Increased Sediment Flux Research Articles

Quantification des flux et des stocks solides holocènes dans le bassin de la Loire (France)

Quantity of solid matter yielded and stored during the Holocene period in the whole Loire catchment was calculated from two little catchments located in the Massif Central and in the Parisian Basin. The Massif Central produced tenfold more solid matter than the Parisian Basin (56.10 9 t versus 5,5.10 9 t) and fourfold less dissolved matter (6,9.10 9 t versus 28,2.10 9 t). Solid matter yielded by the Massif Central was evacuated downward to the main rivers where it was mainly stored. Solid matter yielded by the Parisian Basin was mainly stored in the small catchments and a small part only reached the main rivers. Anthropic increase of sediment yield during the last 1000 years quickly increased sediment flux in the river of the Massif Central while it appears to be differed, because solid matter was stored as colluvium on slopes, in the Parisian Basin.

Hillslope evolution by nonlinear, slope‐dependent transport: Steady state morphology and equilibrium adjustment timescales

Soil‐mantled hillslopes are typically convex near the crest and become increasingly planar downslope, consistent with nonlinear, slope‐dependent sediment transport models. In contrast to the widely used linear transport model (in which sediment flux is proportional to slope angle), nonlinear models imply that sediment flux should increase rapidly as hillslope gradient approaches a critical value. Here we explore how nonlinear transport influences hillslope evolution and introduce a dimensionless parameter ΨLto express the relative importance of nonlinear transport. For steady state hillslopes, with increasing ΨL(i.e., as slope angles approach the threshold angle and the relative magnitude of nonlinear transport increases), the zone of hillslope convexity becomes focused at the hilltop and side slopes become increasingly planar. On steep slopes, rapid increases in sediment flux near the critical gradient limit further steepening, such that hillslope relief and slope angle are not sensitive indicators of erosion rate. Using a one‐dimensional finite difference model, we quantify hillslope response to changes in baselevel lowering and/or climate‐related transport efficiency and use an exponential decay function to describe how rapidly sediment flux and erosion rate approach equilibrium. The exponential timescale for hillslope adjustment decreases rapidly with increasing ΨL. Our results demonstrate that the adjustment timescale for hillslopes characteristic of the Oregon Coast Range and similar steep, soil‐mantled landscapes is relatively rapid (≤50 kyr), less than one quarter of the timescale predicted by the linear transport model.

Cenozoic evolution of the eastern Danish North Sea

This paper provides a review of recent high-resolution and conventional seismic investigations in the eastern Danish North Sea and describes their implications for the development of the eastern North Sea Basin. The results comprise detailed time-structure maps of four major unconformities in the eastern Danish North Sea: the Top Chalk surface (mid-Paleocene), near top Oligocene, the mid-Miocene unconformity, and base Quaternary. The maps show that the eastern Danish North Sea has been affected by faulting and salt diapirism throughout the Cenozoic. Carbonate mounds, erosional valleys and pockmark- or karst-like structures were identified at the top of the Upper Cretaceous–Danian Chalk Group.Strike-parallel erosional features and depositional geometries observed at near top Oligocene and at the mid-Miocene unconformity indicate that these major sequence boundaries can be attributed to large-scale lateral changes in sediment supply directions. Increases in sediment flux to the southeastern North Sea at the Eocene/Oligocene transition and in the post-Middle Miocene appear to correlate with similar events world wide and with long term δ18O increases, indicating forcing by global factors, i.e. eustasy and climate. Stratal geometries observed on the seismic data indicate that the so-called ‘Neogene uplift’ of the eastern Danish North Sea may have been hundreds of metres less than previously suggested. It is argued that late Cenozoic uplift of the basin margin and of mountain peaks in southern Norway may have been caused entirely by isostatic uplift of the crust in response to accelerated late Cenozoic denudation and dissection of topography created in the Paleogene. The late Cenozoic periods of accelerated denudation and incision rates were most likely driven by climatic deterioration and long term eustatic lowering rather than active late Cenozoic tectonics, the cause of which is conjectural.A series of shallow thrust structures and an associated system of deep, buried valleys were mapped. Thrust faulting most likely occurred in response to gravitational loading at the margin of an advancing ice sheet, and it was followed by deep incision due to subglacial melt-water erosion, probably during the Elsterian glaciation.

Climatic and tectonic controls on fluvial incision and aggradation in the Spanish Pyrenees

The influences of tectonic and climatic changes on upland river systems are investigated using data from Plio-Pleistocene terraces of the Rio Cinca river in the southern Pyrenees. This river runs transverse to the main thrust fault structures and is a major conduit for sediment delivery to the Ebro Basin. Detailed field mapping, combined with long-profile and palaeohydraulic reconstructions, yields a comprehensive picture of changes in palaeoriver character during the Plio-Pleistocene. As the area is over 150 km from the basin outlet in the Mediterranean Sea, changes in base level are unlikely to have influenced terrace development. Although tectonic activity has exerted a strong control on the position of the river, the main period of thrust propagation pre-dates the terraces and activity has waned from the Pliocene through to the present. It is concluded that the main control on incision in this area is climate, through its influence on sediment supply. Rivers which are starved of sediment by climate change will have the power to incise, whereas aggradational phases are linked to periods of increased sediment flux.

The influence of recent environmental change on flooding and sediment fluxes in the Yorkshire Ouse basin

This study attempts to establish causes of historical variations in flood frequency and magnitude in a 119-year flood stage record at York, and assess its likely effects on fluxes of fine sediment and associated base metal contaminants. Climatic controls of flooding are evaluated through analysis of synoptic flood generation, and large-scale land use changes are also considered in terms of their influence on food frequency and magnitude and sediment availability. The late nineteenth century was characterized by low flood frequency and magnitude, but, as a consequence of upland metal mining, contaminant concentrations and downstream fine sediment delivery would have been high. A further decline in flood frequency and magnitude between 1904 and 1943, combined with the cessation of base metal mining, resulted in a decline of sediment and contaminant fluxes. Between 1944 and 1968 increases in upland drainage, changes in agricultural practices and high flood frequency and magnitude resulted in significantly enhanced sediment fluxes. Rates of flux declined between 1969 and 1977 owing to extremely low flood frequencies and magnitudes. In the last two decades there have been a series of extreme magnitude floods, which through remobilization of mining-contaminated alluvium have resulted in high pollutant metal mining loads, probably approaching those of the late nineteenth century. In an historical context it appears that the LOIS monitoring programme has been undertaken in a period of extremely high flood frequency and magnitude in the UK, and significantly enhanced sediment and contaminant metal flux. Analysis of synoptic flood generation suggests that any future increase in the frequency and vigour of cyclonic atmospheric circulations may result in a higher frequency of extreme floods and consequently increased sediment fluxes.

Reconstruction of Tertiary progradation and clinoform development on the New Jersey passive margin by 2-D backstripping

We have reconstructed the Oligocene to Middle Miocene paleobathymetry and stratigraphy of the New Jersey margin using a modified backstripping technique. By analyzing the geometry of the margin through time, we investigate its response to fluctuating sea level, changing climate, and variable sediment supply during the Tertiary. The reconstructions reveal a change in the margin morphology from a more steeply dipping (1 : 300 to 1 : 500) carbonate ramp in the Eocene to a flatter shelf with a sharp shelf edge at present. This was accomplished by an increase in the terrigenous sediment supply that filled available accommodation and caused progradation across the margin. We link the increase in sediment flux with climatic cooling rather than tectonic processes. The progradation is evidenced by a series of clinoforms whose formation was modulated by sea level and which extend over 100 km across the shelf. The height and dip of the clinoforms increased as they extended onto the deeper parts of the earlier ramp. The Miocene clinoform rollovers at the New Jersey margin had water depths of ∼60–130 m and are interpreted as the edge of a new continental shelf built over the older ramp. Sea-level fall was probably insufficient to drive the Miocene shorelines past the shelf breaks. Thus, measurements of sea-level amplitude based upon `coastal' onlap over the clinoforms are not reliable.

Influence of cryptogamic crust disturbance to wind erosion on sand and loam rangeland soils

A portable field wind tunnel was used to assess the sediment flux rates of loam and sand textured soils in the Mallee region of southeastern Australia. Three levels of crust disturbance (nil, moderate and severe) simulating stock trampling were investigated. The results demonstrated the importance of cryptogamic crusts in binding the soil surface and providing roughness after the soil was moderately disturbed. On the loamy soil, the crust helped maintain sediment flux rates below the erosion control target to 5 g m−1 s−1 for a 65 km h−1 wind measured at 10 m height. Once the crust was severely disturbed, sediment fluxes increased to 1·6 times the erosion target. On the sandy soil, even with no crust disturbance the sediment flux was 1·6 times the erosion control target. Disturbing the crust increased sediment fluxes to a maximum of 6·7 times the erosion control target. Removal of the crust also decreased the threshold wind velocity that resulted in an increase to the risk of erosion from <5 per cent to 20 per cent. © 1998 John Wiley & Sons, Ltd.

Tectonically versus climatically driven Cenozoic exhumation of the Eurasian plate margin, Svalbard: Fission track analyses

Apatite and zircon fission track (FT) thermochronology, as well as regional geologic data, are used to assess the tectonic and climatic mechanisms responsible for exhumation of Svalbard during the last 70 million years. Four stratigraphic profiles in the Paleogene Central basin spanning ∼0.6–1.1 km vertical relief yield apatite FT ages ranging from 28.3 ± 2.3 Ma to 41.8 ± 3.7 Ma, with a mean age of ∼36 Ma and no apparent vertical trend. Apatite ages from the Paleogene Forlandsundet and Kongsfjorden basins and the Late Cretaceous‐Tertiary Svalbard Orogen range from 26.8 ± 3.7 to 55.7 ± 4.7 Ma, with Kongsfjorden data suggesting cooling earlier than the Central basin. Although 10 detrital zircon samples were analyzed from both the Forlandsundet and Central basins, only one sample (from the basal Paleogene deposits in the northwesternmost Forlandsundet basin) had a reset age of 42.4 ± 1.8 Ma. Nonreset detrital zircons elsewhere retain Caledonian and Proterozoic detrital ages. Thermal‐history models derived from apatite track‐length distributions of six Central and Kongsfjorden basin samples indicate a strikingly consistent five‐phase thermal history. Initial uplift of sediment sources to the north and west is recorded as a 70–50 Ma cooling signature. Following deposition from 63 to 49 Ma, the samples were heated until ∼35 Ma, probably as the result of continued sediment burial. This basin filling coincided with a dextral continental transform connecting spreading ridges in the Arctic and Norwegian‐Greenland oceanic basins. The thermal models are consistent with ∼40°–50°C of cooling from ∼35 to 25 Ma. This cooling, which is consistent with deposition patterns both onshore and offshore, appears to be the result of rift‐related uplift and erosion as the Atlantic spreading ridge propagated northward to separate Greenland from Svalbard. Track‐length models are consistent with thermal stasis from ∼25 Ma to 5 Ma, despite sedimentation in offshore Miocene basins suggesting that continued erosion was occurring. We suggest that an increase in the geothermal gradient (by the “Yermak Hot Spot” and associated onshore volcanism) offset cooling expected at this time as the result of continued erosion. A final rapid post‐5 Ma cooling phase of ∼70°C is attributed to ∼2.1 km of erosion in response to intensified glacial denudation. Pliocene‐Holocene cooling, which occurred despite a persistently elevated geothermal gradient, is consistent with offshore evidence for first ice cover in the Norwegian‐Greenland Sea at 5.5 Ma and a substantially increased sediment flux to continental‐slope basins from 5.0 to 0.44 Ma.

Relations between dune morphology, air flow, and sediment flux on reversing dunes, Silver Peak, Nevada

ABSTRACTRelations between wind speed, sediment flux and dune morphology were measured for two reversing dunes situated in the south‐western part of the Silver Peak dunefield in Clayton Valley, west‐central Nevada. The larger dune was 120 m in length with a height of 12.5 m and the smaller dune 80 m long and 6 m high. Both dunes were sharp crested, aligned approximately E‐W perpendicular to the dominant wind direction, and had slightly concave stoss profiles. Twenty‐seven rotating cup anemometers were placed (0.3 m elevation) along N‐S transects on each of the dunes. At each anemometer site a passive wedge‐shaped sediment trap was used to measure sediment flux. Amplification of wind speed was observed towards the crest on the stoss side of both dunes with speed‐up factors (ucrest/Ubase) ranging from 1·50–3·19, with a corresponding increase in sediment flux by 1–2 orders of magnitude. In general, the ratio of crest flux to base flux (qc/qt,) increased with increasing incident basal wind speed on both dunes. Direct measurements of the stoss slope variation in sediment flux relative to the dune crest are in good agreement with Owen's transport model. Friction speed (u) was approximated from near surface (0·3 m) point wind speed. Although not all assumptions of the Owen model are upheld, the modified model performance is sufficiently robust to predict short‐term variation in stoss sediment flux on the study dunes.Improved models that adequately account for variation in sediment flux under changing air flow and transport conditions are necessary for the prediction of longterm evolution of dunes. In this regard, further progress in model development will require increased understanding of the spatial and temporal variability of airflow and the short term response of sediment flux to these flow conditions.

Sequence stratigraphic interpretations in the southern Dead Sea basin, Israel

A chronostratigraphic framework has been developed to date the late Tertiary and Pleistocene fill of the southern Dead Sea basin. This framework is based on a sequence stratigraphic interpretation of seismic data tied to eustatic sea-level curves. The limited biostratigraphic data available for the Dead Sea basin stratigraphy make this chronostratigraphy tentative, but nine third-order sequence boundaries have been identified and related to climatically driven lake-level falls that appear to correlate with eustatic events. In contrast, higher-order sequence boundaries were interpreted to reflect changes in both climate and local tectonics and the consequent sediment supply. The seismic stratigraphic interpretation indicates that fan-delta sediments accumulated at the southern and northern ends of the basin, while lacustrine clastic sediments filled the rest. Interfingering of southern and northern source sediments has been recognized in the northern part of the basin. The changing position of interfingering was interpreted as a response to lake-level changes, the lake-level falls inducing increased sediment flux from the northern source, and lake-level highstands favoring the influx and progradation of sediments from the south.

Fluvial Response in a Sequence Stratigraphic Framework: Example from the Montserrat Fan Delta, Spain

ABSTRACT Exceptional exposure of the Montserrat fan-delta system (Eocene) in northeastern Spain provides an excellent framework to evaluate the alluvial response to sea-level changes over two different time scales. The alluvial system contains multiple fifth-order cycles ( 104 yr) and eight fourth-order cycles ( 105 yr). Fifth-order cycles are characterized by long-distance shoreline migrations and, occasionally, by incised basal scour surfaces but not by changes in fluvial style, lithofacies, or channel stacking pattern. Fourth-order cycles are composed of stacked fifth-order cycles and have non-erosional basal boundaries. Vertical sedimentation rates and channel-stacking patterns change significantly within fourth-order cycles. The lower parts of these cycles, referred to as the transgressive facies tract, show overall shoreline transgression, and associated alluvial deposits contain abundant overbank materials with isolated (ribbon) channel bodies. During this time the supply of terrigenous material to the shoreline was reduced, as indicated by sediment starvation offshore. The overlying middle part of fourth-order cycles, the lower regressive facies tract, differs only in that shoreline is overall regressive, and there is increased clastic supply to the offshore. In the upper part of hese cycles, the upper regressive facies tract, channel-stacking geometries become denser and more sheet-like, shoreline regression is more pronounced, and vertical aggradation rates are inferred to be reduced. Changes in the alluvial system during fourth-order cycles are most pronounced adjacent to shoreline and die away upstream over just a few kilometers--indicating that the base-level signal decays away over the distance of a few backwater lengths (channel flow depth/slope). Higher-frequency (fifth-order) changes in relative sea level appear to produce the largest shoreline migrations, but lower-frequency (fourth-order) changes have more impact on the channel stacking architecture of the alluvial systems. Observed changes in alluvial stacking pattern may be most commonly found in tectonically active, rapidly subsiding, foreland basins because of their back-tilted geometry. We propose a model in which sediment is trapped in the proximal basin during times of rapid tectonic subsidence and attendant relative sea-level rise. Progradation occurs as erosion rates in the mountain belt increase, and rates of subsidence and relative sea-level rise diminish. Changes in alluvial architecture reflect an increase in sediment flux towards the shoreline as less sediment is trapped upstream. Hence, changes in channel-stacking pattern coincident with transgressions and regressions likely reflect the interplay between subsidence and sediment supply in the proximal part of the basin and are not necessari y driven by eustatic sea-level changes.

Axial–transverse fluvial interactions in half‐graben: Plio‐Pleistocene Palomas Basin, southern Rio Grande Rift, New Mexico, USA

Accurate magnetostratigraphic dating of Plio‐Pleistocene alluvium in the Palomas half‐graben permits correlation of transverse and axial deposits, thus enabling analysis of the movement of alluvial facies belts in time and space for the first time. Northern areas show evidence for basinward progradation of footwall‐sourced Matuyama‐age alluvial fan deposits over axial channel belt deposits of the ancestral Rio Grande, despite both deposits having similar deposition rates. This gradual ‘forced’ westward migration of the axial belt was in opposition to ongoing eastward growth of hangingwall‐sourced fans and tectonic tilt imposed by the bounding Caballo normal fault. Fan growth was coincident with a recently proposed gradual climatic shift that may have increased sediment flux out of transverse catchments. It is also possible that continuing tectonic footwall uplift and divided retreat caused catchment areas to increase, contributing to these trends.Southern areas of the Palomas half‐graben feature late Gilbert/early Gauss deposits indicative of rapid westwards progradation of large low‐gradient, footwall‐sourced fans over axial deposits. This ‘forced’ migration of the ancestral Rio Grande may have occurred due to footwall catchment and fan growth consequent upon initiation and growth of the Red Hills Fault. Subsequent eastward movement of the axial channel belt in late Gauss and Matuyama times overwhelmed these large fans. We attribute this to continued tilting on the Red Hills Fault and to development of the Jornada Fault to the south‐east, the axial river belt avulsing north and eastwards through a developing Red Hills/Jornada crossover transfer zone.We conclude generally that facies architecture of axial and transverse elements in half‐graben must reflect both climatic influences and the effects of fault development. Careful field mapping, accurate dating and palaeoclimatic studies are all necessary to determine the relative importance of these controls. Although adequate as broad guides, previous purely ‘fixist’ tectonosedimentary models allow for no fault growth, decay or climatic modulation of facies trends and are thus generally inadequate to explain important aspects rift basin stratigraphy.

Macrogeomorphic evolution of the post‐Triassic Appalachian mountains determined by deconvolution of the offshore basin sedimentary record

A perplexing macrogeomorphic problem is the persistence of topography in mountain ranges that were initially formed by orogenic events hundreds of millions of years old. In this paper, we deconvolve the post‐Triassic macrogeomorphic history of a portion of one of these ranges, the central and northern Appalachians, using a well‐documented offshore isopach sedimentary record of the US Atlantic margin.Topography is an important signature of tectonic, eustatic and/or geomorphic processes that produces changes in the erodible thickness of the crust (ETC). We define ETC as the total thickness of crust that would have to be consumed by erosion to reduce the mean elevation of a landscape to sea level. We use the term ‘source flux’, designated by ν˙, to describe the rate of change in ETC attributed to deep‐seated geological processes such as crustal thickening, crustal extension, magmatic intrusions or dynamic flow in the mantle. In a mountain belt, the rate of change of mean elevation with respect to a base level, designated by ż′, can be represented as ż′ = c(ν˙ − kd z′ −; ėc ) −&amp; hairsp;l˙, where kd is a proportionality constant relating the mean mechanical erosion rate to mean elevation, ėc is the mean chemcial erosion rate, c is a compensation ratio (held constant for Airy isostasy at 0.21) and l˙ is the rate of eustatic sea‐level change. This equation describes the sum of constructive source terms, two destructive erosion terms and a eustatic sea‐level term.We use this simple linear equation to develop a landscape evolution model based on the concept of a unit response function. The unit response function is analogous to a unit hydrograph which describes the relationship between input (rainfall) and output (discharge) in a hydrological system. In our case, we solve for the general relationship between the source flux into the mountain belt and the erosional flux out of the belt. Offshore sediment volumes are used to determine the erosional flux. Drainage basin area is treated as either a constant (pinned drainage divide) or as increasing through time (migrating drainage divide). We use a third‐order polynomial fit to a global sea‐level curve to account for long‐term eustatically driven changes in ETC and in drainage basin area. Chemical erosion is treated as a constant fixed at 5 m Myr−1.We consider two end‐member models. The first is a ‘tectonic’ model in which the source flux is allowed to vary while kd is assumed to be constant over geological time and equal to its mean Pleistocene value of about 0.07 Myr−1. The second is an ‘erodibility’ model in which kd is allowed to vary, reflecting changes in climatic, climatic variables and rock‐type erodibility, while the source flux is held constant at zero. The ‘tectonic’ model reveals four important increases in the source flux, ranging from 200 to 2000 m Myr−1 that occur over short (&lt;10 Myr) time spans, followed by a protracted period (&gt;25 Myr) where ν˙ drops below zero to values of −1000 to −6000 m Myr−1. The ‘erodibility’ model produces a topography that decays in a step‐like fashion from an initial mean elevation that ranges between ∼1800 and 2300 m.These models cannot unequivocally distinguish the relative importance of tectonic vs. climatic processes in the macrogeomorphic evolution of the post‐rift Appalachians, but they do provide some first‐order quantitative prediction about these two end‐member options. In light of existing stratigraphic, geological and thermochronological data, we favour the tectonic model because most of the events correlate well in time and form with known syn‐ and post‐rift magmatic events. Nevertheless, the most recent episode of increased sediment flux to the offshore basins during the Miocene remains difficult to explain by either model. Limited evidence suggests that this event may reflect asthenospheric flow‐driven uplift and the development of dynamically supported topography at a time when mechanical erosion rates were increasing in response to a cooling terrestrial climate.

Corals to rudists—an environmentally induced assemblage succession

Corals and rudist bivalves commonly coexisted on the shallow carbonate platforms of the Cretaceous Tethyan Realm. Usually, the carbonate platforms show lateral and vertical biotic zonations. In general, the vertical biotic zonation relates directly to a consistent lateral biofacies pattern: coral or coral-rudist assemblages occupy more open zones of the platform, and rudist-dominated assemblages, the more restricted zones. Several corresponding coral-to-rudist assemblage successions in carbonate platforms have previously been widely interpreted as ecological successions. We document some coral-to-rudist assemblage successions, in a well preserved Upper Cretaceous section, in the Tremp area of the South Central Pyrenean Unit, which are interpreted, by contrast, as environmentally-induced faunal replacements. A repeated assemblage succession in the eastern part of the section begins with a coral-dominated assemblage, and passes up gradually to a mixed coral and rudist assemblage that is succeeded by a paucispecific unit of hippuritid elevator congregations. The lower coral-rudist units of such successions represent relatively more open, marine conditions, and the uppermost hippuritid unit relatively more restricted marine conditions. This assemblage succession was formed in response to a change in the depositional setting caused by sediment accumulation. Two consecutive coral-dominated assemblages, overlain by mixed coral and rudist assemblages, constitute a second kind of a sedimentary coral-to-rudist assemblage succession discussed in this paper. Its regular recurrence makes up the uppermost part of the section in the East. The predominance of platy to low domal coral colonies in the lower units suggests that conditions were initially favourable for horizontal coral growth. As the habitat changed, these coral assemblages were replaced by massive-tabular to large domal coral colonies, with the large hippuritid Vaccinites , and were sometimes succeeded, in turn, by clustered hippuritid elevators. The most important factor causing the replacement was probably the increase in sediment flux, associated with turbidity. This change in the depositional environment eventually favoured the establishment of rudist elevators, well adapted to moderate and intermittent to continuous accumulation of sediment.

Development of deflation lag surfaces

ABSTRACTA series of wind tunnel tests were carried out to investigate the development of deflation lags in relation to the non‐erodible roughness element concentration. Glass spheres (18 mm in diameter) were placed along the complete length of the wind tunnel working section in regular staggered arrays using three different spacings (d=18, 30 and 60 mm) and completely covered with a 0.27‐mm erodible sand. A pre‐selected free stream velocity above threshold (8m s−1) was established above the surface and the sediment transport measured at 2‐s intervals using a wedge‐shaped trap in which an electronic balance is incorporated. Throughout each test, the emerging lag surface was periodically photographed from above at two locations upwind of the trap. The photographs were electronically scanned and analysed to calculate the lag element coverage and location, as well as mean height and frontal area for each time period. Test results indicate that lag development has a profound effect on both the sediment flux and wind profile characteristics. Initially, there is an increase in sediment flux above that for a rippled sand bed because of increased erosion around and reduced kinetic energy loss in highly elastic collisions with the emerging roughness elements. With further emergence, a dynamic threshold is reached whereupon the sediment flux decreases rapidly, tending towards zero. At this point, the supply of grains to the air stream through fluid drag follows a reduction in aerodynamic roughness and, therefore, surface shearing stress. At least as important is the lesser potential for grain ejection at impact because of reduced momentum imparted from the air stream during saltation. Although recent shear stress partitioning models indicate when particle movement may commence on varying surfaces, our experimental results demonstrate that this partitioning has a further direct bearing upon the saltation flux ratio.

Does Local Resuspension Maintain the Benthic Nepheloid Layer in Southeastern Lake Michigan?

Time-series observations of water temperature, water transparency, and current velocity at four stations in southeastern Lake Michigan show that the benthic nepheloid layer is probably not maintained by the local resuspension of bottom sediment. Local resuspension may occasionally occur in the deep parts of the lake, but it seems likely that vertical mixing and offshore advection of sediment-laden water maintain the benthic nepheloid layer during most of the year. Although sediment trap data have been interpreted as indicating that local resuspension does occur, it is more likely that the increased sediment fluxes observed near the bottom are due to vertical redistribution of material already in suspension.

Climate Change: Numerical Modelling of Sedimentation and Coastal Processes, Eastern Canadian Arctic

We use three sequential [open quotes]water-sediment transport[close quotes] numerical models to evaluate changes in fluvial sediment outputs for two near-future contrasting climate-change scenarios (warmer summers or warmer and moister winters) for the eastern Canadian Arctic. Our integrated model generates values for deposition into a fiord basin. The effects of climate warming are imposed on 200-yr fluctuations in relative sea level (base level) that in turn have a long-term forcing associated with late Quaternary glacial unloading. Thus some areas are submerging (around the periphery of the former ice sheet) and others are emerging (inland at the heads of fiords). Small, short-term changes in sea level may involve meltwater supply to the world ocean from the melting of glaciers and ice sheets, and sea water expansion with a warming ocean. Alternatively, large ice sheets may also thicken over the short term (200 yr), thereby withdrawing water from the global hydrological cycle. Both climate scenarios result in an increased sediment flux over the next 200 yr. However, warmer summers will have the largest impact by (1) causing ice caps to melt, (2) inducing more expansive and turbid river plumes, (3) increasing progradation of the coastline into the sea, (4) raising relative sea level,more » and by (5) increasing the number and size of turbidity currents generated off river mouths. Warmer and moister winters that may lead to deviations from these impacts include (1) growth of ice caps, (2) fall of relative sea level, (3) stability of coastlines as increased sediment delivery keeps pace with changes in regional relative sea level, and (4) fewer basin turbidity currents, as more of the bedload is trapped on top of sandur surfaces. 55 refs., 11 figs., 2 tabs.« less

The large‐scale dynamics of grain‐size variation in alluvial basins, 2: Application to syntectonic conglomerate

ABSTRACTThe concept of‘syntectonic’ conglomerate is based on the idea that gravel progradation is mainly generated by an increase in tectonic uplift and erosion of a source area with attendant increase in sediment flux supplied to a basin. However, other mechanisms, such as changes in basin subsidence rates, sorting of supplied sediment, and capability of transporting streams, can also lead to progradation and be difficult to distinguish from a syntectonic origin. Here we use our previously developed model to help understand the origin of gravel progradation in three Neogene alluvial basins ‐ the Bermejo Basin of Argentina, the Himalayan Foreland Basin, and the San Pedro Basin of southern Arizona ‐ all of which have available high‐resolution magnetostratigraphy. Interpretation of the origin of gravel progradation in these basins begins with calculation of basin equilibrium time, which is the time‐scale required for the streams to reach a steady‐state profile, assuming constant conditions.We then compare the time‐scale of the observed changes in the basin with the equilibrium time to determine if and how the model can be applied to the stratigraphic record. Most of the changes we have studied occur on time scales longer than the equilibrium time (‘slow variations’), in which case the key to interpretation is the relationship between overall grain‐size change and sedimentation rate in vertical sections.Of the three examples studied only one, the Bermejo Basin, is consistent with the traditional model of syntectonic progradation. Overall progradation in the two other basins is most consistent with a long‐term reduction in basin subsidence rates. In addition, short‐term variation in diffusivity or sediment flux, probably climatically driven, is the most likely control of small‐scale progradation of gravel tongues in the San Pedro Basin. These results, along with observations from other basins, suggest that subsidence is clearly an important control on clastic progradation on ‘slow’ time scales (i.e. generally a million years or more). If subsidence rates are directly linked to tectonic events, then subsidence‐driven progradation marks times of tectonic quiescence and is clearly not syntectonic in the traditional sense.These examples show that the model can be useful in interpreting the rock record, particularly when combined with other traditional basin‐analysis techniques. In particular, our results can be used to help discriminate between clastic progradation due to tectonic origin and progradation resulting from other mechanisms in alluvial basins.

On the manufacture of the carbon elements for bunsen batteries

Mechanisms of forearc basin evolution remain not very well understood, mostly because of insufficient resolution of their temporal evolution and potential drivers. The convergent margin of northwest Colombia where the San Jacinto and Lower Magdalena Valley forearc basins are located, offers a unique opportunity to study such mechanisms. The formation of the Lower Magdalena amagmatic, forearc basin occurred in a stable setting from the Oligocene to the present, characterized by the slow and nearly orthogonal, low-angle subduction of the Caribbean plateau. We use an exceptional regional database to reconstruct the subsidence, extension, sedimentation and paleo-geographic history of the Lower Magdalena forearc basin. We propose possible mechanisms controlling its evolution, in the absence of major changes in plate kinematics and in a low-angle subduction setting. Following the collapse of a pre-Oligocene magmatic arc, late Oligocene to early Miocene fault-controlled subsidence allowed initial basin fill at relatively low sedimentation rates. After the connection of the Lower and Middle Magdalena valleys, the proto-Magdalena river in the north and the proto-Cauca river in the south both started delivering high amounts of sediments in middle Miocene times. Fault controlled subsidence was gradually replaced by subsidence due to increased sedimentary load. Increase in sediment flux would have also caused the formation of an accretionary prism, weakened the plate interface through lubrication with fluid-rich sediment and initiated underplating, with the development of forearc highs in the San Jacinto area. Inherited basement structures allowed the tectonic segmentation of the basin with the formation its two depocenters (Plato and San Jorge). Our results highlight the fundamental role of sediment flux, of the basement structure and of flat subduction on the evolution of forearc basins such as the Lower Magdalena.