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This report presents a series of maps that describe the bathymetry and late Quaternary geology of the San Pedro shelf area as interpreted from seismic-reflection profiles and 3.5-kHz and multibeam bathymetric data. Some of the seismic-reflection profiles were collected with Uniboom and 120-kJ sparker during surveys conducted by the U.S. Geological Survey (USGS) in 1973 (K-2-73-SC), 1978 (S-2-78-SC), and 1979 (S-2a-79-SC). The remaining seismic-reflection profiles were collected in 2000 using Geopulse boomer and minisparker during USGS cruise A-1-00-SC. The report consists of seven oversized sheets: 1. Map of 1978 and 1979 uniboom seismic-reflection and 3.5-kHz tracklines used to map faults and folds on San Pedro Shelf. 2. Maps of multibeam shaded bathymetric relief with faults and folds, and bathymetric contours. 3. Isopach map of unconsolidated sediment, seismic-reflection profile across the San Pedro shelf, seismic-reflection profile across San Gabriel paleo-valley. 4. Seismic-reflection profiles across the Palos Verdes Fault Zone. 5. Geologic map and samples of Uniboom and 120-kJ sparker seismic-reflection profiles used to make the map. 6. Map showing thickness of uppermost (Holocene?) sediment layer. 7. Map of San Gabriel Canyon paleo-valley and associated drainage basins.

The surface morphology of the northern slope of the Brooks Range (North Slope) from the Canning River, Alaska, eastward is dominated by a series of large alluvial fans and braided streams floored by coarse alluvium. On the basis of the absence of sediment on the seasonal sea ice after spring flooding, the measured stability of fan fronts during a 30year period, and the sparsity or absence of Holocene marine sedimentary deposits seaward of the fans, we conclude that the fans are not prograding now nor have they been prograding at any time during the Holocene. In numerous areas, surficial fan alluvium terminates on land at or below 2-m-thick glaciomarine deposits. These deposits, the Flaxman Member of the Gubik Formation, formed during the latest major interglacial transgression (oxygen-isotope stage 5a), about 85-80 ka. We therefore believe that the fans are still older. Our observations suggest that during the latest transgression and the following sea-level highstand, the North Slope depositional environment and climate differed greatly from the present ones.

Northward-flowing rivers of Alaska inundate extensive areas of sea ice during spring breakup. Drainage of large volumes of fresh water through the ice at holes and cracks (strudel) causes scour depressions more than 4 m deep and as much as 20 m or more across in the sea floor below. These modern strudel scours and their filled counterparts were studied by side-scan sonar, echosounder, high-resolution seismic gear, diving observations, and sediment sampling. Strudel scours occur within 30 km of river mouths. Their shapes and distribution patterns correspond to those of strudel seen in the ice canopy. The scours are most numerous in the inner areas of overflow, as many as 25 per kilometer of ship's track. Strudel scours also occur outside of the 1970-72 overflow areas; these must be relatively old. One strudel scour investigated by diving was surrounded by a ridge, had vertical walls exposing a tundra horizon, and terminated at a gravel layer 4 m below a lagoon floor. Another one terminated at a semiconsolidated layer of silty clay. Both the gravel and silty clay are pre-Holocene deposits. From highly irregular bedding in seismic records it is apparent that virtually all of the Holocene deposits and significant parts of underlying older sediments around river mouths have been reworked by strudel scour. This mixing of sediments by vertical strudel flow causes sediment types to vary greatly over small areas. These new observations add further complicating factors to be considered in the interpretation of shallow-water deposits of cold climates.