Lehman and Chatterjee reported in 2005 that Triassic strata of the Dockum Group in Texas comprise two major upward-fining alluvial–lacustrine depositional sequences. The two sequences are represented by the (1) Santa Rosa–Tecovas, and (2) Trujillo–Cooper Canyon Formations. The second sequence is much thicker than the first, and occupies a greater geographic part of the Dockum basin.
In the figure above, the Dockum basin of western Texas and eastern New Mexico, shows positions of stratigraphic sections 1 through 13 given in figure 3. Outcrop (dark patterns) and subsurface extent (light stipple) of Triassic strata in the Dockum basin are modified from Roth (1955), Dane and Bachman (1965), and McGowen et al (1979). Outcrop of Santa Rosa Sandstone (TR s), Tecovas Formation (TR v), Trujillo Sandstone (TR j), Cooper Canyon Formation (TR c), and Redonda Formation (TR r) are shown, as are the locations of their type areas. Cross-section A–A shows generalized subsurface relationships between the Triassic formations.
Each sequence of alluvial and lacustrine sediment accumulation is characterized by sediment derivation from a different source terrain. The unconformable relationship between the two depositional sequences, the change in mineralogical composition and presumed source areas between these units, differences in paleocurrent orientation between units, and evidence for intervening episodes of local deformation indicate that the sequences are of tectonic origin.
These strata are not the product of a single sediment dispersal system, such as the centripetally-drained lacustrine delta complex previously envisioned for the Dockum basin. Both Dockum sequences are comprised largely of two typical alluvial facies associations; stream channel facies, and overbank flood-plain facies, that are similar to those described in nearly all fluvial deposits. In addition, the Dockum Group contains a peculiar lacustrine facies that accumulated in local flood-plain depressions, and probably resulted from subsidence over areas of subsurface salt dissolution.
Vertebrate fossil assemblages are found in all three Dockum facies associations. Five fossiliferous sites in the Dockum are discussed in the context of these three depositional settings. The Dockum tetrapod diversity is reviewed in a hierarchical phylogeny with remarks on the history of collection, stratigraphic distribution of genera, and their taxonomic status. The stratigraphic ranges of tetrapod taxa do not support the recently proposed successive Otischalkian,
Adamanian, Revueltian, and Apachean biochrons within the Dockum Group. Instead, a few index fossils provide a broad framework for correlation of Late Triassic nonmarine strata of the Dockum with the Carnian and Norian Alpine marine stages.
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