3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item 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 CherThe 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.Item The petrology and geochemistry of the impactite sequence and selected target rocks from the Yaxcopoil-1 borehole, Chicxulub Impact Structure, Yucatan Peninsula, Mexico(2008-10-14T11:09:58Z) Tuchscherer, Martin GuillaumeGeological and geophysical investigations of the Chicxulub meteorite impact structure have been ongoing since its scientific recognition in 1991 Hildebrand et al. 1991). The structure is of important significance because it is currently the only known impact crater that is linked to a global catastrophe, the Cretaceous/Tertiary boundary that occurred 65 Ma years ago. Major climatic and biological changes occurred at this interval that include the disappearance of 70% of all living species, in particular the dinosaurs. A global iridium anomaly along with the occurrence of shocked quartz grains characterize a thin clay layer this interval that led to the search for a large meteorite impact crater on continental crust. A large “volcanic” igneous province identified by oil exploration boreholes on the NW region of the Yucatán Peninsula was eventually recognized as a vast impactite deposit associated with a 180 km wide crater. Until 2002, only small grab and chip samples had been described from Chicxulub. This lack of sampling and, thus, poor understanding of the cratering conditions at Chicxulub led the International Continental Drilling Program (ICDP) to fund and drill the Yaxcopoil-1 borehole. The Yaxcopoil-1 (Yax-1) borehole was drilled 60 km south-southwest from the center of the Chicxulub meteoritic impact. It intersected 794.63 m of post-impact cover rocks, 100.31 m of impactites, and 616.03 m of Cretaceous target rocks, terminating at a final depth of 1510.97 m. The impactite interval, as well as several selected samples from the Cretaceous target rocks, is the focus of this scientific investigation. In conjunction with this work, the Yax-1 core was studied by numerous international research groups and is, thus, currently one of the best studied continuous diamond drill core from an impact crater. This petrographic and geochemical investigation provides further understanding on the primary and secondary conditions that influenced the formation of the Yax-1 impactites and selected target rocks. Five units have been recognized in the impactite interval. These subdivisions are based on macro- and microscopic observations and are complemented by geochemical characteristics. Unit 1 (795-822 m) comprises subrounded melt rock particles that are poorly sorted, yet show a progressive gradation with height, are self supported, show perlitic devitrification texture, and are generally fine-grained. Unit 2 (823-846 m) and Unit 3 (846-861 m) are relatively similar, as they both consist of a groundmass-supported breccia with melt rock particles that are angular, fluidal, and vesiculated in texture. The groundmass in both units is pervaded by numerous carbonate-veinlets and decreases in volume towards Unit 3 because of compaction. Unit 2 and Unit 1 are both altered to a predominantly green colour by the pervasive conversion of silicate phases from clay minerals. Unit 3 is of a variegated character and is suggested to be the less altered unit bove Unit 4. Unit 4 (861-885 m) comprises a massive yet brecciated microcrystalline impact melt rock. It is primarily of a silicate composition and contains only minor secondary carbonate crystals. All lithic fragments are of silicate compositions. Unit 5 (885-895 m) shows the greatest variation in the proportion of melt rock particles and lithic fragments. The melt rock particles contain numerous microlites that crystallized below the glass-transition temperature. These are suspended in a carbonate groundmass that is either of a primary impact melt origin or of a secondary nature. Units 1 and 5 both contain foraminifera fossils and greater proportions of carbonate clasts than any other units. All unit show shock metamorphic characteristics, i.e., planar deformation features, ballenquartz, and checkerboard feldspar. Geochemical results have been obtained by various analytical techniques in order to constrain cratering and alteration processes at various sampling scales. Main results reveal that samples from units 1 and 2 have been leached of their alkali elements, show negative Ce anomalies on a microscopic scale, and show less major element variation on a bulk sample scale than lower units. The groundmass in units 1 to 3 comprises a microcrystalline calcite and altered alkali element-, Ca- and Si- rich cement. In units 2, 3, and 5 melt rock particles are of a heterogeneous composition. In Unit 1, melt rock particles are highly altered, therefore volatile rich, and are of a more homogeneous composition than those of other units. On a bulk sampling scale, the silicate component for the whole impactite sequence shows remarkable homogeneity. Major and trace element compositions show that this component and Unit 4 are typical of the upper continental crust. The carbonate component is more calcite rich than dolomitic and most likely represents strong secondary alteration. No significant sulfur content was measured compared to published known target rock values. The contents of the siderophile elements, including Ni, Co, Ir, and Cr, do not indicate the presence of a significant extraterrestrial component in the Yax-1 impactites. Cretaceous rocks were also sampled in order to provide compositional constraints with the impactites and observe any shock related metamorphic features. Petrographic observations indicate that the Cretaceous rocks in the Yaxcopoil-1 drill core likely register a multistage deformation history that spans the period from pre- to post-impact. Contrary to previous studies that claimed evidence for the presence of impact melt breccia injection veins, no evidence was found from samples located between 1347–1348 m depth for the presence of melt breccia. An emplacement mechanism for the impactite sequence is proposed with regards to cratering. Unit 5 is interpreted as an early ejecta deposit that was emplaced following the passage of the initial ejecta curtain during the excavation stage of cratering. Unit 4 is an allogenic siliceous melt rock body that originated primarily from the fusion of the silicate crystalline basement. The origin of Unit 4 is based on geochemical and petrographic arguments, i.e., no carbonate component to the melt could be detected and only igneous/metamorphic mineral/rock fragments were observed in it. It is suggested Unit 4 was emplaced as an outward flow of fused crystalline basement rocks from the collapsing central uplift or it may have also been deposited from the fallback of a large melt bomb. Brecciation occurred post-deposition as fragments fit together like pieces of a jigsaw puzzle. Units 2 and 3 represent unreworked fallback suevite deposits. Vesiculated melt rock particles are a testimony of the volatile rich nature of the collapsing impact plume. Volatiles are believed to have helped disperse the suevite and inhibited the melt rock particles from undergoing compositional homogenization. Unit 1 represents a reworked fallback deposit that formed from the resurge of seawater into the impact basin. Unit 2 is the altered equivalent of Unit 3 and along with Unit 1 underwent significant post-depositional phyllosilicate alteration from circulating fluids at the top of the suevite pile.