Abstract: Oligocene to Miocene Sedimentation and Tectonism Along the Northeast Flanks of Mount Diablo

Raymond Sullivan

The base of the Oligocene and Miocene succession along the northeast flanks of Mount Diablo is marked by a regional unconformity across which a major basinal shift in facies is observed. At the unconformity, volcanic-rich estuarine sandstones of Oligocene age overly mid-Eocene bathyal shales and shelfal sandstones. The Oligocene to Miocene strata can be divided into three depositional sequences that correspond to the Kirker, Cierbo and Neroly formations. Each sequence consists of lowstand/transgressive systems tract, but associated highstand deposits do not appear to be preserved. The Kirker Formation of Oligocene age is approximately 250-310 feet thick and is unconformably overlain by 200-400 feet of mid-Miocene Cierbo. Both sequences are composed of interbedded, subtida channel sandstones and finer grained, fossiliferous, highly bioturbated, intertidal sandstones. The subtidal channel sandstones are particularly well developed in the upper part of the Cierbo Formation. The base of the overlying late Miocene Neroly Formation is marked by local valley incisions and an abrupt change in facies. This sequence is over 600 feet thick and consists of poorly sorted volcaniclastic, fluvial sandstones and interbedded mudstones. The main source of sediment for these sequences was westward directed streams from the Sierra Nevada that flowed into the coastal estuaries in this part of the Coastal Range. Structural control on the deposition of the Tertiary strata is evident by the marked stratigraphic changes that occur when the sequences are traced westwards across t e Kirker pass fault. This fault marks the eastern boundary of the Carquinez high. The absence of the upper part of the middle Eocene strata and the Kirker Formation, coupled with the distinctive thinning of the younger Miocene sequences on this high, is interpreted to reflect continued uplift of this structure in late Tertiary times.

AAPG Search and Discovery Article #90958©1995 AAPG Pacific Section Meeting, San Francisco, California