MICROFOSSILS FROM CHERTS OF THE EDIACARAN SHUURGAT FORMATION, ZAVKHAN TERRANE, SOUTHWESTERN MONGOLIA

Neoproterozoic strata exposed on the Zavkhan Terrane preserve abundant microfossils in the ca. 660-635 Ma Taishir Formation (Fm), that include testate forms in addition to macroscopic organic warty sheets that may represent algae. Here we report microfossils in the overlying Ediacaran strata of the Shuurgat Fm and assess changes in microfossil assemblages across the Marinoan Snowball Earth glaciation.

Black chert nodules, of early diagenetic origin, in the upper Shuurgat Fm preserve a diverse microbiota evident in petrographic thin-section. The Shuurgat Fm was deposited during the early Ediacaran: it conformably overlies a ca. 635 Ma Marinoan cap carbonate in the basal Ol Fm and is overlain unconformably by the Zunne Arts Fm which includes the Precambrian-Cambrian boundary. Strontium isotope values in the Shuurgat Fm are consistent with those of the early Ediacaran. The upper portion of the Shuurgat Fm is comprised predominantly of thinly laminated micrite and calcisiltite, but the fossiliferous chert nodules occur in subsidiary intraclast conglomerate horizons.

The black chert nodules contain abundant fragments of organic material possibly representing isolated pieces of microbial mat. The organic material includes carbonaceous microfossils preserved in three dimensions. The microfossil assemblage is dominated by 5-20 micron spheroids. Some of these spheroids have thick walls with internal detail, and clump together in mm scale accumulations. Filaments are also well represented and a number of different morphologies occur, including branched, unbranched, and septate forms. Unicellular eukaryotes may also be present with a candidate vase-shaped microfossil (VSM) ~100 microns in maximum dimension. VSMs have previously only been reported in ca. 850-720 Ma strata, and if confirmed, the new Shuurgat Fm VSMs represent the youngest Neoproterozoic occurrences described.