bluuebunny/crossref_metadata_2025 huggingface dataset:
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.
Liquefaction, fluidization and erosional structures associated with bituminous sands of the Bracklesham Formation (Middle Eocene) of Dorset, England
Authors: PLINT A.G.
Published: June 2006

ABSTRACT At Hengistbury Head, Dorset, the Boscombe Sands (Middle Eocene, Bracklesham Formation) are of estuarine channel facies. A mud‐filled channel is exposed, the banks and eastern flank of which have a black carbonaceous stain, the degraded remains of a bitumen. At the time of deposition, the bitumen rendered the sediment firm and it was extensively burrowed by a Thalassinoides‐forming organism (crustacean). The bituminous sand on the eastern channel bank suffered brecciation and dilation as a result of liquefaction and flowage of the underlying sediments. This is thought to have been due to rapid expulsion of pore water, possibly as a result of seismic shock. The layers of bituminous sand below the surface were ruptured during water‐escape, resulting in localized zones of rapid flow causing fluidization and the development of dewatering pipes up to 1.2 m long. The estuarine sediments were subsequently transgressed during which the bituminous sand was exposed on the seafloor, when it was eroded into a hummocky topography and heavily burrowed. Blocks of bituminous sand were reworked into the marine basal conglomerate, composed mainly of flints, demonstrating the remarkable strength of the bituminous cement.
Towards a general model of quick clay development
Authors: TORRANCE J. KENNETH
Published: June 2006

ABSTRACT Within the clear definition of a quick clay as being a soil which has a sensitivity greater than 30 and a remoulded strength less than 0.5 kPa, a general model for quick clay development is presented, based on the influences of various material properties and natural processes on the sensitivity and remoulded strength. The influences are separated according to depositional requirements and post‐depositional factors and according to whether the influence is to increase the undisturbed strength or to decrease the remoulded strength. Depositional factors are the requirements for a flocculated structure and the dominance of low activity minerals. Post‐depositional factors are cementation and slow load increase, which increase the undisturbed strength, and little consolidation, leaching and dispersants, which decrease the remoulded strength. The application of the model to marine, brackish water and freshwater sediments is discussed.
The use of the fractal dimension to quantify the morphology of irregular‐shaped particles
Authors: ORFORD J. D. , WHALLEY W. B.
Published: June 2006

For 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

Four 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

Textural, 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

Shells 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

A 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

Sand 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

Basal 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

The 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.
Boundary skin friction and sediment transport about an animal‐tube mimic
Authors: Eckman James E. , Nowell Arthur R. M.
Published: June 2006

ABSTRACT

A flume study was made of fluid flow, boundary skin friction, and sediment transport about an animal‐tube mimic. The effect of a tube on momentum transfer depends most strongly on its height. A tube increases the net boundary skin friction locally and tends to promote sediment entrainment near its base. Flow in this region appears to be governed by the same similarity laws that apply to cylinders with much larger body Reynolds number. Particles travelling as suspended load may be deposited immediately downstream of the tube. The net sedimentological effect of any tube (i.e. deposition or scour) will depend both on its height and on the boundary shear stress imposed by the external flow. The near‐wake region of a tube is dominated by a strong cross‐stream exchange of momentum. Wake perturbations decay downstream seemingly in accordance with similarity laws which also govern mounted, two‐dimensional structures.
Methane‐derived high‐Mg calcite submarine cement in Holocene nodules from the Fraser Delta, British Columbia, Canada
Authors: NELSON CAMPBELL S. , LAWRENCE MARK F.
Published: June 2006

Carbonate nodules and slabs in late Holocene shelly terrigenous deposits of the modern Fraser River delta (∼49°N) are formed close to the seafloor by precipitation from saline pore waters of mainly fibrous to bladed crystals of high‐Mg (∼ 10–20 mol% MgCO3) calcite cement as coalescing isopachous crusts on grains. Previous reports that the cement is low‐Mg calcite are not supported by this study. Highly negative δ13C values of − 7 to − 59‰ for the cements indicate that the bulk of their carbonate carbon was derived from the microbiological degradation of organic matter in the deltaic deposits during shallow burial. In particular, the production of biogenic methane (CH4) by anaerobic bacterial fermentation, its upward migration, chemical or biological oxidation to CO2 and neutralization in the near‐surface sediment, and diffusion to microenvironments relatively enriched in organic components, are a possible set of conditions influencing the process and sites of carbonate cementation. Methane‐derived Mg‐calcite appears also to be the major submarine cement in several other modern occurrences of lithified shallow‐water terrigenous sands and muds at non‐tropical latitudes.
Proximal braidplain deposits in the Middle Devonian Malbaie Formation of Eastern Gaspé, Quebec, Canada
Authors: RUST BRIAN R.
Published: June 2006

The Middle Devonian Malbaie Formation of Eastern Gaspé, Canada, comprises sharply alternating conglomerate and sandstone units. Their petrography suggests derivation from the same source, but palaeocurrents indicate different dispersal systems for the sand and gravel. The principal conglomerate facies is horizontally stratified, with well‐developed imbrication, characteristic of deposition on a high‐energy, proximal braidplain. Minor cross‐stratified conglomerate shows well‐defined size sorting, attributed to avalanching down foresets and sorting within minor bedforms on bar tops. The sandstone units mainly comprise erosion surfaces overlain by mudstone intraclasts, alternating with lineated low‐angle to horizontally stratified sandstone or trough cross‐stratified sandstone. The sandstone units were deposited on a proximal braidplain with highly variable discharge, but the absence of calcrete indicates that dry periods were not prolonged.

The Malbaie is the coarsest, uppermost formation of a Devonian clastic wedge formed during the Acadian Orogeny. The uniformity of facies association and palaeocurrents, particularly in conglomerate units, indicates derivation from a broad upwarp to the south. This implies that the Acadian uplands resulted from straight compression, in contrast to the localized uplifts formed by strike‐slip faulting during the Carboniferous.
Facies model for a sandy ephemeral stream and clay playa complex; the Middle Devonian Trentishoe Formation of North Devon, U.K.
Authors: TUNBRIDGE IAN P.
Published: June 2006

Sediments in the Trentishoe Formation of the Middle Devonian Hangman Sandstone Group (North Devon, U.K.) provide the basis of a model for sandy ephemeral stream and clay playa deposition. Three types of sequence are found, representing proximal, medial and distal areas on an extensive alluvial plain. The Proximal sequence consists of cross‐cutting channel‐fill sandstones which represent the deposits of a network of low sinuosity sand bed streams. The Medial sequence comprises upwards coarsening cyclothems which start with relatively distal, thinly bedded sandstone and siltstone flood sheets cut by complexes of silt draped channel‐fill sandstones and single channel fill sandstones. The flood sheets coarsen and thicken upwards to more proximal multistorey sheet sandstones. The Distal sequences consist of laminated mudstone and sandstone, cut by desiccation and water escape features, alternating with wave rippled sandstones, and represent playa lakes occasionally incised by high sinuosity channels with laterally accreting sandstones. The three sequence types represent the downslope progression from a low sinuosity channel network which passed into an ephemeral flood deposit complex which in turn drained into clay playas.
Sedimentological characteristics of the surficial sediments of the Kuwaiti marine environment, northern Arabian Gulf
Authors: KHALAF F. , AL‐BAKRI D. , AL‐GHADBAN A.
Published: June 2006

The textural characteristics, carbonate content and the coarse fraction components of the Recent bottom sediments of the marine environment off Kuwait are described and the faunal‐sediment associations discussed. The sediments were subdivided into seven textural classes, namely sand, silty sand, muddy sand, sandy silt, sandy mud, silt and mud. Most of the study area is covered with muddy sediments whereas sandy deposits are restricted to the rocky bottoms near the southern flat of Kuwait Bay, the southern coast of Kuwait and around the islands and bathymetric highs. The textural classes, carbonate contents and faunal types of the coarse fraction were used to construct a biolithofacies map of the marine bottom sediments off Kuwait, in which nine facies are identified. The sedimentological characteristics of the Recent marine bottom sediments off Kuwait reflect the interaction between autochthonous calcareous fragments mostly of biogenic origin, lime rock fragments derived from beachrocks and submerged reef flats, and allochthonous terrigenous detritus transported to the area mainly by dust storms. The Kuwaiti offshore area is generally a low energy depositional environment with little sediment transport.
Towards a general model of quick clay development
Authors: SMALLEY I. J. , FORDHAM C. J. , CALLANDER P. F.
Published: June 2006

Torrance (1983) has recently reviewed factors producing high sensitivity in clayey sediments and proposed a general model for quick clay development. He suggested that the mineralogical requirement for quick clay behaviour to develop is that low activity minerals must predominate in the sediment. This is also the essential mineralogical requirement of the inactive‐particle, short‐range‐bond hypothesis of high sensitivity (Smalley, 1971, 1972a, 1976; Cabrera & Smalley 1973; Smalley, Ross & Whitton, 1980). The predominance of low activity minerals causes most of the interparticle bonds to be of the short‐range variety which allow/promote the‘quick’ behaviour—the transition from solid to liquid. In Canadian or Scandinavian quick clays the short‐range bond requirement is largely met by the abundance of finegrained primary mineral particles. The very sensitive clays involved in the Tauranga landslide in New Zealand (Smalley et al., 1980) owed their sensitivities to their content of inactive spheroidal halloysite particles. By many definitions the Tauranga material is a quick clay and its existence provides us with another window through which to view the quick clay problem. Torrance observed that for a quick clay to develop the sediment must have an open structure with a high void ratio. In a classic Canadian quick clay this structure is obtained by the slow sedimentation of fine particles in the salty Champlain Sea; in the Tauranga soil an open structure is obtained by air‐fall sedimentation of the initial volcanic particles.
Longitudinal transport of turbidity currents—–a model study of Horgen events
Authors: SIEGENTHALER CHRISTOPH , HSÜ KENNETH J. , KLEBOTH PETER
Published: June 2006

Experiments were carried out in a 10 × 6 m basin to simulate turbidity currents generated by the Horgen Slumping Events of 1875. The conditions for kinematic similarity were satisfied and the experiments gave further insight into the mechanics of transport triggered by the Horgen slumps. The experimental turbidity currents laid down thick deposits on a subaqueous fan, and thin sheets of turbidite on the floor of the elongate basin through longitudinal transport, comparable with the simulated deposits in Lake Zurich. It is concluded that longitudinal transport is a general phenomenon of turbidity currents.
Experiments on the settling, overturning and entrainment of bivalve shells and related models
Authors: ALLEN J. R. L.
Published: June 2006

An experimental and theoretical examination has been made of the settling, entrainment and overturning of 176 valves representing 16 common Northwest European marine bivalve species, together with a comparative study of 15 plastic models in the form of segments from cylindrical tubes.

Settling behaviour in both stagnant and moving water depends on particle mass, symmetry and concavo‐convexity. Separated empty bivalve shells spin and spiral while settling and, if sufficiently elongated, also pitch. At the observed Reynolds numbers, the shells and models fall concave‐up, the terminal fall velocity increasing as the square root of the unit immersed mass or weight. The drag coefficient is independent of Reynolds number but increases with surface roughness and, particularly, particle elongation. Turbulence slightly lowers the critical elongation for pitching.

A separation vortex lies captive on the upper side of each descending particle. Consequently, an empty bivalve shell traversing a suspension of sand traps grains on its upper side at a rate proportional to their volume concentration and terminal fall velocity. This process, increasing the effective shell mass, is limited only by the capacity of the shell and grain spillage due to the possible onset of pitching.

The ratio (non‐dimensional) of a quantity proportional to the applied fluid force and the particle unit immersed weight consistently describes the entrainment of concave‐up and convex‐up particles, and also the immediate overturning of a valve on settling concave‐up to the bed. These thresholds vary in relative magnitude with bed‐particle friction and particle concavo‐convexity. In general, convex‐up particles are the most stable; the concave‐up entrainment and overturning thresholds are of a substantially lower but similar magnitude. The high frequency of concave‐up bivalve attitudes in turbidites is understandable largely in terms of the ability of a settling valve to increase in effective mass by grain entrapment. Convex‐up attitudes in the lower parts of turbidites may record currents stronger than the overturning threshold.
Stylolite‐controlled layering in an homogeneous limestone: pseudo‐bedding produced by burial diagenesis
Authors: SIMPSON JOHN
Published: June 2006

It is often assumed that layering in many shelf and platform limestones at weathered outcrop is always bedding: the layers always beds, and the surfaces always bedding planes. Recognition of bedding is fundamental to facies and basin analysis.

Examination of part of the Lower Carboniferous High Tor Limestone of the Gower, South Wales, shows that a totally diagenetic layering (pseudo‐bedding) resembling, and parallel to primary depositional layering (true bedding), has been produced by the generation of parallel, more or less evenly‐spaced stylolite surfaces during burial diagenesis. These stylolite surfaces are picked out by weathering at outcrop and resemble bedding planes. Evidence that the stylolite surfaces do not mark original bedding planes includes: a cross‐cutting relationship of a stylolite surface to a primary bedding surface; lack of lithological change across, or sedimentary structures associated with, a stylolite surface; original bedding that is not coincident with stylolite surfaces and has not suffered pressure‐dissolution; and lack of sedimentological evidence that the layers defined by the stylolite surfaces are true primary depositional beds. The generation of parallel and more or less evenly‐spaced stylolite surfaces could be explained by a mathematical‐kinetic model based on the relationship between pressure‐dissolution, diffusion, and reprecipitation.

Recognition of true bedding depends upon presence of lithological changes and/or sedimentary structures. It is suggested that pseudo‐bedding could be confused with true bedding in some open platform/shelf lagoon facies where lithological changes or sedimentary structures are scarce or absent.
A new approach to provenance studies: electron microprobe analysis of detrital garnets from Middle Jurassic sandstones of the northern North Sea
Authors: MORTON ANDREW C.
Published: June 2006

Reading provenance from heavy‐mineral suites is hampered by the depletion of diagnostic, but unstable, heavy minerals through intrastratal solution, a common phenomenon in deep sedimentary basins such as the North Sea. This paper demonstrates the potential of electron microprobe analysis in overcoming this problem, by concentrating on the compositional variations shown by detrital garnets, which are relatively resistant to intrastratal solution.

Studies of the garnets from the Brent Group (Middle Jurassic) of the Murchison and Tern oilfields in the northern North Sea reveal that three distinct areas supplied detritus. Association 1 which characterizes the Broom Formation in both areas and recurs higher in the sequence in the Tern field, is ascribed to an Orkney‐Shetland source. The location of the areas supplying the garnets of Association 2, best represented in Murchison, and Association 3, common to both fields, is less clear. Their ultimate sources clearly include high‐grade metamorphics, and therefore probably lie on the Norwegian landmass, the Orkney‐Shetland Platform, or, conceivably, the Scottish landmass, but the possibility of recycling makes it difficult to judge their immediate provenances. Nevertheless, the presence of three garnet associations indicates that the most widely accepted model of Brent sedimentation, with sands derived from a domal uplift in the outer Moray Firth and channelled northward along the Viking Graben, is untenable.
Reliability of core sampling of soft bottom sediment—an in situ study
Authors: BLOMQVIST SVEN
Published: June 2006

The performances of four light‐weight, open sampling devices intended for use in soft sediments, the Axelsson‐Håkanson gravity corer, the Kajak gravity corer, the Jenkin bottom sampler and the Ekman grab (box corer), were examined in situ by direct observation, measurement and photographic documentation by a SCUBA diver. Restrictions on the reliability of the sediment samples obtained with these devices and sediment coring instruments in general are evaluated.

Separate studies of core shortening show: (1) a positive linear relationship between sediment penetration depth at which shortening of cores starts and coring tube inner diameter, (2) a tube size related shift of curve pattern in the regressions of the core shortening versus sediment depth, and (3) a negative non‐linear relationship of shortening intensity versus increasing coring tube inner diameter. These findings show the great risk, when sampling soft sediments, of obtaining a sample quantitatively unrepresentative of the in situ stratification. An accurate correction factor for the degree of core shortening requires a knowledge of: (1) the sediment depth at which core shortening commences, and (2) the curve describing the relationship of shortening to depth of penetration.
Criteria for the recognition of salt‐pan evaporites
Authors: LOWENSTEIN TIM K. , HARDIE LAWRENCE A.
Published: June 2006

Layered evaporites can accumulate in: (1) ephemeral saline pans, (2) shallow perennial lagoons or lakes, and (3) deep perennial basins. Criteria for recognizing evaporites deposited in these settings have yet to be explicitly formulated. The characteristics of the ephemeral saline pan setting have been determined by examining eight. Holocene halite‐dominated pans (salt pans) and their deposits (marine and non‐marine) from the U.S., Mexico, Egypt and Bolivia. These salt pans are typified by alternating periods of flooding, resulting in a temporary brackish lake, evaporative concentration, when the lake becomes saline, and desiccation, which produces a dry pan fed only by groundwater. The resulting deposits consist of alternating layers (millimetres to decimetres) of halite and mud. The layers of halite are characterized by: (1) vertical and horizontal cavities, rounded crystal edges and horizontal truncation surfaces, due to dissolution during flooding; (2) vertical ‘chevrons’ and ‘cornets’ grown syntaxially on the bottom during the saline lake stage; (3) halite cements (overgrowths and euhedral cavity linings) and disruption of layering into metre‐scale polygons, produced during the desiccation stage. The muddy interbeds are characterized by displacive growth of halite during the desiccation stage. Immediately below the surface of the pan the halite layers are ‘matured’ by repeated episodes of dissolution and diagenetic crystal growth. This results in porous crusts with patches of ‘chevron’ and ‘cornet’ crystals truncated by dissolution, clear diagenetic halite cement, and internal sediment. These layers of ‘mature’ halite closely resemble the patchy cloudy and clear textures of ancient halite deposits. Holocene salt‐pans are known to cover thousands of square kilometres and cap halite deposits hundreds of metres thick, so they are realistic models for ancient evaporites in scale, e.g. Permian Salado Formation of New Mexico‐Texas, which preserves many primary salt‐pan features.
Submarine glide blocks from the Lower Cretaceous of the Antarctic Peninsula
Authors: INESON J. R.
Published: June 2006

Isolated exotic blocks of late Jurassic age occur within an undeformed succession of marine Lower Cretaceous back‐arc basin deposits on the west coast of James Ross Island, Antarctica. These flat, tabular slabs range up to 200 × 800 m in cross‐section and lie concordant with the enclosing Cretaceous strata. Although mainly undeformed, one block displays a range of emplacement‐related deformation structures, comparable in many respects to tectonic fabrics produced by simple shear. Emplacement by submarine block gliding is proposed, possibly as the final phase in the evolution of a composite mass transport event. Derivation of such gigantic slabs requires the existence of a steep, highly unstable basin margin during the early Cretaceous.

In more complex terranes, differentiation between gravity slides and thrust slices can be difficult. Clearly, internal and marginal disruption of an allochthonous unit is not diagnostic since structures developed within a lithified block during submarine gliding may closely mimic tectonic fabrics. Where contact relationships are ambiguous, emplacement by gravity sliding is suggested by the increasing intensity of internal disruption towards the basal margin and by the style of deformation, reflecting simple shear under low overburden.
Analytical method for estimating palaeowave climate and water depth from wave ripple marks
Authors: DIEM B.
Published: June 2006

Analytical expressions are developed for evaluating palaeodepth and palaeowave conditions from wave ripple marks. Palaeodepth and palaeowave climate estimates can be extremely powerful predictors in reconstructions of vertical movements related to basin subsidence, local and regional ancient wave climate trends and the basin shape.

The analytical expressions developed herein are applied to wave ripple marks preserved in the Lower Marine Molasse of Switzerland, western Austria and southern Germany. The results suggest that, in contrast to traditional concepts of an eustatic sea‐level change, the Lower Marine Molasse regression was a result of mainly depositional processes. The subsidence rate greatly exceeded the rate of sea‐level fall over a wide geographical area. Furthermore, the widening of the Lower Marine Molasse basin to the ENE is well documented by the increase of the ancient wave period and wave power, calculated from wave ripple marks found in storm deposits.
Sedimentology of cherts in the Early Proterozoic Wishart Formation, Quebec‐Newfoundland, Canada
Authors: SIMONSON BRUCE M.
Published: June 2006

The siliciclastic Wishart Formation of the Early Proterozoic Labrador trough is a high‐energy shelf deposit. Wishart sandstones contain both interstitial chert with textures of void‐filling cement and thin chert intercalations contaminated with siliciclastic mud. Although volumetrically minor, these cherts occur in several thin, areally extensive stratigraphic intervals. The Wishart contains intraclasts of both the chert‐cemented sandstone and the impure chert layers (as well as several other types of chert sand and gravel). This suggests the cherts formed penecontemporaneously, which is consistent with the absence of any signs of replacement in all but one of the chert types and the clear‐cut distinctions between chert types, even where they are side by side in a single thin section. The origin which appears to be most compatible with available evidence is that the cherts represent silica precipitated from thermal waters that rose through the sediments of the Wishart shelf and discharged into suprajacent seawater. A biogenic origin is unlikely in view of the lack of appropriate organisms during the Early Proterozoic and the rapidity with which the cements formed. A volcanogenic origin is unlikely because volcaniclastic textures are plentiful in associated formations but absent from the Wishart. Precipitation induced by evaporative concentration is unlikely in view of the widespread evidence of tidal currents and the lack of evidence of desiccation in the Wishart. Finally, the cherts are not restricted to the lowest‐energy facies, and therefore they presumably did not accumulate as a background sediment. Deposition of silica above the sediment/water interface was probably made possible by ambient concentrations of silica that were significantly higher than those of Phanerozoic seawater. Cherts with similar textures occur in other Early Proterozoic sediments, most notably arenitic or granular iron‐formations.