摘要:

ABSTRACTA two‐layer model for the flow and sedimentation within a salt wedge or highly stratified hypopycnal fluvial plume emptying into a fiord is developed from basic principles. From this model, it is found that the amount of suspended sediment of a particular grain size decreases as an exponential function of distance, and that the velocity and depth of the plume converge to constant values. The model is consistent with measurements from the plume of the Homathko River in Bute Inlet, British Columbia, Canada, and suggests that certain aspects of current empirical models of plume sedimentation are based upon sound principle

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1995.tb00412.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

ABSTRACTDetailed three‐dimensional (3‐D) observations of sandy point‐bar deposits from the River South Esk in Scotland were made using very closely spaced (metres) vibracores and ground‐penetrating radar (GPR) profiles. In order to explain the origin of the observed patterns of deposition, use was made of previous studies of channel geometry, flow and sediment transport. In addition, the mode and nature of channel migration and point‐bar accretion were determined using published maps, aerial photographs and detailed topographic surveys. Point‐bar deposits accumulated in response to channel‐bend expansion and downstream migration, resulting in preservation of sequences that fine upwards and downstream.Lower‐bardeposits are mainly very‐coarse to coarse sands with medium‐scale trough cross‐strata overlying basal gravels: associated radar facies are generally low‐amplitude, relatively discontinuous inclined reflectors.Upper‐bardeposits are mainly fine to medium sands with medium‐ and small‐scale cross‐strata and vegetation‐rich layers: associated radar facies are generally moderate‐ to high‐amplitude, laterally continuous, inclined reflectors. Large‐scale inclined stratasets seen in GPR profiles resulted from episodic point‐bar accretion. Abrupt lateral changes in inclination of these stratasets, and preservation of distinct unit bars (bar heads, scroll bars), lower‐bar platforms and inner‐bank swale fills, record discrete episodes of erosion and deposition associated with floods with recurrence intervals of decades to centuries. Such detailed 3‐D description and interpretation of these large‐scale features of point‐bar deposits was only possible through the use of GPR profiles tied closely to cores, and through the availability of much previously collected information on channel geometry, wat

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1995.tb00413.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

ABSTRACTThe diagenesis of carbonate platform sediments is controlled by the original facies and mineralogy, climate, sea‐level changes and burial history; these controls are clearly seen in the diagenesis of the Urgonian platform carbonates of SE France. Early diagenesis in the Urgonian platform included the precipitation of marine cements, dissolution of rudist shells and minor karstification. Diagenetic features produced during this phase were controlled by several falls in relative sea‐level during the Barremian to mid‐Aptian punctuating platform sedimentation, the original mineralogy of the sediment and the prevailing semi‐arid/arid climate in the region at this time. Following a relative sea‐level rise and further sedimentation, progressive burial of the platform led to minor compaction, followed by precipitation of coarse, equant, zoned to non‐luminescent, calcite cement. This cement was cut by later stylolites, suggesting a relatively shallow‐burial origin. Stable isotope (mean values ‐ 7.94%δ18O and 0.36%δ13C) and trace element (mean values of Fe 334 ppm, Mn 92 ppm and Sr 213 ppm) data suggest that these cements precipitated from meteoric fluids at temperatures slightly elevated relative to depositional temperatures.A variable thickness of replacive dolomite which occurs preferentially within the shelf‐margin facies of the lower part of the Urgonian post‐dates mechanical fracturing and chemical compaction, but pre‐dates the main phase of stylolitization. It is probable that the dolomitizing fluid was sourced by the early compaction‐driven release of connate fluids held within the underlying muddy units.The burial history of these rocks suggests that calcite cementation and dolomitization took place at relatively shallow burial depths (1–1.5 km). The overall diagenetic history of the Urgonian Limestone Formation is a reflection of the pre‐conditioning of the platform limestones by climate, sea level, tectonics and the shallow burial depths experienced by the platf

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1995.tb00414.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

ABSTRACTIn an effort to gain a better understanding of the relationship between climate, sand supply and aeolian sand deposition, an investigation was undertaken in the Algodones dunefield of south‐eastern California to determine: (1) the source of the sand and (2) the mode of sand supply to the Algodones dunefield. Textural, geochemical and mineralogical analyses show that the dune sands were mainly derived from Colorado River sands during episodes when the Colorado River drained west to the Salton Basin instead of south to the Gulf of California. A model is presented which relates episodic aeolian deposition in the Algodones dunefield to the growth of the resulting lake under a persistent, intense north‐westerly wind regime and to subsequent desiccation of the lake following diversion of the Colorado River to the Gulf of California. Late Holocene radiocarbon ages for relict lake shoreline deposits correlate with documented episodes of extreme flooding and increased winter storminess across the south‐western United States. These conditions are likely to have been the cause of intensified north‐westerly winds which coincided with high stands of Lake Cahuilla. Although diversion of the Colorado River to the Salton Basin occurred at least three times during the late Holocene, it is evident that the river did not change its course in response to the majority of the floods. It appears that the critical control on diversion of the river was the difference in gradient between courses leading to the Salton Basin and Gulf of California, which in turn was influenced by fluvial deposition and tectonic activity. Flooding was able to cause diversion of the Colorado River to the Salton Basin only when the difference in gradient between the two courses was relativel

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1995.tb00415.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

ABSTRACTUpper Jurassic phosphate stromatolites of the Almola Sierra (Southern Spain) encrust macrofossils and hardgrounds, and form oncoids included within pelagic, condensed fossiliferous limestones. Their accretion was determined by bacterially mediated precipitation of phosphate, by the trapping and binding of fine siliciclastics and pelagic biomicrite and by the encrustation of benthonic foraminifera. Phosphorous, trace elements and rare‐earth elements were concentrated from degraded organic matter and seawater by stromatolite‐building communities, which mediated the formation of phosphate‐rich and Fe‐Al‐Si‐rich organic gels under oxic conditions, favouring the precipitation of amorphous mineral precursors (ACP and Fe‐Al‐Si oxyhydroxides). The observed Ce‐enrichment for some stromatolites is explained by oxidative scavenging of Ce4+from seawater by Fe—Mn oxyhydroxides. The bacterially mediated gels were able to migrate and fill the voids of the stromatolite structure, and later changed to carbonate‐fluorapatite, haematite and poorly crystalline Fe‐rich clays under postoxic conditions. Phosphatization of trapped carbonate particles also occurred. The phosphate stromatolites formed on a sediment‐starved pelagic swell, during periods of no carbonate sedimentation and hardground development. Stromatolite lamination provides evidence for rhythmic alternation between bacterially mediated phosphogenesis, sedimentation and erosion, suggesting episodic changes in the sedimentary environment. Although some of the parameters that controlled phosphate precipitation associated with the stromatolites (local high organic productivity, sediment starvation, moderate depth of deposition and physicochemical conditions) were similar to those found in modern and ancient phosphogenic settings, the palaeogeographical framework and the intensity of sedimentary processes were different to those of the World's m

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1995.tb00416.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

ABSTRACTThe K‐Ar system in clay fractions from shallow marine carbonate shelf environments was investigated on silicate fractions (clay minerals, feldspar) separated from 20 Lower Jurassic to Upper Cretaceous sedimentary rocks, deposited in the southern Tethys ocean. The range of lithologies investigated included dolomite and chalk [IR (insoluble residue)90%). The results show that K‐bearing clay fractions often have K‐Ar ages similar to the suggested age of deposition, which means either supply of land‐derived authigenic K‐bearing clays or synsedimentary diagenetic authigenesis, or both. This K‐Ar synsedimentary signal is recorded in clay fractions from the whole range of studied lithologies and stratigraphic units. Among the clay minerals, the synsedimentary K‐Ar signature was recorded and retained in illite/smectite of the<2‐um and10‐um fractions the K‐Ar ages reflect the contribution of detrital mica and feldspar which accompanies the kaolinite‐dominated samples. The overall results differ considerably from K‐Ar age patterns observed in deep‐sea sediments, a difference which may be connected with the occurrence of brines in these shelf deposits. The findings indicate the potential in the K‐Ar dating of fine IR fractions of marine shelf sediments in terms of geochronological‐stratigraphic and palaeogeographical aspects as well as

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1995.tb00417.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

ABSTRACTA new model, which couples fluid and particle dynamics, has been developed to study the motion of the sediment‐water mixture during intense bedload transport, including the velocity profiles of both sediment and water, the roughness length of an upper plane bed and the thickness of moving sediment layers. Standard mixing length theory is used to model the motion of water above the boundary between the overlying water and the sediment‐water mixture. The turbulent flow within the moving sediment layers is described by a shear stress model, in which the effective viscosity of the flowing water is proportional to the velocity difference between the fluid and the sediment. The particle dynamics method, in which the equations of motion of each of many particles are solved directly, is applied to model the movement of sediment particles. The particle‐fluid interaction is expressed by a velocity‐squared fluid drag force exerted on each sediment particle. Both computer simulation results and theoretical analysis have shown that the velocities of both sediment and fluid during intense sediment transport decrease exponentially with depth in the top layers of a fast‐moving sediment—water mixture. The thickness of the moving sediment layers, obtained from the computer simulation results, is proportional to the shear stress, which agrees with previous experimental o

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1995.tb00418.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

ABSTRACTLaboratory experiments indicate that clay particles of the<2‐üm fraction can be considerably degraded by the bivalveMytilus edulis: dickite, kaolinite, smectite, chlorite and illite particles are partially dissolved and rounded by the digestive processes. Structural changes and a decrease in crystallinity of kaolinites, dickites and illites are striking.Investigations of clay minerals from tidal flat sediments highly populated by marine invertebrates, from marine suspensions and samples from profiles acrossMytilis edulisbeds on the North Sea coast of Germany show, however, that these materials have a uniform mineral composition. This uniformity of clay mineralogy can be explained by the hydrodynamic conditions in the nearshore area, where tidal currents and waves cause a periodic resuspension and transport of sediments introduced into the Wadden Sea from different sources (glacial, fluviatile). Processes of bioturbation also redistribute the sedimentary material. These mixing processes effectively erase any evidence of local enrichment of the biogenically degraded clays. However, the disordered clays should react more sensitively (i.e. have higher adsorption capacities for organic substances, hydroxides and metals) in sediments than untreated materials, so that biodegradation is of basic importance for clay diagenesis. During these processes, Pb, V, Ni and Fe are dissolved from the digested particl

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1995.tb00419.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1995.tb00420.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY