Stratigraphy, sedimentology and provenance of the ca. 3.26 Ga Mapepe Formation in the Manzimnyama Syncline, Barberton greenstone belt, South Africa
Keywords: Barberton greenstone belt
Mapepe Formation
Archean
Manzimnyama Syncline
Deep-water depositional systems
Stanford School of Earth Sciences
Department of Geological and Environmental Sciences
Thesis

The Barberton greenstone belt in South Africa contains some of the oldest, well-preserved sedimentary rocks known from mid-Archean time. These rocks represent an important record of surface conditions and sedimentary processes on early Earth. A recent campaign by the International Continental Scientific Drilling Program acquired five cores in the belt, one of which, the BARB4 core, targeted a deep-water sedimentary sequence of banded iron formation (BIF), banded ferruginous chert (BFC) and siliciclastic rocks in the Manzimnyama Syncline. The Manzimnyama Syncline is located in a little studied area in the southeastern extent of the belt near the Swaziland border. Extensive commercial forest cover in the area has obscured the surface stratigraphy, but the acquisition of the ~540 m long BARB4 core permits detailed analysis of a continuous, relatively unweathered succession for the first time. This thesis combines, 1) sedimentological analysis from a high-resolution core description of the lithic sandstone section and along-strike outcrops at the surface with, 2) geochemical analysis of shale sampled along the core length.
Sedimentological analysis reveals a coarse-grained, sand-rich (N/G=0.96) deep-water system dominated by sedimentation from high-density turbidity flows. The beds, which average 16 cm in thickness, have predominately massive, poorly sorted bases with thin, flat-laminated and rarely cross-laminated tops. These beds, which often contain mud rip-up clasts and chert plates, were emplaced primarily via rapid suspension settling of sediment from collapsing, high-density flows. The tabularity of the beds and the lack of large-scale scour suggests the flows probably deposited in an unconfined setting, most likely a frontal lobe location at the terminus of the feeder system. The paucity of mud suggests that the feeder system was likely not a levee-confined conduit, but was instead a canyon or similar incisional feature. Transport distance through the conduit was probably short, based on lack of grain size fractionation expected from flow filtering during a long run-out distance.
The siliciclastic sediments had a coeval lateral facies relationship with the orthochemical deposits. This is attested to by, 1) gradational contacts and inter-bedded nature of the sequence, including siliciclastic turbidite beds within the BIF section, siliciclastic material in chert-plate breccia beds in the BFC section, and centimeter-scale chert horizons in the finest-grained section of the siliciclastic section, and, 2) the deformed chert plates in the turbidite beds that were clearly still soft at the time of deposition and likely represent the Archean equivalent of mud rip-up clasts.

Petrographic examination of the sandstone indicates that, while the grains are pervasively seritized and silicified, grain textures and relict crystal shapes are still largely preserved. Quantitative point counting results show relative modal abundances of polycrystalline quartz (chert) fragments (51.8%) and volcanic lithic fragments (41.0%), with monocrystalline quartz (5.7%) and feldspar (1.5%) as minor components. Geochemical analysis of shale indicates pervasive metasomatism, which remobilized all labile species, depleting the rocks of Na2O, CaO and Sr while enriching the shale with K, Ba and SiO2. Metasomatic overprinting and Al-poor source rocks, such as komatiite and chert, complicate assessment of weathering conditions in the source area, but the preservation of volcanic grains suggests that the weathering environment was not extreme.
Major oxide, trace and rare earth element (REE) abundances in shale indicate a mixed felsic and mafic source, with high Ni and Cr values suggesting some contribution of sediment from ultramafic rocks. Mafic and ultramafic material was likely sourced from proximal uplifts of Onverwacht Group strata. In contrast, the felsic signature likely originates from the tuffaceous products of explosive dacitic volcanism, as the feldspar- and quartz-poor sandstone composition indicates that the plutonic roots of the belt had not been accessed by erosion. REE analyses reveal lower ∑REE, increasing values of Eu/Eu*, and decreasing LaN/YbN ratios with stratigraphic position, pointing to an evolution from a felsic source to a more mafic source over time, which is supported by similar trends in major oxide and trace element ratios with depth. This trend may be the result of either, 1) deeper-level incision into mafic rocks in the source area with time, 2) decrease in explosive dacitic volcanism with time, or 3) a combination of both volcanism and evolution of the source area.
The petrographic and geochemical results support the findings of previous studies that indicate the southern facies Fig Tree Group rocks are distinct from those north of the Inyoka fault. The sandstone composition and shale geochemistry of the Manzimnyama Syncline strata, however, compare favorably with the Mapepe Formation rocks in the Barite Syncline and the Granville Grove fault area of the Central Domain. This suggests, that despite the apparent absence in the study area of impact-related spherule beds characteristic of the basal Fig Tree Group contact, these strata are genetically related to the Mapepe Formation north of the Heights Syncline and south of the Inyoka fault.