bluuebunny/crossref_metadata_2025_split huggingface dataset:
Templates from mainland Africa and the Red Sea for interpreting the early evolution of the South Atlantic
Authors: Crossley R. , Cripps D. 
Published: April 2008

Abstract
Findings drawn from the Tertiary pre-rift, rift and post-rift sequences of Africa and the Red Sea are applied to the South Atlantic margins with a view of aiding hydrocarbon exploration. In relation to gravity and magnetic prediction of top basement, template rifts tend to follow pre-existing lines of weakness and the upper crust underlying these rifts may have geophysical characteristics which are different from those of the rift flanks. Template rifts may or may not be associated with volcanicity. In sparsely volcanic rifts, syn-rift volcanism is located on rift axis structural highs. Extensive volcanism is accompanied by regional doming and erosional loss of the pre-rift section is likely in rift sectors preceded by volcanism. Consequently, it is doubtful whether pre-rift sediments are preserved beneath the major volcanics offshore from the Parana-Etendeka areas in the South Atlantic. The Kalahari Group of southern Africa is identified as a good analogue for pre-rift reservoir objectives in the South Atlantic, and facies and thickness criteria are discussed that help discriminate pre-rift sequences from rift sequences. Where sedimentation has kept pace with rift subsidence, so that fault scarp facies have not developed, then the Okavango Delta provides a useful analogue. Parallels are drawn between major sand dykes interpreted from seismic in the deepwater deposits of Lake Tanganyika and similar units within similar facies in the Recôncavo Basin of Brazil. Direct analogue links between the Red Sea and the South Atlantic are less strong, mainly because marine incursions, and salt deposition, occurred at a later structural stage in the South Atlantic. The onset of salt deposition in the Gulf of Suez-Red Sea occurred whilst rift block faulting was still in progress, and the post-rift sag phase, though partly salt filled, rapidly turned to fully-marine deposition. The earliest part of the post-rift sag phase in the South Atlantic is represented by laterally extensive lacustrine source rocks. Salt deposition in the South Atlantic occurred only after full development of the sag phase.
Upper Cretaceous palaeoenvironments and benthonic foraminiferal assemblages of potential source rocks from the western African margin, Central Atlantic
Authors: Holbourn Ann , Kuhnt Wolfgang , El Albani Abderrazzak , Pletsch Thomas , Luderer Florian , Wagner Thomas 
Published: April 2008

Abstract

Upper Cretaceous benthonic foraminiferal assemblages, clay minerals, kerogen types and carbonate microfacies are examined in organic-rich sediments from western African coastal basins along a latitudinal transect extending from southern Morocco to Nigeria. Samples from onshore sections of the Tarfaya Basin, southern Morocco, from DSDP Site 369, from offshore commercial wells on the Casamance Shelf, Senegal, from DSDP Site 367, from ODP Site 959 and from onshore sections of the Benue Trough in Nigeria, are analysed. Organic-rich sediments from these sites are characterized by smectiterich clay mineral assemblages, high amounts of marine organic matter and a typical benthonic foraminiferal biofacies dominated by buliminids and bolivinids. Benthonic foraminifera provide robust proxy indicators of palaeoproductivity and organic matter flux to the seafloor. The palaeobiogeographic distribution of benthonic foraminiferal biofacies can be used to identify areas and stratigraphic intervals of hydrocarbon accumulation. Lower Turonian benthonic foraminiferal assemblages exhibit extremely low diversity and are strongly dominated by a
Gabonita
biofacies, in contrast to Coniacian to Maastrichtian assemblages, which display higher diversity and contain numerous species of
Buliminella, Praebulimina, Afrobolivina, Bolivina
and
Orthokarstenia
. The palaeobathymetric distribution of these high productivity benthonic foraminiferal biofacies reflects the establishment of a productivity driven oxygen minimum zone along the western African shelf, associated with an upwelling system active off coastal Africa during most of the Late Cretaceous. The extent of the high productivity equatorial belt appears to have shifted latitudinally during the Late Cretaceous as sea level and circulation patterns changed. The zone of highest productivity was broadest in the Early Turonian, when it coincided with the maximum sea-level rise and highest atmospheric temperature in the Late Cretaceous.
Tectono-sedimentary controls on Cretaceous black shale deposition along the opening Equatorial Atlantic Gateway (ODP Leg 159)
Authors: Wagner Thomas , Pletsch Thomas 
Published: April 2008

AbstractThe Cretaceous Equatorial Atlantic Gateway between the Central and South Atlantic basins is of interest not only for palaeoceanographic and palaeoclimatic studies, but also because it provided particularly favourable conditions for the accumulation and preservation of organic-rich sediments. Deposition of carbonaceous sediments along the Côte d’Ivoire-Ghana Transform Margin (Ocean Drilling Program Leg 159) was intimately linked to the plate tectonic and palaeoceanographic evolution of this gateway. Notably, the formation of a marginal basement ridge on the southeastern border of the transform margin provided an efficient shelter of the landward Deep Ivorian Basin against erosive and potentially oxidizing currents. Different subsidence histories across the transform margin were responsible for the development of distinct depositional settings on the crest and on both sides of the basement ridge. Whereas the southern, oceanward flank of the basement ridge was characterized by rapid, continuous deepening since the latest Albian-early Cenomanian, marine sedimentation on the northern, landward flank was interrupted by a period of uplift and erosion in the late Albian, and rapid subsidence started after the early Coniacian.Organic-rich sediments occur throughout almost the entire Cretaceous section, but hydrogen-rich marine black shales were exclusively recovered from core sections above an uplift-related unconformity. These black shales formed when separation of Africa and South America was sufficient to allow permanent oceanic midwater exchange after the late Albian. Four periods of black shale accumulation are recorded, some of them are correlated with the global oceanic anoxic events: in the latest Albian-earliest Cenomanian, at the Cenomanian-Turonian boundary, during the middle Coniacian-early Campanian, and in the mid-Maastrichtian. These periods were characterized by increased carbon fluxes to the seafloor, induced by enhanced palaeoproductivity and intensified supply of terrestrial organic matter. Black shale deposition appears to be intimately linked to periods of rising or maximum eustatic sea level and to the expansion of the oxygen minimum zone, as indicated by foraminiferal biofacies. Intervals between black shale units, in contrast, indicate a shrinking oxygen minimum zone and enhanced detrital flux rates, probably related to lowering sea level.Upper Cretaceous detrital limestones with high porosities may provide excellent hydrocarbon reservoirs, although their areal extent appears to be limited. Palaeogene porcellanites, capped by Neogene pelagic marls and clays, extend over a wider area and may provide another target for hydrocarbon exploration.
Miocene benthonic foraminiferal morphogroups in an oxygen minimum zone, offshore Cabinda
Authors: Preece R. C. , Kaminski M. A. , Dignes T. W. 
Published: April 2008

Abstract

Two hundred and seventy-eight benthonic foraminiferal species were recorded (126 agglutinating and 152 calcareous) from CABGOC 128-3, offshore Angola. This assemblage is comprised of a cosmopolitan fauna named the Agua Salada Fauna, known from the Caribbean, in the Vienna Basin, West Africa, Libya and Borneo. The fluctuating relative abundance of benthonic foraminiferal morphogroups in combination with ‘total organic carbon’ (TOC) data is suggested to reflect variation in the intensity of the oxygen minimum zone as it steadily increases upwards. An infaunal agglutinated ‘opportunistic’ (
r
-selected) group moves from antiphase into phase with TOC upwards in CABGOC 128-3. A morphologically comparative calcareous group displays the opposite trend, moving from phase into antiphase with TOC data. Planispiral agglutinated foraminifera emulate the steady increase in TOC upwards in CABGOC 128-3. This group includes
Cyclammina cyclops
, previously only recorded in the Arctic. Elongate, flattened calcareous morphologies undergo a sudden expansion towards the top of the well and are apparently unaffected by TOC. This overall trend is supported by concurrent poor preservation amongst distinct calcareous groups susceptible to dissolution, as expected in an area of dysaerobic conditions. The disappearance of all agglutinated faunas in the late Miocene is considered to be a reflection of oceanographic change.
Onshore equivalents of the main Kudu gas reservoir in Namibia
Authors: Stanistreet I. G. , Stollhofen H. 
Published: April 2008

Abstract
Radiometric dates of Jurassic and Cretaceous flood basalts of the southern South Atlantic maritimes have thrown the stratigraphic contexts of Karoo and post-Karoo sequences into considerable question. Central to the problem is the status of volcano-sedimentary units, characterized by aeolian-related sedimentary interlayers, containing evaporites and other lacustrine elements. The main Kudu gas reservoir (‘Lower Gas Sand’) offshore Namibia, interleaved with undated flood basalts, is a prime example. A previously proposed onshore correlate is the fluvio-aeolian Etjo Sandstone, with a central Namibian type area, and comparisons have been made, particularly with units traditionally classified as Etjo Sandstone, near the Etendeka Plateau base in northwest Namibia, now recognized to be of younger age. We time-correlate the main Kudu Sandstone reservoir, however, with evaporitic fluvio-lacustrine and aeolian interlayers beneath and interleaved with Early Jurassic Kalkrand flood basalts of southern Namibia, situated onshore almost directly opposite the Kudu Field. The main Kudu Sandstone reservoir is transitional in palaeoenvironmental setting between Early Jurassic onshore analogues. Main Kudu reservoir geometries and associated lithologies can be better constrained with knowledge derived from Kalkrand sedimentary interlayers and their tectonic and volcanic controls. Kalkrand interlayer thicknesses are controlled primarily by down-to-the-east extensional fault system half-graben geometries, antithetic to an inferred down-to-the-west master detachment. Within resulting sub-basinal accommodation space, lava topography controlled thickness variation. When Kalkrand insights are taken into account with radiometric dating and biostratigraphy, a Namibian Mesozoic framework fixes Kudu reservoirs and analogues into a southern Gondwanan context. At least in its type area in central Namibia, the Etjo Formation is Early Jurassic, discounting a previously proposed widespread <100 Ma hiatus along a sub-Etjo disconformity. Instead, sequence boundary hiatuses of Permian-Triassic, Early Jurassic and Middle Jurassic-Early Cretaceous ages represent time-stratigraphic gaps, relating to major faulting episodes, recording extensional events associated with continental margin evolution. Understanding onshore analogues and refining the Namibian Mesozoic framework should help constrain seismic analysis of the Namibian margin and hydrocarbon reservoir geometries. Consequently, exploration focus should widen to include Karoo sequences beneath the basal Early Cretaceous transgression.
Are gold deposits in the crust fractals? A study of gold mines in the Zimbabwe craton
Authors: Blenkinsop Tom G. , Sanderson David J. 
Published: April 2008

Abstract

The hypothesis that gold deposits in the earth are fractal dusts is tested using data on gold mine distribution in the Zimbabwe craton. Mine distribution reflects deposit distribution only indirectly because of undiscovered deposits and economic factors. Nevertheless, box counting shows an approximately linear relationship between the logarithm of the number of squares necessary to cover all mines in the craton and the logarithm of the square size between 10 and 60 km, indicating a fractal distribution. More detailed data from two areas within the craton have a similar relationship between square sizes of 2.5–20 km. Departure from the ideal fractal relationship is termed ‘roll-off’, and is very satisfactorily modelled by random sampling of a fractal dust. The results are consistent with the hypothesis that gold deposits are fractal dusts with fractal dimensions of
c.
1.0 and the distribution of mines can be regarded as a random sample of the distribution of deposits. Making this assumption, regions that have been incompletely but randomly explored can be identified and distinguished from regions containing large subareas that have been completely unexplored, and appropriate exploration strategies can be devised in either case.
Nickel ore troughs in Archaean volcanic rocks, Kambalda, Western Australia: indicators of early extension
Authors: Brown M. A. N. , Jolly R. J. H. , Stone W. , Coward M. P. 
Published: April 2008

Abstract
The Kambalda nickel sulphide deposits are located in the 2700 Ma Eastern Goldfields Greenstone Belt, Yilgarn Craton, Western Australia. The main lithologies are basalts and komatiites. Nickel sulphide ore occurs within the komatiite at the contact with the underlying basalt. This contact is structurally complex, the result of several phases of compressive deformation that post-date ore formation. Ore is confined to trough structures at the basalt-komatiite contact, these may be tens to hundreds of metres wide, several kilometres in length and tens of metres deep. Troughs are subparallel to each other and to the regional structural trend, ore bodies have a characteristic ribbon-like geometry. Various models have been proposed to explain trough formation but none account for the variety of features seen in the area. A new model is presented explaining the Kambalda structures by analogy with present-day graben structures associated with Icelandic-style fissure swarms. It is proposed that the initial geometries of the Kambalda trough structures, prior to compressive deformation, were similar to the present-day structures seen in Iceland. Komatiitic lavas subsequently exploit this graben topography, resulting in distinctive orebody geometries. Thus, it is suggested that the Kambalda troughs reflect an early phase of extension in the region.
The Mediterranean Basins: Tertiary Extension within the Alpine Orogen — an introduction
Authors: Jolivet L. , Frizon de Lamotte D. , Mascle A. , Séranne M. 
Published: April 2008

AbstractThe recent evolution of ideas on the Mediterranean region has been triggered by very active data acquisition over the last 15 years. Seismic tomography provides an unique view of mantle heterogeneities, space geodesy leads to precise determinations of the present strain and velocity fields, the combination of structural geology, radiometric dating and metamorphic petrology allows the description ofP-T-t-Dpaths of exhumed metamorphic rocks, and exploration geophysics, onshore and offshore, gives a detailed view of the crustal geometry. Extension started in the Gulf of Lion and propagated eastwards and southwestwards to form the Liguro-Provençal basin, Tyrrhenian Sea and the Alboran Sea. It also started, at much the same time, in the Panonnian basin as well as in the Aegean back-arc region. Thus a seminal event occurred some 30 Ma ago that produced a sharp change from overall compression to back-arc extension. Although gravitational forces due to the collapse of a thick crust have affected most basins, it is now almost certain that this event ultimately originated in the mantle, either by slab detachment, slab rollback or both processes acting in sequence.
The Gulf of Lion continental margin (NW Mediterranean) revisited by IBS: an overview
Authors: Séranne Michel 
Published: April 2008

AbstractThe Gulf of Lion margin is one of the Tertiary extensional basins of the western Mediterranean that opened during convergence of Africa and Europe. This Oligocene-Aquitanian rifted margin and associated Burdigalian oceanic basin have been used as case study for stretching models of ‘Atlantic-type’ margins. However, when the Integrated Basin Study (IBS) project was initiated, several outstanding questions remained about the present structure and the geodynamic setting of the margin within the Western Mediterranean. IBS-Gulf of Lion research was based on the existing onshore and offshore, industrial and academic data, which were heterogeneous and unevenly distributed. Compilation of the stratigraphic correlations on a regional scale allowed precise calculation of the timing of rifting, and clarification of the relationships with Alpine and Mediterranean geodynamics. Reprocessing of the existing ECORS deep seismic reflection profiles shed new light on the extensional structure and mechanisms of extension of the continental margin. Structural and sedimentological studies onshore led to the definition of new tectonostratigraphic models for extensional basins. Results of structural analyses showed a partitioning of the extensional deformation processes across the continental margin. 3D gravity modelling of the margin and basin area led to the production of a new map of the Moho depth by inversion, and testing several hypotheses for the origin of the present day subsidence. Although the Gulf of Lion margin displays structural and stratigraphic features similar to ‘Atlantic-type’ margins, its structure and evolution corresponds to that of a rifted margin of a large continent formed during the opening of a marginal basin. Integration of the new results of IBS-Gulf of Lion within the geodynamic evolution of the western Mediterranean suggests that the Oligocene rifting of the Gulf of Lion represents the initial stage of a succession of rifting events and back-arc basin formation, due to continuously retreating subduction during convergence of Africa and Europe.
Constraints on the Neogene Mediterranean kinematic evolution along a 1000 km transect from Iberia to Africa
Authors: Vergés J. , Sàbat F. 
Published: April 2008

Abstract
We present in this paper a simple kinematic model to illustrate the evolution of the western Mediterranean since upper Oligocene times. The five-step mass balanced reconstructions of the region are constructed along a 1000 km long crustal-scale transect from Iberia to Africa across the València and Algerian basins. The model uses constant divergence and convergence rates and fits the geological and geophysical data well along the transect. The amount of convergence and divergence along the studied transect can be resolved by the combination of two tectonic mechanisms: the northern motion of the African plate and the southwards retreat of a subducting Tethyan oceanic slab located between the pre-Neogene Iberian and African margins.

The evolution of the western Mediterranean region during the last 30 Ma can be summarized as follows: (a) pre-Neogene (late Oligocene) Iberian and African margins separated by a north-dipping fragment of Tethyan oceanic crust; (b) widespread extension affecting the Iberian margin developing the València trough and the Algerian basin from upper Oligocene to the top of Langhian; (c) end of the formation of the València trough and the collision between the Kabylies and Africa domains at
c.
13 Ma; (d) end of the Algerian basin opening and northern Kabylies and Tellian thrust systems coupled tectonic domains in the upper Tortonian (8 Ma); (e) shortening within the whole domain by the northwards motion of Africa after late Tortonian times. The propagation of stress within the western Mediterranean region seems to affect the whole region as far north as the northern side of the Pyrenean range located a distance of 1200 km from the south Atlas front, the southern limit of the system.
Interaction between faulting, drainage and sedimentation in extensional hanging-wall syncline basins: example of the Oligocene Matelles basin (Gulf of Lion rifted margin, SE France)
Authors: Benedicto A. , Séguret M. , Labaume P. 
Published: April 2008

Abstract
A tectono-sedimentary model for hanging-wall syncline basins emphasizing the role of erosion in the basin architecture is proposed, based on the well-exposed Matelles basin. Detailed geological mapping is the base for the analysis of the extensional double ramp-flat Matelles fault system, hanging-wall deformation and relationships between faulting/drainage/sedimentation. Displacement of the hanging-wall above the fault resulted in the formation of (a) upper and lower rollovers associated with the upper and lower rampflat couples, (b) a hanging-wall ramp syncline above the lower ramp and (c) a half graben associated with the emergent ramp and upper flat couple. Only the hanging-wall ramp syncline and the lower rollover are preserved. Ramp syncline basin-fill architecture is characterized by progressive unconformity and faultward migration of the growth synclines ‘climbing up’ the monoclinal prerift strata corresponding to the ramp-side limb of the ramp syncline. Clast composition analysis of the synrift deposits indicates that sediments were mainly supplied from the hanging-wall flat, and allows to propose palinspastic reconstructions of the hanging wall in order to establish the palaeo-drainage system and its evolution. ‘Upper hanging-wall flat catchments’ and ‘lower rollover catchments’ are proposed as main features of the drainage system in extensional hanging-wall syncline basins.
New insights from
40
Ar/
39
Ar laserprobe dating of white mica fabrics from the Pelion Massif, Pelagonian Zone, Internal Hellenides, Greece: implications for the timing of metamorphic episodes and tectonic events in the Aegean region
Authors: Lips A. L. W. , Wijbrans J. R. , White S. H. 
Published: April 2008

Abstract

40
Ar/
39
Ar laserprobe dating of mylonites fabrics from the Pelion Massif in the Pelagonian Zone of mainland Greece has characterized its Mid-Late Alpine deformation history. Following high pressure (HP) metamorphism, ductile deformation occurred under greenschist-facies conditions from
c.
54 Ma, and continued to affect the Pelion Massif until
c.
15 Ma. The prolonged episode of ductile deformation in the Pelion Massif has resulted in the formation of an Oligocene-Early Miocene ductile domal structure. The new geochronological data obtained for the Pelion contribute to a detailed record of the Alpine kinematic history in the Pelagonian Zone and allow a discussion of
P-T-t
data from Aegean HP rocks to characterize the regional thermotectonic history. Comparison with the
P-T-t
data from the Cycladic region reinforces the point that Mid-Eocene phengite ages, commonly taken as the age of peak HP metamorphism in the Cyclades, do not always reflect the metamorphic culmination, but rather the retrograde paths of the HP rocks. It is shown that, on a regional scale, termination of HP metamorphism is a diachronous process in the Aegean region, being
c.
54 Ma in the north (Pelagonian Zone) and shifting to younger ages, chiefly
c.
40 Ma in the Cyclades, and
c.
20 Ma on Crete, as the present-day subduction zone is approached. In contrast to the diachronous exhumation of Aegean HP assemblages, the well documented Miocene phase of ductile regional extension appears to be synchronous across the whole Aegean region and affected basement rocks until
c.
15 Ma.