Analysis of the Upper Cretaceous "Chico" formation in the Simi Hills of Southern California reveals rock types and megascopic features typically associated with turbidity-current deposits. The sandstone is principally arkosic arenite. The "Chico" formation is over 5,000 ft thick and is considered to be Late Cretaceous (Campanian and Maestrichtian).

Cross-bedding data indicate a mean direction of traction-current flow N21°E with statistical significance of 10^-5. Current data derived from analysis of sandstone grain fabric and from sole marks indicate turbidity-current flow toward the north, northwest, and west.

Many sandstone beds are very coarse grained and thickly bedded. Outcrops in some areas have sandstone to mudstone ratios in excess of 3:1, whereas in other outcrops the ratio is more nearly 1:1. Conglomerate beds are few and are composed mainly of pebble-size clasts.

Pre-Cretaceous granitic and metamorphic rocks make up the basement terrane in the western San Gabriel Mountains, 5 mi east of the Simi Hills, and in the eastern Santa Monica Mountains southeast of the Simi Hills. Collectively the basement terrane on the north, east, and southeast is known as "Mohavia."

The reconstructed submarine-current patterns, the petrology of the sandstone and conglomerate beds, the coarseness and great thickness of the sandstone beds, and the high sandstone-to-mudstone ratio support the suggestion that the source area was nearby Mohavia. Further, it is suggested that the sandstones were deposited as coalescing submarine fans on a topographically smooth north-to-northwesterly surface.

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