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Earliest Triassic Claystone Breccias and Soil-Erosion Crisis

¹ Department of Geological Sciences, University of Oregon, Eugene, Oregon 97403-1272, U.S.A.

Chemostratigraphic methods (¹³C_org) for pinpointing the Permian–Triassic boundary in nonmarine rocks of Antarctica, Australia, and South Africa have drawn attention to unusual claystone breccias at this horizon of the greatest extinction in the history of life on Earth. These rocks differ from other breccias in having a high proportion of clasts with birefringence microfabrics (sepic plasmic fabrics) characteristic of soils, and can be called sepic pedoliths in the terminology of soil science. At many localities, earliest Triassic sepic pedoliths are thin (5–15 cm) beds, but some fill shallow paleochannels, and pinch out laterally. Other non-marine breccias at the Permian–Triassic boundary are dominated by fragments of coal, pedogenic carbonate nodules, and deeply weathered volcanic rock fragments. Earliest Triassic sepic pedoliths record an unusually severe and widespread episode of soil erosion associated with forest dieback at the Permian–Triassic boundary. Comparable sepic pedoliths are found in debris flows after clear-cutting of forests in western Oregon today. Studies of these modern deposits show that sepic soil peds do not withstand weathering for more than a few months. Such clayey clasts are held together mainly by roots and fungi, which decay rapidly in soils and streams. These modern analogs demonstrate that terminal Permian soil erosion was rapid and profound. Sepic pedoliths are also known at other times of mass extinction, such as the Triassic–Jurassic boundary, but are not yet known before the Devonian advent of trees and forested clayey soils.

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