Reassessing evidence of Moon–Earth dynamics from tidal bundles at 3.2 Ga (Moodies Group, Barberton Greenstone Belt, South Africa)

Past orbital parameters of the Moon are difficult to reconstruct from geological records because relevant data sets of tidal strata are scarce or incomplete. The sole Archean data point is from the Moodies Group (ca 3.22 Ga) of the Barberton Greenstone Belt, South Africa. From the time-series analysis of tidal bundles from a well-exposed subaqueous sand wave of this unit, Eriksson and Simpson (Geology, 28, 831) suggested that the Moon’s anomalistic month at 3.2 Ga was closer to 20 days than the present 27.5 days. This is in apparent accordance with models of orbital mechanics which place the Archean Moon in a closer orbit with a shorter period, resulting in stronger tidal action. Although this study’s detailed geological mapping and section measuring of the site confirmed that the sandstone bed in question is likely a migrating dune, the presence of angular mud clasts, channel-margin slumps, laterally aggrading channel fills and bidirectional paleocurrents in overlying and underlying beds suggests that this bedform was likely located in a nearshore channel near lower-intertidal flats and subtidal estuarine bars; it thus carries risk of incomplete preservation. Repeated measurements of foreset thicknesses along the published traverse, measured perpendicular to bedding, failed to show consistent spectral peaks. Larger data sets acquired along traverses measured parallel to bedding along the 20.5 m wide exposure are affected by minor faulting, uneven outcrop weathering, changing illumination, weather, observer bias and show a low reproducibility. The most robust measurements herein confirm the periodicity peak of approximately 14 in the original data of Eriksson and Simpson (Geology, 28, 831). Because laminae may have been eroded, the measurements may represent a lower bound of about 28 lunar days per synodic month. This estimate agrees well with Earth–Moon dynamic models which consider the conservation of angular momentum and place the Archaean Moon in a lower orbit around a faster-spinning Earth

Reassessing evidence of Moon–Earth dynamics from tidal bundles at 3.2 Ga (Moodies Group, Barberton Greenstone Belt, South Africa)

Past orbital parameters of the Moon are difficult to reconstruct from geological records because relevant data sets of tidal strata are scarce or incomplete. The sole Archean data point is from the Moodies Group (ca 3.22 Ga) of the Barberton Greenstone Belt, South Africa. From the time-series analysis of tidal bundles from a well-exposed subaqueous sand wave of this unit, Eriksson and Simpson (Geology, 28, 831) suggested that the Moon’s anomalistic month at 3.2 Ga was closer to 20 days than the present 27.5 days. This is in apparent accordance with models of orbital mechanics which place the Archean Moon in a closer orbit with a shorter period, resulting in stronger tidal action. Although this study’s detailed geological mapping and section measuring of the site confirmed that the sandstone bed in question is likely a migrating dune, the presence of angular mud clasts, channel-margin slumps, laterally aggrading channel fills and bidirectional paleocurrents in overlying and underlying beds suggests that this bedform was likely located in a nearshore channel near lower-intertidal flats and subtidal estuarine bars; it thus carries risk of incomplete preservation. Repeated measurements of foreset thicknesses along the published traverse, measured perpendicular to bedding, failed to show consistent spectral peaks. Larger data sets acquired along traverses measured parallel to bedding along the 20.5 m wide exposure are affected by minor faulting, uneven outcrop weathering, changing illumination, weather, observer bias and show a low reproducibility. The most robust measurements herein confirm the periodicity peak of approximately 14 in the original data of Eriksson and Simpson (Geology, 28, 831). Because laminae may have been eroded, the measurements may represent a lower bound of about 28 lunar days per synodic month. This estimate agrees well with Earth–Moon dynamic models which consider the conservation of angular momentum and place the Archaean Moon in a lower orbit around a faster-spinning Earth

In Situ Fe and S isotope analyses in pyrite from the 3.2 Ga Mendon Formation (Barberton Greenstone Belt, South Africa): Evidence for early microbial iron reduction

International audienceOn the basis of phylogenetic studies and laboratory cultures, it has been proposed that the ability of microbes to metabolize iron has emerged prior to the Archaea/ Bacteria split. However, no unambiguous geochemical data supporting this claim have been put forward in rocks older than 2.7-2.5 giga years (Gyr). In the present work, we report in situ Fe and S isotope composition of pyrite from 3.28-to 3.26-Gyr-old cherts from the upper Mendon Formation, South Africa. We identified three populations of microscopic pyrites showing a wide range of Fe isotope compositions, which cluster around two δ 56 Fe values of −1.8‰ and +1‰. These three pyrite groups can also be distinguished based on the pyrite crystallinity and the S isotope mass-independent signatures. One pyrite group displays poorly crystallized pyrite minerals with positive Δ 33 S values > +3‰, while the other groups display more variable and closer to 0‰ Δ 33 S values with recrystallized pyrite rims. It is worth to note that all the pyrite groups display positive Δ 33 S values in the pyrite core and similar trace element compositions

Provenance and tectonic implications of the 3.28–3.23 Ga Fig Tree Group, central Barberton greenstone belt, South Africa

Sedimentary rocks provide a valuable record of non-uniformitarian Paleoarchean tectonic processes. The rocks of the 3.28–3.23 Ga Mapepe Formation mark the initiation of orogenesis in the Barberton Greenstone Belt, South Africa. However, reconstruction of the Mapepe basin(s) is challenging because the rocks occur in fault-bounded structural belts that are characterized by lateral heterogeneity and whose correlation remains uncertain. We have studied the petrography, geochemistry, and detrital zircon geochronology from outcrop and drill core samples from two adjacent structural belts, the Manzimnyama Syncline and the Eastern Barite Valley area, to evaluate the timing of basin formation, correlate between these two adjacent areas, and provide constraints on the character of tectonic uplift. In the Eastern Barite Valley area sedimentation was initiated at about 3260 Ma. The section includes a lower, deep-water unit of mainly mudstone overlain by a shoaling-upward sequence of fan-delta sandstone and conglomerate. The sandstone throughout the formation was derived by erosion of local uplifts of the underlying greenstone-belt sequence except at the top of the section where sedimentation was dominated by dacitic volcaniclastic sediments dated at 3239 Ma. In contrast, rocks of the Mapepe Formation in the Manzimnyama Syncline area, 1–2 km south of the Eastern Barite Valley area, record the initiation of sedimentation at about 3277 Ma with deposition of a rhyolitic tuffaceous unit followed by persistent deep-water sedimentation of banded iron formation, banded ferruginous chert, and turbiditic sandstone. Siliciclastic sediments are immature and were derived by erosion of older supracrustal greenstone-belt rocks. No Fig Tree age volcanism younger than 3260 Ma is recorded in this belt. Overall, Mapepe sedimentary rocks record the formation of numerous local uplifts of underlying supracrustal rocks with periodic volcanic activity, and the dispersal of debris to form mainly small fan-deltas flanked by deep-water systems. The contrasting ages, lithologies, depositional settings, and provenance of the sedimentary rocks in the now adjacent Eastern Barite Valley and Manzimnyama Syncline belts suggest that they were deposited in either different parts of a large, complex Fig Tree basin or represent separate basins that have been juxtaposed during post-Fig Tree tectonism and shortening

Provenance and tectonic implications of the 3.28–3.23 Ga Fig Tree Group, central Barberton greenstone belt, South Africa

Sedimentary rocks provide a valuable record of non-uniformitarian Paleoarchean tectonic processes. The rocks of the 3.28–3.23 Ga Mapepe Formation mark the initiation of orogenesis in the Barberton Greenstone Belt, South Africa. However, reconstruction of the Mapepe basin(s) is challenging because the rocks occur in fault-bounded structural belts that are characterized by lateral heterogeneity and whose correlation remains uncertain. We have studied the petrography, geochemistry, and detrital zircon geochronology from outcrop and drill core samples from two adjacent structural belts, the Manzimnyama Syncline and the Eastern Barite Valley area, to evaluate the timing of basin formation, correlate between these two adjacent areas, and provide constraints on the character of tectonic uplift. In the Eastern Barite Valley area sedimentation was initiated at about 3260 Ma. The section includes a lower, deep-water unit of mainly mudstone overlain by a shoaling-upward sequence of fan-delta sandstone and conglomerate. The sandstone throughout the formation was derived by erosion of local uplifts of the underlying greenstone-belt sequence except at the top of the section where sedimentation was dominated by dacitic volcaniclastic sediments dated at 3239 Ma. In contrast, rocks of the Mapepe Formation in the Manzimnyama Syncline area, 1–2 km south of the Eastern Barite Valley area, record the initiation of sedimentation at about 3277 Ma with deposition of a rhyolitic tuffaceous unit followed by persistent deep-water sedimentation of banded iron formation, banded ferruginous chert, and turbiditic sandstone. Siliciclastic sediments are immature and were derived by erosion of older supracrustal greenstone-belt rocks. No Fig Tree age volcanism younger than 3260 Ma is recorded in this belt. Overall, Mapepe sedimentary rocks record the formation of numerous local uplifts of underlying supracrustal rocks with periodic volcanic activity, and the dispersal of debris to form mainly small fan-deltas flanked by deep-water systems. The contrasting ages, lithologies, depositional settings, and provenance of the sedimentary rocks in the now adjacent Eastern Barite Valley and Manzimnyama Syncline belts suggest that they were deposited in either different parts of a large, complex Fig Tree basin or represent separate basins that have been juxtaposed during post-Fig Tree tectonism and shortening

Heterogeneous Hadean crust with ambient mantle affinity recorded in detrital zircons of the Green Sandstone Bed, South Africa

The nature of Earth\u27s earliest crust and the processes by which it formed remain major issues in Precambrian geology. Due to the absence of a rock record older than ∼4.02 Ga, the only direct record of the Hadean is from rare detrital zircon and that largely from a single area: the Jack Hills and Mount Narryer region of Western Australia. Here, we report on the geochemistry of Hadean detrital zircons as old as 4.15 Ga from the newly discovered Green Sandstone Bed in the Barberton greenstone belt, South Africa. We demonstrate that the U-Nb-Sc-Yb systematics of the majority of these Hadean zircons show a mantle affinity as seen in zircon from modern plume-type mantle environments and do not resemble zircon from modern continental or oceanic arcs. The zircon trace element compositions furthermore suggest magma compositions ranging from higher temperature, primitive to lower temperature, and more evolved tonalite-trondhjemite-granodiorite (TTG)-like magmas that experienced some reworking of hydrated crust. We propose that the Hadean parental magmas of the Green Sandstone Bed zircons formed from remelting of mafic, mantle-derived crust that experienced some hydrous input during melting but not from the processes seen in modern arc magmatism

Reassessing evidence of Moon–Earth dynamics from tidal bundles at 3.2 Ga (Moodies Group, Barberton Greenstone Belt, South Africa)

Past orbital parameters of the Moon are difficult to reconstruct from geological records because relevant data sets of tidal strata are scarce or incomplete. The sole Archean data point is from the Moodies Group (ca 3.22 Ga) of the Barberton Greenstone Belt, South Africa. From the time‐series analysis of tidal bundles from a well‐exposed subaqueous sand wave of this unit, Eriksson and Simpson (Geology, 28, 831) suggested that the Moon’s anomalistic month at 3.2 Ga was closer to 20 days than the present 27.5 days. This is in apparent accordance with models of orbital mechanics which place the Archean Moon in a closer orbit with a shorter period, resulting in stronger tidal action. Although this study’s detailed geological mapping and section measuring of the site confirmed that the sandstone bed in question is likely a migrating dune, the presence of angular mud clasts, channel‐margin slumps, laterally aggrading channel fills and bidirectional paleocurrents in overlying and underlying beds suggests that this bedform was likely located in a nearshore channel near lower‐intertidal flats and subtidal estuarine bars; it thus carries risk of incomplete preservation. Repeated measurements of foreset thicknesses along the published traverse, measured perpendicular to bedding, failed to show consistent spectral peaks. Larger data sets acquired along traverses measured parallel to bedding along the 20.5 m wide exposure are affected by minor faulting, uneven outcrop weathering, changing illumination, weather, observer bias and show a low reproducibility. The most robust measurements herein confirm the periodicity peak of approximately 14 in the original data of Eriksson and Simpson (Geology, 28, 831). Because laminae may have been eroded, the measurements may represent a lower bound of about 28 lunar days per synodic month. This estimate agrees well with Earth–Moon dynamic models which consider the conservation of angular momentum and place the Archaean Moon in a lower orbit around a faster‐spinning Earth.Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/50110000165

The Sedimentary Geochemistry and Paleoenvironments Project.

Authors thank the donors of The American Chemical Society Petroleum Research Fund for partial support of SGP website development (61017-ND2). EAS is funded by National Science Foundation grant (NSF) EAR-1922966. BGS authors (JE, PW) publish with permission of the Executive Director of the British Geological Survey, UKRI.Publisher PDFPeer reviewe