Kenneth A. Eriksson

Geochemistry of shales from the Archean (~3.0 Ga) Buhwa Greenstone Belt, Zimbabwe: Implications for provenance and source-area weathering

Abstract Phyllites from the Archean (~3.0 Ga) Buhwa Greenstone Belt, Zimbabwe, were deposited on a stable cratonic platform. Analyses of the phyllites generally define a single geochemical group based on major-and trace-element abundances. The phyllites are strongly depleted in CaO, Na2O, and Sr with respect to average Archean upper crust. By contrast, K2O, Ba, and Rb are enriched several times relative to average Archean upper crust, reflecting basin-scale K metasomatism. Transition metals are somewhat depleted, whereas high field strength elements are typically enriched relative to average Archean upper crust. Samples generally have fractionated LREE patterns (average CeN/SmN = 2.4) with small negative Eu anomalies (average Eu/Eu* = 0.78) and generally flat HREEs (average GdN/ YbN = 1.3). Such geochemical characteristics suggest that the source dominantly consisted of tonalite with less common occurrences mafic volcanic rocks and granite. Mixing calculations, which massbalance the REEs and Th/Sc, suggest 70% tonalite, 15% mafic volcanic rocks, and 15% granite as possible proportions for the source of the phyllites. A nearby early Archean (~3.5 Ga) continental nucleus preserves similar lithologies and is the probable source. Intense chemical weathering of the source terrane is indicated by premetasomatized chemical index of alteration values of 95–100, nearly complete depletion of CaO and Na2O, and high Al2O3/Na2O ratios. Fine-grained sediments of comparable age and presumed tectonic setting elsewhere in southern Africa show similar geochemical characteristics, implying that source-area compositions and weathering intensities were similar. These mature platformal deposits suggest a tectonically stable environment where intense chemical weathering took place at ~3.0 Ga across southern Africa.

Geochemistry of Archean shales from the Pilbara Supergroup, Western Australia

Archean clastic sedimentary rocks are well exposed in the Pilbara Block of Western Australia. Shales from turbidites in the Gorge Creek Group (ca. 3.4 Ae) and shales from the Whim Creek Group (ca. 2.7 Ae) have been examined. The Gorge Creek Group samples, characterized by muscovite-quartzchlorite mineralogy, are enriched in incompatible elements (K, Th, U, LREE) by factors of about two, when compared to younger Archean shales from the Yilgarn Block. Alkali and alkaline earth elements are depleted in a systematic fashion, according to size, when compared with an estimate of Archean upper crust abundances. This depletion is less notable in the Whim Creek Group. Such a pattern indicates the source of these rocks underwent a rather severe episode of weathering. The Gorge Creek Group also has fairly high B content (85 ± 29 ppm) which may indicate normal marine conditions during deposition. Rare earth element (REE) patterns for the Pilbara samples are characterized by light REE enrichment (LaNYbN ≥ 7.5) and no or very slight Eu depletion (EuEu∗ = 0.82 – 0.99). A source comprised of about 80% felsic igneous rocks without large negative Eu-anomalies (felsic volcanics, tonalites, trondhjemites) and 20% mafic-ultramafic volcanics is indicated by the trace element data. Very high abundances of Cr and Ni cannot be explained by any reasonable provenance model and a secondary enrichment process is called for.

A new window into Early Archean life: Microbial mats in Earth's oldest siliciclastic tidal deposits (3.2 Ga Moodies Group, South Africa)

Newly discovered sedimentary structures produced by ancient microbial mats in Early Archean sandstones of the 3.2 Ga Moodies Group, South Africa, differ fundamentally in appearance and genesis from Early Archean stromatolites and bacterial cell fossils preserved in chert. Wrinkle structures, desiccation cracks, and roll-up structures record the previous existence of microbial mats that effectively stabilized sediment on the earliest known siliciclastic tidal flats. In thin-section, the sedimentary structures reveal carpet-like, laminated fabrics characteristic of microbial mats. Negative d 13 C isotope ratios of 220.1 to 221.5 6 0.2‰ are consistent with a biological origin for the carbon preserved in laminae. The biogenicity of the sedimentary structures in the Moodies Group is substantiated by comparative studies on identical mat-related features from similar tidal habitats throughout Earth history, including the present day. This study suggests that siliciclastic tidal-flat settings have been the habitat of thriving microbial ecosystems for at least 3.2 billion years. Independently of controversial silicified microfossils and stromatolites, the newly detected microbially induced sedimentary structures in sandstone support the presence of bacterial life in the Early Archean.

Rare earth element patterns in Archean high-grade metasediments and their tectonic significance

Abstract Metasediments from two contrasting types of Archean high-grade terrains are interpreted as being derived from distinct tectonic settings. The Kapuskasing Structural Zone, Canada, represents the deep roots of a typical greenstone belt, whereas the Limpopo Province, southern Africa and Western Gneiss Terrain, Australia, mainly consist of shelf sediments deposited on a granitic basement and then buried to the depths required for granulite fades metamorphism. Upper amphibolite to granulite fades paragneisses from the Kapuskasing Structural Zone have REE patterns similar to those of greenstone belt sediments, except where partial melting has occurred, forming restites with Eu enrichment and melts with Eu depletion. Except in this latter instance, metamorphism has not affected the primary REE patterns. REE patterns in Archean upper amphibolite to granulite facies metasediments from the central Limpopo Province and Western Gneiss Terrain show wide differences, ranging from patterns which resemble those in post-Archean terrigenous sediments, to typical Archean sedimentary rock patterns. The diversity in REE patterns for these shallow shelf metasediments is interpreted as resulting from derivation from local provenances. Samples with “post-Archean” patterns, displaying Eu depletion, are interpreted as being derived from K-rich granitic plutons which were portions of small, stable early Archean terrains, precursors of the widespread late Archean-Proterozoic episode of major cratonic development.

Quantifying the Timing and Rate of Crustal Evolution: Global Compilation of Radiometrically Dated Detrital Zircon Grains

Multiple models (steady state, episodic, and early growth followed by crustal reworking) have been postulated to explain the evolution of Earth’s continental crust. An independent assessment of these models is now possible as a result of the massive numbers of detrital zircon grains that have been dated individually over recent years using U-Pb and Hf isotopes. A compilation of ∼200,000 published zircon dates reveals a global age distribution (U-Pb) with six prominent, statistically significant peaks: 3.2–3.0, 2.7–2.5, 2.0–1.7, 1.2–1.0, 0.7–0.5, and 0.3–0.1 Ga. In most cases, these peaks are recorded on all seven continents. Most of the peaks are detectable also in modern sediments. A comparison of grains with both U-Pb ages and Hf isotope model ages, available for ∼5100 detrital zircons, suggests that the age frequency distribution of detrital grains reflects predominantly episodic crustal recycling rather than crustal growth. The zircon age distributions and associated data provide consistent evidence for the episodic nature of the Earth’s plate tectonics.

Sequence architecture, depositional systems, and controls on development of lacustrine basin fills in part of the Erlian basin, northeast China

Sequence architecture and depositional systems of a Jurassic-Cretaceous lacustrine rift succession in the Wuliyashitai subbasin, and Erlian basin in northeastern China were investigated using seismic profiles, complemented by well logs and cores. Five second-order or composite sequences are identified on the basis of basin-scale unconformities. Constituent third-order sequences are defined by unconformities along the basin margins and correlative conformities within the central basin. Lowstand-transgressive and highstand systems tracts are separated by major lake flooding surfaces. Depositional systems identified in the basin include alluvial fan, incised valley, fan delta, braid delta, and sublacustrine fan. Three types of lacustrine sequences, consisting of distinctive depositional systems, formed during different tectonic stages: (1) alluvial-shallow-lacustrine sequences (type A) developed during the initial rifting stage; (2) shallow-lacustrine to deep-lacustrine sequences (type B) formed during the early and late periods of rifting; and (3) deep-lacustrine sequences (type C) developed in response to rapid tectonic subsidence during the middle, maximum-rifting stage. Uplift following each rifting episode related to changes in the paleostress field appears to have been the major control on the formation of high-order sequence boundaries (second-order and some third-order sequences). Block rotation, coupled with lake level fluctuations, controlled the formation of most third-order sequence boundaries. Forward simulation modeling suggests that development of different lacustrine sequence types can be related to a balance between tectonic subsidence and sediment input. (Begin page 2018) Deltaic sandstone bodies and incised valley fills along the hinged margins within the shallow-lacustrine and deep-lacustrine sequences and sublacustrine-fan sandstones in the central basin are the economically most important reservoir sandstones in the basin. Fan-delta deposits along the escarpment margin constitute another potential reservoir type.

Muddy Roll-up Structures in Siliciclastic Interdune Beds of the c. 1.8 Ga Waterberg Group, South Africa

Abstract Concentrically rolled-up silty mudrock laminae, 1–2 mm thick, are found in the uppermost of four, 40–110 cm thick interdune beds within a thick aeolian succession of the c. 1.8 Ga Makgabeng Formation, Waterberg Group, South Africa. These curved laminae are analogous to previously described “roll-ups,” biogenic structures generally ascribed to soft-sediment deformation or desiccation of microbial mats overlying either carbonate or siliciclastic sediments, within shallow- to deep-marine paleoenvironments. The Makgabeng roll-ups are thought to reflect desiccation of a microbial mat, followed by resedimentation of cohesive, discrete, curled mat fragments. Their alignment is considered to be the result of an extreme precipitation event. The significance of the South African example of roll-ups is that they appear to have formed within a fully terrestrial paleoenvironment, in one of the oldest known Precambrian deserts. Thus, they represent the oldest evidence for microbial colonisation of a terrestrial setting.

Marginal marine depositional processes from the Archaean Moodies Group, Barberton Mountain Land, South Africa: Evidence and significance

Abstract The Moodies Group, which is approximately 3300 Ma in age, contains the oldest recognizable sediments of marginal marine origin. These accumulated in estuarine-deltaic, barrier beach and back-barrier depositional environments. A paleogeographic model recognizes the interplay of different shallow marine processes and the spatial relationships of the depositional environments. Delta plain sediments consist of tidal channel and estuarine tidal flat deposits. Elongate sand shoals, oriented perpendicular to the palaeoshoreline, fronted the delta plain and indicate that sedimentation occurred under macrotidal conditions in a non-barred estuary. Rapid gravity flow sedimentation on the delta front resulted in ubiquitous soft-sediment deformation and water escape. Longshore reworking of riverborne sediments led to the development of barrier island and back-barrier deposits lateral to the delta. Chemical and suspension sedimentation predominated in the offshore, and low-energy bedload and storm-surge sedimentation in the nearshore depositional environments. Shoreface sediments display widespread evidence of rip current activity and were succeeded shorewards by tidal inlet and barrier-spit deposition. Tidal flat as well as flood tidal delta and washover fan sediments accumulated behind the barrier island. Spit, washover fan and abundant rip current deposits, coupled with the absence of extensive ebb tidal delta accumulations, are indicative of a microtidal coastline. The coeval existence of micro- and macrotidal conditions is attributed to irregularities along the coastline and/or variations in width of the continental shelf. While providing no new clues as to the original composition of the earths crust, the nature of the Moodies sediments indicates the existence of widespread exposed granitic terrains by 3300 Ma. In addition, the abundance of marginal marine orthoquartzitic sandstones in the Moodies Group suggests a relatively stable crust on which extensive reworking of sediments occurred. The proposed micro- and only local macrotidal ranges, however, imply a fairly narrow shelf during deposition of the Moodies Group. The conformably underlying Fig Tree sediments, which are considered to be a deep-water facies equivalent of the Moodies Group, are further indicative of a narrow shelf. The above criteria have been used to suggest that the Moodies and Fig Tree Groups accumulated along an ancient continental margin. Marginal marine facies are apparently lacking in other Archaean terrains, most notably in Canada; rather alluvial and turbidite sediments occur and were deposited in localized graben-like basins. These two styles of Archaean sedimentation may reflect responses to different stages of continental rifting and spreading.

Quantifying the oldest tidal record: The 3.2 Ga Moodies Group, Barberton Greenstone Belt, South Africa

The 3.2 Ga Moodies Group in the Barberton Greenstone Belt, South Africa, contains the oldest preserved record of tides. The tidal record is preserved in a tidal sand-wave deposit in the lower Moodies Group as bundles of sandstone foresets separated by mudstone drapes. Detailed analysis of rhythmic foreset bundles permits quantification of the tidal record and reveals a hierarchy of diurnal, fortnightly, and monthly tidal periodicities. Thick-thin pairs of foreset bundles reflect deposition from semidiurnal dominant and subordinate flood-tidal currents, respectively. Cyclic variations in foreset bundle thicknesses record longer period changes in strength of the dominant semidiurnal tidal currents consistent with neap-spring-neap tidal cyclicity. Alternating thicker and thinner neap-spring-neap cycles are comparable to anomalistic, perigean-apogean tidal signatures. This quantitative record of tides in the middle Archean Moodies Group represents, by 2.2 b.y., the oldest such documentation. Tidal cyclicity recognized in the Moodies sand-wave deposit is comparable to that recorded in modern tidal settings and identified in the Carboniferous rock record and is most compatible with a lunar orbital shape similar to that existing today.

Early Archean foredeep sedimentation related to crustal shortening: a reinterpretation of the Barberton Sequence, Southern Africa

Abstract The upper, dominantly terrigenous sedimentary successions of the Early Archean Barberton Sequence, Kaapvaal Province, record an evolution similar to that of Phanerozoic foredeeps and may represent the oldest known foredeep sequence, rather than a passive continental-margin sequence as previously proposed. Pre-foredeep ocean crust, the 3.5 Ga-old Onverwacht Group, consists of bimodal volcanics with intercalated shallow-water sediments. Gravity modeling and boundary shear zones indicate that deformed Onverwacht crust was obducted northward onto sialic continental crust immediately before, during or after burial by the synorogenic terrigenous clastic fill. Foredeep basin deepening due to crustal loading from the south is recorded by basinal iron-formation, chert, and mudstone of the basal Fig Tree Group which accumulated paraconformably on the Onverwacht. Overlying sediments of the Fig Tree Group consist of ~ 2 km of submarine-fan greywackes and mudstones. Shoaling of the basin is indicated by the transition to the overlying braided-alluvial and shallow-marine sediments of the ca. 3.3 Ga-old Moodies Group (~ 3 km thick). Changing REE patterns and relative abundances of Ni, Cr, Sr, Th and Ur in the sediments of the Fig Tree and Moodies Groups indicate a significant ultramafic-mafic volcanic component diluted upward in the stratigraphic sequence by an increasing granitic contribution. A similar trend is reflected in sandstone petrography, in which the ratio of extrusive to intrusive rock fragments decreases upwards in the Fig Tree Group whereas the Moodies Group sediments are enriched in quartz and potassium feldspar. A persistent chert component in both units suggests that the tectonic setting of the basin is analogous to that of Phanerozoic recycled orogens. The distribution of facies in the Fig Tree and Moodies Groups, and paleocurrent, petrographic and geochemical data, indicate progressive unroofing of a southerly source terrane consisting of the Onverwacht volcanics and intercalated sediments, and the 3.5-3.2 Ga-old Ancient Gneiss Complex (AGC). In addition, older components of the Fig Tree Group were recycled into overlying stratigraphie units. The AGC consists of mantle-derived 3.5 Ga-old bimodal metavolcanics and metasediments, a tonalite-gneiss batholith, metabasite dikes, and meta-anorthosites. Its structures record progressive northwestward uplift, beginning around 3.4 Ga ago. Intense homogeneous ductile strains were overprinted by northwest verging major folds rotated by large-scale northwestward simple shear, followed by imbricate faulting and pseudotachylite formation. By 3.3 Ga ago the Fig Tree and Moodies Groups had also shortened, with similar kinematics to those of the AGC, by (partly syndepositional) folding and volume loss during the formation of transecting cleavage. Further folding and slide formation followed during subsolidus diapiric rise of the initially tabular Kaap Valley tonalite by 3.2 Ga and emplacement of trondhjemite plutons from 3.3 to 3.0 Ga ago.