Architecture of an Oligocene fluvial ribbon sandstone in the Ebro Basin,North‐eastern Spain

Fluvial ribbon sandstone bodies are ubiquitous in the Ebro Basin in North‐eastern Spain; their internal organization and the mechanics of deposition are as yet insufficiently known. A quarrying operation in an Oligocene fluvial ribbon sandstone body in the southern Ebro Basin allowed for a three‐dimensional reconstruction of the sedimentary architecture of the deposit. The sandstone is largely a medium‐grained to coarse‐grained, moderately sorted lithic arenite. In cross‐section, the sandstone body is 7 m thick, occupies a 5 m deep incision and wedges out laterally, forming a ‘wing’ that intercalates with horizontal floodplain deposits in the overbank region. Three architectural units were distinguished. The lowest and highest units (Units A and C) mostly consist of medium‐grained to coarse‐grained sandstone with medium‐scale trough cross‐bedding and large‐scale inclined stratasets. Each of Units A and C comprises a fining‐up stratal sequence reflecting deposition during one flood event. The middle unit (Unit B) consists of thinly bedded, fine‐grained sandstone/mudstone couplets and represents a time period when the channel was occupied by low‐discharge flows. The adjoining ‘wing’ consists of fine‐grained sandstone beds, with mudstone interlayers, correlative to strata in Units A and C in the main body of the ribbon sandstone. In plan view, the ribbon sandstone comprises an upstream bend and a downstream straight reach. In the upstream bend, large‐scale inclined stratasets up to 3 m in thickness represent four bank‐attached lateral channel bars, two in each of Units A and C. The lateral bars migrated downflow and did not develop into point bars. In the straight downstream reach, a tabular cross‐set in Unit A represents a mid‐channel transverse bar. In Unit C, a very coarse‐grained, unstratified interval is interpreted as deposited in a riffle zone, and gives way downstream to a large mid‐channel bar. The relatively simple architecture of these bars suggests that they developed as unit bars. Channel margin‐derived slump blocks cover the upper bar. The youngest deposit is fine‐grained sandstone and mudstone that accumulated immediately before avulsion and channel abandonment. Deposition of the studied sandstone body reflects transport‐limited sediment discharges, possibly attaining transient hyperconcentrated conditions.     

Petrography and geochemistry of the late Eocene–early Oligocene submarine fans and shelf deposits on Lemnos Island,NE Greece. Implications for provenance and tectonic setting

Provenance and tectonic history of the late Eocene‐early Oligocene submarine fans and shelf deposits on Lemnos Island, NE Greece, were studied using sandstone framework composition, sedimentological data and sandstone and mudstone geochemistry. The resulting tectonic–sedimentological model is based on the late Eocene–early Oligocene Lemnos Island being in a forearc basin with the outer arc ridge as a major sediment source. Modal petrographic analysis of the studied sandstones shows that the source area comprises sedimentary, metamorphic and plutonic igneous rocks deposited in the studied area in a recycled orogenic environment. Moreover, within the above sediments, the minor occurrence of volcanic fragments suggests little or no influence of a volcanic source. Provenance results, based on major, trace and rare earth element (REE) data, suggest an active continental margin/continental island arc signature. All the samples are LREE, enriched relative to HREE, with a flat HREE pattern and positive Eu anomalies, suggesting that the processes of intracrustal differentiation (involving plagioclase fractionation) were not of great importance. Results derived from the multi‐element diagrams also suggest an active margin character, and a mafic/ultramafic source rock composition, while the positive anomaly of Zr that can be attributed to a passive continental margin source, is most likely associated with reworking and sorting during sediment transfer. Palaeocurrents, with a NE–NNE direction, indicate a northeast flow, towards the location of the late Eocene–early Oligocene magmatic belt in the north‐east Aegean region. Conglomerates are composed of chert, gneiss and igneous fragments, such as basalts and gabbros, suggesting this outer arc ridge as a likely source area. Copyright © 2009 John Wiley & Sons, Ltd.     

The provenance of Western Irish Namurian Basin sedimentary strata inferred using detrital zircon U–Pb LA‐ICP‐MS geochronology

The Western Irish Namurian Basin (WINB) preserves classic examples of basin floor sequences through to slope deposits and deltaic cyclothems. Despite over 50 years of research into the WINB, its sediment provenance remains highly contested. Sedimentological arguments, including palaeocurrent vectors and palaeoslope indicators have been invoked to propose a sediment source from the NW or the west (i.e. from within Laurentia). These same indicators have been subsequently reinterpreted to reflect a southern provenance. It is not clear from sedimentological arguments alone which interpretation more accurately reflects the infilling of the WINB. Regional‐scale constraints on WINB provenance may be obtained with detrital zircon U–Pb geochronology. U–Pb LA‐ICP‐MS detrital zircon analysis was undertaken on samples from three sandstone units at different stratigraphic levels within the WINB siliciclastic sedimentary fill (Ross Formation, Tullig Sandstone, Doonlicky Sandstone). The samples are dominated by 500–700 Ma zircons, which can be correlated with Cadomian–Avalonian orogenic activity within terranes to the south of the WINB (Avalonia/Ganderia, Armorica and Iberia). In contrast, Eastern Laurentia, to the north of the WINB, was devoid of orogenic activity at this time. WINB samples also yield age populations younger than 500 Ma, and older than 700 Ma. These are not diagnostic of a particular source terrane and thus could be derived from terranes north and/or south of the WINB. WINB detrital zircon age spectra can be reconciled by an Avalonian or combined Avalonian–Laurentian provenance for WINB sedimentary strata. Further research is required in order to distinguish between these two possibilities. Copyright © 2011 John Wiley & Sons, Ltd.     

Palaeo‐climate controlled diagenesis of the Westphalian C & D fluvial sandstones in the Campine Basin (north‐east Belgium)

The Westphalian C and D fluvial sandstones in the Campine Basin (north‐east Belgium) are potential reservoirs for the sequestration of CO₂ and interesting analogues of the hydrocarbon reservoirs in the Southern North Sea. Although these sandstones were deposited in a relatively short period of time, their reservoir properties and mineralogical compositions are very different. A petrographic study complemented with stable isotope analyses, fluid inclusion microthermometry and X‐ray diffraction analyses of the clay fractions of the sandstones, which were sampled from deep boreholes (>1000 m) in the Campine Basin, revealed that these differences are related mainly to the climate at the time of deposition. The most important eogenetic processes affecting the Westphalian sandstones were the generation of a pseudomatrix by physical compaction of Al‐silicates and lithic fragments that were strongly altered by extensive meteoric leaching, kaolinitization of unstable silicates and precipitation of siderite. These processes had a detrimental influence on the reservoir properties of Westphalian C sandstones, but their impact on the Westphalian D sandstones was minimal. The difference is assumed to be related to the climate at the time of deposition, which changed from tropical humid in the Westphalian C to semi‐arid/arid during the Late Westphalian D. Both the Westphalian C and D sandstones were affected by similar mesogenetic processes. Mesogenetic quartz cementation resulted from chemical compaction and illitization of kaolinite, K‐feldspar and smectitic clays. Illitization of kaolinite was controlled by the available quantities of co‐existing kaolinite and K‐feldspar and mainly affected the Westphalian D sandstones. Illitization of K‐feldspar was controlled by the K‐feldspar content. It had a much larger impact on the reservoir properties of the Westphalian D as, in these sandstones, K‐feldspar was less affected by eogenetic alteration. The illitization of smectitic clays resulted in illite, quartz and ankerite cementation in both reservoirs. This process had a more important impact on the Westphalian C reservoir, since cementation here also resulted from smectite to illite conversion in the interbedded and underlying shales. The effect of mesogenetic alterations on the reservoir properties was much less drastic than the impact of eodiagenesis. Mesogenetic alterations do exert a significant control on the properties of the Westphalian D. The vast impact of eodiagenesis on the Westphalian C sandstones made them less susceptible to mesogenetic alteration. The effect of telogenetic processes on the porosity and permeability of the Westphalian sandstones was small and restricted to the top reservoir intervals that directly underlie the Cimmerian Unconformity. No significant telogenetic alterations related to the Variscan Unconformity were observed.     

U–Pb Zircon Age Constraining the Source and Provenance of Gem‐Bearing Late Cenozoic Detrital Deposits,Mamfe Basin,SW Cameroon

Zircons and other heavy minerals (corundum, rutile, ilmenite, magnetite, sillimanite) are identified in the Nsanaragati gem corundum placer deposit, in the western part of the Mamfe sedimentary basin, SW Cameroon. These alluvial minerals have different morphological characteristics and zircons, in particular, vary mostly in colour and shape. They are reddish, brownish, yellowish, pink or colourless. These minerals form rounded and sub‐rounded alluvial grains, prismatic, pyramidal or dipyramidal crystals. Reddish zircons retain their original crystallographic shape. Trace element and U–Pb isotopic geochemical analyses of these reddish zircons, using the LA‐ICP‐MS method give significant Hf (4576–6334 ppm), Th (46–1565 ppm) and U (66–687 ppm) contents, with Th/U ratio ranging from 0.6 to 3.0. The ²⁰⁶Pb/²³⁸U corrected mean age gave 12.39 ± 0.55 Ma, which characterizes an Upper Cenozoic (Serravallian) magmatic event. The zircons are probably sourced from a magmatic field in the South eastern boundary of the Cross River Formation. The Cameroon Volcanic Line of basaltic and alkaline lavas and intrusions which lie east of the Mamfe Basin mostly range in age from 37 Ma to <1 Ma. The zircons may also relate to the Mount Bambouto plateau lavas which lie northeast of the Mamfe sedimentary basin and have an eruptive age range of 21–14 Ma. The oldest Nsanaragati reddish zircon ages overlap within error with the end stages of the Bambouto eruptions. This eruptive or a related episode provides a potential source for megacrystic reddish zircons within the Nsanagarati placer deposit.     

Origin of Carboniferous sandstones fringing the northern margin of the Wales‐Brabant Massif: insights from detrital zircon ages

A study of detrital zircon age populations in Namurian–Westphalian (Carboniferous) sandstones in the southern Central Pennine Basin of the UK has revealed considerable complexity in their provenance history. The Pendleian–Marsdenian Morridge Formation, which is known to have been derived from the Wales‐Brabant Massif to the south on the basis of palaeocurrent and petrographic information, is dominated by zircons ultimately derived from the Caledonian belt to the north. These zircons were recycled from sandstones of northern origin that had been previously deposited over the massif during Middle to Late Devonian times. The Morridge Formation also includes Late Neoproterozoic zircons of local Wales‐Brabant Massif origin. The south lobe of the Yeadonian Rough Rock has been previously interpreted as having a complex provenance including sediment of northern origin interbedded with sediment ascribed to a Wales‐Brabant Massif source. However, the zircon spectrum lacks a Late Neoproterozoic component that would have been diagnostic of input from the Wales‐Brabant Massif, and the provenance history of the Rough Rock south lobe therefore remains enigmatic. The Langsettian Ludgbridge Conglomerate is dominated by Late Neoproterozoic zircons of Wales‐Brabant Massif origin, but even in this evidently proximal deposit, the provenance is complex since the main zircon group (ca. 640 Ma) cannot be matched with known local Neoproterozoic basement sources. The data either indicate the presence of hitherto‐unknown magmatic rocks of this age adjacent to the South Staffordshire coalfield or indicate that the zircons were recycled from sediment with a more distal origin. Finally, the Duckmantian Top Hard Rock contains zircons that can be reconciled with a source in the Irish Caledonides, consistent with the palaeocurrent evidence, supplemented by zircons derived from the Wales‐Brabant Massif, possibly including the Monian Composite Terrane of Anglesey. The study reinforces the important message that failure to recognize the presence of recycled zircon could lead to erroneous reconstructions of sediment provenance and transport history. Copyright © 2014 John Wiley & Sons, Ltd.     

U–Pb dating of detrital zircons from Andros,Greece: constraints for the time of sediment accumulation in the northern part of the Cycladic blueschist belt

The Attic–Cycladic Crystalline Belt in the central Aegean region represents a major tectono‐stratigraphic unit of the Hellenides. The essential geological, magmatic and tectono‐metamorphic features are well documented. Unresolved questions concern the time of sediment accumulation and litho‐ and/or tectono‐stratigraphic relationships across the study area. In order to address this issue we have studied siliciclastic metasedimentary rocks from Andros Island, northern Cyclades. The sampling strategy aimed at covering the complete age range recorded by the Andros metamorphic succession. Detrital zircon U–Pb dating of nine samples indicates maximum depositional ages of c. 260 Ma for the topmost part of the metamorphic succession and of c. 160–140 Ma for rock sequences below a prominent serpentinite belt that is interpreted to outline a major tectonic contact. These age constraints are in accordance with interpretations suggesting that the metamorphic rocks of Andros represent different tectonic subunits (Makrotantalon Unit and Lower Unit) that are separated by a thrust fault. Modification of the internal structure of the Lower Unit by tectonic stacking can currently not be ascertained. The new data for the Lower Unit corroborate the importance of Late Jurassic–Early Cretaceous sediment accumulation for the larger study area. In contrast to some of the neighbouring islands, no evidence for transfer of Late Cretaceous (c. 80 Ma) material into the Andros sedimentary environment was found. The most striking feature of the zircon populations of the Lower Unit is a remarkable age cluster at 250–200 Ma that documents the importance of Triassic igneous sediment sources. Copyright © 2015 John Wiley & Sons, Ltd.     

Origin of Lower Cretaceous (‘Nubian’) sandstones of North‐east Africa and Arabia from detrital zircon U‐Pb SHRIMP dating

Lower Cretaceous sandstones of the type exposed in Israel, deposited over much of North Africa and Arabia as widespread sandstone sheets, typically are mineralogically and texturally mature. Previous petrographic examinations suggested that the Lower Cretaceous sandstones are at least partly a product of recycling and the present study supports this notion. The results of U‐Pb Sensitive High Resolution Ion Micro‐Probe (SHRIMP) dating of detrital zircons from the Lower Cretaceous section exposed in Israel indicate that they are dominated by detrital zircons of Neoproterozoic age, mainly concentrated in the 0·55 to 0·65 Ga interval, with various amounts of older (pre‐Neoproterozoic) zircons (of 0·95 to 1·10, 1·7 to 2·0 and 2·6 to 2·65 Ga age groups). The overall age signal is similar to detrital zircon age spectra previously obtained from the Cambrian–Ordovician sections of Israel and Jordan. Remarkably, the detrital zircon spectra remained almost unchanged for nearly 400 Myr. Thus, the most probable provenance of the Lower Cretaceous sandstone is the recycling of relatively proximal Palaeozoic sandstone. Since first unroofed from above pan‐African terranes closer to the secession of orogeny, the ensuing siliciclastics were recycled repeatedly throughout the Phanerozoic with little additional basement denudation. The Lower Cretaceous sandstone comprises quartz sand that was first eroded from above pan‐African orogens ca 400 Myr prior to deposition.     

Facies model of a mixed clastic–carbonate,wave‐dominated open‐coast tidal flat (Tithonian–Berriasian,north‐east Spain)

Detailed logging and analysis of the facies architecture of the upper Tithonian to middle Berriasian Aguilar del Alfambra Formation (Galve sub‐basin, north‐east Spain) have made it possible to characterize a wide variety of clastic, mixed clastic–carbonate and carbonate facies, which were deposited in coastal mudflats to shallow subtidal areas of an open‐coast tidal flat. The sedimentary model proposed improves what is known about mixed coastal systems, both concerning facies and sedimentary processes. This sedimentary system was located in an embayed, non‐protected area of a wide C‐shaped coast that was seasonally dominated by wave storms. Clastic and mixed clastic–carbonate muds accumulated in poorly drained to well‐drained, marine‐influenced coastal mudflat areas, with local fluvial sandstones (tide‐influenced fluvial channels and sheet‐flood deposits) and conglomerate tsunami deposits. Carbonate‐dominated tidal flat areas were the loci of deposition of fenestral‐laminated carbonate muds and grainy (peloidal) sediments with hummocky cross‐stratification. Laterally, the tidal flat was clastic‐dominated and characterized by heterolithic sediments with hummocky cross‐stratification and local tidal sandy bars. Peloidal and heterolithic sediments with hummocky cross‐stratification are the key facies for interpreting the wave (storm) dominance in the tidal flat. Subsidence and high rates of sedimentation controlled the rapid burial of the storm features and thus preserved them from reworking by fair‐weather waves and tides.     

Meganodular anhydritization: a new mechanism of gypsum to anhydrite conversion (Palaeogene–Neogene,Ebro Basin,North‐east Spain)

A number of Palaeogene to Early Neogene gypsum units are located along the southern margins of the Ebro Basin (North‐east Spain). These marginal units, of Eocene to Lower Miocene age, formed and accumulated deposits of Ca sulphates (gypsum and anhydrite) in small, shallow saline lakes of low ionic concentration. The lakes were fed mainly by ground water from deep regional aquifers whose recharge areas were located in the mountain chains bounding the basin, and these aquifers recycled and delivered Ca sulphate and Na chloride from Mesozoic evaporites (Triassic and Lower Jurassic). In outcrop, the marginal sulphate units are largely secondary gypsum after anhydrite and exhibit meganodules (from 0·5 to >5 m across) and large irregular masses. In the sub‐surface these meganodules and masses are mostly made of anhydrite, which replaced the original primary gypsum. The isotopic composition (11·1 to 17·4‰ for δ¹⁸O_VSMOW; 10·7 to 15·3‰ for δ³⁴S_VCDT) of secondary gypsum in this meganodular facies indicates that the precursor anhydrite derived from in situ replacement of an initial primary gypsum. As a result of ascending circulation of deep regional fluid flows through the gypsum units near the basin margins, the gypsum was partly altered to anhydrite within burial conditions from shallow to moderate depths (from some metres to a few hundred metres?). At such depths, the temperatures and solute contents of these regional flows exceeded those of the ground water today. These palaeoflows became anhydritizing solutions and partly altered the subsiding gypsum units before they became totally transformed by deep burial anhydritization. The characteristics of the meganodular anhydritization (for example, size and geometry of the meganodules and irregular masses, spatial arrangement, relations with the associated lithologies and the depositional cycles, presence of an enterolithic vein complex and palaeogeographic distribution) are compared with those of the anhydritization generated both in a sabkha setting or under deep burial conditions, and a number of fundamental differences are highlighted.     

Lithogeochemical,Re–Os and U–Pb Geochronological,Hf–Lu and S–Pb Isotope Data of the Ga'erqiong‐Galale Cu–Au Ore‐Concentrated Area: Evidence for the Late Cretaceous Magmatism and Metallogenic Event in the Bangong‐Nujiang Suture Zone,Northwestern Tibet,China

The Ga'erqiong‐Galale skarn–porphyry copper–gold ore‐concentrated area is located in the western part of the Bangong‐Nujiang suture zone north of the Lhasa Terrane. This paper conducted a systematic study on the magmatism and metallogenic effect in the ore‐concentrated area using techniques of isotopic geochronology, isotopic geochemistry and lithogeochemistry. According to the results, the crystallization age of quartz diorite (ore‐forming mother rock) in the Ga'erqiong deposit is 87.1 ± 0.4 Ma, which is later than the age of granodiorite (ore‐forming mother rock) in the Galale deposit (88.1 ± 1.0 Ma). The crystallization age of granite porphyry (GE granite porphyry) in the Ga'erqiong deposit is 83.2 ± 0.7 Ma, which is later than the age of granite porphyry (GL granite porphyry) in the Galale deposit (84.7 ± 0.8 Ma).The quartz diorite, granodiorite, GE granite porphyry and GL granite porphyry both main shows positive ε_Hf(t) values, suggesting that the magmatic source of the main intrusions in the ore‐concentrated area has the characteristics of mantle source region. The Re–Os isochron age of molybdenite in the Ga'erqiong district is 86.9 ± 0.5 Ma, which is later than the mineralization age of the Galale district (88.6 ± 0.6 Ma). The main intrusive rocks in the ore‐concentrated area have similar lithogeochemical characteristics, for they both show the relative enrichment in large‐ion lithophile elements(LILE: Rb, Ba, K, etc.), more mobile highly incompatible lithophile elements(HILE: U, Th) and relatively depleted in high field strength elements (HFSE: Nb, Ta, Zr, Hf, etc.), and show the characteristics of magmatic arc. The studies on the metal sulfides' S and Pb isotopes and Re content of molybdenite indicate that the metallogenic materials of the deposits in the ore‐concentrated area mainly come from the mantle source with minor crustal source contamination. Based on the regional tectonic evolution process, this paper points out that the Ga'erqiong‐Galale copper–gold ore‐concentrated area is the typical product of the Late Cretaceous magmatism and metallogenic event in the collision stage of the Bangong‐Nujiang suture zone.     

Extreme aridity prior to lake expansion deciphered from facies evolution in the Miocene Ili Basin,south‐east Kazakhstan

Central Asia witnessed progressive aridification during the Miocene, commonly related to mountain uplift, the Paratethys retreat and global climate cooling. However, the formation of Miocene lakes in Central Asia seems to oppose drier conditions, suggesting that the precise timing, extent and forcing of the aridification is still not well constrained. This study presents a facies model for the alluvial–lacustrine part of the Middle to Late Miocene of the Ili Basin, obtained from two successions. The model enables the semi‐quantitative assessment of regional water level and salinity, and characterizes the control of water level on evaporite formation and diagenesis. Both the proximal Kendyrlisai and the distal Aktau successions show an overall increase in water availability from dry mudflat deposits to lacustrine sedimentation with a transitional playa phase. Increasing evaporation rates outpaced the water supply and caused groundwater salinization. Subsequent lake expansion coincided with a basin‐wide desalinization and required a shift to a positive water budget. A climatic control of the hydrological evolution is inferred due to abrupt salinization and a minor tectonic influence. The long‐term water accumulation is probably related to the hydrological closure of the basin in the early Middle Miocene (15·3 Ma). Starting at 14·3 Ma, the step‐wise salinization occurred simultaneously with the global cooling of the Miocene Climate Transition. The Miocene Climate Transition led to extreme aridity in the Ili Basin, highlighted by the early diagenetic formation of displacive anhydrite in the basin centre. The expansion of the freshwater lake (12·7 to 11·5 Ma) was possibly promoted by lower evaporation rates due to decreasing air temperatures in the Ili Basin after the Miocene Climate Transition. The extreme aridity in the Ili Basin is interpreted as a continental counterpart to the Badenian Salinity Crisis in the Central Paratethys. This emphasizes the role of atmospheric forcing on evaporite sedimentation across Eurasia during the Middle Miocene.     

An Eocene/Oligocene blueschist‐/greenschist facies P–T loop from the Cycladic Blueschist Unit on Naxos Island,Greece: Deformation‐related re‐equilibration vs. thermal relaxation

Geothermobarometric and geochronological work indicates a complete Eocene/early Oligocene blueschist/greenschist facies metamorphic cycle of the Cycladic Blueschist Unit on Naxos Island in the Aegean Sea region. Using the average pressure–temperature (P–T) method of thermocalc coupled with detailed textural work, we separate an early blueschist facies event at 576 ± 16 to 619 ± 32°C and 15.5 ± 0.5 to 16.3 ± 0.9 kbar from a subsequent greenschist facies overprint at 384 ± 30°C and 3.8 ± 1.1 kbar. Multi‐mineral Rb–Sr isochron dating yields crystallization ages for near peak‐pressure blueschist facies assemblages between 40.5 ± 1.0 and 38.3 ± 0.5 Ma. The greenschist facies overprint commonly did not result in complete resetting of age signatures. Maximum ages for the end of greenschist facies reworking, obtained from disequilibrium patterns, cluster near c. 32 Ma, with one sample showing rejuvenation at c. 27 Ma. We conclude that the high‐P rocks from south Naxos were exhumed to upper mid‐crustal levels in the late Eocene and early Oligocene at rates of 7.4 ± 4.6 km/Ma, completing a full blueschist‐/greenschist facies metamorphic cycle soon after subduction within c. 8 Ma. The greenschist facies overprint of the blueschist facies rocks from south Naxos resulted from rapid exhumation and associated deformation/fluid‐controlled metamorphic re‐equilibration, and is unrelated to the strong high‐T metamorphism associated with the Miocene formation of the Naxos migmatite dome. It follows that the Miocene thermal overprint had no impact on rock textures or Sr isotopic signatures, and that the rocks of south Naxos underwent three metamorphic events, one more than hitherto envisaged.     

Potential source rocks, organic geochemistry and thermal maturation in the southern depocenter (Kipourio–Grevena) of the Mesohellenic Basin, central Greece

The study area is the southern depocenter (depth > 4200 m) of the Mesohellenic Basin which extends between Kipourio and Grevena, central Greece. The Mesohellenic Basin is a Middle-Tertiary intramontane basin developed within the Hellenide orogen. Previous studies have focused on the depositional environments, configuration and hydrocarbon potential of the basin. In this paper we present additional geochemical and petrographic data from outcrop samples of the basin's southern depocenter, which is considered the most promising area, in terms of hydrocarbon prospectivity. A total number of thirty six samples were analysed: Rock-Eval pyrolysis, maceral analysis, vitrinite reflectance and thermal alteration index, bitumens extraction, liquid chromatography, and GC-MS. The samples were collected from deltaic deposits and submarine fan sediments of Late Eocene to Late Oligocene age. The TOC values of the analysed samples range between rich and very rich and the organic matter consists mainly of type III kerogen and the organic matter consider to be predominately gas prone. The thermal maturity assessed from T_max and vitrinite reflectance shows an immature stage of the organic matter along with the presence of layers having reached the very early mature stage. Vitrinite reflectance measurements and maturity calculations (applying the Lopatin modeling), reveal that the lower part of the depocenter sediments falls within the ‘oil window’. The extractable organic matter (EOM) (mg bitumens/g TOC) indicate the existence of samples (from deltaic deposits) with high ratio of transformation (EOM) (> 100 mg bitumen/g TOC). The GC and GC-MS analyses of the biomarkers indicate mainly the occurrence of terrestrial organic matter reflecting oxidizing conditions and both immature and very early mature stages. The results of the Rock-Eval pyrolysis and the distribution of the isoprenoids support the assumption of the input of an organic matter mixture.     

Provenance implications of Th–U–Pb electron microprobe ages from detrital monazite in the Carboniferous Upper Silesia Coal Basin, Poland

This paper reports the results of CHIME (chemical Th–U–Pb isochron method) dating of detrital monazites from Carboniferous sandstones in the Upper Silesia Coal Basin (USCB). A total of 4739 spots on 863 monazite grains were analyzed from samples of sandstone derived from six stratigraphic units in the sedimentary sequence. Age distributions were identified in detrital monazites from the USCB sequence and correlated with specific dated domains in potential source areas. Most monazites in all samples yielded ca. 300–320 Ma (Variscan) ages; however, eo-Variscan, Caledonian and Cadomian ages were also obtained. The predominant ages are comparable to reported ages of certain tectonostratigraphic domains in the polyorogenic Bohemian Massif (BM), which suggests that various crystalline lithologies in the BM were the dominant sources of USCB sediments.     

Multi‐scale classification of fluvial architecture: An example from the Palaeocene–Eocene Bighorn Basin,Wyoming

Fluvial channel geometry classification schemes are commonly restricted in relation to the scale at which the study took place, often due to outcrop limitations or the need to conduct small‐scale detailed studies. A number of classification schemes are present in the literature; however, there is often limited consistency between them, making application difficult. The aim of this study is to address this key problem by describing channel body geometries across a depositional basin to ensure that a wide range of architectures are documented. This was achieved by studying 28 locations over 4000 m of vertical succession in Palaeocene‐aged and Early Eocene‐aged deposits within the Bighorn Basin, Wyoming, USA. Five different channel body geometries have been defined based on the external geometric form, and internal arrangement and nature of storey contacts. These include the massive channel body geometry, semi‐amalgamated channel body geometry, internally amalgamated channel body geometry and offset stacked channel body geometry, which are considered to be subdivisions of the sheet geometry of many other classifications. An isolated channel body geometry has also been recognized alongside splay channel and sheet sandstone geometries in the floodplain facies associations. Field evidence, including the stacking style of storey surfaces, suggests that the different geometries form a continuum. The nature and degree of amalgamation at the storey scale are important in producing the different geometries and are related to the degree of channel migration. It is speculated that this is the result of differences in sediment supply and available accommodation. In contrast to previous schemes, the classification scheme presented here recognizes the importance of transitional geometries. This geometrical range has been recognized because of the basin‐scale nature of the study.     

Experimental identification of CO2–water–rock interactions caused by sequestration of CO2 in Westphalian and Buntsandstein sandstones of the Campine Basin (NE-Belgium)

Geological sequestration of CO₂ is one of the options studied to reduce greenhouse gas emissions. Although the feasibility of this concept is proven, apart from literature data on modelling still little is known about the CO₂–water–rock interactions induced by CO₂-injection.To evaluate the effect of CO₂–water–rock interactions on three sandstone aquifers in NE-Belgium an experimental setup was built. Eighteen experiments were performed in which sandstones were exposed to supercritical CO₂. CO₂–water–rock interactions were deduced from the evolution of aqueous concentrations of 25 species and a thorough characterisation of the sandstones before and after treatment. The results show that dissolution of ankerite/dolomite and Al-silicates could enhance porosity/permeability. The observed precipitation of end-member carbonates could increase storage capacity if it exceeds carbonate dissolution. Precipitation of the latter and of K-rich clays as observed, however, can hamper the injection.     

A paired metamorphic belt in a subduction‐to‐collision orogen: An example from the South Qilian–North Qaidam orogenic belt,NW China

The early Palaeozoic South Qilian–North Qaidam orogenic belt in northwestern China records a nearly complete history of early‐stage long‐lived oceanic subduction–accretion followed by late‐stage continental collision. Most previous studies have focused on low dT/dP metamorphism (HP–UHP) in this belt whereas the paired high dT/dP belt in the hinterland has received little attention. In this contribution, phase equilibrium modelling is combined with zircon petrochronology to determine the P–T–t evolution of granulites in the North Wulan gneiss complex in the high dT/dP hinterland of the South Qilian–North Qaidam orogen. Granulites record a clockwise P–T path with near‐peak temperatures of ~800–900°C at 5.5–7 kbar. Peak metamorphism was followed by high‐T decompression. Zircon petrochronology reveals protracted zircon growth from c. 474 to 446 Ma during the high‐T portion of the P–T path. High dT/dP metamorphism in the North Wulan gneiss complex was likely the result of heat transfer from the underlying hot asthenosphere and minor coeval magmatism in an arc–back‐arc system during slab retreat and roll‐back of the South Qilian oceanic plate. Broadly contemporaneous but slightly younger HP–UHP metamorphism in the foreland of the South Qilian–North Qaidam orogenic belt indicates that the region records an early Palaeozoic paired metamorphic belt. This early Palaeozoic paired metamorphic belt provides a detailed example of dual thermal regimes in a modern‐style orogenic system that can be applied to understanding the time‐scales and P–T conditions of high dT/dP metamorphism that accompany subduction in Phanerozoic and Precambrian orogenic belts.