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Density-current Strata in Lower Cretaceous Washita Group, North-central Texas
Robert W. Scott, Hooman Laali (2), David W. Fee (3)
Distinctive sedimentary structures and textures characterize flaggy siltstone interbeds sporadically distributed within the predominantly terrigenous clastic strata of the Washita Group, such as the Kiamichi, Denton, and Pawpaw formations. The 1 to 10 cm thick strata consist mainly of silt- and sand-sized grains; and the grain-size distribution is normally graded. Both basal and uppermost surfaces of these strata are in sharp contact with overlying and underlying shale. Burrow casts, tool marks and flute marks are common on the basal surfaces. Upper surfaces display low-amplitude interference ripples crossed by burrows. Internal structures are parallel laminae at the base overlain by climbing ripple or rarely trough cross laminae. Most strata persist at a given stratigraphic position or less than a few kilometers.
These siltstone strata resemble classic distal turbidites. However, the small scale of their properties and the absence of thick, rhythmically bedded sequences indicate shallow-water deposition by rather sporadic agents. Some form of advective density current generated by storms, tides, rip flow, or floods transported this clastic debris from a nearby shelf zone.
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