Abstract Regressive coastal deposits containing internal downlapping surfaces are common on continental shelves of the world. Through theoretical considerations and evaluation of examples from the literature and our own studies in California and Italy, we have examined the conditions that lead both to the formation and preservation of these deposits. Coastal downlapping deposits form by progradation of coastal and deltaic lithosomes during stable and falling sea level. in addition to the rate and direction of sea-level change, the major controls governing the development of downlapping deposits are sediment availability and shelf morphology (gradient, surface irregularity, and depth of shelf break). Continental margins receiving a large, continuous supply of sediment commonly have vertical stacks of thick, laterally extensive deposits; those fed by relatively small coastal streams may have well-developed shelf-margin deposits if accommodation space was available. in the absence of feeder streams, shelf-margin downlapping deposits can form from locally derived sediment. Regressive coastal deposits are preserved in a variety of settings, but they are least likely to be preserved on broad, low-gradient (coastal plain) shelves, where small drops in relative sea level were accompanied by large seaward shifts of the shoreline. Even on high-gradient tectonic margins, downlapping deposits commonly are not preserved on the midshelf. The process of shoreface erosion during a transgression is efficient at planing off deposits from previous depositional cycles. Where the deposits are thin, the deposit is partially or wholly reworked; where they are thick, the basal part may be preserved. Regressive deposits are most likely to be preserved along the shelf margin, where relatively thick sequences form at or below the position of the lowstand sea level. On many continental shelves, the coarse sandy texture of outer-shelf sediment largely reflects original deposition in a regressive coastal environment during a fall and lowstand of relative sea level.

ABSTRACT Analysis of wide-swath side-scan sonar (GLORIA) data from Eel River Basin shows the basin to have a distinctive surface morphologic and structural expression that documents the influence of interaction between the North American and Gorda Plates during the Quaternary. Elongate, north-trending anticlinal folds and thrust faults occur along and west of the basin axis out to the base of the continental slope. The deformation front defined by these structures increases in age to the west and evolves through a sequence of diapiric deformation in the east, followed by broad anticlinal folding, and finally by large-scale thrust faulting to the west. The deformation front curves inland to the southeast near the southern end of the basin. Young anticlinal folds, cored in places by diapirs, break through the seafloor. Additional information from photos, seismic-reflection profiles, and samples of the anticlinal folds show shale flowage and melange. GLORIA images show halos around rising dome structures, indicating active uplift. Submarine drainage patterns visible on the GLORIA sonographs are controlled by structure; channels and canyons follow fault traces in places and elsewhere are diverted by rising anticlines. Eel River Basin has a high rate of sediment input, and deposits are being deformed both by tectonic compression and by an abundance of slope failures that exceeds most continental margins. Debris slides on the seafloor, in association with growing anticlinal ridges, document recency of these processes.

ABSTRACT Eel River Basin, offshore northern California, contains geochemical and geophysical evidence that hydrocarbon gases are present in the sediment column as indicated by 1) measurements of hydrocarbon gas in samples of near-surface sediment; 2) seafloor craters and gas plumes imaged on side-scan sonar records; 3) acoustic anomalies on seismic records of Quaternary and pre-Quaternary strata; and 4) bottom-simulating reflectors, which are acoustic anomalies indicating the possible presence of gas hydrates. This evidence results from four U.S. Geological Survey research cruises to the Eel River Basin and is derived from analysis of short (1-3 m) gravity cores and information from single channel sparker, airgun, and high-resolution boomer and transducer records. The presence of hydrocarbon gases in surface sediment is based on the analyses of forty-two sediment cores, nine of which contained concentrations of methane in excess of 10,000 μL/L of wet sediment. These high concentrations of methane greatly exceed the concentrations of accompanying ethane and propane and indicate a probable microbial origin. One sample, however, contained anomalous amounts of gas- and gasoline-range hydrocarbons, and this mixture was most likely thermogenically derived. Acoustic anomalies suggestive of high concentrations of gas are most common on seismic records of sediment beneath the upper slope and outer shelf, an area where large seep craters have recently been discovered. On the shelf, acoustic anomalies are evident in pre-Quaternary deposits in anticlines and along bedding planes, and in Holocene deposits. Seismic records from the outer portion of the Eel River Basin show a well-defined, bottom-simulating-reflector (BSR). The depth, location, and character of this BSR suggest that it marks the base of an extensive gas hydrate deposit. The combined lines of evidence—direct gas measurements, craters and gas plumes, acoustic anomalies including BSRs—all indicate that the basinal sequence of sediment has variable concentrations of microbial and thermogenic hydrocarbon gas.

Abstract Economically important sandstone bodies enclosed in marine shales have been described from several areas of the Western Interior region in Utah, Colorado, Wyoming, and New Mexico. Many of these sandstone bodies are believed to have been deposited in open marine, mid- to outer-shelf settings. This study in northern Colorado demonstrates that many of these sand bodies were deposited originally in nearshore and shallow marine settings and that their present occurrence and distribution was largely dependent upon eustatic sea level changes and minor transgressive-regressive phases of local origin. Along the western margin of the Upper Cretaceous seaway in Utah, nearshore sedimentation rates apparently were high due to uplift in nearby source areas, and shelf areas were broad and shallow. These factors resulted in discontinuous seaward migration of strandlines into former shelf areas of western Colorado. Basinal areas lay to the east in areas where subsidence was greatest. Discrete sandstone members of the lower Mesaverde Group and Mancos and Pierre shales in northern Colorado include the following depositional environments: estuarine or distributary channels, reworked distributary mouth bars, distributary mouth bars, delta front sheet sands, pro-delta sand sheets, shoreface, and mid- to outer-shelf sand ridges. Variations in texture, sorting, bioturbation, thickness and bedding of these units reflect variations in rate of deposition and in wave and current energy. Evidence of transgressive reworking is present locally at the tops of some units, as well as in discrete sandstones that are completely encased in marine shale. Although some sandstone members (e.g., the Hygiene Sandstone of the Pierre Shale) have been correlated previously over distances exceeding 80 kms, our examination of cross-bedding patterns and composition indicates that several distinct environments of deposition are present, including nearshore to outer shelf environments. Cross-stratification (medium-scale tabular) and lithology of the easternmost exposures of the Hygiene are interpreted to be similar to those in modem sand ridges on the U. S. Atlantic, Bering Sea, and North Sea continental shelves, whereas massive to cross-bedded sandstones to the west were deposited in delta front environments. The sand was derived from the west, transported eastward, and then redistributed by southward-flowing storm and oceanic currents.

ABSTRACT Pacific-style continental margins, such as that of western North America, are marked by large contrasts in the type of shelfedge sedimentary deposits and the processes that form them. The Pacific shelves of the United States are generally much narrower than the Atlantic shelves, and the source areas exhibit more relief. The greater relief of Pacific coast source terranes results in a relatively high rate of sedimentation in humid areas and fluctuating (areally and seasonally) sedimentation patterns and rates in semiarid areas. Sediment shed from the adjacent landmass is discharged, generally seasonally, onto the Pacific Continental Shelf at point sources. Many of the sediment sources of the northwestern United States and southern Alaska feed directly onto swell- and storm-dominated shelves. On such narrow unprotected shelves, sediment has a short residence time in submarine deltaic deposits before being remobilized and dispersed to outer-shelf and upper-slope environments. Through study of high-resolution seismic-reflection profiles, we have identified four principal types of shelfedge deposits: (1) starved, (2) draped, (3) prograded, and (4) upbuilt and outbuilt. Each type of shelfedge deposit results from a characteristic balance between sedimentation rate and distributive energy (waves and currents) and is, therefore, characterized by distinctive seismic facies and bedding patterns. A special type, the cut-and-fill shelfedge, and a composite type consisting of two or more of the main depositional styles supplement the four principal types of shelfedge. Incorporated within each of these facies, especially on the upper slope, are chaotic deposits formed by slumps or slides, which are common along technically active margins.

Abstract The need to expand the search for energy resources in deeper marine environments has intensified the importance of better understanding the nature and origin of continental slope settings and of acquiring a working knowledge of their characteristics. It has been known for a number of years that coarse-grained mass-flow deposits beyond the shelf break can form major petroleum reservoirs (Barbat, 1958), and it is likely that these deep-water environments will continue to be future exploration targets (Hedberg, 1970; Curran et al, 1971; Gardett, 1971; Nagel and Parker, 1971; Yarborough, 1971; Cooke et al, 1972; Schlanger and Combs, 1975; Enos, 1977; Walker, 1978; Wilde et al, 1978). Recently, however, with the concept of plate tectonics, seismic stratigraphy, and advances in seismic-reflection technology, there has emerged a more sophisticated approach to understanding the developments of continental margins. This understanding has placed more emphasis on the geological history and petroleum potential of continental slopes (Burk and Drake, 1974; Weeks, 1974; Bouma et al, 1976; Thompson, 1976; Bloomer, 1977; Schlee et al, 1977; Mattick et al, 1978). This paper presents a practical guide for recognizing continental slope sequences. Criteria are presented that we believe are common, or could be common, to all slopes regardless of whether they are adjacent to active, passive or buttressed continental margins.

Abstract Large-scale sediment slides and slumps, measuring several square kilometers in area, have been described from basin slopes in southern California as well as from many other continental slopes of the world. However, little attention has been focused on smaller features that may reflect significant downslope transport of sediment by small-scale mass movements. High-resolution seismic reflection data, seafloor photographs, and side-scan sonographs obtained using a deeply-towed geophysical instrument package show that at least six such features or zones of submarine gravity transport are present in a relatively small (150 km 2 ) area on an intercanyon slope off San Nicolas Island in the southern California borderland. Each zone is small (hundreds of meters on a side) and somewhat different from the others in age, shape, and inferred origin. These features range from rotational slumps to chaotic slides. Photographs show rounding and extension fractures in transported clasts of sediment, and a difference in the activity of benthic organisms within and outside of the slide zones. The thickening of sedimentary units downslope, coupled with the presence of small features indicative of sediment failure, suggests that gravity-driven processes contribute significantly to the construction of basin slopes in this region.