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Paleontology and stratigraphy of the Miocene Saddleback Valley limestone, Orange County, Southern California

2016; Natural History Museum Los Angeles County; Volume: 524; Linguagem: Inglês

10.5962/p.324168

2165-1868

Carol J. Stadum, Kenneth L. Finger,

Botany, Ecology, and Taxonomy Studies

Although rare in California, Neogene limestone outcrops are well exposed within a 10 km 2 4 area of western Saddleback Valley in southern Orange County.These occur as inconsistent, discontiguous sequences of micrite, calcarenite, and limey sands that are informally referred to herein as the Saddleback Valley limestone.Its principal biotic components are cheilostome bryozoans, coralline red algae, and mollusks characteristic of the middle Miocene "Temblor" California Provincial Molluscan Stage (CPMS); hence, previous workers had referred to these beds as the "Temblor" limestone.In the Los Angeles Basin, the shallow-marine "Temblor" macrofauna also occurs in the Topanga Canyon Formation of the Santa Monica Mountains and the Topanga Formation in the Santa Ana Mountains.Nevertheless, geologists mapping the San Juan Capistrano and El Toro quadrangles placed the Saddleback Valley limestone at the base of the local Monterey Formation, presumably on the basis of undescribed field observations.This study combines lithostratigraphic and paleontologic data, including micropaleontology, to determine whether this recently challenged affiliation is justified.Microfossils (ostracodes, foraminifera, and calcareous nannoplankton) in the Saddleback Valley limestone indicate that its conspicuous shallow-water "Temblor" fauna and associated Topanga-like sediments had been transported downslope in the late Relizian (late early Miocene) at -16 Ma. 8 Sr/ 86 Sr analyses confirm the age at 15.9-16.5 My.It is therefore interpreted that the depositional history of the Saddleback Valley limestone began with a rich subtropical invertebrate community inhabiting the margins of a channel that existed between the mainland and a high-relief peninsula or island to the west.Increasing tectonic activity along the plate margin caused large amounts of schistose rock to slide off the island's eastern slope and into the channel, forming the San Onofre Breccia.Continued subsidence of the area created a deep basin into which turbidity currents transported the unconsolidated sediments, including its "Temblor" biota, which settled on the breccia or where "Monterey" muds had started to accumulate.These displaced sediments filled slope channels that were part of a deep-sea fan complex.In the deep-water Monterey realm, the intermittent deposits of limey sands were buried by hemipelagic muds.Diagenesis transformed isolated calcarenite lenses composed predominately of calcareous algae and invertebrates into nearly pure limestone.As tbe basin emerged in the late Pleistocene, erosion of the overlying strata exposed the Saddleback Valley limestone.Its depositional history and stratigraphy imply that it is the basal "member" of the local Monterey Formation.