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The continental Triassic rocks of Texas are represented by four distinct but similar rock groups that exist both in outcrop and in the subsurface and include the Eagle Mills Formation (south-central and northeast Texas), Sycamore Formation (central Texas), Dockum Group (west Texas), and Bissett Formation (southwest Texas). They are clearly terrigenous in nature derived principally from older Paleozoic sedimentary rocks. The rock groups are composed in part or entirely of mudstone, siltstone, medium to coarse-grained sandstone, and pebble to boulder conglomerate (intrabasinal and extrabasinal). The sediments were deposited in alluvial fans, braided and meandering streams, lobate deltas, fan deltas, and lakes. The coarse sandstone and conglomerate are the products of high-e ergy, short-duration depositional events. Sedimentation was greatly affected by alternating climatic conditions that produced changes in base level, water depth, and lake area as well as the type of streams that flowed into the depositional basins. The character of the rock groups strongly suggests semi-arid to arid deposition typical of the low latitude desert regions of today. Thus, the rocks comprising the Eagle Mills, Sycamore, Dockum, and Bissett Formations appear to be products of continental clastic deposition during a major semi-arid to arid climatic episode, such as that of late Triassic time.
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