摘要:

ABSTRACTField measurements of the vertical structure of near‐bed suspended sediment concentrations were obtained from arrays of fast response optical backscatter suspended solids sensors to examine the time‐dependent response of sediment resuspension to waves and currents and the constraints imposed by bedforms. Data were recorded from both a nonbarred, marine shoreface and a barred lacustrine shoreface, under both shoaling and breaking waves (significant heights of 0·25–1·50m; peak periods of 3 and 8 s) and in water depths of 0·5–5·0 m. Sediment concentrations are positively correlated with increasing elevation above the bed, but lagged in time. The time lag varies directly with separation distance between measurement locations and inversely with the horizontal component of the near‐bed oscillatory velocity.Both the presence of wave groups and the settling velocities of the sediment particules in suspension influence the temporal changes in concentration at a given elevation. Sediment concentrations appear to respond more slowly to the incident wind‐wave forcing with distance away from the bed as a result of two factors: (1) the sequential increase in concentration induced by a succession of large waves in a group; and (ii) the relative increase in finer sediments with smaller settling velocities. Bedforms interact with the near‐bed horizontal currents to impose a distinct constraint upon the timing of suspension events relative to the phase of the fluid motion, and, therefore, the vertical structure of the suspended sediment concentration at a range of time scales. The near‐bed concentrations appear to be strongly dependent upon the vertical convection of sediment associated with the ejection from the wave boundary layer of separation vortices generated in the lee of ripple crests. Concentration gradients in the presence of vortex ripples are large, as are the correlation between concentrations measured at different elevatio

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1993.tb01352.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

ABSTRACTThe Kleszczów Graben in central Poland was formed by late Oligocene to Middle Pleistocene extensional tectonics. During the Pleistocene it was infilled with a 200 m thick sequence of predominantly glacial sediments. Four distinct formations of Elsterian and Saalian age are identified, each containing 15–40 m of glaciolacustrine strata. The boundaries between formations are marked by erosional surfaces and, in part, by angular discordances caused by tectonism. Glaciolacustrine sedimentation was tectonically controlled: the thickness of the sequences in the graben are three to five times greater than outside the area of fault‐controlled subsidence. Deposition in the proglacial lakes was controlled by differential subsidence rates within the basin: deep‐lake facies (varved clays) were deposited in sub‐basins with high subsidence rates and deltaic to shallow‐water facies accumulated in areas of moderate subsidence or occasional uplift. These variations led to the development of a very complex, ‘mosaic’ of lateral facies relationships, suggesting that several sub‐basins with differing subsidence rates were present. The Vertical successions show proximal‐distal sequences typical of glacier‐fed lakes that have limited contact with the ice sheet. However, gravity flow facies are very common, and occur both in the shallow‐ and deep‐water deposits. These deposits are interpreted to have been formed adjacent to active fault scarps which bordered the lake basin. Although several distinct phases of glaciolacustrine sedimentation occurred during the history of trough infilling, the location of the areas of high subsid

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1993.tb01353.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

ABSTRACTAncient stream‐dominated (‘wet’) alluvial fan deposits have received far less attention in the literature than their arid/semi‐arid counterparts. The Cenozoic basin fills along the Denali fault system of the northwestern Canadian Cordillera provide excellent examples of stream‐dominated alluvial fan deposits because they developed during the Eocene‐Oligocene temperate climatic regime in an active strike‐slip orogen. The Amphitheatre Formation filled several strike‐slip basins in Yukon Territory and consists of up to 1200 m of coarse siliciclastic rocks and coal. Detailed facies analysis, conglomerate: sandstone percentages (C:S), maximum particle size (MPS) distribution, and palaeocurrent analysis of the Amphitheatre Formation in two of these strike‐slip basins document the transition from proximal, to middle, to distal and fringing environments within ancient stream‐dominated alluvial‐fan systems.Proximal fan deposits in the Bates Lake Basin are characterized by disorganized, clast‐supported, boulder conglomerate and minor matrix(mud)‐supported conglomerate. Proximal facies are located along the faulted basin margins in areas where C:S = 80 to 100 and where the average MPS ranges from 30 to 60 cm. Proximal fan deposits grade into middle fan, channelized, well organized cobble conglomerates that form upward fining sequences, with an average thickness of 7 m. Middle fan deposits grade basinward into well‐sorted, laterally continuous beds of normally graded sandstone interbedded with trough cross‐stratified sandstone. These distal fan deposits are characteristic of areas where C:S = 20 to 40 and where the average MPS ranges from 5 to 15 cm. Fan fringe deposits consist of lacustrine and axial fluvial facies. Palaeogeographic reconstruction of the Bates Lake Basin indicates that alluvial‐fan sedimentation was concentrated in three parts of the basin. The largest alluvial‐fan system abutted the strike‐slip Duke River fault, and prograded westward across the axis of the basin. Two smaller, coarser grained fans prograded syntaxially northward from the normal‐faulted southern basin margin.Facies analysis of the Burwash Basin indicates a similar transition from proximal to distal, stream‐dominated alluvial fan environments, but with several key differences. Middle‐fan deposits in the Burwash Basin define upward coarsening sequences 50 to 60 m thick composed of fine‐grained lithofacies and coal in the lower part, trough cross‐stratified sandstone in the middle, and conglomerate in the upper part of the sequence. Upward‐coarsening sequences, 90–140 m thick, also are common in the fan fringe lacustrine deposits. These sequences coarsen upward from mudstone, through fine grained, ripple‐laminated sandstone, to coarse grained trough cross‐stratified sandstone. The upward‐coarsening sequences are basinwide, facies independent, and probably represent progradation of stream‐dominated alluvial‐fan depositional systems.Coal distribution in the Amphitheatre Formation is closely coupled with predominant depositional processes on stream‐dominated alluvial fans. The thickest coal seams occur in the most proximal part of the basin fill and in marginal lacustrine deposits. Coal development in the intervening middle and distal fan areas was suppressed by the high fre

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1993.tb01354.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

ABSTRACTThe Lower Eocene Ametlla Formation of the Ager Basin, Spanish Pyrenees, is a rapidly deposited shallow marine unit formed in a setting characterized by syn‐sedimentary tectonic activity. Mapping of the formation over a distance of 25 km was conducted according to sequence stratigraphical principles with emphasis on facies analysis. Twelve facies, grouped in five facies associations, have been recognized in the Ametlla Formation. The studied succession records a vertical transition from deltaic systems prograding onto a sediment‐starved shelf, via estuarine deposits associated with incised valleys, to sandbar complexes in a tidal seaway.In terms of sequence stratigraphy, three scales of genetic sedimentary units were recognized.(1) At the regional scale, elements of two 3rd‐order composite sequences (sensuExxon) have been recognized. These include a 3rd‐order highstand sequence set encompassing the lowermost part of the Ametlla Formation and the underlying Passarella Formation, and a 3rd‐order transgressive sequence set that constitutes the middle parts of the Ametlla Formation. The sequence sets are separated by an unconformity with up to 35 m of incision that is interpreted as a major sequence boundary. It is argued that the incised valleys associated with this unconformity were infilled during landward‐stepping of the shelfal depositional system. Basinwards, the unconformable surface becomes subhorizontal and is overlain by a 2 m thick oyster bed formed in a sediment‐starved setting subsequent to flooding of the incised valleys (which still acted as sediment conduits). Sandstones dominate the transgressive sequence set, whereas the highstand sequence set is dominated by siltstones, particularly in the lower part. In the transgressive sequence set, an upward increase in sand content and calibre is observed, relatable to punctuations of the transgressive trend by high‐frequency sea‐level fluctuations, and to downslope redistribution of sand.(2) At the subregional scale, detailed mapping indicates the presence of five 4th‐order sequences. The 4th‐order sequence boundaries are associated with sediment bypassing and minimal erosional relief, and were created by forced regressions during periods of relative sea‐level fall. Sharp‐based sandstones overlying these unconformities are believed to have accumulated during subsequent rise of relative sea‐level. Where 4th‐order maximum flooding surfaces can be recognized, the sequences may be subdivided into a sandstone‐dominated transgressive systems tract and a siltstone‐dominated highstand systems tract.(3) At the local scale, 2–9 5th‐order parasequences are present within the 4th‐order sequences. Superimposed parasequences are separated by flooding surfaces characterized by bioclastic accumulations, pervasive burrowing and extensive calcite cementation. The parasequences are commonl

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1993.tb01355.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

ABSTRACTThe non‐marine Campins Basin developed in the Oligocene, during a period of early rifting of the Catalan Coastal Ranges. Lacustrine deposits, interbedded between two alluvial units, comprise shallow and deep lacustrine facies. The lower, shallow lacustrine facies are made up of microbialite buildups and thin limestone beds. In the studied area, these facies are overlain by deep lacustrine facies which consist of alternations of several, metre‐thick carbonate‐ and mudstone‐dominated intervals. In addition to calcite, which is characteristic of the shallow lacustrine facies, aragonite and abundant dolomite are present in the deep lacustrine facies. This mineralogical change in the sequence reflects an overall increase in the Mg/Ca ratio of the lake waters. The deep lacustrine sequences are interpreted as having formed in a hydrologically closed basin that was subject to changes in the Mg/Ca ratio of the water, probably related to variations in the evaporation/precipitation rate.The sedimentological, mineralogical and isotopic characteristics of the Campins Basin dolomites suggest that, in general, they are primary in origin. The stable isotope data show an approximate covariance between δ13C and δ18O in the lower shallow lacustrine carbonates (calcite) which suggests that they formed during the onset of closure of the lake. The δ13C and δ18O values of the deep lacustrine carbonates display three different clusters that are roughly related to the carbonate mineralogy. Normalisation with respect to calcite of the isotopic compositions of dolomite and aragonite from the deep lacustrine carbonates allows the integration of all these isotope values into one covariant trend.The sequential appearance of different carbonate minerals and the isotopic covariant trend may indicate an overall evaporative concentration of the lake waters. The change in slope of the covariant trend for the isotope values between the shallow and the deep lacustrine carbonates might reflect the change in the waterbody morphology recorded in the basin fi

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1993.tb01356.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

ABSTRACTThe Lower Miocene Clews Formation at Alvord Mountain, Mojave Desert, California, comprises an upward coarsening sequence of synextensional continental deposits of variable thickness; this sequence pinches out to the west against the Alvord Mountain pre‐Tertiary basement complex and thickens eastward toc.300 m over a distance of 7 km. Two stages of sedimentation are recognized in the formation. During the initial stage of sedimentation, the depocentre was delineated by a lacustrine system that was bounded to the west by small, sheetflood‐dominated alluvial fans and to the north by a southerly flowing fluvial braidplain. Lacustrine mudstone, siltstone and carbonate suggest an initially closed basin. Conglomerate to the west represents small, sheetflood‐dominated fans. Igneous clasts and a south‐east palaeoflow direction suggest a proximal, low‐relief source in the western Alvord Mountain area. Pebbly sandstone of the southward‐prograding braidplain has a metasedimentary provenance in the Paradise Range to the north. The second stage of deposition was dominated by coarse conglomerate and breccia. West to southwest palaeotransport indicators and a distinctive metaigneous petrofacies indicate a provenance in the Cronese Hills, 8 km to the east. These strata reflect rapid westward progradation of sheetflood‐ and debris flow‐dominated alluvial fans that advanced across the axial braidplain and lacustrine system.Basin development is interpreted to have been controlled by regional NE‐directed extension on a detachment fault associated with the central Mojave metamorphic core complex. 30 km to the west. The Alvord Mountain ‐ Cronese Hills region was initially transported as a single hangingwall block on the E‐dipping detachment. Initial lacustrine sedimentation reflects the development of a flexural or sag basin in the hangingwall. Subsequent westward progradation of alluvial fans out of the Cronese Hills is believed to record the propagation of a NW‐striking, SW‐dipping normal fault antithetic to the low‐angle

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1993.tb01357.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

ABSTRACTThe Tamana Formation of the Central Range of Trinidad was studied in order to determine its importance in the stratigraphical and structural development of north‐eastern South America. Biostratigraphical, petrological and mineralogical data, combined with field mapping show that the Tamana sediments are composed of five distinct lithofacies: inner to outer shelf, burrowed shaley mudstone; outer shelf, Fe‐rich sandy limestone; submarine channel, conglomeratic mudstone; middle shelf to nearshore, algal‐foram packstone/grainstone; and intertidal to nearshore, algal‐stromatolite‐coral boundstone with coral bioherms. Maximum thickness of the Tamana Formation is 244 m.Deposition of the Tamana limestones occurred between thePraeorbulina glomerosa(latest early Miocene) andGloborotalia fohsi robusta(middle part of the middle Miocene) planktonic foraminiferal zones, and in a more continuous trend than is seen in the current outcrop belt. Detailed biostratigraphy shows that the Tamana Formation is a facies equivalent of the shallow‐ and deep‐water shales of the Brasso Formation, and the deep water turbidites of the Herrera Member of the Cipero Formation.The early diagenetic history of the Tamana limestones was dominated by the precipitation of authigenic glauconitic smectite, and the dissolution of skeletal grains and carbonate matrix. Late burial diagenesis was dominated by the precipitation of illite and illite/smectite. Comparative mineralogy and textural analyses indicate a minimum range of burial depth for the Tamana Formation at 800–1500m, with a maximum of 2400 m. Alteration of Fe‐bearing minerals to geothite and late fracturing occurred during post‐Pliocene tectonic uplift and unroofing of the Central Range.The Tamana Formation sediments can be used as a structural and stratigraphical event marker within the Late Tertiary geological history of Trinidad. These units record a phase of the tectonic interaction between the Caribbean and South American plates in the south‐eastern Caribbean, and reflect the onset of contractile deformation

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1993.tb01358.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

ABSTRACTThe Udo tuff cone of Cheju Island, South Korea, is a middle Pleistocene basalt tuff cone that has formed by early Surtseyan‐type eruptions and later drier hydroclastic eruptions. The tuff cone comprises steep (20–30°) and planar beds of lapillistone, lapilli tuff and tuff that can be grouped into seven sedimentary facies (A‐G). Facies A and B comprise continuous to lenticular layers of grain‐supported and openwork lapillistone that are inversely graded and coarsen downslope. They suggest emplacement by grain flows that are maintained by gravity‐induced stress and grain collisions. Facies C includes poorly sorted, crudely bedded and locally inversely graded lapilli tuff, also suggestive of rapid deposition from highly concentrated grain flows. Facies D includes thinly stratified and mantle‐bedded tuff that was probably deposited by fallout of wind‐borne ash. Other facies include massive lapilli tuff (Facies E), chaotic lapilli tuff (Facies F) and cross‐bedded tuffaceous sandstone (Facies G) that were deposited by resedimentation processes such as debris flow, slide/slump and stream flow, respectively.The grain flows that produced Facies A, B and C are interpreted to have originated from falling pyroclasts, which initially generated highly dispersed, saltating avalanches, in which momentum was transferred by the particles themselves. This transport mechanism is similar to that of debris fall. As the slope gradient was too low to maintain a highly dispersed flow, the debris fall decelerated and contracted due to a decrease in dispersive pressure. The mode of momentum transfer changed to one of collision because contraction of the debris fall resulted in an increase in particle concentration. This transport mechanism is similar to that of common grain flows. Grain segregation occurred in several ways. Initial segregation of ash from lapilli occurred due to their differing terminal fall velocities, and their contrasting degrees of sliding friction with the bed. Percolation of ash into interstices of lapilli during flow (kinematic sieving) augmented further segregation of ash from lapilli. The latter process, along with a dispersive pressure effect, gave rise to vertical inverse size grading. Downdip inverse grading was produced by part

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1993.tb01359.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

ABSTRACTPermeability measured with a portable probe permeameter on outcrops of cross‐bedded sandstones ranges between 0·9 and 19 D. The highest permeability (2–19 D with an average of 8·5 D) occurs in the coarsest grained foreset laminae (CFL), intermediate values (2–12 D with an average of 5·3 D) occur in finer grained foreset laminae (FFL) and the lowest values (0·9–10 D with an average of 4·8 D) occur in bottomset layers (BL). In the cross‐beds the average grain size ranges from medium grained sand in the CFL to fine grained sand in the FFL and BL. In all three subfacies, the average size of the primary pores is approximately 1φ unit smaller than the average grain size. The abundance of unstable carbonate clasts correlates with increasing average grain size, micritic clasts being most abundant in the CFL. Conversely, quartz content increases with decreasing grain size and is highest in the FFL and BL. Diagenetic destruction of primary porosity by compaction and cementation, as well as generation of secondary porosity through dissolution, were controlled by the original mineralogical composition of the sand. Contrasts in grain size determine the primary pore size contrasts and differences in composition between CFL, FFL and BL. Permeability contrasts reflect variations in average primary pore size rather than differences in total porosity. Probe permeability contrasts between CFL, FFL and BL depend on contrasts in average pore size and contrasts in mineralogical composition betwee

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1993.tb01360.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

ABSTRACTA critique is given of recent methods proposed for the standardized description, classification and interpretation of fluvial deposits in terms of: (1) hierarchies of strata and their bounding surfaces; (2) lithofacies; (3) lithofacies associations (architectural elements); (4) geometry of sedimentary bodies. Much of the descriptive information can be conveyed using clear, explicit diagrams without recourse to complicated terminology. Any classification that is used should be based on easily measurable parameters which are used to define mutually exclusive classes. Terms used to refer to these classes should be explicit. Little agreement exists on the best terms to describe strata, mainly because existing terminology was not designed to account for all the different superimposed stratal scales. A simple terminology is suggested here whereby the relative scales of hierarchies of strata and cross‐strata are specified. Methods of describing hierarchies of strata by numerically ordering stratal bounding surfaces (rather than the strata themselves) are difficult to use in practice. The use of standardized lithofacies codes tends to discourage close observation and recognition of varieties and superpositions of lithofacies, has led to proliferation of acronyms, and to the suggestion that a particular lithofacies has a unique interpretation. In fact, many interpretations associated with these lithofacies codes are misleading or wrong. In a commonly used classification system of lithofacies associations (architectural elements) the different classes are not mutually exclusive, they are referred to using a mixture of descriptive and interpretive terms, and each element is represented graphically by only a single two‐dimensional section. Terms used to describe the three‐dimensional geometry of a sediment body should only be used to refer to two‐dimensional sections if three‐dimensional form can be reconstructed unambiguously. Fluvial ‘facies models’ are constructed to relate lithofacies associations and geometry to parameters such as channel and floodplain geometry, modes of channel migration and deposition rates. Most are of limited use or misleading because insufficient three‐dimensional information is shown. In addition, they commonly incorrectly associate channel planform (e.g. meandering, braided) with a characteristic lithofacies association and geometry, whereas other factors may play

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1993.tb01361.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY