The Makganyene diamictite, South Africa : evidence for an early Proterozoic glacial event
Neumiller, Christine MacKay
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Early Proterozoic chemical sediments (chert, iron-formation, and carbonate) are depleted in ¹⁸O with respect to modem analogues. Two explanations for this phenomenon are: (1) the oceanic waters from which the sediments precipitated were warmer in temperature (+60°C) and similar in ¹⁸O content than present; and (2) the oceanic waters from which the sediments precipitated were similar in temperature and lower in ¹⁸O content than present. Diamond drill core specimens of the Early Proterozoic Makganyene Formation (2400 Ma), Transvaal Supergroup, South Africa, contain evidence supporting the second theory. One sequence contains features typical of an ice-proximal glacimarine environment, including: (1) dropstones and outsized clasts in diamictites and associated sediments; (2) rapid intercalation of debris flow and dropstone diamictites with conglomerate, pelletstone, peloidstone, and felutite; and (3) extrabasinal components in diamictites and associated sediments. Although inferred glacigenic sediments are not in direct contact with chemical iron-formation, they grade into chemical iron-formation, allowing temperature constraints to be placed upon the oceanic waters from which the iron-formation precipitated. Evidence supporting thrust faulting in Griqualand West is also found in the Makganyene cores. Several intervals within the cores have been tectonically repeated, separated by intervals of highly disturbed sediment interpreted to be fault zones.