Lithogenetic Stratigraphy of the Triassic Dockum Formation, Palo Duro Basin, Texas

Four clastic depositional sequences compose the lower part of the Triassic Dockum Formation in the Palo Duro Basin. Each sequence consists of a basal interbedded mudstone and interbedded siltstone succession overlain by an upper succession of sandstone containing minor conglomerate, siltstone, and mudstone beds. Regional cross sections indicate that the lower parts of sequences typically thicken basinward, whereas the upper sandy partsnbecome more discontinuous basinward and upsection. Lithofacies cross sections show that thick sandstones are more common in sequences I and II and in the eastern part of the basin, whereas interbedded sandstones and mudstones are more prevalent basinward and upsection. Core analysis indicates that the lower parts of each sequence consist of lacustrine and prodelta mudstones and siltstones. Delta-front and channel-fill sandstones dominate the upper parts of each sequence. Swamp and eolian lithofacies are rare. Paleosol horizons are commonly preserved in lacustrine and floodplain sediments.Depositional sequences in the lower Dockum represent cycles of lacustrine transgression followed by clastic progradation, sometimes accompanied by a drop in base level. Sequence I was deposited by alluvial fans and fan deltas during a relatively dry climate before an extensive lake had formed, and therefore basal mudstone facies are absent. Sandstones in sequences II, III, and IV were deposited by bed- and mixed-load streams in fluvial and deltaic systems. Sequence II contains a large valley-fill system (IIb) that represents a major lake-level fall: less severe falls are recorded by paleosols in other sequences developed in lacustrine and prodelta mudstones and siltstones. Sequences IIIand IV record deposition in an expanding lake where sedimentation was dominated by lacustrine and deltaic systems: associated fluvial systems have generally been removed by erosion in updip areas. Deposition of the upper part of the Dockum Formation was probably similar to that of sequences III and IV. Comparisons of isopach and percent-sandstone maps of the upper and lower parts of the Dockum Formation and of net-sandstone maps of the lower part of the Dockum with basement-structure maps reveal a coincidence between basement lows and highs and isopach and sandstone thicks and thins, respectively. Well-defined basement structures, for instance the Castro Trough system and the Littlefield positive structure, clearly influenced thickness and sandstone distribution patterns throughout most of the Late Triassic Epoch, probably by slight topographic expression. Other recurring isopach and sandstone thicks and thins may help define basement structures in lesser known areas. The Dockum basin and the sediments in it resemble equivalent deposits in New Mexico and Arizona that were deposited in large structural depressions. Dockum rocks differ from equivalent deposits in rift basins in Mexico and along the Gulf and Atlantic coasts that are much thicker, more conglomeratic, and typically associated with igneous rocks.