Genetic stratigraphy of the Dockum Group (Triassic), Palo Duro Canyon, Panhandle, Texas
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The Triassic Dockum Group is a complex assemblage of lacustrine, deltaic, and fluvial facies. Excellent exposures of these rocks were studied in and around Palo Duro State Park where the Prairie Dog Town Fork of the Red River and its tributaries have carved a network of steep-walled canyons. Three-dimensional exposures of the Dockum have permitted study of lateral as well as vertical facies relationships. Three progradational genetic sequences were delineated within the study area. A genetic sequence consists in ascending order of: lacustrine mudstone, deltaic siltstone-sandstone-conglomerate, and fluvial sandstone-conglomerate. The following three lines of evidence indicate that complex base-level changes, caused by fluctuations in lake area and depth, occurred during deposition of the Dockum Group. Within the first genetic sequence burrowed lacustrine mudstones are interbedded with caliche horizons. These caliche horizons contain discrete burrowed and pisolitic carbonate nodules that are composed of microspar calcite, sparry calcite, and minor dolomite. Silicified evaporite nodules occur at the base of this unit. The second genetic sequence comprises a progradational lacustrine delta system which was truncated by a valley-fill system which formed in response to diminished lake depth and area. The valley-fill system comprises a transgressive fluvial-deltaic-lacustrine sequence encased within an overall progradational sequence. The third genetic sequence is composed of classic "Gilbert-type" lacustrine deltas with prominent foreset beds indicating deposition in 8 to 15 m of water. Ephemeral lakes characterized the first and second progradational sequences. Lacustrine fan deltas formed in water 1 to 10 m deep. The third progradational sequence was deposited in more continuously deep-water (10-20 m) lakes. A vertical sequence through Dockum Group rocks reveals a change from alternating humid and arid conditions of the first and second progradational sequences toward continuously humid conditions which prevailed during deposition of the third progradational sequence.