Pincus, Maya

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    Glacial tillites as proxies for ancient environments: Evidence of impact, erosion, and deposition processes
    (2019-05-14) Pincus, Maya; Santos-Mercado, Hernán; College of Arts and Sciences - Sciences; Cavosie, Aaron; Gibbon, Ryan; Department of Geology; Rodriguez Iglesias, Lizzette
    The Vredefort Dome impact structure in South Africa is the oldest and largest verified impact structure on Earth. Because it is fairly well-preserved for its age, it serves as an integral repository for information about impact cratering in Earth’s early history. The investigation detailed in this thesis tested a new method for reconstructing ancient impacts through the sediments that were eroded away from target rocks: seeking detrital shocked minerals in lithified glacial sediments as evidence of impact. Continental glaciers from the Permo-Carboniferous Dwyka glaciation flowed directly over the Vredefort impact structure, incorporating target minerals into the ice and depositing them elsewhere. Grains of quartz, zircon, and monazite with shock microstructures found in Dwyka tillites demonstrate that shocked minerals can preserve evidence of impact through glacial transportation and deposition. This is the first documented evidence of detrital shocked mineral being transported solely by ice and preserved through lithification in the deep rock record. An additional component of this investigation was a search for evidence of the elusive Vredefort melt sheet. Numerous studies indicate that an impact at the scale of the Vredefort would have generated a melt sheet hundreds of meters thick. However, in the 2000 million years since its crystallization, all superficial remnants of the melt sheet have been eroded. If any vestige of this melt sheet had been exposed on Earth’s surface at the time of the Dwyka glaciation, target minerals in the form of igneous zircons yielding an impact age could have been incorporated into the ice and deposited in glacial till. Three hundred zircons from three Dwyka samples were analyzed for an impact age; none indicated a Vredefort melt sheet origin. This indicates that the melt sheet had already been eroded prior to the Dwyka glaciation. The U-Pb analyses of gem-quality zircons can also provide insight into the timing of deposition of Dwyka sediments in the Vredefort region. Though most studies report glaciation to have occurred between 300 and 290 Ma, a statistically significant number of grains analyzed in this study yield ages between 222 and 290 Ma. The ages of these grains correspond to known igneous events occurring in South Africa at the time, and are therefore indicative that glaciation may have extended through this time frame. As a whole, this thesis demonstrates several ways in which ancient detrital sediments can provide insight into Earth’s complicated and often obscured history. Even though surface and tectonic processes can alter and erase superficial features such as impact structures, the target minerals that have been transported and deposited can preserve a detailed geologic record.