摘要:

ABSTRACTThe Castissent Formation represents a phase of strong fluvial progradation within the fluvio‐deltaic Montanyana Group, probably enhanced by the late Ypresian sea‐level fall. The structural setting is characterized by the emplacement of the South Pyrenean nappes. The major architectural feature of the Formation is the superposition of three multilateral and multistorey sheet sandstone complexes (A, B and C), interbedded in finer floodplain deposits and dark‐grey mudstones of brackish transitional origin. The structure of the sheet sandstone complexes is exemplified by the detailed analysis of the lower sheet sandstone complex (A), 25 m thick, 6 km wide and exposed over 25 km of its length. Three types of channel‐fill sandstones serve as components: (1) lenticular‐bedded bodies, interpreted as braided stream deposits, (2) lateral accretion‐bedded bodies, intepreted as meandering stream deposits, and (3) ribbons, originating from laterally stable and short‐lived offshoot or crevasse channels. By coalescence and/or stacking of these channel‐fill types, higher‐order architectural units (A1, A2, A3) have been formed that together build the sheet sandstone complex. On the basis of downstream variation in proportion and geometrical relation between the channel‐fill components, three depositional zones have been differentiated within the fluvial system. Each zone is characterized by specific architectural processes: (1) alternation of minor aggradational‐degradational cycles in the proximal valley, (2) generation of new alluvial belts in the intermediate nodal avulsion zone, and (3) successive random avulsion processes with development of semipermanent crevasse channels or minor distributaries in the distal distributive zone. The sedimentation of the Castissent Formation was structurally controlled by an interplay of vertical basement movement due to thrust stacking in the hinterland and surficial thrust displacement to the foreland resulting in alternating southward and northward shift

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1988.tb01247.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTThe sandstones and coquinas of the upper 20 m of the Sundance Formation are interpreted as a tidal inlet, back‐barrier shoal and sandy tidal‐flat sequence deposited at the close of marine Jurassic sedimentation in north‐central Wyoming. The barrier strandline maintained a generally E‐W trend as it prograded to the north. The lateral migration of inter‐barrier tidal inlets along the regressive shoreline of the late Sundance sea caused the coquinas and sandstones of the uppermost Sundance Formation to be deposited as tabular, laterally‐extensive units. Tidal bundles, sigmoidal reactivation surfaces, herringbone cross‐lamination and abundant mud drapes within the sandstones are evidence of considerable tidal influence during the deposition of the uppermost Sundance Formation. Earlier models, which attach an offshore environment of deposition to the sequence, do not explain the tabular geometries of the sandstone and coquina units and their conformable stratigraphic relationship with the overlying non‐marine sediments of the Mor

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1988.tb01248.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTPreliminary results are reported from an experimental study of the interaction between turbulence, sediment transport and bedform dynamics over the transition from dunes to upper stage plane beds. Over the transition, typical dunes changed to humpback dunes (mean velocity 0–8 ms‐1, depth 01 m, mean grain size 0.3 mm) to nominally plane beds with low relief bed waves up to a few mm high. All bedforms had a mean length of 0.7–0.8 m. Hot film anemometry and flow visualization clearly show that horizontal and vertical turbulent motions in dune troughs decrease progressively through the transition while horizontal turbulence intensities increase near the bed on dune backs through to a plane bed. Average bedload and suspended load concentrations increase progressively over the transition, and the near‐bed transport rate immediately downstream of flow reattachment increases markedly relative to that near dune crests. This relative increase in sediment transport near reattachment appears to be due to suppression of upward directed turbulence by increased sediment concentration, such that velocity close to the bed can increase more quickly downstream of reattachment. Low‐relief bedwaves on upper‐stage plane beds are ubiquitous and give rise to laterally extensive, mm‐thick planar laminae; however, within such laminae are laminae of more limited lateral extent and thickness, related to the turbulent bursting process over the downstream depositional surface of

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1988.tb01249.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTThe dynamic interpretation of most current‐structure sequences derives directly from experiments on the succession of bedforms produced by flows in flumes. The results of these and related studies have been used to construct stability field diagrams in which the fields of individual bedforms are usually expressed as a function of flow intensity (power, velocity, bed shear stress, etc.) and grain size.The data underlying existing stability‐field diagrams were collected largely from the study of flows carrying coarse‐grained sediment entrained through particle‐by‐particle bed erosion. Many flows, however, do not entrain sediment through simple bed erosion. Most turbidity currents originate by the development of turbulence in slumps, slides, and other slope failures. Such flows generally form with highly concentrated suspended loads and their bed‐load layers derive sediment from the collapsing suspended‐sediment clouds. Because the collapse properties of such clouds may be related as much to suspended particle concentration, size distribution, particle interactions, and other factors as to flow intensity, the stability fields of bedforms developed beneath such flows may differ in flow intensity‐grain‐size relationships from those beneath flows deriving sediment from bed erosion alone.Useful stability‐field diagrams for turbidity currents must include suspended‐load fallout rate as a third variable, independent of flow intensity and mean grain size. A preliminary stability‐field diagram of this type indicates that BoumaTabcsequences may theoretically form with essentially no velocity variation of the attendant flow. This type of analysis may have considerable relevance to the interpretation not only of turbidites but also of other deposits formed where bed‐load layers are fed from above rather than below. These include shallow‐shelf storm units deposited from highly concentrated flows and volcaniclastic layers formed where pyroclastic debris falls

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1988.tb01250.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTCyclically deposited siliciclastic and carbonate rocks of the Late Pennsylvanian (Virgilian, Holder Formation) of south‐central New Mexico were deposited on a narrow shelf between the Pedernal uplift and the Orogrande basin. In the Dry Canyon area, Sacramento Mountains, shelf‐to‐basin palaeotopography is indicated by onlapping beds and primary dips. Calcareous alteration zones are developed on the tops of major carbonate units. These detailed studies confirm earlier interpretations that alteration zones represent subaerial exposure events and provide new evidence for and further documentation of a paleosol environment. The paleosols contain: (1) rhizoliths; (2) tangential needle fibres of calcite; (3) five types of alveolar texture; (4) upward oriented ‘ribbon spar’; (5) brownish and clear isopachous spar; (6) blackened grains; (7) random needle fibres of calcite; (8) coated grains; (9) micritized grains; (10) glaebules; (11) desiccation cracks; (12) possibleMicrocodium; and (13) well‐developed laminated crusts. Isopachous cements, upward‐oriented cements (which mark the former tops of capillary fringes), dark colour and shallow depth of penetration of rhizoliths (less than 1 m) indicate significant paleosol formation in waterlogged (hydromorphic) and drier conditions. Pennsylvanian palaeohydrogeology of the system is reflected in the type of paleosol alteration.Paleosols developed on subtidal strata and paleosols which drape tens of metres of palaeotopography indicate cyclic sedimentation was not controlled by delta lobe switching. Local tectonic up and down movement would be an unlikely mechanism to explain the repeated tens of metres of relative sea‐level fluctuation. Eustatic sea‐level fluctuations, induced by Late Pennsylvanian Gondwana glaciation, probably were of the correct magnitude and frequency to have caused

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1988.tb01251.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTAnalyses of high resolution, seismic reflection profiles and surface sediment samples indicate that the Cat Island shelf is presently in an incipiently drowned state. This small carbonate bank is characterized by a thin (<4 m), coarse‐grained, relict sediment cover, along with limited reef development, and a relatively deep (20–30 m) margin indicating that it has been unable to ‘keep‐up’ with Holocene sea‐level rise.Early flooding at relatively high rates of sea‐level rise (4 m kyr‐1, 5–8 × 103yr BP) in conjunction with small bank size and relatively low elevation, led to a reduced rate of carbonate accumulation and incipient drowning. The shelf edge currently lies beneath the zone of maximum carbonate production and exposes the interior shelf to open marine conditions which may result in permanent drowning if it is unable to ‘catch‐up’ with continued sea‐level rise. Sediment facies patterns are largely oriented perpendicular or oblique to the shelf edge and appear to be controlled by shelf circulation patterns focused by bank‐margin reentrants.In comparison with most of the northern Bahamas, the Cat Island shelf was flooded earlier and at relatively higher rates of Holocene sea‐level rise which led to selective drowning, implying that carbonate platforms need not drown synchronously over widespread areas as commonly thought. The potential rock record of this incipient drowning event would be a thin, open‐marine sand sheet of highly degraded cryptocrystalline and aggregate grains associ

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1988.tb01252.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTThe most ubiquitous syn‐sedimentary cements affecting Mururoa atoll are composed of magnesian calcite. Three main types are distinguished: fibrous, bladed and sparitic on the basis of petrography, morphology and MgCO3concentration of the constituting crystals, while peloid infills, a particular form of HMC chemical precipitation, also exist. Petrographic evidence and isotopic signatures are compatible with marine precipitation.Mururoa atoll was exposed several times to meteoric diagenesis resulting in varied diagenetic alterations including selective dissolution and partial dolomitization of Mg‐calcite cements. These alterations are responsible for substantial modifications of the initial cement fabrics and may introduce unconformities in the diagenetic chronology. The first stage of the partial dissolution of Mg‐calcite induces the development of chalky, white friable zones within the initially crystalline, hard cement layers. At ultrascale, this is due to the creation of micro‐voids along the elongate cement fibres. Advanced dissolution includes total disappearance of cement portions as attested to by large voids within the cement crust and/or between superposed cement layers. Mg‐calcite dissolution is related to meteoric diagenesis during periods of Quaternary exposure. The creation of voids within Mg‐calcite layers is due to the mechanical removal of previously altered calcium carbonate, a process suggesting marine or non‐marine water flow, probably in the vadose environment.Selective dolomitization of Mururoa cements involves alternations of calcite and dolomite which form successive cement‐like rinds within primary cavities. At Mururoa, these alternations are the result of selective dolomitization of the pre‐existing Mg‐calcite cements rather than successive precipitation of calcite and dolomite. Selective dolomitization of Mg‐calcite cements at Mururoa indicates that a given cement succession is not necessarily a simple chronological sequence. Oxygen isotope values of dolomites are enriched in δ186 by about 3‰ PDB within calcite‐dolomite pseudo‐alternations. The dolomitizing fluid at Mururoa seems similar to present marine water although some mixture with meteoric water is probable to favour dissolution as

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1988.tb01253.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTA bored and encrusted late Pleistocene ooid grainstone was recovered from the seafloor at a depth of approximately 40 m on the outer continental shelf of eastern Florida. Ooid cortices are dominantly bimineralic, generally consisting of inner layers of radial magnesian calcite and outer layers of tangential aragonite. Ooid nuclei are dominantly rounded cryptocrystalline grains, although quartz grains and a variety of skeletal grains also occur as nuclei. Ooids are partially cemented by blocky calcite, and interparticle porosity is partially filled by micrite.Radial cortex layers are composed of brightly cathodoluminescent magnesian calcite having a composition of approximately 12 mol% MgCO3and 1000 ppm strontium. The iron and manganese concentrations in radial cortex layers are generally in the range of 500–1000 ppm and 100–250 ppm, respectively. Tangential cortex layers are composed of noncathodoluminescent aragonite containing approximately 11 500 ppm strontium and less than 0.5 mol% MgCO3. Iron concentrations in tangential cortex layers are generally in the range of 150–400 ppm, and manganese concentrations are generally below the detection limit of 100 ppm. Echinoderm skeletal fragments, which are present as accessory grains, are composed of brightly cathodoluminescent magnesian calcite. Some ooid nuclei and the thin outer edges of some blocky calcite cement are cathodoluminescent; micrite matrix and the bulk of blocky calcite cement are noncathodoluminescent. Ooids do not exhibit textural evidence of recrystallization.The ooid grainstone underwent an episode of meteoric diagenesis. but ooid cortices were not affected by the event. We propose a previously unrecognized process by which the magnesian calcite cortex layers underwent diagenetic alteration in oxygen‐depleted seawater. During this diagenesis, magnesium was lost and manganese was incorporated without apparent textural alteration and without mineralogical stabilization. Thus, we Suggest that cathodoluminescence may result from diagenetic alteration on the se

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1988.tb01254.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

Book review in this articleCross‐bedding, Bedforms and Paleocurrents, by D. M. RubinBeach and Nearshore Sediments and Processes, ed. By R. A. Davis Jr.,Sedimentary Processes on the Amazon Continental Shelf, ed. by C. A. Nittrouer and D. J. DeMasterFjords: Processes and Products, by J. P. M. Syvitski, D. C. Burrell, and J. M. SkeiAlluvial Soils, ed. by J. Gerrard,Electron Micrographs (TEM, SEM) of Clays and Clay Minerals, by K.‐H. Henning and M. StörrThe Origins of Angiosperms and their Biological consequences, ed. by E. M. Friis, W. G. Chaloner&P. R. CraneThe Motion of Allochthonous Terranes Across the North Pacific Basin, by M. G. Debiche, A. Cox and D. EngebretsonGeomorphic Systems of North America, ed. by W. L. GrafApproaches to Interpretation of Sedimentary Environments, ed. by D. J. Cant and F. J. HeinCarbonate Depositional Environments: Modern and Ancient. Part I. Reefs.European Dinantian Environments, ed. by J. Miller, A.E. Adams, and V.P. WrightMesozoic and Cenozoic Oceans, ed. by K. J. HsüMarine Minerals: Advances in Research and Resource Assessment, ed. by P.G. Teleki, M.R. Dobson, J.R. Moore and U. von Stackelberg.Sedimentation and Mineral Deposits in the Southwestern Pacific Ocean, ed. by D.S. CronanA Practical Approach to Sedimentology, by Ray LindholmClastic Particles, Scanning Electron Microscopy and Shape Analysis of Sedimentary and Volcanic Claste, ed. by John R. Ma

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1988.tb01255.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1988.tb01256.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY