In Situ Calcite and Aragonite Precipitation as an Important Late Archean Platform-Building Process
Dawn Y. Sumner
1999, AAPG SEPM Annual Meeting, Abstracts
The precipitation of calcite and aragonite crystals directly on the sea floor was a volumetrically important process in the accumulation of Late Archean carbonate platforms. In the 2520-2550 Ma Campbellrand-Malmani carbonate platform, South Africa, laterally continuous, decimeter-thick beds composed solely of calcite and aragonite cement are common. Up to 50 cm-tall aragonite botryoids and crystal fans grew upward/outward from bedding surfaces and stromatolites. Dcm-thick encrustations of herringbone calcite, a marine Mg-calcite cement, isopachously coat depositional surfaces. Supratidal to shallow subtidal lithofacies contain a minimum average of 5-25% in situ precipitated calcite and aragonite with preserved cement textures. Originally, cement proportions were probably much higher, particularly near the platform margin. Deep subtidal microbial lithofacies contain a minimum of 20% calcite cement on average. One 40 m section contains 65% herringbone, bladed, blocky, and microcrystalline cements that precipitated on and in microbial mats. The remaining 35% is recrystallized, but probably also consisted of in situ precipitated calcite. Thus, precipitation of calcite and aragonite on the sea floor was very important in accumulation of the platform.
Other Late Archean carbonates, including those in the Belingwe and Bulawayo greenstone belts, Zimbabwe, and the Steeprock greenstone belt, Canada, also contain abundant sea floor-encrusting calcite and aragonite in a variety of depositional environments. The large volume of marine cement demonstrates that in situ calcite and aragonite precipitation was a more important platform-building process for Late Archean carbonates than it was for younger platforms.
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Dawn Y. Sumner
Department of Geology
University of California
Davis, CA 95616