Earth's earliest known extensive, thick carbonate platform suggested by new age constraints

Abstract

Proterozoic and Phanerozoic carbonate platforms have provided considerable information on how the flora, fauna and water chemistry of warm, shallow seas evolved through time. This contrasts with the relative scarcity of Archean examples of these extensive repositories of biochemical and chemical sediments. Until now the late Neoarchean Campbellrand-Malmani and Hamersley carbonate platforms have provided the only examples of extensive, thick Archean carbonate deposits. Scattered outcrop areas of Mesoarchean carbonate, up to 400 m thick, are present in western Superior Province, but past geochronology has assigned significantly different ages to them. A reappraisal of previously dated felsic volcanic rocks as sandstones, combined with new U-Pb zircon geochronology conducted on intermediate to felsic tuffs, determined that four of these carbonate occurrences, now scattered over 2300 km², were deposited between 2.87 Ga and 2.85 Ga. The realization that an extensive, thick carbonate platform, and deeper water chronostratigraphic equivalents (Slate Bay Assemblage), are probably present in this area provides a basis for future comprehensive studies of the relationships between the various types of depositional processes and compositions of seawater chemistry developed on and above the Mesoarchean seafloor.