3. Electronic Theses and Dissertations (ETDs) - All submissions

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    Petrographic and geochemical analysis of the impactite succession in the Eyreville B drill core, Chesapeake Bay impact structure, Virginia, USA
    (2011-09-19) Jolly, Lauren Cher
    The 35.3 million year old, 85 km diameter, Chesapeake Bay impact structure (CBIS) in Virginia, USA, is one of the best preserved complex marine impact structures on Earth and is associated with the North American tektite strewn field. Three drill cores (Eyreville A, B and C) were obtained from the Chesapeake Bay impact structure during 2005-2006 by the CBIS Deep Drilling Project in conjunction with the International Continental Scientific Drilling Program (ICDP) and the United States Geological Survey (USGS). The drill cores intersected crystalline basement rocks, impactites, and impactrelated and post-impact sediments. This study focuses on the impactite sequence of the Eyreville B drill core. The primary focus has been to examine and understand the conditions and processes involved in the formation of the crater-fill impactite sequence, and the provenance of the impactites, through detailed lithostratigraphic, petrographic and geochemical analysis. The Eyreville B drill core intersected 154 m of impactites between the depths of 1397.16 and 1551.19 m. The impactite sequence is divided into the upper (1397.16 to 1474.05 m) and the lower (1474.05 to 1551.19 m) impactite units. The upper impactites are matrixsupported (23.5 rel% of total clast count) and characterised by suevite, clast-rich impact melt rock and cataclastic gneiss blocks, whereas the lower impactites are clast-supported (19.8 rel% of total clast count) and are dominated by polymict impact breccia and cataclastic gneiss boulders and blocks. The suevites comprise melt and lithic clasts from sedimentary (predominantly shale and sandstone) and igneous (such as granitoid and quartz pegmatoid) target rocks in an unsorted matrix composed of mineral (primarily quartz, feldspar and micas) and lithic clasts. The polymict impact breccias are primarily composed of metamorphic clasts such as phyllite, mica schist and felsic and mafic gneiss, and are largely, but not completely devoid of melt clasts. The majority of clasts in the impactite sequence closely resemble the granitoid, pegmatoid, calc-silicate, amphibolite and mica schist lithologies found in the underlying basement-derived succession and megablocks in the overlying sedimentary clast breccia. Overall, the crystalline (igneous and metamorphic) and sedimentary clasts contribute 62.3 and 20.8 vol%, respectively, of iv the total lithic clast composition which is comparable to 58.2 (crystalline) and 26.0 (sedimentary) vol% for the latest published results. The impactites are generally heterogeneous in terms of their chemical compositions. The impactite samples display enrichment in FeO+MgO in comparison to the target rock lithologies, with smaller abundances of K2O and Na2O, with little to no CaO. Throughout the impactite sequence, the suevites display the largest variety in chemical composition due to the heterogeneity of the clasts. The overall abundance of melt clasts varies from 22.1 vol% (of the total clast population) in the upper impactites to 2.5 vol% (of the total clast population) in the lower impactites. Melt clasts are generally flattened and elongated and display laminar flow structures (schlieren), with fractured terminations. Most melts are highly vesiculated and altered to phyllosilicate minerals. Overall, melt clasts show a general decrease in size with depth. Observations indicate that no coherent melt sheet was intersected; impact melt rock was only noted in the impactite sequence at depths between 1402.02 and 1407.49 m and 1450.22 and 1451.22 m. Melt clasts are heterogeneous in terms of their chemical compositions and are generally SiO2-rich and represent the melting and mixing of different mineral (quartz, feldspar and phyllosilicates) types derived from the target lithologies. This finding is comparable to the observations noted in the recent published literature. On average, 23.6 rel% of all quartz grains in the upper impactite unit display one or more PDF (planar deformation features) sets, with this number decreasing to 13.33 rel% for the lower impactite unit. A general decrease in average shock pressure with depth has been noted, which is consistent with the decrease in other shock features and melt clast abundance from the upper to lower impactites. A maximum of 3 PDF sets in the quartz grains, in the upper impactites, were noted; however, mostly 1 or 2 PDF sets were observed. Diaplectic glass has been noted in the melt clasts and is present predominantly in the upper impactites. No PDFs in feldspar grains have been noted. v A small, low temperature impact-induced hydrothermal system (220 – 300 °C) affected the material within the crater, which is evident from veins and patches of quartz, calcite, secondary phyllosilicate minerals (smectite), zeolites, secondary pyrite and chalcopyrite, as well as other sulphides. The upper and lower impactites show differing petrographic, geochemical and shock characteristics, suggesting that they were formed by different mechanisms. The upper suevites (upper part of the impactite sequence) are composed of fallback debris from the collapsing ejecta plume or curtain, whereas the suevites (S3 and S2) represent a mixture of the ground-surge material and fallback debris from the collapsing ejecta plume. The impact melt rocks are interpreted as either detached remnants of the melt lining the transient crater or piles of melt derived from fallback debris. It is proposed that the lower suevites (S1) and polymict impact breccia represent ground-surge deposits at the base of and behind the advancing ejecta curtain, modified by slumping and mixing of unshocked material from the outer crater walls. The cataclastic gneiss blocks and boulders slumped in from the outer transient crater walls and were incorporated into the ground-surge deposits. This study of the impactite sequence from the Chesapeake Bay impact structure has provided new insights into the formation of the impactite sequence as well as that of the Chesapeake Bay impact structure. Research such as this allows for further understanding and discussions regarding marine cratering processes (impact processes and impact-generated deposits) and emplacement mechanisms for impact craters. Essentially a study such as this provides material for further extensive research into the formation of marine impact craters and comprehensive modelling.
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    The Sub-Kalahari geology and tectronic evolution of the Kalahari basin, Southern Africa
    (2006-02-15) Haddon, Ian Gerald
    Geophysical, borehole and mapped data from the Kalahari Basin were used to create maps of the sub-Kalahari geology, isopachs of the Kalahari Group and basal gravels and a sub-Kalahari topographical surface. These are the first basin-wide maps of this type to be produced. These new data were interpreted with the aid of an extensive literature review as well as data gathered at three localities in the southern part of the Kalahari Basin and enabled several conclusions to be made regarding the tectonic evolution of the area. The sub-Kalahari Geological Map shows that rocks dating from the Archaean to present are exposed on the edges of the basin as well as covered by the Kalahari Group sedimentary rocks. Many of the rocks shown on the sub-Kalahari geological map record a history of rifting and subsequent collision, with the NE and SW trending structures appearing to have been reactivated at various times in the geological past. The extent of Karoo Supergroup rocks is greater than previously thought and Karoo sedimentary and volcanic rocks cover a large percentage of the sub-Kalahari surface. The Karoo Supergroup lithologies have been intruded by dolerite dykes and sills and the massive Botswana Dyke Swarm is shown on the sub-Kalahari map extending in a northwest direction across Botswana. The subtraction of the thicknesses of Kalahari Group sediments from the current topographical digital elevation model (DEM) of Africa in order to prepare a DEM of the sub-Kalahari topographical surface and the preparation of an isopach map of the basal gravels gives some indication of the courses followed by Mid-Cretaceous rivers. Topographic profiles along the proposed courses of these rivers show that the floor of the Kalahari Basin has a particularly low elevation in certain areas suggesting that downwarp of the interior of the basin rather than adjacent uplift was the driving force behind Kalahari Group sedimentation. When down-warp of the Kalahari Basin began in the Late Cretaceous these rivers were back-tilted into the newly formed basin and deposition of the Kalahari Group sediments began. The basal unit of the Kalahari Group consists of gravels deposited by the Cretaceous rivers as well as on scree slopes. As down-warp of the basin continued, so more gravels were deposited as well as the sand and -iifiner sediment carried by the rivers. Thick clay beds accumulated in the lakes that formed by the back-tilted rivers, with sandstone being deposited in braided streams interfingering with the clays and covering them in some areas as the shallow lakes filled up with sediment. During the Mid-Miocene there was a period of tectonic stability that saw the silcretisation and calcretisation of older Kalahari Group lithologies. At the end of the Miocene there was some uplift along the eastern side of southern Africa as well as along certain epeirogenic axes in the interior. In general this uplift was fairly gentle. Later more significant uplift in the Pliocene possibly elevated Kalahari Group and Karoo Supergroup sedimentary rocks above the basin floor and exposed many of them to erosion. The eroded sand was washed into the basin and reworked into dunes during drier periods. This uplift occurred along epeirogenic axes and was greater than the Miocene uplift. The development of the East African Rift System (EARS) in the Late Eocene or Oligocene has had a significant influence on the Kalahari Basin. Reactivation of older NE-SW trends by SWpropagating rifts extending from the main EARS is evident by recent movement along faults along the Damara Belt and those that were associated with Karoo sedimentation and post-Karoo graben formation. The propagating rifts have resulted in uplifting, faulting and in some cases, graben formation. In some cases lakes have formed in the grabens or half-grabens themselves and in other cases they have been formed between the uplifted arches related to parallel rifts. The propagating rifts have had a strong influence on the drainage patterns and shape of the Kalahari Basin, in particular in the middle parts of the basin where they have controlled the formation of the Okavango Delta and the Makgadikgadi pans
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