Search

Greenstone belt dynamics are a key to understanding the formation and evolution of continental crust but the scarcity, commonly poor exposure, strong deformation, and high metamorphic grades of many Archaean supracrustal rocks preserved in greenstone belts often hinder geological insights. Exceptionally, the largely siliciclastic Palaeoarchaean (ca. 3.22 Ga) Moodies Group, uppermost unit of the Barberton Greenstone Belt (BGB) in South Africa and Eswatini, allows a detailed understanding of regional depositional processes and stratigraphic architecture due to its low degree of deformation, considerable thickness, generally good exposure, and diverse sedimentary facies. We here investigate the Powerline Road Syncline (PRS) and the adjacent Maid-of-the-Mists Syncline (MMS), two east-west-trending, tightly to isoclinally folded, locally northwardly overturned synclines in Moodies Group strata in the central BGB. The northern and southeastern margins of the PRS and the southern to southwestern margin of MMS are bounded by the bedding-parallel 24-Hour-Camp Fault. Along this fault, Moodies Group strata and stratigraphically underlying units of the Auber Villiers Formation of the upper Fig Tree Group structurally overlie lower Fig Tree strata above Onverwacht Group strata. Stratigraphic architecture and palaeocurrent analysis of Moodies Group strata in both synclines document a northeastward (depositionally downdip) facies transition from proximal fan delta conglomerates to fluvial- to coastal-plain and estuarine sandstones in which thick foresets likely represent subtidal channel fills. The overall deepening-and fining-upward trend in this sequence, corresponding to an increasing mineralogical maturity of sandstones, is partially obscured by local hydrothermal alteration which was contemporaneous with sedimentation, minor volcanism, and sill intrusion. Moodies Group conglomerates and sandstones were largely shed from the rising Onverwacht Anticline to the southwest. Lithologies and stratigraphy in the PRS-MMS region closely resemble strata of the lower Moodies Group in the Sadddleback Syncline, located nearby to the north but across the Inyoka Fault. If the 24-Hour-Camp Fault was part of a group of extensional faults accompanying the rise of the Onverwacht Anticline and of tonalitic-trondhjemitic-granodioritic plutons around the BGB margins at ca. 3 224 Ma, it would have acted as one of numerous basin-bounding normal faults during early Moodies time. This would place the deposition of Moodies Group strata in the PRS-MMS region in a supra-detachment fault environment which was subsequently tightly folded. [ABSTRACT FROM AUTHOR]

The ~3.22 Ga Moodies Group, Barberton Greenstone Belt (BGB), South Africa, provides a unique window into Archaean sedimentary, magmatic and ecological processes. In the central BGB, a regional mafic complex, consisting of a genetically related major mafic sill, a peperitic dyke stockwork, and extensive basaltic lava flows affected thick quartzose sandstones of the Moodies Group. We argue that epithermal hydrothermalism associated with this magmatic event occurred, at least in part, syndepositionally and in places destroyed, in other places preserved the abundant benthic microbial mats in terrestrial- and coastal-facies sandstone of this unit. We differentiate four principal types of hydrothermal alteration: (1) Sericitization resulted from ubiquitous feldspar breakdown; (2) iron-oxide alteration replaced the original matrix by fine-grained iron oxide; (3) silicification replaced matrix and most nonsilica grains by microcrystalline silica and locally preserved kerogenous microbial mats; and (4) hydraulic fracturing at shallow depth brecciated consolidated Moodies Group sandstone and created closely spaced, randomly oriented fractures and quartz-filled veins. Because stockwork intrusion locally interacted with unconsolidated water-saturated sediment and because the dykes connect the sill with the mafic lava but also follow zones of structural weakness, we suggest that hydrothermalism associated with this magmatic event occurred syndepositionally but was also - within the resolution of radiometric age data - contemporaneous with tight regional folding. We conclude that microbial organisms in Paleoarchaean coastal (tidal, estuarine) environments may have been formerly widespread, possibly even abundant, but are nearly nowhere preserved because they were easily degradable. Preservation of Early Archaean microbial mats in a thermal aureole in the central BGB was controlled by the "just right" degree of heating and very early hydrothermal silicification. [ABSTRACT FROM AUTHOR]