Evolution of a Neoproterzoic continental margin subject to tropical glaciation

Abstract

Abstract by author:

The strength of this structural interpretation lies not only in its explanation of stratal geometries, but also in its ability to account for many of the sedimentologic features of the Tweelingskop area. Because normal faulting would have caused uplift and backrotation of the platform north of the Rockeys fault, Tweelingskop would have been the topographically highest area on the platform. This position has important consequences for the nature and timing of siliciclastic influx into the Tweelingskop area as well as the erosional truncation of some of the syn-tectonic sedimentary units

The Otavi Group contains two glacial units, each overlain directly by 'cap carbonates. ' The younger of the two 'cap carbonates' at Tweelingskop, the Maieberg Fm, contains unique meter-scale sea floor precipitates pseudomorphic after aragonite. These precipitates are reeflike in overall construction and are structurally similar to what has been seen in Archean sediments. Such sedimentation is evidence of extremely rapid precipitation from an ocean that may have been greatly oversaturated with respect to CaCO 3. The aragonite pseudomorphs as well as the micritic sediment surrounding them contain anomalously low Sr contents, demonstrating that that basing interpretations of original carbonate mineralogy on Sr content alone may be misleading. While Tweelingskop's unique topographic setting may account in part for the localization of these structures, it is ultimately the climatic consequences of the high atmospheric CO2 required for termination of the Ghaub glaciation that set the stage for their formation

The Ghaub Fm, one of several Neoproterozoic glaciogenic diamictites capped by carbonate sediments showing immense negative d13C excursions has recently been interpreted to have formed during a world-wide glaciation in which the oceans were covered by more than a kilometer of ice (Hoffman et al, 1998b) : the so called "snowball Earth" hypothesis. d13C data from carbonates in the Tweelingskop area strongly confirm the existence of this negative excursion. The apparent flooding that separates the Maieberg and underlying formations may be the product of millions of years of thermal subsidence during the Ghaub glaciation. The dramatic increase in pCO2 required by the snowball Earth hypothesis to initiate melting would produce rapid transfer of CO2 from the atmosphere to the ocean initiating the rapid carbonate precipitation indicated by the aragonites from Tweelingskop. Ultimately, the unique sedimentology of the Maieberg Fm is best explained by both the structural model of fault block rotation and the climatic model of the snowball Earth