摘要:

ABSTRACTAeolian sand sheets are areas of aeolian sand where dunes with slipfaces are generally absent. Sand sheets are ubiquitous to modern, warm‐climate sand seas, generally occurring marginal to dune fields, although they may exist within the interior of a sand sea or independent of a dune field. Sand‐sheet deposits are recognized in ancient aeolian sequences, where they may account for significant accumulations of low‐angle aeolian stratification. We suggest that the occurrence of sand sheets instead of dunes indicates that conditions are outside the range within which dunes form or that one or more factors interfere with dune development while also favouring the accumulation of sand sheets. A study of six modern sand sheets in North America (located at Great Sand Dunes, Gran Desierto, Dumont, Algodones, Padre Island, and Colorado River delta) indicates that the factors favourable for sand‐sheet development are: (1) a high water table, (2) surface cementation or binding, (3) periodic flooding, (4) a significant coarse‐grained sediment population, and (5) vegetation. These factors are reflected in the nature of stratification and the accessory features of sand‐sheet accumulations within the areas of modern sand sheets as well as in their ancient counterparts in the Triassic Dolores and Pennsylvanian‐Permian Ri

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1986.tb00983.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

ABSTRACTThe influence of syndepositional fault patterns on palaeocurrents is demonstrated in fluvial to shallow‐marine sandstones of Upper Triassic basins in the High Atlas. The synsedimentary nature of faults is deduced from hydroplastic slickensides, thickness variations due to block tilting and dislocation of layers next to the fault scarp. On a regional scale, it is shown that the major normal fault trend of N050–070° controlled the overall palaeocurrent pattern which was directed towards the west‐southwest, i.e. in the direction of the future Atlantic ocean. Some anomalies in the palaeocurrent pattern could be related to an increase in subsidence which induced a general coarsening of sediment towards the top of the Tr

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1986.tb00984.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

ABSTRACTPaleosols in the Lower Carboniferous limestones of South Wales commonly contain needle‐fibre calcite which is an unusual form of calcite recently shown to form by the calcification of fungal hyphae in present day soils. The needle‐fibre calcite occurs in two associations in the paleosols: as coatings on sediment grains and as rhizocretions. The former can be compared with the microbial grain coatings of Quaternary calcretes. The latter represent the sites of fungal coats on roots and are interpreted as probable ectomycorrhizae, a symbiotic fungal sheath‐root association. These findings suggest that biomineralization was important in the formation of soil fabrics during the Carboniferous as it is in present day

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1986.tb00985.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

Mid‐channel bars are common on many active meandering gravel‐bed rivers, but specific information on timescales of development and on the occurrence of bars is lacking. Data from the River Dane in Cheshire, NW England, are presented here which indicate that a sequence of formation from accumulations of coarse gravel on the centre of riffles through to attachment of bars to floodplains is common and takes a period of 5–15 years. The sedimentological and vegetation changes through the sequence are described and the ro̊1e of flow events is discussed. About 10% of bends in this meandering reach contain mid‐channel bars at any time. Bars develop in steeper sections of the channel and downstream of sharp corners and rapidly eroding apices. Many of the bars are caused by excessive erosion of the banks which overwidens the channel. In the longer‐term the bars become incorporated in the developing

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1986.tb00986.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

ABSTRACTA study reach of the Calamus River, Nebraska Sand Hills, has a low sinuosity (less than 1.3) and braiding parameter (less than 1). Depending on sinuosity, the channel is occupied by alternate bars and point bars, the emergent parts of which form nuclei for midstream bars (islands). Channel migration occurs by bend expansion and translation, downstream and lateral growth of islands, and by chute cutoff.Channel‐bed sediment is mainly medium‐grained sand, but gravel and coarser sand sizes occur in thalweg areas adjacent to cutbanks and upstream parts of bars and islands, and finer sands occur on the downstream parts of bars and filling channels. Curved‐crested dunes cover most of the channel bed at most flow stages, with ripples restricted to shallow areas near banks. Bed material is mostly large‐scale cross‐stratified, with small‐scale cross‐strata interbedded with plant debris occurring in topographically high areas near banks.Vibracores through channel bars show a basal erosion surface overlain by large‐scale cross‐stratified sands, in turn overlain by small‐scale cross‐stratified sand interbedded with plant debris. The overall sequence generally fines upwards, but the large‐scale cross‐stratified portion either fines upwards, coarsens upwards, or shows little grain size variation. Lithofacies distributions vary spatially within and between bars depending on position in the bar and local channel curvature/width, in a similar way to unbraided rivers elsewhere. Lithofacies of bar deposits are similar to those in the active channel, and the elevations of the basal erosion surface and adjacent channel thalweg correspond closely.Channels abandoned by chute cutoff are filled progressively from the upstream end, and comprise deposits similar to the downstream parts of bars (i.e. fining upwards). The downstream extremities of channel fills may contain large proportions of peat relative to sand, but little mud due to the paucity of such fine susp

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1986.tb00987.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

ABSTRACTSedimentary organic matter behaves as sedimentary particles and this study attempts to interpret its occurrence in the Palaeocene sands and mudstones of the Forties Field of the North Sea. New nomenclature and classification of this palynodebris are presented and are used to make quantitative counts of the palynodebris from nearly 600 samples of known sedimentary character. The new categories are further refined by objective testing of the data by Principal Components Analysis and Cluster Analysis. It is argued that two types of sedimentary organic matter are of particular value because they are preferentially deposited in different environments:palynowafersare most commonly deposited in the submarine fan lobes and channel complexes andamorphousmatter occurs mostly in the lower energy basin plain sediments.

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1986.tb00988.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

ABSTRACTBed load sediment traps were deployed at two sections across channels in Rangaunu Harbour entrance. Traps were inspected and emptied by divers at hourly intervals through both spring and neap tidal cycles for a total of 292 trap deployments. Current velocities were measured simultaneously with the trap inspections. Transport is concentrated in sandy megaripple fields on the channel banks and sub‐tidal platforms flanking the channels. There, transport is almost continuous throughout the tidal cycle, increasing with flow velocity but lagging by approximately one hour. The channel floors are lined with shell‐gravel lag across which bedload transport rates are low and discontinuous. Tidal asymmetry produces a net seaward transport through the channel troughs and a net landward transport across the channel banks and flanking sub‐tidal platforms. Sediment leaving the harbour recirculates in anticlockwise gyres across the ebb‐tide delta to re‐enter the harbou and maintain the supply of sand to the megaripple field. Transport during spring tides is typically 25–30 times that during neaps. Predictions of transport rates, from a method developed by Black&Healy utilizing the Yalin bedload equation, produced transport rates similar to the traps over sand beds. Transport over shell lag surfaces appears independent of near‐bed velocity and more dependent on the passage of ribbons of sand across th

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1986.tb00989.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

Book review in this articlePrinciples of Physical Sedimentology, by J. R. L. Allen, George Allen and UnwinExperiments in Physical Sedimentology, by J. R. L. Allen, George Allen and UnwinArchaean Geochemistry: The Origin and Evolution of the Archaean Continental Crust, ed. by A Kröner, G. N. Hanson&A. M. GoodwinEstuaries as Sinks for Sediment and Industrial Waste ‐ a Case History from the Massachusetts Coast, by C. P. Summerhayes, J. P. Ellis&P. StoffersSedimentology ‐ Recent Developments and Applied Aspects, ed. by P. J Brenchley&B. J. P. WilliamsEnvironmental Chemistry, by Peter O'

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1986.tb00990.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1986.tb00991.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY