Abstract: Late Miocene Stratigraphic and Paleogeographic Setting of Garnet Hill in the Northwestern Salton Trough, Southern California

Michael J. Rymer, Charles L. Powell II, Andrei M. Sarna-Wojcicki, John A. Barron

New geologic and biostratigraphic data from the Imperial Formation at Garnet Hill (northwestern end of the Salton Trough) better constrain the timing, depositional environment, and paleogeography of a late Miocene marine incursion into the Salton Trough. Fluvial sandstone, possibly of the Hathaway Formation, contains a volcanic ash bed and is interbedded with the marine Imperial Formation. The ash bed correlates with an ash bed in the Modelo Formation, in the Ventura area, on the basis of chemical analyses. The age of the ash in the Modelo Formation is 7.6-8.0 Ma, based on diatom biostratigraphy in the formation.

Imperial Formation deposits above the Hathaway(?) Formation at Garnet Hill contain a bivalve and gastropod assemblage that correlates with the megafossil fauna in the Imperial Formation exposed in the Whitewater-Cabazon sections to the northwest, across the Banning strand of the San Andreas Fault Zone. The Imperial Formation in the Whitewater section is overlain by the Painted Hill Formation, containing basalt flows with two K/Ar dates of 6.0 ± 0.2 Ma, and underlain by the Coachella Fanglomerate with a K/Ar date of 10.1 ± 1.2 Ma. The bracketing dates on the Imperial Formation in the Whitewater section and the correlated volcanic ash age in the interbedded Hathaway(?) Formation at Garnet Hill, along with the molluscan correlations, indicate a late Miocene (6.0-7.6 Ma) incursi n into the Salton Trough.

Palinspastic reconstruction of the Garnet Hill and Whitewater sections for this period indicates deposition about 150 km to the southeast. The Whitewater section was deposited opposite the southern Chocolate Mountains and was later displaced along the south branch of the San Gabriel-Banning Fault, a late Miocene plate-boundary fault predating the San Andreas Fault, and later the modern San Andreas Fault. The Garnet Hill section was displaced by the above faults and additionally by the Banning strand of the San Andreas Fault Zone during the Quaternary.

AAPG Search and Discovery Article #90958©1995 AAPG Pacific Section Meeting, San Francisco, California