Abstract: Gilbert-Type Deltas in a Miocene Marine Sequence, Cuyama Basin Southern California

J. Alan Bartow

The Soda Lake Shale Member of the Vaqueros Formation in the Cuyama Valley-Caliente Range area of the southern Coast Ranges of California consists almost entirely of basinal facies, although in the southeastern Caliente Range, the unit contains prominent sandstone bodies that are interpreted as small delta deposits. The Soda Lake in the southeastern Caliente Range consists of four lithofacies: (1) siltstone and mudstone, interpreted as the deposits of a muddy marine shelf or shallow basin, (2) platy shale, interpreted as deep marine basin deposits, (3) thick-bedded sandstone, interpreted as gravity flow deposits, and (4) large-scale foreset-bedded sandstone, interpreted as the deposits of small deltas. The abundance of graded beds in the foreset-bedded sandstone lenses sug ests that gravity-flow processes were the predominant mode of sediment transport. These deltaic deposits are interpreted as Gilbert-type deltas, not greatly different from the classical Gilbert delta model.

The progradational sequence in the southeastern Caliente Range suggests an initial ramp geometry for the basin margin there, in contrast to what appears to have been a steep basin margin in the northwestern part of the basin. The basin margin evolved from a ramp geometry by a combination of differential compaction of fine-grained basinal deposits versus coarser-grained near-shore deposits and the progradation of small deltas, leading to the formation of a marine shelf and slope.

During Soda Lake Shale Member deposition, the Cuyama Basin was bounded on the west by uplands and had a marine inlet to the south. The small Gilbert-type deltas were deposited along the southeast side of this semi-enclosed marine basin. The basin was, therefore, protected from the strong wave influence of major Pacific storms which allowed the formation of Gilbert-type deltas in relatively quiet waters.

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