A MICROFOSSIL ASSEMBLAGE FROM POST-STURTIAN CAP CARBONATES OF THE RASTHOF FORMATION, NORTHERN NAMIBIA

The Rasthof Formation of northern Namibia hosts a Sturtian cap carbonate (~716 Ma) that directly overlies glacial strata deposited during a Snowball Earth event. Here, we report the first unambiguous microfossil evidence for life that was present in the immediate aftermath of the Sturtian low-latitude glaciation. These assemblages are preserved in the basal 170 m of the Rasthof Formation in microbialaminites at the Okaaru locality (18°45’15”S, 13°42’37”E).

Samples from both thin- and thickly-laminated facies were collected and thin sectioned. We have described three distinct morphologies in these facies: 1) oval-shaped organic walled structures; 2) flat filaments and; 3) organic tubes. Oval-shaped, walled organic structures (69—358 μm long) were abundant in thin sections characterized by good fabric preservation (7 out of 15). More than 150 oval organic structures were identified in acid macerates of thinly-laminated microbialaminite, but fewer than 30 were identified in the thickly-laminated microbialaminite. Most of these structures are hollow and globular with a blunt end, and < 10 % may have necks. The uniformity of shapes and sizes, the hollowness and the presence of 6 ± 2 μm-thick walls (N=15) suggest that these structures are fossil tests. All examined tests contain carbonaceous material, and are covered by quartz, muscovite, hematite and microcline. The heterogeneous presence of these minerals on the surfaces of individual tests indicates that testate organisms preserved at Okaaru agglutinated detrital minerals while alive. The fossil tests are compositionally and morphologically consistent with tests of modern lobose testate amoebae (Arcellinida). Both facies also contain flat, ~ 10 μm wide and >100 μm long filaments that are probable remnants of fossils algae or cyanobacteria. Rare organic-rich agglutinated tubes (N=15) with variable lengths (250-1200 μm) and widths (26 – 123 μm) are also present in both facies. The morphological and the compositional characters of these tubes are consistent with those of modern single-chambered foraminifera. The microfossil assemblages from the Rasthof Formation show that microbially-dominated ecosystems harboring abundant heterotrophic testate amoebae and other test-building organisms flourished in the immediate aftermath of Sturtian glaciation.