bluuebunny/crossref_metadata_2025_split huggingface dataset:
Convolute lamination in modern sands of the estuary of the Oosterschelde, the Netherlands, formed as the result of entrapped air*
Authors: DE BOER P. L. 
Published: June 2006

ABSTRACTSoft‐sediment deformation structures similar to convolute lamination were found at the sandy rim of an intertidal shoal in the Oosterschelde estuary, the Netherlands. Abundant air filled cavities within upward penetrated domes suggest that reversed density stratification resulting from trapped air in some sand layers plays a role in the deformational process.Field observations show that air‐filled cavities are formed within fine‐grained sand; at rising tide the ground water level does not rise quickly enough to replace the interstitial air before flood water covers the sediment surface. Thus, air is trapped in the sand between the ground water level and the sediment‐water interface. Because of the weight of the overlying water column and the slow downward movement of water due to capillary action, the entrapped air is compressed and eventually may attain a pressure which enables it to lift the overlying sediment. At this time bubbles are formed, often to a depth of 20 cm. Observation and experiments show that, in layers of fine sand, bubbles develop preferentially in better sorted and coarser zones. This is probably because capillary forces are greater in finer‐grained and less sorted sand. Thus, water will penetrate by preference into the latter, pressing the interstitial air into the better sorted and coarser sand. The high content of air cavities in certain layers then provides the density instability responsible for the deformational process leading to the formation of convolute lamination. In the intertidal zone this appears to be a slow process that covers a number of ebb and flood cycles.Convolute lamination has been described from a number of ancient sandstones that are thought to have been deposited in shallow water, near‐shore environments. In many of these cases sand size compares with that found in the Oosterschelde. Air entrapment might have played a role in the formation of certain of these occurrences of convolute lamination.
Distinguishing between fine‐grained turbidites and contourites on the Nova Scotian deep water margin
Authors: STOW DORRIK A. V. 
Published: June 2006

ABSTRACTReview of the criteria which have been proposed for distinguishing between the deposits of turbidity currents and bottom currents in deep water sedimentation shows no general agreement on their validity. It is important to compare finegrained turbidites and contourites, to recognize that different turbidity current and bottom current mechanisms exist, and that their deposits may be closely inter‐bedded in a continental rise environment.Interbedded turbidites and contourites have been recognized in cores from the deep‐water margin off Nova Scotia. The most useful criteria for distinguishing between the two deposits were found to be: (1) fining and sorting trends: perpendicular or parallel to the contours; (2) marked textural differences between interbedded turbidites and contourites indicating differences in source and transport distance; (3) mineralogy and textural composition: regional patterns indicating transport perpendicular or parallel to the contours; (4) grain fabric: indication of downslope or along‐slope transport at the time of final deposition; (5) sedimentary structures: turbidites show a structural sequence and evidence of rapid burial; contourites are bioturbated and contain irregular lag concentrations of biogenic sand. Other criteria include grain‐size parameters, and the regional setting, distribution and depositional rate of the various facies. With due care these criteria can be applied to other regions.Previously used characteristics of silt‐laminae abundance, layer thickness, heavy mineral cross lamination, sorting, and the nature of bed contacts are not applicable to fine‐grained turbidites and contourites. Compositional criteria depend on regional features.
Time and space distribution of megaripples and associated bedforms, Middle Atlantic Bight, North American Atlantic Shelf
Authors: SWIFT D. J. P. , FREELAND G. L. , YOUNG R. A. 
Published: June 2006

ABSTRACTThree genetically distinct size classes of lower regime transverse bedforms have long been known from laboratory studies, and from studies of the intertidal zone; ripples, megaripples, and sand waves. These features are also present on the subtidal shelf surface of the Middle Atlantic Bight, and their distribution in time and space allows us to draw inferences concerning the time and space pattern of sediment transport. Transverse bedforms in the Middle Atlantic Bight occur in response to tidal flows at estuary and inlet mouths and on tide‐dominated banks; on the shelf surface, however, they are primarily responses to wind‐driven flows. Ripples are the most widespread of the three classes. They are current‐formed during peak storm flows, but are probably remade as oscillatory wave ripples as the flow wanes. Megaripples are found primarily on the inner shelf, also as responses to peak storm flows. Sand waves of several metres amplitude occur on the inner shelf in the vicinity of topographic highs; low amplitude sand waves (< 2m), solitary or in trains, are widespread on the inner shelf. They survive through many seasons of storm flows. Megaripples are especially interesting as records of specific flow events. They are widespread on the inner shelf during the winter, occurring in fields up to several kilometres in diameter. On a portion of the Long Island inner shelf during December 1976, megaripple fields covered approximately 15% of the shelf surface. They tend to be erased during the succeeding summer months. Both megaripples (short‐term response elements) and sand waves (long‐term response elements) indicate that sand transport in the Middle Atlantic Bight is directed to the southeast, parallel with the regional trend of the isobaths.
A geochemical and mineralogical investigation of some British and other European tonsteins
Authors: SPEARS D. A. , KANARIS‐SOTIRIOU R. 
Published: June 2006

ABSTRACTThe volcanic origin of a number of major tonstein horizons has been established from mineralogical compositions and from trace elements that are quantitatively retained during diagenesis. Discriminant function analysis using the variables Ti/Al, Cr/Al, Zr/Al and Ni/Al allows the tonsteins to be classified according to original composition. Two main groups are recognized amongst the British tonsteins. Those formed from acid volcanic ash are comparable in composition with French and German tonsteins and a common source is postulated, whereas tonsteins formed from basic volcanic ash contain variable amounts of detrital sediment and are thought to originate from local eruptions. Possible lateral equivalents between tonstein horizons in Britain and elsewhere in Europe are suggested.
Non‐annual cycles of varve‐like sedimentation in Walensee, Switzerland*
Authors: LAMBERT† ANDRÉ , HSÜ K. J. 
Published: June 2006

ABSTRACTDating of recent varve‐like sediments from the perialpine Lake of Walenstadt (=Walensee) indicates that the number of laminae deposited in the 165 years between 1811 and 1976 ranges from 300 to 360 depending on sample location. Direct evidence that up to five graded laminae may be deposited during one year was found in 1912 by an engineer of the Swiss Federal Hydrological Agency who was using sediment traps to determine the annual accumulation rate. These layers are considered to be deposits of continuous‐fed turbidity currents generated by hyperpycnal inflow during river‐flood stages. Current measurements revealed that these underflows can occur sporadically throughout the year, but are especially common during the snow‐melting season and after heavy rainfalls. Currents with bottom‐velocities up to 50 cm s−1 were detected during summertime when the lake is thermally stratified, indicating that such stratification is no obstacle to the generation of turbidity underflows. The laminated sediments of the Walensee do not represent deposition of annual cycles and these non‐annual varve‐like sediments seem to be less regularly rhythmic than the annual varves of Lake Zurich.
Sedimentation in a closed trough north of the Iberia Abyssal Plain in the northeast Atlantic†
Authors: ADDY SUNIT KUMAR , KAGAMI HIDEO 
Published: June 2006

ABSTRACTMirrol Trough of the northeast Atlantic contains five NNE‐SSW trending, en echelon, turbidite‐filled basins deeper than 5500 m, each ranging from 4 to 10 km in width and 19 to 65 km in length. Trough deposition has consisted mainly of turbidites from adjacent hills and ridges as indicated by the physiography of the region, sediment isopach map, the nature of the sediments in the trough, and benthic foraminiferal depth indicator species. The sedimentation rate on abyssal hills and ridges, as deduced from palaeomagnetic evidence, is 1.36 cm/103 years. Using this sedimentation rate, it is estimated that Mirrol Trough subsided relative to the surrounding area and began receiving sediments between 8.3 and 11.5 m.y. ago; and the deposition of the most recent turbidite has occurred sometime between 29,000 and 44,000 years b.p. Tilting of the base of the most recent turbidite with respect to the basin floor is observed, and this is attributed to relative sinking of the eastern margin of the trough after the deposition of the most recent turbidite.
Calcification in a coccoid cyanobacterium associated with the formation of desert stromatolites
Authors: KRUMBEIN W. E. , GIELE C. 
Published: June 2006

ABSTRACTIn the Borrego Desert (California) and in the Sinai Desert (Israel) laminated, microbially mediated carbonate crusts have been found and analysed biologically and mineralogically, and further studied with scanning electron microscope methods combined with energy dispersive X‐ray analyses. All morphological and biological features of the extant crusts justify the term ‘desert stromatolite’, a term applied to stromatolites from desert regions which form under permanent exposure to the atmosphere. These stromatolites are never covered by standing water, and running water (heavy rainfall) covers them for only a few hours during the year. Carbonate deposition is achieved principally by the cyanobacterium Pleurocapsa sp. which exhibits characteristic yet different stages of calcification. Calcification occurs in the sheaths of single cells (including baeocytes) as well as in mature colonies. The specificity for calcification in Pleurocapsa sp. is discussed.
A simulation model of alluvial stratigraphy
Authors: BRIDGE JOHN S. , LEEDER MICHAEL R. 
Published: June 2006

ABSTRACTThe quantitative model presented simulates the development of a two‐dimensional alluvial sedimentary succession beneath a floodplain traversed by a single major river. Several inter‐related effects which influence the distribution of channel‐belt sand and gravel bodies within overbank fines are accounted for. These are (a) laterally variable aggradation, (b) compaction of fine sediment, (c) tectonic movement at floodplain margins, and (d) channel avulsion. Selected experiments with the model show how the interconnectedness and areal density of channel‐belt deposits decrease with increasing floodplain width/channel‐belt size, mean avulsion period, and channel‐belt aggradation rate. Separation of stream patterns based on interconnectedness and channel deposit density is difficult. Tectonic movements do not have a significant influence upon the successions unless a preferred direction of tilting is maintained (half‐graben). Then channel‐belt deposits showing offlap tendencies tend to cluster adjacent to the active floodplain margin, leaving dominantly fine‐grained alluvium to accumulate on the inactive side. Individual channel‐belt deposits thicken during aggradation, although a self‐regulating limit to such thickening is likely to operate. ‘Multistorey’features resulting from aggradation may be difficult to tell apart from those arising through superposition of distinct channel‐belt deposits of avulsive origin.
Mechanisms to explain the loss of heavy minerals from the Upper Palaeozoic tillites of South Africa and Australia and the late Precambrian tillites of Australia
Authors: GRAVENOR C. P. , GOSTIN† V. A. 
Published: June 2006

ABSTRACTHeavy mineral studies on Pleistocene tills from North America, Upper Palaeozoic tillites of South Africa and Australia, and late Precambrian tillites of South Australia show that the heavy mineral suites of the Pleistocene tills are dominated by amphiboles, the Upper Palaeozoic tillites by garnet, and the late Precambrian tillites by zircon and tourmaline. About half of the garnets in the Upper Palaeozoic tillites show evidence of having been rounded, and retain delicate surface chattermark trails, which indicates that these garnets have not undergone chemical attack since deposition. Although the remainder of the garnets show, by way of etching, that intrastratal solutions were active in the sediments, it is suggested that amphiboles, pyroxenes and epidote, which must have been present in the original Upper Palaeozoic heavy mineral suites, were lost primarily by the action of sorting and mechanical abrasion in beach environments prior to, and during interglacial periods. The absence of garnet and the etching of tourmaline and zircon in the late Precambrian tillites is attributed to the action of alkaline intrastratal solutions over the long time interval during which the tillites were buried in the Adelaide Geosyncline.
Changes in carbon and oxygen isotope composition during limestone diagenesis
Authors: DICKSON J. A. D. , COLEMAN M. L. 
Published: June 2006

ABSTRACTThe calcite fossils of the Derbyhaven Beds, Isle of Man, have δ13C values (+ 1·8 PDB) similar to modern, shallow‐water marine skeletons, but the δ18O values (−6·1 PDB) are much lighter than modern skeletons. The light oxygen values indicate either re‐equilibration with isotopically light water before cementation started, or Carboniferous sea water with δ18O of −6‰. Aragonite dissolution was followed by precipitation of zoned calcite cement. In this cement, up to six intracrystalline zones, recognized in stained thin sections, show isotopic variation. Carbon varies from + 3‐8 to + 1‐2‰. and oxygen from − 2‐6 to − 12‐4‰. with decreasing age of the cement. This trend is attributed to increasing temperature and to isotopic evolution of the pore waters during burial. The zoned calcite is sequentially followed by dolomite and kaolinite cements which continue the trend towards light isotopic values. This trend is continued with younger, fault‐controlled dolomite, and is terminated by vein‐filling calcite and dolomite. The younger calcite, interpreted as a near‐surface precipitate from meteoric waters, is unrelated to the older sequence of carbonates and has distinctly different carbon isotope ratios: δ13C − 6‐8‰.
Carbonate deposition and facies distribution in a central Michigan marl lake
Authors: MURPHY DAVID H. , WILKINSON BRUCE H. 
Published: June 2006

ABSTRACTRelatively pure lacustrine carbonates referred to as marl are being deposited in Littlefield Lake, central Michigan, a hard‐water lake with little terrigenous clastic influx. Thick accumulations of marl form both progradational marl benches along lake margins, and islands or lakemounts in the lake centre. Marl benches develop flat platforms up to 20 m wide in very shallow water and steeply inclined slopes, up to 30°, extending into deep water. The flat landward platform is frequently covered by algal pisoliths while the upper portion of the lakeward‐sloping bottom is overgrown by Chara which in the summertime becomes thickly encrusted with low‐magnesian calcite. Marl islands are flat‐topped features that formed over relict highs on Pleistocene drift which underlies the lake basin. These are fringed by marl benches identical to those found along lake margins. Marl benches are composed of four units: two thin facies deposited on the shallow‐water bench platform and two thicker faces deposited on the bench slope developed in moderate water depths. These in turn overlie a fifth facies deposited in deep water. A coarsening‐upward sequence is developed in these sediments as a result of both mechanical sorting, and primary production of carbonate sand and gravel in shallow water. In addition to facies sequences and size grading, trends upsection of increasing carbonate content and decreasing insoluble content may serve to identify temperate‐region lacustrine carbonate deposits in the rock record.
Penecontemporaneous folding, sedimentation and erosion in Campanian Chalk near Portsmouth, England
Authors: GALE A. S. 
Published: June 2006

ABSTRACTCampanian (Upper Cretaceous) Chalk exposed in a quarry near Portsmouth is unusual in the occurrence of a contemporaneous anticlinal fold, probably diapiric in origin. Hardgrounds and local slumps developed over the structure, and part of the chalk succession thins considerably over its crest. Erosional channels, sometimes containing bioclastic lag deposits, formed at two levels. Movement on the anticline occurred three times during the part of the Lower Campanian represented in the section; the third phase resulted in extensive fracturing of the crestal region of the fold. Burial of the structure and the onset of normal chalk deposition concluded the Cretaceous phase of its history.
Suitability of monosaccharides as markers for particle identification in carbonate sediments*†
Authors: BÖHM LOTHAR , DAWSON RODGER , LIEBEZEIT GERD , WEFER GEROLD 
Published: June 2006

ABSTRACTThe identification of the remains of organisms contributing to carbonate sediments by means of scanning electron‐microscopy is limited to particles of the 2–20 μm size class. Mineralogy and the content of Mg, Sr and trace elements alone are usually insufficient to solve the problem of identification, especially in the differentiation between algal and coral aragonite. The organic matrix of calcareous organisms consists of stable biopolymers such as polysaccharides and glycoproteins which are intimately associated with the carbonate skeleton. Analysis of these hydrolysed compounds gives rise to characteristic arrays of monosaccharides which provide independent criteria for producer identification. The calcareous green algae Halimeda, Penicillus and Udotea show high xylose and low fucose levels. Xylose and fucose levels are elevated in the red algae Amphiroa but only fucose is prominent in the brown algae Padina. The corals Oculina, Porites, Millipora and Montastrea are relatively rich in fucose and show little or no xylose. In the bivalves Arca, Codakia and in Argopecten mannose may be characteristic. Analysis of artificial and natural sediments demonstrates that coral and algal aragonite can be distinguished on the basis of the total sugar concentration and respective xylose and fucose levels. The applicability of the technique in comparison to geochemical and mineralogical methods has been demonstrated for surface sediments from varying water depths of Harrington Sound, Bermuda.
Early diagenetic, Na‐alunites in Miocene algal mat intertidal facies, Ras Sudar, Sinai
Authors: GOLDBERY RON 
Published: June 2006

ABSTRACTAn early diagenetic non‐hydrothermal deposit of Na‐alunite [(Na,K)Al3(SO4)2(OH)6] is described from the Miocene Ras Maalab Group at Ras Sudar Sinai. The alunite is nodular and occurs along two horizons parallel to bedding. Displacive growth structures and disrupted bedding within the host rock, together with enterolithic growth structures within the nodules, indicate a period of growth prior to compaction. Linear arrays of fabric elements within the nodules, observed under the microscope, further support early diagenetic growth. The host sequence, believed to be the product of sedimentation within barred coastal lagoons has been subdivided into two lithofacies, one reflecting a subtidal open‐lagoonal environment, the other an intertidal restricted‐circulation environment. Anaerobic conditions of early diagenesis conducive to generation of H2S and FeS2 preceded an oxidation event produced either by contact with free‐circulating oxygenated waters or by emergence. The sulphuric acid generated lowered the pH and clay minerals were converted to alunite.
Basin‐wide turbidites in a Miocene, over‐supplied deep‐sea plain: a geometrical analysis
Authors: LUCCHI FRANCO RICCI , VALMORI EMIDIO 
Published: June 2006

ABSTRACTEighteen stratigraphic sections, 200 m thick on average, were logged in basin plain deposits of the Marnoso‐arenacea Formation (Miocene, northern Apennines) over an area of 123 × 27 km. Turbidites form 80–90% of the facies association, hemipelagites the remainder. Thin and thick‐bedded turbidites are separated by an approximate statistical boundary at 40 cm; most prominent beds (> 1 m thick) are qualified as megaturbidites. With reference to the main supply‐dispersal system (NW to SE), the basin plain can be axially subdivided into proximal, intermediate and distal segments by means of the following parameters: bulk sand content, sand/shale ratio in turbidites, mean thickness of individual layers and component beds, and frequency of thick layers. Almost 40% of thick‐bedded turbidites can be traced over the whole study area. These basin‐wide deposits form the bulk of the basin fill. Geometrical reconstruction shows that some sandstone beds taper downcurrent from the proximal plain or the adjacent fan area while others thin upcurrent suggesting sand by pass of the fan. Mudstone beds in general thicken towards the end and the margins of the plain indicating that turbidite mud, besides bypassing the fan as a rule, was affected by ponding in the plain. Thin‐bedded turbidites have a low sand/shale ratio or are completely muddy representing either tails of sandier turbidites of the outer fan (lobe and fringe deposits) or sheets extending to a great part of or to the whole plain. Sandstone lobes advanced from fans into the plain for 40–50 km gradually thinning and shaling out over a transitional zone of 10–20 km. Their internal geometry shows simple and complex growth patterns: end members are defined as progradational and aggradational. Estimates of original length, width and volume of individual turbidites strongly suggest that flows were usually confined and deflected by basin slopes regardless of source location. Basinal deposits are thus characterized by great thickness and volume, abundance of mud and fine sand, extremely low lateral gradients of thickness and grain size (but rapid wedging near the sides). The basin plain developed as a part of an elongated, oversupplied basin with a ‘highly efficient’, probably delta‐fed, dispersal system.