Showing papers in "Journal of the Geological Society in 1990"

Ophiolites in Northeast and East Africa: implications for Proterozoic crustal growth

Abstract: The presence of ophiolite complexes in NE and E Africa has been documented using Landsat, field and geochemical studies. The present work identifies five major ophiolitic sutures in NE Africa, while plate reconstruction of Africa and Madagascar suggests a possible sixth ophiolite belt to the east. The ophiolites are considered to be remnants of supra-subduction zones and back-arc basins. The ophiolites are dismembered, and their mode of occurrence varies widely resulting in different structural relationships. In Western Ethiopia, the Yubdo complex is formed of harzburgite which grades into a cumulate sequence of ultramafic and gabbroic rocks and metabasalts. In Kenya, the Baragoi complex is formed of tectonized harzburgite with dunite and chromite pods, a cumulate sequence of ultramafic and gabbroic units and a dyke unit. Trace element data from the Baragoi complex show a transitional mid-ocean ridge basaltic to island arc tholeiitic affinity, and the presence of boninites suggests a supra-subduction setting, while data from the Adola-Moyale belt (S. Ethiopia-NE Kenya) indicate an island-arc and MORB geochemistry, which developed in a back-arc tectonic setting. In Sudan, the Ingessana complex which has an island arc tholeiitic tectonic affinity indicates development in a supra-subduction setting, while the trace element suggest that the Sol Hamed complex developed in a back-arc basin. These ophiolite belts represent sutures marking the position of island arcs extending to those in Saudi Arabia on a pre-Red Sea drift reconstruction and further south to Mozambique. The terrane delimited by a proposed Ingessana-Port Sudan suture and by the Adola-Moyale ophiolite belts as defined in this study encompasses an area in which the crust was derived from oceanic and island arc material. Relative age reconstruction across the Arabian-Nubian Shield in East Africa broadly indicates eastward younging of the ophiolite belts. Regional geological, tectonic and geochemical studies suggest rifting at c. 1200 Ma and subsequent convergence led to the development of intra-oceanic arcs and associated marginal basins in the north and narrow basins within the sialic basement gneisses further south in Kenya and Tanzania. This was followed by continent-continent collision which led to accretion of island arcs by gentle collision from the northeast in Saudi Arabia and severe crustal shortening in S Sudan, Kenya and SE Ethiopia as compared to Saudi Arabia, NE Sudan and N and W Ethiopia owing to oblique collision from the southeast.

An oceanic model for lithological and faunal changes tested on the Silurian record

Abstract: A model for oceanic cycles is presented. The cycles are expressed as changes between more humid low latitude and cooler high latitude climates (P episodes), and dryer low latitude and warmer high latitude climates (S episodes). The cyclicity may be self-regulating through changes in the CO 2 storage capacity of the deep ocean due to temperature changes and through changes in the oceanic input and output of dissolved carbonate. The model permits over 30 predictions regarding observable changes in both deep water and shelf lithologies and variations in the abundance, diversity and extinction rate of planktic and benthic faunas. Sedimentary changes involve fluctuations in the rate of clay deposition and carbonate production, the advance and retreat of carbonate deposition on the shelf edge, the timing of oolite, reef, and black shale formation, and changes between oxic and anoxic deep water. Faunal effects include both iterative evolution and intermittently widespread taxa (‘Lazarus taxa’). These predictions agree well with changes observed in well known sequence of Wenlock and Ludlow age, as well as changes seen in selected older and younger sequences.

Plate reorganization: a cause of rapid late Neogene subsidence and sedimentation around the North Atlantic?

Abstract: Subsidence analysis of wells in the central North Sea and Labrador-Grand Banks and oft the West Greenland, Scotian shelf and United States Atlantic margin shows distinct quantitative stratigraphic correlation patterns of circum North Atlantic sites. A significant departure from the overall decrease in subsidence for the Pliocene occurs in many wells, when the rate is found to have increased one or more orders of magnitude from Oligocence/Miocene rates. Wells were selected along transects from shore to basin to find if relative basin position is influenced by differential basin subsidence. Although stratigraphic resolution is not detailed, more basinward sites experienced up to four times larger subsidence rates in the late Neogene than in the Oligocene/Miocene, with a peak in the Pliocene. Wells at the basin edge experienced much less subsidence or showed uplift. The observations are consistent with a rapid change of intraplate stress at the cause for this observed transition in Neogene subsidence. Major reorganizations of spreading direction and rate occurred during the Pliocene along the entire Atlantic spreading system, possibly in conjunction with more global changes in plate motions. We propose that the associated changes in intraplate stress caused the excess margin subsidence. Relative uplift along basin edges is consistent with this mechanism of relative movement and may explain apparent eustatic changes in sea level.

Climatic and biotic change in the late Triassic

Abstract: Generally arid conditions that pervaded much of Europe and North America during the late Triassic were interrupted by a wet monsoonal climatic phase during Middle and Late Carnian times. Extensive fluviatile sandstones deposited at this time throughout the region, occur within a thick sequence of playa-lake mudstones. The sandstones occasionally contain kaolinite, suggesting a humid climate. An extreme δ 13 C depletion in a shallow marine sequence of this age in Israel has been interpreted as evidence for an influx of freshwater. A widespread change from carbonates to clastics in marine sequences at this time may also be climate-related. Water-course cave systems in limestone areas exposed during the late Triassic indicate high levels of runoff during the Middle and Late Carnian. The marine invertebrate fauna shows a significant turnover at the end of the Early Carnian. The terrestrial fauna and flora were relatively unaffected at this time but subsequently diversified prior to a major biotic turnover at the Carnian-Norian boundary. These periods of biotic change appear to be synchronous with the onset and cessation of a Carnian humid phase. The change to a monsoonal climate during this interval has been documented over more than 90° of longitude between 5° and 50° north of the Triassic equator. It may have been caused by rising atmospheric CO 2 levels due to volcanism associated with the incipient dispersal of Pangaea.

Geodynamic evolution of the northern and central Andes during early to middle Mesozoic times: a Tethyan model

Abstract: The early to middle Mesozoic sedimentary and magmatic history of the northern and central Andes (i.e. the NNE-trending Colombian-Ecuadorian segment, and the NW-trending Peruvian segment) exhibits a spatially contrasted evolution involving several successive tectonic and geodynamic settings. The late Triassic to late Liassic period began with a widespread marine transgression. From the latest Triassic, the Colombian marine shelf was progressively destroyed by southward propagating extensional tectonic activity but marine sedimentation continued in Peru. During this time, no significant magmatic activity is recorded except in the emerging Colombian area. This period is interpreted as the result of rifting of a Tethyan oceanic arm which separated the Colombian and palaeo-Mexican margins. From latest Liassic, an important calc-alkaline magmatic arc developed along the emergent Colombian segment. Further south, the north-Peruvian shelf probably emerged, but marine sedimentation continued in southern Peru. During middle and early Late Jurassic times, the Colombian segment was characterized by important magmatic activity and by coarse clastic continental sedimentation. Along the Peruvian segment, a turbiditic trough, emergent areas, and continental basins were created, and the scarcity of calc-alkaline magmatism suggests that only very local subduction took place. This period is regarded as one of the southeastward subduction beneath the Colombian segment, and of Tethyan oceanic crust originating in the newly formed ‘Colombian’ oceanic arm. This pattern would have induced a chiefly left-lateral transform motion along the Peruvian segment. By Kimmeridgian-Tithonian times, the palaeogeographic framework had drastically changed. Along the Colombian segment, magmatic activity ceased, and continental accretions occurred along dextral strike-slip sutures. In Peru, tectonic activity was recorded by the creation of a new turbiditic trough and by the resumption of detrital sedimentation. In the coastal area, arc-related volcanism indicated that subduction took place beneath this segment. This geodynamic change is interpreted as the result of a sharp decrease in the spreading activity of the Tethyan ridges and replacement by Pacific spreading centres inducing a roughly northeastward convergence direction.

Late Cretaceous-early Tertiary palaeoclimates of northern high latitudes: a quantitative view

Abstract: Analyses of plant community structure, vegetational and leaf physiognomy, and growth rings and vascular systems in wood provide qualitative and quantitative data that can be combined to define non-marine palaeoclimatic parameters with better resolution than is available from other, principally sedimentological, methods. Application of these techniques to Cenomanian through Paleocene floras from high palaeolatitudes (75°-85°N) indicates a polar light regime similar to that of the present. Plant data suggests Cenomanian sea level mean annual air temperatures (MATs) of 10 °C, and MATs of 13 °C, 5°C and 6-7 °C in the Coniacian, Maastrichtian, and Paleocene respectively. Evapotranspirational stresses at sea level were low and precipitation was in most part uniform throughout the growing season in the Cenomanian, with possible seasonal drying occurring by the Maastrichtian. Maastrichtian winter freezing was likely, but periglacial conditions did not exist at sea level. Permanent ice was likely above 1700 m at 75°N in the Cenomanian, and above 1000 m at 85°N in the Maastrichtian. These near-polar data provide critical constraints on global models of Late Cretaceous to early Tertiary climates.

Late Palaeogene-Neogene evolution of the triple junction region near Maraş, south-central Turkey

Abstract: The Marao region of Turkey includes the Anatolian-Arabian-African triple junction. The African-Arabian margin is marked by left-lateral transtension along the Dead Sea (Kara Su) rift. Motion on the Anatolian-Arabian boundary is partitioned between the left-lateral East Anatolian fault and a fold-thrust belt termed the Engizek zone, equivalent to the Bitlis suture further east. The African-Anatolian boundary lies along the Cyprus-Misis-Andirin trend and is expressed near the triple junction as the Aslantao-Iskenderun zone, now undergoing slow left-lateral transtension. The Neogene evolution of these boundaries is largely recorded in the stratigraphy and structure of the basins that lie along the Anatolian-Arabian (Lice basin) and Anatolian-African (Aslantao-Iskenderun basin) boundaries. These basins appear to have developed in the early Miocene along major strike-slip zones. Large half graben in the pre-Neogene 9basement9 of the western (Misis-Andirin block) developed contemporaneously with motion on the Anatolian-African boundary. These extensional features evolved subsequent to the intense shattering of the Mesozoic carbonate sequence in the Misis-Andirin block and its mixing with panels of deep water Palaeogene strata to form an extensionally disrupted assemblage. Transpression and conversion of both Anatolian-African and Anatolian-Arabian boundaries to fold-thrust belts occurred in the late Miocene. Transpression continues along the Anatolian-Arabian boundary but transtension has recurred along the Anatolian-African boundary since the Pliocene.

Estimation of completeness of stratigraphical sections using empirical data and theoretical models

Abstract: The completeness of a stratigraphical section is the fraction of time intervals of some specified length ( t ) that have left a record. A record is left when some sediment is deposited during the interval and is not subsequently eroded. A complete section contains no hiatuses longer than t . The completeness of a section varies with t and its accumulation rate varies with the length of the time span over which it is measured. Plots of measured accumulation rate against time span are an empirical means of estimating completeness. Simple theoretical models help extrapolate meagre data and identify bias. Completeness at time scale t can depend upon three general properties of the accumulation history: the age of the section, the long term net accumulation rate (drift) and the unsteadiness of the sedimentation rate. The way unsteadiness is measured depends upon the kinds of fluctuations that can be recognized in the accumulation rate. To describe random fluctuations a standard deviation of rates is sufficient. One dimensional Brownian motion is a model of random fluctuations that explains many aspects of stratigraphical completeness. Regular periodic fluctuations in accumulation rate may be described in terms of wavelength and amplitude. Completeness is an increasing function of t, drift and, in the periodic case, the wavelength of fluctuations; it decreases with increasing standard deviation of accumulation rate and the amplitude of periodic fluctuations. The completeness and thickness of stratigraphical sections are weakly positively associated.

Do fossil plants give a climatic signal

Abstract: The agreement in the pattern of major biomes with that of climatic zonation of the Earth gives a strong indication that climate is the overriding influence controlling the distribution of plant communities. Three aspects of plants which may be preserved in the fossil state, give a signal of the climatic conditions under which they grew: (1) the present climatic association of their 9nearest living relative9; (2) leaf physiognomy of arborescent plants; (3) the character of their secondary xylem (9wood9 of ordinary usage) reflecting, by the presence or absence of growth rings, the seasonality (or lack of it) in their environment and the potential for tree growth that it offered. The significance and limitations of these 9palaeoclimatic signals9 as they may be read from the fossil plant record are reviewed and evaluated. The recent demonstration that stomatal frequency of leaves is responsive to changes in ambient carbon dioxide partial pressure offers promise for direct palaeobotanical evidence for past changes in the level of this climatically significant atmospheric constituent.

Extensional and compressional zones in slumps and slides in the Namurian of County Clare, Ireland

Abstract: Extensional and compressional zones in slumps and slides have been studied in a well-exposed Namurian (Carboniferous) base-of-slope and delta slope sedimentary succession in County Clare, Ireland. Extensional zones are in slumps dominantly characterized by single, downslope-dipping normal faults, while in slides they are characterized by normal, listric fault families. These faults sometimes show minor growth. Compressional zones in slumps are dominated by a single reverse fault with upslope-dip. They are only rarely characterized by imbricate fault fans. Compres­sional zones in slides fall into a tri-partite division of thick packages, thin packages and basal zones. Thick packages compare with tectonic imbricate zones, while thin packages are simpler and characterized by very shallow synthetic thrusts. Basal zones compare with blind imbricate fans. Lateral margins of both extensional and compressional zones in slumps and slides show transfer faults with transtensional, transpressional or purely strike-slip motion, depending on the width variability of the slide and slump scars. The similarity in structural style between soft-sediment deformational structures and deformation structures in lithified rocks shows that, on the basis of structural style alone, it is problematical to differentiate between these two groups of structures.

Age of amphibolitic metamorphism in the ophiolitic unit of the Morais allochthon (Portugal): implications for early Hercynian orogenesis in the Iberian Massif

Abstract: Structurally lower portions of the allochthonous Morais complex are comprised of an ophiolitic unit exposed in two contrasting structural units separated by a ductile thrust fault. The lower unit is dominated by variably foliated amphibolite whereas the upper unit is largely comprised of metaperidotite and metagabbro. Most of the ophiolitic unit has recrystallized synkinematically under amphibolite facies metamorphic conditions ( c. 500°C and 5 kbar). Amphiboles formed during this metamorphism are typically magnesio-hornblende. An amphibole concentrate from the lower structural unit records a 39 Ar/ 40 Ar plateau age of 397.3 ± 4 Ma. An amphibole concentrate from the upper structural unit displays an internally discordant 39 Ar/ 40 Ar age spectrum corresponding to a total gas age of 431.1 ± 27.3 Ma. Gas fractions evolved from each amphibole concentrate at intermediate and high experimental temperatures are characterized by similar apparent K/Ca ratios and yield well-defined 36 Ar/ 40 Ar v. 39 Ar/ 40 Ar isotope correlations which result in similar ages of 384.2 ± 5.2 and 392.4 ± 7.3 Ma. These are considered to be more reliable than the plateau ages and are interpreted as dating the last cooling through those temperatures required for intracrystalline retention of argon within constituent amphibole grains. The associated tectonothermal event represented a first stage of Hercynian orogenesis in the Iberian massif. It was probably related to oceanic closure and associated obduction of granulitic/eclogitic massifs which at present structurally overlie the ophiolitic unit.

The Sunnfjord melange, evidence of Silurian ophiolite accretion in the West Norwegian Caledonides

Abstract: A major composite terrane, the Sunnfjord Melange, has been identified in the West Norwegian Caledonides. The rocks of the melange provide a terrane-link between the allochthonous continental rocks of the Dalsfjord Suite with its cover of continental margin deposits and the oceanic terrane of the Solund-Stavfjord Ophiolite Complex. The melange was formed as the ophiolite was emplaced on the the fossiliferous Lower to Middle Silurian continental margin deposits of the Herland Group. This group unconformably overlies older metasedimentary rocks of the Hoyvik Group and the crystalline basement of the Dalsfjord Suite. A structural style characteristic of thin-skinned thrust-foldbelts is locally preserved within the Herland Group on Atloy, and the thrust-foldbelt was developed in the foreland of the continental margin during the ophiolite accretion. A U-Pb zircon age of 443 ± 3 Ma from the ophiolite, the Silurian fossils in the Herland Group and the identification of the Sunnfjord Melange as an obduction melange provide the basis for a well constrained model of ophiolite accretion in the Scandinavian Caledonides.

Changes in marine isotopic composition and the late Ordovician glaciation

Abstract: Well-preserved brachiopod shells and marine cements from limestone coquinas which cap carbonate mudmounds in the Siljan area of central Sweden have the heaviest stable oxygen and carbon isotope values yet reported for Lower Palaeozoic marine sediments. The coquinas formed during the eustatic regression at the onset of the late Ordovician glaciation: the isotopic compositions reflect simultaneous shifts in both carbon and oxygen (more than 5‰ δ 13 C and up to 2o‰ δ 18 O) away from more normal Lower Palaeozoic values in similar carbonates from within the mounds. Oxygen isotope data are consistent with a change in the isotopic composition of the sea water probably accompanied by a decrease in temperature. The changes in carbon values suggest enhanced deposition of organic carbon, a process which would have decreased p CO 2 in the ocean and the atmosphere and thus contributed to rapid global cooling.

Eustatic and tectonic controls upon Quaternary coastal sedimentation in the Corinth Basin, Greece

Abstract: The late Quaternary sedimentology and palaeogeography of the Corinth Isthmus in central Greece is re-interpreted. A series of beach-to-shoreface sub-sequences are seen, each characteristically having an overall fining-up character. These occur within a coastal facies belt which has prograded to the west. Computer modelling illustrates the interaction of tectonic displacements and eustatic sea-level variation as controls of facies patterns. Late Pleistocene-Holocene facies geometries are predicted on the basis of published sea-level data over that time interval. On coastlines undergoing tectonic uplift, such as the Isthmus of Corinth, depositional patterns may be characterized by transgressive wedges. These sub-sequences merge basinwards below sea-level minima. Each transgression relates to a major eustatic sea-level peak. Such first order models are modified by variations in the rate of tectonic deformation, the second control on relative sea-level position. Modelled facies patterns are tested on the basis of new 230 Th/ 234 U dates obtained from sclerac-tinian corals of the Corinth canal area. Dated sub-sequence sediments correspond in age to late Pleistocene eustatic highs and observed facies patterns correlate with the major late Pleistocene-Holocene eustatic transgressions of c . 100 ka periodicity. Episodes of intrabasinal normal faulting are isolated from a structural history otherwise dominated by uplift across the Isthmus. Quantification of post-depositional vertical tectonic displacements is achieved, constraining the eustatic base-level at the time of deposition of a dated horizon by reference to the global late Quaternary sea-level curve. The original depositional level is then subtracted from the present outcrop level. Using available 230 Th/ 234 U dates to establish syn-depositional base-levels, minimum uplift rates of the order of 0.3 m ka −1 are calculated across the area, averaged through the late Pleistocene and Holocene.

Aspects of climatic influence on the clay mineralogy and geochemistry of soils, palaeosols and clastic sedimentary rocks.

Abstract: Climate strongly influences soil clay mineralogy and bulk chemical composition. In humid climates chemical weathering is at its most intense and virtually all ‘primary’ bedrock minerals suffer complete breakdown. They are replaced by authigenic hydrated oxides of Fe and Al together with halloysite and kaolinite in deep profiles. These minerals develop where the residence time of water in the soil is slight. Longer residence times lead to higher solute concentrations and the stabilization of smectite. Authigenic palygorskite and sepiolite may form in highly saline, alkaline soils. The stability of these precipitated minerals is problematic: they may be metastable or even unstable. All other clays found in soils are unstable, being incomplete, solid-state hydrolysis products of substrate minerals. The balance between stable, metastable and unstable minerals and the rate of profile development are both much influenced by temperature, rainfall and activities of soil biota; themselves reflecting climate. Bedrock influence is greatest under conditions of low to moderate precipitation where unstable minerals persist. Local topography, however, also strongly influences both the rate and products of chemical weathering, quite independently of climate. On the grand scale, tectonic factors may constrain the time available for profiles to develop, hence the extent to which bedrock minerals survive or are altered. Mountain soils reflect local climate and bedrock composition much more than latitude. The sediments delivered to major basins today do not reflect global soil distribution patterns. The bulk composition of suspended sediment entering the Indian Ocean, for example, is rather similar to that entering the Arctic Ocean. Sediment dispersal patterns can also confuse climatic signals: a problem not encountered with palaeosols. Authigenic clays formed within sedimentary basins and post-depositional (burial) alteration of both sediments and buried soil profiles must lead to interpretational ambiguities. Climatic signals are likely to survive best in bulk compositional data (although diagenetic reactions at depths in excess of about 2 km do not appear to be isochemical) and in the distribution of more stable yet climatically sensitive minerals (kaolinite as opposed to illite, chlorite).

Pan-African reworked early/middle Proterozoic crust in NE Africa west of the Nile: Sr and Nd isotope evidence

Abstract: Whole-rock Rb-Sr studies on six suites of highly metamorphosed basement rocks from southern Egypt and northern Sudan, west of the River Nile, yield Pan-African ages (562–918 Ma): these ages are interpreted as reset. Orthogneisses give Nd model ages of 1600 to 2600 Ma for their calc-alkaline plutonic precursors. A gneiss derived from sedimentary precursors gave a mean crustal residence age of 2200 Ma. A series of I-type, late-tectonic, granitoids were intruded during a period of uplift, erosion, and wrench faulting at 560–620 Ma. Geochemical characteristics and Nd model ages (1200–1700 Ma) indicate that these rocks were derived from a mixed source of juvenile Pan-African mantle material and older continental crust of mainly early/middle Proterozoic age. Negative ɛNd t , values for granitoid samples (–18.9 to –5.3) show that the Pan-African episode in NE Africa, west of the Nile, involved considerable reworking of pre-existing crust.

Provenance of the pre-Devonian sediments of England and Wales: Sm-Nd isotopic evidence

Abstract: ɛ Nd values and clast petrography of the Longmyndian sediments of Shropshire indicate a provenance dominated by juvenile (high ɛ Nd ( t )) Uriconian volcanic sources, but with an increasing detrital contribution from older (low ɛ Nd ( t )) metamorphic sources up-section. By contrast, most Lower Palaeozoic strata of England and Wales have low ɛ Nd ( t )) values, typical of sediments derived from long established continental crust. The Sm–Nd isotopic evidence suggests that Avalonia received sediment from an adjacent large continent in the Cambrian to Lower Ordovician and in the Silurian. Ordovician and Silurian volcanicity supplied juvenile detritus only during and shortly after active volcanism.

Footwall uplift associated with Late Jurassic normal faulting in the northern North Sea

Abstract: Tilted fault-blocks in the North Sea show varying degrees of erosion. Such footwall erosion is predicted by the domino model of extension, in which rigid fault-blocks rotate as extension proceeds. Profiles of three North Sea fields (Snorre, Brent and Heather) are analysed to estimate the amount of uplift that accompanied Late Jurassic normal faulting, and these estimates are compared with the footwall uplift predicted by the domino model. Agreement between observed and predicted uplift is excellent, varying from >1 km at Snorre to c . 100 m at Heather. For a given amount of extension, large fault-blocks tend to be bounded by large faults and to show a greater degree of erosion. The Brent Province of the northern North Sea is an area of prolific hydrocarbon discoveries. Almost all of the major oil-fields occur in tilted fault-blocks which were formed by Late Jurassic extension and subsequently buried by Cretaceous-Tertiary thermal subsidence (Badley et al. 1988; Marsden et al. 1990). The crests of the tilted blocks are typically eroded or degraded, with varying amounts of Middle-Lower Jurassic and Upper Triassic section removed. This footwall erosion is commonly ascribed in the literature to pre-rifting doming (Ziegler 1982), heterogeneous stretching (Coward 1986), local transpression (Beach 1985) or even regional compression (Frost 1987). The purpose of this paper is to show that the uplift and degradation is a direct consequence of the Late Jurassic normal faulting and fault-block rotation. The degree of uplift is related in a simple way to the size (across strike) of

The influence of a sudden climatic change on marine deposition in the Kimmeridgian of northwest Europe

Abstract: A sudden change from humid style to semi-arid style deposition markedly affected the accumulation of the Upper Kimmeridge Clay in southern England. Many of the changes appear to be related to a change in sedimentation rate at this time. Thus softground faunas are replaced by firmground faunas; diagenetic dolostones formed in the methanogenic zone are replaced by sulphate reduction zone carbonate nodules; and depositional gradients, recorded by lateral biofacies changes, becomes steeper. The evidence available is in accord with a decline in offshore sedimentation rates during this interval. Other changes, such as a decline in kaolinite abundance, were more directly controlled by the ‘drying-out’ of the hinterland. Similar changes, elsewhere in the marine geological record, could be used as climatic indicators. The climatic change is part of a wider, northern hemisphere dry event which affect a broad area in the late Jurassic. The Kimmeridge Clay of southern England was one of the last depositional environments to be influenced by the climatic change at this latitude.

A multiple fluid history recorded in Alpine ophiolites

Abstract: Petrographic and cathode luminescence studies of Alpine ophicalcites are combined with stable isotope data of pelagic sediments and associated ophiolite relicts, to document multiple phases of fluid-rock interaction related to sea-floor processes and Alpine orogeny. Internal sediments and primary carbonate microstructures in ophicalcites provide evidence for early sea-floor fragmentation, cementation and fluid activity in the Jurassic Tethys. Ca-carbonate cementation is comparable to cementation in modern transform setting analogues, where alkaline and Ca-enriched fluids are associated with serpentinites. Local mineralization and isotopic compositions of serpentine in ophicalcites are interpreted as signs of hydrothermal activity at temperatures of 100–150 °C. Differences in the degree of oxygen isotope re-equilibration reflect differences in fluid/rock interaction during Alpine accretionary tectonics and continent-continent collision. Regional-scale homogenization of oxygen isotopes in carbonates and cherts indicate the presence of pervasive metamorphic fluids in the pelagic sediments during early Alpine recrystallization. In contrast, a lack of oxygen isotope re-equilibration in serpentine may reflect lower permeabilities and more limited fluid flow in serpentinites during Alpine metamorphism. Cathode luminescence and preliminary oxygen isotope data of late vein-forming phases suggest that late metamorphic fluids were channelled along discrete brittle fractures in a closed, rock-dominated system.

Sequence stratigraphy of the Palaeozoic Welsh Basin

Abstract: The timing and extent of unconformities in the Welsh Basin are investigated using ‘rock preservation curves’ derived from outcrop stratigraphic logs. Four basin-wide unconformities occur, focussed in late Precambrian, late Tremadoc, Pusgillian (early Ashgill) and mid-Devonian times. These bound three megasequences, equating with newly defined lithostratigraphic units, the Dyfed, Gwynedd and Powys Supergroups. Less extensive unconformities bound 18 component sequences. The majority of the sequence boundaries reflect a component of tectonic or volcanotectonic activity rather than a pure eustatic sea-level change. The megasequence boundaries are attributed to late Precambrian to early Cambrian onset of rifting to form a passive margin, Tremadoc onset of subduction with intra-arc then back-arc extension, late Caradoc end to subduction, and late Early Devonian collisional deformation. The megasequences and controlling events can be tentatively matched with other basins on the Avalonian margin. More generally, this study shows that sequence analysis is feasible in onshore basins lacking well and seismic data, and that the global eustatic interpretation of sequence stratigraphy is only partially applicable to active margin basins.

Structural trends in central Britain from images of gravity and aeromagnetic fields

Abstract: Digital image processing techniques have been used to analyse the regional gravity and aeromagnetic datasets in central Britain. Colour and shaded-relief images have been generated which convey information on both anomaly amplitude (as colour) and anomaly gradient (as relief) and highlight structural trends, lineaments and textural contrasts not easily discernible on standard contour maps. We have identified some of the more important features and have tried to relate these to the evolution of the crust in each region. On a broad scale the images emphasize Caledonoid (ENE) trends to the north of the Solway line and both Tornquist (SE) and Caledonoid (NE to ENE) trends to the south. The pattern of magnetic anomalies over central England seems to define the extent of the shallow late Precambrian–early Palaeozoic basement of the Midlands Microcraton and delineates crustal elements and characteristic trends within it. Magnetic lineaments and the grain of gravity anomalies, trending in northeasterly and northwesterly directions on either side of the microcraton, seem to relate to structures which originated during the evolution of the Welsh and eastern English Caledonides respectively. Where Lower Palaeozoic and Precambrian rocks form the concealed ‘basement’, as in most of England, the gravity lineaments tend to reflect the influence of basement fractures on the subsequent pattern of sedimentation whereas many of the features on the magnetic images are more directly related to structures within the basement itself.

Granite magmas: their sources, initiation and consequences of emplacement

Abstract: Geochemical and other evidence shows that granite magmas are generally derived from a mixture of melted crust and mantle material and the existence of a spectrum of types varying from largely mantle-derived to mainly crustal-derived is emphasized. Each pluton possesses a unique mix or range of mixtures which preserves an adumbrate history of the deeper levels of the crust and mantle at the particular location. Four illustrative granite-forming provinces are considered: (1) Archaean cratons with their voluminous tonalites, (2) magmatic arcs as exemplified by the western Americas, (3) the British Tertiary Province as an example of the results of extensional faulting and (4) the British and Irish Caledonide granites, the main phase (440–390 Ma) of which was produced during major Acadian strike-slip faulting. All these granites provide evidence of mixing of source materials. Geochemistry, especially of the Rare Earth Elements, U-Pb ages and isotopic studies of Sr, Nd, Pb, O, H and in some volcanic rocks Be, is beginning to enable quantification of the source components. The importance of major fracturing of the crust and mantle in providing magma triggers, magma focusses and magma consolidation sites is stressed; major batholiths probably occupy holes created by pulling apart the crust. Geophysical evidence does not suggest that granite plutons are distributed throughout the thickness of the crust, but rather that they are concentrated relatively near the surface. The extant elevation of the whole of the north British crust, its altitude over the last 400 Ma and even in places the present shape of the coastline, are significantly influenced by Caledonian granites, the tops of which are generally within about 3 km of the present landsurface. Thus relatively little erosion of most of these bodies has taken place and the dominant sources of post-Silurian clastic sediments in the area were stripped off volcanic cover rocks and recycled sediments and metasediments. By inference, other continental areas with numerous exposed granite pluton roofs had a similar history. The process of granite formation is an important means of density-stabilization of the crust with high-level emplacement of light sialic granite refined out of the mantle and lower, middle and even upper crust. The refractory mafic residues generally become denser and sink or stay down in the density-stratified crust but the division into residue and magma is not sharp with some granite magmas including significant quantites of restite.

The formation of coastal peat marshes under an upward tendency of relative sea-level

Abstract: The upward growth of a tidal mudflat-marsh within the tidal frame is determined theoretically by the rates of minerogenic sediment supply, organogenic sediment supply, long-range sediment compaction and movement of relative sea-level. Where the movement of relative sea-level is upward, a marsh can achieve a state of dynamic equilibrium in which the composition of the sediment accumulating on its surface is governed by the balance between the rates of organogenic supply and sea-level change. A peat (organogenic) marsh should arise where the former rate roughly equals or exceeds the latter. A minerogenic marsh should form where the rate of sea-level rise is dominant, even though the absolute availability of mineral fine sediment may be so high as to appear to suggest that build-up to extreme tidal levels, where vegetation would dominate, should have occurred.

Constraints on measurement of sea-level movements from salt-marsh accretion rates

Abstract: 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.

The Devonian lacustrine sediments of Orkney, Scotland; implications for climate cyclicity, basin structure and maturation history

Abstract: The Orcadian Basin of northern Scotland (largely of Middle Devonian age) is a major lacustrine rift basin, with the widely correlateable, Achanarras/Sandwick lake bed extending for at least 800 km along the rift Above this lake bed, there are about 540 m of cyclic lacustrine sediments, with lake sedimentation terminated by alluvial fan progadation The Upper Stromness Flagstone Formation and the Rousay Flagstone Formation making up this lacustrine sequence are given revised and precise definitions within four reference sections from the Orkney Islands The sequence consists of about 45 first-order cycles averaging about 12 m in thickness, which resulted from long-term climatic fluctuations in rainfall with a cycle time-scale of about 25 000 years Longer climatic fluctuations of about 100000 and 400000 years are also present These periods correspond to the precession and two eccentricity orbital cycles Detailed knowledge of the stratigraphy constrains the structural and maturation history of the basin The basin formed in a series of extensional half graben, and the Orkney area suffered later inversion The lake sediments first reached the oil window at relatively shallow depths ( c 2000 m), probably in the early Carboniferous, implying very high geothermal gradients during the early part of the basin history as corroborated by the presence of Devonian volcanics in the region In the Orkney area, uplift probably started in the late Carboniferous

Sedimentological response of an alluvial system to Neogene thrust tectonics, Atacama Desert, northern Chile

Abstract: The Llano de la Paciencia is a thrust sheet top basin in which the sedimentological and topographic evolution can be linked to thrust tip propagation. It is an elongate gravel plain which borders the Salar de Atacama, a major intermontane basin in the Andean forearc of northern Chile. The Llano is bounded to the west by the Cerros de Purilactis a Cretaceous–Paleocene sequence uplifted by the Frontal Domeyko Thrust. The eastern margin of the Llano is formed by the Cordillera de la Sal which was uplifted by a linked back thrust-frontal thrust system. The Salar de Atacama is thus divided into a number of discrete sub-basins: the Llano de la Paciencia, the Pampa Visachita and the western sub-Llano which from east-west are bounded by the Cordillera de la Sal, the northern imbricates and the ignimbrite back thrust. Two phases of sedimentological evolution can be distinguished within the Llano on the basis of Quaternary to Recent sediment dispersal patterns. Initially, Phase 1 alluvial fan lobes prograded eastwards into the main Salar de Atacama basin. Subsequently, uplift of the Cordillera de la Sal deflected drainage systems southwards parallel to the structural strike. These Phase 2 alluvial deposits drain into the Salar de Atacama at the lateral termination of the Cordillera de la Sal frontal thrust. In places where thrust tip ramps are emergent within the Llano, gullies have been incised into the drainage pathways. This has resulted in the reworking of the early Phase 1 gravels and progradation of the Phase 2 fan lobes. The evolution of the Llano de la Paciencia illustrates the close link between topography and syntectonic alluvial drainage patterns in an arid intermontane basin.

Equatorial aridity and climatic oscillations during the early Carboniferous, southern Britain

Abstract: Evidence from palaeosols, palaeokarsts and styles of meteoric diagenesis shows that the early Carboniferous climate of southern Britain was dominantly seasonally semi-arid. This semi-aridity occurred while the region was in an equatorial palaeolatitudinal position. The cause of the seasonal aridity was possibly the deflection of easterly winds into a major low pressure zone set up over Gondwana during the southern hemisphere summer.

Palynofacies in a Late Silurian regressive sequence in the Welsh Borderland and Wales

Abstract: The Upper Whitcliffe and lower part of the Downton Castle Sandstone Formations (Ludlow and Pridoli Series) have been sampled from sequences distributed over an area roughly 4800 square km in west-central England and Wales. Ludfordian age rocks have been interpreted as the deposits of shelf and basin environments, and those of the lower Downton Group as near-shore to beach. Sections sampled from the ‘shelf’ area are in the Ludlow type area, and in the Usk, May Hill and Woolhope inliers. ‘Basin’ sediments were sampled at Knighton, the Long Mountain, Capel Horeb and Kington. The samples were analysed for their palynological content, and all the organic-walled microfossils were studied together with the fragmentary remains of larger plants and animals. Using a series of ratios between different groups of microfossils, it is apparent that there are consistent changes through time in both the palynomorph assemblages and in the structured and unstructured organic material. Dominance of some microplanktic groups/taxa is used to define a new microphytoplanktic phase concept and to compare the sequence of palynoevents. Depositional environments inferred from palynofacies interpretation of the Ludlow type area are closely comparable with those proposed previously from sedimentological data. Palynofacies sequences differ for the shelf and basin areas but show similar trends. We have also recognized a dramatic shift in palynofacies at a level within the Downton Castle Sandstone Formation and its correlatives in both shelf and basin areas; this change is tentatively interpreted as a ‘storm’ event.

Structural evolution of Mesozoic Peninsular Malaysia

Abstract: Field evidence from Upper Palaeozoic and Mesozoic strata exposed in Peninsular Malaysia demonstrates that the structural style, degree and orientation of folding, axial-plane cleavage, and faulting in the Triassic, Jurassic and Cretaceous rocks of the central basin are remarkably similar. In contrast, the Upper Palaeozoic strata, especially the Carboniferous rocks exposed in the eastern part of the peninsula, show multiple phase folding and regional metamorphism unlike anything seen in the Mesozoic rocks. These observations demonstrate that two important compressional events affected Peninsular Malaysia, one in Late Permian times and the other in the mid- to late Cretaceous. The Late Palaeozoic compressional event was a major orogenic mountain building phase with associated emplacement of major Permo–Triassic granite plutons that form the eastern and main ranges. No indications were found of the widely reported Triassic orogenic compression. The palaeotectonic and palaeogeographical implications of these discoveries are important for interpreting the evolution of eastern Gondwana and Tethys. The Raub–Bentong line was an important fault zone active during the Mesozoic but does not appear to have been a major tectonic suture since the Late Palaeozoic. We postulate that the Permo–Triassic granites of the eastern belt and those presently exposed in the main range were originally about 30–50 km apart, and that Triassic and Jurassic crustal attenuation and subsidence led to the separation of these two granitic belts by more than 100 km.