2009 Portland GSA Annual Meeting (18-21 October 2009)

Paper No. 2
Presentation Time: 9:00 AM-6:00 PM


PLUDOW, Bryce A.1, GASTALDO, Robert A.1 and NEVELING, Johann2, (1)Department of Geology, Colby College, 5807 Mayflower Hill Drive, Waterville, ME 04901, (2)Council for Geosciences, Private Bag x112, Pretoria, 0001, South Africa, bapludow@colby.edu

The Permian-Triassic mass extinction at the end of the Paleozoic (~252.6 Ma) witnessed the greatest loss of marine and terrestrial life in Earth history. The Karoo Basin, South Africa, exposes Upper Permian and Lower Triassic terrestrial rocks that include the vertebrate extinction event and subsequent recovery. The Triassic Katberg Formation is a quartz-rich feldspathic sandstone in which the recovery phase is documented. Current hypotheses about the causes of extinction focus on climate and there is controversy about the climate state during recovery. Proxies for climate interpretations are based on paleosols that are interpreted to indicate that the Early Triassic was either hotter and more arid or less arid and more seasonal than the Permian. More recent data indicate that the stratigraphic record consists of wetland environments overprinted by more seasonal climates that ended in aridisols or calcisols. But, no arid paleosols are exposed in the Lower Triassic.

This study tests the hypothesis that Early Triassic arid soils once existed in the Karoo Basin based on the presence or absence of pedogenic mud aggregates in the Katberg Sandstone, Carlton Heights. Other modern and ancient studies have used the presence of aggregates in fluvial deposits as an indicator of an arid and highly seasonal climate in which Vertisols were alternately wetted and dried. When eroded, sand-sized mud clasts were transported as bedload in anabranching systems recording the presence of these soil types that may no longer be in the stratigraphic record.

Criteria used in the present study to identify an “aggregate” clast include an unique shape, the inclusion of individual silt clasts within the grain boundary, and pore space surrounding an uncompressed clast. A surrounding network of quartz grains protected aggregate texture during compaction. Results, to date, indicate that aggregates are present in anabranching channel deposits at Carlton Heights. Quartz grains are very fine sand (X=98 µm) whereas aggregates are fine sand (131 µm) size. There is no evidence for Vertisols in the Katberg Formation; rather, evidence exists for Calcisols in the sequence. Hence, these may be the first evidence for pedogenic mudclast aggregates that did not originate in Vertisols.