Formation Of Wedge Research Articles

Composition, age, and depositional rates of shoreface deposits under barriers and beach plains of the Columbia River littoral cell, USA

The Columbia River littoral cell (CRLC) consists of four subcells (totaling 160 km in length) that are unique in the West Coast of the United States, in that they contain prograded barriers and beach plains, reaching 0.5–3 km in width (Fig. 1). The prograded beach deposits (1–5 ka in age) overlie shoreface deposits (1–8 ka in age), as identified in 18 ground penetrating radar profiles, and sampled from 24 boreholes. Two competing hypotheses were initially proposed to account for the origins of these unique, progradative shorelines: (1) cross-shore feeding by onshore wave transport of pre-Holocene sand from the submerged shelf, and (2) longshore dispersal of nearshore sand that was supplied to the littoral system by bedload sediment discharge from the Columbia River during the Holocene. The CRLC sand forming the shoreface deposits is fine (diameter 0.2 ± 0.02 mm) and rich in lithic fragments (20–40% by volume). Gravel and shell lag layers are uncommon in most of the CRLC shoreface deposits, but they show greater abundance locally near ravinement surfaces, tidal inlets, and in the Clatsop subcell, located south of the Columbia River mouth. Gravel and granule layers increase upsection in barriers south of the Columbia River and downsection in barriers at the northern end of the littoral system. These trends suggest different mechanisms of shoreface sediment feeding within the four subcells. However, borehole samples from all four subcells show the same sand provenance, i.e., post-glacial Columbia River sand, which is identified by high ratios of hypersthene:augite in heavy-mineral fractions. Selected shoreface sections were dated (0.5–8 ka) by AMS radiocarbon analysis of articulated-shell and wood fragments recovered from auger flights (3–22 m depth subsurface). Relatively young shoreface deposition (2.5 ka at − 6.5 m elevation NGVD88) in the Clatsop subcell south of the Columbia River shows a net-southward beach transport that fed shoreface and beachface progradation into deeper water. Older and deeper shoreface deposition (4.4 ka at − 7.1 m elevation) in the Long Beach subcell north of the Columbia River was a result of the filling of innermost-shelf accommodation space prior to beachface progradation. The total volume of shoreface sand deposited under the barrier spits and beach plains of the CRLC is estimated to be 6–7 km 3 deposited since 6–8 ka. There was a net-northward transport of littoral sand (∼ 1 × 10 6 m 3 year− 1) along the nearshore and inner-shelf; subsequently some of this sand was transported onshore to feed beaches of the northernmost subcells. Columbia River sand was also the source for the formation of the offshore shelf wedge above the transgressive ravinement surface, and for the filling of major tidal basins located north of the Columbia River. In summary, the unique progradational history of the CRLC barriers and beach plains derives from the combination of (1) longshore dispersal of fine sand discharged from the Columbia River during Holocene time, and (2) across-shore feeding of beaches at the northern end of the littoral system from fine sand carried north along the nearshore and the inner-shelf.

Various Perspectives on Accretionary Wedge Formation and Related Phenomena

The present situation of studies on accretionary wedge formation and related phenomena is briefly summarized from various perspectives, ranging from theories, model experiments, observations on land and submarine, exhumation of high-pressure metamorphic rocks, fluid seepage, stress field, and asperity. Future perspectives are also considered from such recent results with potential areas of study. Gravity acts ubiquitously—everywhere and at all times—on the Earth's materials, so the role of gravity is also accounted for in the wedge development.

Inner shelf paleoenvironmental evolution as a function of land–ocean interactions in the vicinity of the Guadiana River, SW Iberia

This study investigates the land–ocean interactions along the northern margin of the Gulf of Cadiz in the vicinity of the Guadiana River. Benthic foraminifera and sedimentological characteristics were analysed in a sedimentary sequence spanning ca. 5000 years (core 8, 22 m water depth) retrieved from the inner shelf prodeltaic wedge. The analyses were integrated in a temporal framework based on accelerated mass spectrometry radiocarbon dating. Paleoenvironmental changes and sediment transfer mechanisms from the continent to the shelf were investigated and related to climatic oscillations and anthropogenic impact in the region. The results allowed the identification of two main periods of deposition. The first period, from ca. 5150 cal. BP (core base) to 1500–1200 cal. BP, is characterised by a mix of fine and coarse sediments, relatively constant percentages of terrigenous and bioclasts, and benthic foraminifera species characteristic of coastal environments. These features indicate an environment strongly influenced by discharge from the Guadiana River. The second period, from ca. 1500–1200 cal. BP to ca. 200 cal. BP (core top), is characterised by silt-clay fraction dominated sediments, an increase in terrigenous sediment towards the top, and benthic foraminifera species characteristic of environments with low levels of energy. An increase in the level of human occupation associated with changes in climate led to widespread erosion and soil loss to the continental shelf during this period, with possible silting up of the Guadiana eastern distributary, allowing the transport of high amounts of sediment to the shelf by the western distributary that led to the enhancement of fine sedimentation and the formation of the prodeltaic wedge in the area of the studied core.

Analogue modelling of continental collision: Influence of plate coupling on mantle lithosphere subduction, crustal deformation and surface topography

The role of the plate boundary in a continental collisional setting is investigated by lithospheric-scale analogue models. Key variables in this study are the degree of coupling at the plate interface and along the Moho of the lower plate as well as the geometry of the plate contact. They control the onset of intra plate deformation, orogenic architecture, amount of mantle lithosphere subduction and basin development. In all experiments, deformation initiates at the plate interface by the formation of a pop-up structure. A vertical plate boundary with respect to the shortening direction results in buckling of the lithosphere, whereas experiments with an inclined plate boundary show underthrusting and foreland basin development without orogenic wedge formation. Continental collision and coinciding mantle lithosphere subduction may occur only if the lower crust of the foreland plate is weak enough promoting crust–mantle decoupling. During decoupling the weak lower crust beneath the orogen thickens significantly by ductile flow as it detaches from the down going mantle lithosphere. This lower crustal thickening effects the distribution of upper crustal deformation and topography. Subduction of weak lower crust is favored when the weak plate interface has a significant thickness (~ 15 km in nature) and a high amount of shortening is applied. Increasing coupling at the plate interface through time leads to intra plate deformation by thickening and gentle folding and influences surface uplift and subsidence above the plate interface. The transition from a mechanically decoupled plate boundary with a significant amount of mantle lithosphere subduction towards stronger plate coupling resulting in intra plate deformation and topography development can be recorded in for instance the Caucasus, the Colombian Cordillera, the Pyrenees and the Alps. Thickening of the lower crust as portrayed for the Western Alps does not demand a strong, frictional-type behavior of the lower crust, but can also be the consequence of ductile processes.

Particle size effect on abrasion resistance of mottled cast iron with different retained austenite contents

Effects of particle abrasive sizes on wear resistance of mottled cast iron with different retained austenite contents were studied. Abrasive wear tests using a pin test on alumina paper were carried out, using abrasive sizes between 16 μm and 192 μm. Retained austenite content of the matrix was determined by X-ray diffraction. The wear surface of samples and the alumina paper were examined by scanning electron microscopy for identifying the wear micromechanism. The results show that at lower abrasive sizes the mass loss was similar for the iron with different austenite contents. However, at higher abrasive sizes the samples with higher retained austenite content presented higher abrasion resistance. For lower abrasive sizes tested, samples with higher and lower retained austenite content both presented microcutting. On the other hand, the main wear micromechanism for the samples with higher retained austenite content and higher abrasive sizes was microploughing. The samples with lower retained austenite content presented microcutting and wedge formation at higher abrasive sizes. Higher abrasive size induced more microcutting in samples with lower retained austenite. The iron with lower retained austenite content presented wider grooves for the different abrasive sizes measured. SEM on the abrasive paper used on samples with higher retained austenite showed continuous and discontinuous microchips and the samples with lower retained austenite showed discontinuous microchips at 66 and 141 μm. This research demonstrates the relation between abrasive size, wear resistance, groove width and wear micromechanism for mottled cast iron with different retained austenite contents.

The source of D e variability in periglacial sand wedges: Depositional processes versus measurement issues

Relict periglacial wedge structures are widespread in mid-latitude and polar regions. The wedges have a high preservation potential and are often infilled with quartz-rich aeolian sand, making them potentially suitable for luminescence dating. This paper presents initial work from an anti-syngenetic sand wedge in the Tuktoyaktuk Coastlands, Arctic Canada. When samples were measured at the single grain level they showed poor palaeodose (D e) reproducibility and consequent high age uncertainties. The aim of the study was to determine whether this intra-sample D e scatter reflected methodological issues arising from OSL measurement or processes of wedge development. Initial single grain dose recovery tests show that D e scatter was not easily explained by poor recycling, sensitivity changes, variable OSL components, recuperation problems, or large D e uncertainties from dim grains. Single grain preheating tests did show that some D e scatter might be attributable to individual grains requiring different preheat temperatures, whilst dose recovery tests revealed that highly sensitive grains showed less scatter than dim grains. However, selection of these bright grains from natural samples still resulted in scattered D e values. An alternative explanation for the D e scatter relates to the formation of anti-syngenetic sand wedges by thermal contraction cracking over thousands of years in an eroding landscape, which may result in sediment of very different ages being deposited in adjacent cracks. Finite mixture modelling was used to identify D e components within each sample. Ages calculated from these components suggest sand wedge formation at ca. 5, 8.5, 12.5 and 18 ka which correlate well with known cooling events over the last 18 ka and support a multiple phase activity model for anti-syngenetic sand wedges.

FORMATION OF CRACK-LIKE DIFFUSION WEDGES AND COMPRESSIVE STRESS EVOLUTION DURING THIN FILM GROWTH WITH INHOMOGENEOUS GRAIN BOUNDARY DIFFUSIVITY

We consider the effects of inhomogeneous grain boundary (GB) diffusivity on the formation of crack-like grain boundary diffusion wedges and evolution of compressive stress during the growth of a polycrystalline thin film on a substrate with a perfectly bonded film-substrate interface. The problem is formulated as a class of moving boundary diffusion problems with fully coupled elasticity, GB diffusion and film growth. The inhomogeneous GB mobility leads to a set of hypersingular integro-differential equations which are solved by a special numerical scheme. During the film growth, atoms diffuse into the GBs due to the tensile coalescence stress in the film associated with GB formation. We show that, at deposition rates comparable to the diffusion time scale, such GB diffusion leads to temporal decay of GB traction and induces crack-like singular stress concentration at the roots of the GBs. For a slowly growing high mobility thin film, the stress intensity factor near the tip of the diffusion wedge is comparable to that of a GB diffusion wedge under annealing conditions and can lead to nucleation of dislocations, which in turn can influence stress evolution during film growth. Furthermore, the higher chemical potential at the film surface can overdrive adatoms into GBs and result in a final compressive stress in the film. Our simulations on cyclic film deposition and growth interruption processes qualitatively reproduced the experimentally observed behavior of GB mobility on the Stress-Thickness evolution during deposition, the stress relaxation during growth interruption, as well as the dependence of steady state stress on the deposition rate.

THEMIS observations of consecutive bursts of Pi2 pulsations: The 20 April 2007 event

On 20 April 2007, four Pi2 pulsation bursts occurred successively and simultaneously in the premidnight sector at the E spacecraft and ground‐based observatories for the THEMIS mission, while the AE index was less than 100 nT. Especially for the last three onsets, both ground‐based and GOES 12 magnetometers sensed magnetic perturbations, as expected from the formation of a substorm current wedge (SCW). Moreover, LANL 1994–084 detected an enhancement of energetic particle flux. Spectral analysis shows a matched wave frequency ∼6–8 mHz and another harmonic frequency ∼17 mHz for the fourth burst. The orientation of the major axis of the wave polarization hodogram points toward the SCW location. The first burst has both latitudinal and longitudinal polarization changes from counterclockwise (CCW) to clockwise (CW), in contrast to the other three that have a latitudinal reversal only. The longitudinal CW to CCW change at low latitudes signifies that hydromagnetic waves propagate westward and eastward from the longitude of the impulsive source responsible for SCW. The latitudinal CW to CCW reversal is consistent with induction by a westward moving upward field‐aligned current carried by Alfvén waves leading to field line oscillations. Consequently, they can be explained by the coupling of a fast magnetospheric cavity mode driven by fast compressional waves to field line resonances as expected from braking bursty bulk flows, resulting from magnetotail reconnection, triggered by a preceding northward interplanetary magnetic field turning. This event shows that the source mechanism of consecutive Pi2 onsets at times of weak geomagnetic activity is the same as during substorm times.

Rim uplift and crater shape in Meteor Crater: Effects of target heterogeneities and trajectory obliquity

We have analyzed the rim structure of Meteor Crater, Arizona, in order to understand the mechanism of rim uplift in simple craters and the causes of the shape of polygonal impact craters. For this purpose, we systematically determined bedding orientation of the autochthonous crater wall and overturned flap and analyzed the kinematics of major radial faults. We found that rim uplift correlates with the crater shape and increases in the corners of the crater. The two main mechanisms of differential uplift are the formation of horizontal interthrust wedges, leading to the doubling of strata in the rim, and radial corner faults, or tear faults, that vertically displace bedrock. The development of Meteor Crater's quadrangular shape is caused by more effective crater excavation flow parallel to major joint sets. Additionally, we infer the impact direction with a newly developed technique, the two corners model, and review the arguments in favor of an oblique trajectory. While the data set is ambiguous, several indicators suggest an impact direction from the NNW. We conclude that oblique impacts should have an effect on early cratering and excavation flow, whereas target heterogeneities like joints start to play a prominent role in later stages when the stresses induced by the excavation flow are in the same order of strength as the material involved.

Substorm current wedge driven by plasma flow vortices: THEMIS observations

A multipoint analysis of conjugate magnetospheric and ionospheric flow vortices during the formation of the substorm current wedge (SCW) on 19 February 2008 is presented. During the substorm, four Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft were located close to the neutral sheet in the premidnight region between 9 and 12 RE geocentric distance, of which three closely (∼1–2 RE) clustered at ∼23 MLT and one was farther west at ∼21 MLT. The closely clustered spacecraft were engulfed by a counterclockwise plasma flow vortex, while the single spacecraft recorded a clockwise plasma flow vortex. Simultaneously, a pair of conjugate flow vortices with clockwise and counterclockwise rotation appeared in the ionosphere, as inferred from equivalent ionospheric currents. The counterclockwise space vortex, which corresponded to a downward field‐aligned current, was at least 1–2 RE in diameter and had rotational flow speeds of up to 900 km/s. Current density estimates associated with the formation of the space vortex in the first 30 s yielded 2.8 nA/m2 (14 μA/m2 mapped to the ionosphere), or a total current of 1.1 × 105 A. Model calculations based on midlatitude ground magnetometer data show a gradual increase of the field‐aligned current, with 1–2 × 105 A within the first minute and a peak value of 7 × 105 A after 10 min, associated with the SCW, and a matching meridian of the downward current of the SCW and the downward current (counterclockwise) space vortex. The combined ground and space observations, together with the model results, present a scenario in which the space vortices generated the field‐aligned current of the SCW at the beginning of the substorm expansion phase and coupled to the ionosphere, causing the ionospheric vortices.

Near-Earth magnetic signature of magnetospheric substorms and an improved substorm current model

Abstract. Based on a comprehensive catalogue with more than 4000 magnetospheric substorm entries from the years 2000–2005, the spatial distribution of the substorm-related magnetic signatures at mid and low latitudes around local midnight was investigated. Superposed epoch analysis of a larger number of recent observatory data from mid and low latitudes revealed a field strength increase that is consistent with the results of earlier studies. For the first time, the magnetic signature of the substorm current wedge formation is studied also in near-Earth satellite data from CHAMP. The average maximal deflection measured on board the satellite is smaller by a factor of 2 than that determined from ground observations. The recurrence frequency of substorms as well as the amplitude of their magnetic signature depends strongly on the prevailing magnetic activity. The observed average substorm-related magnetic field signatures cannot be described adequately by a simple current wedge model. A satisfactory agreement between model results and observations at satellite height and on ground can be achieved only if the current reconfiguration scenario combines four elements: (1) the gradual decrease of the tail lobe field, (2) the re-routing of a part of the cross-tail current through the ionosphere, (3) eastward ionospheric currents at low and mid latitudes driven by Region-2 field-aligned currents, and (4) a partial ring current connected to these Region-2 FACs.

P-T evolution of metapelites from the Guarguaraz Complex, Argentina: evidence for Devonian crustal thickening close to the western Gondwana margin

In the Guarguaraz Complex of the Argentine Frontal Cordillera (latitude 33.3°S) serpentinite lenses, metabasites, and garnet-bearing metasedimentary rocks occur. The corresponding Devonian metamorphic event was assigned to the formation of an accretionary wedge by previous authors. We have tested this hypothesis by investigating the metamorphic evolution of a pelite from this complex. Garnet in this rock shows a well evolved prograde zoning. Potassic white mica with Si contents between 3.38 (core) and 3.12 (rim) per formula unit, biotite, quartz and plagioclase, which have formed late blasts at the expense of amphibole, coexist with this phase. On the basis of P-T pseudosections and water-absent equilibria, we have estimated the subsequent P-T path for the studied metapelite. The garnet core formed at about 8 kbar and 470-500°C. After nearly isothermal burial to 45 km (13.5 kbar) where Si-rich phengites grew, temperatures increased followed by exhumation and an overprint stage at 8 kbar and 565°C. This P-T path is not compatible with the idea of an accretionary wedge complex. It is more likely that the high-pressure conditions were attained by thickening of continental crust when the hypothetical Chilenia terrane collided with the continental margin of western Gondwana in Devonian times.

The deep seismic reflection MARCONI-3 profile: Role of extensional Mesozoic structure during the Pyrenean contractional deformation at the eastern part of the Bay of Biscay

The new MARCONI-3 deep seismic profile allows recognition of the upper crustal structure of the eastern part of the Bay of Biscay and the main features of its Alpine geodynamic evolution. It denotes that the easternmost part of the Bay of Biscay consists of a thick wedge of uppermost Cretaceous to Cenozoic synorogenic sediments lying unconformably on the top of a thinned continental crust with the Mesozoic Parentis Basin to the north and the coeval Landes High to the south. The Parentis Basin appears as a major half-graben bounded southwards by a north-dipping planar fault. It is filled by a thick sequence of Jurassic–Upper Cretaceous carbonates affected by salt domes and squeezed diapirs made up of Triassic evaporites and mudstones. These salt tectonic structures also affect the overlying uppermost Cretaceous to Lower Miocene synorogenic deposits which are folded upon these structures. The Landes High includes a thin pre-Upper Cretaceous cover tilted to the south. In the Basque shelf, it is deformed by a basement-involving thrust wedge emplaced during the Late Eocene–Miocene that constitutes the North-Pyrenean contractional front. Geometric relationships and thickness variations depict that this overall structure results from the following. • Mesozoic extensional stage which includes a Late Jurassic (?)–Late Aptian syn-rift stage in which the Parentis Basin formed; and an Albian–early Late Cretaceous post-rift stage in which diapirs of Triassic evaporites grew close to this major fault. • Compressive deformational stage coeval to the Pyrenean orogeny which led to (1) the development of a latest Cretaceous up to Middle Miocene foreland basin; and (2) from Late Eocene, the formation of a basement-involving thrust wedge in the innermost foreland basin, the squeezing of the diapiric structures formed previously in the Parentis Basin and, later (Middle Miocene), the partial inversion of some pre-existent faults located in the southern Parentis Basin margin. This geodynamic evolution together with the structure of the area evidences that the extensional structure resulting from the opening of the Bay of Biscay played an important role both in the location of the North-Pyrenean front and in the North-Pyrenean foreland contractional deformation features. Specially, the lack of significant inversion structures in the Parentis Basin, despite that it belongs to a severely thinned crustal area before the Alpine compression, denotes that the Mesozoic Landes High acted as an important buffer for the propagation of the Pyrenean contractional deformation to the north. This deformational buffer was active until Early Miocene and probably vanished afterwards during the last stages of Pyrenean orogen development when some basement faults reactivated in the Parentis Basin.

Experimental Study of the Lubrication Mechanism for Thrust Slide Bearings in Scroll Compressors

This study focuses on the effect that a pressure difference across the orbiting thrust plate of a thrust-slide bearing has on the improved lubrication of the bearing in scroll compressor applications. A thrust slide bearing model submerged in a refrigerant oil, VG-56, was operated under pressurized conditions using R-22 as the pressurizing gas, where the pressure difference across the friction surface of the thrust bearing was adjusted from 0 to 1.0 MPa and the friction force and friction coefficient at the thrust slide bearing were measured over a range of orbital speeds. As a result, significant improvement in lubrication at the thrust slide bearing due to the pressure difference was identified. Furthermore, careful observation of the wear state of the thrust slide bearing revealed the formation of a fluid wedge between the sliding surfaces due to axial loadings. This fluid wedge appears responsible for the observed improvement in lubrication. In addition, the wedge formation was quantitatively investigated using finite element method (FEM) analysis of the elastic deformation of the thrust plate. The FEM analysis was subsequently validated through strain measurement on the thrust plate.

Evolution of the Cretaceous Astrid thrust belt in the ultradeep-water Lower Congo Basin, Gabon

The Lower Congo Basin contains the greatest salt-based fold and thrust belt off Africa's Atlantic margin. Our study area in the Anton Marin and Astrid Marin exploration blocks is in the northern part of the basin. Gravity-driven tectonic shortening began soon after the Aptian salt deposition, forming gentle, west-trending, salt-cored anticlines, which, together with salt diapirs, created a template for later thrusting. In the Late Cretaceous, a thrust front propagated landward into the study area, and thrusts formed above salt anticlines and diapirs. Formation of a hanging-wall wedge of growth strata was recorded when each thrust fault ruptured the seabed. Thrusting began after widespread salt thinning, as autochthonous salt was expelled into older, passive diapirs. Thinning stiffened the detachment, so that thrusts verge strongly seaward. Structural restorations, dip-corrected isochron maps, and fault-activity graphs all show that the landward edge of the thrust belt propagated landward. Three main pulses of shortening episodically reactivated thrust faults as the thrust front broke landward. As thrusting culminated, precursor passive diapirs were squeezed and extruded small allochthonous sheets. Translation culminated in major erosional scouring, from which we infer epeirogenic slope steepening in the Late Cretaceous. As shortening spread updip into the previously extensional domain during the Late Cretaceous to Paleogene, older extensional faults were inverted, and new extensional faults formed orthogonally, parallel to the regional paleoslope. The structural pattern, created in the Late Cretaceous when the paleoslope dipped southward, remains recognizable in the little-deformed Neogene strata, although the present continental slope dips westward.

Physical environmental conditions, spawning and early-life stages of an estuarine fish: climate change implications for recruitment in intermittently open estuaries

Significant variation in the egg and larval survival and juvenile recruitment of estuarine fishes has been linked to fluctuating environmental conditions. This present study compared the distribution and abundance of black bream (Acanthopagrus butcheri) eggs and yolk-sac larvae between two microtidal estuaries of different flow regimes, where the riverine flow into the Glenelg estuary was around eight times the flow volume into the Hopkins estuary. Samples were collected monthly from September to November at sites along each estuary where vertical profiles of temperature, salinity and dissolved oxygen (DO) were measured, and vertically stratified sampling of black bream eggs and yolk-sac larvae was conducted using a Schindler sampler. Salt wedge formation was apparent in both estuaries, with significant de-oxygenation of deeper, saline waters. Eggs occurred in a wide range of DO levels but yolk-sac larvae were less common at the lowest levels. Most eggs and yolk-sac larvae were collected in salinities greater than 10. Results suggested that egg mortality was higher in the Hopkins than the Glenelg estuary, which may be associated with the hypoxic conditions characteristic of low-flow conditions. The results have significant implications in terms of climate change that is predicted to lead to warmer, drier conditions in south-eastern Australia, potentially increasing stratification and subsequent hypoxic zones.

Instantaneous end moraine and sediment wedge formation during the 1890 glacier surge of Brúarjökull, Iceland

Contemporary understanding of the behaviour of surging glaciers and ice streams is hampered by the lack of data on landsystem evolution and sedimentary environments. This study concerns the ice-marginal environment of the surge-type Brúarjökull in Iceland. The sediment distribution in the glacier forefield as well as the morphology, sedimentology and tectonic architecture of the 1890 end moraine is investigated for highlighting the interaction between very dynamic ice and sediment/landform associations. As a result of substrate/bedrock decoupling during the 1890 surge, subglacial sediment was dislocated across the bedrock surface and deformed compressively, leading to gradual substrate thickening and the formation of a sediment wedge in the marginal zone. A drop in subglacial porewater pressure at the very end of the surge led to substrate/bedrock coupling and a stress transfer up into the sediment sequence causing brittle deformation of the substrate. Simultaneously, the glacier toe ploughed into the topmost part of the marginal sediment wedge initiating the moraine-ridge construction. Fine-grained and incompetent sediment deformed in ductile manner, resulting in a narrow moraine dominated by rooted folds, while coarse-grained and competent sediment deformed in brittle fashion, resulting in a wider moraine dominated by thrust blocks. A new sequential model of subglacial and ice-marginal processes operating during a glacier surge is proposed, illustrating the stepwise formation of a marginal sediment wedge and an end moraine—a twofold, inseparable marginal end-product that formed during the last days of the 1890 surge.

Architecture of the Holocene Rhine-Meuse delta (the Netherlands) - A result of changing external controls

AbstractThe Holocene Rhine-Meuse delta is formed under the influence of sea-level rise, tectonics, and variations in discharge and sediment supply. This paper aims to determine the relative importance of these external controls to improve our understanding of the evolution of the Rhine-Meuse fluvio-deltaic system. To do this, the geological and lithological composition of the fluvio-deltaic wedge has to be known in detail, both in space and time. This study presents five cross-valley sections in the Holocene Rhine-Meuse delta, based on almost 2000 shallow borings. Over 130 14C dates provide detailed time control and are used to draw time lines in the sections. Distinct spatio-temporal trends in the composition of the Holocene fluvio-deltaic wedge were found. In the upstream delta, the Holocene succession is characterised by stacked channel belts encased in clastic flood basin deposits through which several palaeo-A-horizon levels are traceable. In a downstream direction, the fluvio-deltaic wedge thickens from 3 to 7 m. The Holocene succession in the downstream cross sections formed from <8000 cal yr BP onwards and is characterised by single channel belts encased in organic flood basin deposits. The main part of the organic beds accumulated between 6000 and 3000 cal yr BP. After 3000 cal yr BP, clastic deposition dominated throughout the delta, indicating an increase in the area of clastic sedimentation. The Holocene fluvio-deltaic wedge is subdivided into three segments based on the relative importance of eustatic sea-level rise, subsidence, and upstream controls (discharge and sediment supply). Before 5000 cal yr BP, eustatic sea-level rise controlled the build-up of the wedge. After eustatic sea-level rise ceased, subsidence was dominant from 5000 to 3000 cal yr BP. From 3000 cal yr BP onwards, increased sediment supply and discharge from the hinterland controlled the formation of the fluvio-deltaic wedge. A significant part of the present-day Rhine-Meuse fluvio-deltaic wedge aggraded after eustatic sea-level rise ceased. We therefore conclude that external controls other than eustatic sea-level rise were also of major importance for the formation of the fluvio-deltaic wedge. Because this is probably true for other aggrading fluvial systems at continental margins as well, all external controls should be addressed to when interpreting (ancient) fluvio-deltaic successions.

Influence of Stochastic Discontinuity Network Parameters on the Formation of Removable Blocks in Rock Slopes

We study the effects of discontinuity network parameters on the formation of removable wedges in rock slopes. Discontinuities are simulated using the Poisson disk model, and removable wedges are identified using block theory. The formation of removable wedges of different sizes is assumed to follow a Poisson process. Poisson regression and Monte Carlo simulation are then used to identify statistically relevant parameters of the model, and to study the effects that variations in their values have on formation of removable blocks. The sensitivity of the results as a function of the mean orientations of the discontinuity sets forming the blocks is also studied by means of a parametric study. The volumetric intensity of discontinuities in the rock mass is found to have a significant impact on the computed estimates of removable block formation. As predicted by theory, our results indicate that, everything else being equal, the expected rate of formation of removable wedges is proportional to the square of the intensity measure. Estimates are also sensitive to changes in discontinuity size, especially in cases in which discontinuities are smaller than one to two times the height of the slope. The interaction between the mean size of discontinuities and the coefficient of variation of discontinuity sizes is found to be significant as well. Finally, results of our sensitivity analysis suggest that the orientation of discontinuity sets significantly affects the rate of formation of removable blocks in rock slopes.

O CONTROLE ESTRUTURAL NO AQÜÍFERO BARREIRAS - ÁREA

The regional structuring, outlined by the overall preferential directions NW, NE and W-W, revealed as exerting strong influence on a diverse array of aspects of the Barreiras Aquifer context. These influences were attested by several aspects, based on the analysis of geological, hydrogeological and geo-electrical data, notably on the aquifer geometry, as witnessed by sudden variations of saturated thicknesses caused by faulting. Quantitative configuration of major transmissivities, resulting from local increase of these saturated thicknesses, was evidenced through statistical correlations carried out between exploitation flows and saturated thicknesses or extensions of filtering sections, where correlation coefficients up to 0.79 were obtained. The remaining influences of the structural framework of the Barreiras Aquifer include from a control of the underground flow to evidences on the potenciometric map, as expressed by sudden changes in spacing or inflexion of the equipotentials. In addition, quantification of permanent reserves was carried out in the area of the Pirangi River basin, taking into consideration variations in the aquifer geometry resulting from local structuring. Other aspects of regional implication were also addressed, among them the probability that some coastal line faults, particularly the distensional and transtensional ones, be associated to the formation of saltwater wedges.