2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014)

Paper No. 220-1
Presentation Time: 9:00 AM

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


ANDERSON, Ross P., Geology and Geophysics, Yale University, 210 Whitney Avenue, New Haven, CT 06511, MACDONALD, Francis A., Department of Earth and Planetary Sciences, Harvard University, 2, Cambridge, MA 02138, TOSCA, Nicholas J., Earth Sciences, University of St Andrews, Irvine Building, St Andrews, Fife, KY16 9AL, United Kingdom, BOSAK, Tanja, Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, BOLD, Uyanga, Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, MA 02138 and BRIGGS, Derek E.G., Geology and Geophysics, Yale University, Kline Geology Laboratory, 210 Whitney Avenue, New Haven, CT 06511, ross.anderson@yale.edu

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.