摘要:

ABSTRACTThe Cumberland Marshes in east‐central Saskatchewan, Canada, occupy over 5000 km2and contain a variety of active and abandoned fluvial features, including straight to sinuous isolated channels, anastomosed channel systems, levees, and crevasse splays in addition to marshes, lakes and bogs. In 1873, an avulsion of the Saskatchewan River diverted most of its flow into a portion of the Cumberland Marshes (locally termed the breakout area), and altered the alluvial terrain as the invaded wetlands adjusted to the influx of sediment and water. These adjustments continue today, and so far over 500 km2of wetlands have been affected by the avulsion.Avulsion‐controlled modification of the wetlands involves the initiation and evolution of crevasse splays and splay complexes. Three intergradational forms are recognized, each associated with characteristic sand‐body geometries. Stage I splays are small, lobate in plan, crossed by unstable distributary channels, and form wedge‐shaped sheets which depositionally overlie fine‐grained, organic‐rich wetland sediments. Stage II splays and splay complexes evolve both spatially and temporally from Stage I splays. They are larger, contain dense networks of anastomosed channels, and form disconnected tabular sand bodies or continuous sand sheets, some of which incise underlying wetland sediments. Stage III splays develop from either Stage I or II splays and contain few but stable anastomosed channels that deposit isolated stringer sands encased in fine‐grained floodplain sediments. Although sand bodies deposited by splays comprise important components of the evolving floodplain, various fine‐grained facies occurring in levees, shallow lakes, abandoned splay channels, and interchannel floodplains dominate the avulsion deposits.The post‐1873 record of deposition and terrain modification in the breakout area suggests four stages of floodplain evolution following avulsion. In theinitial avulsion stage,new channels and splay complexes increase in numbers rapidly as diverted discharge of water and sediment overwhelm the adjacent floodbasin. Theanastomosed stageis characterized by an approximate balance between rates of new channel and splay development, and abandonment of old ones. This stage continues for as long as new floodplain areas are invaded. The rate of new splay development eventually decreases as accessible floodplain becomes aggraded, forcing a higher rate of channel abandonment and concentration of remaining flow into fewer but larger channels (reversion stage). The result of reversion is eventual return to asingle channel stage,completing the avulsive sequence and initiating a new alluvial ridge. For the Saskatchewan River, this final‐stage single channel will likely produce a meander belt which occupies only a portion of the more extensive avulsion bel

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1989.tb00817.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTExcellent exposures and the presence of lithological markers make it possible to reconstruct the fan facies pattern for a narrow (6000–60 000 yr), late Tortonian (7–8 Ma) time slice of a submarine fan complex that developed in the Tabernas basin. Unlike most classical fan models, which seem to refer to a single feeder‐lobe system, the reconstruction from the Tabernas basin reveals at least three distinctive juxtaposed feeder‐lobe systems: (I) a straight feeder valley terminating in a sandstone body consisting of stacked sand‐filled scours (sand‐rich system); (II) a straight feeder channel diverging in a broad front of nested scours in mudstones. These scours terminate in fine grained sheet‐like turbidite deposits (muddy system); (III) a sinuous channel complex extending far into the basin without ‘lobe’ deposits at its mouth (solitary system).Sedimentary features indicate that in the Tabernas basin, initial basin floor morphology and mass‐flow transport behaviour controlled and eventually stopped formation of the sand‐rich system (I), while depositional topography and slope instability controlled formation of the muddy system (II). The unusual narrowness (10–30 m) and length (8 km) of the solitary system (III) points to its confinement, possibly a result of an intrabasinal fault escarpment trending obliquely to the general direction of slope.Flow stripping(Piper&Normark, 1984) could then explain the absence of a lobe

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1989.tb00818.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTA series of laboratory experiments has been carried out in which parallel‐laminated deposits were produced from an upper‐regime plane bed. The laminae had thicknesses of a few mm and could be traced continuously over distances up to the length and width of the depositional area (0–3 m by 1–5m). Fluctuations in bed elevation were measured both during deposition and at equilibrium; much of the bed fluctuation occurs at time scales that are too long to be due directly to turbulence, as most theories for lamina formation would require. We suggest instead that extremely low‐amplitude bed forms are present even on apparently flat beds and that the migration of these bed forms produces laterally continuous lamination.All the lamination produced in the laboratory experiments was normally graded. Using high‐speed photography it was observed that the normal grading results from rapid deposition of a layer of loosely packed coarse sand several grain‐diameters thick followed by the slow sieving‐out of a well packed surface layer of finer sand. The initial deposition is the result of small‐scale turbulent fluctuations in boundary shear stress. The sieving‐out that follows results in a smooth surface whose low friction coefficient temporarily inhibits further deposition; we term this process ‘glazing’. The alignment of small‐scale turbulent scour‐and‐fill structures along the paths traced by migrating bedform troughs produces laterally c

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1989.tb00819.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTMeasurements of the nearbed turbulent current flow and the bedload transport of marine gravel have been made over three tidal cycles. The turbulence in the bottom boundary layer was measured using two electromagnetic current meters, and the gravel transport was measured using a passive acoustic system which monitored the interparticle collision noise of locally mobile material. Visual estimates of bedload were also obtained with an underwater TV camera. The acoustic technique, unlike a conventional bedload sampler, has allowed estimates of transport to be obtained with a temporal resolution comparable with the turbulence data collected. This has enabled a detailed comparison to be made between the turbulent flow and the sediment response to the instantaneous flow conditions. The results of the study show that of the turbulent bursting events which contribute towards the Reynolds stress, only the sweeps and outward interactions play a significant role in the transport of coarse sedimentary material. The measurements show that it is the instantaneous increases in the horizontal turbulent velocity fluctuations that generate excess shear stresses which drive the transport process.

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1989.tb00820.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTUpper Proterozoic carbonate successions from central East Greenland (the Limestone‐Dolomite ‘Series’ of the Eleonore Bay Group) and Svalbard (the Backlundtoppen Formation of the Akademikerbreen Group, Spitsbergen, and the Upper Russö Formation of the Raoldtoppen Group, Nordaustlandet) contain thick sequences dominated by pisolites. These rocks were generated in shallow marine enviroments, and the pisoids are essentially oversized ooids. A marine environment is supported by the thickness and lateral extent of the carbonates; by a sedimentary association of pisolites with stromatolites, flake‐conglomerates, calcarenites, calcilutites, microphytolites, and ooids similar to that found in numerous other Proterozoic carbonate successions; by sedimentary structures, including cross‐beds and megaripples that characterize the pisolitic beds; and by microfossils of endolithic cyanobacteria that are specifically comparable to microorganisms that inhabit modern marine ooids of the Bahama Banks. Petrographic features and strontium abundances suggest that the pisoids were originally aragonitic, but neomorphism, silicification, calcitization, and dolomitization have extensively modified original mineralogies and fabrics. The East Greenland and Svalbard pisolitic carbonates reflect similar depositional environments and diagenetic histories, reinforcing previous bio‐, litho‐, and chemostratigraphic interpretations that the two sequences accumulated contiguously in a coastal zone of pisoid genesis which extended for at least 600, and probably 1000 or mo

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1989.tb00821.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTThis study describes the field and petrographic relationships of a widespread deposit of Pleistocene travertines in the northern Hula Valley, northern Israel. The travertines interfinger with conglomerate deposits and basaltic lava flows. Field relationships and radiometric dating indicate that the travertines accumulated intermittently over the past million years, and their formation virtually stopped 25 000 years ago.The travertines are characteristically highly porous. Some of the pores, as well as some spar‐filled voids, preserve the shapes of stalks and leaves. The abundance of plant material suggests that photosynthesis, rather than bacterial or abiogenic processes, was the main mechanism which induced carbonate precipitation.The reasons for travertine accumulation in the past and for the cessation of its formation today are ascribed to differences in the palaeogeographic setting. In the past, water flow is viewed as having been mostly sluggish, and a widespread and shallow sheet of water was formed. The lush vegetation, combined with the relatively long residence time of the water in the area, led to increased efficiency of the calcium carbonate precipitation. Today, by contrast, water flows rapidly in gorges, precipitating only a small fraction of its load of dissolved calcium carbonate. The conjectured change in hydrological conditions is ascribed to rejuvenation of faulting activity in the Late Quaternar

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1989.tb00822.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTCeylon Lake, a small salt playa located in southern Saskatchewan, is typical of many shallow ephemeral lacustrine basins found in the northern Great Plains of western Canada. The present‐day brine, dominated by magnesium, sodium, and sulphate ions, shows wide variation in composition and concentration on both a temporal and a spatial basis. The modern sediments overall exhibit relatively simple facies relationships. An outer ring of coarse grained shoreline and colluvial clastics surrounds mixed fine grained clastics and salts and, in the centre of the basin, salt pan evaporites composed mainly of mirabilite, thenardite, and bloedite.Coring of the late Pleistocene and Holocene sedimentary fill shows that the lake has evolved from a relatively dilute, deep water, clastic dominated basin through a shallower, brackish water, carbonate‐clastic phase, and finally into the present salt dominated playa. The thick sequence of evaporites preserved in the basin suggests evolution of the brine from a Na‐rich solution to a mixed Mg‐Na system. The most important post‐depositional processes affecting Ceylon Lake sediments are mud diapirism and salt

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1989.tb00823.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTSettling rates of natural sand‐size particles are influenced, to some extent, by their shapes and this may be an important factor in using settling rates to estimate grain size. In order to gauge the sensitivity of this influence, two natural sand populations from the Mesozoic Nubian Sandstones of Southern Israel, with a high probability of being similar in their bulk shape characteristics, were examined in ¼φ sieved fractions for their shape characteristics and settling rates. Fine surface features (roundness and surface roughness) were evaluated using Fourier shape analytical methods. Significant differences in bulk shape were detected and their influence on settling rates was measured empirically in a settling tube. The most marked differences were in the coarse grain sizes and, to a lesser extent, in the intermediate sizes.Sampling of raw settling data at closely‐spaced time intervals yielded high‐resolution grain size frequency plots which were usually polymodal in nature. Subtle shape contrasts, which are an important influence on settling rates, are thus an important consideration when working at this level of sensitivity.Natural sand populations which have followed a more varied provenance or process pathway could be expected to have even greater contrasts in settling rates than the samples analysed here. Thus it is recommended that the bulk shape factor should be taken into account in order to minimize errors in the conversion of settling times to grain size. An easy method, outlined in this paper, is through the establishment of an empirically derived calibration curve for each individual suite of sand undergoing analysis. Sieved ¼φ samples, derived from a split of the total composite sample undergoing analysis, forms the basis of the calibration and hence a correction factor converting sieve diameters to true diameters must be applied. In this research, nominal section diameters were obtained optically through an image

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1989.tb00824.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1989.tb00825.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1989.tb00826.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY