Late Pleistocene to Holocene sedimentation and hydrocarbon seeps on the continental shelf of a steep,tectonically active margin,southern California,USA |
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Authors: | Amy E Draut Patrick E Hart Thomas D Lorenson Holly F Ryan Florence L Wong Ray W Sliter James E Conrad |
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Affiliation: | (1) U.S. Geological Survey, 400 Natural Bridges Drive, Santa Cruz, CA 95060, USA;(2) U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025, USA |
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Abstract: | Small, steep, uplifting coastal watersheds are prolific sediment producers that contribute significantly to the global marine
sediment budget. This study illustrates how sedimentation evolves in one such system where the continental shelf is largely
sediment-starved, with most terrestrial sediment bypassing the shelf in favor of deposition in deeper basins. The Santa Barbara–Ventura
coast of southern California, USA, is considered a classic area for the study of active tectonics and of Tertiary and Quaternary
climatic evolution, interpretations of which depend upon an understanding of sedimentation patterns. High-resolution seismic-reflection
data over >570 km2 of this shelf show that sediment production is concentrated in a few drainage basins, with the Ventura and Santa Clara River
deltas containing most of the upper Pleistocene to Holocene sediment on the shelf. Away from those deltas, the major factor
controlling shelf sedimentation is the interaction of wave energy with coastline geometry. Depocenters containing sediment
5–20 m thick exist opposite broad coastal embayments, whereas relict material (bedrock below a regional unconformity) is exposed
at the sea floor in areas of the shelf opposite coastal headlands. Locally, natural hydrocarbon seeps interact with sediment
deposition either to produce elevated tar-and-sediment mounds or as gas plumes that hinder sediment settling. As much as 80%
of fluvial sediment delivered by the Ventura and Santa Clara Rivers is transported off the shelf (some into the Santa Barbara
Basin and some into the Santa Monica Basin via Hueneme Canyon), leaving a shelf with relatively little recent sediment accumulation.
Understanding factors that control large-scale sediment dispersal along a rapidly uplifting coast that produces substantial
quantities of sediment has implications for interpreting the ancient stratigraphic record of active and transform continental
margins, and for inferring the distribution of hydrocarbon resources in relict shelf deposits. |
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