3.2 Ga ocean sedimentary sequence in the Komati section of the Mapepe Formation in the Barberton Greenstone Belt, South Africa
Abstract
The Mapepe Formation (Heinrich,1980) is the lowermost part of the Fig Tree Group in the Barberton Greenstone Belt, and single zircon U-Pb dating suggests its sedimentary age of 3260 to 3230 Ma (Kroner et al. 1991). Our study area (Komati section) is located along the Komati River near the border to Swaziland. This section preserved more than 300m-long continuous outcrop and consists of well-stratified sedimentary sequence with bedded chert and shale. We performed 1/100 scale detailed geologic mapping to identify stratigraphic continuity. The Komati section is divided into 6 units (B1-, B2-, C-, D1-, D2- and E-unit) bounded by deformed zones. Thickness of each unit is 6.8m, 45m, 22.8m, 19m, 5.7m and 23m, respectively. Total thickness of the studied section reaches 128m. The studied section may be divided into the following four rock types. 1) black shale: It consists of massive type, laminated type that has 50μm size quartz lamina and gradational type that changes its color from black to red-brown. 2) red-brown (ferruginous) shale; (3) white chert (massive); (4) red chert: It consists of laminated type that has magnetite lamina and podded type that changes its color from white to red with sharp boundary and partly with podded structure. In all secions, grading from black to red-ferruginous shales are observed. The Corg contents of black shale from all units are ranging from 0.01 to 8.96 wt.%, with an average of 1.73 wt.% (n=204) and delta13C show -38.92~-19.83 per mil. The B2 and D1 units show large (>15 per mil) shifit in delta13C values within 10m section from -21.48 to -37,34 per mill and from -23.43 to -38.92 per mil respectively. These organics date not identified at 3.2-3.1 Ga Dixon Island-Cleaverville sequence (Kiyokawa et al., 2012). Magnetic susceptibility data indicate that Fe content is increasing upward in each unit. Therefore, such repeated upward lowering in the C isotope compositions and increase in Fe contents suggest repetition of increasing influence of hydrothermal input and associated changes in microbial community or metabolism.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFMOS31B1734T
- Keywords:
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- 0408 BIOGEOSCIENCES / Benthic processes;
- 0428 BIOGEOSCIENCES / Carbon cycling;
- 1041 GEOCHEMISTRY / Stable isotope geochemistry;
- 9623 INFORMATION RELATED TO GEOLOGIC TIME / Archean