bluuebunny/crossref_metadata_2025_split huggingface dataset:
Diagenetic processes in Holocene carbonate sediments: Florida Bay mudbanks and islands
Authors: BURNS STEPHEN J. , SWART PETER K. 
Published: June 2006

ABSTRACTThe sedimentology, mineralogy and pore fluid chemistry of seven cores from the Holocene sediments of Florida Bay were studied to determine the physical processes and diagenetic reactions affecting the sediments. The cores were taken in a transect from a shallow mudbank onto a small adjacent island, Jimmy Key. Steady state models of pore fluid chemistry are used to estimate the rates of various reactions.In the mudbank sediments, little carbonate mineral diagenesis is taking place. No change in sediment mineralogy is detectable and pore water profiles of Ca2+, Mg2+ and Sr2+ show only minor variation. Chloride concentrations indicate substantial biological mixing of seawater from the bay into the sediments in one of the cores. Pore water analyses of sulphate and alkalinity show only a low degree of sulphate depletion and a decreasing extent of sulphate reduction downcore. Models of sulphate reduction in the mudbank show that there is substantial chemical exchange between the sediment pore fluids and water from the bay probably as a result of bio‐irrigation. The sulphate and alkalinity data also suggest that the underlying Pleistocene rocks contain water of near normal seawater composition.Stratigraphic analysis and δ13C analyses of the organic carbon in the sediments of the island cores show that the sediments were primarily deposited in a subtidal mudbank setting; only the upper 20–30 cm is supratidal in origin. Nevertheless, island formation had a significant effect on pore fluid chemistry and the types of diagenetic reactions throughout the sediment column. Chloride in the sediment pore fluids is more than twice the normal seawater concentrations over most of the depth of the cores. The constant, elevated chloride concentrations indicate that hypersaline fluids which formed in ponds on the island are advected downward through the sediments. Models of the chloride profiles yield an estimate of 2·5 cm yr−1 as a minimum advective velocity. Changes in pore water chemistry with depth are interpreted as indicating the following sequence of reactions: (1) minor high‐Mg calcite dissolution and low‐Mg calcite precipitation, from 0 to 35 cm; (2) Ca‐ or Mg‐sulphate dissolution and low‐Mg calcite precipitation, from 5 to 35 cm; (3) dolomite or magnesite precipitation together with sulphate reduction, from 35 to 55 cm; and (4) little reaction below 55 cm. In addition, one or more as yet unidentified reactions must be taking place from 5 to 55 cm depth as an imbalance in possible sources and sinks of alkalinity is observed. The imbalance could be explained if chloride is not completely conservative. Despite the pore fluid chemical evidence for diagenetic reactions involving carbonate minerals, no changes in sediment mineralogy were detected in X‐ray diffraction analyses, probably because of the comparatively young age of the island.
A Holocene calcrete from North Yorkshire, England: implications for interpreting palaeoclimates using calcretes
Authors: STRONG G. E. , GILES J. R. A. , WRIGHT V. P. 
Published: June 2006

ABSTRACTGlacial gravels of Late Devensian Dimlington Stadial age (26 000–13 000 years BP) at West Tanfield, North Yorkshire, England, have been cemented by carbonate‐rich solutions to produce a strongly indurated calcrete horizon. The low‐Mg cements occur as drusy spar, needle fibres, alveolar septal structures, micrite and micropinnacles, indicative of vadose‐zone cementation. Some complex pore partition structures attributed to precipitation along meniscus films also occur. These partitions separate air‐dominated and water‐dominated microenvironments of the vadose zone. The abundance of vadose fabrics shows that the accumulation is not a groundwater calcrete. In addition, much of the carbonate appears to have been precipitated by biological mediation.Carbon and oxygen isotopic data suggest that the carbonate did not form as a result of freezing, as has been suggested for some ‘arctic’soil carbonates. The pollen history of the area since the Devensian suggests that this calcrete precipitated at low temperatures; this contrasts with widely reported occurrences of calcrete in soils of hot arid or semi‐arid regions, and suggests that palaeo‐calcretes should not be used as absolute palaeoclimatic indicators.The unusual occurrence, albeit localized, of a thick calcrete under a cool and wet climate probably reflects the well‐drained nature of the gravels, the abundance of CaCO3 as limestone clasts in the gravel and a high degree of biological activity beneath a forest cover, which created a local environment favouring carbonate precipitation.
Storm‐influenced siliciclastic and carbonate ramp deposits, the Lower Ordovician Dumugol Formation, South Korea
Authors: LEE YONG IL , KIM JEONG CHAN 
Published: June 2006

ABSTRACTThe Dumugol Formation (Lower Ordovician) in the southern part of the Baegunsan syncline, South Korea, contains mixed siliciclastic and carbonate ramp deposits. The ramp sediments were frequently influenced by storm events resulting in tempestites of sandstone‐mudstone couplets, bioclastic grainstones to packstones, flat‐pebble conglomerates, a skeletal lag layer and laminated calcisiltites. All tempestites are characterized by an erosive to sharp base, poor grading and a transitional upper boundary. The difference in lithology of tempestites appears to have been controlled by the nature of substrates and by proximality. For example, laminated calcisiltites have developed on the shallow carbonate ramp, flat‐pebble conglomerates are closely associated with nodular limestones on shallow and deep ramps, and thin skeletal lag layers from fossiliferous argillaceous sediments formed in a basinal setting. The stratigraphic succession of the Dumugol Formation represents an initial transgression followed by a regression. The vertical facies change records the transition from a shallow siliciclastic ramp to a deep carbonate ramp, to a basin, shallowing to a deep carbonate ramp, and to a shallow carbonate ramp. Storm effects are mostly well preserved in shallow to deep ramp deposits.
Groundwater dolocretes from the Upper Triassic of the Paris Basin, France: a case study of an arid, continental diagenetic facies
Authors: SPÖTL CHRISTOPH , WRIGHT V. P. 
Published: June 2006

ABSTRACTThick dolomite‐cemented horizons (dolocretes) occur within a fluvial sandstone‐mudstone sequence of Late Triassic age in the western part of the Paris Basin, France. Two types of dolomites can be distinguished: (a) nodular dolomitic beds less than a few metres thick, which formed within mottled overbank siltstones and mudstones; and (b) massive dolomite up to 16 m thick, which occurs in coarse grained channel sandstones and conglomerates.The majority of the dolomite consists of a finely crystalline groundmass of dolomicrospar and, less commonly, dolomicrite. Glaebules, irregular spar‐filled cracks, spheroidal dolomite, silicification and vuggy porosity are locally abundant in the massive dolomite. In contrast, biologically induced micromorphological features such as rhizocretions and alveolar‐septal fabrics were observed in the thin, nodular dolomite beds.The dolomite is near stoichiometric, well ordered and non‐ferroan. 18O values range from −7·7 to −0·4%o PDB and 18O values range from −5·1 to + 1·8%0 PDB and no obvious difference in the stable isotopic composition between both types of dolomites was observed. Sr isotope ratios range from 0·7101 to 0·7126 and are invariably higher than the contemporary Triassic sea water.A vadose—pedogenic origin for the thin dolocrete layers is indicated by the occurrence of rhizocretions and other biological structures. Several features, however, argue against a pedogenic origin for the massive carbonates, most notably the absence of biologically induced structures, the occurrence in coarse grained channel (and not overbank) deposits, and the great thickness. These units are thus interpreted as groundwater in origin. Phreatic calcretes of Quaternary age, widespread in inland Australia, are regarded as a modern analogue for the Triassic Paris Basin dolocretes.Petrographic observations argue in favour of primary (proto)dolomite precipitation, although early diagenetic replacement of calcite by (proto)dolomite cannot be ruled out. Strontium and carbon isotope data of early diagenetic dolocrete cements and oxygen isotope data of early diagenetic silica indicate an entirely non‐marine, continental origin for the groundwaters. The poorly ordered and non‐stoichiometric protodolomite probably underwent stabilization upon further burial resulting in a near‐stoichiometric, well ordered dolomite that clearly lacks evidence for pervasive recrystallization.
Episodic sedimentation on an early Silurian, storm‐dominated carbonate ramp, Becscie and Merrimack formations, Anticosti Island, Canada
Authors: SAMI TERRY , DESROCHERS ANDRÉ 
Published: June 2006

ABSTRACTThe 150–160 m thick lowermost Silurian (Rhuddanian) Becscie and Merrimack formations of Anticosti Island, Canada, represent continuous deposition on a shallow, open marine carbonate ramp. Several rock types are identified: (a) laminated and homogenous mudstone; (b) laminated and homogenous packstone; (c) argillaceous mudstone and packstone; (d) calcareous shale; (e) laminated calcisiltite; (f) medium‐ to fine‐grained grainstone; and (g) bio/intraclastic rudstone. These rock types are arranged into five distinct lithofacies: (LF1) calcareous mudstone‐shale; (LF2) laminated‐homogenous mudstone; (LF3) calcareous grainstone‐shale; (LF4) laminated mudstone‐grainstone; and (LF5) laminated calcisiltite‐grainstone. The sequence reflects deposition on a low‐energy, muddy, carbonate to argillaceous ramp subject to short‐lived, episodic high‐energy storms. These events produced fining‐upwards storm units 5–80 cm thick, or tempestites, consisting of: a sharp scoured base overlain by intra/bioclastic rudstone grading upwards into medium‐grained grainstone, finely laminated calcisiltite and mudstone, or shale. These are interbedded with low‐energy, fairweather mudstones and calcareous shales.Deposition progressed from a carbonate mud‐dominated ramp in the Becscie Formation to an argillaceous mud‐dominated ramp in the Merrimack Formation. Lateral tempestite proximality trends and lithofacies distribution indicate that the Anticosti Basin deepened to the south‐east into the Iapetus Ocean and shallowed towards a SW—NE‐orientated shoreline to the north‐west. Vertical tempestite proximality trends and lithofacies changes identify third‐order eustatic sea‐level changes. After an initial deepening at the base of the formation, a shallowing‐deepening event dominated the sequence. Several higher order fluctuations, defined by lithofacies and tempestite proximality trends, are superimposed on these changes. The fluctuations identified with the aid of tempestite proximality trends are of an order of magnitude higher than those identified by either lithofacies or palaeontological methods.
Flow characteristics of Metresa: an Oligocene seismoturbidite in the Dukla Unit, Polish Carpathians
Authors: SLACZKA A. , WALTON E. K. 
Published: June 2006

ABSTRACTMetresa is a thick, distinctive turburdite within the Menilite Shales in the Carpathians of south‐east Poland. It was deposited by a SE‐flowing turbidity current derived from the Silesian cordillera and can be traced for 55 km downcurrent; it has a width of at least 15 km. The current contained roughly 3 km3 of sediment and conforms to the definition of a seismoturbidite. Grain‐size analyses of samples from the nine localities where Metresa is exposed indicate average velocities decreasing from 2 m s−1 to less than 1 m s−1 over the 55 km. Massive divisions are poorly developed; instead, the turbidite, although graded, tends to be laminated throughout. Flat lamination predominates with intercalated cross‐lamination and erosion structures at various levels. Small‐scale cross‐lamination, presumably associated with ripples, occurs at some upper levels. An exceptional feature is a large‐scale wave‐like structure somewhat resembling hummocky cross‐stratification. The structures are not consistent with the inferred velocities. It is suggested that the stability fields relating structures to stream power (herein called the ‘Allen fields’) are displaced in respect to deposition from waning turbidity currents.
How round is round? A new approach to the topic ‘roundness’by Fourier grain shape analysis
Authors: DIEPENBROEK MICHAEL , BARTHOLOMÄ ALEXANDER , IBBEKEN HILLERT 
Published: June 2006

ABSTRACTNumerous methods for roundness measurement have been developed. None, however, has been generally accepted, because of conceptual and practical deficiencies. Modern image processing and Fourier grain shape analysis have eliminated the practical shortcomings, but the conceptual ones remained. Single, higher harmonics of the Fourier series, for example, fail to serve as reliable equivalents for roundness evaluation. The concept outlined in this paper recognizes three criteria for the evaluation of roundness. (1) All curvatures, convex as well as concave or plane elements, must be considered. (2) The positions of morphological elements are significant because salient parts of a particle are more easily abraded than protected ones. Consequently, the curvatures have to be weighted by their relative position on the particle. (3) Positions and curvatures of morphological elements have to be compared with the particle's ultimate abraded shape, which is assumed to be an ellipsoid. The ellipsoid reflects the aspect of form or sphericity. The distinction between sphericity and roundness is retained because there is no evidence that sphericity changes significantly during transport. The measurement is based on the outline of a particle's maximum projection plane, which is transformed to a Fourier series. Roundness data are derived from the complete amplitude spectrum. The aspect of sphericity is eliminated by subtracting the amplitude spectrum of the best approximating ellipse from the spectrum of the empirical shape. The residual amplitudes are normalized and summed. In a final step the resulting values are rescaled. This guarantees reasonable boundaries and a normal distribution of roundness values. The procedure is automated and its efficiency permits the calculation of large samples. Tests on fluvial and coastal gravel populations demonstrate that the method is sensitive to abrasional wear during all stages of roundness.
Sediment dynamics and hydrodynamics in the lower course of a river highly regulated by dams: the Ebro River
Authors: GUILLEN JORGE , PALANQUES ALBERT 
Published: June 2006

ABSTRACTThe lowest part of the Ebro River is a microtidal salt‐wedge estuary. Penetration of the salt‐wedge is largely controlled by the fluvial discharge and the morphology of the river bed, although sea level variations caused by tides and atmospheric conditions can also play significant roles. The concentration and distribution of suspended particulate matter in this part of the river and the fluvial sediment discharge are strongly influenced by the dynamics of the salt‐wedge. Damming of the river has caused sediment to be trapped in reservoirs and has regulated the fluvial discharge. Intrusion of the salt wedge has thus also been regulated. At present, sediment discharge is between 1 and 1·5 × 105 tons per year, which is less than 1% of the sediment that the Ebro River discharged into the sea before construction of the dams. This extreme reduction in sediment supply has allowed marine erosional processes to dominate in the delta.
Controls on the accumulation of coal and on the development of anastomosed fluvial systems in the Cretaceous Dakota Formation of southern Utah
Authors: KIRSCHBAUM M. A. , McCABE P. J. 
Published: June 2006

ABSTRACTAlluvial strata of the Cretaceous Dakota Formation of southern Utah are part of a transgressive systems tract associated with a foreland basin developed adjacent to the Sevier orogenic belt. These strata contain valley fill deposits, anastomosed channel systems and widespread coals. The coals constitute a relatively minor part of the Dakota Formation in terms of sediment volume, but may represent a substantial amount of the time represented by the formation. The coals are separated by clastic units up to 20 m thick.The stratigraphically lowest clastic unit of the Dakota Formation lies above an unconformity cut into Jurassic rocks. Incised valleys associated with the unconformity are up to 12 m deep. Two discrete episodes of valley fill sedimentation are recognized, including a lower sandstone unit with conglomerate layers, and an upper, discontinuous, coal‐bearing unit. After the valleys filled, the area became one of low relief where extensive mires formed. Peat accumulation was interrupted at least three times by deposition of clastic sediment derived from the west.The clastic units consist of sandstone, mudstone or heterolithic ribbon bodies, stacked tabular sandstones, and laminated mudstones, and contain minor coal beds less than 0·35 m thick. Ribbon bodies are 1–9 m thick and 15–160 m wide, have pronounced basal scours, and are filled with both lateral and vertical accretion deposits. An anastomosed channel complex is suggested by the large number of coeval channels of varying dimensions, the variation in the structure and grain size of channel fills, and the presence of abundant tabular sandstones interpreted as crevasse splays. Although some sandstone bodies have well developed lateral accretion surfaces, the overall ribbon geometry indicates that mature meandering streams were not well developed. This is in contrast to modern anastomosed systems, which are commonly thought to be a transitional morphology caused by avulsion of a meander belt to a new position on its floodplain. Rather than being a transitional channel pattern related to river avulsion, the anastomosed channels of the Dakota Formation may have formed part of a large inland delta that episodically invaded widespread mires. The mires developed during periods when clastic influx was reduced either by high rates of subsidence close to the thrust belt or by deflection of rivers by emergent thrusts.
Relations between dune generations in the Gran Desierto of Mexico
Authors: LANCASTER N. 
Published: June 2006

ABSTRACTMultiple generations or genetic units of dunes characterized by different morphologies, sand grain size and sorting, and degree of post‐depositional alteration of sediments occur in the Gran Desierto sand sea of Mexico. Dune generations are separated by super bounding surfaces. Episodic accumulation of dunes is a result of a complex response of different sediment sources to regional climatic and eustatic changes. Based on examples from three parts of the sand sea, relations between different generations of dunes suggest that there are two end‐member modes of sand sea accumulation: (1) stacking or superposition of dune generations, and (2) accumulating mosaics of dunes of different morphological units. The mode of accumulation is determined by sand supply and accumulation rate: stacking occurs where sand supply is high and/or the sand sea area is restricted by topography; accumulation of mosaics is the preferred mode of accumulation where sand supply is low.
Dolomitization of Quaternary reef limestone, Aitutaki, Cook Islands
Authors: HEIN JAMES R. , GRAY SARAH C. , RICHMOND BRUCE M. , WHITE L. D. 
Published: June 2006

ABSTRACTSix holes were drilled to depths of 30–69 m in the shallow lagoon of Aitutaki in the southern Cook Islands. One hole encountered pervasively dolomitized reef limestones at 36 m subbottom depth, which extended to the base of the drilled section at 69·3 m. This hole was drilled near the inner edge of the present barrier reef flat on the flank of a seismically defined subsurface ridge. Both the morphology and biofacies indicate that this ridge may represent an outer reef crest. Mineralogy, porosity and cementation change in concert downhole through three zones. Zone 1, 0–9 m, is composed of primary skeletal aragonite and calcite with minor void‐filling aragonite and magnesian calcite cement of marine phreatic origin. Zone 2, 9–36 m, is composed of replacement calcite and calcite cement infilling intergranular, intragranular, mouldic and vuggy porosity. Stable isotopes (mean δ18O=—5·4‰ PDB for carbonate; δD =—50‰ SMOW for fluid inclusions) support the petrographic evidence indicating that sparry calcite cements formed in predominantly freshwater. Carbon isotope values of —4·0 to —11·0‰ for calcite indicate that organic matter and seawater were the sources of carbon. Zone 3, 36–69·3 m, is composed of replacement dolostone, consisting of protodolomite with, on average, 7 mol% excess CaCO3 and broad and weak ordering X‐ray reflections at 2·41 and 2·54 A. The fine‐scale replacement of skeletal grains and freshwater void‐filling cements by dolomite did not significantly reduce porosity. Stable isotopes (mean δ18O=+2·6‰0 PDB for dolomite; maximum δD =—27‰ SMOW for fluid inclusions) and chemical composition indicate that the dolomite probably formed from seawater, although formation in the lower part of a mixed freshwater‐seawater zone, with up to 40% freshwater contribution, cannot be completely ruled out. The carbon (δ13C=2·7‰) and magnesium were derived from seawater.Low‐temperature hydrothermal iron hydroxides and associated transition metals occur in void space in several narrow stratigraphic intervals in the limestone section that was replaced by dolomite. The entire section of dolomite is also enriched in these transition metals. The metals dispersed throughout the dolostone section were introduced at the time of dolomitization by a different and later episode of hydrothermal circulation than the one(s) that produced the localized deposits near the base of the section.The primary reef framework is considered to have been deposited during several highstands of sea level. Following partial to local recrystallization of the limestone, a single episode of dolomitization occurred. Both tidal and thermal pumping drove large quantities of seawater through the porous rocks and perhaps maintained a wide mixing zone. However, the isotopic, geochemical and petrographic data do not clearly indicate the extent of seawater mixing.