Paper No. 19
Presentation Time: 9:00 AM-6:30 PM


SPENCER, Kody, 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, GEISSMAN, J.W., Department of Geosciences, University of Texas at Dallas, 800 West Campbell Road, Richardson, TX 75080 and KAMO, Sandra L., Jack Satterly Geochronology Laboratory, Univ of Toronto, 22 Russell Street, Toronto, ON M5S 3B1, Canada,

The Karoo Basin, South Africa, hosts a terrestrial sedimentary record spanning the late Permian and early Triassic. One of the critical PT boundary localities is Tweefontein in the Eastern Cape Province. This 100+ m section is based on outcrop at S 31º 49.334, W 24̊º48.565 and described as a transition from green siltstone of the Permian to a thick, mostly red siltstone Triassic interval. The current study, complementing one higher in the section, developed a high resolution stratigraphy of the basal 33 m of a section near the reported original coordinates beginning at S 31° 49.230, E 24º 48.818. 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.

The basal part of the Tweefontein section consists of fining up sequences of olive gray (5 Y 4/1) siltstone in which intervals contain carbonate-cemented concretions and burrow structures. Siltstones are well or poorly cemented, resulting in a high weathering state of the latter. A new fossil-plant assemblage dominated by Glossopteris with sphenophyllanean Trizygiawas collected at a stratigraphic height of 18 m. This assemblage is similar to that reported from Wapadsberg Pass and is in close proximity to a very thin tuffite from which no datable zircon grains were recovered. Total Organic Carbon ranges from 0.2 to 0.7%, and C:N ratios vary from 4 to 33, indicating more contribution of organic matter from algal sources.

The upper part consists of a thick fluvial sandstone characterized by very fine to fine grained, subangular to subrounded, yellow-gray lithic wacke organized into trough cross bed sets. Individual bedforms may be several meters thick and show bedding variation from trough to low angle planar beds, which may be rippled. Paleocurrent direction, derived from trough crossbed axial orientations, is to the northeast.

Comparison with previously published Tweefontein stratigraphy indicates green siltstone dominates this part of this section, a similar finding to others. Depositional environments include either poorly developed paleosols or fluvial channel-fill siltstone and sandstone, indicating the sufficient water supply, and presumably wet climate conditions, during deposition of the sequence.