Hydrofracturing in response to the development of an overpressurised subglacial meltwater system during drumlin formation: an example from Anglesey,NW Wales

This paper presents the results of a detailed study of a complex hydrofracture system and host diamictons exposed within a longitudinal section through an elongate drumlin located to the west of Cemlyn Bay, Anglesey, NW Wales. This complex, laterally extensive sand, silt and clay filled hydrofracture system was active over a prolonged period and is thought to have developed beneath the Late Devensian (Weichselian) Irish Sea Ice Stream as it overrode this part of NW Anglesey. The sediment-fill to the hydrofracture system is deformed with kinematic indicators (folds, thrusts, augen) recording a SW-directed sense of shear, consistent with the regional ice flow direction across this part of the island. The lack of any geomorphological evidence for active retreat of the Irish Sea ice across Anglesey has led to the conclusion that hydrofracturing at the Cemlyn Bay site occurred within the bed of the Irish Sea Ice Stream whilst this relatively faster flowing corridor of ice was actively overriding the island. Shear imposed by the overriding ice led to the development of a subglacial shear zone which facilitated the propagation of the hydrofracture system with the laterally extensive feeder sills occurring parallel to Y-type Riedel shears. Although a subglacial setting beneath the active Irish Sea Ice Stream can be argued for the Cemlyn Bay hydrofracture system, its relationship to the formation of the ‘host’ drumlin remains uncertain. However, evidence presented here suggests that hydrofracturing may have occurred during the later stages or post landform development in response to the migration of overpressurised meltwater within the bed of the Irish Sea ice; possibly accompanying the local thinning and shutdown of the Irish Sea Ice Stream on Anglesey.     

Neoproterozoic glaciation in the mid-oceanic realm: An example from hemi-pelagic mudstones on Llanddwyn Island, Anglesey, UK

We report the first occurrence of ice-rafted dropstones in mid-oceanic sediments belonging to an ocean plate stratigraphy within a Neoproterozoic accretionary complex on Llanddwyn Island, Wales, UK. Dropstones of sandstone, chert, and basalt occur as matrix-supported exotic clasts in a 1 m-thick, hemi-pelagic mafic mudstone; the largest clast is 20 × 25 cm across. These dropstones occur specifically in hemi-pelagic mafic mudstone that is located at the structural top of ocean plate stratigraphy that records a ridge-trench transition; they are supplementary to dropstones associated with extensive tillites reported in shallow marine sequences of continental shelf facies and in back-arc basins.     

On the structural age of the Rhoscolyn antiform,Anglesey, North Wales

In the Rhoscolyn area of Anglesey, the late Precambrian interbedded psammites and pelites of the Monian Supergroup are folded into a kilometre‐scale antiform, plunging about 25°NE and with an axial surface dipping about 40°NW. Numerous folds of up to a few tens of metres in wavelength are present on both limbs of this antiform. These smaller‐scale folds also plunge about 25°NE but clearly belong to two separate episodes of folding, and it has become a matter of longstanding controversy as to whether the larger antiform belongs to the first or second of these episodes. Close examination of the cleavage/bedding asymmetries from all the lithologies, however, shows that the large antiform is a second‐generation structure, and that on the gently dipping northwest limb, the sense of cleavage/bedding asymmetry of the earlier cleavage in the psammitic units has been almost uniformly and homogeneously reversed (so that it appears to be axial planar to the antiform), while in the pelitic units the sense of cleavage/bedding asymmetry of the earlier cleavage has been preserved. Many of the small‐scale complexities of the observed cleavage/bedding relationships may be explained by appealing to differences in the timing of the formation of buckling instabilities relative to this reorientation of the early cleavage in the psammites during the second deformation. A first‐order analysis of the finite strains from around the large‐scale antiform shows that the orientation of the first cleavage prior to the second deformation was steeply dipping to the southeast. The second deformation correlates with the southeast‐verging Caledonian deformation affecting the Monian and Ordovician units elsewhere in northwest Anglesey, while the northwest‐verging first deformation event, which is not present in the Ordovician rocks, must have occurred before they were deposited. Copyright © 2004 John Wiley & Sons, Ltd.     

Glacial deposits at Wylfa Head,Anglesey, North Wales: Evidence for Late Devensian deposition in a non-marine environment

The nature and origin of glacial sediments at Wylfa Head are described, and their significance with regard to sedimentary environments during Late Devensian deglaciation of the Irish Sea Basin is discussed. Recent models of deglaciation under glaciomarine conditions are challenged. The Quaternary sequence at Wylfa consists of eroded and glaciotectonically deformed bedrock, locally derived lodgement till, calcareous silt-rich lodgement till containing northern erratics, discontinuous units of orange-brown silty sand of possible aeolian origin, and grey laminated freshwater silts filling a small kettle hole. The till units thicken to the south where the surface is drumlinised. It is concluded that the landforms and deposits result from a warm-based Irish Sea glacier, which moved towards the southwest. Spatial variation in basal water pressure resulted from localised drainage through zones of more heavily jointed bedrock. Rapid glacial erosion occurred in areas where subglacial water pressure was relatively high, while deposition of the resulting basal sediment took place where water pressures were reduced. The glacier also carried basal calcareous silty till onshore, which was deposited by lodgement processes. None of the deposits at Wylfa are interpreted as glaciomarine in origin, and there is no evidence at this site for an isostatically induced marine transgression prior to deglaciation.     

A lithological assessment of the resistivity data acquired during the airborne geophysical survey of Anglesey,North Wales

A recent airborne geophysical survey has provided high resolution estimates of the electrical resistivity of the near-surface and deeper (bedrock) formations found across Anglesey and a portion of the coastal area of North Wales. This single small survey provides new geophysical information on both the complex configuration of the Neoproterozoic and Cambrian bedrock units and the shallow near-surface geology and glacial features.     

Foraminifera from the Irish Sea glacigenic deposits at Aberdaron,western Lleyn,North Wales: Palaeoenvironmental implications

In a sequence of glacigenic sediments at Aberdaron, Foraminifera were obtained from samples located specifically in order to differentiate between opposing models of depositional environment. All the diamict samples yielded remarkably uniform assemblages, with similar numbers of benthic specimens and benthic species per unit weight of sediment, similar planktic : benthic ratios, and similar ratios of clearly allochthonous to possibly autochthonous elements. This is precisely as predicted by the terrestrial model of sedimentation, where all of the sediments are interpreted as being derived from the melting of glacier ice rich in marine debris entrained during passage along the Irish Sea Basin. The results lend no support to a glacial marine model, since no faunal responses to increasingly distal sedimentary environments are observed. However, the fauna is dominated by the Foraminifera Elphidium excavatum (Terquem) forma clavata Cushman, which is commonly assumed to indicate glacial marine conditions. The modern distribution of similar assemblages suggests that it is just as likely to represent the cold, reduced salinity conditions that would have prevailed in the northern Irish Sea Basin for much of the Quaternary.     

On the deformation sequence in the New Harbour Group of Holy Island,Anglesey, North Wales

Four phases of deformation are recorded by minor structures in the New Harbour Group (NHG) of southern Holy Island. The regional schistosity in these rocks is a differentiated crenulation cleavage of D₂ age. An earlier preferred orientation (S₁) is commonly preserved as crenulations within the Q-domain microlithons of the S₂ schistosity and is demonstrably non-parallel to bedding. F₃ folds are widely developed in S₂ and, to a lesser extent, in bedding. S₃ crenulation cleavage is sporadically developed but can be intense locally. A major antiformal fold exists in the NHG near Rhoscolyn. This fold is of D₃ age since it clearly deforms S₂ schistosity and is consistent with the vergence of F₃ minor structures. All planar structures are deformed by folds of D₄ age. © 1997 John Wiley & Sons, Ltd.     

Late Devensian and Holocene relative sea‐level change in North Wales,UK

Deglacial sea‐level index points defining relative sea‐level (RSL) change are critical for testing glacial isostatic adjustment (GIA) model output. Only a few observations are available from North Wales and until recently these provided a poor fit to GIA model output for the British‐Irish Ice Sheet. We present results of an integrated offshore geophysical (seismic reflection), coring (drilling rig), sedimentological, micropalaeontological (foraminifera), biostratigraphical (palynology) and geochronological (AMS ¹⁴C) investigation into a sequence of multiple peat/organic sediment horizons interbedded within a thick estuarine–marine sequence of minerogenic clay‐silts to silty sands from the NE Menai Strait, North Wales. Ten new sea‐level index points and nine new limiting dates from the Devensian Late‐glacial and early Holocene are integrated with twelve pre‐existing Holocene sea‐level index points and one limiting point from North Wales to generate a regional RSL record. This record is similar to the most recent GIA predictions for North Wales RSL change, supporting either greater ice load and later deglaciation than in the GIA predictions generated before 2004, or a modified eustatic function. There is no evidence for a mid‐Holocene highstand. Tidally corrected RSL data indicate initial breaching of the Menai Strait between 8.8 and 8.4 ka BP to form a tidal causeway, with final submergence between 5.8 and 4.6 ka BP. Final breaching converted the NE Menai Strait from a flood‐dominated estuary into a high energy ebb tidal delta with extensive tidal scouring of pre‐existing Late‐glacial and Holocene sequences. The study confirms the value of utilising offshore drilling/coring technology to recover sea‐level records which relate to intervals when rates of both eustatic and isostatic change were at their greatest, and therefore of most value for constraining GIA models. Copyright © 2011 John Wiley & Sons, Ltd.     

Mineral isograds and metamorphic zones of the Anglesey blueschist belt, UK: implications for the metamorphic development of a Neoproterozoic subduction–accretion complex

The 560–550 Ma blueschists and associated rocks in Anglesey, UK were derived from a subduction–accretion complex. The blueschist unit is divided into three mineral zones by two newly mapped metamorphic isograds; zone I sub-greenschist facies, (crossite isograd), zone II blueschist facies, (barroisite isograd), zone III epidote-amphibolite facies. The zones and isograds dip gently to the east, and decrease in metamorphic grade from the central high-pressure zone III to lower grade zones II and I to the west and east. The P – T conditions estimated from zoned amphibole indicate an anticlockwise P – T path following adjustment to a cold geotherm. This path is well preserved in the compositional zoning of Na–Ca amphibole that have a core of barroisite surrounded by a rim of crossite, although this is only locally developed. The sense of subduction was to the east and exhumation to the west, as indicated by the metamorphic isograds. The symmetrical arrangement of the metamorphic zones with the deepest high-pressure rocks in the middle suggests an isoclinal antiformal structure that formed by wedge extrusion during exhumation in the subduction zone.     

Ice-marginal sedimentary, glacitectonic, and morphologic features of Pleistocene drift: An example from Newfoundland

A Pleistocene drift sequence in hummocky terrain along part of the southern Avalon Peninsula of Newfoundland is interpreted to comprise complexly interrelated lodgement till, melt-out till, flow till, supraglacial and proglacial outwash, and supraglacial rhythmites. The gray and tan melt-out tills are stacked in imbricate fashion, giving rise to exceptionally thick stratigraphic sections. Contacts between melt-out tills are interpreted as remnants of shear planes because they are sharp, they dip in the up-ice direction, and they converge toward valley margins. Overlying flow tills interdigitate with supraglacial outwash. The drift sequence was deposited during a single episode of glaciation, rather than by repeated glacier advance, as previously proposed. It is the product of thrusting of englacial debris along ice-marginal shear planes, subsequent melting-out of englacial debris, and formation of supraglacial flow till and outwash. Preservation of this sequence probably is due to high content of englacial debris within the Wisconsinan ice. The sedimentary, glacitectonic, and morphologic features of this sequence are similar to those found at the margins of certain Arctic glaciers of subpolar thermal regime which have recently been the subject of Pleistocene glacial sedimentation models for west-central Canada and Great Britain. Recognition of these distinct elements indicates wisconsinan glacier lobes were of the cold Arctic type in southeastern Newfoundland. Alternative explanations for this sequence, such as deposition by glaciers of temperate thermal regime or by surging glaciers, are discounted. Because the features described here are complex and difficult to recognize, they may be more widespread in Pleistocene drift than has previously been interpreted.     

Segregation of leucogranite microplutons during syn-anatectic deformation: an example from the Taylor Valley, Antarctica

A suite of migmatites in uppermost amphibolite facies schists of the Koettlitz Group exposed in the Taylor Valley, Antarctica, provides direct evidence of the behaviour of partially molten rock during syn-anatectic deformation. The geometry of the migmatites is directly related to their position relative to the hinge of a kilometre-scale antiform. Migmatitic rocks on the fold limbs are characterized by extensional shears and fractures, filled with leucosome material, that intersect the pervasive foliation and millimetre-thick stromatic leucosomes. Vein- and dyke-like leucosomes become more common and thicker from the limb to the hinge region of the antiform. Rocks characterized by high leucosome-to-rock ratios near the antiform hinge are xenolithic in appearance. Major parasitic folds within the hinge contain leucogranite 'microplutons' up to 50 m across beneath refractory 'cap-rock' layers.
Angular boudinage structures in schists surrounded by leucosomes indicate a relatively low yield strength in the leucosome, which is compatible with a molten rather than solid leucosome. Leucogranite-bearing extensional shears and fractures indicate that repeated extensional fracturing and shearing promoted by high fluid (melt) pressure is an important mechanism of melt segregation. Dilation in the hinges of developing folds aids the migration of melt into fold hinges and the development of 10–50-m-wide 'microplutons' of xenolith-rich leucogranite.
Lack of vapour-absent melting and consequent low melt-to-rock ratios allowed the Koettlitz Group to maintain its structural coherency on a kilometre scale. Consequently, leucosome 'microplutons' did not exceed 50 m in width, and therefore observed leucosomes have not contributed to the development of adjacent plutonic-scale granitoids.     

Defining geo-habitats for groundwater ecosystem assessments: an example from England and Wales (UK)

Groundwater ecosystems comprising micro-organisms and metazoans provide an important contribution to global biodiversity. Their complexity depends on geology, which determines the physical habitat available, and the chemical conditions within it. Despite this, methods of classifying groundwater habitats using geological data are not well established and researchers have called for higher resolution habitat frameworks. A novel habitat typology for England and Wales (UK) is proposed, which distinguishes 11 geological habitats (geo-habitats) on hydrogeological principles and maps their distribution. Hydrogeological and hydrochemical data are used to determine the characteristics of each geo-habitat, and demonstrate their differences. Using these abiotic parameters, a new method to determine abiotic habitat quality is then developed. The geo-habitats had significantly different characteristics, validating the classification system. All geo-habitats were highly heterogeneous, containing both high quality habitat patches that are likely to be suitable for fauna, and areas of low quality that may limit faunal distributions. Karstic and porous habitats generally were higher quality than fractured habitats. Overall, 70% of England and Wales are covered by lower quality fractured habitats, with only 13% covered by higher quality habitats. The main areas of high quality habitats occur in central England as north–south trending belts, possibly facilitating dispersal along this axis. They are separated by low quality geo-habitats that may prevent east-west dispersal of fauna. In south-west England and Wales suitable geo-habitats occur as small isolated patches. Overall, this paper provides a new national-scale typology that is adaptable for studies in other geographic areas.     

Origin and significance of soft-sediment deformation in the Old Red Sandstone of central South Wales,UK

Vertically oriented water-escape cusps are the most common type of soft-sediment deformation structure in sandstone-rich intervals of the fluvial Brownstones and Senni Formations (Cosheston Subgroup, Daugleddau Group) of the Lower Old Red Sandstone in the central Brecon Beacons and eastern Black Mountains, South Wales. The structures are widely distributed and occur at several stratigraphical levels. They can be divided into two styles. (1) Small-scale (height less than a single bed), isolated water-escape cusps formed when loosely packed sediment deposited rapidly in flood events liquefied in advance of subsequent flood events or pulses, causing localised fluidization due to the escape of excess pore water. Inclined cusps higher in some beds confirm the relationship of this deformation style to active flood events. (2) Horizons of larger-scale (occupying the entire bed thickness), laterally continuous water-escape cusps and fold trains can be traced for hundreds of metres to kilometres and result from widespread liquefaction in response to earthquakes. A lack of overturning indicates that their formation did not coincide with active flow conditions. Further detailed mapping is needed to clarify the continuity and extent of such structures and their relationship to faults that may have been active during sedimentation. The occurrence of triggers capable of causing liquefaction in granular materials provides a greater control on the occurrence of soft-sediment deformation than do lithological controls such as grain size or interbedding of sandstone and mudstone. The findings are broadly consistent with interpretations of soft-sediment deformation in the Cosheston Subgroup in Pembrokeshire, SW Wales.     

10Be and 26Al exposure‐age dating of bedrock surfaces on the Aran ridge,Wales: evidence for a thick Welsh Ice Cap at the Last Glacial Maximum

This paper presents results of the analysis of paired cosmogenic isotopes (¹⁰Be and ²⁶Al) from eight quartz‐rich samples collected from ice‐moulded bedrock on the Aran ridge, the highest land in the British Isles south of Snowdon. On the Aran ridge, comprising the summits of Aran Fawddwy (905 m a.s.l.) and Aran Benllyn (885 m a.s.l.), ²⁶Al and ¹⁰Be ages indicate complete ice coverage and glacial erosion at the global Last Glacial Maximum (LGM). Six samples from the summit ridge above 750–800 m a.s.l. yielded paired ¹⁰Be and ²⁶Al ages ranging from 17.2 to 34.4 ka, respectively. Four of these samples are very close in age (¹⁰Be ages of 17.5 ± 0.6, 17.5 ± 0.7, 19.7 ± 0.8 and 20.0 ± 0.7 ka) and are interpreted as representing the exposure age of the summit ridge. Two other summit samples are much older (¹⁰Be ages of 27.5 ± 1.0 and 33.9 ± 1.2 ka) and these results may indicate nuclide inheritance. The ²⁶Al/¹⁰Be ratios for all samples are indistinguishable within one‐sigma uncertainty from the production rate ratio line, indicating that there is no evidence for a complex exposure history. These results indicate that the last Welsh Ice Cap was thick enough to completely cover the Aran ridge and achieve glacial erosion at the LGM. However, between c. 20 and 17 ka ridge summits were exposed as nunataks at a time when glacial erosion at lower elevations (below 750–800 m a.s.l.) was achieved by large outlet glaciers in the valleys surrounding the mountains. Copyright © 2011 John Wiley & Sons, Ltd.     

Progressive deformation across a ductile shear zone: an example from the Singhbhum Shear Zone,eastern India

Structural investigations in the Precambrian Singhbhum Shear Zone of eastern India document an intimate relationship between micro- to meso-scale structures and the deformation history. Shear zone rocks are characterized by composite foliation, a well-developed stretching lineation, folds, shear planes, and quartz veins. These structures reflect thrusting of the Proterozoic north Singhbhum hanging wall block over the Archaean south Singhbhum footwall block. Microstructural analysis of multiple foliation and mylonitic rocks within the shear zone helps to define its progressive evolution. During progressive deformation, overprinting of microstructures resulted in incomplete transposition or complete erasing of previously formed structures and mineral assemblages, allowing room for new dynamic equilibrium structures to form. The dominant deformation mechanism was dissolution–recrystallization, with locally important fluid circulation responsible for transformation of the quartzo-feldspathic mass into phyllonite, and quartzites and schists into mylonite. Textural features suggest that the bulk deformation was non-coaxial, evolving from dominant pure shear in the early stage followed by simple shear in a single progressive strain history of the Singhbhum Shear Zone.     

The palynology and provenance of the coal fragments contained in the late-Pleistocene Lleiniog gravels of Anglesey,North Wales

The numerous coal clasts in the late-Pleistocene fluvioglacial Lleiniog Gravels of eastern Anglesey contain a varied assemblage of Westphalian A (and possibly also early Westphalian B) spores. Directional sedimentary structures indicate that the meltwater currents which deposited the gravels came mainly from the north and east, while glacial striae and erratic trains indicate that the parent (? Devensian) glacier flowed from the northeast. Consideration is given to the possibility that the coal clasts were derived from the breaking-up, fairly locally, of far-travelled rafts of Coal Measures. It is concluded, however, that the most probable explanation is still Greenly's (1919) postulate that the source is a yet-to-be-identified submarine outcrop situated only a few kilometres northeast of Lleiniog.     

CORS网和GNSS技术在地面变形监测中的应用

地面变形对工程建设安全稳定具有一定影响,对工程建设规划、地质灾害防治等具有重要意义。本文利用卫星导航定位基准站（CORS）网全球卫星导航系统（GNSS）连续观测,实现了区域地面大地高、地面重力和地倾斜变化面的监测,通过与部分地质灾害信息进行时空分析,确定了用于地面稳定性变化分析的要素和权重,进而定量分析了地面稳定性变化。该方法在浙江省东南部地区进行了测试验证,获得了较好的效果。利用CORS网连续观测能够定量监测地面变形整个面的变化,分析区域地面稳定性变化情况,可为区域规划和地质灾害防治提供技术支持和参考。     

Relation between metamorphic crystallization and deformation: an example from lower Himalayan terrain,India

The metamorphic rocks (Salkhalas) of Kishtwar area bear evidence of four phases of deformation and three episodes of metamorphism. The last deformational phase has however not initiated crystallization of new phase. The metamorphic crystallization was synkinematic to first phase of deformation with the rise in grade during post D₁ phase which continued and reached its culmination in post D₂ phase. The mineral assemblages suggest medium to high pressure Barrovian type metamorphism. This was followed by granitization leading to the formation of migmatites, augen & porphyroblastic gneisses in the late stages persisting till early D₃ phase. The growth of index minerals suggest that the metamorphism was progressive in time and it predates migmatization and thrusting in the area. The metamorphism is not caused by the granite intrusion but is probably related to heat flow from the mantle in its disturbance during Himalayan orogeny.

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Zusammenfassung Die metamorphen Gesteine (Salkhalas) aus dem Kishtwar Gebiet liefern einen Beweis für vier Deformationsphasen und drei Episoden einer Kristallisationsmetamorphose. Die letzte Deformationsphase ist mit keiner Neukristallisation verbunden. Die metamorphe Kristallisation verlief synkinematisch mit der ersten Deformationsphase (D₁), setzte sich zunehmend fort über die zweite Phase (D₂) hinaus und erreichte einen Höhepunkt in einer Post-D₂-Phase. Die Mineralvergesellschaftungen zeigen einen Barrov-Typ von mäßigem bis hohem Druck. Dieser Metamorphose folgte eine Granitisierung, die mit einer Bildung von Migmatiten, und Augen- und porphyroklastischen Gneissen bis in die frühen Stadien der dritten Phase (D₃) hinein verbunden war.Das Wachstum der Indexmineralien zeigt eine zeitlich fortschreitende Metamorphose, die vor der Migmatisierung und der tektonischen Einengung stattfand. Die Metamorphose ist weiterhin kein Resultat der Granitintrusion, sondern wahrscheinlich durch Wärmefluß aus dem Erdmantel verursacht.

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Résumé Les roches métamorphiques (Salkhalas) de la région de Kishtwar mettent en évidence quatre phases de déformation et trois épisodes de métamorphisme. La dernière phase de déformation n'a toutefois pas entraîné de néveristallisatiòn. La cristallisation métamorphique a été syncinématique de la première phase déformative, avec élévation de son degré au cours de la phase consécutive á D₁ qui continua et atteignit son plus haut point dans la phase postérieure á D₂. Les associations minérales suggérent un métamorphisme Barrovien de moyenne à haute pression. Celui-ci a été suivi par une granitisation conduisant à la formation de migmatites, de gneiss oeillés et porphyroblastiques au cours des derniers stades qui ont persisté jusqu'au début de la phase D₃. La croissance des minéraux index suggére que le métamorphisme fut progressif au cours du temps et qu'il ert antérieur à la migmatization et au charriage dans la région. Le métamorphisme n'a pas pour cause l'intrusion granitique, mais est probablement en relation avec le flux de chaleur au cours des perturbations dans le manteau.

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