Origin and paleo-oceanic conditions of Guadalupian bedded chert in the northern margin of the Upper Yangtze region: Implication for marine anoxia and the Permian Chert Event

Unusual accumulations of chert developed worldwide over tens of millions of years during the Permian Period, and they are referred to as the Permian Chert Event. Middle Permian cherts, in the forms of nodules and bedded cherts, are widely distributed in the Yangtze region. A comprehensive study was conducted on middle Permian cherts from the Xibeixiang section on the northern margin of the Upper Yangtze region. This study combined micropaleontology and geochemical analyses to determine the ages, paleo-oceanic conditions, and implications of the Permian Chert Event. Identification of conodonts and brachiopods showed that the Xibeixiang cherts belong to the late Capitanian Jinogondolella granti zone. The Xibeixiang cherts can be divided into two subunits. The lower section (Sc1) consists of a thin interbedded layer of bedded chert, shale, and limestone, which were strongly influenced by upwelling with relatively low temperatures. The upper section (Sc2) comprises a thin interbedded layer of bedded chert and shale. The intermediate chemical index of alteration (CIA) values of Sc2 suggest a temperature increase compared to Sc1 and a weakening of upwelling activity. The Sc2 section of the Xibeixiang cherts is consistent with the end-Capitanian oceanic anoxic event and coincides in time with the end-Guadalupian biotic crisis, supporting a connection between the biotic crisis and marine anoxia. The Permian Chert Event distributions in the Guadalupian Epoch exhibit substantial temporal and spatial heterogeneity on a global scale. Alterations in the oceanic environment likely caused the shoaling of the lysocline and calcite compensation depth (CCD). This allowed cherts to be deposited instead of carbonates, even in relatively shallow-marine areas, such as continental margin environments.