Grain scale simulations of loose sedimentary beds: the example of grain-bed impacts in aeolian saltation   总被引：4，自引：0，他引：4  

PETER K. HAFF  ROBERT S. ANDERSON† 《Sedimentology》1993,40(2):175-198

Sediment transport by wind is one of many processes of interest to the geomorphologist in which grain to grain contacts play an important role. In order to illustrate the modelling of collections of frictional, inelastic sedimentary grains with the particle dynamics method (PDM), we use the grain impact process in aeolian saltation as a specific example. In PDM, all the forces on each particle are evaluated at a sequence of small time-steps, and the Newtonian equations of motion are integrated forward in time. Interparticle forces at grain contacts are treated as springs with prescribed stiffness (normal force) and by a Coulomb friction law (tangential force); particle inelasticity is represented by spring damping. The granular splash resulting from saltation impacts is assessed for sensitivity to the choice of grain properties, and the integration time-step. We find that for the range of impact speeds and impactor masses relevant to aeolian settings, grain splashes are relatively insensitive to grain stiffness, grain inelasticity and grain friction, and that the pattern of ejection from the bed is largely controlled by bed microtopography. A large set of impact realizations involving a variety of impact points on a small set of target beds is used to collect the appropriate statistics for describing the stochastic splash process. The splash function representing these statistics is then available for use in calculations over longer time-scales, such as the evolution of the saltation curtain. The details given here will enable the interested reader to adapt PDM modelling to other types of clastic sedimentary systems.  相似文献   

Porosity evolution of the Cambrian Bonneterre Dolomite, south-eastern Missouri, USA     

JAY M. GREGG  PETER R. LAUDON†  ROBERT E. WOODY  KEVIN L. SHELTON‡ 《Sedimentology》1993,40(6):1153-1169

The Cambrian Bonneterre Dolomite of south-eastern Missouri, USA, hosts the large Mississippi Valley type (MVT) lead-zinc-copper ore deposits of the region. The Bonneterre Dolomite consists of dolomitized algal bioherms, oolitic grainstone and associated lithologies that were deposited on a carbonate platform surrounding the Precambrian age St Francois Mountains. Porosity was determined by point counting thin sections from cores and mines in the Bonneterre Dolomite and by gas porosimetry. Volumes of epigenetic cements were estimated by point counting cement filling micro- and mesoporosity using cathodoluminescence. Cement volumes were added to present porosity to estimate porosities during various stages of mineralization. Prior to the onset of mineralization, micro- and mesoporosity in the Bonneterre Dolomite averaged approximately 19%. Precipitation of early dolomite cement (roughly concurrent with the main period of sulphide deposition) reduced average porosity to approximately 7% and closed off much of the intercrystalline pore space. Later cementation by dolomite (prior to late stage sulphides) reduced porosity to approximately 5%, and late cementation by quartz further reduced porosity to the present average value of <4%. Periods of carbonate dissolution during MVT mineralization enhanced large scale megaporosity associated with fractures and breccias but did not significantly increase smaller scale porosity. Dolomite cementation associated with MVT mineralization, porosity and permeability were facies controlled. Today, as a result of mineralization, large scale fractures and breccias control porosity and permeability. This study indicates that dolomite porosity may undergo significant change during basinal brine migrations associated with MVT mineralization.  相似文献   

The ratio of MSA to non-sea-salt sulphate in Antarctic Peninsula ice cores   总被引：2，自引：0，他引：2  

ROBERT MULVANEY  ELIZABETH C. PASTEUR  DAVID A. PEEL  ERIC S. SALTZMAN  PAI-YEI WHUNG 《Tellus. Series B, Chemical and physical meteorology》1992,44(4):295-303

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Dynamics and facies model of a macrotidal sand-bar complex, Cobequid Bay—Salmon River Estuary (Bay of Fundy)     

ROBERT W. DALRYMPLE  R. JOHN KNIGHT  BRIAN A. ZAITLIN  GERARD V. MIDDLETON 《Sedimentology》1990,37(4):577-612

The 40-km-long, Cobequid Bay—Salmon River estuary has a maximum tidal range of 16·3 m and experiences limited wave action. Sediment, which is derived primarily from areas seaward of the estuary, is accumulating faster than the high-tide elevation is rising, and the system is progradational. The deposits consist of an axial belt of sands, which is flanked by mudflats and salt marshes in the inner half of the estuary where a funnel-shaped geometry is developed, and by erosional or non-depositional foreshores in the outer half where the system is confined by the valley walls. The axial sands are divisible into three facies zones: zone 1—elongate, tidal sand bars at the seaward end; zone 2—sand flats with a braided channel pattern; zone 3—the inner, single-channel, tidal—fluvial transition. Tidal current speeds reach a maximum in zone 2, but grain sizes decrease headward (from medium and coarse sand in zone 1, to fine and very fine sand in zones 2 and 3) because the headward termination of the major flood channels prevents the coarse, traction population from entering the inner part of the estuary. Longitudinal progradation will produce a 20-m-thick, upward-fining succession, the lower 1/2–2/3 of which will consist of cross-bedded, medium to coarse sand deposited on the zone 1 sand bars. The ebb-dominated portion of this unit will be finer grained than the flood-dominated part, and will contain trough crossbedding produced by 3-D megaripples; the flood-dominated areas, by contrast, will consist mainly of compound cross-bedding created by sandwaves with superimposed megaripples. Headward migration of swatchways (oblique channels that link the ebb- and flood-dominated areas) will create packages of ebb cross-bedding that is orientated at a high angle to the long axis of the estuary and that contains headwardinclined, lateral-accretion surfaces. The overlying fine and very fine sands of zones 2 and 3 will be composed mainly of upper-flow-regime parallel lamination. The succession will be capped by a 4-m-thick unit of mixed flat, mudflat and salt marsh sediments. A review of other macrotidal estuaries with tidal ranges greater than 10 m suggests that the major elements of the model have general applicability.  相似文献   

Award presented by the International Association of Sedimentologists at the thirteenth International Sedimentological Congress in Nottingham, England, on 27th August 1990     

ROBERT LOUIS FOLK 《Sedimentology》1991,38(1):191-195

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Studies in the genesis and deformation of migmatites: introduction   总被引：1，自引：0，他引：1  

ROBERT J. TRACY  HOWARD W. DAY 《Journal of Metamorphic Geology》1988,6(4):385-386

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A theoretical model for aeolian impact ripples   总被引：12，自引：0，他引：12  

ROBERT S. ANDERSON 《Sedimentology》1987,34(5):943-956

New insights into the grain-bed impact process arising from both numerical and physical experiments involving single grain impacts lead to a more complete conceptual model of the aeolian saltation process that in turn allows a simple model of aeolian impact ripples to be developed. The saltating population may be idealized as consisting of (1) long trajectory, high impact-energy, constant impact-angle ‘successive saltations’, and (2) short trajectory, low impact-energy ‘reptations’. It is argued that the spatial variations in mass flux due to the reptating population lead to the growth and translation of impact ripples. Using the sediment continuity equation, an expression for the spatial variation in the ejection rate of reptating grains from a sinusoidally perturbed bed, and a probability distribution for the reptation lengths, a simple stability analysis demonstrates that the flat bed is unstable to small amplitude perturbations. A fastest-growing wavelength emerges that is roughly six times the mean reptation length, and is only weakly dependent upon the detailed shape of the probability distribution of reptation lengths. The results match well with the observed initial wavelengths in wind tunnel experiments.  相似文献   

S.E.G. PRESIDENTIAL ADDRESS GIVEN ON THE OCCASION OF THE SILVER ANNIVERSARY MEETING OF THE EUROPEAN ASSOCIATION OF EXPLORATION GEOPHYSICISTS IN THE HAGUE, JUNE 1, 1976     

ROBERT B. RICE 《Geophysical Prospecting》1976,24(3):418-422

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Origin and diagenesis of lacustrine algal bioherms at the margin of the Ries crater, Upper Miocene, southern Germany     

ROBERT RIDING 《Sedimentology》1979,26(5):645-680

The marginal carbonate facies of the Miocene Ries meteorite crater lake in southern Germany contain bioherms up to 7 m high and 15 m across built by the green alga Cladophorites. The algae were externally encrusted during life by micrite, probably precipitated in response to photosynthetic uptake of CO₂, which produced tufts of fine (100 μm diameter), calcareous tubes. Coalescence of tufts, together with incorporation of peloidal and skeletal sand, created nodules and cones of algal tufa which in turn formed larger masses some of which are in the form of compound cones up to 2 m high. The bioherms are constructed by beds and groups of these cones and masses, and are surrounded by poorly cemented peloid, ostracod and gastropod sands. Five depositional and diagenetic stages of development can be distinguished: (1) growth and calcification (probably calcitic) of Cladophorites in shallow fresh- or slightly brackish water; (2) emergence due to a temporary fall in lake level and veneering of the algal tufa and adjacent sediments by laminated sinter; (3) resubmergence and deposition of peloidal and skeletal sands; (4) burial and partial phreatic dolomitization, together with dissolution of aragonite and penecontemporaneous deposition of thin isopachous rims of dolomite rhombs; (5) local vadose cementation by rhombs, spar and spikes of low-magnesian calcite. Stages 1–3 probably occurred several times. Most of the biohermal and surrounding sediments were produced by biological processes in the lake; subaerial sinter deposition and meteoric cementation have contributed relatively minor amounts of material and the majority of the sediments retain porosities of 10–30%.  相似文献   

Surge marks: are they water-escape structures?     

ROBERT W. DALRYMPLE 《Sedimentology》1980,27(3):341-343

In his recent paper on surge mark formation and morphology, Bull (1978) has provided considerable new and interesting information on these peculiar features. Of particular note is his observation that the laminae within surge marks are continuous and not dissected as would be expected if the surge marks were erosional rills: the previously proposed mechanism of formation (High & Picard, 1968; Picard & High, 1973). Consequently, it becomes necessary to formulate an alternative explanation for surge mark origin, and Bull (1978, p. 885) offers the suggestion that: ‘Surge marks may be the result of selective depositional/erosional processes together with external deformation pressures.’  相似文献