Showing papers in "Geological Journal in 2004"

Terrestrial trace fossils from the Lower Old Red Sandstone, southwest Wales

Abstract: An assemblage of terrestrial trace fossils is described from the Lower Old Red Sandstone of Pembrokeshire, southwest Wales, preserved in mainly fine-grained alluvial facies (channel and overbank sediments, deposited by predominantly ephemeral flows within a semi-arid environment), and thin tuffaceous horizons. The ichnofauna is dominated by an extensive, but low diversity Beaconites ichnocoenosis, comprising the meniscate backfilled burrow Beaconites barretti. Concentrations of these burrows (up to 30 per square metre) show normal size distributions, representing periodic colonization events (inferred as a response to seasonal desiccation) of subaerially exposed (partly indurated) sediments, probably by a population of eoarthropleurid myriapods penetrating the substrate to the level of the water table in order to aestivate and/or moult. Arthropod trackways also characterize an active arthropod epifauna of arachnids (Paleohelcura; first Welsh record) and myriapods (two forms of Diplichnites up to 160 mm wide) and Diplopodichnus. Additional ichnotaxa include arthropod foraging and resting traces (bilobed trails, Tumblagoodichnus and Selenichnites), ‘scratch arrays’ and worm burrows (Cochlichnus, and Palaeophycus) and faecal pellets all representative of the Scoyenia ichnofacies. Copyright © 2004 John Wiley & Sons, Ltd.

Silurian and Lower Devonian plant assemblages from the Anglo‐Welsh Basin: a palaeobotanical and palynological synthesis

Abstract: A comprehensive survey of plant assemblages from Upper Silurian (Gorstian–Přidoli) and Lower Devonian (Lochkovian–Pragian) localities in South Wales and the Welsh Borderland is presented, together with some comments on recent improvements in dating and correlation. Spore assemblages provide a stratigraphic framework for plant evolutionary studies and, along with microvertebrates, enable correlation from continental to marine rocks, i.e. near-shore to shelf deposits. While it is recognized that the assemblages provide the most complete and extensive record of the history of vascular plants in a restricted geographical area during the time interval, it seems likely that major evolutionary innovation occurred elsewhere. The plants themselves display a number of types of fossilization. Particularly important are the pyrite permineralizations which have helped in the elucidation of the ultrastructure of tracheids in early vascular plants. Minute, coalified, relatively uncompressed fossils (mesofossils) from the Přidoli Lochkovian of the Welsh Borderland are hypothesized to be the products of wildfire, the earliest records to date. In addition to displaying often exquisite anatomical detail, the sporangia contain spores which have been linked to those in dispersed assemblages. Such relationships are helpful in the reconstruction of local and regional vegetation, even in the absence of meso- and megafossils. However, because these fossils are allochthonous and preserved in fluvial sediments, the habitats of the plants remain uncertain. A new hypothesis suggests that interfluvial areas, where calcretes are interpreted as relicts of vertisols and a seasonally dry climate, provided refugia in the Lower Devonian for the cryptospore-producing plants which dominated Ordovician and Lower Silurian land vegetation, while vascular plants colonized areas in the immediate vicinity of rivers. Variation in regional vegetation is explored via comparisons of composition of spore assemblages in southern Britain and Scotland and timing of appearances of derived taxa, integrated with limited information on the spore producers.

Mud volcanoes of Italy

Abstract: The locations and information about the sizes of 61 mud volcanoes on the Italian mainland and Sicily, plus an area of mud diapirism in the Italian Adriatic Sea, are presented. Data about the emission products are also provided. The majority of these mud volcanoes are found where thick sedimentary sequences occur within a zone of tectonic compression associated with local plate tectonic activity: the movement of the Adriatic microplate between the converging African and Eurasian plates. The principal gas emitted by these mud volcanoes is methane, which probably originates from deep within the sediments. Other mud volcanoes, associated with igneous volcanism, produce mainly carbon dioxide. The mud diapirs in the Adriatic Sea are thought to form as a result of the mobilization of shallow gassy sediments. It has been shown that radon emissions from mud volcanoes are indicators of forthcoming earthquake events. Copyright © 2004 John Wiley & Sons, Ltd.

Major neotectonic features of eastern Marmara region, Turkey: development of the Adapazarı–Karasu corridor and its tectonic significance

Abstract: Eastern Marmara region consists of three different morphotectonic units: Thrace–Kocaeli Peneplain (TKP) and Camdag–Akcakoca Highland (CAH) in the north, and Armutlu–Almacik Highland in the south of the North Anatolian Fault Zone (NAFZ). The geologic-morphologic data and seismic profiles from the Sakarya River offshore indicate that the boundary between the TKP in the west and CAH in the east is a previously unrecognized major NNE–SSW-trending strike-slip fault zone with reverse component. The fault zone is a distinct morphotectonic corridor herein named the Adapazari–Karasu corridor (AKC) that runs along the Sakarya River Valley and extends to its submarine canyon along the southern margin of the Black Sea in the north. It formed as a transfer fault zone between the TKP and CAH during the Late Miocene; the former has been experiencing extensional forces and the latter compressional forces since then. East–West-trending segments of the NAFZ cuts the NE–SW-trending AKC and their activity has resulted in the formation of a distinct fault-bounded morphology, which is characterized by alternating E–W highlands and lowlands in the AKC. Furthermore, this activity has resulted in the downward motion of an ancient delta and submarine canyon of the Sakarya River in the northern block of the NAFZ below sea level so that the waters of the Black Sea invaded them. The NE–SW-trending faults in the AKC were reactivated with the development of the NAFZ in the Late Pliocene, which then caused block motions and microseismic activities throughout the AKC. Copyright © 2004 John Wiley & Sons, Ltd.

Mudrock deposition in an ancient dryland system: Moor Cliffs Formation, Lower Old Red Sandstone, southwest Wales, UK

Abstract: Studies of the lowermost Old Red Sandstone of South Wales (Pridolian–Lochkovian) have revealed that the mudstones that constitute up to 80% of some of the formations have been deposited in a variety of environments. In addition there is a variety of lithofacies associations present in some mud-dominated units that suggests diverse mechanisms of deposition. This paper considers the Moor Cliffs Formation and examines four distinct mudrock facies. The overall depositional environment is considered to have been a multi-stage, multi-channel system, similar to a modern dryland river system, with mud-dominated, moderately sinuous ephemeral rivers that reworked muddy floodplain sediment over a proximal braidplain during seasonal flooding. The alluvial deposits were pedified to varying degrees as calcic Vertisols, which, due to seasonal wetting and drying, provided soil aggregates for reworking by wind and water as sand- or silt-sized pellets. Very infrequent flooding events (superfloods) resulted in extensive stripping of the floodplain surface and the introduction of allochthonous sand sheet deposits over wide areas in this normally sediment-starved system. Depressions (pans or waterholes) on the floodplain formed ephemeral lakes following floods, and acted as sediment traps for dust from the frequent dust storms occurring in the system. Copyright © 2004 John Wiley & Sons, Ltd.

Cambro‐Ordovician trace fossils from the SW‐Norwegian Caledonides

Abstract: A diverse trace fossil association is described for the first time from low-grade metamorphic rocks of the SW-Norwegian Caledonides. The investigated cliff sections with autochthonous to parautochthonous metasediments comprise a coarsening- and thickening-upward succession interpreted as prograding delta deposits. Sedimentary features indicate a tide-influenced environment. Twenty-one ichnospecies have been identified and assigned to the Cruziana and Skolithos ichnofacies, including the oldest record of Beaconites capronus and Macaronichnus segregatis. Cruziana ichnostratigraphy (sensu lato), previously rarely used in other palaeocontinents than Gondwana, allows an age determination for these metasediments of Middle Cambrian to Lower Ordovician as indicated by the presence of Cruziana barbata, C. furcifera, C. rugosa, C. semiplicata, Didymaulichnus rouaulti and Rusophycus ramellensis. Baltica was geographically the most isolated from the other three large continents (Gondwana, Laurentia and Siberia) during Cambro-Ordovician time, and provinciality of faunal assemblages (e.g. brachiopods, conodonts) has been proved and is also supposed for trilobites by some authors. However, although the Cruziana ichnospecies result from a high specialization of their tracemakers, and therefore only a small group of trilobite species is eligible for its origin, the ichnospecies reported from Baltica occur also on other palaeocontinents and do not support the assumption of trilobite provincialism. Copyright © 2004 John Wiley & Sons, Ltd.

Fish trails from the Lower Old Red Sandstone of Tredomen Quarry, Powys, southeast Wales

Abstract: Alluvial deposits of the St Maughans Formation (Lower Old Red Sandstone, Early Devonian) of Tredomen Quarry, near Brecon, southeast Wales, have yielded the oldest known trails of swimming fish as well as body fossils of heterostracans and osteostracans, their inferred producers. Undichna unisulca comprises a single sinusoidal wave (of varying amplitude and wavelength) and is attributed to the caudal lobe or fin of a swimming heterostracan or osteostracan. Variation in the dimensions of U. unisulca trails, together with functional analysis of their inferred producers, suggests different fish sizes and swimming speeds. Undichna cf. simplicitas shows a more complex arrangement of intertwined waves and is interpreted as being produced by a combination of the caudal fin, anal spine and paired pelvic spines of an acanthodian. A new ichnotaxon, Undichna trisulcata isp. nov., consists of three isolated furrows arranged in parallel with associated pectoral fin imprints, and is interpreted as either the trail left by a ‘cruising’ cephalaspid, intermittently pushing off the substrate with its pectoral fins, or a trail made by a cephalaspid landing on the substrate. The presence of Undichna within these relatively proximal fluvial sediments (displaying no evidence of marine influence) is indicative of an in situ vertebrate freshwater community. Taphonomic constraints on Undichna preservation, in combination with sedimentological analysis, suggest weak bottom currents and/or rapid burial, post-trail formation. Arthropod trackways (Diplichnites gouldi Types A and B, Protichnites isp., and Palmichnium pottsae), a bilobed trail (cf. Cruziana), worm burrows (cf. Cochlichnus) and large meniscate backfilled burrows (Beaconites barretti) contribute invertebrate components to this ichnofauna. The association of Diplichnites trackways on the same surfaces as Undichna supports a semi-aquatic habit of their inferred myriapod producers. Copyright © 2004 John Wiley & Sons, Ltd.

A rock avalanche deposit in Burtness Comb, Lake District, northwest England

Abstract: A large tongue-shaped accumulation of coarse rock debris in Burtness Comb, Lake District, northwest England, previously regarded as a glacial moraine or relict rock glacier, has been examined and is reinterpreted as the product of rock-slope failure. The debris tongue has morphological similarities with landforms produced by rock avalanches. This is the first rock avalanche deposit to be reported from the Lake District. Schmidt hammer R-values and morphological contrasts between the upper and lower parts of the debris imply that two stages of slope failure and downslope debris streaming occurred. In the absence of evidence for a Loch Lomond Stade (c. 12.9–11.5 cal. ka BP) glacier in Burtness Comb, the debris may have accumulated at any time since removal of the Dimlington Stade ice cover (c. 16.8 cal. ka BP) and is regarded as a form of paraglacial landscape response to the effects of glaciation. Copyright © 2004 John Wiley & Sons, Ltd.

Pendleian (early Serpukhovian) marine carbonates from SW Spain: sedimentology, biostratigraphy and depositional model

Abstract: The San Antonio–La Juliana tectono-sedimentary unit contains the only Namurian marine carbonates in the southwestern part of the Iberian Peninsula. The analysis of this unit is fundamental in understanding the sedimentary evolution and tectonic movements which operated during the Namurian in this area. Using foraminifera the succession has been assigned to two biozones (Zones 17 and 18), both occurring in the Pendleian (early Namurian). Seven stratigraphic sections have been analysed: San Antonio, Burjadillo, Lavadero de la Mina, Cornuda, Lozana, Caridad and Via Crucis. The stratigraphic succession of the San Antonio–La Juliana Unit consists of olistolites in the basal part, with common debris-flow deposits (mainly of carbonates, with minor siliciclastic rocks), and turbidites, all of them embedded in shales. These rocks, interpeted as slope deposits, pass up into shallow-water platform facies, with sediments characteristic of the inner platform and tidal flats. Above these rocks, terrigenous deltaic deposits occur. Thus, the stratigraphic sections show an overall shallowing-upward trend. The isolation of some outcrops, and the duplication and absence of some parts of the stratigraphic succession are explained by tectonic movements. Overall, tectonic factors seem to be the main control rather than glacio-eustatic or autocyclic processes, and sedimentation took place in a strike-slip regime. Copyright © 2004 John Wiley & Sons, Ltd.

Mantle-derived and crustal melts dichotomy in northern Greece: spatiotemporal and geodynamic implications

Abstract: Two distinct groups of subduction-related (orogenic) granitoid rocks, one Jurassic and the other Tertiary, occur in the area between the Vardar (Axios) Zone and the Rhodope Massif in northern Greece. The two groups of granitoids differ in many respects. The first group shows evolved geochemical characters, it is not associated with mafic facies, and evidence of magmatic interaction between mantle- and crustal-derived melts is lacking. The second group has less evolved geochemical characters, it is associated with larger amount of mafic facies, and magmatic interaction processes between mantle-derived and crustal melts are ubiquitous as evidenced by mafic microgranular enclaves and synplutonic dykes showing different enrichment in K2O, Ti, and incompatible elements. This kind of magmatism can be attributed to the complex geodynamic evolution of the area. In particular, we suggest that two successive subduction events related to the closure of the Vardar and the Pindos oceans, respectively, occurred in the investigated area from Late Jurassic to Tertiary. We relate the genesis of Jurassic granitoids to the first subduction event, whereas Tertiary granitoids are associated with the second subduction. Fluids released by the two subducted slabs induced metasomatic processes generating a ‘leopard skin’ mantle wedge able to produce mafic melts ranging from typical calc-alkaline to ultra-potassic. Such melts interacted in various amounts with crustal calc-alkaline anatectic melts to generate the wide spectrum of Tertiary granitoids occurring in the study area. Copyright © 2004 John Wiley & Sons, Ltd.

Kinematics and strain partitioning in the southeast Hellenides (Greece)

Abstract: Department of Geology, University of Patras, GreeceNew kinematic and structural data from the tectonic windows of eastern Crete and the Dodecanese Islands combined with strainand quartz fabric analysis have enabled us to determine a detailed structural evolution of the region and to present a plate tec-tonic scenario for the southeast Hellenides.During the Early Mesozoic, the southeastern part of Apulia was separated from North Africa and the adjacent microplates byWNW-trending rift zones and NE-trending transfer faults. Displacement along the transfer faults has locally reoriented these riftzones into an ENE–WSW direction. Finite strain and quartz fabric asymmetry data indicate that in Late Cenozoic time, NNW-directed nappe movements caused a nearly coaxial deformation along the ENE–WSW trending rift segments and non coaxialtop-to-the-southeast shearing along the WNW-trending rift segments, as well as along the pre-existing NE-trending transferfaults. Tectonic style along the margin varies in response to the obliquity of the principal shortening direction with respectto the margin. These variations could be due to the pre-convergence geometry of the southern margin of Eurasia and to localstrain partitioning effects.Furthermore, a tectonic model is presented in which syncompressional uplift and vertical buoyancy of the subducted crustalslice caused the rapid exhumation of metamorphic units in the south Hellenides. Copyright # 2004 John Wiley & Sons, Ltd.

Origin, age and petrogenesis of Neoproterozoic composite dikes from the Arabian‐Nubian Shield, SW Jordan

Abstract: The evolution of a Pan-African (c. 900–550 Ma) suite of composite dikes, with latite margins and rhyolite interiors, from southwest Jordan is discussed. The dikes cut the Neoproterozoic calc-alkaline granitoids and high-grade metamorphic rocks (c. 800– 600 Ma) of the northern Arabian-Nubian Shield in Jordan and have been dated by the Rb-Sr isochron method at 566 � 7 Ma. The symmetrically distributed latite margins constitute less than one-quarter of the whole dike thickness. The rhyolite intruded a median fracture within the latite, while the latter was still hot but completely solidified. The dikes are alkaline and bimodal in composition with a gap in SiO2 between 61 and 74 wt%. Both end members display similar chondrite-normalized rare earth element patterns. The rhyolites display the compositional signature of A-type granites. The (La/Lu)N values are 6.02 and 4.91 for latites and rhyolites, respectively, and the rhyolites show a pronounced negative Eu anomaly, in contrast to the slight negative Eu anomaly of the latites. The chemical variability (e.g. Zr/Y, Zr/Nb, K/Rb) within and between latites and rhyolites does not support a fractional crystallization relationship between the felsic and mafic members of the dikes. We interpret the magma genesis of the composite dikes as the result of intrusion of mantle-derived mafic magma into the lower crust in an extensional tectonic regime. The mafic magma underwent extensive fractional crystallization, which supplied the necessary heat for melting of the lower crust. The products of the initial stages of partial melting (5–10%) mixed with the fractionating mafic magma and gave rise to the latite melts. Further partial melting of the lower crust (up to 30%) produced a felsic melt, which upon 50% fractional crystallization (hornblende 15%, biotite 5%, feldspars 60%, and quartz 20%) gave rise to the rhyolitic magma. Copyright # 2004 John Wiley & Sons, Ltd. Received 20 March 2002; revised version received 2 March 2003; accepted 20 March 2003

Devonian-Carboniferous brachiopod zonation in the Tarim Basin, northwest China: implications for biostratigraphy and biogeography

Abstract: Fourteen brachiopod zones are recognized in the Upper Devonian and Carboniferous successions of the Tarim Basin, northwest China. Of these, the Trifidorostellum longhuiense Zone is established from the Famennian, the Eochoristites neipentaiensis Zone in the lower Tournaisian, the Chuiella aitegouensis Zone from the upper Tournaisian to lowest Visean; the Brochocarina kunlunensis, Rugosochonetes hardrensis, Buxtonia pseudoscabricula, Vitiliproductus groberi, and Datangia weiningensis zones from the Visean, the Gigantoproductus edelburgensis Zone from the Serpukhovian age, both Productus productus and Eomarginifera lata zones from the Bashkirian, both Choristites abnormalis and Purdonella artuxensis zones from the Moscov- ian, and the Linoproductus cora Zone from the Kasimovian-Gzhelian. The faunal characteristics and basin-wide distributions of these zones are discussed. Close comparisons of the Tarim brachiopod zones with faunas from other regions of China (South China, North China, Tienshan-Junggar, and Tibet), Thailand, Pakistan (Karakorum), Central Asia (Uzbekistan, Kazakhstan, and Afghanistan), and the Urals-Russian Platform suggest that the Tarim faunas were biogeographically closely related to the South Chinese faunas in the late Famennian-Tournaisian and appear to be of a mixture between South Chinese and Central Asian-Uralian faunas, with elements from West Europe, North America, Siberia, and Gondwanaland in the Visean-Serpukho- vian. The Tarim brachiopods demonstrate stronger links with faunas of the Urals-Russian Platform and Central Asia and, to a lesser extent, with the South Chinese faunas throughout the Bashkirian to Gzhelian. Copyright # 2004 John Wiley & Sons, Ltd.

Acritarchs from the Lower Palaeozoic succession on the south County Wexford coast, Ireland: new age constraints for the Cullenstown Formation and the Cahore and Ribband Groups

Abstract: Lower Palaeozoic sediments crop out on the southern coast of County Wexford, Ireland, comprising three distinctive lithostratigraphical units: from west to east the Cahore Group, the Blackhall Formation of the Ribband Group and the Cullenstown Formation. The three units are largely devoid of macrofossils and thus their ages have to date been uncertain and, in the case of the Cullenstown Formation, speculative. In the Cahore Group, a diverse assemblage of acritarchs composed of seventeen species has been recorded indicating a middle Early Cambrian age. This is similar to the age of the lithologically identical Bray Group to the north, in County Wicklow. In the Ribband Group, two very distinct assemblages have been noted. Both are poorly preserved, but diagnostic species have been determined giving a biostratigrapical range of early Mid-Cambrian to Llanvirn. Palynomorphs have been recorded for the first time from the Cullenstown Formation allowing comparison with eastern Newfoundland where a similar, less diverse assemblage has been recorded. The age indicated is latest Mid-Cambrian to early Late Cambrian. Overall, despite generally poor preservation of the organic matter, some 45 acritarch species have been distinguished, among which one new combination is proposed: Retisphaeridium postae instead of Cymatiosphaera postae. Copyright © 2004 John Wiley & Sons, Ltd.

Variable alluvial sandstone architecture within the Lower Old Red Sandstone, southwest Wales

Abstract: Sandstone bodies within the Lower Old Red Sandstone (ORS) in southern Pembrokeshire exhibit variability in architecture, sediment grade and composition both spatially and temporally. Four architectural styles are observed, namely decimetre- to metre-thick sheets, metre-thick multi-storey amalgamations, inclined-heterolithic units and ribbon geometries. Sandstone bodies in the Freshwater East Formation are sheet-like, heterolithic units several metres thick. An association with lingulids and wave ripples alludes to a marine influence, possibly estuarine tidal flats or storm washovers. Within the Moor Cliffs Formation, the most common sandstone bodies are centimetre- to metre-thick sheets with high width-to-depth ratios. Fine-grained sandstones represent sheet-flood deposition on unconfined, planar surfaces, whereas coarser-grained sandstones constitute distinctive amalgamations of discrete flood events, reflecting either a change in provenance or tectonic influence. Clear incision of coarse-grained, multi-storey units within the Inter-Tuff Moor Cliffs Formation reflects a change in relative sea-level, possibly tectonically induced. The base of the Conigar Pit Sandstone Member (CPSM) is marked by a distinctive, exotic-clast conglomerate defining the base to heterolithic, lateral-accretion bedsets and sandstone sheets. This association defines a significant influx of coarse-grained sediment post-Chapel Point Calcrete formation, an interval of presumed topographic stability across the Anglo-Welsh Basin. This influx must reflect rejuvenation of source regions, with changes in base-level reflecting either eustatic or tectonic influences. Commonly observed in the CPSM are fine-grained, inclined-heterolithic bedsets recording deposition by highly sinuous rivers with flashy discharge. Up-sequence within the CPSM are metre-thick, multi-storey amalgamations of predominatly trough cross-stratified medium- to coarse-grained sandstone. It is likely that these units are genetically related to contemporaneous decimetre-thick sandstone sheets, the latter being ‘splay’ events marginal to the main channel axis. The interbedding of multi-storey sandstones and fine-grained laterally accreted units reflects changes in provenance, slope and/or climate. Thickness variations within the Lower ORS detail significant thickening of all units northward into the Benton Fault. It seems likely that this thickening reflects variable accommodation space development associated with active growth along this and other WNW–ESE-trending faults, and migration of channel belts toward the footwall. Copyright © 2004 John Wiley & Sons, Ltd.

Sedimentation and tectonics; the marine Silurian-basal Lower Old Red Sandstone transition in southwest Wales

Abstract: Both regional and localized tectonic events controlled deposition within the Wenlock and early Ludlow of SW Wales. Estuarine deposits within north–south-tending incised valleys dominate the youngest (Homerian) Gray Sandstone Group, valley incision being probably related to changing base-levels associated with Avalonia/Laurentian collision. Available accommodation space was outpaced by sediment supply, with the Red Cliff Formation (Late Ludfordian) defining a conformable transition from marine to Old Red Sandstone (ORS) deposition within the Marloes Peninsula. Sedimentation was dominated by fine-grained pedified siliciclastics, with subordinate fine-grained ephemeral sheet-flood sandstones. Local palaeocurrents indicate sediment transport from the south and west, though long-distance transport from a distant Laurentian provenance is assumed. A probable tectonically generated sequence boundary marks the base of petrographically distinctive, multi-storey pebbly sandstones of the Albion Sands Formation, deposited within the hangingwall valley of the active east–west-trending Wenall Fault. Sediment accommodation space was controlled by proximity to the tip-point of this important growth fault within the Lower ORS. Debris-flow-dominated fans, shed from both the hangingwall and footwall of the Wenall Fault, deposited the Lindsway Bay Formation, an exotic-clast conglomerate unit sourced predominantly from the south and west. It is uncertain as to whether movement along the Wenall Fault was caused by collision-related transtension, or rifting associated with the southern margin of Avalonia. Copyright © 2004 John Wiley & Sons, Ltd.

Magnitude of lateral displacement on the Crevillente Fault Zone (Betic Cordillera, SE Spain): stratigraphical and sedimentological considerations

Abstract: The Crevillente Fault Zone (CFZ) comprises a system of northeast to southwest oriented dextral faults that extend for some 600 km in the External Zones of the Betic Cordillera (SE Spain). The magnitude of lateral displacement related to this fault zone is not well constrained, and it is considered to be between 20 and 400 km. The stratigraphical and sedimentological criteria used in this work have proven effective in quantifying the magnitude of the displacement along this structure. We have analysed an oolitic turbidite facies in the Middle Jurassic of the Sierra de Ricote (Median Subbetic of Murcia Province). A detailed revision of ooidal limestone outcrops has revealed that the source area of these deposits was to the Internal Subbetic zone, north of Velez Rubio (Almeria Province). These two tectonic units, the Median and Internal Subbetic, are currently 75 km from each other and separated by the CFZ. The conclusions arising from our stratigraphical, petrological and sedimentological studies favour interpretation of a 75–100 km lateral displacement. After restoring the Late Jurassic–Cretaceous anticlockwise rotation of Iberia, the CFZ appears to belong to the E–W palaeofault system that is related to the extension of the South Iberian Continental Margin (SICM). Copyright © 2004 John Wiley & Sons, Ltd.

Sedimentary evidence for the formation mechanism of the Armoy moraine and late Devensian glacial events in the north of Ireland

Abstract: The internal sedimentology of the Armoy moraine, which marks a late Devensian (Weichselian) incursion of Scottish ice into the north of Ireland, is described from a temporary exposure near Armoy village. The exposure (up to 30 m long, 10 m high) comprises folded gravel beds, massive clay grading up to pebbly diamicton, massive gravel, overfolded and deformed sand, and a gravel and massive diamicton cover. These sediments overlie glacially smoothed basalt bedrock. The sediment succession records the interaction of near-touching Scottish and Irish ice margins, and the formation, infilling and drainage of an ice-supported proglacial lake. Scottish ice likely advanced into the north of Ireland by a short-lived glacier surge during the Killard Point Stadial (coeval with Heinrich Event 1 at 14.5 14C ka BP), forming the Armoy moraine at its maximum extent. The presence of rafted sediment blocks within the moraine suggests ice-marginal thrusting and deformation of proglacial sands, and syntectonic resedimentation into low points on the sediment surface. Asynchroneity between Scottish and Irish ice-marginal fluctuations may reflect differing responses to climate forcing during the Killard Point Stadial, or the effects of local factors such as substrate type and basal ice thermal regime. Copyright © 2004 John Wiley & Sons, Ltd.

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

Abstract: 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.

Structural evolution of the southern Rif Cordillera (Morocco): tectonics and synsedimentary fault processes

Abstract: This paper attempts to investigate the tectonics of the southern Rif Cordillera. Hydrogeological and oil well data, together with interpretation of seismic reflection lines help to characterize the architecture of the Rharb–Mamora Basin located in the frontal region of the Gibraltar Arc. The facies map constructed from the drilling data exhibits four main types of Pliocene facies: (i) conglomerates; (ii) limestones; (iii) sandstones and sands more or less rich with shelly remains; (iv) clays. The lateral variation of deposits is accompanied by thickening, which can reach a few tens of metres. Thickening of layers and lithofacies variation indicate synsedimentary faulting processes. Two major fault zones have been identified: Kenitra–Sidi Slimane Fault Zone (K2SFZ) and Rabat–Kenitra Fault Zone (RKFZ). In the western coastal area, the geometrical configuration suggests a partition into horsts and grabens in the southern part, and a system of three geological units in the northern part. The analysis and interpretation of the gravity data reveal an important gravity anomaly, referred to as the Kenitra Gravity Anomaly. It corresponds to the Hercynian faults deduced by the seismic reflection line interpretation: K2SFZ and RKFZ. From Larache to El Jadida cities, the Kenitra area represents the hinge between the positive and negative gravity values, with a major negative anomaly in the eastern part of Kenitra. Copyright © 2004 John Wiley & Sons, Ltd.

An Early Devonian (Lochkovian) microflora from the Freshwater West Formation, Lower Old Red Sandstone, southwest Wales

Abstract: The Lower Old Red Sandstone deposits occurring to the south of the Ritec Fault on the Pembroke Peninsula in southwest Wales are very poorly age-constrained. Two moderately well-preserved spore assemblages have been recovered from the Freshwater West Formation, locally the highest division of the Milford Haven Group. These assemblages contain both cryptospores and trilete spores, with the latter group being dominant. They essentially represent a single microflora which is assigned to the middle part of the Emphanisporites micrornatus–Streelispora newportensis (MN) Spore Zone and is considered to be of Early Devonian (mid-Lochkovian) age. A new cryptospore species Qualisaspora sinuata and a new trilete spore species Dictyotriletes williamsii are described. The microflora is correlated with similar assemblages from the Dittonian of the main Anglo-Welsh Basin. Locally, the new biostratigraphic data constrain the age of the rich ichnofauna of the Freshwater West Formation and also date the influx of coarse clastic sediments into the basin following the sedimentary quiescence represented by the underlying Chapel Point Calcrete. Copyright © 2004 John Wiley & Sons, Ltd.

Inter-tuff correlation and architecture within the Moor Cliffs Formation, Lower Old Red Sandstone, southwest Wales: a multi-disciplinary approach for reservoir characterization

Abstract: The laterally continuous coastal exposures of the Moor Cliffs Formation of the Anglo-Welsh Basin have allowed a highly detailed 2D reconstruction of the floodplain. Whole-rock trace element geochemical and spectral gamma-ray (SGR) correlation are combined with traditional sedimentological analysis allowing outcrop and subsurface techniques to be compared. Micaceous, quartz-rich and well-sorted fluvial channel sandstones form up to 10% of the thickness in this low net-to-gross succession. High Th/K ratios derived from SGR analysis distinguish sandstones from the surrounding fine-grained sediments, but lateral facies variations prevent the correlation of fluvial channel intervals using SGR without outcrop control. Red-brown micaceous siltstone and claystone constitute 65 to 95% of the outcrop thickness and contain abundant pedogenic features characteristic of modern Vertisols. The most mature Vertisol horizons and the non-pedogenically altered siltstones/claystones do not show significant differences in their SGR response. Intraformational conglomerates comprising carbonate and siltstone clasts within a siltstone matrix are common above laterally extensive erosion surfaces, and can be clearly recognized from a sharp spike in Nb/Y and Zr/Y ratios. Major tuff horizons, which form chronostratigraphic marker horizons throughout the Moor Cliffs Formation, are also characterized by a distinctive geochemical signature, enabling calibration of the geochemical and sedimentological datasets. Geochemical and SGR analytical techniques are complementary and assist in enhancing the confidence of correlations created by traditional techniques in the Moor Cliffs Formation. Where multiple correlation scenarios have been interpreted due to a lack of diagnostic marker beds or biostratigraphic control, the SGR and geochemical data can increase confidence in a particular interpretation and hence decrease uncertainty in geological models of low net-to-gross systems in the subsurface. Copyright © 2004 John Wiley & Sons, Ltd.

Implications of dissected drift at Stockdale Head, western Lake District, northern England

Abstract: School of Environmental Sciences, University of Ulster at Coleraine, Co. Londonderry, UKFrom mapping and consideration of prominent drift ridges at Stockdale Head, western Lake District, northern England it isinferred that the ridges are the products of dissection of a glacigenic or soliflual drift sheet rather than landforms constructedat the margins of a Loch Lomond Stade (LLS) valley-head glacier. This proposal has implications for the recognition of LLSglacier limits and, possibly, understanding the dearth of moraine ridges associated with Dimlington ice in Lake District valleys.Copyright # 2004 John Wiley & Sons, Ltd.