INTENSITY OF CONTINENTAL WEATHERING AND NEARSHORE MARINE CARBONATE SEDIMENTATION DURING AND AFTER THE END-PERMIAN MASS EXTINCTION: KHARTAM MEMBER, KHUFF FORMATION, SAUDI ARABIA

The end-Permian mass extinction coincided with eruption of the Siberian Traps flood basalts, the consequent atmospheric emissions of vast amount of CO₂ and, in the oceans, an interval of widespread anoxia.

This project focuses on shallow-marine facies and geochemical evidence for intensity of continental weathering during the end-Permian mass extinction from two stratigraphic sections of the mixed carbonate-siliciclastic Khartam Member of the Khuff Formation, located approximately 10 km northeast of the city of Buraydah in central Saudi Arabia. Both sections show an upward transgressive shift across the Permian-Triassic boundary from mudrock, siltstone, and sandstone succession intercalated with green and dark gray shales, bioturbation, root casts, and anhydrite nodules of the lower Khartam Member to oolitic, peloidal and thin-shell bivalve and microgastropod, lime grainstone intercalated with lime mudstone and bright-red mudrock intervals in the upper Khartam member. Ooids are typically partially to wholly dissolved, generating an extensive oomoldic porosity. Ooids range from 0.2-0.5 mm in diameter. Ooid are well-preserved within the lowermost oolite grainstones and contain radial and tangential cortical fabrics indicating aragonitic and calcitic mineralogy, with nuclei composed of peloids or abraded mollusk fragments.

In both sections, mudrock shows enrichment in Al₂O₃, Fe₂O₃ and kaolinite and depletion in CaO and K₂O with a consequent increase in the Chemical Index of Alteration (CIA) from values of 20 to as high as 90 across the Permian-Triassic boundary (PTB) interval and basal Triassic. The total clay content increases from 45% to as much as 84%, and kaolinite proportion of clay minerals increases from less than 3% to 29%. The abrupt increase of CIA values at the PTB in our sections is consistent with values previously reported from the Khartam Member. Elevated CIA values are typically interpreted to indicate higher humidity and therefore greater chemical alteration of feldspars to clay minerals. In the case of the Khartam Member, however, evidence for aridity (evaporites and red beds) indicates that elevated CO₂ and warm climate in an arid environment resulted in increased silicate weathering, and the increased CIA of mudrock deposited during and immediately after the end-Permian extinction.