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ABSTRACT: Depositional Facies of the Upper Cretaceous Forbes Formation, Sacramento Basin, CA

Douglas P. Imperato, T. H. Nilsen, D. W. Moore

The Upper Cretaceous Forbes Formation was deposited as a mud-rich basin-plain, deep-sea fan, and slope turbidite system that prograded southward down the plunging axis of the Cretaceous Great Valley forearc basin. The Forbes can be subdivided into three members based on depositional facies, each with characteristic well-log response, seismic-reflection signature, and benthic foraminiferal zonations: (1) the Dobbins Shale Member, basin-plain and marginal-slope deposits (G-1 zone foraminifera), (2) the unnamed middle member, deep-sea-fan deposits (F-2 zone foraminifera), (3) the unnamed upper member, axial-slope and marginal-shelf deposits (F-1 zone foraminifera).

The Dobbins Shale Member is a condensed shale interval 100-400 ft thick that has a low-resistivity log response and forms a continuous and prominent seismic reflector. The basin-plain facies locally contains thin, laterally continuous sandstone beds. Shales of the marginal-slope facies drape older Upper Cretaceous strata along the eastern basin margin and subsequently are onlapped by deep-sea fan deposits of the middle member of the Forbes Formation.

Deep-sea fan deposits, between 1000 and 3000 ft thick, of the middle member conformably overlie the Dobbins Shale Member, except along the eastern basin margin where the contact is a basin-margin unconformity. The deep-sea fan is comprised of a series of channel-levee sequences 100-500 ft thick containing channel, channel-margin, levee, and interchannel deposits. These form a coarsening-upward, then fining-upward symmetrical well-log signature and appear as flat, subparallel and relatively continuous seismic reflectors.

Slope deposits of the upper member as thick as 3000 ft conformably overlie the middle member. This shale-rich sequence contains abundant synsedimentary deformation features and locally laterally discontinuous sandstone and conglomerate beds deposited in channels eroded into the slope. The upper member has a relatively transparent seismic signature with short, discontinuous reflectors and local low-angle clinoforms.

Division of the Forbes Formation into members based on depositional facies is critical in subsurface mapping. Regional delineation of members provides an essential framework from which successful exploration strategies can be developed.

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