摘要:

ABSTRACTThe erodibility of natural estuarine sediments was measuredin sitalong a longitudinal transect of Manitounuk Sound, Hudson Bay, using the benthic flume Sea Carousel. Sedimentation processes along the transect varied from continuous, rapid, post‐glacial sedimentation in the inner Sound, to glacial outcrops and seabed reworking of the outer Sound. The grain size and physical bulk properties reflect changes in depositional environment and correlate with sediment erosion threshold stress (τc), erosion rate (E), erosion type and still‐water mass settling rate. There was a steady increase in τc(0·8–2·0 Pa) with distance down the Sound in parallel with the decreasing sedimentation rate (0·003–0·001 m yr−1) and increasing sediment bulk density (1650–2010 kg m−3). The near‐surface friction coefficient varied up to 68° in proportion to the clay content of post‐glacial material. Glacial sediments were characterized by variable results and generally higher friction coefficients. Seabed erosion in Sea Carousel began with surface creep of loose aggregates, pellets and organic debris. This was followed by Type I bed erosion at rates that varied between 0·0002 and 0·0032 kg m−2s−1(mean 0·0015). Type I peak erosion rate was inversely related to applied bed shear stress (τo). Type II erosion succeeded Type I, often after a broad transitional period. Simulations of suspended sediment concentration in Sea Carousel were made using four commonly used erosion (E) algorithms. The best results were obtained using Krone's dimensionless ratio relationship:E=M(τo/τc‐1). Simulations were highly sensitive to the definition of erosion threshold with sediment depth [τc(z)]. Small errors in definition of τc(z) caused large errors in the prediction of suspended sediment concentration which far exceeded

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1996.tb01455.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTHigh‐speed photography was used to record saltating sand grains colliding with a horizontal, noncohesive bed of similarly sized grains. Impacting grain/bed interaction is discussed in general. The process, as observed from the films, is then described in terms of the apparent bed contact length (ABCL) and various parameters of the impacting grains and any ejected grains. Examples are given of typical behaviour of bed grains in response to impacting grains of different sizes. Saltating grains that are large in comparison to the bed grains they encounter at collision can churn up the surface layers of soils and sediments, so that previously buried grains become available for entrainment. This process is discussed in relation to the potential release of dust particles into the airflo

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1996.tb01456.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTFew studies have examined the hydrodynamic behaviour of carbonate sediments. The data presented here are the result of preliminary research on entrainment in well‐ and poorly sorted carbonate sands. Experiments were performed using naturally occurring sediments in a tilting, recirculating freshwater flume. Results indicate that when of similar size, shape and density, the transport threshold of carbonate sands is similar to that of quartz. However, owing to their lower density and often platy or irregular shape, skeletal sands require a lower shear stress to initiate transport. Because the density of carbonate particles may increasingly vary with grain size, the threshold of motion in coarse carbonate grains may differ more markedly from that of quartz. In poorly sorted samples, results show that the coarse‐grained constituents move before the finer‐grained components. Grain properties and boundary‐layer dynamics are believed to explain this phenomenon. Rollability of the larger grains combined with physical trapping and immersion within a low velocity sublayer are believed to prevent finer particles from moving. Given the appropriate sediments and flow conditions, it may therefore be possible to deposit and preserve fine‐grained sediments in a flow regime typically thought to transport such

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1996.tb01457.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTA field experiment was conducted to measure the flux and speed of wind‐blown sand under known conditions in a natural setting. The experiment, run at Pismo Beach, California, involved a tract 100 m long (parallel with the wind) by 20 m wide. The site was instrumented with four arrays of anemometers to obtain wind velocity profiles through the lower atmospheric boundary‐layer, temperature probes to determine atmospheric stability and wind vanes to determine wind direction. From these measurements, wind friction speeds were derived for each experimental run. In order to measure sand saltation flux, a trench 3 m long by 10 m wide (transverse to the wind direction) by 0·5 m deep was placed at the downwind end of the tract and lined with 168 collector bins, forming an ‘egg‐box’ pattern. The mass of particles collected in each bin was determined for four experimental runs. In order to assess various sand‐trap systems used in previous experiments, 12 Leatherman traps, one Fryberger trap and one array of Ames traps were deployed to collect particles concurrently with the trench collection. Particle velocities were determined from analysis of high‐speed (3000 and 5000 frames per second) motion pictures and from a particle velocimeter. Sand samples were collected from the trench bins and the various sand traps and grain size distributions were determined. Fluxes for each run were calculated using various previously published expressions, and then compared with the flux derived from the trench collection. Results show that Bagnold's (1941) model and White's (1979) equation most closely agree with values derived from the trench. Comparison of the various collector systems shows that the Leatherman and Ames traps most closely agree with the flux derived from the trench, although these systems tended to under‐collect particles. Particle speeds were measured from analysis of motion pictures for saltating particles in ascending and descending parts of their trajectories. Results show that particle velocities from the velocimeter are in the range 0·5–7·0 m s−1, compared to a wind friction velocity of 0·32–0·43 m s−1and a wind velocity of 2·7–3·9 m s−1at the height of the particle measurements. Descending particles tended to exceed the speeds of

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1996.tb01458.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTSecondary precipitates (iron oxide, calcite, etc.) are currently observed in cold‐climate Pleistocene deposits. Some have micro‐ and ultrastructures quite different from precipitates of vadose, phreatic and biogenic origin, and seem to have originated by freezing processes. The microstructures of calcite coatings from a Pleistocene cryopediment in the Mendoza Pre‐Cordillera, from a Lower Pleistocene cryogenic slope deposit in Western Transbaikalia and from the present active layer in Antarctica are described. They show similar patterns: fibrous crystals often consisting of piles of platelets, some with internal holes, assembled in millimetre‐scale fringes on the lower face of clasts. Observational (mainly fabric) evidence confirms that such peculiar crystals are formed during freezing. The features are unknown in other climates and, when found in past sediments, can be diagnostic of cryogenic palaeoenvir

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1996.tb01459.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTThe effect of mid‐air collisions on aeolian saltation is investigated using concentration profiles and grain velocities predicted by a numerical saltation model. The probability of a mid‐air collision is found to be greater at high wind speeds. It is also found that mid‐air collisions tend to reduce the number of grain/bed impacts and thus reduce the number of ejecta near the bed; this, in turn, reduces the intensity of mid‐air collisions. It is suggested that this feed‐back mechanism significantly influences transport rates at high wi

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1996.tb01460.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTEarly concretionary and non‐concretionary siderites are common in subsurface Triassic sandstones and mudrocks of the Rewan Group, southern Bowen Basin. A detailed petrological and stable isotopic study was carried out on these siderites in order to provide information on the depositional environment of the host rocks. The siderites are extremely pure, containing 85–97 mol% FeCO3, and are commonly enriched in manganese. δ13C (PDB) values are highly variable, ranging from ‐ 18·4 to +2·9‰, whereas δ18O (PDB) values are very consistent, ranging from ‐ 14·0 to ‐ 10·2‰ (mean= ‐ 11·9 ± 1·0‰). The elemental and oxygen isotopic composition of the siderites indicates that only meteoric porewaters were involved in siderite formation, implying that host rocks accumulated in totally non‐marine environments. The carbon isotopic composition of the siderites is interpreted to reflect mixing of bicarbonate/carbon dioxide generated by methane oxidation and methanogenesis. Very low δ13C values demonstrate that, contrary to current views, highly13C‐depleted siderite can be produced at shallow burial depths

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1996.tb01461.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTComplex structure, poorly understood sedimentology and poor biostratigraphic control make the Upper Jurassic Humber Group of the South Central Graben one of the least understood and most complex hydrocarbon reservoirs of the North Sea. Detailed logging of available core from 19 exploration wells has been combined with an improved understanding of the relevance of trace fossils and a recognition of important base‐level variations to provide a greatly enhanced understanding of the depositional system active within the area at that time. A new sedimentological model, based upon the distribution of facies and facies associations, illustrates that Upper Jurassic structure and consequent basin geometry were the principal controls upon the distribution of depositional environments. Rifting and second‐order transgression controlled the back‐stepping onlap patterns observed and higher frequency base‐level fluctuations controlled the internal architecture of individual sandbodies. The model presented accounts for features of these deposits that were previously considered anomalous, such as the thickness of bioturbated sandstones, paucity of foreshore deposits and complex age relationships o

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1996.tb01462.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTDetailed sedimentological and stratigraphic analyses were carried out on seven Kullenberg cores collected across the Brazilian continental margin during the French cruises Byblos and Apsara III, in order to highlight the factors controlling the sediment flux distribution in the Southern Brazil Basin during the late Quaternary. On the continental slope and upper continental rise above 3000 m depth, sediment fluxes are important and highly variable (4·2–14·2 g cm−210−3yr). The sediments show a pelagic or turbiditic character, depending on the width of the shelf and proximity of canyons. The material is characterized by high kaolinite contents, and originates from the coastal rivers draining the South American continent north of Rio de Janeiro. On the middle continental rise between 3000 and 4000 m depth, sediment fluxes are the lowest observed in the area (0·9 g cm−210−3yr), because terrigenous input is trapped at shallower depths on the São Paulo Plateau. Pelagic settling is the dominant process. In the deep domains, below 4000 m depth, contouritic accumulations are developed on the path of the northwards moving Antarctic bottom water (AABW) currents. The deposits consist of fine‐grained silty‐clayey muds with very low carbonate contents. The sediment fluxes (1·45 g cm−210−3yr) are higher than on the middle continental rise, as a consequence of fine‐grained terrigenous supply derived from higher latitudes (Argentine Basin and Southern Ocean), and transported in the basin through the Vema Channel by the AABW currents. This material is characterized by high smectite and chlorite contents. These data reveal large sediment flux variations which are linked to distinct depth‐related domains. Such a distribution is the consequence of the presence of two available sources of terrigenous sediments: (1) the Brazilian continental areas with a downslope material transport and a sediment distribution controlled by the morphology of the margin, and (2) the Argentine Basin with an alongslope material transport by deep‐sea currents which dominate the sediment

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1996.tb01463.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTA 1600‐m‐thick succession of the Miocene Horse Camp Formation (Member 2) exposed in east‐central Nevada records predominantly terrigenous clastic deposition in subaerial and subaqueous fan‐delta environments and nearshore and offshore lacustrine environments. These four depositional environments are distinguished by particular associations of individual facies (14 defined facies). Subaerial and subaqueous fan‐delta facies associations include: ungraded, matrix‐and clast‐supported conglomerate; normally graded, matrix‐ and clast‐supported conglomerate; ungraded and normally graded sandstone; and massive to poorly laminated mudstone. Subaqueous fan‐delta deposits typically have dewatering structures, distorted bedding and interbedded mudstone. The subaerial fan‐delta environment was characterized by debris flows, hyperconcentrated flows and minor sheetfloods; the subaqueous fan‐delta environment by debris flows, high‐ and low‐density turbidity currents, and suspension fallout.The nearshore lacustrine facies association provides examples of deposits and processes rarely documented in lacustrine environments. High‐energy oscillatory wave currents, probably related to a large fetch, reworked grains as large as 2 cm into horizontally stratified sand and gravel. Offshore‐directed currents produced uncommonly large (typically 1–2 m thick) trough cross‐stratified sandstone. In addition, stromatolitic carbonate interbedded with stratified coarse sandstone and conglomerate suggests a dynamic environment characterized by episodic terrigenous clastic deposition under high‐energy conditions alternating with periods of carbonate precipitation under reduced energy conditions. Massive and normally graded sandstone and massive to poorly laminated mudstone characterize the offshore lacustrine facies association and record deposition by turbidity currents and suspension fallout.A depositional model constructed for the Horse Camp Formation (Member 2) precludes the existence of all four depositional environments at any particular time. Rather, phases characterized by deposition in subaerial fan, nearshore lacustrine and offshore lacustrine environments alternated with phases of subaerial fan‐delta, subaqueous fan‐delta and offshore lacustrine deposition. This model suggests that high‐energy nearshore currents due to deep water along the lake margin reworked sediment of the fan edge, thus preventing development of a subaqueous fan‐delta environment and promoting development of a well‐defined nearshore lacustrine environment. Low‐energy nea

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1996.tb01464.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY