摘要:

ABSTRACTA study of the Middle Jurassic Rannoch Formation in the Gullfaks Field shows that there is no relationship between the content and distribution of kaolinite and location relative to the late Cimmerian unconformity. From petrographic data most of the kaolinite is interpreted to be detrital, and only traces of authigenic kaolinite are observed. Mass‐balance calculations are used to provide guidelines as to the likelihood of kaolinite being formed and preserved in sandstones under the unconformity. The result of the calculations shows that the propagation rate of the dissolution front in the sandstones was probably on average slower than the erosion rates during the formation of the late Cimmerian unconformity. Hence, transformation of significant amounts of feldspar and mica to kaolinite probably did not take place within the sandstones which at present underlie the unconformity in the Gullfaks Field. Periods of exposure during formation of unconformities may thus be less effective causes of kaolinitization in sandstones than has been assume

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb00143.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

ABSTRACTMultiple generations (I‐IV) of radiaxial fibrous calcite (RFC) cement crusts are intercalated with meteoric internal sediments and microsparitic calcite cements within karst dissolution cavities in Upper Triassic Steinplatte and basal Liassic Hirlatz carbonates in the Northern Calcareous Alps of Austria. Stratigraphic, petrographic and geochemical evidence suggest that RFC precipitated as high‐Mg calcite from marine fluids in the shallow‐burial (<180m) environment. Estimated original δ18O and δ13C isotopic compositions of Late Triassic and Early Jurassic marine waters are −1.88 to −2.21%‰ and +2.37 to +2.89%‰ (PDB), respectively. The RFC subsequently was altered to low‐Mg calcite by meteoric or mixed meteoric‐marine fluids introduced during periods of subaerial exposure.RFC cement generations I‐IV in Steinplatte reef‐slope beds, and I‐II in Steinplatte reef and lagoon and Hirlatz samples retained near‐original marine oxygen and carbon isotopic compositions (‐1.88 and +2.89%‰ PDB, −2.11 and +2.37%‰ PDB, respectively) and petrographic identity during the conversion to low‐Mg calcite. In contrast, RFC generations III and IV in the Hirlatz and upper Steinplatte were intensely leached and partly replaced by coarse, blocky calcites of meteoric origin, with attendant modification of original crystal fabrics and isotopic compositions (−4.65 to −5.22%‰ PDB δ18O, +2.54 to +3.14%‰ PDB δ13C). These differences in diagenetic behaviour are related to contrasting processes of fluid‐rock systems and original microfabric variations in the RFC cements. The karst‐affected, phreatic‐marine burial environment of RFC formation and subsequent diagenetic alteration inferred for these rocks have

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb00144.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

ABSTRACTIn the Lower Saxonian Basin, cores from three drill holes provide a cross‐section from the basin centre towards the margin through the upper Barremian/lowermost Aptian pyritic claystones and marls. Microscopic analyses distinguish six types of pyrite (I: ‘cones and tubes’, II: ‘tubes’, III: ‘isometric crystal aggregates’, IV: ‘pseudomorphs of fossils’, V: ‘star‐like concretions’, and VI: ‘filiform pyrite‐marcasite intergrowths’) which are associated with subordinate amounts of Fe carbonate, chalcopyrite, sphalerite and wurtzite. The crystal morphology of these sulphides shows a clear‐cut diagenetic sequence from pyrite crystals dominated by the octahedron to those dominated by the cube.Among these early to late diagenetic pyrites, the conversion of crystal habits is accompanied by a striking variation in trace element contents, some of which have extremely low values (e.g. Au, Se, Te, Tl, Co). In this context, classification of crystal habits and analyses of As and Ni establishes a sequence of pyritization which may help define a basin zonation for these argillaceous sedimentary rocks, categorized as ‘normal facies’—oxygenated bottom waters—interrupted by short episodes of bottom water oxygen depletion (‘bituminous facies’). Substitution of the cube for the octahedron in the Fe bisulphide aggregates led to a continuous removal of As and Ni from the pyrite, attesting to a steady decrease of those elements in the pore fluids with time. By contrast, within each type of pyrite, As and Ni contents increase from the margin towards the centre of the basin, owing to pre‐concentration of both elements in the basinal sediments, which are more abundant in organic matter. Some other elements, however, such as Mn, Cu, Sb and Ag, do not follow this trend and are unrelated to the lattice transformation of the Fe bisulphides. Their sometimes anomalously high quantities in Fe bisulphides are controlled by the host rock chemistry (presence of volcaniclastic material) and accessory minerals intergrown with pyrite (e.g. chalcopyrite).Pyrite I through III formed in tubular hollows of burrowing organisms, and type IV in tests of fossils. Types V and VI did not result from replacement or void filling but were generated by diffusion of sulphate along inhomogeneties, such as bedding planes

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb00145.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

ABSTRACTSiliceous nodules in the upper Eocene to middle Miocene Cypress Hills Formation in southern Saskatchewan formed on the distal portions of an extensive braidplain. The nodules are similar to silcretes described elsewhere in the literature but their morphology and occurrence suggests that they are of a rare variety. The silcretes are discrete equant to disc‐shaped nodules, 1–3 cm thick, and up to 15 cm long. The silicretes form horizontal, discontinous layers parallel to bedding within an unweathered profile. Void spaces and fractures within the nodules are lined or filled with drusy quartz. The silcretes contain less than 0.07% TiO2. The TiO2/SiO2/Al2O3and TiO2/SiO2/Fe2O3ratios are similar to values obtained from nodules formed in an arid to semi‐arid environment based on comparison with modern silcretes. This interpretation is confirmed by independent sedimentological and palaeontological climatic evidence for an arid to semi‐arid climate throughout Oligocene time in the western Canadian

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb00146.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

ABSTRACTCarbonate buildups in the Flinders Ranges of mid‐Early Cambrian age grew during a period of high archaeocyath diversity and are of two types: (1) low‐energy, archaeocyath‐sponge‐spicule mud mounds, and (2) high‐energy, archaeocyath‐calcimicrobe (calcified microbial microfossil) bioherms. Mud mounds are composed of red carbonate mudstone and sparse to abundant archaeocyath floatstone, have a fenestral fabric, display distinct stromatactis, contain abundant sponge spicules and form structures up to 150m wide and 80 m thick. Bioherms are either red or dark grey limestone and occur as isolated small structures 2–20 m in size surrounded by cross‐bedded calcarenites and calcirudites or as complexes of mounds and carbonate sands several hundreds of metres across. Red bioherms comprise masses of whiteEpiphytonwith scattered archaeocyaths and intervening areas of archaeocyath‐rich lime mudstone. Grey bioherms are complex intergrowths of archaeocyaths, encrusting dark greyRenalcisand thick rinds of fibrous calcite cement. The bioherms were prone to synsedimentary fracturing and exhibit large irregular cavities, up to 1.5 m across, lined with fibrous calcite.The buildups are isolated or in contiguous vertical succession. Mud mounds occur alone in low‐energy, frequently nodular, limestone facies. Individual bioherms and bioherm complexes occur in high‐energy on‐shelf and shelf‐margin facies. The two types also form large‐scale, shallowing‐upward sequences composed of basal (deep water) mud mounds grading upward into archaeocyath‐calcimicrobe bioherm complexes and bioherms in cross‐bedded carbonate sands. The uppermost sequence is capped by ooid grainstone and/ or fenestral to stromatolitic mudstone.The calcimicrobe and metazoan associations form the two major biotic elements which were to dominate reefs throughout mu

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb00147.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

ABSTRACTExamination of sediments along the north shore of Lake Erie at Mohawk Bay reveals a relationship between the formation of intensely brecciated diamictons and the presence of sand‐block intraclasts. It is postulated that the sand blocks were subglacially deposited within a meltwater environment, and later frozen prior to being eroded and transported within a mobile subglacial debris layer. On immobilization the frozen sand blocks, encased within the diamicton, acted as a heat sink creating cryostatic stresses within the surrounding diamicton as a result of the advance of a frost front and related frozen ‘fringe’. The effect of these anisotropic stresses resulted in porewater migration to the frost front. Subsequent development of intense brecciation occurred as aureoles around the sand intraclasts due to localized high tensile stresses causing fracturing within the fine‐grained matrix of the di

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb00148.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

ABSTRACTThe Outardes Estuary (Quebec) is a regressive sandy system developed in a sub‐boreal climatic setting. The Outardes River drains an area of 18 780 km2and was dammed for hydroelectric purposes in the 1960s and‘70s. By reconstructing the hydrographs and mapping the morphology of the estuary from aerial photographs over a period of 21 years, we have documented the changes in the estuary resulting from the modification of the hydrologic regime.Until the mid‐1960s, a catastrophic spring freshet (1800–2800 m33s‐1) controlled the morphology of the estuary. The mouth was dominated by a disorganized, braided channel pattern, and large fluvial discharge‐generated transverse bars. Peripheral channel areas in the upper estuary were zones of transport at flood stages. For 18 months in 1968 and‘69 the river discharge was drastically reduced (50 m3s‐1) to accomplish the main reservoir filling. Tidal currents and waves filled and remodelled the morphology of the mouth of the estuary, while the peripheral zones in the upper estuary experienced low‐energy sedimentation. Since the generating plants were brought on‐line in 1969, the fluvial discharge has been unable to modify substantially the sand body geometry in the mouth of the estuary. In the upper estuary the peripheral regions continue to be filled in, but the thalweg is being eroded.Based on the results of our coring programme, we suggest that major environmental changes may leave a sedimentary signature (addition or subtraction of size modes) which can be identified by the detailed analysis of grain

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb00149.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

ABSTRACTMeasurements are described of the geometry of ripples formed on beds of sand exposed to a steady current at right angles to an oscillatory flow. Four different sands were studied. The oscillation was produced by an oscillating tray set into the bed of a steady‐flow flume.It was observed that straight‐crested ripples formed by oscillatory flow would usually develop a ‘serpentine’ form when the superimposed steady current exceeded a certain limit. For amplitudes of the tray velocity U∞less than about 0.38 m s‐1this limit corresponded toU∞/Ū*c>31, whereŪ*cis the shear velocity measured just upstream of the oscillating tray. It is suggested that the serpentine form is caused by the interaction of vortices carried back and forth between adjacent ripples. On this assumption, the wavelength of the serpentine form would be proportional to the product of period of oscillation and near‐bed steady current velocity. The present measurements appear to support this hypothesis although there is also evidence that the wavelength is influenced by preferred spacing patterns between vortices. The measurements also show the ratio of the amplitude of the serpentine form to its wavelength to be approximately constant.Empirical relationships are derived relating ripple geometry to flow and sediment properties. It is observed that the influence of Reynolds number and sediment properties on the geometry is very weak. It is suggested that this is typical of ripples formed with relatively low sediment transport rates. It is also found that, under the present experimental conditions, the ripple spacing in the direction of oscillation is almost independent of the magnitude of the steady current and in close agreement with the wavelengths previously measured in an oscillating water tunnel. This suggests that the additional inertia effects associated with oscillating tray rigs were not sufficient to affect bed geometry under the presen

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb00150.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

ABSTRACTTwo forms of horizontal laminae have been produced under upper‐flow‐regime conditions in a flume by deposition on a plane bed and on low in‐phase waves. Upper plane bed laminae consist of approximately equal numbers of fining‐upward (FU) and coarsening‐upward (CU) textural laminae; heavy minerals, when present, are typically within or form CU laminae. In horizontal laminae formed under low in‐phase waves, FU laminae may predominate and are significantly thicker than CU laminae; heavy minerals are associated with the tops of some FU laminae and are absent from CU laminae. Deposits aggraded on plane beds and low in‐phase waves both display low‐angle (13–14°) upstream grain imbrication. Upper plane‐bed horizontal laminae are attributed to the effects of the burst/sweep cycle on local bedload transport (bursts form FU laminae and sweeps form CU laminae). Some FU laminae in in‐phase wave deposits are thought to form with downstream migration of bed waves on which sand size decreases from trough to crest. Heavy minerals are concentrated in the tops of these FU laminae due to their enrichment on the crests of the migrating bed waves. Other FU and CU laminae in in‐phase wave deposits probably formed by the action of bursts and sweeps, as on Upper plane beds.A sequence of bed phases and stratification formed under upper‐flow‐regime conditions is described based on the observations here combined with descriptions from the literature. With increasing flow strength the gradual sequence of bed phases is: (1) plane bed, (2) low relief, downstream‐migrating inphase waves increasing in amplitude and wavelength with increasing flow strength, (3) stationary in‐phase waves, (4) upstream‐migrating (including breaking wave) antidunes. The sequence of stratification formed under these bed phases is: (1) upper plane bed horizontal lamination, (2) in‐phase wave horizontal lamination to in‐phase wave foreset cross‐laminae, (3) in‐phase wave drape l

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb00151.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

ABSTRACTSmall‐scale hummocky cross‐stratification occurs in Upper Cretaceous calciclastic turbidites exposed in the western Basque Pyrenees; facies associations and microfossil assemblages indicate slope to base‐of‐slope (bathyal) depositional environments. It is developed in the fine‐grained portion of beds and displays spacings mostly between 0.2 and 0.7 m. The beds fine upward with no sharp grain size breaks or mud partings, suggesting that deposition occurred during a single flow event.Hummocky intervals are 0.1–0.8 m thick and consistently grade laterally and vertically into flat, planar laminations of Bouma B divisions suggesting that deposition occurred under upper‐flow‐regime conditions. They have wave‐like geometries with laminae continuous across ‘crests’ and ‘troughs’ and display a ratio of ‘wavelength’ to estimated underflow thickness of 11.3–12.8.Combining the above observations and inferences, these examples of small‐scale hummocky cross‐stratification are interpreted as a form of antidune stratification generated by standing waves along the interface of a thinner, denser underflow (main body/tail of the turbidity current) and an overlying thicker, low‐density layer. This occurrence is further evidence that small‐scale hummocky cross‐stratification is multigenetic and therefore not indicative of a particular f

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb00152.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY