CHANGES IN FACIES DISTRIBUTIONS AS AN INDICATOR OF STROMATOLITE REEF GROWTH IN A NEOARCHEAN CARBONATE PLATFORM

Shifts in facies are associated with development of a rimmed margin in the 2550 Ma Reivilo Formation, South Africa. These facies changes allow identification of stromatolitic reefs that are sedimentologically similar to high energy shallow subtidal to intertidal ramp stromatolitic facies. From NE to SW, the lower Reivilo Fm consists of: 1) Shallow subtidal to intertidal columnar stromatolites of precipitated origin associated with grainstone, trapped and bound stromatolites, and aragonite pseudomorphs; 2) Subtidal 1-10 m wide, several decameter long stromatolites containing fine laminae, cm-scale columnar stromatolites, and aragonite pseudomorphs; 3) Deep subtidal fenestrate microbialites with abundant calcite cements and little detrital carbonate; and 4) Coarsely laminated dolomite with rare interbeds of fenestrate microbialites, shale, and iron-formation. Upward in the Reivilo Fm, facies shift to (NE to SW): 1) Subtidal m-wide stromatolites containing cm-scale columnar stromatolites; 2) Shallow subtidal to intertidal precipitated columnar stromatolites with oolitic grainstones and aragonite pseudomorphs; and 3) Subtidal finely to crinkly laminated dolomite with interbedded carbonate turbidites, diamictites, and fenestrate microbialites. Soft sediment deformation with SW transport was common. 4) Correlations to deeper depositional environments are poorly constrained.

The shift of intertidal facies toward the basin and the development of slope facies document the development of a platform rim in the upper Reivilo Fm. Thus, the formation can be interpreted as a lower high energy carbonate ramp that transitions upward into a rimmed platform due to growth of a stromatolitic reef. Reef facies have not been independently identified because the reefal columnar stromatolites are sedimentologically very similar to those in ramp settings; their identification depends on broader facies distributions and platform geometry. Sedimentary features unique to stromatolitic reefs have not yet been recognized and may not be universally present. Reef growth was promoted by localization of sedimentation and carbonate precipitation at the platform margin, and the hydrodynamics of the margin promoted reef growth in the absence of skeletal organisms as seen in microbial reefs of diverse ages.