The Role of Wrench Fault Tectonics and Relative Changes of Sea Level on Deposition of Upper Miocene - Pliocene Pismo Formation, Pismo Syncline, California

Abstract The Pismo Formation records late Miocene and Pliocene sedimentation on the margin of an extensional wrench-fault basin between the West Huasna Fault Zone and the Hosgri Fault Zone. The formation is made up of three depositional sequences of relatively conformable successions of genetically related strata bound by unconformities. The earliest sequence of late Miocene and early Pliocene age rests unconformably on Monterey highstand deposits and records a major downward shift in coastal onlap associated with a fall of relative sea level. This downward shift in coastal onlap was brought about by uplift associated with formation of the basin and right lateral movement on the faults bounding the basin, the West Huasna system, and by the 6.6 million years before present Messinian global fall in sea level. The fall in global sea level accentuated deformation that resulted from tectonic uplift at the basin margin and led to deposition of lowstand deposits. A subsequent rise in sea level resulted in a rapid landward shift in coastal onlap and the deposition of nearshore inner neritic sandstone of the Edna Member. These sandstones interfinger to the southwest with outer neritic mudstone and diatomaceous strata and step landward as relative sea level continued to rise. Cessation of coastal onlap and rapid seaward progradation of neritic sandstones at the top of the sequence, together with the presence of Thalassinoides hardgrounds and penecontemporaneous dolomite concretions in coeval offshore diatomaceous strata, indicate a relative high-stand of sea level. Maximum water depths indicated by laminated diatomaceous strata in their most landward stratigraphic position probably correspond to highstands of global sea level, but also were influenced by early Pliocene subsidence of the basin margin. Relative fall in sea level and downward shift in coastal onlap is indicated by the unconformity between the upper Miocene-lower Pliocene sequence and overlying upper Pliocene sequences. Both upper Pliocene sequences are thin and made up of transgressive inner neritic sandstone and minor outer neritic mudstones and diatomaceous mud-stone. These younger sequences reflect a progressive decrease in accommodation during highstands of global sea level caused by late Pliocene uplift and folding of the Pismo Syncline. The stratal associations and arrangement within depositional sequences provide a basis for evaluating the role of global sea level changes and tectontism on sedimentation in wrench basins. Both factors strongly influenced deposition of the Pismo Formation in the Pismo Syncline, the northeastern margin of the Pismo Basin.