Presented by Hofmann, H. J..
Key words: Archean, stromatolites, morphology, carbonates
In Session 102 Paleontology/Paleobotany IV: Origin and Diversification of Early Life on Earth Wednesday, 28-Oct-98 AM in Room: 701A at 8:15 AM for 15 minutes.
Abstract: Stromatolites are morphologically circumscribed accretionary growth structures with a primary lamination that is, or may be biologically influenced (biogenic). The earliest records date back to nearly 3.5 Ga, and their worldwide distribution and abundance increase as time progresses: Paleoarchean occurrences (3) are outnumbered by those in the Mesoarchean (7) and Neoarchean (13), while the remaining occurrences are questionably Archean, or questionably stromatolites.
Stromatolites are generally found in Archean sedimentary carbonate rocks, almost always associated with extensive volcanic sequences. Three occurrences were known in 1960; since then, 29 additional ones have been reported from 11 small regional clusters representing the world's principal preserved Archean cratons on five of the modern continents; 16 occurrences are in North America, 7 in Africa, 5 in Australia, 3 in Asia, and 1 in Europe; none are presently known from Archean rocks of South America and Antarctica.
Morphological types include all possible lamina shapes: flat, convex-up, concave-up, and globoidal; stacking patterns generated nodular, columnar (unbranched as well as branched), and oncoidal forms. Ministromatolites with radial-fibrous microstructure, probably almost exclusively the result of chemical precipitation, developed after 3.0 Ga, as did mesoscopic aragonite/calcite crystal fans, indicating carbonate supersaturation of ambient Meso- and Neoarchean ocean waters. The observed diameters of the structures show a gradual increase in size with time, extending over two orders of magnitude in geon 34 (centimetric to decimetric), but ranging over 6 orders of magnitude by geon 25 (sub-millimetric to dekametric). Unlike Proterozoic stromatolites, most are developed in limestones rather than dolostones, with sideritic/ankeritic and cherty types also present. Microfossils are preserved only very rarely.
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