bluuebunny/crossref_metadata_2025 huggingface dataset:
Physical and seismic modelling of listric normal fault geometries
Authors: McClay K. R. , Waltham D. A. , Scott A. D. , Abousetta A.
Published: April 2008

Scaled analogue models of listric fault systems display a characteristic rollover anticline and crestal collapse graben features. Faults nucleate in sequence into the hangingwalls of the crestal collapse graben and towards the footwall of the master fault. Superposition of crestal collapse grabens produces complex fault arrays. When the basal detachment surface is tilted a characteristic panel of unfaulted synfault stratigraphy forms adjacent to the detachment. These experiments produce structures with close similarities to listric growth fault systems on prograding delta slopes. Synthetic seismic sections of the experimental models have been produced using Kirchhoff integral techniques. These illustrate the complexities in trying to image complex structures in listric fault systems but also show good agreement with natural seismic examples.
Geochemistry and provenance of Rhenohercynian synorogenic sandstones: implications for tectonic environment discrimination
Authors: Floyd P. A. , Shail R. , Leveridge B. E. , Franke W.
Published: April 2008

The provenance of synorogenic greywackes from Devonian flysch successions occupying structurally similar positions at opposite ends of the Rhenohercynian zone in SW England and Germany is evaluated. Greywackes from both regions are petrographically and chemically similar, although minor differences are seen in the relative proportions of lithics; the Gramscatho group being richer in volcanic and metavolcanic clasts, whereas the Giessen group is richer in metasedimentary clasts. Absolute abundances of Ni-Cr-V and Zr-Hf-Y vary to a limited extent in each group and reflect variable mafic detritus and heavy mineral inputs respectively. Framework mode parameters and chemical data indicate the Rhenohercynian greywackes were derived mainly from a calc-alkali, acidic, dissected continental arc source, with minor MORB-like and argillaceous metasedimentary components. Upper continental crust-normalized multi-element patterns for the greywackes are characteristic of the continental arc/active margin tectonic environment. However, ‘Mid-Proterozoic’ model Nd ages for the Gramscatho greywackes suggest that an active Devonian arc source is unlikely. Instead the range of chemical and isotopic composition displayed mainly reflects mixing between acidic arc terranes of Proterozoic age and Devonian (Lizard-type) oceanic crust. Petrographic and geochemical discrimination diagrams alone cannot resolve the temporal decoupling between source and basin and may lead to an erroneous interpretation of tectonic setting.
Anoxia in the Northern Adriatic Sea: rapid death, slow recovery
Authors: Stachowitsch Michael
Published: April 2008

The Northern Adriatic Sea is characterized by the full range of features associated with sensitive marine ecosystems. In the Gulf of Trieste, mortalities due to anoxia have been reported in 1974, 1983 and 1988. The anoxic event in September 1983 led to mass mortality in a highly developed and wide-ranging community in the Gulf. This destroyed over 50% of the epifaunal biomass in only two days, and over 90% within four days.

The increased frequency, severity, and range of recent oxygen deficiencies suggest that, in addition to meteorological and hydrographic factors, anthropogenic influences are a contributing or enhancing factor in the onset of anoxic conditions and associated mass mortalities. These anthropogenic influences may be subsumed under the heading eutrophication. If eutrophication only slightly increases the rate of oxygen decrease below the thermocline, it may advance the point at which critical oxygen levels are attained by several days. Since only a few days of anoxic conditions are required to destroy this benthic community (with an equally short time required for reoxygenation) the degree of eutrophication may determine whether an ecological catastrophe occurs or not.

The recolonization process between 1984 and 1987 has been slow: the total biomass and the relative contribution of the designating species have remained distinctly below previously recorded values. Additional anthropogenic influences (intensive dredging) may further impede recolonization. A further, small-scale mortality was registered in 1988.
Sedimentary and geochemical expressions of oxic and anoxic conditions on the Peru Shelf
Authors: Emeis Kay-Christian , Whelan Jean K. , Tarafa Martha
Published: April 2008

Sediments from drill hole transects across the Peru shelf and upper slope show that the extent and intensity of the oxygen minimum zone (OMZ) has varied considerably from the early Pleistocene to the present. Primary sedimentary features, such as laminations and bioturbation, and the hydrogen index of sedimentary organic matter, appear to covary with the presence and intensity of the OMZ. Sediments deposited under dysoxic to anoxic bottom-water conditions are significantly more hydrogen-rich relative to bulk TOC. The molecular composition of pyrolytic hydrocarbons and heterocompounds, however, is surprisingly uniform, and is indicative of neither oxic nor anoxic degradation at either the sediment-water interface or in the water column. The concept of better preservation of organic matter under anoxic conditions in the bottom water is reflected in the bulk parameter of pyrolyzable organic matter content relative to TOC, but not in individual compound classes incorporated into kerogen. The most important control on the hydrogen-richness of organic matter appears to be sediment reworking and redeposition.
Pelletal black shale fabrics: their origin and significance
Authors: Cuomo M. Carmela , Bartholomew Paul R.
Published: April 2008

Recent work has demonstrated that, under certain conditions, laminated mudstones may be produced from the compaction of highly pelletized, organic-rich sediments occupied by populations of marine benthic worms. Such sediments can be found at the most developed part of the dysoxic region in many modern minimum zones. In order to reconstruct this facies accurately in the rock record, it is necessary to discern the true origin of pellets (benthic or planktonic) present in black, laminated shales.

Petrographic techniques, electron microscopy and microprobe analysis were employed on modern benthic and planktonic faecal pellets in order to arrive at inorganic geochemical criteria useful for discriminating between them. Benthic pellets contained more of those elements found in the fine fraction of the sediments (Al, Mg, K) while planktonic pellets contained high amounts of those elements (Si) representing the inorganic remains of the food they consume.

These criteria were successfully applied to pellets and their enclosing sedimentary matrices in ancient laminated black shales. Systematic identification of the origins of pellets found in laminated black shales should allow for detailed identification of palaeooxygen gradients within ancient depositional environments and may reveal that truly anoxic and azoic marine conditions were much less widespread than is commonly thought.
Thin widespread Pennsylvanian black shales of Midcontinent North America: a record of a cyclic succession of widespread pycnoclines in a fluctuating epeiric sea
Authors: Heckel P. H.
Published: April 2008

Within the Middle-Upper Pennsylvanian cyclic sequence of Midcontinent North America (Iowa, Nebraska, Missouri, Kansas, Oklahoma), there are at least 20 horizons of thin (<1 m) phosphatic black shales that extend up to 600 km along outcrop. They overlie transgressive limestones that range from very thin (1–5 cm) shell beds where they lie above coals, to thin (1 m or less) skeletal calcilutites where they lie above fluviodeltaic deposits and palaeosols. The black shales underlie thicker (2–10 m) regressive limestones that are characterized by various types of shallowing-upward sequences and are typically capped by palaeosols on the northern shelf (Iowa to central Kansas). The black shales represent the deepest-water deposits within the succession of widespread eustatic inundations that were responsible for the major cyclothems. Their frequency in the cyclic sequence is estimated to be close to that of the longer of the earth’s orbital parameters that are considered to control the wax and wane of continental glaciation. They apparently formed beneath pycnoclines that were established during interglacial high-stands of sea level when the water was both warm enough (at the surface) and deep enough to maintain a thermocline for perhaps several thousand years. These thermoclines were probably augmented by haloclines in areas of greater fresh-water run-off and possibly in areas accessible to more saline bottom water.
Biofacies, stratigraphic distribution and depositional models of British onshore Jurassic black shales
Authors: Wignall Paul B. , Hallam Anthony
Published: April 2008

The majority of British Jurassic dysaerobic and anaerobic facies (black shales) accumulated in rapidly subsiding basinal areas. Exceptionally, during the earliest stages of transgression, organic-rich facies expanded their distribution into more marginal areas. The biofacies of the black shales display a range of species richness values which, when ranked, record an oxygen gradient that is also reflected by gradients in geochemical and sedimentological data. Truly anaerobic facies (no benthic taxa present) are very rare in the British Jurassic. More common are fissile organic-rich shales with discrete, low-diversity horizons of benthos here defined as lower dysaerobic biofacies. These are interpreted to have formed in a generally anoxic environment punctuated by benthic colonization events in response to ephemeral improvements of oxygen levels. Upper dysaerobic biofacies are characterized by slightly higher diversity faunas scattered throughout poorly fissile strata.

The favoured depositional model for the British Jurassic black shales attributes the high preservational values of organic carbon to accumulation in oxygen-deficient waters trapped beneath a stratified water column in the deepest, basinal areas. We propose an ‘expanding puddle’ model for transgressive black shales. Deep water conditions are envisaged to become proportionally more extensive during early transgression due to the combined effects of subsidence, a rapid rate of sea-level rise and, unique to this portion of the sea-level curve, sediment starvation.
Organic matter and palynomorphs in the ‘Posidonienschiefer’ (Toarcian, Lower Jurassic) of southern Germany
Authors: Prauss Michael , Ligouis Bertrand , Luterbacher Hanspeter
Published: April 2008

The low δ
13
C values observed in beds with high TOC and high hydrogen index in the Toarcian oil shales of the ‘Posidonienschiefer’ (Lower Jurassic) correlate with palynomorph assemblages composed largely by prasinophyte phycomata and sphaeroidal, coccal palynomorphs of ambiguous affinity (‘sphaericals’). The macerals of the same beds are dominated by lamalginite and bituminite. The liptodetrinite content is very variable. Vitrinite, inertinite and sporinite macerals are rare. This correlation is consistent with the model proposed by Riegel
et al.
for the ‘Posidonienschiefer’ of northwestern Germany. The accumulation and preservation of the organic matter were favoured by a density stratification of the water column caused by the influx of low-salinity water. The paleogeographic setting and the affinity of dinocyst assemblages in the Proto-Arctic and in W Europe during Pliensbachian/Toarcian times suggest a high-latitude origin of this lowsalinity water.

Hydrogen indices obtained from Rock—Eval analyses indicate Type-II kerogen for most of the ‘Posidonienschiefer’ oil shale. Reflectance values of vitrinite and bitumen in oil shales are significantly suppressed by the presence of high amounts of liptinite.

The maturity of the organic matter determined by vitrinite reflectance and by Rock—Eval pyrolysis corresponds to the start of the oil window.
Glauconitization episodes in marginal settings as echoes of mid-Cretaceous anoxic events in the Vocontian basin (SE France)
Authors: Bréhéret Jean-G.
Published: April 2008

The formation of mid-Cretaceous glaucony-rich beds on the margins of the Vocontian (SE France) is correlated with the black shales intervals in the adjacent basinal facies. These black shales are a local expression of widespread oceanic anoxic events during the late Lower Aptian, Lower and Middle Albian, and late Upper Albian. An interfingering of glaucony-rich beds and black shales may be seen in some Lower Albian sections in outer shelf facies. Furthermore, one of the most important black shale horizons (the Paquier Level) contains numerous thin layers rich in glaucony grains that often developed at the expense of in situ faecal pellets. This autochthonous origin implies a direct relationship between the laminated black shale facies deposited under anoxic conditions, and glaucony as a subsequent diagenetic product formed under the influence of slightly oxygenated waters. Mineralogical and chemical data acquired on green grains characterize ‘evolved’ assemblages rich in micaceous glauconite, suggesting that the Paquier Level represents a condensed interval. These observations may have considerable significance for mid-Cretaceous sequence stratigraphy and palaeoceanographic investigations.
Micropalaeontological and geochemical evidence of mid-Cretaceous dysoxic-anoxic palaeoenvironments in the Sergipe Basin, northeastern Brazil
Authors: Koutsoukos E. A. M. , Mello M. R. , de Azambuja Filho N. C.
Published: April 2008

Combined micropalaeontological, geochemical and sedimentological studies of the mid-Cretaceous (upper Aptian to Turonian) succession of the Sergipe Basin, northeastern Brazil, indicate that intermittent dysoxic-anoxic events occurred in the basin, most likely associated with the interplay of processes such as restricted physiography in the deep basin, salinity-stratified water masses, increased epipelagic primary productivity and high sea levels. Three maxima in oxygen depletion are recorded in the succession from middle neritic to upper bathyal environments: the late Aptian-earliest Albian, the early Cenomanian, and the latest Cenomanian-earliest Turonian. Improving oxidizing conditions (dysoxic to oxic) are apparent during middle-late Albian and early-middle Turonian times. The existence of a rich and diverse planktonic biota throughout the mid-Cretaceous succession suggests widespread oxygenated epipelagic layers of variable thickness in space and time in the basin.
Modelling of secondary migration and entrapment of a multicomponent hydrocarbon mixture using equation of state and ray-tracing modelling techniques
Authors: Sylta Øyvind
Published: April 2008

Equation of state modelling of multicomponent hydrocarbon mixtures has been incorporated into a program which models the secondary migration of oil and gas. Migration is modelled by means of a ray-tracing technique which operates on backstripped depth maps of a carrier bed. Reservoir beds are represented as rectangular grids (arrays) in the computer, ensuring the high resolution required for proper secondary migration modelling. The resulting software, named SEMI, can be calibrated against the hydrocarbon composition in drilled traps and then used to compute the composition in undrilled prospects. This is especially useful in moderately explored basins where some discoveries have already been made. A model for the loss of hydrocarbons during secondary migration is utilized and, by modelling secondary migration within a synthetic basin, it is shown that compositional differences in traps may result from differences in loss of the vapour and liquid phases. Using a simple model for primary migration, it is shown that the primary and secondary migration losses influence the hydrocarbon composition in traps differently. Thus it is hoped that the proposed methodology can be used to quantify some of the unknown effects of migration and that a decrease in the quantified uncertainty in exploration for oil and gas may result.
Structural controls on fluid migration through the Rencurel thrust zone, Vercors, French Sub-Alpine Chains
Authors: Roberts Gerald
Published: April 2008

The Vercors, French Sub-Alpine Chains, is part of a classic foreland thrust belt which accommodates the last few kilometres of WNW-directed shortening in the Alps. The Rencurel thrust zone locally emplaces Barremian limestones onto Miocene sandstones. The thrust zone is well exposed allowing detailed structural logging and sample collection to be carried out. Calcite veins occur within gouge zones along minor faults occurring pervasively throughout the fault zone. However, at the base of the exposed fault zone, where a large displacement thrust has developed, the dominant vein fill is ferroan calcite, locally accompanied by bitumen. Cross-cutting relationships indicate that the pervasive set of minor faults developed early in the deformation sequence. The lack of ferroan calcite and bitumen within the early set of faults suggests that they were impermeable during later faulting. Fluid migration during deformation along the large displacement thrust was confined to underneath the impermeable zone of early minor faults. This study indicates that the style and location of deformation changes during the incremental development of a foreland thrust zone. Distributed deformation by mesoscale faulting or folding may be characteristic of deformation which occurs before the localization of displacement onto a major fault zone. Deformation probably occurs episodically with diffusive mass transfer and cataclasis as the dominant deformation mechanisms. Microstructures evolve during progressive deformation and this may induce changes in the permeability along a fault zone through time. These changes in the nature of fault zones during incremental deformation must be considered in order to assess fully the role of faulting in controlling syn-tectonic fluid migration.
Updoming, rifting and continental margin development during the Late Palaeozoic in northern Oman
Authors: Blendinger W. , van Vliet A. , Hughes Clarke M. W.
Published: April 2008

Subsurface data from northern interior Oman and new surface data from the Oman Mountains imply that the north Oman passive continental margin developed in the Permian. Late Carboniferous and Permian updoming of an area roughly overlapping the present Oman Mountains is indicated by the northward thinning of the continental clastics of the Haushi Group (Late Carboniferous and Early Permian) in the sub-surface. Well correlations show the thinning as cutouts at internal unconformities proving contemporary uplift along a roughly E-W strike. Deep water sedimentation off northern Oman commenced already in the early Late Permian. Permian deep water sediments of the Hawasina nappes consist of reef-derived sediment gravity flow deposits, cephalopod limestones and radiolarites that were deposited in a slope and basin floor environment north of a coralgal reef tract. Mafic volcanics underlying these deposits show locally tholeiitic characters suggesting that incipient drifting of the Hawasina ocean occurred as early as the Late Permian. Penecontemporaneous flooding of the Arabian shield occurred on an almost unfaulted substratum even in areas close to the inferred shelf edge.

The interpretation of these data in terms of models for continental rifting suggests that north Oman developed on the little faulted side of an asymmetrical rift produced by Permo-Carboniferous low-angle normal faulting of the lithosphere.
Sedimentary aspects of the Eo-Alpine cycle on the northeast edge of the Arabian Platform (Oman Mountains)
Authors: Rabu D. , Le Metour J. , Bechennec F. , Beurrier M. , Villey M. , Bourdillon-Jeudy de Grissac C.
Published: April 2008

The northeast edge of the Arabian Platform in the Oman Mountains is represented by the autochthonous unit that crops out in the two windows of Jabal Akhdar and Saih Hatat, beneath and in front of the Hawasina and Samail nappes.

The Eo-Alpine orogenic cycle began in the Late Permian with extension on the margin of Gondwana and ended in the Campanian with the formation of a subduction-obduction-type mountain range, the Oman Mountains of the Late Cretaceous. During this cycle, long periods of stable shelf sedimentation were interrupted by tensional tectonic episodes, often accompanied by volcanism and subsidence of the platform, passing into the bathyal domain.

Deposition during the Permian to Campanian occurred during five main sedimentary cycles. In the Late Permian (Murghabian), a widespread marine transgression covered the edge of Gondwana in a tensional setting. The ‘Fusulinid Sea’ transgression was followed by the deposition, lasting until the Triassic, of a thick regressive sequence, the Akhdar Group, which was terminated by emergence and weathering in a continental environment. The next sedimentary cycle, represented by the Sahtan Group, began in the Pliensbachian, when the Oman Mountains area formed an inner carbonate shelf, and ended in the early Tithonian. The entire margin of the platform was profoundly affected, in the late Tithonian, by extension, listric faulting and foundering into the bathyal domain, with deposition of the Kahmah Group (Maiolica-type deposits: calpionellid-bearing micrite), and rapid retreat of the continental slope by about 250 km. Renewed carbonate shelf deposition then prograded SSW-NNE across the foundered part, though this was still incomplete when the latest Aptian-Albian transgression took place. At the Cenomanian-Turonian boundary, the shelf underwent profound reorganization with the development of the Muti (intrashelf) Basin, in which the Aruma Group was deposited, and the Masqat-Musandam High which provided the detritus filling the Muti Basin on the continentward side and the Hawasina Basin on the oceanic side. Fragments from the advancing nappe were redeposited in the Muti Basin only from the Campanian on, and it was only during this period that a flysch trough became superimposed on part of the Muti Basin. The closure of the Muti Basin was affected with the emplacement of the nappes on the platform and the subsequent deposition of cover formations over both nappes and the autochthon from the late Maastrichtian onward.
The Lower Kahmah Group of Oman: the carbonate fill of a marginal shelf basin
Authors: Haan E. A. , Corbin S. G. , Hughes Clarke M. W. , Mabillard J. E.
Published: April 2008

Following the rifting and spreading related to the opening of the Tethys Ocean during the Triassic and Early Jurassic, relaxation and subsidence affected its passive margins. In Iran and Oman, such subsidence of the continental margins gave rise to deeper marine, marginal shelf basins, that initially were filled with pelagic sediments, but were progressively infilled by prograding shallow shelf-derived sediments. In Oman the fill sequence is well displayed for study in surface outcrops in the Oman Mountains as well as in subsurface geological and geophysical data. This paper provides an integrated picture of the geometry and facies distribution of the sediments, which filled this marginal shelf basin. The pelagic facies comprises the Rayda Formation and the basin, therefore, is named the Rayda Basin. The prograding, shelf-derived, fill facies comprises the Salil Formation. Surface-type sections of these two formations are described from outcrops in the Oman Mountains. The last stage of the basin fill is represented by a diversified shallow marine facies, referred to as the Habshan Formation, which is defined in Abu Dhabi. This unit prograded over the entire sag basin and almost levelled the pre-existing basin topography in such a way that overlying deposits hardly reflect the original Rayda Basin outline. The Habshan shoal facies is therefore in part coeval with the Rayda and Salil and exhibits a complex interdigitation with those two formations towards the basin margin. The seismostratigraphic expression of the Habshan, Salil and Rayda Formations provides a valuable tool for defining basin outline and mode of infill. A number of seismic lines are presented in relation to well log correlation panels to highlight the complexity of the sedimentary processes affecting these units. The fill of the basin was completed by the Valanginian and the ovelying Lekhwair Formation of Hauterivian to Barremian age represents a renewed marine transgression.
Manganese occurrences in the Al Hammah Range — Wahrah Formation, Oman Mountains
Authors: Kickmaier W. , Peters Tj.
Published: April 2008

The host rocks of the manganese in the Al Hammah Range (125 km S of Muscat) are red radiolarian cherts of Tithonian to Neocomian age, continuously exposed over 1000 km
2
. The stratigraphic sequence starts with fine-grained turbiditic limestones followed by a transition zone of coloured silt-mudstones and cherts which grade into almost CaCO

3
−

free cherts, topped by 1–5 m of silicified limestones. The base of the Wahrah Formation is unknown. Sedimentological and geochemical features indicate that the cherts are of biogenic, non-hydrothermal origin. The stratiform manganese deposits are the result of sedimentary and tectonic enrichment processes. The single Mn-layers formed during periods of condensed sedimentation at the sediment-water interface. The different Mn-chert types (disseminated and layered ore and black Mn-rich cherts) can be explained by varying sedimentation rates of radiolaria, clay and manganese precipitation. Geochemically, the manganese deposits are characterized by high Mn/Fe ratios and by an extremely low minor element (e.g. Ni, Cu, Co) content. Using geochemical discrimination diagrams, the manganese deposits of the Wahrah Formation would be of hydrothermal origin. This contrasts with the sedimentary characteristics of the manganese-bearing cherts and with the field observations. Because of the Tithonian to Neocomian age of the Wahrah Formation cherts, the manganese enrichment can not be related to the formation of the Semail ophiolite. It is proposed that they might relate to a long distance discharge from the Owen fracture zone during the formation of the Masirah ophiolite.
Origin of culminations within the Southeast Oman Mountains at Jebel Ma-jhool and Ibra Dome
Authors: Cawood P. A. , Green F. K. , Calon T. J.
Published: April 2008

Structural culminations within the southeast Oman Mountains at Jebel Ma-jhool and the Ibra Dome developed through a process of multistage thrusting. This involved assembly and emplacement of the Semail and Hawasina thrust sheets over a footwall sequence of shelf carbonates and their pre-Permian basement, followed by progressive footwall collapse and breaching of the Hawasina thrust stack producing a hanging-wall anticline cored by shelf carbonates at Jebel Ma-jhool. At the Ibra Dome, foreland- and hinterland-dipping thrusts developed during the latter stages of thrusting forming a pop-up structure. Structural relations at Jebel Ma-jhool provide conclusive evidence that culminations within the Hawasina Complex are related to a decollement surface which lies at least as deep as the level of the shelf carbonates. Formation of the Ibra Dome and Jebel Ma-jhool structures was part of a widespread re-imbrication and culmination forming episode within the Oman Mountains. Major culminations, such as the Jebel Akhdar—Jebel Nakhl—Saih Hatat structures form significant morphological as well as structural highs. Slippage of the upper level Semail Ophiolite and Oman Melange sheets off these structures has resulted in truncation of the underlying Hawasina Complex thrust stack at Ibra Dome and Jebel Ma-jhool producing a break-back thrust sequence. The timing of initial thrusting, subsequent re-imbrication and culmination formation, and gravity-driven slippage of surficial thrust sheets is constrained to a 5 Ma period between the late Campanian and early Maastrichtian. This is based on the Coniacian to Campanian age of Aruma Group strata, the youngest rock unit affected by thrusting and the presence of the Maastrichtian Qahlah Formation unconformably overlying the flanks of both the Jebel Ma-jhool and Ibra Dome culminations. Broad scale folding and high-angle faulting of Maastrichtian and Tertiary strata in the southern Oman Mountains formed through regional mid-Tertiary compression and reactivation of Late Cretaceous culminations.
Metamorphism in the Oman Mountains in relation to the Semail ophiolite emplacement
Authors: El-Shazly A. K. , Coleman R. G.
Published: April 2008

The metamorphic rocks associated with the Semail ophiolite in the Oman Mountains represent two different facies series. Amphibolites and greenshists at the base of the Semail ophiolite constitute a metamorphic sole formed in a high-temperature, lowpressure environment. Eclogites, blueschists, lawsonite schists and other metasediments occurring at different structural levels in the basement and shelf deposits of the continental margin formed at high pressures and low temperatures. The amphibolites of the metamorphic sole formed as a result of heat transfer from the hot overthrust ophiolitic slab to the underlying oceanic sediments and volcanics shortly following intraoceanic detachment at a collapsed spreading centre about 100–90 Ma. The underlying greenschists represent sediments and volcanics that were metamorphosed farther away from the spreading centre as the ophiolitic slab became cooler and moved closer to the continental margin. Eclogites, blueschists and crossite epidote schists formed in the Saih Hatat window as a result of A-type subduction (crustal thickening) of the basement and shelf units of the Oman continental margin in response to a change in plate motion between Africa and Eurasia and transpression or collision with a fragment of Gondwanaland about 131 Ma. A-type subduction along an east-dipping zone resulted in the burial of the eastern edge of the continental margin to a minimum depth of 30 km. High
P/T
metamorphic rocks formed by this process followed ‘clockwise’
P-T
paths. The Late Cretaceous emplacement of the ophiolite onto the continental margin of Oman resulted in the imbrication of the continental shelf deposits, the development of low-grade, high-
P/T
metamorphic rocks in the ‘internal’ thrust piles due to tectonic overloading and the juxtaposition of the metamorphic rocks representing different facies series. Extensional deformation associated with culmination collapse in the Late Cretaceous and Tertiary led to the exhumation of the various high
P/T
metamorphic rocks and the distortion of the metamorphic zones.
The Maastrichtian transgression onto the northwestern flank of the Proto-Oman Mountains: sequences of rudist-bearing beach to open shelf facies
Authors: Skelton P. W. , Nolan S. C. , Scott R. W.
Published: April 2008

The Maastrichtian Qahlah and Simsima Formations, the first autochthonous deposits on the obducted Oman ophiolite complex, crop out in a gridle of jebels around the Oman Mountains. Those around the NW flank, between Al Dhayd (Sharjah) and Al Ayn-Buraymi (Dubai-Oman) are documented to illustrate: (1) an exposed regional reference section for the widespread Simsima Formation, at Jebel Faiyah (Sharjah); (2) the marked transgressive nature of the sequence onto the obducted complex; and (3) the rich rudist faunas there, associated with various beach to shelf facies, and their palaeoecological and palaeobiogeographical implications.

‘Basement’ (serpentinites in the north and sedimentary nappes in the south) is overlain by locally developed laterites and the dominantly siliciclastic, marginal continental to marine deposits of the Qahlah Formation, followed, with local erosion and overlap, by the carbonate-dominated Simsima Formation. The latter continues the deepening trend, passing from basal rudist-rich inshore facies to open shelf foraminiferal packstones to wackestones. The whole sequence onlaps eastwards onto the obducted complex. Its truncated top is overlain by Palaeogene slope deposits.

In the north (Jebel Faiyah area) the Maastrichtian sequence has an open, relatively high energy aspect. A shelf margin of Simsima Formation rudist limestones briefly developed in the early Maastrichtian around Qarn Murrah. Further transgression to the southeast drowned this and, by ‘mid’-Maastrichtian times, a bouldery beach conglomerate of the Qahlah Formation, with
in situ Durania
, was forming at Jebel Faiyah itself, along a cliff-backed shoreline with sandy embayments. Over this came tidally influenced rudist-rich facies and foraminiferal limestones of the Simsima Formation. In the south (Jebel Huwayyah area), in contrast, marginal fan-delta gravels and sandstones, of early Maastrichtian age, were followed by more restricted shallow marine silty marls and limestones with
Loftusia
, rudists and corals, of early to mid-Maastrichian age. These Qahlah deposits were overlapped by a relatively rudist-poor Simsima Formation sequence.

The obducted complex was thus strongly emergent at the start of the Maastrichtian, and the Aruma Sea transgressed onto its western flank, as the ophiolite subsided through that stage, forming a relatively open shelf in the north, becoming broader and more restricted further south. The rudist assemblages found on this shelf are similar to those of other open Tethyan shelves in the eastern and central Mediterranean and Middle East.
The Batain Melange of NE Oman
Authors: Shackleton R. M. , Ries A. C. , Bird P. R. , Filbrandt J. B. , Lee C. W. , Cunningham G. C.
Published: April 2008

Most of the Batain coast area, previously unmapped or mapped as Hawasina, is shown to be underlain by melange. This consists of blocks, up to 10 km long, of a variety of rocks in a scanty, seldom exposed sheared shaly matrix. Clasts include Triassic and younger Hawasina-type red cherts and shales; Mesozoic calciturbidites; Permian limestones and megabreccias (associated with basalts);
ammonitico rosso
type blocks (Triassic?); a limestone and pillow lava assemblage, probably Jurassic; Cretaceous ophiolite; and Upper Cretaceous sandstones with basement-derived debris flows. Deformation becomes increasingly complex northwards. WNW-vergent thrusts and folds, dominant in the south, appear to have been continuous with the south-vergent structures in the Hawasina, west of the Jebel Ja’alan basement uplift. The melange is interpreted as primarily tectonic but probably composite in origin. It is correlated with the Hawasina (Oman) Melange which underlies the Semail Ophiolite, west of the Jebel Ja’alan Uplift. It is compared with other Tethyan melanges.
A new model for the late Palaeozoic—Mesozoic tectonic evolution of Iran and implications for Oman
Authors: Şengör A. M. C.
Published: April 2008

It has been suggested that in a number of places such as Greece, Turkey, the Central Pamirs, and Thailand Neo-Tethys may have opened as a back-arc basin above a Palaeo-Tethyan subduction zone. The purpose of this paper is to test a similar suggestion for Oman. I review the late Palaeozoic to end-Mesozoic tectonic evolution of the Middle Eastern Tethysides in terms of a new tectonic model, whose main tenet is to regard the late Palaeozoic to Late Triassic basements of the Pontide/Dzirula/Adzharia-Trialeti/Arvin-Karabagh/Sanandaj — Sirjan zones collectively as a NNE-facing Palaeo-Tethyan magmatic arc, here named the ‘Podataksasi arc’ (or ‘zone’) whose Jurassic—Cretaceous movement with respect to Eurasia was responsible for much of the coeval deformation in Iran and Transcaucasia. The late Palaeozoic deformation of the Omani basement is regarded as a part of the retroarc fold and thrust belt of this arc and therefore independent of the Hercynides in Europe and NW Africa. The arc may have been compressional in late Carboniferous to possibly early Permian time and turned extensional in the earlier middle Permian. As a result, it rifted from NE Gondwana-Land, opening, successively, the Hawasina basin in the middle Permian and the main Neo-Tethyan ocean in the Triassic, together forming the ‘Omani Neo-Tethyan back-arc basin complex’. Nearly all of the tectonic and magmatic characteristics of this basin complex are compatible with a back-arc basin interpretation, except perhaps the anomalously far inland location of the initial rift axis. A rather complete Mesozoic tectonic history of the entire Middle Eastern Tethysides is given to justify the model presented.
GLORIA and other geophysical studies of the tectonic pattern and history of the Easter Microplate, southeast Pacific
Authors: Rusby Ruth I.
Published: April 2008

In October–November 1988 the RRS Charles Darwin surveyed the Easter Microplate using the GLORIA (long-range sidescan) sonar system and also obtained magnetic, bathymetric and gravity data. The aim of the cruise was to produce a sidescan mosaic covering most of the microplate, in order that a greater understanding of the region could be achieved. Previous geophysical studies of the area had enabled most of the microplate boundaries to be determined to a reasonable extent, but still little was known about the interior of the microplate, or its tectonic history.

The major objectives of the GLORIA survey were to image the whole microplate including its boundary, discover the type of plate boundary element throughout (e.g. fast or slow spreading, transform or compressional zone), calculate the spreading directions and rotation poles, and determine the complex history of the microplate from the tectonic spreading fabric, fracture zone and propagating rift traces, and magnetic anomaly data.

The GLORIA survey imaged virtually all of the Easter microplate in just 18 days, and revealed unprecedented detail and complexity in its tectonic fabric and history. There is clear evidence of rapid microplate rotation, of rapidly evolving plate boundaries and of complex deformation near the northern and southern microplate boundaries. Examination of GLORIA and magnetic data to the west of the East Rift of the Easter Microplate indicate that spreading may have been taking place for over 6 Ma and the calculation of poles of rotation for different stages of the Easter Microplate evolution have enabled a simplified history to be reconstructed.
Along-axis variations in tectonic extension and accommodation zones in the MARK Area, Mid-Atlantic Ridge 23°N latitude
Authors: Karson Jeffrey A. , Winters Alec T.
Published: April 2008

Multidisciplinary studies of the slow-spreading (<30 mm a
−1
) ridge axis in the MARK Area define a number of discrete spreading cells in each of which the style of seafloor spreading is distinctly different. In some cells, episodic magmatism and asymmetrical detachment faulting occur, whereas in others, a more continuous magma supply and symmetrical, steep, normal faulting dominate. Oblique-slip fault zones that link the cells are a kinematic requirement of such along-strike variations and are similar to the accommodation or transfer zones of continental rifts.
Concretionary cements in Jurassic sandstones, Isle of Eigg, Inner Hebrides
Authors: Wilkinson Mark
Published: April 2008

Minor element and stable isotope data are presented for calcite cements within seven decimentre-scale concretions from a single bedding plane within the Middle Jurassic Valtos Sandstone Formation of Eigg. Stable oxygen isotope data (δ
18
O = −9.3 to −11.1‰) suggest precipitation at burial depths of between 200 and 300 m at temperatures of 31–46°C, within static pore waters of meteoric origin. Mg/Ca and Mn/Ca ratios (2.5 ± 1.2 and 0.062 ± 0.008 mole percent respectively) imply that growth was predominantly transport-controlled. An average sized concretion from the horizon (
r
= 11.8 cm) is predicted to have grown in 0.36–0.84 Ma. Stratigraphic constraints show concretion growth to have been delayed by at least 15 Ma after the deposition of the sediments.

The concretions are shown to be clustered within the bedding plane; concretion nucleation is interpreted to have been controlled by the availability of suitable substrates within the sediment. The concretions probably nucleated upon aragonitic
Neomiodon
shells which had previously transformed to calcite.
Evolution of the Carboniferous Lough Allen Basin, Northwest Ireland
Authors: Philcox M. E. , Baily H. , Clayton G. , Sevastopulo G. D.
Published: April 2008

The Lough Allen Basin contains approximately 2.9 km of Dinantian (Courceyan) to Silesian (Arnsbergian) limestone, mudstone and sandstone, mostly of marine origin. The basin was initiated during the Courceyan Stage as a result of movement of the basin margin fault complex along the southeast side of the Ox Mountains inlier. The main rifting phase, during the late Courceyan, resulted in the deposition of a southeasterly thinning, initially non-marine, clastic wedge. The intrabasinal Dowra—Macnean High and Slisgarrow Trough developed during the Chadian Stage. Movement on the Curlew fault, which bounds the basin to the south, may have begun in the Courceyan but culminated in the Arundian. There is little direct evidence of fault-controlled sedimentation in the Holkerian and much of the Asbian, although the distribution of carbonate mudmounds of Asbian age in part coincides with earlier structural elements. During the late Asbian, there appears to have been renewed tectonic activity, which led to substantial regional differences of thickness of the lower formations of the Leitrim Group. There is little evidence of tectonic control on sedimentation from the Brigantian to the Arnsbergian.

Vitrinite reflectance determinations show that the Carboniferous rocks in the basin are supra-mature for the generation of oil but are mature with respect to the generation of gas. The maturation values for the youngest preserved rocks suggest that they have been buried under 3–5 km of Upper Palaeozoic cover.
The post-Variscan history of ireland
Authors: Naylor David
Published: April 2008

Permian and Mesozoic rocks are of limited distribution in Ireland, except for the Ulster Basin in the northeast of the island. This contrasts with the offshore basins, the widespread Mesozoic and Tertiary sequences of which are briefly reviewed. Onshore Permo-Triassic red-bed rocks occur in the Kingscourt Graben and in Wexford. Jurassic red clays are recorded in karstic solution hollows in Carboniferous limestones in south County Cork, and Upper Cretaceous chalk in a karstic collapse feature at Ballydeenlea, near Killarney. A growing number of Tertiary non-marine fissure and collapse deposits are being found, and where dated are of Oligocene and Neogene age. The stratigraphy and setting of the post-Carboniferous outliers are discussed. Published organic maturation data suggest that high maturation values in the Carboniferous over much of the country can be attributed to high heat flows during the Variscan episode, and to a cover of Silesian rocks. Post-Variscan burial depths appear to have been modest.

Available evidence suggests that Ireland has been a positive and largely emergent feature for much of post-Variscan time. Permo-Triassic deposits were probably never widespread onshore. The Early Jurassic and Late Cretaceous were periods of marine submergence, although rocks of this age were subsequently largely removed by erosion. Low topographic relief and the protective layers of Lias and Chalk have combined to restrict the amount of erosion of the landscape, the essential elements of which had been etched out by the end of the Triassic. The scattering of Mesozoic and Tertiary deposits at low topographic levels argue against the traditional view of the Irish landscape as a product of erosion of a high-level peneplain.
Post-Permian basin history of northeast Ireland
Authors: McCaffrey Robert J. , McCann Noel
Published: April 2008

Basin histories and major structures for Mesozoic and Cenozoic strata in northeast Ireland have been constrained with the help of computer-contoured isopach maps. Hand-drawn maps are included for comparison. The Highland Border Ridge, a well defined structural element since at least Lower Carboniferous times, separates the Rathlin Trough from the Lough Neagh-Larne Basin. The development of these basins was controlled by the pre-existing structures of the area, fault reactivation during late Hercynian movements and also Pangean rifting. A long-lived structural high in the Lough Neagh-Larne Basin, the Knockagh axis, is inferred to be a major block detached from the adjacent Longford-Down Massif.
Lithospheric stretching in the Porcupine Basin, west of Ireland
Authors: White Nicky , Tate Michael , Conroy John-Joe
Published: April 2008

The Porcupine Basin is a N-S trending Mesozoic extensional sedimentary basin situated on the continental shelf west of Ireland. All available datasets have been examined in order to see how well the stretching model accounts for the structure and evolution of the basin. Subsidence data indicates that there have been two phases of stretching: at the end of the Jurassic and during the Palaeogene. The earlier phase is of more significance; stretching factors varying from 1.2 at the northern end of the basin to greater than six at the southern end. Stretching factors for the later phase are generally less than 1.1. Crustal thinning information, determined principally from gravity data, is consistent with the above estimates. The timing of volcanism within the basin is broadly consistent with these stretching phases. Significantly, the location of a 150 km long and 20 km wide median igneous ridge is broadly consistent with the steady increase in stretching southwards.
A syn-rift to post-rift transition sequence in the Main Porcupine Basin, offshore western Ireland
Authors: Moore John G.
Published: April 2008

The evolution of the Main Porcupine Basin can be divided into pre-rift, syn-rift and post-rift stages. The syn-rift/post-rift transition represents a gradual change from extensive fault activity through localized faulting and passive infill to sporadic faulting during post-rift times. This Transition Sequence is present in localized areas (sub-basins). It infills the remnant syn-rift topography after which the post-rift stage begins. Sub-basins developed during the syn-rift stage have a general N-S to NE-SW orientation and range in size from approximately 30 km × 20 km to approximately 12 km × 8 km. Their shapes range from symmetrical fault-bounded depocentres to half-graben. The sub-basins have several internal unconformities which merge into a single composite erosion surface at the margins. Areas in which the Transition Sequence is developed usually have an element of fault control, especially during the early stages. Clastic fans are identified prograding from fault scarps. Later passive onlap during a rising sea-level progressively infills the subbasins. The age of the Transition Sequence is problematical, but borehole data suggests that it spans uppermost Kimmeridgian/Tithonian to end-Berriasian times. The sub-basins appear to be broadly coincident with small Permo-Triassic basins. This may point to a deep-seated fault control on the location of the Transition Sequence sub-basins. Comparable syn- to post-rift Transition Sequences are also present in the Orphan and Jeanne d’Arc Basins, offshore Eastern Canada, and possibly in other North Atlantic rift basins also.