bluuebunny/crossref_metadata_2025_split huggingface dataset:
Late Proterozoic periglacial aeolian deposits on the West African Platform, Taoudeni Basin, western Mali
Authors: DEYNOUX MAX , KOCUREK GARY , PROUST JEAN NOËL 
Published: June 2006

ABSTRACTThe Late Proterozoic Bakoye 3 Formation is a predominantly aeolian unit deposited in the glacially influenced cratonic Taoudeni Basin of western Africa. The Bakoye 3 can be divided into five distal units, two proximal units, and a local upper massive sandstone. The basal Unit 1 shows a complex interfingering of aeolian and subaqueous structures, and is interpreted as the precursor of the overlying erg sequences. Unit 2 consists of compound, trough cosets of aeolian cross‐strata dominated by grain‐flow strata. The unit is interpreted to represent draas with superimposed, small, crescentic dunes. A super bounding surface marks the termination and planation of the erg. Unit 3 is distinguished from the underlying Unit 2 by its larger, overall simple sets of trough cross‐strata, interpreted to represent simple, large, crescentic dunes. Unit 4 occurs only locally in laterally discontinuous, large troughs. In one case the trough is filled by small sets of tabular cross‐strata dominated by grain‐flow deposits. At another section, wedges of coarse‐grained wind‐ripple strata fill the trough. Unit 4 may represent remnants of ergs or, more likely, local deposition in depressions. The depressions, in the latter scenario, formed with the development of a second super surface that truncates Unit 3. Unit 5 consists of very large sets of wind‐ripple cross‐strata with less common sets of grain‐flow deposits. These deposits are believed to represent enormous dunes with large plinths and subordinate slip face development. A third super surface separates Unit 5 from overlying marine deposits. Together, Units 1–5 represent the core of the ergs in a distal position relative to adjacent upland source areas. Proximally, aeolian deposits are simple, smaller, trough sets interpreted as moderate sized crescentic dunes. Coarse‐grained braided stream deposits are prominent. Locally, the top of the Bakoye 3 is marked by channelized mass‐flow deposits containing aeolian blocks, and is believed to have resulted from iceberg grounding. An overall environment for the Bakoye 3 is one of uplands marked by ice sheets, with outwash plains extending distally to aeolian ergs. Super surfaces, all marked by polygonal fractures and coarsegrained sediment, represent periods of erg termination that may be linked to glacial‐fluvial‐aeolian cycles.
Gravelly shoreface deposits: a comparison of modern and ancient facies sequences
Authors: HART B. S. , PLINT A. G. 
Published: June 2006

ABSTRACTThe morphology and dynamics of modern gravel shorefaces are poorly documented. This hinders the interpretation of possible ancient counterparts. A comparative study of a modern (Chesil Beach, England) and an ancient (Baytree Member of the Cardium Formation, Alberta) gravel shoreface shows that the two systems are very similar close to and above sea‐level, with a high (about 1 m) gravel plunge step lying below plane‐bedded sands and gravels of the beachface. The shoreface at Chesil Beach is dominated by asymmetrical gravel wave ripples. These are oriented offshore near the toe of the shoreface, and onshore in shallower depths. This may reflect offshore movement during storms and landward reworking during fair weather. The Baytree Member is over 12 m thick and comprises over 80% conglomerate. Conglomerate is decimetre‐bedded, massive or cross‐bedded, with sets over 60 cm thick produced by gravel bedforms migrating alongshore. It is interbedded with discontinuous cm‐ to dm‐bedded sandstones which may be cross‐bedded. Pebble fabric and cross‐bed orientation both indicate strong alongshore sediment transport. Near the base of the section, pebble orientations suggest that gravel wave‐ripples developed below the zone of strong longshore flows. Differences between these two examples may be attributed to different directions of wave approach.
Small‐scale hummocky cross‐stratification in turbidites: a form of antidune stratification?
Authors: PRAVE A. R. , DUKE W. L. 
Published: June 2006

ABSTRACTSmall‐scale hummocky cross‐stratification occurs in Upper Cretaceous calciclastic turbidites exposed in the western Basque Pyrenees; facies associations and microfossil assemblages indicate slope to base‐of‐slope (bathyal) depositional environments. It is developed in the fine‐grained portion of beds and displays spacings mostly between 0.2 and 0.7 m. The beds fine upward with no sharp grain size breaks or mud partings, suggesting that deposition occurred during a single flow event.Hummocky intervals are 0.1–0.8 m thick and consistently grade laterally and vertically into flat, planar laminations of Bouma B divisions suggesting that deposition occurred under upper‐flow‐regime conditions. They have wave‐like geometries with laminae continuous across ‘crests’ and ‘troughs’ and display a ratio of ‘wavelength’ to estimated underflow thickness of 11.3–12.8.Combining the above observations and inferences, these examples of small‐scale hummocky cross‐stratification are interpreted as a form of antidune stratification generated by standing waves along the interface of a thinner, denser underflow (main body/tail of the turbidity current) and an overlying thicker, low‐density layer. This occurrence is further evidence that small‐scale hummocky cross‐stratification is multigenetic and therefore not indicative of a particular flow condition or depositional environment.
The variability of critical shear stress, friction angle, and grain protrusion in water‐worked sediments
Authors: KIRCHNER JAMES W. , DIETRICH WILLIAM E. , ISEYA FUJIKO , IKEDA HIROSHI 
Published: June 2006

ABSTRACTThe erodibility of a grain on a rough bed is controlled by, among other factors, its relative projection above the mean bed, its exposure relative to upstream grains, and its friction angle. Here we report direct measurements of friction angles, grain projection and exposure, and small‐scale topographic structure on a variety of water‐worked mixed‐grain sediment surfaces. Using a simple analytical model of the force balance on individual grains, we calculate the distribution of critical shear stress for idealized spherical grains on the measured bed topography. The friction angle, projection, and exposure of single grain sizes vary widely from point to point within a given bed surface; the variability within a single surface often exceeds the difference between the mean values of disparate surfaces. As a result, the critical shear stress for a given grain size on a sediment surface is characterized by a probability distribution, rather than a single value. On a given bed, the crtitical shear stress distributions of different grain sizes have similar lower bounds, but above their lower tails they diverge rapidly, with smaller grains having substantially higher median critical shear stresses. Large numbers of fines, trapp.ed within pockets on the bed or shielded by upstream grains, are effectively lost to the flow. Our calculations suggest that critical shear stress, as conventionally measured, is defined by the most erodible grains, entrained during transient shear stress excursions associated with the turbulent flow; this implies a physical basis for the indeterminacy of initial motion. These observations suggest that transport rate/shear stress relationships may be controlled, in part, by the increasing numbers of grains that become available for entrainment as mean shear stress increases. They also suggest that bed textures and grain size distributions may be controlled, within the constraints of an imposed shear stress and sediment supply regime, by the influence of each size fraction on the erodibility of other grain sizes present on the bed.
Tectonic versus chemical‐weathering controls on the composition of fluvial sands in tropical environments
Authors: JOHNSSON MARK J. 
Published: June 2006

ABSTRACTThe compositions of fluvial sands collected in mountainous regions along the volcanic front in Central America and along the Andean front in Venezuela and Colombia correlate with the tectonic setting of their source terrains. The sands derived from the Central American volcanic arc are compositionally distinct from those derived from the Andean terrains, and minor variations in tectonic style can also be discriminated. Subtle differences in tectonic setting and source rock composition are recognizable only when modification by chemical weathering is slight; they are strongest in sands from low‐order streams draining mountainous areas with modest alluvial‐plain development. Sands that have been exposed to weathering for extended periods during alluvial storage are progressively enriched in chemically stable phases. With increased weathering, sands from continental terrains converge toward a supermature quartz arenite end member, and the imprint left by source rock composition and tectonic setting is obscured. In intraoceanic terrains, a similar but much less pronounced enrichment in stable phases relative to less‐stable phases is observed, similarly overprinting the tectonic signal. Although detailed tectonic information is easily lost, sands derived from intraoceanic terrains are distinguishable from those derived from continental terrains even after long exposure to intense chemical‐weathering conditions.
Evaporitic sediments of Early Archaean age from the Warrawoona Group, North Pole, Western Australia
Authors: BUICK ROGER , DUNLOP J. S. R. 
Published: June 2006

ABSTRACTChemical sediments are common and diverse in the c. 3500 Myr old North Pole chert‐barite unit in the Warrawoona Group, Western Australia. Although almost all original minerals were replaced during hydrothermal alteration, metamorphism and deformation, pseudomorphic relics of sedimentary and diagenetic textures and structures show that at least six lithofacies were partly or wholly chemical in origin. These contained five main chemical sedimentary components: primary carbonate mud, diagenetic carbonate crystals, primary sulphate crystals, diagenetic sulphate crystals and diagenetic sulphate nodules. All show a wide range of characteristics consistent only with a marine evaporative origin. Diagenetic carbonate and sulphate crystals, once ferroan dolomite and gypsum, were precipitated within volcanogenic lutites high on littoral mudflats. The other evaporative phases were apparently deposited behind a barrier bar composed of stranded pumice rafts. Primary sulphate crystals, once gypsum and now barite, were precipitated in semi‐permanent pools immediately behind the bar. Primary carbonate mud, originally calcitic or aragonitic but now silicified, was deposited in nearby channels and on surrounding mudflats. Within these sediments, diagenetic carbonate crystals (formerly ferroan dolomite) and diagenetic sulphate nodules and crystals (once gypsum) grew during later desiccation. The existence of these evaporites, and more like them in the sediments of other Early Archaean cratons, suggests that shallow marine and terrestrial conditions prevailed over a small but significant portion of the early Earth, contrary to some models of global tectonic evolution. Their overall similarity with more recent evaporitic deposits indicates that there was greater conformity between conditions in modern and primeval sea‐shore environments than might be expected, given the great age difference. The attitude implicit in many accounts of Earth's early history, that evaporites were either not deposited or not preserved in Archaean sediments, thus seems to be incorrect.
The sedimentary structures of barite: examples from the Chaudfontaine ore deposit, Belgium
Authors: DEJONGHE L. 
Published: June 2006

ABSTRACTThe Chaudfontaine ore deposit is composed mainly of barite and hosted in Frasnian shale and carbonate formations of Belgium. Thirteen sedimentary structures involving barite crystals are described and discussed. Many of these structures appear to have developed under gravity control. The role of mechanical and chemical ore apposition processes are detailed in each case.The study shows that two populations of barite were formed: (1) crystals which grew in brine above the water‐sediment interface and (2) crystals which developed in the sediment during early diagenesis. The two modes of formation have specific sizes, habits and structures. Both populations crystallized under the control of the prevailing physico‐chemical conditions, and the size and abundance of the barite crystals may be expressed as a function of the degree of barium supersaturation. High‐energy currents and unstable sediment caused previously deposited crystals to be reworked. Morphological similarities were found between Chaudfontaine barite and gypsum from evaporites.
Geochemistry of carbonate cements in the Sag River and Shublik Formations (Triassic/Jurassic), North Slope, Alaska: implications for the geochemical evolution of formation waters
Authors: MOZLEY PETER S. , HOERNLE KAJ 
Published: June 2006

ABSTRACTCarbonate cements (calcite, siderite, dolomite, and ankerite) formed throughout the diagenetic history of the Sag River and Shublik Formations. The trace element and isotopic geochemistry of these cements varies as a function of the timing of precipitation. Earliest calcites, formed prior to significant compaction of the sediment, are relatively enriched in Mg (up to 4·4 mol%), and have 87Sr/86Sr values (mean = 0·707898) compatible with the original marine pore waters. Later calcites are relatively Fe‐rich (up to 5·0 mol%) and are characterized by increasing 87Sr/86Sr values (up to 0·712823) and Sr content with decreasing age. The Fe content of zoned siderite and dolomite/ankerite rhombs increases towards the outside of the rhombs (i.e. increasing Fe content with decreasing age).These geochemical variations appear principally to result from changes in pore‐water chemistry during diagenesis. The increase in 87Sr/86 Sr and Sr content of the cements is most likely due to interaction between pore waters and 87 Sr‐rich clay and possibly feldspar in Ellesmerian mudrocks (whole rock 87Sr/86 Sr signatures for the mudrocks are > 0·716). Pore‐water Fe2+ concentration was probably controlled by diagenetic alterations involving Fe‐bearing minerals (e.g. pyrite precipitation). A reconnaissance examination of carbonate cements in the overlying Kingak Shale indicates that similar alterations occurred in the Kingak.The low δ18 O value of some calcite cements (‐11·96% PDB) suggests that an influx of meteoric water may have occurred in the mid‐Neocomian, though the low value could also result from an abnormally high geothermal gradient associated with mid‐Neocomian rifting.
Lithic‐volcanic sandstones derived from oceanic crust in the Franciscan Complex of California: ‘sedimental memories’ of source rock geochemistry
Authors: LARUE D. K. , SAMPAYO MARIA M. 
Published: June 2006

ABSTRACTThe Permanente terrane, part of the Franciscan accretionary complex of California, contains an unusual fault‐bounded packet bearing lithic‐volcanic sandstones and conglomerates. Based on petrographical, sedimentological, stratigraphical and geochemical studies, it is concluded that these rocks were deposited on the flank of an oceanic rise or plateau by sediment gravity flows probably below the carbonate compensation depth. The bulk geochemistry of the sandstones is similar to the chemistry of greenstones in the Permanente terrane, suggesting that the greenstones or rocks similar to them represent the source of the lithic‐volcanic sandstones, and that the sandstones geochemically ‘remember’ the chemistry of their source rocks. This ‘sedimental memory’ is impressive: geochemical variation in the sandstones is co‐linear with presumable magmatic variation trends in the greenstones. Sandstone geochemistry, plotted on Zr/Y vs. Y and V vs. Ti diagrams, shows that compositions occupy similar fields as greenstones in the Permanente terrane and oceanic crustal compositions in general.Because oceanic crustal sequences can become accreted to continental margins, oceanic crust should be considered a potential source of sediment, especially in continental margin settings. However, differentiating sandstones derived from oceanic arcs and from oceanic crust is extremely difficult unless discriminating geochemical criteria are used.
An ornamented mudstone cavity, Boss Point Formation, Sackville, New Brunswick, Canada: evidence for mud intrusion and rheoplasis
Authors: VANDEPOLL H. W. , PATEL I. M. 
Published: June 2006

ABSTRACTAn ornamented, dome‐shaped cavity, partially filled with mudstone, 4 m in diameter at its circular base and 1 m in height occurs in fluvial sandstone of the Carboniferous Boss Point Formation near Sackville, New Brunswick, Canada. The cavity formed by differential weathering and erosion of the mudstone. Its origin is enigmatic though its size, shape and relationship to the underlying mudstone bed on which it rests suggest a diapiric origin for the mudstone that filled it. On the other hand, the ornamentation on the cavity surface includes flute moulds, suggesting an erosional origin for the domal structure.Of the four principal hypotheses for its origin the one preferred by the authors involves formation by diapiric intrusion of semi‐fluid mud into liquefied sand, soon after deposition. The ornamentation on the cavity surface would have formed as part of the intrusion process. Structures akin to flute moulds have been produced experimentally in support of this interpretation by differential flow across a cement‐mix‐mud interface in a flow box and also by diapiric intrusion of mud in a soft cement mix. By analogy the flute moulds on the cavity surface could have been formed in the same way. By this interpretation, primary sedimentary processes need not be invoked to explain their occurrence on either this cavity surface or on the numerous other mudstone cavity surfaces that are ubiquitous in the Permo‐Carboniferous of eastern Canada. Results of this study have important implications with respect to the potential diversity of origins of flute moulds in general.
Anaerobic diagenesis within Recent, Pleistocene, and Eocene marine carbonate frameworks
Authors: SANSONE FRANCIS J. , TRIBBLE GORDON W. , ANDREWS CHRISTINE C. , CHANTON JEFFREY P. 
Published: June 2006

ABSTRACTPorewaters from a variety of Recent, Pleistocene, and Eocene lithified marine carbonate frameworks displayed similar chemical characteristics: highly depleted concentrations of dissolved oxygen (>20 μM), elevated levels of dissolved methane (25‐5000 nM), and near‐seawater sulphate levels. These porewaters also had low pH values (7·5‐7·9), and contained elevated concentrations of sulphide (4–10 μM), dissolved inorganic carbon (2·05–2·46 mM), and inorganic nutrients. Hydrocarbon composition data indicate that the methane is biogenic, whereas the methane δ13C values (–47·4 ± 2·7%0) suggest that it has been subject to oxidation. The porewater dissolved inorganic carbon δ13C values varied from –0·6 to –39%0, suggesting input of carbon dioxide from organic matter oxidation. We conclude that anaerobic diagenesis involving bacterial degradation of organic matter is a common process in lithified marine carbonates and hypothesize that it may be an important factor controlling their carbonate geochemistry.
Calcretes related to phreatophytic vegetation from the Middle Triassic Otter Sandstone of South West England
Authors: PURVIS K. , WRIGHT V. P. 
Published: June 2006

ABSTRACTCarbonate cementation in the Triassic Otter Sandstone of Budleigh Salterton, SW England, occurs in two distinct forms: large, vertical, concentrically zoned cyclinders, and thin subhorizontal sheets. The former represent rhizocretions of the tap roots of phreatophytic plants which colonized bars and abandoned channels on a large braidplain. The sheets represent cementation around ancient water‐tables. The precipitation of the rhizocretions took place, at least in part, during the life of the plants and δ13C and δ18O values support the view that evapo‐transpiration induced carbonate precipitation. Palaeosol profiles are rare in the Otter Sandstone, reflecting the geomorphological instability of the braidplain surfaces. In contrast, rhizocretionary and sheet calcretes, which formed several metres below the active depositional surface, are abundant because they had a high preservation potential.
An undulatory discontinuity surface on a gently inclined muddy ramp: possible evidence of secondary helical flow from the geological record
Authors: MILLER KEITH B. 
Published: June 2006

ABSTRACTAn unusual, somewhat cryptic, undulatory discontinuity has been recognized near the basal contact of the Wanakah Shale Member of the Middle Devonian Ludlowville Formation in western New York. It is in part marked by undulating diagenetic limestone beds and concretionary horizons, but can be traced continuously by subtle surfaces of fossil and pyritic lags. This surface marks the top of parallel, elongate mud ridges trending approximately N‐S, perpendicular to the inferred E‐W depositional strike of the Appalachian Basin in western New York. These ridges are observed in the field as irregularly spaced mud swells which have up to 1.5 m of relief, and vary from 30 to 50 m or more in width. The intervening, broad, flat‘channel floors’ are characterized by dense concentrations of diminutive brachiopods with gastropods and styliolinids, which are gradually replaced by thin styliolinid horizons with abundant pyrite nodules and pyritized burrows over the tops of the swells. Traceable shelly horizons are observed to pinch out over the swells, suggesting a period of relatively undisturbed sediment‐filling following the formation of the channellized surface. The interval of nearly barren shale below this irregular surface, however, contains concretionary horizons and rare styliolinid pavements which are themselves irregular, possibly indicating a changing topography for this surface through time.Previous taphonomic and palaeoecological studies of the lower Wanakah Shale Member have disclosed a well‐defined regressive‐transgressive cycle with more subtle superimposed subcycles. The undulatory surface described here occurs within the early regressive part of this cycle, and appears to record an accentuated regressive kick of a lower‐order subcycle. The formation of this surface is therefore interpreted to be a consequence of regressive submarine erosion and sediment bypass, together with redeposition from suspension. The development of the‘channels’ and parallel longitudinal mud ridges was probably controlled by helical flow cells within offshore‐directed bottom currents. They may be the first recorded instance of sedimentary furrows described from the geological record. Taphonomic and biofacies data indicate relative depths below average storm wave‐base, suggesting that sediment winnowing and redeposition occurred at depths only rarely affected by the direct impact of storm wave agitation.