GSA Annual Meeting in Denver, Colorado, USA - 2016

Paper No. 76-6
Presentation Time: 9:00 AM-5:30 PM


CHURCHILL, Alyson N., Geology, Colby College, 5800 Mayflower Hill, Waterville, ME 04901, GASTALDO, Robert A., Department of Geology, Colby College, 5807 Mayflower Hill Drive, Waterville, ME 04901 and NEVELING, Johann, Council for Geosciences, Private Bag x112, Pretoria, 0001, South Africa,

The Permian-Triassic Boundary (PTB) is interpreted to mark a globally synchronous, rapid extinction event. A few sequences expose the terrestrial PTB, including the Karoo Basin, South Africa. Massive siltstone facies that span the vertebrate-defined PTB in these localities exhibit a color change from greenish-gray to reddish-gray (maroon). Associated with this vertical color change is an interpreted shift from wet conditions to increasingly warm and arid conditions with a less seasonal paleoclimate. Our study, situated at Tweefontein within the Old Lootsberg Pass area, contains sandstone bedload deposits that can be traced for several kilometers. These are overlain by interbedded thin sandstone and thick siltstone intervals exhibiting the vertical color change. The current model identifies most of these siltstone deposits as paleosols, although primary sedimentary and biogenic structures in some are indicative of channel-fill complexes.

Paleosol proxies provide a means to reconstruct paleoclimate, although their accuracy depends on whether siltstones actually represent paleosols. Our research goal is to test the hypothesis that interprets most siltstone deposits surrounding the PTB as paleosols rather than channel-fill deposits. At Tweefontein, a 15 m thick homogenous interval of green-gray to mottled reddish-gray siltstone was studied. Total Organic Carbon (TOC) analysis identified carbon-enriched intervals indicative of potential A horizons. Elemental analysis of these intervals using XRF allowed for the identification of geochemical trends.

Significant trends for several major elements exist in our Tweefontein section. TOC analysis reveals positive excursions at multiple intervals, which may be indicative of paleosol profiles. Parallel trends exist for Na, Mg, and Ti, with Na and Mg exhibiting synchronous positive excursions throughout the profile in concert with negative excursions of Ti. However, as other major elements appear to exhibit no significant geochemical trends and calculated weathering indices are ambiguous, these data are inconclusive for the presence of paleosols.