Microbialite recovery in the aftermath of the Sturtian glaciation: Insights from the Rasthof Formation, Namibia

Abstract

Ice sheet meltback in the aftermath of the Sturtian (mid Cryogenian) glaciation was accompanied or followed by deposition of thick carbonate successions. In northern Namibia, the Rasthof Formation is a 200–400m thick cap carbonate sequence divided into (1) a basal cap dolostone, (2) a microbial member and (3) an epiclastic member. This subdivision applies for >100km along strike at the southern and western edges of the Owambo Basin. In this paper we focus essentially on macrofacies of the cap dolostone and the microbial member. Cap dolostones are commonly interpreted as subtidal to deep water deposits, with delicate mm thick laminae. We describe well-preserved, ungraded hummocky cross-stratification in the cap dolostone, expected to occur no deeper than the offshore transition zone. The overlying microbial member contains thickly laminated microbialites with folded and contorted intervals interpreted as soft-sediment deformation structures. The thickly laminated microbialites are followed by more thinly laminated microbialites, with “roll-up” structures and more unusual individual, vertical stromatolite morphologies. We interpret the vertical growths in the microbial member as a direct response to the changing environment. The new observations and interpretations presented in this paper contrast with previous work on the Rasthof Formation. We recognise a relatively shallower setting associated with a trend in the geometries of the microbial member.