (Fig. 2). Both shore-normal, and shoreparallel depressions occur here, with the shoreparallel depressions in the 10^20 m depth range (Hunter et al., 1988). These shallower depressions, with their coarse-sand £oors, have been temporally sampled with side-scan sonar and demonstrated to have changed shape and location on

time scales of months (Hunter et al., 1988). This contrasts with the deeper depression system shown in Fig. 12, that has been surveyed on four occasions over 2 years and was observed on all surveys to have an identical shape and position. Fig. 12 also shows the symmetrical 1^2-m wavelength ripples that form the £oor of the depression at 59 m water-depth. Ripple heights of 15 cm have been determined by Hunter et al. (1988) for the depressions in 20 m water-depth, north of Seaside. Coarse sand with the same acoustic character as that limited by trough boundaries and discussed above also occurs, unlimited by trough boundaries, over large areas of the sea£oor surrounding the granitic rock outcrops of the Monterey Peninsula (Fig. 2). Due to a lack of data with 1 m or ¢ner resolution in this area, we cannot say for sure if this sand is rippled similar to those discussed above, but because all other characteristics appear the same, we assume that it is. This area is one exposed to large ocean swell, with limited sources of ¢ne sediment and perhaps a

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Fig. 12. Broad ¢eld of coarse-sand rippled-scour depressions, north of the city of Monterey in water depths ranging from 20 to 60 m. High-resolution side-scan image at tip of rippled-scour depression was obtained with Klein 100-kHz system. Pervasive orthogonal pattern in shaded-relief image is due to system noise produced by wave motion. See Fig. 2 for location.

relatively rich source of coarse sand eroding from the granitic rocks of the area. The boundaries between the high-backscatter coarse sand and the low-backscatter mud also consists of, as is the case with the bounded rippled depressions, an approximately 1-m step-up to the level of the mud. Using the same criteria for recognition of coarse sand as above (high and uniform backscatter, delimited from low-backscatter muds at depression boundaries) these same coarse-sand facies is observed further south. South of the Monterey Peninsula on the Sur Platform, broad expanses of coarse sand, bounded by ¢ner sediment at abrupt step-up boundaries, are observed similar to the pattern around the Monterey Peninsula (Fig. 2). The seaward edge of these coarse deposits commonly consist of thin pinchouts or tips that point directly downslope (e.g. Fig. 3f). These tips extend to greater depth, up to 100 m, than the deepest of the rippled depressions referred to above in southeast Monterey Bay. In the area due west of Pt Sur, the ¢ner sediment on top is also formed into £at dune-like structures with crescentic shapes whose tips point o¡shore

(see Fig. 10). The largest expanse of coarse sand, measuring about 3 kmU7 km, is on the north side of the southern-most elliptical outcrop due west of Pt Sur (Fig. 2). Here the 5-km-long northern boundary of the coarse sand consists of a 1-m high headwall scarp with a 300-m wavelength scalloped pattern. High- and uniform-backscatter sea£oor that displays ripples also occurs in some of the erosional troughs cut through the bedrock outcrops of the inner shelf in the region from Santa Cruz to Point An‹o Nuevo and is discussed by Anima et al. (2002). 3.3. Mud Of all the categories of sea£oor bottom type, this occupies the largest area. This category represents sediment being deposited today, at rates averaging 2.7 mm/yr in depositional areas (Lewis et al., 2002). The acoustic characteristic of this bottom type is low and uniform backscatter, and absence of observable relief, where the latter is known from multibeam bathymetric data. There is good agreement between our de¢ned

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areas of this bottom type and the areas of dominantly silt-sized material as measured by textural analysis of bottom samples (Edwards, 2002). Also, these mid-shelf areas are the sites of thickest deposits of Holocene sediment, both on the northern and southern shelves, de¢ned by Greene (1977), Mullins et al. (1985), and Chin et al. (1988) as sediments younger than a prominent widespread erosional unconformity. This erosional unconformity has recently be dated as 14.8 kyr cal BP 14 C, or base of Holocene (Eittreim et al., 2001). Sea£oor video observations have shown that the ¢ne sediment of this category commonly

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has small-scale wave-oscillatory ripples at the surface suggesting a signi¢cant ¢ne-sand component. 3.4. Relict gravels This category describes areas on the outer shelf with elevated backscatter, but with the higher backscatter area bounded by gradational edges rather than sharply de¢ned (e.g. Fig. 4). This category occurs west of the Salinas River and Castroville on the outer shelf, and directly across the canyon on the outer shelf due south of Capitola (Fig. 2). In the area designated with

Fig. 13. Side-scan sonar imagery of low-backscatter lineament, parallel to the northern San Gregorio Fault, interrupted by two isolated high-backscatter targets. This low-backscatter lineament probably represents a small trough ¢lled with low-backscatter muds. See Fig. 2 for location.

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this category west of the Salinas River on the outer shelf, relict gravels with Pleistocene molluscs have been reported by Chin et al. (1988) and gravels and sandy gravels have been reported from samples of Yancey (1968) and Malone (1970). Submersible dive reports by H.G. Greene (1998, personal communication) in this area report similar ¢ndings that suggest relict Pleistocene gravels and sands. Another possible source of the higher backscatter of these deposits, in addition to a coarse gravelly nature, is carbonate cementation, through migration of £uids from below. Such carbonate cemented deposits have been commonly observed by submersible and ROV dives on the outer shelves of Monterey Bay (H.G. Greene, 1998, personal communication; Stakes et al., 1999b). 3.5. Active fault lineations Straight lineations that a¡ect the modern muds of the shelf and that are related to faults are observed in only two locations. In association with the San Gregorio Fault (McHugh et al., 1998) there are short segments on the northern and southern shelves where the mud surface shows distinct lineations parallel to the fault (Fig. 2). At the northern site, the surface expression consists of an approximately 1-m-wide low-backscatter lineament, presumably a small trough ¢lled with low-backscatter Holocene muds, along a 700-m length (Fig. 13). At two locations highbackscatter zones, of a few meters to tens of meters in length, interrupt the low-backscatter lineament. These bright spots have been observed with ROV and discovered to be carbonate-mound deposits associated with £uid £ow (C. Paull, 2001, personal communication). At the southern San Gregorio Fault site, southwest of Pt Lobos, an elongate 10-km-long welt of basement rocks with clearly delineated right-lateral o¡set, de¢ned by accompanying drag folds, stands a few meters above the surrounding recent sediment of the continental shelf (Fig. 9). This clearly de¢ned fault lineament is the boundary between the eastern Salinian crustal block and the western San Simeon block (McCulloch, 1989). In addition to the high-standing welt, a modern or active 10-

km long lineament, in the form of a small step or escarpment (about 1 m or less), is also apparent in the surface of Holocene muds of the shelf, 500 m to the west of the main lineament (Fig. 9). Although numerous other relatively young faults are observed to cut Holocene sediment below the surface (Mullins et al., 1985), the above two sites are the only places we surveyed where the sur¢cial modern mud is apparently disrupted. 3.6. Pipelines The only anthropogenic materials that we mapped on the sea£oor are three wastewater pipelines, all built on footings of gravel and broken rock, that cross the shoreline to depths of about 30 m on the inner shelf (Fig. 14). The three pipelines, ranging in size from 1 to 2 m in diameter, were completed in 1988, 1987 and 1982, respectively, clockwise around the bay at Santa Cruz, Watsonville/Parajo and Marina. Along the eastern side of the 4.2-km-long Santa Cruz pipeline, high-backscatter sands have collected in bands perpendicular to the pipeline trend. Some e¡ect of the Marina pipeline is seen in patterns of sedimentation, with the sediment surface about 1 m higher on the north side than on the south, re£ecting some sediment-redistribution e¡ects since 1982. Also, this pipeline has resulted in an erosional moat, approximately 1 m deep, along both sides of its rock-debris foundation.

4. Discussion The sur¢cial geology and morphology of the Monterey Bay shelf is a product of shoreline transgression and regression over past millenia with the rise and fall of sea level, in combination with local tectonic movements. The most recent lowstand of sea level, which climaxed at about 20 000 yr BP at 3120 m, put the shoreline just beyond the shelf break. Prior to that, sea level £uctuated between about 380 and 320 m, with a rough periodicity of about 20 000 years (Pinter and Gardner, 1989). Thus the continental shelf, particularly the region between 20 and 80 m water-depth, has been subject to extensive shore-

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Fig. 14. Imagery of the three major waste-water pipeline outfalls of Monterey Bay, from upper left, clockwise around the Bay. The two upper images, Santa Cruz and Pajaro, are side-scan sonar imagery and the lower right, Marina, is multibeam EM1000 backscatter (upper) and shaded relief (lower) with sun angle from north. Pervasive orthogonal pattern in shaded-relief image is due to system noise produced by wave motion.

line erosion. During the last 100 000 years the southern £ank of the Santa Cruz Mountains to the north has been uplifted by about 30 m near the shoreline, based on uplifted dated marine terraces (Bradley and Griggs, 1976). This uplift is in response to compression at the restraining bend of the San Andreas Fault along the crest of the Santa Cruz Mountains, about 20 km northeast of the coast, where 90 m of uplift has occurred (Anderson, 1994; Burgmann et al., 1994). The Monterey Peninsula is also undergoing uplift as evidenced by the £ights of marine terraces that festoon its £anks (Dupre¤, 1990). In contrast, in southeastern Monterey Bay from Moss Landing to Seaside, at the terminus of the Salinas Valley, the shoreline is stable (Dupre¤, 1990). The pattern of outcrops and recent sediment distribution on the shelf sea£oor is consistent with the above tectonic variations around the bay. On the inner-shelf, shallow or exposed bedrock occurs in areas that have a history of uplift, with the Purisima Formation and Santa Cruz Mudstone being the youngest of the uplifted

pre-Holocene sedimentary rock units. The Salinian granitic rocks and the Monterey Formation have been similarly uplifted around the Monterey Peninsula. In the southeastern interior of Monterey Bay, farther removed from the uplifting highlands, where less uplift and possible subsidence is occurring, bedrock is more deeply buried and the sea£oor consists of only Holocene sediment. The broad mid-shelf regions, having been eroded down to a level in equilibrium with past sea-level positions, have extra accommodation space that now is rapidly ¢lling with muds at sea£oor depths below the high-energy conditions produced by storm waves (see Lewis et al., 2002). A similar ‘mid-shelf mud belt’ was found on the Washington shelf north of the Columbia River (Nittrouer and Sternberg, 1981). As Mullins et al. (1985) and Greene (1977) have shown, the mid-Santa Cruz shelf region has a covering of sediment, up to 30 m thick, above a prominent erosional unconformity, dated as base of Holocene (Fig. 15). With time this sediment may build out to the shelf break, eventually covering the outer shelf rock

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Fig. 15. Contours of sediment thickness above the basal Holocene erosional unconformity (after Greene, 1977; Mullins et al., 1985; Chin et al., 1988).

outcrops now exposed. An alternative explanation for the mud-free outer-shelf is that the outer shelf is a high-energy zone, caused by internal-wave breaking (Cacchione and Drake, 1986) or other high-energy conditions associated with the shelfbreak that inhibit ¢ne-sediment deposition. An exception to the simple pattern for the Santa Cruz shelf of rock outcrops on the outer and inner shelves, and muds on the mid-shelf, is the region along the west side of the San Gregorio Fault near the northwest boundary of our coverage (Fig. 2). Here, older rocks that generally lie at greater depths under the shelf have been tectoni-

cally raised into a feature that Nagel et al. (1986) termed the Ascension-Monterey High (an example of which is shown in Fig. 8), a feature that interrupts the otherwise continuous mid-shelf mud belt. Similarly, the southern Monterey Bay shelf, north of the Monterey Peninsula, is gradually ¢lling with sediment. The Salinas River is the primary source of sediment in the south bay, and the outer limit of its mud-delta lies at approximately 90 m water-depth, with outcrops of Purisima Formation and relict Pleistocene sand and gravel beyond (Chin et al., 1988). North of Moss Landing

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the Pajaro and San Lorenzo Rivers are signi¢cant sources of mud and, probably of somewhat lesser importance, is cli¡ erosion of the silt-rich Purisima Formation and Santa Cruz Mudstone (Dingler et al., 1985; Best and Griggs, 1991). In southeastern Monterey Bay around Moss Landing and the Salinas River, unlike the area o¡ Santa Cruz, the shelf is not uplifting and the inner-shelf sea£oor consists of Holocene sediment rather than Tertiary outcrops. To reiterate, the pattern of exposed rock outcrops on the inner and outer Santa Cruz shelf, separated by sediment-covered areas between, can be explained as the result of the inner shelves being proximal to tectonically uplifting areas whereas the outer shelves may be beyond the reach of the outward-prograding Holocene sediment. In addition, sedimentation might be inhibited near the shelf break due to high-energy conditions. The pro¢le of Fig. 16 illustrates this pattern for the northern shelf. On the shelf o¡ the Monterey Peninsula and southward, it appears there is no mid-shelf mudbelt as on the Monterey Bay northern and southcentral shelves, perhaps due to both the high-energy wave climate and a lack of ¢ne-grained sources of sediment. On the Sur Platform and on the

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narrow shelf to the southwest of Pt. Lobos (Fig. 2) the sea£oor geology is apparently dominated by translational tectonics, active today, and older Cretaceous accretionary tectonics (McCulloch, 1989; Hall, 1991). The southern San Gregorio Fault has produced an uplifted bedrock welt consisting of a right-lateral strike-slip tectonic zone (Fig. 9). Farther south on the Sur Platform, the rock outcrops consist of distinct bodies of exposed bedrock that appear to consist of three individual plunging fold belts. The largest and southernmost of these bodies consists of high-relief topography with abruptly rising pinnacles (Fig. 10). No samples have been reported from this outcrop, but its proximity to the rocks of the Franciscan Complex onshore (Hall, 1991), combined with its faulted-fold style suggest rocks of the Franciscan Complex. The Pt Lobos-Sur Platform shelf is apparently starved of sediment and, consequently, combined perhaps with the high wave-energy climate here, the sediment is relatively coarse-grained, and pre-Holocene bedrock is relatively uncovered. Vast areas of high-backscatter coarse-sand deposits, measuring a total of 44 km2 , cover the shelf from the northern Monterey Peninsula south to the Sur Platform (Fig. 2). On the high-energy

Fig. 16. Combined high-resolution seismic line and onshore topographic pro¢le through the Sand-Hill Blu¡ coastal region and line-drawing interpretation of seismic section below. Onshore topography from USGS Santa Cruz Quadrangle topographic map, 7.5 minute series. Terrace ages taken from Anderson et al. (1990). Line located in Fig. 2.

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Sur Platform, these ‘coarse sands’ might in places be composed of material up to gravel size (McCulloch et al., 1985). These deposits are remarkable for their wide areal expanse and arcuate-shaped bounding edges. As discussed previously, the coarse-sand/mud boundary is consistently observed as a step-up of approximately 1 m, from coarse sand to mud. This geometry can be explained by a layer 1 m thick, of mud covering a widespread coarse-sand layer in the subsurface that is exposed in some places, and covered in others. Such a widespread underlying coarse-sand source would be expected as a transgressive lag deposit at the base of Holocene sediment and on top of the erosional truncation that is clearly apparent in seismic re£ection data from the Monterey Bay shelves (Greene, 1977; Mullins et al., 1985; Chin et al., 1988). However, some of the coarse-sand ripple depressions in 10^30 m water-depths actively change shape (Hunter et al., 1988) and the ripples on the £oors of depressions (to depths at least to 59 m in southeast Monterey Bay) are in equilibrium with large winter storm waves (Xu and Eittreim, 1997). Consequently, although the source of these coarsesand deposits may be a base-of-Holocene transgressive lag, the coarse sand at the surface is modern in the sense that it is being mobilized today by the oscillatory motion of the large winter storm waves of the shelf. Whether or not there is any net-transport of these sands across the shelf surface is unknown. Understanding the dynamics of the rippled-sand depressions and the mud/sand boundaries is a ¢rst-rank research problem, well beyond the scope of this paper. See Hunter et al. (1988), Mariant (1993), Cacchione et al. (1984) and Xu and Eittreim (1997) for further discussion. The range of coarse-sand deposits on the Monterey Bay shelf and south to the Sur Platform have outer limits that probably re£ect the increasing wave energy conditions from north to south. These outer limits range from 20 m water-depth in the protected northeastern Monterey Bay o¡ Capitola, 35 m depth in the less protected area o¡ Davenport, 59 m depth in the southern Monterey Bay o¡ Seaside, and 100 m depth o¡ the Monterey Peninsula and south to Point Sur.

Modern sources of coarse sand, other than underlying transgressive lag deposits, may also be important in de¢ning where these coarse sands occur. A prime source may be the eroded Salinian granitic rocks of the Monterey Peninsula. South of Monterey there are only small coastal streams and few signi¢cant ¢ne-sediment sources that drain large watersheds (Griggs and Hein, 1980).

5. Conclusions (1) Pre-Holocene rocks, most of which can be con¢dently correlated with rock units that have been mapped onshore, crop out on the inner and outer shelves of the greater Monterey Bay area, with the mid-shelf generally dominated by the mid-shelf mud belt, which forms a Holocene ¢ne-grained sediment wedge. An exception to this generalization is in the area adjacent to and south of the Monterey Peninsula that lacks ¢ne sediment sources, and is an area of high storm-wave energy conditions. (2) Coarse-sand deposits, whose source may be a Pleistocene transgressive lag deposit that underlies most of the continental shelf, lie at the sea£oor where windows through the modern mud deposits are caused by high-energy oscillatory bottom currents. The dynamics of these sand deposits are not well understood. It has been demonstrated by Cacchione et al. (1984) and Xu and Eittreim (1997) that the 1^2-m wavelength symmetrical ripples common in these deposits are in resonance with the benthic oscillations due to the largest winter storm waves that impinge on this coast. (3) The large areas of ¢ne sediment that cover most of the Monterey Bay mid-shelf regions, judging by their geographic coverage, comprise mud-deltas or sea£oor depositional plumes of the principal rivers that enter into the bay, the Salinas, San Lorenzo and Parajo Rivers (also see Edwards, 2002 and Eittreim et al., 2002). The decrease in thickness of this ¢ne sediment cover, from the Santa Cruz area south to Monterey Peninsula and the Sur Platform, and the increasing dominance of coarse sand southward, probably re£ects both the lack of ¢ne sediment

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sources on the Peninsula and the Sur coast, and the large wave-energy conditions there. (4) Despite the fact that the San Gregorio Fault is acknowledged as an active fault, there are only two segments on the continental shelf, one 700 m long, the other 10 km long, where fault lineations that a¡ect modern sediments are seen in sea£oor acoustic imagery. This suggests that Holocene shelf sedimentation has buried evidence of all but the most recent activity.

Acknowledgements We express our sincere thanks to the MBNMS office in Monterey for their logistics and ship support in data gathering and the Research Activities Panel of MBNMS which acts as a scientific communication link and advisory board for scientists involved in MBNMS investigations. We thank Monty Hampton, James Gardner and Sam Clarke for helpful comments in reviewing previous versions of this paper. Florence Wong and Wilson Lee helped with GIS-building and figures and Larry Mayer helped in multibeam data processing.

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