Proterozoic

TECTONOSTRATIGRAPHY OF THE ESPINHAÇO BASIN IN THE NORTHERN PART OF THE SÃO FRANCISCO CRATON: RECORD OF A DISCONTINUOUS POLYHISTORIC EVOLUTION This paper presents the tectonostratigraphic evolution of the northern segment of the Espinhaço Range, located in the northern part of the São Francisco craton. The stratigraphic framework was documented in a systematic way through recognition and characterization of eight synthems that are equivalent to unconformity-bounded units: Algodão, São Simão, Sapiranga, and Pajeú (lower interval), Bom Retiro, São Marcos, and Sítio Novo (intermediate interval) and Santo Onofre (upper interval). The Bom Retiro and São Marcos synthems correspond to intracontinental sag basins. The Pajeú, Sítio Novo and Santo Onofre synthems are related to the evolutionary stages of the rift basin; the first two basins were generated by extensional tectonics, and the last one by transcurrent tectonics. The Algodão and Sapiranga synthems are interpreted as rift-sag basins. Finally, the São Simão synthem is related to intracontinental magmatism. Based on these new stratigraphic subdivisions and their regional tectonic context, we conclude that the 'Espinhaço Basin' records a polycyclic depositional history, alternating episodes of distinct tectonic regimes to the long of time.

Lower and Middle Cambrian carbonate rocks of the Çal Tepe Formation, cropping out in the western Taurus Mountains, yielded a large number of microfossil remains. Small shelly fossils from a single level in the upper Lower Cambrian represent a high diversity biota that could be related to the «Cambrian explosion». Microfossil association from the lower Middle Cambrian sediments of the Çal Tepe Formation is taxonomically very reduced and a dominant taxon is Hadimopanella GEDIK. This sudden change could be attributed to a deepening of the basin during the early Middle Cambrian transgression.

A proposal is put forward to rede®ne the geological time scale for the Precambrian. Flaws of the present, chronometrically de®ned, time scale are discussed and illustrated. It is concluded that we need to go back to the rock record to de®ne a ªnaturalº time scale, in which major divisions (eons, eras, etc.) are de®ned in terms of ®rst-order events and transitions in the observable stratigraphic record. For the earliest part of Earth history, we need a time scale, including a formalized Hadean eon, that is fully consistent with rapidly evolving insights from planetary science. & geological time scale, Precambrian, Proterozoic, Archean, Hadean, rock record.

In the last decades Fe-oxide Cu-Au (IOCG) deposits attracted the interest of exploration geologists and geophysicists. General geophysical descriptions of IOCG deposits have been published in the recent past, but there is still a lack of detailed geophysical investigations. We present a petrophysical study of rock samples from exploration borehole TJ71305, which intersects the Cuprospect in the Tjårrojåkka IOCG mineralised area, northern Sweden. Furthermore geophysical data are compiled and analysed to tentatively define a geophysical signature, at local scale, for this type of deposit. The study area is dominated by intrusive and volcanic rocks of Middle Proterozoic age, the latter hosting the Cu and Fe occurrences.

C. R. Acad. Sci. Paris, Sciences de la Terre et des planètes / Earth and Planetary Sciences 332 (2001) 283-289  2001 Académie des sciences / Éditions scientifiques et médicales Elsevier SAS. Tous droits réservés S1251-8050(01)01542-7/FLA Tectonique / Tectonics Abstract -Geometry and kinematics of Variscan thrusts in the northeastern Armorican Massif (France). Variscan structures of the northeastern Armorican massif consist of folds induced by south-verging thrust faults. This thin-skinned process is controlled by a major décollement that would be expected at the base of the Brioverian flysch. So, the northeastern Armorican domain can be integrated to the tectonic model admitted at the Variscan Orogenesis scale.  2001 Académie des sciences / Éditions scientifiques et médicales Elsevier SAS Variscan orogenesis / Palaeozoic synclines / thin-skin tectonics / décollement / Normandy / France Résumé -La structuration varisque du domaine Nord-Est-Armoricain résulte de plis associés à des chevauchements à vergence sud, organisés suivant une géométrie en « plats et rampes ». La cinématique d'un tel système nécessite un décollement majeur, qui pourrait être recherché à la base de la couverture flyschoïde briovérienne. Le domaine étudié s'intègre ainsi au modèle tectonique admis à l'échelle de l'orogenèse varisque.  2001 Académie des sciences / Éditions scientifiques et médicales Elsevier SAS orogenèse varisque / synclinaux paléozoïques / chevauchements / décollement / Normandie / France Abridged version * Correspondance et tirés à part. Adresses (J. Le Gall).

The geological record of consolidation and uplift of crustal protolith identifies three major groupings of Archaean cratonic nuclei . These comprise 'Ur' (∼3.0 Ga cratons of Southern Africa, Western Australia), 'Arctica' (∼2.5 Ga cratons of Greenland, Fennoscandia, Laurentia and Siberia) and 'Atlantica' (∼2.0 Ga cratons of Western Africa and South America). In this paper the ∼2.9-2.0 Ga record from these cratons is used to develop a palaeogeographic model based on (i) the proximities recognised from geological evidence and (ii) the correlation of ∼2.7-2.2 Ga palaeomagnetic poles between Ur and Arctica . The solution shows that Archaean continental crust had aggregated into a crescent-shaped supercontinent, 'Protopangaea', by mid-Archaean times. The outer wings comprised core elements of Ur and Arctica which appear to have retained internal quasi-integrity with respect to each other until the end of the Proterozoic eon. Crust in between was still consolidating in early Palaeoproterozoic times and would ultimately include the Atlantica cratonic grouping. Geological tracers, including straight belts and mineral provinces, form lineaments parallel to the arcuate shape of the protocontinent and, as presently defined, the reconstruction indicates that a minimum of 35-56% of the present continental crust had been extracted from the mantle by these times. Derivation of the reconstruction is facilitated because the bulk of the ∼2.7-2.2 Ga poles imply very low rates of apparent polar wander (APW, <10 mm/year) during this ∼5 Ga interval. These low APW rates correlate with the later granite-greenstone tectonics and are presumed to reflect dominant small scale convection. In contrast rapid APW after ∼2.2 Ga records a transition to typical Proterozoic tectonics with large scale mobility reflected in the accretion of mobile belts to the core Ur-Atlantica-Arctica assemblage. The reason why continental crust accretes into supercontinents of symmetrical crescent-shaped form is also addressed. By analogy with Phanerozoic Pangaea and the present-day geoid it is interpreted to have resulted from large-scale, presumably whole mantle, convection systems driving the continental crust towards regions of minimum gravitational potential.

Abstract Microscopic phosphatic scales are found in limestones and cherts from the 812–717 million year old Fifteenmile Group of the Yukon Territory. These enigmatic microfossils, which to date have not been identified in any other locality, display a diversity of intricate morphologies. Here we describe six new genera containing 17 new species of scale microfossils obtained from macerated limestone.

A group of less disturbed and unmetamorphosed ‘Proterozoic’ platformal sediment package lying over deformed and metamorphosed Archean to Paleoproterozoic basement in several parts of Peninsular India is informally termed as ‘Purana’ basins in Indian stratigraphy. The trend shift in the understanding of these basin fills in last two decades, often supported by international collaborations, has made significant advancements in respect to unconformity and ‘Sequence’ development, relationship with accretion and break-up of the supercontinents, initiation and evolution of the basins in different tectonic set ups, nature of sediment provenance, paleoclimate and paleo-weathering patterns and basin water chemistry. Geophysical backups encompassing the basins, occasionally extending up to the craton and adjacent mobile belts helped in refining the understanding on the basement structure and tectonic forcings. The present paper is an attempt towards the collation of the database generated in last two decades in order to present the status of current understanding on different ‘Purana’ basins with regard to interactions of physical, chemical and biological parameters of the Proterozoic Earth.

The moderately metamorphosed and deformed rocks exposed in the Hampden Synform, Eastern Fold Belt, in the Mt Isa terrane, underwent complex multiple deformations during the early Mesoproterozoic Isan Orogeny (ca 1590(ca -1500. The earliest deformation elements preserved in the Hampden Synform are first-generation tight to isoclinal folds and an associated axial-planar slaty cleavage. Preservation of recumbent first-generation folds in the hinge zones of second-generation folds, and the approximately northeast-southwest orientation of restored L 1 0 intersection lineation suggest recumbent folding occurred during east-west to northwest-southeast shortening. Firstgeneration folds are refolded by north-south-oriented upright non-cylindrical tight to isoclinal second-generation folds. A differentiated axial-planar cleavage to the second-generation fold is the dominant fabric in the study area. This fabric crenulates an earlier fabric in the hinge zones of second-generation folds, but forms a composite cleavage on the fold limbs. Two weakly developed steeply dipping crenulation cleavages overprint the dominant composite cleavage at a relatively high angle (>45°). These deformations appear to have had little regional effect. The composite cleavage is also overprinted by a subhorizontal crenulation cleavage inferred to have developed during vertical shortening associated with late-orogenic pluton emplacement. We interpret the sequence of deformation events in the Hampden Synform to reflect the progression from thin-skinned crustal shortening during the development of first-generation structures to thick-skinned crustal shortening during subsequent events. The Hampden Synform is interpreted to occur within a progressively deformed thrust slice located in the hangingwall of the Overhang Shear.

The 2.7-2.0 Ga volcano-sedimentary records of the African, Indian and Australian cratons indicate two broadly defined periods of extensive drowning of the emergent continental areas, concomitant with lowered freeboard. Carbonate-banded iron formation (BIF ) platforms characterised the first such event, at ca 2.6-2.4 Ga (Africa and Australia) to 2.7 Ga (India). These earlier globally enhanced sea levels are ascribed to increased mid-ocean ridge activity, possibly related to breakup of a postulated Late Archaean 'southern' supercontinent. Alternatively, a transition from global-scale catastrophic mantle overturn events to the onset of plate tectonics may have occurred in the Late Archaean (Nelson, 1998. Earth Planet. Sci. Lett. 158, 109-119). Both explanations of increased mid-ocean ridge activity are compatible with significant Early to Middle Archaean crustal growth (Armstrong, 1981. Phil. Trans. R Soc. London A 301, 443-472), with the emergent high freeboard cratons being subjected to aggressive weathering and erosion. Enhanced continental crustal growth near the Archaean-Proterozoic boundary (McLennan and Taylor, 1982. J. Geol. 90, 347-361), related to the development of significant island arc complexes, would have resulted in common lowered freeboard-enhanced sea level conditions at the passive margins of the 'southern' cratons. The diachronous nature of these earlier transgressions in the various cratons may reflect the effect of local tectonic movements and/or the thermal state of the cratons. From ca 2.4-2.2 Ga, cratons that make up the present-day continents of India, Africa and Australia had relatively high continental freeboard and lowered sea levels. Glacigenic deposits are preserved on the Kaapvaal (Africa), Singhbhum (India) and Pilbara (Australia) cratons. The second broadly defined drowning event, at ca <2.2 and >2.15 Ga, was probably due to post-glacial climatic amelioration. Freeboard was reduced by the combination of eustatic rise and the reestablishment of aggressive weathering as warmer palaeoclimates returned. In India, carbonates were more prominent than the siliciclastic sediments (including prominent black shales) seen in Africa and Australia.

Life is an inordinately complex unsolved puzzle. Despite significant theoretical progress, experimental anomalies, paradoxes, and enigmas have revealed paradigmatic limitations. Thus, the advancement of scientific understanding requires new models that resolve fundamental problems. Here, I present a theoretical framework that economically fits evidence accumulated from examinations of life. This theory is based upon a straightforward and non-mathematical core model and proposes unique yet empirically consistent explanations for major phenomena including, but not limited to, quantum gravity, phase transitions of water, why living systems are predominantly CHNOPS (carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur), homochirality of sugars and amino acids, homeoviscous adaptation, triplet code, and DNA mutations. The theoretical framework unifies the macrocosmic and microcosmic realms, validates predicted laws of nature, and solves the puzzle of the origin and evolution of cellular life in the universe. Life 2012, 2 2

There are only two kinds of organisms on the Earth: prokaryotes and eukaryotes. Although eukaryotes are considered to have evolved from prokaryotes, there were no previously known intermediate forms between them. The differences in their cellular structures are so vast that the problem of how eukaryotes could have evolved from prokaryotes is one of the greatest enigmas in biology. Here, we report a unique organism with cellular structures appearing to have intermediate features between prokaryotes and eukaryotes, which was discovered in the deep sea off the coast of Japan using electron microscopy and structome analysis. The organism was 10 µm long and 3 µm in diameter, having >100 times the volume of Escherichia coli. It had a large 'nucleoid', consisting of naked DNA fibers, with a single nucleoid membrane and endosymbionts that resemble bacteria, but no mitochondria. Because this organism appears to be a life form distinct from both prokaryotes and eukaryotes but similar to eukaryotes, we named this unique microorganism the 'Myojin parakaryote' with the scientific name of Parakaryon myojinensis ('next to (eu)karyote from Myojin') after the discovery location and its intermediate morphology. The existence of this organism is an indication of a potential evolutionary path between prokaryotes and eukaryotes.

Ongoing discoveries of new rangeomorph fossils from the Ediacaran of Avalonia allow us to put forward a unified and approachable scheme for the description and phylogenetic analysis of frondose genera and their species. This scheme focuses upon the branching morphology of rangeomorph units. Our system has the advantage of being applicable at all visible scales of subdivision and is suitable for the study of isolated fragmentary specimens. The system is also free from hypothesis about biological affinity and avoids tectonically influenced features such as shape metrics. Using a set of twelve character states within this unified scheme, we here present emended diagnoses for Beothukis, Avalofractus, Bradgatia, Hapsidophyllas, Fractofusus, Trepassia and Charnia, together with a more extensive taxonomic treatment of the latter genus. For those forms that fall within the morphological spectrum between Trepassia and Beothukis, we introduce Vinlandia gen. nov. It is hoped that this scheme will provide a robust framework for future studies of rangeomorph ontogeny and evolution.

The Zgounder ore deposit (Anti-Atlas, Morocco), is hosted in a PII-PIII Proterozoic volcanosedimentary series. Disseminated mineralization is dominated by mercuriferous native silver (2 to 30 wt.% Hg), with few silver sulfosalts (acanthite, pearceite), arsenopyrite and base-metal sulfides. Arsenic grade of arsenopyrite and homogenisation temperatures of fluid inclusions indicate initial conditions of high temperature (above 400 °C). Lead isotope compositions comfort a Late-Proterozoic age and a crustal origin for metals. Similarities are obvious with the neighbouring silver ore deposit of Imiter and lead to consider Zgounder as another example of Neoproterozoic epithermal deposit in the Anti-Atlas of Morocco, a region that appears more and more as a silver metallogenic province

Soapstone deposits on the Belcher Islands are a product of intrusion of the Haig gabbro sills as part of 1960 ±160Ma plume-related volcanic events. Multiple sills intruded into wet sedimentary units underlying the Flaherty pillow basalts. Evidence of autohydration of sills include the uralitization and chloritization of pyroxene, epidotization, saussuritization and calcite replacement of plagioclase, and leucoxenization of ilmenite. Autohydration accompanied subsolidus cooling of 1000°C gabbro sills to under 400°C under low 2kbar pressure conditions.

In September of 1869, while studying sponges off the Norwegian island of Gisoe, Ernst Haeckel (1834-1919) discovered a tiny, flagellated ball-shaped organism swimming about in his samples. Appearing first to be the planula larva of an invertebrate marine animal further observation revealed it to be a colony of flagellated cells with a complex life cycle transitioning between multicellular and single-cell stages and several distinct forms of protozoa. Haeckel named it Magosphaera planula (the "magician's ball") and it eventually assumed a central role in his theories of animal evolution, appearing as the modern exemplar of the blastaea stage in his gastraea theory of metazoan evolution. Throughout the latter half of the nineteenth century and into the twentieth it was an object of considerable scientific interest, and yet it was only ever observed by Haeckel himself and then only the once. Eventually it faded altogether from scientific discussion. This paper traces the rise and fall of Magosphaera as an important epistemic object in the theories of Haeckel and other biologists, and an attempt is made to identify what exactly the organism (or organisms!) was that Haeckel observed in the fall of 1869.

Geological history from the late Palaeoproterozoic to early Neoproterozoic is dominated by the formation of the supercontinent Columbia, and its break-up and re-amalgamation into the next supercontinent, Rodinia. On a global scale, major orogenic events have been tied to the formation of either of these supercontinents, and records of extension are commonly linked to break-up events. Presented here is a synopsis of the geological evolution of southwest Fennoscandia during the ca. 1.9–0.9 Ga period. This region records a protracted history of continental growth and reworking in a long-lived accretionary orogen. Three major periods of continental growth are defined by the Transscandinavian Igneous Belt (1.86–1.66 Ga), Gothian (1.66–1.52 Ga), and Telemarkian (1.52–1.48 Ga) domains. The 1.47–1.38 Ga Hallandian–Danopolonian period featured reorganization of the subduction zone and over-riding plates, with limited evidence for continental collision. During the subsequent 1.38–1.15 Ga interval, the region is interpreted as being located inboard of a convergent margin that is not preserved today and hosted magmatism and sedimentation related to inboard extensional events. The 1.15–0.9 Ga period is host to Sveconorwegian orogenesis that marks the end of this long-lived accretionary orogen and features significant crustal deformation, metamorphism, and magmatism. Collision of an indenter, typically Amazonia, is commonly inferred for the cause of widespread Sveconorwegian orogenesis, but this remains inconclusive. An alternative is that orogenesis merely
represents subduction, terrane accretion, crustal thickening, and burial and exhumation of continental crust, along an
accretionary margin. During the Mesoproterozoic, southwest Fennoscandia was part of a much larger accretionary orogen
that grew on the edge of the Columbia supercontinent and included Laurentia and Amazonia amongst other cratons. The
chain of convergent margins along the western Pacific is the best analogue for this setting of Proterozoic crustal growth and
tectonism.

The Olympic Dam polymetallic deposit is located 520 km northnorthwest of Adelaide, South Australia. The deposit has an inferred resource of 2660 Mt at 1.2% Cu, 1.4 kg/t U 3 O 8 , 0.5 g/t Au. The deposit occurs in a hydrothermal breccia complex within Mesoproterozoic crystalline basement of the Stuart Shelf. It is overlain by 260 m of flat-lying sedimentary rocks and has no surface expression. The mineralisation is irregularly distributed within the breccia complex, occurring predominately in haematiterich matrix material. The deposit was located in 1975 by Western Mining Corporation by the drilling of coincident gravity, magnetic and tectonic targets, identified with respect to a geological conceptual model.

The North African section of the Gondwana margin was the site of voluminous, arc-related magmatism during the Late Neoproterozoic (Avalonian–Cadomian orogen). The lower (and older) metasedimentary sequence that constitutes the Basal Units of the Allochthonous Complexes of NW Iberia was deposited in that setting. In these units, sedimentation was followed by the intrusion of tonalites and granodiorites in the late Cambrian (ca. 493–489Ma). In the Late Paleozoic, the collision of Gondwana and Laurussia (Variscan orogeny) deformed and metamorphosed the whole ensemble.

New whole rock geochemical analysis performed in seven samples of metatonalites and fourteen samples of metagranodiorites are characterized by: i) slight enrichment in incompatible elements (Rb, Ba, Th, U), ii) negative anomalies in Nb, Ta, P, and Ti, and iii) negative anomalies in Eu. These chemical features are in agreement with a subduction-related setting for the genesis of both types of magma, which is also supported by chemical discrimination using tectonic setting diagrams. Positive anomalies of Pb suggest a crustal component. The new geochemical data reveal that the convergent orogen that ruled the paleogeography of the Gondwana periphery during the Neoproterozoic (Cadomian orogen) remained active beyond the Neoproterozoic-Cambrian transition, up to at least late Cambrian times.

Analysis of more than 900 wireline logs indicates that the Middle Devonian Marcellus Formation encompasses two thirdorder transgressive-regressive (T-R) sequences, MSS1 and MSS2, in ascending order. Compositional elements of the Marcellus Formation crucial to the successful development of this emerging shale gas play, including quartz, clay, carbonate, pyrite, and organic carbon, vary predictably within the proposed sequencestratigraphic framework. Thickness trends of Marcellus T-R sequences and lithostratigraphic units reflect the interplay of Acadian thrust-load-induced subsidence, short-term base-level fluctuations, and recurrent basement structures. Rapid thickening of both T-R sequences, especially MSS2, toward the northeastern region of the basin preserves a record of greater accommodation space and proximity to clastic sources early in the Acadian orogeny. However, local variations in T-R sequence thickness in the western, more distal, area of the basin may reflect the reactivation of inherited Eocambrian basement structures, including the Rome trough and northwest-striking cross-structural discontinuities, induced by Acadian plate convergence. Episodes of block displacement locally warped the basin into northeast-southwest-trending regions of starved sedimentation and/or erosion adjacent to depocenters in which regressive systems tract deposits were ponded. Block movement

Whereas both meteorite impact and volcanic mechanisms have been proposed to explain the rocks and associated ore deposits within the Sudbury structure (1.85 Ga), few models take into consideration the paleogeographic constraints implied by the history of the region before, and following the "Sudbury event". Huronian depositional style south of the Sudbury structure appears to have been strongly influenced by the early development of transtensional extensional basins. These basins were initiated by sinistral strike-slip movement along part of a Paleoproterozoic continental margin, which later evolved into a passive margin. Transition from a rifted strike-slip margin to a passive margin was accompanied by major slope failure and soft sediment deformation south of the Murray Fault. Diabase dikes were intruded into unconsolidated sediments at considerable depth, producing round-stone breccias, which are texturally similar to Sudbury breccia. Minor compressional folding occurred prior to emplacement of the Nipissing Diabase Suite at around 2.2 Ga. The abundance of diabase intrusions near the Sudbury structure suggests that the area may have been a focal point for magma emplacement.

"Diploblastic eumetazoans of the phylum Cnidaria originated during the Neoproterozoic Era, possibly during the Cryogenian Period. The oldest known fossil cnidarians occur in strata of Ediacaran age and consist of polypoid forms that were either nonbiomineralizing or weakly so. The oldest possible anthozoans, including the genus Ramitubus, may be related to tabulate corals and occur in the Doushantuo Lagerstatte (upper Doushantuo Formation, South China), the age of which is poorly constrained (approximately 585 Ma?). Conulariid scyphozoans may first appear as early as 635–577 Ma (Lantian Formation, South China). A definite conulariid, most similar to Palaeozoic species assigned to the genus Paraconularia, occurs in association with the possible scyphozoan, Corumbella werneri, in the latest Ediacaran (c. 543 Ma) Tamengo Formation of Brazil. Basal Cambrian (c. 540 Ma) phosphorites in the upper Kuanchuanpu Formation (South China) yield solitary polyps of the oldest probable anthozoan (Eolympia pediculata), which appears to have been a stem hexacorallian. This same formation contains fossils interpreted by some authors as pentaradial cubozoan polyps; however, both the oldest known cubozoans and the oldest hydrozoans, all medusae, may actually occur in the Cambrian (Series 3, c. 505 Ma) Marjum Formation (Utah, USA). Although these recently published palaeontological data tend to corroborate the hypothesis that Cnidaria has a relatively deep Neoproterozoic history, the timing of major internal branching events remains poorly constrained, with, for example, the results of some molecular clock analyses indicating that the two cnidarian subphyla (Anthozoaria and Medusozoa) may have originated as many as one billion years ago. Further progress towards elucidating the evolution and early fossil record of cnidarians may accrue from: (1) an intensive search for phosphatized soft parts in possible anthozoans from the Ediacaran Doushantuo Formation; (2) an expanded search for Ediacaran conulariids; and (3) additional detailed analyses of the taphonomy and preservation of Ediacaran and Cambrian cnidarians, including possible pentaradial cubozoan polyps from the Fortunian upper Kuanchuanpu Formation."

The Kitto intrusion is a Mesoproterozoic 1117.7 ±1.8 Ma peridotitic to gabbroic intrusion, that formed as a result of plume magmatism and was emplaced along Mesoproterozoic and Archean faults in the East Nipigon Embayment of northern Ontario. The plume signature for the Kitto intrusion is suggested by five samples with OIB (ocean island basalt)-like primitive mantle normalized multielement patterns in the lherzoliteolivine websterites.

The siliciclastic lacustrine rocks of the~1000 Ma Diabaig Formation, northwest Scotland, contain a remarkable diversity of macroscopic structures on bedding planes that can be compared with various kinds of microbially induced sedimentary structures (MISS). Field sedimentological investigations, combined with laboratory analysis of bedding planes and petrographic study of thin sections have allowed us to characterise a range of depositional environments and document the spectrum of biological structures. MISS are reported from frequently subaerial environments, through commonly submerged facies, and down to permanently sub-wavebase settings. Palaeoenvironmental conditions (water depth, exposure, hydrodynamic energy) control the distribution of MISS within these facies. This demonstrates that mat-forming microbial communities were arguably well adapted to low light levels or periodic exposure. Some MISS from the Diabaig Formation are typical of Precambrian microbial mats, including reticulate fabrics and 'old elephant skin' textures. In addition to these, a number of new and unusual fabrics of putative microbial origin are described, including linear arrays of ridges and grooves (cf. 'Arumberia') and discoidal structures that are comparable with younger Ediacaran fossils such as Beltanelliformis. These observations indicate that benthic microbial ecosystems were thriving in freshwater lake systems~1000 Ma, and indicate how microbially induced sedimentary structures may be applied as facies indicators for Proterozoic lacustrine environments. The discovery of structures closely resembling Ediacaran fossils (cf. Beltanelliformis) also serves to highlight the difficulty of interpreting simple discoidal bedding plane structures as metazoan fossils.

Palaeomagnetic data are used to study the configurations of continents during the Proterozoic. Applying stringent reliability criteria, the positions of the continents at 12 times in the 2.45- to 1.00-Ga period have been constructed. The continents lie predominantly in low to intermediate latitudes. The sedimentological indicators of palaeoclimate are generally consistent with the palaeomagnetic latitudes, with the exception of the Early Proterozoic, when low latitude glaciations took place on several continents.The Proterozoic continental configurations are generally in agreement with current geological models of the evolution of the continents. The data suggest that three large continental landmasses existed during the Proterozoic. The oldest one is the Neoarchaean Kenorland, which comprised at least Laurentia, Baltica, Australia and the Kalahari craton. The protracted breakup of Kenorland during the 2.45- to 2.10-Ga interval is manifested by mafic dykes and sedimentary rift-basins on many continents. The second ‘supercontinental’ landmass is Hudsonland (also known as Columbia). On the basis of purely palaeomagnetic data, this supercontinent consisted of Laurentia, Baltica, Ukraine, Amazonia and Australia and perhaps also Siberia, North China and Kalahari. Hudsonland existed from 1.83 to ca. 1.50–1.25 Ga. The youngest assembly is the Neoproterozoic supercontinent of Rodinia, which was formed by continent–continent collisions during ∼1.10–1.00 Ga and which involved most of the continents. A new model for its assembly and configuration is presented, which suggests that multiple Grenvillian age collisions took place during 1.10–1.00 Ga. The configurations of Kenorland, Hudsonland and Rodinia depart from each other and also from the Pangaea assembly. The tectonic styles of their amalgamation are also different reflecting probable changes in sizes and thicknesses of the cratonic blocks as well as changes in the thermal conditions of the mantle through time.

The study of life remote in space has strong parallels with the study of life remote in time. Both are dependent on decoding those historic phenomena called 'fossils', here taken to include biogenic traces of activity and waste products. There is the shared problem of data restoration from incomplete data sets; the importance of contextual analysis of potentially viable habitats; the centrality of cell theory; the need to reject the null hypothesis of an abiogenic origin for candidate cells via morphospace analysis; the need to demonstrate biology-like behaviour (e.g., association with biofilm-like structures; tendency to form clusters and 'mats'; and a preference for certain substrates), and of metabolism-like behaviour (e.g., within the candidate cell wall; within surrounding 'waste products'; evidence for syntrophy and metabolic cycles; and evidence for metabolic tiers). We combine these ideas into a robust protocol for demonstrating ancient or extra-terrestrial life, drawing examples from Earth's early geological record, in particular from the earliest known freshwater communities of the 1.0 Ga Torridonian of Scotland, from the 1.9 Ga Gunflint Chert of Canada, from the 3.4 Ga Strelley Pool sandstone of Australia, and from the 3.46 Ga Apex Chert also of Australia.

Recent geological studies have provided new concepts concerning the timing of the final break-up of the Archaean craton. Age data on detrital zircons from sedimentary rocks in Sweden and Finland show marked similarities and indicate pervasive homogenization of accumulated material. Geochemical and isotopic studies in Sweden and Finland suggest the existence of two volcanic belts, separated by an area dominated by sedimentary rocks. However, the ages of metamorphism and plutonism differ; in particular, the late (post-l.85 Ga) granitoid rnagmatism was more voluminous in Sweden than in Finland. The Svecofennian crust in Finland is anomalously thick (up to 65 km) and is characterized by a high-velocity lower crust of variable thickness. The evolution of the Svecofennian Orogen is modelled by opening of the pre-Svecofennian ocean at 1.95 Ga and progressive accretion of two arc complexes to the Archaean craton between 1.91-1.87 Ga. Initial accretion at 1.91 Ga led to thrusting of the slices of an island arc complex and Proterozoic marine strata onto the thickening craton margin. Subduction reversal with simultaneous anticlockwise rotation of the converging plate, or convergence of another plate, resulted in subduction below the newly accreted arc complex. The tectonic style at the craton margin changed from thrusting to lateral displacement in the east (in Finland). In the west (in Sweden), the subduction zone retreated along a transform fault. A large sedimentary basin closed in the east 1.89 Ga ago when another arc complex collided against the accreted one. Collision resulted in lithospheric thickening, subsequent delamination of the thickened lithosphere and compensation by a hot mantle underplate, and large-scale granitoid magmatism as a result of mixing between mantle magma and remelted arc complex material. The thick, dense layer that formed in the lower crust as the result of underplating kept the thickened crust in isostatic balance. Thus, the exhumation rate in this area was similar to the rate in areas of thinner crust. Subduction and sediment accumulation continued in the west until at least 1.87 Ga when large areas in the east were already stabilized. An intracratonic transpressional zone developed as the result of continued convergence. Subsequent extensional collapse along the transpressional zone caused melting of Svecofennian sedimentary rocks, migmatization and generation of the 1.84-1.82 Ga anatectic granites of southern Finland and central Sweden. The voluminous post-l.85 Ga granitoid magmatism in Sweden is tentatively associated with an east-west extension that is not related to the extensional collapse.

The Kufra Basin is a large\ underexplored\ Palaeozoic intracratonic sag basin in SE Libya and NE Chad with extensions into NW Sudan and SW Egypt[ The basin _ll consists of shallow marine to~uvial deposits ranging in age from infracambrian to Cretaceous[ Geologically\ the basin is very similar to the Murzuq Basin in SW Libya which recently presented Libya with its largest oil discovery for over a decade[ Most of the hydrocarbon play elements known from the Murzuq Basin also occur in the Kufra Basin] thick\ porous Cambro!Ordovician sandstones are present and would form good reservoirs\ lower Silurian shales may act as e}ective seals\ and there are potential structural traps in seismically de_ned fault blocks[ However\ the source rock availability in the Kufra Basin is currently unclear[ One of the two main source rock candidates in the basin is a lower Silurian shale unit "Tanezzuft Formation#[ The Tanezzuft shales have been described as being up to 029 m thick in outcrops at the basin margins\ but the shales were found to be replaced by siltstones and sandstones in two dry exploration wells drilled in the northern part of the basin by AGIP between 0867 and 0870[ Hot shales developed at the base of this widespread Silurian shale unit form important source rocks in many areas of North Africa and Arabia[ These hot shales are interpreted to have been deposited in palaeodepressions\ such as incised valleys of the preceding lowstand\ or intrashelf basins\ during the initial transgression after the melting of the late Ordovician ice cap[ The areal distribution of the organic!rich unit is\ therefore\ discontinuous[ Fieldwork in the Kufra Basin has shown that the basal Tanezzuft horizon is not exposed on the northern and eastern margins of the basin[ Deep infracambrian rift grabens have been interpreted on seismic lines from the Kufra Basin and\ in analogy to Oman and Algeria\ could contain organic!rich infracambrian deposits[ The infracambrian succession in the Kufra Basin may contain a second major potential source rock and warrants further investigation[ Þ 0888 Elsevier Science Ltd[ All rights reserved[ Keywords] Libya^Sahara^North Africa^Source rock^Silurian^Infracambrian^Intraplate basin Corresponding author[ Tel[] ¦33!060!781!8351^fax] ¦33!060!781! 8164[ E!mail address] sebastian[luningÝlasmo[com[

Planetary climate and surface chemistry are tightly coupled; atmospheric composition affects the transfer of solar and infrared radiation, and therefore climate, whereas the components of climate, including temperature and precipitation, strongly affect the chemical composition of the surface environment and the geochemical cycling of elements through it. An element of climatic, environmental and biological importance is sulfur. The modern biogeochemical sulfur cycle has been extensively studied and is relatively well understood. How the sulfur cycle may have differed early in the evolution of Earth and Mars, when the surface was anoxic and biological activity was geochemically unimportant, is less well understood and much more poorly constrained by data.

The Paleoproterozoic Woodline Formation forms a 50 km long, northeast-trending belt of metasedimentary rocks that unconformably overlies Archean granites and greenstones of the Eastern Goldfields Superterrane near the southeastern margin of the Yilgarn Craton. The Woodline Formation is dominated by mature, quartz-rich sandstone units interbedded with siltstones, and represents the remnants of a broader siliciclastic sequence deposited along a passive margin after c. 1737 Ma. The formation is weakly deformed and metamorphosed to lower greenschist facies, with upright northeast-trending folds resulting from northwest-southeast shortening during the Albany-Fraser Orogeny between 1345 Ma and 1140 Ma. An extensional event at c. 1210 Ma is recorded by the emplacement of the northeast-trending Fraser dyke swarm and formation of the grabens that preserved the Woodline Formation.

The ∼1000 Ma Torridon Group of northwest Scotland are here shown to contain a rich diversity of organic walled microfossils preserved in exceptional detail within sedimentary phosphate. The phosphatic nodules and bands in which they occur are autochthonous, as are the organic fossils they contain. The associated sedimentology of this phosphate is shown to be consistent with a lacustrine setting. Informal taxonomic treatment of the microfossils allows statistical assessment of the relative abundances of different morphotypes across a range of lacustrine environments. Exceptional preservation of pristine organic-walled eukaryotes and prokaryotes, together with a taphonomic spectrum in both physicochemical and biological degradational conditions allows a picture to be built of the earliest lacustrine ecological communities.