TAPHONOMY OF THE EDIACARAN GAOJIASHAN LAGERSTATTE: IMPORTANCE OF OBRUTION AND EARLY DIAGENETIC PYRITIZATION

The upper Ediacaran Gaojiashan Lagerstätte (551–541 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). Of particular interest is the fossiliferous middle Gaojiashan Member at the Gaojiashan section, where preservation of the tubular fossils is dominated by pyritization. Integrated paleoecological, sedimentological, and taphonomic analysis shows that obrution by event deposition played an important role in the biostratinomy of the Gaojiashan Lagerstätte. The Gaojiashan fossils, particularly those that are pyritized, preferentially occur in mm-thick, normally graded calcisiltite-siltstone layers that are interpreted as distal event deposits below the average storm wave base. One of the most abundant Gaojiashan fossils—the benthic tubular fossil Conotubus—shows evidence of rejuvenation, growth interruption, and reorientation after being felled, suggesting that these organisms were buried in situ by distal event deposits. Subsequent to quick burial, Conotubus tubes were filled with early diagenetic minerals, first pyrite and then calcite, that precipitated centripetally within the tubes and centrifugally outside the tubes. The results show that the Gaojiashan Lagerstätte, like many Phanerozoic soft-bodied Lagerstätten, was strongly influenced by event deposition and early mineralization which collectively ensured quick burial, rapid fossil diagenesis, and thus a high fossilization potential. Although some taphonomic processes (e.g., those related to bioturbation and microbial activities) may be restricted to certain geological time intervals, others such as sedimentary obrution and rapid authigenic mineralization (e.g., phosphatization, pyritization, calcification, and silicification) were universally important in exceptional soft-body preservation throughout geologic history.