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Bureau Seminar, Sept 17, 2010

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

Link to streaming video TBA: available 09.17.2010 at 8:55am

Jesus Ochoa
Exploration Geologist
Marathon Oil Corporation
Houston, TX
and Michael H. Gardner
Exploration Geologist, Marathon Oil Corporation, Houston Texas

One dilemma in sedimentology is that multiple depositional processes can produce similar features while one formative process can generate multiple patterns. A comparison of (1) depositional energy trends from grain size and primary structures, (2) placement within a stratigraphic hierarchy, (3) ichnofacies type, and diversity, 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), flow evolution (velocity, run-out length), preservation (bypass, erosion), and reworking (biological, physical).

Cretaceous and Tertiary outcrops in the Mesa San Carlos area expose four different thin-bedded sandstone types (TBS). This study presents a matrix of the most important attributes used to recognize them: (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 turbidites that flank and confine multistory channelbelts up to 90m thick that thin and pinch out within 500m of interdigitated but stacked conglomerate channels, (3) TBS turbidites separating channel bodies form 25m-thick and 230m-wide preserved remnants, and (4) TBS contourites comprising <3m wide sandstone lenses amalgamated laterally to form tabular bedsets. Paleocurrent indicators change from unidirectional offshore during hyperpycnal flow to slope parallel flow during waning energy conditions of this mudstone-rich cycle.

Turbidite and hyperpycnite deposition respond to external controls, whereas contourites are reflecting internal controls in the slope system. Failure-initiated flows dominate the deposition in the third-order growth phase and flood-initiated flows dominate in the third-order initiation and retreat phases. Bottom current rework is the main internal process that affects deposits in the initiation and retreat phases. In the growth phase, the internal processes are more variable. They are controlled by overspilling and superelevation of the flows. Channel and scour bodies deposited by hyperpycnal flows show downstream and vertical changes in grain size, primary sedimentary structures, bed thickness, and sedimentation units that allow recognition of an energy matrix recording variations of the flow magnitude.



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