bluuebunny/crossref_metadata_2025 huggingface dataset:
Triassic carbonate submarine fans along the Arabian platform margin, Sumeini Group, Oman
Authors: WATTS KEITH F.
Published: June 2006

ABSTRACT

The Sumeini Group formed along the passive continental margin slope that bounded the northeastern edge of the Arabian carbonate platform. With the initial development of this passive continental margin in Oman during Early to Middle Triassic time (possibly Permian), small carbonate submarine fans of the C Member of the Maqam Formation developed along a distally steepened slope.

The fan deposits occur as several discrete lenticular sequences of genetically related beds of coarsegrained redeposited carbonate (calciclastic) sediment within a thick interval of basinal lime mudstone and shale. Repeated pulses of calciclastic sediment were derived from ooid shoals on an adjacent carbonate platform and contain coarser intraclasts eroded from the surrounding slope deposits. Sediment gravity flows, primarily turbidites with lesser debris flows and grain flows, transported the coarse sediments to the relatively deep submarine fans. Channel erosion was a major source of intraformational calcirudite.

Two small submarine fan systems were each recurrently supplied with calciclastic sediment derived from point sources, submarine canyons. The northern fan system retrogrades and dies out upsection. The southern fan system was apparently longer‐lived; calciclastic sediments in it are more prevalent and occur throughout the section. The proximal portions of this fan system are dominated by channelized beds of calcirudite which represent inner‐ to mid‐fan channel complexes. The distal portions include mostly lenticular, unchannelized beds of calcarenite, apparently mid‐ to outer‐fan lobes.

Carbonate submarine fans appear to be rare in the geological record in comparison with more laterally continuous slope aprons of coarse redeposited sediment. The carbonate submarine fans of the C Member apparently formed by the funnelling of coarse calciclastic sediment into small submarine canyons which may have developed due to rift and/or transform tectonics. The alternation of discrete sequences of calciclastic sediment with thick intervals of ‘background’ sediment resulted from either sea‐level fluctuations or pulses of tectonic activity.
The pattern of grainfall deposition in the lee of aeolian dunes
Authors: ANDERSON ROBERT S.
Published: June 2006

ABSTRACT

A simple model for the deposition pattern in the lee of aeolian dunes is presented that relies heavily upon a recently developed understanding of aeolian saltation. Grainfall deposition at any position on the lee face is the result of all saltation trajectories that leave any point on the surface of the dune upwind of the brink with sufficient initial velocity to travel the intervening distance. The deposition rate at any position on the lee slope is obtained by integrating over all combinations of initial position and required velocity, the velocity being weighted by its probability density.

The resulting calculated total deposition rate patterns show distinct maxima on the order of one to a few decimetres from the brink, beyond which deposition rates fall off roughly exponentially. An important length scale emerges that characterizes this decay with distance from the brink, the length increasing with wind velocity, and decreasing with grain diameter. It is shown that this length scale is on the order of one metre for typical grain size and wind conditions. That this is typically smaller than the length of the lee slope is what gives rise to the oversteepening and eventual avalanching of the lee sides of aeolian dunes. The position of a pivot point on the lee slope may be predicted, separating source regions from accumulation regions for grainflow avalanche deposits.

The calculated patterns provide not only a means for quantitative interpretation of active and fossil dune grainfall deposits, but they provide the initial geometry for grainflow avalanches. The initial failures should coincide with the steepest gradient in grainfall deposition, slightly downslope from the grainfall maximum.
The impact process in aeolian saltation: two‐dimensional simulations
Authors: WERNER B. T. , HAFF P. K.
Published: June 2006

ABSTRACT

A critical event in the trajectory of a sand grain saltating in air is its interaction with the surface. We examine the phenomenon of grain‐bed impacts in two dimensions using a combination of dynamical computer simulations, analytical models and physical reasoning. The results indicate that the grain‐bed collisions can be treated as two‐body collisions with the bed particle assuming an effective mass greater than its true mass. Also, the presence of geometrical surface irregularities has a strong bearing on the interaction between saltating and surface grain populations, as well as on the formation of small‐scale bedforms.
Late Pleistocene subaerial debris‐flow facies of the Bow Valley, near Banff, Canadian Rocky Mountains
Authors: EYLES NICHOLAS , EYLES CAROLYN H. , McCABE A. MARSHALL
Published: June 2006

A thick late Pleistocene sequence, comprising multiple beds of massive diamict facies resting on outwash gravels, occurs along the Bow River, near Banff, Alberta. Diamicts have a simple sheet‐like geometry which dip downvalley at between 5° and 10°, with largely conformable bedding contacts. The sediments are strongly bimodal in texture, consisting of clasts supported by a silty sand matrix. Prolate clasts show a weaka‐axis alignment parallel and transverse to the trend of the Bow Valley with a weak imbrication. Clasts of soft sediment are common. The diamict sequence has a maximum thickness of 30 m, infills the valley floor over an area of approximately 12 km2and buries a channelled topography cut on the surface of underlying outwash gravels. Abandoned braided channels, veneered by windblown sand containing volcanic tephra dated at 6600 yrbp, occur on the infill surface.

Diamict facies were emplaced as subaerial debris flows derived by the mixing and remobilization of large volumes of outwash and glaciolacustrine sediment. Orientation data from clasts and a few basal grooves indicate that debris was derived upvalley and from adjacent valleyside slopes. A major episode of debris flow sedimentation may have been triggered by the abrupt drainage of lakes in tributary valleys as the Bow Valley glacier retreated. Downslope resedimentation of large volumes of sediment resulted in braided river aggradation downstream along the Bow River and can be dated to between 12 000 and 10 000 yrbp. Similar conditions probably occurred during deglaciation throughout the Canadian Cordillera and many other alpine areas, where poorly‐sorted late glacial debris‐flow facies have been mistaken for tills deposited directly by glaciers. The late glacial setting identified in this paper provides a basis of comparison with ancient continental glacial facies preserved in areas of active tectonism and high relief.
A sand budget for the Alexandria coastal dunefield, South Africa
Authors: ILLENBERGER WERNER K. , RUST IZAK C.
Published: June 2006

The sand in the Alexandria coastal dunefield is derived from the sandy beach which forms the seaward boundary of the dunefield. Sand is blown off the beach onto the dunefield by the high‐energy onshore‐directed dominant wind. The dunefield has been forming over the past 6500 years.

Sand transport rates calculated from dune movement rates and wind data range from 15 to 30 m3 m ‐1 yr‐1 in an ENE direction. The sand transport rate decreases with increasing distance from the sea due to a reduction in wind speed resulting from the higher drag imposed upon the wind by the land surface. Aeolian sand movement rates of this order are typical of dunefields around the world.

The total volume of sand blown into the dunefield is 375 000 m3 yr‐1. Sand is being lost to the sea by wave erosion along the eastern third of the dunefield at a rate of 45 000 m3 yr ‐1. The dunefield thus gains 330 000 m3 of sand per year. This results in dunefield growth by vertical accretion at about 1.5 mm yr‐1 and landward movement at about 0.25 m yr‐1. The dunefield is a significant sand sink in the coastal sand transport system.

The rate of deposition in coastal dunefields can be 10 times as high as rates of deposition in continental sand seas. The higher rate of deposition may result from the abundant sand supply on sandy beaches, and the higher energy of coastal winds.

Wind transport is slow and steady compared to fluvial or longshore drift transport of sediment, and catastrophic aeolian events do not seem to be significant in wind‐laid deposits.
Anomalous geochemical signals from phosphatic Middle Cambrian rocks in the southern Georgina Basin, Australia
Authors: DONNELLY TERRENCE H. , SHERGOLD JOHN H. , SOUTHGATE PETER N.
Published: June 2006

Middle Cambrian rocks of the Georgina Basin contain both phosphatic and organic‐rich sediments, the former often occurring as commercially viable phosphate deposits, and the latter as minor oil shale occurrences. Both kinds of rocks occur in the Hay River Formation penetrated by Bureau of Mineral Resources (BMR) stratigraphic corehole Tobermory 14. Detailed sampling and analysis of this core has been undertaken to assess the chemostratigraphy of the Hay River Formation; to examine the possible use, as a palaeoceanographic tool, of changes in δ13C values of carbonates and kerogens; and to gain an understanding of phosphate deposition in this part of the basin.

The Upper Hay River Formation was deposited in a normal marine environment characterized by oxic bottom conditions. It contains several episodes of significant trace metal enrichment which appear to indicate periodic replacement of the water body. The Lower Hay River Formation, however, was deposited in an abnormal marine environment low in sulphate and Mo and enriched in P. Its carbonate phase has positive δ13C values 1–1.5%0 enriched over the accepted Cambrian marine carbonate value of ‐0.57 ± 0.17%0. The very low sulphate concentration, Mo‐depletion and spiking of carbonate δ18O values suggests there was an influx of deeper ocean water of sufficient volume to maintain prolonged stable anoxic bottom waters. These data support a previous suggestion that phosphogenesis in the Georgina Basin was related to ‘Oceanic Anoxic Events'.
Biogenic laminar calcretes: evidence of calcified root‐mat horizons in paleosols
Authors: WRIGHT V. P. , PLATT N. H. , WIMBLEDON W. A.
Published: June 2006

Laminar calcretes are described from the Lower Carboniferous of South Wales, the Upper Jurassic of southern England and the Upper Jurassic‐Lower Cretaceous of northern Spain. They are interpreted as calcified root‐mats (horizontal root systems) and are compared with other examples in the geological record and with possible modern analogues. All three occurrences consist of virtually identical, centimetre to decimetre‐thick, locally organic carbon‐rich, laminar micrites containing up to 50% by volume of millimetre‐sized typically calcite‐filled, tubular fenestrae set in an irregular but very finely laminated matrix. It is suggested that root‐mat calcretes are probably very common in the geological record in peritidal, lacustrine margin and floodplain deposits, but owing to their crudely biogenic microstructure, they more closely resemble cryptalgal laminites than do other laminar calcretes. The recognition of such root‐mat calcretes in sedimentary sequences not only provides evidence of subaerial exposure and vegetation cover but can also indicate positions of palaeo‐water‐tables in certain circumstances.
Evaluation of aeolian sand transport equations using intertidal zone measurements, Saunton Sands, England
Authors: SARRE R. D.
Published: June 2006

Sand transport rates were measured using a vertical sand trap along the intertidal zone of a beach in North Devon, England, together with simultaneous monitoring of the wind speed on the beach and moisture levels in the surface layers of sand. The results of 88 sand trap samples in a wide range of wind speeds showed that moisture levels up to 14%, in the top 1–2 mm of the beach sand, have no discernible effect on the transport rates. Transport rates measured from areas of the beach where the moisture was below this level are compared with the rates predicted by seven expressions based on theoretical and wind tunnel research together with the empirical results of other published research. Measured transport rates range from 0.0001 to 0.22 kg m‐1 s‐1. The results indicate that expressions based on a power relation between the wind speed and the transport rate, and which include a threshold velocity term, provide the best estimates of the observed transport rates.
Applications of grain‐pivoting and sliding analyses to selective entrapment of gravel and to flow‐competence evaluations
Authors: KOMAR PAUL D. , LI ZHENLIN
Published: June 2006

Models of sediment threshold by grain pivoting or sliding over underlying particles are examined in order to explore their application to evaluations of selective entrainment of gravel by flowing water. Of special interest is whether such process‐based models provide satisfactory evaluations of flow competence and the movement of large clasts by floods. A detailed derivation is undertaken, focusing first on the fluid flow and forces at the particle level. The resulting threshold equation for the particle‐level velocity is then modified to yield the mean entrainment stress for the flow as a whole. This approach is appropriate for considerations of selective entrainment of grains of varying sizes within a deposit, the sorting being due to their relative projection distances above the bed and the dependence of their pivoting angles on grain size and shape. The resulting threshold equations contain a number of coefficients (e.g. drag and lift) whose values are poorly known, but can be constrained by requiring agreement with the Shields curve for the threshold of grains in uniform deposits. If pivoting coefficients based on laboratory measurements with tetrahedral arrangements of particles are used in the models, smaller degrees of selective sorting are predicted than found in the field measurements of gravel entrainment. However, if reasonable modifications of those coefficients are made for expected field conditions, then the models yield good agreement with the data. Sliding models, where sorting is due entirely to projection distances of the grains above the bed, yield somewhat poorer agreement with the field data; however, the sliding models may have support from laboratory experiments on gravel entrainment in that the data and theoretical curves have similar concave trends. The existing measurements lack documentation of the mechanisms of grain movement, so it is not possible to conclusively determine the relative importance of grain pivoting versus sliding. In spite of such uncertainties, the results are encouraging and it is concluded that pivoting and sliding models for grain entrainment do have potential for field computations of selective entrainment and flow competence.
Tidal inlet sequence, Sundance Formation (Upper Jurassic), north‐central Wyoming
Authors: UHLIR DAVID M. , AKERS ARTHUR , VONDRA CARL F.
Published: June 2006

The sandstones and coquinas of the upper 20 m of the Sundance Formation are interpreted as a tidal inlet, back‐barrier shoal and sandy tidal‐flat sequence deposited at the close of marine Jurassic sedimentation in north‐central Wyoming. The barrier strandline maintained a generally E‐W trend as it prograded to the north. The lateral migration of inter‐barrier tidal inlets along the regressive shoreline of the late Sundance sea caused the coquinas and sandstones of the uppermost Sundance Formation to be deposited as tabular, laterally‐extensive units. Tidal bundles, sigmoidal reactivation surfaces, herringbone cross‐lamination and abundant mud drapes within the sandstones are evidence of considerable tidal influence during the deposition of the uppermost Sundance Formation. Earlier models, which attach an offshore environment of deposition to the sequence, do not explain the tabular geometries of the sandstone and coquina units and their conformable stratigraphic relationship with the overlying non‐marine sediments of the Morrison Formation.
Progradational gravel beach sequences in a moderate‐ to high‐energy, microtidal marine environment
Authors: MASSARI F. , PAREA G. C.
Published: June 2006

The anatomy of progradational gravel beaches was analysed in two different tectonic settings: the strongly subsiding Messinian foreland basin of the eastern Southern Alps and the rising Pleistocene marine terraces of the outer margin of the Apenninic chain. Progradation took place near the mouths of fan‐delta or braid‐delta distributaries debouching in microtidal landlocked basins (palaeo‐Adriatic and palaeo‐Ionian seas). A wind‐wave climate, comparable to that of the present‐day, was characterized by periodic intense storm activity. Most of the beachface progradation is thought to have taken place during the post‐storm recovery and fair‐weather stages, whereas the impact of storms is mostly recorded by the cutting of erosional surfaces on the beachface and by emplacement of poorly‐sorted and coarse‐grained gravels on the shoreface by storm‐intensified seaward‐trending flows and behind the highest berms by washover processes. Analysis of the sequences led to the identification of a number of typical divisions which are differently developed in the two settings compared here. The characteristics of the Apenninic sequences suggest a strongly variable wind‐wave climate, with periodic changes from a relatively dissipative barred morphology during storms, accompanied by strong longshore currents and rip development, to a more reflective state, typical of recovery and fair‐weather stages. The Southern Alpine sequences, on the other hand, are characterized by poor evidence of barred morphology and of longshore currents and widespread effects of shear‐ and gravity‐sorting in the lower‐beachface gravels. These features indicate a comparatively more reflective average state, due to location of beaches along the deeply embayed head of the palaeo‐Adriatic, and the very narrow directional width of the incoming wave spectrum, which was almost shore‐normal most of the time. In addition to well‐developed shape zonation of gravels, the common presence of wave‐generated gravel megaripples in the Southern Alpine sequences indicates the greater influence of long‐period waves, due to the greater available fetch distances. In both settings the cyclicity is thought to be genetically related to minor changes of sea‐level.
A Late Pleistocene coarse‐grained spit‐platform sequence in northern Jylland, Denmark
Authors: NIELSEN LARS HENRIK , JOHANNESSEN PETER N. , SURLYK FINN
Published: June 2006

Several well‐preserved Late Pleistocene spit systems occur uplifted in northern Jylland, Denmark. Their present‐day morphological expression allows detailed study of spit growth patterns while the internal sedimentological organisation can be examined in a series of pits distributed along the length of the spits. Two characteristic vertical sequences are recognized in the systems. The first (Sequence I) consists of a giant‐scale cross‐bedded foreset unit, overlain by topset and beach units, while the second (Sequence II) consists of the foreset unit overlain by bar‐trough and beach units. The two sequence types pass laterally into each other with a short overlap zone. They can be interpreted in terms of Meistrell's (1966, 1972) model for spit‐platform growth based on scaled wave tank experiments. The giant‐scale cross‐bedded unit corresponds to prograding of a coarse‐grained subaqueous spit‐platform while the topset, bar‐trough and beach units reflect the growth of the subaerial spit. The alternation between sequence I and II reflects the inversely related growth of the spit and platform structures: when the rate of subaqueous platform progradation declines, the subaerial spit grows uniformly, and when the platform progrades uniformly spit growth declines. The model is probably only valid for relatively coarse‐grained systems because only these deposits would have a relatively steep front. The water depth in which the spit system progrades and thus bottom topography, determines the thickness of the giant‐scale cross‐bedded foreset unit because the water depth over the top of the platform is relatively constant. If the water is less than a few metres deep the spit‐platform is not developed as seen where the Late Pleistocene spit systems prograded over elevations of the sea bottom. Conversely, the correct recognition of spit‐platform sequences allows precise determination of sea‐level and water depth at the time of formation. Finally, the model adds one further mode of formation of giant‐scale cross‐bedding to those already known from fluvial transverse, lateral and point bars, subtidal sand waves and Gilbert deltas.
Algodones dune field of southeastern California: case history of a migrating modern dune field
Authors: SWEET M. L. , NIELSON JAMIE , HAVHOLM K. , FARRELLEY JOHN
Published: June 2006

The Algodones dune field of southeastern California is one of the largest active dune fields in North America. The dune field is migrating in an easterly direction, oblique to the resultant sand flow direction (S 24° E). The migration of the Algodones results from an interaction between regional winds and the dune field. This interaction generates a localized secondary flow that has caused the dune field to migrate in a direction oblique to the resultant sand flow direction.

Four lines of evidence suggest that the Algodones has migrated in an easterly direction: (1) A ramp, interpreted as the trailing edge of the dune field, 35 m thick and 500 m wide composed of aeolian deposits that borders the western edge of the dune field. No similar deposits are found on the eastern (leading edge) margin of the dune field. (2) Leading‐edge sand‐sheet deposits are exposed in interdune areas within the dune field. These deposits are west of the modern leading‐edge sand sheet. (3) Across the breadth of the dune field sands are consistently coarser and more poorly sorted in the west and finer and better sorted in the east. This observation suggests that sand is transported from west to east. (4) Eastward migration of a large compound‐complex crescentic dune.

If the dune field continues to migrate it will deposit a vertical sequence consisting of: a basal sand‐sheet deposit consisting of wind and water‐ripple laminae, small‐scale aeolian cross‐strata, and ephemeral stream (wadi) deposits; aeolian dune deposits consisting of medium‐scale aeolian compound cross‐strata; small‐scale simple sets of aeolian cross‐strata with highly variable dip directions; a sand sheet containing low‐angle wind‐ripple cross‐strata capped by a coarse sand lag super bounding surface.
Facies and origin of shales from the mid‐Proterozoic Newland Formation, Belt Basin, Montana, USA
Authors: SCHIEBER JÜRGEN
Published: June 2006

Shales constitute more than 60% of the world's sediments, yet while facies models for sandstones and carbonates are at a high level of sophistication, the study of shales has clearly lagged behind. In the mid‐Proterozoic Newland Formation six major shale facies types, deposited in nearshore to basinal environments, are distinguished on the basis of bedding characteristics, textural features, and the proportions of silt, clay and carbonate. Textural features of these shale types are related to sedimentary environments as deduced from associated lithologies. The shales are undisturbed by bioturbation, and their textural and sedimentary characteristics reflect subaqueous growth of microbial mats, erosion and deposition by storms, deposition of flocculated vs. dispersed clays, continuous slow background sedimentation, winnowing by waves or currents, and subaerial exposure.
Turbidites and iron formations, Beardmore‐Geraldton, Ontario: application of a combined ramp/fan model to Archaean clastic and chemical sedimentation
Authors: BARRETT T. J. , FRALICK P. W.
Published: June 2006

Sandy turbidites, grain flows, conglomeratic mass‐flows and oxide‐facies iron formation are present in the late Archaean Beardmore‐Geraldton terrain, a metasedimentary belt which extends for at least 80 km in an E‐W direction. The marine portion of this basin contains four lithofacies associations (LA): (1) Thinbedded, iron formation‐clastic sediment association. This association represents a continuum of deposit types containing iron formation; subtypes are defined on the basis of bedding attributes and the proportion of iron formation to sand/silt. (2) Thin‐bedded, turbidite‐dominated association. These sediments consist mostly of silt/sand beds which either show no vertical trends, or thin and fine upwards over a few metres. (3) Medium‐bedded, turbidite‐dominated association. Most of these sediments are medium to coarsegrained, vertically unstructured sand sequences with occasional structured intervals. (4) Thick‐bedded association. This is dominated by poorly graded sands up to 7–8 m in thickness. Sand beds are characterised by a thin basal zone of coarse sand and pebbles, a large central interval containing a mixture of medium and coarse sand, and a thin upper zone of fine sand/silt.

The overall depositional system was initiated by transport of sediment by braided streams to the strand area where it accumulated in distributary mouth bars. We infer a nearshore break in slope, locally with large channels (LA 4) extending from close to the strand line across deltaic surfaces to the deeper portions of submarine fans (structured portions of LA 3). However, many deltaic surfaces probably were not tapped by major channels, but merged downslope into a submarine ramp. Sediment was transported across the ramp by slump events and sheet‐like grain flows (unstructured portions of LA 3). Iron formation and LA 2 sediments probably accumulated both in upper‐mid ramp areas with low sediment delivery rates, and distal to fan‐ramp successions.

As major streams on the braid plain changed position, associated submarine channels and slump‐fed ramp deposits also would have shifted laterally. This produced overlap of different facies associations in both the fan and ramp environments, which may explain why observed vertical trends in bedding are limited to several metres. We suggest that on narrow, active Archaean cratonic margins, additional non‐regular processes such as variations in sediment supply related to periods of heightened pyroclastic activity, and seismic activity associated with the arc, also contributed to the lack of vertically structured sequences. Turbidite sequences in such environments in general may contain important contributions from both submarine ramp and submarine fan sedimentation.
Multiple paleosols of the late Albian Boulder Creek Formation, British Columbia, Canada
Authors: LECKIE DALE , FOX CATHERINE , TARNOCAI CHARLES
Published: June 2006

Fifteen lithified paleosols, closely spaced in vertical sequence, occur in the top 90 m of the late Albian Boulder Creek Formation in the foothills of northeastern British Columbia, Canada. The paleosols have well‐developed profiles 0·5 to 1·5 m thick, including A, B and C horizons. The paleosols are characterized by their grey colour, cutans, vertical roots, peds, spherulitic siderite and absence of sedimentary structures. The paleosols formed during a period when one or more basin wide unconformities occurred as a result of either eustatic sea level fluctuations or local tectonic events. These unconformities represent the terrestrial record of a lowered base level which caused valley incision and decreased rates of sedimentation on the incised flood plain. The climate was humid to subhumid. Overall, the environment in which these soils developed was of low relief and subject to little erosion. The water table was high for part of the year but there is also evidence of periodic drying and oxidation of organic debris. Cumulatively, the paleosols in this interval may represent 150000 yr of non‐deposition.
Relationships between cyclicity and stromatolite form in the Late Proterozoic Bitter Springs Formation, Australia
Authors: SOUTHGATE PETER N.
Published: June 2006

Stromatolite biostromes and bioherms in the lower two units of the Late Proterozoic Loves Creek Member of the Bitter Springs Formation represent shallowing upward and deepening upward sequences. In the central unit stromatolite form is governed by relative position in an asymmetric shallowing upward sequence. Ooid and/or peloid‐intraclast grainstones and small, irregular bulbous and columnar stromatolites characterize the basal, transgressive portion of cycles. Domal, columnar and stratiform stromatolites comprise the bulk of the cycle. These forms accreted in a gradually shallowing epeiric sea. Domal stromatolites predominate in the deeper parts of cycles. Here synoptic relief gradually increases upwards. Columnar and stratiform stromatolites predominate in the shallower parts of cycles, where synoptic relief rapidly diminishes upwards. In thin‐bedded dolo‐mudstones at the tops of cycles the co‐occurrence of desiccation cracks, tepee structures, scalloped dissolution surfaces, gypsum moulds and anhydrite nodule pseudomorphs provides evidence for subaerial exposure. In contrast, stromatolites in a unit at the base of the Loves Creek Member accreted during a gradual rise in sealevel. Stratiform, columnar and domal stromatolitic building blocks of the shallowing upward cycle are present in this deepening sequence, but only the lower half of the shallowing upward cycle is represented. Synoptic relief of the stromatolitic laminae gradually increases upward throughout the basal stromatolitic unit. Recognition of a deepening upward stromatolite sequence at the base of the Loves Creek Member, and a disconformity surface between this sequence and the underlying Gillen Member, permits palaeoenvironmental re‐interpretation of the Loves Creek Member as a single ‘large scale’ sea‐level cycle.
Three distinctive feeder‐lobe systems within one time slice of the Tortonian Tabernas fan, SE Spain
Authors: KLEVERLAAN KICK
Published: June 2006

Excellent exposures and the presence of lithological markers make it possible to reconstruct the fan facies pattern for a narrow (6000–60 000 yr), late Tortonian (7–8 Ma) time slice of a submarine fan complex that developed in the Tabernas basin. Unlike most classical fan models, which seem to refer to a single feeder‐lobe system, the reconstruction from the Tabernas basin reveals at least three distinctive juxtaposed feeder‐lobe systems: (I) a straight feeder valley terminating in a sandstone body consisting of stacked sand‐filled scours (sand‐rich system); (II) a straight feeder channel diverging in a broad front of nested scours in mudstones. These scours terminate in fine grained sheet‐like turbidite deposits (muddy system); (III) a sinuous channel complex extending far into the basin without ‘lobe’ deposits at its mouth (solitary system).

Sedimentary features indicate that in the Tabernas basin, initial basin floor morphology and mass‐flow transport behaviour controlled and eventually stopped formation of the sand‐rich system (I), while depositional topography and slope instability controlled formation of the muddy system (II). The unusual narrowness (10–30 m) and length (8 km) of the solitary system (III) points to its confinement, possibly a result of an intrabasinal fault escarpment trending obliquely to the general direction of slope. Flow stripping (Piper & Normark, 1984) could then explain the absence of a lobe at the channel terminus.
Upper‐regime parallel lamination as the result of turbulent sediment transport and low‐amplitude bed forms
Authors: PAOLA CHRIS , WIELE STEPHEN M. , REINHART MARY A.
Published: June 2006

A series of laboratory experiments has been carried out in which parallel‐laminated deposits were produced from an upper‐regime plane bed. The laminae had thicknesses of a few mm and could be traced continuously over distances up to the length and width of the depositional area (0–3 m by 1–5m). Fluctuations in bed elevation were measured both during deposition and at equilibrium; much of the bed fluctuation occurs at time scales that are too long to be due directly to turbulence, as most theories for lamina formation would require. We suggest instead that extremely low‐amplitude bed forms are present even on apparently flat beds and that the migration of these bed forms produces laterally continuous lamination.

All the lamination produced in the laboratory experiments was normally graded. Using high‐speed photography it was observed that the normal grading results from rapid deposition of a layer of loosely packed coarse sand several grain‐diameters thick followed by the slow sieving‐out of a well packed surface layer of finer sand. The initial deposition is the result of small‐scale turbulent fluctuations in boundary shear stress. The sieving‐out that follows results in a smooth surface whose low friction coefficient temporarily inhibits further deposition; we term this process ‘glazing’. The alignment of small‐scale turbulent scour‐and‐fill structures along the paths traced by migrating bedform troughs produces laterally continuous parallel lamination.
Sedimentology of a saline playa in the northern Great Plains, Canada
Authors: LAST WILLIAM M.
Published: June 2006

Ceylon Lake, a small salt playa located in southern Saskatchewan, is typical of many shallow ephemeral lacustrine basins found in the northern Great Plains of western Canada. The present‐day brine, dominated by magnesium, sodium, and sulphate ions, shows wide variation in composition and concentration on both a temporal and a spatial basis. The modern sediments overall exhibit relatively simple facies relationships. An outer ring of coarse grained shoreline and colluvial clastics surrounds mixed fine grained clastics and salts and, in the centre of the basin, salt pan evaporites composed mainly of mirabilite, thenardite, and bloedite.

Coring of the late Pleistocene and Holocene sedimentary fill shows that the lake has evolved from a relatively dilute, deep water, clastic dominated basin through a shallower, brackish water, carbonate‐clastic phase, and finally into the present salt dominated playa. The thick sequence of evaporites preserved in the basin suggests evolution of the brine from a Na‐rich solution to a mixed Mg‐Na system. The most important post‐depositional processes affecting Ceylon Lake sediments are mud diapirism and salt karsting.
Measurements of turbulence in the benthic boundary layer over a gravel bed
Authors: WILLIAMS JON J. , THORNE PETER D. , HEATHERSHAW ANTHONY D.
Published: June 2006

It is now generally recognized that the mathematical description of sediment dynamics will be improved through an understanding of geophysical turbulence. In this study, therefore, the turbulence characteristics of the benthic boundary layer over coarse sediments are examined as a step towards the realization of such a description.

Examination of the turbulence has been conducted using spectral, correlation, quadrant and statistical analyses, and a number of features have been identified. Mean eddy length scales obtained using spectral and autocorrelation techniques were found to be similar. The interpretation of the duration and interval between the intermittent ‘bursting’ events, scaled using outer flow variables, was found to be dependent upon the sampling interval. Autocorrelation analysis revealed a high correlation between bedload transport and the instantaneous horizontal flow component rather than instantaneous kinematic stress. From the standpoint of developing prognostic transport models based on physically realistic benthic flow processes, it is considered that turbulence measurements will lead to such formulation.
Kinetic energy transfer through impact and its role in entrainment by wind of particles from frozen surfaces
Authors: NEUMAN CHERYL MCKENNA
Published: June 2006

While extensive aeolian deposits are associated with glaciation, little is known of the mechanisms which produced them. Observation of glaciofluvial sediments in Pangnirtung Pass, N.W.T., indicates that peak deflation occurs in the fall and winter months so that the nature of sediment transport involving frozen surfaces greatly differs from warm environments. Laboratory experiments indicate that such surfaces are mobilized through abrasion, and are most susceptible at low volumetric water content (less than 20%) and low temperature (–20°C). Stable surfaces have volumetric water contents in excess of 30% and temperatures of –15°C. Surface stability also increases with decreasing particle size, density and, possibly, transport load. Collisions with frozen surfaces are relatively elastic with significantly greater kinetic energy transfer to ejected surface grains than observed for loose dry surfaces. Saltation transport in cold environments could therefore be sustained at lower windspeeds than for dry surfaces provided that some loose particles are available initially but do not accumulate at the surface.
Late Proterozoic Patsy Springs Canyon, Adelaide Geosyncline: submarine or subaerial origin?
Authors: von der BORCH C. C. , GRADY A. E. , EICKHOFF K. H. , DIBONA P. , CHRISTIEBLICK N.
Published: June 2006

A significant aspect of Late Proterozoic sedimentation in the Adelaide Geosyncline, South Australia, is the presence of kilometre‐deep erosional incisions which have been termed canyons. These structures were formerly described to be of submarine origin, cut and filled in an inferred basin‐slope setting by subaqueous processes. Subsequent detailed research, particularly on a specific incision known as Patsy Springs Canyon, indicates that sedimentary structures within some of the canyon‐filling sediments are indicative of deposition above fair weather wave base. In addition, an unusual carbonate unit, which is observed to veneer upper portions of canyon shoulders and to contribute to carbonate breccias interbedded with canyon‐fill, has a stable isotope signature which may imply a non‐marine origin. The presence of the carbonate veneer, where it is in situ, suggests that at least upper portions of the canyons could have been emergent during the canyon‐filling phase. Considering these observations, and combining them with regional stratigraphical relationships, an alternative model for canyon genesis is proposed involving subaerial erosion and subsequent filling by coastal onlap. Such a model requires base‐level changes of the order of 1 km, in order to account for observed canyon cutting and filling. Vertical movements associated with halokinesis, or thermally‐induced uplift of the order of 1 km, could have resulted in the observed erosional events. Alternatively, a Messinian‐style evaporitic lowering of base‐level is currently receiving serious attention. With present knowledge this mechanism most satisfactorily explains all observations.
Chertification histories of some Late Mesozoic and Middle Palaeozoic platform carbonates
Authors: MALIVA ROBERT G. , SIEVER RAYMOND
Published: June 2006

A consistent pattern for the silica sources, depositional environments and timing of chertification was observed in a diverse suite of five Late Mesozoic and Middle Palaeozoic carbonate sequences; the (1) Upper Greensand (Cretaceous) and (2) Portland Limestone (Jurassic) of southern England, (3) the Ramp Creek Formation (Mississippian) of southern Indiana, and the (4) lower Helderberg Group (Devonian) and (5) Onondaga Limestone (Devonian) of New York State. Nodular chert formation in all five limestone sequences occurred in sediments that were largely uncemented. Ghosts of pre‐chertification carbonate cements are present in some chert nodules but are volumetrically minor. In every limestone sequence except the Upper Greensand, chertification occurred after burial to a depth sufficient for intergranular pressure solution and mechanical grain deformation of carbonate sand.

Nodular chert is most abundant in subtidal, normal marine wackestones and mudstones that were deposited at or below fair‐weather wave base, and is absent or rare in supratidal, intertidal and high‐energy subtidal limestones and dolomites. An intraformational sponge spicule silica source for chert nodules is suggested by direct evidence, such as calcitized sponge spicules in the host limestone, and circumstantial evidence, such as ghosts of sponge spicules in chert nodules and a correlation of chert abundance with depositional environment. Most molds of siliceous sponge spicules were apparently obliterated by post‐chertification intergranular compaction. We propose that these general trends for the depositional environments, silica sources and timing of chertification are representative of most Mesozoic to Middle Palaeozoic platform limestones.
Glacially‐influenced deep‐marine sedimentation of the Late Precambrian Gaskiers Formation, Newfoundland, Canada
Authors: EYLES NICHOLAS , EYLES CAROLYN H.
Published: June 2006

The Late Proterozoic Conception Group, exposed on the Avalon Peninsula in Newfoundland, Canada, is a 4 km thick turbidite succession containing a conformable 300 m thick sequence of diamictites (the Gaskiers Formation) near the base. Massive and crudely‐stratified diamictites form beds up to 25 m thick which have a tabular geometry with slightly erosive basal contacts and are interbedded with mudstones and fine‐grained, thin‐bedded turbidites. These diamictites are interpreted as submarine debris flow deposits. Disrupted diamictites form strongly deformed units that contain large, complexly folded rafts of mudstone and turbidite facies. These diamictite units are interpreted as submarine slumps. Diamictites contain glacially‐striated and faceted clasts; clasts and matrix are predominantly of volcanic provenance. One outcrop shows interbedded volcanic agglomerate and diamictite, and volcanic bombs can also be identified.

The interbedding of diamictites with turbidites and the stratigraphic context provided by the thick sequences of turbidites below (Mall Bay Formation) and above (Drook Formation) indicate a deep marine slope setting of diamictite deposition. Diamictite facies record remobilization and downslope transfer of large volumes of unstable volcanic and glacial debris initially deposited in a shallower water marginal marine zone. The regional tectonic framework suggests the Conception Group accumulated in a deep, southward‐opening ensialic rift basin with active but waning volcanic centres to the north.

The Gaskiers Formation may be representative of other Late Precambrian glacially‐influenced diamictite sequences that were deposited around the North Atlantic region and in Europe. These deep marine diamictite sequences characterized by debris flows, turbidites, and slump deposits, can be contrasted with more extensive shallow marine shelf diamictite sequences found in association with dolomites and tidally influenced shallow water facies in other basinal settings.
Coal measures (Namurian and Westphalian) Blackband Iron Formations: fossil bog iron ores
Authors: BOARDMAN E. L.
Published: June 2006

Blackband iron formations are essentially thin (approximately 0·75 m thick) siderite‐rich (total iron up to 40%), carbonaceous, laminated mudrocks which commonly occur in grey coal bearing sequences in close proximity to coal seams. They exhibit a conspicuous laminated macrotexture made up of carbonaceous and siderite‐rich laminae together with primary textures such as root‐disturbed laminae, sideritized unflattened spores and preservation of plant cell detail. These all point to formation of siderite soon after deposition, in some cases before significant compaction. Their enclosing sediments clearly show that they were deposited in environments intermediate between delta top alluvial floodplains and coastal plain swamps in which small areas were subjected to periods of lacustrine deposition. The presence of varved mudrocks and oil shales in the beds directly above many of the blackband iron formations and the distinctive fine lamination of the blackbands are evidence of such depositional environments. The blackband iron formations are considered to have been formed in a similar way to Recent bog iron ores and are therefore interpreted as fossil bog iron ores. A model for their mode of formation is presented.
Saline lake carbonates within an Upper Jurassic‐Lower Cretaceous continental red bed sequence in the Atacama region of northern Chile
Authors: BELL C. M.
Published: June 2006

The Codocedo Limestone Member is a thin but laterally persistent lacustrine sequence within the red beds of the Upper Jurassic‐Lower Cretaceous Quebrada Monardes Formation, in the Atacama region of northern Chile. The thick succession of clastic terrigenous sediments of the Quebrada Monardes Formation was deposited in an arid to semi‐arid environment. Sedimentary facies are indicative of deposition of aeolian dunes, alluvial fans and braided streams, playa‐lake mudflats, and saline lakes and coastal lagoons. The strata accumulated in a N‐S elongated extensional back‐arc basin on the landward side of an active volcanic arc.

The 3 m thick Codocedo Limestone Member marks striking facies changes within the Quebrada Monardes Formation. It is underlain by a thick sequence of conglomerates and sandstones, deposited on alluvial fans. The limestone itself is characterized by evaporite minerals and laterally continuous laminations, indicative of deposition by vertical accretion in a perennial saline lake. The overlying siltstones and fine sandstones contain geodes and gypsum pseudomorphs and were deposited on playa‐lake mudflats. The limestone therefore represents a relatively short period of lacustrine deposition within an essentially terrigenous succession. The lake was possibly formed quite suddenly, for example by damming of the basin by a lava flow.

Sedimentation in the perennial lake was predominantly cyclical. Seasonal planktonic algal blooms produced millimetre‐scale laminations. Interbedded with these laminites are centimetre‐scale beds of evaporitic gypsum, anhydrite and minor halite. The evaporite minerals have been largely replaced by calcite, chalcedony and quartz. The centimetre‐scale cycles may have resulted from periodic freshwater input into the lake. After a period of about 3000 yr the lake dried up, to be replaced by extensive playa‐lake mudflats.

The Codocedo Limestone Member possibly formed a plane of detachment during an early Tertiary phase of E‐W directed regional compression. The limestones and evaporites were folded and extensively brecciated. This deformation probably resulted from simple shear along the bedding plane of the relatively weak evaporite minerals prior to their replacement by calcite and quartz.
Lacustrine carbonates and pedogenesis: sedimentology and origin of palustrine deposits from the Early Cretaceous Rupelo Formation, W Cameros Basin, N Spain
Authors: PLATT NIGEL H.
Published: June 2006

The Berriasian Rupelo Formation of the W Cameros Basin consists of a 2–200 m thickness of marginal and open lacustrine carbonate and associated deposits. Open lacustrine facies contain a non‐marine biota with abundant charophytes (both stems and gyrogonites), ostracods, gastropods and rare vertebrates. Carbonate production was mainly biogenic. The associated marginal lacustrine (‘palustrine’) facies show strong indications of subaerial exposure and exhibit a wide variety of pedogenic fabrics. Silicified evaporites found near to the top of the sequence reflect a short hypersaline phase in the lake history. The succession was laid down in a low gradient, shallow lake complex characterized by wide fluctuations of the shoreline.

Carbon and oxygen stable isotope analyses from the carbonates show non‐marine values with ranges of δ13 from − 7 to − 11‰and δ18 from − 3 to − 7.5‰. Differences in the isotopic composition of open lacustrine carbonates are consistent with sedimentary evidence of variation in organic productivity within the lake. Analyses from the entire sample suite plot on a linear trend; isotopic compositions become lighter with increasing evidence of pedogenic modification. This suggests progressive vadose zone diagenesis and influence of meteoric waters rich in soil‐derived CO2. The stable isotope data thus support evidence from petrography and facies relations that ‘palustrine’limestones form through pedogenic modification of lake carbonates.