Age and paleoenvironment of the Imperial Formation near San Gorgonio Pass,southern California

Microfossiliferous marine sediments of the Imperial Formation exposed in th e Whitewater and Cabazon areas, near San Gorgonio Pass, southern California , are late Miocene in age and were deposited at intertidal to outer neritic depths, and possibly upper bathyal depths. A late Miocene age of 7.4 to >6 .04 hla is based on the ranges of age-diagnostic benthic foraminifers (Cass idulina delicata and Uvigerina peregrina), planktic foraminifers (Globigeri noides obliquus, G. extremus, and Globigerina nepenthes; zones N17-N19), an d calcareous nannoplankton (Discoaster brouweri, D. aff. D. surculus, Retic ulofenestra pseudoumbilicata, Sphenolithus abies, and S. neoabies; zones CN 9a-CN11) coupled with published K/Ar dates from the underlying Coachella Fo rmation (10.1 +/- 1.2 Ma; Peterson, 1975) and overlying Painted Hill Format ion (6.04 +/- 0.18 and 5.94 +/- 0.18 Ma; J. L. Morton in Matti and others, 1985 and Matti and Morton, 1993), Paleoecologic considerations (sea-level f luctuations and paleotemperature) restrict the age of the Imperial Formatio n to 6.5 through 6.3 Il la. Benthic foraminiferal assemblages indicate that the Imperial Formation in t he Whitewater and Cabazon sections accumulated at inner neritic to outer ne ritic (0-152 m) and possibly upper bathyal (152-244 m) depths. Shallowing t o inner neritic depths occurred as the upper part of the section was deposi ted. This sea-level fluctuation corresponds to a global highstand at 6.3 Ma ;la (Haq and others, 1987), Planktic foraminifers suggest an increase in su rface-water temperatures upsection, A similar increase in paleotemperatures is interpreted for the North Pacific from 6.5 to 6.3 Ma (warm interval W10 of Barren and Keller, 1983), Environmental contrasts between the Whitewater and Cabazon sections of the Imperial Formation provide evidence for right-lateral displacements on the Banning fault, a late Miocene strand of the San Andreas fault system. The C abazon section Lies south of the Banning fault, and has been displaced west relative to the Whitewater sections. The Cabazon section was deposited at greater depths, suggesting that it accumulated farther offshore than the Wh itewater section. If the Salton Trough was a southward-opening, elongated n orthwest-southeast basin similar to the modern Gulf of California, the Caba zon sequence probably has been displaced right-laterally from a position fa rther southeast of the Whitewater sequence. This relation requires late Mio cene displacements greater than the present 12 km cross-fault separation be tween the two Imperial sections in the San Gorgonio Pass area.