Recalibrating the breakup history of SW Gondwana: the first U-Pb chronostratigraphy for the Uitenhage Group, South Africa

• Main Author: Muir, Robert Anthony
• Other Authors: Bordy, Maria Emese
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: Faculty of Science 2019
• Subjects: Antarc*; Antarctica
• Online Access: http://hdl.handle.net/11427/30432; https://open.uct.ac.za/bitstream/11427/30432/1/thesis_sci_2019_muir_robert_anthony.pdf

Syn-rift deposits often provide the only means to determine the processes for initiation and evolution of rift basins and passive margins. The structurally preserved erosional remnants of several rift basins that formed during the Mesozoic breakup of Gondwana are located within the southern Cape region of South Africa. These onshore basins contain the Suurberg and Uitenhage Groups, which are predominantly continental, taphrogenic, fossiliferous strata interbedded with volcaniclastics. Their significance in the Gondwanan breakup events is poorly understood due to a lack of precise and accurate radioisotopic ages. The development of SW Gondwana into the modern passive margins of southern Africa, South America and Antarctica, as well as the evolution of life recorded in the regional strata are difficult to evaluate without a high resolution chronostratigraphic framework. By integrating field observations with U-Pb geochronology of over 4000 detrital and primary volcanic zircons from pyroclastic, mixed-origin volcaniclastic and sedimentary rocks by Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS), this thesis presents the first radioisotopic ages for the Uitenhage Group and provides a new chronostratigraphic framework for the onshore Jurassic – Cretaceous in the southern Cape. To further improve precision and accuracy, a selection of crystals from four pyroclastic deposits in key stratigraphic positions were selected for single-zircon Chemical Abrasion – Thermal Ionisation Mass Spectrometry (CA-TIMS) analysis to minimize the effects of Pb-loss and constrain depositional age uncertainties to < 1%. These new age constraints show that the Suurberg Group was deposited rapidly during the emplacement of the Karoo and Ferrar Large Igneous Province in the Early Jurassic (Pliensbachian) and likely predates the main phase of rifting in the southern Cape, whereas the Uitenhage Group was deposited over a prolonged (> 40 Ma) period from the Early Jurassic into the Early Cretaceous and records two ...