Pristerognathus Assemblage Zone fauna from the Central Free State Province � Support for reciprocal stratigraphy in Karoo Basin Development����

RUBIDGE, B.S.; DAY, M; VIGLIETTI, P.; ABDALA, F.

Johannesburgo

Conferencia; 18th Biennial Conference of the Palaeontological Society of Southern Africad; 2014

Palaeontological Society of southern Africa

The Middle Permian to mid-Triassic rocks of the Beaufort Group were deposited in a variety of fluvial depositional environments. Because of the channelized nature of fluvial deposits, there are relatively few extensive lithological markers which can be used for basin-wide stratigraphic correlation in the Beaufort Group. As a result stratigraphic correlation of strata of the Beaufort Group around the basin has relied largely on the six Permian and two Triassic SACS approved vertebrate biozones. Fossil collecting programmes to targeted stratigraphic horizons of the Beaufort Group over the past two decades have resulted in several proposals for the subdivision of some of the existing biozones. These biostratigraphic refinements have enhanced Pangean-wide correlation of tetrapod-bearing continental successions, and improved models of Karoo basin development. Stratigraphic collecting along the Ecca-Beaufort shoreline contact around the basin has revealed a northward change in the fauna above this contact. North of the town of Phillopolis in the southern Free State Province, the only Permian biozone is the Dicynodon Assemblage Zone (AZ). Recent fossil collecting in the Jagersfontein district of the central Free State resulted in the discovery of faunal elements of the Pristerognathus AZ, a biozone characterized by the presence of scylacosaurid and lycosuchid therocephalians and the absence of dinocephalians. Our collecting in the Jagersfontein district has revealed basal therocephalians and no dinocephalians. The most northerly occurrence of a dinocephalian in the Karoo is from the VictoriaWest district. The only fossils recovered above the basal therocephalians are of Dicynodon, only a few metres higher in the succession. This fact, coupled with the fact that no fossils of the Tropidostoma and Cistecephalus AZs have been found so far north in the basin, indicates that in this part of the basin rocks of the Middle Permian PristerognathusAZare directly overlain by the Late Permian Dicynodon AZ. Recently published dates of the Permian Beaufort biozones consider the lower Tropidostoma AZ to be 259.291 ma, and the uppermost Cistecephalus AZ at 255.24 ma. This implies that the early part of the Late Permian depositional succession, present in the southern part of the Beaufort Basin, is absent in the central Free State Province. There was thus a depositional hiatus of Late Permian Beaufort deposition lasting at least four million years ? or perhaps a later period of erosion ? in this part of the Basin, which lends support to the reciprocal stratigraphic depositional model for the Karoo Basin.