HIGH RESOLUTION STRATIGRAPHY OF THE PERMIAN-TRIASSIC BOUNDARY AT BETHULIE, KAROO BASIN SOUTH AFRICA

The Permian-Triassic extinction in the continental record has focused on the stratigraphy of the Karoo Basin, South Africa. The extinction event has been linked to a short sequence in the upper Balfour Formation of the Karoo Supergroup that includes a concretionary paleosol, referred to as the End Permian Paleosol (EPP), which marks the last occurrence of Dicynodon. The nodule layer is overlain by several meters of interbedded siltstone and mudstone that has been referred to in past works as the “Event Bed” or “Dead Zone.” This interval is considered to be stratigraphically unique. The EPP and “Event Bed” have been defined by the stratigraphy at Bethulie, located in the Free State province and correlated across the entire Karoo Basin.

Lateral variation of the marker horizon on the Bethel and Heldenmoed farms in the Bethulie district was tested for continuity using high resolution stratigraphy assisted by data collected via thin section, XRD, and SEM analyses.

The lowermost 4 m of the EPP interval are composed of greenish grey and maroon siltstones and document a minimum of three aggradational cycles. The base of each cycle is marked by green-gray siltstone plugs that crosscut the maroon strata, some of which are interpreted as infilled vertebrate burrows. Katbergia burrows also were found in several intervals in addition to two distinct carbonate-cemented nodule layers that occuring in the predominantly maroon siltstone. A sequence of interbedded very fine sandstone, sandy coarse siltstone, and mudstone in fining upwards couplets immediately overlies the paleosol.

Tracing the maroon paleosol and overlying laminated siltstone interval across the two farms resulted in the recognition of significant lateral variability particularly in the “Event Bed.” Within several hundred meters, the well defined interbedded siltstone grades to massive siltstone with angular, millimeter-sized mudclasts, overlain by interbedded fine sandstone and siltstone. Two hundred meters northwest, this interval is replaced by massive green-gray siltstone containing pebble-sized mudclasts. Basin-scale correlation of the “Event Bed” seems unlikely due to the lithological variances occurring over the <1 km examined in the present study.