ENVIRONMENTAL DISTRIBUTIONS OF MICROFOSSIL ASSEMBLAGES PRESERVED IN A POST-STURTIAN CAP CARBONATE, THE RASTHOF FORMATION, NORTHERN NAMIBIA

The Rasthof Formation of northern Namibia hosts a Sturtian cap carbonate (716 Ma) that directly overlies glacial deposits of the first Snowball Earth event. Previous work on the Ongongo and Okaaru localities of the lower Rasthof Formation has revealed diverse assemblages of agglutinated tests similar to modern testate amoebae and unilocular foraminifera. Here, we describe microfossil assemblages from two additional localities of the Rasthof Formation, preserved in the basal ~70 m of the Rasthof Formation in thickly- and thinly-laminated microbialaminites at the Ombepera (S18˚45.506', E13˚36.959') and S. Ombepera (S18˚48.129', E13˚38.515') localities.

Oval-shaped walled organic structures were found in thin sections and acid macerates of microbialaminite samples. Elongated, tubular tests were preserved at both localities. In addition, more than 50 previously-unreported round morphologies were identified in the acid macerates of thinly-laminated microbialaminite facies at S. Ombepera but were rare or absent elsewhere. These hollow dorso-ventrally compressed structures have an elliptical horizontal cross-section, with lengths of ~130 – 160 μm, widths of ~120 – 130 μm, and ~15 – 25 μm-thick walls. These structures are morphologically consistent with modern shells of agglutinated organisms, either Arcellinida testate amoebae or foraminifera. The elongated, flattened shape bears great resemblance to both Heleoperiidae and Nebelidae testate amoeba. Tubular tests, both partial and complete (N=7), with lengths of ~300 – 700 μm and widths of ~40 – 90 μm are also present in both thinly- and thickly-laminated microbialaminite facies from both localities. These flexible organic structures are coated with 5 – 20 μm minerals, have a round cross-section, and some have tapering ends. They are morphologically consistent with the tests of modern single-chambered foraminifera. Microfossil assemblages from Ombepera and S. Ombepera expand the morphological diversity of test-building organisms in the immediate aftermath of Sturtian glaciation and confirm the lateral variations in these assemblages. These variations across environments and facies suggest ecological and/or taphonomical influences on assemblages of the Rasthof Formation.