Carbonate precipitation and oxygen stratification in late Archean seawater as deduced from facies and stratigraphy of the Gamohaan and Frisco formations, Transvaal Supergroup, South Africa

The correlative 2521 +/- 3 Ma Gamohaan and Frisco formations, Transvaal Supergroup, South Africa, consist of peritidal and subtidal carbonate lithofacies that pass conformably upward into the deep subtidal Kuruman and Penge iron-formations. The stratigraphic setting and lithofacies transitions demonstrate that the Gamohaan and Frisco formations were deposited in open marine environments during a transgression that resulted in drowning of the underlying Campbellrand-Malmani carbonate platform. The Gamohaan and Frisco formations contain complex microbial structures associated with abundant sea floor-encrusting and void-filling calcite. In a 40 m section of the Gamohaan Formation, more than 35 percent of the rock consists of marine calcite that precipitated as crystals directly on the sea floor or in primary voids. Individual beds of precipitated, sea floor-encrusting calcite are up to 30 cm thick and are laterally continuous for the entire 7000 km(2) of good stratigraphic control. The abundance of precipitated carbonate and the lateral continuity of individual beds demonstrate that deep subtidal seawater was supersaturated with respect to calcite, that carbonate precipitation was controlled by regional seawater chemistry, and that in situ calcite precipitation directly on the sea door was an important rock-forming process in late Archean oceans. Transitions from the Gamohaan Formation to basinal equivalents laterally and to the Kuruman Iron Formation vertically show a progressive change from precipitated calcite to shale to siderite-facies iron-formation mixed with oxide-facies iron-formation deposition. This facies succession can be attributed to an increase in [Fe2+] with depth: As [Fe2+] increases, calcite precipitation slows, and siderite becomes supersaturated resulting in a change from limestone to iron-formation accumulation. This gradient probably ranges from several 100 mu mol/l Fe2+ in deep seawater to a few mu mol/l Fe2+ in the mixed zone of the oceans. The presence or any Fe2+ in the mixed layer of late Archean oceans requires low atmospheric [O-2].