Abstract

The Oligocene stratiform manganese deposits in the Chiatura region (Georgia, Soviet Union) were deposited on stable crystalline basement in a restricted arm of the Paratethys during a transgressive-regressive cycle. Transgression was characterized by, in the early stages, the development of a fining-upward gravel and sand sequence and, with increasing water depth, deposition of organic-rich, fine-grained sediments in waters increasingly depleted in oxygen. Manganese, derived from weathering of recks surrounding the basin, was probably concentrated in these reduced waters to levels well above those of normal sea water. Manganese carbonates were formed in offshore areas subject to reducing conditions; in well-aerated, nearshore parts, manganese oxides were formed. Landward migration of the oxidation/reduction interface during transgression led to further concentration of manganese in nearshore regions and eventually to extensive precipitation and accumulation of manganese oxide ooliths and pisoliths, occasionally in graded units, during peak transgression and early regression when basin waters were reoxygenated. Late regression was characterized by the deposition of sands and spongiolite.

Petrographic studies of manganese-oxide ooliths and pisoliths indicate a concentrically laminated structure in which constituent pyrolusite microcrystals show a characteristic radial and, less frequently, random orientation with respect to the grain center. Ooliths and pisoliths, which also exhibit micro-unconformities and entrapped detrital material, are considered to be analogous to some present-day calcite and aragonite ooliths and were probably formed in a low energy, shallow-marine environment. Graded bedding in oolite and pisolite is comparable to similar beds documented in the Cretaceous stratiform manganese deposit at Groote Eylandt in northern Australia. Grading was probably formed as a result of changes in energy and/or oxygen levels during small-scale fluctuations in sea level.

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