Age determination of the Vendian intrusions in the polar Urals

The Precambrian basement beneath the Pechora Basin of northern Russia is known from deep (up to approx. 4.5 km) drill holes to be largely composed of Neoproterozoic successions, variously deformed and metamorphosed and intruded by magmatic suites of Vendian age. Presented here are new single- zircon, Pb-evaporation (Kober method) ages from eight intrusions across the Izhma, Pechora and Bolshezemel'skaya Zones, all from below the Lower Ordovician (locally Middle Cambrian) unconformity. The majority of the intrusions (six) yield remarkably similar ages of 550-560 Ma, apparently dating a widespread pulse of late- to post-tectonic magmatism. An early Vendian granite (618 Ma) has been identified in the northeasternmost region (Bolshezemel'skaya zone) and a Devonian granodiorite (380 Ma) in the Pechora Zone, where mid to late Palaeozoic magmatism has been previously reported. Evidence of inheritance in the zircon populations suggests the presence of Mesoproterozoic crust beneath the Neoproterozoic complexes.

Tab. 1. Listing of BGR seismic refraction results off West Spitsbergen

Three main depositional sequences have been determined in the seismic records taken off West Spitsbergen (1) a Plio-Pleistocene sequence SPI-I with velocities of 1.7 to 2.8 km/sec; (2) a Pliocene allochthonous sequence SPI-II with velocities of 2.4 to 2.8 km/sec underlying unconformity U1; (3) a pre-Middle Oligocene sequence SPI-III with velocities of 2.9 to 4.8 km/sec underlying a distinct unconformity (U2) and deposited in front of the downfaulted Spitsbergen Platform indicating some opening of the Greenland Sea already before tbe time of magnetic anomaly 13 (36 m.y.b.p.). A marked change in the seismic configuration of the oceanic basement has been observed about 30 to 40 km east of the central Knipovich graben. The transition from the oceanic crust of the Knipovich Ridge to the strongly faulted, continental substratum of the Spitsbergen Platform occurs over a narrow zone and is associated with a pre-Middle Oligocene depocenter.

(Table T4) Final depth vs. velocity and two-way traveltime data, ODP Site 178-1103

Site 1103 was one of a transect of three sites drilled across the Antarctic Peninsula continental shelf during Leg 178. The aim of drilling on the shelf was to determine the age of the sedimentary sequences and to ground truth previous interpretations of the depositional environment (i.e., topsets and foresets) of progradational seismostratigraphic sequences S1, S2, S3, and S4. The ultimate objective was to obtain a better understanding of the history of glacial advances and retreats in this west Antarctic margin. Drilling the topsets of the progradational wedge (0-247 m below seafloor [mbsf]), which consist of unsorted and unconsolidated materials of seismic Unit S1, was very unfavorable, resulting in very low (2.3%) core recovery. Recovery improved (34%) below 247 mbsf, corresponding to sediments of seismic Unit S3, which have a consolidated matrix. Logs were only obtained from the interval between 75 and 244 mbsf, and inconsistencies on the automatic analog picking of the signals received from the sonic log at the array and at the two other receivers prevented accurate shipboard time-depth conversions. This, in turn, limited the capacity for making seismic stratigraphic interpretations at this site and regionally. This study is an attempt to compile all available data sources, perform quality checks, and introduce nonstandard processing techniques for the logging data obtained to arrive at a reliable and continuous depth vs. velocity profile. We defined 13 data categories using differential traveltime information. Polynomial exclusion techniques with various orders and low-pass filtering reduced the noise of the initial data pool and produced a definite velocity depth profile that is synchronous with the resistivity logging data. A comparison of the velocity profile produced with various other logs of Site 1103 further validates the presented data. All major logging units are expressed within the new velocity data. A depth-migrated section with the new velocity data is presented together with the original time section and initial depth estimates published within the Leg 178 Initial Reports volume. The presented data confirms the location of the shelf unconformity at 222 ms two-way traveltime (TWT), or 243 mbsf, and allows its seismic identification as a strong negative and subsequent positive reflection.

U, FE, AND LI ISOTOPE RATIOS IN THE MINERALIZED AND BARREN AREAS OF THE KOMBOLGIE BASIN

The Paleo- to Meso-Proterozoic Jabiluka unconformity related uranium mine is located within the Alligator River Uranium Field, found in the Northern Territories, Australia. The uranium ore is hosted in the late middle Paleoproterozoic Cahill Formation, which is unconformably overlain by a group of unmetamorphosed conglomerates known as the Kombolgie subgroup. The Kombolgie subgroup provided the source for oxidized basinal brines, carrying U as the mobile form U(VI), which interacted with reducing lithologies in the Cahill formation, thus reducing U(VI) to the solid U(IV), and leading to the precipitation of uraninite (UO2). In order to characterize fluid interaction with the ore body and compare that to areas without mineralization, several isotopic tracers were studied on a series of clay samples from drill core at Jabiluka as well as in barren areas throughout the ARUF. Among the potential tracers, three were selected: U (redox sensitive and recent fluid mobilization), Fe (redox sensitive), and Li (fractionated by hydrothermal fluids and adsorption reactions). δ238U values were found to be closely linked to the mineralogy, with samples with higher K/Al ratios (indicating high illite and low chlorite concentrations) having higher δ238U values. This demonstrates that 235U preferentially absorbs onto the surface of chlorite during hydrothermal circulation. In addition, δ234U values lie far from secular equilibrium (δ234U of 30‰), indicating there was addition or removal of 234U from the surface of the samples from recent (<2.5Ma) interactions of mobile fluids. δ57Fe values were found to be related to lithology and spatially to known uranium deposits. Decreasing δ57Fe values were found with increasing depth to the unconformity in a drill hole directly above the ore zone, but not in drill holes in the barren area. Similarly to δ238U, δ7Li is found to correlate with mineralogy, with higher δ7Li values associated with samples with more chlorite. In addition, higher δ7Li values are found at greater depth throughout the basin, indicating that the direction of the mineralizing fluid circulation was upwards from the Cahill formation to the Kombolgie subgroup.

Analysis of Evidence from Core Sample Fractures in Stewardson Lake Uranium Prospect

This study was performed to characterize evidence of potential unconformity-type U mineralizing fluids in drill core fractures from the Stewardson Lake prospect, in the Athabasca Basin, located in Northern Saskatchewan and Alberta, Canada. Fractures were visually classified into eight varieties. This classification scheme was improved with the use of mineralogical characterization through SEM (Scanning Electron Microscope) and XRD analyses of the fracture fills and resulted in the identification of various oxides, hydroxides, sulfides, and clays or clay-sized minerals. Fractures were tallied to a total of ten categories with some commonalities in color. The oxidative, reductive or mixed nature of the fluids interacting with each fracture was determined based on its fill mineralogy. The measured Pb isotopic signature of samples was used to distinguish fractures affected solely by fluids emanating from a U mineralization source, from those affected by mixed fluids. Anomalies in U and U-pathfinder elements detected in fractures assisted with attributing them to the secondary dispersion halo of potential mineralization. Three types of fracture functions (chimney, composite and drain) were defined based on their interpreted flow vector and history. A secondary dispersion halo boundary with a zone of dominance of infiltrating fluids was suggested for two boreholes. The control of fill mineralogy on fracture color was investigated and the indicative and non-indicative colors and minerals, with respect to a secondary dispersion halo, were formally described. The fracture colors and fills indicative of proximity to the basement host of the potential mineralization were also identified. In addition, three zones of interest were delineated in the boreholes with respect to their geochemical dynamics and their relationship to the potential mineralization: a shallow barren overburden zone, a dispersion and alteration zone at intermediate depth, and a second deeper zone of dispersion and alteration.

Heavy Minerals in Soils from the Athabasca Basin and the Implications for Exploration Geochemistry of Uranium Deposits at Depth

The Centennial deposit is a high grade (~8% U3O8), deeply buried (~950m), unconformity-related U deposit located in the south-central region of the Athabasca Basin in northern Saskatchewan, Canada. The mineral chemistry of fine fractions (<63 μm) of soils from grids above the Centennial deposit were examined to understand possible controls on the geochemistry and radiogenic 207Pb/206Pb ratios measured in the clay-size (<2 μm) fractions used for exploration. Soil samples distal and proximal to the deposit projection to the surface and geophysically defined structures were selected. Mineral abundances were determined using the scanning electron microscope and Mineral Liberation Analysis. Zircon was the only U-rich mineral identified with modal abundances >0.02% by weight. Monazite, which can be U-rich, was identified, but not in significant abundances. The source of the zircon and other heavy minerals is interpreted to be from sub-cropping sources that are >100 km up-ice from Centennial. Trace element analysis using laser ablation inductively coupled plasma mass spectrometry of hydroseparated zircon grains indicate that zircon abundances and zircon Pb concentrations in surficial samples have minimal effect on the radiogenic 207Pb/206Pb ratios in the clay-fraction of the samples, with the dominant source of radiogenic Pb being clay mineral surfaces that trapped Pb during secondary dispersion from the Centennial uranium deposit through faults and fractures to the surface. The REE patterns indicate HREE enrichment in the clay-fractions of samples that have higher abundances of zircon in the <20 μm fraction. Immobile elements such as HREE that are concentrated in zircon can be used as indicators of radiogenic Pb being sourced from minerals at the surface rather than being sourced from secondary dispersion from deeply buried U deposits.

Die Solling-Folge (Mittlerer Buntsandstein) im Grenzgebiet Niedersachsen - Thüringen - Hessen

This paper deals with the lithostratigraphic structure of the Solling sequence (Lower Triassic, Middle Buntsandstein) in the area between the Weser river region in the west and the Thuringian Eichsfeld region in the east. Lithologic profile mapping and the gamma-ray logs of several boreholes and 40 exposures have been used to define the lithostratigraphic Classification of the Solling sequence, to mark the facies zones and to find the connection between Sediments of the Thuringian basin in the east and the Weser fault trough via the crest of the Eichsfeld-Altmark Ridge. Tectonically controlled movements of synsedimentary character are the reason for the extreme convergence within the Solling sequence and the extreme Stratigraphie gap at its base (Hardegsen unconformity, Trusheim 1961) in the region of the swells. The discussion also demonstrates the importance of fault bundles active during Triassic and responsible for the thickness pattem of the Solling sequence between the Weser fault trough and the Eichsfeld-Altmark Ridge. The largest Stratigraphie gap is present at the line Brehme (Ohm Mountains) - Beuren - Treffurt where the Solling sequence covers Av/cn/a-bearing layers of the Volpriehausen sequence. In paiticular the Ridge sequences prove the existence of a further erosion unconformity within the Solling sequence (Solling unconformity, Kunz 1965) below the Thuringian Chirotheriensandstein as found by Rohling (1986) in the North German basin at the Stratigraphie level of the Karlshafen layers.

Riffe und fazielle Entwicklung der florigemma-Bank (Korallenoolith, Oxfordium) im Süntel und östlichen Wesergebirge (NW-Deutschland

During the Sedimentation of the platform carbonate deposits of the Korallenoolith Formation (middle Oxfordian to early Kimmeridgian) small buildups ofcorals formed in the Lower Saxony Basin. These bioconstructions are restricted to particular horizons (Untere Korallenbank,ßorigenuna-Bank Member etc.) and represent patch reefs and biostromes. In this study, the development of facies, fossil assemblages, spatial distribution of fossils, and reefs of the ßorigenuna-Bank Member (upper Middle Oxfordian) in the Süntel Mts and the eastern Wesergebirge Mts is described; the formation of reefs is discussed in detail. Twelve facies types are described and interpreted. They vary between high-energy deposits as well winnowed oolites and quiet-water lagoonal mudstones. Owing to the significance of biota, micro- and macrofossils are systematically described. The reefs are preserved in growth position, are characterized by numerous corresponding features and belong to a certain reef type. According to their size, shape and framework, they represent patch reefs, coral knobs (sensu James, 1983), coral thrombolite reefs (sensu Leinfelder et al., 1994) or “Klein- and Mitteldickichte” (sensu Laternser, 2001). Their growth fabric corresponds to the superstratal (dense) pillarstone (sensu Insalaco, 1998). As the top of the ßorigenuna-Bank displays an erosional unconformity (so-called Hauptdiskontinuität), the top of the reefs are erosionally capped. Their maximum height amounts to at least the maximum thickness of the ßorigenuna-Bank which does not exceed 4 metres. The diversity of coral fauna of the reefs is relatively low; a total of 13 species is recorded. The coral community is over- whelmingly dominated by the thin-branched ramose Thamnasteria dendroidea (Lamouroux) that forms aggregations of colonies (77?. dendroidea thickets). Leafy to platy Fungiastrea arachnoides (Parkinson) and Thamnasteria concinna (Goldfuss) occur subordinately, other species are only of minor importance. In a few cases, the reef-core consisting of Th. dendroidea thickets is laterally encrusted by platy F. arachnoides and Th. concinna colonies, and microbial carbonates. This zonation reflects probably a succession of different reef builders as a result of changing environmental conditions (allogenic succession). Moreover, some reefs are overlain by a biostrome made of large Solenopora jurassica nodules passing laterally in a nerinean bed. Mikrobial carbonates promoted reef growth and favoured the preservation of reef organismn in their growth position or in situ. They exhibit a platy, dendroid, or reticulate growth form or occur as downward-facing hemispheroids. According to their microstructure, they consist of a peloidal, clotted, or unstructured fabric (predominately layered and poorly structured thrombolite as well as clotted leiolite) (sensu Schmid, 1996). Abundant endo- and epibiontic organisms (bivalves, gastropods, echinoids, asteroids, ophiuroids, crabs etc) are linked to the reefs. With regard to their guild structure, the reefs represent occurrences at which only a few coral species serve as builder. Moreover, microbial carbonates contribute to both building and binding of the reefs. Additional binder as well as baffler are present, but not abundant. According to the species diversity, the dweller guild comprises by far the highest number of invertebrate taxa. The destroyer guild chiefly encompasses bivalves. The composition of the reef community was influenced by the habitat structure of the Th. dendroidea thickets. Owing to the increase in encrusting organisms and other inhabitants of the thickets, the locational factors changed, since light intensity and hydrodynamic energy level and combined parameters as oxygen supply declined in the crowded habitat. Therefore a characteristic succession of organisms is developed that depends on and responds to changing environmental conditions („community replacement sequence“). The succession allows the differentiation of different stages. It started after the cessation of the polyps with boring organisms and photoautotrophic micro-encrusters (calcareous algae, Lithocodium aggregatum). Following the death of these pioneer organisms, encrusting and adherent organisms (serpulids, „Terebella“ species, bryozoans, foraminifers, thecideidinids, sklerospongid and pharetronid sponges, terebratulids), small mobile organisms (limpets), and microbial induced carbonates developed. The final stage in the community replacement sequence gave rise to small cryptic habitats and organisms that belong to these caves (cryptobionts, coelobites). The habitat conditions especially favoured small non-rigid demosponges (“soft sponges”) that tolerate reduced water circulation. Reef rubble is negligible, so that the reefs are bordered by fossiliferous micritic limestone passing laterally in micritic limestone. Approximately 10% of the study area (outcropping florigemma-Bank) corresponds to reefal deposits whereas the remaining 90% encompass lagoonal inter-reefal deposits. The reef development is a good example for the interaction between reef growth, facies development and sea-level changes. It was initiated by a sea-level rise (transgression) and corresponding decrease in the hydrodynamic energy level. Colonization and reef growth took place on a coarse-grained Substrate composed of oncoids, larger foraminifers and bioclasts. Reef growth took place in a calm marine lagoonal setting. Increasing abundance of spherical coral morphs towards the Northeast (section Kessiehausen, northwestem Süntel Mts) reflects higher turbidity and a facies transition to coral occurrences of the ßorigenuna-Bank Member in the adjacent Deister Mts. The reef growth was neither influenced by stonns nor by input of siliciclastic deposits, and took place in short time - probably in only a thousand years under most probably mesotrophic conditions. The mass appearance of solenoporids and nerineids in the upper part of the ßorigenuna-Bank Member point to enhanced nutrient level as a result of regression. In addition, this scenario of fluctuations in nutrient availability seems to be responsible for the cessation of reef corals. The sea level fall reached its climax in the subaerial exposure and palaeokarst development of the florigemma-Bank. The reef building corals are typical pioneer species. The blade-like, flattened F. amchnoides colonies are characterized by their light porous calcium carbonate skeleton, which is a distinct advantage in soft bottom environment. Thus, they settled on soft bottom exposing the large parts of its surface to the incoming light. On the other hand, in response to their light requirements they were also able to settle shaded canopy structures or reef caves. Th. dendroidea is an opportunistic coral species in very shallow, well illuminated marine environment. Their thin and densely spaced branches led to a very high surface/volume ratio of the colonies that were capable to exploit incoming light due to their small thamasterioid calices characterized by “highly integrated polyps”. In addition, sideward coalescence of branches during colony growth led to a wave-resistant framework and favoured the authochthonous preservation of the reefs. Asexual reproduction by fragmented colonies promoted reef development as Th. dendroidea thickets laterally extend over the sea floor or new reefs have developed from broken fragments of parent colonies. Similar build ups with Th. dendroidea as a dominant or frequent reef building coral species are known from the Paris Basin and elsewhere from the Lower Saxony Basin (Kleiner Deister Mts). These buildups developed in well-illuminated shallow water and encompass coral reefs or coral thrombolite reefs. Intra- and inter-reef deposits vary between well-winnowed reef debris limestone and mudstones representing considerably calmer conditions. Solenoporid, nerineids and diceratides belong to the characteristic fossils of these occurrences. However, diceratides are missing in theflorigemma-Bank Member. Th. dendroidea differs in its colonization of low- to high-energy environment from recent ramose scleractinian corals (e.g., Acropora and Porites sp.). The latter are restricted to agitated water habitats creating coral thickets and carpets. According to the morphologic plasticity of Th. dendroidea, thick-branched colonies developed in a milieu of high water energy, whereas fragile, wide- and thin-branched colonies prevail in low-energy settings. Due to its relatively rapid growth, Th. dendroidea was able to keep pace with increased Sedimentation rates. 68 benthonic foraminiferan species/taxa have been recognized in thin sections. Agglutinated foraminifers (textulariids) predominate when compared with rotaliids and milioliids. Numerous species are restricted to a certain facies type or occur in higher population densities, in particular Everticyclammina sp., a larger agglutinated foraminifer that occurs in rock building amounts. Among the 25 reef dwelling foraminiferal species, a few were so far only known from Late Jurassic sponge reefs. Another striking feature is the frequency of adherent foraminiferal species. Fauna and flora, in particular dasycladaleans and agglutinated foraminifers, document palaeobiogeographic relationships to the Tethys and point to (sub)tropical conditions. Moreover, in Germany this foraminiferan assemblage is yet uncompared. In Southern Germany similar tethyan type assemblages are not present in strata as young as Middle Tithonian.

Contribuição para o conhecimento da geologia do Grupo das Beiras (CXG) na região do Caramulo-Buçaco, Portugal Central

O presente trabalho ocupa-se do estudo do Complexo Xisto-Grauváquico ante-ordovícico (Grupo das Beiras) na região do Caramulo-Buçaco (centro de Portugal). Em termos geológicos, a área estudada pertence à Zona Centro Ibérica e encontra-se limitada a N pelo granito do Caramulo, a S pela bacia meso-cenozóica de Arganil, a W pelo sinclinal paleozóico do Buçaco e pela bacia meso-cenozóica ocidental portuguesa e a E pelo sinclinal paleozóico de Arganil e pelo plutonito granítico de Tábua-Santa Comba Dão; no seio da área estudada encontra-se a bacia meso-cenozóica de Mortágua. Com base nas características litológicas e estruturais distinguem-se no Complexo Xisto Grauváquico 4 grandes conjuntos litológicos concordantes entre si, designados de Unidades I, II, III e IV, que se desenvolvem da base para o topo de N para S. A Unidade I situa-se a N da região. O seu limite inferior é desconhecido, e o superior posiciona-se no último conjunto arenoso com potência decamétrica. É constituída por xistos cinzentos e negros com intercalações de arenitos de espessura não superior a 100 metros e de extensão lateral quilométrica. Apresenta uma espessura mínima de 1000 m. A Unidade II apresenta consideravelmente menor proporção de material arenoso intercalado entre os pelitos comparativamente à unidade inferior. É caracterizada por apresentar um predomínio de material silto-argiloso e escassos níveis arenosos com potência não superior à dezena de metros e escassa continuidade lateral. Cartograficamente esta unidade constitui uma franja alargada de orientação próxima a E-W. Apresenta uma espessura aproximada de 1500 m. A Unidade III é caracterizada pela presença de conjuntos arenosos com extensão lateral quilométrica e espessura de várias dezenas de metros, separados por material silto-argiloso. Os limites inferior e superior estão situados respectivamente abaixo e acima dos principais conjuntos arenosos. Esta unidade apresenta uma espessura máxima estimada na ordem dos 2000 m. A Unidade IV, que é a unidade superior, apresenta um predomínio pelítico, com escassas intercalações de conjuntos arenosos. O seu limite inferior encontra-se no topo do último conjunto arenoso da Unidade III. Apresenta uma espessura mínima de 500 m. As características sedimentológicas das 4 unidades indicam uma sedimentação num ambiente de plataforma externa siliciclástica aberta, com a construção de barras e por vezes sujeita à acção de tempestades, com sucessivos períodos de superficialização e profundização numa bacia de sedimentação bastante subsidente. Em termos estruturais, para além duma deformação pré-ordovícica, que é comprovada pelo forte mergulho e dispersão da orientação dos eixos da 1ª fase varisca e da lineação de intersecção L1, a área estudada foi principalmente afectada pela Orogenia Varisca. A 1ª fase de deformação varisca (F1) gerou dobras com superfícies axiais e xistosidade associada (S1) de direcção WNW-ESE, e forte pendor para NNE. Estas dobras D1 apresentam comprimentos de onda que nunca chegam a ser quilométricos, desenvolvendo-se um grande flanco inverso denunciando a presença de uma antiforma para NNE e uma sinforma para SSW. A 2ª fase de deformação varisca (F2) actuou na parte nordeste da área estudada e é caracterizada por ter gerado dobras de comprimento de onda quilométrico, com planos axiais e xistosidade associada S2 de direcção NW-SE, subverticais ou a pender fortemente para NE. Embora com alguma dispersão, as lineações de intersecção L2 e os eixos das dobras D2 apresentam maioritariamente forte pendor para E. A direcção e tipos de estruturas da F2 sugerem uma correlação com a terceira fase definida em vários pontos da Zona Centro Ibérica e estreitamente relacionada com as intrusões graníticas. Do ponto de vista petrológico, distinguem-se várias rochas sedimentares (pelitos e arenitos) todas elas sujeitas a metamorfismo que não ultrapassa a fácies dos xistos verdes. Dentro das rochas sedimentares mais grosseiras, há a destacar a presença de arenitos vulcânicos cuja composição denuncia, não muito afastados da bacia sedimentar, a presença de aparelhos vulcânicos que estariam em actividade durante a sedimentação. Foram analisadas isotopicamente 27 amostras de metapelitos colhidas em 5 locais diferentes de forma a abranger quase toda a área estudada. Os dados isotópicos de quatro destes locais de amostragem forneceram isócronas Rb-Sr, em rocha total, com valores da ordem dos 400-440 Ma. O granito do Caramulo, datado pela isócrona Rb-Sr em amostras de rocha total, forneceu uma idade de 326±12Ma. As idades modelo Sm-Nd (manto empobrecido) de 5 amostras de metapelitos estão compreendidas entre 1.35 e 1.25 Ga. Este período de tempo pode ser considerado como correspondendo à época de diferenciação mantélica da crusta que deu lugar à maioria das áreas fonte dos metapelitos.