PROVENANCE AND REDOX CHEMISTRY OF THE MAOTIANSHAN SHALE, YUNNAN, CHINA AND IMPLICATIONS FOR THE PRESERVATION OF THE CHENGJIANG BIOTA

The Maotianshan Shale contains the exceptionally preserved fossils of the Chengjiang Biota Lagerstätte and is one of the earliest Burgess Shale-type (BST) deposits. The fossils are largely non-mineralized and are found in fine-grained mudstone event beds. Few studies have been reported on the geochemistry of the sediments because surface exposures are significantly weathered and published data differs about whether the chemical signatures are primary. Here we present the geochemistry and mineralogy of unweathered samples of the Maotianshan Shale from a recently excavated exposure at Jianshan, Yunnan Province, China.

Geochemical and mineralogical analyses of samples collected from this newly exposed and continuous section documented the sediment source along with the chemical conditions in the sediments during and shortly after deposition. The mineralogy of the Maotianshan Shale predominantly consists of quartz, orthoclase, muscovite, rutile, dolomite, illite and chlorite. This mineral assemblage and elemental plots (REE vs C1 Chondrites; LA/Th vs Hf; La/Sm vs Yb/Sm; La-Th-Sc) identified a terrigenous/granitic source for the sediments comprising the Maotianshan Shale. Elevated concentrations of molybdenum (>2ppm) indicated anoxic conditions during deposition of the Maotianshan Shale. Pyrite was also present in many of the samples and may support anoxic conditions or may have formed during diagenesis.

The results presented herein suggest anoxic conditions were present in the Maotianshan Shale sediments, which would have aided in the fossilization of the Chengjiang Biota. Anoxia is an important factor in the preservation of fossil tissues in BST deposits. The granitic/terrigenous sediment source may also have a role in the exceptional preservation of the fossils; several other BST Lagerstätten also have a granitic provenance, suggesting the composition of the sediments is important to this type of fossilization. Further investigations of fresh samples from other BST Lagerstätten will identify what role similar geochemical conditions or provenance play in the taphonomic process.