bluuebunny/crossref_metadata_2025_split huggingface dataset:
Fluvial channel and overbank deposits from the Westphalian of the Durham coalfield, NE England
Authors: FIELDING C. R. 
Published: June 2006

ABSTRACTThe Westphalian (Upper Carboniferous) Coal Measures of the Durham coalfield in NE England were deposited in lower and upper delta plain environments. Distributary channels crossed the plain and were separated by shallow, interdistributary lakes and bays. Detailed observation of three‐dimensional (3‐D) opencast (surface) mine exposures, in collaboration with subsurface borehole analysis, has revealed the existence of five varieties of channel deposits and two associated overbank facies within the Durham Coal Measures.Major distributary channels were the major avenues of sediment transport across the Coal Measures plain, were variably sinuous, mostly 1–2 km wide and deposited elongate belts of sand mostly up to 5 km wide. Proximal, major crevasse splay channels formed by the breaching of major channel banks during flood events, were straight, sand‐filled and up to 400 m wide. Minor distributary channels formed by the sustained operation of such crevasses, varied from straight to highly sinuous, and deposited ‘shoestring’ sand/mud belts up to a few hundred metres wide. Minor crevasse channels, generally straight and up to 50 m wide, were formed through bank breaching of minor, and in a few cases major, distributaries. Distal feeder channels formed down‐palaeocurrent extensions of minor distributaries which supplied interdistributary minor delta subsystems, were generally straight and up to 200 m wide.Of the two types of channel overbank (levee) deposits recognized, one, comprising thinly interbedded fine‐grained sandstone and siltstone/claystone, is mostly, though not exclusively, associated with major distributary channels. The other, consisting of ‘massive’ siltstones with regularly spaced, thin claystone bands, is uniquely developed at the margins of minor distributary channels.The lower part of the Westphalian A succession in the northern Pennines records a change in the depositional environment upwards from a lower to upper delta plain. Through this transition, major channel deposits show evidence of having evolved from being of dominantly low sinuosity to being more variable in morphology. Channel sedimentation was profoundly influenced by regular, possibly seasonal, variations in flow stage and sediment load.
Facetted garnets formed by etching. Examples from sandstones of Late Triassic age, South Germany
Authors: BORG GREGOR 
Published: June 2006

ABSTRACTDetrital garnets from the Upper Triassic (Keuper) of southern Germany have well developed facets where they occur in calcite‐cemented sandstone. The grains belong to the pyralspite series and have a homogeneous chemical composition, without any zonation is revealed by microprobe analyses. It is concluded that the large facets are caused by etching during diagenesis and not by authigenic overgrowth. High calcium contents of the pore fluids may have been important in the formation of these garnet facets.
Numerical computation of co‐oscillating palaeotides in the Catskill epeiric Sea of eastern North America
Authors: SLINGERLAND RUDY 
Published: June 2006

ABSTRACTA numerical model describing two‐dimensional, shallow water, long wave propagation has provided estimates of the co‐oscillating palaeotides in the upper Devonian Catskill Sea. This sea was open to the ocean along the present Gulf Coast and, moving clockwise, was circumscribed by the Transcontinental Arch, the Old Red Sandstone Continent, and the rising Appalachian Orogen. Using the present best estimates of basin bathymetry and open‐ocean tidal range, the model indicates high mesotidal to low macrotidal ranges for the Catskill Shelf along the ancestral Appalachians. The Mid‐Continent Shelf, centred on Iowa, is considered to have been microtidal. These conclusions are not sensitive to reasonable variations in shelf geometry and support the hypothesis that at least some epicontinental seas could have been tide‐dominated.
The role of fungal biomineralization in the formation of Early Carboniferous soil fabrics
Authors: WRIGHT V. PAUL 
Published: June 2006

ABSTRACTPaleosols in the Lower Carboniferous limestones of South Wales commonly contain needle‐fibre calcite which is an unusual form of calcite recently shown to form by the calcification of fungal hyphae in present day soils. The needle‐fibre calcite occurs in two associations in the paleosols: as coatings on sediment grains and as rhizocretions. The former can be compared with the microbial grain coatings of Quaternary calcretes. The latter represent the sites of fungal coats on roots and are interpreted as probable ectomycorrhizae, a symbiotic fungal sheath‐root association. These findings suggest that biomineralization was important in the formation of soil fabrics during the Carboniferous as it is in present day soils.
Push‐moraines and glacier‐contact fans in marine and terrestrial environments
Authors: BOULTON G. S. 
Published: June 2006

ABSTRACT The local climatic regime and the mass balance state are important determinants of the dynamics of terrestrial and marine glacier fronts, which in turn determine the sediments and landforms produced at the glacier front. Many modern glaciers undergoing overall retreat in areas of‘maritime’climate produce winter push moraines during a late winter readvance, followed by a summer retreat, whilst in more‘continental’regions no significant winter readvance occurs and annual push‐moraines are absent. The frontal dynamics which lead to these changes are analysed and the form, structure, sequence and field relations of both terrestrial and marine push‐moraines are described from Iceland, Spitsbergen and Baffin Island.Long‐term changes in mass balance leading to major glacier advances or readvances also generate large push‐moraines. In terrestrial environments push‐moraine formation is accompanied by uplift, rejuvenation and down‐cutting of outwash systems whose sediments become closely associated with glaciotectonic structures, which permit pre‐, syn‐ and post‐tectonic sequences to be identified.The development of ice marginal fan/moraine complexes is modelled as a function of the relative magnitude of two parameters: the velocity of ice movement and the calving rate. A high ice velocity just exceeded by the calving rate gives closely spaced push‐moraines and confluent ice marginal fans. A high velocity far exceeded by the calving rate produces closely spaced moraines but separate ice marginal fans. A low ice velocity in combination with a high calving rate results in well separated and feebly developed push‐moraines, while a low ice velocity and a low calving rate produces feeble push‐moraines and coalescent fans.
Diagenesis of Tertiary playa sandstones of Northern Chile; implications for Andean uplift and metallogeny
Authors: FLINT S. 
Published: June 2006

ABSTRACTThe Oligo‐Miocene Pacencia Group of Antofagasta Province, northern Chile consists of over 2 km of molassic sediments deposited in a series of alluvial fan and playa sub‐environments. Sandstones of the Artolla Member, exposed around San Bartolo, represent playa‐marginal sandflat deposits and locally host stratiform copper mineralization. A detailed study of these sediments has revealed a complex sequence of diagenetic modifications to the arkosic detrital mineral assemblage. Early dissolution of unstable ferro‐magnesian minerals and calcic feldspar resulted in the release of ions into intra‐stratal solution and the subsequent formation of a suite of eogenetic authigenic minerals.These include early hematite coatings on framework grains, pore‐lining zeolite and pore‐filling calcite, gypsum and celestite cements. Syntaxial overgrowths of albite, quartz and less common K‐feldspar are well developed. Following early oxidative intrastratal conditions, the influx of acidic fluids derived from mudrocks during mesogenesis resulted in an important secondary porosity generation and the establishment of an acidic, reducing intrastratal environment. This facilitated the subsequent introduction of low temperature mineralizing solutions and formation of the San Bartolo copper deposit.A return to oxidative diagenetic conditions is recorded by late stage overgrowths of K‐feldspar and quartz and the oxidation of native copper/chalcopyrite cements to cuprite/tenorite and chalcocite/covellite assemblages. This final telogenetic modification to the mineral assemblage is related to flushing by meteoric waters and provides independent evidence for an important phase of Andean deformation and uplift in the middle to upper Miocene.
Proterozoic ministromatolites with radial‐fibrous fabric
Authors: HOFMANN HANS J. , JACKSON GARTH D. 
Published: June 2006

ABSTRACTSmall digitate stromatolites with diameters in the range of 0·2–20 mm (ministromatolites) are common in Early and Middle Proterozoic carbonate sequences, and extend stratigraphically from the Archaean to the Holocene. An occurrence of columnar and stratiform types exhibiting a primary or early diagenetic radial‐fibrous fabric and microscopically crinkled (microcrenate) lamination is described from the ˜ 1·9 Gyr old Belcher Supergroup (McLeary Formation) in southeastern Hudson Bay, Canada. The structures, which can be considered to be a variety of tufa, are unusually well preserved because of early diagenetic silicification. Columnar types are referable to Pseudogymnosolen (Asperia), and are morphologically similar to other occurrences of these taxa in coeval dolostones in northwestern and eastern Canada, where the fabric is normally preserved by a secondary mosaic of dolomite. The textural evidence of angulate cross‐sections and rectilinear divergent patterns indicates that the radial‐fibrous fabric represents primary or very early diagenetic precipitation, and that pseudogymnosolenids with mosaic dolomite originally also had radial‐fibrous structure. The precipitation may have been within, or on, microbial mats.
Calcium‐poor dolomite from the sabkhas of Kuwait
Authors: GUNATILAKA A. , SALEH A. , AL‐TEMEEMI A. , NASSAR N. 
Published: June 2006

ABSTRACTSmall quantities of a rare Ca‐deficient dolomite have been found coexisting in thin layers and alternating with the normal Ca‐rich variety in the Holocene sabkhas of southern Kuwait. The Ca‐poor dolomite has a molar composition of Ca46–49, whereas the Ca‐rich type is Ca51–56. The former type has been found only in two localities of the supratidal zone that are not subjected to tidal flooding today, and mostly within the fluctuating ground‐water table in these zones. The thin layers are either carbonate mud‐rich or pellet‐rich, and the amount of Ca‐poor dolomite is never more than 4 to 6 per cent by weight of the total carbonate fraction. The dolomites vary in size between 2 μm and 5 μm and exhibit characteristic rhombic crystal morphologies.
Initiation of motion in overland flow
Authors: GOVERS G. 
Published: June 2006

ABSTRACTThis article reports the results of an experimental study concerning the initial movement of sediment in overland flow. Experiments were carried out in a small flume using water and quartz materials of various grain sizes.The comparison of our experimental results with classic criteria showed that the Shields‐curve and the criterion of Yang can also be used for turbulent overland flow. Generally, higher shear stresses are necessary to provoke bed‐instability under laminar flow conditions, especially at higher shear‐velocity particle Reynolds numbers. A relative roughness effect also exists in laminar flow. Contrary to the observations by Savat on fixed beds, laminar flow does not show an ‘uncommon selectivity’, when loose, homogeneous beds are considered.
Coarse‐grained sediment gravity flow facies in a large supraglacial lake
Authors: EYLES NICHOLAS , CLARK BRYAN M. , CLAGUE JOHN J. 
Published: June 2006

ABSTRACTNear Williams Lake, in the central interior of British Columbia, the Fraser River exposes long sections of late Pleistocene glaciolacustrine sediments selectively preserved within a bedrock trough. The dominant facies types are thick, normally graded gravels and sands that occupy steeply dipping multistorey channels up to 300 m wide and several tens of metres deep. Channels appear to have been simultaneously cut and filled by high density turbidity currents in a glacial lake floored by stagnant ice. Fining upward sediment gravity flow sequences up to 50 m thick may be the product of quasi‐continuous ‘surging’ turbidity flows triggered by catastrophic meltwater discharges into the trough or retrogressive failure of ice‐cored sediments. Large‐scale post‐depositional deformation structures, such as synclinal folds, normal faults, sedimentary dyke swarms and dewatering structures, record gravitational foundering of sediment and pore‐water expulsion caused by the melt of underlying glacier ice. Melting of buried ice masses along the floor of the trough appears to have controlled the flow paths of turbidity currents by producing sub‐basins within the overlying sediment pile. An idealized model of ‘supraglacial’ lacustrine sedimentation is developed that may be applicable to other glaciated areas with similar bedrock topography.
Subtidal occurrences of ladder‐back ripples: their significance in palaeo‐environmental reconstruction
Authors: REDDERING J. S. V. 
Published: June 2006

ABSTRACTLadder‐back ripples are common on modern intertidal flats and have been thought to be diagnostic of deposition in this environment. Recognition of superimposed, asymmetric ladder‐back ripples in a subtidal back‐barrier estuary and superimposed, symmetric ladder‐back ripples in an embayed, shallow marine environment indicates that these features are not diagnostic of deposition in the intertidal zone only, but can occur in other depositional environments, produced by different mechanisms and on different scales. The presence of ladder‐back ripples in the subtidal zones challenges aspects of a model used to determine the palaeotidal range of ancient tidal sequences.
Hummocky cross‐stratification, tropical hurricanes and intense winter storms
Authors: SWIFT DONALD J. P. , NUMMEDAL DAG 
Published: June 2006

ABSTRACTDuke (1985b) argues that ‘most examples (of hummocky cross‐stratification) were formed by tropical hurricanes.’ His statement is based on the assumption that ‘hurricane‐generated surface gravity waves form powerful oscillating or multidirectional flows at the water‐sediment interface which do not possess a significant unidirectional component.’ It is true, as one of us has previously stated, that hurricanes are rapidly‐moving, short‐lived, localized, and infrequent systems as compared with mid‐latitude storms; midlatitude storms are consequently more efficient in coupling with the shelf water‐column than are hurricanes. However, Duke's argument that hummocky cross‐stratification may be the result of purely oscillatory flow is untenable. His reasoning contradicts established theory about oscillatory bedforms, and his numerous examples of hummocky cross‐stratification come largely from continental shelf settings where the storms (tropical or otherwise) would have created concurrent alongshelf undirectional flow as well as wave oscillatory motion. There is no theoretical or observational basis for the belief that water movement on the sea‐floor during hurricanes is qualitatively different from water movement during mid‐latitude storms. Consequently, hummocks are no more liable to form beneath hurricanes than they are beneath mid‐latitude storms.
The carbonate compaction law: a new tool
Authors: RICKEN WERNER 
Published: June 2006

ABSTRACTCarbonate content, compaction, and porosity are evaluated from a large number of samples using micritic marl‐limestone alternations from Germany, France, and Italy. Compaction is measured directly by utilizing both deformed, originally cylindrical bioturbation tubes and steinkerns of ammonites. Additionally, a method is developed to indirectly evaluate the total compaction of the rock matrix by using early, selectively cemented burrows and fossils. Plots representing measurements of compaction versus carbonate content display distinctly non‐linear relationships which show increasing compaction with decreasing carbonate content. These relationships are found to clearly correspond with the carbonate compaction law. The compaction law is introduced as a theoretical derivation for sediments and rocks which calculates the carbonate and non‐carbonate content, compaction, and porosity. It is based on the assumption that the non‐carbonate fraction standardized to the primary sediment‐volume remains constant during carbonate diagenesis. The compaction law is used to calculate the most commonly found sediment/rock transformations (e.g. mechanical compaction, cementation, and chemical compaction) and to simulate the diagenetic processes of given examples.Decompaction formulas are developed to evaluate the porosity and carbonate content of the primary sediment. An example of calculating decompaction and determining the original composition of the sediment is given utilizing carbonates spanning the Cretaceous‐Tertiary boundary at Gubbio (Italy).
Chert occurrences in carbonate turbidites: examples from the Upper Jurassic of the Betic Mountains (southern Spain)
Authors: BUSTILLO M. A. , RUIZ‐ORTIZ P. A. 
Published: June 2006

ABSTRACTIn Upper Jurassic carbonate turbidites of the Betic mountains (southern Spain), chert occurs in three morphologies: bedded chert, nodular chert and mottled chert. The last refers to a weak dispersed and selective silification which gives a speckled appearance to the rock. The three types of chert are formed by replacement of limestones and are associated with different calcareous facies. Turbidite packstones of Saccocoma and peloids, and turbidite lime mudstones of pelagic material contain bedded and nodular cherts. The silicification textures are mainly micro‐ and cryptocrystalline quartz, with local chalcedonic quartz (both length‐fast and length‐slow) which is more common in the packstones. Only mottled chert is produced where calcareous breccia beds are silicified. Mottled chert consists of micro‐ and cryptocrystalline quartz, length‐slow chalcedonic quartz and mosaics or individual crystals of euhedral megaquartz. Beds and nodules are the result of early diagenetic silicification, with silica derived from the calcitization and dissolution of radiolarians and, subordinately, sponge spicules, whereas mottled chert is the consequence of later silicification in a probably Mg‐rich environment.Early silicification is mainly confined to turbidite beds and only rarely occurs in the interbedded pelagic limestones. Turbidite sedimentation favours silicification because rapid burial of the transported siliceous tests prevents silica from the dissolution of tests passing into overlying sea water. A silica‐rich interstitial fluid develops in the turbidite layer and this migrates to more permeable zones giving rise to bedded and nodular chert.
Authigenic quartz and albite in Devonian limestones: origin and significance
Authors: MOLENAAR N. , de JONG A. F. M. 
Published: June 2006

ABSTRACTEuhedral quartz and albite crystals are common in Devonian (Givetian‐Frasnian) shallow‐marine shelf carbonates from the Belgian Ardennes. Several features such as morphology, the presence of carbonate inclusions, inversion temperatures and occurrence in the insoluble residues of stylolitic surfaces indicate that these crystals have developed authigenically. Oxygen isotope ratios point to an intermediate deep burial realm of origin at temperatures of 60–90°C. The predominance of illite and the almost total absence of smectite clay minerals is interpreted as an indication that illitization produced the silica needed for authigenesis. The mineral composition of inclusions indicates that the carbonate host rock must have consisted of low‐Mg calcite already at the time of authigenesis. These inclusions represent an earlier diagenetic stage than the present carbonate rock, since they were protected from further diagenetic alteration by the surrounding quartz. The calcite inclusions display a higher Sr/Ca and Mg/Ca ratio than the carbonate host‐rock. Because neomorphic diagenesis of the carbonate continued after silicate authigenesis, the contents of Mg and Sr in the calcite of the host carbonate are even lower. The authigenic feldspar mineralogy seems to be determined by the composition of the host‐sediment.
Sedimentology of an ancient erg margin: the Lower Jurassic Aztec Sandstone, southern Nevada and southern California
Authors: PORTER M. L. 
Published: June 2006

ABSTRACTThe Lower Jurassic Aztec Sandstone is an aeolian‐deposited quartzose sandstone that represents the western margin of the southerly‐migrating Navajo‐Nugget sand sea (or erg). Vertical and lateral facies relations suggest that the erg margin encroached upon volcanic highlands, alluvial fan, wadi and sabkha environments.In southern Nevada, 700 m thick facies successions record the arrival of the Aztec sand sea. Initial erg sedimentation in the Valley of Fire consists of lenticular or tongue‐shaped aeolian sand bodies interstratified with fluvially‐deposited coarse sandstone and mudstone. Above, evaporite‐rich fine sandstone and mudstone are overlain by thick, cross‐stratified aeolian sandstone that shows an upsection increase in set thickness. The lithofacies succession represents aeolian sand sheets and small dunes that migrated over a siliciclastic sabkha traversed by ephemeral wadis. These deposits were ultimately buried by large dunes and draas of the erg. In the Spring Mountains, a similar facies succession also contains thin, lenticular volcaniclastic conglomerate and sandstone. These sediments represent the distal margin of an alluvial fan complex sourced from the west.Thin aeolian sequences are interbedded with volcanic flow rocks, ash‐flow tuffs, debris flows, and fluvial deposits in the Mojave Desert of southern California. These aeolian strata represent erg migration up the eastern flanks of a magmatic arc. The westward diminution of aeolian‐deposited units may reflect incomplete erg migration, thin accumulation of aeolian sediment succeptible to erosion, and stratigraphic dilution by arc‐derived sediment.A two‐part division of the Aztec erg is suggested by lithofacies associations, the size and geometry of aeolian cross‐strata, and sediment dispersal data. The leading or downwind margin of the erg, here termed the fore‐erg, is represented by a 10–100 m thick succession of isolated pods, lenses, and tongues of aeolian‐deposited sediment encased in fluvial and sabkha deposits. Continued sand‐sea migration brought large dunes and draas of the erg interior into the study area; these 150–500 m thick central‐erg sediments buried the fore‐erg deposits. The trailing, upwind margin of the erg is represented by back‐erg deposits in northern Utah and Wyoming.