RUI: THE CHENGJIANG SCIENTIFIC DRILLING PROJECT AND AN INTEGRATED MODEL FOR UNDERSTANDING BURGESS SHALE-TYPE DEPOSITS

Pomona College, Claremont CA

Investigators

Abstract

RUI: THE CHENGJIANG SCIENTIFIC DRILLING PROJECT AND AN INTEGRATED MODEL FOR UNDERSTANDING BURGESS SHALE-TYPE DEPOSITS Robert R. Gaines, Geology Department, Pomona College EAR-1046233 ABSTRACT Cambrian Burgess Shale-type (BST) deposits preserve soft-bodied organisms and are central to our understanding of the initial radiation and early evolution of the Metazoa; however, these deposits represent significant deviations from the constraints that govern the typical operation of the fossil record. This problem has remained unexplained since Walcott?s discovery of the Burgess Shale in 1909. By combining high resolution centimeter to micron-scale analyses with modern geochemical approaches, this study will test an integrative model for the origin of enigmatic Burgess Shale-type fossil deposits using the Chengjiang deposit of Yunnan Province, South China. In this project a total of 4 drill cores will be extracted along a proximal-distal transect for micro-scale analysis of the paleoenvironments and geomicrobiological processes that led to exceptional fossilization of the Chengjiang Biota of the Yu?anshan Shale. The Chengjiang occupies a special position in our understanding of the history of animal life as the oldest of nine principal Cambrian BST deposits and the most diverse Cambrian biota yet described. Using the dynamic and geographically widespread strata of the Yu?anshan Shale, this project will test a new hypothesis that conservation of exceptional fossils occurred at discrete loci only when a specific set of paleoenvironmental criteria was satisfied. Data from 8 of the principal BST deposits indicate that BST intervals occur in a specific physical depositional window at the distal margin of scour, where re-working was absent, yet deposition of clay-sized particles was event-driven and rapid. Within this favorable physical environment, it is hypothesized that the chemistry of the benthic environment exerted the next control, with exceptional fossilization occurring only under anoxic benthic conditions. Ultimately, the chemistry of the early burial environment and of early Phanerozoic seawater are proposed to have determined what microbial reactions were favored and the extent to which each could progress. Exceptional preservation is suggested to have resulted from a combination of factors that significantly inhibited normal microbial processes; a hypothesis readily tested using the ä34S system. This work will apply newly-developed, interdisciplinary methods to examine the paleoenvironments, preservation and geomicrobiology of the Chengjiang deposit in a unified context, and to reconstruct the ecological environments in which exceptional fossils were conserved. Accordingly, this work will be conducted at a variety of scales, including formation-scale stratigraphic analyses, lab-based microstratigraphic analyses (mm-cm scale), SEM investigations, and extensive geochemical analyses tied into a microstratigraphic framework.