THE LATE PERMIAN STRATIGRAPHY OF THE UPPER TWEEFONTEIN SECTION, EASTERN CAPE PROVINCE, SOUTH AFRICA

The Karoo Basin of South Africa hosts a terrestrial record spanning the latest Permian and earliest Triassic. One critical boundary locality is Tweefontein in the Eastern Cape Province. This 100+ m section, S 31º 49.334, W 24º 48.565, is reported as a transition from green Permian siltstones to mostly red Triassic siltstone. Our study, complementing one lower in the section, focused on developing a high resolution stratigraphy of the upper 90+ m of a section near the original published coordinates (S 31º 49.089, E 24º 48.121). The project was to develop a high resolution framework for complementary geochronologic and paleomagnetic stratigraphic studies, and provide a model for the depositional environments of paleontological assemblages.

Much of the section is covered but exposures are dominated by olive gray (5 Y 4/1), fining up, coarse to fine siltstone sequences. Carbonate cemented concretions occur in several horizons of olive siltstone from which a skull of Dicynodon sp. was collected. Siltstone color changes to grayish red (5 R 4/2) only near the top of the section. Here, TOC data from subjacent 15 m of olive siltstone, ranging from 0.3 - 6.6 %, indicate more algal than terrestrial biomass contribution. This TOC may reflect an abandoned channel fill complex that underwent pedogenesis and diagenetic overprinting, accounting for sediment reddening.

A zircon-bearing porcellanite occurs in a donga ~500 m to the west and is correlative at a stratigraphic height of ~65 m in our section. A suite of euhedral, sharply faceted grains was analyzed using CA-ID-TIMS with the ET535 EARTHTIME tracer. The 206Pb/238U weighted mean age of a coherent cluster of 5 concordant data for this lithology is 254.73±0.24 Ma (early Changhsingian).

There are few localities in which long stratigraphic sections can be measured and our findings differ from those reported for the Tweefontein section. Previous studies illustrate 40 m of Triassic red, concretionary mudstone interbedded with fluvial sandstone bearing intrabasinal pedogenic conglomerate lags in the upper part of the stratigraphy. In contrast, we record a predominance of olive gray siltstone and non-pedogenic conglomerate-bearing fluvial sandstone, constrained by geochronometric data to the latest Permian.