Provenance and temporal constraints of the Early Cambrian Maotianshan Shale, Yunnan Province, China

The Early Cambrian Maotianshan Shale deposits in Yunnan Province, China are well known for their exceptional preservation of the soft-bodied fossil fauna known as Chengjiang Fauna or Maotianshan Shale fauna. The high abundance and variety of species display a diversification of disparate anatomical architectures yielding critical insight into the evolutionary explosion of animal body plans known as the Cambrian Explosion. The fossil preservation of the labile tissues is widely distributed in central Yunnan but its spatial and stratigraphical restrictions within the Maotianshan Shale, and environmental controls remain a mystery.

Provenance analysis traditionally focuses on the compositional analysis of sand sized grains (e.g. Dickinson and Suczek, 1979, Ingersoll et al., 1984), despite the fact that silt sized grains are the most common grain size fraction in many depositional environments (e.g. Hay, 1998, Allison et al., 2003, Curray et al., 2003). Finer sediment components also have the advantage that they are commonly less permeable than their coarser grained counterparts, minimizing the impact of diagenetic alterations in this grain size fraction, and consequently more reliably preserving information about the original depositional setting (Blatt and Sutherland, 1969, Blatt, 1985). Compositional information from silt and smaller sediment components therefore can't be ignored and is essential to conduct an unbiased provenance study (Cullers, 1988, Garzanti et al., 2011).

Here, we present a multi-proxy approach to decipher the provenance of the Maotianshan Shale and add to the understanding of the taphonomic conditions that lead to the preservation of the Chengjiang Biota in the Chengjiang, Haikou, and Shankou fossil sites.

Geologic setting

The study area is located near the western edge of the Yangtze Block, in Yunnan Province, South China (Fig. 1). The northwestern margin of the Yangtze Block is defined by the Longmenshan fault, separating the Yangtze Block from the Tibetan plateau (Fig. 1).

The Precambrian strata of the Yangtze Block is largely composed of Mesoproterozoic lowgrade metamorphic (greenschist facies) clastic sedimentary rocks, and abundant Neoproterozoic granites that intruded in the Mesoproterozoic strata (Guo et

Methods

Samples were selected from outcrops at five different locations near the city of Kunming (Fig. 1). All locations are known for abundant fossils, and some locations are located in designated geoparks and world heritage sites. The samples vary in color, reflecting the variable degree of weathering in the area. The least weathered sample stems from a deep core from the Maotianshan area. Other unweathered samples were taken from fresh exposures in the Kunyang Phosphate Mine (Jianshan locality). All

Detrital components

Local source area variations, hydraulic sorting, and implications on the taphonomy

The fate of the Changjiang organisms after death depends on physical and biologic factors that influence taphonomy. Provenance is a physical factor that controls, in part, sediment composition in a basin and ultimately might influence the chemical and physical conditions at or near the sediment water interface or during early burial.

The interpretation of the general provenance of the Maotianshan Shale from detrital zircon, whole rock geochemistry, and petrography places the source area for the

Temporal constraint of the Maotianshan Shale deposition

Conclusion/summary

Acknowledgments

We gratefully acknowledge the support of this research by a grant from the National Science Foundation to MHH (NSF-RAPID, EAR-1338442), a grant by the Chinese Ministry of Science and Technology to XHL (grant no. 2013CB835005), grants from the National Science Foundation of China to CJ (grant nos: 41023008 and 41272008), and a graduate student grant from the National Science Foundation to LAM (NSF-EAPSI 0914034). Other funding was provided by the University of Montana, the Geological Society of