Der ICDP-USGS Eyreville Bohrkern wurde in den Jahren 2005-2006 im zentralen Graben der spät-Eozänen Chesapeake Bay Impaktstruktur, nahe der Eyreville Farm (Northampton County, Virginia, USA) erbohrt. Die vorliegende Dissertation präsentiert petrografische und geochemische Analysen von 166 Impaktgesteinsproben der Eyreville-Bohrkerne A und B. Weiters wurden speziell die Intervalle der Impaktbrekzien und der kieseligen Sande des Bohrkerns detailliert untersucht.
Das Impaktbrekzien-Intervall besteht hauptsächlich aus Sueviten, aber es kommen auch zwei dünne Lagen Impaktitschmelzen und große kataklastische Gneisblöcke vor. Dieses Intervall wurde auf Grund der Unterschiede im Matrix-, Schmelzen- und Klastengehalt in sechs Untereinheiten unterteilt. Im Liegenden der Impaktbrekzien kommt eine Dichtestrom-Brekzie vor, wohingegen gegen das Hangende Rückfall (fall-back)- Material häufiger wird. Es wurden verschiedene schockmetamorphe und ähnliche Effekte verzeichnet. In den Gesteinsbruchstücken (Klasten) wurde die Häufigkeitsverteilung geschockter Quarzkörner ermittelt, sowie mittels Universal-Drehtisch die Orientierung der Schocklamellen dieser Quarzkörner bestimmt. Mit zunehmender Tiefe verringert sich die Häufigkeit der stark geschockten Klasten; weiters zeigen Klasten unterschiedlicher Lithologien unterschiedliche Schockintensitäten. Die Zusammensetzung der polymikten Impaktite zeigt mit zunehmender Tiefe einige Unterschiede. Die Konzentration der siderophilen Elemente weist auf keine Präsenz einer extraterrestrischen Komponente hin. Geochemische Mischungsberechnungen lassen vermuten, dass die Hauptkomponenten der polymikten Impaktite aus grundgesteinsbezogenen Gneisen/Schiefern und einer signifikanten sedimentären Komponente bestehen. Basierend auf optischen und elektronenmikroskopischen Beobachtungen, sowie Elektronenmikrosonden-Analysen, konnten die Schmelzpartikel in sechs unterschiedliche Typen gruppiert werden. Diese Beobachtungen, zusammen mit den Mischungsberechnungen, legen einen vorwiegend sedimentären Vorläufer der Schmelzen nahe.
Das Intervall der kieseligen Sande besteht aus schlecht sortierten unkonsolidierten Sanden, hauptsächlich zusammengesetzt aus Quarzen, Kalifeldspäten, Smektiten und Kaoliniten. Die Untersuchungsergebnisse bestätigen die Entstehung der kieseligen Sande aus einer vom Impakt induzierten Gesteinslawine, bestehend aus Material der nichtmarinen Potomac-Formation.The late Eocene Chesapeake Bay impact structure is 35.3 Myr old and ~85 km in diameter. Three stacked cores (A, B, and C) were drilled to a total depth of 1766 m in years 2005–2006 at Eyreville Farm, Northampton County, Virginia, USA, located in the central moat of the impact structure. The project was a joint International Continental Drilling Program (ICDP) and U.S. Geological Survey (USGS) collaboration. This thesis presents investigations of 166 samples of impactites from Eyreville cores A and B. All samples were described macro- and microscopically and their major and trace element composition was determined. Further work was focused on the impact breccia interval (1397–1551 m depth) and the overlying gravelly sand interval (1371–1397 m depth).
The impact breccia interval consists mostly of suevite, but two thin layers of impact melt rock were found in the upper part of the interval and large blocks of cataclastic gneiss occur in the lower part. The impact breccia interval has been divided into six subunits based on the differences in content of matrix, melt, and clasts of different lithologies. Generally the abundance of lithic clasts increases and amount of melt particles decreases with increasing depth. The bottom part is a ground surge breccia, whereas towards the top the fallback material becomes more abundant and is dominant in the uppermost part of the interval. Various shock metamorphic and related effects were noted in the impact breccias, including rare planar fractures (PFs) and abundant planar deformation features (PDFs) in quartz, common “toasted” appearance of quartz, occasional ballen silica, rare PDFs in feldspar, and kink banding in mica.
Shock metamorphic effects were studied in detail. The proportion of shocked quartz grains (grains displaying PFs and/or PDFs) was investigated in clasts of different lithologies from the impact breccia interval. No linear trend with depth was found, although the highly shocked clasts become generally less abundant with depth. More important differences were found among clasts of different lithologies, e.g., mostly minimally shocked clasts from the crystalline basement versus abundant highly shocked clasts from the overlying sedimentary rocks. In addition, the crystallographic orientations of PDFs were determined (using universal stage measurements) in quartz grains of several clasts.
The polymict impactites show a decrease in the SiO2 content and slight increases of the TiO2, Al2O3, and Fe2O3 abundances, with depth. This is in agreement with an increase of the schist/gneiss component with depth. Siderophile element concentrations are lower than in, e.g., the target schists, and do not indicate the presence of an extraterrestrial component. Geochemical harmonic least square mixing (HMX) calculations suggest that the main components of the polymict impactites of the impact breccia interval are basement-derived rocks (gneiss/schist) together with a significant sedimentary component (probably derived mainly from the Cretaceous Potomac Formation) and possibly a pegmatite/granite and amphibolite component.
Melt particles were grouped into six different types, primarily based on their appearance under optical microscope (e.g., color, shape, inclusions). Some common melt types occur over a wide depth range, whereas other types are found only in the impact melt rock intervals. Several melt particles of each type were analyzed by electron microprobe. Average composition of each melt type was determined and possible precursors were discussed. The observations, including HMX calculations, suggest a predominance of sedimentary precursors. Mineral phases in the melt (i.e., undigested clasts, quench crystals, as well as secondary phases) were analyzed by electron microprobe and microRaman spectrometry.
The gravelly sand interval of the Eyreville drill core consists of grayish, poorly sorted and poorly consolidated sand. The matrix comprises 30 to 40 vol% and includes clasts of mostly mono- and polycrystalline quartz and less abundant K-feldspar. Other minerals are only accessory. The main clay fraction components are smectite and kaolinite. The gravelly sand is non-marine, as indicated by the absence of marine microfossils and glauconite. The composition is very silica-rich (>80 wt% of SiO2). Results of our investigations are in agreement with the hypothesis that the gravelly sand interval was formed by an avalanche during the crater modification and the material originated from the non-marine Potomac Formation.
The Eyreville core impactites represent a complex series of depositional events following the shallow-marine Chesapeake Bay impact event. Probably all the lithologies in the core are allochthonous. The deposition mechanism of the impactites changed from ground-surge to fallout, which was soon interrupted by rock avalanches and resurge of the ocean water with sediments. Shock metamorphism effects are present in the impact and resurge breccias, but the allochthonous crystalline blocks are unshocked
