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A single bed at the base of the Jordan Sandstone in a road cut at Arcadia, Wisconsin, yielded an undescribed Late Sunwaptan (Saukia Zone) trilobite fauna that includes at least four species from the families Dikelocephalidae Miller, 1889 and Eurekiidae Hupe, 1953. Arcadiaspis bispinata n. gen. and sp. is a distinctive eurekiid that is characterized by paired occipital and thoracic axial spines, long genal spines, and a nonspinose pygidial margin. Other genera present are the dikelocephalids, Dikelocephalus Owen, 1852 and Calvinella Walcott, 1914, and the eurekiid, Eurekia Walcott, 1916. Type material of Calvinella spiniger (Hall, 1863) is illustrated photographically for the first time. A new eurekiid species, Corbinia burkhalteri from the Fort Sill Formation, Oklahoma, is also described.

A new lyssacinosan hexactinellid, Rossella bromleyi n. sp., is described from the Upper Cretaceous of Arnager Pynt, Bornholm (Denmark). The sponge shows a cup-shaped form with a broad, moderately dense root tuft, which contains three- and four-rayed orthotropal anchors characteristic for Rossella. The skeleton is composed of bundles of diactines, hexactines of two orders, and pentactines. Diactine bundles are mainly in the growth direction or diagonal to it. Prostalia lateralia are pentactines with the paratangential rays protruding beyond the outer surface of the sponge body. Hypodermalia are absent.

Carbonaceous compression fossils in shales of the uppermost Doushantuo Formation (ca. 555-590 Ma) at Miaohe in the Yangtze Gorges area provide a rare Burgess-Shale-type taphonomic window on terminal Proterozoic biology. More than 100 macrofossil species have been described from Miaohe shales, but in an examination of published and new materials, we recognize only about twenty distinct taxa, including Aggregatosphaera miaoheensis new gen. and sp. Most of these fossils can be interpreted unambiguously as colonial prokaryotes or multicellular algae. Phylogenetically derived coenocytic green algae appear to be present, as do regularly bifurcating thalli comparable to red and brown algae. At least five species have been interpreted as metazoans by previous workers. Of these, Protoconites minor and Calyptrina striata most closely resemble animal remains; either or both could be the organic sheaths of cnidarian scyphopolyps, although an algal origin cannot be ruled out for P. minor. Despite exceptional preservation, the Miaohe assemblage contains no macroscopic fossils that can be interpreted with confidence as bilaterian animals. In combination with other late Neoproterozoic and Early Cambrian body fossils and trace fossils, the Doushantuo assemblage supports the view that body-plan diversification within bilaterian phyla was largely a Cambrian event.

The last magmatic stage before the Sveconorwegian orogeny in the Baltic Shield is represented by bimodal, c.1.16 Ga volcanics from the Bandak Group in southern Norway and largely coeval basalts from the Dal Group in SW Sweden. In both areas, the volcanic rocks are intercalated with sedimentary units and the onset of basin development is marked by deposition of clastic sediments. In the lower part of the Bandak Group, the Morgedal basalts are relatively evolved and geochemical signatures suggest assimilation of lower crustal components. In contrast, the subsequent Gjuve basalts are more primitive and record assimilation of upper crustal rocks similar to exposed basement rocks. In the Bandak Group, the chemistry of the basalts changes within the stratigraphy, such that the older Morgedal basalts (MgO = 5.2-8.8 wt%, initial [[epsilon].sub.Nd] 2.81-3.96, Zr/Y = 4.0-5.1, Zr/Nb = 34-50, La/Nb = 2.4-3.7) are more fractionated and have more enriched Nd isotope ratios compared to the Gjuve basalts (MgO = 6.4-12.3 wt%, initial [[epsilon].sub.Nd] 4.05-4.97, Zr/Y = 3.4-4.7, Zr/Nb = 20-30, La/Nb = 1.5-2.4). This suggests that there earliest basalts are more contaminated, and it appears that the composition of the crustal component changed with time. A thin felsic volcanic unit, the Dalen Formation, separates the two basalt sequences. It represents melting of upper crustal rocks, triggered by injection of mafic magmas into crustal magma chambers. In the Dal Group, basalts form a relatively thin unit (< 500 m) where flows with high MgO values (> 9%) were least affected by assimilation of crustal components. Whole-rock geochemistry and Nd isotope ratios for the Bandak and Dal basalts indicate shallow melting of sub-continental lithospheric mantle in response to extension in a continental back-arc setting, related to subduction along the western margin of Baltica. These constraints envision a similar tectonic evolution with that of eastern Laurentia, which support models of a pre-Grenvillian supercontinent with a long-lived, active margin that reached western Baltica. Keywords: Mesoproterozoic, Sveconorwegian Orogeny, Laurentia-Baltica, volcanism, geochemistry.

Major and trace element and Sr-Nd-Pb-O-C isotopic compositions are presented for carbonatites from the Cape Verde (Brava, Fogo, Sao Tiago, Maio and Sao Vicente) and Canary (Fuerteventura) Islands. Carbonatites show pronounced enrichment in Ba, Th, REE, Sr and Pb in comparison to most silicate volcanic rocks and relative depletion in Ti, Zr, Hf, K and Rb. Calcio (calcitic)-carbonatites have primary (mantle-like) stable isotopic compositions and radiogenic isotopic compositions similar to HIMU-type ocean island basalts. Cape Verde carbonatites, however, have more radiogenic Pb isotope ratios (e.g. [sup.206]Pb/[sup.204]Pb = 19.3-20.4) than reported for silicate volcanic rocks from these islands (18.7-19.9; Gerlach et al. 1988; Kokfelt 1998). We interpret calcio-carbonatites to be derived from the melting of recycled carbonated oceanic crust (eclogite) with a recycling age of ~1.6 Ga. Because of the degree of recrystallization, replacement of calcite by secondary dolomite and elevated [delta][sup.13]C and [delta][sup.18]O, the major and trace element compositions of the magnesio (dolomitic)-carbonatites are likely to reflect secondary processes. Compared with Cape Verde calcio-carbonatites, the less radiogenic Nd and Pb isotopic ratios and the negative [Delta]7/4 of the magnesio-carbonatites (also observed in silicate volcanic rocks from the Canary and Cape Verde Islands) cannot be explained through secondary processes or through the assimilation of Cape Verde crust. These isotopic characteristics require the involvement of a mantle component that has thus far only been found in the Smoky Butte lamproites from Montana, which are believed to be derived from subcontinental lithospheric sources. Continental carbonatites show much greater variation in radiogenic isotopic composition than oceanic carbonatites, requiring a HIMU-like component similar to that observed in the oceanic carbonatites and enriched components. We interpret the enriched components to be Phanerozoic through Proterozoic marine carbonate (e.g. limestone) recycled through shallow, subcontinental-lithospheric-mantle and deep, lower-mantle sources.

The stratigraphical and structural continuity of the Late Proterozoic Dalradian rocks of the Scottish Highlands is re-examined in the light of new U--Pb zircon ages on the tuffs belonging to the Tayvallich Volcanic Formation (601 [+ or -] 4 Ma), and on the late Grampian `Newer Gabbros' (470 [+ or -] 9 Ma) of Insch and Morven-Cabrach in Aberdeenshire. These age data, together with the existing 590 [+ or -] 2 Ma age for the Ben Vuirich Granite, provide key radiometric constraints on the evolution of the Dalradian block, and the implications arising from these ages are critically assessed. Three main conclusions are drawn. (1) The entire Caledonian orogeny, although short-lived, is unlikely to have affected sediments of Arenig age and a break probably occurs between those Dalradian sediments of late Proterozoic ( (2) A period of crustal thickening probably affected some Dalradian rocks prior to 590 Ma. Such an event is indicated by both the polymetamorphic histories of the lower parts of the Dalradian pile and the contact metamorphic assemblages within the aureole of the Ben Vuirich Granite, which are incompatible with sedimentary thicknesses. (3) Age constraints on global Late Proterozoic glacial activity also suggest that the Dalradian stratigraphy is broken into discrete smaller units. Models involving continuous deposition of Dalradian sediments from pre-750 Ma to 470 Ma are rejected. Keywords: Dalradian Orogeny, Neoproterozoic, glaciation, U--Pb, zircons.

Ediacara-type fossils represent a group of soft-bodied organisms, mainly known from imprints in Proterozoic coarse-grained siliciclastic sedimentary rocks. Circular compressions of Beltanelliformis brunsae and remains related to Ediacara-type fossils, such as Cucullus fraudulentus, and Mucuplagum primitivum are reported here in an organic mode of preservation from the Neoproterozoic Doushantuo and Liulaobei Formations of China. They can be interpreted as prokaryotic colonies. A charnid fossil with circular attachment disc and stalk, but torn-off frond, is documented in a three-dimensional and partly organic mode of preservation from the Neoproterozoic Ust-Pinega Formation (White Sea coast, Russia). According to their morphology and structure, the Charniidae are not regarded by us as pennatulaceans. Modern Myxobacteria illustrate that macroscopic size, complexity, and even compartmentalization can also be developed by prokaryotic colonies. Part of the Ediacara-type fossils may therefore represent prokaryotic colonies or symbiotic organisms involving prokaryotes. Finally, direct evidence indicates that biofilms with associated prokaryotic sheaths, preserved in both organic and pyritic fashion, form the wrinkled surfaces ('elephant skin') that were preferentially colonized by Ediacara-type fossils. This finding supports previous interpretations, based on comparative morphological and sedimentological approaches, that ancient wrinkle structures were microbial mats. Keywords: Proterozoic, fossil, prokaryotes, Ediacaran, organic residues.

SHRIMP analyses of detrital zircon cores from high-grade metasediments from the Mid-German Crystalline Rise, which is situated between Eastern Avalonia in the NW and Saxo-Thuringia in the SE, yield mostly ages of c. 550 Ma and c. 2.06 Ga, with minor c. 1.0 and 2.4-2.9 Ga components. The sedimentary protoliths were deposited during the Late Proterozoic to Early Cambrian, probably prior to the break up of the northern Gondwana margin at 460-500 Ma. These data are consistent with the sediments' high end values (0.9 to -3.0), which are comparable to those of well-documented Late Proterozoic sediments from other parts of Europe. The combined isotopic data suggest derivation of the sediments from at least three distinct crustal source regions. Dominant sources were the Avalonian-Cadomian orogenic belt (c. 45%), situated at the northern margin of Gondwana during the Neoproterozoic, and the West African and/or eastern Amazonian cratons (c. 45%). The Grenvillian belt was a minor source (c. 10%). Keywords: SHRIMP data, Sm-Nd, Mid-German Crystalline Rise, Gondwana, provenance.

Strong affinities between New Hampshire's Massabesic Gneiss Complex and southern New England's Avalonian rocks provide evidence of continental rifting in the late Proterozoic era. The Complex's dominant rock type, migmatized paragneiss, has both trace and major element compositions that correspond with continental arc graywackes.

LITHOPROBE seismic reflection data acquired in 1994 within the Trans-Hudson orogen include a 180 km profile that crosses the boundary between a collapsed marginal basin (Kisseynew domain) and an older volcanic-plutonic arc terrane (Lynn Lake belt). The seismic data image a prominent band of northeast-dipping anastomosing reflections that extend to 10 s two-way traveltime from the surface location of a geological boundary where interpreted oceanic crust is preserved, and separate thick crust (16 s) of the Kisseynew domain from the abruptly thinner crust (12 s) of the Lynn Lake belt. Highly reflective lower crust (10-16 s) of the Kisseynew domain dips gently northward and extends laterally for at least 40 km beneath the adjacent Lynn Lake belt in a manner reminiscent of the western Alps. Complementary gravity modeling shows that the thickness of metasedimentary rocks within the Kisseynew domain is 20 +/- 3 km. The seismic image is interpreted as portraying a crustal-scale collisional suture that formed as the end result of northward subduction of backarc basin crust (ancestral Kisseynew basin floor), and subsequent contraction and thrusting of the remnant Kisseynew basin fill under the margin of the Lynn Lake belt. Keywords: suture, seismic, gravity, Proterozoic, subduction.

Thousands of cubic kilometers of high-K calc-alkalic magmas intruded the Borborema Province (northeastern Brazil) during the Neoproterozoic Brasiliano orogeny. They make up large batholiths in which mantle-derived mafic to intermediate rocks coexist with a larger amount of granitoids. The relatively low silica contents (61-70 wt% SiO(sub 2)) and moderate to high compatible element concentrations (0.3-3.5 wt% MgO, 1.5-3.8 wt% CaO, as much as 150 ppm of Cr) of the granitoids indicate that they contain an appreciable mantle component. The similar trace element geochemical (high contents of incompatible trace elements) and isotopic (strongly negative (epsilon)(sub Nd) values) signatures of mafic and felsic rocks combined with geochemical modeling suggest that (1) the mafic and felsic rocks are genetically linked, (2) the granitic magmas were produced by 20%-30% partial melting from a source having geochemical characteristics similar to the mafic rocks, and (3) mingling and mixing of felsic magmas with subsequent batches of mafic magmas yielded the silica-poor granitoids. Isotopic data preclude involvement of the asthenosphere in the genesis of the mafic melts and instead indicate their derivation from an old, enriched lithospheric mantle. Therefore, addition of mantle material to the crust occurred through internal lithospheric differentiation, in contrast with conventional crustal-growth models. Keywords: high-K, calc-alkalic, geochemistry, Nd isotopes, Proterozoic crustal growth.

Ediacaran fossils from the southwestern Great Basin may help constrain regional Vendian-Cambrian biostratigraphy and provide biogeographic links between facies in this region and elsewhere. Locally, trace fossils suggest the Vendian-Cambrian boundary occurs within or below the upper third of the lower member of the Wood Canyon Formation. Ediacaran soft-bodied and tubular fossils, including the frondlike fossil Swartpuntia and tubular, mineralized or agglutinated fossils similar to Archaeichnium, Cloudina, Corumbella, and Onuphionella occur in the lowermost Wood Canyon Formation. Discoidal forms referred to Nimbia occur in both the lowermost Wood Canyon Formation and the underlying strata of the Stirling Quartzite. These fossils occur directly below Lower Cambrian trace fossils, including Treptichnus pedum, and confirm the persistence of the Ediacaran biota to near the base of the Cambrian. These faunas may also help strengthen previously proposed correlation schemes between the two main facies belts of the southwestern Great Basin (the Death Valley and White-Inyo facies), because a nearly identical Vendian-lowest Cambrian succession of faunas occurs in both regions. Lastly, lack of cosmopolitan Ediacaran faunas in these strata suggests a paleobiogeographic link between the southwestern U.S. and southern Africa in Vendian time.

Research presented concerns the geochemistry of Proterozoic metasedimentary rocks formed under high temperatures in Halls Creek Orogen of northern Australia. Techniques in geochronology are discussed, including the analysis of zircon crystals using Sensitive High-mass Resolution Ion Micro Probe.

A laterally extensive, Neoproterozoic extensional detachment (the Betsileo shear zone) is recognized in central Madagascar separating the Itremo sheet (consisting of Palaeoproterozoic to Mesoproterozoic sediments and underlying basement rocks) from the Antananarivo block (Archaean/Palaeoproterozoic crust re-metamorphosed in the Neoproterozoic). Non-coaxial deformation gradually increases to a maximum at a lithological contrast between the granitoids and gneisses of the footwall and the metasedimentary rocks of the hangingwall. Ultramylonites at this highest-strained zone show mineral-elongation lineations that plunge to the southwest. (Sigma), (beta) and C/S-type fabrics imply top-to-the-southwest extensional shear sense. Contrasting metamorphic grades are found either side of the shear zone. In the north, where this contrast is greatest, amphibolite-grade footwall rocks are juxtaposed with lower-greenschist-grade hangingwall rocks. The metamorphic grade in the hangingwall increases to the south, suggesting that a crustal section is preserved. The Betsileo shear zone facilitated crustal-scale extensional collapse of the East African Orogeny, and thus represents a previously poorly recognized structural phase in the story of Gondwanan amalgamation. Granitic magmatism and granulite/amphibolite-grade metamorphism in the footwall are all associated with formation of the Betsileo shear zone, making recognition of this detachment important in any attempt to understand the tectonic evolution of central Gondwana.

Arrested-type charnockite formation occurs in an assemblage of high-grade gneisses at several localities of the Chilka Lake area that belongs to the Proterozoic Eastern Ghats Belt of India. The isolated ellipsoidal domains are found exclusively in leucogranite (leptynite) bands that intruded litpar-lit interbanded granulite-grade supracrustal and intermediate igneous rocks (khondalite-enderbite). Macrostructures and microfabrics document a multiple deformation of the rock assemblage under high-grade conditions. The intrusion of the leucogranitic melts separates a first episode of deformation, D(sub 1), from a younger progressive deformation, D(sub 2)-D(sub 4). A transpressive regime and inhomogeneous deformation is indicated for D(sub 2)-D(sub 4) by the associated structures and fabrics. But quartz caxis patterns show that pure shear prevailed during the closing stages of deformation. The spatial distribution and orientation of the ellipsoidal charnockite domains within the host leptynite and the orientation pattern of orthopyroxene c-axes inside the domains provide evidence for a synkinematic in situ formation of the domains during D(sub 3), through partial breakdown of the leptynite assemblage (Bt + Grt + Qtz + Fl(sub 1) < > Opx + Fsp + Ilm + Fl(sub 2)/L). Local fluid migration along steep foliation planes associated with large-scale D(sub 3) folds triggered the reaction. Orthopyroxene blastesis was confined to the centre of the domains, and an envelope formed in which the residing fluid caused secondary intergranular formation of chlorite, ore and carbonate, imparting the domains' typical greenish-brown charnockite colour. The shape of the envelope, which varies from prolate in limbs to oblate in hinges of D(sub 3) folds, is responsive to the local stress field. Comparison of chemical rock compositions supports the in situ formation of charnockite in leptynite. Subtle compositional differences are controlled by the changing mineralogy. Compared to the host leptynite, the charnockite domains are enriched in K(sub 2)O, Ba, Rb and Sr, but depleted in FeO*, MnO, Y and Zr. The data obtained in this study provide conclusive evidence that the ellipsoidal charnockite domains do not represent remnants of stretched enderbite layers as proposed by Bhattacharya, Sen & Acharyya, but formed in situ in the leptynite as a result of localized synkinematic fluid migration late in the deformation history.

The age distributions of four detrital zircon samples from Western Australia placer deposits are found to be dominated by Neoproterozoic and Mesoproterozic ages; this outcome challenges a long-held assumption about derivation from the nearby Archean Yilgarn craton. The dominant ages are consistent with derivation from Proterozoic orogens marginal to the Yilgarn craton, including the Pinjarra orogen and Leeuwin block to the west and the Albany-Fraser orogen to the south. The results suggest that these marginal orogens were preferentially uplifted in the Jurassic-Cretaceous rifting event that formed the Perth basin and subsequently dominated the sedimentary contribution to this basin. These sediments have since been recycled to form the modern beach-sand placer deposits that reflect their non-Archean provenance despite proximity to the Yilgarn craton. The lack of a sedimentary contribution from the Yilgarn craton to the Perth basin implies that peneplanation was complete before the Mesozoic and that denudation rates have been minimal ever since.

Widespread quartz pseudomorphs after evaporitic minerals are interbedded with stromatolites in 2.2 Ga sedimentary rocks in the Yerrida rift basin of Western Australia. These deposits preserve diverse original crystal morphologies that grew displacively either as individuals or as clusters within stromatolitic horizons and associated fine-grained siliciclastic beds. The stromatolites were deposited in shallow-marine, restricted enviroments. Although evaporitic minerals are largely replaced by quartz, their crystal shapes include lenticular, rosette, needle, and nodular forms, suggesting that they formed from calcium sulfate-rich brines. Textures suggest that replacement by quartz occurred during early alteration of the sediments. Original calcium sulfate minerals such as gypsum or anhydrite are indicated by the morphologies of the pseudomorphs; this concept is supported by anhydrite inclusions in the quartz crystals that form the pseudomorphs, and this finding indicates that calcium sulfate existed in Early Proterozoic sedimentary environments.

A shallow-water shale unit from the early Neoproterozoic Wynniatt Formation, arctic Canada, preserves an unusually high diversity of organic-walled fossils, including abundant cyanobacteria, several multicellular protists and/or problematica, and more than 30 distinct acritarch species. Recognition of 13 new acritarchs, based on novel ornamentation, excystment structures, and/or wall structure, substantially increases their known diversity for this interval and points to a severe undersampling of the Proterozoic fossil record. Three of these new acritarchs exhibit features characteristic of dinoflagellate cysts and are reasonable candidates for early representatives of the clade, particularly in light of recent molecular phylogenetic analyses and biomarker data. The high diversity of acritarchs in the Wynniatt Formation also bolsters the potential for biostratigraphic resolution in the Neoproterozoic.

Permineralized fossils of the terminal Proterozoic (600-550 Ma) Doushantuo Formation, China, provide an unusually clear window on biological diversity just before the Ediacaran radiation. In the eastern Yangtze Gorges region, cherts in lower and upper Doushantuo carbonates preserve prokaryotes and protists from subtidal marine environments below and above fair weather wave base, respectively. Phosphorites in the Weng'an district to the south contain diverse acanthomorphic acritarchs as well as cellularly preserved thalloid algae. Twelve taxa of probable cyanobacteria are recognized. None is endemic to the Doushantuo Formation, and most have long stratigraphic ranges. The apparent restriction of two species to late Neoproterozoic and Cambrian rocks may reflect secular variation in taphonomic circumstance rather than evolution. Thirty-one species of spheromorphic and acanthomorphic acritarchs are recognized, about half of which occur elsewhere in rocks of the same approximate age. At least some of the eight formally described species of multicellular algae can be assigned with confidence to the Rhodophyta; these fossils provide a glimpse of structural and reproductive diversity in Neoproterozoic algae that is, to date, unique. Several reports of Doushantuo animal fossils have been published; most compelling are triact spicules identified in chert nodules. Along with more than two dozen taxa of compressed macrofossils preserved in carbonaceous shales from the top of the formation, Doushantuo permineralizations indicate that large animals radiated into a world rich in prokaryotic, protistan, and, even, multicellular diversity.

The 18 O/16 O ratio and 13 C/12 C ratios of the calcite shells of living non-symbiotic and symbiotic plankton foraminifera decline when seawater increases. Standard glacial foraminiferal stable-isotope information does not predict this finding, nor does it support the hypothesis that a transfer of terrestrial carbon into the sea could have produced the glacial-interglacial change in the 13 C/12 C ratio. The extreme stable-isotope differences in the Phanerozoic and Proterozoic aeons may be explained by the carbonate-concentration effect.

The timing of the evolution of non-photosynthetic sulphur (S)-oxidizing bacteria and changes in stable S isotopes indicates that these events were caused by a large increase in the atmospheric oxygen during late Proterozoic. The oxygen levels increased to more than 5 to 18% of the present atmospheric level and this permitted the development of an oxidative S cycle and bacterial evolution. Analysis of the 16S rRNA of gamma-proteobacteria shows that they evolved almost 0.76 billion years (Gyr) ago. A significant change in the S isotope composition took place between 0.64 to 1.05 Gyr ago.

New fossils from the Neoproterozoic Ediacara fauna of South Australia are interpreted as the oldest known hexactinellid sponges. They occur within the Ediacara Member of the Rawnsley Quartzite (Pound Subgroup) from several locations in the Flinders Ranges. The new genus, Palaeophragmodictya, is characterized by disc-shaped impressions preserving characteristic spicular networks and is reconstructed as a convex sponge with a peripheral frill and an oscular disc at the apex.

Geochronological data on metamorphic minerals within and outside the Bancroft shear zone, a large-scale Proterozoic shear zone, provide evidence for extensional displacement over a period of 150 m.y. This is the longest time interval yet demonstrated for any fault responding to one orogenic phase. Extensional displacement rates along the Bancroft shear zone, averaged over 150 m.y. from 1040 to 893 Ma, are 0.06 to 0.13 km/m.y. During extension, unroofing of the footwall was accompanied by rotation as 5 to 15 km of overlying crust was unloaded. Similar lower-plate rotation has been inferred for many of the Tertiary metamorphic core complexes exposed in the western United States. The protracted displacement and rotation history of the Proterozoic Bancroft shear zone contrasts with the much faster extensional unroofing of the younger metamorphic core complexes, suggesting inherent differences in the tectonic processes operating in these two geologic settings. Similarities, however, indicate that both settings may share common aspects of tectonic deformation. These findings underscore the geologic importance of large, deep-crustal faults and indicate that present-day zones of active extension, such as in the Himalayas, could remain active many millions of years into the future.

Electromagnetic studies of the Precambrian basement beneath the Western Canada Sedimentary Basin in Alberta indicate a narrow linear conductivity anomaly spatially correlated with a strong positive magnetic feature, the Red Deer high. The conductor is located below sedimentary cover near the top of the crystalline basement and has limited depth extent. We propose that this anomalous feature represents graphitic metasedimentary rocks in the euxinic-flysch facies of a Proterozoic foredeep sequence. The strong magnetic anomaly results from an associated iron formation deposited on the outer ramp of the foredeep. This model explains the geophysical anomalies, has analogues on the exposed shield, and is consistent with the timing, deformation history, and known geology of the Precambrian basement.

Possible new evidence for the origin of metazoans prior to 1 Ga comes from sediment-filled tubes preserved on a bedding-plane parting in a chert in the Allamoore Formation (Mesoproterozoic) in a talc quarry in the Millican Hills, near Van Horn, Trans-Pecos Texas. Available data for the Van Horn region indicate that the age of the Allamoore Formation is [approximately] 1250 Ma. The sediment-filled tubes are most likely the feeding traces of animals grazing on algal mats in shallow subtidal or intertidal environments.

The Grampian and Appin groups of the southwestern Monadhliath Mountains form the earliest known syn-rift sequences of the Scottish central Highlands. They were likely to have formed in an intracontinental setting and represent deposition of mixed clastic and carbonate shallow and deep marine strata. The Grampian Group of the southern Monadhliath Mountains was deposited during a period of initial basin rifting (NW-SE extension) followed by a phase of thermal subsidence. Syn-rift sediments comprise a 2.5-6km thick turbidite system. Thermal subsidence brought about the basinward progradation of shallow marine shelf sediments resulting in the infilling of pre-existing basin topography. The overlying Appin Group commenced with deposition of a shallow marine sequence alternating between nearshore tidal sand and offshore mud deposition. This formed in response to renewed rifting and concomitant subsidence. Accelerated rifting resulted in localized footwall uplift and erosion while sedimentation continued in the hanging-wall areas. Resultant subsidence, perhaps partly thermally driven, caused gradual basin widening and produced an onlapping marine sequence. There followed a period of progressive clastic deprivation when carbonates were precipitated, and at the onset of anoxic conditions, deposition of organic muds. The fundamental structural elements responsible for the formation of the Grampian and Appin group basins were also influential in the orogenic evolution of the basin-fill. Half-graben fills were deformed to produce regionally extensive folds such as the Stob Ban-Craig a' Chail Synform. Keywords: Proterozoic, basins, sedimentology, synsedimentary processes, orogeny.

Comparison of a new [approximately]1700-1600 Ma segment of the Proterozoic apparent polar wander path (APWP) for Australia with the time-equivalent segment of the North American APWP that has been rotated clockwise by 117 [degrees] about the Euler pole, long 100 [degrees] E, lat 38 [degrees] N, superposes the North American APWP onto the Australian APWP and shows the segments to be similar. The same rotation makes the Pacific margins of the North American and Australian cratons adjacent, as predicted by the Southwest U.S.-East Antarctic (SWEAT) hypothesis, but North America is located farther north relative to Australia than originally suggested. However, the reconstruction is consistent with the identification of western sediment sources for the Belt-Purcell basin in western North America, with matching of basement provinces, and with correlation of major lineaments of the two continents. The rotation gives only very broad agreement between the younger Proterozoic pole sets. This is probably partly due to the sparseness of poles on the Australian APWP whereby prominent features that are found on the North American APWP, such as the Grenville loop, are eliminated on the Australian path by smoothing. Nevertheless, some discrepancies between the younger Proterozoic poles cannot be accounted for without revision of the APWPs.

Silicified peritidal carbonates of the Mesoproterozoic Kotuikan and Yusmastakh Formations, Anabar Uplift, northeastern Siberia, contain exceptionally well-preserved microfossils. The assemblage is dominated by ellipsoidal akinetes of nostocalean cyanobacteria (Archaeoellipsoides) and problematic spheroidal unicells (Myxococcoides); both are allochthonous and presumably planktonic. The assemblage also includes distinctive mat-forming scytonematacean and entophysalidacean cyanobacteria, diverse short trichomes interpreted as cyanobacterial hormogonia or germinated akinetes, rare longer trichomes, and several types of colonial unicells. Although many taxa in the Kotuikan-Yusmastakh assemblage are long-ranging prokaryotes, the overall character of the assemblage is distinctly Mesoproterozoic, with its major features shared by broadly coeval floras from Canada, China, India, and elsewhere in Siberia. Microfossils also occur in middle to inner shelf shales of the Ust'-Il'ya and lower Kotuikan Formations. Leiosphaerid acritarchs (up to several hundred microns in diameter) characterize this facies. As in other Mesoproterozoic acritarch assemblages, acanthomorphic and other complex forms that typify Neoproterozoic assemblages are absent. The combination in Billyakh assemblages of exceptional preservation and low eukaryotic diversity supports the hypothesis that nucleated organisms diversified markedly near the Mesoproterozoic-Neoproterozoic boundary. The assemblages also demonstrate the antiquity of cyanobacteria capable of cell differentiation and suggest the importance of both changing peritidal substrates and evolving eukaryotes in determining stratigraphic patterns of Proterozoic prokaryotes. The permineralized assemblage contains 33 species belonging to 17 genera. Ten new species or new combinations are proposed: Archaeoellipsoides costatus n. sp., A elongatus n. comb., A. dolichos n. comb., A. minor n. nom., A. crassus n. comb., A. major n. comb., A. bactroformis n. sp., Veteronostocale medium n. sp., Filiconstrictosus cephalon n. sp., and Partitiofilum yakschinii n. sp.

In the Neoproterozoic, NW Scotland was located in a tract of extending continental lithosphere that evolved into the eastern passive margin of Laurentia when Iapetus opened in the Vendian. On the NW Caledonian foreland, major rift events are inferred in the Riphean and Vendian, corresponding to the intra-Torridonian and sub-Cambrian unconformities respectively. In this context, current views that the Moine and Dalradian rocks of the Scottish Highlands were subjected to repeated orogeny during the same period become difficult to sustain. We propose a tectonostratigraphic history for the two areas in which Neoproterozoic orogeny plays no part. Keywords: Scotland, Proterozoic, tectonostratigraphy, rifting.

Analysis of the isotopic delta 18O and delta 13C compositions of primary and secondary graphites and calcites from the Bamble Sector, Southeastern Norway, to determine the nature of amphibolite to granulite facies transitions indicates that the primary calcites display a reverse isotopic trend partly due to prograde metamorphic decarbonation reactions, while primary graphites do not display any such trends. Amphibolite grade primary calcites have low delta 13C values unlike granulite grade primary calcites that preserve their sedimentary composition. Reyleigh fractionation modeling suggests that low grade and specific diagenetic processes account for the stable delta 13C compositions and the absence of any trend in primary graphites during metamorphic zoning.

The thick, richly fossiliferous succession of the upper Windermere Supergroup, Mackenzie Mountains, northwestern Canada, provides a test of integrated biostratigraphic and chemostratigraphic frameworks in terminal Proterozoic correlation. The C-and Sr-isotopic abundances of lower Keele Formation carbonates approximate those for other pre-Varanger samples, confirming that the simple disc-like fossils of the underlying Twitya Formation predate all known diverse Ediacaran faunas. 'Tepee' and Sheepbed carbonates record strong post-glacial isotopic excursions; in contrast, [Delta]13C values for Gametrail through Risky carbonates vary only within the narrow range of about +1% to +2%. A second negative excursion occurs in Ingta Formation carbonates that immediately underlie the paleontologically determined Precambrian-Cambrian boundary. The upper Windermere profile as a whole compares closely with curves determined for other terminal Proterozoic successions. The lowermost diverse Ediacaran assemblages in the Sheepbed Formation correlate chemostratigraphically with the oldest fauna in Namibia, but the two assemblages differ in taxonomic composition. Blueflower assemblages correlate both chemostratigraphically and taxonomically with faunas from Australia, China, Siberia, and elsewhere. Increasing data support the hypothesis that paleontological and geochemical data together provide a reliable means of correlating terminal Proterozoic sedimentary rocks throughout the world.

The oldest known tidal rhythmites, identified in the Big Cottonwood Formation, Utah, are Late to Middle Proterozoic in age (800 Ma to 1.0 Ga), [approximately] 250 to 400 m.y. older than the previously oldest known tidal rhythmites. Four tidally forced cycles and one nontidal (seasonal) cycle control lamina thickness patterns. All of these cycles are recognized in outcrop and core and include cycles associated with daily, semimonthly (synodic), monthly (anomalistic), semiyearly, and yearly (seasonal) events. These features form the oldest geological record of lunar-solar tidal forcing and show that the middle to late Precambrian lunar- and solar-generated tides behaved in a manner very similar to that of today. The analysis also suggests that the Big Cottonwood Formation may have undergone a seasonal climate.

The early Proterozoic Polmak-Pasvik-Pechenga-Imandra/Varzuga-Ust'Ponoy Greenstone Belt is situated in the northeastern part of the Baltic Shield and is discontinuously developed over a length of ca. 1000 km. It comprises five main zones, which are separated by longitudinal and a number of transverse syndepositional faults. Each zone contains a number of lithostratigraphic units at the rank of Groups that are subdivided into several sedimentary and volcanic formations with a total thickness over 16,000 m. The sequence comprises a series of sedimentary-volcanic rhythms usually separated by stratigraphic breaks marked by palaeo-weathering. The Groups are separated by major disconformities which may mark/record rift inversion or orogenic episodes. At least seven distinctive lithostratigraphic markers occur in the Belt including two komatiitic volcanic series separated by immature conglomerates, two red-coloured proto-evaporitic formations separated by the alkaline volcanics, and two different 'black shale' formations. Perhaps the main characteristic of the belt is the apparently long history of sedimentary/volcanic development, at the same site, spanning a time-period of 2500 to 1800 Ma. A three stage rift development model is considered: (1) an intracontinental rift stage (2500-2100 Ma), separated by the major inversion (2400-2330 Ma) and comparable with present-day Afar Triangle and East African Rifts; (2) a transitional from intracontinental to intercontinental rift stage (2100-1970 Ma) with possible short-lived spreading (about 1990-1970 Ma); (3) a collision related intercontinental rift stage (1970-1800 Ma) followed by the Svecofenian orogeny (1800-1700 Ma). The observed distribution of platinum group elements (PGE)-mineralization is related to layered gabbro-norite complexes (Fedorova and Panskie Tundry, Gora General'skaya intrusions) dated at 2470 Ma which are apparently co-magmatic and coeval with the first komatiitic volcanic series (Seidorechka Formation) found in the eastern part of the Belt only. The major chromite occurrences are connected with a layered gabbro-norite intrusion (Imandra Lopoloth) which is co-magmatic and coeval between 2440-2330 Ma with the second komatiitic volcanic series (Rusinga, Polisarka, Akhmalahti and Malbekk Formations). The Ni-Cu deposits of the belt are connected with a differentiated gabbro-werhlite complex which is co-magmatic and coeval (ca. 1970 Ma) with the specific Fe- and Ti-rich ferropicrites with komatiitic affinities.

The deposition of thick marine deposits during the Neoproterozoic period along the margins of Laurentia created geodynamic and geotectonic problems. The deposition of earlier thicker sediments and thinner Vendian deposits, interpreted as effects of continental rift, occurred before plate separation. Studies reveal that the thick Neoproterozoic sediments originated at a triple rift junction between Laurentia, Baltica and Amazonia.

The low diversity neoproterozoic ichnofossils in the Lower Alcudian strata of central Spain are formed due to oxygen intake of marine invertebrates with colonization of deep waters. The ichnofossil is similar to the Vendian of the Lower Alcudian megaunit. The fossil consists of different ichnospecies. The pore Palaeozoic structures of the Alcudian strata is constrained to Cadomian deformations.

Analysis of paramagnetically well-characterized uraninite and pitchblende minerals from Proterozoic, high-grade U deposits of the unconformity and complex vein type in northern Saskatchewan, Canada, involved estimations of isotopic and chemical compositions. Significant inconsistencies between U-Pb ages hint at variable recrystallization of U minerals, leading to subsequent Pb loss.

The Nakasib suture is a prominent structural belt in the central Red Sea Hills, Sudan. It is one of the ophiolite-decorated sutures along which the late Proterozoic (Pan-African) island arc/back arc terranes and continental micro-plates of the Arabian-Nubian Shield were welded together. It juxtaposes the 900-800 Ma Haya terrane in the south with the 830-700 Ma Gebeit terrane to the north. New structural data from the Nakasib suture show that the suture had evolved through three phases of deformation which gave rise to a fold and thrust belt. During the early two phases of deformation, SE-verging tight folds and thrusts were developed. The third phase of deformation refolded the earlier structures into NE-trending, upright, and horizontal antiforms and synforms and culminated in the development of NW-verging thrusts. The structural data are used, together with previous stratigraphic and geochemical data, to suggest a tectonic model for the evolution of the Nakasib suture. The suture followed a Wilson Cycle. This started with rifting of the Haya terrane, extrusion of rift volcanics and deposition of passive margin sediments, and development of a NW-dipping subduction zone over which the arc volcanics of the Gebeit terrane erupted. The closing of the oceanic basin gave rise to the Nakasib suture with the present structural configuration. The structural styles of other sutures of the Arabian-Nubian Shield are examined in light of structural data from the Nakasib suture. These sutures show a common feature of steepening of the early subhorizontal ophiolite-obduction structures during a late event of upright folding which may be related to collision between terranes.

Two major mylonite zones are exposed in the southeastern Archean Wyoming province: the Cheyenne belt, which marks the boundary between the Wyoming and Colorado provinces, and the Laramie Peak shear zone, across which Archean rocks have been uplifted differentially at least 10 km. Between the two mylonite zones is a 60-100-km-wide belt of Archean migmatitic gneiss, which has been intruded by diabase dikes and peridotites and contains enclaves of high-grade metasedimentary rocks. We interpret this belt of high-grade rocks as a tectonically reactivated block of Archean crust, uplifted during the development of the Cheyenne belt collisional zone at ca. 1.8 Ga. This block does not exhibit a decreasing metamorphic gradient away from the mylonite zone, but is at uniformly high grade. Thus, rather than a thick-skinned thrust-ramp model for uplift, we invoke vertical uplift along a high-angle reverse fault without rotation. This style of uplift may be a tectonic response to transpression, underplating, or tectonic interfingering at depth during collisional orogeny, and it may be a more common feature of basement uplifts within foreland fold-and-thrust belts than has been previously recognized.

The Mid-Proterozoic-Early Palaeozoic tectonic development of western Dronning Maud Land is examined in the light of recent proposals that suggest geological continuity between Laurentia and Gondwana during the Middle Proterozoic (the SWEAT hypothesis). Gondwana amalgamation from the remaining constituents followed fragmentation and rifting of Laurentia from this supercontinent, culminating in a Late Cambrian orogeny at c. 500 Ma. The Mid-Proterozoic-Early Palaeozoic tectonic development of western Dronning Maud Land was significantly different to that observed in the Transantarctic Mountains, and the c. 500 Ma event in particular is characterized by: (1) a regional isotopic resetting event at c. 470 Ma, (2) scarce coeval magmatism, comprising only one major intrusion of alkaline affinity, and volumetrically small 'S-type' granitic sheets derived from local anatexis of gneisses, (3) structural deformation considerably less intense than during the preceding Kibaran (1100 Ma) orogeny, comprising relatively open to gentle folding, (4) reactivation of thrusting initiated during the Kibaran orogeny, with associated metasomatism. It is concluded that the effects of the Late Cambrian (c. 500 Ma) event are consistent with crustal thickening through large-scale underplating, rather than subduction-related processes as seen in the Ross orogeny of the Transantarctic Mountains. It is also concluded that Gondwana amalgamation occurred along suture lines not currently exposed in outcrop in Dronning Maud Land. These data neither contradict nor support the SWEAT hypothesis as currently argued.

New deep seismic reflection data outline a major crustal transition, from thick craton on the east to thin basinal crust on the west, that is buried beneath the Phanerozoic rocks of the Western Canada sedimentary basin. The transition formed prior to, or during, deposition of westward-thickening Mesoproterozoic sedimentary rocks between about 1.85 and 0.9 Ga and appears to be the southward continuation of the Fort Simpson structural trend, which is crossed by reflection data nearly 700 km to the north. It may be analogous to crustal ramps that are now commonly observed in many Phanerozoic orogens and, along with an associated decrease in Moho depth, has been preserved at this location for more than 1.0 b.y.

Of the sixteen known Oklo and the Bangombe natural fission reactors (hydrothermally altered clastic sedimentary rocks that contain abundant uraninite and authigenic clay minerals), reactors 1 to 6 at Oklo contain only traces of organic matter, but the others are rich in organic substances. Reactors 7 to 9 are the subjects of this study. These organic-rich reactors may serve as time-tested analogues for anthropogenic nuclear-waste containment strategies. Organic matter helped to concentrate quantities of uranium sufficient to initiate the nuclear chain reactions. Liquid bitumen was generated from organic matter by hydrothermal reactions during nuclear criticality. The bitumen soon became a solid, consisting of polycyclic aromatic hydrocarbons and an intimate mixture of cryptocrystalline graphite, which enclosed and immobilized uraninite and the fission-generated isotopes entrapped in uraninite. This mechanism prevented major loss of uranium and fission products from the natural nuclear reactors for 1.2 b.y.

There is a major belt of Proterozoic, so-called anorogenic, igneous rocks that extends from the Baltic Shield through southern Greenland and eastern Canada to southern USA. All rocks are situated within a zone of 1.9-1.6 Ga arc-accretionary orogens that consist of juvenile mantle-derived material especially accretionary prism sediments that provided the fertile source for abundant granitic crustal melts. There were two main periods of formation of the anorogenic rocks. 1.76-1.55 Ga rocks are mainly rapakivi granites and rhyolites that formed about 60-200 Ma after the last compressional events in the Svecofennian, Ketilidian and Penokean orogens, the time-lag being caused by the slow thermal relaxation of thickened lower crust that had commonly been depleted by the extraction of earlier granitic crustal melts. 1.45-1.4 Ga anorogenic rocks are mainly rhyolites and peraluminous granites in central/southern USA and anorthosites in eastern Canada that formed in the continental margin of the Grenvillian ocean, the subduction of which led to the Grenville orogen. These anorogenic igneous rocks formed in extensional regimes during the formation and breakup of a 1.5 Ga supercontinent.

Paleoproterozoic basement occurs on the islands of Islay and Colonsay in western Scotland and on Inishtrahull off the north coast of Ireland. The basement is composed of a deformed alkalic igneous association, collectively refeffed to as the Rhinns complex. Isotopic data indicate that the complex represents new addition of mantle-derived material to the crust at ca. 1.8 Ga. The Rhinns complex may link with rocks of similar age in North America, Greenland, and Scandinavia, which formed part of the southern margin of the Laurentia-Baltica supercontinent., Paleoproterozoic basement has recently been discovered on islands in western Scotland and north Ireland. Called the Rhinns complex, it is composed of deformed alkalic igneous rock representing new crust derived from the mantle at ca 1.8 Ga. This complex may be associated with the southern portion of the Laurentia-Baltica supercontinent, remains of which form the continental crust of North America, Greenland and Scandinavia.

The stratigraphic record contained in several foredeep basins is incompatible with the formation of these basins by flexure of a homogeneously strong foreland lithosphere. For example, the stratigraphy of the Early Proterozoic Kilohigok foredeep basin of northern Canada indicates that the inner part of the basin deepened in time without migration of the flexural bulge toward the foreland, and that the flexural bulge remained fixed in time relative to the downgoing plate. One mechanism that may be responsible for the localization of the flexural bulge in the Kilohigok and similar basins is that of lateral strength variations in the flexing lithosphere. Our results show that lateral strength variations within the foreland lithosphere can produce temporal variations in basin geometry and also serve to localize the position of the flexural bulge relative to the foreland for extended periods of time. This produces basin episodicity on a time scale of millions to tens of millions of years. Perfectly elastic models require a large strength contrast (about 50% in elastic plate thickness), but a brittle-elastic-ductile lithosphere rheology reduces the magnitude of the strength contrast required for episodicity to about 10%-15% of the initial effective elastic plate thickness. We suggest that basin episodicity may induce comparable episodicity in the coeval thrust belt, because theoretical studies of thrust mechanics suggest that the deformation in thrust belts is highly sensitive to the dip of the basal thrust angle. Through mechanisms such as this, the laterally heterogeneous continental lithosphere may provide a natural filter that transforms the presumably continuous and smoothly varying processes of the mantle into the spatially irregular and temporally episodic deformation observed in the crust at time scales of tens of millions of years.

The Quadrilatero Ferrifero is a metallogenic district (Au, Fe, Mn) located in the southernmost Sao Francisco craton in eastern Brazil. It is composed of Archean granitoids, gneisses, and a greenstone belt sequence (Rio das Velhas Supergroup), overlain by a Proterozoic clastic-chemical sedimentary sequence hosting a thick Lake Superior-type banded iron formation (Minas Supergroup). A U-Pb study of the main units leads to the following conclusions: granitoid emplacement was dated at 2776 +7/-6 Ma and 2721 plus or minus 3 Ma; two felsic volcanic rocks from the Rio das Velhas Supergroup yielded ages of 2776 +23/-10 Ma and 2772 plus or minus 6 Ma. These data indicate that greenstone-belt volcanism was coeval with granitoid emplacement, a common feature of greenstone-belt volcanism was coeval with granitoid emplacement, a common feature of greenstone-belt terrains, but so far undocumented in the southern Sao Francisco shield. The younger granitoid intrusion marks the stabilization of the shield in Late Archean time. Zircon cores older than 2776 Ma, together with other data, indicate the presence of continental crust 2.88-3.1 Ga old in the Quadrilatero Ferrifero. A detrital zircon from the uppermost formation in the Minas Supergroup (Sabara Formation) is concordant at 2125 plus or minus 4 Ma and provides the first age constraint on the deposition of the Supergroup. Monazite from two pegmatites and titanite from an amphibolite enclave in gneisses from Bacao Complex, in central Quadrilatero Ferrifero, yielded ages between 2060 Ma and 2030 Ma. These data may constrain the minimum age for the deposition of the Minas Supergroup. In addition, they indicate that the Early Proterozoic Transamazonian orogeny is represented by amphibolite-facies metamorphism and partial melting of Archean crust.

The Fort Simpson structural trend is a series of north-northwest - striking Middle Proterozoic anticlinal culminations buried beneath Paleozoic and Mesozoic cover in northwestern Canada. Interpretations of seismic reflection profiles coupled with drilling data and outcrop and potential-field observations show that the culminations are cored by equivalent to 1.85 Ga crystalline rocks and include overlying sedimentary rocks of the equivalent to 1.7-1.3 Ga Hornby Bay and Dismal Lakes Groups. Formation of the structures probably occurred along west-dipping thrust faults following deposition of the Dismal Lakes Group strata and prior to deposition of the equivalent to 0.8-0.7 Ga Mackenzie Mountains Supergroup strata that unconformably overlie the Dismal Lakes rocks. This time interval coincides with major east-southeast - directed thrusting in the same rocks beneath the Anderson Plains to the north.