bluuebunny/crossref_metadata_2025 huggingface dataset:
The confrontation of structural styles and the evolution of a foreland basin in central SW England
Authors: SEAGO R. D. , CHAPMAN T. J.
Published: December 2007

Detailed mapping of the rocks to the north and south of Plymouth reveals a sedimentary sequence deformed by a series of folds and thrusts. Two structural zones exist each with a different style of deformation. In the Lower to Upper Devonian rocks of the southern zone, slip vectors, vergence and facing of D1 folds indicate that the transport direction is to the north-west. An associated first phase cleavage dips to the south-east. The northern zone, of Upper Devonian and Lower Carboniferous strata, extends into central Devon and east Cornwall and the geometry and facing of the early folds in these rocks indicate a transport direction to the south or south-east. D1 folds generally verge north but are downward facing towards the south. A flat-lying, penetrative slaty cleavage occurs in the mudrocks and also indicates southward facing. Generally the Carboniferous rocks in this area are inverted. The two structural zones confront each other at an E–W-trending line which passes through Cargreen, 8 km north of Plymouth. Here the deformation is more intense as the cleavage, common to the southern zone, overprints that of the northern zone, resulting in the formation of D2 folds. The confrontation is interpreted as a northerly dipping backthrust produced by large-scale underthrusting. It is proposed that the northward-advancing Variscan thrust belt progressively underthrust the Carboniferous foreland basin flysch deposits which became inverted and backthrust towards the south.
Structure of the Musandam culmination (Sultanate of Oman and United Arab Emirates) and the Straits of Hormuz syntaxis
Authors: SEARLE M. P.
Published: December 2007

The Musandam Mountains form a link, both in time and space, between the late Cretaceous ophiolite obduction tectonics of the Oman Mountains and the late Tertiary continental collision tectonics of the Zagros Fold belt of Iran. Culmination of the Musandam shelf carbonates occurred after the Turonian-Maastrichtian thin-skinned thrusting of the Oman Mountains was completed. The basal Hagab thrust cuts up-section to the west from at least mid-Permian level up to the late Cretaceous Aruma Group and breached up into the overlying previously allochthonous (Cretaceous) Hawasina cherts. In the Hagil Window (Ras al Khaimah), the shelf carbonates show from 4 to 15 km of westward translation. Extrapolation of foreland sequences suggests that Upper Cretaceous Aruma Group and Hawasina complex sediments could occur in the subsurface along the footwall east of the mountain front. The Hagab thrust and Musandam structures plunge south-southwest at increasingly greater depth (and with decreasing amount of translation) and carry all overlying late Cretaceous thrust sheets of the Oman Mountains in a piggy-back fashion. The Eocene-Lr. Oligocene Pabdeh foredeep developed as a result of loading in front of the Musandam thrust sheets wedging out to the south. The trailing (south-eastern) edge of the Musandam culmination is a hinged listric normal fault downthrowing to the south-east up to 2.5 km at Bayah and less than 500 m at the western end of Batha Mahani, and is interpreted as a dorsal culmination collapse feature.
Dips of normal faults in British Coal Measures and other sedimentary sequences
Authors: WALSH J. J. , WATTERSON J.
Published: December 2007

Fault dip measurements (n = 3291) from 122 faults in British Coalfields are presented. Two classes of normal fault are recognized: (a) gently dipping (<55°) synsedimentary faults of gravity driven or, less commonly, tectonic origin and (b) steeply dipping (>55°) post-coalification tectonic faults formed at depths of about 2.5 km which comprise 90% of the faults analysed. Dip data from the steeply dipping faults are variable (standard deviation = 9°), both on single faults and between faults, and 70° is a better approximation of fault dip than the commonly accepted value of 60°. Data on tectonic normal faults, with comparable depths of formation (<3–4 km), from other sedimentary sequences show that dips of greater than 60° are quite usual. Systematic changes of dip on single fault surfaces have predictable effects on the curvature of fault traces.

Using the modified Griffith fracture criterion the dips of faults at varying depths (up to 4 km) are calculated for both unsaturated (dry) and saturated rocks. The dips of normal faults are expected to vary from 90° at the surface to 60° at 3–4 km and this variation is consistent with the limited field data available. The coalfield data are consistent with fault formation at standard hydrostatic pore fluid pressures at depths of about 2.5 km. Below depths of a few kilometres normal faults are expected to flatten out gradually to a dip of 45°, but such flattening is not necessarily an indication of an underlying detachment.
Mixing-zone dolomites in the Gully Oolite, Lower Carboniferous, South Wales
Authors: SEARL A.
Published: December 2007

The Gully Oolite at St Lythams, Glamorgan, is extensively dolomitized towards its base. The dolomite is non-ferroan and is both replacive and void-filling in nature. δ
18
O varies from −6.06%0 to −2.96%0 (PDB) and δ
13
C from +2.32%0 to 3.55%0 (PDB) with a marked covariation of δ
18
O and δ
13
C. Dolomitization is believed to have occurred within a mixing-zone environment at salinities up to 50% of that of seawater. The variation of dolomite isotopic composition versus depth in the sediment is explained in terms of successive upwards migration of the mixing-zone and associated multiple phases of dolomitization. The tops of these successive mixing-zones coincide with evidence of sediment emergence, and the presence of dolomite intraclasts implies that dolomitization occurred at shallow depths within the sediment. This suggests dolomitization occurred in the absence of an overlying meteoric-phreatic lens. The high degree of dolomitization at St Lythams as opposed to that occurring in modern mixing-zones is related to: (1) longer duration of the mixing-zone environment in the sediment; (2) the proximity of the mixing-zone to the sediment surface; (3) possible chemical differences between Carboniferous and modern seawater, particularly with respect to
pco
2.
Structural controls on Upper Palaeozoic sedimentation in south-east Wales
Authors: WILSON D. , DAVIES J. R. , SMITH M. , WATERS R. A.
Published: December 2007

Marked thickness changes in early Dinantian (Courceyan-Chadian) sediments occur across the Cardiff-Cowbridge Anticline, a major east-west Variscan fold in the Vale of Glamorgan. They resulted from differential subsidence over an active concealed basement fault zone, the Vale of Glamorgan Axis, which is coincident with the hinge of the Anticline; the latter formed in response to inversion on this basement fault. A Caledonian ancestry for the Vale of Glamorgan Axis is indicated by pronounced Upper Old Red Sandstone overstep across it.

Across the Severn Estuary, between South Wales and the Bristol-Mendips region, there is a significant offset of isopachs and facies belts in both the Courceyan-Chadian and Arundian sequences. It is suggested that these result from intra-Carboniferous dextral strike-slip along a major fault zone underlying the Severn Estuary (the Severn Estuary Fault Zone), and that the Vale of Glamorgan Axis was a synthetic structure, developed in conjunction with this strike-slip movement. By inference, the Severn Estuary Fault Zone shares a common history with the Vale of Glamorgan Axis, extending back to the Devonian and possibly earlier.

Late Dinantian (Holkerian) facies distribution reflects uplift and emergence on the north-south Usk Anticline and Malvern Lineament. Continued uplift is recorded in Silesian sequences in south-east Wales and the Bristol-Mendips area. The orientation of structures bordering the Severn Estuary is consistent with their development in a right-lateral strike-slip zone.
A model for the development of the Irish Variscides
Authors: PRICE C. A. , TODD S. P.
Published: December 2007

A clear correlation exists in the southern Irish Variscides between extensional structures which controlled late Palaeozoic sedimentation in the Munster Basin, and contractional thrusts and folds of end-Carboniferous age. This correlation is interpreted to indicate that the extensional structural framework of the basin, including a basal low-angle detachment and an array of superjacent normal faults, was reused during transpressive inversion of the basin to produce the fold-thrust belt. The extensional framework, which in turn was to some extent derived by reactivation of Caledonian structures, therefore, exerted a fundamental control on the geometry and style of contractional deformation. It is not evident that the detachment had a discrete link to the fold belt in south Wales or the main Variscan orogen in Europe.
Ophiolitic thrust nappes in western Nevada: implications for the Cordilleran Orogen
Authors: DILEK Y. , MOORES E. M. , ERSKINE M. C.
Published: December 2007

New field and geophysical evidence from Jurassic mafic and ultramafic rocks in western Nevada indicate the presence of two major ophiolite complexes tectonically overlying the rocks of the North American continental margin. Coupled with the existence of Jurassic and older island arc-marginal basin complexes and overthrust relations in the adjacent Sierran-Klamath region, these ophiolitic thrust nappes in Nevada support collision models for the middle Jurassic deformation. The geometry of the observed structures and their relative age relationships suggest that this deformation might have resulted from the collision of the North American continent with a west-dipping subduction zone which resulted in ophiolite emplacement and continental underthrusting. Continued westward motion of North America beneath the allochthonous oceanic terranes during the late Mesozoic developed extensive thrust and possible strike-slip faulting in the western US Cordillera and caused migration of deformation towards the east. This eastward propagation of deformation is interpreted to have resulted in the development of the Sevier foreland thrust belt of latest Jurassic to latest Cretaceous age. These collisional processes and the geometry of the proposed collision orogen are reminiscent of the alpine-type collision belts.
Late Precambrian tidal rhythmites in South Australia and the history of the Earth's rotation
Authors: WILLIAMS G. E.
Published: December 2007

Sedimentary rhythmites of siltstone and fine sandstone from late Precambrian (c. 650–800 Ma) glaciogenic formations in South Australia are interpreted as distal ebb-tidal deposits that record variability in the velocity and range of palaeo-ebb tides. Variations in lamina thickness encode a full spectrum of palaeotidal cycles, including semidiurnal, diurnal, fortnightly and monthly tidal cycles as well as the lunar apsides (perigee) and nodal cycles. A half-yearly oscillation is attributable largely to a beat between the fortnightly tidal cycles of luni-solar conjunction and lunar declination; the lunar nodal cycle is discernible as an amplitude modulation of this beat oscillation. The data allow determination of the Earth's palaeorotation and the past dynamics of the Earth-Moon system with an accuracy previously unattainable for the Precambrian. The late Precambrian (c. 650 Ma) year contained 13.1 (±0.5) lunar months and
c.
400 (±20) days, and the late Precambrian lunar month c. 30.5 (±1.5) days. These values suggest an average equivalent phase lag near 3
°
since late Precambrian time rather than the present value of 6
°
. The period of 19.5 (±0.5) years determined for the lunar nodal cycle
c
. 650 Ma ago indicates a lunar distance 96.9 (±1.7)% of the present distance. The low rate of lunar recession since late Precambrian time revealed by the rhythmite data militates against a close approach of the Moon during the Proterozoic. Precambrian sedimentary rhythmites may hold a key to the early history of the Earth's rotation.
The outgassing history of the Earth’s atmosphere
Authors: TURNER G.
Published: December 2007

The principal source of information concerning the outgassing history of the atmosphere is the isotopic composition of helium, argon and xenon in the modern atmosphere and emerging from the upper mantle. Helium provides the best indication of current noble-gas outgassing rates which are seen to be extremely low in comparison to the mean rates over geological time. Extremely high (
40
Ar/
36
Ar) ratios in MORB (Mid-Ocean Ridge Basalt) indicate conclusively that the major release of primordial volatiles occurred before there had been significant growth of radiogenic
40
Ar in the mantle, i.e. within the first 500 Ma or so of Earth history. High (
129
Xe/
130
Xe) ratios in MORB combined with the argon data can be used to indicate that 80% or more of the release occurred in the first 50 Ma of Earth history, though alternative interpretations are possible. The data can be used to constrain a narrow range of acceptable mathematical models. However, the major conclusion that outgassing occurred early in Earth history is inescapable and model independent. Further information on the detailed dependence of outgassing rates with time may come from a study of the isotopic composition of palaeoatmospheres.
Carbon flows in the biosphere: present and future
Authors: HALL D. O.
Published: December 2007

The atmosphere, biosphere and ocean surface layers contain approximately equal amounts of carbon (the deep ocean contains over 50 times as much). The flux of carbon between these pools is dominated by the process of photosynthesis which is itself influenced by the CO
2
concentration of the atmosphere. Over the last 200 years deforestation and now fossil fuel combustion have added CO
2
to the atmosphere to increase the concentration by 27%—half the increase occurred over the last 30 years. The resulting greenhouse effect has been accompanied by an increase in the global average temperature of about 0.5°; other 'greenhouse gases' (N
2
O
1
O
3
, CFCs, CH
4
) are also contributing to the warming. Present uncertainties with estimating carbon flows result from the disputed contribution of deforestation, the level of increased plant productivity and carbon storage due to CO
2
fertilization, and problems with estimating net primary production. Uncertainties in estimating future carbon balances arise from the extent of fossil fuel use especially by USA, USSR and China, the degree of control of greenhouse gas emissions, the CO
2
fertilization effect on plant productivity and vegetation responses (including moisture availability) and the rate of change of the climatic factors resulting from changing carbon fluxes.
Samarium-neodymium isotope stratigraphy of the Lunde and Statfjord Formations of Snorre Oil Field, northern North Sea
Authors: MEARNS E. W. , KNARUD R. , RÆSTAD N. , STANLEY K. O. , STOCKBRIDGE C. P.
Published: December 2007

Two sequences of fluviatile sediments from Snorre Oil Field are investigated by the Sm-Nd isotopic method. The results illustrate distinctly different source areas for the (Upper Triassic) Upper Member of the Lunde Formation and for the (Lower Jurassic) Statfjord Formation. The Lunde Formation has Sm-Nd provenance ages mainly in the range 1500 to 1700 Ma, which is not diagnostic of a specific source area. The Statfjord Formation has provenance ages in the range 1700 to 2150 Ma. This is old within the context of source areas in the hinterland of the northern North Sea and provenance partly from the Archaean Lewisian shield to the northwest is suggested.

Intensive sampling through the Statfjord Formation in one well illustrates that the provenance ages evolve systematically with time. The pattern provides a detailed record of how source areas evolved during deposition of these strata. The possibility is recognized of correlating changes or trends in provenance between wells.

The data also suggest that Sm/Nd ratios and isotopic compositions of these rocks are not significantly fractionated between fine- and coarse-grained sediments by mechanical or diagenetic processes.
Sedimentary dynamics of the Kimmeridge Clay: tempests and earthquakes
Authors: WIGNALL PAUL B.
Published: December 2007

Storms are suggested to be an important influence upon the formation of epicontinental black shales, controlling both their distribution and facies types. A variety of storm-produced event beds are described from the Kimmeridge Clay of Dorset. These include horizons of graded rip-up clasts, silt laminae, thin graded mud horizons and shell pavements. Biostratinomy and population dynamics indicate that the pavements represent brief benthic colonization events in a predominantly anaerobic-dysaerobic environment. Differential near-surface lithification has also affected the sediments, leading to the formation of flow phenomena. These include loop structures, microfolding and brecciation. Earthquake-induced fissuring also occurs at a number of horizons.

The Kimmeridge Clay shows well-developed cyclically alternating organic-rich shales and mudstones on a decimetre scale. A feedback mechanism of storm-induced benthic oxygenation and temperature-stratified inhibition of storm mixing may account for the abrupt nature of the cyclicity. Subsidence and sedimentation rate were probably important factors controlling the thickness and organic-richness of the sediments whilst the influence of storms was important in controlling the distribution and facies types of these important source rocks. The model proposed for the Kimmeridge Clay may be of general application to many epicontinental black shales in the geological record.
Lignite and associated deposits of the Tertiary Lough Neagh Basin, Northern Ireland
Authors: PARNELL J. , MEIGHAN I. G.
Published: December 2007

Report of a meeting of the British Sedimentological Research Group of the Geological Society held at the Queen's University of Belfast on 27–28 September 1988. The organizers were J. Parnell, I. G. Meighan and I. C. Legg.

The existence of lignite in the Tertiary deposits of the Lough Neagh Basin has been known since the eighteenth century. Recent drilling by the Geological Survey of Northern Ireland and several exploration companies has shown that the lignite deposits occur on a substantial scale and, therefore, may provide a major source of energy, particularly as a fuel for electricity generation. The meeting was convened as a forum for the presentation of results by a multi-disciplinary group of geologists, chemists, civil engineers, chemical engineers and economists concerned with lignite deposits, its extraction and upgrading. Fourteen pagers were presented, followed by a one-day field excursion to North Antrim and the exploration office of BP Coal Ltd.

The geological background to the deposits was described by
H. Cherry,
who showed that the Oligocene Lough Neagh basin developed on the irregular surface of the Palaeocene Antrim Lava Group and explained the allocation of licenses for lignite exploration by the N.I. Department of Economic Development.
I. C. Legg
outlined the distribution of lignite deposits within the basin, and gave details of particularly rich deposits in the regions of Crumlin, Co. Antrim; Coagh, Co. Tyrone; and Ballymoney, Counties Antrim and Londonderry. Lignite deposits also occur to the south of Lough Neagh in Co. Armagh but not at a
Short paper: The Silurian of Clew Bay, Ireland: part of the Midland Valley of Scotland?
Authors: PALMER D. , JOHNSTON J. D. , DOOLEY T. , MAGUIRE K.
Published: December 2007

A new agnathan-dominated biota found near Louisburgh, Co. Mayo in the west of Ireland is correlated with post-Llandovery biotas of the Midland Valley of Scotland (MVS). It is argued that this biotic similarity implies a 500 km elongate aquatic connection between the two areas. An alternative hypothesis, that the fault-bounded Irish outcrop is a displaced terrane, originally in closer contiguity with the MVS inliers is rejected. This results from a detailed structural analysis of the bounding fault (Emlagh Thrust), which constrains the post-Llandovery strike-slip motion to less than 25 km.
Short Pape: Movements on the Southern Upland fault
Authors: McKERROW W. S. , ELDERS C. F.
Published: December 2007

Consideration of the distinct sequences within the Northern Belt suggests that the Southern Upland fault (SUF) lies to the north of the ‘conventional’ line in E Scotland, SW Scotland and NE Ireland. Comparisons of clasts in mass flow conglomerates in the Southern Uplands with granites in western Newfoundland suggest large Ordovician and Silurian strike-slip movements, while southerly-derived clasts in Late Silurian Midland Valley conglomerates suggest post-Silurian movements. In western Ireland, the pre-Devonian movements on the SUF probably continued along the Skird Rocks Fault. Post-Silurian movements on the SUF may have extended along the Clew Bay fault zone or the Fergus shear zone.
Short Paper: Detrital mineral ages from the Southern Uplands using
40
Ar-
39
Ar laser probe
Authors: KELLEY S. , BLUCK B. J.
Published: December 2007

Detrital mica ages in the Southern Uplands range from 458 to 502 Ma. The evidence favours a source in a basement which was part of an arc complex to the north rather than from the Dalradian block which also has mica ages of this range. No Grenville ages have yet been recorded. Detrital volcanic clasts dispersed from the south have cooling ages of 560 and 530 Ma indicating that they did not come from a contemporary arc, but an older arc terrane of uncertain affinities.
Short Paper: Palaeomagnetic constraints on the geological terrane models of the Scottish Caledonides
Authors: TRENCH A. , DENTITH M. C. , BLUCK B. J. , WATTS D. R. , FLOYD J. D.
Published: December 2007

A compilation of new and published palaeomagnetic data from the Scottish Caledonides allows construction of Apparent Polar Wander Paths (APWPs) for the Grampian Highlands, Midland Valley and Southern Uplands terranes. Geological incompatibilities between terranes are the result of predominantly palaeolongitudinal movements prior to late Silurian times. A common late Silurian-early Devonian ‘corner’ in each APWP suggests the terranes to have docked (to within the errors of the individual determinations) by this time.
Extended Abstract: Correlation of the Monian Supergroup in NW Anglesey with the Cahore Group in SE Ireland
Authors: TIETZSCH-TYLER D. , PHILLIPS E.
Published: December 2007

Due to their similarity and proximity along strike the rocks of Anglesey and the southeast corner of Ireland have long been the subject of comparison. The most frequent correlation is between the sedimentary and metasedimentary successions which lie northwest of the central gneisses in Anglesey and the Rosslare Complex in southeast Ireland.
Crimes & Dhonau (1967)
considered the Cullens-town Formation to have a correlative in the South Stack Group of Holy Island due to their lithological similarity and comparable complex deformation history. However, Barber & Max (1979) considered the style of deformation and metamorphic grade of the Cullenstown Formation to be more reminiscent of the New Harbour Group.

Until recently the Monian Supergroup in Anglesey was believed to be Precambrian and as such could only be correlated with Precambrian rocks in southeast Ireland. With the recognition of Lower Cambrian microfossils (

Muir
et al.
1979

) in the Gwna Group, there emerged the possibility that much of the Monian Supergroup is Lower Palaeozoic, and thus that correlation may be attempted with younger rocks in southeast Ireland. Recent work in southeast Ireland makes a more detailed comparison between the two areas possible. Much of the stratigraphic terminology used here is newly introduced and will be formally defined in forthcoming papers.

On the basis of lithological correlation it is possible to demonstrate that the Cullenstown Formation is an integral part of the Cambrian Cahore Group (Crimes & Crossley 1968), and that its structure is not incompatable with that developed elsewhere in that group. The
Palaeomagnetic evidence for rotations and deformation in the Nogueras Zone, Central Southern Pyrenees, Spain
Authors: BATES MARTIN P.
Published: December 2007

A multicomponent palaeomagnetic remanence from the Lower Triassic Buntsandstein redbeds of the Central Southern Pyrenees has been interpreted as being composed of three components; a low blocking temperature recent viscous magnetization (L), an intermediate blocking temperature Eocene remagnetization (I), and a high blocking temperature Lower Triassic remanence (H).

These palaeomagnetic results can be used in conjunction with structural evidence to identify poles of rotation during the development of the Nogueras Zone antiformal stack. Poles from individual sites reflect local deformation and folding, including vertical rotation due to shearing between adjacent thrust sheets. The I component was sometimes acquired syn-tectonically during thrust motion, and can be used to define intermediate stages of structural development.

Results from sites located on a single thrust sheet (‘No. 7’) indicate that it rotated by 45° clockwise during emplacement. This is reflected in an easterly increase in displacement estimated from the construction of balanced cross-sections. Thrust sheets appear to have been pinned close to the present day location of Pont de Suert during the later stages of structural development. The arcuate path of thrust sheet motions, and the potential movements in and out of cross-section, can lead to over- and under-estimates of minimum displacement when using line-length cross-section balancing techniques.
Climatic versus tectonic controls of fan sequences: lessons from the Dead Sea, Israel
Authors: FROSTICK LYNNE E. , REID I. A. N.
Published: December 2007

Tectonic activity is commonly held responsible for abrupt coarsening of alluvial fan sequences and the role of climate is often relegated. The Pleistocene fans of the Dead Sea rift indicate clearly the problems of differentiating the sedimentary signatures of climatic and tectonic adjustments in a closed lake basin. Exposures are excellent. The rivers are currently incising in response to the low lake levels of the Holocene, producing cliffs >50m high. These reveal an interdigitation of lake deposits (dominantly fine grained carbonates) and the coarse river gravels which characterize the fan deposits. The bounds of each facies are sharp: river sediments are superimposed on lake sediments with little evidence of either reworking or loading; in turn, aragonite crusts are draped over alluvial gravels indicating the sudden return of still-water conditions. Neither can be explained by gradual adjustment of the hinterland river system after tectonic shock. Instead, it is clear that the coarse alluvium was deposited by discrete high magnitude floods that occurred only sporadically because of the infrequent nature of rainfall in this arid environment. In interpreting basin development in ancient settings, it is always tempting to assume that coarse deposits are indicative of tectonism. However, the Dead Sea fans demonstrate that climate is just as likely to be responsible for the conglomerates in a sedimentary sequence.
The metagabbros, orthogneisses and paragneisses of the Connemara complex, western Ireland
Authors: Leake Bernard Elgey
Published: December 2007

Although now largely disappeared within the 400 Ma-old Galway Granite, the syntectonic Connemara metagabbro–gneiss complex, which crystallized at 490 Ma (Arenig), is the largest intrusion into the Dalradian rocks. It is part of widespread Cambro–Ordovician volcanism and plutonic injection into continental crust in the Appalachian–Caledonide orogenic belt, being synchronous in crystallization with the widely obducted 490 Ma ophiolites within the orogeny. The peridotites and hornblende gabbros were injected, broken up, and shouldered aside by quartz diorite, granodiorite, granite and trondhjemite gneisses, and it is shown that these orthogneisses are not the in situ differentiation products of the gabbro magma. The hornblende in the metagabbros is mainly igneous but is partly metamorphic and replaces olivine (Fo
75–92
), orthopyroxene (En
65–92
), clinopyroxene (Wo
48–5
En
50–39
FS
5–14
) and plagioclase typically An
80–92
) (range An
50–96
) An aureole at least 10 km wide in the sillimanite zone was imposed, and cordierite-garnet-sillimanite Dalradian hornfelses were partially melted at temperatures up to 1000
o
C near to the gabbros at 4.5 ± 1 kbar corresponding to c. 18 km depth. This suggests that the complex forms part of the root of a now eroded batholith of calc-alkaline affinities, the gabbros corresponding to the transition zone to a basic bottom which is often supposed to underlie many batholiths. Within the orthogneisses are sheets of paragneiss formed from Dalradian metasediments which have been folded up from the floor and down into sides of the magma chamber, which process could be illustrative of the means by which basic magma can intimately interact with crustal material. The
18
O/
16
O, D/H, Nd and Sr isotope evidence, and the existence of inherited zircons in the gneisses, indicate that the gabbro and gneiss magmas were significantly contaminated with crustal, possibly Dalradian, material both
before
and
after
intrusion, with the lowest initial
87
Sr/
86
Sr at 490 Ma being 0.708 even in samples far removed from metasediment. The metagabbros are tholeiitic to high alumina basalt in character but the gneisses have distinct calc-alkaline geochemistry. The chemistry, igneous hornblende and highly calcic plagioclase match many arc plutons in the western USA. The syntectonic nature and the ubiquitous gneissic fabric in the quartz diorites, trondhjemites and granites is a consequence of early movements which were probably in part connected with the separation of the Connemara massif, and which culminated in slicing off the largely inverted complex and carrying it southwards on the Mannin Thrust which underlies the whole of Connemara. The complex is thus a fragment of a now disrupted Cambro-Ordovician continental magmatic arc or continent-island arc environment which can be traced along the SE edge of the Laurentian continent.
Short Paper: Fossil vertebrate tracks and indenter mechanics
Authors: ALLEN J. R. L.
Published: December 2007

A reconnaissance study is outlined showing how an improved understanding of the formation and character of fossil vertebrate tracks in soft sediments is provided by the application of the mechanical engineer’s indenter theory, supported by complementary scaled laboratory experiments. Only the footprint at the bottom of the shaft cut by the animal's limbs in general offers the best preservation of the shape of the underside of the foot. The character of the foot is less well preserved in the deformed bedding beneath the footprint and among laminae deposited in the shaft after the passage of the animal. This understanding will help to reduce errors in the use of tracks in taxonomic, biostratigraphic, palaeoecological, behavioural and environmental studies.

When a limbed vertebrate traverses yielding sediment, the limbs deform the substrate into tracks which vary with the nature, size and gait of the animal, the forcefulness of its limb movements, and the sediment properties. Most previous geological work on tracks and trackways has emphasized their taxonomic, biostratigraphical, palaeocological and behavioural significance (
Lockley 1986
;
Leonardi 1987
), but without an adequate understanding of their environmental implications (
Lockley 1986
) and, in particular, their genesis and preservation as trace fossils.

The response of an elastic-plastic material to an indenter or punch, both in terms of theory (
Calladine 1969
;
Hill 1971
;

Johnson
et al.
1982

) and in the light of qualitative laboratory experiments, provides a general mechanical model for track formation and a basis for the assessment of track preservation in sediments. The indenter represents the animal's
Short Paper: Lyell’s pillars and uniformitarianism
Authors: AGER DEREK
Published: December 2007

It is suggested that the Roman pillars at Pozzuoli, near Naples, the great symbols of uniformitarianism, record not gradual but very sudden changes in sea level and a more catastrophic, or at least episodic, concept of changes in the recent geological past than was envisaged by Charles Lyell.

The great symbols of uniformitarianism were the marine borings in the Roman pillars of Serapeo (Serapis) at Pozzuoli near Naples. Charles Lyell used the pillars as the frontispiece of his great book
Principles of Geology
1831, which was to have such a fundamental effect on geological thought in the last century. They also appear on the reverse side of the Lyell Medal of the Geological Society of London. The importance of these pillars was that they proved that changes in sea level had occurred in historic times. They were the proof, it might be said, that the present may be used as the key to the past. If sea level changes could occur so recently, then all the complex pattern of transgressions and regressions in the geological past could be explained by reference to what was seen happening today. They put an end, it was thought, to catastrophism and the doctrine of sudden, short-lived episodes in earth history, which was always associated with the thought of that great Frenchman Georges Cuvier. But the Lyell and Cuvier of standard text-books are cardboard figures, as Stephen Gould called them (1987), Cuvier is particularly maligned and blamed for all the later excesses of catastrophism. It
Short Paper: Plagiogranitic differentiates in MORB-type sills of the Faeroe–Shetland Basin
Authors: KANARIS-SOTIRIOU RAYMOND , GIBB FERGUS G. F.
Published: December 2007

The association of certain types of low-potassium acid igneous rocks known as plagiogranites with oceanic crust and ophiolite complexes is well known. Both this association and experimental studies have suggested that oceanic plagiogranites could have originated by differentiation of magmas of mid-ocean ridge basalt (MORB) type. However, no actual example appears to have been recorded in which the progressive differentiation of MORB-type magma can be demonstrated to generate
integrally associated
plagiogranite. The occurrence in the Faeroe–Shetland sill complex of intrusions containing plagiogranite within tholeiitic olivine dolerite of MORB-type composition provides direct evidence of this process for the first time.

The occurrence and composition of oceanic plagiogranites have been reviewed by Coleman & Donato (1979). They describe plagiogranites as leucocratic rocks which occur within ophiolite complexes (in the upper parts of cumulate gabbros and within the sheeted dyke complexes) and which have also been occasionally recovered by sea-floor dredging and drilling (see also
Hekinian 1982
). Plagiogranites characteristically differ from most continental granitic rocks by having very low K
2
O and Rb contents, having Rare Earth element (REE) distributions that resemble those of MORB (light REE-depleted), and typically exhibiting interstitial vermicular and/or micrographic intergrowths of quartz and plagioclase.

The main models proposed for the origin of plagiogranites are (1) fractional crystallization of MORB-type magma, (2) liquid immiscibility, and (3) anatexis of basic rocks. Some workers (e.g.

Saunders
et al.
1979

;

Ashley et al
. 1983

) have concluded that although fractionation of basic liquids was the dominant process by which plagiogranites originated in the ophiolites
A sulphur isotope study of Ni-Cu mineralization in the Huntly-Knock Caledonian mafic and ultramafic intrusions of northeast Scotland
Authors: FLETCHER T. A. , BOYCE A. J. , FALLICK A. E.
Published: December 2007

The Caledonian mafic and ultramfic intrusions of the Grampian region of northeast Scotland are synorogenic tholeiitic plutons of middle Ordovician age. They include layered cumulates and xenolithic, contaminated and granular gabbroic varieties. The structurally complex Huntly–Knock intrusions contain locally significant quantities of Fe–Ni–Cu sulphide, while the associated country rock metasediments are sporadically enriched in Fe-sulphide.

Sulphur isotope analyses on sulphide from within and around the intrusions give the following ranges of δ
34
S; – 0.1 to —1.7‰ for disseminated to massive sulphides in the complex and deformed Littlemill-Auchencrieve contact zone; +0.7 to +4.3‰ for disseminated interstitial sulphides within cumulate and granular rocks; + 1.7 to +6.0‰ for graphitic and sulphidic pyroxenitic pegmatites; –6.0 to +16.5‰ for disseminated sulphide from country rock metasediments; –4.0 to +8.2‰ for sulphides in partially melted sediments.

δ
34
S of sulphides in the igneous rocks (χ̄ = +0.5 ± 2.4‰ (1
σ
),
n =
36) lie within the range usually indicated for primary magmatic sulphur, i.e. 0 ±3‰, so that the sulphide system was probably dominated by magmatic sulphur. There is, however, a distinct difference between the isotopically heavier cumulate and granular rocks ( =±2.4 ± 1.2‰ (1
σ
)
n
= 9) and the lighter sulphide of the contact zone (χ̄ = –1.1 ± 0.4‰ (l
σ
), n = 21
)
. The possibility that the slightly negative δ
34
S values of the contact zone are due to a contribution of
32
S rich sulphur from sulphidic calcareous units is considered unlikely, due to the homogeneity of the contact zone δ
34
S
values, and so the variation between the two groups is attributed to processes operative within the magma. Locally, an input from country rock sulphur has occurred as suggested by the δ
34
S values for xenolithic gabbro (+6.5‰), some of the graphitic and sulphidic pyroxenitic pegmatites (+5.9, +6.0‰) and possibly a basal olivine cumulate (+4.3‰). Although the data from the Littlemill-Auchencrieve contact zone are isotopically distinct from those Ni-Cu deposits dominated by crustal sulphur, petrographic evidence suggests that crustal involvement may have been important in the siting of the ore.
A high precision U-Pb age for the Ben Vuirich granite: implications for the evolution of the Scottish Dalradian Supergroup
Authors: ROGERS G. , DEMPSTER T. J. , BLUCK B. J. , TANNER P. W. G.
Published: December 2007

High precision/high accuracy U-Pb analyses of zircons from the Ben Vuirich granite in the Scottish Dalradian succession were performed on carefully picked, high integrity, abraded fractions of specific morphologies. Zircon needles, which grew during crystallization, give an age of 590 ± 2 Ma. Stubby prisms show inheritance with an upper concordia intercept of 1448 ± 7 Ma. The age of intrusion at 590 Ma contrasts strongly with a previous U-Pb zircon age of 514
+6
−7
Ma, determined on bulk-fraction unabraded grains, which, it is argued, must have suffered significant Pb loss. As the Ben Vuirich granite was emplaced between the regional D2 and D3 deformation events in the Dalradian block, the new data imply that all Dalradian sedimentation was Precambrian. Supposed Lower Cambrian fauna1 links between the Dalradian succession and Laurentia are thus discounted, and the possibility that the Dalradian Supergroup has non-Laurentian affinities must be considered. The early D1/D2 nappe-forming Grampian structures are also Precambrian, and may be associated with terrane docking along a tectonically active ocean margin. Later D3 structures and associated peak Barrovian metamorphism suggest a subsequent thickening event, possibly
c
. 520 Ma ago.
The geology and structural evolution of a Caledonian fold and ductile thrust zone, Kyle of Tongue region, Sutherland, northern Scotland
Authors: HOLDSWORTH ROBERT E.
Published: December 2007

The Middle to Upper Proterozoic metasedimentary rocks of the northern Highland Moine in Sutherland are disposed in a series of major Caledonian (
c
. 470–430 Ma) metamorphic thrust nappes produced by NW/WNW-directed shortening and reworking of parts of a pre-existing Precambrian (c. 1000 Ma) orogen. Lewisian inliers occur throughout Sutherland and represent tectonically interleaved units of the original basement gneisses upon which the Moine cover sequence was deposited.

In the western Moine Nappe of the Kyle of Tongue region, the rocks are deformed by two main episodes, D1 and D2, which are thought to represent the effects of Precambrian and Caledonian orogenesis respectively. Little basement-cover interleaving or large-scale folding occurred during the D1 event in the Moine Nappe, but the associated metamorphism (to
ut leust
garnet grade) produced peak metamorphic mineral assemblages. Subsequent Caledonian reworking (D2) produced a foreland-propagating system of WNW-directed ductile thrusts and related major F2 sheath folds, both of which resulted in much of the observed Moine-Lewisian interleaving. Metamorphism associated with D2 decreases in grade westwards, occurring at similar, or slightly lower grades, than the earlier D1 event; this retrogressive overprint is most obvious at the W margin of the Moine Nappe. The D2 folds and ductile thrusts are geometrically and kinematically similar to the Caledonian structures recognized within the Moine Thrust Zone to the west, the very low (≫5°) ramp angles suggesting that the thrusts must have very shallow-dipping trajectories through the crust. F2 fold formation is attributed to an early phase of WNW-directed overthrusting compression which was then progressively superseded by a regional ductile extensional strain contemporaneous with continued WNW-directed thrusting. The occurrence of internal extension in the orogenic wedge during active thrust-related foreshortening implies that the shear strength of the material close to the basal detatchment(s) was continuously reduced, i.e. wedge motion became
supercritical
. In this context, strain softening processes within the ductile thrust mylonites were probably of major significance.
Short Paper: Hummocky cross-stratification in the Downton of the Welsh borderland
Authors: SMITH R. D. A. , AINSWORTH R. B.
Published: December 2007

Hummocky cross-stratification is recorded from the classic Upper Silurian Whitcliffe Group (Ludlow Series)/Downton Group (Pridoli Series) boundary section at Ludlow, Shropshire. This necessitates a revision of previous interpretations of palaeoenvironments which infer either a tidal mudflat depositional setting. Evidence for tidal influence is lacking. Instead the importance of storm activity, probably on a shoreface, is emphasized. The absence of articulate brachiopods and other open marine faunal elements in the Downton strata is attributed to changes in salinity in the restricted Downton sea shortly before its elimination by Acadian uplift.

The boundary between the Upper Whitcliffe Formation (Ludlow) and the overlying Downton Castle Sandstone Formation (Pridoli), formerly proposed as the stratotype section for the base of the fourth Series of the Silurian System, is designated at Ludford Corner, Ludlow, Shropshire [SO 51237413] (Fig. 1,

Bassett
et al.
1982

and references therein). It has received considerable attention, with particular emphasis on faunal changes through the section. In addition to its stratigraphic significance, the section occurs at a crucial position in the transition from marine to non-marine environments associated with the Acadian (late Caledonian) inversion of the Welsh Basin (

Woodcock
et al.
1988

).

The Downton Castle Sandstone Formation in the Ludlow area overlies shelly calcareous siltstones of the top Ludlow, Upper Whitcliffe Formation and comprises a coarsening and cleaning-upward sequence, about 15 to 20 m thick (

Bassett
et al.
1982

). Coastal mudflat deposits of the Temeside Shale Formation lie directly above but are not seen at Ludford corner. The basal part of the
Short Paper; Terrane rotations in the Irish Caledonides
Authors: BRIDEN J. C. , ROBERTSON D. J. , SMETHURST M. A.
Published: December 2007

Palaeozoic palaeomagnetic data from Scotland and Ireland have been compared on a terrane model, and early Caledonide clockwise rotation of a terrane through 65º ± 30° (Connemara Massif) and later anticlockwise rotation of up to 50° (in the vicinity of Clew Bay) have been deduced. These are the firfirst estimates of terrane rotations in the European part of the Caledonian–Appalachian orogen.

Terrane analysis of orogenic belts is currently a major field of study. To a large extent this has been stimulated by studies in the North American Cordillera where large rotations and translations have been determined palaeomagnetically (see

Beck
et al.
1986

and references therein). Terrane analysis may be applied to older orogenic belts such as the Caledonides, which have been particularly intensively studied. Previously published palaeomagnetic data from Connemara (Morris & Tanner 1977) and NW Galway (

Morris
et al.
1973

) had indicated local block rotations, but furnished few constraints on either their age and magnitude, or on the identities of the blocks. This paper relates the currently available palaeomagnetic dataseto a terrane model of W. Ireland (Hutton & Dewey 1986), see Fig. 1.

New data.
New palaeomagnetic data from the Irish Caledonides comprise the following.

Connemara Gabbro (
Robertson 1988
).

The Dalradian rocks of the Connemara Massif are unique in the British Caledonides in lying to the south of a Caledonian ophiolite and also south of a thick sequence of Ordovician shales and volcanics of low metamorphic grade (in Clew Bay and the South Mayo Trough respectively). The structural
Snowdon basalts and the cessation of Caledonian subduction by the Longvillian
Authors: LEAT P. T. , THORPE R. S.
Published: December 2007

Basalts of the Bedded Pyroclastic Formation, Snowdon Volcanic Group, North Wales of Longvillian (Caradoc) age, are heterogeneous in trace element composition, forming a mixing trend between ocean island basalt (OIB)-type and magmatic arc basalt. The two end-member compositions were partial melts of two distinct upper mantle sources. The OIB end-member is similar to Icelandic OIB, and is the only known example of basalt of this type in the Ordovician-Silurian Welsh Basin. Since ocean island basalt magma type is thought never to erupt above well-established subduction zones, the eruption of the Longvillian OIB provides a minimum age for the end of subduction of Iapetus oceanic lithosphere below the Welsh Basin. A model for the evolution of the mantle below the Welsh Basin is proposed which accounts for the geochemistry of magmas erupted from the Tremadoc to the early Silurian.
Constraints on measurement of sea-level movements from salt-marsh accretion rates
Authors: ALLEN J. R. L.
Published: December 2007

The use of sediment accretion rates measured on coastal and estuarine salt marshes as a surrogate for rates of sea-level change rests on the (usually implicit) assumption that marsh maturity is unchanged over the measurement period. This assumption is inherently difficult to test empirically, but a simple mathematical analysis shows that it is true only in a severely restricted case. Under most practical conditions, measured accretion rates either over-estimate or under-estimate the rate of sea-level change. Under certain circumstances, they may indicate an upward trend when the tendency is actually downward. A knowledge of marsh history is critical to the evaluation of such measurements. Sea-level movements estimated from accretion rates are theoretically most reliable in the case of relatively old marshes and comparatively short measurement periods.