MSc, Faculty of Science, University of the Witwatersrand, 2011 / 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.

A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of requirements for the degree of Master of Science. Johannesburg, 2015. / The ca. 36 Ma Chesapeake Bay impact event on the east coast of Virginia, USA, formed an 85 km complex crater in Cretaceous to Eocene sediments and underlying crystalline basement rocks belonging to the Appalachian orogen. Appalachian rocks are well exposed along the Appalachian Mountains to the west, however, little is known of the basement along the Atlantic Coastal Plain owing to the covering sedimentary sequence. This study investigates the crystalline rocks intersected by the 2006 ICDP (International Continental Scientific Drilling Program) – USGS (United States Geological Survey) drilling of the Chesapeake Bay impact structure (CBIS) on the Eyreville Farm near Cape Charles, Virginia. The crystalline rocks of the Eyreville-B borehole core are found in the lower basement-derived section (between 1551.19 m and 1766.32 m depth), in the amphibolite megablock (between 1376.38 m and 1389.35 m depth) and in the upper granite megablock (between 1095.74 m and 1371.11 m depth). The lower basement-derived section consists of foliated metasediments, which include mica schist, amphibolite and calc-silicate rock, and coarse-grained to pegmatitic granite. The amphibolite megablock is a black to dark grey to dark green, fine- to medium-grained, locally foliated, relatively homogenous, lithic block. The upper granite megablock is divided into gneissic and massive varieties, with a minor component of biotite schist xenoliths. The crystalline rocks contain foliations and related structures, fractures and breccias, microstructures and porphyroblast microstructures; however, none of the three lithic blocks is in situ and, consequently, structural measurements cannot be fully interpreted tectonically. Mineral assemblages and microstructural evidence in the mica schists suggest the rocks in the lower basement-derived section experienced a syn-D1 amphibolite facies peak metamorphic event (M1a) followed by retrograde metamorphic conditions (M1b) limited to D1b mylonitic and D2 brittle deformation. Similar metamorphic conditions in the upper megablocks suggest that the three sections likely formed part of a single metamorphic terrane. iv Geochemistry in the lower basement-derived mica schists revealed a strong intermediate igneous provenance, whereas the upper megablock biotite schist xenoliths showed a quartzose sedimentary provenance; the precursors to both appear to have been deposited in active continental margin settings. The lower basement-derived amphibolite appears to be derived from a sedimentary source. The precursor to the upper amphibolite megablock, on the other hand, was probably a tholeittic gabbro generated in an island arc setting. The peraluminous, S-type nature of the lower basement-derived granite suggests it was most likely generated in a within-plate tectonic setting. In contrast, the massive and gneissic granites from the upper megablock are metaluminous, I-type granites that were most likely generated in a syn-collisional environment. Metamorphic conditions of the M1 event were constrained using mineral assemblages mainly from the lower basement-derived section, which limited the X(H2O) value to 0.8, P to >0.4 GPa and the T range to 600-670°C. Using the 0.4 GPa pressure constraint, Zr-in-rutile thermometry revealed a peak metamorphic temperature for the M1 event of 606 ± 18°C, which is consistent with mid-amphibolite facies metamorphism. These estimates suggest a very steep geothermal gradient approaching ~44°C/km. Rutile U/Pb geochronology revealed that the M1 event recorded in the lower basement-derived metasediments occurred at 259 ± 13 Ma, with Ar/Ar geochronology indicating the cooling path through to greenschist metamorphic conditions. Zircon U/Pb SHRIMP geochronology performed by Horton et al. (2009b) on the massive and gneissic megablock granites dated their crystallisation ages at 254 ± 3 Ma and 615 ± 7 Ma, respectively, with the former age in agreement with the rutile U/Pb peak metamorphism results from the lower basement-derived section. These ages, together with petrography, structural observations, geochemistry and geothermobarometry suggests that the amphibolite and granite megablocks form part of the same metamorphic terrane as the lower basement-derived section and that the D1 and M1 events recorded in the lower basement-derived section and upper megablocks of the Eyreville-B borehole core likely occurred during the late stages of the Alleghanian orogeny. v Based on mineralogy, geochemistry, metamorphic grade and structural evidence, comparisons with the neighbouring terranes within the Appalachian basement beneath the Atlantic Coastal Plain sediments suggest that the lower basement-derived and upper amphibolite and granite megablocks of the Eyreville-B borehole core most likely formed part of the Hatteras terrane prior to the Chesapeake Bay impact event. This terrane, together with 5 other terranes, forms part of the Carolina Zone, a peri-Gondwanan micro-continent formed by the amalgamation of magmatic arcs during the Penobscottian and Taconian orogenies, which was then accreted onto the Laurentian margin during the Salinic and Acadian orogenies.

Thesis (Ph. D.)--University of Southern California, 2004. / Includes bibliographical references (leaves 208-224).

Call number: LD2668 .T4 1985 A42 / Master of Science

Thesis (Ph. D.)--University of Missouri-Columbia, 2003. / Typescript. Vita. Includes bibliographical references. Also available on the Internet.

Thesis (Ph. D.)--University of Missouri-Columbia, 2003. / Typescript. Vita. Includes bibliographical references. Also available on the Internet.

Thesis (Ph. D.)--University of California, San Diego, 1997. / Vita. Includes bibliographical references (leaves 271-300).

Thesis (Ph. D.)--Earth and Atmospheric Sciences, Georgia Institute of Technology, 2006. / Taillefert, Martial, Committee Member ; Weber, Rodney, Committee Member ; Stack, Andrew, Committee Member ; Benner, Ronald, Committee Member ; Ingall, Ellery, Committee Chair. Includes bibliographical references.

Thesis (M.A.)--Memorial University of Newfoundland, 2003. / Bibliography: leaves 120-129. Also available online.

Thesis (M.S.)--University of Puerto Rico, Mayagüez Campus, 2005. / Tables. Printout. Includes bibliographical references (leaves 130-135).