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.

Abstract The Miocene Monterey Formation in the Pismo Syncline, San Luis Obispo County, California, is characterized by the following lithofacies: 1) diat-omite, 2) carbonate, 3) phosphate, and 4) siliciclas-tic siltstone. The Pismo Syncline is a relatively simple fold that served as a depositional basin during most of the Miocene and Pliocene. During deposition of the Monterey Formation, the Pismo Basin was separated into two parts by a positive volcanic high running parallel to the San Miguelito fault along the western edge of the Basin. That portion of the Basin northeast of the topographic high received 1500 meters of Monterey sediment, whereas the portion of the Basin to the southwest of the high received 300 to 600 meters of sediment. The Monterey Formation in the central portion of the Pismo Syncline consists of a thick section of silty diatomaceous sediments. In contrast, the Monterey Formation along the western edge of the Basin (west of the topographic high) is characterized by carbonate and phosphatic-rich rocks, as well as diatomaceous rocks. The volcanic ridge acted as an effective barrier to siliciclastic sediments entering the Pismo Basin from the east. Current directional features in the Monterey rocks in the western portion of the Pismo Basin suggest that much of the calcareous, phosphatic and diatomaceous material was originally deposited or formed in situ on the slopes of the topographic high and was later swept downslope into the lower portions of the Basin. In the Monterey Formation along the western side of the Pismo Syncline there is an upward trend of carbonate → phosphate → diatomite sedimentation. This vertical trend in sedimentation can be correlated to the Miocene sea level changes described by Vail and Hardenbol (1979).

Abstract Kerogen in the Miocene Monterey Formation in the Pismo syncline consists of amorphous material of algal origin. Most of the rocks presently contain 1-5 wt. % total organic carbon. Thermal alteration index determinations and pyrolysis (thermal extract) data suggest that the organic material at the center of the fold is mature, whereas the organic material on the limbs of the fold is immature. The Monterey Formation in this region entered the liquid hydrocarbon window at a present day depth of approximately 1700 meters. The liquid window appears to be coincident with the opal CT to quartz reaction (80 ± 10°C). Hydrocarbons were expelled from the Monterey rocks deep in the center of the fold and began migrating as a result of microfracturing. The hydrocarbons migrated up into the southwest limb of the fold through macrofractures. This limb of the fold is characterized by brittle dolomitic and opal CT rich rocks that were intensely fractured prior to hydrocarbon migration. The potential reservoirs on this limb of the fold are in fractured Monterey. In contrast, on the northeast limb of the fold Monterey rocks consist of silty and sandy siliceous rocks that tend to be more resistant to fracturing; hydrocarbons migrate up into this limb of the fold through relatively low angle faults and spread into structural traps along the fault in the adjacent Pliocene Pismo Formation through conjugate shears. The potential reservoirs on this limb are in structural traps in the Pismo Formation. A third potential hydrocarbon target is the basal sand in the Pismo Formation; almost everywhere in the fold this unit is highly bituminous.

ABSTRACT The Miocene Monterey Formation in the Pismo Syncline, San Luis Obispo County, California, is characterized by the following lithofacies: 1) diatomite, 2) carbonate, 3) phosphate, and 4) siliciclastic siltstone. The Pismo Syncline is a relatively simple fold that served as a depositional basin during most of the Miocene and Pliocene. During deposition of the Monterey Formation, the Pismo Basin was separated into two parts by a positive volcanic high running parallel to the San Miguelito fault along the western edge of the Basin. That portion of the Basin northeast of the topographic high received 1500 meters of Monterey sediment, whereas the portion of the Basin to the southwest of the high received 300 to 600 meters of sediment. The Monterey Formation in the central portion of the Pismo Syncline consists of a thick section of silty diatomaceous sediments. In contrast, the Monterey Formation along the western edge of the Basin (west of the topographic high) is characterized by carbonate and phosphatic-rich rocks, as well as diatomaceous rocks. The volcanic ridge acted as an effective barrier to siliciclastic sediments entering the Pismo Basin from the east. Current directional features in the Monterey rocks in the western portion of the Pismo Basin suggest that much of the calcareous, phosphatic and diatomaceous material was originally deposited or formed in situ on the slopes of the topographic high and was later swept downslope into the lower portions of the Basin. In the Monterey Formation along the western side of the Pismo Syncline there is an upward trend of carbonate→phosphate→diatomite sedimentation. This vertical trend in sedimentation can be correlated to the Miocene sea level changes described by Vail and Hardenbol ( 1979 ).

Abstract The Cenozoic High Plains sequence is a semiarid alluvial and eolian complex in which low temperature diagenetic processes were primarily the result of reactions of unconsolidated sediment and dilute aqueous solutions. Post-depositional modifications of arkosic and vitric sandstone include intrastratal alteration and dissolution of chemically unstable grains and the precipitation of cement in pore space. These processes, however, have only slightly altered the original fabric and mineralogy of High Plains sandstone, although chemically unstable heavy minerals and volcanic glass are abundant. Thermochemical calculations suggest that present-day groundwater is in possible equilibrium with calcite, montmorillonite, kaolinite, and a silica phase. Cement in sandstone is commonly montmorillonite, but can be calcite, opal, chert, and rarely clinoptilolite. Most sandstone is friable and cemented with montmorillonite. Interspersed in this sandstone are calcite concretions; opal and chert cemented sandstone occurs sporadically in High Plains rocks on Cretaceous shale and associated with old land surfaces in the Tertiary rocks. Commonly, the order of precipitation of cement is calcite, montmorillonite, and opal. All cemented sandstone shows some dissolution of grain surfaces, but only friable sandstone shows extensive intrastratal dissolution of grains, peripheral alteration of pyroxene and amphibole grains, and hollow montmorillonite coatings where chemically unstable grains were dissolved. These montmorillonite coatings are up to 100 micrometers thick and consist of a reticulate arrangement of relatively porous clay particles oriented normal to the grain's surface. Most of the cements and intrastratal alteration features can be explained by pedogenic processes, evaporative concentration of groundwater in the capillary zone, or by groundwater reaction with sediment.