bluuebunny/crossref_metadata_2025 huggingface dataset:
The Woolhope and Usk Basins: Silurian rift basins revealed by subsurface mapping of the southern Welsh Borderland
Authors: BUTLER A. J. , WOODCOCK N. H. , STEWART D. M.
Published: December 2007

Subsurface mapping of the southern Welsh Borderland has utilized over 600 km of seismic data and three exploration boreholes to investigate the subsurface of this classic geological area. The survey reveals two Silurian basins, each about 400 km
2
in area, in the western part of the Midland Platform.

The Woolhope Basin underlies the Silurian inlier of the Woolhope Anticline. The basin fill is bounded to the northwest by faults of the Neath Disturbance, and was also influenced by WNW-striking extensional growth faults. The fill thins eastward towards the Malvern Line. Lower Silurian sequences thin southward before thickening again, particularly across WNW-striking faults. The resulting Usk Basin, underlying the Usk Inlier, has ill-defined margins, but some additional N-S fault control.

The Fownhope-1 and Usk-1 wells indicate that rapid subsidence occurred in both basins during Rhuddanian and Aeronian (Early to Mid-Llandovery) time. Modelling of this subsidence, assuming the finite duration lithospheric extension model, yields stretching factors of between 1.3 and 1.6. The later part of the fill to both basins is marine, but red conglomerates, sandstones and mudstones in their Early Llandovery intervals suggest continental fluvial environments in the initial stages of basin development.

The Woolhope and Usk Basins show that the western sector of the Midland Platform rifted during Early to Mid-Llandovery time, perhaps during regional dextral transtension. The rifting is synchronous with, and perhaps genetically related to, an episode of mafic volcanism in the southwest part of the platform. However, this rifting apparently predates the Late Llandovery (Telychian) fault-controlled subsidence seen in the Welsh Basin to the northwest.
Evolution of seasonal stratification in the Celtic Sea during the Holocene
Authors: AUSTIN W. E. N. , SCOURSE J. D.
Published: December 2007

Seasonal thermocline development in the shelf seas of NW Europe results in the formation of pronounced frontal regions which separate stratified from well-mixed water. Numerical
M

2

palaeotidal modelling of the NW European continental shelf during successive stages of the Holocene transgression indicates that seasonal stratification started to develop in the central Celtic Sea once a critical threshold water depth had been exceeded at around 9000 years BP. This hypothesis has been tested through faunal, isotopic and geochemical analysis of an AMS
14
C-dated British Geological Survey vibrocore from the Celtic Deep bathymetric basin; a record which spans the last 12 000 years. Marked positive trends in the stable oxygen isotopic data from the benthic foraminifera
Ammonia batavus
and
Quinqueloculina seminulum
between 9000 and about 5000 years BP represent a 4–5°C cooling in summer bottom water temperature caused by the progressive replacement of tidally well-mixed by stratified water. Stable carbon isotopic data from the same species, measured through the same core intervals, show a negative trend consistent with net oxygen utilization and decomposition of organic matter beneath the productive frontal zone. These geological data indicate the progressive evolution of seasonally stratified water in the Celtic Sea during the earlier part of the Holocene. Modern shelf sea frontal and thermocline development are known to influence biological productivity, sedimentation rates, the organic content of shelf sediments and the climate of surrounding landmasses. Hence these palaeodata have significance in defining the role of shelf sea environments in the climate system.
The Cretaceous-Tertiary biotic transition
Authors: MACLEOD N. , RAWSON P. F. , FOREY P. L. , BANNER F. T. , BOUDAGHER-FADEL M. K. , BOWN P. R. , BURNETT J. A. , CHAMBERS P. , CULVER S. , EVANS S. E. , JEFFERY C. , KAMINSKI M. A. , LORD A. R. , MILNER A. C. , MILNER A. R. , MORRIS N. , OWEN E. , ROSEN B. R. , SMITH A. B. , TAYLOR P. D. , URQUHART E. , YOUNG J. R.
Published: December 2007

Mass extinctions are recognized through the study of fossil groups across event horizons, and from analyses of long-term trends in taxonomic richness and diversity. Both approaches have inherent flaws, and data that once seemed reliable can be readily superseded by the discovery of new fossils and/or the application of new analytical techniques. Herein the current state of the Cretaceous-Tertiary (K-T) biostratigraphical record is reviewed for most major fossil clades, including: calcareous nannoplankton, dinoflagellates, diatoms, radiolaria, foraminifera, ostracodes, scleractinian corals, bryozoans, brachio-pods, molluscs, echinoderms, fish, amphibians, reptiles and terrestrial plants (macrofossils and palynomorphs). These reviews take account of possible biasing factors in the fossil record in order to extract the most comprehensive picture of the K-T biotic crisis available. Results suggest that many faunal and floral groups (ostracodes, bryozoa, ammonite cephalopods, bivalves, archosaurs) were in decline throughout the latest Maastrichtian while others (diatoms, radiolaria, benthic foraminifera, brachiopods, gastropods, fish, amphibians, lepidosaurs, terrestrial plants) passed through the K-T event horizon with only minor taxonomic richness and/or diversity changes. A few microfossil groups (calcareous nannoplankton, dinoflagellates, planktonic foraminifera) did experience a turnover of varying magnitudes in the latest Maastrichtian-earliest Danian. However, many of these turnovers, along with changes in ecological dominance patterns among benthic foraminifera, began in the latest Maastrichtian. Improved taxonomic estimates of the overall pattern and magnitude of the K-T extinction event must await the development of more reliable systematic and phylogenetic data for all Upper Cretaceous clades.
Stable isotopic evidence for the sympatric divergence of Globigerinoides trilobus and Orbulina universa (planktonic foraminifera)
Authors: PEARSON P. N. , SHACKLETON N. J. , HALL M. A.
Published: December 2007

Two extant species of spinose planktonic foraminifera,
Globigerinoides trilobus
and
Orbulina universa,
shared a common ancestor in the early Miocene, as studied here in well-preserved fossil material from Limalok Guyot in the tropical Pacific (Ocean Drilling Program Site 871). The first appearance of
O. universa
(15.1 Ma) was preceded by an increase in the morphological variance of the ancestral lineage, including the origin of several new and short-ranging
Praeorbulina
morphospecies. Biological speciation (cladogenesis) probably occurred before 15.1 Ma. New oxygen (δ
18
O) and carbon (δ
13
C) stable isotopic results are compared with analyses of two reference species,
Globigerinoides ruber
(shallow water) and
Globoquadrina venezuelana
(deep water). Oxygen isotopic ratios of
G. trilobus, Praeorbulina
spp., and
O. universa
indicate that the entire evolutionary transition took place within mixed layer habitats similar to those occupied by modern
G. trilobus.
A slight separation in the δ
18
O of
O. universa
and
G. trilobus
later in the mid-Miocene may indicate subsequent habitat partitioning. Carbon isotopes suggest that no significant change in the carbon metabolism or degree of photosymbiosis occurred. The origin of
Orbulina
therefore appears to have been a case of sympatric speciation at shallow depths in the open ocean. The causes of the speciation and morphological transition are unknown.
Interpreting environmental and biological signals from the stable carbon isotope composition of fossilized organic and inorganic carbon
Authors: BEERLING D. J.
Published: December 2007

Stable carbon isotope studies on marine and terrestrial organic and inorganic carbon provide a means for detecting global climate change and for reconstructing past concentrations of atmospheric CO
2
. Comparison between the CO
2
estimates reconstructed from carbon isotope studies for the past 150Ma show good agreement with the predictions of a long-term carbon-cycle model based on mass-balance studies. Further, the CO
2
estimates from these sources over the entire Phanerozoic show agreement with the fossil record of leaf stomatal density change—a feature inversely related to the concentration of atmospheric CO
2
. Isotopic studies on temporal sequences of fossilized terrestrial organic matter have contributed to palaeoecological studies on shifts in the dominance of plants with the C
4
photosynthetic pathway in ecosystems and historical changes in the metabolic processes of leaves of individual species. The long-term perspective offered by these studies provides critical information for assessing the responses of biological systems to future global environmental change.
Are oxygen and carbon isotopes of mollusc shells reliable palaeosalinity indicators in marginal marine environments? A case study from the Middle Jurassic of England
Authors: HENDRY JAMES P. , KALIN ROBERT M.
Published: December 2007

Stable isotope analyses of non-luminescent, texturally-pristine oyster calcites
(Praexogyra hebridica)
are presented from Bathonian inner-ramp limestones in central-eastern England. A relatively large range of values (δ
18
O= –2.0 to +2.3‰ PDB; δ
13
C= +0.9 to +3.7‰ PDB) define a crude positive covariance, which can be regionally correlated with an offshore tonearshore increase in faunal restriction. This palaeoenvironmental stress gradient has previously been interpreted as a consequence of fluctuating nearshore palaeo-salinities, due to episodic or seasonal runoff from surrounding hinterlands.

Covariant δ
18
O-δ
13
C trends from euryhaline bivalves are commonly ascribed to regional salinity gradients, the most isotopically depleted examples representing the greatest influence of fresh water. However, Bathonian palaeogeography indictates that the most isotopically-depleted oysters inhabited relatively offshore settings. Assuming an average growth temperature of 25°C, these shells were secreted in near-normal sea water (δ
18
O=+0.4‰ SMOW). In contrast, isotopically heavy oyster calcites were produced in highly evaporated, nearshore lagoonal waters (δ
18
O up to +4.8‰ SMOW). Modelling the oxygen isotope and chemical evolution of residual waters during evaporation suggests that up to 65% of the nearshore water mass was lost, probably in a complex of hydrodynamically choked lagoons. Evaporites were not precipitated in the sediment, and geochemical modelling suggests that the lagoon waters may have been brackish rather than marine in origin. The regional δ
13
C trend is contrary to that predicted on the basis of palaeosalinity and may have resulted from variations in organic carbon production, decay and fixation in early diagenesis.
Mid-Tertiary rifting and magmatism in the Traill Ø region, East Greenland
Authors: PRICE SIMON , BRODIE JAMES , WHITHAM ANDREW , KENT R.A.Y.
Published: December 2007

A number of planar normal faults displace Tertiary basaltic intrusions in the Traill Ø region of East Greenland. This 'post-magmatic' extension occurs over an along-strike length of >350 km and over an across-strike width of >100 km with fault displacements and fault-block rotations indicating that (3 increases in magnitude eastwards from near unity to β ≈ 1.1. The stretching direction was approximately E-W, orthogonal to the strike of major faults in the region, most of which are reactivated Mesozoic structures. Two periods of Tertiary basaltic magmatism are recognized in the area.
40
Ar–
39
Ar step heating suggests that the first event, which gave rise to tholeiitic sills with a composite exposed thickness of
c.
1 km, occurred at
c.
54 Ma, and the second, which formed smaller volumes of alkaline dykes and two large syenite complexes, occurred at
c.
36 Ma. Field observations indicate that 'post-magmatic' extension occurred after 54 Ma and possibly continued after 36 Ma. The tholeiitic intrusions are probably related to a stretching event (β ≤ 1.1) prior to the onset of sea-floor spreading in the Norway Basin, whilst the alkaline basalts and 'post-magmatic' rifting are associated with the separation of the Jan Mayen continental block from the East Greenland margin during the Eocene-Oligocene. Both tholeiitic and alkaline basalts formed by the partial melting of garnet-spinel peridotite with greater degrees of partial melting (8–16%) for the earlier tholeiitic group. Rifting in East Greenland ceased with the development of an oceanic ridge between Jan Mayen and East Greenland in late Oligocene time. Subsequent minor compression, probably the result of ridge-push, is suggested by the development of broad wavelength, low amplitude folds in the Traill Ø region.
South Atlantic volcanic margins
Authors: GLADCZENKO T. P. , HINZ K. , ELDHOLM O. , MEYER H. , NEBEN S. , SKOGSEID J.
Published: December 2007

The Early Cretaceous South Atlantic continental break-up and initial sea-floor spreading were accompanied by large-scale, transient volcanism emplacing the Paraná-Etendeka continental flood basalts and voluminous extrusive constructions on the conjugate margins south of the Torres Arch–Abutment Plateau. On the North Namibia margin we interpret four main tectono-magmatic crustal units: (1) oceanic crust; (2) thickened oceanic crust covered by huge seaward-dipping wedges; (3) a
c.
150 km wide break-up related rift zone partly covered by the dipping wedges; and (4) thicker continental crust, partly deformed by Palaeozoic extension, east of the Early Cretaceous rift. Similar settings also characterize other South Atlantic margin segments. We infer an up to 300 km wide and 2400 km long rift zone representing lithospheric extension leading to breakup and formation of the South Atlantic volcanic margins. Comparison with other volcanic margins demonstrates, in spite of local and regional differences, gross similarities in tectono-magmatic style, crustal units and dimensions.
Structural segmentation and the influence of basement structure on the Namibian passive margin
Authors: CLEMSON J. , CARTWRIGHT J. , BOOTH J.
Published: December 2007

Passive margins are segmented into discrete elements by segment boundaries exhibiting a variety of structural styles. The Namibian margin is bounded by a Hinge Line that is offset by segment boundaries. The most important is the Cape Cross Segment Boundary. Here, pre-existing basement lineaments and ‘Karoo’ rift elements were reactivated during late Mesozoic rifting. It marks the northern limit of the earliest of three rift sequences and relates to a temporal change in break-up and formation of the South Atlantic. A similar change occurs across other important segment boundaries but smaller examples lack this. Major segment boundaries with a temporal change in rifting and break-up exhibit a strong connection to the basement province.
Transform tectonics and thermal rejuvenation on the Côte d’Ivoire-Ghana margin, west Africa
Authors: CLIFT P. D. , LORENZO J. , CARTER A. , HURFORD A. J. ,   ODP LEG 159 SCIENTIFIC PARTY
Published: December 2007

Formation of a pronounced basement ridge along many transform continental margins has been attributed to a variety of processes during continental break-up, including transpressional crustal thickening, and thermal rejuvenation and igneous underplating during passage of a spreading ridge. ODP drill holes on the Côte d’Ivoire-Ghana margin now provide the first opportunity to quantify the vertical motions along this type of margin. Apatite fission-track dating of detrital sands suggests that large amounts of erosion occurred on the flanks of an intra-continental wrench zone that predated margin formation. Rapid cooling of >120°C at 120–115 Ma corresponds to erosion of 3.5–5 km along the conjugate Brazilian margin, reflecting
c.
1 km of tectonically driven uplift, subaerial erosion, and isostatic uplift due to unloading. Following rift initiation at 120 Ma (Aptian), an oceanic spreading axis passed adjacent to this part of the margin at 90 Ma (Cenomanian). Maximum uplift during the ridge-transform intersection was 390 m, considerably less than the 2000+ m predicted by heat conduction models in local isostatic equilibrium. The modern ridge is partially the product of thicker crust (22 km) underlying the ridge than the adjacent Deep Ivorian Basin (19 km), and partially related to flexural unloading of the transform ridge between the end of intra-continental wrenching and ridge-transform intersection. Flexural coupling between the continental and oceanic plates since ridge-transform intersection has caused a progressive depression of the offshore margin, estimated at about 650 m in the study area.
Thermal evolution of the Central Atlantic passive margins: continental break-up above a Mesozoic super-plume
Authors: WILSON MARJORIE
Published: December 2007

A near synchronous Early Jurassic (200 Ma) magmatic event along the North American and West African passive continental margins can be explained by the upwelling of a large-scale mantle plume (super-plume) beneath the West African craton. Lateral deflection of the outflow from the plume to the NE by the ambient sub-lithospheric mantle flow could account for tholeiitic basaltic magmatism extending over a distance of nearly 5000 km from Brazil to southern Spain. Continental break-up is considered to occur above the stagnation streamline along which cooler material eroded from the base of the continental lithosphere by the plume is returned to the convecting mantle. The dynamic model is a scaled version of that recently proposed for the Hawaiian plume and satisfactorily explains the apparent lack of involvement of thermally anomalous mantle in the generation of Central Atlantic oceanic crust.
Volcanism on the Rockall continental margin
Authors: BARTON A. J. , WHITE R. S.
Published: December 2007

Continental break-up in the northern North Atlantic was accompanied by massive volcanism. This produced sequences of basalt flows erupted close to sea-level, which are imaged as seaward-dipping reflectors on seismic reflection profiles. They comprise the upper 4-5 km of transitional crust in both the Edoras Bank and Hatton Bank areas of the Rockall continental margin. Detailed surveys show that the reflectors are convex-upward on dip lines but are sub-horizontal on strike lines. They are probably produced by aerially extensive basalt flows extruded from linear fissure vents aligned parallel to the rift axis. Along the Rockall margin two sequences of seaward-dipping reflectors occur, separated by an 'outer high' near the foot of the continental margin. The landward set of basalt flows are emplaced above stretched and intruded continental crust, while the seaward set span the region from highly distended continental crust to fully oceanic crust. The outer high is capped by basalt flows but exhibits little coherent internal reflectivity. We discuss possible origins of the outer high as a residual block of thinned and intruded continental crust or as a central volcano cut by fissure vents similar to those found in the neovolcanic zones of Iceland.
A tectonic model for the Norwegian passive margin with implications for the NE Atlantic: Early Cretaceous to break-up
Authors: LUNDIN E. R. , DORÉ A. G.
Published: December 2007

A sub-regional tectonic study of the mid-Norwegian margin has been assimilated into a broader, regional study of the NE Atlantic margin. The NE Atlantic margin experienced several extensional events between Devonian post-orogenic collapse and break-up in Early Tertiary. This paper focuses on the extensional events between Early Cretaceous and Early Tertiary. It is suggested that NE-trending Early Cretaceous rifting followed the entire margin from the southern Rockall Trough to the Bjørnøya Basin of the SW Barents Sea. This rift had a markedly different trend than the preceding N-trending Late Jurassic rift of, e.g., the North Sea. Seamounts may have been emplaced in the highly thinned basin axes during Early Cretaceous rifting. Subsequent rift events followed the Early Cretaceous rift trend. An enigmatic 'mid'-Cretaceous event, probably extensional, is observed in the northern part of the mid-Norwegian margin. The event is expressed by faulting in the inner Vøring Basin, and by uplift of the outer Vøring and Lofoten margins. Uplift is associated with a widespread unconformity. Uplift of the Gjallar Ridge in the Vøring Basin culminated during the latest Cretaceous to Early Tertiary rifting, with structural geometries broadly similar to those seen in core complexes. A new tectonic model for NE Atlantic margin is proposed based on these observations, and builds on analogues from the modern rift system in the Red Sea and Woodlark Basin of Papua New Guinea.
The mineralization of dinosaur soft tissue in the Lower Cretaceous of Las Hoyas, Spain
Authors: BRIGGS DEREK E. G. , WILBY PHILIP R. , PÉREZ-MORENO BERNARDINO P. , SANZ JOSÉ LUIS , FREGENAL-MARTÍNEZ MARIAN
Published: December 2007

The mineralized soft tissue of a dinosaur, only the second discovery of its kind is reported from the Lower Cretaceous of Las Hoyas, Spam. Cellular details of mineralized skin and muscle of
Pelecanimimus
are replicated in an iron carbonate. The outline of this ornithomimosaur is preserved by a phosphatized microbial mat that enshrouded the carcass. It confirms the existence of either a throat pouch or dewlap, and soft occipital crest. This study confirms the importance of microbial mats hi the fossilization of soft tissues and emphasises the need for careful monitoring of vertebrate remains for mineralized soft tissues prior to acid preparation.

The multitoothed ornithomimosaur dinosaur
Pelecanimimus polyodon
was recently described from the Lower Cretaceous lacustrine lithographic limestones of Las Hoyas (Cuenca, Spain) (Perez-Moreno
et al.
1994). The only known specimen of this, the first ornithomimosaur theropod found in Europe, preserves what were originally described as 'integumentary impressions'. These occur in the throat region, flanking the more proximal neck vertebrae, in the vicinity of the ribs, and behind the elbow. In addition a small triangular area at the rear of the skull was interpreted as a possible occipital crest (Perez-Moreno
et al.
1994). Analyses confirm that these features represent mineralized traces of soft tissues. They are preserved both as replicas of the original skin and muscle tissue, and as impressions on microbial mats or veils that grew over them.

In addition to
Pelecanimimus,
the lacustrine limestones of Las Hoyas (Melendez 1995) have yielded a well preserved diverse biota of plants,
Australia-Banda Are collision as an analogue for early stages in Iapetus closure
Authors: SNYDER D. B. , BARBER A. J.
Published: December 2007

A comparison of structures formed since the Pliocene during Australia-Banda Are collision with structures hi the central British Isles provides insights into early tectonic events and later structures formed along the Laurentian margin of the Iapetus Ocean between Ordovician and Devonian tunes. In particular, the timing of ophiolite emplacement, incorporation of a volcanic are into a continental margin and early stages hi nappe formation can be recognized by comparing seismic reflectors and mapped geological structures hi both regions. In this comparison the Midland Valley of Scotland is interpreted as a compressed and composite volcanic are, formed hi two stages as the result of are reversal and becoming more mafic than adjacent blocks which were formed from accretionary prisms.

Uniformitarian principles are used to interpret geochemical processes and tectonic events throughout the Phanerozoic and have also been used to interpret events in the Proterozoic and Archaean (e.g. Taylor & McLennan 1995). The Tonga-Kermadec, Aleutian, Andean and Banda-Sunda ares are often cited as present-day analogues of ancient convergent plate boundaries. Similar logic motivated BIRPS to acquire two deep seismic reflection profiles across the currently active Australia-Banda are collision zone to the east of Timor (Figs 1 & 2a) in order to provide a better understanding of the tectonic events represented by structures that formed along the margin of Laurentia during the closure of the Iapetus Ocean c. 450 Ma (Fig. 2) and that are observed in the crust of the British Isles today.

The Banda Arcs mark the site of subduction of
Preservation of a subcritical wedge in the south Urals foreland thrust and fold belt
Authors: BROWN D. , ALVAREZ-MARRÓN J. , PÉREZ-ESTAÚN A.
Published: December 2007

The geometric and kinematic evolution of the south Urals foreland thrust and fold belt appears to have been quite simple. Despite this simplicity, there are several key differences when compared with foreland thrust and fold belts developed in other orogens, in particular the anomalously small amount of shortening (14 %), and the location and geometry of the basal thrust in the basement. To understand the possible reasons that led to its evolution, the critical wedge model was applied to try and determine the relative contribution of the mechanical factors that acted on the wedge during its development. The south Urals foreland thrust and fold belt appears to be a 'snapshot' of a developing wedge that is preserved in the subcritical field.

In recent years significant advances have been made in the understanding of the geometric, kinematic, and mechanical development of foreland thrust and fold belts. These advances have largely come about due to the development of section balancing techniques (e.g.
Dahlstrom 1969
;
Hossack 1979
), kinematic modelling (e.g.
Geiser 1988
), and critical wedge modelling (

Davis
et al.
1983

;

Dahlen
et al.
1984

). Whether applied individually or together these techniques are powerful tools for investigating the frontal parts of orogenic belts. They have demonstrated that a number of general features are found in many of the worlds foreland thrust and fold belts (e.g.
Rodgers 1990
) that reflect their mechanical development, although there are many notable exceptions (e.g.
Rodgers 1991
). Section balancing and kinematic modelling have shown that foreland thrust and fold belts
δ34
S of Lower Carboniferous anhydrite, Cumbria and its implications for barite mineralization in the northern Pennines
Authors: CROWLEY S. F. , BOTTRELL S. H. , MCCARTHY M. D. B. , WARD J. , YOUNG B.
Published: December 2007

Recent hydrocarbon exploration in the Solway Basin has revealed the existence of Lower Carboniferous paralic sediments which contain numerous discrete anhydrite beds. Anhydrite S isotope ratios from the Easton-1 Well are compared with new isotopic data from barite mineralization that occurs along the faulted southern margin of the Northumberland-Solway Basin and published data from the Northern Pennine orefield. Similarities betw
een the δ

34

S of anhydrite and barites from major mines at Haydon Bridge and the Alston district of the Northern Pennine orefield supports a Lower Carboniferous marine sulphate source for mineralization at these sites. Identification of extensive evaporite deposits hi the Solway Basin resolves several longstanding problems associated with mineralization hi the Northern Pennines including the source of barite sulphate and the origin of hypersaline mineralizing fluids.

In an earlier investigation of the S isotope geochemistry of Carboniferous-hosted barite mineralization in and around the Alston district of theNothern Pennine orefield (NPO, Fig. 1a), Solomon
et al.
1971 drew attention to the similarity between the δ
34
S of Lower Carboniferous marine sulphate and the sulphur isotopic composition of barite deposits. A model for barite mineralization was proposed in which deep, circulating, BaCl
2
brines mixed with shallow (< 1 km depth), sulphate-rich, connate fluids composed of Lower Carboniferous sea water. Although the model proposed by Solomon
et al.
1971 is consistent with aspects of barite mineralization, more recent studies of the evolution of pore fluid geochemistry in sedimentary basins (
Land 1987
;
Hanor 1994a
,

b

) argue against a marine connate origin for the sulphate component
Geological constraints on geophysical models of relative sea-level change during deglaciation of the western Irish Sea Basin
Authors: McCABE A. MARSHALL
Published: December 2007

Numerical model predictions of low Late Devensian sea levels around Britain are inconsistent with dated sea-level indicators contained in the geological record. Stratigraphies from the western Irish Sea Basin record high sea levels throughout the last deglacial cycle (21–13 ka
bp)
and onshore ice-marginal glaciomarine and marine sedimentation. Sedimentary evidence emphasizes the role played by delayed isostatic uplift coupled with rapid tidewater evacuation of ice from the basin. Drumli-nization and marine downdraw in the northern Irish Sea Basin records subglacial sediment transfer towards tidewater ice margins and headward extension of ice streams deep into the ice sheet around 14 ka
bp.
The application of sequence stratigraphy to Upper Carboniferous fluvio-deltaic strata of the onshore UK and Ireland: implications for the southern North Sea
Authors: HAMPSON GARY J. , ELLIOTT TREVOR , DAVIES SARAH J.
Published: December 2007

Stratigraphical correlations and fades interpretations of Upper Carboniferous fluvio-deltaic strata have been based traditionally on cyclothems bound by marine flooding surfaces (marine bands). The recent recognition of major, regionally extensive erosional unconformities (Exxon-style sequence boundaries) within selected cyclothems questions their validity as units of genetically related strata. U sing examples from the Carboniferous of the onshore UK and Ireland, we present sedimentological criteria for the recognition of sequence boundaries, placing particular emphasis on the regional context of these surfaces. Sequence boundaries comprise widespread, deeply eroded surfaces at the base of major fluvial sandstone complexes, and laterally equivalent palaeosols developed on interfluves at the margins of the fluvial complexes. These sequence boundaries define units of genetically related strata (sequences) which contain other key surfaces of time-stratigraphic significance, including marine bands and regionally extensive coals. The recognition of key surfaces enables the construction of a high resolution stratigraphic framework within which coeval facies relationships can be interpreted.

Sequence boundaries can be correlated between individual basins in the onshore UK, by reference to their position in relation to a particular marine band. For example, the sequence boundary at the base of the Farewell Rock in the South Wales Basin can be correlated with that at the base of the Rough Rock in the Pennine Basin, northern England, since both these sandstone bodies are directly overlain by the Subcrenatum Marine Band. Interbasinal correlations of this nature imply that potential fluvial sandstone reservoirs within major incised valley fills in the Upper Carboniferous strata of the southern North Sea can be predicted by correlation with the onshore UK. The stratigraphical framework can be extended and tested using core and well-log data, particularly spectral gamma-ray data, which are able to identify key sequence stratigraphic surfaces.
Mid-Jurassic volcanic structures in the Outer Moray Firth Basin, UK
Authors: STEWART S. A.
Published: December 2007

3D seismic mapping in the southern part of Quadrant 15 in the Outer Moray Firth Basin reveals an array of circular to oval downthrown blocks bound by steep ring faults. These lows affect Triassic and older strata, are approximately 1.5–2 km in diameter and 0.5–1 km deep. The footwall crests of the structures now lie at a depth of
c.
2.5 km. Several alternative causal mechanisms for these circular fault blocks are examined including salt dissolution, strike-slip pull-apart and impact cratering, but in terms of geological and geometrical criteria, the most likely mechanism appears to be small calderas or diatreme-conduit collapse. Seismic data indicates that the circular structures cut Triassic, Permian and Devono-Carboniferous strata and are infilled with pre-Upper Oxfordian sediments, therefore they are broadly synchronous with Mid-Jurassic volcanism–local well control demonstrates the presence of several thousand feet of basalt tuffs and lavas. The model proposed here to account for the circular structures consists of an initial phase of diatreme/maar cratering as magmas associated with the Rattray Volcanics Member intruded shallow aquifers in the Pentland Formation fluviodeltaic rocks. These maars enlarged and deepened via conduit wall collapse, supplemented by downsag-faulting due to magma withdrawal. The post-volcanic evolution of the structures included (1) differential erosion creating accommodation space which may have been infilled with Oxfordian reservoir sands and (2) local reactivation by underlying basement faults in the Kimmeridgian.
Upper Pleistocene loess stratigraphy in the southern Yenisei Siberia area
Authors: FRECHEN MANFRED , YAMSKIKH ANATOLY F.
Published: December 2007

The loess-like sediments along the river Yenisei provide a detailed palaeoclimatic archive of the last interglacial/glacial cycle for the southern part of Central Siberia. The loess-like sediments were formed by aeolian, alluvial, lacustrine–alluvial, slope wash and flood processes. Chernozem or chernozem-like palaeosols formed during the interglacial and interstadials. The loess-like sediments and palaeosols at the sections of Tatysh and Cholnokovo near Krasnoyarsk and Berezov were investigated with thermolumi-nescence and infrared optically stimulated luminescence dating methods. Luminescence dating suggests that the oldest sediments below the lowermost chernozem formed during the penultimate glaciation. It is likely that the lowermost chernozem at the Tatysh, Cholnokovo and Berezov sections formed during oxygen isotope stage 5. A large hiatus occurs above and below this lowermost chernozem, indicating that most of the last interglacial and early glacial record is missing in the sections investigated.

The intercalated duplicate chernozem-like palaeosols in the upper part of the section formed during an interstadial within oxygen isotope stage 3, indicating forest-steppe climatic conditions between 25 and 35 ka BP. The youngest loess-like sediments, overlying the duplicate palaeosol horizon, apparently have a very high sediment-accumulation rate.
High-precision
40
Ar/
39
Ar spectrum dating on sanidine from the Donets Basin, Ukraine: evidence for correlation problems in the Upper Carboniferous
Authors: HESS JÜRGEN C. , LIPPOLT HANS J. , BURGER KURT
Published: December 2007

Two
40
Ar/
39
Ar spectrum measurements on well-preserved volcanic feldspar (sanidine) from the coal tonstein horizon 1
3
1
from the lower Kashirsky group (middle Moscovian stage) of the Donets Basin Upper Carboniferous (Ukraine) yielded well-defined plateaus with an average age of 305.5 ± 1.5 Ma. The age is to be interpreted as sedimentation age of the coal tonstein and thus defines a chronostratigraphic time mark in the East European Carboniferous. Taking the correlation of the 1
3
1
horizon and the recently published time scales for the Upper Carboniferous as a basis for comparison, the age of 305.5 Ma is about 6 Ma too young. This age discrepancy obviously points to inaccurate or even erroneous correlation, and we therefore are compelled to suggest a reconsideration of the validity of the current biostratigraphic correlation between the Carboniferous of western and East Europe.
Lake-level changes, sedimentation and faunas in a Middle Devonian basin-margin fish bed
Authors: TREWIN N. H. , DAVIDSON R. G.
Published: December 2007

The classic Devonian fossil fish site at Tynet Burn, Fochabers, NE Scotland has been investigated following recent excavations. The fish assemblage consists of placoderms, acanthodians, an actinopterygian, porolepiforms, osteolepiforms and a dipnoan. The assemblage is typical of the 'Achanarras Fauna' of Eifelian age which is widespread in the Orcadian basin, representing a time of generally high lake levels. The most diverse fauna occurs in the Upper Nodule Bed which can be divided into top and bottom levels, where fish are mainly preserved in calcareous nodules, and a middle level with fish in laminated limestone.

Three fluvial to lacustrine cycles are exposed in the Tynet Burn in 27 m of strata. Fish are abundant in the Upper Nodule Bed, and are also sparsely represented in lacustrine strata both above and below this level. The lacustrine laminites of the Upper Nodule Bed were deposited following a rapid rise in lake level. Subsequent fall in lake level resulted in deposition of a thin shallow water massive limestone, prior to exposure and desiccation of the lake floor. Disruption of laminites by sediment sliding and sand intrusion was followed by compaction and concretion formation. Chemotrophic bacteria later invaded open cracks and porosity and partially oxidized the fish remains; the bacterial oxidation event being responsible for the characteristic pink, red and purple colours of fossil fish from Tynet Burn. Subsequent lake level rise resulted in deposition of shales with abundant diagenetic concretions, rarely containing fish. With increasing clastic input alluvial sands prograded over the area from the south.

The rapid deepening event associated with the base of the Upper Nodule Bed at Tynet is considered to be synchronous with similar events represented in other Achanarras-equivalent fish beds. However, elements of the Achanarras fish assemblage can be encountered in less persistent lacustrine intervals both above and below this level.

Minor cyclicity, superimposed on the main Achanarras event, resulted in repetition of fluvial to lacustrine transitions. The main control on cyclicity is considered to have been climatic, but subsidence and sediment supply rates were also a significant influence.
Rare earth geochemistry of Arenig cherts from the Ballantrae Ophiolite and Leadhills Imbricate Zone, southern Scotland: implications for origin and significance to the Caledonian Orogeny
Authors: ARMSTRONG H. A. , OWEN A. W. , FLOYD J. D.
Published: December 2007

Rare earth element (REE) data from low to mid-Arenig cherts are used to test competing models for the early Ordovician evolution of the Laurentian margin in the northern British Isles. Cherts from the Ballantrae Ophiolite Complex have chondrite-normalized REE patterns typical of continental margin settings with LREE enrichment, a slight negative Eu
anom
and shale and chondrite-normalized La/Yb values of 0.97–1.41 and 7.78–11.4 respectively. This pattern, together with a large positive chondrite-normalized Ce
anom
(1.44–1.70), is virtually identical to that found in radiolarian chert of the Gascoyne Abyssal Plain, in the Timor Sea. Cherts from the Raven Gill Formation within the Leadhills Imbricate Zone, Northern Belt, Southern Uplands have typical continental margin REE patterns, chondrite-normalized Ce
anom
(0.9–1.21) and Eu
anom
(0.61–0.79) values indicating that they formed closer to the continental margin than those from Ballantrae. Shale and chondrite-normalized and La/Yb values of 0.95–1.27 and 4.92–13.88 respectively confirm this interpretation.

It is concluded that the Ballantrae ophiolite formed in a rifted–arc basin above a northwards dipping, intra-oceanic subduction zone. The modest depth of burial of the Raven Gill Formation precludes it being part of a marginal basin which was subsequently trapped as the Ballantrae Ophiolite was obducted in the late Arenig. The Arenig rocks of the Leadhills Imbricate Zone represent an allochthonous terrane accreted to the western extension of the Midland Valley in Ireland in pre-Caradoc times. Here it formed the basement to the Southern Uplands basin. Palaeontological evidence places this basin adjacent to Pomeroy, Co. Tyrone in the early Caradoc. Sinistral strike-slip faulting, from the late Ashgill transported the Southern Uplands Terrane to its present location, a distance of less than 250 km.
Structural evolution of shale diapirs from reactive rise to mud volcanism: 3D seismic data from the Baram delta, offshore Brunei Darussalam
Authors: VAN RENSBERGEN P. , MORLEY C. K. , ANG D. W. , HOAN T. Q. , LAM N. T.
Published: December 2007

Two areas of shale diapirism in a deltaic setting were examined using 3D seismic data from offshore Brunei to determine the origin, evolution and geometry of shale diapirs. The study areas cover parts of the Egret and Ampa fields within the Baram Delta province. The Ampa diapir consists of a small bulge of the mobile Setap shale around which lies a zone of chaotically disrupted country-rock reflections. At the crest of this chaotic zone, vertical, cylindrical shale pipes arise that end in a wider head. The diapir heads wear halos of high amplitude reflections, probably due to the presence of methane gas. The Egret data show the same features, except that the bulge of the source layer is not detected due to low reflectivity and chaotic reflections at the base of the overburden. The bulging of the Setap shale on the Ampa data is interpreted as modified reactive diapirism in response to differential loading. The initial reactive diapir shape was modified by lateral or oblique upward migration of fluidized shales into the hanging wall of the fault controlling the reactive diapir. This probably occurred as a network of dykes and sills. The presence of some preserved bedding is indicated by weak but coherent reflectivity from the area initially defined as chaotic shale.

The spreading of overpressure and the injection of shale into the overburden is thought to be a late phenomenon, possibly Late Pliocene or Early Pleistocene and the mobility of the shale is attributed to overpressure increase due to gas generation within the Setap shale and maybe the lower part of the overburden. Gas migration, perhaps ahead of the diapir intrusion, probably facilitated the mud injection. The Ampa and Egret diapirs formed in two diapiric phases, an early reactive diapirism and a later active diapirism. But the phases have little in common; they are seperated by about 9 million years of inactivity and differ in process and probably differ in degree of overpressure, in overpressure generating mechanism, and in properties of the diapiric mud.
Mesozoic-Cenozoic evolution of the North Patagonian Batholith in Aysen, southern Chile
Authors: PANKHURST R. J. , WEAVER S. D. , HERVÉ F. , LARRONDO P.
Published: December 2007

A detailed Rb-Sr geochronological and geochemical study has been carried out on granitoids of the North Patagonian batholith in Aysén. The results for 25 plutonic bodies reveal a complex age pattern. The principal zones defined are, from west to east: Late Cretaceous (a single 76 Ma pluton), Early Cretaceous (
c.
135 Ma), Eocene (
c.
45 Ma), and Early Miocene (25-15 Ma), reverting to mid-Cretaceous (120-90 Ma) in the Main Cordillera. The igneous suite is typically metaluminous and calc-alkaline, with hornblende-biotite granodiorite and tonalite dominant, although small bodies of Late Miocene/Pliocene (
c.
10-5 Ma) peraluminous leucogranite occur sporadically. Tertiary plutonism extends to gabbroic compositions and is concentrated in the vicinity of the dextral strike-slip Liquine-Ofqui fault zone. The highest initial
87
Sr/
86
Sr ratios (
c.
0.7050) occur in the Early Cretaceous group, with a sharp decrease to 0.7034–0.7045 that persists until a Late Miocene reversal to higher values (0.7040–0.7048). These variations are reflected by Nd isotopes, but trends in the ɛSr, v. ɛNd, diagram show that this is not due to contamination from the accretionary complex into which the batholith was emplaced. An origin by melting of mafic crustal underplate and lower crust is suggested for the main magma suite. The discrete episodes of magmatism correlate with significant changes in subduction kinematics.
U–Pb ages from the Harts Range, central Australia: evidence for early Ordovician extension and constraints on Carboniferous metamorphism
Authors: HAND MARTIN , MAWBY J.O. , KINNY PETER , FODEN JOHN
Published: December 2007

Sensitive high-resolution ion microprobe (SHRIMP) U–Pb analyses of monazite and zircon from the Harts Range indicate the southeastern Arunta Inlier has been affected by two major Palaeozoic tectonothermal events. During the early Ordovician (467 ± 8 Ma), regional deformation produced a sub-horizontal upper amphibolite-facies (> 700°C and 7 kbar) shear fabric that overprinted
c.
800°C migmatitic assemblages. In the intracratonic Amadeus Basin south of the Harts Range region, subsidence rates increased in the early Ordovician with the accumulation of easterly, and southerly, derived mature sediments. This is contrary to what would be expected if deformation in the Harts Range region occurred in a compressional setting, and suggests that the early Ordovician deformation may have occurred in an extensional intraplate setting. The terrain remained at depth until the mid-Carboniferous when up to 20 km of exhumation occurred during the latter stages of the intracratonic Devonian to Carboniferous (400–300 Ma) Alice Springs Orogeny. Monazite U–Pb data indicates the structurally lowest parts of the terrain were at
c.
6 bar and
≥c.
600°C at 343 ± 8 Ma, and subsequently underwent high-temperature decompression, resulting in overprinting of the kyanite-grade assemblages with sillimanite-bearing assemblages during south-directed thrusting. During exhumation, pegmatite emplaced in the southeastern Harts Range at 330 ± 6 Ma (zircon U–Pb) was strongly deformed at temperatures around 560°C and depths around 15 km. The U–Pb data from the Harts Range indicate that the record of intraplate deformation in central Australia was more complex than previously thought.
Unravelling tectonic and climatic signals in sedimentary successions
Authors: PEDLEY H. M. , FROSTICK L.
Published: December 2007

During the past two decades scientists and governments have progressively been drawn towards a search for a fuller understanding of the mechanisms of climatic change. In recent times attention of the public has been drawn to an apparent increase in adverse weather conditions leading to such processes as increasing mean world sea levels and accelerated desertification. The leading question was and still is 'how much of this is small oscillations on a longer term climatic trend'. Equally importantly, 'how much is related directly to man's activities'.

Less easy to speculate upon are tectonic signals. Many, such as the Mount St Helens eruption, leave a legacy of atmospheric repercussions felt for many years, yet these are effectively instantaneous in a geological context. In contrast, longer term plate and polar wanderings, and mountain-building episodes, are insignificant when measured in a human life span.

The mainly observational data set upon which scientists base all predictions for future change spans little more than a few centuries. Only by accurately interpreting the sedimentary record will it be possible to extend the 'data set', thereby providing the spring-board upon which to base forward projections. There is, therefore, a pressing fundamental requirement to establish a set of tools which will tease out the tectonic and climatic strands from the depositional record.

A two day meeting was held at Burlington House jointly by the British Sedimentological Research Group and the Geological Society (26–27 June 1997) specifically to address some of these problems by examining a range of