The Origin of Erosional Confinement in Deepwater Conglomerate Channels: Upper El Rosario Formation (Paleocene), San Carlos, Baja California, Mexico

Bryan J. Anderson, Michael H. Gardner, and James G. Schmitt
Department of Earth Science, Montana State University, Bozeman, MT

Upper slope conglomerates of the El Rosario Formation (41% mudstone, 28% conglomerate, 18% sandstone) are restricted to erosional depressions. Conglomerate beds (up to 15m long; up to 2m thick) stack as multilateral composite channels-fills (5-15m thick; 100-300m wide) within larger erosional to levee-confined channel-belts (200m thick; 2km wide).

Six conglomerate facies are recognized based on variations in clast-size. Five are massive, clast-supported, and contain poorly sorted, fine- to medium-grained sand matrix. The sixth is mud matrix-supported. Lack of stratification, poorly sorted matrix, outsized clasts, and paucity of grading within these facies indicates deposition from debris flows, a flow type not commonly characterized by significant erosion.

The conspicuous origin of erosion at the base of these debrites can be related to 1) formative processes capable of operating within a debris flow (cohesion, bedload transport, or both); 2) timing of erosion and deposition (different flow events, or surging within the same event); and 3) one or multiple flow types (debris flow and/or turbidity flow).

Lack of evidence of sand matrix infiltration suggests gravel and sand deposition en masse, with no hydraulic sorting as expected with bedload transport. Under-capacity flows, such as hyperpycnal flows, could be the erosional agent providing the erosional depressions for subsequent debris flow back-filling and plugging. The debris flow head may act as a bulldozer-like plow, displacing preceding channel-fill and driving lateral offset of successive channel thalweg-fills stacking to form multilateral composite channel-fills. The latter observation provides a consistent explanation for the association of facies types, facies distributions, and body architecture within this upper slope setting.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas