MICROFOSSILS IN THE 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 deposits of the Chuos Formation. The basal 170 m of the Rasthof Formation is characterized by thinly (<1 mm) and thickly (1—4 mm) laminated microbialaminites, which preserve the first microfossil evidence for life that was present in the immediate aftermath of the Sturtian low-latitude glaciation. Here, we describe the microfossils present in the microbialaminites of the lower Rasthof Formation at the Okaaru locality (18°45’15”S, 13°42’37”E).

At Okaaru, the basal 40 m of the Rasthof Formation alternates between allodapic carbonates and thickly laminated microbialaminite. Lamination size decreases up-section, and at ~60 m, thinly laminated microbialaminite becomes prevalent. Samples from both thin and thickly laminated facies were collected and thin-sectioned. Oval-shaped walled organic structures (69—358 μm long) were abundant in 7 out of 15 thin sections. More than 150 oval, elongate organic structures were also identified in the acid macerates of thinly laminated microbialaminite, but were rarer in the thickly laminated microbialaminite (<50 specimens). Most of these structures are globular with a blunt end, but <10% are vase-shaped with distinct necks. All of these contain agglutinated phyllosilicate grains, and some are hollow. These forms are consistent with shells constructed by Arcellinid testate amoebae. Both facies also contain flat, ~10 μm wide and >100 μm long filaments that may be the remnants of fossils algae or cyanobacteria. Rare organic-rich agglutinated tubes (N=12) with variable lengths (250—890 μm) and widths (26—123 μm) are also present in both facies. The common types of microfossils found in both facies, fossil testate amoebae and agglutinated tubes, are similar. The prevalence of fossil testate amoebae in the thinly laminated microbialaminites and apparent paucity in thickly laminated microbialaminites may be due both to ecological and taphonomic variations between these two morphologically distinct deep water facies. Overall, the preservation of abundant testate amoebae and other shell-building organisms in both facies demonstrates the importance of diverse agglutinated organisms in deep-water microbial ecosystems after the Sturtian glaciation.