Lithostratigraphy, chemostratigraphy, and vertebrate biostratigraphy of the Dockum Group (Upper Triassic) of southern Garza County, West Texas

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Title: Lithostratigraphy, chemostratigraphy, and vertebrate biostratigraphy of the Dockum Group (Upper Triassic) of southern Garza County, West Texas
Author: Martz, Jeffrey Warren
Abstract: 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 .
URI: http : / /hdl .handle .net /2346 /20495
Date: 2008-05

Citation

Lithostratigraphy, chemostratigraphy, and vertebrate biostratigraphy of the Dockum Group (Upper Triassic) of southern Garza County, West Texas. Doctoral dissertation, Texas Tech University. Available electronically from http : / /hdl .handle .net /2346 /20495 .

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