bluuebunny/crossref_metadata_2025_split huggingface dataset:
Anatomy and origin of a Cretaceous phosphorite‐greensand giant, Egypt
Authors: GLENN CRAIG R. , ARTHUR MICHAEL A. 
Published: June 2006

ABSTRACTLate Cretaceous epicontinental phosphorites, porcelanites/cherts, dark‐coloured shales, glauconitic sandstones and bioclastic and fine‐grained carbonate rocks in Egypt are examined in terms of their overall depositional and diagenetic framework and stable isotopic and organic geochemical characteristics. Two main depositional realms are interpreted and correlated through sequence stratigraphic analysis: (1) a shallow hemipelagic environment accompanying initial stages of marine transgression and conducive to the formation of organic carbon‐rich shales, biosiliceous sediments and thick phosphorites, and (2) a relatively high energy depositional regime accompanying sea‐level fall during which deltas advanced, glauconites were reworked seaward and prograding oyster banks became periodically exposed to episodes of fresh water diagenesis, thereby promoting solution‐collapse phenomena in associated cherts. Lenticular to massive phosphorites are viewed as the result of current winnowing and concentration of authigenic grains initially precipitated in associated reducing shales and biosiliceous sediments. In eastern Egypt the phosphorites form winnowed lag layers, some of which may have been redeposited down slope in structural lows. In the west, these sands were concentrated into giant phosphorite sand waves built by reworking of penecontemporaneously deposited phosphatic muds during marine transgression. Carbon isotopic results substantiate interpretations from modern deposits for limitation of phosphate mineral precipitation with depth in sediments as a result of lattice poisoning. However, direct desorption of phosphorus to pore waters from detrital iron‐oxyhydroxide phases also may have been important in the Cretaceous setting, the iron reduced in this process being available for incorporation in glauconites. The main locus for authigenic glauconite precipitation appears to be where iron fluxes from regions of lateritic weathering were highest and near the boundary between oxygenated and reduced waters.This study suggests a model for the common coexistence of glauconites and phosphorites in the geological record. Although upwelling is often advocated as the origin of nearly all giant phosphorite deposits, we suggest that some of these may have been strongly influenced by fluvially derived phosphorus borne on particulates and desorbed from these compounds upon flocculation and/or reduction in bottom waters or pore waters.
Diagenetic alunite in clastic sequences, Kuwait, Arabian Gulf
Authors: KHALAF FIKRY I. 
Published: June 2006

ABSTRACTDiagenetic alunite occurs with calcrete, gypcrete and silcrete deposits in a Mio‐Pleistocene clastic sequence at several locations in southern Kuwait, Arabian Gulf. Based on their physical properties and textural characteristics the alunite deposits were divided into (1) chalky quartzitic alunite, (2) chalky quartz‐free alunite, (3) hard pink alunitic sandstone. The chalky alunite is composed mainly of hydronium‐alunite (H3O)Al3(SO4)2(OH)6, while the hard pink alunite is composed solely of well developed potassium alunite KAl3(SO4)2(OH)6. These minerals resulted from the action of sulphuric acid on clays and K‐feldspars in the muddy sandstone and mudstone host sediments. The sulphuric acid is most probably produced by the oxidation of hydrogen sulphide that might have seeped in from the oil fields of this area. Four diagenetic stages are suggested for the genesis of the studied alunites: gypsification, sulphuratization, silicification and alunitization. In oil field areas, the occurrences of alunite would serve as an indicator for the presence at depth of hydrocarbons.
Neptunian dykes and associated breccias (Southern Alps, Italy and Switzerland): role of gravity sliding in open and closed systems
Authors: WINTERER E. L. , METZLER C. V. , SART M. 
Published: June 2006

ABSTRACTNeptunian dykes and sills of Middle Jurassic pelagic limestone within Lower Jurassic shallow‐water carbonate host rocks occur at many localities in the Southern Alps of Italy and Switzerland, especially on what were the upper slopes of tilted half‐grabens created during the Early Jurassic rifting stage of a passive margin that faced the Middle and Late Jurassic Tethyan Ocean. The host rocks were dilated by cracking, folding, and brecciation during movements of shallow‐based gravity‐driven slides and slumps of semibrittle platform strata, commonly along décollement contacts between layers of different competence. In most places, the network of cavities in the dilated strata connected to the sea floor, and pelagic sediments trickled from above into the open spaces. In other places, the brittle strata were overlain by somewhat impermeable sediments that formed a partial seal. Sudden dilation of the brittle beds resulted in forceful injection of the overlying weakly consolidated or plastic sediments into open spaces. The filling in both open and closed systems was commonly episodic, resulting in complex internal‐sediment stratigraphy and cross‐cutting dykes. Stable isotopic data on internal sediments and early‐formed cement lie within the field of normal sea water, and none of the sedimentological or stable isotopic data supports a subaerial, dissolution (karst) origin for the Jurassic neptunian dykes of this region.
The shelf sand‐plume model: a critique
Authors: SCHEIHING MARK H. , GAYNOR GERARD C. 
Published: June 2006

ABSTRACTIn recent years, a new model for deposition of sand bodies in a shelf environment has appeared. This model, known as the shelf sand‐plume model, is hypothesized to result from storm‐driven currents that are deflected around a deltaic headland, stripping sand from the headland and redepositing it in a downcurrent ‘plume’ on the inner shelf. The modern analogue for this model is considered to be an arcuate shelf sand body located off the Damietta branch of the Nile Delta. However, the distribution of older deltaic and shoreline sands probably controls the arcuate outline of the sand body. The present current system has certainly reworked these sands into ridges and large‐scale bedforms but is not responsible for the overall outline of the sand body. Grain‐size range and distribution of sand on the shelf demonstrate that the source of sand in the Nile shelf sand body is not the modern Damietta headland as postulated by the shelf sandplume model.In our view, the shelf sand‐plume model is presently unsubstantiated and has orginated as a misapplication of the original Nile example. As a geological model, the shelf sand‐plume model lacks a set of observable, consistently applicable criteria. The only common denominator to the model is the ‘plume’ geometry of a sand body located off a deltaic promontory. However, workers postulating the existence of shelf sand‐plumes have neither clearly established a ‘plume’ geometry nor shown the juxtaposition of these bodies with respect to coeval deltaic headlands in their outcrop or subsurface examples. The model does not provide criteria to distinguish a ‘shelf sand‐plume’ from other classes of shelf sand bodies, notably sand ridges and storm‐generated sheet‐like sands. Its application to the rock record should be re‐evaluated.
Eustatically‐controlled sedimentation in the Hettangian‐Sinemurian (Early Jurassic) of Poland and Sweden
Authors: PIEŃKOWSKI GRZEGORZ 
Published: June 2006

ABSTRACTIn earliest Jurassic times, terrigenous, continental and marginal marine deposition occurred in a large epeiric basin along the Tornquist Line in Europe. Detailed sedimentological studies allow recognition of palaeoenvironmental fluctuations in space and time. The main earliest Jurassic transgressions occurred in the early Hettangian, early Sinemurian, mid‐Sinemurian and latest Sinemurian and formed bounding discontinuities (transgressive surfaces) of considerable correlative significance. There is a step‐wise trend of increasing marine extension and influence during the early Hettangian, early Sinemurian, mid‐Sinemurian and latest Sinemurian‐earliest Pliensbachian transgressions. Four sequences, four transgressive systems tracts, three highstand systems tracts and three levels regarded as equivalents of maximum flooding surfaces are distinguished. In the case of type 2 sequences, when incised valley‐fill deposits are not developed and regional erosion is less common, it may be rather difficult to define the sequence boundaries, which are often concealed within the amalgamated fluvial deposits occurring in the neighbouring parts of two adjacent sequences (fluvial/deltaic sediments terminate the highstand systems tracts and in this setting the transgressive systems tracts start with continental deposits prior to the transgressive surfaces). Generally, an exact correlation can be achieved between the sequence stratigraphy of the northeast and northwest European Lower Jurassic and the eustatic curve proposed by EPR (assuming some changes proposed by A. Hallam). The establishment of this correlation hopefully will stimulate future studies of the sequence stratigraphy of poorly dated siliciclastic deposits of marginal basins. In this setting even minor changes in sea‐level may cause major changes in facies development over large areas.
Transverse and linear dunes in an Upper Pleistocene marine sequence, Corinth Basin, Greece
Authors: COLLIER RICHARD E. LL. , THOMPSON JILL 
Published: June 2006

ABSTRACTA range of large‐scale dunes of oolitic calcarenite composition are exposed in the Corinth Basin of central Greece. These transverse dunes and a very large linear dune (> 15 m high) lie within an Upper Pleistocene, transgressive marine sequence. Tidal flow, accelerated by constriction through a narrow, fault‐bounded seaway, is interpreted to have generated the current velocities necessary to produce the dunes.Marine facies in the Upper Pleistocene sequence include beach to offshore conglomerates and sandstones with wave‐modified sedimentary structures and herringbone cross‐stratification. An offshore facies association comprises variably bioturbated siltstones and sandstones with a varied marine fauna that includes thermophile species such as scleractinian corals and Strombus bubonius. Oolitic sandstone facies also occur.Oolitic sands were apparently produced in shoal environments subject to tidal (and wave) action, and transported by dominant southerly currents over the southern part of the basin. Oolites accumulated in a linear dune 2.7 km long and 15–20 m high and in three‐dimensional transverse dunes up to 10 m high having a variety of compound and simple internal geometries. The isolated, WSW‐ENE‐trending linear form exhibits angle of repose sedimentary dips (up to 35°) of avalanche sets on its SE flank and sets typically with dips of 15–20° to the NW. Internal high‐angle discontinuities are developed in the SE‐dipping lee face. It is proposed that a dominant north‐to‐south flow crossed over the crest obliquely, resulting in both net erosional and depositional processes on the lee flank. A subordinate (?tidal) current may have locally and or periodically crossed the dune crest in a westwards direction. A string of transverse dunes, which were located adjacent to a fault/marine terrace scarp, is interpreted to have originally coalesced to form the linear dune.The distribution of transverse and linear dunes together with the palaeogeographical reconstruction suggest that a marine connection periodically existed across the Corinth Isthmus during the Late Pleistocene due to a combination of active faulting and glacio‐eustatic highstands of sea level.
Formation of 18O‐depleted dolomite within a marine evaporitic sequence, Triassic Reichenhall Formation, Austria
Authors: SPÖTL CHRISTOPH , BURNS STEPHEN J. 
Published: June 2006

ABSTRACTDark grey, bituminous dolostones interbedded with marine‐derived anhydrite horizons occur in the Triassic Reichenhall Formation of western Austria. Fossils are rare and indicate a hostile, hypersaline depositional environment. The dolomites are finely crystalline, fairly stoichiometric, well ordered and non‐ferroan. Closely spaced samples (94 in total) of individual dolomite units have been analysed for their carbon and oxygen isotopic composition. The data indicate surprisingly low δ18O values (‐5.7 to ‐2.1%0 PDB), whereas the δ13C values are comparable to the contemporary Triassic seawater (+0.2 to +2.6%0 PDB). Sedimentological evidence, including (i) lack of any evidence for extensive dissolution, (ii) distinct oxygen and carbon isotope ratios of individual dolomite units, (iii) covariance of carbon and oxygen isotopes within some dolomite layers and (iv) inclusions of celestite in dolomite, indicates a nearly closed system after early diagenesis. Combining this information with water‐rock interaction calculations suggests that the lightest oxygen isotope compositions are the result of freshwater influx into the basin during very early dolomite formation. A secondary factor may be dolomite recrystallization at elevated temperatures during burial.
Rock varnish in the Sonoran Desert: microbiologically mediated accumulation of manganiferous sediments
Authors: NAGY BARTHOLOMEW , NAGY LOIS ANNE , RIGALI MARK J. , JONES WILLIAM D. , KRINSLEY DAVID H. , SINCLAIR NORVAL A. 
Published: June 2006

ABSTRACTRock varnish occurs in virtually all environments, most commonly in arid and semi‐arid climates, including Antarctica. Rock varnish consists of thin layers of intimately mixed aeolian and chemical sediments often showing botryoidal and more rarely stromatolite‐like morphologies. Typical rock varnish samples collected at Twin Peak Mountain Park, near Phoenix, Arizona, consist of abundant quartz, with plagioclase, illite and a mixed layer, Fe‐clay mineral, probably corrensite. EDS, SEM (BSE) and TEM analyses revealed that the typical Mn, Fe minerals occur as minute particles; some of these particles and other mineral grains are attached to filaments. XRD and electron diffraction showed that the Mn.Fe‐bearing particles are poorly crystalline. The filaments, based on morphological criteria, are virtually indistinguishable from fungal filaments. Most filaments are fragments, probably broken by scraping during sample collection. Coccoid and rod‐shaped forms, resembling cyanobacteria and other bacteria, respectively, are also present. Unlike definitive minerals, these filaments disintegrated in the concentrated energy of the SEM electron beam at the instrumental and experimental conditions used. In addition, no filamentous, rod‐shaped or coccoid forms were observed in samples hydrolysed with 6 N HCl for 24 h at 100°C. Bacteria and fungi in powdered rock varnish were cultured on four media, incubated aerobically in the dark at 25°C. The culture media yielded dense growths of spore‐forming bacteria and filamentous fungi. One fungus and two Bacillus isolates oxidized and concentrated manganese. Control experiments revealed that fungi and bacteria are present on and below the surfaces of rock varnish. Free and hydrolysed, peptide/protein‐bound amino acids were identified in the rock varnish. Amino acids showed virtually no racemization with the exception of D/L asp = 0.1. Relatively high molecular weight humic matter was also separated from the rock varnish. High‐resolution mass spectrometry revealed non‐hydrocarbon moieties, similar to a Suwannee River (FL) humic acid standard. Micro‐organisms and their original biochemical compounds do not seem to be preserved for long in the accreting varnish layer. The studies showed that the filaments helped to trap mineral particles of rock varnish, and that bacteria and fungi abetted Mn concentration. Some structures in the layers of rock varnish resemble stromatolites and present definitions would allow them to be termed as such.
Controls on synorogenic alluvial‐fan architecture, Beartooth Conglomerate (Palaeocene), Wyoming and Montana
Authors: DeCELLES P. G. , GRAY M. B. , RIDGWAY K. D. , COLE R. B. , PIVNIK D. A. , PEQUERA N. , SRIVASTAVA P. 
Published: June 2006

ABSTRACTLate Palaeocene uplift of the Beartooth Range in northwestern Wyoming and southwestern Montana generated the Beartooth Conglomerate along the eastern and northeastern flanks of the range. Systematic unroofing sequences and intraformational unconformities, folds, and faults in the conglomerate attest to deposition during uplift. Along the eastern flank, at least three ancient alluvial‐fan systems and a braidplain system can be distinguished on the bases of petrofacies and lithofacies. The two southern fans consist of 700+ m of sedimentary‐clast conglomerate and subordinate sandstone, dominated by hyperconcentrated‐flow and stream‐flow facies. The next fan to the north is dominated by plutonic and metamorphic clasts and contains abundant mud‐matrix‐supported debris‐flow facies, as well as stream‐flow facies. The northernmost depositional system consists of arkosic, channellized fluvial conglomerate and sandstone, overbank mudstone, and crevasse‐splay sandstone units. Palaeocurrent data indicate eastward dispersal, away from the Beartooth Range.Outstanding exposure of the Beartooth Conglomerate allows facies to be mapped on lateral photographic mosaics. A seven‐fold hierarchy of bounding surfaces and enclosed lithosomes exists in the Beartooth Conglomerate. First‐ through fourth‐order surfaces are analogous to first‐ through fourth‐order surfaces that recently have been documented in sandy fluvial facies, with one exception: sediment gravity flows are bounded by first‐order surfaces. Fifth‐order surfaces are either erosional (e.g. lateral migration of fanhead trench) or accretionary (e.g. aggradation of fan surface during backfilling of trench, and construction of lobes on lower fan during entrenchment on upper fan). Some fifth‐order surfaces coincide with intraformational angular unconformities and are thus the result of long‐term fanhead entrenchment following uplift of the upper part of the fan. Sixth‐order surfaces bound individual fan packages that are several hundred metres thick and ∼ 10 km2 in area. The enclosed sixth‐order lithosomes are distinguishable in terms of petrofacies and lithofacies. A single seventh‐order surface bounds the entire Beartooth Conglomerate. Lower‐order lithosomes are produced by intrinsic processes of fan construction. Fifth‐order lithosomes can be attributed to both extrinsic and intrinsic controls. Sixth‐ and seventh‐order lithosomes are generated by extrinsic controls.
Environmental gradients in carbonate sediments and rocks detected by correspondence analysis: examples from the Recent of Norway and the Dinantian of southwest England
Authors: HENNEBERT M. , LEES A. 
Published: June 2006

ABSTRACTContinuous sedimentary gradients are only crudely expressed by standard facies and microfacies methods which are more appropriate to situations where changes occur in relatively discrete steps. In carbonate sediments and rocks, continuous gradients are often represented by the arrangement of component grain types in a relay, that is, a systematic shifting of the relative importance of the components. Subdivision of such relays into (micro)facies can only be arbitrary.Correspondence analysis is shown to be useful for detecting, isolating and describing relays. Particular use is made of the arch effect in which samples and components from data sets with a strong unidimensional structure (a relay) plot in the form of an arch in the plane of the first two factor axes. A relay index, indicating the position of samples in the relay, can be extracted from the analysis and plotted on maps and stratigraphic logs to reveal details of the sedimentary gradient in areal and/or stratigraphic context.Examples are given from: (i) Recent shallow‐marine carbonate sediments from northern Norway, illustrating a relatively simple depositional setting where surface sediments are viewed in plan; and (ii) Lower Carboniferous carbonates of southwest England, representing a more complex regional study of a particular stratigraphic interval viewed in cross‐section. In both examples the relays can be related to identifiable environmental gradients.
Syntectonic carbonate veins from the Barbados accretionary prism (ODP Leg 110): record of palaeohydrology
Authors: VROLIJK PETER , SHEPPARD SIMON M. F. 
Published: June 2006

ABSTRACTSyntectonic carbonate veins were found along the décollement zone between the Atlantic and Caribbean plates and in two sites 12 and 17 km landward of the deformation front of the Barbados accretionary prism (ODP Leg 110). Both calcite and lesser amounts of rhodochrosite (MnCO3) occur in microcrystalline, fibrous, and blocky mosaic forms in veins deposited along scaly shear zones. Descriptions of the occurrences, textures, mineralogy, and stable isotope geochemistry of these veins presented herein help trace the hydrological evolution of the Barbados accretionary prism, which has seen the impact of more deeply derived fluids on elevated sediment temperatures and altered geochemistry of interstitial fluids. At one site, for example, vein‐forming fluids have been as much as 10°C warmer and 1‰ more enriched in 18O than expected.Carbon isotope ratios of veins indicate a mixture of carbon derived from seawater and from oxidized organic matter during sulphate reduction, although dissolution of calcareous skeletons may be an additional component. The presence of methane is recorded in one vein sample in the accretionary prism, indicating that either the plumbing of the prism has changed with time, allowing more fluid to leak into the prism in the past, or that materials currently 1 km above the décollement zone were once deformed in the décollement.Using oxygen isotope analyses of rhodochrosite samples from two sites in conjunction with measurements of fluid temperature and δ18O values of interstitial fluids, it is proposed that the fractionation of oxygen isotopes between rhodochrosite and water (1000 1nα)∼38‰ at 25°C, 10‰ greater than the fractionation between calcite and water. However, it appears that substitution of 10‐20 mol.% Ca into rhodochrosite reduces the fractionation factor by 2‐9‰ so impure diagenetic rhodochrosite samples from the deep ocean will rarely be this enriched in 18O.
Sheet slides and rotational failures on a convergent margin: the Kidnappers Slide, New Zealand
Authors: BARNES PHILIP M. , LEWIS KEITH B. 
Published: June 2006

ABSTRACTThe Kidnappers Slide, on the upper continental slope of the convergent margin off eastern North Island, New Zealand, has been re‐examined using a grid of high‐resolution seismic reflection profiles. The slide is not a single feature but a complex of sheet slides and rotational failures ranging from 20 to 140 m thick, and covering a total area of 720 km2. Failures occurred in several phases, on slopes of 1–5°, in late Quaternary, muddy, shelf‐edge clinoforms that have prograded into an accretionary, trench‐slope basin. Piston cores and seismic stratigraphy show that the main failure probably occurred in early Holocene times but that movements ranged from mid last glacial to late Holocene times.The sheet slides exhibit tensional collapse via numerous listric normal faults that sole out on glide planes; there is no clear evidence of compressional structures anywhere within the complex. The glide planes occur at progressively deeper stratigraphic levels towards the northeastern end of the complex, and near the steep slope that defines the seaward edge of the trench‐slope basin. There is retrogressive failure at the top of the slope. The surficial slides are being deformed by growth of active tectonic faults and folds associated with the convergent plate margin.This type of slope failure may be partially related to metastable sandy layers within the last glacial age progradational sequence, and possibly to formation of bubble phase gas at shallow depths. Failure was probably triggered by earthquake loading of sediments in this highly seismic region.