bluuebunny/crossref_metadata_2025_split huggingface dataset:
Vertical exchange of coarse bedload in desert streams
Authors: Schick A. P. , Lekach J. , Hassan M. A. 
Published: April 2008

Summary
The tracing of coarse particles, transported by flood events in two ephemeral streams in the Negev Desert, Israel, reveals that a size-independent, highly skewed longitudinal dispersion pattern is accompanied by vertical distribution throughout the scour layer. An analysis of the intermittent burial and resurfacing of these particles from one event to the next leads to the formulation of a concept of equilibrium in vertical exchange. This equilibrium requires that, for the ‘dominant’ flood event (frequency 1–2 years), the number of previously buried particles exposed by that event is balanced by an identical number of previously surficial particles being buried. Such an equilibrium can be maintained only if the scour layer reaches sufficiently deep into the ‘sub-armour’ layer to entrain a requisite number of particles of armour-layer size. The resulting functional relationship between (1) the size of coarse particles in relation to the matrix; (2) the thickness of the armour and ‘sub-armour’ layers and the distribution of coarse particles; and (3) aspects of the flow regime, enables the derivation of hydrological information from studies of the three-dimensional distribution of coarse particles in modern channel bed sediments of arid streams. It also helps to explain the vertical distribution of pebbles in ancient continental clastic sediments.
Plio-Pleistocene raised channel systems of the western Sharqiya (Wahiba), Oman
Authors: Maizels J. K. 
Published: April 2008

Summary
Extensive alluvial fans bounding the western edge of the Sharqiya (or Wahiba) Sands are characterized by complex palaeochannel systems now forming a series of superimposed gravel ridges. The oldest, most extensive fan system is crossed by numerous sinuous, superimposed palaeochannel courses comprising highly weathered, chert-rich gravels cemented by clear crystalline calcite. Many of these old fan sediments have been chemically altered to depths of over 200 m to form a pink dolomitic clayey deposit here termed barzamanite. The formation of this calcrete-like deposit was probably associated with rising water tables during long-term fan growth. The younger, more limited fan deposit, by contrast, comprises thin, coarse-grained, broad spreads and terrace veneers of weakly cemented ophiolitic gravels. The fans have been subject to entrenchment and deflation, resulting in extensive land-surface lowering and the exhumation of buried palaeochannels. Although no dates are yet available, the older channel systems are likely to date from more humid phases during the Pliocene-Early Pleistocene, whereas the younger, terraced deposits, and the period of subsequent channel exhumation, may date from semi-arid periods of lower sea level during the Pleistocene.
Ancient ephemeral stream deposits: a local terminal fan model from the Bunter Sandstone Formation (L. Triassic) in the Tønder-3, -4 and -5 wells, Denmark
Authors: Olsen H. 
Published: April 2008

Summary

Facies studies of a
c.
30 m thick fluvial sand body in the Tønder area, Denmark indicate that the sand body was formed by an ephemeral stream system comparable to modern terminal fans. The sediments are grouped into three genetic facies associations (A, B, C) reflecting two styles of channel flow (A and B) and overbank sheet flow (C). The sediments are dominated by parallel laminations (28%) and large-scale cross-bedding (27%) in association with small-scale cross-bedding (17%), massive bedding (14%) and intraformational conglomerates (5%). Mudstones and siltstones form only 9% of the deposits. Anhydrite (formerly gypsum) is associated with the fine-grained deposits which are mainly of overbank origin. It is commonly observed as intraformational clasts in the channel deposits. The inferred ephemeral stream complex probably formed a link between a more permanent braided stream system towards the N (the Skagerrak Formation) and a sabkha system towards the S (the Middle Buntsandstein in northern Germany).
Alluvial fan dissection: relationships between morphology and sedimentation
Authors: Harvey A. M. 
Published: April 2008

Summary
Aggradation and dissection on alluvial fans influence the coupling between sediment source areas and main channels. Previous alluvial fan models emphasize the influence of drainage area and sediment character on overall fan morphometry and proximal-distal sedimentary variations. A model is proposed for the morphology and sedimentology of dry-region alluvial fans, based on crusted and non-crusted Quaternary fans in SE Spain, differentiating between aggradational and dissectional tendencies. Many fans show fan-head trenching, with the channel emerging on the fan surface at a mid-fan intersection point. Distally either aggradation or headcut development may occur, with headcut development and trenching leading ultimately to the total dissection of the fan surface. Three sets of factors determine intersection-point behaviour: fan surface crusting, discrepancy between fan and channel slopes, and channel morphology. Multiple regression models are developed to account for variations in fan and channel slopes and channel morphology, taking into account source area and sediment variations. Intersection point trenching is most common on steep crusted fans, where the discrepancy between fan slope and channel slope is greatest, and where channel widths are limited. Arid region fans from the American Southwest are contrasted with the Spanish fans. Neither the non-crusted nor the crusted American fans examined show any tendency towards intersection-point trenching. Their fan slope/channel slope discrepancies are less and their channels are wider, the result of both differing Quaternary sequences and contemporary environments between Spain and the American Southwest. The implications are important for the continuity of dry-region fluvial systems and for the stratigraphy of alluvial fan sequences. Simple fans with distal area aggradation show a simple stacked stratigraphy, but fans with a propensity for mid-fan and distal trenching may show a complex inset stratigraphy.
Grain-size variation on a ‘complex’ linear dune in the Namib Desert
Authors: Livingstone I. 
Published: April 2008

Summary
Against a background of increasing knowledge of regional grain-size variations, particularly in the Namib Desert, this paper reports an investigation of small-scale grain-size variations using 25 sample points on the cross-profile of a single linear dune. The results suggest that a discernible pattern of grain size across the dune does exist, and that changes are gradual rather than discrete. From an examination of the near neighbours of the main study dune there is also some evidence of measurable differences between dunes in the same area. The fact that grain sizes are not randomly distributed across the width of the dune indicates a response to spatial variations of process, but grain size does not seem to control dune form in the way that some earlier workers have envisaged. Furthermore, monthly sampling provides preliminary evidence that grain-size values on this dune respond to a seasonal wind regime, and that time of sampling may therefore be a crucial determinant of the results obtained.
Silica and calcium carbonate replacement of plant roots in tropical dune sands, SE India
Authors: Hendry D. A. 
Published: April 2008

Summary

The Quaternary coastal sediments of SE India are composed of raised marine deposits capped by aeolianite and red weathered dune sands (Teri). The red weathering associated with these sands is irregular in depth and unusual in the fast rate at which it has developed. Associated at depth with this red weathering is precipitated silica and calcium carbonate which replace plant roots and act as a cement. In places the plant root cells are prefectly replaced to form rhizoliths which occur in the soil profile with and without surrounding cement. Ultimately, the silica source is silicic acid (H
4
SiO
4
) that is a product of the
in situ
weathering of unstable minerals in the overlying dune sands during the last 20 000 years. The calcium carbonate is thought to have had a much more localized source. Plants and the litter they produce play at least two possible roles in the silica cycle. First, silica-organic complexing occurs as the Teri sands are leached by organic acids derived from decomposing litter. Secondly, the plant roots create a favourable micro-environment for silica and calcium carbonate precipitation and replacement by altering partial gas pressures and hence the pH of soil solutions through their respiratory activity.
The application of a solution-mineral equilibrium model to the diagenesis of Carboniferous sandstones, Bothamsall oilfield, East Midlands, England
Authors: Warren E. A. 
Published: April 2008

Summary
Within the fluviatile reservoir sandstones of the Bothamsall oilfield, authigenic cements of early quartz, kaolinite and illite predominate. Detrital feldspars show varying degrees of alteration and muscovite micas are commonly altered to intergrowths of kaolinite and illite.

Solution-mineral equilibria in the idealized system of K
2
O-Al
2
O
3
-SiO
2
-H
2
O are used to relate the simple silicate mineral assemblage observed to possible pore-fluid compositions. Previous studies considered aluminium to be immobile in this system. Petrographic evidence of the paragenetic sequence at Bothamsall indicates that aluminium is mobile, albeit over short distances. This study therefore considers aluminium mobility in the solution-mineral equilibria. Graphical plots of total aluminium versus pH for given potassium ion activity, at a temperature of 298 K, are constructed to illustrate the stability fields of the phases identified. These show that aluminium can be an important component, together with pH and potassium-ion activity, in affecting mineral stabilities in this system.

The existence of several co-stable phases at Bothamsall considerably reduces the possible permutations for pore-fluid chemistry; kaolinite-illite alteration of mica proves particularly sensitive in this respect. The relative merits of two different pore-fluid models are compared: one, where the pore-fluid composition is considered to be constant in an open system, and another where the pore-fluid composition is assumed to vary in response to
in situ
mineralogy in a closed system. The former would require a very restricted range of pore-fluid compositions to result in the diagenetic modifications observed, whereas a wide range of pore fluids could evolve in a closed system driven by feldspar dissolution. This pore-fluid evolutionary model is preferred.
Petrology (including fluorescence microscopy) of cherts from the Portlandian of Wiltshire, UK—evidence of an episode of meteoric water circulation
Authors: Astin T. R. 
Published: April 2008

Summary
Chert nodules from the Wockley Member of the Portland Stone (Upper Jurassic) in Wiltshire, England are comprised of three stages of chert cementation and sediment replacement: (1) opal-CT lepisphere cement and grain replacement, (2) length fast chalcedony grain and matrix replacement and cement, and (3) quartz cement. These are readily distinguished by their fluorescence, the first and last stages being non-fluorescent, the second stage showing internal zonation changing from moderate to bright fluorescence with time.
Some carbonate grain dissolution took place prior to silica precipitation. The first stage silica cement formed mainly as pore-linings, with minor replacement of grain margins. Its botryoidal, lepisphere habit implies it was opal-CT, now inverted to quartz. Extensive grain replacement took place while chalcedony cements formed. Chalcedony cements show an increase in inclusions of brown organic particles with time, joined at a late stage by iron oxide inclusions.
Silicification and carbonate dissolution occurred during a period of meteoric water penetration, probably as a result of soil formation at the top of the Portland Stone. Silica was apparently derived as opal-A from adjacent marine limestones, redistributed by groundwater flow, and precipitated as opal-CT and chalcedony. The organic inclusions probably come from the developing soil horizon, the iron oxides marking the migration of an oxidation front away from the soil horizon into the sediment. The later quartz cement lacks inclusions and was formed during later burial after stagnant groundwater conditions were re-established.
Diagenesis of carbonate cements in Permo-Triassic sandstones in the Wessex and East Yorkshire-Lincolnshire Basins, UK: a stable isotope study
Authors: Bath A. H. , Milodowski A. E. , Spiro B. 
Published: April 2008

Summary

The
13
C/
12
C and
18
O/
16
O ratios of carbonate cements in Permo-Triassic fluvio-deltaic sandstones differ between the Wessex and East Yorkshire-Lincolnshire Basins. The isotopic data indicate different conditions of deposition and early post-depositional carbonate precipitation which supports petrological and mineralogical studies of the same sandstones focussed on the development of reservoir properties. Isotopic compositions of carbonate cements vary across the Wessex Basin; they indicate early derivation of dissolved carbonate during recrystallization of detrital limestone. Within the basin, calcite cements show little variation in δ
13
C but considerable differences in δ
18
O between early calcrete calcites and isotopically-light later calcites; δ
18
O decreases to values close to isotopic equilibrium with present water while δ
13
C is not in isotopic equilibrium with bicarbonate in present-day brines. Two distinct generations of replacive dolomite, early non-ferroan and later ferroan, are found in the basin centre and have different δ
18
O but similar δ
13
C values.
34
S/
32
S and
18
O/
16
O ratios of anhydrite cements could indicate a remote Zechstein source of sulphate or a mixed marine sulphate-sulphide source. The Permo-Triassic sandstones of the East Yorkshire-Lincolnshire Basin, in contrast, contain dominantly dolomite cement. δ
18
O and δ
13
C values are relatively heavy and indicate early dolomitization of calcite over large areas. δ
18
O values of minor calcites indicate that dedolomitization is continuing in the present-day groundwater regime. The isotopic differences between carbonate cements in the two basins is tentatively attributed to the differing early post-depositional conditions and to the differences in carbonate chemistry between calcite dissolution/precipitation reactions and dolomitization reactions.
Secondary porosity in hydrocarbon-bearing transgressive sandstones on an unstable Lower Palaeozoic continental shelf, Welsh Borderland
Authors: Parnell J. 
Published: April 2008

Summary
Coarse sandstones were deposited during each of four transgressions across the shelf of the Lower Palaeozoic back-arc Welsh Basin. Hydrocarbon shows record porosity in the deposits of the Cambrian, Arenig, Caradoc and Llandovery transgressions. Hydrocarbons migrated from two sources: from Lower Palaeozoic source rocks within the basin, and later from Carboniferous rocks.
Diagenesis and reservoir potential is related to the mineralogy of the sandstones, which reflects a balance between reworking of shelf sediment and an input from the eroded substrate. The substrate included Precambrian volcanic and plutonic rocks formed at the southern margin of the Iapetus Ocean. Quartz arenites are quartz-cemented or exhibit thick pore-linings of clay which restrict permeability. They have only minor reservoir potential. Arkoses do not exhibit macroporosity but have effective microporosity in the immediate vicinity of source rocks. Subarkoses have optimal reservoir potential, consisting of primary porosity about non-quartz grains (feldspars, glauconite) and secondary (intragranular) porosity in feldspars and lithics. The secondary porosity formed by leaching of clay alteration products in feldspars. Subarkoses retained sufficient framework stability to preserve the secondary porosity, whereas arkoses experienced widespread alteration and framework collapse. Limited secondary porosity after calcite dissolution may be due to surficial weathering of the Lower Palaeozoic rocks during Devonian/Lower Carboniferous exposure.
Deposition and diagenesis in an extensional basin: the Corallian Formation (Jurassic) near Oxford, England
Authors: de Wet C. B. 
Published: April 2008

Summary
The mixed carbonate-siliclastic Corallian Formation was deposited on horsts and in grabens which were produced during Jurassic extensional tectonics. The high areas became sites for sandstone and carbonate deposition in shallow subtidal environments while clays and allochthonous debris accumulated in the depressions. Tectonic activity, perhaps confined to single fault blocks, produced depositional variation distinct to each area.
The basin has been subdivided into four major east-west trending fault blocks. The most northerly of these is the Oxford Shallows, interpreted as a small accretionary-margin slope. The depositional facies and diagnesis of this area are discussed relative to the basin as a whole.
The diagenesis of the Corallian sediments has been divided into three phases of cementation: pre-burial, shallow burial and deep burial. Phase 1 spars include marine precipitates and non-ferroan, neomorphic and drusy spars. Phase 2 cements are ferroan, precipitating during shallow burial, including an intermediate ferroan (mauve) spar which may represent development of a redox front. Phase 3 spar post-dates vuggy dissolution which probably took place during Barremian uplift. The cement precipitated during the ensuing burial phase.

Geochemistry and stable isotope data show an overall fluid evolution path for the first two spars, although Sr
2+
values in replaced corals reveal variability in the degree of meteoric flushing. The chemistry of the late spars suggests low water/rock ratios, where local dissolution provided the cations for the cement. The oxygen isotope values suggest, however, that meteoric and connate waters were mixed at slightly elevated temperatures.
A hot-spot model for early Tertiary volcanism in the N Atlantic
Authors: White R. S. 
Published: April 2008

Summary
The initial stages of rifting of the N Atlantic were accompanied by massive extrusion of volcanic rocks and the accretion to the lower crust of even greater volumes of igneous rock. Emplacement of the bulk of the igneous rocks along the 2000 km long continental rift occurred in as little as 2–3 My. The pattern and timing of magmatic activity can be explained by partial melting of anomalously hot asthenosphere as it welled up to fill the space left by the stretched and thinned continental lithosphere. Asthenosphere temperatures were elevated regionally by 100–150°C over a 2000 km diameter region by the mushroom-shaped head of hot material fed by a mantle plume, which at present is centred beneath Iceland.
Deep crustal structure and magmatic processes: the inception of seafloor spreading in the Norwegian-Greenland Sea
Authors: Mutter J. C. , Zehnder C. M. 
Published: April 2008

Summary
An extensive investigation into the deep crustal structure of the conjugate Norwegian and E Greenland margins utilizing two-ship Expanded Spread and Wide Aperture CDP Profiling has led to the identification of a unique suite of crustal structures developed during the onset of seafloor spreading. On both margins, a region of low velocity crust was found proximal to marginal escarpments. Immediately seaward of the low velocity crustal block seaward-dipping reflectors occur within thick crust that is interpreted to be wholly igneous, a product of voluminous melt production enhanced by convective partial melting processes. This earliest formed oceanic crust is actually several kilometres thicker than the low velocity crust or the adjacent continental crust. Normal oceanic crustal thicknesses are not produced until about 5 My after the initiation of spreading when convective partial melting has abated and the magma budget has reduced. The extrusive unit reduces to normal thicknesses long before the intrusive and plutonic section, indicating that the thickening or thinning of oceanic crust is accomplished primarily by variations in the latter.
Early Cainozoic evolution of the Norwegian volcanic passive margin and the formation of marginal highs
Authors: Skogseid J. , Eldholm O. 
Published: April 2008

Summary
The final continental break-up between Greenland and Norway which took place during the late Palaeocene/early Eocene was accompanied by extensive subaerial volcanism along parts of the plate boundary lasting for about 3 My. Subsequent subsidence and change to normal seafloor spreading have left behind marginal highs partly underlain by seaward-dipping reflector sequences composed of basaltic extrusives. We propose a model in which the formation of the marginal highs and the emplacement of the dipping series are related to the crustal configuration prior to the early Cainozoic plate break-up, causing an initial uplift if the rifting occurs within much thinned continental crust. This forms a ‘volcanic’ type rifted margin characterized by a small amount of crustal extension compared with a ‘normal’ margin segment that has experienced a large amount of extension and rapid initial subsidence. Recent drilling of the Vøring marginal high suggests that the plate boundary volcanism and the N Atlantic Volcanic Province may be manifestations of the same regional volcanic surge.
Dipping reflector styles in the NE Atlantic Ocean
Authors: Parson L. M. ,   the ODP Leg 104 Scientific Party
Published: April 2008

Summary
The geometries and structure of volcanic rifted continental margins, that is, those with an ocean-continent transition masked by thick sequences of inclined tholeiitic basaltic flows, are examined. Small- and large-scale differences in sequence seismostratigraphy along strike, down-dip, and between conjugate configurations are discussed. Detailed appraisals of deep drilling results and seismic data for the Vøring Plateau and western Rockall Plateau are used to contrast margin architecture and demonstrate difficulties in interpolating data between sites of dipping reflector formation.
Tectonic and volcanic events at the Jan Mayen Ridge microcontinent
Authors: Gudlaugsson S. T. , Gunnarsson K. , Sand M. , Skogseid J. 
Published: April 2008

Summary
Two main tectonic phases were responsible for the formation of the Jan Mayen Ridge microcontinent: (1) the opening of the Norway Basin in late Palaeocene/early Eocene times, and (2) subsequent rifting within the Greenland margin by which complete separation was achieved in early Miocene times. During the first phase the eastern ridge flank developed as a volcanic passive margin. The initial break-up was associated with flexuring and the formation of sequences of eastward-dipping basalt flows, which are considered equivalent to similar features beneath the Vøring and Faeroe-Shetland marginal highs off Norway. Rifting along the Greenland margin during the second phase was accompanied by uplift, listric normal faulting and the formation of large extensional fault blocks. To the W and S of the ridge a flat volcanic marker of probable earliest Miocene age covers the subsided rift and masks the ocean-continent transition. It was formed by a volcanic event of large magnitude, either as submarine lava flows or as a sill complex.