PERMIAN-TRIASSIC BOUNDARY CLAYSTONE BRECCIAS AND THEIR MODERN ANALOGUES

A dramatic perturbation in the carbon isotopic composition of organic matter demarcates the Permian-Triassic boundary in nonmarine rocks. Distinctive claystone breccias coincident with the d¹³C_org minimum, and with the extinction of up to 97% of fossil leaf species, have been found across the Gondwana supercontinent, including three localities in Antarctica, two in Australia, and three in South Africa. The claystone breccias are dominated by granule-size clayey clasts and are unusual lithologies for their sequences because other claystone breccias are composed of much larger (2-10 cm), chemically resistant mineral grains with lower sphericity and greater angularity. Moreover, the claystone breccias lack elongate, curved, or rounded clay chips similar to those redeposited from desiccation cracks in mudflats. Petrographic studies of the claystone breccias reveal that the clayey clasts are similar in color, mineralogy, and texture to underlying paleosols. To investigate the sedimentary processes involved in their formation, the sedimentology and petrology of modern soils eroded and redeposited after recent landscape disturbances are documented. Redeposited soils were collected in the Oregon Coast Range from: a rill on a clear-cut (ca. 1 year old), an alluvial fan on a road cut and a debris cone formed after a wildfire (< 1 year old), and three debris flow deposits formed on a clear cut (< 6 months old). The redeposited soils contain coarse sand- to granule-size spheroidal peds displaying mosepic, clinobimasepic, or insepic soil microfabrics; they are petrographically similar to Permian-Triassic claystone breccias. Thus, provenance studies and comparison with modern analogues suggest that the Permian-Triassic boundary claystone breccias reflect widespread soil erosion following the abrupt extinction of Permian plants.