The Impact of Differential Subsidence on Turbidite Deposition, Upper Miocene Bakersfield Arch, California
Consulting geologist, 609 Elsey St., Bakersfield, CA 93309
Upper Stevens sandstones on the Bakersfield Arch, San Joaquin basin, California represent an Upper Miocene, line-sourced, sand-rich turbidite complex deposited in a tectonically active setting characterized by differential subsidence. These sandstones at North and South Coles Levee anticlines near the west end of the Arch contain two depositional sequences 330-500 m thick. The upper sequence contains sandstones deposited by sediment-gravity flows at neritic (300-500 m) water depths. Backstepping (retrogradational) reservoir compartments in this sequence overlie vertically stacked to prograding compartments. Although the backsteps are the better reservoirs with higher permeabilities and porosities, the vertically stacked to prograding compartments are volumetrically more important. Curiously, the upper section is sand-rich (60-80% ss) on structural highs that were also paleotopographic highs. Thicker sections with less sand characterize the flanks. These sand distributions are anomalous since sediment-gravity flows preferentially deposit in bathymetric lows. Apparently, the flanks of these paleohighs subsided faster than the crests. Consequently, greater accommodation on the flanks resulted in aggradation with preservation of section. Less accommodation on the crests resulted in progradation with erosion and reworking. Through lobe-switching processes some sand reached all parts of the depocenter. Subsequent progradation with erosion of shale interbeds and attendant reworking by density currents created sand-rich sections in slowly subsiding areas (crests). Concurrent aggradation preserved shale interbeds and created shale-dominated sections in rapidly subsiding areas (flanks). Greater compaction of thicker, shale-dominated flanks equalized thicknesses of flank and crest sections. Thus, the reservoir architecture represents the integrated effects of compensation (lobe-switching), accommodation (subsidence + sea level), and compaction.
AAPG Search and Discovery Article #90076©2008 AAPG Pacific Section, Bakersfield, California