Enhancement in wind-driven sand transport by electric fields

Aeolian grain transport is a powerful erosion mechanism and a significant factor affecting atmospheric dynamics by the creation of particulate aerosols. Electric fields, both natural and man-made, occur widely at the Earths surface in environments where granular material is found. Such electric fields may induce electrification of granular material and affect the transport dynamics of the grains. In this laboratory study we show that the influence of such electric fields is significant at field strengths well below that at which breakdown occurs and present a simple semi-empirical expression which allows this mechanism to be quantified.     

川东北地区异常高压形成的地温场背景

利用四川东北地区现有的钻井测温数据及测定的岩石样品热导率数据,计算了12口钻井的大地热流值. 结果表明,川东北地区现今地温梯度为18～25 ℃/km,平均21 ℃/km,大地热流值介于41～57 mW/m2之间,平均为49 mW/m2. 在此基础上,利用镜质体反射率(Ro)资料对研究区热史进行了恢复,认为研究区在255Ma左右古热流达到最高值(62～70 mW/m2),此后热流持续降低直到现今;研究区中-新生界不整合面的剥蚀厚度最大,可达约2100 m;异常高压是在低热流的地温场背景下形成的.     

OVERVIEW OF DAM GULLY EROSION RESEARCH

Traditionally gully erosion has been identified with the dissection of the landscape in agricultural settings but it is also recognized as a prevalent erosion feature in earthen dam auxiliary spillways and embankments. Flows through earthen spillways and over dam embankments, due to large rainfall events, have the potential to erode and breach the dam or spillway and result in catastrophic releases from the reservoir. The gully erosion process in an earthen spillway or on an embankment can be characterized by stages of initiation, development, and migration of a headcut. A headcut is defmed as a near vertical drop at the upstream end of a gully. The rate of headcut migration is important in determining the breach potential of an earthen spillway and dam embankment. A research program is being conducted to examine the gully erosion processes of earthen dam auxiliary spillways and embankments. This paper describes： l ） the unique test facilities constructed to examine the dominant factors affecting the erosion of earthen spillways and embankments; 2） the observations of the erosion processes and results to date; and 3） the predictive relationships that have been developed for dam gully erosion research at the ARS Hydraulic Engineering Research Unit laboratory in Stillwater, OK.     

An incomplete correlation between pre-salt topography,top reservoir erosion,and salt deformation in deep-water Santos Basin (SE Brazil)

In deep-water Santos Basin, SE Brazil, hypersaline conditions during the Aptian resulted in the accumulation of halite and carnallite over which stratified evaporites, carbonates and shales were folded, translated downslope and thrusted above syn-rift structures. As a result, high-quality 3D seismic data reveal an incomplete relationship between pre-salt topography and the development of folds and thrusts in Aptian salt and younger units. In the study area, three characteristics contrast with known postulates on passive margins’ fold-and-thrust belts: a) the largest thrusts do not necessarily occur where the salt is thicker, b) synthetic-to-antithetic fault ratios are atypically high on the distal margin, and c) regions of intense folding do not necessarily coincide with the position of the larger syn-rift horsts and ramps below the salt. Regions marked by important erosion and truncation of pre-salt strata, uplifted and exposed sub-aerially before the deposition of Aptian salt, can form structural lows at present or be part of horsts uplifted after the Aptian. This is an observation that suggests significant intra-salt shear drag above pre-salt structural highs during Aptian-Late Cretaceous gravitational gliding, but not on younger horsts and ramps reactivated after the main phase of salt movement. Either formed by drag or sub-aerial erosion, strata truncation below the Aptian salt does not correlate with the present-day pre-salt structure in terms of its magnitude and distribution. In addition, there is a marked increase in deformation towards the distal margin, where low-angle thrusts are ubiquitous on seismic data. The geometry and large synthetic-to-antithetic fault ratios of post-salt strata on the distal margin lead us to consider a combination of gravitational gliding of salt from the northwest and ridge push from the east as responsible for the observed styles of salt deformation.     

Land degradation assessment based on environmental geoindicators in the Fortaleza metropolitan region,state of Ceará, Brazil

This paper summarizes the results obtained by land degradation assessment in the Fortaleza Metropolitan Region, in the state of Ceará, Brazil. Area assessment was done in a two-phase study: the characterization of the environmental components by field and laboratory work, and a more detailed study in the degraded sites. Environmental geoindicators were used to classify the degradation level for each drainage basin as low, intermediate, or high. Coastal erosion, gravitational mass movements, dune movements, water erosion, sedimentation, water pollution, sanitary landfills in inappropriate sites, caves and abandoned mines of aggregates exploitation and occupation in swamp areas are the main land degradation sources registered in the region. Among the drainage basins for degradation level, 5 were classified as high, 3 as intermediate, and 4 as low. These problems have affected the people living in the region and demanded heavy investments to rehabilitate degraded areas.     

Emergence of the Canadian Rockies and adjacent plains: A comparison of physiography between end-of-Laramide time and the present day

The landscape of the Canadian Rockies in southern Alberta is not a direct result of constructional processes; that is, the ridges and peaks have not been pushed into the positions in which we see them today. Tectonic activity provided original elevation but not mountains: at the end of Laramide time, what are now the front ranges and foothills of the Rockies comprised a high-elevation upland of relatively low relief. The present mountain physiography is the result of 55–60 million years of post-orogenic differential erosion, in which more resistant rocks have been left at higher elevations than less-resistant rocks.The Canadian Rockies and the foothills are developed in a thin-skinned, thrust-and-fold belt created during the Laramide Orogeny; the adjacent Interior Plains cut across foreland basin sediments derived from the mountains. The mountains currently consist of large parts of ridges of well-indurated Paleozoic and, locally, Proterozoic rock alternating with valleys developed in soft Mesozoic clastic rock. In the foothills, where the soft Mesozoic rock is at the surface, relief is subdued, but ridges of more-resistant sandstone rise above shaley lowlands. The plains are relatively flat but also contain erosional outliers of higher paleo-plains-surfaces.Numerous lines of evidence suggest that the mountains and foothills have lost several kilometers of overburden since the end of the Laramide Orogeny, while the western plains have lost at least 2 km, requiring that the local relief of the mountains and foothills that we see is erosional in origin. Local physiography is adjusted to lithology: the mountains have high relief because the exposed sub-Mesozoic rocks can hold up high, steep slopes, whereas the foothills have low relief because the underlying Cretaceous rocks cannot hold up high, steep slopes. The east-facing escarpment at the mountain front is a fault-line scarp along a low-angle thrust.Mesozoic rocks involved in the deformation originally extended all the way across the thrust and fold belt, and physiography of the belt at the end of Laramide time (60–55 Ma) depended mainly on whether Mesozoic or Paleozoic/Proterozoic rocks were exposed at the surface at that time. A reconstruction using critical-taper theory generally agrees with reconstructions from earlier stratigraphic and paleothermometry studies: what are now the front ranges at the eastern edge of the Rocky Mountains were mostly or perhaps entirely covered with Mesozoic rocks and despite that high elevation had a hilly, not mountainous, character. The main ranges, in the central Rocky Mountains, were in part stripped of Mesozoic cover by then and more mountainous. Treeline was higher then, and the thrust belt may have been largely or entirely vegetated. Generation of modern relief in the front ranges, including the escarpment at the mountain front, had to await stripping of Mesozoic rocks and incision of rivers into harder substrates in post-Laramide time.The Interior Plains are an erosional surface that was cut 1 to 3 km below the aggradational top of the foreland basin sediments. Although some of the present low local relief of the plains results from weakness of underlying Cretaceous/Tertiary rocks, the low relief is probably largely related to the process of denudation.     

Reconstructing ancient topography through erosion modelling

One of the main aims of geomorphology is to understand how geomorphic processes change topography over long time scales. Over the last decades several landscape evolution models have been developed in order to study this question. However, evaluation of such models has often been very limited due to the lack of necessary field data. In this study we present a topography based hillslope erosion and deposition model that is based on the WATEM/SEDEM model structure and works on a millennial time scale. Soil erosion, transport and deposition are calculated using slope and unit contributing area. The topography is iteratively rejuvenated by taking into account modelled erosion and deposition rates, thereby simulating topographic development backwards in time. A first attempt has been made to spatially evaluate the model, using detailed estimates for historical soil erosion and sediment deposition volumes, obtained from an augering campaign in a small catchment in the Belgian Loess Belt. The results show that the model can simulate realistic soil redistribution patterns. However, further research is necessary in order to deal with artificial flaws that cause routing problems and significantly influence results. Common problems and issues related to this type of backward modelling are also discussed.     

Cenozoic uplift of Nuussuaq and Disko, West Greenland—elevated erosion surfaces as uplift markers of a passive margin

Remnants of a high plateau have been identified on Nuussuaq and Disko, central West Greenland. We interpret the plateau as an erosion surface (the summit erosion surface) formed mainly by a fluvial system and graded close to its former base level and subsequently uplifted to its present elevation. It extends over 150 km east–west, being of low relative relief, broken along faults, tilted westwards in the west and eastwards in the east, and having a maximum elevation of ca. 2 km in central Nuussuaq and Disko. The summit erosion surface cuts across Precambrian basement rocks and Paleocene–Eocene lavas, constraining its age to being substantially younger than the last rift event in the Nuussuaq Basin, which took place during the late Maastrichtian and Danian. The geological record shows that the Nuussuaq Basin was subjected to subsidence of several kilometres during Paleocene–Eocene volcanism and was transgressed by the sea later during the Eocene. By comparing with results from apatite fission track analysis and vitrinite reflectance maturity data, it is suggested that formation of the erosion surface was probably triggered by an uplift and erosion event starting between 40 and 30 Ma. Surface formation was completed prior to an uplift event that started between 11 and 10 Ma and caused valley incision. This generation of valleys graded to the new base level and formed a lower erosion surface, at most 1 km below the summit erosion surface, thus indicating the magnitude of its uplift. Formation of this generation of valleys was interrupted by a third uplift event also with a magnitude of 1 km that lifted the landscape to near its present position. Correlation with the fission-track record suggests that this uplift event started between 7 and 2 Ma. Uplift must have been caused initially by tectonism. Isostatic compensation due to erosion and loading and unloading of ice sheets has added to the magnitude of uplift but have not significantly altered the configuration of the surface. It is concluded that the elevations of palaeosurfaces (surfaces not in accordance with present climate or tectonic conditions) on West Greenland's passive margin can be used to define the magnitude and lateral variations of Neogene uplift events. The striking similarity between the landforms in West Greenland and those on many other passive margins is also noted.     

Thin-sheet modelling of lithospheric deformation and surface mass transport

We study the effects of incorporating surface mass transport and the gravitational potential energy of both crust and lithospheric mantle to the viscous thin sheet approach. Recent 2D (cross-section) numerical models show that surface erosion and sediment transport can play a major role in shaping the large-scale deformation of the crust. In order to study these effects in 3D (planform view), we develop a numerical model in which both the dynamics of lithospheric deformation and surface processes are fully coupled. Deformation is calculated as a thin viscous layer with a vertically-averaged rheology and subjected to plane stresses. The coupled system of equations for momentum and energy conservation is solved numerically. This model accounts for the isostatic and potential-energy effects due to crustal and lithospheric thickness variations. The results show that the variations of gravitational potential energy due to the lateral changes of the lithosphere–asthenosphere boundary can modify the mode of deformation of the lithosphere. Surface processes, incorporated to the model via a diffusive transport equation, rather than just passively reacting to changes in topography, play an active role in controlling the lateral variations of the effective viscosity and hence of the deformation of the lithosphere.     

Modelling differential catchment response to environmental change

The CAESAR (Cellular Automaton Evolutionary Slope And River) model is used to demonstrate significant differences in coarse sediment transfer and alluviation in medium sized catchments when responding to identical Holocene environmental changes. Simulations for four U.K. basins (the Rivers Swale, Ure, Nidd and Wharfe) shows that catchment response, driven by climate and conditioned by land cover changes, is synchronous but varies in magnitude. There are bursts of sediment transfer activity, generally of rapid removal but with some sediment accumulation ‘spikes’, with longer periods of slow removal or accumulation of sediment in different valley reaches. Within catchments, reach sensitivity to environmental change varies considerably: some periods are only recorded in some reaches, whilst higher potential sensitivity typically occurs in the piedmont areas of the catchments modelled here. These differential responses appear to be highly non-linear and may relate to the passage of sediment waves, by variable local sediment storage and availability, and by large- and small-scale thresholds for sediment transfer within each catchment. Differential response has major implications for modelling fluvial systems and the interpretation of field data. Model results are compared with the record of dated alluvial deposits in the modelled catchments.