Contrasting Active Margin Slope Channel and Intra-Slope Fan Complexes, Eocene Forearc Basin, California

Abstract

Sandy slope channel and intra-slope fan complexes, common depositional settings on active margin continental slopes, can be substantial reservoirs. This paper aims to compare these two slope system types, what concerns their sedimentary facies, bypass signatures and architecture, and discuss the forcing factors controlling and the respective reservoir properties. Coeval slope channel and intra-slope fan complexes are widely exposed in the Californian middle Eocene forearc basin, as Torrey, Ardath, and Scripps Fm in beach cliffs, north of La Jolla, and as Juncal Fm in the Pine Mountain Fault block of the western Transverse and southern Coastal Ranges, respectively. Their mutual characteristics, such as the presence of coarse conglomeratic sediment and ubiquitous bypass indicators are assigned to deposition on active margin slopes, characterized by steep gradients from hinterland to deepwater. Yet, there are distinct differences in the two studied successions, what concerns their sedimentary facies, depositional trends, specific bypass indicators and architecture. The confined slope channel complexes, studied by 10 measured sections (410m), helicopter photomosaics, and GPS and laser surveys, exhibit 2 major upward-fining amalgamated channel complexes (overall 180m thick), based by major erosion surfaces. The basal erosion surfaces are overlain by conglomerates (or sandstones), followed by thick sandy and then by heterolithic to mud-filled channel complexes. The individual sandy channels are 10's to 400 m wide and 10–45 m thick, and display relatively low aspect ratio. The sandy channel fills are amalgamated and commonly contain 10–30 m thick upward fining successions. The thick sandstone beds indicate high deposition rates, and sediment bypass is only indicated by channel-bounding erosion surfaces. The heterolithic and mud-filled channels are also amalgamated, but the channel fills exhibit in-channel bypass features in addition to the channel bases. The intra-slope fan lobes, studied by 9 measured sections (535 m), photomosaics and a GPS survey, are 1200–2000 m wide and 10–25 m thick. They consist of 5–10 m thick coarsening upward successions, where coarse sandstone facies with sharp or erosive bases commonly contain cross strata and are interbedded with heterolithic facies, in a decimeter to a few meter scale. Sediment bypass here is indicated by the erosive base of the sandstone succession, and heterolithic facies commonly with lenticular lamination.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015