ABSTRACT

The 3.28 to 3.23 Ga Mapepe Formation in the Barberton greenstone belt, South Africa, marks the initiation of widespread tectonic uplift and deformation after nearly 300 million years of predominantly basaltic and komatiitic magmatism under largely anorogenic conditions. This rapid transition is recorded in the eastern Barite Valley area by the buildup of a fan delta. Well-exposed sections there reach about 450 m thick and can be divided (from base to top) into five informal members: Member 1 is dominated by mudstone with subordinate banded ferruginous chert and turbiditic sandstone representing a deep-water basinal environment. Member 2 is composed of siltstone and fine-grained sandstone reworked by currents to form laminated, cross-laminated, and low-angle cross-stratified sediments in an off-shore or possibly subtidal fan-delta-front setting. Member 3 overlies member 2 unconformably; it is composed of predominantly coarse-grained, cross-bedded sandstone interbedded with laminated mudstone deposited on shallow-subtidal to intertidal flats along the fringe of a small fan delta in which putative microbial mats covered low-energy upper tidal flats. Fan-delta sedimentation was subsequently overwhelmed by the influx of dacitic pyroclastic sediments of member 4. Orthochemical sedimentary rocks including barite, jaspilite, and chert deposited on top of this shallow-water bank. Mappable facies changes towards the northeast and southwest document the transition from bank top into major mass-transport deposits of fan-delta slope facies and then into basinal deposits. Subsequent relative sea-level rise resulted in the return to below-wave-base deposition of turbiditic sandstone, mudstone, and banded ferruginous chert of member 5.

The lenticular geometry of units in cross section, mineralogical immaturity, and high variability in provenance of the coarse-grained units imply short-distance transport of sediment derived from strata of the underlying Onverwacht Group and from local penecontemporaneous dacitic volcanism. Throughout the greenstone belt, Mapepe rocks in several structural belts display fan deltas developed adjacent to small, local uplifts. While the cause of these uplifts has generally been associated with the initiation of geodynamically driven tectonic activity in the BGB, it is possible that a cluster of large meteorite impacts may have directly or indirectly triggered the crustal deformation.

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