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