Abstract
M.Sc.
Certain carbonate bearing formations in the Paleoproterozoic Pretoria Group and its
Griqualand West equivalent exhibit remarkable geochemical and stable isotopic
signatures. The 8'3Ccarb isotopic signatures from the Duitschland and Silverton Formations
exhibit large positive excursions, which seemingly coincide with a significant increase in
atmospheric oxygen between 2.4 and 2.0 Ga.
The Duitschland Formation with its distinctive basal unconformity is composed primarily
of limestone and dolomite units, interbedded with two compositionally different shale units
and quartzite. Toward the base of the formation there is a distinct conglomeratic quartzite
which forms a sequence boundary above which isotopic and geochemical signatures
change dramatically. Normal marine isotopic signatures characterize the lower portion of
the succession while above the sequence boundari, the carbonates are enriched in "C. This
enrichment, however, appears to be the result of local processes occurring within a closed
basin. Furthermore it is apparent that the Duitschland Formation (with its three distinct
marker beds) is the equivalent of the Rooihoogte Formation and therefore constitutes the
base of the Pretoria Group. The Mooidraai Dolomite Formation which outcrops only locally in the Northern Cape
Province, is characterized by fenestral and microbially laminated dolomite. The
geochemical properties are relatively homogeneous with increases in the FeO and MnO
concentrations, resulting from post depositional diagenesis. The stable isotope signatures
of these dolomites represent normal marine signatures. There is, however, a depletion in
the 813C and 8180 signatures in the ankeritic and sideritic lithofacies, which suggests that
this succession was deposited from a stratified water column with respect to the total
dissolved CO2.
The positive 6 13C excursion present in the carbonates of the Lucknow Formation in
Griqualand West, traditionally grouped with the Olifantshoek Group can be correlated with carbonates near the top of the Silverton Formation in the Transvaal area. The latter
also displays distinctly positive 6 43C values. One possibility is that if these successions
were deposited in closed anoxic basins and that the isotopic anomalies are the result of
local processes such as fermentive diagenesis and methanogenesis. However, the close
association of the carbonates with shallow marine orthoquartzites suggests that these were
deposited in an open marine system and that the positive 8 !3C values reflect a shift in the
composition of the ocean water at the time of deposition of the carbonates at 2.2 Ga.
Other carbonates present in the Pretoria Group, namely from the Vermont and Houtenbek
Formations, display normal open marine 8' 3C values of close to zero. A systematic
stratigraphic compilation of all 6 43C values available from the Transvaal Supergroup
indicates that two clear-cut positive 5' 3C excursions are present. These excursions were
apparently short-lived and well defined and did not occur over an extended period of time
as suggested by earlier studies based on global compilations with large uncertainties in
radiometric ages of deposits.