摘要:

ABSTRACTPurbeckian carbonates in the Swiss and French Jura (Goldberg Formation, lower Berriasian) comprise shallow‐subtidal, intertidal, supratidal, low‐energy, high‐energy, marine, brackish, freshwater, and hypersaline facies. These facies are arranged in small (0–2–1.5 m thick) sequences which display a dominant shallowing‐upward component, and which form the fundamental units of the highly structured Purbeckian sedimentary record.Six types of small‐scale sequences can be recognized. A: intertidal to supratidal overprinting of shallow lagoonal facies; B: algal‐marsh sequence with frequent dolomitization; C: sabkha sequence, often associated with collapse breccia; D: tidal‐flat sequence with desiccation features; E: lacustrine sequence; F: terrestrial overprinting of subtidal or intertidal facies. Episodic event deposits such as tempestites are superimposed] Thin transgressive beds which rework elements of the underlying facies are frequently found at the base of the sequences. Green marls and black pebbles are common at the top and indicate long subaerial exposure. The sequences are often incomplete, as subtidal facies may be absent, or their upper part can be eroded. Lateral facies changes are common, which is due to the very shallow and partly emergent Purbeckian platform where various depositional environments were juxtaposed. However, many sequence boundaries are well developed and can be correlated over large parts of the study area.The Purbeckian shallowing‐upward sequences were generated by climatically controlled sea‐level changes. Autocyclic processes occurred locally, but were overprinted by drops of sea‐level affecting the entire platform. The small‐scale sequences are most probably related to the 20 000‐year cycle of the precession of the equinoxes. Larger sequences with usually well‐developed emersion surfaces are attributed to the 100 000 and 400 000‐year eccentricity cycles of the Earth's orbit. Identification and correlation of sequence boundaries makes it possible to set up a framework of isochronous surfaces (which often cut across facies boundaries), and thus to interpret in detail the palaeogeographic, sedimentological and diagenetic evolution of the Purbeckian p

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1988.tb00992.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTDolomitized intervals of a core from San Salvador Island, Bahamas, exhibit variations of two texturally and geochemically distinct end‐members. In the Pliocene section of the dolomitized interval, the two end‐members alternate in a pattern that may reflect originally and/or diagenetically modified depositional facies. Formerly mud‐free intervals, locally capped by exposure surfaces are massive crystalline, mimetic dolomites (CM). Muddier sediments are replaced by friable microsucrosic dolomites (MS). CM and MS dolomites also differ in porosity (30%), permeability (100 md), mol% MgCO3(44–9 vs 47–7) mol%), oxygen isotopic composition (1–7 vs 2–7‰) and strontium content (241 vs 106 ppm).These data indicate that depositional and diagenetic fabric are the principal controls governing the distribution of dolomite types. Differences in texture and geochemistry are suggested as arising through differential rates of crystallization produced as a result of variations in permeability and reactivity of the precursor sedi

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1988.tb00993.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTNodular limestones have been studied from the Lower Chasmops Shale (Middle Ordovician) and the Rytteráker Formation (Lower Silurian). Observations on nodule‐host‐rock relations and variations of ferroan/non‐ferroan calcite cements help explain the role of precipitation, dissolution and redistribution of carbonate.Distribution and frequency of nodules depends on environmental parameters such as carbonate/clay ratio, grain size distribution and bioturbation, though final shapes are the result of pressure‐dissolution and

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1988.tb00994.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTBy far the commonest consituents of insular phosphate deposits are calcium phosphates of the apatite series of minerals, especially varieties that contain structural carbonate in their crystal lattices, e.g. francolite and dahllite. This reflects the fact that the vast majority of described insular phosphate deposits occur in low latitudes, where they have formed, in a tropical environment, by metasomatic replacement of carbonate in coral substrates by phosphate derived either from avian excrement or from phosphorus‐enriched lagoonal/lacustrine waters.Thin, recently‐formed guano accumulations on islands of the subantarctic Bounty group differ radically in that they consist principally of struvite, a hydrated ammonium magnesium phosphate that is better known as a component of human/mammalian urinary stones and enteroliths, and of bat guano in sheltered speleothems. Apatite occurs only in an accessory role in the Bounty Islands guano. This contrast in mineralogy, and the somewhat anomalous survival of soft, unstable struvite in a totally exposed environment on the Bounty Islands, are attributed to climatic factors and to the nature of the granodioritic rock substr

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1988.tb00995.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTThe Upper Ordovician Martinsburg Formation of eastern Pennsylvania consists of mudstone, siltstone, and sandstone turbidites that accumulated in a tectonically active foreland basin. The mudstone‐rich Bushkill Member, the stratigraphically lowest unit of the Martinsburg in this area, grades upward into approximately equal proportions of mudstone, siltstone, and sandstone of the Ramseyburg Member. Many of the turbidites of these units are arranged in small‐scale (1–9 m) fining‐upward sequences that are interpreted as reflecting the influence of external or allocyclic controls such as variations in the local rate of sea‐level rise and/or variations in the intensity of tectonic activity in shelf/nearshore or hinterland areas rather than more commonly cited autocyclic mechanisms. The thick (approximately 2000 m) Bushkill‐Ramseyburg coarsening‐upward sequence records progradation of a muddy turbidite depositional system along the axis of the foreland basin. Although this sequence accumulated during a Caradocian eustatic rise in sea‐level, sedimentation rates landward of the shoreline were apparently great enough to allow for long‐term seaward progradation of the shelf source. The paucity of depositional lobe‐like facies (coarsening‐upward sequences) in the Bushkill Member allows for tentative comparison of the progradational Bushkill‐Ramseyburg system with the active fan lobe of the Mississippi Fan.Progradation of the Bushkill‐Ramseyburg system ceased abruptly when mudstone turbidites and laminated black shale of the upper unit of the Martinsburg, the Pen Argyl Member, accumulated. The great thickness of some mudstone turbidite beds of the Pen Argyl Member is interpreted to record topographic confinement of the central Appalachian foreland basin, which may have helped to preclude continued progradation of the Bushkill‐

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1988.tb00996.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTAnalysis of an 18‐day time‐lapse film record of shoreface ripple development, with concurrent measurements of near‐bottom flow and surface waves, provides new insight on equilibrium bedform conditions, adjustment of ripple planform to variable hydrodynamics, and ripple migration behaviour. The study was conducted in approximately 10 m water depth, 1 km off Martinique Beach on the Atlantic coast of Nova Scotia (Canada), under low‐energy summer wave conditions. Significant wave‐height and peak period during the study averaged 0–7 m and 8 s, respectively, with extremes up to 1–7 m and 11 s during passage of three weak weather disturbances. Six mutually exclusive ripple types have been defined: (1) short‐wavelength regular ripples; (2) variable bifurcated ripples; (3) variable terminated ripples; (4) short‐crested ripples; (5) long‐wavelength regular ripples; and (6) chaotic ripples. Ripple wavelength ranged from 0–07 m to 0–24 m and displayed a strong Reynolds number dependence. Together with other published field data, the results suggest a lower limit of γ=0–06 m for the wavelength of wave ripples in ocean shoreface environments. Ripple orientation ranged through 38° and responded rapidly to changes in wave approach direction, but did not conform to the orientation of the adjacent shoreline. Ripples were observed to migrate both on‐ and off‐shore (with and against the wave advance direction) at rates up to ±0–1 m h‐1, associated with net flows other than wave‐induced onshore asymmetry and mass transport. Migration (mainly of ripple types 1 and 2) occurred during the peak of storm events, but showed no obvious correlation with measured near‐bottom flow magnitude or direction. Ripple behaviour demonstrates equilibrium with prevailing dynamic conditions when straight‐crested rippie types 1 and 5 are present. Disequilibrium in orientation or dimensions is expressed by increasing sinuosity, bifurcation

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1988.tb00997.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTA thick late Pleistocene sequence, comprising multiple beds of massive diamict facies resting on outwash gravels, occurs along the Bow River, near Banff, Alberta. Diamicts have a simple sheet‐like geometry which dip downvalley at between 5° and 10°, with largely conformable bedding contacts. The sediments are strongly bimodal in texture, consisting of clasts supported by a silty sand matrix. Prolate clasts show a weaka‐axis alignment parallel and transverse to the trend of the Bow Valley with a weak imbrication. Clasts of soft sediment are common. The diamict sequence has a maximum thickness of 30 m, infills the valley floor over an area of approximately 12 km2and buries a channelled topography cut on the surface of underlying outwash gravels. Abandoned braided channels, veneered by windblown sand containing volcanic tephra dated at 6600 yrbp, occur on the infill surface.Diamict facies were emplaced as subaerial debris flows derived by the mixing and remobilization of large volumes of outwash and glaciolacustrine sediment. Orientation data from clasts and a few basal grooves indicate that debris was derived upvalley and from adjacent valleyside slopes. A major episode of debris flow sedimentation may have been triggered by the abrupt drainage of lakes in tributary valleys as the Bow Valley glacier retreated. Downslope resedimentation of large volumes of sediment resulted in braided river aggradation downstream along the Bow River and can be dated to between 12 000 and 10 000 yrbp. Similar conditions probably occurred during deglaciation throughout the Canadian Cordillera and many other alpine areas, where poorly‐sorted late glacial debris‐flow facies have been mistaken for tills deposited directly by glaciers. The late glacial setting identified in this paper provides a basis of comparison with ancient continental glacial facies preserved in areas of active tectonism and hi

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1988.tb00998.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTRiver channel confluences form important morphological elements of every river system, being points at which rapid changes in flow, sediment discharge and hydraulic geometry must be accommodated. This article presents results of a quantitative investigation of sediment transport at channel confluences accomplished through both scaled laboratory flume simulation and complementary monitoring of a natural channel confluence.Bed morphology at channel confluences is characterized by three distinct elements: avalanche faces at the mouth of each confluent channel, a deep central scour and a bar within the separation zone formed at the downstream junction corner. These elements are controlled predominantly by the confluence angle and the ratio of discharges between the tributary and mainstream channels. As confluence angle and discharge ratio increase, the sediment contributions from the confluent channels are progressively segregated in their paths through the junction, with sediment being transportedaroundrather thanthroughthe centre of the confluence. This segregation of sediment loads is accompanied by the retreat of the main channel avalanche face from the confluence, an increase in the scour depth, a change in the orientation of the scour and an increase in the size of the separation zone bar. Field measurements closely replicate the flume simulation.A model of sediment transport and bed morphology links these features to the fluid dynamics of these sites. An understanding of confluence dynamics is important not only in considerations of channel morphology and design criteria but must form the basis for the interpretation of confluence sediments in the ancient record.

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1988.tb00999.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTWhen air blows across the surface of dry, loose sand, a critical shear velocity (fluid threshold,ut), must be achieved to initiate motion. However, since most natural sediments consist of a range of grain sizes, fluid threshold for any sediment cannot be defined by a finite value but should be viewed as a threshold range which is a function of the size, shape, sorting and packing of the surface sediment. In order to investigate the initiation of particle movement by wind a series of wind‐tunnel tests was carried out on a range of pre‐screened fluvial sands and commercially available glass beads with differing mean sizes and sorting characteristics. A sensitive laser‐monitoring system was used in conjunction with a high speed counter to detect initial grain motion and to count individual grain movements. Test results indicate that when velocity is slowly increased over the sediment surface the smaller or more exposed grains are first entrained by the fluid drag and lift forces either in surface creep (rolling) or in saltation (bouncing or hopping downwind). As velocity continues to rise, larger or less exposed grains may also be moved by fluid drag. On striking the surface saltating grains impart momentum to stationary grains. This impact may result in the rebound of the original grain as well as the ejection of one or more stationary grains into the air stream at shear velocities lower than that required to entrain a stationary particle by direct fluid pressure.As a result, there is a cascade effect with a few grains of varying size initially moving over a range of shear velocities (the fluid threshold range) and setting in motion a rapidly increasing number of grains. Results of the tests showed that the progression from fluid to dynamic threshold, based on grain movement, can be characterized by a power function, the coefficients of which are directly related to the mean size and sorting characteristics of the sed

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1988.tb01000.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTThe sand in the Alexandria coastal dunefield is derived from the sandy beach which forms the seaward boundary of the dunefield. Sand is blown off the beach onto the dunefield by the high‐energy onshore‐directed dominant wind. The dunefield has been forming over the past 6500 years.Sand transport rates calculated from dune movement rates and wind data range from 15 to 30 m3m‐1yr‐1in an ENE direction. The sand transport rate decreases with increasing distance from the sea due to a reduction in wind speed resulting from the higher drag imposed upon the wind by the land surface. Aeolian sand movement rates of this order are typical of dunefields around the world.The total volume of sand blown into the dunefield is 375 000 m3yr‐1. Sand is being lost to the sea by wave erosion along the eastern third of the dunefield at a rate of 45 000 m3yr‐1. The dunefield thus gains 330 000 m3of sand per year. This results in dunefield growth by vertical accretion at about 1.5 mm yr‐1and landward movement at about 0.25 m yr‐1. The dunefield is a significant sand sink in the coastal sand transport system.The rate of deposition in coastal dunefields can be 10 times as high as rates of deposition in continental sand seas. The higher rate of deposition may result from the abundant sand supply on sandy beaches, and the higher energy of coastal winds.Wind transport is slow and steady compared to fluvial or longshore drift transport of sediment, and catastrophic aeolian events do not seem to be significant in win

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1988.tb01001.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY