ABSTRACT The deposits of the upper Neoproterozoic Zerrissene Group of central‐western Namibia represent a large siliciclastic deep‐water depositional system that showcases the intricacies of facies and architectural relationships from bed‐scale to fan‐system‐scale. The lack of vegetation in the Namib Desert and regular east–west repetition of folded stratigraphy (reflecting ca 50% tectonic shortening) provides quasi‐three‐dimensional exposure over a current area of approximately 2700 square kilometres. The Brak River Formation, the middle sand‐rich unit of the Zerrissene Group, consists of nearly 600 m of strata exposed in multiple parallel continuous outcrops up to ca 10 km in length and oriented obliquely to depositional dip. Ten stratigraphic sections are correlated ca 32 km ( ca 64 km restored) across the basin and offer exposure comparable in scale to modern submarine fans. Six sedimentary facies are identified and grouped into four facies associations that represent axial‐to‐marginal portions of deep‐water lobes in an unconfined submarine fan system. Spatial facies patterns, regional thickness variations, and palaeocurrents indicate that Brak River Formation sediments were transported primarily from the north to south–south‐west through a trough‐like basin, and deposited within an unconfined basin plain at the junction of the Adamastor and Khomas oceans. The unique outcrop exposure and extent permits the documentation of system‐scale architecture and basin configuration of the Brak River submarine fan system. A transition from the sand‐rich lower Brak River Formation to more intercalated mudstone‐dominated intervals in the middle and upper Brak River Formation is interpreted to record a change from aggradational to compensational stacking of lobe deposits. This records the evolution of a large submarine fan as it filled the subtle seafloor topography and became less confined at the system‐scale. The documentation of these deep‐water deposits from centimetre‐scale to basin‐scale provides a new model for a system with extensive long‐distance transport of sand‐rich sediment gravity flows to submarine lobes without apparent channelization.

Large meteorite impacts must have strongly affected the habitability of the early Earth. Rocks of the Archean Eon record at least 16 major impact events, involving bolides larger than 10 km in diameter. These impacts probably had severe, albeit temporary, consequences for surface environments. However, their effect on early life is not well understood. Here, we analyze the sedimentology, petrography, and carbon isotope geochemistry of sedimentary rocks across the S2 impact event (37 to 58 km carbonaceous chondrite) forming part of the 3.26 Ga Fig Tree Group, South Africa, to evaluate its environmental effects and biological consequences. The impact initiated 1) a giant tsunami that mixed Fe 2+ -rich deep waters into the Fe 2+ -poor shallow waters and washed debris into coastal areas, 2) heating that caused partial evaporation of surface ocean waters and likely a short-term increase in weathering and erosion on land, and 3) injection of P from vaporization of the S2 bolide. Strata immediately above the S2 impact event contain abundant siderites, which are associated with organic matter and exhibit light and variable δ 13 C carb values. This is consistent with microbial iron cycling in the wake of the impact event. Thus, the S2 impact likely had regional, if not global, positive and negative effects on life. The tsunami, atmospheric heating, and darkness would likely have decimated phototrophic microbes in the shallow water column. However, the biosphere likely recovered rapidly, and, in the medium term, the increase in nutrients and iron likely facilitated microbial blooms, especially of iron-cycling microbes.

The 3.55-3.26 Ga Onverwacht Group, Barberton Greenstone Belt, South Africa and Eswatini, records nearly 300 m.y. of Paleoarchean history dominated by basaltic, komatiitic, and felsic volcanism. It provides the oldest well-preserved record of crustal development and the nature and evolution of associated surface environments, ocean, atmosphere, and biosphere on early Earth. Sedimentary layers within this 10- to 12 -kmthick sequence represent a range of proximal to distal sedimentary environments relative to the centers of volcanism. Proximal deposits include coarse felsic breccias, conglomerates, and sandstones and mafic to komatiitic lapillistones that often show evidence for deposition in relatively shallow water. Distal deposits are composed of fine pyroclastic debris, chemical sediments, and biogenic materials deposited under subaqueous conditions during local volcanic quiescence. They show abundant current -produced features and are interpreted to have formed at water depths of a few hundred meters or less under the influence of tidal and/or ocean -circulation currents. Terrigenous clastic sediments formed by the weathering and erosion of older rocks are essentially absent. The Onverwacht Group was deposited under marine conditions on what appears to have been a water world with little evidence of large land areas and no evidence of active tectonism. It is interpreted to represent the upper part of a Paleoarchean stagnant lid overlying but decoupled from an active mantle. The Barberton Greenstone Belt and other Paleoarchean terranes offer a view of this lid over the past 300 m.y. of its development and during the early stages of its fragmentation and disruption.

The base of the ICS (International Commission on Stratigraphy) Geological Time Scale was ratified in 2022 by defining a new Global Stratigraphic Standard Age (GSSA) for the lower boundary of the Hadean Eon (formerly 4000-3600 Ma); the age of the Solar System based on the oldest solids, calcium-aluminium inclusions (CAIs), generated in the protoplanetary disk. The formal GSSA for the Hadean base is the oldest reliable, weighted mean Ucorrected Pb-Pb age of 4567.30 +/- 0.16 Ma obtained for CAIs in primitive meteorites Allende and Efremovka. This age is supported by the 4568-4567 Ma U-corrected Pb- Pb ages of chondrules in Northwest African meteorites. The boundary sets an upper lifetime for the protoplanetary disk and timing of planet formation. The Hadean Eon encloses the accretion and differentiation of the Earth and other planets, the Moon-forming Giant Impact, the beginning of the suggested Late Heavy Bombardment, and the formation of the Earths' protocrust. Due to the Moon- forming Giant Impact that occurred after the differentiation of the proto-Earth and the fact that Earth's ' s first crust has been destroyed, the age of the planet Earth itself remains an open question. However, many pieces of astronomical, chemical, physical, and chronological evidence point to the very fast formation of the Solar System and rapid accretion and differentiation of the proto-Earth in only a few million years. Compared to the half-billion-year duration of the Hadean, it is reasonable to set the age of the Earth at the beginning of the formation of the Solar System. This communication explains and justifies the selection of the GSSA for the Hadean base.

The eastern Barberton Greenstone Belt (BGB) includes four stratigraphic and structural divisions: from northwest to southeast, the Mlumati, Manzimnyama, and Paulus Synclines and the Emlembe Belt. All are made up largely of sedimentary rocks of the Fig Tree Group separated by antiformal belts of sheared Onverwacht Group komatiitic rocks. Fig Tree rocks in the Mlumati and Manzimnyama Synclines are mostly chemically precipitated banded iron formation (BIF) and banded ferruginous chert (BFC) with a major siliciclastic unit, the Gelegela Grit, composed of quartz-poor (<5% monocrystalline quartz, Qm) volcaniclastic sandstone showing abundant ~3.445–3.455 Ga detrital zircons. The Paulus Syncline is dominated by shale containing in the upper half chert-clast conglomerate and sparse lithic sandstone (Qm<10%) but includes near the middle lenticular units up to unit 30 m thick of quartz-rich (Qm >50%) sandstone. The Emlembe Belt consists largely of chert-clast conglomerate and quartz-bearing (Qm = 10–50%) sandstone. The Paulus and Emlembe belts show detrital zircon age peaks at ~3.295–3.275 Ga and ~3.445–3.455 Ga. While exhibiting overall similar stratigraphic development and detrital zircon ages, Fig Tree rocks in these belts show contrasting compositions and sediment sources. They do not represent parts a single basin or fairway of sediment transport and deposition. Fig Tree siliciclastic rocks mark the first deformation, uplift, and erosion in the BGB. However, the thinness of Fig Tree strata, mostly less than 1000 m, and rapid facies changes argue that deformation involved local uplifts and small basins that accumulated thin sedimentary sequences. We suggest that early Fig Tree deformation is consistent with crustal disruption triggered by large meteor impacts, starting perhaps as early as ~3.277 Ga but certainly by ~3.260 Ga. The Fig Tree Group may record a cluster of impacts that fragmented and destabilized a long-lived crust followed during later or post-Fig Tree time by tectonic uplift and orogeny.

Silicate spherules have been identified from the ca. 3.4 Ga-old Strelley Pool Formation (SPF) in the Pilbara Craton, Western Australia. Their origins and geochemical characteristics, including the Re and platinum-group elements of their host clastic layer and the overlying and underlying microfossil-bearing finely laminated carbonaceous cherts, were examined. The spherules have various morphologies (completely spherical to angular), sizes (∼20 to >500 μm), textures (layered, non-layered, and fibrous), mineralogy (various proportions of microcrystalline quartz, sericite, anatase and Fe-oxides), and chemistry (enriched in Ni and/or Cr), commonly with thin anatase-rich walls. Their host clastic layer is characterized by rip-up clasts, suggesting a suddenly occurring high-energy depositional environment, such as tsunamis. Although various origins other than asteroid impact were considered, none could unequivocally explain the features of the spherules. In contrast, non-layered spherical spherules that occur as individual framework grains or collectively comprise angular-shaped rock fragments appear to be more consistent with the asteroid impact origin. The calculated Re-Os age of the cherts (3331 ± 220 Ma) was consistent with the established age of the SPF (3426-3350 Ma), suggesting that the Re-Os system was not significantly disturbed by later metamorphic and weathering events.

As Mars transitioned from an early Earth-like state to the cold desert planet it is today, it preserved a near pristine record of surface environments in a world without plate tectonics and complex life. The records of Mars' Earth-like surfaces have remained largely untouched for billions of years, allowing space exploration to provide critical insights about the early days of our own planet. Here, we first review what Mars has taught us about volcanic, tectonic and metamorphic processes in the absence of discrete plates, drawing comparisons with the terrestrial and venusian records. Then, we summarize advances in understanding its early surface environments, including impact cratering, hydrological, sedimentary and geochemical processes. Altogether, the martian record provides a picture of early environments that were similar to modern terrestrial ones in many respects, with sediment and geochemical cycling, hydrothermal systems capable of hosting life, but with the exception that topography, sediment and heat sources were provided by volcanoes and impact cratering rather than plate tectonics. Mars thus offers a lens through which one might catch a glimpse of Earth's infancy, provided exploration efforts continue to refine our understanding of the similarities between Earth and Mars as well as the specificities of each planet.

The nature of Earth's earliest crust and crustal processes remain unresolved questions in Precambrian geology. While some hypotheses suggest that plate tectonics began in the Hadean, others suggest that the Hadean was characterized by long‐lived protocrust and an absence of significant plate tectonic processes. Recently proposed trace‐element proxies for the tectono‐magmatic settings in which zircons formed are a relatively novel tool to understand crustal processes in the past. Here, we present high‐spatial resolution zircon trace and rare earth element geochemical data along with Hf and O isotope data of a new location with Hadean materials, 4.1–3.3 Ga detrital zircons from the 3.31 Ga Green Sandstone Bed, Barberton Greenstone Belt. Together, the hafnium isotope and trace element geochemistry of the detrital zircons record a major transition in crustal processes. Zircons older than 3.8 Ga show evidence for isolated, long‐lived protocrust derived by reworking of relatively undepleted mantle sources with limited remelting of surface‐altered material. After 3.8 Ga, Hf isotopic evidence for this protocrust is muted while relatively juvenile source components for the zircon's parental magmas and flux‐like melting signatures become more prominent. This shift mirrors changes in Hf isotopes and trace element geochemistry in other Archean terranes between ∼3.8 and 3.6 Ga and supports the notion that the global onset of pervasive crustal instability and recycling—A possible sign for mobile‐lid tectonics—Occurred in that time period.

The existence of a core dynamo during the first billion years of Earth history is closely related to the thermal state of the Earth's interior and composition of the early atmosphere. The scarcity of well-preserved rock units older than 3.5 billion years (Ga) has motivated the paleomagnetic analysis of detrital zircons. Studies of zircons from Jack Hills, Australia, however, have found the pervasive occurrence of secondary ferromagnetic minerals, casting doubt on the ability of these zircons to record a >3.5Ga geomagnetic field. Here we report paleomagnetic analyses on a set of 19 zircons with crystallization age 3.5-4.0Ga from the Barberton Greenstone Belt (BGB) of South Africa, which have undergone lower grade metamorphism compared to all other known >3.5Ga detrital zircon localities. We find that BGB zircons have magnetic moments nearly one order of magnitude weaker than Jack Hills zircons, precluding the retention of primary paleomagnetic information. This result corroborates findings from the Jack Hills and other Archean zircon populations that primary ferromagnetic inclusions are readily eliminated from zircons during sedimentary transport and metamorphism, likely facilitated by radiation damage-induced permeability. Paleomagnetic determination of geodynamo activity prior to 3.5 Ga may require investigation of other detrital grains with lower radiation damage potential or whole-rock samples that have escaped high degree metamorphism. (C) 2021 Elsevier B.V. All rights reserved.

One of the major challenges in early Earth geology is the interpretation of the nature of the crust and tectonic processes due to the limited exposures of Archean rocks. This question is predominantly addressed by numerical modeling, structural geology, geochemical analyses, and petrological approaches. Here we report on the reconstruction of one of the oldest, well-preserved volcano-sedimentary sequences on Earth, the 3.28-3.22 Ga Fig Tree Group in the Barberton Greenstone Belt, South Africa, based on geochronology, provenance, and stratigraphy to provide new constraints on the nature of tectonic processes in the Archean. The Fig Tree basin was asymmetric and the onset of deposition varied across the greenstone belt. The Fig Tree Group is now preserved in east-west oriented bands of fault-bounded structural belts with those preserved in the southern parts of the greenstone belt showing an onset of deposition at 3.28 Ga, those in the center at 3.26 Ga, and those in the north at 3.24 Ga. Stratigraphically, the rocks display a general up-section trend from deeper to shallower-water deposition and/or from finer-to coarser-grained sedimentary rocks. Associated with this up-section stratigraphic trend, the sedimentary rocks show a change in provenance from more regionally similar to more locally variable, and an increase in felsic volcanic activity, especially toward the closure of Fig Tree deposition. The data is consistent with formation of the Fig Tree Group in a compressional tectonic setting by deposition in a foreland basin that experienced progressive accretion of crustal terranes onto a northward prograding fold-and thrust belt.

The 3.6-3.2 Ga Barberton Greenstone Belt, South Africa, is a complex terrain divided into multiple structural blocks. The structural deformation and poor geochronological constraints often make correlation among blocks difficult. To overcome structural complexities, provenance proxies including sandstone petrography, shale geochemistry, and detrital zircon geochronology are here used to compare source terrain signatures among multiple structural blocks in the East-Central Domain of the Barberton Greenstone Belt. This will expand our understanding of Paleoarchean paleogeography and the nature of crustal uplift in the southeastern part of the Barberton Greenstone Belt. The Manzimnyama and Mlumati Synclines are composed of 600 m-thick sections of the Fig Tree Group, Mapepe Formation. These synclines have high sedimentary and volcanic lithic contents in sandstones, an upward transition from felsic to mafic signatures in shales, and a detrital zircon age distribution transition from a basal member with a main peak at 3.28 Ga to members with a peak at about 3.45 Ga. There are three other EastCentral Domain, fault-bounded blocks made up mostly of Mapepe Formation strata. The Eastern Barite Valley lies northwest of the Manzimnyama Syncline. From older to younger strata, the Eastern Barite Valley progresses from high lithics to more quartz and feldspar in sandstones, from more mafic to felsic signatures in shales, and from a unimodal 3.45 Ga detrital zircon signature at the base to complexly mixed age distributions with a strong 3.24 Ga peak at the top. The Paulus Syncline, southeast of the Manzimnyama Syncline, has a high chert content, mafic to ultramafic geochemistry signatures, and a main detrital zircon peak at 3.28-3.30 Ga with a minor peak at 3.45 Ga. The Emlembe Syncline, southeast of the Paulus Syncline, has a high monocrystalline quartz content, mixed geochemistry signatures, and unimodal 3.28-3.30 Ga detrital zircon ages. The proximity and sedimentology of these structural blocks could suggest deposition in a single basin; however, contrasts in provenance suggests sediments were deposited in different basins, at different times, or in one basin with provenance partitioning. The blocks likely had locally different sources, implying the sequences were not part of a single large source, transport, and depositional fairway.

Supplemental File S1: Additional figures; Supplemental File S2: List of dated tuffs of the Fig Tree Group; Supplemental File S3: U-Pb geochronological data; Supplemental File S4: Mudstone geochemical data.

Significance The nature of Earth’s earliest crust is enigmatic due to the lack of a rock record for most of Earth’s first ∼600 My, the Hadean Eon. Studies have thus turned to scarce sites where Hadean detrital zircons have been discovered. The geochemistry of Hadean detrital zircon from a newly discovered site in South Africa suggests that the parental melts formed from variably hydrous melting of crust derived from the ambient mantle and show little evidence for an origin in arc-like settings. These results suggest that crust derived from ambient mantle played an important role during crust formation in the Hadean.

In the absence of a rock record older than about 4.03 Ga, Hadean (>4.0 billion-years-old) zircons provide a unique window into crustal development and surface conditions on the Earth within a few hundred million years of its formation. Recently discovered Hadean detrital zircons, 4.0-4.2 Ga, in the Barberton Greenstone Belt, South Africa, occur in a 3 to 5 m thick layer termed the Green Sandstone Bed (GSB) that is composed largely of mafic to komatiitic volcaniclastic debris and trace amounts of sand-sized quartz, chromite, and zircon. The youngest concordant zircons in the GSB and ages of zircons in immediately underlying strata suggest a depositional age of about 3.306 +/- 6 Ga. Hadean zircons are concentrated in the lower 200 cm of the unit: higher parts of the GSB lack Hadean zircons and the uppermost 150 cm contain zircons 3.312 +/- 5 Ga. The quartz and heavy minerals in the lowest 200 cm of the GSB appear to have been transported as windblown sediments in a subaerial environment. They were subsequently incorporated into mass flows represented locally by the lowest 200 cm of the GSB. The bulk of the overlying GSB sediments were worked and deposited in wave- and current-active, shallow-marine to intertidal settings. Cathodoluminescence studies suggest that a high proportion of the detrital GSB quartz is of plutonic origin although the GSB is underlain by 10-12 km of volcanic and volcaniclastic silicified sedimentary rocks with no evidence of uplift or terrigenous sediment sources. The GSB lies <2 m above impact layer S6 and we infer that much of the windblown sediment was derived by erosion of crustal rocks that were uplifted and emplaced at the surface during the S6 impact and of komatiites exposed subaerially between the impact and depositional sites.

Recent studies have suggested that similar to 3.445 Ga felsic volcaniclastic rocks of member H6 of the Hooggeneoeg Formation in the Barberton Greenstone Belt, South Africa, show evidence for deposition above a major erosional unconformity under glacial to periglacial conditions and contain debris eroded from older, granitic continental crust. These conclusions represent fundamental re-interpretations of this sequence, its crustal setting, and processes of deposition. We here present evidence that these felsic strata represent terrestrial to shallow water mass flows deposits, storm-influenced shallow-marine units, and turbidity-current-like, storm-driven submarine flows, all of which lack any evidence of a glacial influence during sedimentation. They accumulated as part of a conformable stratigraphic sequence and show no evidence of mantling an erosional surface. The debris consists entirely of volcanic-derived materials. No clasts of plutonic rock are found in the conglomerates. Low-temperature diagenesis and potash metasomatism have altered primary volcanic feldspar in the volcaniclastic debris to tartan twinned microcline, previously thought to be characteristic of granitic plutonic rocks. This potassium feldspar is quite distinctive and unlike the microcline found in plutons surrounding the greenstone belt. This succession is part of a classic felsic volcaniclastic sequence deposited as an apron flanking a major felsic volcanic center.

ABSTRACT Studies of Earth's surface temperature before 3.0 Ga have focused heavily on the oxygen isotopic composition of silica-rich sedimentary rocks called cherts. Interpretation of the results have suggested early surface temperatures ranging from as high as 70 ± 15 °C down to those that differ little from modern values. A major controversy centers on whether differences in the oxygen isotopic compositions of cherts over time reflect changing surface temperatures, changing ocean isotopic composition, or post-depositional diagenetic and metamorphic effects. We here present results of triple oxygen measurements of 3.472 Ga to 3.239 Ga cherts from the Barberton Greenstone Belt, South Africa. The best preserved samples based on geological evidence have Δ'17O and δ'18O values that plot generally on or near the equilibrium fractionation line for silica precipitated out of modern, ice-free sea water. Geologic considerations allow many potentially useful samples to be eliminated for paleotemperature analysis because of proximity to younger mafic intrusions or interactions with meteoric waters during deposition, both of which tend to lower preserved isotopic values. Our results of triple-O isotopic analyses of a suite of samples representing deposition under open marine, shallow shelf conditions suggest that Archean surface temperatures were well above those of the present day, perhaps as high as 66 to 76 °C. They demonstrate that geologic context, including depositional setting and post-depositional history, requires careful assessment before the significance of oxygen isotopic results can be evaluated.