bluuebunny/crossref_metadata_2025_split huggingface dataset:
Late Quaternary mass movement on the lower continental rise and abyssal plain off Western Sahara
Authors: SIMM R. W. , WEAVER P. P. E. , KIDD R. B. , JONES E. J. W. 
Published: June 2006

ABSTRACTThe late Quaternary development of part of the lower continental rise off Western Sahara has been determined from an investigation of short (< 2 m) gravity cores collected from a deep‐sea channel, the interchannel areas and an abyssal hill, between 30 and 33°N. Stratigraphic analysis is based on systematic variations in abundances of particular coccolith species and pelagic sediment types, referenced to the oxygen isotope time‐scale. During the last 73 000 years deposition in the channel has included volcaniclastic sand/silt turbidites and minor marl turbidites as well as pelagic sediments. The interchannel area has fewer turbidites, and the sands present were probably deposited from turbidity currents which spilt over the channel sides. The last‘event’ to give rise to sands in the channel and interchannel area occurred about 45 000 years ago. Although the channel has been inactive as an area of turbidity current deposition for the last 20 000 years, sands were deposited elsewhere on the lower rise, indicating that turbidity current transport routes have varied in time.Turbidity current deposition on the abyssal plain and low‐lying continental rise appears to be related to distinct sliding events involving transport of material from various sources. Thin marl turbidites are interbedded with pelagic sediments in the area of sediment drape. There is a strong correlation between these and the thick marl turbidites on the abyssal plain, suggesting that the same turbidity current‘events’, occurring about once every 25 000 years, gave rise to both sets of deposits. The thinner units probably represent deposition from the outer parts or tails of the large turbidity flows. The turbidites occur at glacial/interglacial transitions, suggesting that the slides that created them were triggered by mechanisms related to climatic change. Several volcaniclastic sand/silt units within the channel and in interchannel areas occupy mid‐stage stratigraphic positions, perhaps indicating a different triggering mechanism for slides around volcanic islands. A debris flow deposit (debrite), between 30°N, 21°W and 31°N, 24°W, is related to the Saharan Sediment Slide, a major mass movement feature on the continental slope over 1000 km to the southeast. Stratigraphic correlations indicate that this slide produced a large turbidity current as well as a debris flow.
The geochemistry of concretions from the Kimmeridge Clay Formation of southern and eastern England
Authors: SCOTCHMAN I. C. 
Published: June 2006

ABSTRACTConcretions from the Kimmeridge Clay Formation are of three types: calcareous concretions, septarian calcareous concretions and pyrite/calcite concretions and nodules, which occur within different mudstone facies. Isotopic and chemical analysis of the concretionary carbonates indicate growth in the Fe‐reduction, sulphate‐reduction and decarboxylation zones.The septarian concretions show a long and complex history, with early initiation of growth and development spanning several phases of burial, each often resulting in the formation of septaria. Growth apparently ceased in the transitional zone between the sulphate‐reduction and the methanogenesis zones. Very early growth in the Fe‐reduction zones is also seen in one sample. The non‐septarian concretions began growth later within the sulphate‐reduction zone and have had a simpler burial history while the pyrite/calcite concretions show carbonate cementation in the sulphate‐reduction‐methanogenesis transition zone. A ferroan dolomite/calcite septarian nodule with decarboxylation zone characteristics also occurs.Development of concretions appears to be indirectly controlled by the sedimentation rate and depositional environment, the latter determining the organic matter input to the sediments. Calcareous concretions predominate in swell areas and during periods of low sedimentation rate in the basins with poor organic matter preservation and deposition of calcareous mudstones. Pyrite/calcite concretions occur in organic‐rich mudstones deposited under higher sedimentation rates in the basins, while the ferroan dolomite nodule grew under very high sedimentation rates.
Morphogenetic impact of microbial mats on surface structures of Kimmeridgian micritic limestones (Cerin, France)
Authors: BERNIER P. , GAILLARD C. , GALL J. C. , BARALE G. , BOURSEAU J. P. , BUFFETAUT E. , WENZ S. 
Published: June 2006

ABSTRACTFor several years, palaeoecological research has been conducted on micritic limestones of late Kimmeridgian age in the southern Jura Mountains. The sedimentary environment is that of a lagoon with an irregular bottom, which was repeatedly exposed. Between two stages of lime deposition, a microbial mat grew over the muddy surface, giving cohesion to the sediment, restraining erosion and preserving fossil remains and reptile tracks. Various structures at the microbial mat surface can be observed: crescentic wavy, radial wavy, torn, petee and mixed structures. They imply the presence of desiccation periods and a slight bottom slope leading to a downward sliding of the microbial mat. Such features may also be generalized to explain superficial microbial structures of other shallow carbonate‐mud environments.
Coral‐stromatolite reef framework, Upper Miocene, Almería, Spain
Authors: RIDING ROBERT , MARTIN JOSÉ M. , BRAGA JUAN CARLOS 
Published: June 2006

ABSTRACTStromatolitic crusts on stick‐like and platy Porites corals forming Messinian reefs in Almería played an important role in supporting and binding the brittle corals. The crusts were previously regarded as probable marine cements. However, their clotted, peloidal, and micritic fabrics are directly comparable with those of stromatolites. They accreted allochthonous grains even on vertical faces, and include bushy fabrics closely comparable with those produced by cyanobacterial calcification. They contain numerous fenestrae, exhibit rapid fabric variation, and locally form micro‐columns and laminated domes. Their similarities to peloid micrite crusts in Recent reefs suggest that at least some of these Recent crusts are microbial in origin, even though they have widely been interpreted as marine cements.The sedimentary effects of crust development substantially affected both the morphology and relief of the reefs and the generation of reefal clasts. Binding of the reef‐frame in the Pinnacle and Thicket zones in the lower and middle parts of the reef created a rigid margin which shed large (commonly up to 5 m) cuboidal blocks of coral‐stromatolite frame. The blocks broke along planes of weakness provided by the vertical Porites sticks and were deposited on the Fore‐Reef Slope. In the uppermost parts of the reefs crusts dominate the structure, constituting 80% or more of the rock. Veneers up to 15 cm thick encrust thin irregular Porites plates to create a solid Reef Crest Zone which has not been recognized before.The coral‐stromatolite framework is associated with echinoids, crustose corallines and halimedacean algae which, together with the scleractinians, indicate normal marine salinity throughout reef growth.
Sedimentation from gravity currents generated by turbulent plumes
Authors: SPARKS R. S. J. , CAREY S. N. , SIGURDSSON H. 
Published: June 2006

ABSTRACTSedimentation from radially spreading gravity currents generated at the top of ascending sediment‐laden plumes is described by a model which assumes that sediment is dispersed homogeneously by turbulence in the gravity current, resulting in an exponential decrease in the concentration of sediment with time as particles settle out of the lower boundary of the current. For radial spreading this model predicts a Gaussian distribution of sediment accumulation away from the source with an exponential constant, B, which depends on flow rate, Q, and particle settling velocity, v (B=nv/Q). In the experiments described, sedimentation occurs from gravity currents generated by ascent of buoyant, particle‐laden plumes of fresh water in a tank of salty water. The sediment accumulation shows close agreement with the theoretical model, and the Gaussian decay constant, B, can be determined from a maximum in the accumulated mass of sediment per unit distance and from the slope of the line In(S/S0) = ‐Br2, where r is the radial distance, S is the sediment mass flux per unit area and S0 is the value of S at r=0. Data from the dispersal of volcanic ejecta from a large (c. 24 km high) plinian eruption column in the Azores also show good agreement with the theory, confirming that it is general and independent of scale and the nature of the fluid. The experimental data also show a change in sedimentation behaviour at distances from the source corresponding to the corner of the plume where it diverts into a lateral gravity current and there is an abrupt decrease in vertical velocity. Sedimentation of coarse grain sizes, between the source and the corner, occurs from the inclined plume margins and does not behave as predicted by the theoretical model.
Interpretation of Holocene sea level tendency and intertidal sedimentation in the Tees estuary using sediment luminescence techniques: a viability study
Authors: PLATER A. J. , POOLTON N. R. J. 
Published: June 2006

ABSTRACTIt is demonstrated that detailed examination of the photo‐ and thermoluminescence (PL, TL) of Holocene intertidal sediments can reveal important information regarding their depositional history in the context of sea level tendency and storm surge activity. The technique is particularly applicable to thick deposits of relatively uniform composition, and is demonstrated with a 1·7‐m core of marine to brackish water silty clay taken from Cowpen Marsh in the Tees estuary, north‐east England. This sedimentary unit is intercalated with an upper and a lower terrestrial peat bed, which have been radiocarbon dated to 5250 ± 45 and 7065 ± 4514C years BP respectively. An investigation of the mineral composition and diatom assemblage of the silty clay facies reveals a marked hiatus in deposition towards the top of the minerogenic unit. Analysis of the luminescence profiles of the sediment illustrates that, below this hiatus, sedimentation was slow and continuous, with approximately 1·0 m of sediment being deposited in 1800 ± 400 years, whilst above, sedimentation was rapid, with 0·7 m of sediment being deposited in 0 ± 200 years. It is shown that variations in the luminescence of the sediments can be interpreted in terms of the onset of a positive sea level tendency period coupled with the occurrence of a low‐frequency extreme water level event.
Flow and sediment dynamics in a low sinuosity, braided river: Calamus River, Nebraska Sandhills
Authors: BRIDGE JOHN S. , GABEL SHARON L. 
Published: June 2006

ABSTRACTThe interaction between channel geometry, flow, sediment transport and deposition associated with a midstream island was studied in a braided to meandering reach of the Calamus River, Nebraska Sandhills. Hydraulic and sediment transport measurements were made over a large discharge range using equipment operated from catwalk bridges. The relatively low sinuosity channel on the right‐hand side of the island carries over 70% of the water discharge at high flow stages and 50–60% at low flow stages. As a result, mean velocity, depth, bed shear stress and sediment transport rate tend to be greater here than in the more strongly curved left‐hand channel. The loci of maximum flow velocity, depth and bed shear stress are near the centre of the channel upstream of the island, but then split and move towards the outer banks of both channels downstream. Variations in these loci depend on the flow stage. Topographically induced across‐stream flows are generally stronger than the weak, curvature‐induced secondary circulations. Water surface topography is controlled mainly by centrifugal accelerations and local changes in downstream flow velocity. The averaged water surface slope of the study reach varies very little with discharge, having values between 0·00075 and 0·00090. As bed shear stress generally varies in a similar way to mean velocity, friction coefficients vary little, normally being in the range 0·07–0·13. These values are similar to those in straight channels with sandy dune‐covered beds. Bedload is moved mainly as dunes at all flow stages. Grain size is mainly medium sand with coarse sand moved in thalwegs adjacent to the cut banks, and with fine sand at the downstream end of the island. These patterns of flow velocity, depth, water surface topography, bed shear stress, bedload transport rate and mean grain size can be accurately predicted using theoretical models of flow, bed topography and sediment transport rate in single river bends, applied separately to the left and right channels.During high flow stages deposition occurs persistently near the downstream end of the island, and cut banks are eroded. Otherwise, erosion and deposition occurs only locally within the channel as discharge varies. Abandonment and filling of a strongly curved channel segment may occur by migration of an upstream bar into the channel entrance at a high flow stage.
Early diagenetic recrystallization of Holocene (<3000 years old) peritidal dolomites, Ambergris Cay, Belize
Authors: GREGG JAY M. , HOWARD SCOTT A. , MAZZULLO S. J. 
Published: June 2006

ABSTRACTThree peritidal carbonate crusts and associated intercrust sediments (total thickness of ∼30cm; aged <3000 years BP) on Ambergris Cay, Belize, contain 32–100% calcian dolomite (δx=72·5% dolomite) ranging in composition from 40 to 46 mol% MgCO3 (δx=43·3). Dolomite replaced high Mg calcite foraminiferal muds penecontemporaneously with sedimentation, forming partially dolomitized sediments and lithified crusts. Dolomitization probably occurred in normal to moderately evaporated seawater and is apparently continuing at the present.Detailed scanning electron microscope analysis shows a linear increase in mean dolomite crystal size with depth; 0·4 μm near the top of the section to 1·0 μm near the base of the dolomitized section. This size increase is not accompanied by any significant decrease in porosity. Crystal size distributions appear to be log‐normal and become increasingly broad and flat with depth. Rietveld X‐ray pattern‐fitting structure refinements indicate increasing Ca and Mg concentrations on their respective sites (cation ordering) as a function of increasing depth. Most of the ordering occurs within the first 15 cm of the surface. Stoichiometry does not increase with depth indicating no relationship between the Ca/Mg ratio and cation ordering. Strong geochemical trends were observed down‐section in the dolomite, including: (1) increasing Mn content (44 to 274 ppm), and (2) decreasing δ13C values (−0·9 to −5·5‰ PDB). Oxygen isotope values range from δ18O = 1·3‰ PDB in the upper part of the section to 2·6‰ PDB in the lower part of the section and are interpreted to represent two distinct groups of values rather than a continuous trend.Down‐section dolomite crystal size increase and shapes of crystal size distributions are consistent with recrystallization via a surface energy‐driven dissolution‐reprecipitation process (Ostwald ripening). The observed trends in carbon isotopes and Mn content probably result from geochemical re‐equilibration during recrystallization and reflect reducing conditions and an isotopically light, organically derived, carbon source. Oxygen isotope compositions probably reflect relict original dolomite values and are a result of decreasing evaporation due to rising sea level.
Genesis of diamicton in the Oak Creek Formation of south‐east Wisconsin, USA
Authors: RONNERT LARS 
Published: June 2006

ABSTRACTThe clayey diamicton of the Oak Creek Formation was deposited by the Lake Michigan Lobe of the Laurentide Ice Sheet during the late Wisconsin deglaciation. Rapid changes in the position of the ice margin may have been facilitated by a subglacial deforming bed. However, although it is difficult to find proof either for or against a subglacial deforming bed, most observations from this investigation are more easily explained by transportation within and deposition from the ice itself. Good exposures of the diamicton units, especially of the lower contacts, along the shore of Lake Michigan, are the basis for the interpretation of basal till genesis. Basal deposition occurred mainly by stagnation and stacking block by block followed by melting out, as opposed to lodgement grain by grain. This interpretation of basal deposition is applicable in other areas and in most topographic environments.
Shingled Quaternary debris flow lenses on the north‐east Newfoundland Slope
Authors: AKSU A. E. , HISCOTT R. N. 
Published: June 2006

ABSTRACTDebris flow deposits are the principal component of Quaternary continental slope sediments between the north‐east Newfoundland Shelf and central Orphan Basin. In seismic profiles, these deposits occur as shingled, elongate, acoustically transparent lenses with their long axes orientated downslope. Deposits of individual flows form positive mounds on the sea floor; subsequent flows were diverted by the pre‐existing topography into bathymetric lows between older debris flow deposits. These deposits show a large variation in the area of sea floor covered by individual flows (about 60–1000 km2), average thickness of deposits (9–37 m) and volume of sediment displaced (1–27 km3). The ratio of average thickness to a measure of deposit diameter, termed the aspect ratio, has a threefold variation from 0·0006 to 0·0021. Very low depositional slopes and low aspect ratios suggest relatively low viscosities, probably due to inmixing of water during downslope transport. Stratified sediments form three distinct horizons and are locally interbedded with the debris flow deposits. These are mainly hemipelagic deposits.The slope and rise to the west of the Orphan Basin are constructional in character. The apparent absence of upper slope erosional features and the abundance of debris flow deposits on the slope suggest that the supply of sediment to the continental slope occurred predominantly during times of maximum extent of Quaternary glacial ice. The ice sheet grounding line during several glacial maxima must have been situated at or near the present shelf break, supplying vast amounts of sediment directly to the upper slope. Oversteepening and subsequent slope failures fed material into deeper water.
Palaeo‐yardangs: wind‐scoured desert landforms at the Permo‐Triassic unconformity
Authors: TEWES DAVID W. , LOOPE DAVID B. 
Published: June 2006

ABSTRACTIn Canyonlands National Park, south‐east Utah, at least 29 partly exhumed, aligned sandstone ridges trending generally N20°W occur at the upper unconformable surface of the Lower Permian (Leonardian) White Rim Sandstone. The ridges are at least 1·5 km long, 250 m wide and have up to 14 m of vertical relief (mean of 9 m). A thin lag of coarse sandstone that contains wind‐ripple laminae and granule ripples directly overlies the ridges. Angular blocks of sandstone within the lag and sand‐filled fissures immediately below the lag, within the ridges, attest to early cementation of the ridge‐forming material. SE‐dipping aeolian cross‐strata within the White Rim Sandstone and within the lag closely parallel the ridge trend. The ridges are interpreted as wind‐sculpted desert landforms (yardangs) that developed on the lithified upper surface of the White Rim Sandstone during an extended period of hyperaridity towards the end of the Permian.
The grain/bed collision in sand transport by wind
Authors: McEWAN I. K. , WILLETTS B. B. , RICE M. A. 
Published: June 2006

ABSTRACTIn the last decade much progress has been made toward the development of a comprehensive model of aeolian sediment transport in which the grain/bed collision has been identified as having a significant role. The grain/bed collision has been studied by both physical experiments and numerical simulation. A principal objective of these studies has been to gather sufficient data to characterize the collision in order that it may be represented as an empirical function in numerical models of the sand transport system. Thus the study of the exact physical mechanism of the collision has to some extent been neglected.The transport of larger particles in saltation over a loose surface is known to promote the release of finer particles from that surface into suspension. Thus the precise physics of the grain/bed collision become highly significant with regard to the physics of dust release from a surface. This paper re‐examines previous collision data and compares them with a simple collision model. This model proposed that the impinging grain strikes a single surface grain and rebounds: an alternative mechanism might be that the impinging grain ploughs through the surface striking a number of bed grains before rebounding. The collision data are shown to support the model: first, because the duration of the collisions observed on the high speed film supports a short contact time with the surface and, second, because the data fit well with the model.
Geometry and facies of Tertiary clinothems, Spitsbergen
Authors: HELLAND‐HANSEN WILLIAM 
Published: June 2006

ABSTRACTSandstone clinothems from the Battfjellet Formation (Palaeogene) on Spitsbergen are locally exceptionally well preserved along depositional dip‐parallel mountainsides. The clinothems are more than 1 km wide and more than 100 m thick. Superposition of several sandstone clinothems separated by mudstones reflects repeated shoreline progradation and transgression. Deposition took place partly on‘post‐transgressional’ depositional shelves, and partly by contributing seaward‐sloping wedges, or clinothems, to a ramp progradation. Shorelines dominated both by mouth bar and shoreface environments have been identified. The clinothems are organized into an overall progradational architecture with a geometry having features in common with progradational seismic facies.