ORIGINAL RESEARCH PAPER Late Pleistocene to Holocene sedimentation and hydrocarbon seeps on the continental shelf of a steep, tectonically active margin, southern California, USA Amy E. Draut • Patrick E. Hart • Thomas D. Lorenson • Holly F. Ryan • Florence L. Wong • Ray W. Sliter • James E. Conrad Received: 21 May 2009 / Accepted: 28 October 2009 / Published online: 21 November 2009 Ó Springer Science+Business Media B.V. 2009 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 depo- sition 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 Qua- ternary climatic evolution, interpretations of which depend upon an understanding of sedimentation patterns. High- resolution seismic-reflection data over [ 570 km 2 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. De- pocenters containing sediment 5–20 m thick exist opposite broad coastal embayments, whereas relict material (bed- rock 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. Under- standing 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 conti- nental margins, and for inferring the distribution of hydrocarbon resources in relict shelf deposits. Keywords Continental shelf  Fluvial sediment  Hydrocarbon seeps  Southern California Introduction Styles of sedimentation on continental shelves influence the economic potential, geohazard risk, and vulnerability to anthropogenic environmental contamination of many coastal regions worldwide. Understanding patterns of sed- iment dispersal and deposition from small, steep, moun- tainous watersheds is particularly important in regional and global sedimentology because 10 1 –10 4 km 2 watersheds along mountainous coasts collectively have a proportion- ately greater importance for global sediment production than do larger rivers (Milliman and Syvitski 1992). In this paper, we examine a new, high-resolution seismic record of continental-shelf sedimentation along *120 km of moun- tainous, tectonically active coastline in southern California, USA. These data, collected as part of the U.S. Geological Survey (USGS) California seafloor mapping program and a collaborative study of hydrocarbon seeps by the USGS and U.S. Minerals Management Service, elucidate 10s-km- scale sedimentation patterns that have proceeded since late A. E. Draut (&) U.S. Geological Survey, 400 Natural Bridges Drive, Santa Cruz, CA 95060, USA e-mail: adraut@usgs.gov P. E. Hart  T. D. Lorenson  H. F. Ryan  F. L. Wong  R. W. Sliter  J. E. Conrad U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025, USA 123 Mar Geophys Res (2009) 30:193–206 DOI 10.1007/s11001-009-9076-y