Abstract

Superbly preserved reefs of Ediacaran age in Namibia give clues to environmental controls on stromatolite-thrombolite growth morphology and nucleation. Digital mapping and Measured stratigraphic sections reveal parasequences featuring meter-scale alternation of shallow marine shale and stromatolitic-thrombolitic microbialites associated with other clastic carbonates (grainstones, mudstones). Stromatolite-thrombolite column width, spacing and height vary systematically with the type of sediment being deposited, with growth inhibited during shale deposition. Columns are generally wider and more closely spaced during carbonate sediment deposition and narrower and more widely spaced during shale deposition. While stromatolite growth should be sensitive to sediment type explicitly, we also interpret sediment type as a general proxy for changing environmental conditions (e.g. water depth, turbidity) that may directly affect reef growth. A simple rule-based numerical simulation of microbialite growth is formulated based on the field interpretations of sedimentological and topographic growth controls. The model cannot explain detailed morphologic attributes, but can recreate correlations between stromatolite column widths, column spacing and layer bed thickness as a function of sediment type.

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