Multiple Origins of Thin-Bedded Deepwater Slope Sandstones: El Rosario Formation (Upper Cretaceous - Paleocene) Baja California, Mexico

Jesus Ochoa, Michael Gardner, and James Schmitt
Earth Sciences, Montana State University, Bozeman, MT

One dilemma in sedimentology is that multiple depositional processes produce similar features while one formative process generates multiple patterns. Comparison of (1) depositional energy trends from grain size and primary structures, (2) placement within a stratigraphic hierarchy, (3) ichnofacies type, and (4) sedimentary body type and associated architectural changes are used to assess the causal mechanism. Variations of these attributes reflect flow initiation processes (flood vs. failure), depositional processes (flow stripping, overspilling or bottom current reworking), and preservation (thin-beds bounded by erosional channels).

El Rosario outcrops expose five different thin-bedded sandstone types (TBS): (1) Hyperpycnite successions (4-15m thick) are interbedded with slope mudstone deposits that together form tabular (85m thick; >1km wide) successions, with sandstone channels and scours common at the base and mass transport deposits present at the top. (2) Wedge-shaped TBS flank and confine multistory channelbelts up to 90m thick that thin and pinch out within 500m of interdigitated but stacked conglomerate channels. (3) TBS separating channel bodies form 25m-thick and 230m-wide preserved remnants. Unidirectional current ripple laminations are most common in upward-thicking successions within these channellized regions. (4) TBS successions (9m thick; 120m wide) also cap upward-fining channelbelt cycles; their stratigraphic position suggests waning energy conditions within these cycles. Similarly, (5) contourites comprise <3m wide sandstone lenses amalgamated laterally to form (4m thick; >1km wide) tabular bedsets. Structureless sandstones capped by highly bioturbated mudstone are common. Paleocurrent indicators change from unidirectional offshore during hyperpycnal flow to slope parallel flow during waning energy conditions of this mudstone-rich cycle.

AAPG Search and Discovery Article #90092©2009 AAPG Rocky Mountain Section, July 9-11, 2008, Denver, Colorado