Stratigraphic Models and General Criteria for Recognition of Alluvial, Barrier-Bar, and Turbidity-Current Sand Deposits

Abstract

The depositional framework for sandstone bodies is most accurately determined by comparing their geometry and internal features with those of stratigraphic models (sand units of known depositional environments). Alluvial, barrier-bar, and turbidity-current sandstones are each characterized by a combination, or group, of distinguishing features. The Lower Cretaceous Cut Bank Sandstone of Cut Bank field in northwest Montana is an example of alluvial valley deposition south of an arctic sea. Basal and lateral contacts of the sandstone are sharp, erosional boundaries; the sedimentary structural and textural sequences are quite similar to modern point-bar sequences. Massive beds at the base of the Cut Bank Sandstone are overlain by beds having medium-scale cross-bedding. Grain size decreases upward. The lowermost sandstone unit of the Upper Cretaceous Eagle Sandstone at Billings, Montana, was deposited as a barrier-bar feature some distance east of the mainland. Its features are very similar to those of the modern Galveston Island sand. The lower and lateral contacts are gradational whereas the upper contact is sharp. The width-thickness ratio is high. Low-angle inclined bedding overlies mottled bedding, and the grain size increases upward. The lower Pliocene Repetto Formation of the Los Angeles basin is characterized by a great thickness of turbidity-current sandstone, deposited in a deep-marine orogenic basin. The depositional basin was very similar to modern basins offshore from southern California. Gross sandstone thickness trends parallel the length of the basin. Each sandstone unit is composed of several graded sandstone beds with sharp basal contacts. The sandstones are poorly to moderately sorted. The foraminiferal assemblages in interbedded shale indicate bathyal-abyssal depths during deposition.