Microbial Structures in An Early Cryogenian Cap Carbonate, Northern Namibia

In northern Namibia, the Rasthof Formation is a laterally-extensive carbonate unit that caps glacial strata of presumed Early Cryogenian age. Unlike basal Ediacaran cap carbonates, Rasthof carbonates do not record initial transgression; instead, basal carbonates were deposited in deep-water settings. A new high-resolution carbon isotope record from the Rasthof Formation shows that a positive isotope excursion (-4 to +3) occurs across the transition from a 17 m-thick mechanically-laminated rhythmite/grainstone unit at the base of the section to a 35-m thick stromatolite facies.

On the Otavi Platform, the Rasthof Formation is 200-400 m thick. Microbially-influenced facies are abundant in the Rasthof Formation. The stromatolite facies consists of light and dark mm-sized laminae and contains abundant syndepositional voids. A possible flooding surface, characterized by cross-bedded silty dolomite, caps the stromatolite facies. Overlying this unit, an 80-m thick deep-water microbialaminite facies is exposed and contains abundant unusual roll up structures. These roll up structures consist of several folded sub-mm-sized laminae. The folding of these laminae indicates that they were cohesive, perhaps microbially-bound, during disruption. In outcrop, the roll ups show no preferential orientation and occur only in the microbialaminite facies. The absence of storm deposits and bedforms in the microbialaminite facies suggests roll up structures formed below storm wave base. Roll up structures are commonly flanked by small cm-sized cement-filled voids. The carbon isotopic composition of void cements does not differ significantly from the host rock, suggesting that the DIC of seawater exerted a strong control on the isotopic signature of Rasthof carbonates. Formation of the roll ups may be related to the growth and decay (gas release) of microbial mat communities. Future work, including modeling of these microbial structures in the lab, will provide insights into the environmental conditions that fostered the formation of unusual microbial structures in Neoproterozoic cap carbonates.