bluuebunny/crossref_metadata_2025 huggingface dataset:
Middle and Late Quaternary depositional sequences and cycles in the eastern Mediterranean
Authors: MURAT A. , GOT H.
Published: June 2006

Although most of the Quaternary sediments of the eastern Mediterranean appear reworked, extensive areas of hemipelagic sediments occur, mainly in the central part of the basin (Mediterranean Ridge). The most obvious character of these muddy sediments is the numerous changes in colour. They result from the organic carbon content as well as from concentration and chemical form of metallic elements (Mn, Fe, Cu, Zn). The succession of coloured layers is arranged into sequences developed between two sharp contacts; a complete sequence involves five lithofacies, from an oxidized layer at the base to a reduced sapropel at the top, and displays progressive internal changes of colour from ochre, beige, greyish‐beige, grey and black layers: several sequence‐variants (top‐truncated, base‐truncated, recurrent) are identified. These sequences are a consequence of changes in oceanographic conditions, i.e. from oxidizing to reduced environments, resulting in different organic and mineral contents.

Based on detailed studies of Quaternary sediments collected by coring, a synthesized standard succession representative of the ordered sequences in the eastern Mediterranean is proposed, from about 650 000 yr as deduced from the calculated rate of sedimentation of 1·7 mm 1000yr−1. This succession appears modulated by cyclic events: large‐scale periods (200 000 yr) of reduced and oxidized sequences correlate with major alternate phases of flooding and reduced flow of the Nile River during the Quaternary. Subsequent sequences of shorter time‐scale (20 000–50 000 yr) results from two factors: heaviest monsoons leading to an increase of the influx of the Nile River and global climatic changes (Eurasian ice sheet meltwater); these two factors act synchronously or independently along the studied time‐span.

These interplaying factors result in periodic stages of increasingly restricted conditions in the basin and explain the complexity of the eastern Mediterranean succession, at least since 650 000 yr ago.
Mass transport in Eocene pelagic chalk on the northwestern edge of the Arabian platform, Shefela area, Israel
Authors: BUCHBINDER B. , BENJAMINI C. , MIMRAN Y. , GVIRTZMAN G.
Published: June 2006

ABSTRACT

Glides, rotational slumps, debris‐ and mud‐flows occur in the Eocene pelagic chalk of the Shefela region, Israel. The pelagic lithofacies are present from the beginning of the Eocene, but mass transport is initiated only in the latest early Eocene and the early middle Eocene. Slight steepening of the palaeoslope, coupled with increased sedimentation rate and early diagenetic changes lowering inherent strength, led to gravity‐induced mass transport in the poorly consolidated pelagic sediments.

Instability of the sediment pile was enhanced by underconsolidation and development of spacious packing associated with a relatively high rate of deposition and low initial strength due to high content of siliceous micro‐biota.

The palaeogeographic position indicated by the mass‐transport features is the steepened, distal part of the ramp‐like continental margin of the Arabian Platform. Distal steepening of the ramp as part of regional downwarping is coincident with reactivation of Syrian Arc folding, reflecting northward convergence of the Arabian Plate during the Eocene.
Mineralogy of nitrogenous guano on the Bounty Islands, SW Pacific Ocean
Authors: CULLEN DAVID J.
Published: June 2006

By far the commonest consituents of insular phosphate deposits are calcium phosphates of the apatite series of minerals, especially varieties that contain structural carbonate in their crystal lattices, e.g. francolite and dahllite. This reflects the fact that the vast majority of described insular phosphate deposits occur in low latitudes, where they have formed, in a tropical environment, by metasomatic replacement of carbonate in coral substrates by phosphate derived either from avian excrement or from phosphorus‐enriched lagoonal/lacustrine waters.

Thin, recently‐formed guano accumulations on islands of the subantarctic Bounty group differ radically in that they consist principally of struvite, a hydrated ammonium magnesium phosphate that is better known as a component of human/mammalian urinary stones and enteroliths, and of bat guano in sheltered speleothems. Apatite occurs only in an accessory role in the Bounty Islands guano. This contrast in mineralogy, and the somewhat anomalous survival of soft, unstable struvite in a totally exposed environment on the Bounty Islands, are attributed to climatic factors and to the nature of the granodioritic rock substrate.
Sedimentology and evolution of the Martinsburg Formation (Upper Ordovician) fine‐grained turbidite depositional system, central Appalachians
Authors: LASH GARY G.
Published: June 2006

The Upper Ordovician Martinsburg Formation of eastern Pennsylvania consists of mudstone, siltstone, and sandstone turbidites that accumulated in a tectonically active foreland basin. The mudstone‐rich Bushkill Member, the stratigraphically lowest unit of the Martinsburg in this area, grades upward into approximately equal proportions of mudstone, siltstone, and sandstone of the Ramseyburg Member. Many of the turbidites of these units are arranged in small‐scale (1–9 m) fining‐upward sequences that are interpreted as reflecting the influence of external or allocyclic controls such as variations in the local rate of sea‐level rise and/or variations in the intensity of tectonic activity in shelf/nearshore or hinterland areas rather than more commonly cited autocyclic mechanisms. The thick (approximately 2000 m) Bushkill‐Ramseyburg coarsening‐upward sequence records progradation of a muddy turbidite depositional system along the axis of the foreland basin. Although this sequence accumulated during a Caradocian eustatic rise in sea‐level, sedimentation rates landward of the shoreline were apparently great enough to allow for long‐term seaward progradation of the shelf source. The paucity of depositional lobe‐like facies (coarsening‐upward sequences) in the Bushkill Member allows for tentative comparison of the progradational Bushkill‐Ramseyburg system with the active fan lobe of the Mississippi Fan.

Progradation of the Bushkill‐Ramseyburg system ceased abruptly when mudstone turbidites and laminated black shale of the upper unit of the Martinsburg, the Pen Argyl Member, accumulated. The great thickness of some mudstone turbidite beds of the Pen Argyl Member is interpreted to record topographic confinement of the central Appalachian foreland basin, which may have helped to preclude continued progradation of the Bushkill‐Ramseyburg turbidite system.
Time‐sequence observations of wave‐formed sand ripples on an ocean shoreface
Authors: BOYD R. , FORBES D. L. , HEFFLER D. E.
Published: June 2006

Analysis of an 18‐day time‐lapse film record of shoreface ripple development, with concurrent measurements of near‐bottom flow and surface waves, provides new insight on equilibrium bedform conditions, adjustment of ripple planform to variable hydrodynamics, and ripple migration behaviour. The study was conducted in approximately 10 m water depth, 1 km off Martinique Beach on the Atlantic coast of Nova Scotia (Canada), under low‐energy summer wave conditions. Significant wave‐height and peak period during the study averaged 0–7 m and 8 s, respectively, with extremes up to 1–7 m and 11 s during passage of three weak weather disturbances. Six mutually exclusive ripple types have been defined: (1) short‐wavelength regular ripples; (2) variable bifurcated ripples; (3) variable terminated ripples; (4) short‐crested ripples; (5) long‐wavelength regular ripples; and (6) chaotic ripples. Ripple wavelength ranged from 0–07 m to 0–24 m and displayed a strong Reynolds number dependence. Together with other published field data, the results suggest a lower limit of γ=0–06 m for the wavelength of wave ripples in ocean shoreface environments. Ripple orientation ranged through 38° and responded rapidly to changes in wave approach direction, but did not conform to the orientation of the adjacent shoreline. Ripples were observed to migrate both on‐ and off‐shore (with and against the wave advance direction) at rates up to ±0–1 m h‐1, associated with net flows other than wave‐induced onshore asymmetry and mass transport. Migration (mainly of ripple types 1 and 2) occurred during the peak of storm events, but showed no obvious correlation with measured near‐bottom flow magnitude or direction. Ripple behaviour demonstrates equilibrium with prevailing dynamic conditions when straight‐crested rippie types 1 and 5 are present. Disequilibrium in orientation or dimensions is expressed by increasing sinuosity, bifurcation and crest termination in types 2,3,4 and 6.
The initiation of particle movement by wind
Authors: NICKLING W. G.
Published: June 2006

When air blows across the surface of dry, loose sand, a critical shear velocity (fluid threshold, ut), must be achieved to initiate motion. However, since most natural sediments consist of a range of grain sizes, fluid threshold for any sediment cannot be defined by a finite value but should be viewed as a threshold range which is a function of the size, shape, sorting and packing of the surface sediment. In order to investigate the initiation of particle movement by wind a series of wind‐tunnel tests was carried out on a range of pre‐screened fluvial sands and commercially available glass beads with differing mean sizes and sorting characteristics. A sensitive laser‐monitoring system was used in conjunction with a high speed counter to detect initial grain motion and to count individual grain movements. Test results indicate that when velocity is slowly increased over the sediment surface the smaller or more exposed grains are first entrained by the fluid drag and lift forces either in surface creep (rolling) or in saltation (bouncing or hopping downwind). As velocity continues to rise, larger or less exposed grains may also be moved by fluid drag. On striking the surface saltating grains impart momentum to stationary grains. This impact may result in the rebound of the original grain as well as the ejection of one or more stationary grains into the air stream at shear velocities lower than that required to entrain a stationary particle by direct fluid pressure.

As a result, there is a cascade effect with a few grains of varying size initially moving over a range of shear velocities (the fluid threshold range) and setting in motion a rapidly increasing number of grains. Results of the tests showed that the progression from fluid to dynamic threshold, based on grain movement, can be characterized by a power function, the coefficients of which are directly related to the mean size and sorting characteristics of the sediment.
Stratigraphy and remanence acquisition of a palaeomagnetic reversal in alluvial Siwalik rocks of Pakistan
Authors: TAUXE LISA , BADGLEY CATHERINE
Published: June 2006

A high‐resolution record of a palaeomagnetic reversal is documented in Miocene alluvial rocks of Pakistan. We examined lateral variability of lithostratigraphy and palaeomagnetic stratigraphy through the same palaeomagnetic reversal in six correlated sections. Each section contains one or more palaeomagnetic sites with directions between fully reversed and fully normal. The position of the reversal illustrates local relief in the study area and the time‐transgressive nature of certain stratigraphic units. Variability in the thickness of the transitional interval indicates contemporaneous variability in sediment accumulation rates.

Different characteristics of palaeomagnetic remanence are associated with the depositional and post‐depositional history of these sediments. Variability in patterns of remanence behaviour is the basis for inferences about post‐depositional processes. We discuss two magnetic parameters that express coherency of palaeomagnetic samples—the maximum angle of deviation and the circular standard deviation. Of particular interest are samples with incoherent palaeomagnetic signals. The incoherency of samples is inversely correlated with the thickness of the transitional interval. A low rate of sediment accumulation, suggested by a thin transitional interval, may facilitate a prolonged period of remagnetization through pedogenic or hydrological processes. Alternatively, transition intervals, denned by coherent magnetization, may be thin as a result of pedogenically induced incoherency.
Snowball structures and their slide origin in the Jebel Tekzim Limestone (Palaeozoic of the Jebilet Range, Morocco)
Authors: ANDRE JEAN‐PIERRE
Published: June 2006

Sedimentary ball structures observed in the Visean series of Jebel Tekzim, in the Palaeozoic massif of the eastern Jebilet Range, Morocco, are described and interpreted.

The metre‐sized balls are composed of carbonate. They are texturally akin to wackestone or packstone, and some display a spiralled internal organization. The balls are encased in a calcarenitic to conglomeratic matrix and directly overlie a shale unit.

The origin of the structures is thought to be analogous to the mechanism whereby an avalanche of wet snow gives rise to the deposition of snowballs at the base of a slope. A layer of carbonate sediment detaches along an organic‐rich horizon and rolls up during downslope movement under gravity.
Faceted garnets from sandstones of the Munising Formation (Cambrian), northern Michigan: petrographic evidence for their origin by intrastratal dissolution
Authors: SALVINO JOHN F. , VELBEL MICHAEL ANTHONY
Published: June 2006

Imbricate wedge marks (facets) on garnets in sandstones of the Cambrian Munising Formation of northern Michigan are associated with mouldic secondary porosity developed at the expense of garnet. Mouldic pores surrounding faceted garnets indicate that garnets in these sandstones have been affected by intrastratal dissolution (retreat of the mineral surface from its original boundaries) rather than by grain enlargement, which would be expected if garnet overgrowths had formed. The association of garnet facets with textural evidence of garnet dissolution proves that garnet facets form by intrastratal dissolution. These results confirm similar findings in other recent studies, and extend the geographic and stratigraphic range of proven occurrences of facet formation by intrastratal dissolution.
Hummocky cross‐stratification and post‐vortex ripples: length scales and hydraulic analysis
Authors: SHERMAN DOUGLAS J. , GREENWOOD BRIAN
Published: June 2006

Duke (1987) argues that the structure reported by Greenwood & Sherman (1986) as hummocky cross‐stratification from a shallow lacustrine environment is, instead, bedding formed by simple post‐vortex oscillation ripples. His primary concern is with the length scale of the structures identified in box cores. However, using measured flow data, it is shown here that existing models of bedform generation based upon wave orbital velocity, orbital diameter and grain diameter do not support a simple post‐vortex ripple interpretation. In particular, the relative steepness of the structure refutes such an origin. The structural characteristics of the lacustrine cores most closely match those associated with classic hummocky cross‐stratification and were produced under a combined oscillatory/quasi‐steady flow field.
Characterizing grain size distributions: evaluation of a new approach using a multivariate extension of entropy analysis
Authors: FORREST J. , CLARK N. R.
Published: June 2006

In the search for environment‐diagnostic descriptors of grain size distributions, the use of both least squares statistical procedures and single index or summary measures to characterize sediment distributions have come under increasing criticism. Tests of these approaches highlight the validity of the criticisms. Analysis of sieved samples using the entropy concept, noted in the sedimentological literature as a potentially powerful tool for granulometric analysis, gives a much sharper result. Used previously only in its univariate form, the multivariate extension to the entropy procedure proposed here overcomes the problem of numbers of intervals and interval width inherent in the univariate application. It groups samples in terms of the whole shape of their grain size distributions, allows for variation in the optimal number of intervals for particular samples, and permits the use of more than one set of descriptor variables to be incorporated.
In situ accretion mechanism of concavo‐convex lacustrine oncoids (‘swallow nests’) from the Oligocene of the Mainz Basin, Rhineland, FRG
Authors: LEINFELDER REINHOLD R. , HARTKOPF‐FRöDER CHRISTOPH
Published: June 2006

Oligocene oncoids from the lacustrine Süβwasserschichten of the Mainz Basin exhibit a pustular surface and a characteristic cavity on their undersides, which gave rise to their designation as ‘swallow nests’. Oncoids grew in situ, under low‐energy conditions during a pause in sedimentation. Three features indicate that they were never overturned: (1) oncoids can be placed in a stable position on a smooth surface; (2) laminae, which are mostly concentric around the entire oncoids, display polar thickenings on the upper side during all growth stages of oncoids; (3) the depression is always situated on the stable side of oncoids. Development of this cavity is favoured by unionid shells, in the stable position, serving as oncoid nuclei, but occurs also under flat nuclei. It is interpreted as a product of reduced growth of cyanobacteria due to oligophotic conditions, on a fine‐grained substrate, and is considered as diagnostic of the in situ growth of oncoids.

Oncoid cortices are mainly formed by a Scytonema‐like cyanobacterium, whose carbonaceous organic remnants are occasionally preserved. Growth stages of individual oncoids were image‐processed and compared by means of shape parameters. Due to optimum growth conditions, cyanobacteria try to occupy the entire available space; hence, oncoids tend to achieve a spherical form even during in situ growth. Renewed background sedimentation triggered the formation of pustular surfaces and finally caused the cessation of cyanobacterial growth.
Modern and Holocene carbonate sedimentology of two saline volcanic maar lakes, southern Australia
Authors: LAST WILLIAM M. , DE DECKKER PATRICK
Published: June 2006

The Basin Lakes are two adjacent maar lakes located in the centre of the Western Volcanic Plains District of Victoria, Australia. Both lakes are saline and alkaline; West Basin Lake is meromictic whereas East Basin is a warm monomictic lake. The carbonate mineral suite of the modern offshore bottom sediments of these Basins consists mainly of dolomite and calcite, with smaller amounts of hydromagnesite and magnesite in West Basin and monohydrocalcite in East Basin. The dolomite, hydromagnesite, magnesite, and monohydrocalcite are endogenic in origin, being derived by primary inorganic precipitation within the water columns of the lakes or at the sediment‐water interface. The calcite is biologically precipitated as ostracod valves.

In addition to the carbonates in the modern offshore (deep‐water) sediments, the lakes also contain a girdle of nearshore carbonate hardgrounds. Both beachrock and microbialites (algal boundstones) are present. These modern lithified carbonate units exhibit a wide range of depositional and diagenetic fabrics, morphologies and compositions. In West Basin, the hardgrounds are composed mainly of dolomite, hydromagnesite, and magnesite, whereas dolomite and monohydrocalcite dominate the East Basin sediments. Aragonite, high‐Mg calcite, kutnahorite, siderite, and protohydromagnesite also occur in these lithified carbonate units.

Stratigraphic variations in the carbonate mineralogy of the Holocene sediment record in the lakes were used to help decipher the palaeochemistry and palaeohydrology of the Basins. These changes, in conjunction with fluctuations in organic remains and fossil content, indicate a pattern of lake level histories similar to that deciphered from other maar lakes in western Victoria.
Floodplain sedimentology of the Squamish River, British Columbia: relevance of element analysis
Authors: BRIERLEY GARY J.
Published: June 2006

Element analysis of modern‐day floodplains provides a framework for characterizing associations amongst depositional forms, the processes responsible for them and their local depositional environment. From interpretation of the spatial association of elements, mechanisms of floodplain evolution can be analysed. The Squamish River, in southwestern British Columbia, is a high‐energy, gravel‐based river, which exhibits a distinct downstream gradation in channel planform type. The floodplain sedimentology of this river is evaluated using an element approach. Five elements, defined on the basis of their morphological outline, position within sediment sequences and sedimentological character, describe the floodplain sedimentology: (i) top‐stratum, (ii) chute channel; (iii) ridge; (iv) bar platform; (v) basal channel gravels. The sedimentological composition of each element is described. Each of these units relates directly to morphostratigraphic units which make up contemporary bars of the Squamish River. Associations among facies defined at the bedform scale, morphostratigraphic units on bar surfaces and elemental floodplain features are described and explained.

The vertical stacking arrangement of elements is analysed in trenches (dug perpendicular to the main channel) and in bank exposures. Two elemental sedimentology models are proposed. In the first model, bar platform sands are discontinuous above basal channel gravels. Chute channel, ridge and proximal topstratum elements form thick sequences above. The second model is characterized by sequences in which distal top‐stratum deposits are observed. In these instances, bar platform sands are better preserved beneath the distal top‐stratum element, with proximal top‐stratum elements above. The applicability of these models is determined primarily by position on the floodplain. Chute channel reworking of floodplain sediments and replacement by top‐stratum elements is the dominant process marginal to contemporary bars. Sites in which channel avulsion has resulted in preservation of distal top‐stratum deposits in the midsequence of the present‐day channel banks determine the occurrence of the second model. Although channel planform style changes down‐valley in the study reach from braided to meandering, these two models apply in each reach. It is concluded that processes operative at the element scale, rather than the channel planform scale, determine floodplain sedimentology.
The relationships between sedimentary structures, transport directions and dune types in Mesozoic aeolian sandstones, Atacama Region, Chile
Authors: BELL C. M.
Published: June 2006

Extensive occurrences of Upper Jurassic to Lower Cretaceous aeolian sandstones form part of a thick sequence of terrigenous red beds in northern Chile. These sediments accumulated in a N‐S‐elongated intermontane back‐arc basin on the landward side of an active volcanic chain. This volcanic chain, which was produced by subduction beneath the continental margin, provided the source of almost all the clastic sediments.

The aeolian sands formed isolated dune fields covering only part of the depositional basin at any one time. Cosets of cross‐bedded sandstone up to 40 m thick represent the deposits of linear draas. Cross‐bedded sets, which average between 0.5 and 1 m in thickness, were produced by the migration of small dunes down and along the gentle lee slopes of the draas. The majority of foreset laminations in the cross‐bedded sets are thin, parallel and persistent. They are the product of the lateral migration of dunes which developed oblique to the prevailing winds. Foreset laminations dip at low angles (averaging 19°), well below the angle of repose of dry sand. Most of the laminations originated by tractional processes, rather than by grainflow (avalanching) or grainfall. This conclusion indicates that the dunes were gently undulating with no slip faces.

The orientation of the direction of maximum dip of foreset laminations does not provide a reliable indicator of the palaeowind direction. Although individual sets or cosets commonly show a unidirectional pattern, significant variations have been recorded between sets of the same age at nearby locations and vertically between one coset and the next. Variations in vertical sections were produced by the superimposition of draas with different orientations.
Dolomitization of the Devonian Swan Hills Formation, Rosevear Field, Alberta, Canada
Authors: KAUFMAN JONATHAN , HANSON GILBERT N. , MEYERS WILLIAM J.
Published: June 2006

The Swan Hills Formation (Middle‐Upper Devonian) of the Western Canada Basin is host to several NW‐SE‐trending gas fields developed in massive replacement dolostone. One of these, the Rosevear Field, contains two major dolostone trends along opposing margins of a marine channel that penetrates into a platform‐reef complex. Dolostones consist predominantly of branching and bulbous strdmatoporoid floatstones and rudstones with well‐developed moldic and vuggy porosity. Replacement dolomite is coarsely crystalline (100‐600 μm), inclusion‐rich, composed of euhedral through anhedral crystals and has a blotchy to homogeneous red cathodoluminescence. Geochemically, replacement dolomite is characterized by (i) nearly stoichiometric composition (50.1‐51.1 mol% CaCO3), (ii) negative δ18O values (mean=‐7.5‰, PDB) and (iii) variable 87Sr/86Sr ratios ranging from values similar to Late Devonian‐Early Mississippian seawater (∼0.7082) to radiogenic compositions comparable to saddle dolomite cements (>0.7100).

Dolomitization began after widespread precipitation of early, equant calcite spar and after the onset of pressure solution, implying that replacement dolomite formed in a burial environment. Oxygen isotope data suggest that dolomite formed at 35‐75°C, temperatures reached during burial in Late Devonian through Jurassic time, at minimum depths of 450 m. The linear NW‐SE orientation of most dolomite fields in the Swan Hills Formation is suggestive of fault control on fluid circulation. Two models are proposed for fault‐controlled circulation of dolomitizing fluids at the Rosevear Field. In the first, compaction‐driven, updip fluid migration occurred in response to basin tilting commencing in the Late Palaeozoic. Deep basinal fluids migrating updip were focused into channel‐margin sediments along fault conduits. The second model calls upon fault‐controlled convective circulation of (i) warm Devonian‐Mississippian seawater or (ii) Middle Devonian residual evaporitic brines.

The overlap in 87Sr/86Sr and δ18O compositions, and similar cathodoluminescence properties between replacement and saddle dolomites provide evidence for neomorphism of some replacement dolomite. Quantitative modelling of Sr and O isotopes and Sr abundances suggests partial equilibration of some replacement dolomite with hot radiogenic brines derived during deep burial of the Swan Hills Formation in the Late Cretaceous‐Palaeocene. Interaction of replacement dolomite with deep brines led to enrichment in 87Sr while leaving δ18O similar to pre‐neomorphism values.
Stoss‐side mud‐drapes: deposits of interdune pond margins
Authors: PURVIS KEVIN
Published: June 2006

Arcuate mud‐drapes have been observed in an interdune area in southern Tunisia. They are believed to have been formed by deposition from ponded water around aeolian dunes during flood events. They are preferentially preserved on the stoss‐side of the dunes as a result of subsequent dune migration, and exhibit extensive evidence of animal tracks. They are useful in illustrating dune‐interdune dynamics and recording the type of animals present.
Mega‐debris flow deposits from the Oligo‐Miocene Pindos foreland basin, western mainland Greece: implications for transport mechanisms in ancient deep marine basins
Authors: LEIGH SEBASTIAN , HARTLEY ADRIAN J.
Published: June 2006

The turbidite dominated, Oligo‐Miocene Pindos foreland basin of western mainland Greece contains two thick (60–72 m), matrix supported conglomerates. The conglomerates are ungraded and contain three clast types: (1) polymict, rounded, extrabasinal clasts (long axes 3–50 cm); (2) tightly folded, intrabasinal clasts (long axes 1–10 m); and (3) tabular, largely undeformed, intrabasinal blocks (long axes 18–300 m). Clasts are isolated within a slit dominated matrix. These chaotic, matrix supported conglomerates are interpreted as mega‐debris flow deposits. During transport, extrabasinal clasts were supported by a combination of matrix cohesion and clast dispersive pressure, folded intrabasinal clasts were supported by a combination of buoyancy (Archimedes principle) and clast dispersive pressure. The large tabular clasts were transported by gravity sliding/gliding within the flow on films at high pore fluid pressure. These different clast support mechanisms were active simultaneously within the Pindos mega‐debris flow deposits. As a result, the deposits have no systemic vertical stratigraphy, in contrast to many described large scale mass flow deposits. The mega‐debris flow deposits are significantly thicker than most described ancient siliciclastic debris flow deposits and provide an ancient analogue for the thick Recent siliciclastic debris flow deposits on continental margins.
Diastasis cracks: mechanically generated synaeresis‐like cracks in Upper Cambrian shallow water oolite and ribbon carbonates
Authors: COWAN CLINTON A. , JAMES NOEL P.
Published: June 2006

Upper Cambrian limestones and dolostones of western Newfoundland, Canada, display conspicuous synsedimentary mud cracks. Cracks occur in carbonate mudstone interbedded with ooid and peloid grainstone (unwashed oolite and ribbon rock lithofacies). The traditional interpretation is that these are desiccation cracks. The weight of evidence supports an alternative explanation: cracks resulted from the differential mechanical behaviour under stress of stiff mud interlayered with loose ooid/peloid sand. The processes envisaged to cause such diastasis cracks may be applicable to a wide variety of both carbonate and terrigenous clastic deposits composed of interlayered sediments of contrasting material properties, and may be a viable alternative to synaeresis. Diastasis cracks are not depth limited, and may form in any subtidal environment from the beach zone to below wave base. If this interpretation is correct, there may not be nearly as many intertidal lithofacies in the rock record as are presently assumed.
Geochemistry and origin of a black mudstone in a volcaniclastic environment, Ordovician Lower Rhyolitic Tuff Formation, North Wales, UK
Authors: FRITZ WILLIAM J. , VANKO DAVID A.
Published: June 2006

Black mudstones in marine volcaniclastic environments have been interpreted both as non‐volcanic ‘background’sedimentation and as fine grained vitric dust from the waning stages of primary volcanism. Although difficult to distinguish by standard petrographic techniques, differentiation between the two is crucial when attempting to determine sedimentation rates or to infer periods of volcanic quiescence. In the Ordovician Lower Rhyolitic Tuff Formation of North Wales such a fine grained black unit at Cwm Idwal is geochemically similar to an underlying rhyolite ash flow tuff. Its chemical index of alteration (CIA) is identical to that of the tuff. These data suggest that the black mudstone unit is a vitric tuff related to the underlying ash flow tuff. Use of a CIA in addition to trace element geochemistry should, in most cases, serve to distinguish tuffs from silicified mudstones.
Effect of source width and tidal elevation changes on aeolian transport on an estuarine beach
Authors: NORDSTROM KARL F. , JACKSON NANCY L.
Published: June 2006

Data from a moderate energy, meso‐tidal beach on the east side of Delaware Bay, New Jersey, USA, revealed the significance of both beach width as a source for aeolian transport and the effect of tidal rise on source width. Wind speeds averaged over 17·1 min, recorded 6 m above the crest of a 0·5 m high dune, ranged from 11·6 to 12·7 m s−1 during the experiment. The highest observed rate of transport on the beach was 0·0085 kg m−1 s−1, monitored at rising low tide when the average wind speed was 11·6 m s−1 across 0·35 mm diameter surface sediments. The wind direction was oblique to the shoreline, creating a source width of 34 m. The reduction in the width of the beach as a source for aeolian transport during rising tide was approximately arithmetic, whereas the reduction in volume of sediment trapped was exponential. Aeolian transport effectively ceased when source width was less than 8 m. Wind conditions, moisture content of the surface sediments and presence of binding salts did not appear to vary dramatically, and no coarse grained lag deposit formed on the surface of the beach. The decrease in rate of sediment trapped through time in the tidal cycle is attributed to differences in source width. Sediment deposited in the litter behind the active beach by strong winds during the rising tide was eroded during the high water period by the high waves and storm surge generated by these winds, and net losses of sediment were observed despite initial aeolian accretion.
Calcite from the Quaternary spring waters at Tylicz, Krynica, Polish Carpathians
Authors: KOSTECKA ALEKSANDRA
Published: June 2006

At Tylicz, near Krynica Spa (Polish Carpathians), spelean deposits fill fissures and caverns in Eocene flysch rocks. They occur as: (1) clastic cave sediments transformed into hard crusts due to cementation by finely crystalline low‐Mg calcite, (2) drusy calcite that covers crust surfaces and fills voids in the crust and (3) colloform calcite.

Two varieties of drusy calcite are distinguished: acicular and columnar. The acicular calcite is built up of crystallites forming spherulitic fans or cones. In places it is syntaxially covered with colloform calcite. The drusy calcite is low‐Mg ferroan calcite with non‐ferroan subzones, whereas the colloform calcite is a low‐Mg non‐ferroan variety.

The columnar calcite crystals form fan‐like bundles. Cross‐sections cut perpendicular to the c‐axes of columnar crystals are equilateral triangular in shape, although some have slightly curved edges. The columnar crystals have steep rhombic terminations and most have curved triangular faces, i.e. gothic‐arch calcite. Saddle crystals have also been observed.

The columnar crystals are composed of radially orientated crystallites whose long dimension is parallel to the c‐axis. The curved crystal faces of such polycrystals are interpreted as a result of differential growth rates of the crystallites.

The spelean calcites precipitated from CO2‐saturated water. The high rate of CaCO3 precipitation is thought to be responsible for the formation of radial structures.

Finely crystalline calcite formed within pore spaces of clastic sediments close to the water‐air interface, drusy calcite crystallized beneath the water‐air interface, and colloform calcite precipitated from thin films of water.
Anomalous pebble size variation in an erosive, Late Archaean braided stream: the Middelvlei gold placer, Witwatersrand, South Africa
Authors: ELS B. G.
Published: June 2006

A characteristic of fluvial conglomerates is a gradual decrease in pebble size in the direction of the palaeocurrents. However, for the ancient braided stream investigated in this study, a deviation from this norm was found.

The Middelvlei gold placer is a quartz pebble conglomerate that is mined in a South African goldfield, known as the West Wits Line, situated west of Johannesburg. Stratigraphically the placer occurs near the base of the Central Rand Group of the Witwatersrand Supergroup.

In previous studies, the placer and the underlying sequence have been shown to be fluvial in origin, constituting a coarsening upward genetic increment of strata. The complete conformable succession, however, is only preserved in the distal parts of the depositional environment. In most parts of the goldfield the placer is separated from the underlying sequence by a progressive unconformity, which disappears deeper into the basin. This unconformity is thought to have formed during a progradation.

Palaeocurrents of the Middelvlei placer are towards the south‐southeast in the western parts of the study area and towards the south‐east in the east. Maximum pebble size, however, decreases towards the west‐southwest, that is in a direction roughly perpendicular to the palaeocurrents.

A reconstruction of the palaeotopography of the base of the placer in the western parts of the study area revealed considerable variations in palaeoheight with a gradient of about 0·002 along depositional strike.

The anomalous lateral pebble size variation of the placer is explained by postulating an eastward migration of the depositing braided stream, simultaneous with its progradation. During the eastward migration, erosion of the underlying sequence became progressively more severe. This created an eastward‐thinning wedge of underlying strata.

The results of the study do not invalidate lateral pebble size variation as a regional palaeoslope indicator. However, the findings suggest that caution should be exercised in the application of this indicator.
Carboniferous calcretes in the Canadian Arctic
Authors: THERIAULT P. , DESROCHERS A.
Published: June 2006

Calcrete palaeosols have been found in the Upper Carboniferous Canyon Fiord Formation of southwestern Ellesmere Island, Canadian Arctic. These calcretes were developed in fluvial and shallow marine sediments that accumulated within two adjacent subbasins, in which the tectono‐sedimentary environment led to the deposition of five different sedimentary facies: (i) floodbasin sandstones; (ii) alluvial fan sandstones; (iii) alluvial fan conglomerates; (iv) braided fluvial sandstones; and (v) shallow marine limestones.

Nodular/massive palaeosol profiles, consisting of cryptic, nodular, massive and laminar horizons, occur within the floodbasin sandstone and alluvial fan sandstone facies. Plugged palaeosol profiles, consisting of cryptic, plugged and laminar horizons, are restricted to the alluvial fan conglomerate facies. Massive/brecciated palaeosol profiles, consisting of cryptic, massive/brecciated and laminar horizons, occur essentially within the shallow marine limestone facies.

The relationships between calcrete profiles and sedimentary facies suggest that profile types were controlled mainly by the texture and composition of the parent material: nodular/massive profiles are restricted to silicate‐rich sandstone hosts, plugged profiles are restricted to carbonate‐rich conglomerate hosts and massive/brecciated profiles are restricted to limestone hosts. Important relationships also exist between the maturity levels of nodular/massive profiles and sedimentary facies: profiles are generally mature in the floodbasin sandstones, invariably immature in the alluvial fan sandstones and absent from the braided fluvial sandstones. These different maturity levels were probably controlled mainly by exposure time, vegetation and substrate composition.
Sedimentology and isotope geochemistry of lacustrine carbonates of the Oligocene Campins Basin, north‐east Spain
Authors: ANADÓN P. , UTRILLA R.
Published: June 2006

The non‐marine Campins Basin developed in the Oligocene, during a period of early rifting of the Catalan Coastal Ranges. Lacustrine deposits, interbedded between two alluvial units, comprise shallow and deep lacustrine facies. The lower, shallow lacustrine facies are made up of microbialite buildups and thin limestone beds. In the studied area, these facies are overlain by deep lacustrine facies which consist of alternations of several, metre‐thick carbonate‐ and mudstone‐dominated intervals. In addition to calcite, which is characteristic of the shallow lacustrine facies, aragonite and abundant dolomite are present in the deep lacustrine facies. This mineralogical change in the sequence reflects an overall increase in the Mg/Ca ratio of the lake waters. The deep lacustrine sequences are interpreted as having formed in a hydrologically closed basin that was subject to changes in the Mg/Ca ratio of the water, probably related to variations in the evaporation/precipitation rate.

The sedimentological, mineralogical and isotopic characteristics of the Campins Basin dolomites suggest that, in general, they are primary in origin. The stable isotope data show an approximate covariance between δ13C and δ18O in the lower shallow lacustrine carbonates (calcite) which suggests that they formed during the onset of closure of the lake. The δ13C and δ18O values of the deep lacustrine carbonates display three different clusters that are roughly related to the carbonate mineralogy. Normalisation with respect to calcite of the isotopic compositions of dolomite and aragonite from the deep lacustrine carbonates allows the integration of all these isotope values into one covariant trend.

The sequential appearance of different carbonate minerals and the isotopic covariant trend may indicate an overall evaporative concentration of the lake waters. The change in slope of the covariant trend for the isotope values between the shallow and the deep lacustrine carbonates might reflect the change in the waterbody morphology recorded in the basin fill sequence.
The Udo tuff cone, Cheju Island, South Korea: transformation of pyroclastic fall into debris fall and grain flow on a steep volcanic cone slope
Authors: SOHN YOUNG KWAN , CHOUGH SUNG KWUN
Published: June 2006

The Udo tuff cone of Cheju Island, South Korea, is a middle Pleistocene basalt tuff cone that has formed by early Surtseyan‐type eruptions and later drier hydroclastic eruptions. The tuff cone comprises steep (20–30°) and planar beds of lapillistone, lapilli tuff and tuff that can be grouped into seven sedimentary facies (A‐G). Facies A and B comprise continuous to lenticular layers of grain‐supported and openwork lapillistone that are inversely graded and coarsen downslope. They suggest emplacement by grain flows that are maintained by gravity‐induced stress and grain collisions. Facies C includes poorly sorted, crudely bedded and locally inversely graded lapilli tuff, also suggestive of rapid deposition from highly concentrated grain flows. Facies D includes thinly stratified and mantle‐bedded tuff that was probably deposited by fallout of wind‐borne ash. Other facies include massive lapilli tuff (Facies E), chaotic lapilli tuff (Facies F) and cross‐bedded tuffaceous sandstone (Facies G) that were deposited by resedimentation processes such as debris flow, slide/slump and stream flow, respectively.

The grain flows that produced Facies A, B and C are interpreted to have originated from falling pyroclasts, which initially generated highly dispersed, saltating avalanches, in which momentum was transferred by the particles themselves. This transport mechanism is similar to that of debris fall. As the slope gradient was too low to maintain a highly dispersed flow, the debris fall decelerated and contracted due to a decrease in dispersive pressure. The mode of momentum transfer changed to one of collision because contraction of the debris fall resulted in an increase in particle concentration. This transport mechanism is similar to that of common grain flows. Grain segregation occurred in several ways. Initial segregation of ash from lapilli occurred due to their differing terminal fall velocities, and their contrasting degrees of sliding friction with the bed. Percolation of ash into interstices of lapilli during flow (kinematic sieving) augmented further segregation of ash from lapilli. The latter process, along with a dispersive pressure effect, gave rise to vertical inverse size grading. Downdip inverse grading was produced by particle overpassing.
Shallow seismic stratigraphy and pockmarks of a hydrothermally influenced lake, Lake Rotoiti, New Zealand
Authors: PICKRILL R. A.
Published: June 2006

Lake Rotoiti formed 11 850–20 000 years ago when lava flows dammed the drainage system through the Okataina caldera, one of the major rhyolitic centres of the intracontinental, back‐arc, Taupo Volcanic Zone of North Island, New Zealand. The lake morphology reflects this complex origin, with remnants of the old caldera rim, the lava dam and relict river system being preserved. High resolution seismic reflection profiles (7 kHz) show the drowned river system channelled inflowing water into the lake basin, scouring and depositing sediments and maintaining an active sublacustrine channel in the former river valley. Airfall tephras are preserved in lake sediments. The tephrachronology, established from the surrounding catchments, can be correlated with the siesmic stratigraphy for eight major eruptions over the last 11 850 years. Hydrothermal fields underlying the lake generate gaseous sediments which mask seismic penetration. This is indistinguishable from seismic masking caused by biogenic gas accumulation within the sediments. Venting of hydrothermal gases at the surface creates pockmarks. From the seismic stratigraphy beneath pockmarks seven stages of pockmark genesis have been identified which show pockmarks grow, decay, migrate and persist over several thousand years.
Sedimentology of subaqueous volcaniclastic sediment gravity flows in the Neogene Santa Maria Basin, California
Authors: COLE RONALD B. , STANLEY RICHARD G.
Published: June 2006

Subaqueous tuff deposits within the lower Miocene Lospe Formation of the Santa Maria Basin, California, are up to 20 m thick and were deposited by high density turbidity flows after large volumes of ash were supplied to the basin and remobilized. Tuff units in the Lospe Formation include a lower lithofacies assemblage of planar bedded tuff that grades upward into massive tuff, which in turn is overlain by an upper lithofacies assemblage of alternating thin bedded, coarse grained tuff beds and tuffaceous mudstone. The planar bedded tuff ranges from 0.3 to 3 m thick and contains 1‐8 cm thick beds that exhibit inverse grading, and low angle and planar laminations. The overlying massive tuff ranges from 1 to 10 m thick and includes large intraclasts of pumiceous tuff and stringers of pumice grains aligned parallel to bedding. The upper lithofacies assemblage of thin bedded tuff ranges from 0.4 to 3 m thick; individual beds are 6‐30 cm thick and display planar laminae and dewatering structures. Pumice is generally concentrated in the upper halves of beds in the thin bedded tuff interval.

The association of sedimentary structures combined with semi‐quantitative analysis for dispersive and hydraulic equivalence of bubble‐wall vitric shards and pumice grains reveals that particles in the planar bedded lithofacies are in dispersive, not settling, equivalence. This suggests deposition under dispersive pressures in a tractive flow. Grains in the overlying massive tuff are more closely in settling equivalence as opposed to dispersive equivalence, which suggests rapid deposition from a suspended sediment load. The set of lithofacies that comprises the lower lithofacies assemblage of each of the Lospe Formation tuff units is analogous to those of traction carpets and subsequent suspension sedimentation deposits often attributed to high density turbidity flows. Grain distributions in the upper thin bedded lithofacies do not reveal a clear relation for dispersive or settling equivalence. This information, together with the association of sedimentary features in the thin bedded lithofacies, including dewatering structures, suggests a combination of tractive and liquefied flows.

Absence of evidence for elevated emplacement temperatures (e.g. eutaxitic texture or shattered crystàls) suggests emplacement of the Lospe Formation tuff deposits in a cold state closely following pyroclastic eruptions. The tuff deposits are not only a result of primary volcanic processes which supplied the detritus, but also of processes which involved remobilization of unconsolidated ash as subaqueous sediment gravity flows. These deposits provide an opportunity to study the sedimentation processes that may occur during subaqueous volcaniclastic flows and demonstrate similarities with existing models for sediment gravity flow processes.