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Downhole nuclear magnetic resonance technology was applied in four boreholes intersecting glaciomarine sediments of the Ottawa Valley, Ontario. The study evaluated the ability of slim‐hole nuclear magnetic resonance tools to measure in situ volumetric water contents (porosities in saturated sediments) for geohazard and hydrogeological applications. The sediments are composed of clay‐ and silt‐sized grains of glacially eroded rock flour derived from the Precambrian Shield containing trace amounts of magnetic minerals, and porosities ranging from 40 to 74 porosity units (PU, 1% porosity = 1 PU). Two nuclear magnetic resonance instruments were deployed with echo times of 0.5 and 1.0 ms, and diameters of investigation ranging from 14.0 to 30.5 cm. Quantitative nuclear magnetic resonance porosities in the sediments were typically within ±5 PU (95% within ±10 PU) of core calibration data sets in the silty clays where threshold bulk magnetic susceptibility values were <30 × 10−4 SI. This was found to deviate, however, where the concentration and mineralogy of magnetic particles changed, interpreted to be shortening relaxation times which led to underestimation of true water contents. This effect is correlated with a change in depth from magnetite to superparamagnetic nanoparticles of greigite (low‐temperature iron monosulphide) magnetism, interpreted to occur at the sulphate‐methane interface. Clay mineralogy and pore water chemistry were also examined as contributing factors, but were not found to significantly shorten nuclear magnetic resonance relaxation responses. Very short T2 times (<2 ms) are typical in these particular silty clays, requiring a tool with an echo spacing of <1.0 ms. Due to increased potential for sediment disturbance around a well caused by the geotechnical sensitivity of the sediments, a nuclear magnetic resonance instrument with multiple frequencies provided signal penetration out to various diameters around the tool, giving needed information about the size of the disturbed zone surrounding the casing. [ABSTRACT FROM AUTHOR]

Correlations within and between Precambrian basins are heavily reliant on precise dating of volcanic units (i.e., tuff beds and lava flows) in the absence of biostratigraphy. However, felsic tuffs and lavas are rare or absent in many basins, and direct age determinations of Precambrian basaltic lavas have proven to be challenging. In this paper, we report the first successful application of 40Ar/39Ar dating to pyroxene from a Neoproterozoic basalt unit, the Keene Basalt in the Officer Basin of central Australia. 40Ar/39Ar analyses of igneous pyroxene crystals yielded an age of 752 ± 4 Ma (mean squared weighted deviation = 0.69, p = 72%), which is underpinned by 40Ar/39Ar plagioclase age (753.04 ± 0.84 Ma) from the basalt. This age is significant because the Keene Basalt is one of the very few extrusive igneous rocks identified within the Neoproterozoic successions of central Australia and is potentially an important time marker for correlating the Neoproterozoic stratigraphy within, and beyond, the central Australian basins. Our geochronological and geochemical data show that the Keene Basalt, which is characterized by enriched elemental and Nd‐Pb isotopic signatures, is strikingly similar to, and coeval with, the 755 ± 3 Ma Mundine Well Dolerite in northwestern Australia. Here we suggest that both are part of the same large igneous province (~6.5 × 105 km2) related to breakup of the supercontinent Rodinia. This study demonstrates the potential of pyroxene 40Ar/39Ar geochronology to date ancient flood basalts and to provide pivotal time constraints for stratigraphic correlations of Precambrian basins. Plain Language Summary: Ancient basaltic lavas erupted on Earth's crust hold critical clues about the tectonic and igneous history of geologic past and are important for correlating Precambrian stratigraphy and reconstructing supercontinents. However, constraining the crystallization age of basaltic lavas proves challenging due to the dearth of commonly used uranium‐lead geochronometers such as zircon in such rocks. In this research, we applied 40Ar/39Ar dating of pyroxene and plagioclase to establish the eruption age of a critical basalt lava suite, the Keene Basalt, in the Neoproterozoic Officer Basin in central Australia. The new age, combined with geochemical constraints, contributes to conclusively defining the genetic affinity and geodynamic setting of the basalt unit. Application of pyroxene 40Ar/39Ar dating to terrestrial basalts represents an important technical breakthrough, and we report the first successful result for a Precambrian basalt. The robustly dated Keene Basalt is an important time marker for correlating the Neoproterozoic successions and associated glacial deposits in Australia and worldwide. The new age and geochemistry of the basalt are used to test its relationship to known Neoproterozoic igneous events. Our data, integrated with existing evidence, reveals a 755‐ to 750‐Ma large igneous province related to Rodinia breakup. Key Points: We report the first application of pyroxene 40Ar/39Ar dating to a Precambrian basaltic lava, the Keene Basalt in central AustraliaThe Keene Basalt is correlated with the Mundine Well Dolerite, both being part of a 755‐ to 750‐Ma LIPThe robustly dated Keene Basalt is an important time marker for correlating Neoproterozoic successions in Australia and worldwide [ABSTRACT FROM AUTHOR]

Highlights • Three Precambrian phases of igneous activities in southern Tarim identified. • Tectonic evolution of Tarim from breakup of Columbia to assemblage of Rodinia. • Precambrian basement of Tarim was composed by two distinct terranes. Abstract The Tarim block in NW China consists of a Precambrian basement and late Neoproterozoic to Paleozoic cover sequences. In order to better understand its roles in Precambrian supercontinent evolution, we used SHRIMP and LA-ICPMS U-Pb methods to date late Paleoproterozoic rifting-related mafic dykes and Mesoproterozoic igneous activity in the Precambrian of the southwestern Tarim terrane (STT): 1) WNW trending mafic dyke swarm emplaced at ca. 1785 Ma; 2) Mesoproterozoic bimodal volcanic sequence (the lower member) and marble- clastic-quartzite sequences (the upper member) deposited at ca. 1525 Ma and 1500–1400 Ma, respectively; 3) late Mesoproterozoic granodiorite-augen granite-leucogranite complex emplaced at ca. 1117 Ma. Geochemistry revealed that the 1784 ± 12 Ma mafic dykes were derived from an E-MORB – like mantle source suggesting an extensional setting, and likely linked with coeval major large igneous provinces (LIPs) on several crustal blocks which collectively suggest a pulse of rifting associated with assembly of the Columbia supercontinent. The obtained U-Pb age of 1525 ± 4 Ma from a meta-rhyolite in the bimodal sequence matches a recently discovered 1525–1529 Ma Large Igneous Province from the southern West African craton (Essakane-Sambarabougou swarm) and formerly adjacent Amazonian craton (Kayser event). Geochemistry of the ca. 1117 Ma granitic intrusions defines their A-type signature. These may be linked to coeval intraplate mafic magmatism of a reconstructed 1110 Ma LIP involving Kalahari, Congo, India (Bundelkhand block), Amazonia cratons. In line with these three newly recognized pulses of Precambrian igneous activities in STT and the significant diachroneity of the Precambrian tectono-magnetic events between the STT and the Northern Tarim terrane (NTT), we argue that the Precambrian basement of the Tarim were most likely composed of two distinct terranes (i.e., the STT and the NTT). These originated from different Precambrian nuclei and they did not amalgamate as the Tarim block until to the early middle Neoproterozoic. [ABSTRACT FROM AUTHOR]

Abstract The Paraná Basin, one of the most important Brazilian sedimentary basins, was created during the formation of Western Gondwana. Until recently, its tectonic framework has been studied mainly through indirect studies using geophysical data such as gravity, magnetic, as well as seismic reflection, and bore hole data. However, the last years, new geophysical data with better resolution became available, enabling new studies on the Brazilian sedimentary basins. To this end, the objective of this work is to contribute with new information on regarding the central and northern areas of the Parana basin and to investigate the major tectonic structures of its basement, as to understand the role of the Precambrian basement structures on its reactivation and geodynamics. For this purpose, quantitative and qualitative analysis of gravity and magnetic data were conducted using the Matched-Filter technique associated with enhancement filters, such as Total Horizontal Gradient and Tilt Derivative of the Total Horizontal Gradient. These geophysical products allowed interpreting the main geophysical lineaments and their depth. We identified structural patterns related to basement reactivation associated with the evolution of Western Gondwana. The NNE-SSW trending structures are related to reactivations of the Transbrasiliano and Paraíba do Sul Strike-slip Systems in the Late Ordovician. The NW-SE trending lineaments were generated with the tectonic growth of the Western Gondwana in the Triassic to the Late Jurassic, which started in the region of the Ponta Grossa Arch. This event was followed by the intracontinental magmatism in the Early Cretaceous. The E-W trending structures are related to the drifting of the South American Plate in the Late Cretaceous-Neogene and the faults originated from it, forming dextral binaries inside the plate pertaining to the Neotectonic Strike-slip System. The NNE-SSW, NE-SW, and NW-SE trending lineaments close to the border between the Paranapanema Block, the Goiano Domain, and the Araguaia, Paraguay, Brasília and Ribeira Belts are associated with suture zones. Thus, this work provides additional information related to the continuity of fault systems and crustal domain limits, as well as the extent of the crustal blocks in the basement of the Paraná Basin. It highlights the reactivation influence of the structures inherited from the Precambrian basement based on the results from both, gravity and magnetic data. Highlights • Magnetic and gravity data reveled tectonic framework of the study area. • The Matched-filter technique results allowed interpreting the tectonic lineaments and domains at different crustal depths. • This work unveiled the continuity of the outcropping fault systems in the basement of the Paraná Basin. • New information on the boundaries of the Paranapanema Block and Brasiliano Orogenic Systems are presented. [ABSTRACT FROM AUTHOR]

In the central part of the semi-arid Dargol Basin of southwestern Niger, most of the groundwater resource is contained in the fractured aquifers of the Precambrian basement. The groundwater resource is poorly characterized and this study is the first attempt to better describe the recharge mechanisms and hydrogeochemical behaviour of the aquifers. Hydrogeochemical and piezometric methods were combined to determine changes in recharge rate and origin of groundwaters for the shallow weathered aquifer and the deep fissured/fractured aquifer. At the basin scale, the groundwater fluxes towards the Niger River are influenced mainly by topography, with no visual long-term trend in groundwater levels (1980-2009). The hydro-geochemical signature is dominated by the calcic-bicarbonate to magnesian (70%) type. It shows evolution from an open environment with CO2 and low mineralized water (granitoids, alterites) towards a more confined environment with more mineralized waters (schists). Stable water isotopes (δ18O, δ2H) analysis suggests two main groundwater recharge mechanisms: (1) direct recharge with nearly no post-rainfall fractionation signature and (2) indirect recharge from evaporated surface waters and/or stream-channel beds. Groundwater tritium content indicates that recharge is mostly recent, with an age less than 50 years (3H > 3 TU), with only 10% indicating low or even no recharge for the past decades. A median value of the groundwater renewal rate estimated from individual values of tritium is equivalent to 1.3% year−1, close to the one determined for groundwater samples dating to the early 1980s, thus indicating no measurable long-term change. [ABSTRACT FROM AUTHOR]

The results of U–Pb dating of detrital zircons extracted from rocks of the Ai Formation are presented. A provenance-signal of a local source with an age of about 2.07 Ga has been documented, and the effect of the Ordovician magmatic episode on the sedimentary sequence has been revealed. [ABSTRACT FROM AUTHOR]

The results of U–Pb-isotopic dating of detrital zircons from sandstones of the Proterozoic Pogor'uy Formation, occurring to the north and south of the East Angara block of Yenisei Ridge, are presented. The sandstones of the northern part of the East Angara block are dominated by Archean and Paleoproterozoic populations of detrital zircons, while the samples from the southern part of the East Angara block have been revealed up to 25% of Mesoproterozoic age grains. The detrital zircon ages allow limiting the maximum sedimentary age of the Pogor'uy Fm. and the entire interval of its formation to 1.2–0.9 Ga. The paleogeographic features of the Pogor'uy Fm. deposition in the context of the existing opinions on the location of Siberia within the Rodinia supercontinent were examined. [ABSTRACT FROM AUTHOR]

The Kushk Series is a distinctive late Ediacaran succession occurring in the Bafq and Behabad regions of central Iran that has yielded an extensive community of diverse Ediacara biota, including several Precambrian (Chuaria) and Ediacaran (Cloudina, Corumbella) index fossils. The Kushk Series reaches a thickness of up to 518 m and consists of shallow to deep subtidal deposits along a homoclinal carbonate ramp during a transgression–regression cycle. These deposits accumulated as part of an extensional rift basin complex resulting from the opening of the Proto–Paleotethys Ocean in northeastern Gondwana. Latest Ediacaran fossils occur in the deep-water calcareous marine shales and represent a thriving community at the dawn of animal life. [ABSTRACT FROM AUTHOR]

Analysis of the morphology of the recent uplifts on Precambrian cratons and geological-geophysical data on the structure of the crust and mantle indicate that these structures were formed due to expansion of the crustal rocks as a result of retrograde metamorphism. This occurred due to the contribution of large volumes of deep fluids to the complexes of the Early Proterozoic rocks, which underwent high-grade metamorphism in the lower crust. Later, these complexes were moved to the shallower depths after denudation of thick overlap sequences from the craton’s surfaces. The calculation of the volumetric expansion effects using P-T diagrams for the main types of metamorphic rocks shows that this mechanism could have prompted the uplifts of the Precambrian crust in recent time with amplitudes from 100-200 to 1000-1500 m. [ABSTRACT FROM AUTHOR]

The Lessel and Ihugh areas are underlain by Precambrian rocks comprising of banded gneiss, medium-grained muscovite granite, coarse-grained biotite muscovite granite, and Turonian arenaceous Lessel Sandstone in the Lower Benue Trough, southeastern Nigeria. Baryte mineralization occurs at Lessel-Mbagwa, Ihugh and Bunde commonly as vein and cavity type deposits within NE-SW and NW-SE trending fracture zones. Major oxides and trace element analysis of baryte and the host rocks was undertaken using Inductively Coupled Plasma Mass Spectrometry (ICP-MS), in order to establish the geochemical characteristics and to constrain the origin of the baryte mineralization. The data revealed enrichment in Ba and Sr and depletion in SiO 2, TiO 2, Al 2 O 3, Fe 2 O 3, MgO, CaO, Na 2 O, K 2 O, LILEs, HFSEs relative to background values obtained from sandstones and granitic rocks in the area. Baryte mineralization from Lessel-Mbagwa, Bunde and Ihugh is marked by positive Gd (17.1–21.4) and negative Eu (0.02–0.05) anomalies. Negative Ce anomaly (0.06) is evident in barytes from Lessel-Mbagwa and Bunde whereas baryte from Ihugh is characterized by positive Ce anomaly (1.3). The structurally controlled nature of the mineralization, depleted major and trace elements concentration in the barytes in addition to REE anomalies indicate that mixing of a strongly differentiated residual Ba-bearing fluid and sulphur-rich seawater occurred under low temperature and oxidizing-reducing conditions. The tectonic event which occurred during the Santonian to Early Campanian reactivated the NE-SW, NW-SE structural pathways in the basement gneiss and also created similar structures in the Turonian Lessel Sandstone. [ABSTRACT FROM AUTHOR]

Mapping of the structural pattern by remote sensing methods (Landsat, SPOT, aerial photography, potential field data) and field study of selected structural elements shows that the cratonic basement of South Norway is strongly affected by a regular lineament pattern that encompasses fault swarms of different orientation, age, style, attitude and frequency. Albeit counting numerous fault and fracture populations, the faults are not evenly distributed and N-S to NNE-SSW/NNW-SSE and NE-SE/ENE-WSW-systems are spatially dominant. N-S to NNW-SSE structures can be traced underneath the Caledonian nappes to the Western Gneiss Region in western and central South Norway, emphasizing their ancient roots. Dyke swarms of different ages are found within most of these zones. Also, the Østfold, Oslo-Trondheim and the Mandal-Molde lineament zones coincide with trends of Sveconorwegian post-collision granites. We conclude that the N-S-trend includes the most ancient structural elements, and that the trend can be traced back to the Proterozoic (Svecofennian and Sveconorwegian) orogenic events. Some of the faults may have been active in Neoproterozoic times as marginal faults of rift basins at the western margin of Baltica. Remnants of such fault activity have survived in the cores of many of the faults belonging to this system. The ancient systems of lineaments were passively overridden by the Caledonian fold-and-thrust system and remained mostly, but note entirely inactive throughout the Sub-Cambrian peneplanation and the Caledonian orogenic collapse in the Silurian-Devonian. The system was reactivated in extension from Carboniferous times, particularly in the Permian with the formation of the Oslo Rift and parts of it remain active to the Present, albeit by decreasing extension and fault activity. [ABSTRACT FROM AUTHOR]

This paper aims to investigate the genesis of clastic dykes and soft-sediment deformation structures in the Mamfe Basin, South West-Region, Cameroon. Results from this study portray the following: (1) The clastic dykes are extrusive and were generated from preexisting soft-sediments that penetrate fissures caused by seismic activity. It can be concluded that clastic dykes originate from seismic shacking, probably induced by volcanic-tectonic activity and magmatic dykes that cut across the Precambrian and Cretaceous formations of the Mamfe Basin. (2) The soft-sediment deformation structures (flexures, sheared foliations, anticlinal folds, load casts, and flame structures) are likely triggered by seismic shocks. The inferred influence of seismic activity results from rifting and rapid subsidence of basin-fill during the Cretaceous. Some of these soft-sediment deformation structures (SSDS) are induced by fluidization and liquefaction triggered by rapid sedimentation within tectonically active settings, as well as density variations illustrated by local occurrence of load casts with weak lateral extensions. (3) The synsedimentary features (joints, faults, filled fractures) are related to local stress triggered by gravitational sliding, because the fractures were filled by unconsolidated clastic materials. [ABSTRACT FROM AUTHOR]

Abstract: The Precambrian Dengying Formation is a set of large‐scale, extensively dolomitized, carbonate reservoirs occurring within the Sichuan Basin. Petrographic and geochemical studies reveal dolomitization was a direct result of precipitation by chemically distinct fluids occurring at different times and at different intensities. Based on this evidence, dolomitization and multiple fluid flow events are analyzed, and three types of fluid evolution models are proposed. Results of analysis show that Precambrian Dengying Formation carbonates were deposited in a restricted peritidal environment (630–542 Ma). A high temperature and high Mg2+ concentration seawater was a direct result of dolomitization for the micrite matrix, and for fibrous aragonite in primary pores. Geochemical evidence shows low δ18O values of micritic dolomite varying from −1.29± to −4.52± PDB, abundant light rare earth elements (REEs), and low dolomite order degrees. Microbes and meteoric water significantly altered dolomite original chemical signatures, resulting in algal micritic dolomite and the fine‐grained, granular, dolosparite dolomite having very negative δ18O values. Finely crystalline cement dolomite (536.3–280 Ma) and coarsely crystalline cement dolomite have a higher crystallization degree and higher order degree. The diagenetic sequence and fluid inclusion evidence imply a linear correlation between their burial depth and homogenization temperatures, which closely resemble the temperature of generated hydrocarbon. Compared with finely crystalline dolomite, precipitation of coarsely crystalline dolomite was more affected by restricted basinal fluids. In addition, there is a trend toward a more negative δ18O value, higher salinity, higher Fe and Mn concentrations, REE‐rich. Two periods of hydrothermal fluids are identified, as the exceptionally high temperatures as opposed to the temperatures of burial history, in addition to the presence of high salinity fluid inclusions. The early hydrothermal fluid flow event was characterized by hot magnesium‐ and silicon‐rich fluids, as demonstrated by the recrystallized matrix dolomite that is intimately associated with flint, opal, and microcrystalline quartz in intergranular or intercrystalline pores. This event was likely the result of a seafloor hydrothermal chimney eruption during Episode I of the Tongwan Movement (536.3±5.5 Ma). In contrast, later hydrothermal fluids, which caused precipitation of saddle dolomite, were characterized by high salinity (15–16.05wt% NaCl equivalent) and homogenization temperatures (250 to 265°C), δ18O values that were more enriched, and REE signatures. Geochemical data and the paragenetic sequence indicate that this hydrothermal fluid was related to extensive Permian large igneous province activity (360–280 Ma). This study demonstrates the presence of complicated dolomitization processes occurring during various paleoclimates, tectonic cycles, and basinal fluids flow; results are a useful reference for these dolomitized Precambrian carbonates reservoirs. [ABSTRACT FROM AUTHOR]

The fold-and-thrust belt of Yenisey Ridge is a key to understand the Precambrian tectonic evolution of the Siberian Craton as well as crustal evolution in the Central Asian Orogenic Belt. Here we report the occurrence of felsic and mafic dyke swarms in the Yenisey Ridge providing evidence for rift-related magmatism. The dikes and sills occur in narrow linear zones along faults, and show bimodal composition with geochemical features indicating intraplate settings. Zircon SHRIMP U-Pb analyses constrain the timing of emplacement of the dykes as 797–792 Ma. The magmatic event at c. 800 Ma along the western margin of the Siberian Craton and other continental blocks can be correlated with the onset of the breakup of the Neoproterozoic Rodinia supercontinent. Post-Grenville episodes of regional crustal evolution are correlated with the synchronous successions and similar style within the Valhalla orogen along the Arctic margin of Rodinia and supports the spatial proximity of Siberia and North Atlantic cratons (Laurentia, Baltica, Svalbard) at c. 800 Ma, as proposed for the Neoproterozoic paleogeographic reconstructions for the Rodinia supercontinent and as robustly constrained from large igneous province (LIP) record. [ABSTRACT FROM AUTHOR]

The stratabound Cu deposits in the Zhongtiao Mountains district contain good records about the Precambrian tectonic evolution in the central North China Craton. Five stages of metamorphic and metallogenic processes are identified and the P-T conditions are constrained using mineral geothermobarometry and P-T pseudosection. Results revealed a prograde metamorphism (stage M − 1) from 565 to 630 °C, and peak metamorphism of 6.5 ± 0.5 kbar and 610 ± 20 °C (stage M − 2). The post-peak metamorphism consisted of a continued retrogression (stage M − 3) at 545 ± 5 °C and 6.0 ± 0.1 kbar, and Cu mineralizations (stages M − 4 and M − 5). Two distinct mineral assemblages of arsenopyrite-pyrrhotite-pyrite-chalcopyrite and bornite-chalcopyrite-parkerite- wittichenite-bismuth are observed in veinlets. The Cu mineralization occurred from 446 to 341 °C at a pressure of ∼5.0 to 4.4 kbar. We obtained laser ablation ICPMS monazite U-Pb ages of 1881 ± 15 Ma and 1863 ± 17 Ma for staurolite schist and mineralized veins, respectively. A Nano-secondary ion mass spectrometer zircon U-Pb age of 1815 ± 32 Ma for fluid-modified zircon rims from host schist. These ages are interpreted as the timing for the stages M − 2, M − 4 and M − 5, respectively. These results define a clockwise P-T-t path that displays nearly parallel prograde and retrograde trajectory (Franciscan type). The retrograde portion is similar to near-isobaric cooling (IBC) stage, which supports a relatively slow exhumation following a continental collision. It is proposed that the ore materials were mainly derived from the host rocks, which might have been pre-enriched. Cu mineralization formed during the retrograde cooling stage at greenschist facies, and was likely related to a post-orogenic sluggish exhumation at a rate of 0.31–0.05 mm/yr. [ABSTRACT FROM AUTHOR]

In this study, we present a field-based structural analysis of the unmetamorphosed Precambrian volcaniclastic sequences of the west-central Arabian Shield. The study area is known as the Fatima fold-and-thrust belt, which is an overturned synclinorium that developed during the Neoproterozoic era. This belt is composed primarily of green mudstone, green sandstone, an andesite flow, limestone, red mudstone and pyroclastic units. This stratigraphic succession, which presents different rheological multilayers, offers significant mesoscale folding and thrust-related structures. Mechanical anisotropy and thickness contrasts have played significant roles in controlling the style of the deformation. Deformed hinge zones, a simple duplex, a domino-style duplex, and imbricated and antiformal stacks are among the thrust-related structures presented and analyzed. The domino-style duplex observed on the backlimbs of the overturned anticlines formed a unique pattern that developed during thrust propagation. The results of this study indicate that the thrust duplex developed according to a thick-skinned model, and it represents a newly recognized tectonic regime in the Arabian Shield. Comprehensive field mapping and structural analyses revealed that the zone under study area was affected by four phases of deformation (D 1 –D 4 ). The D 1 and D 2 phases present ductile deformation that developed during the final cratonization and assembly of the Arabian Shield, and they can be recognized at both the map and outcrop scales. The D 1 phase represents a progressive regime and is indicated by a NW-SE stress orientation and the formation of a series of coaxial symmetrical NE-SW-plunging folds. NNW-directed thrust-related structures progressively developed during the D 2 phase. An approximately fifty-three percent tectonic shortening can be calculated based on the restored structures. D 3 and D 4 have a brittle nature and are indicated by shearing and normal faulting, respectively. [ABSTRACT FROM AUTHOR]

Constraining the age of strata is a fundamental source of uncertainty in the study of sedimentary rocks, particularly in restricted basins that generally lack index fossils. An illustrative example of this is the evaporite-bearing Salina Group in the Michigan Basin. Our integrated study of facies, paleontology, and stable isotope geochemistry from the base of the Salina Group in Wisconsin addresses long-standing chronostratigraphic uncertainty surrounding these units. Conodont samples from the basal boundary interval ( Racine- Waubakee formation contact) produced non-diagnostic 'disaster' and 'recovery' faunas typical of both the Mulde ( Homerian) and Lau ( Ludfordian) events. Strontium isotope analysis (87Sr/86Sr) of these conodonts from five horizons just below the boundary yield values between 0.70844 and 0.70850 confirming a Homerian age. Multiple carbon isotope profiles through this interval confirm the presence of a 2.5-3‰ positive excursion. Cumulatively these data constrain the base of the Salina Group in Wisconsin to the Mulde Excursion interval (late Homerian). This integrated study provides a sound initial step towards a deeper understanding of the processes of Silurian evaporite formation in the Michigan Basin. [ABSTRACT FROM AUTHOR]

Basaltoids of the Vendian-Cambrian Kulbyurstyug volcanic complex in the eastern part of Kuznetsk Alatau have a high content of titanium (TiO 3-4 wt %). They are relatively enriched with LILE and HFSE (Ba 360-900, Zr 160-726, Nb 66-101, ΣREE up to 225-329 ppm), and demonstrate a fractionated spectrum of REEs (La/Yb ∼ 13-17), high Nb/U (44-66), and low Th/Ta and Th/Nb. These features are comparable with derivatives of OIB (Ocean Island Basalts) magmatism. The rock geochemistry suggests the possible formation of an initial mafic melt in the garnet-bearing peridotite mantle with 2-4% degree of melting and the presence of a small amount of spinel. By the age and composition, the studied volcanics are correlated with the OIB magmatism products, occurring in the adjacent Gorny Altai and also considered to be derivatives of intraplate magmatism of Riphean-Early Cambrian age. This magmatism was caused by plume activity during formation of the Paleo-Asian Ocean. [ABSTRACT FROM AUTHOR]

A new Pt-He method for dating isoferroplatinum has been developed at the Institute of Precambrian Geology and Geochronology, Russian Academy of Sciences. Here we publish the first results of dating of isoferroplatinum from the main mineralogical and geochemical types of PGE mineralization in dunite. The obtained Pt-He age of isoferroplatinum is 129 ± 6 Ma. The gained Pt-He age of isoferroplatinum specimens of different genesis (magmatic, fluid-metamorphogenic, and metasomatic) from the Kondyor Massif indicates that the PGM mineralization took place synchronously and successively with evolution of primarily picrite, followed by subalkaline and alkaline melts of the Mesozoic tectonic-magmatic activation of the Aldan Shield. [ABSTRACT FROM AUTHOR]

The current paradigm on Precambrian fluvial sedimentology assumes that pre-vegetation environments did not allow for the establishment of deep, stable channels. However, few studies have documented a continuum of fluvial-depositional architectures along km-scale transects where clusters of channel bodies can be observed in their entirety. The Stoer Group consists of a 1.2 Ga rift-basin fill, its type area occurring at Stoer Peninsula, NW Scotland. Ground observations on classic coastal sections are integrated with remote sensing on a restored transect 7 km wide, representing 400 m of stratigraphy. The transect spans a set of palaeovalleys carved on high-relief gneissic basement. Remote sensing and ground-based sedimentology unveil a set of depositional domains, including: (i) perennial channels filled with downstream-lateral accretionary bars; (ii) poorly drained muddy floodbasins; (iii) well-drained sandy floodbasins containing splays and distributary channels, at times meandering; (iv) gravelly piedmonts composed of talus cones and fluvial-fan deposits; and (v) aeolian fields comprising dunes, ponded interdunes, and sandsheets. These depositional domains reveal an original geomorphic complexity higher than previously recognized for Precambrian terrestrial environments. The occurrence of clustered channel bodies alongside or within both muddy and sandy depositional domains indicates that stable fluvial channels developed independently from the cohesive nature of the substrate. Their development was possibly aided by drainage focusing along valley thalwegs. Three types of fluvial channels have been identified: laterally extensive, multistorey channels akin to mobile, weakly sinuous rivers; vertically stacked, multistorey channels reminiscent of confined, weakly sinuous rivers; and laterally extensive, multilateral channels, indicative of highly mobile, moderately sinuous rivers. Eighty-four preserved channel fills display width-to-thickness ratios fully overlapping with those of post-vegetation and modern braided rivers, disproving the commonly held notion that pre-vegetation rivers could only generate sheet-like sandbodies. This study provides new insights on Precambrian fluvial styles, and underscores the potential of remote-sensing methods to analyze very large exposures of fluvial rocks. [ABSTRACT FROM AUTHOR]

The Central Manitoba mine trend is one of the most important lode gold districts in the Rice Lake greenstone–granitoid belt of the western Uchi subprovince within western Superior Province, Manitoba, Canada. Neoarchean host rocks consist of a south-facing metavolcano-sedimentary succession (2.75–2.73 Ga) intruded by voluminous gabbroic sills and tonalitic–granodioritic plutons (2.73–2.72 Ga), as well as late aplite dikes (2.73–2.72 Ga) and quartz-feldspar porphyry dikes (2.73–2.71 Ga). Five generations (G 1 –G 5 ) of deformation structures have been recognized through detailed geological mapping. The entire succession was folded during G 1 and G 2 prior to rare late aplite dike emplacement. All fault-fill veins and extension veins cut all lithologic units, and are structurally governed by late (G 3 and G 4 ) conjugate shear zones. Main gold mineralization occurs within fault-fill veins hosted by west-trending steeply-dipping (G 4 ) dextral brittle–ductile and ductile shear zones, which occur along or across contacts of metabasalt, metagreywacke and metagabbro or entirely within metagabbro. Microstructural and paragenetic analyses on main gold-bearing veins have revealed that gold is intimately associated with quartz, pyrrhotite and tellurobismuthite. Main gold introduction is interpreted to have taken place contemporaneously with pyrrhotite and tellurobismuthite deposition early during G 4 shearing. These results in structural and mineralogical aspects at the Central Manitoba mine trend provide new constraints on structural evolution and gold mineralization of the southeastern Rice Lake belt, and can be used to improve exploration models for gold deposits in Archean terranes. [ABSTRACT FROM AUTHOR]

A migmatite-gneiss complex made up of paraand orthogneisses and crystalline schists with bodies of ultrametagenic tonalites is distinguished in the basement of the northwestern part of the Chuya-Kendyktas Massif. ID-TIMS dating of accessory zircon from orthogneisses and ultrametagenic tonalites in combination with LA-ICP-MS analyses of detrital zircons from garnet-biotite paragneisses showed that the migmatite-gneiss complex was formed after protoliths with an age within 800-770 Ma and completed its evolution in the first half of the Neoproterozoic. [ABSTRACT FROM AUTHOR]

Cherty marbles of Hasan-Robat area, northwest of Isfahan, in the Sanandaj-Sirjan Zone of Iran preserves evidences of multiple deformational events. The Sanandaj-Sirjan Zone is the inner crystalline zone of the Zagros Orogen, which has been highly deformed and exhumed during continental collision between the Arabian Plate and Central Iran. The Hasan-Robat area is an example of the exposed Precambrian-Paleozoic basement rocks that stretched along two NW-SE-trending faults and located in the inner part of the HasanRobat positive flower strcuture. The Hasan-Robat marbles record a complex shortening and shearing history. This lead to the development of disharmonic ptygmatic folds with vertical to sub-vertical axes and some interference patterns of folding that may have been created from deformations during the Pan-African Orogeny and later phases. Based on this research, tectonic evolution of the Hasan-Robat area is interpreted as the product of three major geotectonic events that have been started after Precambrian to Quaternary: (1) old deformation phases (2) contractional movements and (3) strike-slip movements. Different sets and distributions of joints, faults and folds are confirmed with effect of several deformational stages of the area and formation of the flower structure. [ABSTRACT FROM AUTHOR]

Major geotectonic elements of the reviewed territory of the Arctic part of the North American continent are the Hyperborean Precambrian Platform, the Franklin folding belt, the northern part of the Precambrian Canadian platform, and the Mesozoic folding belt of Canada and Alaska. The rise of the Arctic slope of Alaska, the Beaufort Sea, and the Sverdrup basin are located in the southern margins of the Hyperborean Platform. The structure and peculiarities of development of these structural elements are genetically related to the evolution of this platform, as well as the current state of petroleum potential of the most promising exploration region of Arctic in the 21st century. The forced exploration of the Arctic regions of the United States and Canada has become an important milestone in the current development of the world energetics. Up to 100 oil, gas, and gas condensate fields have been discovered as a result of violent studies, and the potential oil and gas reserves in the Arctic part of the North American continent have been estimated to 30 billiion t and 50 trillion cubic meters, respectively. Many prospects are related to the continental slopes of all three above-mentioned basins; the total potential reserves of slopes are estimated as 10-12 billion t of oil and 20-25 trillion cubic meters of gas. [ABSTRACT FROM AUTHOR]

Recrystallization of zircons under the influence of fluids was studied using examples from Precambrian rocks (microcline granites, metasedimentary, and mafic rocks) of the Kola Peninsula. All zircon crystals showed complex internal textures visible by cathodoluminescence and backscattered electron (BSE) imaging. Detailed mineralogical and geochemical studies with subsequent secondary ion mass spectrometer U–Pb dating of different zircon domains show that secondary texture formation can be interpreted in terms of metasomatic replacement of zircon crystals on the base of crystallogenetic experimental models. Mechanisms of zircon replacement and interpretation of U–Pb ages for secondary zircon domains are dependent on the degree of damage of the zircon structure and the fluid composition. The recrystallization of metamict zircon without additional supply of new zircon substance (Zr, SiO2) goes with the dissolution of amorphous domains and precipitation of new polycrystalline zircon, which preserves the U–Pb initial age, but loses radiogenic lead, and the lower intercept of Discordia lines with the Concordia curve determines the time of fluid influence. The recrystallization of metamict zircon or crystalline zircon with high contents of impurities with additional supply of Si and Zr forms monocrystalline replacements. Dissolution of primary zircon is accompanied by growth of new zircon domains differing in the composition of isomorphic impurities and zones of transitional composition, whose ages have no geological sense. The study is of particular importance for zircons from Precambrian rocks with long and complex histories. [ABSTRACT FROM AUTHOR]

Two discriminant-function-based multidimensional major-element diagrams for the tectonic discrimination of siliciclastic sediments were recently published from a coherent statistical methodology of loge-ratio transformation and linear discriminant analysis. These diagrams were constructed based on worldwide examples of Neogene–Quaternary siliciclastic sediments from known tectonic settings. In this work, these two tectonic discrimination diagrams were first successfully tested from Holocene (<0.0117–0 Ma) beach and deep-sea sediments from the Gulf of Mexico. These diagrams were used to decipher tectonic settings of 11 case studies of the Precambrian clastic sedimentary rocks (~512–2800 Ma) from Argentina, USA, Ghana, Spain, Norway, India, China, and Australia. The test and application results obtained from these discrimination diagrams were generally consistent with the geology of the Precambrian source areas. Therefore, the two multidimensional diagrams can be considered as a useful tool for successfully discriminating the tectonic setting of older sedimentary basins, which may consist of one or more tectonic assemblages. Comparison of results of this study with the previously published tectonic discrimination diagrams is illustrated and the probable reasons for some inconsistent inferences were also discussed. [ABSTRACT FROM AUTHOR]

The New England orogen of eastern Australia is characterized by tight orogenic curvatures (oroclines). Oroclinal bending commenced in the Early Permian during a period of extension that involved crustal melting, widespread emplacement of S-type granitoids, high-temperature metamorphism, exhumation of metamorphic complexes, extensional faulting, and development of rift basins. One of these basins is the Early Permian Nambucca block, which is situated in the "core" of the oroclinal structure, but its origin and time of deposition are poorly constrained. Here, we present new U-Pb ages of detrital zircons from the Nambucca block, which include age populations as young as 299 and 285 Ma, confirming the Early Permian deposition of the succession. Additional Devonian-Carboniferous and Precambrian ages indicate that detritus was mainly derived from the New England sub-duction complex and cratonic Gondwana. The range of ages suggests that the Nambucca Basin received detritus from both arc and continent and that deposition occurred in a back-arc setting. Given the coeval formation of the Nambucca Basin and the New England oroclines, we propose that this back-arc extensional basin was controlled by trench retreat, which resulted in "Mediterranean-style" orogenic curvatures along the plate boundary of eastern Gondwana. The recognition of a genetic link between oroclinal bending and back-arc extension may explain how accretionary orogens, such as the eastern Australian Tasmanides, were able to obtain an anomalous width without a substantial contribution of accreted exotic terranes. A similar mode of tectonism may have played an important role in other accretionary orogens. [ABSTRACT FROM AUTHOR]

Purpose. To determine the relation between geochemical composition of Precambrian granitoid massifs of Azov region and its petrostructural organization in order to highlight the tectonic conditions of the formation period of these objects. To identify, based on this, the main (most intense) Precambrian regional tectonic pulse from several appeared in the Azov megablock of Ukrainian Shield which makes the frame of structural ensemble of the territory. Methodology. Structural-paragenetic analysis and geochemical correlation techniques make the base of the work. The former is the concept of paragenic-based dislocation process taking into account the PT-conditions of geological environment. The latter provides a comparison of qualitative and quantitative features of chemical composition of the granitoids from different time slices according to their geological and structural position in the Precambrian basement of Azov region. Findings. We have analyzed the structures at different hierarchical levels of the crystalline basement of the west Azov megablock of Ukrainian Shield and the distribution of major and small chemical elements of granitoids of different age that form the structural skeleton of the territory. The megablock was identified by the set of data as a dislocation shear-system which had been formed by several tectonic pulses. Originality. Linear components of the dislocation shear-system (Korsakskaya and Mangushskaya structures) finally formed in transpression, but in the periods 2.9-2.7 and 2.0-1.9 Ga they experienced transtension. Subcircular components of the system (Saltychanskaya and Gulyaypolskaya structures) formed during transtension in the periods 2.9-2.7 and 2.0-1.9 Ga were under the influence of transpression. During the period 2.2-2.0 Ga the distribution of chemical elements of the studied granitoids corresponds to the current dynamo-kinematic status of components of shear-system of Western Azov. That is, the structural ensemble of the studied area as a permanently functioning dislocation system, especially in regard to kinematic-dynamo aspect, in general formed in the period 2.2-2.0 Ga. Practical value. The results could be used for specific minerals prospecting. [ABSTRACT FROM AUTHOR]

The article discusses the role of mineralogical geochemical lineage of ore matter. Topics discussed include metallogenic dynamics of the marginal sea lithosphere, precambrian basement mineralization in different terranes and favorable combinations that can be derived from analysis of ore formation through the geochemical isotopic properties.

New data are reported on the content of radioactive elements in the Precambrian Na-K granitoids from the southwestern margin of the Siberian Craton, Aldan and Ukrainian shields, and Kursk-Voronezh Massif. Analytical data on other regions were generalized for comparison. Two global epochs of Na-K granitoid magmatism bearing elevated contents of radioactive elements (U, Th, K) were distinguished in the Early Precambrian (in Ga): Neoarchean (2.8-2.6) and Late Paleoproterozoic (1.9-1.75). Mesoarchean (3.1-2.8 Ga) epoch of Na-K granite formation has been additionally distinguished at the Australian, South African, and Canadian shields. These epochs of granitization provided high maturity of the crust: geochemical differentiation of the oldest continental blocks and their geochemical and metallogenic specialization for trace elements and RAE. In the southern margin of the Siberian Craton, the most intense granite formation occurred in the Late Paleoproterozoic. The extended South Siberian belt of collisional and within-plate Na-K granitoids is characterized by intense influx of RAE and other trace elements in the upper crustal shell. The southwestern margin of the craton (Yenisei Range) was spanned by repeated Late Neoproterozoic Na-K granite formation, with wide development of collisional and within-plate Na-K granites having elevated Th content and [Th]/[U] ratio. The higher RAE concentrations are typical of within-plate Paleo and Neoproterozoic granitoids. The highest uranium content was found in the postcollisional and within-plate Na-K granites and subalkaline leucogranites. Uranium ore concentrations were formed at the riftogenic stages of evolution of these crustal blocks, when within-plate subalkaline acid magmatism and accompanying hydrothermal metamorphism overprinted granitized crystalline massifs, including high-U sedimentary and volcanic complexes. Areas with the most favorable geological-geochemical environments for the formation of uranium mineralization were distinguished in the southern margin of the Siberian Craton and its nearest folded framing. [ABSTRACT FROM AUTHOR]

Zircon (ZFT) and apatite (AFT) fission-track low-temperature thermochronology was applied at the Brazilian passive continental margin in order to understand and reconstruct the post-rift evolution since the breakup of southwestern Gondwana. The thermochronological data obtained from samples of both the Precambrian basement and the Paleogene to Neogene sedimentary rocks from the continental rift of southeastern Brazil provided ZFT ages between 148 (15) and 64 (6) Ma, and AFT ages of 81 (8)-29 (3) Ma. These data clearly indicate syn- and post-rift reactivations during the Early Cretaceous, with great emphasis on Paleogene to Neogene times. Integrating the results of older thermochronological studies, the reactivation of the southeastern Brazilian margin can be described in three main phases related to the rift to post-rift evolution of SE Brazil. In general, ZFT and AFT data yield spread values that become younger as samples are closer to the reactivated Neoproterozoic shear zones and might reflect source area exhumation. The analysis of ZFT and AFT data allowed interpretations regarding the main phases that occurred in the study area related to the thermotectonic and tectono-stratigraphic evolution in southeastern Brazil. [ABSTRACT FROM AUTHOR]

Modern evaporitic microbial ecosystems are important analogs for understanding the record of earliest life on Earth. Although mineral-depositing shallow-marine environments were prevalent during the Precambrian, few such environments are now available today for study. We investigated the molecular and lipid biomarker composition of an endoevaporitic gypsarenite microbial mat community in Guerrero Negro, Mexico. The 16 S ribosomal RNA gene-based phylogenetic analyses of this mat corroborate prior observations indicating that characteristic layered microbial communities colonize gypsum deposits world-wide despite considerable textural and morphological variability. Membrane fatty acid analysis of the surface tan/orange and lower green mat crust layers indicated cell densities of 1.6 × 109 and 4.2 × 109 cells cm−3, respectively. Several biomarker fatty acids, ∆7,10-hexadecadienoic, iso-heptadecenoic, 10-methylhexadecanoic, and a ∆12-methyloctadecenoic, correlated well with distributions of Euhalothece, Stenotrophomonas, Desulfohalobium, and Rhodobacterales, respectively, revealed by the phylogenetic analyses. Chlorophyll ( Chl) a and cyanobacterial phylotypes were present at all depths in the mat. Bacteriochlorophyl ( Bchl) a and Bchl c were first detected in the oxic-anoxic transition zone and increased with depth. A series of monomethylalkanes ( MMA), 8-methylhexadecane, 8-methylheptadecane, and 9-methyloctadecane were present in the surface crust but increased in abundance in the lower anoxic layers. The MMA structures are similar to those identified previously in cultures of the marine Chloroflexus-like organism ' Candidatus Chlorothrix halophila' gen. nov., sp. nov., and may represent the Bchl c community. Novel 3-methylhopanoids were identified in cultures of marine purple non-sulfur bacteria and serve as a probable biomarker for this group in the lower anoxic purple and olive-black layers. Together microbial culture and environmental analyses support novel sources for lipid biomarkers in gypsum crust mats. [ABSTRACT FROM AUTHOR]

Abstract: U–Pb ages of zircons from Precambrian granitoids and major rivers are grouped into a series of major peaks at about 2.7, 2.5, 2.1, 1.9 and 1.1Ga. Recently these peaks have been interpreted as times of enhanced preservation of the continental crust associated with the assembly of supercontinents. An older interpretation, which we support, is that they correspond instead to periods of accelerated crustal growth related to episodic convection of the mantle. In this paper we use fluid mechanics experiments to develop a new model of mantle convection and crustal growth. A dense layer at the base of the mantle persists until 2.7Ga when it destabilizes and generates large domes that rise into the upper mantle. There they cause a large increase in the rate of subduction which leads to enhanced granite magmatism at convergent margins and thus to a pulse of crustal growth. The domes heat the upper mantle which partially melts at mid-ocean ridges to produce thick oceanic crust that resists subduction. The subsequent period of subdued plate motion is broken by the next generation of mantle domes. Before and after the Archean–Proterozoic period of episodic crustal growth, plate tectonics operated quasi-continuously. [Copyright &y& Elsevier]

Abstract: We review the thickness and shear wave velocity structure of Precambrian crust in Africa and Arabia, where over 90% of the landmass is comprised of Archean and Proterozoic terranes, and examine the data for evidence of secular variation. The data come from many published 1D shear wave velocity profiles obtained by jointly inverting receiver functions and surface wave dispersion measurements, 35 new 1D shear wave velocity profiles for locations in eastern Africa, and a new map of crustal thickness for Africa and Arabia derived from modeling satellite gravity data. We find for both Archean and Proterozoic terranes a similar range of crustal thicknesses (~33–45km), similar mean crustal shear wave velocities (~3.6–3.7km/s), and similar amounts of heterogeneity in lower crustal structure, as reflected in the thickness of lowermost crust with shear wave velocities ≥4.0km/s. There is little evidence for secular variation in crustal structure, indicating that there may have been few changes over much of Earth's history in the processes that form the continental crust. Post-formation tectonic events also may have modified many of the terranes to such an extent that secular trends arising from crustal genesis may be difficult to recognize. [Copyright &y& Elsevier]

Abstract: The Phanerozoic Hudson Bay Basin is a large intracratonic basin that is almost completely encircled by Precambrian rocks of the Canadian Shield. The preserved sedimentary succession is up to 2500m thick and consists mainly of Upper Ordovician to Upper Devonian limestones, dolostones, evaporites and minor siliciclastics that were deposited in shallow marine conditions. Backstripping, based on new paleontological data and well correlations, reveals an irregular subsidence history marked by several periods of exhumation. In seismic data, the Hudson Bay Basin appears to have a relatively simple geometry, characterized by a lower sedimentary package cut by high-angle faults, overlain by a saucer-shape, essentially underformed upper sedimentary package. Normal (or transtensional) faults imaged on seismic reflection profiles provide clear evidence for crustal extension during deposition of the older sedimentary packages or slightly later, indicating that the basin is, at least partly, extensional in nature. However, significant changes in the depocenter location during the Paleozoic and variable exhumation values required by new maturation data indicate that other mechanisms influenced the subsidence/exhumation history of the basin. In particular, the influence of far-field events and dynamic topography transmitted by large-scale mantle flow in the continental interior (creating long-wavelength tilting and unconformities) is suspected but not yet proven. [Copyright &y& Elsevier]

Microbialites are the most abundant macrofossils of the Precambrian. Decline in microbialite abundance and diversity during the terminal Proterozoic and early Phanerozoic has historically been attributed to the concurrent radiation of complex metazoans. Similarly, the apparent resurgence of microbialites in the wake of Paleozoic and Mesozoic mass extinctions is frequently linked to drastic declines in metazoan diversity and abundance. However, it has become increasing clear that microbialites are relatively common in certain modern shallow, normal marine carbonate environments-foremost the Bahamas. For the first time, we present data, collected from the Exuma Cays, the Bahamas, systematically characterizing the relationship between framework-building cyanobacteria, microbialite fabrics, and microbialite-associated metazoan abundance and diversity. We document the coexistence of diverse microbialite and infaunal metazoan communities and demonstrate that the predominant control upon both microbialite fabric and metazoan community structure is microbial mat type. These findings necessitate that we rethink prevalent interpretations of microbialite-metazoan interactions and imply that microbialites are not passive recipients of metazoan-mediated alteration. Additionally, this work provides support for the theory that certain Precambrian microbialites may have been havens of early complex metazoan life, rather than bereft of metazoans, as has been traditionally envisaged. [ABSTRACT FROM AUTHOR]

The Precambrian-Cambrian boundary is iconic, marking the first appearance of shelly fauna in the fossil record, and opening the Phanerozoic Eon. In England and Wales, the transition from predominantly Precambrian igneous rocks to Cambrian sedimentary strata is generally unconformable. An exceptional exposure of this transition can be observed in the Ercall Quarries in Shropshire, a classic locality in all senses of the word. [ABSTRACT FROM AUTHOR]

The extensive matgrounds in Carboniferous-Permian open-marine deposits of western Argentina constitute an anachronistic facies, because with the onset of penetrative bioturbation during the early Paleozoic microbial mats essentially disappeared from these settings. Abundant microbially induced sedimentary structures in the Argentinean deposits are coincident with the disappearance of trace and body fossils in the succession and with a landward facies shift indicative of transgressive conditions. Deposits of the Late Carboniferous-Early Permian glacial event are well developed in adjacent basins in eastern Argentina, Brazil, South Africa and Antarctica, but do not occur in the western Andean basins of Argentina. However, the deglaciation phase is indirectly recorded in the studied region by a rapid rise in sea level referred to as the Stephanian-Asselian transgression. We suggest that an unusual release of meltwater during the final deglaciation episode of the Gondwana Ice Age may have dramatically freshened peri-Gondwanan seas, impacting negatively on coastal and shallow-marine benthic faunas. Suppression of bioturbation was therefore conducive to a brief re-appearance of matground-dominated ecosystems, reminiscent of those in the precambrian. Bioturbation is essential for ecosystem performance and plays a major role in ocean and sediment geochemistry. Accordingly, the decimation of the mixed layer during deglaciation in the Gondwana basins may have altered ecosystem functioning and geochemical cycling. [ABSTRACT FROM AUTHOR]

Here, we explore enrichments in paleomarine Zn as recorded by authigenic iron oxides including Precambrian iron formations, ironstones, and Phanerozoic hydrothermal exhalites. This compilation of new and literature-based iron formation analyses track dissolved Zn abundances and constrain the magnitude of the marine reservoir over geological time. Overall, the iron formation record is characterized by a fairly static range in Zn/Fe ratios throughout the Precambrian, consistent with the shale record (Scott et al., 2013, Nature Geoscience, 6, 125-128). When hypothetical partitioning scenarios are applied to this record, paleomarine Zn concentrations within about an order of magnitude of modern are indicated. We couple this examination with new chemical speciation models to interpret the iron formation record. We present two scenarios: first, under all but the most sulfidic conditions and with Zn-binding organic ligand concentrations similar to modern oceans, the amount of bioavailable Zn remained relatively unchanged through time. Late proliferation of Zn in eukaryotic metallomes has previously been linked to marine Zn biolimitation, but under this scenario the expansion in eukaryotic Zn metallomes may be better linked to biologically intrinsic evolutionary factors. In this case, zinc's geochemical and biological evolution may be decoupled and viewed as a function of increasing need for genome regulation and diversification of Zn-binding transcription factors. In the second scenario, we consider Archean organic ligand complexation in such excess that it may render Zn bioavailability low. However, this is dependent on Zn-organic ligand complexes not being bioavailable, which remains unclear. In this case, although bioavailability may be low, sphalerite precipitation is prevented, thereby maintaining a constant Zn inventory throughout both ferruginous and euxinic conditions. These results provide new perspectives and constraints on potential couplings between the trajectory of biological and marine geochemical coevolution. [ABSTRACT FROM AUTHOR]

The paper presents data on the mineralogy, petrography, and lithogeochemistry of Riphean and Vendian sandstones in the Volga-Ural region. The studied rocks generally differ in several parameters. The Zr/Sc and Th/Sc values typical of psammites suggest that the rocks are dominated by the petrogenic clastic material. This conclusion is supported by median KO/AlO values (from 0.39 to 0.45). The fine-grained clastic rocks associated with sandstones are also characterized by sufficiently high KO/AlO values, indicating the prevalence of the first cycle material therein. It is shown that the Prikamsk and Tukaevo sandstones include, in addition to the petrogenic quartz, a significant amount of lithogenic (multifold redeposited) quartz, whereas ratio of these rock types is approximately equal in the Leonidovo and Baikibashevo sandstones. Sandstones of the Kairovo and Shkapovo groups are dominated by the petrogenic quartz. Analysis of the ln(Q/L + CE)-ln(Q/F) diagram shows that the Vendian psammites are dominated by disintegration products of plutonic rocks, whereas the Riphean psammites contain a significant portion of clastic material (related to the erosion of metamorphic rocks) along with felsic and intermediate-felsic igneous rocks. Relationships of feldspars, rock clasts, polycrystalline quartz, and quartz, in general, reflected in the Q/(F + R)-Q/(F + R) diagram indicate that the Riphean psammites were deposited in a humid setting; the Vendian psammites, in a semihumid/semiarid setting. [ABSTRACT FROM AUTHOR]

A significant body of evidence suggests that the marine environment remained largely anoxic throughout most of the Precambrian. In contrast, the oxygenation history of terrestrial aquatic environments has received little attention, despite the significance of such settings for early eukaryote evolution. To address this, we provide here a geochemical and isotopic assessment of sediments from the late Mesoproterozoic Nonesuch Formation of central North America. We utilize rhenium-osmium (Re-Os) geochronology to yield a depositional age of 1078 ± 24 Ma, while Os isotope compositions support existing evidence for a lacustrine setting. Fe-S-C systematics suggest that the Nonesuch Formation was deposited from an anoxic Fe-rich (ferruginous) water column. Thus, similar to the marine realm, anoxia persisted in terrestrial aquatic environments in the Middle to Late Proterozoic, but sulfidic water column conditions were not ubiquitous. Our data suggest that oxygenation of the terrestrial realm was not pervasive at this time and may not have preceded oxygenation of the marine environment, signifying a major requirement for further investigation of links between the oxygenation state of terrestrial aquatic environments and eukaryote evolution. [ABSTRACT FROM AUTHOR]

A regional truncation surface developed during the Oligocene in the northern Red Sea and southern Levant as a result of the combined effects: (1) the formation of an ~3000 km x 1500 km crustal dome affecting the northeastern sector of the Afro-Arabian continent above the Afar plume, and (2) widespread fluvial erosion that truncated the northern edge of this dome. The regional truncation surface separates two main stratigraphic divisions: (1) middle Eocene--early Oligocene prerift clastics preserved below the regional truncation surface and (2) late Oligo cene--Holocene Red Sea Supergroup deposited after the regional truncation surface developed. The prerifting sequence is composed of two major units: (1) Eocene marine intraformational debris flows (Themed clastics), and (2) fluviolacustrine continental conglomerates, most of them monomictic (Mahatardi Conglomerate). The Red Sea Supergroup is composed of polymictic conglomerates, which include fragments of older rock units, down to the Precambrian. The reconstructed regional truncation surface and its subcrop map shed new light on the initial stages of the Afro-Arabian breakup, indicating two principal phases: (1) an Oligocene slow crustal doming, with reactivation of preexisting faults and the development of the regional truncation surface, all triggered by the upwelling of the Afar plume; and (2) late Oligo cene--early Miocene rifting of the Red Sea and Suez rift, which refaulted the region, including the initiation of the Dead Sea Transform along a preexisting suture line. This two-stage model indicates the mutual genetic relationship between the early Oligocene phase of tectonic development following the penetration of the Afar plume and the post-Oligocene rifting and continental breakup of the Afro-Arabian continent. [ABSTRACT FROM AUTHOR]

Abstract: The low δ18O values of Precambrian cherts have been widely used to infer that temperatures were higher and/or seawater δ18O was lower compared to today''s oceans. However, the Precambrian cherts presented as evidence for these temperatures are neomorphosed from amorphous precursors that originally precipitated from ocean water, suggesting diagenetic alteration is an important cause of the widespread low-δ18O values. Many pores between sand-size clasts in granular iron formations (GIFs) are filled with coarsely crystalline quartz cements that are texturally primary. In unmetamorphosed GIF samples from the 1.9Ga Gunflint and Sokoman iron formations, synsedimentary clasts of GIF and high minus-cement porosities (the volume of pore space occupied by recognizable cement, inversely proportional to the degree of compaction) indicate such cements precipitated directly from pore water near the depositional interface. Growth bands identified in quartz cement crystals using cathodoluminescence (CL) imaging further confirm their void-filling and unrecrystallized nature. In order to test the effect of neomorphism on the δ18O of silica in these rocks, unrecrystallized primary quartz cements and neomorphic quartz in adjacent clasts were both analyzed by ion microprobe. Values of δ18O range from 23.5 to 26.4‰ VSMOW in cements and 21.3–26.8‰ in clasts in samples from the Gunflint Iron Formation. Samples from the Sokomon Iron Formation record deeper, more evolved pore waters and in some cases contain lower δ18O cements. In individual pores of Gunflint samples, cement values vary only slightly more than analytical uncertainty from edge to center and no consistent increasing or decreasing trends were observed. Overall, the δ18O of the cements are similar to the most elevated δ18O reported from cherts of similar age, but span a substantially smaller range. This suggests most of the lower values reported previously reflect late-diagenetic conditions at greater burial depths. The shallow, possibly restricted seas above the Gunflint and Sokoman GIFs appear to have been warmer and may have been lower in δ18O than present-day seawater, but they may not have been representative of global seawater. [Copyright &y& Elsevier]

Abstract: Stratigraphic architecture of the fluvial interval at the base of the Neoproterozoic Sonia Sandstone, Rajasthan, India has been evaluated in consideration of the vegetationless Precambrian depositional environment. For its critical appreciation, facies, architectural elements, bounding surfaces, deductive palaeohydraulics, palaeocurrents, distribution of soft sediment deformation structures, as well as sequence stratigraphic status of the interval have been taken into account. Confined between an unconformity at the base and an overlying transgressive lag, the interval is considered a lowstand product. Temporal shifts in base profile, nevertheless, affected the tripartite division of the LST, systems tract developed during relative sea level lowstand. Palaeocurrent diversions across division boundaries as well as preferred concentration of soft sediment deformation structures along the top of the middle division make the role of tectonics evident. Facies, architectural elements, palaeohydraulic derivations as well as the degree of palaeocurrent consistency depict significant differences in channel patterns between the divisions. The axiom regarding braided patterns, frequent avulsion, and the ephemeral nature and flashy behavior of Precambrian rivers find conformity only in Division II. The river systems in Divisions I and III had frequently become meandering, due to valley constriction in the former and bank stability in the latter. Division I accumulated through sediment aggradation within a constricted valley incised deeply into the bedrock during the preceding course of base profile fall. Channel amalgamation characterizes Division I and more profoundly, Division II. The sharp upward transition from Division II to III records rapid enhancement in the rate of rise of base profile due to basin subsidence leading to encroachment of the contemporary sea. Mud settling was encouraged because of ponding at the downstream end turning the river channels fixed. Eventually the depositional site was drowned under the sea. [Copyright &y& Elsevier]