bluuebunny/crossref_metadata_2025 huggingface dataset:
Structural and metamorphic history of a segment of the Sesia-Lanzo zone, and its bearing on the kinematics of Alpine deformation in the western Alps
Authors: Ridley J.
Published: April 2008

The Sesia-Lanzo zone of the western Alps is a zone of heterogeneous crystalline basement that was reworked and remobilised during the Alpine orogeny, partly under eclogite-facies metamorphic conditions. Reworking took place in three separable phases of deformation, each under different metamorphic conditions and each localised predominantly within a particular zone in the crust. The structures and fabrics produced by the phase of deformation post-dating the early eclogite-facies event, and associated with glaucophane-albite parageneses, suggest a strike-slip tectonic setting. Within the Sesia-Lanzo zone, up to a few tens of kilometres of N-S strike-slip movement is inferred for the late Cretaceous-early Tertiary. Movement within a strike-slip system may have been responsible for much of the uplift of the eclogitic rocks from 60 km depth.
On the age of deep crustal extensional faulting in the Ivrea zone, northern Italy
Authors: Brodie K. H. , Rutter E. H. , Rex D.
Published: April 2008

High temperature fault zones in the Valle d’Ossola region of the Ivrea zone in northern Italy appear to accommodate extension of the deep crustal section.
40
Ar-
39
Ar age spectra indicate that this extensional event occurred prior to 280 Ma, and initiated a period of crustal thinning and cooling. Interpretation of the closure temperature in this system is dependent on grain size and cooling rate.
Kinematics of the western Mediterranean
Authors: Dewey J. F. , Helman M. L. , Knott S. D. , Turco E. , Hutton D. H. W.
Published: April 2008

The kinematic understanding of the relationship between relative plate motion and the structure of orogenic belts depends upon a knowledge of relative plate motion across the plate boundary system, the relative motion of small blocks and flakes within the system, an evaluation of orogenic body forces, and an understanding of the thermomechanical evolution of the upper part of the orogenic lithosphere in determining strength and detachment levels. We have built a preliminary model for the Cenozoic kinematic evolution of the western Mediterranean oceanic basins and their peripheral orogens that integrates (1) the motion of Africa relative to Europe based upon a new study of Atlantic fracture zones using SEASAT data and the Lamont-Doherty magnetic anomaly database, (2) a new interpretation of the rotation of Corsica/Sardinia and the opening of the Balearic and Tyrrhenian oceanic basins, (3) sedimentary facies sequences in the Apennines, Calabria, and Sicily, and (4) Apennine/Calabrian structure and structural sequence.
Alpine thermal evolution in the central and the western Alps
Authors: Hunziker J. C. , Desmons J. , Martinotti G.
Published: April 2008

In the western and central Alps, high-pressure assemblages dated as Eoalpine are distributed in the Pennine, Piemonte and Austro-Alpine units. The starting point of metamorphism must have been at low temperatures as both pre-Alpine metamorphic basement and Mesozoic sediments have been involved. In those parts of the Sesia-Lanzo zone where no later Tertiary high temperature overprint modified or obliterated the early Alpine history, the Eoalpine peak of metamorphic pressure was reached at about 110 Ma, the temperature peak at 85 Ma and a cooling down to 300°C at 60 Ma. The radiometric evidence gives insight into the Cretaceous orogenic events which also include penetrative deformation, nappe thrusting and uplift. The Pennine and Austro-Alpine units are both considered to have been located south of the Jurassic oceanic gap and to have formed parts of the fragmented northern margin of Gondwana.

The Tertiary Mesoalpine and Neoalpine metamorphic events in the western Alps are separated in time by 32–30 Ma magmatic intrusions. In the central Alps both events are distinguished on the basis of an additional schistosity connected with the second event. The Neoalpine ages are considered as resulting from the Mesoalpine thermal conditions outlasting the Mesoalpine time and, in the Simplon region, as recrystallization ages related to late mylonitization.

The convergence rates during the Cretaceous orogenesis were one order of magnitude greater than during the Tertiary orogenesis. Sedimentation in some Pennine units continued through Eoalpine times until Palaeogene; this is interpreted as resulting from the imbrication of blocks of different evolution in Tertiary times.
Unravelling the thermo-tectonic evolution of the Alps: a contribution from fission track analysis and mica dating
Authors: Hurford A. J. , Flisch M. , Jäger E.
Published: April 2008

Fission track analysis of apatite and zircon, and K-Ar and Rb-Sr dating of white mica and biotite provide time and temperature constraints for direct dating of deformation phases and in establishing cooling and uplift rates and patterns. In the western Alps, timing of the Sesia-Lanzo zone Eoalpine metamorphic temperature peak is confirmed between 110 and 85 Ma, possibly preceded by the pressure maximum. The slowly cooling Sesia zone reached 200–250°C by ∼33 Ma and ∼100°C by ∼25 Ma, with fast uplift during the late Oligocene to Miocene Insubric phase; cooling in the adjacent Gran Paradiso Massif occurred slightly later, with fission track zircon and apatite ages of 30±2 and ∼22 Ma respectively. No evidence to support an inverted metamorphism of the Gran Paradiso was found. In the Silvretta nappe (E Alps) three different uplift styles and periods are distinguished: uparching between 110 and 35 Ma, homogeneous uplift during 35 and 2 Ma and recent eastward tilting. The data imply that updoming of the Engadine Window occurred before ∼31 Ma. In the central Lepontine Alps, a uniform late Neogene to recent mean uplift rate of ∼0.5 mm/a succeeded early Miocene uplift at ∼2.2 mm/a in the southern steep belt (root zone), the latter resulting from a major phase of backthrusting of the central Alps along the Insubric Line. Clear-cut time-temperature paths are not found for the Suretta Nappe (central Alps): fission track data delimit the Miocene cooling, but speculative interpretation of phengite-mica and K-Ar amphibole ages suggests that the main deformation (possibly Ferrera phase) occurred between 55 and 36 Ma. Schematic models are advanced to describe the asynchronous thermal evolution of different parts of the central Lepontine Alps.
Time and place in Alpine orogenesis — the Fermor Lecture
Authors: Hsü K. J.
Published: April 2008

Alpine geologists have suggested that the main structures of the Alps have been formed by a late Eocene event, which lasted at most a few million years. Data on earlier and later deformations have been interpreted as the evidence of Eoalpine and Neoalpine phases. The supposedly episodic nature of the Alpine orogenesis has been considered to be a fundamental contradiction of the seafloor spreading which predicts slow and gradual movement.

The purpose of this contemplation on time and place in Alpine orogenesis is to point out that the paradox is man-made: the interpretation of the episodic nature of orogeny is a relic of the defunct catastrophic theory of orogeny of Elie de Beaumont (1831). Divorced from that discredited assumption, the orogenic data in the Alps are consistent with a postulate of continuous orogenic deformation since the early Cretaceous.

The Alps is a collision-type of orogen. Using the late Eocene collision-event as a time marker, we can recognize two major stages of deformation: the Eoalpine subduction, and the Neoalpine post-collisional. The geological evidence is consistent with the postulate of a gradual build-up of the Alpine edifice, consequent upon the approach between two continental blocks since the early Cretaceous and continuing deformation after the late Eocene collision.

The Eoalpine subduction should have started 130 Ma ago, some 50 Ma before the approach of the African and European Plates. This and other geological evidence necessitates an assumption of relative movements among 3 plates to explain the Alpine orogenesis; the middle Mediterranean Plate (M-plate) had movements independent of Africa since 130 Ma
bp
. The kinematics of plate displacements in the Alpine-Mediterranean region have been reconstructed, using the data on Atlantic magnetic lineations to fix the positions of Africa and Europe, and geological evidence to guide the movement of the M-Plate. This kinematic analysis, harmonized with geological data, indicates that the Alpine Orogenesis resulted from the relative movement between the European- and M-Plates at rates of about 1 centimetre per year. There is no contradiction between Alpine geology and the plate-tectonic theory.
Phanerozoic ironstones: an introduction and review
Authors: Young T. P.
Published: April 2008

Ironstones, particularly ooidal ironstones, have long fascinated sedimentary geologists and have generated an enormous variety of interpretations, but have remained poorly understood. A recent upsurge in interest in this group of rocks has generated much new information derived from many different disciplines within the earth sciences.

The first part of this introduction examines recent advances, and attempts to produce a synthesis of them, with particular reference to the genesis of marine ooidal ironstones. Current models for the formation of marine oolitic ironstone-formations are discussed, and a working model based on various lines of recent research is proposed. This model invokes the intrasedimentary formation of berthierine ooids in marine environments during post-oxic diagenesis. The ooids may be mechanically modified and mineralogically transformed by reworking, as well as by later diagenesis. Goethite ooids may be oxidized berthierine ooids, reworked lateritic ooids or primary grains. The formation of oolitic ironstone-formations is favoured by a break in clastic sediment supply, continued supply of iron and physical reworking. Many sedimentary environments may supply these parameters, but changes in sea level leading to the flooding of land masses and the reworking of suitable terrestrial soils may be both an important source for the iron and provide a break in clastic sediment supply.

In the second part of this introduction the terminology and classification are discussed, and conventions proposed. The term
berthieroid
is introduced as a non-specific term for material of undetermined berthierine or chamosite composition. The term
ironstone
is proposed as a petrological term and as an informal lithostratigraphic term. A deposit composed of
ironstones
may be termed an
ironstone-formation
. It is recommended that a ‘Dunham’ style of classification is employed petrographically, with -
ironstone
replacing the -
stone
of the original carbonate classification. The nomenclature of allochems in ironstones is also discussed. A glossary of ironstone terminology is provided.
Aspects of iron incorporation into sediments with special reference to the Yorkshire Ironstones
Authors: Spears D. A.
Published: April 2008

Ironstones of former economic value occur in both Coal Measures and Jurassic sequences in Yorkshire. The distribution of pyrite and siderite in the Coal Measures sediments is explained by the stability relationships between the two minerals and the controls on the rate of pyrite formation, particularly the sulphate availability, but also involving the amount and quality of metabolizable organic matter and the iron availability. In marine black shales early diagenetic pyrite formation may have immobilized much of the available iron, whereas in non-marine sediments iron was available over much longer diagenetic periods to form siderite in the mudrocks and pyrite in the coals. The qualitatively more important siderite forms clay ironstones by a process of diagenetic redistribution and concentration of original iron. In the marine Jurassic sediments pyrite and siderite again show a cyclic distribution. The same explanation holds for pyrite in the marine black shales, but in the grey shales loss of metabolizable organic matter under oxic conditions mainly allows post-oxic reactions, which includes reduction of iron but little sulphate reduction. In the oolitic ironstones efficient loss of organic matter in an important oxic zone is seen as a key factor. The oolitic ironstones do represent an original enrichment of iron in contrast to the other ironstones considered. Post-oxic reactions are thought to be important and produced berthierine and siderite. The possibility of some diagenetic enrichment of iron in the oolitic ironstones should be recognized.
Temporal patterns among Phanerozoic oolitic ironstones and oceanic anoxia
Authors: Van Houten F. B. , Arthur M. A.
Published: April 2008

The stratigraphic distribution of Phanerozoic oolitic ironstones corresponds to temporal patterns recorded in common detrital deposits. These patterns, in which ironstones share significant associations with black shales, comprise successively smaller-scale sequences, each defined mainly by major transgressions and regressions, changing rates of erosion and sedimentation, and inter-regional unconformities.

Two 300 Ma episodes of continental dispersal and Pangaeic assemblage of cratons encompassed the maximum rise and fall of sea level. Deposition of ironstones and black shales was essentially limited to 150–170 Ma phases (Ordovician-Devonian; Jurassic-Palaeogene) marked by extensive epicontinental seas and ineffective oxygenation of deep-water masses.

Smaller-scale tectonically distinct phases of cratonic submergence, several to many tens of Ma long, were separated by briefer erosional intervals, or by longer episodes of oscillating conditions during which several interregional unconformities developed. Widespread submergence generally favoured major production of ironstones and black shales. Within this framework both were especially common during the Ordovician and Jurassic periods. A hypothetical quasiperiodic (c. 32 Ma) pattern of global climate change was accompanied by recurring relatively high sea level and expansion of oxygen-depleted water masses. Many of these phases led to widespread development of both ironstones and black shales.

Episodes of fluctuating coastal onlap one to several Ma long reflect variations in sea level, sediment supply, and/or subsidence. Major successions of ironstones repeated on a similar time scale were limited to ten tectonic provinces comprising foreland basins, cratonic margins and unstable cratons, and intracratonic basins. During the favourable phases black shales and ironstones were deposited alternately in some successions.

In many of the major successions an ironstone developed between each of several repeated small-scale sequences in a cadence of several hundred thousand years. Ironstones in these asymmetric units record a long lapse in normal sedimentation during the initial stage of renewed transgressions that commonly spread organic-rich mud across the shelf. The repeated association of ironstones and black shales with sequences of common sedimentary rocks helps constrain speculation about conditions controlling their origin, and suggests that ironstones reflect the more local development of productive conditions.
Eustatically controlled ooidal ironstone deposition: facies relationships of the Ordovician open-shelf ironstones of Western Europe
Authors: Young T. P.
Published: April 2008

Ooidal ironstones are a characteristic feature of the Ordovician successions of south-west Europe and adjacent areas. They occur in shallow marine clastic sequences deposited on a broad low-topography shelf. Three stratigraphic intervals have a particularly widespread development of ironstones: the early Llanvirn, the early Caradoc and the early Ashgill. These intervals, corresponding to periods of major global eustatic sea-level rise, are marked by the development of thin, but persistent, ooidal ironstones even in relatively offshore areas. The ironstones produced by these three events have been studied in the previously contiguous areas of central Portugal and Armorica.

The development of the ironstones, and the nature of the successions within which they occur, show remarkable similarities among the three horizons; this recurrence of similar sedimentary facies defines a cyclicity. In each of these ‘first-order’ cycles the upper part is characterised by the occurrence of a second order of cyclicity, represented by asymmetric coarsening— and thickening—upwards sequences of siltstone and sandstone tempestites, commonly capped by phosphatic conglomerates. The latest ‘second-order’ cycle in each case is characterized by a more distal sequence of dark mudstones with graptolites. The chamositic ooidal ironstones lie disconformably above these upper mudstone-dominated cycles, and are interpreted as forming during ‘second-order’ transgressions late in major global transgressive events. Both ‘first-’ and ‘second-order’ sedimentary cyclicity is attributed to eustatic variation. The ‘first-order’ eustatic cycles have a period of 10–15 Ma, and largely correspond to the international stratigraphic series, while the period of the ‘second-order’ cycles is estimated as 0.2–0.5 Ma.
Stratigraphic and environmental patterns of ironstone deposits
Authors: Bayer Ulf
Published: April 2008

Evaluation of the accumulation of Minette- and Clinton-type ironstones in terms of stratigraphic successions and regional facies patterns provides a sequence of types from condensed ‘iron shot’ muddy carbonates over barrier bars to distal tempestite deposits. A lagoonal origin of iron-oolites is derived and discussed. Biogenetic carbonate production and a biogenic contribution to iron-oolite formation, as well as replacement patterns of carbonate by iron-minerals indicate that (1) ironstone formation is related to environmental changes in the course of transgressive-regressive cycles and (2) formation of iron-ooids occured in environments with short term fluctuations between oxidizing and reducing conditions. Adopting this dynamic viewpoint allows dissolution and precipitation of iron ‘side-by-side’ without contradicting classical models.
The application of analytical transmission electron microscopy to the study of oolitic ironstones: a preliminary study
Authors: Hughes C. R.
Published: April 2008

Analytical transmission electron microscopy (ATEM) has been used by the author to obtain textural, crystallographic and quantitative chemical information from examples of berthierine-rich and chamosite-rich oolitic ironstones. Individual phases were often found to be too small to have been readily analysed by other techniques. Berthierine laths (of the order of 20 × 500 to 1000 nm) and goethite grains (of the order of 25 nm in diameter) were found to exhibit unorientated microtextures, which were independent of both depositional and oolitic fabrics. Chemical analyses of berthierines revealed fairly uniform compositions regardless of textural association. Chemical data from the chamosite-rich samples appeared analogous to that of the berthierine-rich material, although average crystal sizes were found to be much larger (chamosite laths varied from 0.1 × 1.5 µm to 0.5 × 2 mµ). The compositional data suggest that the chamosite-rich oolitic ironstones are simply the late stage diagenetic (or low grade metamorphic) recrystallized equivalents of berthierine-rich precursors. No direct evidence indicative of the mode of origin for berthierine and goethite ooliths was found. It is suggested that ancient ironstone ooliths were probably formed via similar mechanisms to those which generate modern and ancient carbonate ooliths, although obviously the original sediment and interstitial fluids would have been very different.
Iron-rich ooids, their mineralogy and microfabric: clues to their origin and evolution
Authors: Kearsley A. T.
Published: April 2008

The controversy surrounding oolitic ironstone origins has been prolonged by a lack of published discriminatory characters which could be applied to determine the sedimentary processes responsible for the genesis of individual iron-rich ooids. This is in part due to the limitations of optical petrography and has led to confusion between authors as to the nature of the ooids each is describing. A new and simple descriptive system which documents ooid mineralogy and fine structure has been used to distinguish ooids of different origins and a classification of iron-rich ooids into three broad mineralogical categories is proposed in this paper. These Fe-oxide/hydroxide, berthierine/chlorite and siderite dominated groups are further divided by characteristic textural criteria into 15 minor categories. The abundance of ooids which easily fit the three bulk mineralogical categories probably reflects common diagenetic modifications, rather than preservation of discrete primary phases. The wide variety of ooid microfabrics observed by transmitted light microscopy, secondary electron, backscattered electron and X-ray emission imagery reflect differing modes of ooid accretion and early diagenesis, in addition to later diagenetic and weathering processes.

Combined mineralogical and textural classification of individual ooids can indicate the relative importance of individual processes (such as lateritic ooid/pisolith growth, replacement of carbonates by berthierine or primary berthierine precipitation) in the generation of a particular ironstone.
The origin of the Lower Jurassic Cleveland Ironstone Formation of North-East England: evidence from portable gamma-ray spectrometry
Authors: Myers K. J.
Published: April 2008

The distribution of K, U and Th in the Cleveland Ironstone Formation, measured by gamma-ray spectrometry, is found to be useful in investigating the origin of oolitic ironstones. The oolitic ironstone seams are enriched in Th and depleted in K relative to the interbedded mudstones which show a close correlation between Th and K. The enrichment of Th in the ironstone seams represents a primary depositional rather than a diagenetic process, and the Th-enriched material must have been transported to the site of ironstone deposition from elsewhere. The values of Th (20–45 ppm) and the Th/K ratios (20–100) recorded in the ironstone seams are consistent with the original Th-enriched material being kaolinitic/bauxitic clay from a lateritic weathering source.
Antarctica’s place within Cambrian to Devonian Gondwana
Authors: Cocks L. R. M.
Published: April 2008

Analysis of various fossil groups, in particular brachiopods, graptolites and trilobites, from Gondwana as a whole, which included much of what is now southern Europe, southern Asia, Africa, South America, Australasia and Antarctica, has determined not only the margins of the Gondwanan palaeocontinent in the early and middle Palaeozoic, but also the relative palaeotemperatures and the probable palaeo-latitudes. The patchy faunal record from Antarctica is reviewed from Cambrian to Devonian times, and this consists of well-dated Early, Middle and Late Cambrian shelly faunas with eastern Gondwanan affinities and deposited under tropical conditions. By contrast, there are no proven Ordovician fossils known and only one disputed Silurian record of two coral specimens from the Beardmore Glacier moraine. In early Devonian times, well-dated Emsian shelly faunas of Malvinokaffric affinity are known from the Horlick Range and there is a diverse Middle Devonian fish fauna from Victoria Land. From these records it can be deduced that, as the large Gondwana plate slid over the contemporary South Pole, the palaeo-latitude of Antarctica slowly increased from tropical to high temperate from Cambrian to Devonian times.
Antarctic belemnite biogeography and the break-up of Gondwana
Authors: Doyle Peter , Howlett Philip
Published: April 2008

In the late Jurassic, the belemnite genera
Hibolithes
and
Belemnopsis
were abundant and widespread in Tethys, characterizing a Tethyan Realm that extended south from southern Europe and Asia to Antarctica and the rest of Gondwana. Although a distinct Southern Hemisphere ‘Austral’ belemnite realm counterbalancing the northern Boreal Realm was absent, it is clear that a significant degree of endemicity existed at the species level, with distinct species groups in Indonesia, Madagascar, Australasia, South America and so on.

Trans-Gondwanan faunal links first developed in the late Jurassic shelf seaway between Antarctica, Madagascar and India.
Belemnopsis
became extinct in southern Europe, and was left as a relict, endemic to this trans-Gondwanan seaway. In the Aptian the Belemnopseidae were eventually replaced around Gondwana by the endemic family Dimitobelidae, which developed as the Antarctic—Australasian core of Gondwana began to drift south. In Tethys few genera remained, the Tethyan Realm finally breaking down in the late Cretaceous. Initially the Dimitobelidae were widespread in Gondwanan seas, the trans-Gondwanan links developed in the Jurassic being maintained. However, as Gondwana fragmented further in the late Cretaceous, these links broke down and the Dimitobelidae survived only in the Antarctic-Australasian region which was retreating southwards.
Evolutionary patterns in macrurous decapod crustaceans from Cretaceous to early Cenozoic rocks of the James Ross Island region, Antarctica
Authors: Feldmann Rodney M. , Tshudy Dale M.
Published: April 2008

The fossil record of macrurous decapod crustaceans is unusually rich in the Late Cretaceous to Paleocene rocks of the James Ross Island region, Antarctica. Four species of lobsters, contained in four genera, are known from the region.
Hoploparia stokesi
(Weller) is most abundant and has been collected on James Ross, Vega, Humps, Cockburn, Seymour, and Snow Hill islands from early Campanian through Paleocene rocks. Whereas morphometric analysis of the species shows no evolutionary trends in shape, carapace ornamentation and claw ornamentation become more variable higher in the section.
Metanephrops jenkinsi
Feldmann, has also been identified.
Metanephrops
has previously been known from only one fossil occurrence, in the Pliocene of New Zealand. Its occurrence on Seymour Island may lie within the region of origin of this taxon which currently inhabits deep water, lower latitude habitats in the Atlantic and Indo-Pacific oceans.
Metanephrops jenkinsi
is known in the early Campanian Santa Marta Formation, in the Maastrichtian portion of the Lopez de Bertodano Formation, and in the Paleocene Sobral Formation. Other taxa are less abundant. In early to middle Campanian rocks on James Ross Island,
Meyeria crofti
Ball represents a relatively late occurrence of a conservative group.
Linuparus macellarii
Tshudy & Feldmann represents the highest latitude occurrence of a species within this genus, which is known from numerous Cretaceous and Paleogene sites in the Northern Hemisphere but from only two other fossil occurrences in the Southern Hemisphere. This species ranges from the early to middle Campanian into the Paleocene. There is no apparent effect of the K/T boundary ‘event’ on the decapod fauna. New taxa appear below the K/T boundary and none of the Antarctic species disappear from the record at that time.
Antarctica: the effect of high latitude heterochroneity on the origin of the Australian marsupials
Authors: Case Judd A.
Published: April 2008

The record of the Antarctic marine fauna during the Paleogene indicates the occurrence of taxa in high latitude regions (>60°) before these same taxa are known in midto low latitudes. It has been hypothesized that high latitude regions serve both as ‘holding tanks’ for taxa and as regions in which novel adaptations leading to new lineages within a taxon can arise. These features of high latitude regions appear to have had a major impact on the origin of the Australian marsupial fauna. The australidelphian marsupial clade (all Australian marsupials plus South American microbiotheriids) and the ameridelphian marsupial clade (all South American marsupials except microbiotheriids) diverged in the Late Cretaceous. The South American marsupial radiation occurred in mid- to low latitude regions during the Late Cretaceous to early Paleocene. The Australian marsupial radiation probably did not occur until the medial or late Eocene. If accurate, this represents a time differential of 20 to 25 Ma between the two marsupial radiations. The heterochroneity of the marsupial radiations results from the fact that during the Late Cretaceous to Eocene, Australia resided in a high latitude region, the cool temperate Weddellian Biogeographical Province. The radiation of Australian marsupials only reaches a level of taxonomic diversity comparable to that in South America after the continent drifted northward into lower latitudes and habitat diversity increased. This is deduced from changes in floral diversity and the projected timing of the diprotodontian divergences based on DNA hybridization data.
Antarctica as an evolutionary incubator: evidence from the cladistic biogeography of the amphipod Family Iphimediidae
Authors: Watling Les , Thurston Michael H.
Published: April 2008

The modern Antarctic marine fauna is quite distinct, with levels of endemism reaching 90% at the species level and 70% at the generic level. Since the modern Antarctic waters are very cold and isolated from all of the worlds oceans except the deep-sea, the origin of the modern Antarctic fauna has remained in doubt. The amphipod Family Iphimediidae is represented in Antarctic and Sub-Antarctic waters by 16 of its 21 genera and 490f its species. A phylogenetic analysis of the family indicates that the most primitive genera are distributed primarily outside Antarctic waters. It is suggested that these are relicts of a former global distribution, and that once the Antarctic began to cool (38 Ma ago), a radiation of the iphimediids occurred in the Southern Ocean. This radiation was furthered by the reorientation of the mandible from one where the incisor cut in the horizontal transverse plane to cutting in the vertical frontal plane. With this advance, the family spread outward from the Antarctic, principally through the genus
Iphimedia
, which is today represented by 35 known species worldwide. The tracking of features in
Iphimedia
suggests that its initial spread beyond Antarctic waters occurred before the continent was finally thermally isolated 23 Ma ago. It is proposed that the cooling Antarctic waters acted as an incubator for this family, producing an evolutionary advance that allowed it to be a successful colonizer of the thermally changing global ocean.
Gamma-ray log shape used as a facies indicator: critical analysis of an oversimplified methodology
Authors: Rider M. H.
Published: April 2008

Gamma-ray log shapes are often used by geologists to determine sandstone grain size trends and hence depositional facies. However, for the simple methodology of relating shapes to facies to be valid, there must be consistent relationships between gamma-ray log values and clay content and between clay content and grain size. Log to core comparisons and sandstone textural analyses show that these relationships are naturally variable. Consequently, correlation of log shape with grain size trend is tenable only under very limited (and defineable) conditions. A universal application of gamma-ray log shape to grain size trend and depositional facies is wrong.
Stochastic reservoir description: a methodology
Authors: Moss B. P.
Published: April 2008

Stochastic log interpretation methods are shown to provide more detailed information regarding the geology encountered in a complex formation than can be obtained by conventional interpretation methods. The next stage is to combine these data with qualitative data from other sources than wireline logs to provide information about the ‘rock types’ present. Such rock type definition provides the link between descriptive geological data and the likely hydrodynamic model of interest to reservoir engineers. The final stage in the reservoir description process is to relate the vertical sequence, observed and described in the well bore, to the distribution of the rock types in three dimensions within the reservoir body. This paper presents the methodologies behind each of these three stages in building a more complete reservoir description, and demonstrates their application on actual data from a producing field.
Natural gamma-ray spectrometry in hydrocarbon-bearing sandstones from the Norwegian Continental Shelf
Authors: Hurst Andrew
Published: April 2008

Concentrations of thorium (Th), uranium (U) and potassium (K) are measured using natural gamma-ray spectrometry (NGS) logs. The precision of NGS measurements in sandstones varies: K measurement is in general moderately good; Th measurement varies with the service company involved; U measurement is unreliable. Concentrations of Th, U and K in standard tables of minerals used commonly in log evaluation are not representative of the actual elemental concentrations found in sandstone minerals. Th/K cross-plots are shown to be unsuitable for the identification of clay minerals in sandstones. Illite cement cannot under normal geological circumstances be identified from the K content of the rock. Authigenic kaolinite has no particular affinity for concentration of Th. Th in sandstones is concentrated predominantly in detrital heavy minerals with up to 200f the total Th present in silt-sized and finer authigenic REE-phosphates and Th salts. Variation in the K content of micas from some Jurassic sandstones demonstrates the need for using formation-specific chemical data for the minerals of interest (micas, feldspars, clay minerals) as input parameters in normative calculations. Appropriate statistical control of the validity of Th, U and K contents for specific minerals is required.
Structural interpretation of dipmeter results in the Gullfaks Field
Authors: Rønningsland T. M.
Published: April 2008

Dipmeter interpretation has been carried out on seven wells from the Gullfaks Field. Orientation and style of major and associated minor faults have been investigated using dipmeter results. Major faults, as inferred from dipmeter data, are in fairly good agreement with faults defined from seismic interpretation. Several less significant dipmeter-defined faults were below the resolution of seismic data. The orientation of these faults probably corresponds with that expected of antithetic and synthetic faults associated with the main fault system. Analyses of dipmeter results indicate little or no influence of synsedimentary tectonism. Thickness variations of the different formations can be explained by post-depositional faulting (fault cut-out) and erosion. The differences in structural dip between the seven wells support the interpretation that they are located on separate fault blocks. The density of detected faults, one per 140 m (from dipmeter results), proves the presence of an intensely faulted area. This study therefore emphasises the importance of using dipmeter results as a supplement to seismic for the purpose of structural interpretation.
Example of fault identification using dipmeter data
Authors: Devilliers M. C. , Werner Ph.
Published: April 2008

Dip results obtained by conventional processing techniques are not always suitable for the identification of faults. Dipmeter identification of faults is usually based on the recognition of increasing angles of dip associated with drag zones adjacent to fault zones. Several case studies confirm this relationship. Interpreting faults in this way is, however, risky, and can lead to erroneous data being integrated into structural interpretation. By reviewing original log quality and processing methods, much erroneous information can be eliminated. The influence of faulting on raw dipmeter curves is studied, and actual fault geometries from outcrops are compared with dipmeter patterns. Outcrop data are taken from several tectonic contexts in order to provide a significant amount of information, in spite of the restricted vertical perspective of the outcropping faults. Reliable detailed fault interpretation from dipmeter data is enhanced by reconstruction of dip profiles from field examples, in particular where similar data are available from other wells and local structural geological knowledge is available.
Main aspects of petroleum exploration in the Apure area of Southwestern Venezuela, 1985–1987
Authors: Chigne Nestor , Hernandez Leroy
Published: April 2008

This paper gives an update of exploration advances made in Apure during the years 1985–1987. It begins with a brief history of exploratory activities in the area, and then reviews present concepts as to its tectonic-sedimentary evolution. New seismic surveys have notably extended structural knowledge and recent geochemical studies have given new insights into the origin and migration of hydrocarbons; these, together with results of all exploratory drilling, allow new ideas as to additional prospects in the area.
Petroleum geology of the Campos Basin, offshore Brazil
Authors: Mohriak W. U. , Mello M. R. , Dewey J. F. , Maxwell J. R.
Published: April 2008

The Campos Basin, offshore Brazil, is the most prolific basin in the western South Atlantic, with more than thirty hydrocarbon accumulations currently accounting for about 60% of Brazilian oil production.

Intensive drilling and seismic, gravity and magnetic data have contributed to the recognition of four tectono-stratigraphic units related to the rifting and break-up of Pangea. The lowest sequence consists of Neocomian clastics deposited on basalt dated at 120–130 Ma, and reflects the fault-controlled subsidence associated with the stretching that preceded the emplacement of oceanic crust. The Aptian proto-oceanic stage is characterized by a sequence of evaporitic rocks that have undergone intense diapiric activity in deep water. An open-marine environment begins with a thick sequence of Albian/Cenomanian limestones, locally with clastic input, which grades upwards and basinwards into deep water marls and shales. This section is structurally associated with detached listric normal faults that sole out on the Aptian evaporites. Finally, the marine Upper Cretaceous to Recent clastic section is characterized by a more quiescent phase of thermal subsidence, with some residual halokinetic activity that increases in intensity towards deeper waters.

The hydrocarbon accumulations are distributed throughout the stratigraphic column of the basin from Neocomian to Miocene. The reservoirs range from fractured basalts and porous bioclastic limestone (coquinas) in the Lagoa Feia Formation, to limestones and sandstones in the Macaé Formation, and sandstones in the Campos Formation.

Detailed geochemical analyses undertaken on cutting, core and oil samples show that almost all the hydrocarbon accumulations discovered to date are sourced mainly from lacustrine calcareous black shales deposited in a closed Upper Neocomian lake system, having saline to hypersaline waters of alkaline affinities. The extreme anoxic conditions in this lacustrine environment resulted in the deposition of fine, well laminated organic-rich (TOC up to 9%) calcareous black shales, with high-quality organic matter composed almost entirely of low-sulphur type-I kerogen, originating from lipid-rich algal and bacterially-derived material. The excellent hydrocarbon source potential of these sediments, combined with the appropriate thermal history, produced the necessary conditions to yield low-density oil (around 30° API) characterized by a low sulphur content (around 0.30%), and significant quantities of alkanes (up to 70%). Diagnostic features in the biological markers from this depositional environment include: low concentration of steranes, presence of β-carotane, gammacerane and 28,30-bisnorhopane, very high concentrations of hopanes and high relative abundances of tricyclic terpanes up to C
34
.
Geochemistry of crude oils in Oman
Authors: Grantham P. J. , Lijmbach G. W. M. , Posthuma J.
Published: April 2008

The petroleum geochemistry of Oman provides a picture of considerable variety since crude oils and their source rocks are found both throughout the country and throughout the stratigraphic column from the Infra-Cambrian to the Tertiary. The oils can be geochemically classified into five groups. Three of them can be related to good oil source rocks found in the Precambrian Huqf Group, the Silurian Safiq and the Cretaceous Natih Formations. Another group of oils probably originates from the Upper Jurassic Diyab Formation whilst the fifth group of crudes (name ‘Q’) cannot be correlated to a known source rock but is inferred to have originated from an unpenetrated Huqf level.

The ‘Huqf oils’ are those that have been correlated to known Infra-Cambrian Iuqf source rocks and are characterized by a strong C
29
sterane predominance and very light carbon isotope values (δ C
13
) of around −36.0‰. In contrast, the ‘Q’ crudes, derived from the unknown source are characterized by a C
27
sterane predominance and carbon isotope ratios of around −30.5‰. Both the Huqf and ‘Q’ crudes also contain a series of characteristic compounds referred to as the ‘X’ compounds (all isomers of methyl and dimethyl alkanes). Oils thought to originate from Silurian Safiq source rocks have a weak C
29
sterane predominance, a significant content of rearranged steranes and carbon isotope ratios of approx. −30.5‰. The oils correlatable to the Jurassic Diyab Formation have a similar sterane distribution but heavier carbon isotope values of around −26.5‰. Finally, the crude oils from the mid-Cretaceous Natih Formation source rocks are characterized by steranes with an equivalent distribution of C
27
, C
28
and C
29
isomers and carbon istope values of around −26.9‰.
Zechstein reservoirs in The Netherlands
Authors: Baan Dietert Van Der
Published: April 2008

Thick, shallow-water gypsum banks, formed during Zechstein times at the site of regional palaeohighs along the margins of the Southern Permian Basin, acted as foundations for shallow-water carbonate platforms. The carbonate platform margins, in particular their northern to northeastern windward margins, were characterized by deposition of grainstones dominated by aragonitic ooids, pisoids, intraclasts and bioclasts, while muddy, stromatolitic carbonates and nodular anhydrites accumulated on tidal flats and in lagoons behind the platform margins.

Usually the platforms are surrounded by thick slope deposits consisting of carbonate mudstone and redeposited intervals, but locally the platforms deepened gradually and changed, without intervening slope deposits, into a shallow-marine facies devoid of ooids. The slope aprons grade basinwards into thin successions of finely laminated carbonate.

Porosity creation in Zechstein carbonates was due to meteoric-water leaching and to subsurface leaching by carbon dioxyde-enriched formation waters during burial. Porosity destruction was mainly caused by anhydrite pore plugging and subsurface calcitization. The best reservoir properties are found in the unrestricted platform facies and in parts of the slope facies. Fractures are important in Zechstein carbonates; their presence is a prerequisite for high production rates in gasfield wells.
Hydrocarbon plays in the northern North Sea
Authors: Pegrum R. M. , Spencer A. M.
Published: April 2008

The hydrocarbon finds of the Norwegian and British sectors of the North Sea, north of 56°, can be grouped into six discrete plays. The key to all the plays is the presence of organic-rich Upper Jurassic shales (source rocks) and a rift system of the same age. The rifting provided the structures (the traps) and post-rift cooling caused the subsidence necessary for hydrocarbon generation. The pre-Jurassic Play is of least importance. The Lower-Middle Jurassic Play, with about 40% of the resources, results from pre-rift uplift in the south providing clastic input for a delta system in the north, the thick sandstone reservoirs of which were preserved during the subsequent rifting. In late Jurassic times graben formation by rift collapse was accompanied by erosion of marginal uplifts, resulting in thick sand sequences, which interfinger with graben shales containing the source rocks. This ideal relationship provides the Upper Jurassic Play which contains 30% of the resources. The Lower Cretaceous Play occurs in submarine fan sandstones and is of minor importance. Chalk deposition in a tectonically unstable environment, with subsequent rapid burial beneath Tertiary muds and clays, is responsible for the Chalk Play. Although geographically limited, it is prolific and accounts for nearly 10% of the hydrocarbon resources. Geologically youngest is the Paleogene Play. Uplift of the Orkney-Shetland Platform in early Tertiary times was a consequence of sea-floor spreading in the North Atlantic. Great quantities of sand derived from this uplift spilled eastwards into the northern North Sea, blanketing the western flank of the Tertiary basin and extending axially far south along the Central Graben. These sands have captured vertically migrating hydrocarbons and have allowed extensive lateral migration. Although important volumes of oil and gas, about 20% of the total resource, are trapped within these sands, large volumes have probably been lost to the basin via their outcrop in the west.
Haltenbanken hydrocarbon province (off-shore Mid-Norway)
Authors: Heum O. R. , Larsen R. M.
Published: April 2008

Exploration drilling at Haltenbanken started in 1980 and to date 11 significant hydrocarbon discoveries have been made. The estimated total recoverable discovered reserves are 310 million SM3 of oil and condens and 305 billion SM3 of gas. Only 14 out of 25 proper wildcat wells turned out dry, giving a pure exploration success ratio of 44%. The major structural elements at Haltenbanken are (from East to West) the Troendelag Platform, the Halten Terrace and the Deep Moere Basin. The major hydrocarbon discoveries are located on the Halten Terrace and on the western margin of the Troendelas Platform.

After a long period of regional passive subsidence during Triassic and most of Jurassic the Halten Terrace was formed as a consequence of Late Jurassic east-west rifting and early/middle Cretaceous north-south Dextral wrenching. During late Cretaceous and Tertiary again regional passive subsidence dominated. In Miocene very deep waters had developed at Haltenbanken as well as over most of the Mid-Norwegian Shelf. In Pliocene uplift of the Fennoscandian Shield resulted in rapid westward shelf progradation and deposition of up to 1000 m of sediments.

This Pliocene burial had a great impact on the formation and entrapment of hydrocarbons at Haltenbanken. As the temperatures increased the rich Upper Jurassic marine and Lower Jurassic terrestrial source rocks passed through the oil window and are currently still producing large amounts of oil, condensates and gas.

The rapid Pliocene sediment loading also led to great overpressure in the Upper Jurassic to palaeocene line-grained sequence. In the Eastern areas this overpressure resulted in a perfect seal over the Middle and Lower Jurassic sandstone reservoirs. In the Western areas also the Jurassic reservoir sequence is severely overpressured leading to less favourable sealing conditions. On the other hand, the overpressure in the reservoirs in the Western areas seems to have reduced the rate of compaction, hence preserving the porosity and permeability better at great depths. Seven of the most important discoveries at Haltenbanken are discussed as case examples. The variations in type of hydrocarbons (oil, vs. gas or gas/condensates) entrapped in the individual fields are caused by subtle, but important variations in source rock maturity, migration conditions (including drainage area and storeage capacity), trapping conditions (fill-spill or fill-leak), timing and
PVT
-conditions.