backthrusting in accretionary wedges backthrusting of the rear of the accretionary wedge arcward over the rocks of the forearc basin is a common aspect of accretionary tectonics an older assumption that backstops of accretionary wedges dip back toward the arc and that accreted material is emplaced below such backstops is contradicted by observations from many active forearcs that indicate 1 backthrusting is common 2 forearc basins are nearly ubiquitous associates of accretionary wedges and 3 forearc basement where imaged appears to diverge from the sedimentary package dipping under the wedge while the overlying sediments are often lifted up against it backthrusting may be favored where relief is high between the crest of the wedge and the surface of the forearc basin because the relief must be supported by shear stress along the backthrust we find ratios of lt lb averaging 2 64 0 8 where lt is the horizontal distance from the crest of the wedge to the front of the toe and lb is the distance from the crest of the wedge to the surface projection of the backthrust this ratio is predicted to increase with total convergence and with sedimentation because the toe undergoes higher deformation rates than the backthrust and sedimentation in the trench favors outbuilding of the toe whereas sedimentation in the forearc basin tends to cancel the gravitational component of the driving stress for the backthrust

implications of estimated magmatic additions and recycling losses at the subduction zones of accretionary non collisional and collisional suturing orogens

palaeogene and neogene cold seep communities in barbados trinidad and venezuela an overview palaeogene and neogene fossiliferous carbonates from barbados trinidad and northern venezuela are interpreted to have formed at ancient cold seep sites the hydrocarbon seepage that fuelled these chemosymbiotic ecosystems was related to tectonic activity in the southern caribbean region particularly the subduction of the caribbean plate beneath the north atlantic plate the carbonates and fossils from the scotland district north eastern barbados are eocene miocene in age and are associated with two distinct tectonic units the sub oceanic fault zone and a diapiric melange the sub oceanic fault zone is the tectonic junction between accretionary prism sediments and over lying thrust sheets of fore arc basin sediments the loading of the thrust sheets caused methane rich fluids to be expelled from the accretionary prism sediments and channelled to the sea floor via the sub oceanic fault zone where it supported chemosymbiotic invertebrate communities containing vesicomyid lucinid thyasirid solemyid and nuculanid bivalves a variety of gastropods and possibly vestimentiferan tube worms the diapiric melange is considered to represent sediment that failed under pressure in the accretionary prism and was remobilised as a diapir that extruded onto the sea floor providing a conduit for methane and other hydrocarbons that sustained a chemosynthesis based community of vesicomyid lucinid and nuculanid bivalves and various gastropods the geological setting of fossiliferous carbonates known as freeman s bay limestone in southwest trinidad has been less fully investigated the freeman s bay limestone is a member of the miocene lengua formation which is believed to have formed in a fore deep basin on lapping onto an accretionary prism formed by the subduction of proto caribbean crust beneath the south american plate the carbonates and fossils of the fbl including the bivalves pleurophopsis unioides van winkle 1919 and thyasira adoccasa van winkle 1919 lucinids nuculanids and bathymodiolins and provannid and other gastropods are interpreted to have formed at palaeo seep sites on the accretionary prism little is currently known about the geological setting of the fossils collected from the miocene huso member of the pozon formation from northern venezuela but a seep origin is strongly suspected based on the taxa present like the fossil assemblages from barbados and trinidad the venezuelan material is dominated by vesicomyid bivalves together with lucinid thyasirid bathymodiolin and solemyid bivalves and various gastropods these taxa are characteristics of modern cold seep communities most comparisons between the fossil caribbean and modern seep fauna are necessarily at the generic or family level although in some cases individual species are found at both ancient and modern seep sites for example the gastropod cataegis meroglypta is found in seep carbonate from the sofz of barbados and the fbl of trinidad as well as at modern seeps on the barbados prism at the generic level this study has revealed the first fossil occurrences of abyssochrysos and provanna in the caribbean such temporal and spatial links further the understanding of both local and global patterns of biogeographic distribution of cold seep fauna

the role of fission track dating in discrimination of provenance determination of single crystal ages by fission track dating analysis of zircons and subordinately apatites within a detrital heavy mineral assemblage together with frequency distribution and radial plot analysis of the measured ages permits identification of different age modes synopses are presented of published fission track provenance studies in north central victoria northeastern tasmania barbados the rockall trough the english wealden mexico utah and the central alps from authors

uranium series evidence on diagenesis and hydrology in pleistocene carbonates of barbados west indies uranium series isotope measurements were made by mass spectrometry on aragonite dolomite and groundwater samples from pleistocene coral reef terraces on barbados in order to evaluate the behavior of u th isotopes during water rock interaction in carbonate systems well preserved aragonitic corals from the golden grove terrace yield precise 230 th ages of 219 3 and 224 6 ky and have high and variable initial 234 u relative to values for modern seawater these high 234 u values are common to pleistocene carbonates from many localities and indicate that the u series has not followed closed system behavior since the time of deposition pervasively dolomitized samples from the golden grove terrace have significantly lower 234 u than the unreplaced aragonite and 230 th 238 u act activity ratios in excess of values for secular equilibrium groundwaters sampled from 125 460 ky old portions of the pleistocene carbonate aquifer have 234 u values close to secular equilibrium and low 230 th 238 u act of less than 4 10 3 the pronounced differences in the u th isotope systematics between the aragonitic corals dolomites and groundwaters reflect the enhanced mobility of u relative to th during water rock interaction these results demonstrate that diagenesis can profoundly affect the u th isotopic composition of marine carbonates and therefore the accuracy of high precision 230 th ages determined on such samples however diagenesis involving fluids such as the present day groundwaters on barbados or those which formed the dolomites cannot account for the elevated 234u values of the corals the mechanism by which apparently well preserved corals become enriched in 234 u remains to be identified low 234 u for the dolomites and the groundwaters records the transport of dissolved u from portions of the aquifer that are significantly older e g 10 6 yrs than their present host carbonate rocks the preservation of these distinct u isotopic compositions during fluid migration through younger carbonates indicates that interaction between groundwaters and these carbonates with accompanying 234 u enrichment in the waters from dissolution or alpha recoil has been extremely limited

comment new osl dating of uk loess indications of two phases of late glacial dust accretion in se england and climate implications michele l clarke antoni e milodowski jon e bouch melanie j leng and kevin j northmore 2007 journal of quaternary science 22 361 371

macroroughness and variations in reach averaged flow resistance in steep mountain streams 1 steep mountain streams typically feature macroroughness elements like large immobile boulders or channel spanning bedforms such as step pool sequences the effects of macroroughness on resistance and flow velocity are not well understood and appropriate field parameters for representing macroroughness in flow velocity equations have not been identified the prediction of flow velocity in rough and steep streams therefore remains challenging we measured flow velocity and several macroroughness parameters i e boulder concentration boulder diameter and protrusion and roughness of longitudinal channel profiles in six reaches of steep mountain streams with plane bed riffle step pool and cascade channel morphologies the between site variations in flow resistance can be explained to a large degree by nondimensionalization of discharge and flow velocity using channel slope and a characteristic roughness length using any of our roughness parameters as the characteristic roughness length this nondimensionalization leads to a similarity collapse of the entire data set the remaining differences in flow resistance among the streams are related to dimensionless measures of macroroughness that describe the concentration of boulders or step density in a reach boulder concentration represents the measure best describing the data and is used in a simple regression equation for flow velocity the predictions were better than predictions by the variable power law equation proposed by ferguson although the regression might not be statistically significant the observed trends suggest that boulder concentration partly explains the residual variance of between site variation of flow resistance

influence of bed patchiness slope grain hiding and form drag on gravel mobilization in very steep streams steep streams are a major portion of channel networks and provide a link to transport sediment from hillslopes to lower gradient rivers despite their importance key unknowns remain perhaps foremost of which is evaluating in steep streams empirical laws for fluvial sediment transport developed for low gradient rivers to address this knowledge gap we painted sediment in situ over 3 years to monitor incipient sediment motion and sediment patch development in five small drainage areas of 0 04 2 km 2 and steep slopes of 5 37 tributaries of elder creek california united states we found that channel beds organized into size sorted sediment patches which displayed active fluvial transport of gravel annually consistent year to year patch median grain sizes partial transport of bed material and significantly higher values of critical shields stress for incipient sediment motion compared to that observed for lower gradient rivers the high critical shields stresses up to 0 5 for the median grain size agree within a factor of 3 to theoretical predictions which account for slope dependent hydraulics grain hiding and sediment patches for grains of approximately the same size as the roughness length scale slope dependent hydraulics and bed patchiness are the dominant controls on critical shields stress values while grain hiding is important for grains larger or smaller than the roughness length scale form drag exists in our monitored tributaries but has a smaller influence than the above effects our field observations show fluvial processes contribute to sediment mobilization in steep channels which are often considered to be dominated by debris flows

magmatic helium in subduction zone natural gases abstract he isotope ratios provide important constraints on the input of magmatic gases into crustal systems natural gas associated with active continental margins from taiwan r o c new zealand the philippines indonesia thailand and alaska u s a contain a magmatic he component based on measured 3 he 4 he ratios r ranging from 0 5 to 3 8 times the atmospheric value ra the methane helium results can be interpreted as the result of a mixture between a low 3 he 4 he 0 01r a high ch 4 3 he crustal end member and a high 3 he 4 he 0 6 3 9r a low ch 4 3 he 6 magmatic component the presence of mantle he does not require significant amounts of mantle methane a better fit to the data is obtained using a zero methane magmatic end member the worldwide association of 3he and magmatic activity suggests that volcanism plays a role in methane generation by supplying the heat needed to convert organic matter to methane thermogenically only gases from the accretionary prism of barbados west indies appear totally devoid of magmatic he 3 he 4 he ratio 0 02ra the he is completely of crustal origin there is no evidence that faults within the accretionary prism of this fore arc act as conduits for the migration of mantle he many of the gases studied contain significant quantities of co2 which appear to result from two processes 3 he 4 he high co 2 3 he gases 0 01ra 1011 possibly result from decarbonation reactions in the crust while the high 3 he 4 he low co 2 3 he gases 2 4ra

forearc basin evolution in the pan african damara belt central namibia the hureb formation of the khomas zone abstract the hureb formation in central namibia is one of the uppermost stratigraphic units of the precambrian pan african damara sequence the hureb formation occurs only in a part of the khomas zone a forearc basin which evolved complexly with time the formation is composed of the well graded upward fining schists of the quartzitic keises member the graphitic and calcitic waterval member and the altered scapolite bearing dagbreek member these units accumulated in an elongate turbidite fan system longitudinally deposited on the northern slopes of a wide forearc basin the active continental margin initiated when the kalahari plate subducted oceanic crust underneath the passive continental margin of the congo craton the development and deposition of the turbiditic hureb formation in the foredeep occurred during subduction of oceanic crust simultaneous with d 1 deformation and tholeiitic to andesitic arc volcanism on the continental margin the basin was bound to the north by the rising volcanically active continental margin and to the south by the accretionary prism of obducted slivers of oceanic crust and pelagic sediments above the subduction zone with continental collision the sedimentation in the forearc and continental margin arc volcanism ceased the forearc basin was uplifted and shortened horizontally by synclinoria and anticlinoria which propagated northwards and died out at the edge of the forearc basin the deformation slightly affected the underlying foliated fahlwater schist from the passive margin setting further uplift of the basin and re adjustment between the oblique sutured cratons simultaneous with the d 3 doming in the congo continental margin arc occurred with the underplating of the thin passive margin of the kalahari craton as no more crustal shortening could be absorbed by the upright folding dk 1 reactivation of the thrusts in the accretionary prism leads to the producing of southeast verging with a right lateral sense of movement thrusts dk 2 in the folded forearc basin the dk 2 thrusts propagated and died out northwards and extended towards the south the stacking of the d k2 thrusts southwards thickened the crust creating a low heatflow environment with higher pressure conditions it leads to the preservation of chlorite and production of sillimanite garnet staurolite and kyanite to the south of the accretionary prism collision related deformation created a peripheral foreland thrust zone outlasting orogenic activities in the overriding congo continental margin where isostatic rebound already had occurred

tectonic evolution of the tobago trough forearc basin the tobago trough is a modern marine forearc basin of the lesser antilles arc it lies above an undrilled mainly conformable sedimentary succession at least 10 km thick and probable oceanic basement the strata occur in a forearc basin belt between the crystalline arc platform and the accretionary forearc for hundreds of kilometers north and southwest of the tobago trough seismic data permit division of tobago trough strata into four sequences a ab bc and c a is quaternary ab early pleistocene to late miocene bc miocene and c includes miocene probably through middle eocene and perhaps rocks as old as mid cretaceous or older thickness maps of the four sequences indicate major configurational changes in basins that preceded the modern one a southern c basin extended southwest to the carupano basin north of venezuela the northern c basin extended and perhaps thickened northeast of its current limit of recognition at the crosscutting inner forearc deformation belt ifdb the southern c basin continued unchanged through bc time whereas the northern bc basin was confined to the region west of the ifdb the depoaxis of the bc basin lay close to the western basin flank in contrast sequences ab and a accumulated in an asymmetric basin with maximum subsidence at its eastern margin at the foot of the ifdb a stratigraphic model of eocene and younger layers in the tobago trough is derived from seismic attributes cenozoic rocks on nearby islands well data in the carupano basin and piston cores the model proposes that 1 sequences c and lower bc are volcanogenic hemipelagite including ashfall and fine turbidite to the north and possibly continent derived turbidite to the south 2 upper bc and ab are mainly proximal basin flank derived turbidite to the north perhaps grading south to mainly continent derived hemipelagite and 3 a is mainly continent derived hemipelagite and volcanogenic ashfall vertical velocity distributions in tobago trough strata suggest mainly compaction related increases at an undercompacted rate for the upper 3 4 km local increases in velocity gradient at these depths could represent defluidization possibly at layer parallel detachments extending from the ifdb depositional rates in the tobago trough may have increased monotonically through later cenozoic time they have always greatly exceeded deep oceanic rates the northwestern margin of the c basin was a preexisting irregular edifice that included the arc platform and the st lucia ridge the flank edifice and deep basin floor were covered by onlap of forearc basin strata in a water filled basin and at the same time underwent southeast down rotation the c tilting was without evident local deformation and was perhaps due to intrusion at a currently incompletely defined locus to the northwest the initiation of the bc depoaxis at the western edge of the northern basin records the miocene development of a clastic wedge of arc platform provenance perhaps due to the onset of neogene volcanism and or emergence of the arc platform the shift of the depoaxis in ab and a times to the eastern basin margin reflects loading of the lithosphere by the tectonically thickened ifdb the miocene and younger features of the tobago trough probably reflect a superposition of tectonism associated with the development of the neogene lesser antilles arc on an older arc system the neogene structures crosscut the older ones and are not simply the result of progressive growth of the accretionary forearc

early jurassic evolution of the northern stikinian arc evidence from the laberge group northwestern british columbia this study resolves fundamental questions concerning the age provenance and depositional history of laberge group strata in the whitehorse trough the jurassic inklin formation straddles the stikine and cache creek terranes along much of the length of the whitehorse trough ammonite biochronology indicates an age range of early sinemurian to late pliensbachian and provides the temporal framework for interpreting basin history strong temporal trends in both paleocurrent patterns and sandstone conglomerate petrofacies allow definition of three discrete phases in basin fill history stable tectonics characterized by relative volcanic quiescence and low sedimentation rates prevailed during the sinemurian sinemurian sandstone conglomerate petrofacies record a transitional arc provenance derived from erosion of the upper triassic volcanic pile flanking coastal sediments and arc roots of stikinia to the southwest during the early pliensbachian arc dissection was interrupted by a major magmatic episode wi

petrology of the bitumen manjak deposits of barbados hydrocarbon migration in an accretionary prism abstract bitumen manjak deposits in barbados are located particularly along fracture planes including faults and also in calcite veins as veinlets vug fillings and bedding parallel seams in sandstones and as matrix to diagenetic carbonates they occur particularly near the margins of the diapiric joes river formation and in immediately adjacent rocks reflecting the role of diapirism in the migration of hydrocarbon bearing fluids the larger bitumen veins trend between e w and ne sw following the strike of fault bounded packets of the accretionary prism the predominant fabric in the basal complex of barbados that is hydrocarbon migration followed the structural grain petrographic relationships of bitumen with chemosynthetic diagenetic carbonates suggest that hydrocarbon migration was a multi stage process the host sediments to the bitumens were not well consolidated and appear to have been in an overpressured state hydrocarbon generation may have contributed to overpressuring and buoyancy enhanced diapirism the association of bitumens with faults and decollement surfaces suggests that the lubricating action of the bitumen helped to accommodate deformation during diapirism and other processes related to the evolution of the accretionary prism widespread impregnation of sandstones in the joes river formation suggests that hydrocarbon migration was primarily by intergranular flow in a state of low confining pressure and did not depend on migration along the fault planes which may be of a later date

theoretical development on the effects of changing flow hydraulics on incipient bed load motion 1 several decades of flume and field measurements have indicated that in rough turbulent flows the critical shields stress increases with increasing slope and associated decreasing relative depth this result contradicts the usual consideration of a decreased critical shields value on very steep slopes because of increased gravitational effects however recent studies have demonstrated that these experimental results could be reproduced with a force balance model if the classical logarithmic velocity profile was replaced with a velocity profile that was more compatible with available velocity measurements over gravel beds these measurements indicate the existence of a roughness layer that is a zone of almost constant velocity close to the bed whose properties mean velocity and turbulence depended on the flow s relative depth unfortunately velocity profile measurements for low relative depth associated with steep slopes are scarce and it is still difficult to include such flow properties in a force balance model flow resistance data on the basis of depth average velocity measurements are very common and cover a wide range of slopes and relative depths in this paper these data are used to fit a velocity profile including a roughness layer when used in a force balance model for incipient motion it adequately reproduced a data set composed of 270 critical shields values measured in a flume with near uniform sediments the relevance of this research to field problems is discussed using a data set composed of 92 critical shields stresses obtained from field measurements finally a model is proposed for field applications taking into account the slope effect

influence of boulder concentration on turbulence and sediment transport in open channel flow over submerged boulders in this paper the effects of boulder concentration on hydrodynamics and local and reach averaged sediment transport properties with a flow over submerged boulder arrays are investigated four numerical simulations are performed in which the boulders streamwise spacings are varied statistics of near bed velocity reynolds shear stresses and turbulent events are collected and used to predict bed load transport rates the results demonstrate that the presence of boulders at various interboulder spacings altered the flow field in their vicinity causing 1 flow deceleration wake formation and vortex shedding 2 enhanced outward and inward interaction turbulence events downstream of the boulders and 3 a redistribution of the local bed shear stress around the boulder consisting of pockets of high and low bed shear stresses the spatial variety of the predicted bed load transport rate qs based on local bed shear stress is visualized and is shown to depend greatly on the boulder concentration quantitative bed load transport calculations demonstrate that the reach averaged bed load transport rate may be overestimated by up to 25 times when including the form drag generated shear stress of the immobile boulders in the chosen bed load formula further the reach averaged bed load transport rate may be underestimated by 11 if the local variability of the bed shear stress is not accounted for finally it is shown that for the small spaced boulder array the bed load transport rates should no longer be predicted using a normal distribution with standard deviation of the shear stress distribution

neodymium and strontium isotopic constraints on soil sources in barbados west indies abstract neodymium and strontium isotopic compositions and sm nd ratios are used to constrain the sources of silicate rich soils developed on uplifted pleistocene coral reef limestones on barbados west indies the geographic and geologic setting of barbados facilitates the application of these tracers to the evaluation of the following soil sources 1 pleistocene reef limestone regolith 2 tertiary carbonate rocks sandstones and mudstones that are exposed in northeastern barbados 3 volcanic ash erupted from the lesser antilles arc 4 saharan dust transported by trade winds and 5 fertilizer the soils have nd values that range from 6 6 to 1 9 87 sr 86 sr values of 0 70890 to 0 71067 and sm nd ratios of 0 223 0 260 the pleistocene limestone component is the most significant source of sr in the soils and a negligible source of nd comparison of sm and nd concentrations and neodymium isotopic compositions of soil samples that are weathered to varying extents indicates that sm and nd are relatively unfractionated and retained in the soils during weathering nd and sm nd variations in the soils therefore primarily reflect the compositions and proportions of the silicate sources mass balance calculations based on sm nd systematics require that the silicate soil components contain between 30 85 volcanic ash with the remaining silicate fraction comprised of old continentally derived sediment in contrast to sm and nd sr is mobilized and removed from the soils during weathering strontium from volcanic and carbonate sources is preferentially removed relative to continental silicate sources the strontium isotopic compositions of the soils therefore reflect the combined effects of the degree of weathering and the compositions and proportions of the soil sources mass balance calculations indicate that at least 35 60 of the initial sr in the soils has been removed by weathering these results illustrate 1 the utility of radiogenic isotopes in identifying and quantifying soil sources and weathering processes 2 the compositional influence of numerous sources on soils even those developed in a relatively isolated area such as barbados and 3 the domination of barbados soil sm nd systematics by nonregolith eolian components

source rock and maturation evaluation of the barbados accretionary prism abstract organic material in specimens from surface and downhole sites and samples of oil and gas from barbadian wells were studied for source potential and maturation history the study involved 1 visual examination of the kerogen 2 measurement of total organic carbon content 3 determination of vitrinite reflectance 4 rock eval pyrolysis assay 5 geochemical measurement of pooled and extracted bitumens including carbon isotopic ratios 13 c and 6 gas chromatographic analysis of saturated gaseous hydrocarbons two types of kerogen occur in barbados terrestrial presumably of south american origin and marine planktonic the two types are mixed in most samples the terrestrial kerogens are dominant and occur in all wells in the basal complex and most of its outcrops the planktonic organic matter occurs in exposed melange hemipelagic rocks and rarely in terrigenous turbidites the well specimens do not show significant maturation within the drilled depths suggesting that pooled hydrocarbons originated from deeper levels of the prism and accumulated as a result of migration faults bounding the intraprism thrust sheets probably provided migratory paths for hydrocarbons and water juxtaposed rocks of variable maturity assisted in alteration and biodegradation and led to the formation of structural traps the composition of gases indicates that maturation conditions for gas generation may exist at great depths within the barbados accretionary prism

tectonic relationships between forearc basin strata and the accretionary complex at bath barbados the island of barbados exposes the crestal zone of the accretionary prism of the lesser antilles forearc tertiary rocks at bath barbados include four lithic suites 1 inter bedded volcanogenic and mainly calcareous pelagic rocks the oceanic beds 2 radiolarite 3 siliceous hemipelagic rocks and 4 melange suites 1 3 are in structurally discrete groups of fault packets melange occurs in apparently diapiric crosscutting bodies the oceanic beds occupy a pre pleistocene nappe complex that underlies much of barbados radiolarite and hemipelagic rocks belong to a thick accretionary basal complex and constitute the relative autochthon to the oceanic nappes fault rocks of the sub oceanic fault zone developed during emplacement of the oceanic nappes the lower portion of the tectonic stack is deformed in a major south verging fold couple with wavelength of 250 m lower early emplaced oceanic nappes are restricted to the core of the major synform and are increasingly deformed with depth structures suggest easterly transport of oceanic nappes and upward younging of nappe emplacement indicating that the deformation of oceanic beds propagated westward the oceanic nappes provided a seal to rising fluids transport of nappes may have been facilitated by elevated fluid pressure beneath their sole thrust pre and post nappe folds record north south shortening about shallow west plunging axes consistent with folded accretionary strata elsewhere on barbados deformation occurred under shallow nonmetamorphic conditions the oceanic beds are interpreted as being trapped outer forearc basin strata that have been tectonically shouldered by the arcward migrating prism flank or crest the hemipelagic and radiolarite suites may be offscraped abyssal plain and trench wedge accumulations paleogene lower miocene oceanic strata were deposited at depths between 2 0 and 4 8 km middle miocene oceanic beds accumulated at shallower depths 1 1 5 km thus early miocene was the probable time of inception of major uplift and arcward shouldering in the crestal zone of the accretionary prism accretion of the basal complex of barbados probably occurred in late eocene time

sedimentology and sequence stratigraphy of upper pleistocene carbonates of southeastern barbados west indies 1 upper pleistocene reef associated carbonates of southeastern barbados have been studied in outcrop and core reef terraces formed during glacio eustatic sea level highstands and subsequently uplifted are characterized by thick and areally extensive sequences of allochthonous and autochthonous fore reef calcarenites depositional textures are primarily packstones and grainstones wackestones and coral floatstones are volumetrically less significant sediments are coarse to fine grained reef derived allochems and micrite and autochthonous benthic foraminifera and coralline red algae rates of sediment accumulation of fore reef calcarenites range from about 1 to 4 m 1000 yr although of relatively small scale the carbonate terraces of southeastern barbados provide excellent analogs for sequence stratigraphic concepts in carbonate settings the terraces are primarily highstand systems tract deposits separated by type 1 unconformities these highstand deposits are characterized by reef development and the progradation of fore reef calcarenites extensive fore reef deposits resulted from mechanical erosion of the reef framework on this high energy windward coastline type 1 unconformities are characterized by thin caliche layers developed during lowstand subaerial exposure thin basal transgressive systems tract deposits are characterized by incorporation of extraformational clasts derived from the underlying sequence during sea level rise slope front ero ion vertical shift in the position of freshwater lens and shift in the position of coastal onlap are all consequences of the interplay between eustasy and tectonics these effects and the development of facies geometries on barbados are primarily controlled by the glacio eustatic component inasmuch as rates of eustatic changes of sea level are at least two orders of magnitude greater than the maximum average rates of tectonic uplift

volume loss and defluidization history of barbados the basal complex of barbados island is a tectonic amalgam of eocene strata accreted at an outer deformation front at the end of eocene time the structures and porosity depth function of such sediments give clues to the succession of pore volume loss episodes each of which must have been accompanied by defluidization the basal complex evolved as a perpetually shallow low temperature mass which underwent little or no unroofing in its 36 m y history in the accretionary prism in its early phase of deformation the accretionary structure formed by thrust imbrication perhaps with little concurrent volume loss burial compaction followed tectonic thickening during accretion causing probable large pore volume loss within limits of 0 70 of the initial volume of the imbricate stack a late phase of tectonics has affected barbados and a wide region of the structural high since 20 ma such tectonics reactivated faults in the basal complex which acted as channelways for deep seated and perhaps shallow fluids to emerge from the structural high the late stage tectonics also caused local basins for deposition of sediment cover and emplacement of thrust nappes of forearc basin strata above the basal complex such top loading caused a renewed phase of burial compaction and loss of about 37 or more of the pore volume that existed at the onset of late stage tectonics

mud diapirism and subcretion in the barbados ridge accretionary complex the role of fluids in accretionary processes mud diapirism has recently been recognized in several modern accretionary wedges it provides an important means of dewatering accretionary wedges and should be regarded as an important process for producing the melanges found in both modern and ancient accretionary terranes mud diapirism affects a large area of the barbados ridge accretionary complex the distribution of the mud diapirs appears to be primarily controlled by the presence of underconsolidated terrigenous submarine fan deposits that are being accreted to the complex the frequency of diapir occurrence decreases northward as the fan becomes thinner mud diapirs are absent from the very eastern most part of the complex formed from sediments accreted at its toe with the exception of a few mud volcanoes on the ocean floor in front of the complex the initiation of diapirism appears to be spatially coincident with the onset of subcretion or underplating of sediment to the base of the complex at a ramp between two levels of decollement it is proposed that the release of mud and pore water from the subcreted sediments is a direct or indirect cause of most of the mud diapirism in the accretionary complex there is a range of diapiric form dependent on the viscosity of the mud from mud volcanoes fed by low viscosity mud to higher viscosity mud ridges the diapirs in the eastern areas of the complex are generally mud volcanoes with narrow conduits feeding a surface mound mud ridges are prominent in the western parts of the complex this is interpreted as reflecting a general westward decrease in the fluid content of the accretionary complex bottom simulating seismic reflectors formed by gas hydrate are commonly developed in the areas of mud volcano occurrences the presence of the hydrate indicates that large volumes of methane are being generated at depth in these regions the generation of methane may be contributing to zones of overpressuring in the wedge methane may also be partly responsible for driving the diapiric material to the surface to form mud volcanoes ridges in the subducting oceanic crust beneath the accretionary complex locally enhance diapirism above their crests and southern flanks faults formed later in the development of the complex are more commonly associated with diapirism than those resulting from accretion at the toe of the wedge these later faults play an important role in controlling the sites of individual mud volcanoes chains of mud volcanoes and mud ridges

late pleistocene mixing zone dolomitization southeastern barbados west indies field geochemical and petrographic data for late pleistocene dolomites from southeastern barbados suggest that the dolomite precipitated in the zone of mixing between a coastal meteoric phreatic lens and normal marine waters the dolomite is localized in packstones and wackestones from the algalamphistegina fore reef calcarenite facies stable isotopic evidence suggests that meteoric water dominated the diagenetic fluids responsible for dolomitization carbon isotopes in pure dolomite phases average about 15 0 pdb this light carbon is attributed to the influence of soil gas co2 and precludes substantial mixing with seawater a narrow range of oxygen isotopic compositions coupled with a wide range of carbon compositions attest to the meteoric diagenetic overprint dolomitization likely occurred with as little as a five per cent admixture of seawater strontium compositions of the dolomites indicate probable replacement dolomitization of original unstable mineralogy the dolomite is characterized by low sodium values low concentrations of divalent manganese and iron suggest oxidizing conditions at the time of dolomitization a sequence of petrographic features suggests a progression of diagenetic fluids from more marine to more meteoric early marine diagenesis was followed by replacement dolomitization of skeletal grains and matrix limpid euhedral dolomite cements precipitated in primary intra and interparticle porosity subsequent to replacement dolomitization as waters became progressively less saline dolomite cements alternated with thin bands of syntaxial calcite cement the final diagenetic phase precipitated was a blocky calcite spar cement representing diagenesis in a fresh water lens this sequence of diagenetic features arose as the result of a single fall in eustatic sea level following deposition a stratigraphic eustatic diagenetic model constrains both the timing and rate of dolomitization in southeastern barbados dolomitization initiated as sea level began to fall immediately following the oxygen isotope stage 7 3 high stand some 216 000 yr bp due to the rapidity of late pleistocene glacio eustasy dolomitization locally complete is constrained to have occurred within about 5000 yr

high resolution temporal record of holocene ground water chemistry tracing links between climate and hydrology strontium isotope analysis of precisely dated calcite growth layers in holocene speleothems from barbados west indies reveals high resolution temporal variations in ground water composition and may provide a new approach to documenting the links between climate variability andfluctuations in the hydrologic cycle such as recharge rates andflow paths the speleothems grew in a cave that developed in a fresh water aquifer in upliftedpleistocenereeflimestones threeperiodsofground watersrisotopeevolutionare observed 87 sr 86 srvaluesdecreasedfrom6to4ka increasedfrom4to1ka anddecreased again after 1 ka the sr isotope oscillations appear to record periodic variations in the relative sr fluxes to ground water from exchangeable soil sites vs carbonate mineral reactions asreflectedin 87 sr 86 srvaluesofmodernbarbadosgroundwaters ahydrologic model that explains changes in ground waterflow routes in karst aquifers as a function of amountofrainfallrechargecanaccountforthespeleothemsrisotoperecord independent holoceneclimaterecordsthatindicateamajorperiodofaridityataround1 3 1 1kainthe american tropics correspond with periodic variations in rainfall on barbados that are predicted by this hydrologic model

effects of macro scale bed roughness geometry on flow resistance 1 flow field over macro roughness elements as for example in flow conditions such as of steep mountain streams is investigated by means of a theoretical model and a laboratory study with the main aim of understanding the influence of concentration and planimetric disposition of grain roughness on flow resistance a theoretical model for the average velocity distribution profile over a bed with macro roughness elements is developed herein the model is based on a two layer approach which takes into account the displacement of the main flow due to the drag around the roughness the laboratory study is composed of an extensive set of flume experiments in the case of steep slopes and large relative roughness conditions for a given flow discharge and bed slope various experiments have been carried out with different pattern and spacing of the roughness elements constituted by river pebbles application of the theoretical model to the experimental results enables the determination of how flow resistance is affected by the macro roughness in particular it appears that when the concentration of the roughness is within an optimal range flow resistance is maximum

palaeoclimate sedimentation and soil development during the last glacial stage devensian heathrow airport london uk abstract the langley silt complex overlying the taplow terrace of the river thames at prospect park near heathrow airport west of london is described in terms of lithology carbonate content organic carbon content particle size characteristic macrostructure micromorphology and stratigraphic position the langley silts were deposited by wind or water in cold climate conditions and include evidence for the development of periglacial structures and for devensian lateglacial and holocene soil formation the langley silts rest upon a last interglacial ipswichian ois 5e soil developed in the taplow gravel deposited during the previous cold stage ois 6 during the early devensian and the dimlington stadial and younger dryas of the late devensian wind deposited the silts and during the middle devensian wind and surface wash built up a laminated silt and sand during the middle devensian seasonal dessication formed non sorted polygonal patterns as this laminated unit aggraded but this process ceased with the return to loess deposition in the late devensian argillic soil development occurred during the windermere interstadial only to be disrupted by ground ice processes during the succeeding younger dryas cold phase the site provides one of the most complete sedimentary sequences through the last glacial stage in britain and confirms recent interpretations of soil development and evidence for aeolian deposition in britain during the lateglacial

magnetic properties of k t and e o microspherules origin by combustion abstract magnetic spherules and spheroids are found in association with ir anomalies at the cretaceous tertiary k t boundary and near to the eocene oligocene e o boundary detailed magnetic analyses have been performed on these particles and on bulk k t boundary clay from marine and continental sections coercivity spectra and blocking and curie temperatures of the spheroids and k t boundary clays have been determined and natural remanence and susceptibility measured for the bulk clay and associated strata marine k t boundary clays are strongly magnetic with remanent coercivities similar to those of the spheroids in contrast the continental clays are diamagnetic except for the thin overlying impact layer which contains abundant paramagnetic jarosite along with shocked quartz and feldspar the morphologies mineralogies and magnetic properties of the k t magnetic spheroids are closely analogous to magnetic fly ash spherules this similarity may be related to the presence of carbonaceous soot particles in some marine k t boundary clays and the recent identification of plerospheres and cenospheres in continental boundary clay in contrast tektites and microtektites differ from the k t magnetic spherules in a number of fundamental ways the late eocene particles are magnetically distinct from the k t spheroids both in their coercivity and thermal spectra inaa results on upper cretaceous combustible shales indicate that the anomalous abundances of certain chalcophile and lithophile elements at the k t boundary may be explained in terms of the natural combustion of bituminous sediments

the tectonic fabric of the barbados ridge accretionary complex abstract a map showing near surface structure and other surficial geological features on the barbados ridge accretionary complex has been constructed from gloria long range side scan sonographs detailed bathymetric surveys using seabeam and single and multichannel seismic reflection sections the map shows major variations in the size orientation and style of structures across the accretionary complex that are principally related to the increasing thickness of the complex and the sediment drape upon it and variations along the complex that derive from the northward thinning of sediment accreted from the ocean floor and the effects of ridges in the oceanic basement the development of crossfaulting is mainly associated with basement ridges mud diapirism is abundant in the southern part of the complex and is characteristic of that part of the complex into which turbidites from the orinoco submarine fan are accreted slumping on a variety of scales is another feature of the complex the map is presented at a scale of 1 1063415 4 inches per degree of longitude and also shows bathymetry at 200 m contour intervals

resistance equation for large scale roughness a theory of flow resistance for large scale roughness is developed the main points of which are supported by a flume study from which the outline of a process based resistance equation is determined rivers with large scale roughness have steep slopes and depths of the same order of magnitude as the bed material size flow resistance depends on the form drag of the roughness elements and their disposition in the channel theoretical considerations show resistance to be a function of reynolds number froude number roughness geometry and channel geometry these processes are examined using the results of flume experiments based on five different roughness beds and a wide range of flows semi empirical analysis supports the theory and allows the development of a theoretical power law resistance equation which accounts for most of the processes a summary of the data is presented for use in other research studies

multibeam bathymetric survey and high resolution seismic investigations on the barbados ridge complex eastern caribbean a key to the knowledge and interpretation of an accretionary wedge abstract an important problem of active margin processes is the way accretionary wedges form in response to oceanic crust subduction new data have been obtained along the barbados ridge in front of the lesser antilles tertiary island arc they include high resolution multichannel seismic data and accurate sea beam bathymetric maps both have been used to propose a 3 dimensional interpretation of tectonic and sedimentary processes related to subduction such as oceanic off scraping ductile folding relative uplift and subsidence fore arc basin evolution and mud diapirism they all improved our knowledge of the evolution of accretionary prisms and more specifically of the barbados ridge development in plio quaternary time previous tectonic events are deduced from recently published geological data from barbados and the lesser antilles islands

abrupt last glacial dust fall over southeast england associated with dynamics of the british irish ice sheet loess deposits in southern britain contain a record of dust climate and landscape dynamics over the last glacial yet their age and accumulation rate remain poorly known furthermore the environm

fission track evidence for the source of accreted sandstones barbados a part of the accretionary wedge associated with the lesser antilles arc system is exposed in an area of approximately 50 km in northeastern barbados zircon separates from sandstones collected from the scotland district barbados w i were analyzed using the fission track technique in order to assess the spectrum of source areas from which these terrigenous sediments were derived and to better constrain the timing of deposition results yielded a mixture of ages with strong groupings from 25 80 ma 200 350 ma and greater than 500 ma the youngest population indicates that some of the scotland beds previously dated by paleontologic methods as eocene may actually be as young as late oligocene possible source areas include the lesser antilles arc the netherlands venezuelan antilles arc and the caribbean mountains of venezuela the 200 350 ma population may reflect partially annealed cratonic material an andean component and or material associated with a triassic rifting event the oldest zircons 500 ma and metamict zircons were likely derived from the south american craton detrital feldspars separated from sandstone at the base of chalky mount were analyzed by the 40ar 39ar age spectrum technique results yield what is interpreted to be a slow cooling gradient from 1350 ma to 925 ma and provides additional evidence of a cratonic source for the accreted sediments based on results from this study and paleogeographical constraints it is proposed that source areas for the scotland sandstones of barbados may have included the guayana shield central cordillera uplifted areas of coastal south america and the lesser antilles volcanic arc we speculate that in late oligocene time an immense proto orinoco river system flowed in a northeasterly direction from the shield through the northern venezuelan mountain system cordillera de la costa and araya paria finally emerging into the caribbean sea in the area now occupied by unare depression sediments comprising this deltaic complex and deep sea fan were either deposited directly into the trench or onto the sea floor where they were soon after caught up in the accretionary wedge of the lesser antilles arc system this article contains supplementary material