bluuebunny/crossref_metadata_2025_split huggingface dataset:
Variations in the surface texture of suspended quartz grains in the Loire River: an SEM study
Authors: MANICKAM S. , BARBAROUX L. 
Published: June 2006

ABSTRACTStatistical size distribution and scanning electron microscopic studies of suspended sand grains in the Loire River at Montjean, France were carried out over a period of a year (hydrological cycle) to discern seasonal variations. The sand fraction in suspension is better sorted during winter (average mean = 0.69 mm, median = 0.65–0.95 mm, sorting value, σ= 1.1–1.35) and is dominated by quartzo‐feldspathic minerals. During summer, on the contrary, this fraction is rich in mica minerals and is poorly sorted (average mean = 1.21 mm, median = 0.33–0.95 mm, sorting value, σ= 1.4–2.0). The grain size of the coarser fraction shows a tendency to increase with river discharge up to an optimum discharge of about 1000 m3 s‐1 and thereafter decreases.Scanning electron photomicroscopy of quartz grains from the suspended sand population indicates that both mechanical and chemical features occur. The former (conchoidal fractures, mechanical fractures and breakage) dominate in the samples collected near the water surface during winter floods and the latter (solution pits, vermicular features, silica flowers, neogene silica and diatoms) in the samples from the bottom during low summer flows. During summer, chemical action takes place on sediments prior to or upon their deposition. Sediments that are resuspended during winter floods undergo mainly physical processes (attrition/abrasion). Mixed surface features are, therefore, observed during average and low river discharge. Inheritance of these surface features from the source area is, however, not completely excluded.Thus, the history of quartz grains in suspension can be reconstructed from SEM exoscopic studies: therefore, it can be proposed that the fluvial quartz grain surface textures result from a combination of alternating chemical and physical processes.
Depositional environments of marine‐dominated bedded halite, Permian San Andres Formation, Texas
Authors: HOVORKA SUSAN 
Published: June 2006

ABSTRACTThe interpretation of the depositional environments in which bedded halite is formed is a frontier of evaporite sedimentology. Facies studies, supplemented by geochemical data, define an evaporite shelf depositional environment for the halite of the San Andres Formation of the Palo Duro Basin, Texas Panhandle, that is significantly different from modern and previously described ancient halite‐precipitating environments such as playas, salinas, sabkhas, and barred basins.A coastal palaeoenvironmental setting for the San Andres halite is defined by its facies position between normal marine‐shelf carbonates and aeolian deposits. The San Andres brine pool extended over more than 10 000 km2 but was very shallow. Perennial brine‐pool conditions (thick sequences of halite) alternated with ephemeral brine‐pool conditions (halite, haloturbated mudstone‐halite, mudstone interbeds, and microkarst). The depositional sequence in the perennial brine pool includes (1) flooding and minor dissolution of previously deposited halite, (2) precipitation of a thin lamina of gypsum, and (3) evaporation and precipitation of halite. Bottom‐growth forms of halite, including chevrons and vertically elongated crystals, are well preserved. Cumulates of foundered, floating crystals and rafts were probably abundant but have been almost completely recrystallized.Mudstone interbeds in halite and microkarst features cut into bedded halite record the episodic interruption of brine‐pool deposition and the establishment of subaerial conditions. Vertically elongated karst pits originate at the palaeosurfaces, cross‐cut primary brine‐pool fabrics, and are filled with geopetal sediment and coarse halite cement. Mudstone interbeds and mixtures of mudstone and halite have been haloturbated by the alternating growth and dissolution of halite reflecting conditions of fluctuating salinity. The geochemistry of halite from both ephemeral and perennial brine‐pool environments reflects the frequent influx of marine brine, indicating that communication with the marine environments to the south was ‘good’ considering that restriction was sufficient to precipitate halite.
Upper Cretaceous Muti Formation: transition of a Mesozoic nate platform to a foreland basin in the Oman Mountains
Authors: ROBERTSON A. H. F. 
Published: June 2006

ABSTRACTThe Upper Cretaceous (Turonian‐Campanian) Muti Formation (Sayja Member) documents the transition from a passive continental margin to a foreland basin, related to overthrusting of continental margin and ophiolitic nappes derived from the Tethys ocean. Upper Cretaceous northeastward subduction culminated in collision of a trench with the Arabian margin. As the trench docked with the margin the lithosphere was flexed, forming a peripheral bulge that migrated cratonward with time. The platform edge was initially uplifted (Turonian) and deeply eroded, creating the ‘Wasia‐Aruma break’. After passage of the peripheral bulge subsidence began, with accumulation first of ferruginous crusts on hardgrounds. Lime‐muds were then deposited on a deepening unstable sea‐floor, along with phosphatic nodules and crusts (Turonian‐Coniacian). Passage of the overthrust load over the Arabian continental‐margin edge downflexed the lithosphere (Santonian‐Campanian), resulting in drastic foundering of the old shelf edge to form a foredeep. Upper platform horizons collapsed as slump‐sheets and debris‐flows. Limestone blocks and lithoclastic debris‐flows were shed by mass‐wasting of the already deeply eroded old platform edge. Mud and silt were derived from the uplifted Arabian continent and deposited by mainly gravitational processes in a foredeep below the C.C.D. Subsidence of the Arabian platform edge allowed the Semail ophiolite nappe finally to override the Muti basin (late Campanian) with little internal deformation. Submarine emplacement is suggested by the absence of ophiolitic detritus in the Muti Formation. The stratigraphic evolution of the Muti Formation is in good general agreement with a model of the transition of an old, thermally mature, passive continental margin to a foreland basin, where the emplaced load is submerged.
Reverse flow in turbidity currents: the role of internal solitons
Authors: PANTIN H. M. , LEEDER M. R. 
Published: June 2006

ABSTRACTPickering & Hiscott, (1985) have demonstrated amply the presence of reverse‐flow units within the thick‐bedded calcareous wacke (TCW) beds of the turbiditic Cloridorme Formation (Middle Ordovician, Gaspé Peninsula, Quebec, Canada). These reverse‐flow units are underlain and overlain by units which reveal flow in the primary (obverse) direction.In this paper, a model is proposed for this reverse flow, based on the probable nature of the primary turbidity flow. It appears that the initial flow was highly elongated (thickness h≪ length L), with h∼ 500 m, velocity U∼ 2 m s‐1 and sediment concentration C∼ 1·25%o. The rate of momentum loss of the flow is estimated by means of a useful parameter which we call the ‘drag distance’, symbol dD, defined by
image

where h and L are the thickness and length of the flow, respectively; cCd is a combined drag coefficient representing friction on the bottom and at the upper interface; and fCd is a form‐drag coefficient related to the shape and size of the head. dD is the distance travelled by a current of constant h and L, flowing over a horizontal bottom and obeying a quadratic friction law, for an e‐fold reduction in velocity.Simple considerations, confirmed by our own experiments (described in this paper), show that such an elongated turbidity current cannot be reflected as a whole from an adverse slope: when the nose of the current reaches the slope, it forms a hump, which surges backwards and sooner or later breaks up into a series of internal solitons. The latter, probably numbering 4–7, will cause reverse flow at a given point as they pass by, provided that the residual velocity in the tail is not too great. Flow in the original (obverse) direction will be re‐established after the passage of the solitons. Quiescent periods in front of, between and behind the solitons, when soliton‐associated currents cancelled out the residual obverse flow, would allow the deposition of thin mud‐drapes.Additional flow reversals observed in a few of the TCW beds cannot be explained readily by the re‐passage of solitons, since wave breaking at the ends of the basin would cause massive energy loss; internal seiches are the preferred explanation for these later reversals.
Oxygen and carbon isotope compositions of Upper Cretaceous chalk from the Danish sub‐basin and the North Sea Central Graben
Authors: JØRGENSEN NIELS OLUF 
Published: June 2006

ABSTRACTThe oxygen and carbon isotopic compositions of Upper Cretaceous chalk have been studied from a large number of outcrops and boreholes in the Danish sub‐basin and the North Sea Central Graben. The carbon isotopic compositions vary from +0.50%o to +3.00%oδ13C which correspond to the expected carbon isotopic composition of carbonate precipitated from normal sea water. The oxygen isotopic compositions of samples from outcrops and near‐surface drilled sections fall in the range from —0.50%o to — 2.00%oδ18O, which is close to the expected values for carbonate produced by coccoliths in the Late Cretaceous sea. A geographic distribution exists in the Danish sub‐basin showing relatively heavy oxygen isotope values in the centre of the basin and slightly more negative values in the eastern part of the basin. Primarily the phenomenon is thought to reflect minor variations in the oceanographic parameters in the Late Cretaceous sea.The deep subsurface sections represent a depth interval of 0–3100 m. Mechanical compaction in the uppermost part of the sequences apparently has no influence on the isotopic composition. Chemical compaction dominates at greater depth, leading to temperature‐induced isotopic re‐equilibration in the water‐rock system. In these sequences the oxygen isotope values range from − 1.50%o to — 7.50%oδ18O and are significantly correlated with depth of burial, cementation and porosity. The data indicate that pressure‐dissolution, recrystallization, reprecipitation and ion‐exchange between solid carbonate phase and pore water, are all actively involved in the oxygen isotope re‐equilibration process. This process is believed to take place in a diagenetically closed system and is tentatively divided into two phases: 1) an early diagenetic phase which takes place at porosities down to approximately 20% in which the chalk possesses a pore‐water controlled isotopic re‐equilibration system and 2) a late diagenetic phase at porosities below 20% in which the re‐equilibration process increasingly becomes influenced by the rock‐introduced isotopic change in the composition of the formation water.
The use of burrows to detect compaction and sliding in fine‐grained sediments: an example from the Cretaceous of S.E. France
Authors: GAILLARD CHRISTIAN , JAUTEE ERIC 
Published: June 2006

ABSTRACTIn the French Southern Alps, alternating pelagic limestones and marls of Hauterivian age contain a dense network of burrows. Differences between the original and preserved forms of the burrows provide evidence of sediment deformation. (a) Total compaction, from early dewatering of sediment to deep burial compression, caused flattening of burrows parallel to bedding. Its relation to carbonate is measured, and a tentative mathematical relation is proposed, (b) Synsedimentary slumping is shown by total disordering of the bioturbation network in the displaced layers. (c) A more characterized deformation is occasionally visible only on the upper part of some limestone beds. The sliding is indicated by the dip of burrows in the same direction as the flow. (d) Similar embryonic flows occur in the middle of rare limestone beds. It is thus suggested that this can account for some of the double beds that occur in pelagic alternations of calcareous and marly beds.
Travertine formation in Plitvice National Park, Yugoslavia: chemical versus biological control
Authors: EMEIS K.‐C. , RICHNOW H.‐H. , KEMPE S. 
Published: June 2006

ABSTRACTHydrochemical studies of the Plitvice Lakes and their tributaries (Croatia/Yugoslavia) were coupled with micromorphological investigations on carbonate lake sediments and recent travertines. Karst springs discharge water from aquifers in Triassic and Jurassic dolomites and limestones and collect in lakes, which are ponded behind accreting travertine dams. Waters at springs have a high CO2 partial‐pressure (greater than 7000 ppm) and are slightly undersaturated with respect to calcite (saturation index less than —0·03). CO2 partial pressure is quickly reduced in swift running streams, leading to very high supersaturation with carbonate minerals (saturation indices between 0·74 and 0·53). Calcite deposition, however, is restricted to the lake bottoms (formation of lake marl) and to the tufa dams. The annual carbonate precipitating capacity of the system based on water balance and downstream loss of dissolved ions is estimated to be on the order of 10 000 t CaCO3 as cascade deposits (tufa dams) or as micrite in lakes behind the travertine dams. The initial stages of travertine formation as a result of morphological, biological, and chemical factors are (i) moss settling on small ridges in the creek courses, (ii) epiphytes (diatoms and cyanobacteria) settling on the moss surface, (iii) micrite particles resuspending from lake bottoms and being trapped on mucous excretions from bacteria and diatoms, and (iv) inorganic calcite precipitating as sparite at nucleation sites provided by these crystal seeds. Geochemical studies of the lake marl and tufa dams show that amino acids are dominated by aspartic acid. Carbohydrates come from structural polysaccharides of diatoms. The sticky excretions, rich in aspartic acid, are necessary for the initiation of calcite precipitation. They may be a response of algal and bacterial metabolism to environmental stress by either nutrient depletion or high calcium concentrations in ambient waters. The formation of tufa and micrite (lake marl) appears to be initiated by localized biological factors and is not governed by mere calcite supersaturation of the water. Oligotrophy may be an essential precondition for the formation of fresh water carbonate deposits.
Aeolian saltation threshold: the effect of density ratio
Authors: IVERSEN JAMES D. , GREELEY RONALD , MARSHALL JOHN R. , POLLACK JAMES B. 
Published: June 2006

ABSTRACTSaltation threshold data from three wind tunnels and from hydraulic flumes are presented to show that the dimensionless threshold friction speed for small particles is a continuous function of particle‐to‐fluid‐density ratio. In addition, the dimensionless threshold speed is a function of the grain‐friction Reynolds number and an interparticle force term. The variation with density ratio seems to be due to the relative energy with which particles impact other particles to initiate saltation.
The anatomy of a deep water mud‐mound complex to the southwest of the Dinantian platform in Derbyshire, UK
Authors: BRIDGES PAUL H. , CHAPMAN AMANDA J. 
Published: June 2006

ABSTRACT An elongate Waulsortian mud‐mound complex developed at Dovedale on a ramp to the southwest of a developing carbonate platform in Derbyshire during Chadian (early Viséan) times. The complex occupied an area of approximately 6 km2 and grew to a maximum relief of 80 m with longitudinal and transverse valleys developed near the southern margin.Five mound associated facies have been identified: mound core, mound flank (fine), mound flank (coarse), intermound (fine) and intermound (coarse). The mound core facies is a massive skeletal wackestone with comminuted sponge debris, foraminifera, ostracodes and crinoid debris set in a matrix of clotted micrite. The mound flank sediments display moderately inclined bedding surfaces. While the mound flank (fine) contains sponge debris, the mound flank (coarse) is dominated by articulated crinoid columnals, and includes algal‐encrusted micritized intraclasts and coarse peloids. The well‐bedded intermound (fine) facies is bituminous and micritic while the intermound (coarse) facies is composed of skeletal‐peloidal‐intraclast grainstones which locally contain calcified algae.Although the fauna is diverse, the density of colonization by metazoans was low and the supply of macrofossil debris modest. The clotted micrite texture is interpreted as the product of micro‐organisms which precipitated and trapped fine‐grained sediment. The mud‐mound complex is dominated by the bathymetric assemblages B and C proposed by Lees, Hallet & Hibo which on their model of the Belgian Waulsortian, indicate depths of between 220 and 280 m. Intercalation of assemblages B/C and C/D on the northern margin of the complex is interpreted as the result of local storm disturbance. A deep water drift is postulated to explain the NW‐SE alignment of the complex which probably fitted the ‘export model’ of Bosence, Rowlands & Quine. Beneath the sediment surface, phreatic flow eroded unlithified sediments and developed interconnected cavities which were filled by cement and sediment relatively eariy. Mound instability triggered the opening of fissures which filled with crinoid debris, peloids, indurated lithoclasts and micrite.
Sand waves, Echinocardium traces and their bathyal depositional setting (Monte Torre Palaeostrait, Plio‐Pleistocene, southern Italy).
Authors: COLELLA A. , D'ALESSANDRO A. 
Published: June 2006

ABSTRACT Stacked cross‐sets, up to 2.5 m thick, produced by sand wave migration and meniscate trace fossils produced by Echinocardium cordatum, both considered in the literature as typical of shallow‐water marine depositional settings, commonly occur in the bathyal Plio‐Pleistocene deposits of Monte Torre (Calabria, southern Italy).The Plio‐Pleistocene sediments form two coarsening‐upward depositional sequences, separated by an unconformity and by a palaeobathymetric gap of at least 300 m. The lower sequence passes upwards from hemipelagic marls and thin‐bedded turbidites to thick‐bedded sandy turbidites, then to sand wave deposits alternated with sandy turbidites, and finally to base‐of‐slope megabreccias. Facies characteristics and relationships, and the occurrence of deep‐sea faunal associations, indicate deposition in the bathyal zone. The facies of the upper sequence reflect a fan‐delta environment, no deeper than a few tens of metres.The depositional setting of the lower sequence, where the sand wave deposits and meniscate trace fossils occur, appears to have been a tectonically controlled seaway, connecting the Tyrrhenian and Ionian Seas. This seaway became progressively narrower with time, evolving into a strait. The overall coarsening‐upward trend reflects the upward transition from a low to a high‐energy environment, possibly caused by the tectonic narrowing of the seaway. Deposition and erosion from high‐concentration turbidity currents and from tidal bottom currents were important processes. Periods of tectonic activity, producing first the uplift of the seaway margins and culminating with the uplift of the strait sequence itself, are marked by‐scattered rockfall deposits.The strait setting, causing the development of powerful, oxygenated bottom currents, produced optimal conditions in the bathyal zone for the colonization of sandy bottoms by a single infaunal r‐selected species, Echinocardium sp.
Eocene spheroidal dolomite from the western Sirte Basin, Libya
Authors: AMIRI‐GARROUSSI K. 
Published: June 2006

ABSTRACTScanning electron micrographs of crypto‐crystalline (< 4 μm) dolomites found in cores taken from the lower Eocene Facha Member reservoir of the Zella and Aswad fields in Libya show that they consist of tiny globular bodies mixed with anhedral crystals, producing a chalky texture. The sedimentary sequence indicates deposition in upper intertidal/supratidal arid environments and in saline lagoons. The crypto‐crystalline dolomite is facies‐controlled and probably formed as a primary precipitate, similar to those reported from present‐day subaqueous environments in the Coorong of South Australia and Holocene supratidal crusts off Florida.Such chalky‐textured dolomites could form local fluid‐migration barriers within hydrocarbon reservoirs.
Cycle of earthquake‐induced aggradation and related tidal channel shifting, upper Turnagain Arm, Alaska, USA
Authors: BARTSCH‐WINKLER SUSAN 
Published: June 2006

ABSTRACTAggradation of intertidal sediment in upper Turnagain Arm, a macrotidal estuary located in southern Alaska near Anchorage, began because of subsidence caused by sediment consolidation and tectonic lowering of the land during the earthquake of March 27, 1964. At the head of the Arm, decrease in percent sand of surface sediments with time and return of rodents, pulmonate gastropods, trees, bushes, and grasses to the intertidal region, indicate that the rate of sediment deposition has slowed and the Upper Turnagain Arm area has returned to a state of preearthquake equilibrium only 17 years after the event. Comparison of channel locations before and after the earthquake indicates the possibility of regional postseismic tilting of the estuary.
A preliminary study of the dynamics of a modern draa, Algodones, southeastern California, USA
Authors: HAVHOLM KAREN G. , KOCUREK GARY 
Published: June 2006

ABSTRACTA transverse crescentic draa in the Algodones dune field, California, was monitored for a year using surface process mapping, aerial photography and supplemental wind measurement. The draa is oriented by the long‐term resultant wind, whereas its superimposed features are in equilibrium with the bedform‐modified secondary airflow. Surface airflow and the movement of superimposed bedforms is typically oblique or parallel to the draa brinkline, particularly on the lee slope. Comparison of measurements of draa movement and sand deposition on the lee slope, with expected rates calculated from wind data and draa size, confirm that there is a significant component of sand flow parallel to the draa brinkline. The internal structure being generated at the base of the draa lee slope is inferred from the surface processes active there. Within the space of a kilometer two types of compound cross‐strata, separated by an area of simple cross‐strata, are being produced. This has significant implications for interpretations of ancient aeolian strata. Variations in internal structure types found in lateral sequence may be generated by one complex bedform, and these cross‐strata may be simple or compound. Second‐order bounding surface orientations indicate resultant primary palaeowind directions; compound cross‐strata dip directions indicate secondary flow conditions. The existence of cross‐strata dip directions oblique or perpendicular to the second‐order surface indicates longitudinal secondary flow on the lee face, but not necessarily a longitudinal or oblique draa. Without further detailed knowledge about various draa configurations and behaviour, stratification attributed to draas can be used only to interpret activity on the lower draa lee face.