Abstract: Sequence Stratigraphy and Forearc Basin Fill Architecture, Upper Cretaceous Great Valley Group, North-Central Sacramento Basin, California

WILLIAMS, THOMAS A.

Seismic stratigraphic examination of Turonian through Santonian Great Valley Group rocks of the north-central Sacramento Basin reveals seven sequence boundaries and three major onlap surfaces developed in a dominantly very deep-marine stratigraphic section. All exhibit angularity indicating active syndepositional tectonic tilting of the basin floor.

Two distinct types of sequence boundaries are observed: 1) The 'beveling-type' represents relatively planar erosion of the basin floor, with incision of shallow (<300m) submarine valleys. On seismic data, these sequence boundaries typically are overlain by a concordant package of high-amplitude, highly-continuous parallel reflectors which exhibit minor onlap of the sequence boundary. In outcrop, beveling-type sequence boundaries lie at the base of laterally-persistent sandstone intervals. 2) The 'entrenchment-type' exhibits localized, high-angle incision of deep (500-1500 m) submarine canyons or valleys into the underlying strata. Outside of the submarine canyons only disconformity is observed. Coarse-grained thalweg facies are present in places. Elsewhere, these sequence boundaries are overlain by shale and are often difficult to recognize without seismic data. One submarine canyon's fill crops out on the east flank of Sites Anticline exposing lower and middle Forbes sandstones and shales filling a submarine canyon up to 1.5 km deep, that incised into the Venado sandstone. Blanketing of the basin by hemipelagic drape in the absence of major turbidite systems, with subsequent onlap of the draped topography, creates another important type of bounding surface, the 'draped onlap surface'.

Both entrenchment-type sequence boundaries and draped onlap surfaces are associated with seaward tilting of the basin, and are interpreted as periods of turbidite bypass of the forearc basin resulting from oversteepening of the basin or filling to the spill-point provided by the accretionary prism sediment dam. Beveling-type sequence boundaries are interpreted as reflecting erosion during re-establishment of high-density turbidite system ponding in the forearc basin.

Search and Discovery Article #90945©1997 AAPG Pacific Section Meeting, Bakersfield, California