Browsing by Subject "Dockum Group"
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Item Depositional and diagenetic history of the late Triassic Dockum Group, Young Ranch, Nolan County, Texas(Texas Tech University, 2002-05) Slayden, Jason PThe Late Triassic Dockum Group is a series of continental sediments comprised of lacustrine, deltaic, and fluvial deposits. Dockum sediments are found in Eastern New Mexico, Colorado, and West Texas, Oklahoma, and Kansas. The Santa Rosa and Trujillo Formations represent the Dockum Group on the Young Ranch, Nolan County, Texas. Santa Rosa and Trujillo Formations are fluvial deposits consisting of alternating layers of sandstone and conglomerate. These formations are found on the same topographic level on the ranch. Identification of the Santa Rosa and Trujillo formations can be made through petrographic and paleocurrent analysis. The Santa Rosa Formation tends to be more quartzose and arkosic, while the younger Trujillo Formation is more lithic, with an abundance of metamorphic rock fragments and rare volcanic rock fragments. The Santa Rosa Formation has a reported sediment provenance in the present day Wichita-Arbuckle Mountains. Santa Rosa paleocurrent direction on the Young Ranch trend toward the south. The Trujillo Formation has a reported provenance in the lower Ouachita Tectonic Belt. Trujillo paleocurrent directions on the Young Ranch trend toward the north. Therefore, the paleocurrent analysis on the Young Ranch supports the reported Santa Rosa and Trujillo sediment provenances. Lithofacies types and vertical sequences along with a muitistoried channel stacking architecture indicate a braided fluvial system for the deposition of the Santa Rosa Formation. The presence of the Trujillo Formation at the same topographic level on the ranch leads to the conclusion of deposition in two incised valleys. Diagenetic history of the Dockum Group is marked by a variety of cementation events. Kaolinite and iron oxide occurred as early, shallow burial cementation. Zoned dolomite cement was emplaced during burial. Quartz overgrowths and microcrystalline quartz cement formed during Early Cretaceous Edwards exposure due to pedogenic silerete formation in the overiying Lower Cretaceous Antlers Sandstone. Dedolomite/Poikilitopic calcite formed because of dissolution of the Kirsehberg Evaporite during Early Cretaceous Edwards exposure. Desiccation of recent freshwater springs precipitated gypsum as the final diagenetic event.Item Lithostratigraphy, chemostratigraphy, and vertebrate biostratigraphy of the Dockum Group (Upper Triassic) of southern Garza County, West Texas(Texas Tech University, 2008-05) Martz, Jeffrey W.; Chatterjee, SankarThe 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.Item Petrography, geochemistry, and clay mineralogy of a paleosol in the Dockum Group (Triassic), Texas Panhandle(Texas Tech University, 1997-12) Kanhalangsy, KhamchanhA Triassic paleosol, informally known as the "Palo Duro geosol" in Texas and New Mexico, is the lowermost unit of the Triassic Dockum Group. This buried soil developed on sands of the Permian Quartermaster Formation in Texas. An excellent section of the paleosol is exposed at Caprock Canyons State Park, in Briscoe County, Texas. This paleosol marker bed is found around the Southern High Plains in Texas and New Mexico. A correlative unit, the "purple mottled" horizon, is also found in Triassic strata of Arizona and Colorado. The Palo Duro geosol is an Ultisol with two presumed major horizons identified here as A and B horizons. The A horizon consists of white sandstone with local accumulations of nodular and lamellar carbonate and chert layers at the top of the horizon. The B horizon consists of red, purple, and yellow mottled sandstone, siltstone, and mudstone. Petrographic analysis indicate that quartz, and to a lesser amount, sedimentary rock fragments are the major detrital grains in the entire profile. Iron oxide and chert cements predominate throughout the profile. Micromorphologic features observed in the profile include illuviation channels, voids, cutans, calcrete, and silcrete. Elemental enrichment and depletion trends in the Palo Duro geosol are correlated with those of modem soils with AI2O3 and Ti02 retained in the profile. Most of the elemental constituents are depleted from the profile relative to AI2O3 and Ti02. The clay minerals present in the profile are kaolinite, smectite, and illite. The Palo Duro geosol profile developed as a consequence of intense leaching under well drained environments in a climate with high precipitation rates. The occurrences of calcrete and silcrete samples suggests a climatic change from humid to dryer condition occurred later during soil formation. Isotopic data of several peodogenic calcrete and silcrete suggest that these micromorphologic features formed at low temperature («9 to 38°C) compatible with their origin as a product of soil formation.Item The morphology and ontogeny of Typothorax coccinarum (Archosauria, Stagonolepididae) from the upper triassic of the American Southwest(Texas Tech University, 2002-12) Martz, Jeffrey WarrenThe aetosaur Typothorax coccinarum in known from abundant material from the Late Triassic Chinle Formation and Dockum Group of the American southwest, particularly in Arizona, New Mexico, and west Texas. Redondasuchus reseri is a smaller aetosaur taxon with several similarities to Typothorax that may indicate it is a juvenile. The purposes of this thesis are to provide a thorough, well-illustrated description of Typothorax coccinarum, describe the ontogeny of the taxon, and to determine if Redondasuchus is a valid taxon distinct from Typothorax. The Canjilon Quarry (Upper Petrified Forest Formation, Chinle Formation, north-central New Mexico) Typothorax material includes three large concentrations of scutes and skeletal material (probably representing individuals) and other isolated elements, collected from two distinct stratagraphic layers. The original field maps and field numbers recorded on the bones allow the association of much of the material to be established. The Post Quarry (Cooper Canyon Formation, Dockum Group, west Texas) yielded a subadult specimen of Typothorax (TTUP 9214). Although detailed quarry maps and field notes for the Post Quarry are currently unavailable, the general consistency in the material supports its assignment to a single, unusually small individual of Typothorax. The Apache Canyon Quarry (Redonda Formation, Dockum Group, eastern New Mexico) material of Redondasuchus consists entirely of isolated scutes probably found in at least two different stratigraphic levels within the quarry. The scutes of TTUP 9214 possess the diagnostic features of Typothorax coccinarum, while its small size and the incomplete ossification of some elements suggest it is a subadult. There are no major differences between the scutes of Typothorax and Redondasuchus apart from size; arching is seen in the presacral as well as caudal dorsal paramedian scutes of Typothorax, and the alleged down-turned lateral edge of the holotype scute of Redondasuchus is in fact is the medial edge, placing arching at the center of ossification as in Typothorax. Redondasuchus reseri is assigned to Typothorax, but kept as a separate, smaller taxon, Typothorax reseri, due to the absence of T coccinarum material from the Redonda Formation. The only axial skeletal material described here is from TTUP 9214, which includes a squamosal, quadrate, braincase, dentary, and vertebrae from all regions of the column except for posterior caudals. The braincase has an incompletely enclosed trigeminal foramen and basal tubera that are deeply split by the basioccipitalbasisphenoid suture. The dentary is extremely shallow, with ten aveoli. The transverse processes of the vertebrae grow extremely wide in the dorsal region. Appendicular skeletal material is well represented, and many areas of muscle attachment present in crocodilians can be plausibly inferred. The scapulocoracoid, clavicle, humerus, illium pubis, ischium, femur, tibia, fibula, astragalus, calcaneum, metapodials, and phalanges are all known from the Canjilon Quarry material and/or TTUP 9214. The dorsal paramedian and lateral scutes of Typothorax are the best-known elements, and show variation relating to their anteroposterior placement on the body. The ordering of these scutes is inferred based on aetosaur specimens of other taxa with articulated scutes, and two blocks of articulated Typothorax scutes from the pelvic and caudal region of one of the Canjilon Quarry specimens. All dorsal paramedians have circular or oval ornamentation in a random pattern, a raised anterior bar, and most have a thick ventral keel and are extremely wide. The lateral scutes in the precaudal region are angulated, have a triangular dorsal flange, and lack a horn. In the caudal region the lateral scutes are flat plates with the flanges lying in roughly the same plane. Appendicular and ventral scutes of Typothorax are known, but their arrangement on the body is not understood due to the lack of articulated material. Bivariate ontogenetic allometry was explored for the femur and tibia of Typothorax coccinarum. The femur of Typothorax may have lengthened more rapidly then the tibia. Log squared measurements of femur and tibia length were plotted against each other, yielding a good correlation, but failing to pass the p-test (probably due to the small sample size of individuals having both elements). Log-squared measures for the femur and tibia were then plotted against size scores on the first principal component (PCI) to describe the relative growth of various dimensions of these bones. Several of the femur measures for Typothorax coccinarum have good correlation coefficients that pass the p-test for, generally indicating that the bone grew robust more rapidly distally and at the midshaft then it lengthened. All tibia measures showed good correlation coefficients that passed the p-test, generally indicating that the robustness of the tibia increased faster then its length in all areas of the bone, especially proximally and at the midshaft. Many ontogenetic changes in Typothorax coccinarum seem to represent recapitulations, with likely plesiomorphic traits such as a less compact neck, slightly narrower transverse processes on the vertebrae, femora that are not so long relative to the crus, a narrower carapace, and a raised ridge on the dorsal paramedian scutes being lost with age and increase in size. The young of Typothorax may have been more active then the older individuals. There is insufficient evidence that posture became more erect in Typothorax ontogenetically, or between aetosaur taxa phylogenetically, as a graviportal adaptation. However, this possibility cannot be entirely ruled out for aetosaurs as a group, and it is possible that erect posture evolved in archosaurs in response to more then one selective pressure.