Browsing by Subject "Geology, Stratigraphic--Cambrian"
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Item The geology of the Backbone Ridge area, Llano and Burnet counties, Texas(1948-08) Barrow, Thomas D.; Bybee, Halbert Pleasant, 1888-1957The purpose of the author is to present a geological survey of the Backbone Ridge area in Burnet and Llano counties, Texas. During the summer of 1947 while enrolled in a geologic field course in McCullough County, it was noted that a new classification of the Cambrian and Ordovician formations In central Texas had been presented In the literature. It was noted at the same time that the Paleozoic rocks of the Llano uplift are more highly faulted than had been shown on previous geologic maps of the region. The writer concluded from field observations that the Backbone Ridge area was more complexly faulted then had been previously shown, and it was decided to test this conclusion by making a detailed geologic map of the area using the stratigraphic subdivisions recently established by Bridge, Barnes, and Cloud. A detailed study was made of these subdivisions and a large number of the type sections were visited. It was necessary to study the complete geologic history of the region in order that the events which involved the complex structural pattern and the present physiographic forms might be properly understood. The material contained in this report consists of data obtained from the literature and from field observations which were made in the area during the months of June and July of 1948.Item A revision of helicoplacoids and other early Cambrian echinoderms of North America(2005) Wilbur, Bryan Charles; Sprinkle, JamesThe Lower Cambrian rocks of North America contain the remains of several species of echinoderms, including helicoplacoids, the eocrinoids Gogia, Lepidocystis, and Kinzercystis, and the edrioasteroids Camptostroma and Stromatocystites. Camptostroma roddyi Ruedemann, 1933 is a large echinoderm from the Lower Cambrian Kinzers Formation of southeastern Pennsylvania. The animal has a unique dual-layered oral surface, and a large aboral suction pad used for attachment to skeletal debris. Thought by some authors to be ancestral to the edrioasteroids, it clearly belongs in a clade with the other early edrioasteroids. Helicoplacoids are from the Lower Cambrian Poleta Formation and equivalent units across the western edge of North America. The group was thought to be represented by nine species; this number is reduced here to three species, with the remainder of helicoplacoid disparity accounted for by vii ontogenetic variation. These triradiate spindle-shaped echinoderms were capable of expansion and contraction by means of cloacal pumping, and attached to skeletal debris or semi-lithified matgrounds by means of a suction pad. Rather than representing an ancestral triradiate form that served as the template for a “2- 1-2” ambulacral arrangement, these disparate echinoderms are derived from the ancestral pentameral edrioasteroid bauplan. Gogia is well represented in the Lower Cambrian rocks of California and Nevada by five species, four of which are new and are described here. This is now understood to be the most diverse echinoderm genus from the Early Cambrian. Members of this group attached to skeletal debris by using a suction pad, and fed by employing retractable spiraled brachioles. Stromatocystites walcotti Schuchert, 1919 is from Lower Cambrian rocks from Bonne Bay, Newfoundland. It is a flat edrioasteroid with a retractable suctorial aboral surface devoid of a pad. This morphologic feature delineates the group from other Middle Cambrian Stromatocystites, warranting renaming herein. A tree generated for these groups shows that helicoplacoids, Gogia, Imbricata, and the eocrinoids (Gogia + Imbricata) all constitute monophyletic groups, while edrioasteroids are polyphyletic. Early Cambrian echinoderms are found with only one or (rarely) two species occupying the same morphospace, a pattern repeated by Middle Cambrian groups.Item Sedimentary petrography and sedimentary structures of the Cambrian Hickory Sandstone member, Central Texas(1962) Wilson, William Feathergail; Not availableThe basal Cambrian Hickory Sandstone marks the beginnings of Paleozoic sedimentation in Central Texas. It is a bimodal well rounded, poorly sorted, fine and coarse sandstone. About half the samples are orthoquartzites, the remainder being divided between subarkose and quartzose subgraywacke. Well developed cross-beds in the lower Hickory exhibit a very uniform southeasterly direction, and were produced by marine currents controlled by northwest-southeast Pre-Cambrian ridges on a southeast paleoslope. Metamorphic quartz increases to the southeast. Approximately 75% of the feldspar found is orthoclase; the remainder is twinned microcline. The Hickory Sandstone is largely marine, but was derived from desert dune sands which were in turn derived from the Llanoria land mass to the southeast and the Texas Craton to the northwestItem Tidally influenced deposits of the Hickory Sandstone, Cambrian, Central Texas(1975-12) Cornish, Frank Gary; Scott, Alan JohnsonThe Hickory Sandstone Member of the Riley Formation is dominantly quartz sandstone up to 167 m thick which crops out in the Llano Uplift region of central Texas and dips away in all directions. It lies unconformably upon the irregular surface of the Precambrian Texas craton. The association of isopach thicks and thins over cratonic lows and highs demonstrates topographic control of Hickory deposition. Regional subsurface studies delineate the extent of the overlying Cap Mountain Limestone. Beyond the limits of the Cap Mountain, the Hickory grades into the Lion Mountain Sandstone laterally and vertically so that correlations are difficult. The six lithofacies of the Hickory Sandstone were deposited as nonbarred tidally-influenced or estuarine-related equivalents to deposits of Holocene environments. Outer estuarine tidal channel-shoal deposits display abundant channel fills of large-scale foresets, parallel bedded sandstone, and minor siltstone. Trilobite trackways (Cruziana) and resting traces (Rusophycus) occur in these deposits, associated with U-shape burrows (Diplocraterion and Corophioides). Deposits of open coast sandy tidal flats display upward-fining character, medium-to large-scale festoon crossbedding, abundant small-scale ripple bedforms of all types, and mudcracks. These deposits include the U-shape burrows, Corophiodes, and the trackway, Climactichnites. Deposits of inner estuarine tidal channels and tidal flats display upward-fining character, wavy-lenticular bedding, bimodal paleocurrent patterns, and the resting trace, Pelecypodichnus. All of these deposits prograded as a unit until sea level rise shut off sediment supply. Progradation of tidal channel and shoal sediments was renewed. These deposits are festoon crossbedded hematitic sandstone with wavy-lenticular bedding and abundant fossil debris. Storm energy funneled through tidal channels deposited crossbedded sandstone onto the nearshore inlet-influenced shelf. Final Hickory deposits and initial Cap Mountain deposits were storm-dominated, burrowed and laminated calcitic shelf sands.