FACIES HETEROGENEITY AND SEDIMENT DISPERSAL PATTERNS IN THE MAOTIANSHAN SHALE, CHINA

The Early Cambrian Maotianshan Shale near Kunming, Yunnan, China, contains the Chengjiang biota, a soft-bodied fossil assemblage documenting early metazoan evolution and diversification during the Cambrian Explosion. The fossils show excellent detail and have a large non-mineralizing component preserving soft tissues. Many of the organisms are found buried in situ and have a three-dimensional component, suggesting unique preservational pathways. The high fidelity of the organisms suggests rapid burial under dysaerobic conditions, although much is still unknown about the specific sedimentological processes responsible for the exceptional preservation of the Chengjiang biota.

In this study we present new results from recent fieldwork in the Shankou, Haikou and Maotianshan areas, focusing on the sedimentology and facies distributions of the Maotianshan Shale. Deposits of the Maotianshan Shale consist of interbedded mudstones, siltstones and sandstones, and can best be described by six lithofacies. These facies represent a variety of depositional processes, including oscillatory flows, unidirectional gravity flows and passive, hemipelagic sedimentation.

Facies distribution within individual measured sections is not homogeneous throughout but alternates between intervals dominated by fine-grained deposits and intervals dominated by coarser facies. These stacking patterns are regionally correlative, suggesting that deposition in the basin, at large, has been controlled by extrinsic factors.

The richest soft-body fossil beds in all measured sections are found in the upper parts of the Maotianshan Shale. Most of the soft-bodied fossils are associated with structureless, fine-grained mudstones. This rock fabric indicates rapid deposition of these beds concealing the soft-bodied fossils within. Following this high burst of sediment, sedimentation rate rapidly decreased as evident by laminated hemipelagic mudstones with algal filaments that overlay the fossiliferous beds.

This detailed, sedimentological study of the Maotianshan Shale provides a tool for regional correlation of the Maotianshan Shale in general and a process-oriented depositional model that might have contributed as a primary taphonomic pathway responsible for the preservation of the Chengjiang biota.