FACIES AND EVOLUTION OF PRECAMBRIAN CARBONATE DEPOSITIONAL SYSTEMS: EMERGENCE OF THE MODERN PLATFORM ARCHETYPE

Abstract

Abstract Precambrian carbonates generally have not been examined from the perspective of platform evolution. The major facies and stratigraphic relations of several carbonate sequences ranging from the early Archean through late Proterozoic are examined and discussed in terms of platform construction and the dominant factors that influenced patterns of sedimentation and the production of carbonate. Early to middle Archean carbonate sedimentation was rare and generally restricted to brief interludes between episodes of tectonism and volcanism. Inferred carbonate facies are largely replaced by chert, and individual occurrences were less than a few meters thick. Most accumulated in deeper water settings, although local shallow-marine (or lacustrine) evaporitic settings also were present. Carbonates of the middle Archean Nsuze Group contain the first evidence for deposition on cratonic masses and form a thin (30 m) veneer of shallow-water facies. Late Archean carbonate sedimentation was locally substantial, forming units as thick as 500 m that may have been areally extensive, but which have since been mostly removed by uplift and erosion. Significant facies include major stromatolite buildups, grainstone belts, lagoonal to peritidal cyclic facies, and calcite pseudomorphs after possible giant aragonite fans. Proterozoic carbonates formed platforms that are strikingly similar to Phanerozoic platforms despite the absence of metazoans. Homoclinal and distally steepened ramps developed fringing stromatolite reefs and ooid shoals, stromatolitic barrier reefs, and isolated buildups including pinnacle reefs as thick as 300 m. Rimmed shelves also formed, including accretionary margins, bypass margins with both gullied slopes and escarpments, and intra-shelf basins. Incipient and terminal drowning, rim backstepping, and subaerial exposure were also important in the evolution of Proterozoic platforms. Globally diachronous, cratonic stabilization associated with the Archean/Proterozoic "transition" strongly influenced the long-term evolution of carbonate platforms by creating spatially extensive, stable continental masses on which carbonates formed. Widespread early Proterozoic carbonate platforms document this event. Ramps generally formed in Proterozoic extensional and foredeep basins, and often preceded development of rimmed shelves in more stable, possibly thermally subsiding basins. Rimmed shelves locally formed margins with as much as 1,000 m of relief relative to the adjacent basin and contained stromatolitic barrier reefs that were extensive for hundreds of kilometers. The composition of Precambrian sea water was favorable for calcium carbonate production, and evidence for saturation with respect to calcite and aragonite includes well-preserved pseudomorphs of "abiotic" aragonite and calcite (high Mg?) precipitated in open-marine (non-evaporitic) shallow-water settings. Saturation probably existed to depths of at least 1 km as shown by the presence of carbonate pseudomorphs after (high Mg?) calcite in toe-of-slope sheet cracks and breccias. Evidence for possibly elevated Archean and early Proterozoic saturation values (relative to average Phanerozoic values) includes development of giant aragonite fans in the late Archean and development of extensive tidal-flat tufas in the early Proterozoic. Proterozoic carbonate production may have been strongly regulated by the presence of stromatolite-forming benthic microbial communities. A possible mechanism was removal of CO2 from the water column during photosynthesis. This might have induced in situ carbonate precipitation on and adjacent to stromatolites and provides a mechanism for carbonate mud production, which forms the major non-stromatolitic facies of many Proterozoic platforms. Thick successions of grainstone and other coarse carbonate sediments, comparable to the Jurassic of the United States Gulf Coast and Saudi Arabia, are uncommon in most platforms. Other controls on Proterozoic platform development possibly included the effects of local stromatolite zonation according to paleoenvironment. Conical stromatolites tended to occur in basinal, slope, and deeper ramp settings and may have retarded shelf/basin differentiation and have influenced the progradation of tidal flats, promoting advance in situations where subtidal carbonate production was impeded. Eustasy apparently influenced the stratigraphic architecture of many platforms, causing rim backstepping, incipient shelf drowning, and the development of third-order sequences or "Grand Cycles." It also was the likely cause of parasequences or "small-scale cycles" that characterize peritidal environments in late Archean through late Proterozoic sequences.