bluuebunny/crossref_metadata_2025_split huggingface dataset:
Stratigraphy and remanence acquisition of a palaeomagnetic reversal in alluvial Siwalik rocks of Pakistan
Authors: TAUXE LISA , BADGLEY CATHERINE 
Published: June 2006

ABSTRACTA 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

ABSTRACTSedimentary 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

ABSTRACTImbricate 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

ABSTRACTDuke (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

ABSTRACTIn 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

ABSTRACTOligocene 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

ABSTRACTThe 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

ABSTRACTElement 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

ABSTRACTExtensive 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

ABSTRACTThe 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

ABSTRACTArcuate 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

ABSTRACTThe 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

ABSTRACTUpper 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

ABSTRACTBlack 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

ABSTRACTData 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.