Lithostratigraphy, chemostratigraphy, and vertebrate biostratigraphy of the Dockum Group (Upper Triassic) of southern Garza County, West Texas
Martz, Jeffrey W.
The Dockum Group of southern Garza County, West Texas, consists of a basal siliceous conglomeritic sandstone (the Santa Rosa Sandstone) with pedogenic alteration at the top (“mottled beds”) which is truncated by an unconformity. The Boren Ranch Sandstone and Boren Ranch beds overly this unconformity, the latter may indicate lacustrine deposition. The overlying Cooper Canyon Formation is divided into a Lower Mudstone Member and the Upper Cooper Canyon Formation. The Cooper Canyon Formation is dominated by mudstone, but contains micaceous and feldspathic litharenite sandstones and intrabasinal conglomerates identical to the Boren Ranch Sandstone, several of which can be traced and mapped for several kilometers. Measured sections were measured and correlated to construct a lithostratigraphic framework for vertebrate biostratigraphy. In the Texas Panhandle to the north of the study area, the Tecovas Formation probably correlates with the Lower Mudstone Member and possibly the Boren Ranch Sandstone/beds (which have been erroneously assigned to the Trujillo Formation), the Trujillo Formation correlates with the Dalby Ranch Sandstone and/or Miller Ranch Sandstone just above the Lower Mudstone Member, and the Bull Canyon Formation correlates with most of the Upper Cooper Canyon Formation. In the drainages of the Colorado River and Brazos River to the south, the “Colorado City Member” may mostly correlate with the Lower Mudstone Member, which is suggested by a southwesterly thickening of the Lower Mudstone Member in southern Garza County which can be inferred from well logs. In the Midland Basin, sediments began to be derived from the Ouachita-Marathon Orogenic Belt early in the deposition of the Dockum Group, and extended further north during deposition of the Upper Cooper Canyon Formation, Trujillo Formation, and Bull Canyon Formation. A Tr-4 regional unconformity probably does not exist. Chemostratigraphy, the application of bulk major and trace element geochemistry to stratigraphic subdivision of the Dockum Group, was able to identify broad geochemical differences between different lithostratigraphic units, but could not subdivide them at a fine scale. The Boren Ranch Sandstone/beds and Lower Mudstone Member have geochemical characteristics that tend to cluster samples in both bivariate and multivariate analyses, but they also tend to have overlaps with the Upper Cooper Canyon Formation, which is difficult to subdivide geochemically. However, the Santa Rosa Sandstone can be distinguished from these samples geochemically (especially with multivariate analysis), and samples from the Tecovas Formation of the Panhandle are also very distinct geochemically from the southern Garza County samples. This suggests that bulk geochemistry may be useful for distinguishing units of different lithology and provenance, although the particular discriminant plots the samples were applied to have limited success in identifying plausible tectonic environments of deposition and provenance. The systematics of fossil vertebrates from southern Garza County is discussed in some detail based on ongoing research by myself and others. The localities from which these fossils were collected were placed in superpositional order using the lithostratigraphic framework, and used to establish the known biostratigraphic ranges of individual taxa. The stratigraphic distribution of vertebrate fossils is biased by a few localities with particularly rich and diverse faunas (especially the Boren Quarry, Post Quarry, Headquarters localities, and Patricia Site), and by uneven depositional sampling of macrovertebrates and microvertebrates at these localities, and it must therefore be remembered that the known vertebrate ranges are biased and probably conservative. The veracity of the Late Triassic land vertebrate faunachrons was tested using this data. Recognizing the dependence of biochronology on biostratigraphy, the Late Triassic lvfs were treated as lowest occurrence interval biozones rather than biochrons, using the boundaries based on phytosaur first (lowest) occurrences advocated by Lucas (1998). The lowest occurrences of Paleorhinus, Leptosuchus, Pseudopalatus, and Redondasaurus-grade Pseudopalatus occur in the expected stratigraphic order, and were used to bound the Otischalkian, Adamanian, Revueltian, and Apachean land vertebrate faunachrons. However, the distinctness of the Otischalkian and Adamanian, on the basis of either the superpositonal relationships of phytosaur taxa, or overall faunal distinctiveness, is difficult to establish in western North America. Many biostratigraphic trends are similar to those established for the faunachrons in eastern Arizona and New Mexico. These often involve a decline in diversity from the Adamanian through the Revueltian. For aetosaurs, Stagonolepis may be present in the Otischalkian or early Adamanian, Desmatosuchus, cf. Rioarribasuchus, Typothorax, and Paratypothorax are present in the Adamanian with only the latter two being extending into the Revueltian, and only Typothorax is present in the Apachean. Large metoposaurs are abundant in the Adamanian but rare afterwards. “Metoposaurus” bakeri and the enigmatic archosauriform Doswellia may be restricted to the Otischalkian or lower Adamanian, but they have little use for correlation. The rauisuchids and shuvosaurids have extremely long ranges of Otischalkian?-Adamanian through Apachean, although the poposauroid Poposaurus is restricted to the Otischalkian?-Adamanian. Dicynodonts are absent after the Adamanian. Microvertebrates also show a decline and overturn in diversity, with sphenodontids, Malerisaurus, Trilophosaurus, “Procoelosaurus” and “Pteromimus” being extremely common in the Adamanian, while only drepanosaurs and leptopleurine procolophonids are common in the Revueltian. Dinosauromorphs are highly diverse in the Adamanian but undergo a decline in diversity in the Revueltian, until only theropods remain in the Apachean. The replacement of pseudosuchian archosaurs by dinosaurs in the Late Triassic may have therefore been neither a competitive replacement nor an opportunistic replacement, but the surviving members of the dinosauromorph lineage being the best suited to survive whatever environmental changes caused all these groups to decline in diversity. The correlation of the Upper Triassic in western North America to the Carnian, Norian, and Rhaetian stages of the Upper Triassic is currently controversial, but recent evidence suggests that all four faunachrons may be Norian. Between this and the limited sample sizes available for establishing the biostratigraphic ranges of taxa, not much can be said about the presence or absence of a mass extinction for terrestrial tetrapods at the Carnian-Norian boundary, or during the Norian.