Showing papers in "Journal of the Geological Society in 1987"

Tectonic environment of the Devonian Gramscatho basin, south Cornwall: framework mode and geochemical evidence from turbiditic sandstones

Abstract: The Portscatho Formation, within the allochthonous unit of the Middle and Upper Devonian Gramscatho Group, is a thick sequence of deep-water sandstones and interbedded slates deposited by southerly-derived turbidity currents into the Gramscatho basin of south Cornwall. Throughout an approximately 3.5 km thick sequence, the Portscatho Formation is petrographically and chemically coherent, except that the upper section shows a higher proportion of metamorphic clasts, high, but variable Cr, and low, uniform Zr abundances. Complementary framework mode and bulk geochemistry indicate that the sandstones were derived from a dissected continental magmatic arc of predominantly acidic composition, similar to average upper continental crust, but with an admixture of minor intermediate/basic material. Flysch deposition took place in a fore-arc setting. The presence of an arc to the south of Cornwall during the Devonian implies that there was subduction at the margin of the Gramscatho basin, whose ultimate closure was accommodated by the northward stacking of flysch–ophiolite nappes.

The movement and entrapment of petroleum fluids in the subsurface

Abstract: This paper discusses the migration of petroleum from its formation in a source rock to its subsequent possible entrapment in a reservoir. The chemical and physical properties of petroleum gases and liquids are stressed, particularly their phase behaviour under subsurface conditions which is shown to be a very important factor in determining migration behaviour. Engineering correlations are presented for estimating the properties of petroleum fluids under geologically realistic conditions. The directions and magnitudes of the forces acting on migrating petroleum are deduced from the combined effects of buoyancy and water flow in compacting sediments. These forces are combined, using a fluid potential description. This procedure allows the direction of migration to be denned. The rate of migration is then estimated from the properties of the sediments involved, allowing a distinction to be made between ‘lateral’ and ‘vertical’ carrier beds. This simplified approach is suitable for rapid predictive calculations in petroleum exploration. It is compared with the more complex 3-D computer modelling approaches which are currently becoming available. Migration losses are related to the cumulative pore volume employed by the petroleum in establishing a migration pathway. The petroleum migration mechanism is shown to be predominantly by bulk flow, with a small diffusive contribution for light hydrocarbons over distances less than c. 100 m. The loss factors involved in secondary migration are estimated from field evidence. The mechanism of reservoir filling is presented as a logical extension to those described for migration. This, together with the inefficiency of in-reservoir mixing by diffusion or convection, is shown to tend to cause significant lateral compostional gradients in reservoirs over and above the gravitationally induced vertical gradients described by other workers.

The segregation and emplacement of granitic magmas

Abstract: The segregation of granitic magma from residual crystals at low melt-fraction is strongly dependent on the viscosity of the melt. Theoretical considerations imply that for the typical range of granitic melt viscosities (10 4 Pas to 10 11 Pas) only very limited separation will be possible by a compaction mechanism over the typical duration of a crustal melting event ( c . 10 6 years). Small-scale segregations (millimetre to metre) of the type observed in migmatite terranes may be generated by compaction (possibly assisted by continuous deformation), or by flow of melt into extensional fractures, but low melt-fraction liquids are unlikely to be extracted to form large (kilometre-size) granitic plutons because of the limited separation efficiency. At higher melt-fractions (>30%) the rapid decrease in strength and effective viscosity during partial melting allows other segregation processes to operate. Calculations and experiments indicate that in granitic systems the effective viscosity of partially melted rocks, having a very narrow melt fraction range of 30–50% will fall rapidly to levels at which convective overturn of kilometre-thick zones can occur. Convective motion within anatectic regions is capable of generating large (kilometre-size) homogeneous, high crystal-fraction, crustally-derived magma bodies, which are orders of magnitude greater in size than low melt-fraction segregates. Before convective instability is reached, small (centimetre- to metre-sized) pods of granitic liquid may rise buoyantly through, and pond at the top of such partly molten zones; such a process is consistent with the observation that some granulites appear to be residue rocks, chemically depleted in a minimum melt component. The effective viscosity (and hence the susceptibility to convection) of a partially melted zone within the crust, is strongly dependent on the water content of the system at a given pressure and temperature, because this controls both the quantity of melt generated and also the viscosity of the melt. The intrinsic water content of most crustal lithologies is incapable of promoting the high percentages of partial melting, or the low liquid viscosities, required to form large kilometre-sized granitic plutons by convective homogenization, at typical crustal temperatures. This suggests that the anatexis involved in the generation of large crustally-derived magma bodies has in many cases been promoted by an influx of externally derived aqueous fluid. These magma segregation processes are illustrated with respect to the petrogenesis of three different types of granitoid pluton from a Hercynian low-pressure, metamorphic-anatectic terrane in the Pyrenees.

The history of Quaternary sedimentation in the UK sector of the North Sea Basin

Abstract: Reconnaissance seismic and sampling surveys have shown that up to 500 m of Lower and early Middle Pleistocene deltaic sediments are preserved in the UK sector of the North Sea. The distributions of delta-related facies record the progressive expansion of peripheral delta systems northwards across the southern North Sea during the early Pleistocene. A late Middle Pleistocene unconformity is present throughout the UK sector, and sedimentation above the unconformity was dominated by glacial erosional and depositional processes. A complex system of deeply incised valleys was eroded during each of three regional Middle and Upper Pleistocene glaciations and the valleys have been filled or partially filled by glaciolacustrine or glaciomarine and interglacial marine sediments. The valleys of the earliest (Elsterian) glaciation are up to 400 m deep in the southern North Sea. The Saalian and Weichselian valleys are commonly more than 100 m deep, they are eroded through their contemporary tills nearshore but occur well beyond the limits of these tills offshore. The origins of these valleys are attributed to a combination of fluvial, subglacial and periglacial processes

Geometrical control of subduction-related magmatism: the Mesozoic and Cenozoic plutonic history of Western Colombia

Abstract: Radiometric age data from Western Colombia, combined with geological evidence, allow five main plutonic episodes, ranging in age from Triassic to Tertiary, to be recognized. It is suggested that variation in the convergence angle of the oceanic plate relative to the continental edge was an important factor affecting the timing and spatial distribution of plutonic activity. On a regional scale, major breaks in activity are probably best attributed to either low-angle/parallel convergence or periods of accretion along the convergent margin.

Dispersal, concentration and deposition of suspended matter in the North Sea

Abstract: Suspended matter comes into the North Sea from the Atlantic Ocean, from the Channel, rivers, seafloor erosion, coastal erosion, the atmosphere, and from primary production. The contribution of the last source is temporarily large when phytoplankton growth is abundant, but is soon decomposed, mineralized or consumed so that over the year the net effect is small. Accumulation and deposition of suspended matter occurs predominantly in only a few areas (the Waddensea, the German Bight, the Skagerrak/Norwegian Channel). The remainder of the North Sea floor consists of Pleistocene and Early Holocene relict deposits and of reworked sands and gravels. The mechanisms resulting in accumulation and deposition are discussed. There are strong indications that the amount of suspended matter flowing out from the North Sea into the Norwegian Sea along the Norwegian coast is only little more, or the same, as the amount coming in from the North Atlantic, although its composition may be different.

The tectonic history of Kohistan and its implications for Himalayan structure

Abstract: The tectonic history of Kohistan, northern Pakistan, involves two collisional events. Cleavage and folding developed at 90-100 Ma along the northern suture between the Kohistan island arc and the Asian plate. At the same time there was major folding and shearing of the lower part of the Kohistan arc, approximately 100 km south of the suture. This deformation was followed by ocean subduction south of the Kohistan arc, generating the Kohistan calc-alkaline batholith, with subsequent ocean closure during the Eocene and obduction of the Kohistan arc, together with the adjacent part of the Asian plate, over the Indian continental crust. The construction of balanced cross-sections through the imbricated upper part of the Indian continental crust, in the footwall to this southern suture indicates a minimum displacement of 470 km, requiring the western Himalayan hinterland to be underlain by a large wedge of Indian middle to lower crust. There is some shortening of the overriding Kohistan and Asian plates by thrusts and shear zones, but it is insufficient to satisfy the palaeomagnetic data; there must be major crustal shortening, involving thrusts, in the Hindu Kush and Pamirs north of Kohistan. The post-Eocene thrust direction, which for most of Pakistan is towards 160°, is almost perpendicular to that immediately to the east in the Himalayan belt, generating complex refolded thrust patterns in the Hazara syntaxis and large scale folding and rapid uplift with associated brittle faulting and seismic activity adjacent to the Nanga Parbat syntaxis. These different thrust trends indicate that major thrust movement as well as the folds and deformation fabrics, cannot always be related to plate movement vectors, but are modified by, or develop from, complex rotations during place collision or from the gravitational spreading of a thickened crust. A regional approach is required to recognize and correctly attribute the various components in thrust displacements.

A sequential back-arc and foreland basin thrust duplex model for the Southern Uplands of Scotland

Abstract: Thrust imbrication of Ordovician and Silurian submarine fan sequences overlapping pelagic deposits in the Southern Uplands has been interpreted in terms of an accretionary prism formed above a NW-directed subduction zone. Structural features invoked to support accretion are not definitive and could be explained in terms of a thin-skinned thrusting model. New palaeocurrent and compositional evidence from Llandeilo to mid-Llandovery age turbidites in the northern part of the Southern Uplands proves interdigitation of sediments with strongly contrasting petrography. Turbidites derived from the south contain significant quantities of fresh andesitic detritus whereas those from the north form more mature quartz-rich formations. This implies a back-arc situation; the turbidites being deposited in a basin with a relatively mature continental landmass to the north and a rifted continental fragment containing an active volcanic arc to the south. Oblique collision of the opposing continental margins of the Iapetus Ocean during the Llandovery caused the cessation of subduction. Underthrusting of the southern margin initiated a SE-propagating thrust stack which deformed the back-arc basin sequence and may eventually have ramped over the eroded and faulted remains of the volcanic arc. A southward-migrating foreland basin formed ahead of the rising thrust stack and is now represented by the late Llandovery Hawick Group and Wenlock sequences. Mid- to end-Silurian sinistral strike-slip resulted from oblique collision and produced a transpressional regime during which reactivation of deep-seated structures allowed the intrusion of lamprophyre dykes and granites.

Age of Feiran basement rocks, Sinai: implications for late Precambrian crustal evolution in the northern Arabian–Nubian Shield

Abstract: Basement exposed on the perimeter of the Red Sea was created during the Pan-African event at the end of the Precambrian. Pre-Pan-African crust in the northern part of this region has not yet been identified. This paper reports the results of Rb–Sr whole-rock and U–Pb zircon dating of gneisses and related basement units from the Wadi Feiran area in the Sinai peninsula, where the existence of such older basement has previously been suggested. A post-tectonic extensional dyke gives a Rb–Sr age of 591 ± 9 Ma with an initial 87 Sr/ 86 Sr ratio of 0.7034 ± 0.0002. Rb–Sr whole-rock and thin slab dating of paragneisses gives ages of c . 610 Ma with an initial 87 Sr/ 86 Sr ratio of 0.7035. A U–Pb zircon age of 632 ± 3 Ma is interpreted as either the time of formation of these gneisses or the age of the crust sampled by protolith sediments. Granodiorite to the east gives a U–Pb zircon age of 782 ± 7 Ma and is interpreted as representing the westernmost extent of a 780 ± 50 Ma terrane that extends across Sinai into Jordan. Uplift and erosion of the 780 ± 50 Ma terrane supplied detritus to flanking terranes in N and SE Sinai. This region thus acted as a foreland to the younger accretionary and extensional units to the south and west that were active later in the Pan-African event. There is still no evidence for pre-Pan-African basement in the Precambrian units around the northern Red Sea east of the Nile.

Geology, geochronology and geodynamic implications of the Cenozoic magmatic province in W and SE Ethiopia

Abstract: New K-Ar dates are presented for areas in W and SE Ethiopia. In the west, the dates distinguish the Geba Basalts of 40 to 32 Ma (previously determined by Merla et al. 1979) from the Welega Shield Volcanics which are shown to range from 11.2 ± 2.2 to 7.8 ± 1.6 Ma. In SE Ethiopia, the Lower Stratoid flood basalts range from 30 ± 4.5 to 23.5 ± 4.5 Ma and are unconformably overlain by the Reira-Sanete shield volcanics which range from c. 15 to c. 2 Ma. The unconformity is marked by a palaeosol as are several of the intervals between the major volcanic stages of Ethiopia. Using new field observations, together with previously published results and unpublished data from the Ethiopian Institute of Geological Surveys, it is suggested that the Ethiopian Flood Basalts were erupted in three major stages. Stage 1, which is mainly older than 40 Ma is separated from Stage 2, 34 to 30 Ma for NW Ethiopia and 40 to 30 Ma for SW Ethiopia, by erosional unconformities. Stage 3 spans 30 to 26 Ma in NW Ethiopia, and 30 to 21 Ma in SW Ethiopia and both are marked by the incoming of silicic volcanism. In W Ethiopia, Stages 1 and 3 are not developed, whilst in SE Ethiopia the Tertiary volcanism commences with Stage 3 flood basalts. The overlying shield volcanics; (25 to 13 Ma in NW Ethiopia and 15 to 7 Ma in W Ethiopia) represent a localized terminal episode built on the Plateau and are considered a fourth stage. The earliest volcanism is restricted to two areas: in SW Ethiopia, where the Akobo Basalts give ages as old as 49.4 Ma (Davidson & Rex 1980), and in NW Ethiopia where the Ashange Basalts underlie the Aiba Basalts which are dated at 34 to 30 Ma. The Ashange volcanism may have begun in the Palaeocene—Eocene but radiometric dates are ambiguous and inconsistent. These two volcanic centres show lithological and geochemical differences, and can be related to two separate rift zones north of Lake Turkana and further north in the Tana Graben. Volcanism appears to have spread by the expansion of these areas, and to have migrated eastwards with time towards the Main Ethiopian Rift which begun to form about 14 Ma ago. There is a complex pattern of domal uplifts within these two volcanic centres with volcanism either preceding doming, or the two being contemporaneous.

The compaction of igneous and sedimentary rocks

Abstract: In many problems of interest to geology and soil mechanics a fluid moves through a solid matrix which is also being deformed. In most cases of interest the effective viscosity of the matrix is many orders of magnitude greater than that of the percolating fluid, and both form interconnected networks in three dimensions. The partial differential equations governing such behaviour can now be obtained and a variety of simple model problems have been investigated. Some of the more surprising solutions contain solitary waves and compaction fronts. Some of the less exotic solutions are presently of more geological interest. The mobility of volatile rich melts at melt fractions as small as 0.1% has important consequences for trace element geochemistry. The geometry of layered intrusives appears to result from differential compaction of the crystal mush, and the rates required can be used to estimate a viscosity of 3 x 10 18 Pa s for an olivine matrix. This value agrees excellently with laboratory experiments when proper account is taken of the grain size dependence. Compaction in sedimentary rocks results in overpressure when the porosity becomes sufficiently small, and can also lead to the development of secondary porosity.

The Rb-Sr geochronological evolution of the Ras Gharib segment of the northern Nubian Shield

Abstract: Rb-Sr whole-rock isochron ages have been determined for six igneous rock suites from the Ras Gharib. These represent all major crustal components of the northern Nubian Shield and are, from oldest to youngest: diorite-tonalite, extrusive rocks, granodiorite-adamellite and leucogranite, muscovite trondhjemite (orogenic Pan-African suites), dyke swarms and anorogenic peralkaline granites. The ages obtained are: 881 ± 58 Ma, 620 ± 16 Ma, 552 ± 7 Ma, 516 ± 7 Ma, 493 ± 7 Ma and 476 ± 2 Ma. The initial 87 Sr/ 86 Sr ratios of the Pan-African suites and the dykes are in the range 0.7039 to 0.7047. The initial ratio of the anorogenic peralkaline granites is 0.7110 ±0.0012, higher than any previously reported for igneous rocks from the Egyptian belt. The 493 ± 7 Ma age of the dyke swarms represents a younger limit for the Pan-African compression–accretion event and an upper (older) limit for an extensional tectonic regime. Linked with this is the anorogenic peralkaline granite magmatism (476 ±2 Ma), which is the last manifestation of igneous activity in this region. The low initial ratios of the Pan-African suites provide no support for the existence of an older (Archaen) sialic continental crust in the region. The anorogenic peralkaline granite with the high initial ratio was produced by partial melting of earlier Pan-African suites. The age data are used to construct a tentative correlation scheme for shield rocks of Egypt, Sudan and Saudi Arabia.

Late-Archaean tectonics in the Færingehavn–Tre Brødre area, south of Buksefjorden, southern West Greenland

Abstract: The Faeringehavn–Tre Brodre area consists of three distinct terranes tectonically jux-taposed by a previously unrecognized event. Contacts between the terranes are mylonitic shear zones truncating lithological units in adjacent terranes. The terrenes are: (1) Faeringehavn terrane, largely composed of early-Archaean Amitsoq gneiss cut by younger Archaean granitic gneiss defined here as the Satut gneiss; (2) Tre Brodre terrane comprising mid-Archaean Malene supracrustal rocks, anorthosite complex and polyphase Nuk gneisses; (3) Tasiusarsuaq terrane largely comprising mid-Archaean Nuk gneisses affected by c. 2800 Ma granulite facies metamorphism. The Tasiusarsuaq terrane is structurally above both the Faeringehavn and Tre Brodre terranes which at c. 2800 Ma experienced a lower grade of metamorphism. Juxtaposition of the terranes took place between 2800 and 2500 Ma and involved thrusting and crustal thickening. Subsequent re-equilibration involved folding, steeply inclined shear belts, intrusion of synkinematic granitoids under amphibolite facies conditions and retrogression of granulite facies assemblages. This thrusting post-dates the 2800 Ma granulite facies metamorphism. It is younger and distinct from the thrusting postulated to explain the intercalation of the early-Archaean Amitsoq gneisses and the mid-Archaean Malene supracrustal rocks, associated with intrusion of the Nuk gneiss precursors described from Godthabsfjord. The tectonic breaks provide, for the first time, marker horizons which can be used to assess the amount and type of late Archaean deformation in the southern Godthabsfjord region.

Tectono-sedimentary evolution of the Bowland Basin, N England, during the Dinantian

Abstract: The Bowland Basin is a NE–SW-trending sedimentary basin, lying between the Askrigg–Bowland High to the N and NW and the Central Pennine High to the SE. The basin was initiated in ? late Devonian–Tournaisian times. Sediment thickness and facies variations within the Dinantian succession show marked asymmetry on both a basin-wide and intra-basin scale, as a result of approximate NW–SE to N–S extension across normal faults. Thickening of the sedimentary sequence from NE to SW over much of the basin resulted from fault-controlled extension along the SE margin of the basin with downthrow to the NW, giving the basin a half-graben character. Extension occurred in two distinct episodes, marked by basin-wide debris flow and sedimentary slide/slump deposits. During the late Chadian/early Arundian the simple half-graben became modified by a series of NE–SW-trending antithetic and NW–SE-trending transfer faults associated with local unconformities and marked lateral thickness and facies variations. This interval marks the transition from carbonate ramp to slope sedimentation and a temporary halt in carbonate supply to the basin. The second episode of extension, the late Asbian/early Brigantian, marks the break up of the northern basin margin, ending major carbonate deposition within the basin. Reactivation of these structures at the end of the Carboniferous created the Ribblesdale Fold Belt as a result of NW–SE to N–S compression/transpression. Application of this research to areas surrounding the Bowland Basin has led to the recognition of additional shelf basins, the orientation of which suggests control by normal and transfer fault systems. Discussion and

The Southern Uplands as an accretionary prism: the importance of analogues in reconstructing palaeogeography

Abstract: Fore-arc regions, whether accretionary, erosional or composite, vary greatly both along strike and through time, in the geometries which they assume in response to subduction. There is no well-studied modern/Neogene margin which can be established as the 9type9 accretionary prism. All are individual in some respect or other. Nor is there an example of a well-studied modern/Neogene accretionary margin which can be compared closely to the Southern Uplands, though there are several good analogues for particular stages of its history. Accretion took place along the Southern Uplands margin for c. 50 million years. The range of geometries on modern/Neogene accretionary active margins, and the extent to which the behaviour of those margins can change during periods as long as 50 million years, is reviewed, emphasizing the accretionary prisms of Mexico, SW Japan and the Makran. The interpretation of the Southern Uplands as an accretionary prism, as presented in the late 1970s and early 1980s, is reassessed in the light of advances made subsequently in understanding modern/Neogene prisms. The recent alternative model of Stone et al. for the Southern Uplands envisages that the terrane evolved by the opening and closure (by arc-subduction) of a marginal basin. The new model draws heavily on arc-type detritus derived from the oceanward side of the ancient Southern Uplands trough. There are areas along the SW Japan fore-arc where oceanward-derived tubidites are accumulating in a trench (Nankai Trough), and where active submarine arc-type volcanism is occurring outboard of a currently growing accretionary prism. Such features in an ancient terrane like the Southern Uplands do not necessarily mean that an alternative model must be sought involving whole new arc systems. Furthermore, analysis of the life-spans and sedimentary fills of modern/Neogene marginal basins begs the conclusion that the Southern Uplands9 strata were not deposited in a marginal basin. It is most unlikely that the c. 50 Ma history of pelagic sedimentation preserved there can be manifesting anything other than the closure of a sizeable ocean basin, of a scale exceeding any known western Pacific-type marginal basin, and with a different sedimentation history from any such marginal basin.

Fine sediment sources and sinks around the East Anglian Coast (UK)

Abstract: The cliffs of Norfolk and Suffolk yield an average of 785 000 tonnes (t) of fine sediments to the coastal zone annually. East Anglian rivers yield very little (≈5500 t a −1 ) but those of the Wash produce about 100 000 t a 1 . By contrast the region is dominated by 1.4 × l0 6 ta −1 input from cliff erosion in Holderness. Accumulation in vertical accretion alone of the coastal marshes of East Anglia, the Wash, north Lincolnshire and the Humber requires 1.12 × 10 6 t a −1 of fines. Progradation around the Wash probably accounts for another 1.1 × 10 6 t a −1 . The East Anglian area (including the Wash) is thus sufficient to trap over half the regional mud input from Flamborough Head to the north side of the Thames estuary, and could take it all, depending on assumptions involved in calculation.

The Structure of the West Orkney Basin, northern Scotland

Abstract: The West Orkney Basin developed in Devonian times, as the western part of the Orcadian intermontane basin. It has been studied using commercial speculative seismic reflection data and the MOIST deep seismic data. The NW edge of the West Orkney Basin is formed by listric faults which are also strongly arcuate in plan, while the SE part is composed of straight domino-type faults which formed parallel to earlier (Caledonian) layering in the basement. Fault restoration and balancing suggest that initial extension in the basin occurred on low-angle reactivated Caledonian thrust faults. Steeper breaching faults cut the low-angle set, forming planar (domino-type) faults in the centre of the basin but listric faults at the NW margin. The maximum extension is about 45% in the basin centre, most of this being taken up on the later breaching fault system. This extension decreases to the SW, where fault tips occur on-shore, but some may transfer to fault systems in the Minches. The faults apparently decouple at a depth of 18-20 km and the extension suggests an initial post-Caledonian crustal thickness of up to 40 km. However, the sedimentary thickness is an average of only 3 km in the basin centre, much less than would be expected had the lithosphere thinned homogeneously, and there is no evidence of a thermal subsidence phase to the basin. This suggests that the extension shown by the West Orkney Basin was transferred to lower lithospheric levels to the east along the deep decoupling zone. The Devonian sediments on-land show facies changes and periods of uplift and erosion which may be related to extension during basin development. They also show a phase of pre-Late-Permian tectonic inversion where the beds are locally folded and thrusted, probably related to the Hercynian events further south. The West Orkney Basin is capped by Mesozoic sediments and was probably reactivated during Mesozoic basin development in the Minches and Moray Firth. The shape of the faults, their orientation and decoupling levels are strongly controlled by the earlier Caledonian structure, in particular by the layering and crustal anisotropy developed along and above the Moine thrust. The West Orkney Basin with its 20 km deep decoupling level formed by extension of Caledonian thickened crust. It is notable that the major basin-bounding faults to the NW, the Outer Isles and Flannan faults, which developed where the crust was thinner and hence less ductile at depth, decoupled at much deeper structural levels, at the Moho or below.

The Silurian of the Southern Uplands and Ireland as a successor basin to the end-Ordovician closure of Iapetus

Abstract: A suite of microconglomerates is recognized in Silurian rocks which occur on both sides of the proposed line of the Iapetus suture in Ireland. Clast composition and palaeocurrent data show that these conglomerates, which grade into the typical quartz-rich Silurian turbidites, were derived from two compositionally similar magmatic arc terranes which lay on either side of the present Silurian outcrop. In the Llandovery, derivation was from both the south and the north. In the Wenlock, derivation was from the north and sedimentation prograded southwards across the ‘suture’ and onto the southern margin. The source terrain in the south was probably the Ordovician Wexford–Lake District arc. We identify the northern source as another arc (Cockburnland) which has since been cut out by sinistral strike-slip against the Ordovician Northern Belt. These data imply that arc activity ceased synchronously on either side of Iapetus during the late Ordovician and this leads us to speculate that subduction of oceanic crust ended at that time. Closure was associated with deformation and uplift of the bounding Ordovician terrains. These rocks then contributed detritus to the Silurian infill of a successor basin. Regional sinistral transpression finally deformed and reorganized these units between the end Silurian and the early Devonian and led to the complete closure of the remaining Silurian seaway.

Rapid deposition of sand and gravel mixtures downstream of a negative step: the role of matrix-infilling and particle-overpassing in the process of bar-front accretion

Abstract: An experimental flow-channel was used to investigate the rapid deposition of sand and gravel mixtures downstream of a negative step. Hydraulic processes give rise to the quasi-simultaneous accumulation of openwork gravels, matrix-filled contact framework and dilated framework gravels. The resultant structure is analogous to a bar-front in gravel-bed streams. Flow conditions were non-uniform but steady in time, whereas open and closed gravel fabrics are usually interpreted as representing deposition from unsteady flows—for example a varying discharge regime. Consideration of the percentage of sand in the deposits indicated that, in terms of packing, the sediments may be considered as a simple binary mixture with partial framework dilation occurring with sand percentages in excess of c. 32%. Despite dilation, for sand percentages less than 50%, structural stability of the foreset gravels was maintained so that the bar-front exhibited a distinct brinkpoint, and a planar slip-face (at the angle of initial yield) which made an angular contact with the base of the flume. For sand percentages in excess of 80%, individual gravel clasts were completely supported by sand. The original structural stability was lost so that the bar-front, eroded by reverse currents, assumed a tangential contact with the flume base below the angle of initial yield. The brinkpoint was rounded and indistinct. In addition to altering the bar-front slope, increasing the quantity of sand in the bed sediments had a major influence on the dip of gravel particles on the foresets. In all experimental runs a surface sand-seal (with openwork gravels beneath) developed upstream of the brinkpoint. The mechanics of sand-seal formation require further investigation. However, an overpassing mechanism was noted to be very important in sorting gravel particles rolling over the brinkpoint. A published mathematical model describing the overpassing mechanism was found to be appropriate to the present experimental observations.

Sedimentary and botanical factors influencing peat accumulation in the Mississippi Delta

Abstract: The Holocene Mississippi Delta System consists of six cyclic deltaic packages. Associated peat deposits are planar and eutrophic, and accumulate in areas abandoned by deltaic sedimentation. Peats that formed in large-scale interlobe basins are the subject of this paper. True peats (>75% organic matter) average 81.7% organic matter on a dry-weight basis. Controls on quality and lateral distribution of peat deposits are: botanical parent material, the balance between subsidence and accretion rates, detrital clastic influx and marine inundation. Botanical parent material, inferred from 13 C signatures and microtome sections, is always freshwater, originating either in forested swamps, or under herbaceous (floating) mats. Sphagnum spp. is essentially absent. In order for peat to accumulate, rates of accretion and subsidence must balance. Long-term subsidence rates in the study area increase from about 0.10 to 0.30 cm a -1 over a distance of 40 km from upper to lower delta plain. Subsurface accretion rates for true peats at a depth of 2-3 m, however, average about 0.05 cm a -1 . Renewed coarse detrital clastic influx terminates peat accumulation. The largest quantities of true peat are found together with relatively large amounts of organic-poor clastic sediments (mostly clays). Delta lobe abandonment and consequent coastal erosion causes salt water intrusion into freshwater environments. Resulting saline marsh sediments have maximum organic matter contents of 35%. In a transgressive setting, these may overlie older high-quality (low-ash) freshwater peats. Leaching experiments reveal the presence of water-soluble salts in both transgressed and fresh peats. Leaching reduces ash values by 1/5 to 1/3, suggesting that such losses may occur during early diagenesis. It is possible that some commercial coal seams originated in analogous interlobe environments, where leaching and possibly silica mobilization reduced the originally high ash contents to acceptable levels.

The Northern Belt of the Longford-Down Inlier, Ireland and Southern Uplands, Scotland: an Ordovician back-arc basin

Abstract: The Northern Belt of the Southern Uplands/Longford-Down zone contains two compositionally distinct clastic petrofacies suites each ranging from mid- to late Ordovician age. They extend throughout the length of the belt and are generally separated by a strike fault. The northwestern Metaclast Petrofacies suite is characterized by felsic igneous and metamorphic detritus derived from a continental source to the NW, whereas the southeastern Basic Clast Petrofacies suite contains abundant calc-alkaline detritus derived from an island arc and plutonic/metamorphic root zone complex to the S or SE. A similar source, but located to the north, is indicated for younger clastic sequences of similar composition in the northern part of the Central Belt immediately south of a major sinistral shear zone which defines the boundary between the Northern and Central Belts. It is concluded, in light of other facets of regional geology and possible palaeogeographic configurations, that this pattern reflects a back-arc basin in the Northern Belt, margined by a volcanic arc massif which once occupied the space now represented by the shear zone, and a fore-arc basin to the SE (Central and Southern Belts). The Northern Belt back-arc basin is postulated to have been closed and deformed at the end Ordovician and subsequently overthrust northwestward by the allochthonous, thrust stack imbricated Central and Southern Belts fore-arc sequence at the end Silurian. This, in the Northern Belt, resulted in the development of a major recumbent structure, downward facing on regional (end Silurian) cleavage.

Age and origin of the New Zealand Orocline in relation to Alpine Fault movement

Abstract: The Alpine Fault, and the recurved arc structures (orocline) in the late Palaeozoic-Mesozoic Rangitata Orogen, are two of the first-order structures of the New Zealand subcontinent. A commonly held view is that the orocline originated as a gigantic drag fold with Alpine Fault movement. In view of the post-Oligocene age of the Alpine Fault, however, this concept, involving a single recurved arc, cannot reconcile the two published sets of palaeomagnetic data on tectonic rotations within the Rangitata Orogen and its cover rocks. These data indicate at least 62° of dextral rotation during each of the Mesozoic Rangitata Orogeny and the late Cenozoic Kaikoura Orogeny. A new model presented here postulates that there are actually two dextral recurved arcs. A Western Arc, which lies mainly outside the Australia–Pacific plate boundary zone, has a multiple origin having formed chiefly during the late Triassic phase of the Rangitata Orogeny by the subduction accretion of the exotic Rakaia terrane. An Eastern Arc coincides with the area of the present plate boundary zone. It formed as a mega, brittle–ductile, shear zone with the late Cenozoic development of the modern Australia–Pacific plate boundary, which includes the Alpine Fault. This solves the problem of the nature and location, north of the Alpine Fault, of the relative plate movement evident as 480 km of displacement on the Alpine Fault. A major implication of this model for plate tectonic reconstruc­tions of the SW Pacific is that only approximately 500 km of dextral displacement has occurred through New Zealand during the late Cretaceous–Cenozoic compared with the commonly adopted figure of 1000 km.

Syn-orogenic alkaline magmatism and its relationship to the Moine Thrust Zone and the thermal state of the Lithosphere in NW Scotland

Abstract: The alkaline complexes of NW Scotland lie in a zone sub-parallel to the Moine thrust and represent the NW edge of the otherwise overwhelmingly calc-alkaline late-orogenic magmatism of the Caledonian fold-belt. The emplacement of intrusives in Assynt overlapped the thrust movements and new U-Pb zircon ages for the Loch Ailsh intrusion (439 ± 4 Ma) provide are liable bracket, when combined with existing age data for the Loch Borralan intrusion (430 ± 4 Ma), for the main movements on the Ben More thrust plane. A detailed history of the main tectonic and igneous events in the Moine Thrust Zone is tabulated. Zircons from the undeformed Cnoc-nan-Cuilean syenite, emplaced in Moine rocks near Loch Loyal, give an age of 426 ± 9 Ma, in contrast to the deformed Glen Dessary syenite for which earlier work gave 456 ± 5 Ma. The linear distribution of the alkaline complexes to the west of the calc-alkaline plutons, and the ultra-potassic character of the Loch Borralan complex (the most westerly), are consistent with, but do not prove, generation related to a W-dipping subduction zone. The persistence of the magmatism, over 30 Ma, in a belt being shortened by thrusting, suggests a source in the mantle lithosphere. The absence of magmatism further to the NW and the production of calc-alkaline magmas to the SE of this zone, is compatible with a model in which the thermal state of the lithosphere exercises the most important control on the nature of Late Caledonian magmatism.

Coal deposition in carbonate-rich shallow lacustrine systems: the Calaf and Mequinenza sequences (Oligocene, eastern Ebro Basin, NE Spain)

Abstract: Two main coal-bearing sequences developed during the Oligocene in the Tertiary Ebro Basin, the Calaf (early Oligocene) and Mequinenza (late Oligocene) coal basins. Coal deposition took place in shallow marsh-swamp-lake complexes which sometimes became closed and evolved under warm climatic conditions with fluctuating humidity. These shallow lacustrine systems are closely interrelated with the terminal parts of the distributive fluvial systems which spread from the tectonically active Ebro basin margins. Laterally extensive lignite-bearing sequences, including rather thin, lenticular autochthonous and/or hypautochthonous coal seams with high ash and sulphur contents, characterized coal deposition in the shallow lacustrine systems. Coal seam geometry, which makes them nearly subeconomic, resulted from the tectonic instability during basin margin evolution and the sometimes closed, arid conditions under which the lacustrine systems evolved. High ash and sulphur contents resulted from the inadequate isolation of peat forming environments from clastic influx and from the very low acidity and sometimes high sulphate contents of the lacustrine waters. Coal exploration in shallow lacustrine sequences similar to those described here must take into account that the spread of coal-forming environments and maxima of coal deposition are usually coincident with lake expansions and retraction or shifting of the terminal fluvial zones interrelated with the lacustrine areas.

The provenance of granite boulders in conglomerates of the Northern and Central Belts of the Southern Uplands of Scotland

Abstract: Rb–Sr whole rock isochrons from several suites of granite boulders which occur in Ordovician and Silurian conglomerates in the Southern Uplands of Scotland yield ages of around 1200 Ma, 600 to 700 Ma and 470 to 490 Ma. The plutonic and tectonic history recorded by these clasts corresponds more closely with that of NW Newfoundland than with that of Scotland. The petrography of the clasts can also be matched with exposed granite plutons in Newfoundland. Since the conglomerates were derived from the NW it is concluded that the Southern Uplands lay to the SE of Newfoundland. A total of 1500 km strike-slip displacement occurred between the Caradoc and the Early Devonian, consistent with oblique subduction of the Iapetus Ocean below the North American continental margin.

Sediment exchanges between the Seine estuary and its adjacent shelf

Abstract: The study of sediment dynamics within the Seine estuary and its adjacent shelf permits some conclusions regarding sediment exchanges in this coastal zone. On the shelf, sediment transport is directed toward the Seine estuary and contributes to the infilling of the estuary by marine sands and muds; these were originally derived from land carried to the shelf at times of lower sea level, and subsequently transported shoreward by dominant tidal and wave-induced currents. In the estuarine section, tidal processes form a turbidity maximum both by inducing strong bed resuspension and by trapping fluvial and marine sediments due to flood current predominance. Occasionally this turbid zone extends seaward during periods of river floods, and sediment is redispersed onto the shelf. This phenomenon has been amplified by man-made modifications of the estuarine morphology. However, currents patterns tend to retard the natural seaward escape of fluvial material, mainly due to dredging operations at the present time. Thus, man has changed the natural role of the estuary from a sink to a source of fluvial sediments for the shelf. A tentative sedimentary budget is proposed, taking into account the various sources of material and the complexity of sediment dynamics.

The age of blueschist metamorphism in Anglesey, North Wales: evidence from 40Ar/39Ar mineral dates of the Penmynydd schists

Abstract: The 9Penmynydd9 schists of SE Anglesey include several high P /low T lithologies including: (1) metasedimentary quartz–phengite schist; and (2) mafic blueschist with barroisite/crossite assemblages that appear to have developed from an earlier actinolitic greenschist facies protolith. One phengite and three amphibole concentrates have been analysed by incremental-release 40 Ar/ 39 Ar dating methods and display discordant age spectra with anomalously young apparent ages recorded in low T increments. Ages increase systematically throughout intermediate T portions of the analyses to define high T plateaux of c . 550–560 Ma (barroisite/crossite-rich concentrates) and c. 580–590 Ma (actinolite-rich concentrate). The phengite concentrate also displays an internally discordant spectrum with intermediate T increments defining ages similar to those recorded by barroisite/crossite and high T increments giving ages similar to those of actinolite. The amphibole and phengite ages are interpreted as dating post-metamorphic cooling following a regional low- P Ml greenschist metamorphism ( c . 580–590 Ma) and a high- P M2 metamorphism ( c . 550–560 Ma). 40 Ar/ 39 Ar ages suggest multiple metamorphism of oceanic crust before the arrival of the blueschist terrane and its accretion to the late Precambrian basement of southern Britain immediately prior to development of the Welsh Basin. An age of c. 550–560 Ma for blueschist metamorphism on Anglesey provides a likely maximum age for ignimbrites within the pre-late Lower Cambrian Arfon Group exposed in NW Wales.

Constraints on the age of movement of the Montsech and Cotiella Thrusts, south central Pyrenees, Spain

Abstract: Palaeocene to Oligocene thrusting in the Pyrenees was synchronous with the development of the South Pyrenean foreland basin (collectively the Tremp–Graus, Ainsa and Jaca basins). Sedimentological and stratigraphical evidence from the foreland basin is used to constrain the age of movement of the Montsech, Cotiella and External Sierras thrusts. Movement on the emergent imbricate fan at the Montsech front began in the Early Eocene (Illerdian) during an early phase of thrusting that detached in the Triassic evaporites in that region. This phase of thrusting defined the Tremp–Graus basin as a thrust-sheet-top basin. Following the early thrusting, Cuisian age tectonics in the Tremp–Graus and Ainsa basins was dominated by minor normal fault movements during flexural subsidence associated with thrust sheet loading. Growth of the Boltana anticline to the west of the Ainsa basin indicates that the Ainsa basin had become detached as a thrust-sheet-top basin in the Lutetian. Movement on the Pena Montanesa thrust, and by inference on the Cotiella thrust, began in post-Lutetian times during the propagation of lower thrusts that involved pre-Triassic (Variscan) basement. Thrusting in the External Sierras caused folding and faulting of Oligocene sediments, indicating the continued southward propagation of basement-involved thrusts in the Upper Eocene and Oligocene. Many faults in the Southern Pyrenees have evidence of early normal and later reverse senses of movement indicating reactivation of normal faults during thrusting.

Relationships between chemical and convective layering in the Earth William Smith Lecture

Abstract: This review principally addresses the question of convective isolation is the Earth9s mantle as deduced from chemical and isotopic data. In particular it is shown that the isolation is imperfect and the interactions that occur account for a variety of geochemical phenomena.

Improved discrimination of rock units using Landsat Thematic Mapper imagery of the Oman ophiolite

Abstract: The short wavelength infrared channels recorded by the Landsat Thematic Mapper make this instrument more suitable than the Landsat Multispectral Scanner for lithological mapping using enhanced imagery. The weathering mineralogy of igneous rocks causes absorption features in their reflectance spectra which are particularly characteristic in the near infrared. A Landsat Thematic Mapper band 7,5,4 red, green, blue false colour composite of part of the Oman ophiolite confirms much of the previous mapping and indicates certain hitherto unrecorded features. These include a new mafic-ultramafic late-intrusive complex and differences between deeper and near-crustal levels in the mantle sequence, which may be related to the environment in which serpentinization took place. Decorrelation stretching is an excellent means of accentuating colour variations within an image as an aid to visual interpretation.