Careaga Sandstone

Careaga Sandstone
Name	Careaga Sandstone
Type	Geological formation
Age	Oligocene–Miocene (approx. 28–16 Ma)
Period	Neogene
Primary lithology	Sandstone
Other lithology	Conglomerate, siltstone, shale
Region	Central California Coast Ranges
Country	United States
Underlies	Lower Miocene strata
Overlies	Marine and nonmarine Paleogene units

Careaga Sandstone is a Neogene sedimentary rock unit recognized in the Central California Coast Ranges and adjacent coastal basins. It has been correlated with regional Oligocene–Miocene successions studied by investigators associated with institutions such as United States Geological Survey, Stanford University, University of California, Berkeley, California Geological Survey, and Scripps Institution of Oceanography. The unit figures in regional tectonic and stratigraphic syntheses tied to events like the development of the San Andreas Fault system, the evolution of the Salinas Valley, and uplift related to the Coast Ranges (California).

Description

The Careaga Sandstone is typically medium- to coarse-grained, well-cemented strata forming cliffs, bluffs, and ridges across parts of Monterey County, California, San Luis Obispo County, California, and the northern Santa Barbara County, California. Field descriptions and mapping by geologists from the USGS and the California Division of Mines and Geology document its exposure in coastal sections near communities such as Morro Bay, Cambria, California, and Pismo Beach. The formation’s appearance and physical properties have been summarized in regional geologic maps produced by teams affiliated with California State University, San Luis Obispo and the Natural Resources Defense Council in land-use planning reports.

Stratigraphy and Age

Regional biostratigraphic and radiometric studies link the Careaga Sandstone to the late Oligocene through early Miocene, comparable in age to units mapped in the Santa Maria and Salinas basins. Correlative frameworks reference chronostratigraphic work published by researchers at UC Santa Barbara, USGS Menlo Park, and international collaborators from institutions such as the Smithsonian Institution. Stratigraphic relationships show the Careaga commonly overlies Paleogene marine deposits and is overlain by younger Miocene marine and nonmarine units that have been correlated with faunal assemblages used by paleontologists associated with the American Museum of Natural History.

Lithology and Sedimentology

Lithologically, the Careaga comprises dominantly quartz-rich sandstone with subordinate pebble- to cobble-conglomerate lenses, siltstone partings, and thin shale interbeds. Petrographic and heavy-mineral analyses performed by investigators from California Institute of Technology and University of Southern California indicate provenance signatures consistent with uplifted plutonic and metamorphic sources tied to the Sierra Nevada (U.S.) and local basement outcrops. Grain-size trends, cross-bedding, and imbrication measured in sections cataloged by field teams from USGS and Stanford University support interpretations of variable transport energy and sediment supply controlled by regional tectonics associated with the Pacific Plate–North American Plate boundary.

Depositional Environment

Sedimentological evidence supports deposition in dominantly fluvial to coastal-plain settings with episodic high-energy braided-stream channels, deltaic lobes, and marginal marine shoreface incursions. Interpretations draw on comparisons with modern and ancient analogs studied at institutions such as Woods Hole Oceanographic Institution and referenced in syntheses by researchers from Caltech and UC Santa Cruz. The depositional model integrates regional tectonic activity tied to the initiation and reorganization of the San Andreas Fault system, eustatic sea-level fluctuations recorded in contemporaneous units, and sediment supply from orogenic source areas like the Santa Lucia Mountains.

Geographic Distribution and Outcrops

Key outcrops occur along coastal cliffs and inland exposures in the Central Coast region, including mapped occurrences near Morro Bay State Park, the Los Padres National Forest boundary, and coastal terraces seaward of the Carrizo Plain. Geological mapping undertaken by teams from the USGS and state geological surveys provides the primary spatial framework, supplemented by university-led field studies from Cal Poly San Luis Obispo and UC Santa Barbara. The unit’s spatial distribution is important in regional hazard and land-use assessments conducted by agencies such as California Geological Survey and local county planning departments.

Economic Importance and Uses

The Careaga Sandstone has local economic significance as a building and dimension stone in historical construction projects in communities like San Luis Obispo and Paso Robles, California. Sand and gravel derived from its conglomeratic horizons have been exploited for aggregate by contractors regulated by county public works departments and firms engaged with the California Department of Transportation. Its porosity and permeability have also been evaluated in hydrogeologic studies by teams from USGS and California State University for groundwater resource assessment and in basin analyses for petroleum exploration by energy companies with regulatory oversight from the California Geologic Energy Management Division.

Paleontology and Fossil Content

Although predominantly siliciclastic, the Careaga contains trace fossils and sparse macrofossil remains in marine-influenced intervals, including bivalves and gastropods comparable to faunas documented by paleontologists at UC Berkeley and the Santa Barbara Museum of Natural History. Ichnological assemblages and palynological samples analyzed by researchers at University of California, Davis and Oregon State University contribute to paleoenvironmental reconstructions and correlation with contemporaneous Neogene sequences studied by international teams associated with the International Commission on Stratigraphy.

Category:Geologic formations of California