Stratigraphy and sedimentary environments of the Late Permian Dicynodon Assemblage Zone (Karoo Supergroup, South Africa) and implications for basin development
Viglietti, Pia Alexa
The Dicynodon Assemblage Zone (DiAZ) spans the last three million years of the Late Permian (Lopingian) Beaufort Group (Karoo Supergroup). Fluvio-lacustrine conditions covered the entire Karoo Basin during this period, preserved as the rocks of the Balfour, Teekloof, and Normandien formations. However widely separated exposures and few dateable horizons make correlating between lithostratigraphic subdivisions difficult. Here a revised litho- and biostratigraphic framework is provided for the Upper Permian DiAZ. The Balfour Formation’s Barberskrans Member (BM) is renamed due to identifying the Oudeberg Member and not the BM at the current type locality (Barberskrans Cliffs). It is renamed Ripplemead member (RM) after Ripplemead farm 20 km north of Nieu Bethesda where it outcrops. The Teekloof Formation’s Javanerskop member and Musgrave Grit unit in the central Free State Province are regarded mappable units whereas the Boomplaas sandstone (BS) may represent a unit that is a lateral equivalent to the Oudeberg Member. Palaeontological and detrital zircon data suggest none of these locally persistent sandstone horizons correlate temporally. Three index fossils that currently define the DiAZ (Dicynodon lacerticeps, Theriognathus microps, and Procynosuchus delaharpeae) appear below its lower boundary and disappear below the Permo-Triassic Boundary (PTB), coincidentally with the appearance of Lystrosaurus maccaigi. The base of the DiAZ is redefined, with the revived Daptocephalus leoniceps and T. microps re-established as the index fossil for the newly proposed Daptocephalus Assemblage Zone (DaAZ), and is subdivided into two subzones. Da. leoniceps and T. microps’ appearance define the lower and L. maccaigi defines the base of the upper subzone. The same patterns of disappearance are observed at the same stratigraphic interval throughout the basin, despite the thinning of strata northward. Additionally wetter floodplain conditions prevailed in the Lower DaAZ than in the Upper DaAZ which likely reflects climatic changes associated with the Permo-Triassic mass extinction (PTME). Palaeocurrent and detrital zircon data demonstrate a southerly source area, and recycled orogen petrography indicates the Cape Supergroup is the source of Upper Permian strata. Dominant late Permian zircon population supports the foreland nature of the Karoo Basin. Orogenic loading/unloading events are identified by two fining-upward cycles, separated by a diachronous third-order subaerial unconformity at the base of the RM and Javanerskop members. Sediment progradation northwards was out-of-phase with the south and wedge-shaped. Distributive fluvial systems depositing sediment within a retroarc foreland basin best explains these observations. Lithostratigraphic beds and members are recommended for use as local marker horizons only in conjunction with other proxies, such as index fossils or radiometric dates in future studies.
A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg in fulfilment of the requirements for the degree of Doctor of Philosophy. June 2016.
Viglietti, Pia Alexa (2016) Stratigraphy and sedimentary environments of the Late Permian Dicynodon Assemblage Zone (Karoo Supergroup, South Africa) and implications for basin development, University of Witwatersrand, Johannesburg, <http://wiredspace.wits.ac.za/handle/10539/21691>