Showing papers in "Journal of the Geological Society in 2000"

Mudrock-hosted carbonate concretions: a review of growth mechanisms and their influence on chemical and isotopic composition

Abstract: Existing interpretations of cement textures and isotopic compositions may significantly under‐estimate the depth and duration of concretionary growth. Minus‐cement porosities can commonly under‐estimate depths of concretionary growth for some, or all, of the following reasons; (i) cements might not passively replace host sediment porosity, (ii) non‐cement carbonate phases (such as replaced bioclastic carbonate) can be significant, (iii) sediment compaction models over‐estimate rates of porosity loss at shallow ( Carbon isotope data from modern sediments show that dissolved carbonate in the methanogenic zone has a continuum of values from −30‰ to +15‰, and thus overlaps 13 C‐depleted values normally considered characteristic of sulphate reduction. Many concretions previously thought to have grown entirely during sulphate reduction may therefore have continued cementation during methanogenesis, indicating a deeper and more prolonged cementation history. The necessary carbonate supersaturation for concretionary growth could either occur throughout the porewaters (the equilibrium model), or be generated in situ by organic matter decay (the local‐equilibrium model), or created where external fluids are introduced (the fluid‐mixing model).

Environmental fate and toxicology of organophosphate pesticides

Abstract: Organophosphate pesticides (OPs) are generally regarded as safe for use on crops and animals due to their relatively fast degradation rates. Their degradation varies as a function of microbial composition, pH, temperature, and availability of sunlight. Under laboratory conditions (25°C and pH 7) biodegradation is about one order of magnitude faster than chemical hydrolysis, which in turn is roughly ten times faster than photolysis. Microbial biomass often needs a lengthy adaptation period in which soil bacteria mutate to be able to metabolize OPs. Biodegradation is thus in general an order of magnitude faster in soils that have had repeated applications of OPs compared to control soils which have never had OP applications. Because OPs are relatively soluble, they often enter surface and groundwaters. In the latter OPs are primarily broken down through chemical hydrolysis, which is pH dependent. Hydrolysis half-life of an OP pesticide of 10 days in the laboratory increases to one year if the pH of the water is 6 and the temperature 5°C, suggesting that OPs can persist in the environment for long periods of time. Indeed, OPs are detected in soils years after application. Why this environmental persistence occurs is not clear, but it may be due to sorption of the OPs to soil particles, making them unavailable for microbial metabolism. Example calculations and literature data show that conditions can occur in soil where OPs are preserved and transferred to humans through food. A review of the literature shows that OPs are highly toxic and that human exposure is undesirable. Evidence suggests that OPs are mutagenic and teratogenic and that a large number of modern-day diseases of the nervous and immune system of mammals can be linked to these pesticides. These include BSE (mad cows disease), CJD, Gulf War syndrome, Parkinson’s disease and multiple sclerosis, arguing for a thorough examination of the environmental fate and toxicology of OPs as well as their use.

The Caledonian Orogeny redefined

Abstract: Recent advances in our understanding of Palaeozoic tectonics, and in the precise dating of tectonic events require exact definitions of terminology. The Caledonian Orogeny is here redefined to include all the Cambrian, Ordovician, Silurian and Devonian tectonic events associated with the development and closure of those parts of the Iapetus Ocean, which were situated between Laurentia (to the NW) and Baltica and Avalonia (to the SE and east). We suggest that the term ‘Caledonian Orogeny’ be restricted in this geographic sense, but that (as in modern usage) it continues to encompass a series of tectonic, or orogenic, phases (related to arc–arc, arc–continent and continent–continent collisions as Iapetus was closing). Many of these phases have been named; these and many more unnamed events are defined as orogenic phases (local components) of the Caledonian Orogeny. Some of these phases were synchronous over long distances, whereas others were diachronous. The whole Caledonian Orogeny occupied a time interval of around 200 Ma. Thus the term Caledonian should not be used to indicate age.

Late Proterozoic extensional collapse in the Arabian–Nubian Shield

Abstract: A structural and petrological study of the Late Proterozoic rocks in the Wadi Kid area, Sinai, Egypt indicates the presence of an extensional metamorphic core complex in the northern Arabian–Nubian Shield. Gneissic domes throughout the Arabian–Nubian Shield resemble the core complex of the Wadi Kid area and as a result, they are interpreted as extensional metamorphic core complexes. The presence of a widespread phase of extension at the end of the Pan‐African period in the Arabian–Nubian Shield requires a new interpretation of the tectonic history of this shield. Three main tectonic phases are recognized in the Late Proterozoic of the Arabian–Nubian Shield. Ophiolites and island‐arc remnants are relicts of an oceanic phase, the oldest one. This phase was followed by arc‐accretion, well established in the Arabian–Nubian Shield from the presence of individual terranes bordered by sutures, which was responsible for lithospheric thickening. The Late Proterozoic ended with widespread NW–SE extension. The metamorphic core complexes, late‐orogenic extensional basins and large strike slip zones were formed during this phase. Similarity of the tectonic evolution of the Arabian–Nubian Shield with the Mesozoic and Early Cenozoic evolution of western North America lead us to conclude that gravitational instability at the final stages of the arc‐accretion phase caused the collapse and resulted in extension at the latest stages of the Pan‐African orogeny in the Arabian–Nubian Shield.

The Australian Stress Map

Abstract: Knowledge of the in situ stress field of the Australian continent has increased greatly since compilation of the World Stress Map in 1992, principally by analysis of borehole breakouts and drilling-induced tensile fractures in petroleum wells. Stress orientations are variable across the Australian continent as a whole. However, within 15 of 16 individual stress provinces defined in the Australian continent (of one to a few hundred kilometres scale), mean stress orientations are statistically significant. The stress provinces, and stress trajectory mapping, reveal that there are systematic, continental-scale rotations of stress orientation within Australia. Unlike many other continental areas, stress orientations do not parallel the direction of absolute plate motion. Nonetheless, the regional pattern of stress orientation is consistent with control by plate boundary forces, if the complex nature of the convergent northeastern boundary of the Indo-Australian plate, and stress focusing by collisional segments of the boundary, is recognized.

Marine biodiversity through the Late Cenomanian–Early Turonian: palaeoceanographic controls and sequence stratigraphic biases

Abstract: Changes in the marine macro- and microfauna, sedimentary geochemistry and surface-water palaeoproductivity through the last 500 000 years of the Cenomanian and first 300 000 years of the Turonian are documented. These are based on the succession at Eastbourne, the thickest and most complete section through the Late Cenomanian and Early Turonian in the Anglo-Paris Basin. Two levels of rapid faunal and geochemical change are identified, one coincident with a significant increase in siliciclastic input at the base of the Plenus Marls Member, and the other with a marked drop in surface water productivity near the top of the same unit. Faunal change is demonstrated to be largely a pattern of immigration–emigration rather than true extinction, and our sequence stratigraphical analysis shows that it was coincident with major sea-level changes. No evidence is found to support the hypothesis that reduced bottom water oxygenation developed and was responsible for extinctions amongst the benthos in mid-shelf environments. The onset of pure chalk facies is interpreted to mark the breakdown of shelf-break fronts and the spread of oligotrophic oceanic waters over much of the continental shelf, initiated by rising sea-level. The Cenomanian–Turonian event, far from recording a mass extinction of shelf fauna, is most probably an artifact caused by a significant switch in the nature of the surviving sedimentary record as a result of a major, but perfectly ordinary, oceanographic change.

Variscan collisional magmatism and deformation in NW Iberia : constraints from U-Pb geochronology of granitoids

Abstract: U–Pb geochronology of Variscan granitoid rocks from the West Asturian Leonese Zone of the NW Iberian belt documents the episodic nature of magmatism in this section of the western European Variscides. Each magmatic episode is characterized by granitoids with distinct features and has a duration on the scale of several millions of years. The ages of these granitoids place new constraints on the age and duration of magmatic and tectonic events, that are consistent with previous structural studies and proposed models for the tectonic evolution and migration of deformation in the NW Iberian Variscan belt. Granitoid rocks in this zone belong to two main magmatic episodes (syn‐ and post‐tectonic relative to the Variscan Orogeny) and are broadly representative of the granitoid types found in the NW Iberian Variscan belt. The syntectonic association is formed by: (i) tonalite–granodiorite–monzogranite intrusions emplaced synchronously with the main phase of crustal deformation (D 2 ) at c. 325 Ma and (ii) younger leucogranite intrusions emplaced synchronously with syn‐convergence extensional structures at c. 320–310 Ma. The post‐tectonic association is composed of: (i) volumetrically dominant tonalite–granodiorite–monzogranite intrusions (and associated minor mafic‐intermediate rocks) emplaced at c. 295–290 Ma; and (ii) scarce leucogranite intrusions emplaced at c. 290–285 Ma.

Post‐collisional granite generation and HT–LP metamorphism by radiogenic heating: the Variscan South Bohemian Batholith

Abstract: The Palaeozoic Variscan Orogen of Europe is a well‐documented example of a collision zone characterized by widespread late‐orogenic high‐temperature metamorphism and associated crustal magmatism. However, the heat source is still under debate. Based on the Bohemian Massif in the internal zone of the Variscides as case study, we present geological, geochemical, petrological and geochronological data arguing against a substantial mantle involvement in metamorphism and magma genesis in the area of the South Bohemian Batholith. In order to provide an alternative explanation consistent with heat transfer mechanism, we apply a two‐dimensional thermal–kinematic modelling approach. The model calculates the transient lithospheric temperature field during crustal thickening and subsequent thinning by erosiorf from material parameters and boundary conditions specific to the study area. Model results show that the increased contribution of radiogenic heat in the thickened crust can indeed cause a substantial temperature increase in the middle and lower crust. Model predictions are in good agreement with observations, e.g. the P–T–t evolution of the country rocks, the formation of syn‐kinematic migmatites, the large volumes of peraluminous granites derived from dehydration melting of metasediments and the small volumes of lamprophyric melts from the mantle lithosphere. The results of this study emphasize the importance of radiogenic heat as the source for high‐temperature metamorphism and granite petrogenesis in the Bohemian Massif and potentially in other areas of the Variscan Orogen.

Continental rift to back‐arc basin: Jurassic–Cretaceous stratigraphical and structural evolution of the Larsen Basin, Antarctic Peninsula

Abstract: The Larsen Basin developed in Jurassic times as a result of continental rifting during the early stages of Gondwana break‐up. Lower‐?Upper Jurassic non‐marine sedimentary and volcanic rocks constitute a syn‐rift megasequence recording initial largely amagmatic extension and subsequent widespread extension‐related silicic volcanism. A succeeding, Kimmeridgian–early Berriasian transgressive megasequence, consisting largely of anoxic‐dysoxic hemipelagic mudstones, is thought to have been deposited during a thermal subsidence phase when relative magmatic quiescence and peak Jurassic eustatic sea levels served to maximize sediment starvation. The fragmentary record for late Berriasian–Barremian times suggests that a ?regressive megasequence may have developed in the earlier part of this period, recording increased sediment yield to the Larsen Basin from the increasingly emergent Antarctic Peninsula arc. Subsequently, strata in the southern, but not the northern, part of the basin underwent relatively intense eastward‐verging deformation, possibly during the formation of a retro‐arc fold‐thrust belt. Where exposed, the lower part of the succeeding Aptian–Eocene megasequence consists of a deep‐marine clastic wedge deposited along the fault‐bounded western basin margin during a phase of arc uplift and related differential subsidence. Following partial basin inversion in Late Cretaceous times, regression took place as reduced basinal subsidence rates allowed shallow marine facies to prograde basinward.

A Miocene collisional belt in north Borneo: uplift mechanism and isostatic adjustment quantified by thermochronology

Abstract: Subduction followed by underthrusting of continental lithosphere, driven by Oligocene–Early Miocene spreading in the South China Sea marginal basin, account for the tectonic features of Sabah. Isostatic rebound then caused Late Miocene uplift of the Western Cordillera. The strata were buried under 4–8 km of overburden then rapidly exhumed and cooled at >10°C Ma −1 . A rate of exhumation of 0.5–0.7 mm a −1 is deduced from thermochronology. The same order of uplift in the Labuk Highlands has exposed metamorphic rocks of the epidote-glaucophane facies. Rapid erosion of the Western Cordillera supplied abundant clastic sediments to the Miocene–Pliocene Baram Delta oil-bearing basin and to the Eastern Lowlands and Sulu Sea. The Eastern Lowlands were affected by Miocene rifting of the Sulu Sea marginal basin. In contrast to the Western Cordillera, the strata contain apatite crystals whose fission track ages pre-date the containing rocks, indicating burial by only about 2–3 km of overburden. The terrain has been isostatically stable. Some apatite and all zircon crystals, extracted from Tertiary strata, yield Cretaceous fission track provenance ages.

The eastern Palmer Land shear zone: a new terrane accretion model for the Mesozoic development of the Antarctic Peninsula

Abstract: A major ductile fault zone, the eastern Palmer Land shear zone, has been identified east of the spine of the southern Antarctic Peninsula. This shear zone separates newly identified geological domains, and indicates that during Late Jurassic terrane accretion and collision, two and possibly three separate terranes collided, resulting in the Palmer Land orogeny. The orogeny is best developed in eastern Palmer Land and eastern Ellsworth Land. There, shallow-marine sedimentary rocks of the Latady Formation, and a metamorphic and igneous basement complex of possible Lower Palaeozoic to pre-Early Jurassic age, are thrust and folded. This forms an arcuate, east-directed, foreland, fold and thrust belt up to 100 km wide and 750 km long, parallel to the axis of the Antarctic Peninsula. The newly identified Antarctic Peninsula domains include: (1) a parautochthonous Eastern Domain that represents part of the margin of the Gondwana continent, comparable to the Western Province of New Zealand, the Ross Province superterrane of Marie Byrd Land, the Eastern Series of south-central Chile, the Pampa de Agnia and Tepuel rocks of north Patagonia, and the Cordillera Darwin rocks of Tierra del Fuego, (2) a suspect Central Domain that represents an allochthonous, microcontinental, magmatic arc terrane, comparable to the Median Tectonic Zone of New Zealand, the Amundsen Province superterrane of Marie Byrd Land, and Coastal Cordillera of north Chile and (3) a suspect Western Domain, with strong similarities to the Eastern Province of New Zealand, Western Series of south-central Chile, and Chonos metamorphic complex of north Patagonia, that represents either a subduction–accretion complex to the Central Domain, or another separate crustal fragment. Although an allochthonous terrane hypothesis for the Antarctic Peninsula remains to be fully tested, this has much in common with models for the New Zealand and South American parts of the Pacific margin of Gondwana. The identification of a potential allochthonous terrane–continent collision zone allows us to define the edge of the Gondwana continent in the Antarctic Peninsula sector of the supercontinent margin, which has implications for Mesozoic reconstructions of Gondwana.

A model for a continental accretionary wedge developed by oblique collision: the NE Bohemian Massif

Abstract: Structural and metamorphic investigations of the northeastern margin of the Bohemian Massif indicate three main sequential Devonian–Carboniferous tectonic events: (1) Devonian rifting; (2) Early Carboniferous oblique underthrusting and formation of a continental accretionary wedge; (3) eduction of the wedge and Late Carboniferous transpression. Devonian rifting of the Brunia microcontinent resulted in the formation of two crustal‐scale boudins associated with the development of two syn‐rift Devonian basins. This extensional template strongly influenced the nature of the ensuing Variscan contractional deformation. Early Carboniferous (350–330 Ma), progressive, highly oblique underthrusting of the two crustal boudins beneath the Lugian terrane to the west, generated syn‐deformational Barrovian metamorphism and the formation of a continental accretionary wedge. The wedge was further compressed by continued underthrusting of Brunia which resulted in the successive vertical extrusion (eduction) of an upper and lower allochthon, derived from the more deeply underthrust crustal boudin. The eduction was terminated by a Late Carboniferous (330–310 Ma) transpressional event resulting from continued plate convergence. Release of mantle‐derived magma during late‐stage eduction thermally softened the transpressional zones in the more external parts of the wedge. The resultant differential displacements gave rise to extensional unroofing of the internal part of the wedge.

New U–Pb ages for Early Ordovician magmatism in Central Spain

Abstract: Felsic orthogneisses occur widely in the Ollo de Sapo Domain of the Central Iberian Zone, most of which were previously considered to represent a Precambrian basement to the Palaeozoic sequences of this domain. However, new U–Pb dating across the Berzosa‐Riaza shear zone (Sierra de Guadarrama, Central Spain) indicates an Early Ordovician age for the most representative types of orthogneisses in the eastern part of the Ollo de Sapo Domain. Dated rocks include the volcaniclastic Cardoso gneiss (480±2 Ma); from the low–medium‐grade hanging wall; the Riaza gneiss (468 −8 +16 Ma, mylonitic granite) in the Berzosa‐Riaza shear zone; three types of ‘leucogneiss’ (Buitrago gneiss; 488 −8 +10 Ma, megacrystic granite; 482 −11 +14 Ma, aplitic vein; and 482 −8 +9 Ma, gneissic leucogranite); and the La Morcuera granitic augen gneiss (477±4 Ma) in the high‐grade footwall. The new age of the Cardoso gneiss brackets to the Mid–Late Arenig, the so‐called Early Ordovician Sardic unconformity, characteristic of the Central Iberian Zone. These new ages suggest that the broadly coeval volcanism and plutonism were closely associated with the Sardic events, and that these orthogneisses were part of a felsic magmatic belt which extended along the Ollo de Sapo Domain of the Central Iberian Zone. This magmatic belt is interpreted to have been active during the Early Ordovician break‐up of the peri‐Gondwanan margin of the Iapetus Ocean.

The isotopic signature of planktonic foraminifera from NE Atlantic surface sediments: implications for the reconstruction of past oceanic conditions

Abstract: The stable isotope compositions of the planktonic foraminifera Globigerina bulloides, Globigerinoides ruber (white and pink varieties), Globigerinoides trilobus, Globorotalia inflata and Globorotalia truncatulinoides (right‐ and left‐coiling types) were examined as recorders of North Atlantic surface water properties based on 40 box‐core surface sediments between 60° and 30°N. While G. ruber (white and pink varieties) and G. trilobus mainly reflect summer surface water conditions in their oxygen isotope composition, G. bulloides reflects temperatures of the northward‐migrating spring bloom, February–March in the south to May–June in the north. Our data show that G. bulloides cannot be regarded as an indicator for summer temperatures as deduced from Duplessy et al. ’s data. Gt. inflata and Gt. truncatulinoides (right‐ and left‐coiling) build their shells in the coldest waters compared with the other species and reflect temperatures between 100 and 400 m water depth. The difference in oxygen isotope composition between G. bulloides and G. inflata serves as a proxy for water mass stratification. G. bulloides is the only species that gives a distinct pattern in its carbon isotopic composition showing a high correlation with the surface water phosphate values along the transect and may serve as a proxy for palaeonutrients and/or productivity.

Late Cretaceous sea‐level changes in Tunisia: a multi‐disciplinary approach

Abstract: A multi‐disciplinary study of sea‐level and climate proxies, including bulk rock and clay mineral compositions, carbon isotopes, total organic carbon (TOC), Sr/Ca ratios, and macro‐ and microfaunal associations, reveals seven major sea‐level regressions in the southwestern Tethys during the last 10 million years of the Cretaceous: late Campanian ( c. 74.2 Ma, 73.4–72.5 Ma and 72.2–71.7 Ma), early Maastrichtian (70.7–70.3 Ma, 69.6–69.3 Ma, and 68.9–68.3 Ma), and late Maastrichtian (65.45–65.3 Ma). Low sea levels are generally associated with increased terrigenous influx, low kaolinite/chlorite + mica ratios, high TOC and high Sr/Ca ratios, whereas high sea levels are generally associated with the reverse conditions. These sea‐level changes may be interpreted as eustatic as suggested by the global recognition of at least four of the seven major regressions identified (74.2 Ma, 70.7–70.3 Ma, 68.9–68.3 Ma and 65.45–65.3 Ma). Climatic changes inferred from clay mineral contents correlate with sea‐level changes: warm or humid climates accompany high sea levels and cooler or arid climates generally accompany low sea levels.

Neoproterozoic chemostratigraphy and correlation of the Port Askaig glaciation, Dalradian Supergroup of Scotland

Abstract: The δ 13 C of preglacial carbonates beneath the glaciogenic Port Askaig Formation falls from +5‰ to negative values, while least-altered 87 Sr/ 86 Sr values are close to 0.7067. Postglacial ‘cap carbonates’ of the Bonahaven Formation begin with negative δ 13 C carb values, jumping sharply to +11.7‰ near the top, which we correlate with excursions above the Elbobreen–Wilsonbreen glaciations of Svalbard and putative Sturtian (not Varanger) glaciations elsewhere, dated at c. 720 Ma. Much of the succeeding Terminal Neoproterozoic is missing in NE Svalbard and East Greenland, raising doubts about their use for global Sr and C isotope curves in the Neoproterozoic.

Fractional crystallization of mantle‐derived melts as a mechanism for some I‐type granite petrogenesis: an example from Lachlan Fold Belt, Australia

Abstract: The Mt Buller igneous suite in the southeastern Lachlan Fold Belt encompasses a large variety of rock types, from gabbros through diorites to granites. Mafic rocks and enclaves have primitive, mantle‐like initial 87 Sr/ 86 Sr values, 0.7037–0.7045, and ϵ Nd values +5.6 to +4.1. The granites do not show distinctively more radiogenic initial 87 Sr/ 86 Sr values, suggesting they are derived from the same parent as the gabbros. Most of the Mt Buller rocks do not show a positive correlation between silica and initial 87 Sr/ 86 Sr, or between silica and 143 Nd/ 144 Nd values, indicating that neither crustal contamination nor mixing between mantle‐ and crustal derived melts has been important and that these rock types are likely to have formed by fractional crystallization from a common mantle‐derived parent. Some scatter in compatible trace elements, particularly in granites and diorites, may be explained by mixing. Geochemical modelling confirms a comagmatic origin of the suite. Magma generation probably took place in the subduction‐modified upper mantle. This mechanism may play an important role in I‐type granite petrogenesis in particular tectonic settings such as during an efficient slab roll‐back or sinking of oceanic crust in divergent subduction conditions.

Active normal fault evolution in Greece revealed by geomorphology and drainage patterns

Abstract: Active normal fault systems in Greece are associated with characteristic geomorphology and drainage patterns that are recognizable in satellite images and digital topography. Some allowance must be made for the nature of the footwall rocks, which can exert an important influence on these patterns, and we examine the three main types in Greece: hard Mesozoic limestone, relatively soft schist, volcanic rocks or flysch, and very soft Neogene sediments. Drainage and geomorphology can be used to identify active faults that were previously unknown, but can also reveal information about Plio-Quaternary fault evolution and interaction that may be inaccessible by other means. We illustrate such processes with examples showing the migration of normal fault systems into their hanging walls, and the change in normal faulting activity from NW–SE faults to E–W faults, both of which are common in Greece.

Late Mesoproterozoic to early Neoproterozoic history of the East Greenland Caledonides: evidence for Grenvillian orogenesis?

Abstract: Detrital zircons from high-grade metasedimentary rocks (Krummedal supracrustal sequence) in the East Greenland Caledonian orogen yield ion-microprobe U–Pb ages mainly in the range 1100–1800 Ma but with a few grains of c . 1000 Ma, different from zircon ages (mainly 1800–2800 Ma) obtained from the crystalline basement that underlies the metasedimentary rocks. These results indicate that original deposition of the Krummedal sequence took place after 1000–1100 Ma ago, and that the sediment was not derived from the underlying crystalline basement, but from younger, at present unknown sources. High-grade metamorphism of the Krummedal sequence and formation of anatectic granites took place around 930 Ma ago. Caledonian granites are also present in the region, but cannot be distinguished on visual criteria in the field from the older granites, unless emplaced into a younger (900–600 Ma) sequence of sedimentary rocks, the Eleonore Bay Supergroup. It is not yet certain whether the high-grade metamorphism and granite formation at c . 930 Ma are related to a ‘Grenvillian’ or slightly younger collisional event, or to an episode of rifting and crustal thinning. If present at all, a ‘Grenvillian’ orogen in East Greenland would be of very different character than that in North America and southern Scandinavia.

U–Pb geochronology of Neoproterozoic and Caledonian tectonothermal events in the East Greenland Caledonides

Abstract: The East Greenland Caledonides occupy a crucial position in plate-tectonic reconstructions of the Late Mesoproterozoic to Early Neoproterozoic Grenville–Sveconorwegian belt. We present new field and isotopic data from the northern Stauning Alper which indicate that the 1050–930 Ma history of the area was characterized by deposition of extensive clastic sequences. Sources of detritus were dominated by rocks of Mesoproterozoic age, with only limited contributions from Archaean sources, suggesting deposition at a distance from the present Caledonian foreland. A Neoproterozoic granite (938±13 Ma) provides evidence for thermal perturbation at a time of extensional collapse and uplift recorded in NW Scotland, the Grenville Belt of Canada, Labrador and the Sveconorwegian of SW Sweden and southern Norway. Widespread anatexis in the northern Stauning Alper at c. 430–425 Ma resulted from both collisional melting and decompression melting on uplift contemporaneous with the early part of orogenic collapse of the Caledonian Fold Belt. Caledonian deformation was focused at the zone of most extensive granite emplacement. Isotopic evidence suggests that Caledonian granites, previously thought to be entirely post-kinematic, actually predate late Caledonian extension.

Sequence stratigraphy of a tidally dominated carbonate–siliciclastic ramp; the Tithonian–Early Berriasian of the Southern Neuquén Basin, Argentina

Abstract: The Tithonian–Berriasian Vaca Muerta, Carrin Cura and Picun Leufu formations in the southern Neuquen Basin were deposited on a tidally dominated, mixed carbonate–siliciclastic ramp. Basinal, outer, middle, shallow and back ramp facies associations are recognized and a sequence stratigraphic analysis reveals that the ramp record consists of three shallowing‐upwards sequences (Ti1, Ti2 and Ti3) set within a lower‐order progradational cycle. A higher order of cyclicity is superimposed on to the middle (Ti2) sequence. The majority of the ramp facies belong to the transgressive and highstand systems tracts; however, at the base of Ti2, a lowstand systems tract is identified, characterized by a basal unconformity and an abrupt basinward shift of the shallow marine lithofacies. Transgressive systems tracts were characterized by slow sedimentation rates and rapid sea‐level rises that affected carbonate productivity. Highstand systems tracts show the greatest carbonate productivity and an increased progradation rate on account of a reduction in accommodation space generation. Palaeogeography played a major role in the development of the depositional systems. Partial isolation from the Pacific Ocean reflecting the growth of the Andean magmatic arc and geographic restriction due to tectonic inversion in the central part of the basin resulted in a meso‐macrotidal regime that produced a tidally dominated sedimentary record in the shallow and back ramp environments. Coeval anoxic conditions in the central part of the Neuquen Basin favoured distal ramp and basinal black shale deposition during episodes of relative sea‐level rise.

The Early Palaeozoic geography of Europe

Abstract: In the early Ordovician three main continents are recognized by faunas in Europe, Laurentia (northwest of the closed suture of the Iapetus Ocean), which was tropical; Baltica (north of the general area of the Trans‐European Suture Zone), which was temperate; and Gondwana (south of the TESZ), which was high latitude. As the Ordovician progressed, various terranes separated and drifted away from the Gondwana supercontinent at different times, namely Avalonia (which was probably originally part of Gondwana near South America), Iberia–Armorica, Perunica (Bohemia) and various Alpine fragments. Each terrane developed progressively different faunas as time went by. Avalonia collided with Baltica near the end of Ordovician, confirmed by faunas, tectonics and palaeomagnetism, and subsequently Avalonia–Baltica collided with Laurentia to form Laurussia from the mid‐Silurian to the early Devonian. The Ibero‐Armorica, Perunica and Alpine terranes did not join the European collage until the Devonian. The modern sites of the terrane boundaries bear little direct relationships to the original palaeogeographical boundaries in the Early Palaeozoic.

Ludlow (late Silurian) oceanic episodes and events

Abstract: Biotic and sedimentological cycles in the late Silurian are closely similar to those recognized in the early Silurian. They are explained here by alternations between primo and secundo conditions, the former being characterized by cold dense oceanic bottom waters and high nutrient supply, and the latter by salinity-dense bottom waters and diminished nutrient supply. The lithological pattern, together with the turnover of conodont faunas and of other fossil groups, permits the identification of a sequence of episodes and events in the Ludlow following the Klinte Secundo Episode, which developed in the latest Wenlock: The Sproge Primo Episode, the Linde Event, the Etelhem Secundo Episode, the Havdhem Primo Episode, the Lau Event, the Hoburgen Secundo Episode and the Klev Secundo–Primo Event, continuing into the earliest Přidoli. Testing of this model against others that invoke a primary sea-level control is marred by the current imprecisions of biostratigraphical correlation.

Isotopic evidence for temperature variation during the early Cretaceous (late Ryazanian-mid-Hauterivian)

Abstract: Oxygen and carbon isotopic compositions have been determined from the belemnite genera Acroteuthis and Hibolites sampled from the early Cretaceous (Ryazanian–Hauterivian) interval of the Speeton Clay Formation, Filey Bay, England. The Speeton Clay Formation consists of a series of claystones and calcareous mudrocks deposited in an epicontinental sea. δ 18 O values from belemnites, which met petrographic and chemical criteria for well preserved skeletal carbonate, indicate warm marine palaeotemperatures (c. 12–15°C) for much of the early Valanginian whilst cool temperatures (

Geometry and origin of a polygonal fault system

Abstract: A fault array in South Australia, interpreted from a 3D onshore seismic survey, shows fault traces on the lowermost mapped horizon of a shale‐dominated sequence which outline polygonal cells averaging 1.4 km in diameter. The cell boundaries coincide approximately with the downward terminations and near convergence of conjugate pairs of normal faults. The pattern becomes less spatially ordered on higher horizons where faults still show a near‐isotropic strike distribution. Maximum throws, c. 80 m, occur c. 400 m above the downward terminations of the faults. The faults have a systematic geometric relationship with folds, with anticlines in the mutual hanging walls of fault pairs and broader footwall synclines that define the shallow dish forms of the polygons. Polygon boundaries coincide with anticlinal ridges on the interface between the faulted sequence and an underlying 35 m thick low velocity, low density, overpressured layer. Although the pattern of ridges defining the polygon boundaries is strikingly similar to experimental spoke and hub patterns formed at the boundaries between viscous materials with density inversion, the data do not exclude the possibility of lateral extension.

Late Quaternary evolution of a hyperactive rhyolite magmatic system: Taupo volcanic centre, New Zealand

Abstract: Taupo volcanic centre has been active for c. 300 000 years. Since the c. 26.5 ka caldera‐forming Oruanui eruption 28 eruptions have occurred from Taupo, varying between 0.1 and >45 km 3 in minimum bulk volume, and with repose periods ranging from c. 20 to 6000 years. All magma erupted post‐26.5 ka is compositionally and mineralogically distinct from pre‐Oruanui and Oruanui eruptives, and is inferred to have formed at or after 26.5 ka. Four post‐Oruanui magma types are identified on the basis of whole rock and mineral compositions: one dacitic, forming three eruptions between 20.5 ka and 17 ka, and three subtly distinct rhyolite compositions erupted in discrete periods from 11.8 to 9.95, 7.05 to 2.75 and 2.15 to 1.74 ka. Stepwise compositional variations between, and limited variation within, rhyolite groups suggest emplacement of three petrogenetically separate batches of magma within only 10 000 years. The 15–35 km 3 of magma erupted at 1.77 ka evidently appeared in 3 years; this short residence time may have contributed to the lack of zonation within this magma chamber. Taupo is unusual amongst large rhyolite volcanoes in terms of the high frequency of activity since 26.5 ka, rapid stepwise changes in rhyolite compositions, and insignificant differentiation within individual subgroups. These traits are attributed to the combined effects of the extensional arc setting, thermal energy from mafic magma, and the shallow slope of the plagioclase‐saturated rhyolite liquidus.

Death of a sand sea: an active aeolian erg systematically buried by the Etendeka flood basalts of NW Namibia

Abstract: Here we report on a ‘fossilized’ sand sea that was progressively engulfed by the basal Etendeka flood basalts in NW Namibia. Preserved relict aeolian landforms include transverse barchanoid dunes and isolated barchan dunes. Present‐day preferential erosion of the lava flows exhumes relict aeolian bedforms preserved in the position in which they were migrating at the time of burial ( c. 133 Ma). A passive eruption style of inflated pahoehoe flows has preserved the bedforms without significant deformation. The sediment interlayers record a decrease in sand supply and a change in palaeowind direction, which may have been driven by the ongoing break‐up of west Gondwana, or may be a direct result of the widespread volcanism itself.

Isotopic evidence for anoxic pyrite oxidation and stimulation of bacterial sulphate reduction in marine sediments

Abstract: Pore-water sulphate concentrations show marked increases at depths >50 m at ODP sites 888 and 890/889 from the Cascadia Margin accretionary wedge. In the uppermost 10 m sulphate concentrations decrease with depth and sulphate δ 34 S and δ 18 O increase as sulphate is removed by bacterial sulphate reduction. Isotopic data show that sulphate formed below 50 m results from oxidation of early diagenetic pyrite and that oxygen in the sulphate molecules is derived from pore water. Fe 3+ in the sediment is the probable oxidizing agent. The increased sulphate concentrations stimulate bacterial sulphate reduction at depths of 70–250 m and are thus important in sustaining deep bacterial activity.

The eruption environment of multi-tiered columnar basalt lava flows

Abstract: Field studies on the eruption environment and prevailing cooling conditions of multi-tiered lava flows of the type exemplified by the Giant’s Causeway were carried out at localities on the Columbia River Plateau, southern Iceland, on the islands of Mull and Staffa and around the Giant’s Causeway, NE Ireland. Common features can be recognized in the volcanology of such flows in all the localities examined. These include palaeotopographic evidence of river valleys giving conditions conducive to the ponding of flows and the damming of existing drainage; evidence of high rainfall regimes or abundant surface water; association of interlava lacustrine and fluviatile sediments; association of hyaloclastite flow foot breccias within multi-tiered basalt lava successions and evidence of large volume lava flows and high eruption rates. Examination of the petrography of the upper curvi-columnar parts of the lava (the entablature) shows textural signs of quenching, compared with the lower more regular columnar zone (the colonnade). These common field and petrographic features confirm the model of multi-tiered flow formation by modification of the cooling regime in the upper part of the flow following inundation of the hot lava surface by displaced river drainage.

Extensional folding in the Alaşehir (Gediz) graben, western Turkey

Abstract: The broad folds exposed in the syn-extensional sedimentary rocks of the Alasehir graben, western Turkey, have been recently interpreted as contractional in origin. Our field mapping of these folds suggests that they are the products of extensional tectonics. The field evidence includes the presence of several longitudinal extensional folds such as a rollover anticline and drag folds that are related to normal faults in the area. The extensional nature of these folds negates the previous interpretation and consequently suggests extensional tectonics in the region was not interrupted by short-lived contractional tectonics.