摘要:

ABSTRACTThe Haystack Mountains Formation (Campanian, Mesaverde Group, US Western Interior Basin, Wyoming) contains a series of shallow‐marine sandbodies, extending tens of kilometres out from a basin margin. The study succession (around 200 m thick) is composed of eight major sandstone tongues (Bolten Ranch, O'Brien Spring, Seminoe 1–2–3–4, Hatfield 1 and 2 members), each partially encased within marine shale intervals. The Formation is ‘sequential’at several scales. At the largest scale, the whole succession presents an aggradational to basinward‐stepping stacking pattern of the sandstone tongues. At a lower level, each tongue (member) is characterized internally by two different types of lithosome: the first represents shoreface progradation with hummocky cross‐strata passing up to swaley and trough cross‐stratified sandstones. This lithosome is erosively truncated at its top in most cases, and has a general sheet‐like geometry along strike, whereas down dip it displays a series of sharp‐bounded clinothems. The latter sometimes indicate a downward as well as a basinward shift through time, as suggested by the occurrence of coarser and/or shallower facies at a lower level in the shoreface profile. The second type of lithosome is sheet‐ or wedge‐like and sharply overlies the shoreface deposits. The lithosome consists of laterally widespread units of planar tabular to trough cross‐bedded medium sandstones passing laterally (in a dip direction) into bioturbated sandstones. The lower part of this lithosome is progradational, becoming retrogradational into the overlying shales. The facies within the cross‐bedded lithosome suggest a tidally dominated delta front to estuarine depositional setting.The two types of lithosome are not related genetically. The erosion surface separating the two lithosomes is a sequence boundary separating forced‐regressive (relative sea‐level fall) shoreface deposits from lowstand to transgressive (early relative sea‐level rise), cross‐bedded deposits. The uppermost part of the cross‐stratified lithosome shows a landward‐stepping of component parasequences and is abruptly blanketed by open‐marine shales.The most widespread cross‐bedded lithosomes are apparently best developed in the lowermost members of the Haystack Mountains Formation, i.e. in the aggradational part of the large‐scale progradational succession. In the uppermost, highly progradational sandstone tongues, the shoaling‐upward shoreface lithosome dominates, whereas the cross‐bedded lithosome occurs in narrow, lensoid belts, or is absent. The middle portion of the succession shows intermediate characteristics. The vertical variation in geometry, thickness and progradational extent of successive cross‐bedded lithosomes results from greater confinement of the incised nearshore systems both in space (landward direction) and in time (from the aggradation to the progradation architecture). The latter is a consequence of a d

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1995.tb00392.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

ABSTRACTThe continental terrace of Israel consists of Nile‐derived sediments. They are classified into three major groups, according to their structures: (1) irregularly to wavy laminated, coarse‐grained sediments (quartz and skeletal debris) of the flat outer shelf and upper bend of the slope (10–20% of the bulk) which are influenced by currents. These sediments are bioturbated to various degrees. Smectitic silty clays (80–90%) form two structural groups: (2) laminated, turbiditic sediments which accumulate on the slope, particularly in the canyons of the northern slope. Irregular, wavy, thickly laminated, coarse‐grained sediments of the upper slope and canyon heads merge downslope with parallel, thinly laminated, fine‐grained sediments. Toward the foot of the slope and on the adjacent deep‐sea floor lamination becomes indistinct and the sediment is visually homogeneous. (3) Slump‐generated mud lumps of various size which accumulate on the lower slope and along the transport axes of the canyons. These are the most visible evidence for large‐scale slumping mass movement.Transportational and depositional processes are far more intensive over the steep northern continental slope of Israel, and especially in its canyons than over the gentle southern slope.Very stiff overconsolidated sediments unconformably overlain by the ubiquitous recent silty‐clayey sediment were cored on steep sections of the middle continental slope and along the canyon walls. Their preconsolidation stress values suggest that these sections were formerly overlain by more than 40 m of sediment and now are exposed by slumping. The downsliding slabs usually disintegrated into small fragments although several huge fragments could be identified al

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1995.tb00393.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

ABSTRACTAn experiment was conducted to study megaripple morpho dynamics on a sandy intertidal shoal in a mesotidal mangrove creek (Gordon Creek, Townsville, Australia). Tidal current velocity and depth were recorded with S4 current meters over a period of 35 tides. The tidal megaripples were 0.06–0.2 m in height and 1–2 m in wavelength, and their movement was monitored by (1) electromagnetic bed‐elevation probes (which automatically recorded bed level every 2 min at three positions along the survey transect) and (2) daily surveying for 8 days around spring tidals.The tidal currents in Gordon Creek are ebb‐dominated, with maximum depth‐mean current velocities for the flood and ebb tides of 0.62 and 0.98 m s−1respectively. Significant bedload transport occurs only during spring tides, and only on the larger of the unequal semi‐diurnal tides. Bedload transport is overwhelmingly in the ebb direction. Megaripple migration rates reach 5.6 m per tide in the ebb direction and up to 0.1 m min−1within individual tides. Within‐tide ‘bedform transport rates’are up to 0.29 kg m−1s−1.The results suggest that for reconstruction of palaeoflows from deposits of preserved fine‐ to medium‐grained sandy tidal megaripples, it is valid to use a depth‐averaged velocity of 0.5–0.6 m s−1as the migration threshold. Velocity thresholds associated with partial or complete reversal

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1995.tb00394.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

ABSTRACTThe origin of massive sands in turbidite successions has commonly been attributed to the rapid dumping of sand due to flow unsteadiness in collapsing, single surge‐type, high‐density turbidity currents. The general applicability of this model is questioned here, and we propose that rapid deposition of massive sands also occurs due to non‐uniformity in prolonged, quasi‐steady high‐density turbidity currents. We attempt to eliminate ambiguity in the use of the terms ‘deceleration’and ‘unsteadiness’with respect to non‐uniform sediment gravity flows, and stress that, as with any particulate current, unsteadiness is not a prerequisite of sediment deposition. We propose a mechanism of gradual aggradation of sand beneath a sustained steady or quasi‐steady current, and upward‐migration of a depositional flow boundary that is dominated by grain hyperconcentration and hindered settling. Formation of tractional structures is prevented by the absence of a sharp rheological interface between the lowest parts of the flow and the just‐formed dewatering deposit. Deposition continues as long as the downward grain flux to the depositional flow boundary is balanced by grain supply from above or from upcurrent. Massive sand deposited in this way is not, strictly, a result of ‘direct suspension sedimentation’in that it is characterized by grain interactions, hindered settling, shear and, possibly, by interlocking of grains. The thickness of the resulting massive sand bears no relation to the thickness of the parental current, and the vertical variation within the deposit may reveal little about the vertical structure of the current, even during deposition. Thin, normally graded tops or mud drapes represent the eventual

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1995.tb00395.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

ABSTRACTThe Tiber Delta lies on the passive continental margin of central western Italy. The Tiber Delta is a Late Pleistocene—Holocene sedimentary succession and constitutes an ‘incomplete’high‐frequency depositional sequence (4th or 5th order), which developed from the last post‐glacial rise in sea level to the present stillstand. It is bound by a type 1 basal unconformity which cuts Pleistocene deposits.Five depositional systems have been identified: (1) coastal barrier—lagoon; (2) shelf‐transition; (3) fluvial braided; (4) Tiber lagoonal deltas (T1, T2, T3); and (5) Tiber wave‐dominated delta. The first four systems constitute the transgressive system tract (TST) which is characterized by a retrogradational parasequences set, while the fifth system represents the highstand system tract (HST) characterized by only one parasequence (progradational). The evolution of these depositional systems has been mostly controlled by sea‐level rise, which was not continuous, but punctuated by eight minor stillstands and rapid rises. This mechanism of sea‐level change led to the formation of parasequences that developed within the depositional systems. From a palaeogeographical point of view the present Tiber Delta area was mainly characterized by a coastal barrier, lagoon and Tiber lagoonal deltas (T1, T2, T3), which migrated landwards during the sea‐level rise. Throughout the time of the present stillstand, the T3 lagoonal delta prograded rapidly into the lagoon, reaching the coastal barrier, and the lagoon was replaced first by marshes and then by the alluvial plain. Finally, the Tiber River overcame the littoral barrier supplying the coast, giving rise to the present

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1995.tb00396.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

ABSTRACTA new quantitative method for characterizing quartz grain shape is presented. The method employs a harmonic analysis based upon Fourier descriptors which is a distinct variation of the traditional and widely used Fourier series. Quartz grain images from a scanning electron microscope were ‘frame grabbed’and converted to a digitized grey‐level image. The image processing techniques of enhancement, segmentation and boundary tracking were applied to remove all features except the image boundary. This boundary was sampled at uniform intervals of are length and represented mathematically on the complex plane. In this way problems associated with the location of particle centroid and re‐entrant values were avoided. The resulting data was standardized relative to scale, rotation and starting position. Hence the discrete Fourier transform was applied using modern fast Fourier transform techniques and the modulus of the resulting harmonic amplitude used to characterize the grain shape. The technique was applied to a sample of 0–5‐m quartz grains from three distinct populations: desert quartz, beach grains (Fire Island, New York) and Brazilian crushed quartz. Whilst plots of average amplitude vs. harmonic number for each population appeared similar, discriminant analysis applied to each grain sample distinguished characteristic grain shape with an excellent degree of success. The problems of location of the centroid and re‐entrant values were eliminated. This allowed the technique to be applied to a much wider group of irregularly shaped sedimentary particles

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1995.tb00397.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

ABSTRACTA late Pleistocene morainal bank is sited in a depocentre to the lee of a major rock ridge, near Greystones, in the western Irish Sea Basin. During deglaciation the ridge provided a pinning point during tidewater wastage northwards. Sedimentation patterns and palaeocurrent data show morainal bank growth by discharge from a single basal efflux located to the east or south‐east of the ridge during ice marginal re‐equilibration.The four lithofacies associations which are recognized from the western part of the formerly more extensive apron are related largely to variable jet and plume sedimentation. At the base of the 1.6 km long exposure, Lithofacies association 1 (massive mud, muddy diamict and laminated mud) was deposited from turbid plumes, variable ice rafting and traction current activity. Lenticular units of gravels within this mud bank record high energy pulses and sediment fluxes from the efflux jet. Lithofacies association 2 (sands, laminated muds and muddy diamict) is discontinuous and occurs within basins along a marked erosion surface cut in Lithofacies association 1. It is associated with a decrease in jet strength, traction currents and suspension sedimentation. Lithofacies association 3 is a tabular body of interbedded diamicts and gravels which is present along the entire section. It documents the decay phase of re‐equilibration as the ice margin disintegrated catastrophically and released large volumes of heterogeneous sediment which was resedimented by quasicontinuous mass flow. Lithofacies association 4 consists of stratified and massive gravels within distributary channels cut into underlying facies and represents the last phase of meltwater activity.Sediment geometries, particularly sedimentary contrasts representing erosion surfaces at a variety of scales and abrupt textural contrasts are attributed to jet switching. Lithofacies association 1 (60%) and Lithofacies association 3 (30%) are the dominant facies. In favourable topographic settings this stratigraphic couplet is a signature for re‐equilibrated ice margins in isostatically depressed basins dominated by tidewater fronts, rapid ice flux and high relative sea level.Morainal banks document rapid environmental change and in the Irish Sea Basin they form part of a deglacial event stratigraphy related to unstable tidewater margins and high relative sea level. Deglaciation was therefore controlled primarily by high relative sea level rather than climatic forcing. Facies variations should therefore not be used for stratigraphic correlations in place of direct stratigraphy. This type of situation may be more common than hitherto realized in Late Pleistocene, mid‐latitude shelves where most of the preserved stratigraphy is characterized by complex, interbedded sequences formed when isostatic depression exceeded sea

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1995.tb00398.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

ABSTRACTFollowing the Late Aquitanian sea‐level fall, tide‐influenced deposition started in the North Hungarian Bay, an embayment in the Paratethys open to the north‐east. The relatively narrow, funnel shape of the bay supported amplification of tidal movements, resulting in the generation of strong tidal currents. The length and the depth of the North Hungarian Bay and the connecting seaway through East Slovakia fell into the ‘Tidal Amplification Window’and thus fulfilled the conditions needed for resonant amplification of semidiurnal (M2) tides. Tide‐influenced deposits were formed at both sides of the North Hungarian Bay. They reflect dominant currents in opposite directions and of different strengths at either side of the basin. This difference was the result of bottom‐tide interactions. Cyclonic (anticlockwise) residual currents were induced above the asymmetrical central depression in the bay and were superimposed upon the tidal currents, producing an anticlockwise, time‐and‐velocity asymmetrical current system.The North Hungarian Bay and other examples show that amplification of tidal motions and formation of tide‐influenced deposits may occur if basin dimensions pass through the ‘Tidal Amplification Window’. This window represents ideal conditions for resonant or amphidromic amplification of tidal currents. It determines an ideal length/depth or width/depth ratio relative to the wavelength of the astronomical tides. Thus signs of strong tidal influence in fossil basin fills can be used to reconstruct the dimensions (length, depth an

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1995.tb00399.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

ABSTRACTAn experimental study was undertaken in a large‐scale wind tunnel to investigate sand particle dislodgement by wind over time in the absence of grain‐bed collisions. Aerodynamic dislodgement was measured for six groups of sand particles under two known wind velocity profiles. The results show that the dislodgement rate for both fine and coarse particles decreases rapidly during the transition of the particle surface from a non‐wind‐worked condition to a wind‐worked condition, and that the dislodgement rate continues to decay under a wind‐worked condition even though the mean grain size of surface particles remains nearly the same. A previously developed theoretical method for calculating the number of particles left on the bed by wind was developed further. The derived method was used to calculate the time‐decay of the dislodgement rate and the length of time required for the dislodgement rate to reach an equilibrium. The length of time for dislodgement rate to reach an equilibrium in this study is of the order of 10–15 min. This not only provides further observation of the second, long stage of aeolian sediment transport system development reported previously but also indicates a potentially large variation in the time‐decay of transport rate under different conditions. The results indicate that the time‐decay of the particle dislodgement rate is related to sorting processes. Because of the artificial method of preparation of the grain surface and the wind velocity profiles, the results of this study should be applied with caution to

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1995.tb00400.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

ABSTRACTCollision data are presented from coloured high‐speed films of three size fractions of sand grains saltating over a bed of the total grain population. Each fraction was colour tagged and the proportion of each size ejected by grains colliding with the surface was recorded on a number of films taken as the bed was progressively eroded. The results confirm earlier findings that V3/V1≅0.5–0.6, Vn/V1≅.08 and the rebound angle increases with decreasing grain size. Ejected grains are examined in relation to their size, the impactor size, ejection speed and angle and the number of ejecta per collision. In addition, changes in grain parameters are observed with time. For fine impactors, ejection speeds generally increase with a decrease in ejecta size, but the fine fraction does not follow this trend for the coarse and medium impactors. Ejection angles are typically between 40° and 60°, with coarse grains having shallower mean angles than fine ejecta. The number of ejections per collision increases with a decrease in particle size for each impactor size. The general tendency for coarse particles to be ejected at lower speeds and shallower angles than fine particles will lead to sorting of the grain sizes. There is poor correlation between the forward momentum loss of the saltating grams at collision and both the forward momentum of the ejected grains and the number of ejected grains. Much of the forward momentum of the saltating grains is transfered to creeping grains. The composition and geometry of the bed are considered to be important factors in the evolution of the saltat

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1995.tb00401.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY