Neoproterozoic ironstones in northern Namibia: Biogenic precipitation and Cryogenian glaciation

Abstract

The precipitation of Cryogenian ironstones has been attributed to a spectrum of mechanisms ranging from virtually instantaneous “rusting of the seas” in response to post-snowball Earth ice meltback, to localised hydrothermal activity during fragmentation of Rodinia. The former model presupposes that ironstone deposition took place following peak glaciation. In the Chuos Formation of the Otavi Mountain Land, northern Namibia, ironstone facies precede, and are vertically gradational into, diamictites. Evidence for glaciation in the diamictites includes 1) dropstone textures, 2) subglacially deformed and attenuated, thinly stratified diamictites, supported by 3) the co-occurrence of soft-sediment striations. The underlying ironstones contain evidence for tractional processes (large-scale cross bedding) and biogenic growth (stromatolites) in strata rich in magnetite and hematite. Using the analogy of acidophile biomats in modern acid mine drainage environments, where photosynthetic bacteria construct stromatolites, fixing CO2 and Fe intracellularly, it is suggested that Cryogenian acidophile biomats did likewise, triggering ironstone precipitation and local CO2 drawdown, thereby facilitating concomitant glaciation.