Santa Margarita Sandstone

Santa Margarita Sandstone
Name	Santa Margarita Sandstone
Type	Geological formation
Period	Miocene
Lithology	Sandstone, conglomerate
Region	Central and Southern California
Country	United States

Santa Margarita Sandstone The Santa Margarita Sandstone is a Miocene sedimentary formation in California notable for its coarse-grained sandstone, conglomerate lenses, and fossil content. It occurs within the coastal and transgressive sequences of the Pacific margin and has been studied by regional geologists, paleontologists, and petroleum geoscientists from institutions such as the United States Geological Survey, Stanford University, and the Natural History Museum of Los Angeles County. The unit is important for understanding Neogene stratigraphy, basin evolution, and resource potential in basins like the San Joaquin and Santa Maria.

Geology and Lithology

The formation consists predominantly of well-sorted to poorly sorted quartzose sandstone, pebbly to cobbly conglomerate, and lesser siltstone and shale, described in studies by the United States Geological Survey, California Geological Survey, and researchers at the University of California, Berkeley. Field descriptions from Santa Barbara County, San Luis Obispo County, Monterey County, and Ventura County emphasize cross-bedding, planar bedding, and localized bioturbation noted by academics at Stanford University, UC Santa Cruz, and Caltech. Petrographic analysis undertaken by petrographers affiliated with the American Geophysical Union, Geological Society of America, and American Association of Petroleum Geologists shows abundant feldspar, lithic fragments, and heavy-mineral suites comparable to sediments in the Santa Maria River and Salinas River drainages. Structural relationships with the San Andreas Fault, Hosgri Fault, Nacimiento Fault, and Rinconada Fault were detailed in mapping by the USGS and county geologists, and sediment provenance studies link sources to the Sierra Nevada, Transverse Ranges, and the Peninsular Ranges as reported in journals such as Geology and the Journal of Sedimentary Research.

Stratigraphy and Age

The Santa Margarita Sandstone unconformably overlies older formations including the Vaqueros Formation, Monterey Formation, and older Oligocene units, with overlying relations to the Temblor Formation, Paso Robles Formation, and Pliocene deposits documented by the California Geological Survey and researchers at UCSB. Biostratigraphic and magnetostratigraphic work by teams from UC Davis, UC Riverside, and the Smithsonian Institution constrain the age to the middle to late Miocene, contemporaneous with regional units correlated to the Monterey Formation and Astoria Formation sequences discussed in papers in the Geological Society of America Bulletin. Chronostratigraphic frameworks incorporate zircon U-Pb and Ar-Ar geochronology applied by laboratories at MIT, Caltech, and the University of Arizona, and correlate with North American Land Mammal Ages and marine isotope stages recognized by the International Commission on Stratigraphy.

Depositional Environment

Sedimentological and facies interpretations from investigators at Stanford, UCSB, and CSUN indicate deposition in shoreface to shallow marine environments influenced by high-energy wave and storm processes along the Pacific margin, with fluvial input from paleorivers draining the Sierra Nevada and Coast Ranges similar to documented systems in the Sacramento–San Joaquin drainage. Comparisons have been drawn to modern systems studied by the Woods Hole Oceanographic Institution, Scripps Institution of Oceanography, and the Naval Research Laboratory, and to ancient analogs like the Pisco Formation and the coeval Monterey Basin sequences. Sequence stratigraphic analyses published by the American Association of Petroleum Geologists highlight transgressive-regressive cycles, parasequences, and incised valleys connected to sea-level changes recorded in the Neogene by the International Ocean Discovery Program and Deep Sea Drilling Project.

Paleontology

Fossil assemblages recovered from the unit include marine mollusks, echinoids, decapod crustaceans, and occasional vertebrate remains such as cetacean and pinniped bones documented by paleontologists at the Natural History Museum of Los Angeles County, the Santa Barbara Museum of Natural History, UC Berkeley Museum of Paleontology, and the Los Angeles County Museum. Microfossil work on foraminifera and ostracods by researchers connected to the Smithsonian Institution, Caltech, and Oregon State University has been used for paleoenvironmental reconstruction and correlation with planktonic foraminiferal zones established by the International Commission on Stratigraphy. Faunal lists referenced in monographs by the Paleontological Society and journal articles in Palaeogeography, Palaeoclimatology, Palaeoecology show affinities with Miocene assemblages from the North Pacific, including comparisons to the Astoria Formation and the Pisco Formation.

Geographic Distribution and Outcrops

Prominent outcrops occur along the central and southern California coast in Monterey County, San Luis Obispo County, Santa Barbara County, Ventura County, and parts of Los Angeles County, with inland exposures in the Salinas Valley and fragments in the San Joaquin Valley mapped by the USGS, California Geological Survey, and county geologists. Notable localities include coastal cliffs near Point Sal, the Santa Maria Basin, and roadcuts along U.S. Route 101 and California State Route 1 that are cited in field guides from the Geological Society of America, the Sierra Club, and university field courses at UCSB and Cal Poly San Luis Obispo. Mapping campaigns and GIS datasets maintained by the USGS, California Geological Survey, and academic research groups provide spatial distribution, thickness variations, and structural context relative to basins like the Santa Maria Basin and Ventura Basin.

Economic Uses and Resource Significance

The coarse-grained reservoirs and permeable strata have been evaluated for hydrocarbon potential by industry groups including Chevron, ExxonMobil, and Occidental Petroleum alongside academic studies at Stanford and UC Santa Barbara; analogous reservoir studies are published in the Journal of Petroleum Technology and AAPG Memoirs. Local quarrying for construction aggregate and use as a source of dimension stone has been noted in county planning documents and economic reports from the California Department of Conservation and the U.S. Bureau of Land Management. Groundwater recharge characteristics and engineering properties have been the subject of studies conducted by the USGS, California Department of Water Resources, and municipal water agencies in Santa Barbara and San Luis Obispo counties. Environmental assessments by the National Park Service and California Department of Fish and Wildlife consider the formation in habitat and coastal management planning.

History of Investigation and Naming

Early mapping and description were undertaken by 19th and early 20th century geologists associated with the USGS, California State Mining Bureau, and academic institutions such as Stanford University and UC Berkeley; later detailed stratigraphic and paleontological treatments were published by the California Division of Mines and Geology and scholars at UCSB, Caltech, and UCLA. The formation name entered regional geologic lexicons through county-level mapping, professional societies including the Geological Society of America and the American Association of Petroleum Geologists, and standardization efforts by the United States Geological Survey and the California Geological Survey.

Category:Geologic formations of California