bluuebunny/crossref_metadata_2025 huggingface dataset:
The early diagenetic origin of Lower Carboniferous mottled limestones (pseudobreccias)
Authors: SOLOMON STEPHEN T.
Published: June 2006

Dark mottles are a prominent and widespread feature of the regressive, cyclic, shallow marine limestones which form the late Asbian succession in many parts of Britain.

The colour difference which defines mottles in outcrop is caused by distinct but often subtle petrographic differences in the limestone fabric. Specifically, mottles contain a light brown coloured opaque inclusion‐rich calcite spar with characteristic dull brown luminescence; ‘mottle spar’. Outside mottle margins this calcite spar is absent, with clear inequant blocky cements forming the pore filling phase. ‘Mottle spar’ comprises a fabric of irregular crystals predominantly 5–40 μm in diameter, with more regular crystals up to 100 μm diameter often occurring in intraparticle and large interparticle pores. Under cathodoluminescence, ‘mottle spar’ displays crystal morphologies and growth patterns which indicate that both localized neomorphism and patchy cementation contributed to mottle formation.

Cathodoluminescence cement stratigraphy shows that ‘mottle spar’ pre‐dates all other major pore filling cements in the local Asbian succession, but post‐dates marine micritization. ‘Mottle spar’ sharply defines the moulds of former aragonitic allochems which are now filled by the later clear, inequant spar cements. This shows that aragonite dissolution occurred after the formation of ‘mottle spar’. Mottles in calcretes contain unaltered allochems which have been protected from the effects of subaerial micritization by ‘mottle spar’, although mottles are often affected by subaerial brecciation. This evidence shows that mottles formed during early diagenesis; after marine micritization, but before dissolution of aragonite, subaerial exposure and meteoritic phreatic cementation.

Mottles represented lithified patches of very low porosity which are interpreted to have formed in the marine/freshwater mixing zone, during the repeated phases of regression and emergence in the late Asbian.
The influence of bulk shape factors on settling velocities of natural sand‐sized sedimentary suites
Authors: GOLDBERY RON , RICHARDSON DAVID
Published: June 2006

Settling rates of natural sand‐size particles are influenced, to some extent, by their shapes and this may be an important factor in using settling rates to estimate grain size. In order to gauge the sensitivity of this influence, two natural sand populations from the Mesozoic Nubian Sandstones of Southern Israel, with a high probability of being similar in their bulk shape characteristics, were examined in ¼φ sieved fractions for their shape characteristics and settling rates. Fine surface features (roundness and surface roughness) were evaluated using Fourier shape analytical methods. Significant differences in bulk shape were detected and their influence on settling rates was measured empirically in a settling tube. The most marked differences were in the coarse grain sizes and, to a lesser extent, in the intermediate sizes.

Sampling of raw settling data at closely‐spaced time intervals yielded high‐resolution grain size frequency plots which were usually polymodal in nature. Subtle shape contrasts, which are an important influence on settling rates, are thus an important consideration when working at this level of sensitivity.

Natural sand populations which have followed a more varied provenance or process pathway could be expected to have even greater contrasts in settling rates than the samples analysed here. Thus it is recommended that the bulk shape factor should be taken into account in order to minimize errors in the conversion of settling times to grain size. An easy method, outlined in this paper, is through the establishment of an empirically derived calibration curve for each individual suite of sand undergoing analysis. Sieved ¼φ samples, derived from a split of the total composite sample undergoing analysis, forms the basis of the calibration and hence a correction factor converting sieve diameters to true diameters must be applied. In this research, nominal section diameters were obtained optically through an image analyser.
Grain fabric of a laboratory antidune
Authors: YAGISHITA KOJI , TAIRA ASAHIKO
Published: June 2006

The grain fabric of a laboratory antidune composed of coarse‐ to medium‐grained sand shows a high‐angle upcurrent imbrication both on the upstream and downstream sides. Such a grain fabric differs from a normal dune or a megaripple, where grain alignment displays an upcurrent imbrication relative to the depositional slope in the foreset lamina and/or near parallelism with the slope surface in the toe‐set lamina. Recognition of antidunes in ancient rocks is difficult, but detailed examination of grain fabric of cross‐stratification makes it possible to distinguish antidunes or ‘backset’ bedding from normal dunes or megaripples.
Temporal trends in grain‐size measures on a linear sand dune
Authors: LIVINGSTONE IAN
Published: June 2006

Within aeolian dune systems spatial patterns of grain‐size variation have been recognized, but little has been said about temporal changes. Increasingly it is becoming clear that linear dunes are associated with bi‐directional wind regimes which are often seasonal. In the Namib Sand Sea, where linear dunes are aligned roughly north‐south, winds blow from the west in summer and from the east in winter. In response to this regime, sand is eroded from the west slopes and deposited on the east slopes in summer, and eroded from the east slopes and deposited on the west slopes in winter. Preliminary evidence from a study of a single Namib linear dune reported here confirms that this seasonal aeolian regime induces seasonal responses in some grain‐size measurements due to the dynamics of sand transport on the dune, the characteristics of the sand source immediately upwind of the sample point and the nature of the deposit. Thus, time of sampling is crucial to the results obtained.
Injection clastic dykes in the Lower Oxford Clay (Jurassic) of central England: relationship to compaction and concretion formation
Authors: MARTILL DAVID M. , HUDSON JOHN D.
Published: June 2006

An injection dyke of fine‐grained sandstone derived from the Kellaways Sand Formation intrudes overlying organic‐rich shales and shell beds of the Lower Oxford Clay. The dyke shows cross‐cutting relationships with early carbonate concretions, and fills uncompacted kosmoceratid ammonite shells both within the concretions and surrounding shales. Internally the dyke displays flow‐like features, and the walls show lobate flow structures. Clasts of uncompacted Lower Oxford Clay and fragments of pyrite‐rich concretions occur within the sandstone intrusions. The sandstone of the dyke was cemented by calcite identical to that precipitated in septarian cracks in the concretions. This cementation took place prior to final compaction of the Oxford Clay. The dyke has a sub‐parallel relationship to the nearby Tinwell‐Marholm fault suggesting that the dyke may be related to local tectonic events during the Middle Jurassic.
A vertical record of different tidal cyclicities: an example from the Miocene Marine Molasse of Digne (Haute Provence, France)
Authors: TESSIER BERNADETTE , GIGOT PATRICK
Published: June 2006

The Miocene marine molasse of Digne is thought to have been deposited in a structural embayment with a gulf‐like morphology. This gulf was closed toward the alpine front and opened on to the Rhodano‐Provençal gulf, part of the Tethys.

The outcrop of Auribeau, located on the northern margin of the gulf of Digne, displays a conspicuous vertical record of different tidal cyclicities indicating the importance of tidal dynamics on the Miocene marine molasse deposits. For the first time these tidal cyclicities are analysed from a vertical section (deposited by vertical accretion) and these are similar to the cyclicities described from megaripples or sand waves (deposited by lateral accretion). By means of thickness measurements and time series analysis, four main tidal cyclicities have been recognized: (1) the diurnal cyclicity (as it exists in the Atlantic Ocean); (2) the semi‐lunar month cyclicity of neap to spring; (3) the lunar month cyclicity of neap, high spring, neap to low spring and (4) the semi‐annual cyclicity of solstice to equinox.
Coarse‐grained submarine fan and slope apron deposits in a Cretaceous back‐arc basin, Antarctica
Authors: INESON J. R.
Published: June 2006

The Cretaceous of west James Ross Island, Antarctica represents the proximal fill of a late Mesozoic back‐arc basin that was probably initiated by oblique extension during the early development of the Weddell Sea. The succession records sedimentation in two contrasting depositional systems: a laterally persistent slope apron flanking the faulted basin margin interrupted both spatially and temporally by coarse‐grained submarine fans. Slope apron deposits are dominated by thinly interbedded turbiditic sandstones and mudstones (mudstone association), interspersed with non‐channelized chaotic boulder beds, intraformational slump sheets and isolated exotic blocks representing a spectrum of mass‐flow processes from debris flow to submarine gliding. Localized sand‐rich sequences (sandstone‐breccia association) represent sandy debris lobes at the mouths of active slope chutes. The submarine fan sediments (conglomerate association) are typified by coarse conglomerates and pebbly sandstones, interpreted as the deposits of high‐density turbidity currents and non‐cohesive debris flows. Three assemblages are recognized and are suggested to represent components of the inner channelled zone of coarse‐grained submarine fans, from major fan channels through ephemeral, marginal channels or terraces to levee or interchannel environments.

The occurrence of both slope apron and submarine fan depositional systems during the Early and Mid‐Cretaceous is attributed to localized input of coarse arc‐derived sediment along a tectonically active basin margin. Periods of extensive fan development were probably linked to regional tectonic uplift and rejuvenation of the arc source region; cyclicity within individual fan sequences is attributed to migration or switching of fan channels or canyons. Slope apron sedimentation was controlled largely by intrabasinal tectonics. Local unconformities and packets of amalgamated slide sheets and debris flow deposits probably reflect episodic movement on basin margin faults. Differential subsidence across the basin margin anchored the basin slope for at least 20 Myr and precluded basinward progradation of shallow marine environments.
Stream‐driven, high‐density gravelly traction carpets: possible deposits in the Trabeg Conglomerate Formation, SW Ireland and some theoretical considerations of their origin
Authors: TODD SIMON P.
Published: June 2006

Within high‐density flood flows a prominent mechanism of gravel transport and deposition is by stream‐driven, high‐density traction carpet (with a rheology similar to grain flow). These gravel carpets are envisaged to form the basal portion of a bipartite high‐density flood flow, decoupled from an overlying sand‐ and silt‐laden turbulent flow. Several examples already documented in the literature are reviewed and an additional case from the Lower Old Red Sandstone of southwest Ireland is presented. Two mechanisms of traction carpet initiation are discussed: by rapid entrainment of gravel into suspension on rising stage, followed by settling into the gravel traction carpet at peak and falling stage; and by overconcentration of a ‘normal’, low‐density bedload. Gravel entrainment, suspension and traction carpet development are significantly easier if the flood water already carries a high concentration of sand and silt in suspension. Theoretical consideration further shows that gravelly traction carpets can be maintained in channels of relatively low gradient by the shear stress exerted by the high‐density, sand‐bearing turbulent flood flow above. This tangential shear stress is converted to dispersive pressure, which aids buoyancy and quasi‐static grain‐to‐grain contacts in the support of the clasts within the gravel carpet. The carpet is thought to have a quasi‐plastic rheology but behave much like a viscous fluid at high shear rates. Stream‐driven gravelly traction carpets are expected to produce sheet‐like units of clast‐ to matrix‐supported conglomerate, characterized by a parallel or an a(p)a(i) clast fabric. These units may be ungraded, normally or inversely graded, depending on the rate of shear, the viscosity of the flow and the celerity of deposition.
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

The 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

The 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

Small‐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

The 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

The 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

Chemical 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

The 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

Carbonate 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

The 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

An 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

Porewaters 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

Carbonate 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

An 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.
Reconstructing random topography from preserved stratification
Authors: PAOLA CHRIS , BORGMAN LEON
Published: June 2006

One of the fundamental problems of interpreting the sedimentary record is reconstructing the original heights of palaeotopographical features such as bedforms or river channels. This requires an understanding of the relationship between topography and set thickness, but at present an exact theory exists only for periodic topography of uniform height. The applicability of this simple theory is severely limited by the random variability characteristic of many sedimentary systems. In this paper, we develop an exact theory for the probability‐density function (PDF) of sets generated from topography of random height. We focus on the limiting case of zero net deposition in order to provide a lower bound for the set thickness, and derive an analytical set‐thickness PDF that is determined by one parameter of the PDF for topographical height. This parameter, β, measures the breadth of the tail of the topographical PDF. The mean set thickness is 0.8225 β for bedforms and 1.645 β for river channels. If the topographical height is gamma distributed, the preservation ratio, defined as (mean thickness of preserved sets)/(mean topographical height), is 0.8225 r2 for bedforms and 1.645 r2 for river channels, where r is the coefficient of variation (standard deviation/mean) of the generating topography.

In a comparison with data from laboratory current ripples, our analytical predictions compare well with observations of both mean set thickness and thickness distribution. The preservation ratio for the ripples is about 0.5, whilst measured dune heights give a preservation ratio of about 0.12. Depth data from two modern braided streams yield preservation ratios ranging from 0.4 to 0.75. As more data on the distributions of topographical height in modern environments become available, calculations such as these should help provide reliable error bounds for quantitative topographical reconstruction.
Dolomite‐rock textures and secondary porosity development in Ellenburger Group carbonates (Lower Ordovician), west Texas and southeastern New Mexico
Authors: AMTHOR JOACHIM E. , FRIEDMAN GERALD M.
Published: June 2006

Pervasive early‐ to late‐stage dolomitization of Lower Ordovician Ellenburger Group carbonates in the deep Permian Basin of west Texas and southeastern New Mexico is recorded in core samples having present‐day burial depths of 1.5–7.0 km. Seven dolomite‐rock textures are recognized and classified according to crystal‐size distribution and crystal‐boundary shape. Unimodal and polymodal planar‐s (subhedral) mosaic dolomite is the most widespread type, and it replaced allochems and matrix or occurs as void‐filling cement. Planar‐e (euhedral) dolomite crystals line pore spaces and/or fractures, or form mosaics of medium to coarse euhedral crystals. This kind of occurrence relates to significant intercrystalline porosity. Non‐planar‐a (anhedral) dolomite replaced a precursor limestone/dolostone only in zones that are characterized by original high porosity and permeability. Non‐planar dolomite cement (saddle dolomite) is the latest generation and is responsible for occlusion of fractures and pore space.

Dolomitization is closely associated with the development of secondary porosity; dolomitization pre‐and post‐dates dissolution and corrosion and no secondary porosity generation is present in the associated limestones. The most common porosity types are non‐fabric selective moldic and vuggy porosity and intercrystalline porosity. Up to 12% effective porosity is recorded in the deep (6477 m) Delaware basin. These porous zones are characterized by late‐diagenetic coarse‐crystalline dolomite, whereas the non‐porous intervals are composed of dense mosaics of early‐diagenetic dolomites. The distribution of dolomite rock textures indicates that porous zones were preserved as limestone until late in the diagenetic history, and were then subjected to late‐stage dolomitization in a deep burial environment, resulting in coarse‐crystalline porous dolomites. In addition to karst horizons at the top of the Ellenburger Group, exploration for Ellenburger Group reservoirs should consider the presence of such porous zones within other Ellenburger Group dolomites.
Lake Hoare, Antarctica: sedimentation through a thick perennial ice cover
Authors: SQUYRES STEVEN W. , ANDERSEN DAVID W. , NEDELL SUSAN S. , WHARTON ROBERT A.
Published: June 2006

Lake Hoare in the Dry Valleys of Antarctica is covered with a perennial ice cover more than 3 m thick, yet there is a complex record of sedimentation and of growth of microbial mats on the lake bottom. Rough topography on the ice covering the lake surface traps sand that is transported by the wind. In late summer, vertical conduits form by melting and fracturing, making the ice permeable to both liquid water and gases. Cross‐sections of the ice cover show that sand is able to penetrate into and apparently through it by descending through these conduits. This is the primary sedimentation mechanism in the lake. Sediment traps retrieved from the lake bottom indicate that rates of deposition can vary by large amounts over lateral scales as small as 1 m. This conclusion is supported by cores taken in a 3 × 3 grid with a spacing of 1.5 m. Despite the close spacing of the cores, the poor stratigraphic correlation that is observed indicates substantial lateral variability in sedimentation rate. Apparently, sand descends into the lake from discrete, highly localized sources in the ice that may in some cases deposit a large amount of sand into the lake in a very short time. In some locations on the lake bottom, distinctive sand mounds have been formed by this process. They are primary sedimentary structures and appear unique to the perennially ice‐covered lacustrine environment. In some locations they are tens of centimetres high and gently rounded with stable slopes; in others they reach ∼ 1 m in height and have a conical shape with slopes at angle of repose. A simple formation model suggests that these differences can be explained by local variations in water depth and sedimentation rate. Rapid colonization and stabilization of fresh sand surfaces by microbial mats composed of cyanobacteria, eukaryotic algae, and heterotrophic bacteria produces a complex intercalation of organic and sandy layers that are a distinctive form of modern stromatolites.
Origin of manganese carbonates in Jurassic red shale, central Japan
Authors: MINOURA KOJI , NAKAYA SHU , TAKEMURA ATSUSHI
Published: June 2006

Manganese carbonate deposits in Japanese Jurassic sedimentary rocks were studied petrogeochemically. The deposits are characteristically composed of spheroidal micronodules, up to 1 mm in diameter, and always contain well‐preserved radiolarian shells. Chemical elemental composition and mineralogical characteristics indicate that the micronodules contain rhodochrosite in a mixed carbonate phase composition (Mn86.7−92.2Ca2.2−2.9Mg2.6−6.7Fe2.6−5.6)CO3 Carbon and oxygen isotope values, which range from −7.99 to −4.78‰ and −4.05 to 0.28‰ relative to PDB, respectively, suggest that the manganese carbonate was precipitated in a suboxic zone. The micronodules closely resemble agglutinated benthic foraminifera in shape. We suggest that agglutinated foraminiferal tests composed of radiolarian shells accumulated selectively on the sediment surface during redeposition of bottom sediments and were replaced by manganese carbonate in suboxic diagenetic conditions of manganese reduction.
Diagenesis and reservoir quality of the Aldebaran Sandstone, Denison Trough, east‐central Queensland, Australia
Authors: BAKER JULIAN C.
Published: June 2006

The Lower Permian Aldebaran Sandstone is the principal hydrocarbon reservoir in the Denison Trough (Bowen Basin), east‐central Queensland, Australia. It accumulated in a wide range of fluvio‐deltaic and nearshore marine environments. Detailed petrological study of the unit by thin section, X‐ray diffraction, scanning electron microscopy, electron microprobe and isotopic analysis reveals a complex diagenetic history which can be directly related to depositional environment, initial composition and burial‐temperature history. Early diagenetic effects included the precipitation of pyrite, siderite and illite‐smectite rims (δ18O (SMOW) =+8.9 to + 11.3‰). Deep burial effects included physico‐chemical compaction and the formation of quartz overgrowths, siderite (δ13C(PDB) =−34.0 to + 11.5‰, δ18O =−0.7 to +22.7‰), illite/illite‐smectite and ankerite (δ13C=−9.3 to −4.9‰) δ18O=+ 7.6 to + 14.4‰). Involved fluids were in part ‘connate meteoric’ water derived from compaction of the underlying freshwater Reids Dome beds. Important post‐maximum burial effects, controlled by deep meteoric influx from the surface, were ankerite and labile grain dissolution and formation of kaolinite (δ18O=+7.8 to +8.9‰, δD=−115 to −99‰), calcite (δ13C=−9.5 to +0.9‰, δ18O=+9.0 to +20.0‰) and dawsonite (δ13C=−4.0 to +2.3‰, δ18O=+9.8 to +19.8‰), the formation of dawsonite reflecting eventual stagnation of the aquifer. Entrapment of contained hydrocarbons was a relatively recent event which may be continuing today. Reservoir quality varies from marginal to good in the west to poor in the east, with predictable trends being directly linked to depositional environment and diagenesis.
Microbial micritic carbonates in uppermost Permian reefs, Sichuan Basin, southern China: some similarities with Recent travertines
Authors: GUO LI , RIDING ROBERT
Published: June 2006

The uppermost Permian Chuenmuping reef formed on the western side of a carbonate platform situated in the eastern Sichuan Basin, southern China. Contrasts exist between this western patch reef and the better known Jiantianba barrier reef complex on the eastern platform margin in western Hubei Province, 250 km to the east. Microbial micritic carbonates form significant components of the Chuenmuping reef limestones. Three types of fabric can be identified: irregular, clotted and scalloped‐laminated. An additional, cuneiform fabric has been found only in fore‐reef facies at Jiantianba.

Irregular fabric is comparable with the surface of presently forming travertines at Rapolano Terme, central Italy, which are produced by calcification of cyanobacteria. Clotted fabric shows indistinct peloids which tend to lack rims, and are amalgamated into irregular masses, some of which exhibit bush‐like arrangements where traces of filaments can be seen. Traces of filaments are observed in both dark and light layers in the scalloped‐laminated fabric, again suggesting a microbial origin. Cuneiform fabric consists of peloids and filaments, and closely resembles deposits in Quaternary travertines which may have been caused by bacterial action.

Chuenmuping reef shows a shallowing sequence in normal marine conditions. The micritic fabrics interpreted here as microbial occur more commonly in the upper part of the reef. This indicates that microbial micrites, with a cementing role, formed coevally with reef growth within the upper part of the photic zone.