Abstract: Depositional Framework of Lower Dockum Group (Triassic), Texas Panhandle
J. H. McGowen, G. E. Granata, S. J. Seni
The Dockum Group of Texas and New Mexico consists of up to 2,000 ft (600 m) of complexly interstratified terrigenous clastic red-bed facies. The lower part of the Dockum is 200 to 1,100 ft (60 to 335 m) thick. At least two lacustrine subbasins existed during Dockum deposition--one north and west of the Amarillo uplift-Bravo Dome and another (the subject of this report) between the Amarillo uplift-Bravo Dome and the Glass Mountains. The relict Matador arch crosses the area but it exerted little influence on Dockum sedimentation. Depositional trends inferred from outcrop and subsurface studies indicate that the southern basin was marginally filled. Sediment input in Texas was from the south, east, and north. Sediment also entered the basin from the west through Lea and Roos velt Counties, New Mexico. The geologic setting of the basin during Dockum deposition included structural highs on the north and south, and broad lowlands on the east and west.
Previous workers assigned a Late Triassic age to the Dockum. An abrupt change in depositional style is present between Permian and Triassic deposits in some areas, such as eastern New Mexico and the Palo Duro Canyon area in Texas. Elsewhere there is no evidence of an unconformity. Permian and Triassic strata are unconformable near basin margins and adjacent structural highs, and conformable toward the basin center.
Deposition was influenced by relict Paleozoic structural elements and inferred Triassic climatic cycles. Arid conditions in the Permian changed slowly as rainfall increased into the Triassic. Deposition by braided and meandering streams resulted in associated alluvial fans, fan deltas, high-constructive lobate deltas, and lacustrine deposits. Alluvial fans and fan deltas developed chiefly in northern and southern parts of the basin adjacent to relict Paleozoic structural highs. High-constructive lobate deltas were developed best in the central basin areas. Lowered lake level caused periods when valleys ranging from 50 to 200 ft (15 to 60 m) were incised across the depositional elements.
Several depositional cycles, controlled by base-level (Lake) fluctuations, are recognized in Dickens, Crosby, Kent, and Garza Counties. A cycle comprises a sequence of facies that accumulated during a humid highstand and an arid low-stand of lake level. Progradational delta sequences overlain by meander-belt systems were produced under high-stand conditions of relatively stable base-level. Delta sequences are thin and commonly were partly eroded by meandering streams that cut downward and migrated laterally. Progradational sequences are composed of sediment derived primarily from beyond the basin; these sediments are predominantly siliciclastics that range from mudstone to sandstone; conglomerate is uncommon. A typical progradational sequence consists of lacustrine and prodelta mudsto e to siltstone, delta-front siltstone to sandstone, channel-mouth-bar and distributary sandstone, and meander-belt sandstone to conglomerate. Interdistributary and flood-plain areas were sites of crevasse-splay deposition. Splays comprise poorly sorted sandstone to boulder conglomerate (large clasts are characteristically mudstone and siltstone derived from older Triassic deposits).
A shift from humid to arid conditions was accompanied by a lowering of base level and erosion of older Dockum sediments. Small fan deltas were constructed at low stand by sediment transported to lake margins through short, high-gradient, headwardly eroding streams; fan-delta, fluvial, and lacustrine facies represent low-stand deposition. These reddish-brown lowstand deposits (red beds) range from mudstone to boulder conglomerate and are typified by abrupt vertical and lateral changes in facies. Red-bed facies in the lower Dockum consist of lacustrine mudstone, prodelta mudstone to siltstone, delta-foreset siltstone to conglomerate, delta-platform sandstone to conglomerate, valley-fill mudstone to conglomerate, and interdeltaic mudstone with desiccation features, sparse chert, salt hop ers, and gypsum.
Preliminary data indicate that radioactive elements are most common in sandstones of the high-constructive delta-meander-belt sequence of meander-belt, distributary-channel, delta-front, and crevasse sandstone bodies.
AAPG Search and Discovery Article #90967©1977 GCAGS and GC Section SEPM 27th Annual Meeting, Austin, Texas