bluuebunny/crossref_metadata_2025_split huggingface dataset:
The role of the late Cimmerian unconformity for the distribution of kaolinite in the Gullfaks Field, northern North Sea
Authors: BJØRKUM PER ARNE , MJØS RUNE , WALDERHAUG OLAV , HURST ANDREW 
Published: June 2006

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 assumed.
Diagenesis of radiaxial fibrous calcites in a subunconformity, shallow‐burial setting: Upper Triassic and Liassic, Northern Calcareous Alps, Austria
Authors: MAZZULLO S. J. , BISCHOFF W. D. , LOBITZER H. 
Published: June 2006

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 not been previously documented in other ancient rocks.
Recent evolution of the Outardes Estuary, Quebec, Canada: consequences of dam construction on the river
Authors: HART B. S. , LONG B. F. 
Published: June 2006

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 km2 and 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 size distributions.
Ripple formation in combined transdirectional steady and oscillatory flow
Authors: YOUNG J. S. LEE , SLEATH J. F. A. 
Published: June 2006

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‐1 this limit corresponded to U∞/Ū*c>31, where Ū*c is 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 present test conditions.
Dynamics and facies model of a macrotidal sand‐bar complex, Cobequid Bay—Salmon River Estuary (Bay of Fundy)
Authors: DALRYMPLE ROBERT W. , KNIGHT R. JOHN , ZAITLIN BRIAN A. , MIDDLETON GERARD V. 
Published: June 2006

ABSTRACTThe 40‐km‐long, Cobequid Bay—Salmon River estuary has a maximum tidal range of 16·3 m and experiences limited wave action. Sediment, which is derived primarily from areas seaward of the estuary, is accumulating faster than the high‐tide elevation is rising, and the system is progradational. The deposits consist of an axial belt of sands, which is flanked by mudflats and salt marshes in the inner half of the estuary where a funnel‐shaped geometry is developed, and by erosional or non‐depositional foreshores in the outer half where the system is confined by the valley walls. The axial sands are divisible into three facies zones: zone 1—elongate, tidal sand bars at the seaward end; zone 2—sand flats with a braided channel pattern; zone 3—the inner, single‐channel, tidal—fluvial transition. Tidal current speeds reach a maximum in zone 2, but grain sizes decrease headward (from medium and coarse sand in zone 1, to fine and very fine sand in zones 2 and 3) because the headward termination of the major flood channels prevents the coarse, traction population from entering the inner part of the estuary.Longitudinal progradation will produce a 20‐m‐thick, upward‐fining succession, the lower 1/2–2/3 of which will consist of cross‐bedded, medium to coarse sand deposited on the zone 1 sand bars. The ebb‐dominated portion of this unit will be finer grained than the flood‐dominated part, and will contain trough crossbedding produced by 3‐D megaripples; the flood‐dominated areas, by contrast, will consist mainly of compound cross‐bedding created by sandwaves with superimposed megaripples. Headward migration of swatchways (oblique channels that link the ebb‐ and flood‐dominated areas) will create packages of ebb cross‐bedding that is orientated at a high angle to the long axis of the estuary and that contains headwardinclined, lateral‐accretion surfaces. The overlying fine and very fine sands of zones 2 and 3 will be composed mainly of upper‐flow‐regime parallel lamination. The succession will be capped by a 4‐m‐thick unit of mixed flat, mudflat and salt marsh sediments. A review of other macrotidal estuaries with tidal ranges greater than 10 m suggests that the major elements of the model have general applicability.
Carbonate mineral cycles generated by foraminiferal and pteropod response to Pleistocene climate: west Florida ramp slope
Authors: GARDULSKI ANNE F. , MULLINS HENRY T. , WEITERMAN SANDRA 
Published: June 2006

ABSTRACTCarbonate ramps are gently sloping depositional surfaces where shallow‐water, coarse‐grained facies pass basinward into fine‐grained, deep‐water sediments, with no abrupt change in slope. The objectives of this study are: (i) to integrate the depositional processes recorded in the Pleistocene stratigraphy of the west Florida outer ramp into a palaeoclimatic and palaeoceanographic framework for the eastern Gulf of Mexico; and (ii) to examine the origin of mineralogical and sedimentary cycles in the light of pteropod and planktonic foraminiferal populations corresponding to climatic oscillations.Aragonitic, pteropod‐rich sediments with large amounts of insoluble residue occur in sediments deposited during glacial intervals; sandy calcitic sediments with abundant planktonic foraminifera accumulate during interglacials. These cycles reflect variations in biological productivity of pelagic pteropods and planktonic foraminifera, rather than preferential dissolution of either aragonitic or calcitic fractions. Species assemblages suggest that the productivity cycles are linked to changes in upwelling intensity at the margins of the Loop Current and variations in water mass salinities, as well as terrigenous dilution from the Mississippi Delta. These cycles are the response to Pleistocene glacial‐interglacial oscillations, controlled by Milankovitch orbital parameters.Although the organisms contributing to deep‐water carbonate environments have changed through geological time, facies patterns, as well as sedimentary textures and structures, identified in the west Florida sediments provide criteria for recognition of ancient ramps. An understanding of the processes on a modern ramp slope, such as west Florida, may prove valuable in palaeoclimatic and palaeoenvironmental analysis of epicontinental carbonate sequences and ramps in the rock record.
Hydraulic controls of grain‐size distributions of bedload gravels in Oak Creek, Oregon, USA
Authors: SHIH SHYUER‐MING , KOMAR PAUL D. 
Published: June 2006

ABSTRACTGrain‐size distributions of gravels transported as bedload in Oak Creek, Oregon, show systematic variations with changing flow discharges. At low discharges the gravel distributions are nearly symmetrical and Gaussian. As discharges increase, the distributions become more skewed and follow the ideal Rosin distribution. The patterns of variations are established by goodness‐of‐fit comparisons between the measured and theoretical distributions, and by Q‐mode factor analysis. Two end members are obtained in the factor analysis, having (respectively) almost perfect Gaussian and Rosin distributions, and the percentages of the two end members within individual samples vary systematically with discharge.Transformation from Gaussian to Rosin distribution with increasing discharge may be explained by processes of selective entrainment of grains from a bed of mixed sizes. Samples of bed material in Oak Creek follow the Rosin distribution. At high discharges, the transported bedload approaches the grain sizes of that bed‐material source and mimics its Rosin distribution. Random‐selection processes must be more important to grain entrainment at lower discharges, so that the resulting Gaussian distributions of transported bedload reflect similar distributions of bed stresses exerted by the stream flow.The results from Oak Creek demonstrate that the competence of the flow is reflected in the entire distribution of transported gravel sizes. A sequence of layers of fluvial gravels, modern or ancient, might show systematic variations between coarse Rosin and finer‐grained Gaussian distributions, and these could be used to infer frequencies of various discharges and to establish a relationship to the source sediment. With further study, analyses of changing bedload grain‐size distributions and their transport rates will lead to a better understanding of downstream variations in grain sizes of bed sediments and how their distributions reflect the progressive development of textural maturity.
Cyclic sedimentation in the Southern Alpine Rhaetic: the importance of climate and eustasy in controlling platform‐basin interactions
Authors: BURCHELL MARK T. , STEFANI MARCO , MASETTI DANIELE 
Published: June 2006

ABSTRACTUppermost Triassic (Rhaetic) facies, as developed in the Southern Alpine region of Northern Italy, were deposited in a rapidly subsiding, fault‐dissected trough (the Lombardy Basin) bounded by carbonate platforms. The main part of the Rhaetic succession consists of 10‐m‐scale asymmetric cycles, each divided into three parts: a lower shale portion; a central rhythmic portion consisting of repeated marl‐limestone couplets, the limestone parts of which thicken upward; and an upper, wholly carbonate unit. A study of the diagenetic history of the series demonstrates that both the major asymmetric cyclicity and the limestone‐marl couplets of the central rhythmic member (together constituting a ‘compound’ cyclic form) are fundamentally depositional in nature. It is suggested that this compound cyclicity resulted from the superposition of a low‐frequency (approximately 100 000‐year periodicity) asymmetric carbonate mud signal with a higher‐frequency terrigenous mud signal.Field, petrographic, and geochemical investigations suggest that the basinal carbonate is predominantly allochthonous in origin, having been derived as relatively pure aragonitic mud from adjacent carbonate platforms. It is postulated that the asymmetric carbonate signal was linked to the ecological effects of eustatic fluctuation on platform carbonate systems. Repeated subaerial exposure of subtidal muds in shallow areas indicates that such sea‐level variations occurred. A model is presented in which the basinward export of carbonate was negligible in the deepening phase, increased to a maximum during shallowing and was finally halted by the emergence of large platform flats. In contrast, the higher frequency terrigenous mud signal of the basin is thought to have been climatically modulated; fluctuations of a shorter period than those predicted by the Milankovitch theory affected hinterland precipitation and runoff. Particularly rapid subsidence and high depositional rates may have allowed the preservation of this signal.
Authigenic aluminium phosphate‐sulphates in sandstones of the Mitterberg Formation, Northern Calcareous Alps, Austria
Authors: SPÖTL CHRISTOPH 
Published: June 2006

ABSTRACTComplex aluminium phosphate‐sulphate minerals of the hinsdalite group have been identified as early diagenetic precipitates in Late Permian sandstones of the Northern Calcareous Alps, western Austria. According to their chemical composition they can be regarded as solid solutions between woodhouseite, svanbergite, crandallite and goyazite. The model proposed for their origin involves the dissolution of detrital apatite in a low‐pH environment and subsequent precipitation of aluminium phosphate‐sulphate minerals, which clearly pre‐date syntaxial quartz cementation of the sediment. Their occurrence probably has been overlooked in other sandstones showing diagenetic apatite dissolution. The recognition of such minerals could provide a significant insight into early phosphate diagenesis.
Late Holocene estuarine phosphogenesis in Raglan Harbour, New Zealand
Authors: CULLEN DAVID J. , CHALLIS G. A. , DRUMMOND G. W. 
Published: June 2006

ABSTRACTA unique instance of Holocene estuarine phosphogenesis has been investigated in Raglan Harbour, on the west coast of North Island, New Zealand. Nodular calcareous concretions, which formed within surficial estuarine muds between 6500 and 3600 years BP, were superficially impregnated by carbonate fluorapatite between 3600 and 1600 years BP. The phosphatized nodules lie beneath a late Quaternary shore platform, which is reinforced by a mass of intertwined bioturbation structures and tubular rootlet casts impregnated by limonite and goethite. Large polygonal cracks, indicative of a desiccatory regime, are locally conspicuous on the platform surface and in the immediately subjacent muds.The phosphogenesis in Raglan Harbour is here related to an episode of lowered sea‐level, during which a paludal environment succeeded the estuarine, to be followed, in turn, by a positively and interlude. The subsequent rise of sea‐level to its present position over the last 1600 years or so has reinstated the shore platform in its present intertidal situation.
Sedimentation by river‐induced turbidity currents: field measurements and interpretation
Authors: CHIKITA KAZUHISA 
Published: June 2006

ABSTRACTTurbidity currents, initiated from spring runoffs of an influent river, were observed in the upper region of a reservoir in Hokkaido, Japan, by measuring water temperature, velocity and suspended‐sediment concentration. Their profiles offer some physical parameters for the sedimentary conditions, assuming the turbidity currents to be quasi‐uniform. The bottom sediment deposited by the turbidity currents was then collected by a portable core sampler. The bottom sediment consists of more than 90% silt and clay, and thus offers a hydraulically smooth bed for shear flow; a plane bed as a bed configuration was formed on the reservoir bed, probably because of the low shear velocity and small grain size of sediment. Using a graphic method with log‐normal probability paper, the bottom sediment is divided into several overlapping log‐normal subpopulations. Grain‐size analysis indicates that the bottom sediment may be regarded as cohesionless; criteria for ‘complete deposition’ of transported grains can then be incorporated into the ‘extended Shields diagram’ giving the minimum shear stress to erode bottom sediment. Applying the new diagram to the grain size distribution of the bottom sediment, it is suggested that each of the log‐normal subpopulations was deposited in each of four different ‘modes of deposition’, i.e. ‘traction’, ‘saltation (or intermittent suspension)’, ‘suspension’ and ‘suspension under equilibrium’. The last mode may be observed under a sedimentary condition where upward flux of suspended sediment by eddy diffusion is almost equal to its depositional flux due to gravity. The mean and critical grain sizes for bottom sediment and each of the corresponding subpopulations decrease consistently with an increase of Ψ=Fd2 log10Re (Fd is the densimetric Froude number and Re is the flow Reynolds number). Ψ correlates inversely with shear velocity, which bears a linear relationship to mean velocity. These results lead to the conclusion that relatively fine suspended sediment is deposited as a result of decreasing bottom friction with a relative decrease of turbulent energy.
An empirical model of aeolian dune lee‐face airflow
Authors: SWEET M. L. , KOCUREK GARY 
Published: June 2006

ABSTRACTAirflow data, gathered over dunes ranging from 60‐m tall complex‐crescentic dunes to 2‐m tall simplecrescentic dunes, were used to develop an empirical model of dune lee‐face airflow for straight‐crested dunes. The nature of lee‐face flow varies and was found to be controlled by the interaction of at least three factors (dune shape, the incidence angle between the primary wind direction and the dune brinkline and atmospheric thermal stability). Three types of lee‐face flow (separated, attached and deflected along slope, or attached and undeflected) were found to occur. Separated flows, characterized by a zone of low‐speed (0–3O% of crestal speed) back‐eddy flow, typically occur leeward of steep‐sided dunes in transverse flow conditions. Unstable atmospheric thermal stability also favours flow separation. Attached flows, characterized by higher flow speeds (up to 84% of crestal speed) that are a cosine function of the incidence angle, typically occur leeward of dunes that have a lower average lee slope and are subject to oblique flow conditions. Depending on the slope of the lee face, attached flow may be either deflected along slope (lee slopes greater than about 20°), or have the same direction as the primary flow (lee slopes less than about 20°). Neutral atmospheric thermal stability also favours flow attachment.As each of the three types of lee‐face flow is defined by a range of wind speeds and directions, the nature of lee‐face flow is intimately tied to the type of aeolian depositional process (i.e. wind ripple or superimposed dune migration, grainflow, or grainfall) that occurs on the lee slope and the resulting pattern of dune deposits. Therefore, the model presented in this paper can be used to enhance the interpretation of palaeowind regime and dune type from aeolian cross‐strata.
Clarification of some misconceptions about antidune geometry and flow character
Authors: PRAVE ANTHONY R. 
Published: June 2006

ABSTRACTEstimates of palaeoflow conditions based on antidune geometries generally are obtained using equations derived from potential flow analysis. Unfortunately, in several previous studies and review articles, misunderstanding and improper substitution of density terms has resulted in two misconceptions (a third is discussed in detail elsewhere). Both concern the commonly derived ratio of antidune wavelength to critical flow depth (L/D). Previously published results which suggest that, for a given set of flow conditions in a two‐layer system, L/D increases with increasing density contrast (Δρ) are wrong. Correct substitutions indicate the opposite—there is a slightly inverse relationship between the value of L/D and Δρ. The second misconception concerns the two values reported for critical flows: L/D≥ 6·3 (or ≥ 2γ) and L/D≥ 12·6 (or ≥ 4γ). Despite insinuations that the latter is wrong by a factor of two, both are correct. In this case, what has been overlooked is that the former value is derived for antidunes on free surface flows (a ‘two‐layer’ system in which ρ2 << ρ1) and the latter for antidunes at the interface of a two‐layer system in which Δp→0 (i.e. ρ2SIMEρ1).