Bypass Dominated Slope Channel Complexes Exhibit Progradational Channel Fill Style, Eocene Forearc Basin, California

Abstract

Slope channel complexes, which are common depositional elements on active margin continental slopes, can be substantial reservoirs. This paper aims to describe the sedimentary facies, bypass signatures, architecture, and depositional processes of channel fills. It also discusses the controlling forcing factors and the respective reservoir properties. The continental slope succession, mainly composed of channel complexes, is widely exposed in the Californian middle Eocene Forearc Basin as the Juncal Formation in the Pine Mountain Fault block of the southern Coastal Range. The dataset includes measured sections, photomosaics and a GPS survey. The channel complexes consist of three types of channel fills; 1) heterolithic fill, 2) heterolithic fill with basal sandstone and 3) thick sandstone fill. The individual channels are 200 to 900m wide and 20 to 100m thick, and display a relatively high aspect ratio. The widely distributed heterolithic fill with basal sandstone is a focus of this paper in order to understand the depositional processes. The heterolithic part of the fill is composed of thinly bedded fine-grained sandstone and siltstone interbedded with erosionally based coarser-grained lenticular sandstone beds. These beds are interpreted as deposits of mainly bypassing turbidity currents. The basal sandstone (10 to 20 m thick) is mainly composed of strucrureless sandstone and scour-fill and convex-up low angle stratified sandstone, indicating deposition from supercritical flow. The boundary between the heterolithic beds and the basal sandstone is relatively sharp, which indicates a drastic change of process regime from sandstone dominated deposition to heterolithic deposition. We hypothesize that initial sandstone deposition is confined to the deeper parts of channel axes where the overpassing supercritical flow experiences hydraulic jumps or flow expansion and causes rapid local deposition. This results in smoothing of the channel base and enhances bypass of the successive turbidity currents. Then, channels are filled with heterolithic faces formed by the tails of the bypassing turbidity currents. This progradational channel-fill model contrasts with the well-known backfill model and give valuable insight to the understanding of the slope channel fill processes and the channel reservoir properties.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016