Origin of Facies Architecture in Stacked Gilbert-Type Fan Deltas, Loreto Basin, Baja California Sur, Mexico

Peter D. Falk, Paul J. Umhoefer, and Rebecca J. Dorsey

The Pliocene Loreto basin (Baja California Sur, Mexico) provides an opportunity to study the 2-dimensional facies architecture of stacked shelf-type and Gilbert-type fan-deltas. Seven primary facies are interpreted as deposits of the following environments: alluvial fan, fan-delta plain, Gilbert-delta topsets, Gilbert-delta foresets, Gilbert-delta bottomsets, background shallow marine, and hiatal drowning surfaces (shell beds). Facies associations record repeated cycles of fan-delta progradation punctuated by abrupt drowning episodes that resulted in shoreline retreat. Each cycle begins after a fault-related drowning event, and typically is represented by a laterally continuous, sandy to pebbly shell bed containing pectens, clams, gastropods and other shallow-marine fauna Progradation begins with a shelf-type fan delta that in many cases evolves down-transport into a Gilbert-type fan delta having foresets that dip 15°-30°. Gilbert-delta foresets have initial, proximal thicknesses of only 1-2 meters that suggest growth of Gilbert-type fan deltas from localized delta-mouth bars on the shelf-type fan-delta front. Foreset thicknesses increase down-transport with continued progradation to a thickness of 10 to 28 meters. Controls on increased foreset thickness are twofold: (1) Gilbert-delta progradation into deeper water due to pre-existing bathymetry, and (2) sudden, episodic tectonic subsidence from faulting during Gilbert-delta progradation, which produces sigmoidal topset/foreset geometries and thinning of topsets units down transport. Fan-delta systems typically prograde a distance of 0.75 to > 1.25 km. Previous studies in the area have estimated sediment accumulation rates of 8 ± 4 mm/yr, which yields an estimated rate of fan-delta progradation of about 0.4 m/yr.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California