Paleontology and biostratigraphy of the Plio -Pleistocene lower Saugus Formation, Santa Susana Mountains, southern California

Abstract

Richly fossiliferous marine deposits of the Plio-Pleistocene lower Saugus Formation, Santa Susana Mountains, southern California are herein documented to contain 106 macrofossil species from 15 localities. A faunal survey yielded two bryozoans, five brachiopods, 43 bivalves, 49 gastropods, one scaphopod, two barnacles, one echinoid, two sharks, and one angiosperm. Depositional sequences and analysis of macrofossils of the lower Saugus Formation indicate deposition in shoreface, wave dominated delta-front, and tidal-channel environments in Browns Canyon, embayedchannel deposits and storm-accumulation deposits in Las Llajas Canyon, and embayed-channel deposits in the Gillibrand Quarry area. Bathymetric and latitudinal analyses of this fauna reveals that a majority of the extant species live at study area latitude (34.18' N) between 20 and 30 m depth. Bathymetrically anomalous deeper water species of mollusks in the same beds with shallow-water species, and extralimital northern and southern mollusks found in the lower Saugus Formation, indicate the probability of currents mixing closely adjacent warm: and cold-water habitats. An example would be coldwater upwelling centers in subtropical regions or warrn-water embayments in temperate regions. Southern California Pliocene index macrofossils found in the lower Saugus Formation indicate deposition near the Plio-Pleistocene boundary. Extinct species of terminal Pliocene mollusks, a barnacle, and an echinoid define the uppermost boundary of the Pliocene "San Joaquin Stage" within the study area. The commonest of these index species in the study area are Patlnopecten bealeyi and Dendostrea vespertina. Microfossils present in the lower Saugus are not diagnostic of geologic age. An unconformity within the upper part of the lower Saugus Formation marks the uppermost extent of the Pliocene index macrofossils Pecten lecontei, Patinopecten healeyi, Pycnodonte erici, Lopha veatcbii, Opalia varlcostata, Calicantbarus humerosus, C. kettlemanensis, Musashia (Nipponomelon) oregonense, Terebra martini, Balanus gregarius, and Dendraster asbleyi. Stratigraphically above the unconformity is a calcareous nannofossil assemblage that indicates a Pleistocene age. Plio-Pleistocene formations correlative with the lower Saugus Formation in central and southern California are the San Joaquin Formation, the Careaga Formation, the Las Posas Formation, the Potato Harbor Formation, the Niguel Formation, unnamed Pliocene strata on San Clemente Island, and the San Diego Formation. The San Diego Formation contains more Pliocene index species common to the lower Saugus Formation than do any of the other correlative formations.