bluuebunny/crossref_metadata_2025_split huggingface dataset:
Deep distributary channels and giant bedforms in the Upper Carboniferous of the Central Pennines, northern England
Authors: McCABE PETER J. 
Published: June 2006

ABSTRACTChannels, with maximum depths of over 40 m, have been cut into the top of a coarsening upward sequence, in the Upper Carboniferous of northern England. The channels are interpreted as deep, distributaries cut into delta slope deposits.Giant cross‐bed sets, up to 40 m thick, are the major channel infilling facies. Internal erosion surfaces occur within the giant cross‐beds and some are draped by micaceous, silty sandstone. Previously interpreted as deltaic sedimentation units, the cross‐beds are now thought to have been produced mainly by side attached, alternate bars and the drapes are interpreted as low stage deposits.Cosets of medium scale cross‐beds overlie the giant cross‐beds. These are thought to be deposits of dunes and smaller bars on top of the transverse bars and in the shallower parts of the distributary. The giant cross‐beds are commonly underlain by unlaminated sandstone but may be underlain by undulatory bedding. The latter was apparently produced by ridges, parallel to the current, spaced between 9 and 23 m apart.The north of England appears to have had a major river by present day standards, for at least part of the upper Carboniferous.
Significance and origin of surface textures on broken sand grains in deep‐sea sediments
Authors: KRINSLEY D. H. , McCOY F. W. 
Published: June 2006

ABSTRACTThe surface textures of fine sand particles from the Deep Sea Drilling Project Leg 39 have been studied via scanning electron microscopy. A large portion of the coarsest fraction of these grains from deep sea cores were formerly fragments of larger sand‐sized grains that had been mechanically broken. Surface textures characteristic of previous aeolian and subaqueous environments were preserved in fragments broken from the original larger grain surfaces, thus making palaeo‐environmental reconstruction possible.Previously, characteristic mechanical markings had not been observed on deep sea sands; the momentum of grain to grain collisions with respect to fine sands is generally insufficient to initiate mechanical breakage.
Lower Cambrian patch reefs and associated sediments: southern Labrador, Canada
Authors: JAMES NOEL P. , KOBLUK DAVID R. 
Published: June 2006

ABSTRACTThe complex pattern of biological accretion, internal sedimentation, early lithification, and biological destruction, that characterizes modern reefs and many fossil reefs has been recognized in archaeocyathid‐rich patch reefs of Lower Cambrian age in the Forteau Formation, southern Labrador.Patch reefs occur as isolated masses or complex associations of many discrete masses of archaeocyathid‐rich limestone and skeletal lime sands, surrounded by well‐bedded skeletal limestones and shales. Each reef is composed of many loafshaped mounds stacked on top of one another. The limestone of each mound comprises archaeocyathids and Renalcis or Renalcis‐like structures in a matrix of argillaceous lime mud rich in sponge spicules, trilobite and salterellid skeletons. Numerous growth cavities roofed by pendant Renalcis‐like organisms and Renalcis are partially to completely filled with geopetal sediment indicating that much of the matrix was deposited as internal sediment.Two stages of diagenetic alteration are recognized: (1) syn‐depositional, which affected only the reefs, and (2) post‐depositional, which affected both reefs and inter‐reef sediments. On the sea floor reef sediments were pervasively cemented and fibrous carbonate was precipitated in intraskeletal and growth cavities. These limestones and cements as well as archaeocyathid skeletons, were subsequently bored by endolithic organisms. Later post‐depositional subaerial diagenesis resulted first in dissolution of certain skeletons and precipitation of calcite cement above the water table, followed by extensive precipitation of pore‐filling calcite below the water table.These carbonate reefs are similar in structure to the basal pioneer accumulations of much younger lower and middle Palaeozoic reefs. They did not develop into massive ‘ecologic’ reefs because archaeocyathids never developed the necessary large, massive, hemispherical skeletons.This occurrence indicates that reefs developed more or less coincident with, and not long after, the appearance of skeletal metazoans in the Lower Cambrian.
An equation of state for biologically active lake sediments and its implications for interpretations of sediment data
Authors: HÅKANSON LARS , KÄLLSTRÖM ANDERS 
Published: June 2006

ABSTRACTThe aim of this work is to link physical sediment parameters to biological parameters by an equation of state, which describes how the given variables interact in biologically active deposits from accumulation areas, i.e. lake areas where fine material is being continuously deposited. In the model the following parameters are utilized: sediment depth, rate of deposition, degree of compaction, bulk density, water content, net biotransport, upward biotransport, downward biotransport, and substrate decomposition. The equation of state has been empirically tested with data from Lake Ekoln and Lake Vänern, Sweden. The model enables determinations of age frequency distributions for arbitrary sediment layers, and it has been shown that, for example, the sediment layer 12‐13 cm in Lake Ekoln has a median age of 15.3 years and that the deposits from the median year only constitutes about 15% of the total amount of material in this particular sediment layer. The spread due to bioturbation is considerable and the range at this sediment layer is 22 years. A mechanism to explain secondary lamination is introduced and discussed in the light of the results from the model.
Giant‐scale hydroplastic deformation structures formed by the loading of basalt onto water‐saturated sand, Middle Proterozoic, Northern Territory, Australia
Authors: NEEDHAM R. S. 
Published: June 2006

ABSTRACTLarge‐scale circular (up to 250 m diameter) and rectilinear (up to 50 m across) depressions form regular ‘honeycomb’ patterns on pavements exhumed from below basalt flows in Carpentarian (1800‐1400 m.y.) sandstone of northern Australia. They are likened to soft‐sediment deformation structures produced experimentally in systems with reverse density gradients. The structures represent an intermediate scale between soft‐sediment deformation (small‐scale) and orogenic deformation (large‐scale).
Inter‐particle cementation in Canadian post‐glacial clays and the problem of high sensitivity (St > 50)
Authors: BENTLEY S. P. , SMALLEY I. J. 
Published: June 2006

ABSTRACTA new explanation of the sensitivity exhibited by Canadian clay sediments is outlined. The mechanism involves cementation of the microstructure and is based on the interaction of primary mineral particles with amorphous matter. The particles appear to be coated with amorphous matter and during sedimentation links develop as the particles come into contact. The speed of the sedimentation, the formation of cementitious links at low stresses and the post‐depositional rise in pH enhance the development of the high porosity structures.
Sand transport model of barchan dune equilibrium
Authors: HOWARD A. D. , MORTON J. B. , GAD‐EL‐HAK MOHAMED , PIERCE DEBORAH B. 
Published: June 2006

ABSTRACTErosion and deposition over a barchan dune near the Salton Sea, California, is modelled by book‐keeping the quantity of sand in saltation following streamlines of transport. Field observations of near‐surface wind velocity and direction plus supplemental measurements of the velocity distribution over a scale model of the dune are combined as input to Bagnold‐type sand‐transport formulae corrected for slope effects. A unidirectional wind is assumed. The resulting patterns of erosion and deposition compare closely with those observed in the field and those predicted by the assumption of equilibrium (downwind translation of the dune without change in size or geometry). Discrepancies between the simulated results and the observed or predicted erosional patterns appear to be largely due to natural fluctuation in the wind direction. Although the model includes a provision for a lag in response of the transport rate to downwind changes in applied shear stress, the best results are obtained when no delay is assumed.The shape of barchan dunes is a function of grain size, velocity, degree of saturation of the oncoming flow, and the variability in the direction of the oncoming wind. Smaller grain size or higher wind speed produce a steeper and more blunt stoss‐side. Low saturation of the inter‐dune sandflow produces open crescent‐moon‐shaped dunes, whereas high saturation produces a whaleback form with a small slip face. Dunes subject to winds of variable direction are blunter than those under unidirectional winds.The size of barchans could be proportional to natural atmospheric scales, to the age of the dune, or to the upwind roughness. The upwind roughness can be controlled by fixed elements or by the sand is saltation. In the latter case, dune scale may be proportional to wind velocity and inversely proportional to grain size. However, because the effective velocity for transport increases with grain size, dune scale may increase with grain size as observed by Wilson (1972).
Fluvial processes and facies sequences in the sandy braided South Saskatchewan River, Canada
Authors: CANT DOUGLAS J. , WALKER ROGER G. 
Published: June 2006

ABSTRACTThe South Saskatchewan River has a long term average discharge of 275 m3/sec, with flood peaks in the range of 1500 to 3800 m3/sec. South of Saskatoon, the four major types of geomorphological elements recognised are channels, slipface‐bounded bars, sand flats and vegetated islands and floodplains. Major channels are 3‐5 m deep, up to 200 m wide, and flow around sand flats which are 50‐2000 m long, and around vegetated islands up to 1 km long. At areas of flow expansion, long straight‐crested cross‐channel bars form. During falling stage, a small part of the crest of the cross‐channel bar may become emergent, and act as a nucleus for downstream and lateral growth of a new sand flat.The dominant channel bedforms are dunes, which deposit trough cross bedding. Cross‐channel bars deposit large sets of planar tabular cross bedding. Sand flats that grow from a nucleus on a cross‐channel bar are mostly composed of smaller planar tabular sets, with some parallel lamination, trough cross‐bedding, and ripple cross‐lamination. A typical facies sequence related to sand flat growth would consist of in‐channel trough cross‐bedding, overlain by a large (1‐2 m) planar tabular set (cross‐channel bar), overlain in turn by a complex association mostly of small planar tabular cross‐beds, trough cross‐beds and ripple cross‐lamination.By contrast, a second stratigraphic sequence can be proposed, related only to channel aggradation. It would consist dominantly of trough cross‐beds, decreasing in scale upward, and possible interrupted by isolated sets of planar tabular cross‐bedding if a cross‐channel bar formed, but failed to grow into a sand flat. During final filling of the channel, ripple cross‐lamination and thin clay layers may be deposited. In the S. Saskatchewan, these sequences are a minimum of 5 m thick, and are overlain by 0.5‐1 m of silty and muddy vertical accretion deposits.
Surge mark formation and morphology
Authors: BULL PETER A. 
Published: June 2006

ABSTRACTSurge marks have previously been described as hieroglyphs and more recently as erosional rills caused by the turbulent bank‐swashing of storm waters upstream of obstructions. This paper presents evidence that surge marks have various forms directly related to the steepness of sediment slope. These features are due, at least in part, to deformation and not necessarily to erosion by surging water. The previous term dendritic surge mark may now only apply to one member of the surge mark formation.
Biolithogenesis of Microcodium: elucidation
Authors: KLAPPA COLIN F. 
Published: June 2006

ABSTRACTPetrographic studies of Tertiary and Pleistocene caliche from the western Mediterranean show some unusual calcite structures. These structures were designated Microcodium elegans Glück 1912. New data are presented which question earlier interpretations with regard to the origin of this structure. The new discovery of Microcodium in Recent soils extends its stratigraphic range into the Holocene. Retention of fine detail in Recent samples, revealed by light microscopy and SEM, has suggested an origin hitherto unconsidered, calcification of mycorrhizal associations. Ancient and Recent Microcodium fabrics are compared; sufficient preservation of ultrastructure in the Ancient indicates a homologous origin.Environmental, stratigraphic and palaeoecological significance of Microcodium is discussed: correct recognition indicates existence of a palaeosol, and hence is a valuable criterion for recognition of continental conditions, cessation of sedimentation, subaerial exposure, and time‐equivalent horizons. In particular. Microcodium is a characteristic component of caliche in the western Mediterranean. A review of the literature suggests that its presence may have been overlooked or misinterpreted in other parts of the world and, thus, may be more widespread than hitherto suspected.This study, in its embryonic stage of development. illumines the potential importance of biolithogenesis within terrestial carbonates.
Strontium isotope composition of marine carbonates of Middle Triassic to Early Jurassic age, Lombardic Alps, Italy*
Authors: FAURE G. , ASSERETO R. , TREMBA E. L. 
Published: June 2006

ABSTRACTThe 87Sr/86Sr ratios and strontium concentrations for thirty‐three samples of marine carbonate rocks of Middle Triassic to Early Jurassic age have been determined. The samples were collected from four measured sections in the areas of Val Camonica in northern Italy. The strontium concentrations vary from 40 to 7000 ppm. Most of the samples are calcitic limestones containing less than 10% of non‐carbonate residues. Dolomitic samples and those containing appreciable non‐carbonate residues have significantly diminished strontium concentrations. 87Sr/86Sr ratios of the carbonate phases of these rocks appear to be unaffected by dolomitization and by the presence of non‐carbonate minerals. The average 87Sr/86Sr ratios of the formations vary systematically in a stratigraphic sense. The ratio increased from Early Anisian to Early‐Middle Ladinian, declined during Late Ladinian and Carnian, rose again during the Norian and then declined throughout the Late Norian (Rhaetian), Hettangian, Sinemurian and Pliens‐bachian ages. The average 87Sr/86Sr ratios, relative to 0.7080 for the Eimer and Amend standard, are: Anisian: 0.70805 ± 00019; Early Ladinian: 0.7085 ± 0.00038; Late Ladinian: 0.70791 ± 0.00013; Carnian: 0.70776 ± 0.00015; Norian and Rhaetian: 0.70791 ± 0.00014; Hettangian: 0.70762 ± 0.00021; Sinemurian: 0.7070 ± 0.00038; Pliensbachian: 0.7070 ± 0.00015. These variations reflect changes in the isotopic composition of Sr entering the oceans in early Mesozoic time due to varying rates of weathering and erosion of young volcanic rocks (low 87Sr/86Sr) and old granitic rocks (high 87Sr/86Sr). The data presented in this report contribute to a growing body of information regarding the changes that have occurred in the 87Sr/86Sr ratio of the oceans in Phanerozoic time.
A trough cross‐stratified glaucarenite: a Cambrian tidal inlet accumulation
Authors: CHAFETZ H. S. 
Published: June 2006

ABSTRACTThe upper part of the Riley Formation, Cambrian of central Texas, is primarily composed of a sequence of thoroughly trough cross‐stratified deposits. The dominant lithologies range from fossiliferous glaucarenite to highly glauconitic bio‐sparrudite. These cross‐stratified deposits accumulated within a tidal inlet and associated lagoonal tributary and distributary channels. Tidal inlet‐fill strata are underlain by shallow, open marine oosparites and biomicrites and are overlain by parallel bedded glaucarenites which accumulated as part of a barrier island complex. The parallel bedded deposits exhibit large scale, gently inclined strata, ripple cross‐stratification, and a minor amount of vertical burrows.Some glaucarenite units within the tidal inlet‐fill have local concentrations of skeletal material, primarily trilobite carapaces. These concentrations are most abundant in the bottoms of troughs. Cementation by bladed to fibrous spar between the carapaces has resulted in the nodular appearance of these skeletal accumulations. Calcite clasts, with relict evaporite textures, occur within the carbonate nodules and surrounding glaucarenite. These clasts were eroded from the shallow subsurface of the barrier island as the tidal inlet migrated. The presence of the former evaporite clasts attest to an arid climate at the time of their formation.
A note on the estimation of sedimentation rates in the Gulf of Elat (= Gulf of Aqaba), by the use of Foraminifera
Authors: LARSEN ARNE ROSENKRANDS 
Published: June 2006

ABSTRACTAbundance fluctuations of Amphistegina in the upper 4–5 cm of a few small cores from the Gulf of Elat look somewhat like rainfall fluctuations in the hinterland. To correlate the two patterns, a sedimentation rate of 2.2–3.3 mm/year must be assumed. A similar rate is obtained from estimates of annual production of Amphistegina.
The use of slump structures as palaeoslope orientation estimators
Authors: WOODCOCK N. H. 
Published: June 2006

ABSTRACTThe two main methods of estimating palaeoslope direction from slump structure orientation data are reviewed. The mean axis method takes the downslope direction as the perpendicular to the mean slump fold axis. The separation are method estimates the downslope direction by the bisector of a planar separation angle between groups of slump folds with opposite downplunge asymmetry. The advantages and disadvantages of each method are considered in detail, by reference first to idealized slump fold distributions and then to a compilation of sixty natural slump fold axis distributions, mostly from published data.Among the advantages of the mean axis method are the ease of deriving confidence limits for estimates and its applicability where fold asymmetry data are unavailable. The main advantage of the separation are method is its validity for strongly skewed fold axis samples. The statistical robustness of the mean axis method, however, due to its reliance on average properties of data, makes it more generally reliable than the separation arc method which depends on extreme properties of data. A procedure for determining palaeoslope directions is suggested which reflects the respective strengths of the two methods.
Storm surge turbation
Authors: LINDSTRÖM MAURITS 
Published: June 2006

ABSTRACTSand ridges along the coast of northern Brittany contain alternating beds of coarse and fine to medium sand 4–10 m above normal reach of highest spring tides. They were deposited in the Holocene, largely during the last centuries, by sand charged storm surges. Deformation structures at many levels in these beds have amplitudes of 2–25 cm. Fine‐grained beds were folded in a manner that suggests relative competency of transmitting horizontally directed stress. Coarse‐grained beds, by yielding, permitted folding to die out vertically. It is suggested that a transitional zone existed between the storm surge and the underlying sediment. This zone consisted of a mixture of water and sand. Deformation occurred at the moment movement in this zone was arrested. Comparison with published data on deformed bedding indicates that the described structures may be environment specific.
Patch or pinnacle reefs of Cretaceous age exposed on western margin of Dead Sea (Israel)
Authors: FRIEDMAN GERALD M. , ARKIN YAACOV , AHARONI E. 
Published: June 2006

ABSTRACTFor a petroleum geologist knowledge of the density of the distribution of subsurface Cretaceous reefs is a matter of practical interest. Hence, the discovery in the Judean Desert of Israel, near the western margin of the Dead Sea, of an exhumed sea bottom below an erosional unconformity is of particular interest, because it reveals the original distribution of such reefs, and thus may provide a clue to their subsurface distribution. In this desert, newly described Turonian (Upper Cretaceous) reefs are the dominant geomorphic features, as in their original habitat on the Cretaceous sea bottom. These reefs can be traced on aerial photographs: they are generally 5‐15 m in diameter and 1‐2.5 m in vertical dimension. Generally, reefs occur within 100‐200 m of one another; in many places they are less than 50 m apart. The shapes of the patch or pinnacle reefs are almost circular, as are those occurring in the Edwards Limestone (Comanchean, Lower Cretaceous) in central Texas (Roberson, 1972).The hard, resistant, ring‐like outer rims of these exposed reefs weather out as raised rims. A central depression within such structures consists, in places, of Senonian soft chalky or friable material that has been interpreted as a diagenetic product of the vadose zone. The massive reef core consists of porous dolomite. The flanking strata which dip away from the reef core at angles of approximately 15‐25° are composed of a probable original grainstone which has been diagenetically changed to a micritic fabric.
Errors in measuring uniform aeolian sand flow by means of an adjustable trap
Authors: JONES J. R. , WILLETTS B. B. 
Published: June 2006

ABSTRACTA vertical‐slot sand trap was constructed incorporating a device to adjust the freedom of flow through the trap. Using a dune‐sand of equivalent diameter 0.25 mm, measurements of sand transport were made with this flow so adjusted that no change of sand surface geometry detectable by eye occurred upstream of the trap; this was considered to be a correct procedure. Further measurements were taken with the flow through the trap deliberately set to be (a) more vigorous and (b) less vigorous than the correct setting. In the latter case very significant differences were observed in measured transport rates of sand compared with the ‘correct’measurements. The disparities are of interest because it is commonly assumed that sand grains have enough ‘penetration’to enter a slot trap freely despite a degree of stagnation upstream of it. This does not appear to be a prudent assumption and substantial errors may result from lack of attention to the provision of free air flow through the trap.
Sedimentation in the latest Messinian at Capo Rossello (Sicily)
Authors: CITA M. B. , COLOMBO L. 
Published: June 2006

ABSTRACTTen samples from a Late Messinian section measured at Capo Rossello, a few hundred metres laterally from and stratigraphically immediately below the Miocene/ Pliocene boundary stratotype, have been investigated by various techniques: carbonate content, grain‐size analysis, loss on ignition, clay mineral composition, frequency of euryhaline ostracods. Two lithological sub‐units. with different palaeoenvironmental implications, are recognized. Both sub‐units were deposited after the termination of evaporitic conditions, and immediately after deposition of an ash fall tuff. The lower unit is the Congeria marl which yields the ‘lago‐mare’faunal assemblage, the autochthony of which is supported by biometrical analysis of Cyprides agrigentina, indicating brackish, shallow‐water conditions with Paratethyan affinities, and the upper, the Arenazzolo, a thin sandy unit indicating a higher energy environment whose duration is estimated at some thousand years. Marine faunas yielded by the Arenazzolo are interpreted as allochthonous. Sedimentary structures suggest a littoral setting at the edge of the lake, or a delta lobe.The main environmental change occurs at the base of the immediately overlying Trubi Formation, where open marine conditions indicative of oceanic depths mark the termination of the Messinian salinity crisis.