Biological, depositional, microspherule, and geochemical records of the Frasnian/Famennian boundary beds, South China

Abstract

The investigation of shallow subtidal, inter-reef depression, and inter-reef platform sections from central Hunan of China shows that a transgression–regression cycle occurred in the Late Frasnian ( linguiformis Zone?). The lowest triangularis Zone to Middle triangularis Zone deposits are either very thin or variously missing in some shallow-water sections of Hunan. The uppermost Frasnian (upper linguiformis zone) black-shale interval (including black shale and carbonates) was not ubiquitously developed and probably formed in a regressive, shallower water and anoxic to dysoxic conditions suggested from higher Ce/La ratios, penecontemporaneous dolomitization of carbonates, and low faunal diversity. Two steps of the Frasnian/Famennian (F/F) mass extinction are postulated. The first is the extinction of the benthos near the end of the Frasnian which was probably caused by the sudden onset of anoxic environments. Shallow-water rugose corals and ostracods experienced sudden biomass loss right at the deposition of the black shale. The second is the extinction of pelagic fauna at the end of the Frasnian. Microspherules of probable impact origin are found in several layers in the Xikuangshan F/F boundary section, including two major peaks, respectively, at the Upper rhenana Zone and upper linguiformis zone, and two minor peaks, respectively, near the top of the linguiformis Zone and Lower triangularis Zone. The microtektites are white or brownish in color, characterized by high contents of Si, Al, and intermediate CaO, MnO, and FeO, and minor K 2O and Na 2O. Occurrences of these microtektites do not seem to be directly related to the F/F mass extinction. From the Lower rhenana Zone through the Upper crepida Zone there are four elemental anomalous layers. These elemental anomalies are interpreted to have resulted from the contemporaneous active rifting process and hydrothermal activities in South China.