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
Dawn Y. Sumner
1997 American Journal of Science, v. 297, p. 455-487 (May issue)
(The links "*" connect to definitions of the preceeding word.)
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 km2 of good stratigraphic control (Figure). 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 floor 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 Ámol/l Fe2+ in deep seawater to a few mol/l Fe2+ in the mixed zone of the oceans. The presence of any Fe2+ in the mixed layer of late Archean oceans requires low atmospheric [O2].
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Dawn Y. Sumner
Department of Geology
University of California
Davis, CA 95616