Abstract
Alluvial fan conglomerates of the middle Cenozoic Simmler Formation, in the Cuyama Gorge and southern La Panza Range of west-central California, were deposited in two half-graben basins. On the basis of paleocurrents and down-fan fining of sediment, both to the northeast, the basins are thought to have been adjacent to the then-active Nacimiento and La Panza faults. Three contiguous conglomerate lithofacies, from most proximal to most distal, are: (1) clast-supported, crudely stratified, coarse conglomerate with imbrication — proximal talus and powerful sheet flow; (2) clast-supported, well-bedded, internally unstratified conglomerate, with common imbrication, less common grading, and thin, interbedded, coarse sands — mid- to upper-fan deposits of highly sediment-charged, density-modified, grain-flow sheet flows; (3) stratified sandy conglomerate with imbrication — distal alluvial sheet flows. Two additional lithofacies cut across the first three: (4) parallel-bedded, graded, coarse sands — turbidity currents in floodbasin lakes; and (5) matrix-supported, very poorly sorted conglomerates—cohesive debris flows. The alluvial fan deposits of the more northeasterly basin are overlain directly by the marine Vaqueros Formation. The fans were dominated by sheet flow events. The lack of cross-bedding and minor occurrence of channels indicate that permanent streams were not established. Cohesive debris flows were unimportant. The relationship of fan gravels both to the lake turbidites and to the conformably overlying marine sands indicates the existence of fan deltas. As a result of late Cenozoic transcurrent movement on the La Panza fault, a strand of the early San Andreas system, the two half-graben basins have been moved so as to create a sedimentary mismatch.
Published Version
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