bluuebunny/crossref_metadata_2025_split huggingface dataset:
Isotopic and trace element evidence for submarine lithification of hardgrounds in the Jurassic of eastern England
Authors: MARSHALL JAMES D. , ASHTON MICHAEL 
Published: June 2006

ABSTRACTGeochemical and petrographic data suggest early submarine cementation of hardgrounds from the Lincolnshire Limestone Formation, Middle Jurassic, England. The three hardgrounds, from Cowthick, Castle Bytham and Leadenham quarries, developed in tidal‐inlet, on‐barrier and lagoonal sub‐environments of a carbonate barrier‐island complex. At Cowthick early composite (acicular‐bladed) radial‐fibrous cements, which pre‐date aragonite dissolution, completely fill intergranular pore‐space at the hardground surface; away from it isopachous fringing cements decrease in thickness. Microprobe analyses demonstrate zoning within the fringes with magnesium concentrations (> 2 wt % MgCO3) higher than those in allochems or later, ferroan cement (≅0.5 wt % MgCO3, 1.7 wt % FeCO3). At Castle Bytham early granular isopachous cements, which post‐date aragonite dissolution, occur within 5 cm of the surface. At Leadenham early lithification is superficial and represented by ferruginous crusts and micritic internal sediment. Late blocky cement fills residual pore‐space in all three examples. Carbon and oxygen isotopic composition of whole‐rock samples taken at intervals away from each hardground surface demonstrate the increasing proportion of late 18O depleted cements (δ18O – 8 to – 10). Early cements must have a marine isotopic composition; different δ18O values from each hardground reflect the intensity of early lithification and exclusion of late cements at the hardened surface. There is no isotopic evidence for subaerial cement precipitation during possible emergence at Castle Bytham. Oyster samples (with δ18O, – 2.9 and δ13C, 2.4) give estimated palaeotemperatures of 22–25°C. Early cements from Cowthick are enriched in 18O and 13C (δ18O = 0 δ13C ≅ 3‰) compared to the oyster values. In conjunction with trace element data this is interpreted as evidence for high‐magnesium calcite precursor cements which underwent replacement in a system with a low water: rock ratio. The intensity of early lithification is related to depositional environment: maximum circulation of sea‐water producing the most lithified hardground (Cowthick). This is directly analogous to the formation of Recent hardgrounds.
Participation of fresh water in chert diagenesis: evidence from oxygen isotopes and boron α‐track mapping
Authors: KOLODNY YEHOSHUA , TARABOULOS ALBERT , FRIESLANDER URI 
Published: June 2006

ABSTRACTSedimentary chert breccias are the product of two stages of silicification: one which produced ‘fragments’, the other, during which the ‘matrix’ crystallized. Fragments and matrix of twenty‐five breccias from the Senonian Mishash Formation in Southern Israel were separated and analysed for δ18O in quartz. The matrix is significantly depleted in 18O in comparison with fragments (δ18O of fragments ranges between +29.4 and +34.7‰, for matrix the range is +21.1–32.6‰). Boron has been mapped in the same rocks using the 10B(n, α)7Li reaction and α‐track etching. Fragments are significantly enriched in boron in comparison to the matrix. The correspondence between boron maps and rock texture is perfect.These results when combined with Steinitz's (1977) report of evaporites and evaporite ghosts in the same rocks are interpreted as indicating two different environments of chert diagenesis: a first, normal marine to hypersaline stage during which the microquartz in fragments crystallized, and a second stage of matrix chertification which in most cases occurred when fresh water was interacting with the crystallizing sediment. These conclusions are corroborated by isotopic analysis of carbonates which coexist with the cherts, as well as stratigraphical and palaeontological analysis.
Clay orientation in some Upper Carboniferous mudrocks, its relationship to quartz content and some inferences about fissility, porosity and compactional history
Authors: CURTIS C. D. , LIPSHIE S. R. , OERTEL G. , PEARSON M. J. 
Published: June 2006

ABSTRACTThe degree of preferred orientation of clay minerals in a series of fifteen fine‐grained sediments has been precisely measured by transmitted X‐ray goniometry. This varies systematically with quartz content. By contrast it is apparently unrelated to fissility since the one fissile sample (a ‘paper shale’) occurs at the mid‐point of the sample series ranked in order of degree of preferred orientation. In this series fissility corresponds with the presence of fine‐scale lamination. Estimates of compaction strain using the theory of March (1932) and assuming random initial clay orientation are consistent with the view that these samples started life with porosities similar to those of present‐day muddy sediments. On this basis it is concluded that preferred orientation in clay‐rich sediments results almost entirely from compaction strain. The degree to which this can be attained is limited by the presence of nonplaty particles (such as quartz grains) which prevent planar fabric development in their immediate vicinity.
Petrology of recent caliche pisolites, spherulites, and speleothem deposits from central Texas
Authors: CHAFETZ HENRY S. , BUTLER JOHN C. 
Published: June 2006

ABSTRACTRecent caliche, including nodules, pisolites, crusts, internal sediment, speleothem deposits, and spherulites, has formed within the dolomitic Cretaceous Edwards Formation of central Texas. As weathering altered the host strata, rhombic crystals of calcite were precipitated concomitantly with dissolution of the dolomite, thereby forming nodules. The highly altered dolomite (i.e. pulverulite) was then removed and spar, internal sediment, and travertine accumulated in the internodular voids. Nodular masses of calcite and dolomite are the most prominent constituent of the caliche. Some of the nodules have a well developed concentric structure as well as other characteristics similar to hypersaline pisolites. Features which appear to be useful in distinguishing caliche from hypersaline pisolites are: regional geological setting, association with other caliche and palaeosoil deposits, types of fossils present, and the presence of rhombic calcite and/or bladed sparry calcite with triangular shaped cross‐section. A brick‐like calcite texture and relict aragonite rays characterize hypersaline pisolites. Incipient neomorphism of the nodules and pisolites has resulted in the development of a radial pattern of spar within these structures. Geopetal deposits of internal sediment, including terra rossa soil, inhibited spar growth in the upward direction; consequently, spar is much better developed on the undersides of pisolites. Crusts and travertine flowstone (speleothem) deposits are intimately associated with the nodular masses and internal sediment. The brecciated thin crusts and travertine flowstone are end products of the same processes. The crusts formed during times of periodic desiccation of the growing surface while the flowstone formed when water was relatively abundant. Spherulitic bodies within the caliche, commonly 1–2 mm in diameter, display a radial texture and yet are composed of single crystals of calcite. The structures are the product of neomorphosed Microcodium or Microcodium‐like globular bodies.
Texture and structure of resedimented conglomerates: examples from Książ Formation (Famennian—Tournaisian), southwestern Poland
Authors: NEMEC W. , PORĘBSKI S. J. , STEEL R. J. 
Published: June 2006

ABSTRACTThe Famennian‐Tournaisian conglomerates and sandstones of the Ksiaz Formation are interpreted as marine resedimented deposits. Matrix‐ and clast‐supported conglomerate beds are equally common, and two textural sequences (motifs) have been recognized: (I) matrix‐rich conglomerate → pebbly sandstone → sandstone, and (II) clast‐supported conglomerate → sandstone. Variation in clast type partly controls motif type, and therefore, to some extent, matrix percentage in the conglomerates generally. Grading is extremely common in both clast‐ and matrix‐supported conglomerates: inverse (19%), inverse‐to‐normal (14%) and normal (26%). The studied succession, itself part of a 4 km thick, fan delta, basin‐fill sequence, is organized into large (110–150 m) and small‐scale (5–30 m) sequences, both of which show (1) upward coarsening and thickening, (2) upward trend of sandstones and pebbly sandstone → matrix‐rich conglomerates → clast‐supported conglomerates and (3) a less clear upward tendency of massive and normally graded beds → inversely graded beds. Variation in matrix percentage in beds is therefore also partly controlled by fan processes, during the progradation of fan bodies and lobes. It is predicted that individual resedimented conglomerate beds or motifs show general downfan trends in thickness, texture and structure opposite to those evident in the vertical sequences.
A late Precambrian tidal shelf deposit, the Lower Sandfjord Formation, Finnmark, North Norway
Authors: LEVELL B. K. 
Published: June 2006

ABSTRACTThe Lower Sandfjord Formation is a 1.5 km thick late Precambrian sandstone. It is a remarkably homogeneous unit consisting largely (98%) of cross‐bedded, texturally and mineralogically mature, coarse or medium sandstone, and is interpreted as a shallow marine deposit. This interpretation is based on the maturity, the exclusively tabular bed geometry, occasional sets of herring‐bone cross‐bedding and most importantly, the abundance of sheet‐like pebble layers only 1–5 grain diameters thick and sometimes overlain by thin siltstone drapes. Various different types of compound cross‐bedding, all of which show evidence of reversing currents, are interpreted as sub‐tidal sandwaves. These sets range in thickness from 0.5 to 14 m, and in conjunction with the overall abundance of cross‐bedding probably indicate strong tidal currents. A tide‐dominated current regime is also considered essential to explain the derivation of such large quantities of sand from the contemporary coasts. It is suggested that sand transport offshore took place during the erosional transgression of abandoned delta lobes. However, the predominance of a single, easterly, mode in the palaeocurrent patterns suggests that the tidal currents were reinforced by some other current system. The predominantly unimodal palaeocurrent patterns and the coarse, sand‐rich nature of the succession, taken together with the thickness do not superficially seem likely characteristics for a shallow marine sequence. Nevertheless this study appears to demonstrate that such deposits were formed on tidal shelves in at least late Precambrian time.
Pellets, ooids, sepiolite and silica in three calcretes of the southwestern United States
Authors: HAY R. L. , WIGGINS BRIAN 
Published: June 2006

ABSTRACTPellets and ooids are widespread and locally abundant in mature calcrete profiles in the Argus Range, California; near Wickieup, Arizona; and in Kyle Canyon, Nevada. Most concentrations of pellets and ooids either overlie laminar calcrete at various levels in the calcrete profile or fill subhorizontal fractures in the petrocalcic horizon. In all three profiles the petrocalcic horizon has been thickened by the pelletal, chemically deposited fracture fillings. Pellets range from 0.02 to 8.0 mm in diameter and consist principally of micritic calcite and sepiolite. Ooid coatings are chiefly calcite and opal or calcite and sepiolite. The pellets represent small concretions, some of which grew by accretion, either in void space or by displacing adjacent sediment, and the others of which were formed by cementation of pellet‐shaped bodies of porous micrite. Ooid coatings with opal or sepiolite may have been deposited as a gel with sufficient strength for surface tension to thin the coatings over angular corners of nuclei so as to increase the roundness and sphericity of the particles. Major problems in calcrete genesis are (1) the cause of subhorizontal fractures and the mechanism for widening a fracture as sediment accumulates in it and (2) what determines the deposition of calcite, sepiolite, and opal as pellets and ooid coatings or as laminar layers.
Rhizoliths in terrestrial carbonates: classification, recognition, genesis and significance
Authors: KLAPPA COLIN F. 
Published: June 2006

ABSTRACTRhizoliths are defined as organosedimentary structures resulting in the preservation of roots of higher plants, or remains thereof, in mineral matter. They are abundant and characteristic features of Quaternary terrestrial carbonates (calcretes and aeolianites) from coastal regions of the western Mediterranean. Field and petrographic observations indicate that five basic types of rhizoliths can be recognized: (1) root moulds, which are tubular voids that outline positions of former, now decayed roots; (2) root casts, which are sediment‐ and/or cement‐filled root moulds; (3) root tubules, which are cemented cylinders around root moulds; (4) rhizocretions s.s., which are pedodiagenetic mineral accumulations (here low magnesian calcite) around living or dead plant roots; and (5) root petrifactions, which are mineral impregnations or mineral replacements of organic matter whereby anatomical features of roots have been preserved partially or totally. Apart from rhizoliths themselves, roots of higher plants are responsible for the formation of numerous and characteristic features of pedogenetically affected terrestrial carbonates. Plant roots are responsible for, or contribute to, the formation of alveolar textures, in situ brecciation (rhizobrecciation) textures, horizontal sheet cracks, vertically elongate glaebules (concretionary soil structures) and micritization (rhizomicritization) within terrestrial carbonates. Rhizoliths, together with the above features, are products of pedodiagenesis. More significantly, rhizoliths and related features are indicators of palaeosols and hence of subaerial vadose environments in ancient (post‐Silurian) successions.
Stages in a Precambrian dolomitization, Scotland: cementing versus replacement textures
Authors: FAIRCHILD IAN J. 
Published: June 2006

ABSTRACTDolomitization in member 1 of the Dalradian Bonahaven Formation of Islay was sequential. The sediments studied are siliciclastic tidal flat deposits, which originally contained pebbles of ferroan dolomicrite and sandy dolomicrite (type 1 dolomite). Following burial, both dolomicrite and clay‐stone pebbles, and possibly other carbonates, were replaced by coarser, fluid‐inclusion‐bearing, chemically zoned (type 2) dolomite. Ferroan, inclusion‐free, intergranular dolomite (type 3) in sandstones may be synchronous with late zones of type 2 dolomite which it closely resembles. Cathodoluminescence and microprobe analyses of type 2 dolomite characterize four ubiquitous growth stages within which there was considerable fluctuation in Fe and Mn content. There was much variation between and within samples in terms of timing of nucleation, absolute speed of growth and relative growth rate of crystals. Type 2 dolomite displays two types of intracrystalline truncation surfaces explicable by dissolution followed by overgrowth, or by replacement by a moving solution film. Type A is characterized by a single later zone cutting across earlier ones along the line of an irregular truncation surface which developed from the edges of the crystals. In type B truncations it is the interior, rather than the edges of crystals which show truncation, the truncations forming smooth closed surfaces which enclose areas filled centripetally with new zones. In general, dolomite has a pronounced tendency to form crystal faces, and thus compromise crystal boundaries, during replacement as well as during cementation. Thus, if nucleation of crystals is restricted to the margins of an area to be replaced, replacive dolomite is capable of completely mimicking a vug‐filling texture.
Quaternary pedogenic calcretes from the Kalahari (southern Africa): mineralogy, genesis and diagenesis
Authors: WATTS N. L. 
Published: June 2006

ABSTRACTThe calcretes of the Kalahari are amongst the thickest in the world representing pedogenic episodes in a semi‐arid climate during Pliocene to Recent times. The descriptive morphological terminology of Netterberg is used to describe the calcrete types and a differentiation into simple and composite profiles is made. A pedogenic/diagenetic scheme has been constructed using all available data. Early calcite cementation is induced by two mechanisms. Slow evaporation and/or CO2 loss causes the precipitation of low‐Mg calcite, whereas rapid evaporation and/or CO2 loss precipitates predominantly high‐Mg calcite, in thermodynamic disequilibrium with the low Mg/Ca ratio vadose water, and minor aragonite (which transforms rapidly to low‐Mg calcite). High‐Mg calcite is also precipitated from high Mg/Ca ratio vadose waters in calcretes developed on Mg‐rich host lithologies and by capillary rise from shallow groundwaters in saline depressions. Calcite precipitation may be passive (cement), displacive or replacive, in the latter released silica migrating down‐profile to precipitate length‐slow chalcedony, clinoptilolite (saline conditions), length‐fast chalcedony and megaquartz (non‐saline conditions). Displacive introduction of calcite takes place from highly supersaturated solutions due to rapid evaporation (with CO2 loss) of vadose waters. During low‐Mg calcite precipitation (in a ‘closed’ system) the Mg concentration of the resulting solution increases. This, combined with Mg released during high‐Mg to low‐Mg calcite transformation, induces precipitation of authigenic palygorskite, sepiolite and minor dolomite. Vadose dolomite is often present whilst some dolomite may be precipitated at the mixing‐zone of vadose and phreatic waters. The proposed model applies to the Kalahari calcretes, although it may perhaps be extrapolated to other areas. Further detailed studies, involving analyses of pore water chemistry, soil microclimate, and trace element and isotopic analyses of individual cements are necessary.
A meander‐belt sandstone of the Lower Cretaceous of Southern England
Authors: STEWART D. J. 
Published: June 2006

ABSTRACTThe meander‐belt deposit comprises a sandstone resting on an erosion surface and bounded above and below by massive varicoloured mudstones with rootlet traces. The sandstone unit is composed of six bodies separated from one another, horizontally, by erosion surfaces; together the bodies form a single multilateral sand body. Internally each body is composed of lateral accretion units inclined at up to 6° from the horizontal. Vertical sequences of facies show significant variations but the grain size generally fines upwards. The principal lithofacies within the sandstones are, in common ascending order, intraformational conglomerate, large‐scale cross‐bedded, horizontal bedded and small‐scale cross‐laminated sandstone, and alternate sandstones and mudstones. Current directions are normal to the true slope of accretion surfaces and show insignificant scatter within individual bodies but are very diverse overall. Five of the sand bodies are believed to represent individual point bars, and one body an abandoned channel. Together they comprise the meander belt. The river was subject to very variable discharges and carried high suspended loads. Analysis of vertical profiles indicates that grain size segregation along the length of the point bars caused differentiation of the bars into coarse‐grained heads and sandy tails.
Some new aspects of two‐dimensional turbidity currents*
Authors: LÜTHI STEFAN 
Published: June 2006

ABSTRACTA theoretical consideration of two dimensional underflows and surge‐type turbidity currents results in a general momentum equation. A number of formulae in current use are special cases of this equation, among which are the modified Chézy equation and Bagnold's criterion for autosuspension. Five dimensionless parameters are included: the Richardson number Ri (defined as the inverse square of the Froude number), the friction coefficient cf, the slope β, the dimensionless settling velocity of the sediment Vs/u and the changes in flow height with distance dD/dx. The latter is mainly a measure of the dilution by entrainment of ambient water.For chalk powder experiments on surge type turbidity currents and on the initial front of continuous underflows the momentum equation is shown to be correct. Values for Ri range from about 1.5 at 0° slope to about 0.75 at 5° and are slightly to substantially lower than values from earlier authors. The two types of turbidity currents investigated show close similarity. A surprising attribute is their strong dilution even at very low‐angle slopes. Pelitic sedimentation is possible from the upper, dilute part of the currents, graded intervals found at the base of turbidites can be explained as bedload deposits from the lowermost, concentrated layer of the current; hydraulic jumps are expected to be rare in surge‐type turbidity currents and fronts of incipient underflows.
Particulate matter grain‐size characteristics and flocculation in a partially mixed estuary
Authors: KRANCK KATE 
Published: June 2006

ABSTRACTThe concentration and grain size of the natural and deflocculated inorganic suspended particulate matter were measured along the length of the Miramichi Estuary and interpreted with respect to flocculation and transport properties. Changes in particulate matter concentration are associated with regular changes in grain‐size characteristics. In the turbidity maximum region of the estuary the suspended matter occurs mostly as large flocculated particles whereas, in the waters with lower particle concentrations, a larger proportion of the material occurs as fine material. At higher concentrations natural floc modes and inorganic grain modes vary simultaneously but at low concentrations the two modes vary inversely. This modal relationship and the variation in organic matter within the estuary is proposed to result from variation in inorganic—organic composition of flocs. Increase in settling rates due to flocculation is believed to increase the trapping effect of the estuarine circulation that produces the turbidity maximum.
Comparison of methods of size analysis for sands of the Amazon‐Solimões Rivers, Brazil and Peru
Authors: SEMINAR SEDIMENTATION 
Published: June 2006

ABSTRACTCumulative frequency curves and bivariate textural plots of sands collected and sieved from the medium‐grained (1.94), well‐sorted (0.46) Amazon‐Solimões Rivers are not environmentally sensitive. Only 13 % of the samples plot as fluvial using cumulative frequency curves, and only one of the nine bivariate plots successfully identified the fluvial environment. Thus, identification of depositional environments of sands by size distribution based only on sieving is not a reliable technique—even when the results of diagenesis are not considered.