TAPHONOMY OF EUKARYOTIC MICROFOSSILS BETWEEN CRYOGENIAN ICE AGES EXPLORED IN THE ZAVKHAN TERRANE, SOUTHWESTERN MONGOLIA

The Neoproterozoic Era witnessed the radiation of the crown-group eukaryotes accompanied by environmental perturbations. Few fossils have been described from the critical interval between the Cryogenian “snowball Earth” ice ages. The barren nature of this interval has been attributed to extinction associated with the persistence of biologically unfavorable environments. Recently, however, interglacial fossils have been discovered in various localities including the Zavkhan basin of southwestern Mongolia. Here we explore the role of clay minerals in the preservation of these fossils. We report a preliminary stratigraphic record of fossil abundance from petrographic bedding-perpendicular thin-sections and rock macerations combined with bulk-rock clay mineralogy from X-ray diffraction for carbonate rocks of the Mongolian inter-glacial sequence (Taishir Formation, Tsagaan Olom Group). Clay mineral assemblages vary stratigraphically from berthierine (lowest ~150 m) through to talc (middle ~25 m) and kaolinite (highest ~225 m). This record suggests that shifts in fossil abundance coincide with changes in the clay mineral assemblages. Future work will use samples already collected from shale and chert horizons to explore facies bias, and will address any metamorphic effects on clay mineral assemblages.