Shelf-Edge Conduit: Channelized Sediment Transport Across Eocene Fore-Arc Basin Margin, Southern California: ABSTRACT

Abstract

A complete shallow-to-deep marine transition is exposed in Eocene rocks at San Diego, California. Well-exhibited facies relations, precise biostratigraphic control, and paleobathymetric indicators allow comparison of these continental margin outcrops with similar sequences most commonly observed only by seismic-stratigraphic methods. Even in this active tectonic setting, eustasy appears to be the dominant factor in cutting shelf-edge unconformities, forming and reactivating submarine canyons, and controlling the distribution Figure of facies down the shoreline-to-basin conduit. Broad, shallow outer-shelf channels funneled coarse-grained littoral sediments to the gullied upper slope and canyon head. These channels were filled with massive to laminated sandstone and shell lag, bioturbated mudstone, and rare slump-folded mudstone. An early Eocene canyon head was cut at the shelf edge during a eustatic lowstand, then eroded landward during subsequent sea level rise. The canyon is floored with a thick, massive sandstone that may be amalgamated or crudely graded, containing clasts of canyon-wall debris up to 6 m long in cross section, as well as conglomeratic to pebbly or gravelly sandstone. It exhibits convolutions, flame structures, and other evidence of rapid End_Page 954------------------------------ deposition and dewatering. Deposits above the basal sand accumulated in broad, anastomosing channels 10 to 15 m deep with variegated fill; some still contain sand and conglomerate, but most were evacuated and filled with lower energy thin-bedded sands and muds or massive hemipelagic muds, indicating significant volumes of bypassed sand. At the base of slope, a major 100-m deep leveed channel was floored with conglomerates and large canyon-wall clasts, and filled with massive to convoluted sandstone. The channel fed a system of shallow, crosscutting, conglomeratic channels, interpreted as inner fan that extended into the canyon mouth. Paleobathymetric relief exhibited across this ancient shelf break is minimally 600 m (outer shelf to mid-bathyal or deeper) within a lateral outcrop distance of 3,000 m. End_of_Article - Last_Page 955------------