WHAT DOES GEOCHEMISTRY TELL US ABOUT HOW THE CHENGJIANG BIOTA WAS PRESERVED?

The Chengjiang biota occupy a unique position in the geological record, just post-dating the Precambrian-Cambrian transition, and pre-dating the earliest evidence of land-dwelling organisms. The deposit represents one of the few fossiliferous deposits exhibiting extraordinarily well-preserved examples of biota including soft-bodied animals and soft body parts of animals, joining the Upper Cambrian “Orsten” fauna, the Middle Cambrian Burgess Shale, and the Lower Cambrian Sirius Passet deposits. According to Hagadorn (2002), the fossils are found in shallow marine sediments exposed within Early Cambrian phosphorites. With continued flooding of the Platform, the gray to black siltstone-dominated unit, the Shiyantou Member of the Heilinpu Formation was deposited. Overlying this unit, is the Yu'anshan Member of the Heilinpu Formation, a buff to gray thinly bedded mudstone, that bears the Chengjiang biota. The excellent preservation of the Chengjiang biota requires a unique combination of thermal, geochemical, and sedimentological conditions. We report results from chemical, mineralogical and microscopy studies of samples of fossil-bearing mudstone, and associated black siltstones. REE chemistry suggests an andesitic origin for the terrigenous component of the fossil-bearing samples when compared to values reported from throughout the Sinian-Early Cambrian. Mineralogical and chemical data suggest a material consisting mainly of silica and illite with significant quantities of iron oxide and possibly an iron silicate clay mineral. This composition is somewhat unusual for marine shale, and may indicate a volcanic origin for some of the material. The black siltstones show more variability in REE composition, suggesting a different origin of the material or the presence of depositional or diagenetic overprinting. Most likely the siltstones represent deposition under more normal marine conditions. Recognizing that weathering may have significantly overprinted the transition element patterns, we discuss the evidence for suboxic versus anoxic depositional environments. The overarching problem of the excellent preservation of the Chengjiang biota can be addressed by considering the geochemical conditions of deposition.