CHEMOSTRATIGRAPHY OF EARLY PALEOPROTEROZOIC CARBONATES OF SOUTH AFRICA

The Tongwane Fm. of the Chunniespoort Gr. is locally preserved above the 2.48 Ga Penge Iron Fm. and below the unconformably overlying glacially-influenced Duitschland Fm. in the northeastern part of the Transvaal Basin. Dolomites of the Tongwane Fm. have δ¹³C values ranging between +1.6 and +3.1 permil, consistent with earlier published data (Swart, 1999; Bekker et al., 2001). δ¹³C values of carbonates from the upper Duitschland Fm. collected on the Duitschland Farm and from the drillcore BH2(B) support a previously reported positive δ¹³C excursion (Bekker et al., 2001) but also indicate large and gradual variations within this formation. This formation provides evidence for the positive carbon isotope excursion between two Paleoproterozoic glacial diamictites that are likely correlative with the second and third glacial diamictites of the Huronian Supergroup, Ontario. Since carbonates are lacking in a similar stratigraphic position wordwide, there is some uncertainty whether these carbonates represent a global or local signal.

Carbonates of the Silverton Fm., Pretoria Gr., Transvaal Basin and Lucknow Fm., Olifantshoek Gr., Griqualand West Basin show a similar range of δ¹³C values: +8.3 - +11.2 permil and +8.4 - +10.3 permil, respectively, and are considered correlative based on chemo- and lithostratigraphy (Beukes et al., 2001). Carbonates of the Vermont and Houtenbek fms. of the Upper Pretoria Gr. have δ¹³C values ranging between +1.5 and +3.3 permil and –0.6 and –3.3 permil, respectively. While carbonates of the Vermont Fm. could have been deposited either during the late stage or after the end of the ca. 2.22-2.1 Ga carbon isotope excursion, carbonates of the Houtenbek Fm. were definitely deposited after the end of the carbon isotope excursion. Combined carbon isotope data for the Silverton, Vermont, and Houtenbek fms. suggest that the ca. 2.22-2.1 Ga carbon isotope excursion ended during deposition of the upper Pretoria Gr. The Upper Pretoria Gr. is intruded by the 2061 ± 2 Ma Bushveld Complex. The end of the carbon isotope excursion was previously constrained in Finland (Karhu, 1993) between 2.11 and 2.06 Ga, but based on the present study it is likely closer to 2.11 Ga. Presently available data are consistent with two positive carbon isotope excursions in the Paleoproterozoic, one between two glacial diamictites and another stratigraphically above the last glacial diamictite.