Biostratinomy of the late Ediacaran pyritized Gaojiashan Lagersttte from Southern SHAANXI, South China: Importance of event deposits

摘要

The upper Ediacaran Gaojiashan Lagersttte (551541 Ma) in southern Shaanxi Province, South China, hosts a variety of soft-bodied or lightly biomineralized tubular fossils (Shaanxilithes, Conotubus, Gaojiashania, Sinotubulites, and Cloudina) and calcareous microfossils (Protolagena). This study focuses on the fossiliferous middle Gaojiashan Member at the Gaojiashan section, where pyritization of soft-bodied or lightly skeletonized tubular fossils is the primary mode of preservation. Integrated paleoecological, sedimentological, and taphonomic analysis shows that event deposition played an important role in the biostratinomy of the Gaojiashan Lagersttte. Gaojiashan fossils, particularly pyritized fossils, preferentially occur in mm-thick, normally graded calcisiltite-siltstone layers, interpreted as distal event deposits, but are rarely present in calcilutite-mudstone beds. Two taphofacies are recognized in the middle Gaojiashan calcisiltites-siltstones, on the basis of biostratinomic data (e.g., fossil fragmentation and disarticulation). Both taphofacies contain fossils (Conotubus, Gaojiashania, and Protolagena) that were buried in situ by distal event deposits below the average storm wave base; evidence for in situ burial includes oblique orientation, rejuvenation, growth interruption, and reorientation. This research represents one of the few biostratinomic studies of Precambrian Lagersttten. Results show that the Gaojiashan Lagersttte, like many Phanerozoic Konservat-Lagersttten, was influenced strongly by event deposition that provides an obrution mechanism for quick burial. Although some taphonomic processesthose responsible for Ediacara-type and Burgess Shale-type preservationmay be restricted to certain geological time intervals, others such as sedimentary obrution and rapid burial may have been continuously and universally important in exceptional preservation throughout geologic history.