RECOGNITION OF THE KELLWASSER EVENT IN THE LATE DEVONIAN HONGGULELENG FORMATION OF NORTHWESTERN CHINA

The Late Paleozoic, specifically the Late Devonian, was a time of notable changes in earth history due to the colonization of terrestrial environments by land plants, their rapid evolution, and the subsequent reductions in atmospheric CO₂. These environmental fluxes resulted in a series of ocean anoxia and extinction events that eliminated 21% of families and 50% of genera of marine organisms. In particular, the Kellwasser anoxia event borders the Late Devonian Frasnian-Famennian boundary and is typically identified as a thick black shale deposit. Black shale deposits of the Kellwasser Event have mainly been found in subequatorial, continental shelf environments. However, Frasnian-Famennian sediments from the Hongguleleng Formation in the Xinjiang Uygur Autonomous Region of northwestern China record the Kellwasser Event, despite the absence of a black shale deposit. The Hongguleleng Formation is located in an island arc setting in the mid-latitudes, in contrast to a subequatorial continental shelf.

The Kellwasser Event in the Hongguleleng is recorded in beds of siltstone and fossiliferous limestone (exceeding 100 genera). The biostratigraphic position of the formation is constrained by conodonts from the late-Frasnian linguiformis to the mid-Famennian trachytera biozones, thus encapsulating the correct time interval for the Kellwasser Event to have been recorded. Siltstones of the Hongguleleng Formation have uniform composition, indicating that sedimentation was continuous across the Frasnian-Fammenian boundary and that no major temporal gaps are present. Geochemical proxies (major element data) and mineralogical data (biogenic apatite and pyrite framboids) indicate the presence of the Kellwasser Event at the Frasnian-Famennian boundary despite the lack of black shale. Limestone and siltstone beds in this interval also contain evidence of storm deposits, similar to those seen in previous studies of the Kellwasser Event on continental shelves, further indicating a global event. Studies of Late Devonian sections in unique localities such as the Hongguleleng Formation, which exhibits evidence of biotic crisis in shallow marine environments, yet does not contain black shale, are increasingly important in the development of our understanding of mass extinction events on a global scale.