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The Vedder-Jewett sands represent a retrogradational parasequence set forming a seismically defined, 1500-ft thick transgressive systems tract (TST) that was deposited in a ramp setting on the eastern margin of the San Joaquin basin. This interval has produced 657.5 MMBO and 221.9 BCFG, largely from fault-bounded traps. Due to basin subsidence, the underlying highstand nonmarine Walker was transgressed and overlain by nonmarine and marine deposits of the Vedder and Jewett sands. Seismic, well-log, and core data indicate that parasequences in the upward-deepening TST are comprised of alluvial, fluvio-deltaic, and shallow-marine facies. Each parasequence is punctuated by marine-transgressive episodes. This TST is manifested as a succession of backstepping and aggradational seismic refle tions becoming progressively more transparent upward.
Geologically defined parasequences and stratigraphic discontinuities are seismically detectable because of impedance contrasts between marine and nonmarine sediments at marine-flooding surfaces. Within the TST, these impedance contrasts correspond to (1) the basal sequence boundary, (2) marine-flooding surfaces, and (3) the maximum-flooding surface. Laterally along individual reflections, amplitude and phase changes imply depositional facies changes within parasequences. Reflection amplitude loss coincides with a sandstone-to-shale facies change within a parasequence and, therefore, is a good estimate of lithosome continuity.
Parasequences and their bounding surfaces in the TST have diagnostic sedimentologic and biostratigraphic signatures. Low benthonic foraminiferal abundances characterize shallow-marine deposition; abundances increase to a maximum at the seismic downlap surface (maximum flooding surface). Dinoflagellate occurrences accurately record the transgressive episodes, especially in the predominantly nonmarine lower TST.
AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)