Lake margin depositional systems of the Dockum Group (Upper Triassic) in Tule Canyon, Texas Panhandle




Boone, John Lawrence

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The Dockum Group (Upper Triassic) in the Tule Canyon area is a complex assemblage of terriginous clastic rocks deposited within a broad, shallow intracratonic basin. Lakes were present in the basin through most of Dockum time. Lacustrine character is suggested by faunal content, lithofacies and evidence of rapid changes in base level. Depositional systems identified in the section are: 1) a basal alluvial fan/fan-delta, 2) local valley fill of pebble conglomerate and sandstone, 3) fluvial coarse-grained (low-sinuosity) meanderbelt sandstones and 4) lobate, Gilbert-type deltas that occur in an imbricated series of 3 to 4 progradational cycles. Dockum rocks are composed of approximately 50% fine-grained sandstone, 40% mudstone and siltstone and 10% chert-pebble conglomerate in the valley-fill. Lower Dockum systems are mostly sandstone, whereas the upper Dockum contains thick mudstone and siltstone intervals. Dominance of aquatic vertebrate fossil types, lack of in situ plant fossils and ubiquitous caliche in Dockum rocks suggest that paleoenvironments of the Tule Canyon vicinity were warm and arid to semiarid. However, vigorous currents and abundant sediment supply in large Dockum rivers indicate that pluvial conditions existed in tributary headwaters east of the outcrop. Lower and middle Dockum depositional environments were associated with shallow lakes of variable depth and extent. Delta front thicknesses suggest depths of a few meters or less at the base of the Dockum. Valley entrenchment and soil development accompanied significant changes in climate, lake geometry and depositional conditions after progradation of the basal fan system. Lobate deltas of the upper Dockum record more humid, deeper water lacustrine conditions. Thicknesses of delta front foreset sequences show that water depths near deltas were in the range of 5-8 m, and rarely more than 10 m. Transgressive, fining-upward patterns in delta front deposits and cycles of entrenchment and fill in delta distributaries suggest the occurrence of lake level changes in the upper Dockum of 10 m or less.