ABSTRACT: Oligocene-Middle Miocene Evolution of the Southeastern San Joaquin Basin, CA

Hilary Clement Olson

Oligocene-middle Miocene strata of the southeastern San Joaquin basin of California provide clear evidence of basin subsidence, tectonics, and relative sea level changes. The Walker Formation records the earliest nonmarine history, including late Eocene to Oligocene lahar and fluvial deposits that interfinger basinward with shelf sands of the Vedder Sand as marine environments migrated farther eastward during the Oligocene. A late Oligocene rapid fall in relative sea level produced a major erosional unconformity across the shelf margin.

The early Miocene was characterized by transgression (approximately 23 Ma), rapid subsidence, and basin filling by the Jewett Sand on the shelf and Freeman Silt in the deep basin; the lower Olcese Sand records shoaling and continued basin-filling by shelf sands. Early Miocene tectonic uplift of the Sierra Nevadas to the east resulted in major lithologic changes and the predominance of nonmarine deposits of the middle Olcese Sand. The return to normal marine conditions during the early Miocene is recorded by the transition from braided streams to deltaic deposits of the middle Olcese Sand, culminating in inner shelf sands and outer shelf silts of the upper Olcese Sand. The initial phases of middle Miocene basin evolution consisted of deposition of fine-grained silts of the Round Mounta n Silt, followed by basin subsidence and accumulation of pelagic, diatomaceous slope deposits. The final stages of middle Miocene development of the southeastern basin included shoaling of the basin to shelf depths and deposition of silty sands of the Round Mountain Silt.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990