A reinterpretation of the Wagendrift Quarry, Estcourt, KwaZulu-Natal Province, and its implications for Karoo Basin Paleogeography

Abstract

The South African Karoo Basin preserves a continental record across the Permian/Triassic Boundary in which both plant and vertebrate fossil assemblages may co-occur. The Upper Permian part of the Beaufort Group, a shallow-water fluvial succession spans this boundary. The Estcourt Formation previously was interpreted to be part of this group representing shallow bay fill, the result of overbank deposition. This interpretation implies that sedimentation occurred along the margins of a large, shallowing basin during the Late Permian. Recent work in a series of outcrops at Wagendrift Dam, near the town of Estcourt, requires a reinterpretation of this setting. Three outcrops consist of eight facies, six of which are characteristic of a submarine turbidite system. Basal-most massively bedded sandstones, interpreted as submarine channel deposits are overlain by millimeter-scale, fining upwards couplets of siltstone to mudstone that are distal turbidite in origin. Millimeter-scale bedding is undisturbed and bioturbation is limited, indicating deposition within a low oxygen zone. Ripple cross laminated and ball-and-pillow sandstones, overlain by a finer, massive siltstone are indicative of more proximal turbidites, which is further supported by the presence of large, localized olistoliths (slump blocks) in which internal bedding is preserved. Millimeter-to-centimeter scale upwards-fining successions of planar to ripple cross laminated beds of coarser (fine sand, coarse silt) sedimentary rocks overlain by finer siltstone represent a continuation of proximal turbidite conditions. Thick, ripple-bedded sandstones cap the section, representing a major change in depositional setting into a turbidite fan channel system. The sedimentological characteristics of this succession suggest that although these rocks are assigned to the Latest Permian Beaufort Group, they are, in fact, part of the Ecca Group. Palynomorphs recovered from the quarry are of Late Permian age and consist of taeniate and non-taeniate bisaccate pollen, and spores. Assignment of the assemblage, based on partial canonical correspondence analysis, places the Wagendrift palynoflora in the lower part of Aitken’s Biozone VI (Volkrust Formation; Wujiapingian). These results indicate that turbidite sedimentation similar to that in the southern and western part of the basin persisted into the Late Permian of KwaZulu-Natal.