Facies analysis of the Mapepe Foreland Basin, Barberton Greenstone Belt, by well-to-well correlation
Final Report Abstract
Rock strata recording the environmental conditions from the beginning of Earth history are exceedingly rare, all but a very few having been recycled by Earth’s relentless plate tectonics. By and large, only two places on Earth preserve rocks well enough that they can be studies by geoscientists for their information on the nature of volcanism, the growth of continental crusts, the consequences of meteorite impacts and the gradual spreading of life in a seemingly inhospitable environment. All these strata were laid down in oxygen-free atmosphere and water bodies; however, they are being altered now by weathering in an oxygenrich atmosphere so that only drill cores can recover the original composition. Research reported here focused on the BARB-5 drillhole which was brought down in 2012 in a remote part of the Barberton Mountain Land, South Africa, in strata of the Fig Tree Group, about 3250 Ma old. Together with the strata in another research drillhole, BARB-4, a few km distant, scientists could correlate the rock units encountered in the drillhole and reconstruct the basin geometry. Deposition occurred on a submarine slope that was fed from the south by turbidity currents delivering sand and silt; in the long breaks between sedimentation, silica and iron oxide precipitated from the photic-zone seawater, likely as a result of bacterial action. Zircons extracted from the sandstones are in age all comparable to country rocks and lack evidence to support the existence of an even older continent that was eroded, as had been postulated by other scientists. Where submarine hot springs occurred along the margins of some shallow-water banks, barite, chert, and even some carbonate was deposited and reworked by currents and waves. The drillcore also penetrated a bed consisting of solidified droplets of molten rock which had been generated by a large meteorite impact far away, transported through the atmosphere and rained down into the sedimentary basin examined here. Our field work draws constant attention. For at least one day each field season, we are accompanied by a TV crew or some journalists. Some aspects of our work attracts the attention of the South African public and have been incorporated in public talks, roadside panels, and popular-science guidebooks.
Publications
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2015, Geo-chemistry of a Palaeoarchean shallow shelf environment: unravelling sediment provenance, alter-ation history and redox tracers. Goldschmidt Conf. (Prague) Abstr. Vol.
Mason, P.R.D., A. Galic, P.Z. Vroon, H. Strauss, A. Montinaro, C. Heubeck, N. Drabon
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2015, Sedimentology of the “Tsunami”- chert-slab conglomer-ate of the Archean Fig Tree Group (3.23 Ga), Barberton Greenstone Belt, South Africa: DGGV / DMG Annual Meeting, Berlin, Abstract Volume, p. 145
Fugmann, P., C. Heubeck, E. Profitis
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2016, Sedimentology, Stratigraphy and Depositional Environments of the 3.2 Ga Mapepe Formation in the BARB5 drill core, Barberton Greenstone Belt, South Africa, 35th International Geological Congress, Cape Town, Aug. 27 – Sept. 4
Drabon, N., D.R. Lowe, and C.E. Heubeck
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2017, Detrital zircon geochronology of sandstones of the 3.6-3.2 Ga Barberton greenstone belt: No evidence for older continental crust. Geology 45, 803-806
Drabon, N., D.R. Lowe, G.R. Byerly, and J. Harrington
