Paper No. 20
Presentation Time: 9:00 AM-6:30 PM


LANGWENYA, Mduduzi, Department of Geology, Colby College, 5800 Mayflower Hill Drive, Waterville, ME 04901, GASTALDO, Robert A., Department of Geology, Colby College, 5807 Mayflower Hill Drive, Waterville, ME 04901, NEVELING, Johann, Council for Geosciences, Private Bag x112, Pretoria, 0001, South Africa, PREVEC, Rose, Albany Museum, Rhodes University, Grahamstown, 6410, South Africa, KAMO, Sandra L., Jack Satterly Geochronology Laboratory, Univ of Toronto, 22 Russell Street, Toronto, ON M5S 3B1, Canada and GEISSMAN, J.W., Department of Geosciences, University of Texas at Dallas, 800 West Campbell Road, Richardson, TX 75080,

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.