Abstract
Carbonate facies of the upper Tongwane Formation preserve a largely overlooked record of pre-GOE Paleoproterozoic seawater. This inventory has survived despite a complex paragenetic history involving both diagenetic and contact metamorphic processes. BIF mineral assemblages are dominated by a medium-grade grunerite overprint and the formation of prograde and retrograde riebeckite. The massive dolomite member, a platform top carbonate which caps the succession, is characterized by marine REYSN patterns lacking significant negative CeSN anomalies (<10%) and δ13Ccarb values of between +1.83 and +2.95 ‰VPDB that are slightly elevated above Paleoproterozoic ‘normal marine’ values. Systematically decreasing δ13Ccarb values measured in monotonously recrystallized, talc bearing, slope carbonates reflect interaction with devolatilization fluids during contact metamorphism caused by the intrusion of the Bushveld Igneous Complex; slope carbonates offer no insight into marine paleoredox conditions. Thus, despite contact metamorphism, Paleoproterozoic marine signals are retained in platform-top dolomites, however they do not support claims for widespread oxygen accumulation in shallow depositional environments of the Transvaal Basin on the eve of the GOE.
Highlights
The late Neoarchean to late Paleoproterozoic rise in oxygen, known as the “Great Oxidation Event” (GOE; Holland, 2002; 2006), was arguably one of the most important paleoenvironmental changes in Earth history
The sedimentological microfacies of the Tongwane Formation record a shallowing-upward trend from banded iron formations (LF1) to massive dolomites deposited above wave base (LF6) (Fig. 3; Table 1; Schröder and Warke, 2016)
The preservation of a representative Paleoproterozoic seawater signal within the Tongwane Formation is dependent upon the absence of any overprint imparted during diagenesis and contact metamorphism from the nearby Bushveld intrusion
Summary
The late Neoarchean to late Paleoproterozoic rise in oxygen, known as the “Great Oxidation Event” (GOE; Holland, 2002; 2006), was arguably one of the most important paleoenvironmental changes in Earth history. Evidence for this rise in oxygen is recorded in numerous Neoarchean-Paleoproterozoic supracrustal successions that are preserved in modern-day North America, Western Australia, Fennoscandia, Brazil, and southern Africa (Martin et al, 2013; Gumsley et al, 2017). The middle and upper portions of the succession are dominated by a distally-steepened progradational carbonate ramp which underwent periods of platform margin collapse
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