bluuebunny/crossref_metadata_2025_split huggingface dataset:
Regional dolomitization of subtidal shelf carbonates: Burlington and Keokuk Formations (Mississippian), Iowa and Illinois
Authors: Harris David C. , Meyers William J. 
Published: April 2008

Summary
Cathodoluminescent petrography of crinoidal limestones and dolomites from the Mississippian (Osagean) Burlington and Keokuk Formations in Iowa and Illinois has revealed a complex diagenetic history of calcite cementation, dolomitization, chertification and compaction. Dolomite occurs abundantly in subtidal, open-marine facies throughout the study area. Three luminescently and chemically distinct generations of dolomite can be recognized regionally. Dolomite I, the oldest generation, is luminescent, thinly zoned, and occurs mainly as a replacement of lime mud. Dolomite II has dull red unzoned luminescence, and occurs mainly as a replacement of dolomite I rhombs. Dolomite III is non-luminescent, and occurs as a syntaxial cement on, and replacement of, older dolomite I and II rhombs.
Petrography of these dolomite generations, integrating calcite cement stratigraphy, chertification and compaction histories has established the diagenetic sequence. Dolomites I and II pre-date all calcite cements, most chert, intergranular compaction and styloites. Dolomite III precipitation occurred within the calcite cement sequence, after all chert, and after at least some stylolitization. The stratigraphic limit of these dolomites to rocks older than the St Louis Limestone (Meramecian) suggests that dolomitization took place before or during a regional mid-Meramecian subaerial unconformity.
A single dolomitization model cannot reasonably explain all three generations of dolomite in the Burlington and Keokuk limestones. Petrographic and geochemical characteristics coupled with timing constraints suggest that dolomite I formed in a sea water-fresh water mixing zone associated with a meteoric groundwater system established beneath the pre-St Louis unconformity. Dolomite II and III may have formed from externally sourced warm brines that replaced precursor dolomite at shallow burial depths. These models therefore suggest that the required Mg for dolomite I was derived mainly from sea water, whereas that for dolomites II and III was derived mainly from precursor Burlington—Keokuk dolomites through replacement or pressure solution.
Carbonate cements in clastic reservoir rocks from offshore Norway—relationships between isotopic composition, textural development and burial depth
Authors: Saigal G. C. , Bjørlykke K. 
Published: April 2008

Summary
Many Jurassic and Lower Cretaceous sandstone reservoir rocks from offshore Norway contain carbonate-cemented intervals which typically make up 1–10% of the total sequence. Being nearly impermeable to fluid flow, their distribution may strongly influence reservoir properties. It is therefore important to understand the genesis of these carbonate cements.

Studies of the Jurassic and Lower Cretaceous reservoir rocks from the Haltenbanken and Viking Graben area show that the sandstones contain dominantly two types of calcite cements: (i) microsparry and sparry calcite (Type I) and (ii) poikilotopic calcite (Type II). Type I calcite with δ
18
O values between −4.5 and −8.4‰ PDB precipitated relatively rapidly at shallow burial and at temperatures between 15 and 40°C, possibly from porewater of meteoric origin. Type II calcite with δ
18
O values between −10 and −12‰ PDB have precipitated later at temperatures corresponding to 50–60°C (at ∼ 1.5 km burial depth). Fluid inclusion data also indicate that Type II calcites have crystallized at temperatures between 56 and 68°C.

Silicate grains are often corroded by Type I calcite while grains contained in Type II calcite are uncorroded, suggesting that the porewater was in near equilibrium with the main silicate phases (quartz and feldspar) during the precipitation of Type II calcite.
The degree of supersaturation in the porewater with respect to low Mg calcite probably decreased with increasing burial depth as sources of more soluble carbonate (aragonite, high Mg calcite) were exhausted, causing slower precipitation rates and increased crystal size. The Type II (poikilotopic) calcite is thought to have formed at the expense of only slightly more soluble low Mg calcite crystals.

The fact that the Type II calcite shows nearly the same δ
18
O value (−10 to −12‰ PDB) in sandstones ranging in present burial depth from 1.6 to 5 km suggests that this type of carbonate cementation probably took place early (at temperatures less than 60–70°C), and that very little dissolution and reprecipitation occurred at higher temperatures (greater burial depth).
The early Tertiary stress regime in NW Britain: evidence from the patterns of volcanic activity
Authors: England R. W. 
Published: April 2008

Summary
Major dyke swarms emplaced throughout the Palaeocene volcanic episode in the British Tertiary Volcanic Province indicate NE-SW extension occurred perpendicular to the evolving NE Atlantic continental margin. Sigmoidal N-S orientated secondary swarms cutting Lewisian basement structures indicate a possible dextral shear component within the regional NE-SW extensional stress field, rather than a structural control on dyke emplacement. Both groups of swarms are considered to be the surface expression of linear intrusions emplaced in the lower to middle crust under the influence of the regional extension. Patterns of faulting within the province are consistent with the stresses recorded by the dyke swarms, indicating the stress field persisted into the Eocene during which a change to NW-SE extension occurred.
Palaeogene volcanism: the sedimentary record in Denmark
Authors: Nielsen O. B. , Heilmann-Clausen C. 
Published: April 2008

Summary
Approximately 200 volcanic ash layers are known from Palaeocene and Eocene marine clays in Denmark. The oldest layers are present in the upper part of the Upper Palaeocene and are sporadic, thin and acidic in composition. The main volcanic phase took place at the Palaeocene-Eocene transition. During the main phase, a supply from the N to NE is indicated, while a few layers probably had a source to the NW or W. A later volcanic phase is represented by thin and more sporadic ashes in the Lower to Middle Eocene. These are almost totally transformed to smectite.
In the Upper Palaeocene, and partly in the Lower Eocene, smectite is a highly dominant clay mineral in the interbedded clays. Zeolites of the heulandite-clinoptilolite type are also present, while cristobalite is sporadically present in the lower part. A major supply of smectite from the N Atlantic may be indicated. Below the ash-bearing sequence the only evidence for volcanic activity is the presence of very smectite-rich, zeolitic clays with scattered opal-CT and slightly silicified horizons. Calculated chemical compositions of smectites from both ash layers and clay layers between ash layers and below the ash-bearing sequence are almost identical, indicating a volcanic source for the smectite.
Clastic and carbonate lacustrine systems: an organic geochemical comparison (Green River Formation and East African lake sediments)
Authors: Katz B. J. 
Published: April 2008

Summary

Source rock properties were determined on samples obtained from three modern African lakes—Tanganyika, Kivu and Edward—and on thermally immature material from Eocene Lake Uinta of Utah and Colorado (Green River Formation). The modern rift lakes represent largely clastic lacustrine systems, whereas Lake Uinta is a carbonate-dominated system. The sediments from both systems exhibit a wide range of organic enrichment: up to ∼ 13 wt.% TOC in the clastic lake samples and sometimes greater than 30 wt.% TOC in the carbonate-dominated system. Total hydrocarbon generation potentials, as determined by ‘Rock-Eval’ pyrolyses, also indicate higher yields in the carbonate system (up to ∼200 mg HC g
−1
rock) as compared to the clastic systems (up to ∼ 90 mg HC g
−1
rock). Differences in hydrocarbon yield are greater than can be explained by the organic content. They appear to be the combined result of mineral matrix effects and kerogen character. Elemental and visual analyses of isolated kerogen also support the idea of differences in organic character. The kerogen from the Green River Formation was used to define the Type I reference curve. The kerogen isolated from the clastic lakes appears intermediate between the Type I and Type II reference curves and contains varying and, in some cases, very significant proportions of Type III and/or residual (inert) organic material. In both cases, the principal product will be a high-wax crude oil. The primary differences in organic enrichment and character appear to be related to the relative quantity of allochthonous sedimentary material, both organic and inorganic.
Interactions of iron, carbon and sulphur in marine and lacustrine sediments
Authors: Davison W. 
Published: April 2008

Summary
Sulphur occurs in marine sediments chiefly as pyrite, which can form both directly or diagenetically within the sediment, or directly at the surface of the sediment. Direct formation occurs when reducing and oxidizing conditions co-exist, whereas diagenesis of monosulphides to pyrite involves oxidative transfer of electrons by, for example, decomposition of organic matter. The limited data available indicate that pyrite occurs in low concentrations in freshwater sediments and is formed directly at a redox boundary. Although most of the sulphur is bound within organic compounds, there is relatively little compared with that present in marine sediments. The absolute concentrations of pyrite, and the ratios of carbon to total sulphur, carbon to pyrite sulphur and organic sulphur to pyrite sulphur all have their uses as palaeosalinity indicators. Although their use is soundly based due to the elevated concentrations of sulphate in sea compared with fresh waters, their validity is not proven. There are sufficient gaps in our knowledge, particularly about pyrite concentrations and formation processes in freshwater environments, to suggest that at present they should be used cautiously.
The lacustrine Burdiehouse Limestone Formation—a key to the deposition of the Dinantian Oil Shales of Scotland
Authors: Loftus G. W. F. , Greensmith J. T. 
Published: April 2008

Summary

Oil shales, and the less common limestones associated with them, represent freshwater lacustrine phases within the Dinantian Oil Shale Group at the eastern end of the Midland Valley of Scotland. The understanding of the areally extensive Burdiehouse Limestone Formation is fundamental to an appreciation of the palaeolimnology of the lake in which algal oozes (consisting mainly of planktonic algae analogous to the modern form
Botryococcus braunii
), later to become oil shales, accumulated. The lake, referred to as Lake Cadell, was shallow (< 100 m) with gently sloping margins. It was meromictic and formed a closed system during deposition of the limestones, draining eastwards to the sea during oil shale deposition. The climate was wet tropical ensuring ideal conditions for a prolific flora of algae and vascular plants both around and within the lake, and providing a regular freshwater input. The lacustrine phase forms part of the cyclical Oil Shale Group succession in which each sedimentary cycle can be resolved into four phases indicating long-term lake expansion and contraction.
Characteristics of Mesozoic and Cenozoic non-marine source rocks in north-west China
Authors: Binjie Luo , Xinghua Yang , Hejie Lin , Guodong Zheng 
Published: April 2008

Summary

The formation and distribution of Mesozoic and Cenozoic lacustrine source rocks in north-west China is determined by climatic factors and the tectonics of terrestrial sedimentary basins. The most suitable condition for the formation of non-marine source rocks was a humid to semi-humid climate and a deep to intermediate lacustrine basin during the expansion stage of the lakes. Dominantly fresh to brackish water bodies with sluggish hydrodynamic bottom environments were conducive to the accumulation and preservation of organic matter. The non-marine source rocks show low to high organic matter contents, mainly from a mixture of terrestrial and aquatic sources. Thus, they are characterized by high wax, low sulphur, high nickel and low vanadium. They also have a series of corresponding organic-geochemical signatures. All of these are important indicators in studies of terrestrial source rocks, crude oil and oil-source correlations. A chart with the δ
13
C value of the crude oil against the log plot of pristane/phytane values can be used to discriminate the environment of deposition of the source rock of the oil.
Delta or coastal plain? With an example of the controversy from the Middle Jurassic of Yorkshire
Authors: Alexander J. 
Published: April 2008

Summary
Deltaic and coastal plain deposits can be distinguished from each other in the rock record where enough data are available to interpret the three-dimensional distribution of facies and unconformities over a wide area. This distinction is based on the distribution of facies rather than purely on a marine-to-alluvial transition as has been the case in the past. The distinction between deposits of the two environments is often very difficult or impossible, and if based on limited data may lead to inaccurate and even incorrect models that can cause problems with prediction of reservoir rocks or coal location in the subsurface. Models of the gross facies distribution for tectonically, eustatically and autocyclically controlled coastal plains are seen as the end-members of a continuum between coastline depositional styles.
The Middle Jurassic sedimentary rocks of the Yorkshire Basin are used to illustrate the controversy that can arise from imprecise definitions and the application of incorrect facies models. The Middle Jurassic Ravenscar Group is interpreted here as the deposits of a tectonically and eustatically influenced coastal plain. Repeated fluctuations between shallow-marine and non-marine conditions are interpreted as mainly caused by vertical tectonic movements and sea-level changes rather than by autocyclic processes.
Deltaic reservoir prediction from rotational dipmeter patterns
Authors: Selley R. C. 
Published: April 2008

Summary
The dipmeter is a useful logging tool for predicting the distribution of petroleum reservoirs in deltaic sediments, particularly in the Tertiary deltas of the Gulf Coast petroleum province of the USA. Conventionally upward-coarsening genetic increments are defined on gamma or sonic potential logs. Then progradational dip motifs are back-plotted to define the apex of the delta lobe and hence the highest sand-to-shale ratio of the reservoir. This method can also be applied to lobate deposits of alluvial and submarine fan origin.
The dipmeter technique can be extended by noting how depositional dips rotate as a fan lobe progrades. When viewed down-fan the depositional dip slope rotates clockwise on the right flank and anticlockwise on the left flank. These rotational dips may be detectable on the dipmeter log. Rotating dips are poorly displayed on the conventional tadpole plot but are readily apparent on graphic vector plots. Thus it may be possible to determine which side of a fan lobe is well penetrated. Dip data from two wells may define the origin and termination of the fan axis and hence locate the parent distributary channel that may contain the best reservoir sand development.
The influence of basin subsidence and depositional environment on regional patterns of coal thickness within the Namurian fluvio-deltaic sedimentary fill of the Kincardine Basin, Scotland
Authors: Read W. A. 
Published: April 2008

Summary
Quantitative studies of a varied spectrum of Namurian fluvio-deltaic sediments deposited in Scotland’s most rapidly subsiding Namurian basin, the Kincardine Basin, have revealed the following three simple regional geometrical patterns of coal thickness: (i) basin patterns, (ii) inverse basin patterns and (iii) wedge patterns. These characterize both the overall thickness of coal within a given stratigraphical interval and the thickness of some individual coal seams. The patterns become clearer once local sedimentary and tectonic effects have been filtered out by trend-surface analysis.
In a basin pattern coal thickness is closely related to the net subsidence of the basin. In contrast, in an inverse basin pattern coal-forming peat was selectively preserved on the flanks of the basin and vegetation growth was inhibited in the centre by repeated flooding and channel erosion. A wedge pattern shows coal thickness increasing fairly regularly right across the basin, so that it is directly linked to net subsidence on only one flank. Such a pattern can be linked to saline-water incursions from one direction or to optimal peat growth in a locality sheltered from repeated flooding. Similar coal-thickness patterns seem to be present in some other basins, of various ages, scattered over the world.
Genesis of mid-ocean ridge basalts
Authors: Fujii Toshitsugu 
Published: April 2008

Summary
Analyses of the compositions of liquids from various melting experiments indicate that the liquids coexisting with magnesian olivine, orthopyroxene, clinopyroxene and spinel at 10 kbar occupy a large volume in the normative projection OL-CPX-PL-SIL and that the application of the invariant melting concept analogous to the simple system CMAS is not warranted in the natural peridotite system. The compositions of magnesian mid-ocean ridge basalts (MORBs) are similar to those of liquids coexisting with olivine, orthopyroxene, clinopyroxene and spinel at 10 kbar. The relationships between Cr/(Cr+Al) in spinel and Mg/(Mg+Fe) in olivine or orthopyroxene in both abyssal peridotites and the solid phases of the melting experiments of peridotites indicate that abyssal peridotites may represent the residual materials after the extraction of liquids formed at pressures around 10 kbar. Reconstructed modal compositions of abyssal peridotites indicate that most of them included calcium-rich clinopyroxene at their solidus temperatures. It is suggested that many magnesian MORBs represent primary magmas formed at pressures around 10 kbar, leaving residual materials composed of olivine, orthopyroxene, calcium-rich clinopyroxene and spinel.
Origin of hybrid lavas from Agua de Pau volcano, Sao Miguel, Azores
Authors: Storey M. , Wolff J. A. , Norry M. J. , Marriner G. F. 
Published: April 2008

Summary
The island of Sao Miguel in the Azores exhibits bimodal volcanism: eruptives from Agua de Pau volcano and the ‘Waist’ consist almost entirely of basalt or trachyte with rare intermediate products. Although some intermediate lavas are fractionates from basalt, the majority are mixed-magma hybrids anomalously enriched in europium, barium and potassium. These features clearly indicate that they have also accumulated alkali feldspar. The source of the alkali feldspar contaminant must be the least evolved trachyte in the Agua de Pau system, because barium, europium and strontium are rapidly depleted in liquids and crystals with differentiation to more evolved trachytes. Many less evolved trachytes are also accumulative in alkali feldspar, suggesting a genetic association with the hybrid lavas. Geochemical modelling indicates that the hybrid lavas are the result of contamination of basalt by accumulative trachyte containing up to 75% alkali feldspar.
Field and petrological evidence points to the episodic injection and ponding of basalt beneath trachytic magma at Agua de Pau volcano. One trachytic lava, with alkali feldspar-contaminated basaltic inclusions, represents a sample of this interface. Density calculations indicate that highly accumulative trachyte can have a neutral or negative buoyancy with respect to basalt and could descend through the interface. Resorption of alkali feldspar will lower the bulk density of the contaminated basalt, with the effect of favouring even greater contamination by overlying accumulative trachyte. The most attractive mechanism for achieving the accumulative trachyte compositions is the slumping of side-wall cumulates and the formation of magmatic density currents which flowed along the compositional interface between trachyte and basalt. Flowage differentiation and crystal settling would result in local ‘parcels’ of denser highly accumulative trachytic magma. The process is illustrated with results from a simple laboratory experiment.
The influence of pre-existing basin structure on thrust system evolution in the Western Alps
Authors: Butler R. W. H. 
Published: April 2008

Summary
Collision mountain belts have recently been interpreted as being large scale stacked sedimentary basins, where the wholesale reactivation of pre-existing normal faults exerts the primary control on orogenic style. This model is explored here using examples from the Western Alps. The use of stratigraphic separation across faults, in the absence of corroborative sedimentological data, provides an ambiguous criterion to determinate fault reactivation. Variations between ‘compressional’ and ‘extensional’ stratigraphic separations across the Frontal Pennine thrust are more likely to be due to regional unconformity patterns developed in lower Tertiary times rather than normal fault reactivation along the eastern margin of the external zones. Mesozoic-Tertiary basin structure has been important in controlling the sites of major thrust ramps where stratigraphic horizons suitable for activation as thrust detachment zones have been offset. Examples include the thrust front in the Jura (Triassic detachment dropped to W by faults bounding the Bresse basin) and Vercors (basement uplifted to W at Faille de l’Isere, bounding the Mesozoic sub-Alpine basin to W). Buttressing structures, where thrust displacement is converted into vertical pure shear, may indicate buried normal faults but can occur at either hinterland or foreland-directed basin faults. The overall structural style in the sub-Alpine thrust belt indicates detachment between basement and cover with basement involvement along the Sole thrust which carries the external basement massifs. There is no indication from surface geology that this thrust originates from reactivation of a Mesozoic basin fault although this process may have occurred at depth. Similar styles of detachment-dominated thrusting with large horizontal relative to vertical displacements also characterize internal Alpine structure. Preliminary results from the ECORS-CROP deep seismic reflection profile across the Western Alps support the detachment thrusting model although they do not rule out minor normal fault reactivation.
The Lewis Thrust/Rocky Mountain trench fault system in Northwest Montana, USA: an example of negative inversion tectonics?
Authors: Powell C. M. , Williams G. D. 
Published: April 2008

Summary
In the Rocky Mountains of Northwest Montana, USA, Oligocene basins occur within the Laramide thrust system. During emplacement of the Laramide Lewis thrust sheet in the Glacier National Park area, a penetrative cleavage was developed with an asymmetry suggestive of NE directed movement. This cleavage can be used as a pre-extensional marker to reinterpret the geometry of the Rocky Mountain Trench extensional fault system. The cleavage is confined to a zone of constant thickness above the thrust. Thus its upper limit can be considered a mappable surface horizon from which the depth to the Lewis Thrust can be interpolated. Sequentially restored sections from the Rocky Mountain Trench to the leading edge of the Lewis Thrust illustrate the interpreted geometries and the interaction of the extensional and contractional fault systems.