TUBULAR MICROFOSSILS PRESERVED BY AL-SI CLAY IN SHALLOW WATER CARBONATE FACIES OF THE POST-STURTIAN RASTHOF FORMATION (~660 MA), NORTHERN NAMIBIA

The Rasthof Formation (~660 Ma), a cap carbonate and highstand deposit, overlies the Sturtian glacial strata of northern Namibia. Previous studies of laminated deep-water microbialites reported agglutinated spheroidal, oval, and tubular microfossils and interpreted these fossils as the remnants of agglutinated eukaryotes such as testate amoebae and monothalamous foraminifera. Recent investigations of the Rasthof Formation at Panorama Farm on the Fransfontein Ridge of northern Namibia reveal specimens with two distinct morphologies: oval testate structures (~100-275µm long, ~20-30 µm wide) and tubular forms (~100-275µm long, ~20-30 µm wide). These microfossils are preserved in microbial laminites and columnar stromatolites, respectively, facies that were deposited on a topographic high during the highstand systems tract of the uppermost Rasthof Formation. The testate specimens are oval and elongate along a single axis with an aspect ratio of 1:3. Tubular specimens found from the Fransfontein Ridge are sinuous and straight, with branches or small processes, and with bulbous structures occasionally at one terminus. Analyses of these structures using energy dispersive X-ray spectroscopy (SEM-EDS) show an abundance of aluminum, silicon, and oxygen, with trace amounts of potassium, magnesium, and iron. The elemental composition of the tube-shaped specimens, along with the structure of the minerals viewed under a scanning electron microscope suggest that these microorganisms were preserved by internal molds of an Al-Si clay. These tubular morphologies reveal a new mode of soft-tissue preservation by clay in the most shallow-water microbial facies of the Rasthof Formation that have been investigated to date. Further work will determine whether this mode of preservation required special chemical conditions, redox gradients or microbial activity within decaying tubular organisms. The discovery of varying preservation styles and morphotypes in different facies further illustrates the potential of coupled stratigraphic and paleontological studies of Cryogenian carbonate strata to reconstruct ecosystems in the aftermath of global glaciation.