SMALL SHELLY FOSSIL BIOSTRATIGRAPHY AND PRESERVATION IN THE DZABKHAN BASIN IN SOUTHWEST MONGOLIA

Understanding origination rates and evolutionary changes during the earliest Cambrian depends on accurate documentation and interpretation of first appearances (FADs) and ranges of small shelly fossils (SSF). The Dzabkhan Basin in Southwest Mongolia hosts a rich SSF fossil record within a thick carbonate succession, providing an opportunity to more precisely calibrate evolutionary changes in a chemo-stratigraphic context.

Here we present refined SSF FADs and carbon isotope chemo-stratigraphy in the Dzabkhan Basin. Large lateral facies changes occur over very short distances across the basin, confounding lithostratigraphic correlations, so we reinterpret the biostratigraphy in the context of a facies model and sequence stratigraphy. We present refined biostratigraphic data from both from previously described localities and from new, undescribed sections within the basin. Particularly, we reinterpret the FAD of Anabarites trisulcatus as being in the upper part of the Zunne Arts member (upper member of the Tsagaan Oloom Formation) rather than at the base of the Zunne Arts member, which is likely Cambrian in age, and not early Ediacaran as previously suggested. These data further highlight the strong taphonomic biases and facies dependence for preservation of SSFs. Taken together, these data point to a radically revised record of the Cambrian explosion.