bluuebunny/crossref_metadata_2025_split huggingface dataset:
Environmental setting and microstructure of subfossil lithified stromatolites associated with evaporites, Marion Lake, South Australia
Authors: VON DER BORCH C. C. , BOLTON B. , WARREN J. K. 
Published: June 2006

ABSTRACTA variety of finely laminated, subfossil, aragonitic stromatolites and oncolites occur on a regressive marginal flat surrounding Marion Lake, South Australia. These algal forms overlie a substrate of coarse, highly porous, moldic aragonitic limestone which passes progressively towards the take centre through a zone of interstatified aragonite and gypsum and ultimately to pure crystalline gypsum. All of these facies overlie Holocene marine carbonate bank sediments which unconformably overlie at least one upper Pleistocene marine unit. Detailed petrographic and stratigraphic studies, combined with comparative studies of related nearby lakes containing a variety of living aragonitic cryptalgalaminates, provide a model for development of the Holocene sedimentary sequence. Marion Lake last became inundated by the sea around 6500 years ago during the Holocene transgression, when a protected marine environment was initiated. Lateral sediment accretion sealed marine passes into the resulting lagoon system soon after sealvel stabilized, and a variety of gypsum and gypsum‐carbonate‐algal facies evolved. Pure gypsum was deposited in waters 2–3 m deep in the central basin area concurrently with formation of seasonally alternating gypsum and aragonite layers towards basin margins. Blue‐green filamentous algae thrived in the shallower marginal areas and at least partly controlled carbonate deposition, which must have occurred during seasonal outflow of carbonate‐rich ground water from the calcareous dune aquifer over denser gypsum‐saturated waters. These systems eventually migrated towards the centre of the lake to produce the relationships preserved today. The fresher waters also leached the gypsum from the marginal gypsum‐carbonate facies. Collapse due to gypsum dissolution, along with aragonite crystallization, combined to form a lake‐marginal mega‐polygonal facies. Teepee structures formed around polygon margins, with optimum conditions for stromatolite development occurring on the teepee crests. The actual stromatolites which occur around Marion Lake are strongly indurated and involve a variety of morphologies, the most common of which are laterally linked hemispheroids. Stacked hemispheroids and oncolites are also relatively common, along with irregular forms, many of which encrust a variety of substrate irregularities. Vertical relief of the stromatolites varies from centimetres to tens of centimetres and all forms are characterized by extremely fine internal interlaminations of alternate light and dark grey laminae which typically occur several per millimetre. The microstructure comprises micritic aragonite crystals with fibrous habit associated with organic matter, and occasional zones of abundant algal filament molds which are generally oriented normal to the laminae.
Analytical study of turbidites, Otadai Formation, Boso Peninsula, Japan
Authors: HIRAYAMA JIRO , NAKAJIMA TERUMASA 
Published: June 2006

ABSTRACTOn the Boso Peninsula, southeast of Tokyo, individual turbidite layers have been traced over about 38 km with the aid of tuff marker‐beds.Sandstones are generally much more variable in thickness than the interbedded mudstones, showing lenticular shapes with great lateral continuities up to 30 km or more. But the latter have greater extents than the former, forming extensive sheets with a slight upslope thickening. Most sandstones show vertical and lateral grading, while mudstones gradually coarsen upslope. Sandstones have a variety of sedimentary structures typical of turbidites, whereas mudstones are virtually devoid of them throughout. Sandstones contain intensely abraded shallow‐water fauna in contrast with mudstones with well‐preserved deep‐water fauna. Thus, mudstones are mostly of hemipelagic origin, and turbiditic mudstones, if present, are of negligible amounts.In general, individual sandstone beds are much more variable in thickness on the upstream side than on the downstream side. This is probably because of the existence of some minor channels and the adjacent levee‐like embankments on the upslope side. At the base of thicker sandstone beds local penecontemporaneous erosion has usually taken place, giving rise to a few minor channels bordered by small levee‐like embankments on both sides. The downslope side of the largest channel is commonly flanked by the greatest embankment that is coincident with the thickest part of each sandstone layer. Sandstones in the minor channels and levee‐like embankments contain a lot of mud lumps torn out of the substratum, which decrease in size and amount and improve in roundness with increasing distance from the channels. In general, thicker sandstones show Bouma abcde sequences around the thickest part. Bouma d‐division and convolution are developed preferentially on the downstream side of the thickest part. With decreasing bed thickness laterally, the complete depositional sequences arc successively replaced by Bouma bcde, then cde, and de sequences downslope, while by Bouma bc/c and then c/e sequences upslope.The facies transition of the horizon studied probably represents a section extending from the downslope end of the channelized portion of a suprafan to the outer portion with a smooth surface.
Sandstone mounds and associated facies sequences in some Late Precambrian and Cambro‐Ordovician inshore tidal flat/lagoonal deposits
Authors: BALDWIN CHRISTOPHER T. , JOHNSON HOWARD D. 
Published: June 2006

ABSTRACTSandstone mounds occur in some shallow marine heterolithic deposits from the Late Precambrian Stangenes Formation (N. Norway) and the Cambro‐Ordovician Crozon Formation (N. W. France) and Cabos Series (N.W. Spain). The sediments displaying the mounds accumulated in partially protected tidal flat/ lagoonal environments immediately before and during major transgressions.The mounds are erosional features typically occurring on the tops of sheet sandstones (ca. 50–500 mm thick) some of which may have a storm washover origin. Mound genesis related to periodic emergence and late stage run‐off is supported by their intimate association with mudcracks and other very shallow water features (e.g. bidirectional current‐formed structures, wave ripples, ladder and interference patterns, mudflakes, etc.). Variation in mound morphology suggests that post‐depositional dissection began as elongate ridge‐gully couplets with secondary erosion of the ridge flanks leading to the development of more characteristic hemispherical geometries. Emergence may have been a function of tidal fluctuations and/or subsidence of storm surge events.Facies sequences point to the repeated filling of these inshore environments by storm washovers superimposed on ambient tidal conditions which possibly resulted from the progressive decay of beach barriers during transgression.
Genesis and diagenesis of hiatus‐ and breccia‐concretions from the mid‐Cretaceous of Texas and northern Mexico
Authors: KENNEDY W. J. , LINDHOLM R. C. , HELMOLD K. P. , HANCOCK J. M. 
Published: June 2006

ABSTRACTA study of septarian concretions from late Cretaceous shale sequences of Texas and northern Mexico reveals complex burial‐exhumation histories. First generation concretions and pre‐fossilized moulds formed in silty clays before significant compaction occurred. Syneresis cracks developed and were filled by length slow fibrous calcite with a decrease in iron content toward the centre of each vein; in some cases this was succeeded by growth of equant or bladed calcite. Subsequent erosion of enclosing sediment caused collapse and break up of concretions, leaving fragments and some unbroken concretions exposed to encrustation and boring on the sea floor. These were subsequently buried and enclosed in a second generation of concretions, which also developed syneresis cracks. The calcite that filled these latter cracks was generally bladed (sometimes followed by equant calcite) and contains more iron toward the centre of the vein. Differences between the calcite filling first and second generation septarian veins indicates changing diagenetic regimes during burial in the marine environment, without a major shift in diagenetic conditions, such as the introduction of meteoric waters. These conclusions have significant implications to cementation in ancient limestones, which show textural and chemical sequences similar to those present in the septarian concretions described.
The sedimentary history of a debris‐flow dominated, Devonian alluvial fan–a study of textural inversion
Authors: LARSEN V. , STEEL R. J. 
Published: June 2006

ABSTRACTKarlskaret fan, with a radius of less than 11/2 km and dominated by debris‐flow conglomerates, is one of numerous alluvial fans built out from the fault margins of Hornelen Basin (Devonian, Norway). The fan body is more than 170 m thick proximally, consists of four main coarsening‐upwards segments and thins distally by a rising of its base and by a vigorous interfingering with very fine‐grained sediments originating from an adjacent, impinging floodbasin system.Within the entire fan body, and within individual lobes, is a proximal‐distal (and vertical) facies change from sheet‐like, polymodal debris‐flow conglomerates through matrix‐rich conglomerates that are commonly distorted by loading, slumping and faulting, to remarkably sheet‐like, matrix‐rich granule sandstone of subaqueous debris‐flow origin.Because the alluvial fan prograded into an actively aggrading floodbasin the primary fanglomerates, themselves having been subject to some sorting on the fan surface, incorporated large quantitites of very fine sediments. This inclusion of fines, effectively a textural inversion on the lower fan reaches, frequently led to remobilization and resedimentation of material beyond the fan toe. Anomalous maximum particle size/bed thickness relationships and a variety of graded textures within these resedimented beds suggest deposition in lacustrine areas of the adjacent floodbasin.
Stable isotopes of cherts and carbonate cements in the Lake Valley Formation (Mississippian), Sacramento Mts, New Mexico
Authors: MEYERS W. J. , JAMES A. T. 
Published: June 2006

ABSTRACTOxygen isotopic compositions of chert and calcite cements in the Lake Valley Formation indicate that these diagenetic features cannot be equilibrium co‐precipitates in spite of their coexistence in the same interstices. Petrography of megaquartz and non‐ferroan calcite cements indicates that both are original precipitates that formed during pre‐Pennsylvanian time at shallow burial depths (< 215m) implying precipitation temperatures less than 30°C. Under these constraints the δ18Os of megaquartz (mean =+27.00/00 SMOW; range =+ 24.8 to + 28.90/00) and calcite (mean =+ 28.00/00 SMOW; range =+ 27.3 to + 28.40/00) are best interpreted as unaltered since precipitation; thus, they must reflect the oxygen isotopic composition of pre‐Pennsylvanian pore waters. Microquartz and chalcedony are interpreted to have formed from recrystallization of pre‐Pennsylvanian opal‐CT precursors, and therefore probably re‐equilibrated during recrystallization in late or post‐Mississippian time.We propose a model integrating the isotopic data with regional petrographic and sedimentological data that explains the greater consistency and generally greater δ18Os values of the calcites compared to those of the cherts. This model is one of chertification and calcite cementation in a regional meteoric phreatic ground‐water system, the seaward terminus of which moved southward during lowering of pre‐Pennsylvanian sea level. The calcite cements and some of the opal‐CT precursor to microquartz and chalcedony are interpreted to have formed in the more seaward portions of the groundwater system. The megaquartz precipitated in the more inland parts of the phreatic groundwater system where rainfall was isotopically lighter and more variable. As such, the δ18Os of the megaquartz reflect the isotopic composition of groundwaters in areas undersaturated with respect to calcite.
Diagenesis of late Cenozoic diatomaceous deposits and formation of the bottom simulating reflector in the southern Bering Sea*
Authors: HEIN JAMES R. , SCHOLL DAVID W. , BARRON JOHN A. , JONES MARJORIE G. , MILLER JACQUELYN 
Published: June 2006

ABSTRACTDiatom ooze and diatomaceous mudstone overlie terrigenous mudstone beds at Leg 19 Deep Sea Drilling Project sites. The diatomaceous units are 300‐725 m thick but most commonly are about 600 m. Diagenesis of diatom frustules follows a predictable series of physical and chemical changes that are related primarily to temperature (depth of burial and local geothermal gradient). During the first 300‐400 m of burial frustules are fragmented and undergo mild dissolution. By 600 m dissolution of opal‐A (biogenic silica) is widespread. Silica reprecipitates abundantly as inorganic opal‐A between 600 and 700 m sub‐bottom depth. Inorganic opal‐A is rapidly transformed by crystal growth to opal‐CT. The result is formation of silica cemented mudstone and porcelanite beds.A regional acoustic reflector (called the bottom‐simulating reflector, or BSR) occurs near 600 m depth in the sections. This acoustic event marks the upper surface where silicification (cementation) is active. In Bering Sea deposits, opal‐A is transformed to opal‐CT at temperatures between 35° and 50°C. This temperature range corresponds to a sub‐bottom depth of about 600 m and is the area where silicification is most active. Thus, the BSR represents an isothermal surface; the temperature it records is that required to transform opal‐A to opal‐CT. Deposition of at least 500 m of diatomaceous sediment was required before the temperature at the base of the diatomaceous section was appropriate (35°‐50°C) for silica diagenesis to occur. Accordingly, silica diagenesis did not begin until Pleistocene time. Once silicification began, in response to sediment accumulation during the Quaternary, the diagenetic front (the BSR) moved upsection in pace with the upward migrating thermal boundary.X‐ray diffractograms and SEM photographs show three silica phases, biogenic opal‐A, inorganic opal‐A’, and opal‐CT. These have crystallite sizes of 11‐16 A, 20‐27 A, and 40‐81 A, respectively, normal to 101. The d(101) reflection of opal‐CT decreases with depth of burial at DSDP Site 192. This occurs by solid‐state ordering and requires at least 700 m of burial.Most clinoptilolite in Leg 19 cores forms from the diagenesis of siliceous debris rather than from the alteration of volcanic debris as is commonly reported.
Calcretes of Olduvai Gorge and the Ndolanya Beds of northern Tanzania
Authors: HAY R. L. , REEDER R. J. 
Published: June 2006

ABSTRACTPedogenic calcretes are closely associated with Pliocene to Holocene wind‐worked deposits of volcanic ash in the Olduvai and Ndolanya Beds of northern Tanzania. The typical profile with calcrete consists of an unconsolidated sediment layer, an underlying laminar calcrete, and a lowermost massive calcrete. The laminar calcrete is a relatively pure limestone, whereas massive calcrete is aeolian tuff cemented and replaced by calcite. An Olduvai calcrete profile can develop to a mature stage in only a few thousand years. Carbonatite ash was the dominant source for most of the calcite in the calcretes.Replacement was a major process in formation of the massive calcretes, and oolitic textures have resulted from micrite replacing pelletoid clay coatings around sand grains. Phillipsite and possible other zeolites were extensively replaced in the massive calcretes. Replacement of clay by micrite in the Olduvai calcretes is accompanied by dissolution or leaching of phengitic illite and the formation of clay approaching the composition of halloysite or kaolinite. In the upper calcrete of the Ndolanya Beds, montmorillonite was altered to a kaolinite‐type mineral and to dioctahedral chlorite. Authigenic dolomite, zeolite, and dawsonite in the Olduvai calcretes probably received at least some of their components from replaced materials.
Low‐magnesian calcite limestones forming at the deep‐sea floor, Tongue of the Ocean, Bahamas
Authors: SCHLAGER W. , JAMES N. P. 
Published: June 2006

ABSTRACTCarbonate ooze in the deep troughs between the Bahama Banks is a mixture of pelagic and bank‐derived material. It consists of aragonite, calcite and magnesium calcite in a ratio of about 3:2:1. Where exposed in erosional cuts at the sea floor, this ooze lithifies within 100,000 years and is transformed into calcite micrite of only 3.5‐5 mol % MgCO3. Where buried, the ooze maintains its original composition for at least 200,000‐400,000 years and remains unlithified for tens of millions of years.Quite unexpectedly, the path of sea‐floor diagenesis of peri‐platform ooze was found to be the same as that of freshwater diagenesis. Most of the aragonite is leached, pteropod shells often leaving cement‐lined moulds behind; magnesian calcite recrystallizes and loses magnesium; polyhedral calcite of 2‐4 μm size appears as cement. The setting and the carbon‐oxygen isotope ratios rule out any freshwater influence. Carbon isotope ratios remain heavy, oxygen ratios shift towards equilibrium with the cold bottom water.The calcite cement has 3.5‐5 mol % MgCO3 and can be interpreted as the least soluble form of calcite emerging from alteration at the sea floor or, alternatively, as a direct precipitate from cold sea water. The change in the composition of calcite cements with water depth supports the second interpretation. In the Bahamas and elsewhere in the world ocean, magnesium in calcite cements decreases from the warm surface waters down to 700‐1200 m, i.e. the boundary between intermediate and cold deep‐water masses. Below this level, calcite prevails and magnesian calcite and aragonite cements are restricted to semi‐enclosed seas with exceptionally warm bottom waters.
Boulder shapes and grain‐size distributions of debris as indicators of transport paths through a glacier and till genesis
Authors: BOULTON G. S. 
Published: June 2006

ABSTRACTDebris transported by glacier is derived either supraglacially from nunataks and valley sides or from erosion of the subglacial bed. Debris produced above the glacier by fracturing of rock walls has a dominant coarse fraction with angular boulders. Subsequent englacial or supraglacial transport is relatively passive and little comminution occurs. Debris eroded from subglacial bedrock is initially transported in a basal zone of traction, where particles frequently come into contact with the glacier bed and are retarded by it so that large forces may be generated between particles and the bed and at interparticle contacts. The material introduced into this tractional zone may be subglacial bedrock which has undergone a crushing‐plucking event and which has a dominant coarse fraction, or supraglacially derived material which finds its way to the glacier bed. These parent debris assemblages are further comminuted by failure in response to locally concentrated compressive stresses, and attrition at shearing interfaces. Boulders transported through the tractional zone will tend to be rounded and bear several directions of striation. Large boulders embedded in lodgement till will tend to be streamlined with striae parallel to glacier flow and with an abruptly truncated distal extremity, rather like a roche moutonnée. Textural and boulder shape characteristics can be used to help distinguish different types of till.
Alluvial fan and lacustrine sediments from the Stephanian A and B (La Magdalena, Ciñera—Matallana and Sabero) coalfields, northern Spain
Authors: HEWARD ALAN P. 
Published: June 2006

ABSTRACTVertical sequence analysis within 1500‐2500 m thick coarse‐grained coalfield successions allows six sedimentary associations to be distinguished. These are interpreted in terms of depositional environments on, or related to alluvial fans which fringed a fault bounded source region. (i) Topographic valley and fanhead canyon fills: occurring at the bases of the coalfield successions and comprising sporadically reddened, scree, conglomeratic thinning and fining upward sequences, and fine‐grained coal‐bearing sediments. (ii) Alluvial fan channels: conglomerate and sandstone filled. (iii) Mid‐fan conglomeratic and sandstone lobes: laterally extensive, thickly bedded (1‐25 m) and varying from structureless coarse conglomerates and pebbly sandstones, to stratified fine conglomerates and cross‐bedded sandstones. (iv) Interlobe and interchannel: siltstones, fine‐grained sheet sandstones, abundant floras, thin coals and upright trees. (v) Distal fan: 10 cm‐1.5 m thick sheet sandstones which preserve numerous upright trees, separated by silt‐stones and mudstones with abundant floras, and coal seams. The sheet sandstones and normally arranged in sequences of beds which become thicker and coarser or thinner and finer upwards. These trends also occur in combination. (vi) Lacustrine: coals, limestones, and fine‐grained, low‐energy, regressive, coarsening upward sequences.Proximal fan sediments are only preserved in certain basal deposits of these coalfields. The majority of the successions comprise mid and distal alluvial fan and lacustrine sediments. Mid‐fan depositional processes consisted of debris flows and turbulent streamflows, whilst sheetfloods dominated active distal areas. A tropical and seasonal climate allowed vegetation to colonize abandoned fan surfaces and perhaps resulted in localized diagenetic reddening. Worked coals, from 10s cm‐20 m thick, occur in the distal fan and lacustrine environments.These alluvial fan deposits infill‘California‐like’basins developed and preserved along major structural zones. In many of their characteristics, in particular the occurrence of thinning and fining, and thickening and coarsening upward sequences and megasequences, these sediments have similarities to documented ancient submarine fan deposits.
Grain‐size distribution of suspended load in relation to bed materials and flow velocity
Authors: SENGUPTA SUPRIYA 
Published: June 2006

ABSTRACTGrain‐size frequency distributions of suspended loads at different flow velocities and over sand beds of four different grain‐size patterns were studied in a laboratory flume. The proportion of bed material which went into suspension increased with decrease of grain‐size in each case, but the modes of the suspended loads occurred in the size classes intermediate between the coarsest and the finest. With increase of flow velocity, as also with decrease of the bed's mean grain‐size, the total amount of material in suspension markedly increased, mainly due to addition of particles to the medium size classes.The coarsest grains in the bed resisted erosion due to their weight, whereas the finest ones were either not available in sufficient quantities or resisted erosion due to their homogeneity. The finest of the erodible grains which were abundantly available in bed were therefore, lifted up in large quantities. This size sorting took place at or near the bed surface and was closely related to the process of bed form migration.Large accumulation of medium sized particles in suspension at high velocities led to lognormal grain‐size distributions when the nature of the bed (source) material was suitable. At lower velocities, or over other types of bed materials, the phi (log)‐probability plots of cumulative grain‐size distributions of the suspended loads resolved into a number of straight lines. Mixtures of linear segments on phi‐probability graphs therefore, need not necessarily indicate different modes of sediment transportation, as is commonly believed, but might reflect the conditions of flow and the nature of the source material.