THE FLUVIAL SEQUENCE OF A LATEST CHANGHSINGIAN, PRE-PT EXTINCTION RIVER COMPLEX, EASTERN CAPE PROVINCE, SOUTH AFRICA

The Permian–Triassic mass extinction, dated at 252.25 ± 0.06 Ma, is well documented in the marine record, but details of the event on land are still ambiguous and widely debated within the scientific community. This project focuses on the stratigraphic record leading up to the PT boundary at one of the critical sections in the Karoo Basin of South Africa, Wapadsberg Pass. A transition in river styles is reported to have occurred across the landscape from meandering Permian sandstone-dominated channels to Triassic braided complexes. This transition can be recorded by architectural elements, which reflect sensitive variations in river flow and composition that could be driven by climate change. A road cut along the R61 at New Wapadsberg Pass exposes a vertical cross section through a channel complex, providing an opportunity to evaluate a latest Changhsingian landscape.

Field work undertaken in January 2012 included: construction of a stratigraphic section; standard lithologic description including grain size, color, and primary structures; and acquisition of several photomosaics from which fluvial architecture could be evaluated. Subsequent methods included a refinement of sandstone classification based on optically measured grain size, QFL ratios, and elemental CHN analyses. Panoramic photomosaics were used to determine bounding surfaces of main channels and internal channel geometries which can be correlated to specific depositional environments.

Based on photomosaic analysis, five imbricated channels occur in a 40 m thick stratigraphic section. Towards the section base, bounding surfaces are overlain by thin (< 2 m) sandstone or sandy coarse siltstone bedforms, with an overlying thick channel-fill sequence of coarse and fine siltstone. However, at this stratigraphic level, the river system shows a higher proportion of suspension load relative to bedload. Bedload deposits change to coarse sandy siltstone higher in the section where little sandstone is encountered. Here, there is a significantly higher proportion of bedload, relative to suspension load, with finer grained material making up the channel bed and fill sequences. Hence, the very fine-grained nature of these latest Permian channel fills may be confused with thick paleosol sequences if lateral facies relationships are not taken into account.