Browsing by Subject "Carboniferous"
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Item A new Pachytesta ovule from Iowa(2009-05-15) McCarty, Mary RachelA new species of the medullosan seed fern genus, Pachytesta, is described from specimens discovered in coal balls from Iowa. The specimens used in this study were located in coal balls, which are concretions of ancient permineralized peat. Anatomical study of the new species was made possible through the use of mounted cellulose acetate peels and photomicroscopy. The ovule is approximately 5.0-5.5 cm long and 2.4 -2.8 cm wide at its mid point. The integument contains a sarcotesta with 12 vascular bundles, a sclerotesta of radiating fibers and variable thickness, and an endotesta consisting of a thin cuticle. The primary ribs of the ovule are commissured. Based on the anatomical features studied, Pachytesta sp. A. is most similar to Taylor?s (1965) P. olivaeformis, P. composita, P. illinoensis group and is potentially a member of this group.Item Analysis of the effects of carbonate mounds on associated stratal geometry and fracture development, Sacramento Mountains, New Mexico, USA(2016-12) Tinker, Nathan Scott; Janson, Xavier; Zahm, Christopher Kent; Kerans, Charles; Fisher, William LThe objective of this research is an integrated structural‐stratigraphic analysis of compaction‐related fracturing in carbonate mounds and associated cover strata. The influence of early-cemented carbonate mounds on subsequent sediment deposition (such as creation of hard substrates and topographic relief) is relatively well-understood. The effect of early-cemented carbonate mounds during burial, however, has not been studied in detail. Early marine cementation of mounds enhances mechanical rigidity, which reduces mound compaction during burial as compared to less-resistant sediments surrounding and overlying the mound. This rigidity difference facilitates differential compaction of sediments overlying the mound, which are warped over the inflection point created by the mound topography. This study hypothesizes that there is a measurable increase in fracture intensity associated with differential compaction above early-lithified carbonate mounds. Thus, this work analyzes and quantifies the effects of differential compaction on stratal geometry, mechanical stratigraphy, and fracture development in Mississippian strata overlying carbonate mounds which are well-exposed in the Sacramento Mountains in southeast New Mexico. Methods employed in this study are drawn from structural geology, sedimentology, petrography, and remote sensing in an effort to adequately determine facies, examine fracture characteristics (e.g. size, orientation, and intensity), and to better understand which process(es) most directly control those characteristics (e.g. host rock facies type, diagenesis, bed thickness, mound proximity, mound size). Innovative methods of outcrop characterization such as high-resolution gigapan photography and unmanned aerial vehicle (UAV) photography were combined with photogrammetric techniques to create photo-realistic 3D outcrop models. The resulting models enabled a cost-effective, more detailed, less-distorted, and more comprehensive interpretation compared to previous methods, and improved understanding of the relationship between stratigraphy, rock mechanical evolution, and structural deformation in carbonate mound systems. Field work documented facies, stratal geometries, folds, faults, and fracture sets which validated observations and characterizations made using high-resolution field photographs and 3D outcrop models. Results of this work show that paleotopographic relief which has been early lithified (in this instance, Mississippian carbonate mounds) directly controls fracture development and overlying stratal geometry, in that there is a significant increase in tension fracture (mode 1) intensity above pre-existing rigid structures and over-steepening of bed dips beyond an expected and reasonable angle of repose. Additionally, this work outlines a multi-stage tectonostratigraphic sequence of the development of the stratigraphically complex Teepee Mound assemblage based on field observations of facies, fractures, stratal geometries, and diagenetic effects (e.g. cementation, compaction, and chertification), which includes new evidence of late-Mississippian tectonic compression. This result emphasizes the importance of understanding both syndepositional and post-depositional processes in outcrop characterization. Specifically, syndepositional processes establish the original mechanical stratigraphy and control the formation and propagation of early mechanical discontinuities, which in turn set up the fabric of weaknesses preferentially utilized by later fracture development. Post-depositional mechanical and diagenetic processes alter mechanical stratigraphy and rock brittleness, and thus influence fracture propagation through time.Item High-Resolution Carbon Isotope Stratigraphy, Pennsylvanian Snaky Canyon Formation, East-Central Idaho: Implications for Regional and Global Correlations(2012-07-16) Jolley, CaseyNearly 550 samples of fine grained carbonates, collected every 0.5 to 1.0 m from the Bloom Member of the Snaky Canyon Formation at Gallagher Peak, Idaho, were analyzed to determine the high-resolution carbon isotope stratigraphy. To constrain for diagenesis, thin sections were petrographically analyzed and viewed using cathodoluminescence microscopy. Chemical analyses were performed using an electron microprobe. Average delta18O and delta13C values from the Bloom Member are -4.5% +/- 1.6% (1 sigma) and 2.1% +/- 1.1%, respectively. Maximum delta13C values are about 1% higher for the Desmoinesian and Missourian than the Morrowan and Atokan, similar to results from the Yukon Territory. delta18O and delta13C values are lowest for crystalline mosaic limestones and siltstones, moderate for packstones, wackestones, and mudstones, and highest for boundstones and grainstones. The delta13C profile from Gallagher Peak consists of high frequency 1% oscillations with several larger excursions. No large delta13C increase at the base of the section suggests the Mid-Carboniferous boundary is in the underlying Bluebird Mountain formation. delta13C of Gallagher Peak and Arrow Canyon, NV, correlate well from 318 to 310 Ma, but correlation becomes more difficult around 310 Ma. This may result from increased restriction of the Snaky Canyon platform beginning in the Desmoinesian. Most of the short term (<1 Ma) isotopic excursions are the result of diagenesis. Two of the largest negative excursions at Gallagher Peak correlate with two large negative excursions at Big Hatchet Peak, NM, possibly due to sea level lowstands of the Desmoinesian. Phylloid algal mounds at Gallagher Peak are associated with positive excursions because of original aragonite composition and increased open marine influence. Positive excursions related to other facies characteristics also result from increased marine influence. The delta13C curve for the upper half of Gallagher Peak contains three repeated cycles of increasing delta13C over 1-1.5 Ma, which are possibly related to long-term sea level fluctuations. Given the complexity of each local environment, without detailed biostratigraphy, detailed rock descriptions, and analysis of the various rock components, delta13C stratigraphy of whole rocks can be misinterpreted.Item Oceanic and atmospheric response to climate change over varying geologic timescales(2012-07-16) Woodard, Stella C.Global climate is controlled by two factors, the amount of heat energy received from the sun (solar insolation) and the way that heat is distributed Earth's surface. Solar insolation varies on timescales of 10s to 100s of thousands of years due to changes in the path of Earth's orbit about the sun (Milankovitch cycles). Earth's internal boundary conditions, such as paleogeography, the presence/absence of polar icecaps, atmospheric/oceanic chemistry and sea level, provide distribution and feedback mechanisms for the incoming heat. Variations in these internal boundary conditions may happen abruptly or, as in the case of plate tectonics, take millions of years. We use geochemical and sedimentological techniques to investigate the response of ocean chemistry, regional aridity and atmospheric and oceanic circulation patterns to climate change during both greenhouse and icehouse climates. To explore the connection between orbitally-forced changes in solar insolation, continental aridity and wind, we generated a high-resolution dust record for ~58 Myr old deep-sea sediments from Shatsky Rise. Our data provide the first evidence of a correlation between dust flux to the deep sea and orbital cycles during the Early Paleogene, indicating dust supply (regional aridity) responded to orbital forcing during the last major interval of greenhouse climate. The change in dust flux was comparable to that during icehouse climates implying subtle variations in solar insolation have a similar impact on climate during intervals of over-all warmth as they do during glacial-interglacial states. The Carboniferous Period (359-299 Ma) marks a critical time in Earth's history when a series of tectonic and biological events caused a shift in the mean climate state from a global "greenhouse" to an "icehouse". Geochemical records extracted from sedimentary rocks deposited in shallow epicontinental seaways are increasingly being used to infer relationships between tectonism, carbon cycling and climate and therefore are assumed to reflect global ocean processes. We analyzed radiogenic isotopes in biogenic apatite along a North American transect to constrain the degree of geochemical coupling between the epicontinental seas and the open ocean. Our results argue strongly for decoupling of North American seaways from the open ocean by latest Mississippian time.Item Seasonality and Environmental Heterogeneity during a Late Carboniferous Highstand: Brachiopod Shell Geochemical Records from the Pangean Tropics(2014-12-10) Roark, AndrewThe relationship between Permo-Carboniferous glacial cycles and low-latitude climate remains a subject of vigorous debate. This study investigated seasonality and regional environmental variability in a portion of central equatorial Pangea during a late Pennsylvanian highstand using stable isotope and trace element analyses of brachiopod shells from the Virgilian Ames Member of the Conemaugh Group in the Appalachian Basin, U.S.A. Well-preserved, thick-shelled Neospirifer dunbari specimens were serially sampled across growth bands to elucidate a record of seasonal variability during the life of the organisms. Because Neospirifer only colonized this marginal basin during near-maximum highstands when stable marine salinities were established, these data are a direct proxy for intra-annual climate fluctuations during interglacial times. Additionally, well-preserved specimens of the thinner-shelled brachiopod Crurithyris planoconvexa, which has a wider spatial and stratigraphic distribution within the basin, were analyzed individually. Neospirifer specimens show remarkably little internal chemical variability, with ?^18O generally fluctuating by 0.4? or less and ?^13C by less than 1.5? within a single specimen. Moreover, total variability between all specimens is only ~1.5?. This lack of variation reflects a homogenous, nonseasonal to weakly seasonal climate during the Ames highstand. Both ?^18O and ?^13C are ~1.5 lower than those of other Virgilian specimens from regions with a more proximal connection to the open ocean, suggesting relatively high freshwater influence in the Appalachian Basin during this time, although salinities remained close to marine levels. Thus, brachiopod seasonal records indicate normal moist tropical conditions during this penultimate Carboniferous interglacial, with no evidence for strong monsoonal variations in temperature or rainfall. Crurithyris specimens show similarly homogenous isotopic values that are slightly depleted relative to more nearly marine specimens. There is a weak gradient towards increasing ?^18O and ?^13C in Crurithyris specimens to the west, consistent with decreasing influence of Appalachian runoff waters in that direction. Crurithyris from two sites show anomalously enriched isotopic values, potentially indicating rare, sporadic periods of net evaporation. However, because of Crurithyris? wider temporal distribution, these events may not have occurred during the highstand.