DECREASING CHEMICAL WEATHERING PROXIES OF DRILLED CORE AND PALEOSOLS IN EASTERN YUNNAN, SW CHINA: DIAGENETIC IMPRINT OR WEAKENED WEATHERING FROM LATE PERMIAN TO THE EARLIEST TRIASSIC?

Chemical weathering proxies (e.g., CIA, CIW, and PIA) based on the concentrations of alkali and earth alkaline elements are widely used as potential measurements of paleo-weathering conditions. After checking the same proxies from a drilled core and paleosols in outcrops from the Taoshujing Section, located in Eastern Yunnan, Southwest China, we found dramatically decreasing chemical weathering index values upwards. Most of our samples likely originate from mafic sources with low Th/Sc ratios (<1), indicating that the Emeishan Basalt Plateau supplied a dominant portion of the sediments into the surrounding alluvial plain during most the Late Permian to Early Triassic. Meanwhile, the physical sedimentation processes, such as recycling or hydraulic sorting by size, shape or density during transport and deposition, were not an important factor in regard to changing chemical weathering index values. However, diagenesis significantly altered chemical compositions of the drill core stratigraphy. One of the most common mechanisms for chemical diagenesis is potassium enrichment by illitization of kaolinite or smectite. Moreover, calcite cements from vadose- or groundwater also altered the initial compositions of sediments.

Paleosols in the core change from Histosols and Argillisols developed in the lower section to Vertisol with common syn-depositional mud clasts conglomerates in sandstone bodies further upwards in the section. These stratigraphic changes signify intensified chemical weathering during the transitional period of Late Permian to Early Triassic. These decreasing chemical weathering proxies may be the result of mass wasting due to much flashier sedimentation in the aftermath of terrestrial vascular plant extinction(s) or tectonic changes in sedimentary sources related to the Indosinina orogeny across southern China.