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The AAPG/Datapages Combined Publications Database

Pacific Section of AAPG


Geology of the Midway-Sunset Oil Field and Adjacent Temblor Range, San Joaquin Basin, California, 2001
Pages 115-150
Miocene Sandstone Units

Sandstones of the Antelope Shale Member, Monterey Formation, Midway-Sunset Oil Field, California

Glenn J. Gregory


During the late Miocene, 8.9 to 7.6 Ma, the Gabilan Range shed sediments northeast of the San Andreas fault as debris and turbidity flows depositing eight submarine-fan complexes in the southwestern San Joaquin basin encapsulated within the diatomaceous Antelope Shale Member of the Monterey Formation at Midway-Sunset Oil Field (a major mechanism causing these turbidity flows was probably movement along the San Andreas fault, in other words, the flows were seismically induced). The fans were generally time-transgressive northwestward, as the Gabilan Range was transported right laterally. The larger submarine fan members of the Monterey Formation in ascending stratigraphic order, are the Leutholtz-Metson, Williams, and Republic Sandstone Members. Minor submarine fans sourced from the southern Gabilan Range and deposited synchronously with Republic Sandstone Member deposition are the Sub-Moco, Moco T, Obispo, Uvigerina ā€œCā€ and Rass. The fan morphology was controlled by 1) sand and gravel rich turbidites quickly depositing, thick sand lobes, 2) syndepositional growth folds with thicker sandstones being restricted to intraslope basin synforms, 3) lateral fan deposition, time- transgressively, due to convex fan shape and poor levees, 4) limited sediment supply between seismic and other triggering events, and 5) upper slope channel switching. The Antelope Shale sandstones are arkosic in composition, fine to coarse grained, and locally conglomeratic, depending upon whether distal, mid or proximal in original fan location. At Midway-Sunset Oil Field these submarine fan sandstones have produced more than 80 million barrels of Monterey Formation sourced oils from anticlinal, pinchout, truncation and fluid level traps.

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