Facies, Paleogeography, and Carbonate Precipitation on the Archean (2520 Ma) Campbellrand-Malmani Carbonate Platform, Transvaal Supergroup, South Africa
Dawn Yvonne Sumner
Thesis Advisor: John P. Grotzinger, Associate Professor of Geology
Submitted to the Department of Earth, Atmospheric and Planetary Sciences in
Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy at the Massachusetts Institute of Technology, September, 1995.
(The links "*" connect to definitions of the preceeding word.)
The 2520 Ma Campbellrand and Malmani subgroups contain a >1 km-thick carbonate* platform that is preserved over 190,000 km2 on the Kaapvaal Craton, South Africa. Carbonate rocks are preserved from basinal to supratidal* depositional environments allowing detailed analysis of depositional facies and evaluation of the modes and mechanisms of carbonate precipitation. Results document that the in situ precipitation of carbonate on the sea floor was an important rock-forming process in deposition of the platform. Precipitation of both aragonite* and calcite* crusts on the sea floor was common in supratidal to above wave-base* subtidal* depositional environments. Up to 50 cm-diameter fans of fibrous aragonite (preserved as calcite) grew directly on the sea floor, as well as forming cm-thick crusts on stromatolites (Figures 2-13, 2-14, and 2-18). Herringbone calcite (a new calcite cement morphology described here) precipitation was very abundant in deep subtidal depositional environments composing more than 15% of the rock in a 40 m-thick interval preserved for 140 x 50 km. It is associated with deep subtidal microbialites* that are encased in precipitated carbonate. The microbialites have complex morphologies that are defined by the geometrical arrangement of three components (Figures 3-5 to 3-23) : 1) fine laminae interpreted as the remnants of microbial mats; 2) two dimensional, vertically oriented supports also interpreted as microbial in origin; and 3) voids filled with calcite cements. Calcite preferentially precipitated on the supports over the well developed mats, suggesting that the mat inhibited nucleation of calcite.
Changes in carbonate textures through time, such as a decline in herringbone calcite and thick beds of precipitated carbonate, suggest that an inhibitor to calcite precipitation, possibly ferrous iron, may have caused the herringbone calcite microtexture, promoted the in situ precipitation of thick beds of carbonate on the sea floor, and inhibited micrite precipitation in Archean oceans (more details). An increase in ferrous iron concentration with depth in the oceans also may have caused the observed transition from limestone to siderite precipitates with depth by increasing siderite saturation.
See also: Geological Setting
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Dawn Y. Sumner
Department of Geology
University of California
Davis, CA 95616