Date of Graduation
Eberly College of Arts and Sciences
Geology and Geography
Amy L Weislogel
U-Pb detrital zircon geochronology, in addition to sandstone petrography and heavy mineral analysis are used to constrain the provenance of the Ecca and Beaufort lithologic Groups within the Tanqua, Laingsburg, and Ripon sub-basins of the southern Gondwanan Karoo Basin. Zircon ages were acquired using Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) on 553 grains from seven hand samples. Results show two major age populations exist in all samples including an abrupt 245-295 Ma population and a broad 350-750 Ma population. Other minor age populations are present between 300-350 Ma, 850-1200 Ma, and > 2000 Ma in some samples. These age populations coincide with the age of regional source terranes and tectonic events which consist of: Gondwanide (southern magmatic arc: 245-290 Ma), Pan African (488-1100 Ma), and Pampean (525-550 Ma) Orogenies as well as the Cape Supergroup (400-2700 Ma), Dwyka Glacial Group (500-3500 Ma), North Patagonian Massif (235-580 Ma), Deseado Massif (344-521 Ma), Namaqua-Natal Province (800-2200 Ma), and the Kaapvaal/Kalahari Craton (> 1800 Ma).;U-Pb zircon ages within the Tanqua, Laingsburg, and Ripon sub-basins are relatively uniform throughout the middle to upper Ecca Group succession. Middle Ecca Group zircon samples contains the same major age populations as other samples, however, the proportion of Permian zircons are more prevalent than samples collected in the Upper Ecca and Beaufort Groups whereas >1000 Ma zircon are rare and only exist as either minor populations or single grains. The Cambrian to Neoproterozoic age population also varies within samples with an overall increase in grain abundance upwards into the Beaufort Group.;The CFB-Karoo basin has been identified as a fold-thrust belt/foreland basin system; however, the Karoo basin sediment composition is inconsistent with derivation from the fold-belt leading to the hypothesis that the fold-belt may not have existed during Karoo basin development (300-250 Ma). Thin-section petrography of 18 sandstone samples does not fully support some previously proposed paleogeographic models. All analyzed samples generally lie in the mixed affinity and dissected arc QmFLt diagram fields. This is interpreted to indicate that the Karoo basin fill was sourced by multiple eroding source terranes causing data to plot as a mixture of multiple contributing sediment sources.;In order to further understand sediment distribution within the Tanqua, Laingsburg and, Ripon sub-basins based on north-northeast Permian paleocurrent indicators acquired from previous studies and paleogeographic reconstruction, two primary sediment pathways were hypothesized. The Ceres syntaxis, which separates the N-S and E-W trending branches, may serve as a sediment pathway connecting the hinterland and the main depocenter within the Karoo basin The formation of the Ceres syntaxis is likely responsible for the formation of and deposition into the Tanqua and Laingsburg sub-basins. The other sediment pathway, the Port Elizabeth antitaxis, may be associated with convergence of the Deseado Massif and southern Gondwanna and later associate with the Gulhas-Falkland fracture zone. The formation of this antitaxis was likely responsible for routing sediment into the Karoo Basin. The north-northeast paleocurrent direction also allows northern source terranes surrounding the Kalahari Craton can be eliminated as potential primary sediment sources, therefore grains matching the age of Kalahari Craton rocks that are found in Karoo basin-fill were likely the result of erosional recycling of the Cape Supergroup.
Dean, Justin Raymond, "U-Pb Detrital Zircon Geochronology within the Cape Fold BeltKaroo Basin System" (2014). Graduate Theses, Dissertations, and Problem Reports. 5456.