Microfacies sedimentological analysis of the Chengjiang fossil bearing interval of the Masotianshan Shale at Ercai, Yunnan Province, China
This research is a continuation of a macrofacies analysis of the area. The Ercai is the first locality that we have analyzed with a microfacies analysis.
Abstract
The Early Cambrian Maotianshan Shale, exposed in multiple localities in Yunnan Province, China, contains the exceptionally preserved fossils of the Chengjiang Biota. The sedimentary facies of the Maotianshan Shale overall represent a series of transgressive-regressive cycles deposited within the lower shoreface and the offshore transition zone. The exceptional fossils are usually preserved in situ and are always found in fine-grained, structureless mudstones representing rapid depositional events. This fine-grained mudstone facies is present throughout the Maotianshan Shale, but the high-fidelity fossils are only found in the uppermost parts of the depositional sequence, suggesting additional taphonomical factors at play. This research aims to uncover some of these factors by conducting a microfacies sedimentological analysis of the Maotianshan Shale. For this we focused on the fossil-bearing interval of the Maotianshan Shale exposed at the Ercai section, one of the most prolific localities for soft-bodied fossils. A continuous ~1.4 m section from this locality was collected, the samples were vacuum impregnated in resin and polished to reveal the microfacies sedimentology. The fossils-bearing layers were identified in the field and marked in the collected samples, allowing for differentiation between the fossil-rich and fossil-poor mudstone facies. A detailed (mm-scale) analysis of this section will reveal the small-scale sedimentary changes that occurred in this interval that buried and preserved the fossils of the Chengjiang Biota. These data can be compared with other fossil-bearing localities to try to identify some of the sedimentary changes responsible for the different exposures of the Chengjiang fossils.
