bluuebunny/crossref_metadata_2025_split huggingface dataset:
Discrimination of depositional environments using sedimentary characteristics in the Mega Kalahari, central southern Africa
Authors: Thomas D. S. G. 
Published: April 2008

Summary

Unconsolidated Kalahari sand covers 2.5 million km
2
of central southern Africa, the ‘Mega Kalahari’, extending far beyond the Kalahari Desert of today. Relic aeolian, lacustrine and fluvial landforms indicate that in the Quaternary the Kalahari sand has been deposited and reworked by a variety of processes. Large areas of the sand have no identifiable landform associations, and because of this, sedimentological studies are necessary for a better interpretation of the palaeoenvironmental history of the Mega Kalahari. To this end, almost 200 Kalahari sand samples, in and not in identifiable landform associations, were collected and analysed in order to: investigate sedimentological characteristics; determine whether different processes have imparted distinct sedimentological characteristics to the sand; and thereby attempt to determine the depositional environments of samples without prior knowledge of the landform associations. Three methods have been used: (1) standard sedimentological techniques of grain size and shape determination; (2) multivariate discriminant analyses, to determine whether depositional environments could be better distinguished by considering grain size and shape parameters in combination; and (3) scanning electron microscope analyses of samples which lie centrally in any group identified by statistical analyses. Results indicate that aeolian processes have dominated the environmental history of the Kalahari sand, with only limited modification by subsequent processes. Multivariate discriminant analysis achieved only limited success in differentiating process environments. SEM investigations best identified the subtle modification of sand characteristics by non-aeolian processes, because individual grain texstures are more readily adjusted than overall sample fabrics, and may therefore be the best technique for elucidating palaeoenvironments from sediment characteristics.
Remote sensing of sediment transfer processes in playa basins
Authors: Millington A. C. , Jones A. R. , Quarmby N. , Townshend J. R. G. 
Published: April 2008

Summary

Landsat
Thematic Mapper and Multispectral Scanner data have been used to detect changes in the spectral reflectance and absorption of three playas in south-central Tunisia. Change detection images have been used to analyse changes between wet and dry seasons and between different years. Changes in geomorphological phenomena are interpreted from changes in soil and foliar moisture levels, differences in reflectance between different salts and sediments, and the spatial expression of geomorphological features. The intra-annual changes that can be detected from multidate imagery include surface moisture, texture and chemical composition, vegetation cover and the extent of aeolian activity. Detected inter-annual changes are divisible into those restricted to playa margins—sedimentation from sheetwash and alluvial fans, erosion from surface runoff, and cliff retreat—and those found at central playa locations which are related to the redistribution of water, salt and sediment.
Controls on the geometry and distribution of carbonate cements in Jurassic sandstones: Bridport Sands, southern England and Viking Group, Troll Field, Norway
Authors: Kantorowicz J. D. , Bryant I. D. , Dawans J. M. 
Published: April 2008

Summary
The petrography and diagenesis of calcite cements in the Lower Jurassic, Bridport Sands (southern England) and Upper Jurassic, Viking Group sandstones (Troll Field, offshore Norway) have been investigated in order to assess their geometry and effect on hydrocarbon recovery.
In the Bridport Sands, sediment texture and mineralogy controlled carbonate cementation. Clay-rich fairweather sediments were weakly cemented and are now compacted. Bioclast-rich storm deposits were stabilized mechanically by early fringing cements. During burial bioclasts and fringing cements were replaced or dissolved, and pores were filled by simultaneously precipitated ferroan calcite. Thus, cemented beds are laterally continuous for several kilometres in the Bridport Sands as a consequence of the sheet-like geometry of the storm beds in which they developed.
In the Viking Group sandstones, carbonate cementation was controlled by rate of burial. Fringing cements formed locally during non-deposition or emergence. Cementation continued with non-ferroan calcite incorporating bacterially derived bicarbonate generated during prolonged residence in near surface zones of bacterial activity. Cement geometries will reflect the distribution of emergent surfaces and the longevity of residence near the surface.
These two cases demonstrate the potential for laterally extensive carbonate cements to develop in shelf sandstones. The cements in these examples have different origins but in both cases their distribution is related to the episodic nature of deposition in the shelf environment.
Trace-element source and mobility during limestone burial diagenesis—an example from the Middle Jurassic of eastern England
Authors: Emery D. 
Published: April 2008

Summary
Two phases of burial calcite from the Lincolnshire Limestone (Bajocian) of central Lincolnshire have been analysed for their trace element (Fe, Mn, Mg, Sr) and strontium isotopic compositions.

The earliest, non-ferroan, brightly luminescent phase has the following mean trace element concentrations: Fe- 919 ppm; Mn- 116 ppm: 3 1778 ppm; Sr- 327 ppm. The later, ferroan, dully luminescent phase has the following mean compositions: Fe- 3580 ppm; Mn- 225 ppm; Mg- 1400 ppm; Sr- 318 ppm. The strontium isotopic composition of the two phases is indistinguishable, with a mean
87
Sr/
86
Sr ratio of 0.70820. This value is considerably more radiogenic than host Bajocian marine carbonate (
87
Sr/
86
Sr = 0.70725).

The Fe and Mn of the burial calcites were sourced by iron and manganese oxyhydroxides associated with shales bounding (and internal to) the Lincolnshire Limestone. The relatively shallow maximum burial depths (550 m) and low temperatures (maximum of 42°C) attained by the formation in the area of study preclude major clay or sandstone diagenetic transformations as potential sources for Fe and Mn. These trace elements moved into the Limestone in solution via compaction driven cross-formational flow from bounding shale aquicludes.

The Sr and Mg trace element components of the burial cements were derived from: (i) remobilized Bajocian marine carbonate, and (ii) remobilized Carboniferous marine carbonate. These sources satisfy the requirements of a linear Sr versus Mg relationship observed for the burial cements, that is, a constant Sr/Mg ratio with time, but highly variable absolute Sr and Mg contents. These sources also satisfy the near constant
87
Sr/
86
Sr composition of the burial calcites.

The burial hydrology required by the trace element budget above is a convective flow system with cross-formational elements as normal bleeder faults connecting the Carboniferous Limestone and the Lincolnshire Limestone. Additional compaction-driven flow occurred from bounding shale formations (chiefly the underlying Grantham Formation and to a lesser extent the Upper Estuarine ‘Series’) to the Lincolnshire Limestone. Up-dip flow within the fully confined Limestone provides an enformational limb to the convective system.
Oxygen-isotope studies of clastic diagenesis in the Lower Cretaceous Viking Formation, Alberta: implications for the role of meteoric water
Authors: Longstaffe Fred J. , Ayalon Avner 
Published: April 2008

Summary
The oxygen-isotope compositions and paragenetic sequence of diagenetic minerals from the Lower Cretaceous Viking sandstone and conglomerate from south-central Alberta, Canada have been used to identify changes in porewater composition during diagenesis and to relate these changes to major geological events within the western Canada sedimentary basin. Large-scale influx of meteoric water has played an important role in diagenesis of this unit, especially following uplift of the basin in early Eocene time.

Diagenetic phases include early kaolinite (
δ
18
O SMOW = + 24.9 to +28.2 ‰), siderite (
δ
18
O SMOW = + 22.0 to + 23.9 ‰;
δ
13
C PDB = −6.3 to −1.3 ‰), calcite (
δ
18
O SMOW = + 26.0 ‰;
δ
13
C PDB = −0.4 ‰) and chlorite (
δ
18
O SMOW = + 10.7 − + 13.4 ‰), followed in order by dolomite (
δ
18
O SMOW = + 19.3 − + 22.1 ‰;
δ
13
C PDB = −5.9 to −2.6 ‰), calcite (
δ
18
O SMOW = + 13.9 − + 15.9 ‰;
δ
13
C PDB = −8.4 to −6.1 ‰), ankerite (
δ
18
O SMOW = + 16.3 − + 17.9 ‰;
δ
13
C PDB = −12.4 to −3.5 ‰), kaolin group minerals (
δ
18
O SMOW = + 12.0 − + 15.7 ‰), and illite (
δ
18
O SMOW = + 14.2 − + 15.7 ‰) and illite/smectite (
δ
18
O SMOW = + 13.8 − + 16.5 ‰). Quartz overgrowths (
δ
18
O SMOW = + 16.2 − + 27.4 ‰) began crystallizing relatively early during burial diagenesis and continued to form at least up to the onset of late diagenetic formation of clay minerals.

The interpretation of these results is that shallow diagenesis, early in the burial history (glauconite, pyrite, calcite, chlorite), occurred largely in the presence of seawater-derived fluids, although a freshwater influence is indicated where siderite and(or) early kaolinite cements are abundant. As compaction and burial diagenesis proceeded (diagenetic chlorite, quartz overgrowths, dolomite), the porewaters became enriched in
18
O due to water/rock interaction. Burial diagenesis was terminated in the early Eocene by uplift related to the major Laramide Orogeny. Recharge of the basin by low
18
O meteoric water occurred at this time. The meteoric water then became involved in the formation of diagenetic quartz, calcite and ankerite (and the dissolution and albitization of feldspar) at or near maximum burial temperatures, and in the crystallization at lower temperatures of kaolin group minerals, illite and illite/smectite as the post-Eocene erosion progressed.
Distribution, crustal properties and significance of seawards-dipping sub-basement reflectors off E Greenland
Authors: Larsen H. C. , Jakobsdóttir S. 
Published: April 2008

SummaryWe report in this paper the existence of seawards-dipping sub-basement reflectors along the entire E Greenland margin. The study is based on 8000 km of multichannel seismic data and a sonobuoy refraction seismic study providing information on the geographical and stratigraphical extension, internal geometry and crustal structure of the E Greenland dipping reflector sequence. A basaltic, subaerial seafloor-spreading origin of the reflector sequence is concluded from seismic stratigraphic analysis, including well information from the Rockall Plateau and the Vøring Plateau. Formation of the basaltic dipping reflector sequence off E Greenland took place within a period of a few million years along the axis of opening within the NE Atlantic. Duration of spreading above sea-level was relatively short (2 My) in areas of present-day deep basement, as opposed to 5–8 My in areas of present-day more shallow basement. On the highly elevated Iceland-Greenland Ridge, subaerial seafloor spreading continued into the Neogene and most likely into present-day subaerial spreading in Iceland. Following the mid-Tertiary westward shift of spreading towards Greenland, N of Iceland, spreading again took place above sea-level along this part of the Greenland margin until late Miocene, but this development only caused an erratic and shallow development of seawards-dipping reflectors.Application of the kinematic model for crustal formation in Iceland (Pálmason 1980) onto the E Greenland dipping reflector sequence demonstrates a striking similarity between the two structures. However, volcanic productivity rate within the oldest part of the E Greenland dipping reflector sequence may be as much as three times the volcanic productivity rate recorded in Iceland with an original rift width equal to, or somewhat less than, that in Iceland. The high volcanic productivity rate caused the development of a thick extrusive upper crust (> 5–6 km) dominated by seawards-dipping reflections arising from lava flows or groups of flows which acquired their dip through postdepositional differential subsidence towards the rift zone. Refraction seismology defines a fairly flat-lying velocity zonation of the igneous crust with an anomalously thick layer 2 (3–5.5 km) in areas of well-developed dipping reflectors. The layer 2/3 boundary is seen to cut strongly across the dipping reflectors suggesting a metamorphic origin of this boundary. Initiation of seafloor spreading above sea-level is seen as a result of early upwelling of anomalously hot asthenospheric material that was able to ascend through a relatively mechanically unstretched crust and lithosphere and create a thick extrusive upper crust. Formation of a thick extrusive upper crust above sea-level only continued beyond the early spreading phase in the area of the Icelandic hot-spot (Iceland-Greenland Ridge).
Some properties of basalt lava sequences and volcanic centres in a plate-boundary environment
Authors: Kristjansson L. , Helgason J. 
Published: April 2008

SummaryThis paper discusses the implications that two current studies in Iceland may have in the interpretation of results from marine deep drilling and geophysical surveys in marginal volcanic areas of the N Atlantic. One study, concerning the magnetic remanence of lava flows, shows that the rate of geomagnetic reversals in the Tertiary is higher than is generally expected, and that caution must be exercised in tracing magnetic lineations between coeval submarine and subaerial formations. The other study, which deals with the distribution of major volcanic complexes within the Icelandic lava pile, demonstrates that this distribution is dependent upon various tectonic factors, especially any lateral shifts or direction changes of the active plate boundary that may have occurred.
Volcanic ash in a cored borehole W of the Shetland Islands: evidence for Selandian (late Palaeocene) volcanism in the Faeroes region
Authors: Morton A. C. , Evans D. , Harland R. , King C. , Ritchie D. K. 
Published: April 2008

Summary
A volcanic ash layer, containing basic glass shards compositionally comparable to the basalts of the Faeroe-East Greenland Province, has been discovered in early Selandin sediments from a borehole W of the Shetland Isles. The available magnetostratigraphical, biostratigraphical and tephrachronological information indicates that basaltic volcanism in the province began during magnetic anomaly chron C26R, earlier than the currently accepted C24R interval, but correlating with the main period of basaltic volcanism in the British Tertiary Igneous Province.
Distribution of early Tertiary lavas in the NE Rockall Trough
Authors: Wood M. V. , Hall J. , Doody J. J. 
Published: April 2008

Summary

A study of lavas of early Tertiary age in NE Rockall Trough has revealed that they can be classified on the basis of their seismic reflection character and their relationship with the overlying Tertiary sediments. The lavas have been mapped using commercial seismic reflection data. Our interpretation of the areal distribution of the subaerial flows has major implications for the subsidence history of the area. Three seismic facies are recognized:

(1)
High amplitude parallel reflections in laterally continuous sheets with steep edges.

(2)
Basinward-thinning wedges of high-amplitude discontinuous reflections.

(3)
A single high-amplitude reflection underlain by a zone of small diffractions.

These are interpreted respectively as subaerial flows, hyaloclastite deposits derived from the basalts, and tuffs.
Throughout the sedimentary sequence below the lavas, high amplitude, subhorizontal reflection packages occur. These are interpreted as sills of early Tertiary age intruding Mesozoic sediments. In places, the lavas themselves are intruded by seismically translucent bodies of up to a few kilometres across. These may represent small centres of lava extrusion.
Significance of lacustrine cherts for the environment of source-rock deposition in the Orcadian Basin, Scotland
Authors: Parnell J. 
Published: April 2008

Summary
Middle Old Red Sandstone lacustrine sequences in the Orcadian Basin include rocks of petroleum source-rock potential, most extensively exposed onshore in Orkney and Caithness. The richest source rocks are carbonate laminites, in which carbonate and carbonaceous laminae are believed to be the deposits of algal blooming and mortality in a stratified lake (Donovan 1980). Many carbonate laminites and some other organic-rich laminites contain chert nodules, interpreted as magadi-type chert enclosing pseudomorphs after (?) trona. The cherts and evaporites were precipitated in sub-lacustrine sediment from groundwaters rich in dissolved sodium carbonate/bicarbonate. This implies that the source rocks were deposited from saline, alkaline lake waters.
Biological markers in lacustrine Chinese oil shales
Authors: Brassell S. C. , Eglinton G. , Sheng Guoying , Fu Jiamo 
Published: April 2008

Summary

A major objective in molecular organic geochemistry is the assessment and characterization of sedimentary depositional environments from the occurrence of specific biological marker compounds related to particular source organisms. The aliphatic hydrocarbon distributions of petroleums and/or sediments from Chinese freshwater and hypersaline lacustrine environments are described and contrasted in the search for diagnostic features among their biological marker distributions. The Eocene Maoming oil shale, which crops out in Guangdong Province, shows major differences in hydrocarbon composition through its stratigraphic succession. These variations reflect changes in lithology and in the origins of the sedimentary organic matter, as seen from petrographic examination. The lower lignite and vitrinite layers contain significant contributions of biological markers from bacterial sources, plus components derived from terrigenous higher plants, such as C
29
steroids. In contrast, the overlying shales show a dominance of dinoflagellate-derived 4-methylsteroids and culminate in an horizon with botryococcane homologues as their dominant alkanes. Thus, the geochemical profile suggests that a swampy environment, in which peats accumulated, deepened to support populations of dinoflagellate and, finally, blooms of
Botryococcus
green algae. The aliphatic hydrocarbons of a series of sediments and petroleums from the Eocene hypersaline sequence of the Jianghan Basin in southern Hubei Province shows various features that may be typical of such facies. These include a dominance of even-numbered n-alkanes, high amounts of phytane and gammacerane, and an abundance of C
35
-hopanes. Similar characteristics are observed in shales from the Eocene Green River Formation and in Rozel Point crude oil, a petroleum thought to originate from sediments deposited under hypersaline conditions. The same features also appear in a lower Tertiary evaporitic mudstone from south of the Renqui oil field in Hebei Province, together with 4-methylsteranes, β-carotane and 18α(H)-oleanane, a marker for terrigenous higher plants. These data illustrate the potential of biological markers in the differentiation of different lacustrine environments and suggest that such characteristics may provide signatures for environmentally similar, but geographically distant, deposits.
Upper Proterozoic passive margin deltaic complex, Finnmark, N Norway
Authors: Siedlecka A. , Pickering K. T. , Edwards M. B. 
Published: April 2008

Summary

The Upper Proterozoic Båsnaering Formation in the 9000 m thick Barents Sea Group is a succession of deltaic siliciclastics, 2500–3500 m thick, with an exposed extent covering an area of 3000 km
2
. The Båsnaering Formation is underlain by up to 3200 m of deep-marine submarine fan deposits, while above the deltaics there are up to 1500 m of shallow- and marginal-marine (intertidal and supratidal) carbonates and siliciclastics. The lower part of the Båsnaering Formation records the change from deep- to shallow-marine, slope and prodelta environments to delta-front and delta-top environments. The middle and upper parts of the formation comprise delta-top, fluvial, marginal-marine, interdistributary bay and related environments. The Båsnaering Formation appears to record the northeastward progradation of a fluvially dominated wave-modified deltaic system. Tidal processes influenced the delta development, but the importance and magnitude of such tides remains uncertain. Gravity-controlled sediment instability was prevalent in the prodelta and delta-front environments.
Reservoir geology of the Sirikit oilfield, Thailand: lacustrine deltaic sedimentation in a Tertiary intermontane basin
Authors: Flint S. , Stewart D. J. , van Riessen E. D. 
Published: April 2008

Summary
The Sirikit oilfield, situated some 400 km N of Bangkok, Thailand, is a fault-bounded structure within a major half-graben basin, comprising a relatively undeformed eastern sector and a western area characterized by a complex pattern of isolated fault blocks. The 8 km thick basin-fill succession includes alluvial fan-fluvial deposits overlain by the fluvio-deltaic Lan Krabu Formation, which contains two main oil reservoirs.

A reservoir geological model has been produced based on cores and wireline log suites from some 45 wells. The geology is characterized by cycles of regressive coarsening-upward sequences, up to 15 m thick, which reflect cyclic progradation of fluvial-dominated deltas into a large relatively shallow lake in a humid tropical climatic regime. The cycles are commonly topped by subaerial flood-plain claystones. Distributary channel sandstones show evidence of stage fluctuation and intermittent abandonment. Individual mouth-bar reservoir sandstones range in thickness from 1 to 5 m and form complex sheet-like bodies of lateral extent up to 20 km
2
. Thus the Sirikit deltas have little inferred geometrical similarity with contemporary bar-finger models postulated for river-dominated lacustrine deltas. The sheet-delta geometry may be due to a combination of high frictional forces related to similar river and lake water densities, shallow lake depth and a large but variable fluvial sediment load. This combination of factors resulted in rapid dumping of sediment, the plugging of channel mouths and subsequent channel bifurcation.