摘要:

ABSTRACTSubaqueous dunes are formed on the KwaZulu‐Natal outer‐shelf due to sediment transport by the Agulhas Current (geostrophic current). These dunes occur within two dune fields at depths of − 35 to − 70 m. The net sediment transport direction is south, but short‐period reversals form northward‐migrating bedforms. The dune fields are physically bounded by late Pleistocene beachrock and aeolianite ledges. A bedform hierarchy has been recognized in the dune fields comprising a system of three generations of climbing bedforms. The outer dunefield has given rise to a sand ridge (H=12 m;L=4 km;W=1.1 km; and an 8° lee slope) whereas the inner dune fields have achieved large‐scale dune status. Bedload parting zones within the dune fields occur where the sediment transport direction switches from north to south due to reversals in the geostrophic flow; these zones occur at depths of − 60, − 47 and − 45 m. An interpretative stratigraphic model is presented on what suchgeostrophitedeposits would look like in the ancien

ISSN:0037-0746    

DOI:10.1046/j.1365-3091.1996.d01-17.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTThe lower part of the Jangki Group (Miocene), SE Korea consists of pyroclastic mass‐flow‐dominated facies and epiclastic stream‐flow‐dominated facies which reflect sedimentation during syn‐ and intereruption periods, respectively. On the basis of pyroclastic composition, sedimentary structures and bed geometry, they are organized into two facies associations: (1) dacitic and basaltic debris‐flow and hyperconcentrated‐flood‐flow deposits of eruption periods, and (2) epiclastic stream‐flow and interchannel deposits of intereruption periods. The lateral relationship between the syn‐ and intereruption deposits varies significantly over short distances (2 km). In the western part of the study area, syneruption deposits are predominant, and fluvial deposits occur as small‐scale channel‐fill gravelstone bodies encased within dacitic debris flow deposits. In the eastern part, however, intereruption deposits are dominated with thick sequences of interbedded channel and interchannel deposits. The abrupt lateral change indicates alternation of epiclastic axial fluvial system with pyroclastic‐rich volcaniclastic aprons. The syneruption deposits are enriched in vitric ash but lack contemporary volcanic rock fragments (dacitic or basaltic). They are sharply differentiated from intereruption deposits that mostly consist of epiclasts and are deficient in vitric ash. The vertical transition suggests that streams drained a hinterland of igneous basement rocks during intereruption periods and became bulked with pyroclasts d

ISSN:0037-0746    

DOI:10.1046/j.1365-3091.1996.d01-19.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTPrevious research suggests that the turbulence‐driven suspension process in sand‐bed channels is dominated by intermittent, energetic eddies with length scales of the order of channel depth. Because of the scarcity of data on the turbulent suspension process in alluvial channels, the possible variability in suspension intermittency and turbulent frequency content due to contrasts in flow depth, velocity or bedforms remains unclear however. The present study analyses eddy correlation suspension signals from seven deployments in varied flow conditions around a sandy meander bend. Deployment depths at near‐bankfull flow stages varied from 2 to 5.5 m, velocities at 0.75–1 m height from 0.6 to 0.9 m s−1and local mean suspended sand concentrations ranged from 30 to 150 mg L−1in the intermittence and spectral content of sand suspension between the various deployments are analysed and results are compared with previously published findings. Study data suggest that the dominant eddy sizes involved in sediment mixing across the sensor level are consistently of the order of 1–5 times flow depth and lie within the ‘energy‐bearing’ turbulent range. When sand suspension is analysed in the time domain in the various deployments, energetic, burst‐like suspension events occupying only 1‐5% of the record duration account for 20‐90% of the suspension work. The degree of intermittence in the suspension process was observed to increase in deeper flows, where mixing events contributing extreme vertical sediment fluxes appear to be

ISSN:0037-0746    

DOI:10.1046/j.1365-3091.1996.d01-18.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTThe interaction between dunes and the primary wind results in a complex pattern of secondary airflow on the lee side of dunes. From 15 dunes studied during transverse flow conditions at Padre Island in Texas, White Sands in New Mexico, and the Algodones in California, distinct flow regions can generally be recognized, with the overall flow structure comparing favourably to that proposed for subaqueous bedforms. Downwind of dunes with flow separation is a back‐flow eddy that extends about four dune‐brink heights downwind from the brink of the dune. Beyond the separation cell, the velocity profiles can be divided into regions based upon segments separated by ‘kinks’ in the velocity profiles. Theinterioris an area above the dunes of relative high wind speed but low velocity gradient. Beneath the interior is thewake, which consists of two layers. Theupper wakeexhibits an uppermost portion where the flow decelerates while the remainder exhibits accelerating flow, so that the overall velocity gradient decreases downwind. Thelower wakeexhibits low velocity gradients and wind speeds that accelerate downwind at all heights, but primarily near the top of the layer, thereby causing the velocity gradient to increase downwind. At about eight dune heights downwind, the upper and lower wakes equilibrate to a single profile with the kink between them no longer apparent. The lowest recognizable region is theinternal boundary layer. It is recognized by a relatively steep velocity gradient below the wake, and never exceeds a few tens of centimetres in height for our data set. Because of acceleration and increasing shear stress within this layer, interdune flats are at least potentially erosional. Overall, the wake and internal boundary layer show a downward transfer of momentum from upper regions so that the flow recovers. Where flow separation does not occur, simple flow expansion down the lee‐face causes flow dec

ISSN:0037-0746    

DOI:10.1046/j.1365-3091.1996.d01-20.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTSeven mud‐filled incised valleys (MFIVs) in the paralic facies of the Dinosaur Park and Horseshoe Canyon formations (Upper Cretaceous) of southern Alberta were studied to better understand their morphology, geometry and depositional histories in an estuarine context. Two preservational geometries occur: simple, U‐shaped forms; and internally complex forms. Both types of MFIV record deposition in the central zone of low energy (turbidity) in an estuarine setting.Simple, U‐shaped MFIVs have sharp basal erosional surfaces and consist of mudstone‐dominated heterolithic fills of channel‐wide, concave‐up laminae. Associated fossil assemblages are marine to brackish. Each simple MFIV records a cut‐and‐fill history associated with a cycle of relative sea‐level drop and rise. Low‐energy depositional settings, loss of channel form during infilling, and associated shoreface deposits, as well as the absence of clear tidal indicators suggest a coastal plain estuarine setting, along a wave‐dominated, barred coastline.Complex MFIVs are rarer, and consist of imbricated, wedge‐shaped sets of inclined‐to‐horizontal heterolithic strata. Tidal deposits and/or nonmarine‐to‐marine macrofossils occur locally. Complex MFIVs were infilled in meandering reaches of the central zone of low energy in tide‐dominated estuaries. Their rarity compared to simple MFIVs and their freshwater palaeontological content suggest that they were contiguous landward with extensive fluvial channels. A complex MFIV near Onefour comprises three in‐channel depositional cycles. Each cycle consists of an erosional surface overlain by lateral accretion bedding and a conformable transition to vertically aggraded strata. Each cycle reflects a cut‐and‐fill event under the control of changes in relative sea‐level that culminated in overbank flooding.All MFIVs formed in low‐gradient settings (≤0.03%) where estuarine zones were stretched out over many tens of kilometres. Tide‐dominated estuaries apparently exhibited simple, straight‐to‐meandering upstream transitions and extensive landward penetration (≥200 km) of tidal backwater effects. Few modern estuaries serve as adequate modern analogues to these ancient, tide‐dominated estuaries.Radiometric data indicate that MFIV cut‐and‐fill cycles were 100 000‐400 000 years in maximum duration and thus, equivalent to 4th order sea‐level cycles. However, negative evidence tentatively suggests that these cycles took place over time intervals 1‐2 o

ISSN:0037-0746    

DOI:10.1046/j.1365-3091.1996.d01-15.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTA detailed investigation of floodbasin facies architecture was undertaken in the Upper Triassic (Carnian‐Rhaetian) Callide Coal Measures in east‐central Queensland, Australia, using extensive highwall and exploration borehole data from ongoing mining activities. The composite Callide Seam Member varies up to 23 m in thickness and is locally split by a number of clastic partings up to several metres thick, ranging from claystone to coarse sandstone. A subset of the nine lithofacies recognized in surface exposures was identified from geologists' logs of cored and uncored drillholes through the Callide Seam Member. Facies mapping of each clastic parting (split) was then undertaken using all available highwall and drilling data. Sequential maps of facies and interval thickness for each coal body and clastic parting over the mine area (6000 × 2500 m) record sediment accumulation in alluvial channel and floodbasin environments (including levees, splays and splay complexes, and mires). The maps indicate that the numerous splays have dominantly elongate plan geometry (up to 4 km long), with lesser irregular and rare lobate shapes. Small, elongate splays were evidently formed during single flood events, whereas larger, elongate bodies and more irregularly shaped complexes were the product of longer‐term splay construction over several flood cycles. Quantitative summaries of splay dimensions indicate a wide variety of shape and size. The distribution of splay orientations is similar to the palaeocurrent distribution in major alluvial channel deposits as established from cross‐bedding. Alluvial channels that sourced the splays and other clastic sediments within seam splits were of low sinuosity, braided planform, constructed sediment bodies up to 2800 m wide and were dominantly loaded rather than incised into underlying peat‐rich

ISSN:0037-0746    

DOI:10.1046/j.1365-3091.1996.d01-25.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTProblems of vertical alignment and vibrations of disposable coffee cups used in a modified version of the De Ploey saltating sand catcher were solved. The new version was tested in a sediment wind tunnel. Its catches appeared linearly related to amounts of eroded sand, largely independent of wind speed and wind direction, and depended logarithmically on height. The catch efficiency may therefore be taken to be approximately independent of wind speed and direction. The instrument performed well under conditions of inhomogeneously saltating sand in a strongly desertified environment in Central Sudan. Use and results show the improved simple catcher to be easy to assemble, reproducible and cheap, suitable for multipoint use to cover all inhomogeneities in outdoor saltating sand fields.

ISSN:0037-0746    

DOI:10.1046/j.1365-3091.1996.d01-24.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTThe dynamics and geomorphological development of a trough blowout located at Fiona Beach in the Myall Lakes National Park in NSW, Australia, are examined. Wind speeds, velocities and flow structure were measured utilizing an array of miniature Rimco cup anemometers, Gill bivane and UVW instruments, and wind vanes. Flow measurements indicate that when the wind approaches the trough blowout parallel to the throat orientation, jets occur both in the deflation basin and along the erosional walls, relative flow deceleration and expansion occurs up the depositional lobe, jets are formed over the depositional lobe crest accompanied by downwind flow separation on the leeward side of the lobe, and flow separation and the formation of corkscrew vortices occur over the crests of the erosional walls. Maximum erosion and transport occurs up the deflation basin and onto the depositional lobe. Trough blowout morphologies are explained as a function of these flow patterns.When the wind approaches the blowout obliquely, the flow is steered considerably within the blowout, and the degree and complexity of topographic steering is dependent on the blowout topography. The flow is usually extremely turbulent and large corkscrew vortices are common. The local topography of a blowout can be very important in determining overall sand transport and blowout evolutionary conditions and paths.Estimates of potential sand transport within the blowout may be up to two orders of magnitude lower than actual rates if remotely sensed wind data are used.

ISSN:0037-0746    

DOI:10.1046/j.1365-3091.1996.d01-22.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTExperimental turbidity currents entering two‐layer density‐stratified water behave differently from similar currents flowing over the same topography into non‐stratified water. Experiments were designed as analogues for flows entering Mediterranean hypersaline pools. In both the hypersaline pools and the experiments, the water density changes abruptly across a pycnocline. Turbidity currents generated on a platform at the level of the pycnocline behaved in one of three ways as they flowed from the platform into deeper stratified water. (1) When the bulk density of the current was less than the dense water layer, the current spread at the pycnocline. The head of the current advanced rapidly when it lost contact with the bed. Grains settling out of the current fell through the dense water layer forming an extensive deposit. In nature this behaviour will lead to ‘turbidites’ with sharp but non‐erosive bases, strongly developed grading and no traction features. (2) When the bulk density of the current was greater than the dense water layer, the current continued as an underflow, plunging into the deeper water. Sedimentation lowered the bulk density of the current and the low‐density interstitial fluid caused the head to loft. Low‐density interstitial fluid convected from the body of the current, lofting particles into the water column. These particles were hydraulically sorted during upward transport and subsequent settling to the floor. The resulting turbidites had a more limited extent than the deposits of either non‐lofting underflows or interflows. By inference from the experiments, natural deposits of this type may have local (proximal) erosion and traction features at the base and strongly graded tops. (3) In some of the currents with high bulk density, the rising turbid water reached the pycnocline and spread at that level as a secondary interflow. The tail of the turbidity current, which was less dense than the head and body of the current, flowed above the pycnocline adding momentum to the secondary interflow. The thin non‐erosive graded deposit from the secondary interflow may extend beyond the deposits of the primary underflow. In all three cases (but more pronounced in cases 2 and 3) the interaction of the current with the pycnocline displaced that surface and generated a wave that was reflected back and forth from each end of the pool. The waves remobilized s

ISSN:0037-0746    

DOI:10.1046/j.1365-3091.1996.d01-21.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTThe morphology and migration rate of tidal bedforms are important because of their use in interpretation of modern and ancient sediment transport regimes. Tidal flow, megaripple morphology and migration were studied in the mesotidal Mawddach Estuary, North Wales, to examine the veracity of published flow‐bedform relationships, quantify spatial variations in migration and assess consequences for palaeoflow reconstruction. Two transects were surveyed along a megarippled intertidal shoal (mean grain size 280 μm) for a period of 22 semi‐diurnal tidal cycles. A vertical array of current meters recorded tidal current profiles at the centre of one of the transects.Flood tidal currents dominate at Fegla Fach shoal, with peak velocities over 1 m s−1at spring tides, and 0.5 m s−1at neaps, and bed sediment transport was also flood‐dominated. Over the lunar cycle, the morphology of the megaripples on the survey lines was divisible into three phases:1 the neap mode‐consisting of near‐moribund two‐dimensional (2‐D) flood‐orientated megaripples of wavelengthc. 6 m and heightc. 0.2 m;2 a transitional mode‐where, on rising tidal ranges, scour pits formed and developed into 3‐D megaripples which underwent net migration with the flood tide;3 the spring mode‐consisting of 3‐D megaripples of wavelengthc. 4 m and heightc. 0.2 m. Despite complete re‐orientation by the ebb tide, these were recognizable from one low water survey to the next, and net migration wasc. 1 m per tide with the flood tide. We infer the presence of the equilibrium ‘spring tidal form’ occurring as flood‐orientated megaripples during the flood tide.The data support previously reported separation of 2‐D and 3‐D megaripples at a depth to grain size ratio of 8000, and at a depth‐mean velocity of the dominant tide (Umaxdom) of 0.75‐0.8 m s−1. A migration threshold exists atUmaxdomofc. 0.53‐0.57 m s−1.Measures of migration which might be used on preserved sections have been applied to the data. These measures systematically overestimated bedform migration at most stages of the lunar cycle (by<25% at spring tides and<140% in the post‐spring transition period), but were accurate when the megaripples had developed into their 'spring tidal form’. There is significant variation of migration rates within the survey populations. We conclude that whilst the occurrence of megaripple cross‐sets may be used as a palaeoflow indicator, and sedimentary structures associated with 2‐D to 3‐D transitions may also be indicative of palaeoflows, there are likely to be significant uncertainties involved in usin

ISSN:0037-0746    

DOI:10.1046/j.1365-3091.1996.d01-16.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY