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, Transvaa l Supergroup, South Africa, consist of peritidal and subtidal carbonat e lithofacies that pass conformably upward into the deep subtidal Kuru man and Penge iron-formations. The stratigraphic setting and lithofaci es transitions demonstrate that the Gamohaan and Frisco formations wer e deposited in open marine environments during a transgression that re sulted in drowning of the underlying Campbellrand-Malmani carbonate pl atform. The Gamohaan and Frisco formations contain complex microbial s tructures associated with abundant sea floor-encrusting and void-filli ng calcite. In a 40 m section of the Gamohaan Formation, more than 35 percent of the rock consists of marine calcite that precipitated as cr ystals directly on the sea floor or in primary voids. Individual beds of precipitated, sea floor-encrusting calcite are up to 30 cm thick an d are laterally continuous for the entire 7000 km(2) of good stratigra phic 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 precipitati on was controlled by regional seawater chemistry, and that in situ cal cite precipitation directly on the sea door was an important rock-form ing process in late Archean oceans. Transitions from the Gamohaan Form ation to basinal equivalents laterally and to the Kuruman Iron Formati on vertically show a progressive change from precipitated calcite to s hale to siderite-facies iron-formation mixed with oxide-facies iron-fo rmation deposition. This facies succession can be attributed to an inc rease 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 rang es from several 100 mu mol/l Fe2+ in deep seawater to a few mu mol/l F e2+ in the mixed zone of the oceans. The presence or any Fe2+ in the m ixed layer of late Archean oceans requires low atmospheric [O-2].