Environmental geology of the Wilcox Group lignite belt, East Texas

The Upper Triassic Dockum Group of Texas and New Mexico is composed of 200 to 2,000 feet of complexly interrelated terrigenous clastic facies ranging from mudstone to conglomerate. The lower 200 to 1,000 feet of the Dockum accumulated in a fluvial-lacustrine basin defined by the Amarillo Uplift - Bravo Dome on the north and the Glass Mountains on the south. Outcrop and subsurface data indicate that (1) the basin was filled peripherally, (2) the sediment sources were in Oklahoma, Texas, and New Mexico, and (3) the relict Paleozoic structures in concert with alternating humid and arid climatic cycles exerted considerable influence on the depositional style of the Dockum. An unconformity between the Permian and Triassic is obvious in the northern part of the basin, but physical evidence of an unconformity is lacking in the central basin area. Arid Permian conditions gave way gradually to more humid conditions of the Triassic. Initial deposits of the Dockum, which record these humid conditions, accumulated in (1) braided and meandering streams, (2) alluvial fans and fan deltas, (3) high-constructive lobate deltas, and (4) lakes. Alluvial fans and fan deltas were best developed in northern and southern parts of the basin, whereas high-constructive lobate deltas dominated central basin areas. A change from humid to arid conditions produced (1) lowering of base level, (2) erosion (cannibalization) of older Dockum deposits, (3) replacement of meandering fluvial systems by headwardly eroding valleys and braided streams, and (4) development of small fan deltas. Several depositional cycles are recognized in the area defined by Dickens, Crosby, Kent, and Garza Counties, Texas. A cycle comprises facies that accumulated during one high stand and one low stand of lake level. Thin progradational delta and attendant meanderbelt systems were deposited during high-stand, relatively stable base-level conditions. Progradational delta sequences are composed of extrabasinal sediments ranging in texture from clay to gravel. A typical delta sequence consists of lacustrine and prodelta mudstone-siltstone, delta-front siltstone-sandstone, channel-mouth bar and distributary sandstone, and meanderbelt sandstone-conglomerate. Splay units, consisting of poorly sorted intrabasinal sandstone and conglomerate, are constituents of interdistributary and floodplain deposits. Most delta sequences were partly cannibalized by superimposed meandering streams that migrated across the area. With a shift toward arid conditions there was a lowering of base level accompanied by erosion of subjacent Dockum deposits. Sediment that composes the low-stand facies association ranges from reddish-brown mudstone to conglomerate. Abrupt vertical and lateral textural changes characterize these low-stand deposits. Lower Dockum red beds consist of (1) lacustrine mudstone, (2) prodelta mudstone-siltstone, (3) delta-front (delta foresets) siltstone to conglomerate, (4) delta-platform sandstone and conglomerate, and (5) interdeltaic mudstone exhibiting desiccation features, rare gypsum, salt hoppers, and chert.