TUBESTONE STROMATOLITES AS ‘GEOPLUMB' INDICATORS, WITH APPLICATION TO MARINOAN (635 MA) GLACIOEUSTASY ON THE FORESLOPE OF THE OTAVI CARBONATE PLATFORM, NAMIBIA

Tubestone stromatolites are distinguished by the presence of tubes filled by micrite or cement that assume paleovertical (‘geoplumb') orientation irrespective of the inclination of host lamination. They occur in Marinoan (635 Ma) syndeglacial cap dolostones in California (Noonday Dolomite), northern and southern Namibia, SW Brazil, NE Alaska and SW Mongolia. The tubes are commonly attributed to gas or fluid-escape, but there is disagreement over the gas composition, CH4, CO2 or O2? In northern Namibia, tubestone stromatolite occurs continuously across the Otavi platform and the upper part of its foreslope to a projected distance of ~5 km seaward of the slope break. The structures formed diachronously in shallow water as the foreslope and platform were progressively flooded by glacioeustatic rise. In order to constrain the amplitude of glacioeustasy, we measured the orientations of 260 geoplumb axes and host laminations as a means of estimating the inclination of the foreslope. The average slope inclination was determined to be 15°, implying that the former ice-sheet grounding-line wedge formed at a paleodepth of ~1.34 km (5 km x tan 15°) below the rim of the platform. As the grounding-line wedge is draped by a transgressive ‘cap' dolostone that was deposited above wave-base, their depth on the foreslope must be attributed largely to sea-level fall. After appropriate corrections for glacioisostatic loading of the platform, glaciohydroisostatic unloading of the sea floor, gravitational ‘pull' by the ice sheet on the adjacent ocean, and tectonic subsidence during the glacial period; the glacioeustatic lowering during the glacial was at least 1.5 km, equivalent to a global average ice-sheet thickness of ~3.0 km on all continents and oceanic plateaus. This is consistent with geological evidence concerning the dynamic behavior and extent of Marinoan ice sheets inferred from glacial sedimentology. In order to be wet-based and flow, grounded ice sheets must have been thick.