MICROFOSSIL ASSEMBLAGES IN CRYOGENIAN CAP CARBONATES OF NAMIBIA, ZAMBIA AND MONGOLIA

Cryogenian cap carbonates provide a critical record of evolution during one of Earth’s most dynamic intervals of environmental change. Recent analyses of cap carbonates from multiple continents reveal the presence of fossil groups immediately after the ca. 716-663 Ma Sturtian Glaciation. Previous work on the cap carbonates of the Rasthof Formation of northern Namibia, has yielded diverse assemblages of agglutinated testate microfossils. More recently, 10 out of 19 limestone samples of drill core material of the Rasthof-equivalent Kakontwe Formation of Zambia have also produced abundant agglutinated testate microfossils similar to some forms from the Rasthof Formation and some rounded forms from the post-Sturtian Taishir Formation in northern Mongolia. SEM imaging, petrographic analysis, and EDS analysis reveal morphologic and mineralogical consistencies across assemblages from these three different basins; microfossils at all locations are spherical and ovoid, sometimes having blunt ends or containing slit-like apertures. Fossils found in the Kakontwe and Rasthof formations are generally more varied (ranging from spherical to ovoid with size ranges of ~50 to 120 microns), whereas Mongolian fossils are typically smaller in size (~50 to 90 microns) and spherical. EDS analysis demonstrates the presence of aluminosilicates, quartz, and iron oxides on the surfaces of all tests, and petrographic analysis confirms that the microfossils are preserved in situ at all locations. These microfossils indicate that similar, shell-making eukaryotes were thriving globally in carbonate systems in the immediate aftermath of glaciation.