Lithofacies and Depositional Environments of Etchegoin Group: Model for Late Neogene Sedimentation in Western San Joaquin Basin

Karen B. Loomis

During the late Neogene, the San Joaquin Valley of central California was covered by a shallow sea representing the final marine phase in the history of the San Joaquin basin. High rates of sedimentation and subsidence in the western part of the basin resulted in deposition of a thick (maximum 8,500 ft or 2,590 m) sequence of shallow-marine to nonmarine siltstone, sandstone, and conglomerate, which comprise the Etchegoin Group (i.e., Jacalitos, Etchegoin, and San Joaquin formations).

The Etchegoin Group is characterized by a complex and laterally variable association of lithofacies that include pebbly blue sandstone, blue sandstone with silty laminations, pebbly to conglomeratic brown channelized sandstone, and clay-rich brown sandstone. During the late Miocene and Pliocene, the western San Joaquin basin was a protected bay or estuarine environment characterized by numerous sand shoals and tidal channels. Thick (50 ft or 15 m) pebbly blue sandstone units characterized by trough cross-stratification and southerly (120° to 200°) paleocurrent directions are interpreted as flood-dominated tidal channel deposits. Thinner (30 ft or 9 m) silty blue sandstone units characterized by flaser bedding, low-angle cross-stratification, and variable (90° to 350 ;) paleocurrent directions represent migrating tidal channels in a muddy sandflat environment. Pebbly to conglomeratic brown sandstones containing fossil plant debris were deposited in channels in the upper reaches of the bay or estuary. Fine-grained brown sandstone units characterized by low-angle cross-beds with mud-draped foresets and variable (90° to 270°) paleocurrent directions are interpreted as tidal creek and runoff-channel deposits in an upper intertidal sandflat and mudflat environment.

AAPG Search and Discovery Article #91024©1989 AAPG Pacific Section, May 10-12, 1989, Palm Springs, California.