bluuebunny/crossref_metadata_2025_split huggingface dataset:
Geochemistry and Petrology of Metavolcanic Rocks of the Early Proterozoic Mazaruni Greenstone Belt, Northern Guyana
Authors: Renner R. , Gibbs A. K. 
Published: April 2008

Summary
The dominantly metavolcanic Issineru Formation (IF) forms the lower 5–7 km of the stratotype for the early Proterozoic Mazaruni greenstone belt, N Guyana. Basalts and gabbros comprise the lower part of the section. The upper half includes some basalts, but is predominantly intermediate and felsic rocks. Most of the Issineru Formation volcanics form a tholeiitic series but minor calcalkaline hornblende andesites and two-pyroxene basaltic andesites with alkaline affinities also occur. Issineru Formation tholeiitic basalts have flat, and slightly LREE-depleted patterns with overall abundances of about 10 × chondrites. Some porphyritic basalts have LREE-enriched patterns and may be calcalkaline. The basalts have similar high field strength element (HFS) patterns on multielement MORB-normalized diagrams, with fairly flat patterns and depletion in P and Ti. Their large ion lithophile element (LIL) contents are more variable, due in part to the LIL’s susceptibility to alteration, but range from slightly less than MORB to about 8 × MORB. These patterns are transitional between MORB and island arc tholeiites. Tholeiitic basaltic andesites and andesites have higher REE levels than the basalts and slightly LREE-enriched patterns (Ce/ Yb = 1.3–2). Calcalkaline hornblende basaltic-andesites and andesites have REE patterns with steeper slopes (Ce/Yb=4.2). All of the Issineru felsic rocks have high REE levels (La = 60) and relatively flat patterns with pronounced negative Eu anomalies. Elsewhere in N Guyana there are rhyolites with steeply-sloping REE patterns, which are considered part of the calcalkaline series. The two-pyroxene basaltic andesites with alkaline affinity have very steep REE patterns (Ce/Yb= 10). They may be sills associated with post Trans-Amazonian continental volcanics, rather than coeval members of the greenstone belt volcanism. The IF tholeiitic series can be qualitatively explained by fractional crystallization from a mafic parent with a flat REE pattern.
Alteration of Late Middle Proterozoic Volcanics and its Relation to Stratabound Copper-Silver-Gold Mineralization Along the Margin of the Kalahari Craton in Swa/Namibia and Botswana
Authors: Borg G. , Maiden K. J. 
Published: April 2008

Summary
Metavolcanic and metasedimentary rocks of late Middle Proterozoic age are preserved in basins along the northern margin of the Kalahari Craton. Three broad stratigraphic divisions are recognized: a lower acid extrusive unit, a basic extrusive/red bed unit and a shallow marine unit.
The basalts of the basic extrusive/red bed unit are interpreted to be continental within-plate basalts. Copper mineralization occurs as native copper in amygdaloidal altered basalt, and as sediment-hosted, stratiform sulphide deposits at the base of the shallow marine unit.
The rocks have been strongly altered during regional metamorphism which reached temperatures up to 350°C. Massive domains were metamorphosed to oligoclase-chlorite-epidote-haematite rocks. In permeable zones such as flow top breccias, amygdaloidal zones, and regions of secondary, tectonically induced fracturing, the basalts have been altered to epidote-quartz-chlorite-haematite-muscovite rocks. During this epidote alteration copper, zinc and cobalt were leached from the basalts, thus providing a possible metal source for the sediment-hosted deposits.
A model for multiphase ore genesis is presented in which basalt alteration is the main metal source for epigenetic mineralization.
The Geochemistry of Upper Proterozoic Lavas From the Red Sea Hills, NE Sudan
Authors: Klemenic P. M. 
Published: April 2008

Summary

The geochemistry of the Kadewēb and Homogar Volcanic Groups from the Red Sea Hills of north-east Sudan is discussed. Rb-Sr studies show that these lavas are Upper Proterozoic in age (720 Ma and 670 Ma respectively). The lavas of both groups display a calcalkaline character and are broadly similar in terms of their lithology, phenocryst mineralogy and major element chemistry. Geochemically the Kadawēb and Homogar volcanic rocks are similar to modern arc lavas associated with converging plate margins, although they tend to have higher concentrations of some HFS elements. Despite overall similarity, a distinction can be made between the two groups on the basis of the geochemistry of the basaltic components of the suites. The Homogar basalts are enriched in LIL and REE, having high La/Nb, (Ce/Yb)

n

and Ta/Yb ratios compared with those of the Kadawēb group. It is suggested that the older Kadawēb lavas were erupted along an intra-oceanic island arc system, while the younger Homogar lavas were extruded along a more evolved, possibly continental, arc margin. Differences between these Upper Proterozoic rocks and modern arc lavas may be explained as resulting from differences in the nature of the Proterozoic mantle.
Geochemical and Geochronological Signatures in Australian Proterozoic Igneous Rocks
Authors: Wyborn L. A. I. , Page R. W. , Parker A. J. 
Published: April 2008

Summary

Most igneous activity in Australian Proterozoic domains is related to major orogenic events, or else to crustal extensional events in which there is an earlier volcanogenic rift phase overlain by a non-volcanic thermal subsidence phase. The igneous suites are either predominantly mafic, felsic or bimodal, and there are no major suites of intermediate igneous rocks. With time, the amount of igneous material decreases volumetrically in the Proterozoic sedimentary sequences and there are also distinct compositional changes. Depleted tholeiites are more common in the early Proterozoic sequences, whereas enriched tholeiites and alkaline rocks are dominant after 1800 Ma. Felsic igneous rocks are rare in the earliest Proterozoic sequences, but dominate in the period 1880-1400 Ma. They are also compositionally time controlled, and certain major magmatic felsic suites at
c
. 1860, 1790, 1740, 1670, 1500 Ma have distinct chemical parameters which are common to most domains. All the major suites of felsic igneous rocks show enrichment in K, Rb, Th, U, La, and Ce relative to Phanerozoic analogues. The felsic rocks are predominantly I-types, derived from Proterozoic infracrustal rather than Archaean precursors; rare S-types also appear to have Proterozoic sources. Our preferred model has the felsic igneous rocks derived from underplated sources which are progressively accreted to the base of the crust, as a result of a series of extensional episodes. There are no direct compositional equivalents of modern subduction-related igneous sequences suggesting that subduction as it is known today was not a dominant process in the production of the igneous suites. There are also no recognizable ophiolites, ocean-floor crust, or other evidence of a Wilson cycle.
Geochemical and Tectonic Evolution of the Proterozoic Telemark Supracrustals, Southern Norway
Authors: Brewer T. S. , Atkin B. P. 
Published: April 2008

Summary
The Telemark supracrustals represent the only low-grade metamorphic lithologies within the Sveconorwegian Province of southern Norway. These supracrustals outcrop in five regions and are composed of metabasalts, metarhyolites and metasediments.
In each of the supracrustal regions three metamorphic events can be identified—burial, regional and thermal metamorphism. All of these metamorphic events have had variable effects on the major and trace element compositions of the acid and basic volcanics. Thus, in each region a ‘screened’ data set of relatively unaltered samples has been produced.
In all of the regions the volcanics are strongly bimodal and all possess geochemical signatures indicative of destructive plate margin volcanism. Thus, by combining the new chemical, lithological and structural data a new tectonic model is proposed, which suggests that the supracrustals were formed at a destructive plate margin. This model also satisfies the data for the Middle Proterozoic volcanic suites from Canada, which supports the hypothesis that the Sveconorwegian-Grenville orogen was developed along a continental margin and driven by plate-tectonic processes.
Upper Proterozoic Basaltic Volcanism in the Northern Massif Armoricain, France
Authors: Lees G. J. , Roach R. A. , Shufflebotham M. M. , Griffiths N. H. 
Published: April 2008

Summary
A 2 km thick sequence of subaqueous basaltic rocks—the Hillion-Erquy Volcanic Formation (HEVF)—forms the lower part of the late Proterozoic Brioverian supracrustal sequence around the Baie de St. Brieuc, N Brittany. Basic dykes cutting the adjacent Pre-Brioverian crystalline basement (the Pentevrian) are interpreted as feeders to the volcanic sequence. As a result of the end Precambrian-early Palaeozoic Cadomian orogenic event, the basic volcanics and feeder dykes experienced a regional metamorphism varying from prehnite-pumpellyite facies in the east to upper greenschist (or amphibolite) in the west of the Baie de St. Brieuc region.
The compositions of the basic rocks are evolved, ranging from basalts to basaltic andesites. The suite is subalkaline and tholeiitic in character. Both major and trace element discriminants indicate the suite to be of continental flood basalt affinity. Incompatible element diagrams show evidence for a certain degree of crustal contamination during the evolution of the suite.
It is proposed that the HEVF was erupted in a rapidly subsiding ensialic basin, formed during a phase of crustal extension. Volcanism ceased abruptly and the basin rapidly filled with sediment.
A brief review is given of several other broadly similar basic sequences occurring in other parts of N Brittany. Their chemistry is compared with that of the HEVF and the implications for a geotectonic reconstruction of the region in late Proterozoic times discussed.
Geochemical Study of the Brioverian (late Proterozoic) Volcanic Rocks in the Northern Armorican Massif (France). Implications for Geodynamic Evolution During the Cadomian
Authors: Cabanis B. , Chantraine J. , Rabu D. 
Published: April 2008

Summary
A major and trace element geochemical study has been made of the Brioverian volcanic rocks in the NW of Brittany. They are compared with recent and modern volcanic rocks to identify their magmatic characteristics and thus to relate them to different tectonic contexts and integrate them into an overall geodynamic interpretation.

Three complexes in different locations across the Cadomian orogenic belt have been studied:

The volcanic rocks in Lannion Bay are typical of calcalkaline rocks of frontal volcanic arcs associated with subduction.

The volcanic rocks of St Brieuc Bay are similar to tholeiitic within-arc or back-arc volcanic rocks, associated with internal extension at a convergent plate margin.

The Fresnaye Bay volcanic rocks, however, resemble those from extensional within-plate settings and are quite different from the other two volcanic associations.

These various volcanic settings are integrated into a general model of geodynamic evolution in the Armorican Massif during the Cadomian, to which features of the regional geology lend support.
Fluid flow in sedimentary basins: a geologist’s perspective
Authors: Chapman Richard E. 
Published: April 2008

SummaryWater begins to flow from higher-energy to lower-energy positions as soon as sediment begins to accumulate in a new sedimentary basin, and flow continues during the sedimentary development of the basin. Water expelled by compaction from the compactible less-permeable beds displaces water in the less-compactible more-permeable beds. Compaction flow also causes subsidence that contributes to the sedimentary development of a basin.The pattern of flow is determined by the stratigraphy. In transgressive sequences, it is downwards in mudstone to the basal permeable unit and then lateral towards the land of the time. In regressive sequences, it is upwards and downwards in mudstones towards the intercalated sandstones and then lateral in these towards the land. In compacting sequences of alternating sandstones and mudstones, the sandstones drain distinct isolated hydraulic units that are related to the stratigraphy.When a sedimentary basin ceases to accumulate sediment and compaction has reached equilibrium, the flow patterns change and the main energy comes from elevation of the intake areas.In rift basins the main source of compaction water is the thick mudstone in the post-unconformity-disconformity sequence. Abnormal pressures in the pre-unconformity-disconformity sequence indicate lack of flow in this part of the basin.
Fluid flow in the Chalk of England
Authors: Price Michael 
Published: April 2008

Summary

The Chalk aquifer of England can be thought of as a multi-porosity medium. The matrix is a fine-grained limestone which generally has high porosity but small pore throats, so that its permeability is typically only 0.1 to 10 millidarcys (10
−4
to 10
−2
m day
−1
). A fairly uniform fracture system imparts a secondary permeability, which appears to be about 100 to 1000 mD (0.1 to 1 m day
−1
). Where the Chalk forms a major aquifer, most of the transmissivity results from the enlargement of fractures, by solution, to form a few highly permeable zones. Over much of the outcrop, weathering leads to the development of shallow permeable layers to form an additional component. Each of these permeability systems influences some aspect of subsurface water movement, with implications for resources, quality and construction.
The role of low-permeability rocks in regional flow
Authors: Alexander J. , Black J. H. , Brightman M. A. 
Published: April 2008

Summary
The role of low-permeability rocks in determining fluid migration within sedimentary basins has been shown to be crucial for deriving the direction and rate of fluid movement. The magnitude of the flows depends on the geometry of the sedimentary basin and on the bulk hydrogeological properties of the various strata and their relative values. Measurements of head and pore-water compositions of argillaceous formations show separate evidence for the existence of cross-formational flow resulting in groundwaters of mixed origins. Numerical and analytical models have been developed and applied to some basinal systems in England. They have been used to assess the directions of groundwater flow likely to be encountered by repository site investigation programmes and to aid in the interpretation of groundwater head and chemistry data. Such flow models still require detailed validation. Our knowledge of the flow mechanism and the physical properties of mudrocks at depth is still elementary, but the indications are that the role of low-permeability rocks in controlling fluid migration has been much underestimated.
The flow of water and displacement of hydrocarbons in fractured chalk reservoirs
Authors: Brown David A. 
Published: April 2008

Summary
The fractured chalk reservoirs of the southern part of the Central Graben do not exhibit the flow characteristics more usually encountered in continuous sandstone formations. The Danian and Maastrichtian age formations of the Ekofisk Field are typical of these fractured chalk reservoirs. The Ekofisk Field has been extensively studied to understand better the geology and controlling fluid flow mechanisms prior to commencing enhanced oil recovery projects. During waterflooding the imbibition of water from the fracture network into the rock matrix will control the displacement of hydrocarbons. Extensive laboratory studies have been carried out to quantify the imbibition characteristics of the Danian and Maastrichtian formations. Several parameters that control the amount of water imbibed have been found. A field water-injection pilot project has been implemented to verify the mechanisms and laboratory results and to prove numerical techniques suitable for the prediction of recoveries under a full-field waterflood.
Fluviatile deposits and morphology of alluvial surfaces as indicators of Quaternary environmental changes in the southern Negev, Israel
Authors: Grossman S. , Gerson R. 
Published: April 2008

Summary
Quaternary environmental changes in the southern Negev are recorded in the alluvial deposits and the geometry of depositional surfaces in the Timna Valley. There are some distinct differences between the alluvial deposits of the Holocene to modern flood plains and earlier (Pleistocene) ones. The Holocene to modern alluvium is characterized by sand, fine- to medium-sized gravel and very coarse gravel in large gravel bars. The sediments indicate environments ranging between extremely arid and semi-arid dominated by floods of low to moderate magnitude but with occasional short large floods. The Pleistocene sediments include: (1) debris-flow deposits indicating talus activity under a moderately arid to semi-arid climate; (2) alternating fine-grained and gravelly layers reflecting floods of low to moderate magnitudes. The nature of the soils supports these conclusions. Thick and argillic Reg soils are typical of the Pleistocene surfaces whereas shallow loamy ones are characteristic of the Holocene terraces. The alluvial surfaces usually converge downstream. The intersection points migrate upstream, following stages in the recession of a major escarpment. Broad pedimented surfaces and bajadas are characteristic of the Pleistocene period, whereas long narrow terraces of gentle declivity were formed during the Holocene. The latter suggest depletion of the available debris and in the effectiveness of large floods.
Rates and modes of dust accretion and deposition in an arid region—the Negev, Israel
Authors: Gerson R. , Amit R. 
Published: April 2008

SummaryRates of dust accretion and deposition are dependent on the amount of available dust and the trap efficiency of a particular site. Several types of dust-trapping terrains are widespread in deserts: (1) Gravelly (Serir) surfaces that turn with time into Reg soils; (2) vegetated surfaces in the desert fringe that may turn into löessial terrains; (3) stabilized sand dunes; (4) playa surfaces. Loessial terrains exhibit a high rate of dust accretion during the late Pleistocene—0.07–0.15 mm a−1on the interfluves and ≤0.5 mm a−1along the flood plains. Gravelly surfaces usually trap about 0.1 mm a−1of dust initially but the rates decrease to several μm a−1due to plugging with dust and salts and may ultimately remain constant as a gravel-free B horizon develops. The amounts of imported dust, from both local and distant sources, have changed during the Quaternary due to climatic fluctuations. Roofless ancient building—most efficient dust traps—show that although large amounts of dust were available (much of it from local sources) during the late Holocene, there was not intensive dust accretion during this period due to increasing aridity and decreasing trap efficiency. Wash and gullying led to destruction of the once widespread efficient trapping terrains.
Complex star dunes and associated aeolian bedforms, Hopeman Sandstone (Permo-Triassic), Moray Firth Basin, Scotland
Authors: Clemmensen L. B. 
Published: April 2008

Summary
The Hopeman Sandstone, which is exposed at the southern edge of the Moray Firth Basin, is composed of a number of aeolian sandstone units. These units, which have been mapped continuously for 4.5 km, represent part of an ancient sand sea in which complex star dunes alternated with sand-filled star-dune corridors. The palaeowind regime was complex with prevailing winds from the NNE, secondary winds from the SSE and subordinate winds from the NW. The complex star dunes are represented by two types of deposits: Type I—large-scale or giant-scale mainly trough-formed high- to medium-angle cross-bedding, which constitute the slip-face deposits of actively migrating star dunes; and Type II—bimodally dipping more wedge-shaped large-scale or giant-scale sets, which apparently formed on the opposed flanks of relatively stationary star arm segments. The exposed star arms trend in three main directions being roughly parallel with the palaeowind directions. The dune-dune corridors contain small- and medium-sized crescentic or reversing dunes which frequently overlap the star-dune flanks, interdune flats, and rare ephemeral streams. The palaeowind pattern inferred from the Hopeman Sandstone is very similar to that inferred from early Triassic reversing dune deposits in E Greenland, suggesting the existence of a regional Trade Wind system in the period near the Permo-Triassic boundary.
Aeolian sedimentation from the Galtymore Formation (Devonian), Ireland
Authors: Carruthers R. A. 
Published: April 2008

Summary
The Galtymore Erg (Devonian) of the Galtee Mountains, southern Ireland can be reconstructed by the coupling of field relationships with current hypotheses and observations from other ancient aeolian sequences and modern ergs. Stratigraphical relationships show that the Galtymore Erg formed at the northern, fault-controlled, margin of the Munster Basin. Northern sequences, close to the basin margin, are dominated by northerly derived, proximal braided stream deposits. Two types of aeolian-dominant sequences occur within the Galtymore Formation. A marginal erg facies, characterized by simple crescentic dunes showing S-N palaeocurrents is interbedded with fluvial deposits. To the S an erg facies is developed which is characterized by the vertical repetition of cosets of large-scale cross-strata and thin horizontal deposits. The cosets are interpreted as representing the downwind migration and climbing of large dunes and interdune areas, respectively. The geometrical relationship of the hierarchy of bounding surfaces suggests that the dunes of the erg facies were compound crescentic in form, with enclosed interdune areas. The distribution of stratification types and foreset dip dispersion similarly suggest crescentic dunes. Estimates of compound dune wavelength and some dune and interdune dimensions can be made. Crescentic dunes (simple and compound) were moved by south-southeasterly palaeowinds towards the foot of alluvial fans.
Sand sheets of the eastern Sahara and ripple blankets on Mars
Authors: Breed C. S. , McCauley J. F. , Davis P. A. 
Published: April 2008

Summary

Landsat
images of the eastern Sahara (North Africa) show widespread sandy plains covered by broad, gentle undulations (not dunes) that are interpreted as giant aeolian ripples. This regionally extensive bedform pattern embodies the entire upper unit of the Selima Sand Sheet. This unit, which we interpret as modern and presently active, contains very coarse sand, granules, and small pebbles, medium to very fine sand, and silt in horizontally laminated pairs that form a tabular deposit 1 cm to 10 m thick. It is commonly underlain by a stabilized sand sheet unit, and one or both units are disconformable on an erosion surface of truncated alluvium, mixed alluvial-colluvial-aeolian deposits, or bedrock. Sediment in the sand sheet is derived from alluvium, eroded mostly from sandstones in the Nubia Formation and widely distributed in dry, relict valleys. The upper surface of the sand sheet is a layer of pebbles and granules identical to coarse particles in the internal layers and in the truncated alluvial valley fill. Elongate, thin ripples composed of small pebbles are driven across this surface by prevailing north winds. Fine to medium sand is widely dispersed across the sand sheet from isolated dune fields. Silt particles are added seasonally by dusty winds (Khamsins) from the S. The Selima Sand Sheet may provide an analogue for some thin, horizontally-bedded aeolian sandstones. It offers an explanation for ripple blankets on Mars, where locally-derived coarse particles transported by surface creep and saltation are mixed with globally-derived fine particles deposited from dust storms.