摘要:

ABSTRACTThe 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

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb00624.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

ABSTRACTNew observations concerning the degree of current‐induced erosion and deposition in the path of the 1929 Grand Banks turbidity current are presented. Most of the observations are available from Eastern Valley, Laurentian Fan. Seabeam and SeaMARC I data reveal widespread current erosion along the valley over a distance of 200 km from the shelfbreak. Erosional valley‐floor channels are preferentially developed adjacent to the valley margins and the flanks of intravalley highs. Asymmetric transverse bedforms (herein termed gravel waves) are moulded in a deflationary pebble and cobble lag that overlies the eroded valley floor. In contrast, at the distal limit of Eastern Valley, thick deposits of massive granule gravel indicate deposition beneath a decelerating turbidity current. Symmetrical transverse bedforms (herein termed macrodunes) are developed within these granule gravel sediments.The spatial distribution of both bedforms and the areas of erosive excavation suggest that the turbidity current in 1929 was accelerating over the first 100 km from the shelfbreak and was eroding and entraining sediment from the valley floor over a distance of at least 200 km. With the loss of lateral constraint at the distal limit of Eastern Valley the turbidity current spread laterally and started depositing sediment as it decelerated. Current‐induced erosion of the valley floor represented a potential source of between 50 and 100 km3 of sediment for incorporation into the resulting turb

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb00625.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

ABSTRACTErosional features on the floor of Eastern Valley of the Laurentian Fan, in 2800 m water depth, have been mapped with SeaMARC I side‐scan sonar images and Seabeam multi‐beam echo‐soundings, and were directly observed during a dive with the deep submersible Alvin. The most spectacular feature is a 100‐m‐deep flute‐shaped scour, more than 1 km long. The surrounding valley is floored by an unconsolidated coarse conglomerate, which was moulded into transverse bedforms by the turbidity current that was triggered by the 1929 Grand Banks earthquake. Direct observations and seismic‐reflection profiles show that the flute‐shaped scour cuts through this conglomerate and into Plio‐Pleistocene valley‐floor sediments, thereby exposing a section through the 1929 deposit. Application of the Allen defect theory suggests that the flute is unusually deep because general channel‐floor erosion was inhibited by the conglomerate veneer.Valley‐floor channels typically 1 km wide and 10m deep contain series of closed depressions that occasionally deepen to 30 m. These are also interpreted as erosional scours, analogous to pools cut on the beds of bedrock rivers. The large flute was probably formed by detached flow enlarging an initial scour depression. Such scours probably play an important role in channel‐floor erosion, increasing the volume of sediment transported by

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb00626.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

ABSTRACTThe erodibility of a grain on a rough bed is controlled by, among other factors, its relative projection above the mean bed, its exposure relative to upstream grains, and its friction angle. Here we report direct measurements of friction angles, grain projection and exposure, and small‐scale topographic structure on a variety of water‐worked mixed‐grain sediment surfaces. Using a simple analytical model of the force balance on individual grains, we calculate the distribution of critical shear stress for idealized spherical grains on the measured bed topography. The friction angle, projection, and exposure of single grain sizes vary widely from point to point within a given bed surface; the variability within a single surface often exceeds the difference between the mean values of disparate surfaces. As a result, the critical shear stress for a given grain size on a sediment surface is characterized by a probability distribution, rather than a single value. On a given bed, the crtitical shear stress distributions of different grain sizes have similar lower bounds, but above their lower tails they diverge rapidly, with smaller grains having substantially higher median critical shear stresses. Large numbers of fines, trapp.ed within pockets on the bed or shielded by upstream grains, are effectively lost to the flow. Our calculations suggest that critical shear stress, as conventionally measured, is defined by the most erodible grains, entrained during transient shear stress excursions associated with the turbulent flow; this implies a physical basis for the indeterminacy of initial motion. These observations suggest that transport rate/shear stress relationships may be controlled, in part, by the increasing numbers of grains that become available for entrainment as mean shear stress increases. They also suggest that bed textures and grain size distributions may be controlled, within the constraints of an imposed shear stress and sediment supply regime, by the influence of each size fraction on the erodibility of other grain sizes present on th

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb00627.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

ABSTRACTFor more than a century geologists have wondered why some bedforms are orientated roughly transverse to flow, whereas others are parallel or oblique to flow. This problem of bedform alignment was studied experimentally using subaqueous dunes on a 3–6‐m‐diameter sand‐covered turntable on the floor of a 4‐m‐wide flume.In each experiment, two flow directions (relative to the bed) were produced by alternating the turntable between two orientations. The turntable was held in each orientation for a short time relative to the reconstitution time of the bedforms; the resulting bedforms were in equilibrium with the time‐averaged conditions of the bimodal flows. Dune alignment was studied for five divergence angles (the angle between the two flow directions): 45°, 67–5°, 90°, 112–5° and 135°. The flow depth during all experiments was approximately 30 cm; mean velocity was approximately 50 cm s‐1and mean grain diameter was 0–6 mm. Each experiment continued for 30–75 min, during which time the flume flow was steady and the turntable position changed every 2 min. At the end of each experiment, water was slowly drained from the flume and dune alignment was measured. Transverse dunes (defined relative to the resultant transport direction) were created when the divergence angle was 45° and 67–5°, and longitudinal dunes were created when the divergence angle was 135°. At intermediate divergence angles, dunes with both orientations were produced, but transverse dunes were dominant at 90°, and longitudinal dunes were dominant at 112–5°.One experiment was conducted with a divergence angle of 135° and with unequal amounts of transport in the two flow directions. This was achieved by changing the orientation of the turntable at unequal time intervals, thereby causing the amount of transport to be unequal in the two directions. The dunes formed during this experiment were oblique to the resultant transport direction.These experimental dunes follow the same rule of alignment as wind ripples studied in previous turntable experiments. In both sets of experiments, the bedforms developed with the orientation having the maximum gross bedform‐normal transport (the orientation at which the sum of the bedform‐normal components of the two transport vectors reaches its maximum value). In other words, the bedforms develop with an orientation that is as transverse as possible to the two flows. In those cases where the two flows diverge by more than 90° and transport equal amounts of sand, bedforms that are as transverse as possible to the two separate flows will be parallel to the resultant of the two flow vectors. Although such bedforms have been defined by previous work as longitudinal bedforms, they are intrinsically the

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb00628.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

ABSTRACTPost‐Caledonian southern Ireland witnessed the development of a NE‐SW orientated half‐graben known as the Munster Basin. More than 7 km of non‐marine sediments accumulated in the basin during the late Middle and Late Devonian. Marine conditions became established in the southern part of the basin at the end of the Devonian. In this paper, a model for the evolving style of sedimentation in the basin and its periphery is constructed with the aim of identifying the major factors which controlled sedimentation patterns and the architecture of the basin fill.The depositional history of the basin is considered in terms of four successive episodes. During Episode I, gravelly alluvial fans flanked upland areas around the northeastern and northern basin perimeter. These graded southwestwards to a floodplain dominated by sheet‐floods. In the western part of the basin, the first of three major fluvial influxes into the basin commenced. During Episode II, the first influx developed into a large sandy braided complex. The sediment was derived from a distant source area located to the north and west of the basin and was transported diagonally across the basin towards the southeast. Episode III witnessed a second influx which drained into the basin from the northeast and north. River channels were of low sinuosity and graded distally to an ephemeral playa lake. Episode IV was marked by a third fluvial influx from the west and northwest. This was confined to the southern half of the basin and drainage was directed towards the east. The fluvial distributaries were flanked by permanently flooded overbank areas. This influx coincided with the first marine transgression which advanced westwards. The end of Episode IV coincided with the beginning of the Carboniferous and was marked by a major marine transgression.Sediment input to the basin was influenced by stable areas occupied by granitic plutons on either side of the basin and a southward downthrowing fault along its northern margin. The drainage direction was principally controlled by E‐W trending within‐basin faults and an E‐W trending stable area located to the south. The basin was fundamentally of the axial transport type, the main drainage having been directed towards the east though there was also a strong lateral influx from the north, northwest and northeast. Stable areas around the, basin periphery resulted in either no sediment preservation or sequences of multistorey channel deposits while thick sequences dominated by fine‐grained floodplain or overbank deposits characterized areas of higher subsidence rate within the basin. Movement on the northern basin‐margin fault was probably the major cause of the first fluvial influx, while regional subsidence of the basin and its northern periphery resulted in the second influx. The third influx was a response to local subsidence in the southern part of the Munster Basin. This also contributed to the simultaneous westward marine transgression in this area towards the end of the Devonian. Source area denudation and retreat in association with a sea‐level rise were ultimately responsible for terminating the alluvial regime

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb00629.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

ABSTRACTThe compositions of fluvial sands collected in mountainous regions along the volcanic front in Central America and along the Andean front in Venezuela and Colombia correlate with the tectonic setting of their source terrains. The sands derived from the Central American volcanic arc are compositionally distinct from those derived from the Andean terrains, and minor variations in tectonic style can also be discriminated. Subtle differences in tectonic setting and source rock composition are recognizable only when modification by chemical weathering is slight; they are strongest in sands from low‐order streams draining mountainous areas with modest alluvial‐plain development. Sands that have been exposed to weathering for extended periods during alluvial storage are progressively enriched in chemically stable phases. With increased weathering, sands from continental terrains converge toward a supermature quartz arenite end member, and the imprint left by source rock composition and tectonic setting is obscured. In intraoceanic terrains, a similar but much less pronounced enrichment in stable phases relative to less‐stable phases is observed, similarly overprinting the tectonic signal. Although detailed tectonic information is easily lost, sands derived from intraoceanic terrains are distinguishable from those derived from continental terrains even after long exposure to intense chemical‐weathering con

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb00630.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

ABSTRACTCarbonate ramps are gently sloping depositional surfaces where shallow‐water, coarse‐grained facies pass basinward into fine‐grained, deep‐water sediments, with no abrupt change in slope. The objectives of this study are: (i) to integrate the depositional processes recorded in the Pleistocene stratigraphy of the west Florida outer ramp into a palaeoclimatic and palaeoceanographic framework for the eastern Gulf of Mexico; and (ii) to examine the origin of mineralogical and sedimentary cycles in the light of pteropod and planktonic foraminiferal populations corresponding to climatic oscillations.Aragonitic, pteropod‐rich sediments with large amounts of insoluble residue occur in sediments deposited during glacial intervals; sandy calcitic sediments with abundant planktonic foraminifera accumulate during interglacials. These cycles reflect variations in biological productivity of pelagic pteropods and planktonic foraminifera, rather than preferential dissolution of either aragonitic or calcitic fractions. Species assemblages suggest that the productivity cycles are linked to changes in upwelling intensity at the margins of the Loop Current and variations in water mass salinities, as well as terrigenous dilution from the Mississippi Delta. These cycles are the response to Pleistocene glacial‐interglacial oscillations, controlled by Milankovitch orbital parameters.Although the organisms contributing to deep‐water carbonate environments have changed through geological time, facies patterns, as well as sedimentary textures and structures, identified in the west Florida sediments provide criteria for recognition of ancient ramps. An understanding of the processes on a modern ramp slope, such as west Florida, may prove valuable in palaeoclimatic and palaeoenvironmental analysis of epicontinental carbonate sequences and ramps in th

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb00631.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

ABSTRACTDolomitization of the Zechstein (Late Permian) Main Dolomite carbonates of northern Poland was penecontemporaneous and/or very early diagenetic. Well‐ordered, stoichiometric dolomites are associated with the basinal facies. The platform dolomites are relatively poorly ordered and usually non‐stoichiometric. Most samples are highly enriched in13C, as in other Zechstein carbonates. δ18O values show large variations from ‐5·1%0 to + 7·4%. There is an isotope zonation of the examined dolomites. The isotope signature indicates that dolomites formed from variable solutions of meteoric water, seawater, and evaporitic brines of possible marine or continental origin. Once initiated, dolomitization proceeded despite the evolution of dolomitizing brines. This evolution explains the occurrence of lagoonal dolomites with common evidence for dissolution in the lower part of sections compared with well‐developed rhombohedra in the upper part. Crystal zoning suggests the initiation of dolomite growth in hypersaline water and progressive dilution by fresh water. There is isotopic evidence for migration of continental waters into the basin, presumably following sea‐level fall at the end of the deposition of the Main Dolomite. Influence of fresh water on syndepositional dolomitization, well established in the Main Dolomite, strongly suggests that similar relationships may be characteristic for other evaporite‐associated dol

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb00632.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

ABSTRACTA unique association of beds containing fenestral pore‐filling medium crystalline dolomite with beds of dolospar and quartz sandstone occurs in the Lower Devonian Camsell Formation. Many of these sandstones consist almost entirely of dolospar sand. Sedimentological, petrographic, cathodoluminescent and chemical data indicate that the dolospar sands consist of grains that have been eroded and reworked during Camsell deposition from fenestral dolostone beds that directly underlie these sandstones. Erosional truncation of individual dolomite crystals and of intracrystalline cathodoluminescent zonations along the edges of sand grains indicates that either dolomitization, or dolomite cementation of the pore‐filling carbonate cements within fenestrae of the fenestral fabrics, occurred before erosion and deposition of dolospar sand. The fine details of the cathodoluminescent zonations suggest that the pore‐filling fenestral dolomite originated as primary cements that underwent some annealing recrystallization under the influence of high subsurface temperatures. This unique occurrence may document synsedimentary Devonian cementation by medium to coarsely crystalline dol

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb00633.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY