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ABSTRACT: Stratification of Shelf and Basinal Turbidites: Recognition of Deltaic vs. Fan Deposits

Scott R. Morgan, Kirt M. Campion

Submarine fans of the Jurassic-Cretaceous Great Valley Sequence exhibit a distinctive vertical and lateral association of lithofacies that serves as a general model of deep-marine strata. Other turbidite-bearing deposits of the California coast include the Wheeler Gorge turbidites, the Matilija and Sacate formations of the Transverse Ranges, and the Point Loma Formation of the San Diego basin. These formations have all been identified as submarine fans because they are dominated by turbidite lithofacies, despite substantial differences in vertical succession and lateral continuity of facies and lithologies.

Basinal-turbidites of the Great Valley Sequence, Sacramento basin, consist of interstratified sandstone- and shale-prone units (e.g., Venado and Yolo formations) that represent alternating coarse-grained lowstand, submarine-fan and fine-grained lowstand wedge and highstand deposits. Typically, fans exhibit a sharp base, thin- and fine-upsection, and stack retrogradationally. Cretaceous turbidites at Wheeler Gorge exhibit a similar sharp-based, fining-upward stacking pattern compatible with a lowstand-fan interpretation.

In contrast, the Eocene Matilija and Sacate formations and Cretaceous Point Loma Formation, which exhibit gradational basal contacts and progradational stacking patterns, represent fluvial-dominated turbidite-bearing deltas. In the Matilija and Sacate, thin-bedded, fine-grained turbidites are replaced upsection by thick-bedded turbidites, trough cross-bedded sandstone, and fossiliferous, lignitic mudstone. Similarly, Point Loma turbidites conformably overlie estuarine sandstones and thicken and coarsen upsection in a progradational succession that contains abundant Ophiomorpha and Thalassinoides burrows. Lithofacies associations represented by these formations are incompatible with known basin-fan deposits, but are comparable to fluvial-dominated deltaic systems with progradation into moderately deep water.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990