bluuebunny/crossref_metadata_2025_split huggingface dataset:
Aeolian to marine transition in Cambro—Ordovician cratonic sheet sandstones of the northern Mississippi valley, U.S.A.
Authors: JR R. H. DOTT , BYERS C. W. , FIELDER G. W. , STENZEL S. R. , WINFREE K. E. 
Published: June 2006

ABSTRACTCratonic quartz sandstones have presented several intractable problems. Besides their extreme textural and compositional maturity and paucity of shale, their sheet‐like geometry is particularly notable. If the sandstones were entirely marine, as long supposed, such geometry is difficult to explain in terms of modern shelf sediments, which are generally held to be either relict or only slightly reworked by the Holocene transgression (palimpsest). Re‐study of two quartz sandstones in the northern Mississippi Valley region reveals evidence for significant non‐marine deposition followed by varying degrees of marine reworking during transgressions.Facies patterns are similar in the Cambrian Wonewoc and Ordovician St Peter sandstones, both of which overlie unconformities. In both, a large‐scale cross‐stratified facies believed to represent aeolian ergs passes laterally into a planar‐and‐channelled facies inferred to represent sand plains composed of braided fluvial and aeolian sand sheet deposits. Criteria of aeolian deposition in both facies include adhesion structures, large ripple index, fine climbing translatent lamination, grainfall and grainflow stratification. Criteria of braided fluvial deposition include shallow channels containing sequences of thinning‐upward sets of trough cross‐stratification, reactivation surfaces, low‐index ripples, and polygonal cracks. Probable aeolian sand sheets contain flat bedding punctuated by small channels, adhesion structures, and coarse‐sand ripples with large index. There is a conspicuous absence of trace and body fossils from these inferred non‐marine deposits. In contrast is a burrowed and trough cross‐stratified facies characterized by medium‐scale cross‐bedding alternating with bioturbated intervals and rare brachiopod or trilobite‐mould coquinas, which is interpreted as shallow marine. In both formations, this last facies replaces laterally and overlaps the other two, reflecting transgression and variable reworking. The main areas of non‐marine deposits in both formations are capped by a thin, burrowed subfacies that represents the culmination of each transgression; that is, a stillstand during which sediment influx ceased and both physical winnowing and bioturbation were intense.It is suggested that the sheet‐like geometry of many cratonic quartz sandstones is due primarily to initial sand dispersal by aeolian and fluvial processes. That such processes must have been orders‐of‐magnitude more important on pre‐Devonian, vegetation‐free landscapes than they have been since, not only helps to explain the sheet‐like character but also the exceptional maturity of the older cratonic sandstones.
A model to estimate the depositional brine depths of ancient halite rocks: Implications for ancient subaqueous evaporite depositional environments
Authors: TUCKER ROGER M. , CANN J. R. 
Published: June 2006

ABSTRACTA model is derived which allows a more general approach to the interpretation of bromine profiles in halite rocks than is possible with the models of Kuhn or Holser. Allowing for an open system, with influx and reflux, reduces the depth of brine necessary to generate a given salt deposit. Even very small and regular bromine gradients, which historically have been interpreted as the result of deposition from deep brine bodies, can be generated in an open system from brine no more than a few tens of metres deep. The bromine gradient produced from a given depth of brine is strongly dependent on the composition and amount of influx, but less so on the composition of brine already in the basin.A Zechstein profile is analysed which in a closed basin would require brine 2665 m deep. It can equally be modelled by influx of normal seawater which fully replenishes a basin no more than 140 m deep. A somewhat irregular profile from the Paradox Basin, instead of requiring a basin 100–400 m deep, can be modelled as being produced by deposition from brines only 10 m deep. Reflux in both cases can only be a few per cent of influx at a maximum.
The possible role of benthic microbial mats during the formation of carbonaceous shales in shallow Mid‐Proterozoic basins
Authors: SCHIEBER JÜRGEN 
Published: June 2006

ABSTRACTA considerable portion of the upper member of the Mid‐Proterozoic Newland Formation, Meagher County, Montana, consists of carbonaceous silty shales (striped shale facies). This type of shale facies is common in Proterozoic basins and is host to several major base metal deposits. The striped shales were deposited in a subtidal setting, basinward of carbonates characterized by cryptalgal laminites, mudcracks and flat pebble conglomerates. The carbonaceous silty shales are considered remnants of benthic microbial mats. Irregular internal laminae, patterns of particle trapping, mechanical deformation during penecontemporaneous soft‐sediment deformation and filamentous microbiota provide evidence for this interpretation. The dolomitic clayey shale contains graded silt‐clay couplets, and these are interpreted as storm layers. Modern subtidal microbial mats can only survive under special conditions, but in the Proterozoic, it is suggested that benthic microbial mats colonized the shallow seafloor during periods of low sediment input, leading to the formation of carbonaceous shales.
Thin section and S.E.M. textural criteria for the recognition of cement‐dissolution porosity in sandstones
Authors: BURLEY S. D. , KANTOROWICZ J. D. 
Published: June 2006

ABSTRACTCement‐porosity relationships are described from the Lower Triassic Sherwood Sandstone Group and the Middle Jurassic Ravenscar Group in the United Kingdom. Calcite cemented sandstones display a variety of replacement textures, with preferential replacement of grains and of overgrowth faces with high free‐surface energy. Dolomite and siderite cemented sandstones display similar textures but replacement is less specific and euhedral overgrowth surfaces are commonly embayed by carbonates. Examination of the more porous sandstones with the scanning electron microscope reveals a range of pitting and embayment textures in authigenic overgrowths and in detrital grains. These range from small ‘v’‐shaped notches and pits, through regular and irregular shaped embayments, into large depressions. These textures appear to be morphologically similar to the quartz surfaces seen in thin sections of carbonate cemented sandstones, and are interpreted to have been formed by the dissolution of pore‐filling and grain replacive authigenic carbonates. This is confirmed by examination of experimentally exhumed overgrowth surfaces from carbonate cemented sandstones. These textures indicate that part of the intergranular porosity in these sediments is secondary in origin, and has been generated by the dissolution of carbonate cements. The identification of such textures may lead to a more confident interpretation of the nature of intergranular porosity in the subsurface.
Synsedimentary faulting and palaeocurrent patterns in the Triassic sandstones of the High Atlas (Morocco)
Authors: PETIT JEAN‐PIERRE , BEAUCHAMP JACQUES 
Published: June 2006

ABSTRACTThe influence of syndepositional fault patterns on palaeocurrents is demonstrated in fluvial to shallow‐marine sandstones of Upper Triassic basins in the High Atlas. The synsedimentary nature of faults is deduced from hydroplastic slickensides, thickness variations due to block tilting and dislocation of layers next to the fault scarp. On a regional scale, it is shown that the major normal fault trend of N050–070° controlled the overall palaeocurrent pattern which was directed towards the west‐southwest, i.e. in the direction of the future Atlantic ocean. Some anomalies in the palaeocurrent pattern could be related to an increase in subsidence which induced a general coarsening of sediment towards the top of the Triassic.
Sediment pathways and transport rates through a tide‐dominated entrance, Rangaunu Harbour, New Zealand
Authors: PICKRILL R.A. 
Published: June 2006

ABSTRACTBed load sediment traps were deployed at two sections across channels in Rangaunu Harbour entrance. Traps were inspected and emptied by divers at hourly intervals through both spring and neap tidal cycles for a total of 292 trap deployments. Current velocities were measured simultaneously with the trap inspections. Transport is concentrated in sandy megaripple fields on the channel banks and sub‐tidal platforms flanking the channels. There, transport is almost continuous throughout the tidal cycle, increasing with flow velocity but lagging by approximately one hour. The channel floors are lined with shell‐gravel lag across which bedload transport rates are low and discontinuous. Tidal asymmetry produces a net seaward transport through the channel troughs and a net landward transport across the channel banks and flanking sub‐tidal platforms. Sediment leaving the harbour recirculates in anticlockwise gyres across the ebb‐tide delta to re‐enter the harbou and maintain the supply of sand to the megaripple field. Transport during spring tides is typically 25–30 times that during neaps. Predictions of transport rates, from a method developed by Black & Healy utilizing the Yalin bedload equation, produced transport rates similar to the traps over sand beds. Transport over shell lag surfaces appears independent of near‐bed velocity and more dependent on the passage of ribbons of sand across the lag surface.
Diagenesis of sandstones in the back‐arc basins of the western Pacific Ocean
Authors: LEE YONG IL , KLEIN GEORGE DeVRIES 
Published: June 2006

ABSTRACT Sandstones occur in back‐arc basins of the western Pacific at DSDP sites 299 (Sea of Japan), 297 (northern Shikoku Basin), 445 and 446 (Daito‐Ridge‐and‐Basin Province), 453 (Mariana Trough), 286 (New Hebrides Basin) and 285 (South Fiji Basin). These sandstones are dominantly volcaniclastic arenites derived from andesitic island arcs. The degree of sandstone diagenesis is dependent on original composition, burial rate, heat flow history of the basin, and timing of sandstone deposition with respect to rifting processes and associated high heat flow.Sandstones containing a larger proportion of volcaniclastic components showed more diagenetic effects than sandstones containing a significant volume of other rock fragments and mineral components. Sandstones deposited during early stages of rifting (sites 445, 446) with a slow burial rate and high crustal heat flow showed the greatest degree of downhole diagenetic change. These diagenetic changes include early pore‐space reduction and rim cementation by clay minerals followed later by calcite, and subsequent pore‐fill cementation by clinoptilolite, heulandite, analcite and later calcite. Replacement of recognizable volcanic rock fragments by chert, calcite and zeolites was observed in the deepest part of the hole. Sandstones deposited after rifting under conditions of associated lower heat flow showed considerably less diagenetic changes, particularly if burial was rapid.The high heat flow associated with earliest rifting, associated fluid circulation driven by thermal convection, and slow burial rate controlled the diagenetic history of these sandstones. Thus, timing of sandstone deposition with rifting stage and associated burial rates were key factors in controlling sandstone diagenesis in back‐arc basins.
Cross‐strata and bedforms of probable transitional dune to upper‐stage plane‐bed origin from a Late Precambrian fluvial sandstone, northern Norway.
Authors: RØE SIGNE‐LINE 
Published: June 2006

ABSTRACTA Late Precambrian fluvial sandstone sequence in northern Norway is dominated by large‐scale cross‐sets that show either lenticular or tabular geometries in the streamwise sections. The lenticular sets interdigitate and in places show nearly symmetrical formsets. The tabular sets are in places solitary, but are mainly grouped in cosets. In both cross‐set types, the cross‐strata range from concave‐up to sigmoidal in shape, with the latter variety comprising subhorizontal to gently inclined topset strata (with parting lineation) that merge uninterruptedly downflow into the steeper (10–2°) foresets. Within the cross‐sets the geometry and dip azimuths of the foresets are conspicuously consistent, although the concave‐up and sigmoidal strata commonly alternate downcurrent.The cross‐strata characteristics suggest flood stage deposition from relatively high velocity steady currents heavily laden with suspended sand. Both cross‐set types are interpreted as representing bedforms generated by flow in the dune to upper‐stage plane‐bed transition. The lenticular cross‐sets probably represent periodic dunes, but it is far less clear whether the long bedforms represented by the tabular sets should be classified as dunes, or rather as solitary to quasi‐periodic bars.
A recent turbidity current event, Hueneme Fan, California: reconstruction of flow properties
Authors: REYNOLDS SUZANNE 
Published: June 2006

ABSTRACTA recent (100 yr old) turbidite is described from Hueneme Fan, California Continental Borderland. Dense sampling over the fan surface has allowed excellent delineation of the characteristics of this deposit. It exhibits Bouna DE sequences and has a distinctly bimodal, sandy silt grain size distribution. Through the use of generalized fluid dynamics equations, it is possible to reconstruct original flow properties of the current which deposited this material. The calculated velocities ranged from 10–90 cm s‐1 and excess density (above ambient seawater) from 0·001–0·005 g cm‐3 in the lower midfan and upper fan channel regions, respectively. Height of the current ranged from 5–15 m, on slopes from 1·5 to 0·15°. A total of 107 m3 of sediment was deposited during 10 days. The turbidity current is conjectured to have originated from direct river input during the floods of 1884.An older event is also described, which has distinctly different properties and origins. The grain sizes of this older deposit are much coarser, and sedimentary structures suggest higher flow regimes. This turbidite is conjectured to have been deposited from a higher density, faster current thought to have been generated by slumping. The need for a better understanding of the controls on the characteristics of turbidity currents and their effect on fan morphology is emphasized.
Dust transport and the question of desert loess formation
Authors: TSOAR H. , PYE K. 
Published: June 2006

ABSTRACTAlthough contemporary dust storms are frequent in arid and semi‐arid areas, desert loess deposits are poorly developed. Much of the World's loess occurs in mid‐latitude areas which experienced glaciation during the Pleistocene. Ocean core evidence indicates that dust transport from sub‐tropical deserts increased during cold stages of the Pleistocene, but loess formed only on certain desert margins, for reasons which have not been fully explained. This paper re‐examines the mechanisms of dust transport and deposition, and the circumstances leading to the accumulation of thick loess. Typical loess is composed mainly of medium silt grains which are transported in short‐term suspension a few metres above the ground. Significant thicknesses of loess form only when dust is trapped within a limited area, often relatively close to the source. Dust particles finer than 20 μm are transported mainly in long‐term suspension over a greater height range and may be widely dispersed.The availability of silt and the frequency, magnitude and direction of dust‐transporting winds are important factors governing the potential for loess formation, but the existence of a suitable dust trap is particularly important. Traps may be formed by topographic obstacles, areas of moist ground, or vegetated surfaces. Vegetation adjacent to glacial outwash plains and braided meltwater streams trapped dust in mid‐latitudes during the Pleistocene. Dust blown during glacial periods from certain deserts, notably in Sinai, Soviet Central Asia and China, accumulated as loess in neighbouring semi‐arid regions. On the margins of other deserts loess formation was inhibited partly by the absence of vegetation traps. During most of the Holocene net dust deposition rates in all desert‐marginal areas have been too low for significant loess accumulation. This is mainly due to a reduction in silt availability and a tendency towards landscape stability. Reported dust storm frequencies during the past 50 years over‐estimate the longer‐term Holocene dust flux due to the effects of human activities. Much modern dust owes its origin to erosion of cultivated soils in semi‐arid areas and is finer than typical loess.
Sedimentation at the margins of a late Pleistocene ice‐lobe terminating in shallow marine environments, Dundalk Bay, eastern Ireland
Authors: McCABE A. MARSHALL , DARDIS GEORGE F. , HANVEY PATRICIA M. 
Published: June 2006

ABSTRACTLate Pleistocene morainic sequences around Dundalk Bay, eastern Ireland, were deposited in a variety of shallow, glaciomarine environments at the margins of a grounded ice lobe. The deposits are essentially ice‐proximal delta‐fan and ‐apron sequences and are divided into two lithofacies associations. Lithofacies association 1 occurs as a series of morainal banks formed at the southern margin of the ice lobe in a body of water open to influences from the Irish Sea. The morainal banks consist mainly of diamictic muds deposited from turbid plumes and by ice‐rafting with minor occurrences of turbidites, cross‐bedded gravels (subaqueous outwash) and massive boulder gravels (high‐density debris flows). Lithofacies association 2 was deposited in a narrow arm of the sea at the north‐eastern margin of the ice lobe. The deposits consist mainly of a series of coalescing, ice‐proximal Gilbert‐type fan deltas which are interbedded distally with tabular and lens‐shaped subaqueous deposits. The latter are mainly ice‐rafted diamictons, debris‐flow deposits and subaqueous sands and gravels. Both lithofacies associations are draped by diamictons formed by a combination of rain‐out, debris flow and traction‐current activity. At a few localities the upper parts of the sequence have been sheared by minor oscillations of the ice sheet margin.These sequences form part of an extensive belt of glaciomarine deposits which border the drumlin swarms of east‐central Ireland. Lithostratigraphic variability is partially related to the arrival of large volumes of debris at the ice lobe margin when the main lowland ice sheet surged during drumlin formation. Complex depositional continua of this type lack any major erosional breaks and should not be used either as climatic proxies or for stratigraphic correlations.
Petrological and isotopic implications of some contrasting Late Precambrian carbonates, NE Spitsbergen
Authors: FAIRCHILD IAN J. , SPIRO BARUCH 
Published: June 2006

ABSTRACTThe dominantly shallow‐marine Vendian succession of NE Spitsbergen contains distinctive types of carbonate rock. Limestones deposited before Vendian glaciation resemble those described from other Upper Proterozoic successions, being high in Sr and inferred to have been originally aragonitic, including the distinctive 5–10 Jim equant polygonal calcite of cemented shrinkage cracks. In contrast, manganoan stromatolitic limestones within marginal‐marine glacial‐outwash deposits, and consisting of micrite, microspar and fascicular‐optic calcite are interpreted as originally calcitic. The restriction of primary marine calcite to cold seawater is comparable with Recent and Permian carbonates, although the Precambrian example formed in a sea diluted with meltwater.There is good textural preservation of relatively 18O‐rich oolitic dolostones which were cemented in a supratidal environment by artesian fluids. Nevertheless, early diagenetic replacement is inferred, immediately prior to a glacial episode. Post‐glacial dolostones are either replacive marine, or evaporative lacustrine, but share rather more negative δ18O values, closer to the mean of Late Precambrian dolostones.The heaviest oxygen isotope values constrain seawater δ18O to no more negative than — 2 to — 4SMOW. The main reason for the pronounced oxygen isotopic depletion of most Late Precambrian carbonates is their initial metastable mineralogy. The possibility of determining palaeolatitudes of the enigmatic widespread Late Proterozoic glaciations by isotopic analysis of freshwater periglacial calcareous precipitates is raised. Significant carbon isotope variations reflect changes in depositional water chemistry: some of these could be global in extent.
Petrographic and geochemical analysis of caliche profiles in a Bahamian Pleistocene dune
Authors: BEIER J. A. 
Published: June 2006

ABSTRACTTwo caliche profiles from a Pleistocene carbonate dune on San Salvador Island, Bahamas, were examined by petrographic and geochemical analysis. Profile A is an immature buried caliche profile characterized by caliche pisolites, a friable crust and abundant Cerion. Profile B is a more well‐developed caliche profile at the top of the dune which contains abundant pisolites, rhizomorphs, laminated calcrete, a breccia and abundant Cerion.Geochemical changes in caliche profiles relative to the host rock are an increase in Al2O3, Fe2O3 and total organic carbon, a decrease in Mg and Sr, and a decrease in δ13Ccarb, δ18Ocarb and δ13Corg. The magnitude of these changes is probably a function of the duration of subaerial exposure and resultant colonization by dune plants and associated microflora.Abundance of calcified filaments and needle‐fibre crystals in profile A attests to the importance of microbial processes in the early development of caliche profiles. Biogenic structures are largely destroyed in profile B due to recrystallization, but indirect evidence of biological activity is retained in the form of carbon isotope values.
Sedimentation in ice‐covered Lake Hoare, Antarctica
Authors: NEDELL SUSAN S. , ANDERSEN DAVID W. , SQUYRES STEVEN W. , LOVE F. GORDON 
Published: June 2006

ABSTRACTThe sedimentation mechanisms that occur in ice‐covered Lake Hoare, Antarctica are examined, to determine how sediment enters the lake, and how the sedimentation pattern affects blue‐green algal growth at the lake bottom. The 3 m‐thick ice cover contains pebbly sand as much as 2 m below the surface. Sediment with similar texture and mineralogy is found at the lake bottom. This evidence, together with the lack of sediment in the inflowing stream and the markedly different texture of sediment from the other terrains around the lake suggest that most of the sediment at the lake bottom comes in through the ice cover. Sand grains intermittently migrate through porous ice on the surface, water‐filled vertical gas‐channels penetrating two‐thirds of the ice cover, and possibly through cracks in the ice that act as conduits. The algae at the lake bottom are able to survive in part because sediment that comes through the ice cover does not obliterate them.