Clay mineralogical characteristics at the Permian–Triassic Shangsi section and their paleoenvironmental and/or paleoclimatic significance

Abstract

High–resolution clay mineralogical investigations were conducted on the Permian–Triassic (P–Tr) carbonate– and siliceous rocks from the Shangsi section, South China. The clay mineralogy is dominated by illite and illite/smectite mixed–layers (I/S) throughout the sampled section with subordinate chlorite and vermiculite emerging from bed 24 (approaching the End–Permian mass extinction horizon–EPME) onwards. Positive correlations of clay fraction with mainly land–derived TiO2 and magnetic susceptibility (MS) indicate a primary continental origin of the clay minerals. Rock–Eval pyrolysis analysis, palynomorph coloration, scanning electron microscope (SEM) and Transmission Electron Microscope (TEM) observations, all suggest that the rocks experienced thermal diagenesis which mainly promotes illitization processes of smectite clay minerals while discrete clay minerals are merely affected. Despite this diagenetic effect, clay minerals still carry valuable paleoenvironmental and paleoclimatic information in the Shangsi section. The overall illitic composition of the clay minerals and nearly absence of kaolinite suggest a general semi–arid condition during the P–Tr interval. Appearance of chlorite from bed 24 onwards suggests further aridification compared to the Late Permian. Occurrence of vermiculite from beds 24–28 in association with abundant volcanogenic materials, most negative δ13Ccarb values as well as enhanced soil–derived moretane abundance, suggests catastrophic soil erosion and likely soil acidification during the P–Tr transition in the source area(s) in relation with the active volcanism in that time. Further, decrease of chemical weathering and increase of physical weathering are indicated by increased input of primary clay minerals such as chlorite, a better illite crystallinity and increase of Al2O3/TiO2 ratios. A climate cooling period corresponding to the Clarkina changxingensis–Clarkina deflecta conodont zone to the lower Clarkina zhangi conodont zone is indicated by higher abundance of illite, lower terrigenous TiO2, MS and Al2O3/TiO2 in comparison with the overlying uppermost Permian strata.