--> Climate of Mid-Latitude Northeast Pangea in Early Permian, Southern Bogda Mountains, NW China: Evidence From Sedimentology and Organic Geochemistry

AAPG ACE 2018

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Climate of Mid-Latitude Northeast Pangea in Early Permian, Southern Bogda Mountains, NW China: Evidence From Sedimentology and Organic Geochemistry

Abstract

Climate of mid-latitude northeast Pangea during late Sakmarian to early Kungurian has been interpreted from 140 samples in 6 sections in southern Bogda Mountains, NW China through sedimentary and geochemical analyses. Recent chronostratigraphy places the Lucaogou low-order cycle (LCG LC) between the upper Sakmarian and basal Artinskian and the Hongyanchi (HYC) LC between basal Artinskian and lower Kungurian. The LCG LC mainly contains well laminated organic-rich black shale, dolomitic shale, and stromatolitic limestone, interpreted as fluctuating profundal environments. The lacustrine deposits in LCG LC have averaging TOC value of 2.28%, medium organic carbon isotopic value (δ13Corg) of -25.94‰, medium pristane/phytane ratio (Pr/Ph) of 1.60, high Ts/Tm ratio (C27 18α-trisnor-hopane/C27 17α-trisnor-hopane) of 0.55, and high abundance of gammacerane and C35 hopane with minor β-carotane. The biomarker proxies and abundance of evaporitic minerals suggest that LCG shale deposited in anoxic saline-stratified profundal environments, in arid-semiarid climate. The HYC LC mainly contains laminated shale and siltstone with upward-coarsening lithic wacke, subarenite, arenite, and conglomerate, interpreted as lacustrine and deltaic environments. The HYC deposits are characterized by averaging TOC value of 2.36%, heavy δ13Corg value of -22.18‰, high Pr/Ph value of 2.91, low Ts/Tm value of 0.24, and absence of gammacerane, C35 hopane, and β-carotane. The molecular proxies and absence of evaporitic minerals suggest that the HYC deposits formed in oxic fresh water environments, in humid-subhumid climate. The shift from anoxic saline LCG lake to oxic fresh HYC lake suggests that the climate changed from arid to humid during late Sakmarian-Early Kungurian. The climate changes may have been caused by the shifts of the interglacial-glacial from late Sakmarian to early Kungurian, during which atmospheric pCO2 decreased. This is supported by a major increase of δ13Corg of deposits in LCG to HYC LC.