bluuebunny/crossref_metadata_2025 huggingface dataset:
Depositional magnetization of Carboniferous Limestones from the Craven Basin of northern England
Authors: TURNER PETER
Published: June 2006

Carboniferous Limestones in the Craven Basin of northern England carry a stable natural remanent magnetization (NRM) the intensity of which is facies dependent. Dark argillaceous limestones are most strongly magnetized and pure, pale coloured limestones most weakly magnetized. Partial thermal and alternating field demagnetization suggest that magnetite is the principal carrier of the remanence although some haematite is present in the limestone. The presence of magnetite is confirmed by the low temperature transition, isothermal remanent magnetization (IRM build‐up curves) and microprobe analysis.

Partial demagnetization of IRM and ARM suggest that the magnetite is relatively coarse grained and in the multidomain state. There are no indications of pseudo‐single domain behaviour but magnetite of this type cannot be excluded as a possible remanence carrier. A grain size estimate of 10–20 μm based on coercive force and remanent coercive force is compatible with the theoretical consideration of grain size.

The limestones show a weak but marked magnetic susceptibility anisotropy. This anisotropy defines a depositional fabric which indicates that the magnetization is a depositional remanent magnetization (DRM). A DRM was acquired by each specimen before compaction and cementation and was preserved because of the reducing conditions which prevailed in the early diagenetic environments of the limestones.
Tide‐ and wave‐dominated inshore and shoreline sequences from the late Precambrian, Finnmark, North Norway
Authors: JOHNSON HOWARD D.
Published: June 2006

The Late Precambrian Ekkerøy Formation of Finnmark, northern Norway represents a 40 m thick coarsening upward, prograding deltaic sequence. The upper 6 m of this formation consists of two small‐scale fining upward sequences representing two contrasting shoreline environments. These two sequences have been the focus of a detailed facies analysis based on grain size, bed thickness and sedimentary structures. The study area comprises a series of horizontally bedded rocks which are continuously exposed for 3 km enabling individual facies and sedimentary structures be to traced‐out laterally.

The lower sequence exhibits a wide variety of tidal features including directional bimodality of both large‐scale and small‐scale cross‐stratification, herringbone cross‐stratification, various sandstone‐mudstone alternations including lenticular and wavy bedding, and a general fining upward tendency. A detailed field analysis of the internal structures within the large‐scale cross‐stratified unit is presented and is interpreted as a sub‐tidal channel complex. The associated sandstone‐mudstone facies represents intertidal flats built up by tidal and storm‐generated currents.

The upper sequence displays a wide variety of wave‐formed structures including wave‐ripples and swash‐lamination. Palaeocurrent data indicates a dominance of shoreward migrating megaripples and wave‐ripple crests oriented approximately parallel to the inferred shoreline with a distinct longshore component. The sequence is interpreted as a beach‐shoreface environment. In terms of sedimentary structures, palaeocurrent patterns and facies distributions, the sequence bears a remarkable similarity to the high‐energy, non‐barred shoreline of the modern Oregon coast U.S.A., described by Clifton, Hunter & Phillips (1971).
Water escape structures in coarse‐grained sediments
Authors: LOWE DONALD R.
Published: June 2006

Three processes of water escape characterize the consolidation of silt‐, sand‐and gravel‐sized sediments. Seepage involves the slow upward movement of pore fluids within existing voids or rapid flow within compact and confined sediments. Liquefaction is marked by the sudden breakdown of a metastable, loosely packed grain framework, the grains becoming temporarily suspended in the pore fluid and settling rapidly through the fluid until a grain‐supported structure is re‐established. Fluidization occurs when the drag exerted by moving pore fluids exceeds the effective weight of the grains; the particles are lifted, the grain framework destroyed, and the sediment strength reduced to nearly zero. Diagenetic sedimentary structures formed in direct response to processes of fluid escape are here termed water escape structures.

Four main types of water escape structures form during the fluidization and liquefaction of sands: (1) soft‐sediment mixing bodies, (2) soft‐sedimsnt intrusions, (3) consolidation laminations, and (4) soft‐sediment folds. These structures represent both the direct rearrangement of sediment grains by escaping fluids and the deformation of hydroplastic, liquefied, or fluidized sediment in response to external stresses.

Fundamental controls on sediment consolidation are exerted by the bulk sediment properties of grain size, packing, permeability, and strength, which together determine whether consolidation will occur and, if so the course it follows, and by external disturbances which act to trigger liquefaction and fluidization. The liquefaction and fluidization of natural sands usually accompanies the collapse of loosely packed cross‐bedded deposits. This collapse is commonly initiated by water forced into the units as underlying beds, especially muds and clays, consolidate. The consolidation of subjacent units is often triggered by the rapid deposition of the sand itself, although earthquakes or other disturbances are probably influential in some instances.

Water escape structures most commonly form in fine‐ to medium‐grained sands deposited at high instantaneous and mean sedimentation rates; they are particularly abundant in cross‐laminated deposits but rare in units deposited under upper flow regime plane bed conditions. Their development is favoured by upward decreasing permeability within sedimentation units such as normally graded turbidites. They are especially common in sequences made up of alternating fine‐(clay and mud) and coarse‐grained (sand) units such as deep‐sea flysch prodelta, and, to a lesser extent, fluvial point bar, levee, and proximal overbank deposits.
The origin of authigenic quartz in the Helvetic Siliceous Limestone (Helvetischer Kieselkalk), Switzerland
Authors: FUNK HANSPETER
Published: June 2006

Helvetic Siliceous Limestone (Lower Cretaceous) is characterized by up to 40% of very fine (1‐10 μm), evenly distributed authigenic quartz crystals. Early diagenetic dissolution of opaline sponge spicules led to silica enrichment of interstitial waters, which reprecipitated silica in the overlying horizons, forming tiny quartz crystals in pore spaces.
Discrimination of particle zonation on a pebble beach
Authors: ORFORD JULIAN D.
Published: June 2006

The extent to which pebble shapes are zonally sorted on a pebble beach is examined. On the basis of comprehensive point samples from a pebble beach in west Wales, and using factor analysis, a pebble shape index for each point sample is derived. It is proposed that the sum of squares (%RSS) explained by a linear trend surface of shape index values, approximates to the degree to which the second derivative of pebble shape with respect to distance downbeach, tends towards zero. If the second derivative is zero there is no zonation of sample shape values but, rather, a uniform trend. Strength of zonation on one beach is calculated for eight beach samples during 1971. A heterogeneity index (Ht) is introduced to show the degree of difference between clustered point samples on the basis of shape and size. On the basis of %RSS and Ht value, discrimination between pebble facies type on the sample beach over time can be made.
Regressive diagenesis in Pleistocene eolianites from Fuerteventura, Canary Islands
Authors: MULLER GERMAN , TIETZ GERD
Published: June 2006

Evidence from a Late Pleistocene eolianite in Fuerteventura, Canary Islands, demonstrates that red algal clasts replaced by low‐magnesian calcite in the subaerial environment show a partial re‐constitution of their original high‐magnesian calcite mineralogy when they are exposed subsequently to sea water. This process only affects the cell walls of the red algae, which, owing to their specific microarchitecture (extremely small crystal size, very large specific surface area), offer the most favourable conditions for dissolution‐reprecipitation or diffusion processes.

High‐magnesian caicite precipitated as a second generation cement on fresh water calcitic cement is an equivalent of the high‐magnesian calcite cement which at many locations on the island leads to the formation of beachrock in Holocene intertidal sediments.
Sedimentology of very thick calcarenite‐marlstone beds in a flysch succession, southwestern Pyrenees
Authors: RUPKE N. A.
Published: June 2006

Single beds of up to 41 m thick are exposed for 16 km along the strike in an Eocene flysch, Spanish Pyrenees. These mega‐beds consist of a lower calcarenite (up to 25 m) and an upper marlstone (up to 16 m). Their volume is minimally of the order of 1 km3. The mega‐beds are underlain by slump sheets which in places exceed 100 m in thickness.

The calcarenites show erosional sole markings, no internal amalgamation, a graded texture expressed by matrix percent, coarsest quartz grains, and coarsest foram tests. The marlstones are burrowed from their top, they are graded as expressed by matrix percent, coarsest quartz grains, and carbonate percent. Their grading continues the upward size decline in the underlying calcarenites, and their thickness and carbonate percent vary with those of the underlying calcarenites.

Single calcarenite‐marlstone beds are interpreted as deposited by turbidity currents. The great thickness and other uncommon features (e.g. consistent association with an underlying slump sheet, distal thickening, locally repetitive grading, compositional inhomogeneity) can be accounted for by (1) widespread slumping initiating voluminous turbidity currents, (2) concurrence of tributary turbidity currents to deposit a single mega‐bed, and (3) ponding of the turbidity currents behind what may have been a local, palaeobasin floor high. Slumping and simultaneous turbidity currents were probably triggered by earthquakes of great magnitude. The basin floor high may have formed by basement faulting.

The mega‐beds do not occupy a particular niche in a facies sequence and their great thickness does not reflect a particular environment of deposition. Instead, they more likely reflect the seismic regime (periodic earthquakes of great magnitude) and tectonic style (block faulting) of the flysch basin.
The ability of debris, heavily freighted with coarse clastic materials, to flow on gentle slopes
Authors: RODINE JAMES D. , JOHNSON ARVID M.
Published: June 2006

Observations of many debris‐flow deposits on gently‐sloping alluvial fans have disclosed that debris commonly is heavily loaded with coarse clastic material and contains large isolated blocks. The paper describes how debris charged with coarse granular material can transport large blocks, yet flow on gentle slopes.

Experimental results of mixing sand‐sized particles with a slurry of clay plus water indicate that 45–55 vol. % of a single size, and up to 64% of two selected sizes, can be added before interlocking occurs. Theoretical analysis of multi‐size classes suggest that 89 to more than 95 vol. % debris can be clastic materials without significant particle interlocking.

The clay fraction, even if minor, plays a critical role in determining strength properties of debris. The mixture of clay plus water provides a cohesive slurry that supports fine‐grained particles within the debris, as well as reduces the effective normal stresses between the particles. The increased unit weight of the clay plus water plus fine‐grained particles allows the support of coarser grained particles. The pyramiding upon the clay‐water slurry continues until the entire debris mass is supported in a virtually frictionless position because of the reduced effective normal stress and the lack of particle interlocking.

Thus, the ability of debris flows to support large blocks can be understood in terms of the high unit weight of the displaced debris plus the strength of the fluid phase; that is, the blocks float in the debris as a result of a small density difference between the blocks and the debris, plus the cohesive strength of the clay‐water slurry.

Also, the ability of coarse clastic debris to flow on gentle slopes probably is a result of poor sorting of debris‐flow materials which contain minor amounts of clay. The poor sorting allows the debris to have a high density yet have essentially no interlocking of clasts. The high density of the debris reduces effective normal stresses between clasts, thereby reducing apparent friction of the mixture.
The Rio Dell Formation: a Plio‐Pleistocene basin slope deposit in Northern California
Authors: PIPER DAVID J. W. , NORMARK WILLIAM R. , INGLE JAMES C.
Published: June 2006

The Rio Dell Formation (Pleistocene and Pliocene), exposed south of Eureka, California, is a prograded sequence of basinal turbidites overlain by basin slope and shelf deposits. The slope deposits studied in the Centerville Beach section accumulated in a steadily shallowing environment delineated by analysis of palaeobathymetrically significant benthonic foraminiferal biofacies in turn suggesting deposition at depths of 1000–100 m. Lower slope deposits interfinger with basinal turbidites derived from the Eel River delta to the north. Slumped blocks of silty mudstone, and associated silt and mud beds, are common. The middle slope deposits are mudstones; coarser sediments bypassed this zone. Mudstones and muddy siltstones alternate on the upper slope. Shallow depressions, probably slump scars, that have been rapidly filled by upper slope sediment are common. The transition to shelf deposits is marked by an increase in sediment grain size, in the degree of oxidation, and in the abundance of megafossils. High percentages of benthonic foraminifera displaced from shelf depths indicate that resedimentation processes are most important on the upper slope.
Rare‐earth elements in Lower Palaeozoic epicontinental and eugeosynclinal sediments from the Oslo and Trondheim regions
Authors: DYPVIK H. , BRUNFELT A. O.
Published: June 2006

Eight rare‐earth elements (REE) have been determined in Cambro‐Silurian sediments and volcanics from nine stratigraphic units in the Trondheim Region and ten stratigraphic units in the Oslo Region by neutron‐activation analyses.

In the eugeosynclinal sediments from the Trondheim Region the REE variations are related to variations in source material, which mainly are volcanic debris of local origin. In the foreland sediments from the Oslo Region significant development of clays has occurred and therefore the REE variations reflect the deposition environment.
A note on the significance of lamination in stromatolites
Authors: PARK ROBERT
Published: June 2006

The lamination found in recent stromatolites, an intrinsic feature of many of them, comprises a number of sediment‐rich and organic‐rich couplets. The significance of this lamination has figured in at least two avenues of research. In one is its possible influential role upon geochemical and diagenetic trends, in the other its possible calendar role. Algal growth follows a diurnal pattern but sedimentation, which is largely marine, is far less regular or predictable. The monitoring of modern algal mat surfaces has yielded much useful and accurate information concerning rates of mat accretion. Millimetre‐scale lamination may be produced on a number of interacting processes but as yet no one has been able to determine with any great assurance whether it represents a daily, monthly, annual or some other regular periodic growth. The evidence from the Trucial Coast tends to show, rather, that burial, subsequent compaction, distortion and, in the case of sabkha‐type sections, destruction owing to evaporite mineral growth, all tend to obliterate or at best mask any regular patterns of growth. In conclusion, one is forced to acknowledge that any computations concerning the calendar role of stromatolites are to be regarded with the utmost suspicion.
Log‐normal interpolation in grain size analysis
Authors: BURGER HEINZ
Published: June 2006

A computer program is described for the display of grain size distributions by log‐probability plots. These plots are useful to recognize and interpret log‐normal subpopulations. Because log‐probability plots of the cumulative curves often show two or more straight line segments we suggest a method of ‘log‐normal’interpolation for use in computing statistical parameters. The method combines advantages of graphical methods with those of mathematical moments. Finally, the frequency curve is derived by approximating the log‐probability plot of the cumulative curve with spline functions.
Sedimentary model for intertidal mudflat channels, with examples from the Solway Firth, Scotland
Authors: BRIDGES PAUL H. , LEEDER MICHAEL R.
Published: June 2006

Intertidal mudflat channels (gullies) in the Solway Firth, Scotland possess width/depth ratios similar to meandering rivers. Most channels deeper than 1 m show cut‐bank slides, but narrow, deep channels also have rotational slides on the point‐bar slopes. The channels display two types of point‐bar. The first, associated with gently curved meanders, is sigmoidal in profile. The second, associated with tight meander bends, possesses a pronounced lower platform. The onset of flow separation in meander bends, a phenomenon which enhances cut‐bank erosion and point‐bar deposition, is a direct function of meander‐bend tightness and Froude number. The effects of flow separation are greatest on tight meander bends at times of high velocity during late spring ebb and also during rainfall run‐off at low tide. These events appear to be responsible for the growth of the point‐bar platforms. A model, predicting the type of point‐bar development to be expected in different channel meanders, is used to reconstruct the sedimentary history of active and fossil point‐bars.
Dissolution and transport of manganese by organic acids and their role in sedimentary Mn ore formation
Authors: BROCKAMP OLAF
Published: June 2006

Dissolution experiments of Mn and Fe under natural conditions from fresh basalt, weathered basalt and Mn laterite by different organic acids show that Mn is highly enriched over Fe in solutions from the weathered rocks but that more Fe than Mn is dissolved from the fresh basalt. The enrichment of Mn is caused by sparingly soluble Fe‐oxides and hydroxides and more soluble Mn‐oxides. In addition from the weathered rocks the Mn concentration dissolved by the organic acids is up to 1000 times higher than in inorganic solutions. Mn enrichment is caused by acid attack, organic reduction of Mn4+ to Mn2+ and complexing by the organic acids. The complexed Mn is not attacked as easily by oxidation as free Mn ions. Higher concentrations of manganese in the organic dissolved stage can therefore be transported by rivers over greater distances.

Organic complexed Mn, derived from lateritic weathered rocks may therefore contribute to the formation of low iron marine sedimentary Mn deposits.
Spherular modern dolomite from the Coorong area, South Australia
Authors: VON DER BORCH C. C. , JONES J. B.
Published: June 2006

Scanning electron micrographs show that the youngest and apparently least altered of the Coorong dolomite is in the form of spherular bodies about 0.2–1.0 μm in size which themselves are composed of spherules about 100 nm in diameter. Older and more lithified sediments show sharply defined dolomite crystals suggesting an origin as primary dolomite spherules followed by aggregation and diagenetic alteration to well crystallized dolomites.
Largescale ripples of the lower Wabash River
Authors: JACKSON ROSCOE G.
Published: June 2006

Largescale ripples in the meandering lower Wabash River of Illinois and Indiana, U.S.A., include scroll bars and three dunelike bed forms (dunes, sand waves, and transverse bars). Scroll bars are lobate crested, asymmetrical in stream‐wise vertical profile, usually solitary, and oriented approximately normal to local channel strike. They form by passive flow expansion downchannel from locally emergent topographic highs, face and lie near inner banks of meander bends, enjoy a high preservation potential as leveelike ridges of ridge‐and‐swale topography, and migrate only during relatively low stream discharges, when water depth over bar crests is less than 0·5 m.

Dunes correspond to dunes of the flow‐regime classification and rarely are solitary or superimposed. Sand waves may be symmetrical or asymmetrical, are always superimposed by dunes, occur in depths greater than 4 m and in bed material coarser than 1 mm mean size, and develop at bankfull and flood flows. Transverse bars migrate in depths less than 5 m in straight reaches and near inner banks of bends, display crestal dunes, and correspond to the bars of Costello (1974) and to the sand waves of Boothroyd (1969). Hydrodynamic regimes of scroll bars and transverse bars differ from that of dunes. The omnipresence of dunes upon stoss‐sides of sand waves confirms the existence of an equilibrium superimposition of dunelike largescale ripples.

Depth‐velocity‐size diagrams appear to be a valid representation of empirical stability fields of dunelike largescale ripples in deep unsteady nonuniform aqueous flows. Stability fields of dunes and sand waves overlap greatly. Velocity profiles demonstrate an absence of leeside flow separation over dunes and an appearance (rare) over transverse bars only when the ratio of trough depth to crest depth exceeds two.

Dune stratification displays (1) largescale trough cross‐strata, (2) thinning of sets as bed‐material size increases, and (3) an orientation within 20° of local channel strike. Transverse bars show avalanche sets up to 2 m thick, with reactivation surfaces. Scroll bars display thick avalanche sets separated by reactivation structures consisting of erratically oriented smallscale trough cross‐strata. Avalanche sets of scroll bars and of transverse bars are oriented 50–150° from and within 50° of, respectively, local channel strike.
A cine‐camera technique for process measurement on a ridge and runnel beach
Authors: WRIGHT PETER
Published: June 2006

Previous attempts to quantify beach processes with techniques involving direct measurement have met with varying degrees of success. Problems encountered can be attributed to three main areas of difficulty: (1) the rapidity of change in both process operation and the subsequent production and development of bed forms; (2) the inclement nature of the weather conditions during which the most rapid beach changes take place; (3) interference with the flows being measured, by the equipment. A photographic method is described here, which to a great extent overcomes these problems, and can be operated cheaply by a solitary field investigator.
The production and recognition of aeolian features on sand grains by silt abrasion
Authors: NIETER WILLIAM M. , KRINSLEY DAVID H.
Published: June 2006

Several size fractions of natural loess from Long Island, New York including sand and ventifacts were studied with the scanning electron microscope to determine if distinguishing surface features could be found. Long Island loess compares very closely to descriptions of a number of European loess deposits except for the inclusion of carbonate not found on Long Island. Ventifact and sand grain surfaces were experimentally prepared using natural loess as an abrasive agent, and compared well with those surfaces found in the natural materials. Given favourable circumstances, it is possible via scanning electron microscopy to distinguish surface features created by aeolian action in loess deposits and associated materials.
Man‐made turbidity currents in Lake Superior
Authors: NORMARK WILLIAM R. , DICKSON FRANKLIN H.
Published: June 2006

The discharge of taconite tailings into Lake Superior at Silver Bay, Minnesota, produces turbidity current flow. The silty fine‐sand tailings fraction transported to the deepest part of the lake has formed a small fan with valleys similar in gross morphology to a submarine fan. Current meters anchored 5 m above the lake floor over the wall and over the levee of a distributary valley on the fan recorded intermittent turbidity current flows during 30 weeks in 1972–73. At least twenty‐five discrete periods of observation of turbidity current flow were obtained; single episodes lasted 4−328+ h. Only flows thick enough to overflow the eastern levee of the valley could be observed, and this accounts for the intermittent nature of our observations, as flow within the valleys is expected to be continuous as long as tailings are discharged. Flow velocities were higher near the valley axis where the flow is thicker. Velocities measured over the valley wall averaged 10.8 cm/s for eleven episodes; velocities measured over the levee, more than 1/2 km from the valley axis, only 3.3 cm/s. The maximum velocity during 1300 h of observation did not exceed 31 cm/s. This agrees reasonably well with velocities calculated from channel properties, as commonly done for turbidity currents on deep‐sea fans. Current meters tethered above the bottom meters indicate that lake currents normally parallel the shore throughout the water column. With the onset of a turbidity current, currents higher in the water column remain unchanged but velocities near the bottom go to zero, currents then change azimuth by 90° to parallel the downslope (down‐valley) direction of the fan, then increase in velocity. During a turbidity current episode, the direction of bottom flow stays relatively constant (± 20° of the down‐valley trend) but the velocity oscillates (commonly with 10 cm/s amplitude), periods being of 1/2 h or less to several hours. Turbidity currents generated on Reserve Mining Company's delta are effective in carrying essentially all tailings discharged into the lake into deeper water, where they are deposited.
The origin and diagenesis of cherts from Cyprus
Authors: ROBERTSON A. H. F.
Published: June 2006

The Troodos Massif of Cyprus is overlain by a variety of cherts in pelagic chalks, volcanogenic sediments, radiolarites and radiolarian mudstones, all of Campanian to Upper Eocene age.

There are two chert types, granular chert and vitreous chert. X‐ray diffraction (XRD) reveals the silica polymorphs, disordered cristobalite and quartz.

Silicification of the chalks varies from incipient, to bedded, granular cherts, all with disordered cristobalite as the main silica phase. Quartzitic cherts are restricted to the base of Upper Palaeocene and Lower Eocene calciturbidite beds. Disordered cristobalite predominates in the radiolarian mudstones at the foot of the sequence.

The form of disordered cristobalite in cavities ranges from microspherules of radiating bladed crystals, the ‘lepispheres’ of the Deep Sea Drilling Project (DSDP) to bladed overgrowths, and fibrous silica. In contrast, within the fine grained matrix, the disordered cristobalite takes the form of partly coalescent crude microgranules and microspherules.

Most of the chalcedonic quartz in Cyprus is derived by recrystallization of previously inorganically precipitated disordered cristobalite rather than by direct precipitation.

According to the concept of impurity‐controlled maturation the composition of host sediment controls the incorporation of exchangeable cations and other impurities into inorganically precipitated disordered cristobalite. With time (up to 100 million years) internal solid state reorganization of the disordered cristobalite is accompanied by gradual expulsion of impurities, until the cristobalite dissolves followed by quartz precipitation. Complete conversion to quartz takes place first in porous calcareous sediments free of impurities, as in the Cyprus calciturbidites; in fine grained clay‐rich sediments, like Cyprus radiolarian mudstones, disordered cristobalite persists much longer. Impurity‐controlled maturation also helps explain the diagenesis of Cyprus chert nodules.
Grain‐size parameters of insoluble residues in mixed terrigenous‐skeletal carbonate sediments and sedimentary rocks: some New Zealand examples
Authors: NELSON CAMPBELL S.
Published: June 2006

Grain‐size parameters derived from the mechanical analysis of acid‐insoluble residues from mixed terrigenous‐shell beach sediments from Auckland, New Zealand, are more effective for interpreting the depositional processes, and for characterizing the beach environment on textural scatter plots, than are parameters based on the analysis of the total beach samples. This results mainly from the wide variations in size of hydraulically equivalent skeletal carbonate grains in the sediment, especially in the coarser size grades, but may also be accentuated by the local origin and susceptibility to alteration of the shell material. Compared with the total sediment grain‐size distribution, the insoluble residues from the beach deposits are characteristically finer‐grained, better sorted, and more consistently coarse‐skewed and leptokurtic.

The grain‐size distribution characteristics of insoluble residues from several stratigraphic units in a sequence of Oligocene mixed terrigenous‐skeletal shelf sediments in the South Auckland region distinguish depositional mechanisms and environmental energy levels for each unit that are consistent with interpretations made on the basis of carbonate petrography and palaeontology. With regard to the bulk content of terrigenous mud in the insoluble residues, the grain‐size distribution of the fraction coarser than 4φ is alone diagnostic of the energy‐time trends in these sediments.

The grain‐size parameters of the acid‐insoluble residues in modern and ancient mixed terrigenous‐skeletal carbonate sediments may provide more reliable criteria for distinguishing and characterizing the depositional environment of these deposits than do the parameters obtained from the size distribution of the total grain population.
Basic types of stratification in small eolian dunes
Authors: HUNTER RALPH E.
Published: June 2006

The thinnest recognizable strata in modern eolian dune sands can be grouped into six classes. They are herein named planebed laminae, rippleform laminae, ripple‐foreset crosslaminae, climbing translatent strata, grainfall laminae, and sandflow cross‐strata.

Planebed laminae are formed by tractional deposition on smooth surfaces at high wind velocities. They are very rare in the deposits studied. Grainfall laminae are also formed on smooth surfaces, largely by grainfall deposition in zones of flow separation. They are much more common than planebed laminae, which they closely resemble.

Eolian climbing‐ripple structures are composed primarily of climbing trans‐latent strata, each of which is the depositional product of a single climbing ripple. Climbing translatent strata that formed at relatively high or supercritical angles of ripple climb are typically accompanied by rippleform laminae, which are wavy layers parallel to the rippled depositional surfaces. Ripple‐foreset crosslaminae, which are incomplete rippleform laminae produced when the angle of ripple climb is relatively low or subcritical, are rarely visible in eolian sands.

Sandflow cross‐strata are formed by the avalanching of noncohesive sand on dune slipfaces. Their form varies with slipface height and with other factors.
Deep distributary channels and giant bedforms in the Upper Carboniferous of the Central Pennines, northern England
Authors: McCABE PETER J.
Published: June 2006

Channels, 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

The 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

The 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

The 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

Large‐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

A 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.
A new aeolian sand trap design
Authors: LEATHERMAN S. P.
Published: June 2006

A vertical rod sand trap has been constructed to acquire a volumetric measure of wind‐blown sand. The design is simple, construction cost is minimal, and collection efficiency is quite high. This device can be used effectively for short‐term monitoring of aeolian transport.
Sand transport model of barchan dune equilibrium
Authors: HOWARD A. D. , MORTON J. B. , GAD‐EL‐HAK MOHAMED , PIERCE DEBORAH B.
Published: June 2006

Erosion 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

The 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

Surge 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

Petrographic 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

The 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.