bluuebunny/crossref_metadata_2025_split huggingface dataset:
Deep‐water resedimentation of anhydrite and gypsum deposits in the Middle Miocene (Belayim Formation) of the Red Sea, Egypt
Authors: ROUCHY JEAN MARIE , PIERRE CATHERINE , SOMMER FREDERIC 
Published: June 2006

ABSTRACTThe Middle Miocene evaporites in the Red Sea rift were deposited within a complex system of fault‐bounded basins that were episodically active during sedimentation. Such a tectonic framework is known to be highly favourable to resedimentation processes. An offshore petroleum well in the north‐western Red Sea has cored, below a massive salt unit, an anhydrite‐bearing succession which provides an excellent opportunity to study the processes of gravity induced redeposition of Ca‐sulphates in a deep basin. Anhydrite deposits, interbedded with siliciclastic layers and thin halite layers, are composed of resedimented facies ranging from fine‐grained laminated sediments to coarse‐grained breccias. The components derive from the reworking of shelf sediments deposited initially in shallow water to supratidal settings on the surface and edges of structural highs bordering depressions: proximal siliciclastic deposits with interstitial anhydrite (cement patches, nodules) or gypsum and dolostones with early diagenetic anhydrite facies (nodular, chicken‐wire) formed in sabkha conditions, interstitially grown gypsum crystals and subaqueous gypsum crusts precipitated in hypersaline ponds, and diatom‐rich oozes formed in marine, shallow‐water conditions. The homogeneity of the stable isotope composition and petrography of sulphates argue for the initial crystallization of Ca‐sulphates within brines of the same origin and in closely interconnected sedimentary settings. The unconsolidated sediments redeposited as slope‐foot accumulations were carried both as anhydrite (nodules, soft masses, various fragments, individual grains or crystals released by disintegration of large masses) and gypsum (crystalline aggregates or single crystals) later converted to anhydrite during burial. Layers of chaotic breccia are interpreted as the result of seismic events, whereas the fine‐grained deposits could be related to redistribution by nepheloid layers of suspensions of finer grains released by disintegration of the soft anhydrite masses during downslope transport, or of in situ deposits removed by the turbiditic flows.
Frequency distribution of river bed materials
Authors: KOTHYARI U. C. 
Published: June 2006

ABSTRACTThe size distribution of river bed material plays an important role in the study of fluvial hydraulic processes. It is not always easy to sample the river bed material for the determination of its size distribution, but knowledge of the frequency distribution and of bed material sizes can be used to predict its size distribution. Therefore, data on bed material size distributions of 63 streams from 10 different countries have been analysed to determine the frequency distribution which can be best fitted to them. The log normal distribution cannot be fitted to the bed material mixtures when the entire range of sizes is considered. The method of power transformation is found to normalize these mixtures. Relationships have been proposed between the parameters of the normalization and the stream slope. These relations can be used to predict the size distribution of the river bed material.
Modelling of porosity evolution and mechanical compaction of calcareous sediments
Authors: AUDET D. MARC 
Published: June 2006

ABSTRACTA continuum mechanics model for the gravitational compaction of sediments is derived by assuming that the sediments are normally pressured and in a one‐dimensional state of stress. Sediment strength is characterized in terms of effective stress laws adopted from soil mechanics. The model is a relatively simple mathematical formula that gives the porosity as a function of burial depth. The shape of the porosity profile is controlled by two mechanical parameters, the compression index and the void ratio at an effective stress of 100 kPa. The model was verified by analysing the porosity—depth data of oozes and chalk from the Ontong Java Plateau, gathered during Leg 130 of the Ocean Drilling Program. The mechanical parameters of the sediments were estimated using a least‐squares method to fit the theoretical profile to the porosity data. The theoretical profile described accurately the ooze porosity data over depth ranges of 100 m or more. However, over smaller length‐scales of 10–50 m there were systematic deviations between the theoretical porosity values and the ooze porosity data. The porosity deviations correlated with variations in the mean grain size of the sediments, due in part to changes in the foraminifera abundance. In the case of the oozes, the estimated mechanical parameters were consistent with published values obtained from one‐dimensional compression tests. In contrast, the estimated mechanical properties for the chalks differed from published values. The chalk porosities were lower than could be explained by mechanical compaction. This explanation is supported by the compressional (P‐wave) velocity data. In the chalk sections, the P‐wave velocity increases more rapidly with burial depth than it does in the ooze sections, suggesting that sediment elastic properties are increasing due to interparticle binding.
The stability of sublittoral, fine‐grained sediments in a subarctic estuary
Authors: AMOS CARL L. , SUTHERLAND T. F. , ZEVENHUIZEN J. 
Published: June 2006

ABSTRACTThe erodibility of natural estuarine sediments was measured in sit along a longitudinal transect of Manitounuk Sound, Hudson Bay, using the benthic flume Sea Carousel. Sedimentation processes along the transect varied from continuous, rapid, post‐glacial sedimentation in the inner Sound, to glacial outcrops and seabed reworking of the outer Sound. The grain size and physical bulk properties reflect changes in depositional environment and correlate with sediment erosion threshold stress (τc), erosion rate (E), erosion type and still‐water mass settling rate. There was a steady increase in τc (0·8–2·0 Pa) with distance down the Sound in parallel with the decreasing sedimentation rate (0·003–0·001 m yr−1) and increasing sediment bulk density (1650–2010 kg m−3). The near‐surface friction coefficient varied up to 68° in proportion to the clay content of post‐glacial material. Glacial sediments were characterized by variable results and generally higher friction coefficients. Seabed erosion in Sea Carousel began with surface creep of loose aggregates, pellets and organic debris. This was followed by Type I bed erosion at rates that varied between 0·0002 and 0·0032 kg m−2 s−1 (mean 0·0015). Type I peak erosion rate was inversely related to applied bed shear stress (τo). Type II erosion succeeded Type I, often after a broad transitional period. Simulations of suspended sediment concentration in Sea Carousel were made using four commonly used erosion (E) algorithms. The best results were obtained using Krone's dimensionless ratio relationship: E=M(τo/τc‐1). Simulations were highly sensitive to the definition of erosion threshold with sediment depth [τc(z)]. Small errors in definition of τc(z) caused large errors in the prediction of suspended sediment concentration which far exceeded differences between the methods tested.
Secondary precipitates in Pleistocene and present cryogenic environments (Mendoza Precordillera, Argentina, Transbaikalia, Siberia, and Seymour Island, Antarctica)
Authors: VOGT THEA , CORTE ARTURO E. 
Published: June 2006

ABSTRACTSecondary precipitates (iron oxide, calcite, etc.) are currently observed in cold‐climate Pleistocene deposits. Some have micro‐ and ultrastructures quite different from precipitates of vadose, phreatic and biogenic origin, and seem to have originated by freezing processes. The microstructures of calcite coatings from a Pleistocene cryopediment in the Mendoza Pre‐Cordillera, from a Lower Pleistocene cryogenic slope deposit in Western Transbaikalia and from the present active layer in Antarctica are described. They show similar patterns: fibrous crystals often consisting of piles of platelets, some with internal holes, assembled in millimetre‐scale fringes on the lower face of clasts. Observational (mainly fabric) evidence confirms that such peculiar crystals are formed during freezing. The features are unknown in other climates and, when found in past sediments, can be diagnostic of cryogenic palaeoenvironments.
Sedimentological aspects of the Humber Group (Upper Jurassic) of the South Central Graben, UK North Sea
Authors: HOWELL JOHN A. , FLINT STEPHEN S. , HUNT CATHERINE 
Published: June 2006

ABSTRACTComplex structure, poorly understood sedimentology and poor biostratigraphic control make the Upper Jurassic Humber Group of the South Central Graben one of the least understood and most complex hydrocarbon reservoirs of the North Sea. Detailed logging of available core from 19 exploration wells has been combined with an improved understanding of the relevance of trace fossils and a recognition of important base‐level variations to provide a greatly enhanced understanding of the depositional system active within the area at that time. A new sedimentological model, based upon the distribution of facies and facies associations, illustrates that Upper Jurassic structure and consequent basin geometry were the principal controls upon the distribution of depositional environments. Rifting and second‐order transgression controlled the back‐stepping onlap patterns observed and higher frequency base‐level fluctuations controlled the internal architecture of individual sandbodies. The model presented accounts for features of these deposits that were previously considered anomalous, such as the thickness of bioturbated sandstones, paucity of foreshore deposits and complex age relationships of sands.
Sedimentology of a lacustrine fan‐delta system, Miocene Horse Camp Formation, Nevada, USA
Authors: HORTON BRIAN K. , SCHMITT JAMES G. 
Published: June 2006

ABSTRACTA 1600‐m‐thick succession of the Miocene Horse Camp Formation (Member 2) exposed in east‐central Nevada records predominantly terrigenous clastic deposition in subaerial and subaqueous fan‐delta environments and nearshore and offshore lacustrine environments. These four depositional environments are distinguished by particular associations of individual facies (14 defined facies). Subaerial and subaqueous fan‐delta facies associations include: ungraded, matrix‐and clast‐supported conglomerate; normally graded, matrix‐ and clast‐supported conglomerate; ungraded and normally graded sandstone; and massive to poorly laminated mudstone. Subaqueous fan‐delta deposits typically have dewatering structures, distorted bedding and interbedded mudstone. The subaerial fan‐delta environment was characterized by debris flows, hyperconcentrated flows and minor sheetfloods; the subaqueous fan‐delta environment by debris flows, high‐ and low‐density turbidity currents, and suspension fallout.The nearshore lacustrine facies association provides examples of deposits and processes rarely documented in lacustrine environments. High‐energy oscillatory wave currents, probably related to a large fetch, reworked grains as large as 2 cm into horizontally stratified sand and gravel. Offshore‐directed currents produced uncommonly large (typically 1–2 m thick) trough cross‐stratified sandstone. In addition, stromatolitic carbonate interbedded with stratified coarse sandstone and conglomerate suggests a dynamic environment characterized by episodic terrigenous clastic deposition under high‐energy conditions alternating with periods of carbonate precipitation under reduced energy conditions. Massive and normally graded sandstone and massive to poorly laminated mudstone characterize the offshore lacustrine facies association and record deposition by turbidity currents and suspension fallout.A depositional model constructed for the Horse Camp Formation (Member 2) precludes the existence of all four depositional environments at any particular time. Rather, phases characterized by deposition in subaerial fan, nearshore lacustrine and offshore lacustrine environments alternated with phases of subaerial fan‐delta, subaqueous fan‐delta and offshore lacustrine deposition. This model suggests that high‐energy nearshore currents due to deep water along the lake margin reworked sediment of the fan edge, thus preventing development of a subaqueous fan‐delta environment and promoting development of a well‐defined nearshore lacustrine environment. Low‐energy nearshore currents induced by shallow water along the
Hydrodynamics and sediment dynamics of The Wash embayment, eastern England
Authors: KE X. , EVANS G. , COLLINS M. B. 
Published: June 2006

ABSTRACTWater and sediment movement in The Wash embayment has been determined from an extensive set of data, consisting of tidal current readings and suspended sediment concentration measurements. Instantaneous and residual currents in the embayment show a distinct lateral inhomogeneity, whereas vertically the water column is almost homogenous. The central deep water area (30–40 m) of the embayment is dominated by a residual landward water movement, whilst on the margins, the residual movement is seaward. Sediment is supplied predominantly in suspension from the north, through the northern extremity of Boston Deep. Suspended sediment pathways are coincident with the spring tide water movements and the subtidal channels act as the main conduits. Approximately 6·8 × 106 tonnes yr−1 of suspended sediments are supplied to the embayment from offshore areas. Bedload sediment supply is of lesser importance, ∼ 1·4 × 104 tonnes yr−1. Whereas suspended sediment movement appears to be the dominant mode of transport throughout the embayment, bedload transport is important in reforming the sea bed into a variety of bedforms which are particularly well developed on the margins of channels and shoals.
Bedload transport of aggregated muddy alluvium from Cooper Creek, central Australia: a flume study
Authors: MAROULIS J. C. , NANSON G. C. 
Published: June 2006

ABSTRACTThe ability of mud aggregates to form depositional bedforms is of considerable sedimentological importance for explaining the geomorphology of the Channel Country of central Australia as well as for understanding the depositional environment of certain argillaceous fluvial sequences in the rock record. The sediment transport and bedform development of mud aggregates from the floodplain of Cooper Creek, central Australia, was examined in a laboratory flume over a range of flow conditions. The aggregates were found to be clay‐rich (>60% clay), nonsaline (<0·02%), fine sand‐sized (mean d50=0·13 mm), low density (2300 kg m−3) and water‐stable. Three wetting rates were applied to the sediment in the laboratory prior to wet sieving to replicate various field conditions and results in three mean aggregate sizes. Immersion wetting (no tension) represents inundation of the sediment by overland flow and results in aggregates of 0·13 mm. Tension wetting at 20 and 50 mm corresponds to high‐ and low‐intensity rainfall and results in mean d50 sizes of 0·75 and 0·70 mm, respectively. Immersion wetting is the most applicable wetting mode for hydraulic transport of aggregated sediment on the Cooper Creek floodplain. Considerable variability in sediment transport rates in the field could result from differences in pre‐wetting of the aggregated sediment. The dominance of smectite in the clay mineralogy of the sediment is an important factor in the development of the aggregates; disaggregated sediment reaggregated in a laboratory after 2–3 wetting/drying cycles. In flume experiments, bedforms of aggregated mud ranging from lower‐regime plane beds to upper‐regime antidunes were observed. The aggregates moved predominantly as bedload with measured peak bedload concentrations being high compared with other flume studies. The highly mobile nature of this sediment in the field is due to the ready entrainment of low‐density aggregates in the form of self‐mulching vertisols across extensive floodplains. The occurrence of low‐sinuosity braid‐like channels on this extensive low‐gradient semi‐arid floodplain can be attributed to: (a) the passage of floodwaters across a floodplain with steeper gradients than adjacent more sinuous anastomosing channels; (b) the highly mobile nature of the low‐density sediment aggregates; (c) the ability of the aggregates to be transported as bedload; and (d) their durable nature during transport.
Mesoproterozoic deep‐water reefs from Borden Peninsula, Arctic Canada
Authors: NARBONNE GUY M. , JAMES NOEL P. 
Published: June 2006

ABSTRACTA superbly exposed stromatolite reef complex occurs in the Victor Bay Formation near Strathcona River on northern Baffin Island. Individual reefs are up to 130 m thick and nearly 1 km in length, and their development was clearly related to their position in the facies spectrum and to sea‐level dynamics. In the first sea‐level cycle, metre‐scale reefs grew amongst mid‐ramp calcarenites and outer‐ramp shales during slow sea‐level rise; a 25‐m‐thick oblate reef tract, separating mid‐ramp and outer‐ramp facies, formed during the highstand. The greatest period of reef growth was during the second sea‐level cycle. Pinnacle reefs nucleated on the karsted upper surface of the oblate reef tract and aggraded rapidly in response to rising sea‐level, producing structures with more than 75 m of depositional relief. A gradual symmetrical succession of stromatolite growth forms, from stratiform to cylindrical columns to conical columns and then back through cylindrical columns to stratiform, is mirrored by evidence in offreef deposits for deepening to a maximum flooding surface and then shallowing. The tops of these high‐standing reefs were karsted during the following regression, while dolomite ‘cryptodomes’ grew as sheets on their submerged flanks and as progradational tongues extending basinward of the reefs. Continued sea‐level fall resulted in subaerial exposure of the entire reef complex and the extensive formation of surface and subsurface karst.These Proterozoic slope buildups are similar to Phanerozoic deep‐water reefs in size, shape, prevalence of synsedimentary lithification, presence of Neptunian dykes and in their well‐developed vertical zonation of reefbuilders. However, they differ in being constructed exclusively by stromatolites rather than being mud mounds with small skeletal elements, and in lacking halos of perireefal sand‐ and gravel‐sized calcareous debris. Their responses to changes in sea‐level were strikingly similar to those shown by their younger counterparts, and suggest that sequence‐stratigraphic concepts derived from studies of Phanerozoic reefs can also be applied to the Proterozoic.
Low‐Mg calcite marine cement in Cretaceous turbidites: origin, spatial distribution and relationship to seawater chemistry
Authors: HENDRY JAMES P. , TREWIN NIGEL H. , FALLICK ANTHONY E. 
Published: June 2006

ABSTRACTLower Cretaceous (Hauterivian) bioclastic sandstone turbidites in the Scapa Member (North Sea Basin) were extensively cemented by low‐Mg calcite spars, initially as rim cements and subsequently as concretions. Five petrographically distinct cement stages form a consistent paragenetic sequence across the Scapa Field. The dominant and pervasive second cement stage accounts for the majority of concretions, and is the focus of this study. Stable‐isotope characterization of the cement is hampered by the presence of calcitic bioclasts and of later cements in sponge spicule moulds throughout the concretions. Nevertheless, trends from whole‐rock data, augmented by cement separates from synlithification fractures, indicate an early calcite δ18O value of+0·5 to ‐1·5‰ PDB. As such, the calcite probably precipitated from marine pore fluids shortly after turbidite deposition. Carbon isotopes (δ13C=0 to ‐2‰ PDB) and petrographic data indicate that calcite formed as a consequence of bioclastic aragonite dissolution. Textural integrity of calcitic nannoplankton in the sandstones demonstrates that pore fluids remained at or above calcite saturation, as expected for a mineral‐controlled transformation.Electron probe microanalyses demonstrate that early calcite cement contains <2 mol% MgCO3, despite its marine parentage. Production of this cement is ascribed to a combination of an elevated aragonite saturation depth and a lowered marine Mg2+/Ca2+ ratio in early Cretaceous ‘calcite seas’, relative to modern oceans. Scapa cement compositions concur with published models in suggesting that Hauterivian ocean water had a Mg2+/Ca2+ ratio of ≤1. This is also supported by consideration of the spatial distribution of early calcite cement in terms of concretion growth kinetics. In contrast to the dominant early cement, late‐stage ferroan, 18O‐depleted calcites were sourced outwith the Scapa Member and precipitated after 1–2 km of burial.Our results emphasize that bioclast dissolution and low‐Mg calcite cementation in sandstone reservoirs should not automatically be regarded as evidence for uplift and meteoric diagenesis.
Was Phanerozoic reef history controlled by the distribution of non‐enzymatically secreted reef carbonates (microbial carbonate and biologically induced cement)?
Authors: WEBB GREGORY E. 
Published: June 2006

ABSTRACTThroughout most of the Phanerozoic, reef rigidity resulted as much, or more, from early lithification by microbial carbonates and biologically induced cements (non‐enzymatic carbonates) than from biological encrustation of, or by, large, enzymatically secreted metazoan skeletons. Reef framework is divided into four categories: (1) skeletal metazoan; (2) non‐skeletal microbialite (stromatolite and thrombolite); (3) calcimicrobe; and (4) biocementstone, in which small or delicate organisms serve as scaffolds for rigid cement crusts. The last three categories are dominated by non‐enzymatic carbonates. Skeletal framework and non‐skeletal microbialite framework were the most abundant framework types through the Phanerozoic. The composition and abundance of skeletal framework was controlled largely by mass extinction events, but most reefs consisted of both microbialite and skeletal organisms in a mutually beneficial relationship. Microbialite framework was abundant throughout the Palaeozoic and early Mesozoic, but declined after the Jurassic. Calcimicrobe framework was important during the Cambrian‐Early Ordovician and Devonian and biocementstone framework was important from the late Mississippian to the Late Triassic.The Phanerozoic history of reefs does not correlate well with the stratigraphic distribution of large, skeletal ‘reef builders’, or with a variety of physicochemical parameters, including sea‐level history, Wilson Cycle or global climate cycles. Because non‐enzymatic carbonates result from induction by non‐obligate calcifiers, and not enzymatic precipitation by obligate calcifiers, the distribution of these carbonates was controlled to a larger extent by temporal changes in physicochemical parameters affecting the saturation state of sea water with respect to carbonate minerals. Changes in pCO2, Ca/Mg ratios, cation concentrations and temperature may have affected the abundance of non‐enzymatic carbonates and, hence, reefs, independently from the effects of these same parameters and mass extinction events on skeletal reef biota. The decline in abundance of reefal microbialite and absence of calcimicrobe and biocementstone reef framework after the Jurassic may be a result of relatively low saturation states of sea water owing to increased removal and sequestration of finite marine carbonate resources by calcareous plankton since the Jurassic. Reef history is difficult to correlate with temporal changes in specific global parameters because these parameters affect skeletal biota and biologically induced carbonate precipitation independently. Hence, reef history was regulated not just by skeletal reef biota, but by parameters governing non‐enzymatic carbonates.