Lower Cretaceous Shelf Storm Deposits, North Texas: ABSTRACT

Many of the important features of shelf storm deposits are displayed in the Lower Cretaceous Washita Group in Grayson County, Texas. Proximal shelf sands are up to 2 m thick. They show channeled tops and abrupt lateral thickness variation and are stacked vertically or separated by thin, sparsely fossiliferous marine muds. Each sand unit has a scoured base and comprises a structureless lower interval that grades upward into hummocky cross-stratification. These features in turn are overlain by subhorizontal lamination, with an uppermost interval consisting of small-scale wave ripples containing abundant horizontal burrows and feeding trails. Distal shelf sands are thin (10 to 40 cm) compared to the intervening muds, and are comparable to modern storm deposits of the Texas s elf. A basal shelly lag, with a scoured base of upward-convex, disarticulated bivalves or current-aligned high-spired gastropods merges upward into subdued hummocky cross-stratification and subhorizontal lamination. Powerful bottom return currents, that in the modern Gulf of Mexico are generated by wind immediately prior to storm landfall, were probably the dominant mechanism of offshore sand transport. Storm waves operated contemporaneously and probably contributed toward sediment entrainment, as evidenced by hummocky cross-stratification. However, the disposition of fossils suggests that initially, at least, unidirectional currents were the dominant transport mechanism. Relatively shallow-water depths (50 m or less) are indicated by associated regressive deltaic sequences. End_of_Article - Last_Page 723------------