摘要:

ABSTRACTThe open‐ocean carbonate slope north of Little Bahama Bank consists of a relatively steep (4°) upper slope between water depths of 200 and 900 m, and a more gentle (1–2°) lower slope between depths of 900 and 1300+ m. The upper slope is dissected by numerous, small, submarine canyons (50–150 m in relief) that act as a line source for the downslope transport of coarse‐grained carbonate debris. The lower slope is devoid of any well‐defined canyons but does contain numerous, small (1–5 m) hummocks of uncertain origin and numerous, larger (5–40 m), patchily distributed, ahermatypic coral mounds.Sediments along the upper slope have prograded seaward during the Cenozoic as a slope‐front‐fill seismic facies of fine‐grained peri‐platform ooze. Surface sediments show lateral gradation of both grain size and carbonate mineralogy, with the fine fraction derived largely from the adjacent shallow‐water platform. Near‐surface sedimentary facies along the upper slope display a gradual downslope decrease in the degree of submarine cementation from well‐lithified hardgrounds to patchily cemented nodular ooze to unlithified peri‐platform ooze, controlled by lateral variations in diagenetic potential and/or winnowing by bottom currents. Submarine cementation stabilizes the upper part of the slope, allowing upbuilding of the platform margin, and controls the distribution of submarine slides, as well as the headward extent of submarine canyons. Where unlithified, sediments are heavily bioturbated and are locally undergoing dolomitization. Upper slope sediments are also ‘conditioned’eustatically, resulting in vertical, cyclic sequences of diagenetically unstable (aragonite and magnesian calcite‐rich) and stable (calcite‐rich) carbonates that may explain the well‐bedded nature of ancient peri‐platform ooze sequences.Lower slope sediments have prograded seaward during the Cenozoic as a chaotic‐fill seismic facies of coarse‐grained carbonate turbidites and debris flow deposits with subordinate amounts of peri‐platform ooze. Coarse clasts are ‘internally’derived from fine‐grained upper slope sediments via incipient cementation, submarine sliding and the generation of sediment gravity flows. Gravity flows bypass the upper slope via a multitude of canyons and are deposited along the lower slope as a wedge‐shaped apron of debris, parallel to the adjacent shelf edge, consisting of a complex spatial arrangement of localized turbidites and debris flow deposits. A proximal apron facies of thick, mud‐supported debris flow deposits plus thick, coarse‐grained, Ta turbidites, grades seaward into a distal apron facies of thinner, grain‐supported debris flow deposits and thinner, finer grained Ta‐b turbidites with increasing proportions of peri‐platform ooze. Both the geomorphology and sedimentary facies relationships of the carbonate apron north of Little Bahama Bank differ significantly from the classic submarine fan model. As such, a carbonate apron model offers an alternative to the fan model for palaeoenvi

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1984.tb01956.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY

摘要:

ABSTRACTSeveral Holocene turbidites can be correlated across much of Navy Fan through more than 100 sediment core localities. The uppermost muddy turbidite unit is mapped throughout the northern half of the fan; its volume, grain‐size distribution and the maximum height of deposition on the basin slopes are known. These parameters can be related to the precise channel morphology and mesotopography revealed by deep‐tow surveys. Thus there is sufficient information to estimate detailed flow characteristics for this turbidity current as it moved from fan valley to distal basin plain.On the upper fan, the gradient and the increasing downstream width of the channel and only limited flow overspill suggest that the flow had a Froude number close to 1.0. The sediment associated with the channel indicates friction velocities of about 0.06 m s−1and flow velocities of about 0.75 m s−1. Using this flow velocity and channel dimensions, sediment concentration (∼2×10−3) and discharge are estimated, and from a knowledge of the total volume of sediment deposited, the flow duration is estimated to be from 2 to 9 days. It is shown that the estimates of Froude number, drag coefficient, and sediment concentration are not likely to vary by more than a factor of 2.On the mid‐fan, the flow was much thicker than the height of the surface relief of the fan and it spread rapidly. The cross‐flow slope, determined from the horizontal extent of turbidite sediment, is used to estimate flow velocity, which is confirmed by consideration of both sediment grain size and rate of deposition. This again allows sediment concentration and discharge to be estimated. The requirements of flow continuity, entrainment of water during flow expansion, and observed sediment deposition provide checks on all these estimates, and provide an integrated picture of the evolution of the flow. The flow characteristics of this muddy turbidity current are well constrained compared to those for more sand‐rich late Pleistocene and early Holocene turbidity c

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1984.tb01957.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY

摘要:

ABSTRACTExperiments were carried out in a 10 × 6 m basin to simulate turbidity currents generated by the Horgen Slumping Events of 1875. The conditions for kinematic similarity were satisfied and the experiments gave further insight into the mechanics of transport triggered by the Horgen slumps. The experimental turbidity currents laid down thick deposits on a subaqueous fan, and thin sheets of turbidite on the floor of the elongate basin through longitudinal transport, comparable with the simulated deposits in Lake Zurich. It is concluded that longitudinal transport is a general phenomenon of turbidity currents

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1984.tb01958.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY

摘要:

ABSTRACTThe oolitic ironstones of the Minette were deposited during Toarcian/Aalenian times in a nearshore environment of the Paris Basin. The sedimentary sequence comprises up to 13 coarsening upward depositional cycles. The development of the cycles and different facies types are described. The iron ooids accumulated in a subtidal environment under the effects of tidal currents. Sand waves, which form an important part of the Minette sediments, were deposited under both time‐velocity symmetrical and time‐velocity asymmetrical tides. From the study of their internal structures the time‐velocity patterns of the currents, current speeds (up to 0±9m s−1) and net transport rates have been estimated (29 g m−1s−1in the case of large‐scale sand waves controlled by time‐velocity symmetrical tides). Sand wave heights and calculated near‐bed current speeds suggest a mesotidal regime. Sedimentary facies include large‐scale and waves, subtidal shoals with channels and an offshore muddy shelf. Conclusions as to bathymetry are drawn from ichnofaunal associations and from the study of wave ripple marks. Finally a depositional model of the Minette iron formation is proposed which may be applicable to othe

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1984.tb01959.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY

摘要:

ABSTRACTThe Lower Headon and Upper Barton Beds of Hampshire, southern England, consist of fine sands, silts and clays, often fossiliferous, with lignitic and carbonate horizons. They accumulated in a coastal environment following deposition of the marine Lower and Middle Barton Beds. A variety of distinctive facies can be defined on faunal and lithological grounds, and these permit palaeoenvironments to be defined with some precision. Littoral marine, barrier island shoreface, storm washover and barrier flat, brackish lagoon, distributary channel and floodplain lake environments are recognized. The evidence suggests that a barrier island or spit developed offshore, enclosing a sheltered inshore region of lagoons in which deposition of relatively fine‐grained sediments took place. Lagoonal sediments show a general trend towards reduction of salinity with time. With the eventual exclusion of marine influence, the area underwent a gradual transition to river‐dominated sedimentation in shallow flood‐plain lakes. While the sequence as a whole shows a progressive reduction in salinity, several brief periods of increased salinity are recognized and these reflect the very low topography of the region and its susceptibility to marine incu

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1984.tb01960.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY

摘要:

ABSTRACTAn experimental and theoretical examination has been made of the settling, entrainment and overturning of 176 valves representing 16 common Northwest European marine bivalve species, together with a comparative study of 15 plastic models in the form of segments from cylindrical tubes.Settling behaviour in both stagnant and moving water depends on particle mass, symmetry and concavo‐convexity. Separated empty bivalve shells spin and spiral while settling and, if sufficiently elongated, also pitch. At the observed Reynolds numbers, the shells and models fall concave‐up, the terminal fall velocity increasing as the square root of the unit immersed mass or weight. The drag coefficient is independent of Reynolds number but increases with surface roughness and, particularly, particle elongation. Turbulence slightly lowers the critical elongation for pitching.A separation vortex lies captive on the upper side of each descending particle. Consequently, an empty bivalve shell traversing a suspension of sand traps grains on its upper side at a rate proportional to their volume concentration and terminal fall velocity. This process, increasing the effective shell mass, is limited only by the capacity of the shell and grain spillage due to the possible onset of pitching.The ratio (non‐dimensional) of a quantity proportional to the applied fluid force and the particle unit immersed weight consistently describes the entrainment of concave‐up and convex‐up particles, and also the immediate overturning of a valve on settling concave‐up to the bed. These thresholds vary in relative magnitude with bed‐particle friction and particle concavo‐convexity. In general, convex‐up particles are the most stable; the concave‐up entrainment and overturning thresholds are of a substantially lower but similar magnitude. The high frequency of concave‐up bivalve attitudes in turbidites is understandable largely in terms of the ability of a settling valve to increase in effective mass by grain entrapment. Convex‐up attitudes in the lower parts of turbidites may record currents stronger than th

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1984.tb01961.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY

摘要:

ABSTRACTLower Carboniferous shallow water limestones of Asbian and Brigantian age in Britain commonly contain abundant interparticle micrite and characteristically display large syntaxial calcite overgrowths on crinoid and echinoid grains. These overgrowths appear to have developed at the expense of the micrite and are widely regarded as neomorphic replacements. However, cathodoluminescence of these has revealed growth features which indicate that they are not neomorphic but originated as passive cement fills of solution voids surrounding echinoderm grains.We introduce the term solution corona for these grain selective voids and consider that three processes may have contributed to their development, namely: high‐Mg calcite stabilization of host grains, crystal ripening, and meteoric dissolution. Cyclic subaerial emergence was a critical factor in these processes, and we contrast the morphology of the overgrowths in question with forms produced in basinal limestones which never experienced comparable early meteoric conditions.These early‐formed solution coronas around echinoderm grains are therefore a useful indicator of meteoric diagenesis and have important implications for porosity evolution. The syntaxial cements which fill the solution coronas show distinct phases of growth in cathodoluminescence which reflect a progression from near‐surface meteoric conditions to deep burial with pressure sol

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1984.tb01962.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY

摘要:

ABSTRACTA new sensor for the continuous and unmanned detection and recording of bedload motion is described. The sensor acts in the same fashion as a conventional metal detector but is elongate and installed permanently in the bed of an alluvial channel. Artificial clasts are labelled with short lengths of ferrite rod and are seeded upstream from the sensor, replacing bed particles. The entrainment of seeded clasts takes them over the sensor where they distort the magnetic field and produce a change in inductance that is detected and recorded. A field installation demonstrates the value of the sensor by revealing for the first time in coarse‐grained alluvium the spasmodic nature of particle motion reminiscent of kinematic waves. It also illustrates the importance of pebble clusters in delaying particle entrainmen

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1984.tb01963.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY

摘要:

ABSTRACTA previous proposal (Leeder, 1977) to test the magnitude of the solid‐transmitted stresses due to bedload transport failed to confirm Bagnold's theory because the applied fluid stress contribution was not corrected for sidewall drag. Once this correction is made there is a reasonable correspondence of theory and experimen

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1984.tb01964.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY

摘要:

Book Review in this article:Introduction to Geomagnetism, by W. D. Parkinson.Climatic Geomorphology, by Julius Büdel, translated by Lenore Fischer and Detlef Busche.Contributions to Sedimentology of Some Carbonate and Clastic Units of the Coastal Dinarides. Excursion Guidebook, ed. by L. J. Babić and V. JelaskaContinental Carbonate Sedimentation and Pedogenesis, by P. Freytet and J. C. Plaziat, edited by B. H. PurserRivers—Form and Process in Alluvial Channels, by Keith RichardsAt the Sea's Edge: an introduction to coastal oceanography for the amateur naturalist, by William T. Fox, illustrated by Clare Walker Leslie.Changing Sea‐level Along the North Coast of Kuwait Bay, by Taiba A. Al‐Asfour.Carbonate Depositional Environments, ed. by P. A. Scholle, D. G. Bebout&C. H. Moore.Dynamic Models in Earth‐Science Instruction, by W.

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1984.tb01965.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY