Basin Margin Failure, Sturzstroms, and Turbidite Channelization in the Fish Creek-Vallecito Basin, Salton Trough, California

Abstract

The Fish Creek-Vallecito Basin, part of the larger Salton Trough region of southern California, contains a stratigraphic section that records the opening of the basin, flooding by the Gulf of California, and the arrival of the Colorado River. Multiple times during the earliest filling of the basin unstable alluvial slopes and basin walls collapsed to form fast, far travelling sturzstroms - a type of mass transport flow defined as “a stream of very rapidly moving debris derived from the disintegration of a fallen rock mass of very large size” (Hsu, 1975). The Fish Creek-Vallecito sturzstroms have previously been interpreted as both subaerial and subaqueous debris flows formed at a time when marine water first appears in the basin. Currently there are no documented sedimentological differences that distinguish subaerial from subaqueous sturzstroms, or subaerial dry from saturated flows. Evidence that the rocks were shattered during transport with no internal mixing suggests emplacement by laminar flow, resulting in little difference within the sturzstrom core between subaerial and subaqueous emplacement. To identify differences between subaerial and subaqueous sturzstrom deposits, 10 vertical sections were measured through deposits that crop out over 6 kms, with goals to (1) describe the nature of the contacts with underlying and overlying strata, and how textures and fabrics change vertically within deposits previously identified as either subaerial or subaqueous, (2) determine how the mass transport deposits vary laterally, both source-proximal to source-distal and from the axial core to the flanks of the flows, (3) document microscopic and macroscopic shear fabrics created as a result of emplacement through dry laminar flow, and how such fabrics might differ from the base to the top of the flow, and (4) compare data of subaerial dry flow deposits from this study with descriptions in the literature of documented subaerial wet flows to determine similarities and differences. The results of this study show that subaerial and subaqueous mass transport deposits can be distinguished from one another through their interaction with underlying deposits, and by the grain sorting and depositional structures found at the top and margins of the flow. This study also shows that the irregular paleotopography created by the top of the sturzstrom deposits served subsequently as channel conduits for the turbidity currents that later flowed over them.

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