LATE CRETACEOUS MOLLUSCAN FAUNAS OF THE SANTA MONICA MOUNTAINS, SANTA ANA MOUNTAINS, AND SIMI HILLS, SOUTHERN CALIFORNIA: A COMPARISON

Upper Cretaceous strata in the Santa Monica Mountains (SMM) and Santa Ana Mountains (SAM) of southern California do not share identical lithologies and faunas as has been suggested. The diverse, late Turonian through early Campanian, mainly shallow-water marine molluscan faunas of the SAM do not occur in the SMM. Although sparse faunas dominated by ammonites are known from approximately the same time interval (late Turonian through ?Santonian) in the SMM, the SAM section contains older beds, characterized by Trochacteon and Liopistha.

Lenticular gray to black shale beds with plant fragments and a low diversity marine molluscan fauna characterized by abundant scaphitid ammonite specimens are widely distributed in the lower part of the Tuna Canyon Formation (TCF) in the SMM, but are not common in the SAM, except in the undivided Ladd Formation (LF) northeast of the Elsinore Fault near Corona. In the SMM these beds have yielded numerous aptychi associated with Scaphites cf. S. planus.

Campanian shell beds in the lower part of the Chatsworth Formation (CF) in the Bell Canyon and Dayton Canyon areas of the Simi Hills (SH) were deposited during an interval represented by a hiatus at the base of the Schultz Member of the Williams Formation in the SAM. A hiatus of similar duration might be present in the SMM. Thickly bedded turbidite sandstone beds in the upper part of the CF of the SH contain green siltstone beds with crushed Baculites specimens. Similar beds are present in the uppermost part of the TCF in the SMM west of Rustic Canyon. However, molluscan faunas equivalent to those from near the top of the CF in the SH do not occur in the SMM and SAM.

Although Metaplacenticeras is present in all three areas, only the SMM and SAM faunas match well. At Dayton Canyon in the SH, Hoplitoplacenticeras is present, but the typical large Metaplacenticeras pacificum is unknown.