Sedimentology, fluvial paleohydrology, and paleogeomorphology of the Dockum Formation (triassic), West Texas
Frelier, Andrew P
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Dockum Formation (Late Triassic) sediments are exposed along the eastem High Plains escarpment of west Texas, and consist of fluvial channel sandstone and overbank deposits. Dockum channel sands are classified into 5 orders which represent fluvial deposits of differing stream types. The largest channel sands (first and second order) contain thalweg facies, lower point bar facies, and upper point bar facies which exhibit grain size and bed form structure trends corresponding to channel facies in fully developed meander bend flow. Channel sands of first, second, and third order represent deposits of high sinuousity (P > 1.65), freely-meandering streams. Paleohydrological reconstruction of these channel orders is facilitated by abandoned channels and well defined lateral accretion bedding. Paleocurrent and petrographic analyses indicate trunk streams (analagous to first order sands) flowed from south to north with headwaters reaching into the Ouachita metamorphic core complex approximately 400 km to the south. Multiple phases of floodplain incision and aggradation are documented by disconformities in the overbank deposits which define hills and gullies produced during incisional phases. Fifth order channel gravels (smallest classified order) often occupy the bottoms of paleogullies which formed as headward-eroding fifth order channels dissected elevated floodplains during phases of trunk stream incision. Fourth and fifth order streams were low sinuousity, high bed load mixed load channels that derived their loads exclusively through floodplain erosion. A hot, semiarid to subhumid. Late Triassic climate is inferred on the basis of widespread development of caliche horizons and predominance of montmorillonite clays in overbank deposits, and their production in Dockum paleosols.