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J. L. Chin

Publications and source records attributed to J. L. Chin.

12 recordsLinked to original sources
USGS1997

Shallow subsurface geology of the continental shelf, Gulf of the Farallones, California, and its relationship to surficial seafloor characteristics

The Gulf of the Farallones is located on a continental margin that is tectonically active and has experienced eustatic fluctuations of sea level throughout the Quaternary. Bathymetry of the Gulf suggests that it is different from the average continental shelf off California in that instead of a seafloor that slopes gently seaward from the shoreline to the shelf edge (dominantly west to southwest), much of the Gulf shelf slopes to the northwest, a direction that is subparallel to that of the adjacent mainland shoreline. Isobaths are oblique to shore-normal for much of the Gulf shelf as opposed to the more typical shore-parallel isobaths. This northwest trend is parallel to that of the offshore granitic outcrops, the Farallon Island ridge and Cordell Bank. 3.5 kHz shallow subsurface profiles show a regional unconformity (basal unconformity of this study) that truncates bedrock and typically is overlain by a very thin veneer (1-2 m or less) of acoustically transparent unconsolidated sediment. In places, the basal unconformity appears to be coincident with the seafloor. Where the basal unconformity is within 1-2 m of the seafloor, side-scan sonar reveals that either bedrock pierces the seafloor or numerous linear depressions dissect the shelf seafloor. Side-scan sonar shows that the central part of the study area is characterized by numerous linear depressions that are on the order of 1-3 m deep, several meters to 5 km in length, and from 250 m to 2 km in width. The only Holocene unconsolidated sediment in excess of 1-2 m that has accumulated on the Gulf shelf is located where the bedrock surface is topographically depressed. The topographic lows are situated south of the Point Reyes headland and southwest of the Golden Gate. The sediment deposits that overlie these lows attain thicknesses up to 15-20 m; however, their average thickness is less than 10 m. Structure contours on the basal unconformity surface, essentially the top of the bedrock platform, also show that it is topographically high at its southern end and low at its northern end; the slope of the platform is therefore to the northwest. The seafloor mimics the attitude of the bedrock platform owing to the lack of appreciable sediment cover and hence also slopes northwest. This accounts for the oblique to shore-normal orientation of isobaths in the Gulf and the northwestward slope of the seafloor. The results of this investigation suggest that eustatic sea level fluctuations in conjunction with local tectonics had a profound effect on the deposition, erosion, and preservation of sediment on the Gulf shelf throughout the Quaternary and that the shallow subsurface greatly influences many aspects of the present surficial morphology.

Marine Geology↗
USGS1988

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

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 that has surface expression. High-resolution seismic-reflection profiles reveal that an angular unconformity (Quaternary?) underlies the entire shelf and separates undeformed strata above it from deformed strata below 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, suggests that a general interpretation can be formulated for seismic stratigraphic patterns. Depositional sequences are interpreted to represent shallowing-upwards progradational sequences of marine to nonmarine coastal deposits formed during interglacial highstands and/or during early stages of falling sea level. Acoustic foresets within the intermediate sequence are evidence of seaward progradation. Acoustic unconformities that separate depositional sequences are interpreted as having formed largely by shoreface planation and may be the only record of the intervening transgressions.

Marine Geology↗