ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1988.tb00900.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.tb00901.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTBedforms of the Surtainville area, off the Cherbourg peninsula, include subtidal sandwaves, which are a good example of mega‐structures associated with the strong tidal currents prevailing in the English Channel. A fine‐scale study using a high‐accuracy echo‐sounder and side‐scan sonar shows that some of these sandwaves have a crescentic shape and a strong asymmetry indicating a sand movement toward the north. The sandwaves range in height from about 3.5 m to 7.5 m, in width from 100 m to 500 m and in length from 70 m to 200 m; their internal structure, revealed by the simultaneous use of a high‐resolution seismic source, is characterized by large ‘foreset’ beds dipping in the same direction as the lee sides of the sandwaves. Groups of foresets are limited by reactivation surfaces which we interpret as erosional surfaces created by subordinate tides. The presence of horizontal erosional reflectors inside the sandwaves and the truncation of the present‐day profiles may reflect the effects of storms. The asymmetry of the tide in the area studied, shown by long‐term current measurements, indicates that these sandwaves belong to classes III or IV of Allen's (1980a) classification; the observed structures correspond very well to the prediction of Allen's conceptual model, but we suggest that long term phenomena like equinox cyclicity, associated with storms, may be responsible for their origin rather than the neap‐spring‐neap tidal cycles responsible for the internal structure

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1988.tb00902.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTStokes surfaces in aeolian deposits are caused by wind scour of unconsolidated material to a roughly planar horizon controlled by near‐surface water‐tables (Stokes, 1968). A water‐table forms a downward limit of scour through the cohesion of damp or wet sand near water‐table, and through early cementation by evaporites precipitated in the sediments as water evaporates near the sand‐air interface. Study of modern analogues reveals that Stokes surfaces exist in a variety of depositional settings, including a coastal offshore prograding sand sea (Jafurah, Saudi Arabia); a coastal onshore prograding sand sea (Guerrero Negro, Mexico) and a continental sand sea (White Sands, New Mexico, USA). These modern analogues indicate that our concept of Stokes surfaces must be broadened to include the following: (i) modern analogues for Stokes surfaces described here cover areas on the order of 25 km2. These may be as representative of similar surfaces in ancient rocks as hypothesized plains of deflation requiring removal of entire sand seas; (ii) Stokes surfaces occupy a continuum in scale from local to extensive, and erosional surfaces of different magnitude may be stacked closely in the sediments; (iii) Stokes surfaces, although erosional in nature, are commonly associated with deposits both above and below the Stokes bounding surface which plainly reveal the influence of a near‐surface groundwater control on wind sedimentation. Moreover, the erosional relief of the bounding surface itself (as well as other features) reveals the influence of a groundwater‐table; (iv) Stokes surfaces may be diachronous, representing the lateral shift of a zone of scour within a sand sea rather than simultaneous removal of all dunes from the area encompassed by the erosional surface; (v) Stokes surfaces and associated deposits are often laterally transitional to surfaces and deposits of adjacent depositional environments, including interdunes, tidal flats, lagoons, beaches, lakes and non‐aeolian sabkhas. Finally, modern examples from different depositional settings suggest that while most Stokes surfaces have many features in common (such as erosional ridges due to early cementation), there are some features which may, with further study, be revealed to be distinctive of an individual depos

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1988.tb00903.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTThe Sumeini Group formed along the passive continental margin slope that bounded the northeastern edge of the Arabian carbonate platform. With the initial development of this passive continental margin in Oman during Early to Middle Triassic time (possibly Permian), small carbonate submarine fans of the C Member of the Maqam Formation developed along a distally steepened slope.The fan deposits occur as several discrete lenticular sequences of genetically related beds of coarsegrained redeposited carbonate (calciclastic) sediment within a thick interval of basinal lime mudstone and shale. Repeated pulses of calciclastic sediment were derived from ooid shoals on an adjacent carbonate platform and contain coarser intraclasts eroded from the surrounding slope deposits. Sediment gravity flows, primarily turbidites with lesser debris flows and grain flows, transported the coarse sediments to the relatively deep submarine fans. Channel erosion was a major source of intraformational calcirudite.Two small submarine fan systems were each recurrently supplied with calciclastic sediment derived from point sources, submarine canyons. The northern fan system retrogrades and dies out upsection. The southern fan system was apparently longer‐lived; calciclastic sediments in it are more prevalent and occur throughout the section. The proximal portions of this fan system are dominated by channelized beds of calcirudite which represent inner‐ to mid‐fan channel complexes. The distal portions include mostly lenticular, unchannelized beds of calcarenite, apparently mid‐ to outer‐fan lobes.Carbonate submarine fans appear to be rare in the geological record in comparison with more laterally continuous slope aprons of coarse redeposited sediment. The carbonate submarine fans of the C Member apparently formed by the funnelling of coarse calciclastic sediment into small submarine canyons which may have developed due to rift and/or transform tectonics. The alternation of discrete sequences of calciclastic sediment with thick intervals of ‘background’ sediment resulted from either sea‐level fluctuations or pulses of te

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1988.tb00904.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTSome new aspects of the development of density surges in channels are developed from recent experimental results. A density surge is characterized by the flow of a finite amount of dense fluid. This dense fluid may be a saline‐water solution, a saline‐water solution with tracers (sand in suspension) or a pure suspension (sand in suspension in fresh water).The experimental results show in particular that: (1) the velocity of the surge is effectively proportional to the square root of the initial volume, (2) the velocity of the surge increases with the increase of the initial density of the heavy fluid, proportionally to the square root of the ratio of density difference to the ambient fluid density, (3) the speed of the surge increases again with an increase of the slope.For pure suspensions the experiments show that velocities slow down much more quickly than for saline solutions with the same initial density, due to loss of negative buoyancy.The effects of the initial density and of the size of the grains on the distribution of sediments are descri

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1988.tb00905.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTCretaceous to Palaeogene sedimentary series in two long, continously recovered boreholes from the Senegal coastal basin and adjacent Cape Verde deep‐sea basin have been compared by mineralogical, geochemical, electron‐microscope and microprobe investigations. The transition between the diagenetic influences related to the depth of burial and the palaeoenvironmental influences, identified in the thick coastal‐basin successions, is complicated by the tectonic instability of the margin during periods of high subsidence rates and crustal thinning. As the deep‐sea basin sediments are only slightly affected by diagenetic changes, the comparison between the deep sea and coastal series allows the relative effects of lithostatic pressure, tectonics and other palaeoenvironmental factors to be evaluated. The disappearance of synsedimentary signatures (tectonics, climate, depositional conditions, etc.) expressed by clay mineralogy occurs below 4.5 km; there are no significant diagenetic changes in sediments at less than 2 km depth. By comparing identical time slices in coastal and deep‐sea basins, it is possible to recognize from the clay stratigraphic record the main African detrital sources, the hot and often semi‐arid character of the continental climate during the Cretaceous, the existence of minor but extensive stages of tectonic activity (Barremian‐late Aptian, late Albian‐early Cenomanian), the temporary existence of semi‐closed basins on the shelf (Late Cretaceous, early Palaeogene) and the complexity of transport and sedimentation relationships between proximal and di

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1988.tb00906.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTGravity cores of Holocene sediments from a shallow ephemeral lake in the Coorong region (Pellet Lake, southeastern coastal Australia) show a mineral assemblage and sequence particular to its hydrology. The mineralogical sequence above an initial dolomitic siliciclastic sand reflects conditions of increasing salinity in the lower portions of the core (i.e. organic‐rich aragonite to magnesite + hydromagnesite + aragonite) followed by a relative decrease in salinity (i.e. magnesite + aragonite + hydromagnesite to aragonite + hydromagnesite) in the upper portions of the core. This sequence is capped by ˜ 0.4 m of micritic dolomite and minor amounts of hydromagnesite, with the relative abundance of dolomite increasing upwards. Three stratigraphically and spatially distinct dolomite units (upper, lower and margin) are recognized using stable carbon and oxygen isotope data, unit cell calculations and MgCO3mole per cent data of the dolomite.Detailed X‐ray diffraction (XRD) analyses of samples with more than 80% dolomite shows that the dolomite is ordered. Average unit cell parameters, calculated from the XRD patterns, indicate that the upper dolomite unit has crystal lattices expanded in thecodirection (co= 16.09 Å) relative to ideal dolomite (co= 16.02 Å) and contracted in theaodirection (ao= 4.796 Å) relative to ideal dolomite (ao= 4.812 Å). The mol fraction of MgCO3in the upper dolomite shows up to 4.0 ±M 2.0 mole per cent excess Mg in the dolomite crystal lattice (calculated from XRD). This unusual dolomite crystal chemistry is probably generated by rapid precipitation from solutions which have greatly elevated Mg/Ca ratios. Transmission electron microscopy reveals that the upper dolomite has a heterogeneous microstructure which also suggests rapid precipitation from solution. The modulated microstructure found in calcium‐rich dolomite is completely lacking. Dolomite ordering reflections are present in electron diffraction patterns, but are weak.Stable oxygen and carbon isotope values of the upper dolomite are tightly grouped (ave. δ18O ∼+ 7.55%o, δ13C ∼+ 4.10%o), yet show three upward‐lightening oxygen cycles. The oxygen cycles correlate with three upward decreases in the calculated Mg content of the dolomite zone. These cycles may indicate the increased importance of rain‐water dilution of the brine at times when the water in the lake was at its shallowest levels.Analyses of the lower dolomite and the margin dolomite suggest that these units precipitated more slowly from less evaporitic brines than the upper dolomite unit. The lower dolomite is close to stoichiometric, has less evaporitic stable isotope values than the upper dolomite, and has only a slightly expandedco‐axis. The margin dolomite is Ca‐rich, has a more homogeneous microstructure, and has expandedaoandcoaxes.The abundance of relatively soluble Mg‐bearing phases, such as hydromagnesite and magnesite, may supply additional magnesium for the dolomitization of aragonite and calcite during subsequent diagenesis and burial of the sediment. This process may leave a finely laminated dolomicrite deposit which retains little, if any, evi

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1988.tb00907.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTMaastrichtian strata from the Pachino area (SE Sicily) provide a model of association between rudist‐coral frameworks and submarine volcanic activity.Two successive carbonate units are distinguished: (a) Coral‐rudist bioherms and biostromes developed in a high‐energy environment, (b) Hippuritid build‐ups and banks overlain by rudist‐coral clusters which grew under harsher ecological conditions, in a weaker current regime.At the top of the Maastrichtian sequence, the growth of rudist‐coral frameworks ceased as ecological conditions shifted toward restricted environments. The inferred succession reflects a gradual decrease in current strengths and water depths, related to the Maastrichtian regressive phase.The frameworks display consistent evolutionary sequences reflecting progressive changes in their biota, structure, size and shape.Occurrences of rudist‐coral frameworks are clearly linked with submarine volcanoes which provided opportunities for their development; they sometimes grew in the vicinity of active‐vent centres, a feature reported for the first time in the U

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1988.tb00908.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTAn elongate Waulsortian mud‐mound complex developed at Dovedale on a ramp to the southwest of a developing carbonate platform in Derbyshire during Chadian (early Viséan) times. The complex occupied an area of approximately 6 km2and grew to a maximum relief of 80 m with longitudinal and transverse valleys developed near the southern margin.Five mound associated facies have been identified: mound core, mound flank (fine), mound flank (coarse), intermound (fine) and intermound (coarse). The mound core facies is a massive skeletal wackestone with comminuted sponge debris, foraminifera, ostracodes and crinoid debris set in a matrix of clotted micrite. The mound flank sediments display moderately inclined bedding surfaces. While the mound flank (fine) contains sponge debris, the mound flank (coarse) is dominated by articulated crinoid columnals, and includes algal‐encrusted micritized intraclasts and coarse peloids. The well‐bedded intermound (fine) facies is bituminous and micritic while the intermound (coarse) facies is composed of skeletal‐peloidal‐intraclast grainstones which locally contain calcified algae.Although the fauna is diverse, the density of colonization by metazoans was low and the supply of macrofossil debris modest. The clotted micrite texture is interpreted as the product of micro‐organisms which precipitated and trapped fine‐grained sediment. The mud‐mound complex is dominated by the bathymetric assemblages B and C proposed by Lees, Hallet&Hibo which on their model of the Belgian Waulsortian, indicate depths of between 220 and 280 m. Intercalation of assemblages B/C and C/D on the northern margin of the complex is interpreted as the result of local storm disturbance. A deep water drift is postulated to explain the NW‐SE alignment of the complex which probably fitted the ‘export model’ of Bosence, Rowlands&Quine. Beneath the sediment surface, phreatic flow eroded unlithified sediments and developed interconnected cavities which were filled by cement and sediment relatively eariy. Mound instability triggered the opening of fissures which filled with crinoid debris, peloids, indurated l

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1988.tb00909.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY