摘要:

ABSTRACTMultistorey sandstone bodies described from the Upper Devonian‐Lower Carboniferous of Kerry Head (Ireland) are interpreted as deposits of aggrading, perennial, river channels migrating laterally across alluvial plains. Point bars displayed surface features such as scroll bars, chute channels and chute bars. Relatively uncommon channel fills are both coarse‐ and fine‐grained.Quantitative interpretation of the sandstone bodies was accomplished by comparison with a physical model that predicts the sedimentology of single point bar deposits developed in channels of prescribed geometry and hydraulics. This analysis reveals that the separate storeys (point bars) in each sandstone body were deposited in a single channel belt in which channel geometry and hydraulics varied little with time (order of 103yr) and space (order of 103m). Two southerly flowing rivers of markedly different size were responsible for all sandstone bodies: bankfull widths, depths and mean velocities of both rivers varied little with time (order of 105yr), implying a stable climatic setting. Channel sinuosities were usually 1.15–1.2 throughout the succession. Both rivers decreased in mean channel slope as time progressed, in association with a rising base‐level and a shoreline encroaching from the south.Using Bridge&Leeder's (1979) alluvial stratigraphy model, the nature and distribution of channel sandstone bodies relative to overbank deposits in the succession can be explained by an average (compacted) floodplain deposition rate of about 0.005 m yr−1, if avulsion occurred with a frequency of about once every 103yr. Local variation in the relative amount of channel sandstone in the succession is probably due to local tectonic control of

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1983.tb00698.x

出版商:Blackwell Publishing Ltd    

年代:1983

数据来源: WILEY

摘要:

ABSTRACTThe Bracklesham Formation is of Middle Eocene age and occurs throughout the Hampshire Basin of southern England. The basin is elongated east‐west and filled with Lower Tertiary sediments. Its southern margin is marked by either large, northward‐facing monoclines, or faults, both of which underwent differential movement, with uplift of the southern side throughout the Middle Eocene. The Bracklesham Formation, which is up to 240 m thick, shows pronounced lateral facies changes with dominantly marine sediments in the east passing to alluvial sediments in the west. Four principal sedimentary environments: marine, lagoonal, estuarine and alluvial are distinguished. Marine sediments comprise six facies including offshore silty clays and glauconitic silty sands, beach and aeolian dune sands, and flint conglomerates formed on pebble beaches. Offshore sediments predominate in the eastern part of the basin, as far west as Alum Bay, where they are replaced by nearshore sediments. Lagoonal sediments comprise four facies and formed in back‐barrier lagoons, coastal marshes and, on occasions, were deposited over much of the basin during periods of low salinity and restricted tidal motion. Five estuarine facies represent tidal channels, channel mouth‐bars and abandoned channels. These sediments suggest that much of the Bracklesham Formation was deposited under micro‐ to meso‐tidal conditions. Alluvial sediments dominate the formation to the west of Alum Bay. They comprise coarse to fine sands deposited on the point‐bars of meandering rivers, interbedded with thick sequences of laminated interchannel mudstones, deposited in marshes, swamps and lakes. Extensive layers of ball clay were periodically deposited in a lake occupying much of the alluvial basin. In alluvial areas, fault movement exposed Mesozoic rocks along the southern margin of the basin, the erosion of which generated fault‐scarp alluvial fan gravels. Locally, pisolitic limestone formed in pools fed by springs emerging at the faulted Chalk‐Tertiary contact. In marine areas, flint pebbles were eroded from coastal exposures of chalk and accumulated on pebble beaches and in estuaries. From other evidence it is suggested that older Tertiary sediments were also reworked. The Bracklesham Formation is strongly cyclic and was deposited during five marine transgressions, the effects of which can be recognized throughout the basin in both marine and alluvial areas. Each of the five transgressive cycles is a few tens of metres thick and contains little evidence of intervening major regression. The cycles are thought to represent small‐scale eustatic sea‐level rises (‘paracycles’) superimposed upon a major transgressive ‘cycle’ that began at the base of the Bracklesham Formation, following a major regression, and was terminated, at the top of the Barton Formation by another major regression. This major cycle can be recognized world‐wide and may reflect a period of rapid northward exte

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1983.tb00699.x

出版商:Blackwell Publishing Ltd    

年代:1983

数据来源: WILEY

摘要:

ABSTRACTFor several decades, sedimentologists have had difficulty in obtaining an efficient index of particle form that can be used to specify adequately irregular morphology of sedimentary particles. Mandelbrot has suggested the use of the fractal dimension as a single value estimate of form, in order to characterize morphologically closed loops of an irregular nature. The concept of fractal dimension derives from Richardson's unpublished suggestion that a stable linear relationship appears when the logarithm of the perimeter estimate of an irregular outline is plotted against the logarithm of the unit of measurement (step length). Decreases in step length result in an increase in perimeter by a constant weight (b) for particles whose morphological variations are the same at all measurement scales (self‐similarity). The fractal dimension (D) equals 1.0‐(b), where b is the slope coefficient of the best‐fitting linear regression of the plot. The value of D lies between 1.0 and 2.0, with increasing values of D correlating with increasing irregularity of the outline. In practice, particle outline morphology is not always self‐similar, such that two or possibly more fractal elements can occur for many outlines. Two fractal elements reflect the morphological difference between micro‐scale edge textural effects (D1) and macro‐scale particle structural effects (D2) generated by the presence of crenellate‐edge morphology (re‐entrants). Fractal calibration on a range of regular/irregular particle outline morphologies, plus examination of carbonate beach, pyroclastic and weathered quartz particles indicates that this type of analysis is best suited for morphological characterization of irregular and crenellate particles. In this respect, fractal analysis appears as the complementary analytical technique to harmonic form analysis in order to achieve an adequate specification of all types of particles on a continuum of irregular to reg

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1983.tb00700.x

出版商:Blackwell Publishing Ltd    

年代:1983

数据来源: WILEY

摘要:

ABSTRACTAeolian transport rates were measured for three sands: a quartz sand (relative density 2.68, sphericity 0.73), a shelly sand (relative density 2.64, sphericity 0.59, carbonate content 67%), and granular aluminium oxide (relative density 3.95, sphericity 0.67). Low sphericity depresses the transport rate, particularly at high wind speeds; high density also does so but the effect is more marked at low transporting wind speeds.The sand of low sphericity undergoes longer saltations than the other materials, but is dislodged less frequently than they are and is transported less freely in a given wind. Of the more spherical materials, the denser was the least effective at extracting energy from the wind for grain transport in the range of our experiments. Both of the more spherical materials showed evidence of a fairly sudden transition of transport behaviour at modest wind speed. It is inferred that this marks the transfer of the function of grain dislodgement from direct wind action to inter‐granular collision. The transition did not occur in experiments on the material of lower sphericity.For a given transport rate, wind speed near the bed is highest for the grains of low sphericity (by a considerable margin) and lowest for the more compact quartz grains. Propensity for transport by wind is greatest for the quartz sand, less so for the heavy material and least for the material of low sphericit

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1983.tb00701.x

出版商:Blackwell Publishing Ltd    

年代:1983

数据来源: WILEY

摘要:

ABSTRACTThe late Pleistocene and Holocene stratigraphy of Navy Fan is mapped in detail from more than 100 cores. Thirteen14C dates of plant detritus and of organic‐rich mud beds show that a marked change in sediment supply from sandy to muddy turbidites occurred between 9000 and 12,000 years ago. They also confirm the correlation of several individual depositional units. The sediment dispersal pattern is primarily controlled by basin configuration and fan morphology, particularly the geometry of distributary channels, which show abrupt 60° bends related to the Pleistocene history of lobe progradation. The Holocene turbidity currents are depositing on, and modifying only slightly, a relict Pleistocene morphology.The uppermost turbidite is a thin sand to mud bed on the upper‐fan valley levées and on parts of the mid‐fan. Most of its sediment volume is in a mud bed on the lower fan and basin plain downslope from a sharp bend in the mid‐fan distributary system. Little sediment occurs farther downstream within this distributary system. It appears that most of the turbidity current overtopped the levée at the channel bend, a process referred to as flow stripping. The muddy upper part of the flow continued straight down to the basin plain. The residual more sandy base of the flow in the distributary channel was not thick enough to maintain itself as gradient decreased and the channel opened out on to the mid‐fan lobe.Flow stripping may occur in any turbidity current that is thick relative to channel depth and that flows in a channel with sharp bends. Where thick sandy currents are stripped, levée and mid‐fan erosion may occur, but the residual current in the channel will lose much of its power and deposit rapidly. In thick muddy currents, progressive overflow of mud will cause less declaration of the residual channelised current. Thus both size and sand‐to‐mud ratio of turbidity currents feeding a fan are important factors controlling morphologic features and depositional areas on fans. The size‐frequency variation for different types of turbidity currents is estimated from the literature and related to the evolut

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1983.tb00702.x

出版商:Blackwell Publishing Ltd    

年代:1983

数据来源: WILEY

摘要:

ABSTRACTFour bedform provinces have been identified on Georges Bank using sidescan‐sonar and echo‐sounding techniques: large sand waves superimposed on sand ridges, small sand waves, megaripples, and featureless seafloor. The large sand waves and sand ridges are found on the bank crest where the surface tidal currents are strongest. Areas of small sand waves and megaripples, formed where tidal currents are moderate in strength, border the area of large sand waves to the north and south. Featureless seafloor is found farthest from the bank crest where surface tidal currents are weakest.Sand‐wave asymmetry and surface‐sediment texture have been used to infer bedload transport paths on Georges Bank. In the large sand‐wave area, bedforms indicate a clockwise transport around each of the linear north‐west‐striking sand ridges with slight convergence of the sand waves on the ridge crests. This transport pattern implies erosion from the troughs and accumulation on the sand ridges. The asymmetry of the small sand waves along the south side of Georges Bank indicates that sand is also transported southward away from the linear sand ridges on top of the bank. Although the asymmetry of megaripples could not be determined, the occurrence of megaripples between the small sand‐wave province and areas of featureless seafloor suggests a decreasing effectiveness of sand transport away from the bank crest. This sand dispersal pattern is further supported by the surface sediments which become progessively finer to the north and SW away from the crest

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1983.tb00703.x

出版商:Blackwell Publishing Ltd    

年代:1983

数据来源: WILEY

摘要:

ABSTRACTPseudomorphs after skeletal halite occur in a bed of Lower Jurassic lacustrine shale at Holyoke, Massachusetts. The pseudomorphs include pagoda types similar to those described by Southgate and show dissolution furrows. However the shales were deposited in a permanently subaqueous rather than an emergent environment, and the pseudomorphs accordingly grew in brine‐saturated sediment. Evaporation to dryness is not therefore essential to the genesis of skeletal halit

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1983.tb00704.x

出版商:Blackwell Publishing Ltd    

年代:1983

数据来源: WILEY

摘要:

ABSTRACTRippled flanks of longitudinal dunes and the slip faces of various dune types can be distinguished granulometrically by comparing the top to bottom trend of changes of sorting in the sand population of each individual slope. Flank sands, which are prone to sorting processes through migration of ripples alongslope, are always better sorted upslope whereas slipface sands, which are controlled by avalanche and grainfall of sands, become better sorted downslope. Considering the absolute values of the bottom samples, the sands of the slip faces are both better sorted and the size distribution more symmetrical than those of the rippled flanks.Applying this approach in investigating ancient sediments could improve palaeogeographical interpretations.

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1983.tb00705.x

出版商:Blackwell Publishing Ltd    

年代:1983

数据来源: WILEY

摘要:

ABSTRACTThe fluvial sandstone beneath the Mill Coal in the Westphalian ‘A’ of Scotland erosively overlies a lake mudstone. Slightly erosive surfaces within the sandstone, traceable for over 200 m, are used to divide it into two types of major sedimentary units termed type A and type B.Type A sand units are approximately 200 m wide, up to 7 m thick, convex upward, and lenticular in all directions. The constituent cosets overlap to the ENE and dip mainly at 1–2° downcurrent (NNW), but locally at 10–15°. Where thickest, type A sand units display a vertical facies sequence commencing with trough cross‐bedded and massive sandstone, overlain by a thick zone of ripple cross‐lamination, a thin zone of trough cross‐beds, and a variably eroded silt drape up to 0.4 m thick. Attenuated lateral margins are dominated by flat bedded sandstone with primary current lineation. Type A sand units are interpreted as deposits which were accreted on to a large fluvial bar during successive flood events. The bar is thought to have had a similar topographic significance to sand waves described from the Brahmaputra and slip face bounded bars observed in the South Saskatchewan river.Palaeocurrents measured from trough cross‐bed sets 0.3–1.0 m thick within type B sand units indicate flow to the WSW, perpendicular to the palaeoflow direction measured from type A units. In sections perpendicular to the WSW flow direction type B units are lenticular, and in ENE‐WSW trending sections they can be traced for over 80 m at a constant thickness. Type B sand units are interpreted as the product of low stage channels which flowed across bar fronts and tops.The sandstone described herein is interpreted as a braided‐type river deposit but is atypical, because it is fine grained and has an internal structure dominated by ripple cross‐lamination and upper phase plane beds. The palaeoriver is thought to have been of low sinuosity, 7–10 m deep, with a high suspended load and large rapidly fluctuating discharge. At low stage a braided‐type flow pattern developed around submerged bars. The regime of the palaeoriver was probably controlled by the fine sediment grain si

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1983.tb00706.x

出版商:Blackwell Publishing Ltd    

年代:1983

数据来源: WILEY

摘要:

Exercises in Sedimentology, by G. M. Friedman and K. G. JohnsonReview of Numerical Dating in Stratigraphy, ed. by G. S. Odin.Phosphorites on the Seafloor; origin, composition and distribution, by G. N. Baturin.Animal‐Sediment Relations. The biogenic alteration of sediments, ed. by P. L. McCall&M. J. S. Tevesz.Offshore Tidal Sands‐processes and deposits, ed. by A. H. Str

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1983.tb00707.x

出版商:Blackwell Publishing Ltd    

年代:1983

数据来源: WILEY