bluuebunny/crossref_metadata_2025_split huggingface dataset:
An inlet sequence produced by migration of a small microtidal inlet against longshore drift: the Keurbooms Inlet, South Africa
Authors: REDDERING J. S. V. 
Published: June 2006

ABSTRACT Lateral migration of the Keurbooms Inlet along its barrier causes stratigraphic stacking of the different sedimentary units in the inlet area thus accumulating an inlet sequence under the newly formed barrier. Deposition in the inlet is controlled by wave and tidal action. Migration is caused by sediment accretion on the downdrift side of the inlet and erosion on its updrift side. The migration direction is therefore in the opposite direction to the longshore current. This is comparatively rare as most inlets with a tendency to migrate do so in the same direction as longshore drift. Tidal Current directions in the inlet are highly variable and not bipolar.The inlet‐associated environment is divided into a seaward wave‐dominated zone and a landward tide‐dominated zone. These zones probably have laterally interfingering facies. Only the lower part of the sequence is likely to be preserved. Recognition of ancient analogues in the rock record may be obscured by the unidirectional palaeocurrent pattern of the lower part of the inlet sequence.
Structures and textures in Triassic braided stream conglomerates (‘Bunter’ Pebble Beds) in the Sherwood Sandstone Group, North Staffordshire, England
Authors: STEEL R. J. , THOMPSON D. B. 
Published: June 2006

ABSTRACT In the northern parts of the Needwood and Stafford/Eccleshall Basins, England, the Pebble Beds of the Sherwood Sandstone Group contain thick successions of texturally mature, fluvial pebble/cobble conglomerates which are organized into either horizontal or cross‐stratified sets. The horizontally lying sets, generally coarser grained and more poorly sorted than the cross‐bedded sets, are usually disorganized and either matrix‐ or clast‐supported, although thin lenses of well‐sorted, occasionally openwork units, interpreted as falling stage phenomena, are often present. The cross‐stratified conglomerates have foresets exhibiting remarkable textural organization, with a coarse, bimodal (sometimes matrix‐supported) part grading upwards or being abruptly overlain by a finer, well‐sorted (occasionally openwork) part and finally capped by sandstone. These rhythmic textural changes are attributed partly to an avalanche process at high stage and partly to falling stage conditions. The most common types of vertical association are thick successions of horizontally bedded conglomerates (up to 20 m) and sequences of an upwards coarsening nature (2‐12 m) in which cross‐stratified sets are overlain by flat‐lying sets.The environment of deposition of the gravels is interpreted as one in which water depths at high stage were greater than depths in most modern braided stream plains (proglacial or alluvial fan) but shallower than depths associated with the Pleistocene catastrophic floods from which texturally mature, giant gravel bars have been recorded. Recent braided streams with relatively confined channels and considerable bar/channel relief are better analogues. In particular, medial or mid‐channel bars with a two‐tier structure (subaqueous and partly emergent portions) may explain the upward‐coarsening sequences in which horizontally lying conglomerates overlie cross‐stratified conglomerates. The thicker sequences of horizontally stratified conglomerates represent proximal, longitudinal bar deposits.Sheets of pebbly sandstone and argillaceous sandstone lying between the conglomerates, and commonly occurring towards the top of the succession, largely represent deposition from sandwaves and dunes. Finer, interbedded, argillaceous sandstones, siltstones and mudstones are interpreted as overbank and waning‐flood deposits.Basin‐forming tectonism of increasing intensity probably caused the initial coarsening upwards of the lower part of the succession, whilst more stable tectonic conditions and decreasing relief on the margins of the basins and in the areas of provenance in the Midlands and the Hercynides, account for the upwards‐fining of the upper part of the succession.
Deep channel sedimentation in the Lower Cretaceous McMurray Formation, Athabasca Oil Sands, Alberta
Authors: MOSSOP GRANT D. , FLACH PETER D. 
Published: June 2006

ABSTRACT In the region of the Athabasca Oil Sands, Alberta, the Lower Cretaceous McMurray Formation comprises 50‐80 m of uncemented quartz sand and associated shale, saturated throughout by bitumen. The sediments are dominantly of continental origin, except in the uppermost parts of the formation where sedimentation was influenced by the encroaching boreal sea.In most outcrop and mine face exposures of the McMurray Formation, a sequence of three facies is recognized. In ascending order these are: (1) an erosionally based thick‐bedded sand facies, 2‐20 m thick, dominated by large‐scale trough cross‐beds; (2) an epsilon cross‐stratified facies with solitary sets up to 25 m in thickness, consisting of decimetre to metre thick couplets of sand/mud, with depositional slopes of 8‐12° and palaeocurrent indications parallel to the strike of the epsilon cross‐set; and (3) a horizontally bedded argillaceous sand facies up to a few metres thick. The three‐fold sequence is interpreted as a single upward‐fining cycle of channel sedimentation, the trough cross‐bedded sands resulting from channel bottom deposition, the epsilon cross‐strata accumulating by lateral accretion of channel point bars, and the upper argillaceous sand representing floodplain sedimentation. Where the McMurray Formation is relatively thin (less than 50 m), virtually the entire formation is commonly composed of a single upward‐fining channel deposit.Details of the size and physiographic setting of the channels are somewhat uncertain, but the present evidence suggests that the epsilon‐dominated McMurray Formation sequence in the Athabasca Deposit region represents the coastal plain culmination of a very large fluvial drainage system.
The use of the fractal dimension to quantify the morphology of irregular‐shaped particles
Authors: ORFORD J. D. , WHALLEY W. B. 
Published: June 2006

ABSTRACTFor several decades, sedimentologists have had difficulty in obtaining an efficient index of particle form that can be used to specify adequately irregular morphology of sedimentary particles. Mandelbrot has suggested the use of the fractal dimension as a single value estimate of form, in order to characterize morphologically closed loops of an irregular nature. The concept of fractal dimension derives from Richardson's unpublished suggestion that a stable linear relationship appears when the logarithm of the perimeter estimate of an irregular outline is plotted against the logarithm of the unit of measurement (step length). Decreases in step length result in an increase in perimeter by a constant weight (b) for particles whose morphological variations are the same at all measurement scales (self‐similarity). The fractal dimension (D) equals 1.0‐(b), where b is the slope coefficient of the best‐fitting linear regression of the plot. The value of D lies between 1.0 and 2.0, with increasing values of D correlating with increasing irregularity of the outline. In practice, particle outline morphology is not always self‐similar, such that two or possibly more fractal elements can occur for many outlines. Two fractal elements reflect the morphological difference between micro‐scale edge textural effects (D1) and macro‐scale particle structural effects (D2) generated by the presence of crenellate‐edge morphology (re‐entrants). Fractal calibration on a range of regular/irregular particle outline morphologies, plus examination of carbonate beach, pyroclastic and weathered quartz particles indicates that this type of analysis is best suited for morphological characterization of irregular and crenellate particles. In this respect, fractal analysis appears as the complementary analytical technique to harmonic form analysis in order to achieve an adequate specification of all types of particles on a continuum of irregular to regular morphology.
Bedform distribution and inferred sand transport on Georges Bank, United States Atlantic Continental Shelf
Authors: TWICHELL DAVID C. 
Published: June 2006

ABSTRACTFour bedform provinces have been identified on Georges Bank using sidescan‐sonar and echo‐sounding techniques: large sand waves superimposed on sand ridges, small sand waves, megaripples, and featureless seafloor. The large sand waves and sand ridges are found on the bank crest where the surface tidal currents are strongest. Areas of small sand waves and megaripples, formed where tidal currents are moderate in strength, border the area of large sand waves to the north and south. Featureless seafloor is found farthest from the bank crest where surface tidal currents are weakest.Sand‐wave asymmetry and surface‐sediment texture have been used to infer bedload transport paths on Georges Bank. In the large sand‐wave area, bedforms indicate a clockwise transport around each of the linear north‐west‐striking sand ridges with slight convergence of the sand waves on the ridge crests. This transport pattern implies erosion from the troughs and accumulation on the sand ridges. The asymmetry of the small sand waves along the south side of Georges Bank indicates that sand is also transported southward away from the linear sand ridges on top of the bank. Although the asymmetry of megaripples could not be determined, the occurrence of megaripples between the small sand‐wave province and areas of featureless seafloor suggests a decreasing effectiveness of sand transport away from the bank crest. This sand dispersal pattern is further supported by the surface sediments which become progessively finer to the north and SW away from the crest of Georges Bank.
Compositional variation within some glauconites and illites and implications for their stability and origins
Authors: IRELAND B. J. , CURTIS C. D. , WHITEMAN J. A. 
Published: June 2006

ABSTRACTTextural, mineralogical and chemical data are presented for glauconites and illites from the Lower Cretaceous Glauconitic Sand Formation, Alberta, Canada. Single crystal analyses by scanning transmission electron microscopy (STEM) indicate that both glauconite (29 analyses) and illite (44 analyses) correspond very closely to ideal dioctahedral structures. Al dominates the octahedral sheet in illite. Very extensive A1 for FeIII substitution was also found in some of the glauconites, much more than previously recorded. The dioctahedral mica structure is clearly very stable and flexible with respect to A1 for FeIII substitution. Iron‐poor glauconites were found associated with pyrite. We believe that glauconites tend to lose iron progressively during burial diagenesis, especially where pore water iron activities are low (high HS− activity and low Eh). It seems probable that A1 for Si tetrahedral substitution also takes place during diagenesis with consequent loss of swelling properties. This is the same trend which is responsible for conversion of smectite to illite.Glauconite forms only where sediments reside for lengthy periods at the interface between oxidizing and reducing environments, where Fe3+ is transiently available in solution. Related marine and terrestrial nontronites also form only in similar ‘interface’ environments.
Aragonite relic preservation in Jurassic calcite‐replaced bivalves
Authors: SANDBERG PHILIP A. , HUDSON JOHN D. 
Published: June 2006

ABSTRACTShells of the aragonite bivalve Neomiodon (Great Estuarine Group, Jurassic, Scotland) replaced by coarse neomorphic calcite contain oriented relics of the original aragonite ultrastructure. The presence of these relics in such old altered shells, as well as the high Sr content of the replacement calcite, indicate that the process of calcite replacement of aragonite is not a cumulative slow process involving repeated alteration events, but rather a rapid, one‐step process. Aragonite relics, once encased in neomorphic spar, will survive as unequivocal evidence of original aragonite mineralogy, barring total remobilization of the enclosing stable calcite, a generally unlikely event. The retention of this residual aragonite and high‐Sr calcite supports recent isotopic studies which suggest that the multiple phases of alteration (‘recrystallization’) invoked in earlier literature are unlikely events in the diagenesis of most undolomitized limestones. Retention of aragonite relics appears to be independent of whether alteration occurs in shallow meteoric or, as in the case of our Neomiodon material, deeper burial environments. Pseudopleochroism of the replaced Neomiodon shells appears to be due to organic, largely graphitic, relics, not to the aragonite relics.
Evolution and mechanics of a Miocene tidal sandwave
Authors: ALLEN P. A. , HOMEWOOD P. 
Published: June 2006

ABSTRACTA remarkable exposure of Miocene marine molasse in western Switzerland records the evolution of a tidal sandwave over a period of approximately 2 1/2 months. The sandwave is composed of tidal ‘bundles’ in which a sandwave reactivation stage and full vortex stage can be recognized for the dominant flow (ebb tide) and a rippled flood apron overlain by high water drape for the reversed flow. Bundle thicknesses vary systematically through neap–spring cycles, with a periodicity of 27 demonstrating the semi‐diurnal lunar control of sedimentation. Waves were an additional component, especially when superimposed on flood tides, producing near‐symmetrical combined‐flow ripple marks in the flood apron.Tidal current velocities are estimated using critical shear velocities for entrainment, the ripple‐dune transition and the dune‐plane bed transition. Using appropriate estimates of roughness lengths and a logarithmic velocity law, maximum tidal speeds at 1 m above the bed were approximately 0·6 m sec−1 for ebbs and up to 0·5 m sec−1 for floods. The enhancement by waves of bed shear stress (τwc/τ of approximately 2 for 1 m high waves) under flood currents implies flood tidal velocities closer to 0·2–0·3 m sec−1.Peak instantaneous bedload sediment transport rates using a modified Bagnold equation are nearly 5 times greater under ebb tides than floods. The average net sediment transport rate at springs (0·04 kg m−1 sec−1) is over 10 times greater than at neaps (0·002 kg m−1 sec−1). Comparison with transport rates in modern tidal environments suggests that the marine molasse of Switzerland was deposited under spatially confined and relatively swift tidal flows not dissimilar to those of the present Dutch tidal estuaries.
Petrography of modern marine sands from the Peru‐Chile Trench and adjacent areas†
Authors: YERINO L. N. , MAYNARD J. B. 
Published: June 2006

ABSTRACTSand samples from piston cores taken in the Peru‐Chile Trench and adjacent areas were examined petrographically in an attempt to characterize sands from the oceanward side of a continent‐margin volcanic arc. Framework composition averaged Q18F44L38, close to the average reported for this type of setting, but showed wide variation in the ratio of feldspar to lithic fragments; quartz was uniformly low, less than 30%. Samples rich in lithic grains were from sites that lie off areas of recent volcanism on the adjacent continent. Samples from off southern Central America had compositions more like those found in island‐arc settings, averaging Q3F21L76, whereas those from the Straits of Magellan increased in quartz and decreased in lithic fragments toward the Atlantic. Plagioclase was greatly predominant over K‐feldspar in all samples, and averaged An28. A considerable portion of the plagioclase was close to end‐member albite, suggesting recycling from pre‐existing sedimentary rocks. Climate had no obvious effect on the framework proportions or the feldspar composition, but the volume of sediment in the trench increases with increasing rainfall on the continent.
Lower Cantabrian basin margin deposits in NE Leon, Spain—a model for valley‐fill sedimentation in a tectonically active, humid climatic setting
Authors: IWANIW E. 
Published: June 2006

ABSTRACTBasal Stephanian deposits of early Cantabrian age infill a deeply incised fossil topography at the western edge of the post‐Leonian basin of NE Leon. Seven low sinuosity palaeovalleys, trending NNW–SSE range in depth of incision from 25 to 450 m. The fill is composed of: (i) Red bed succession: a proximal alluvial valley‐fill association of debris flow and stream deposits in which seven mesosequences and two fining upwards megasequences are recognized on the basis of maximum clast sizes. (ii) Grey bed succession: two megasequences are distinguished. The lower megasequence comprises distal alluvial valley‐fill deposits (sheet sandstone, lacustrine, swamp deposits). The upper megasequence comprises a mid‐fan association of debris flow and interlobe/floodplain deposits.The palaeovalleys are considered in the context of previously described palaeovalleys and fill. These are categorized according to the degree of tectonic control and climatic setting.The lower Cantabrian examples may serve as a model for valley‐fill sedimentation in a tectonically active area and a humid climatic setting.
Wind sedimentation in the Jafurah sand sea, Saudi Arabia
Authors: FRYBERGER STEVEN G. , AL‐SARI ABDULKADER M. , CLISHAM THOMAS J. , RIZVI SYED A. R. , AL‐HINAI KHATTAB G. 
Published: June 2006

ABSTRACTThe Jafurah sand sea of the Eastern Province of Saudi Arabia extends along the Arabian Gulf coastline from Kuwait in the north to the Rub Al Khali in the south, a distance of about 800 km. Sand drifts southward to south‐eastward from regions of high wind energy in the north to low wind energy in the south.The aeolian landscape is zoned, with areas of deflation, transport and deposition from north to south. Drift rates in the zone of transport, near Abqaiq, range from 2 m3 m‐w‐1 yr‐1 on sabkhas, to 29 m3 m‐w‐1 yr‐1 on the crests of dunes. Average drift rates of approximately 18 m3 m‐w‐1 yr‐1 observed during the study can cause about 1 m of accumulation per 5500 yr in a 100 km zone of deposition downwind, not including the bulk transport represented by the forward advance of dunes. Dune advance ranged from 23 m (2.9 m high dune) to 3 m (23 m high dune) during April‐October 1980.The study area consists of dune, interdune, sand sheet and siliciclastic sabkha terrains, each of which is characterized by differing drift rates, and differing rates of erosion or deposition. Sedimentation occurs by lateral movement of dunes and interdunes, and vertical accretion by sand sheets and sabkhas.
Characteristics of coarse‐grained sequences deposited in nearshore, wave‐dominated environments—examples from the Miocene of south‐west Oregon
Authors: LEITHOLD ELANA L. , BOURGEOIS JOANNE 
Published: June 2006

ABSTRACT The conglomeratic sandstone of the Sandstone of Floras Lake (Miocene) is a superbly exposed example of a coarse‐grained sequence deposited in a high‐energy, storm‐wave‐dominated, shallow‐marine environment. The features of the unit suggest deposition on a shoreface in proximity to a fluvial source. Distinguishing characteristics include amalgamated hummocky‐stratified sandstone, low‐angle cross‐stratified conglomerate beds, lenticular conglomerate beds, tabular conglomerate beds, high‐angle scours, trough cross‐bedded sandstone, organic‐rich beds, and burrows of several sizes. These features are interpreted in terms of their depths of deposition, generation by fair weather, storm, and/or flood processes, and as indicators of relative rates of sedimentation.The unit records complex interaction of tectonic, wave, fluvial and tidal processes. Relative depth changes indicated in the sequence probably reflect changes in the balances between sediment supply, basin subsidence, and eustatic sea‐level changes. Wave processes are manifest in the presence of structures deposited both above and below fair‐weather wave base. Deposition and reworking by chaotic seas associated with local storms and by long‐period swell associated with distant storms may be indicated by the interbedding of amalgamated hummocky‐stratified sandstone and conglomerate beds. The effects of the lateral migration of a fluvial source with time are reflected by the changing nature and apparent rates of supply of sediments deposited in the nearshore zone. Tidal processes are manifest largely as superimposed features on wave‐ and current‐generated structures. Biogenic structures are an important clue to the rates and frequencies of all these processes.The conglomeratic sandstone was deposited on a high‐energy, tectonically active continental margin, similar to the present setting of the southern Oregon coast. It was deposited at varying depths in a subsiding basin during a time of world‐wide sea‐level rise. The sequence provides an opportunity for the study of coarse‐grained nearshore‐marine sediments and the delineation of the interaction of various processes during deposition. Detailed sedimentologic and palaeoecologic analysis provides insight into the nature of storm and flood processes that are difficult to observe in the modern environment.
An ancient submarine canyon in the Oligocene‐Miocene of the western Niger Delta
Authors: PETTERS S. W. 
Published: June 2006

ABSTRACT An anomalous sequence of thick fossiliferous marine shale of late Oligocene‐early Miocene age, here termed the Opuama Shale Member, occurs within the paralic Agbada Formation in the subsurface of the western Niger Delta. The Opuama Shale fills a deep palaeochannel which was cut into Eocene‐early Oligocene paralic beds. Planktonic and benthic foraminifera suggest that deposition in the channel began in the late Oligocene‐early Miocene at outer shelf‐slope depths, and that by the late early Miocene the channel had filled to shallow neritic depths. The channel, which is believed to be of submarine origin, is termed the Opuama canyon. The Opuama canyon originated during the pronounced early‐Middle Oligocene drop in sea‐level, and was cut by turbidity currents. It was filled, during the major late Oligocene‐early Miocene rise of sea‐level.The location of the Opuama canyon in the western re‐entrant of the Niger Delta supports Burke's (1972) conclusion that the western and eastern (where ancient submarine canyon fills are known) re‐entrants of the delta are potential sites for ancient submarine canyons. Burke based his conclusion on the observation that the eastern and western re‐entrants of the modern delta are areas where opposing longshore drifts converge and generate turbidity currents which cut submarine canyons.
Muddy deltas in freshwater lakes, and tectonism in the Upper Carboniferous Coalfield of NE England
Authors: HASZELDINE R. S. 
Published: June 2006

ABSTRACT The Westphalian B sediments of NE England were deposited on part of a coastal plain with subdued relief. Most of the fine‐grained sediments were deposited by small fluvially dominated muddy deltas which fed into shallow freshwater lakes in tectonic depressions a few metres deep and a few tens of kilometres across.Coalfield lakes contained an abundant low‐diversity bivalve fauna and existed for thousands of years before delta infill. Pro‐delta clay rhythmites, formed by flood turbidites, coarsened up into wide silty mouth bars, which merged up into channel mouth sands, inversely‐graded subaqueous levee deposits and sandy channel fills. Delta complexes were either abandoned below water level leaving a lake, or were colonized by plants, which resulted in trapping of suspended sediment and build‐up of a seatearth to near water level when peats could accumulate. These swamps were analogous to the present raised bogs of Indonesia and had a lateral zonation of vegetation to a central low‐diversity flora. Upwards reversal of this zonation and the succession of coals by lake sediments show that peat floras were killed by drowning due to rapid tectonic subsidence. Subsurface data show that seam splits and sand‐body locations were tectonically controlled, and that the lake deltas in this area were distant from any major sand distributary.