Intrusive Igneous Activity Research Articles

Evaluation of Aeromagnetic Data Over Some Parts of Lower Benue Trough, Nigeria Using Spectral Analysis

We obtained, processed, and interpreted spectral analyses of aeromagnetic data across a portion of the Lower Benue Trough with the goal of estimating the depth to magnetic basement, modeling a conspicuous magnetic anomaly, identifying the basement topography, and evaluating basin configurations. Qualitative interpretation based on visual examination of the total magnetic intensive anomalies map, residual contour map, and first vertical derivative map reveals major faults trending east-west (E-W) and minor faults trending northeast-southwest (NE-SW), which is consistent with the structure of the lower Benue Trough. The western portion of the research region indicates structural change near the towns of Afikpo and Ishiagu, indicating areas of documented intrusive igneous body activity. According to the quantitative interpretation of the aeromagmatic data, the average sedimentary thickness is between (2.3 and 3.2 kilometers) and Quantitative interpretation utilizing spectral analysis identifies two distinct depth sources in the region: shallower sources ranging from 1.09 to 1.6 kilometers in the southern portion of the research area, and deeper sources ranging from 1.61 to 4.90 kilometers in the northern section. Additionally, the research reveals that the depths to the centroid and magnetic bodies (sedimentary thicknesses) vary between 5.02 and 10.65 kilometers and 0.23 and 3.5 kilometers, respectively. It was recommended that a ground magnetic survey be used in conjunction with this work to provide a more conclusive result.

Synrift magmatism in a Cenozoic rift basin, from 3D seismic data, Wichianburi Sub-basin, Phetchabun Basin, Thailand: part 1. Distribution and timing of intrusions and forced folds

The Late Oligocene–Pliocene Wichianburi Sub-basin, Phetchabun Rift, Thailand has undergone three phases of development: Late Oligocene–Early Miocene ( c. 16 Ma) east–west extension on predominantly NNW–SSE- to north–south-striking faults; 16 Ma– c. 3 Ma NE extension on NNW–SSE-, north–south- and NE–SW-trending faults; and 3 Ma to Present post-rift subsidence. The basin was also affected by intrusive and extrusive igneous activity from 24 to 2 Ma. The relationships among the structure, rift basin stratigraphy and sheet intrusions are unusually well documented as a result of extensive hydrocarbon exploration drilling and 3D seismic data, together with the production of hydrocarbons from igneous sills. The oldest igneous activity ( c. 24–13 Ma) lies SW of the Wichianburi Sub-basin, where extrusive rocks exposed at the surface have been dated by K–Ar and 40 Ar/ 39 Ar methods. The overall igneous activity youngs from the south of the basin to the north (11–9 Ma). Numerous domal folds occur across the basin; the sill and laccolith dimensions, the rate of fold development, the variable timing of fold formation and the fold geometry all indicate that the folds are related to magma emplacement, not tectonic inversion. Fold formation occurred at c. 16–15 and 12–11 Ma in single or multiple stages. The timing of folding, coupled with the radiometric dating of some sills (K–Ar, 39 Ar/ 40 Ar), indicates that the greatest amount of intrusive activity occurred between 16 and 11 Ma. The 16 Ma date coincides with the switch from east–west to NW–SE extension, which promoted dilatancy within the upper crust along a range of synrift fault orientations and some pre-rift trends. The burial history of the main depocentre shows that Miocene sill emplacement occurred at depths <1 km, but was still able to cause the local maturation of hydrocarbon source rocks. Sill emplacement affected the petroleum system by forming migration pathways, important reservoirs and also creating forced fold traps.

Geometry and controls on the development of igneous sill-related forced folds: A 2-D seismic reflection case study from offshore southern Australia

Emplacement of magma in the shallow subsurface can result in the development of dome-shaped at the Earth's sur- face. These so-called folds have been described in the fi eld and in subsur- face data sets, although the exact geometry of the and the nature of their relation- ship to under lying sills remain unclear and, in some cases, controversial. In this study we use high-quality, two-dimensional (2-D) seismic refl ection and borehole data from the Ceduna sub-basin, offshore southern Australia, to describe the structure and infer the evolution of igneous sill-related forced in the Bight Basin igneous complex. Thirty-three igneous sills, which were em- placed 200-1500 m below the paleo-seabed in Upper Cretaceous rocks, are mapped in the Ceduna sub-basin. The intrusions are expressed as packages of high-amplitude refl ections, which are 32-250 m thick and 7-19 km in diameter. We observe fi ve main types of intrusion: type 1, strata-concordant sills; type 2, weakly strata-discordant, trans- gressive sills; type 3, saucer-shaped sills; type 4, laccoliths; and type 5, hybrid intru- sions, which have geometric characteristics of intrusion types 1-3. These intrusions are overlain by dome-shaped folds, which are up to 17 km wide and display up to 210 m of re- lief. The edges of these coincide with the margins of the underlying sills and the display the greatest relief where the underly- ing sills are thickest; the are therefore interpreted as forced that formed in response to emplacement of magma in the shallow subsurface. The are onlapped by Lutetian (middle Eocene) strata, indicat- ing they formed and the intrusions were em- placed during the latest Ypresian (ca. 48 Ma). We demonstrate that fold amplitude is typi- cally less than sill thickness even for sills with very large diameter-to-depth ratios, suggest- ing that pure elastic bending (forced folding) of the overburden is not the only process ac- commodating magma emplacement, and that supra-sill compaction may be important even at relatively shallow depths. Based on the ob- servation that the sills intruded a shallowly buried succession, the discrepancy between fold amplitude and sill thickness may refl ect loss of host rock volume by fland pore fl uid expulsion from poorly lithifi ed, water-rich beds. This study indicates that host rock composition, emplacement depth, and deformation mechanisms are important controls on the style of deformation that oc- curs during intrusive igneous activity, and that forced fold amplitude may not in all cases refl ect the thickness of an underlying igneous intrusion. In addition, the results of this study suggest that physical and numeri- cal models need to model more complex host rock stratigraphies and rheologies if they are to capture the full range of deformation mechanisms that occur during magma em- placement in the Earth's shallow subsurface.

Seismic analysis of igneous systems in sedimentary basins and their impacts on hydrocarbon prospectivity: examples from the southern Australian margin

The increasing availability of 3D seismic data from sedimentary basins at volcanic and non-volcanic continental margins has provided fundamental new insights into both the storage and transport of magma in the continental crust. As global hydrocarbon exploration increasingly focuses on passive margin basins with evidence for past intrusive and extrusive igneous activity, constraining the distribution, timing and pathways of magmatism in these basins is essential to reduce exploration risk. Producing and prospective Australian passive margin basins where igneous systems have been identified include the Bight, Otway, Bass, Gippsland and Sorell basins of the southern margin. This paper reviews both the impacts of volcanic activity on sedimentary basin hydrocarbon prospectivity (e.g. advective heating, reservoir compartmentalisation and diagenesis), and the styles, distribution and timing of late Cretaceous–Recent extrusive and intrusive igneous activity along basins of the southern Australian margin, providing illustrative examples based on 2D and 3D seismic reflection data.

Neoproterozoic extension in the Greater Dharwar Craton: a reevaluation of the “Betsimisaraka suture” in MadagascarThis article is one of a series of papers published in this Special Issue on the theme ofGeochronologyin honour of Tom Krogh.

The Precambrian shield of Madagascar is reevaluated with recently compiled geological data and new U–Pb sensitive high-resolution ion microprobe (SHRIMP) geochronology. Two Archean domains are recognized: the eastern Antongil–Masora domain and the central Antananarivo domain, the latter with distinctive belts of metamafic gneiss and schist (Tsaratanana Complex). In the eastern domain, the period of early crust formation is extended to the Paleo–Mesoarchean (3.32–3.15 Ga) and a supracrustal sequence (Fenerivo Group), deposited at 3.18 Ga and metamorphosed at 2.55 Ga, is identified. In the central domain, a Neoarchean period of high-grade metamorphism and anatexis that affected both felsic (Betsiboka Suite) and mafic gneisses (Tsaratanana Complex) is documented. We propose, therefore, that the Antananarivo domain was amalgamated within the Greater Dharwar Craton (India + Madagascar) by a Neoarchean accretion event (2.55–2.48 Ga), involving emplacement of juvenile igneous rocks, high-grade metamorphism, and the juxtaposition of disparate belts of mafic gneiss and schist (metagreenstones). The concept of the “Betsimisaraka suture” is dispelled and the zone is redefined as a domain of Neoproterozoic metasedimentary (Manampotsy Group) and metaigneous rocks (Itsindro–Imorona Suite) formed during a period of continental extension and intrusive igneous activity between 840 and 760 Ma. Younger orogenic convergence (560–520 Ma) resulted in east-directed overthrusting throughout south Madagascar and steepening with local inversion of the domain in central Madagascar. Along part of its length, the Manampotsy Group covers the boundary between the eastern and central Archean domains and is overprinted by the Angavo–Ifanadiana high-strain zone that served as a zone of crustal weakness throughout Cretaceous to Recent times.

Central England's journey through time: the geological evolution of Warwickshire

The central English county of Warwickshire has been shaped by a 600 million‐year history of deposition, erosion, tectonism, continental drift, environmental change and biological evolution. The Neoproterozoic and Lower Palaeozoic rocks indicate island arc accretion, marine transgression and subduction‐related intrusive igneous activity. Upper Palaeozoic through to Triassic times witnessed mainly continental environments, as central England drifted north across the equator. Late Triassic marine transgression led to deposition of richly fossiliferous Jurassic sediments. The bedrock geology shows evidence for the influence of deep‐seated structural lineaments within the central English basement. Influenced by these structures, the modern landscape has been shaped largely by Palaeogene and Neogene uplift and erosion, further alteration by Quaternary erosion and weathering, and glacial and fluvial deposition.

Geochemical and Geochronological Constraints on Mineralization within the Hilltop, Lewis, and Bullion Mining Districts, Battle Mountain-Eureka Trend, Nevada

The Hilltop, Lewis, and Bullion mining districts, located within the northern Shoshone Range, Lander County, Nevada, are part of the greater Battle Mountain-Eureka trend. Historic production of mostly Ag with Au, Cu, Pb, and Zn from all three districts exploited ≤ 3-m-wide quartz veins hosted mainly within upper plate Ordovician-Devonian siliceous and siliciclastic rocks of the Roberts Mountains allochthon. Eocene felsic intrusive rocks, located both proximal and distal to the vein deposits, were emplaced throughout the northern Shoshone Range along a west-northwest trend; molybdenite mineralization is associated with some intrusive rocks, and some intrusive rocks host vein-type mineralization. New and previously published radiometric ages for primary igneous minerals from unaltered rocks within and/or proximal to mineralized areas are nearly coincident with published Re-Os ages for several northern Shoshone Range molybdenite occurrences (40.1 ± 0.6 Ma, avg), supporting a relationship between pluton emplacement and molybdenite mineralization. Age differences between quartz vein-hosted gold ± base metal mineralization (e.g., 38.3 ± 0.1 Ma: Gray Eagle mine, Bullion district) and nearby intrusive rocks (e.g., 39.0 ± 0.1 Ma: Granite Mountain) imply that vein mineralization probably formed during prolonged hydrothermal activity, synchronous with or subsequent to igneous rock emplacement, or from heat associated with a second, slightly younger phase of intrusive igneous activity. Primary fluid inclusion salinity (0–6.4 wt % NaCl equiv) and Th (115°–425°C) data from ore-bearing vein quartz, in conjunction with measured δ18OVSMOW data (−0.97 to +17.3‰), indicate that vein-forming fluids contained variable mixtures of magmatic and meteoric components (calculated δ18OVSMOW values of −16.0 to +13.3‰) and spanned the epithermal-porphyry temperature continuum. New and previously published stable isotope (C, O, and S) data from vein-hosted ore and gangue minerals, unaltered and altered siliceous (upper plate) host rocks, and unaltered carbonate (lower plate and upper plate) rocks indicate mixed and variable sources of vein-hosted ore and gangue components and variable interaction between some vein-forming fluids and nonsiliceous rock types (carbonates, bedded barite) that underlie the northern Shoshone Range. Large vein widths, high vein densities, and lack of evidence supporting fluid boiling in primary fluid inclusions suggest that vein deposits of the northern Shoshone Range probably formed at shallow depths (≤1 km) under lithostatic conditions. Geochronology and stable isotope data support a temporal and genetic link between Eocene magmatism, quartz vein-hosted precious and base metal mineralization, and molybdenite (porphyry) mineralization within the Hilltop, Lewis, and Bullion mining districts.

Late Mesozoic–Cenozoic structural and stratigraphic correlations between the conjugate mid-Norway and NE Greenland continental margins

Late Mesozoic–Cenozoic tectonostratigraphic correlations between the conjugate mid-Norway and NE Greenland continental margins are provided, based on the recently refined structural and stratigraphic framework off mid-Norway, new early opening plate reconstructions of Eurasia versus Greenland between the Jan Mayen and Senja fracture zones, and a sparse regional grid of seismic reflection profiles off NE Greenland. The Norwegian margin exhibits a distinct along-strike margin segmentation governed by across-margin transfer systems. In particular, the Bivrost Fracture Zone and its landward transfer zone prolongation are well-defined features. Corresponding features are here interpreted on the conjugate NE Greenland margin. Together, these conjugate transfer/fracture zones represent a first-order, across-margin tectonomagmatic boundary of prolonged structural inheritance. Regional transects across both margins reveal important vertical and lateral variations in crustal configuration and composition resulting from a complex history of rifting prior to and during the last rift episode in Late Cretaceous–Early Tertiary time, leading to break-up and volcanic passive margin formation. Although the composite Late Jurassic–earliest Cretaceous rifting was the dominant tectonic episode, one also observes structural and stratigraphic relations that indicate an Aptian–?Albian rift phase, probably co-eval with similar events elsewhere on the NE Atlantic margins. Late Cretaceous rifting, with onset in middle Campanian time, was characterized by low-angle detachment faulting, culminating in regional uplift, intrusive igneous activity and subsequent erosion towards the end of the Paleocene. Thick seaward-dipping reflector sequences indicate massive eruptions of lavas during break-up at the Paleocene–Eocene transition. The post-break-up passive margin development was characterized by the transport and deposition of large amounts of sediment in response to margin subsidence and continental uplift, particularly during two distinct phases of outbuilding in Oligocene?/Miocene and Plio-Pleistocene times. Of special interest are a number of mid-Cenozoic intra-basin inversion features recognized both off Norway and off Greenland, revealing a regional compressive regime.

Late Cretaceous–Paleocene tectonic development of the NW Vøring Basin

The Late Cretaceous–Paleocene rifting in the NW Vøring Basin is characterized by four main fault complexes and pronounced upper-crustal structural segmentation. The fault complexes are linked by accommodation zones, which separate fault systems of different polarities and thick from thinner coeval sedimentary successions. Structural and stratigraphic analyses suggest that the early rift phase (∼81 to 65 Ma) was characterized by large-scale normal faulting, along-margin segmentation and varying structural styles; whereas the late rift phase (∼65 to 55 Ma) was associated with continued extension, regional uplift, intrusive igneous activity and subsequent erosion. The rifting ended with breakup at ∼55 Ma accompanied by massive, but gradually waning extrusive igneous activity over the next 3 Myr. The mode of rifting appears to have changed from brittle to more ductile extensional deformation from the early to late rift phase. The changing rift rheology is probably related to the arrival of the Iceland mantle plume and initiation of associated igneous activity. Hence, the NW Vøring Basin provides an example of complex interaction of structural and magmatic relationships during rifting and breakup.

Mesozoic and Cenozoic igneous activity and its tectonic control in northeastern Brazil

The northeastern region of the South American Platform corresponds to a peculiar Brazilian tectonic unit, the Borborema Province. It is a highly complex region due to its tectonic setting, as well as to extensive intrusive and extrusive igneous activity that took place during the Mesozoic and Cenozoic. A regional tectonic analysis based on the spatial and temporal distribution of geochronological data (K/Ar ages) led to the recognition of two main magmatic stages. The earlier events (Upper Jurassic to Lower Cretaceous) are related to the origin of several sedimentary rift basins in the area. The Tertiary alkaline magmatism presents a characteristic distribution whose linear trend suggest the track of an active hot spot in the region.

Zircon ages constrain the timing of deformation events in the the Granites‐Tanami region, northwest Australia*

Dating of extensive granitoids in the The Granites‐Tanami Block of northern Australia has enabled time limitations on deformational events, including five Palaeoproterozoic events (D1‐D5) and one older, almost Neoarchaean event. D1 is present in the basal Tanami Group, D2 to D5 help define other stratigraphic groups in the region. The intrusive rocks have been dated by traditional zircon and stepwise lead evaporation techniques. A post‐D2/pre‐D3 foliated and boudinaged leucocratic tonalite gave 1870 ± 3 Ma. A syn‐D3 foliated and boudinaged tonalite 1858 ± 3 Ma and a post‐D3/pre‐D4 foliated tonalite 1840 ± 11 Ma. Two unmetamorphosed and post‐D5 granitoids gave 1797 ± 1 Ma and 1787 ± 3 Ma, respectively. Some deformation events are separated by as little as 10 to 20 million years. These times of intrusive igneous activity exactly replicate other igneous events, many of which are volcanic, in the Halls Creek Province 400 km to the northwest, which strengthens proposed geological relationships between the two regions. A granitic gneiss in a basement inlier gives 2487 ± 3 Ma, a Neoarchaean age. Tectonic uplift in the Carboniferous is deduced from concordia lower intercept data that is similar to the eastern Arunta Block. Further age definition of D1 D2 and D5 is under current investigation.

97/02551 Variation in the chemistry of macerals in coals of the Mist Mountain Formation, Elk Valley coalfield, British Columbia, Canada

Vitrinite reflectance profiles have been constructed for twelve borehole sequences from the Carboniferous of the eastern and central Midland Valley of Scotland. These successions are intruded by the early Stephanian, tholeiitic, (quartz-dolerite) ‘Midland Valley Sill’, or by Namurian/early Westphalian alkaline (olivine-dolerite or teschenite) sills, or by both alkaline and tholeiitic sill types. Patterns of reflectance variation in sequences invaded by the Midland Valley Sill are similar to those observed in Carboniferous successions in the north of England, where they are intruded by the ‘Great Whin Sill’, which is comagmatic with the Midland Valley Sill. The reflectance patterns all reveal extensive thermal aureoles that have modified organic matter for many tens of metres above and below the sill-sediment contacts. In contrast, no aureoles of similar scale appear to have developed, either above or below the alkaline sills. Reflectance values only rise to substantial levels in the immediate vicinity of the sediment-intrusion contacts. The most likely explanation for the radical contrast between the reflectance patterns produced by the two sill types, must be sought in the physical state of the sediments at the time of intrusion. The degree of sediment compaction, the volume of pore water in the sediments and the initial maturation level of the organic matter at the time the magma invaded the sediments will be the most significant factors, as opposed to the temperature levels of the magmas, which were probably similar (T ≈ 1000 °C), but which, because of the physical condition of the sediments, produced little rise in temperature of the organic matter even very close to the alkaline sills. The data suggest that relationships between reflectance (thermal) gradients, stratigraphic level of sill intrusion and the timing of intrusion should be carefully examined to ensure successful geothermal evaluation of sedimentary basins in which the burial history has been interrupted by intrusive igneous activity. Geochemical/petrological appraisals of sedimentary basins for hydrocarbon prospects must also be influenced by these results.

97/02548 Surface spectroscopic studies of factors influencing xanthate adsorption on coal pyrite surfaces

Vitrinite reflectance profiles have been constructed for twelve borehole sequences from the Carboniferous of the eastern and central Midland Valley of Scotland. These successions are intruded by the early Stephanian, tholeiitic, (quartz-dolerite) ‘Midland Valley Sill’, or by Namurian/early Westphalian alkaline (olivine-dolerite or teschenite) sills, or by both alkaline and tholeiitic sill types. Patterns of reflectance variation in sequences invaded by the Midland Valley Sill are similar to those observed in Carboniferous successions in the north of England, where they are intruded by the ‘Great Whin Sill’, which is comagmatic with the Midland Valley Sill. The reflectance patterns all reveal extensive thermal aureoles that have modified organic matter for many tens of metres above and below the sill-sediment contacts. In contrast, no aureoles of similar scale appear to have developed, either above or below the alkaline sills. Reflectance values only rise to substantial levels in the immediate vicinity of the sediment-intrusion contacts. The most likely explanation for the radical contrast between the reflectance patterns produced by the two sill types, must be sought in the physical state of the sediments at the time of intrusion. The degree of sediment compaction, the volume of pore water in the sediments and the initial maturation level of the organic matter at the time the magma invaded the sediments will be the most significant factors, as opposed to the temperature levels of the magmas, which were probably similar (T ≈ 1000 °C), but which, because of the physical condition of the sediments, produced little rise in temperature of the organic matter even very close to the alkaline sills. The data suggest that relationships between reflectance (thermal) gradients, stratigraphic level of sill intrusion and the timing of intrusion should be carefully examined to ensure successful geothermal evaluation of sedimentary basins in which the burial history has been interrupted by intrusive igneous activity. Geochemical/petrological appraisals of sedimentary basins for hydrocarbon prospects must also be influenced by these results.

A New Look at the Old Problem of Breccias

The typical introductory treatment of breccias is inadequate, often wrong, and needs improvement. Breccias are not conveniently contained within the classical three-fold genetic classification of rocks (igneous, sedimentary, metamorphic) because the myriad of recognized breccia types transcend all the traditional boundaries. A revised breccia definition is proposed: A breccia is a rock composed predominantly of angular fragments greater than 2 mm in diameter that can form in a variety of ways including: sedimentary processes, volcanic activity, intrusive igneous activity, tectonism, hydrothermal processes, and meteorite impact. Most breccia clasts are surrounded by a mineral cement and/or a matrix of finer-grained particles or igneous rock. Geology teachers can use breccia variety to show important connections between different subdisciplines of geology and to demonstrate the importance of field context.

Development of organic maturation in the thermal aureoles of sills and its relation to sediment compaction

Vitrinite reflectance profiles have been constructed for twelve borehole sequences from the Carboniferous of the eastern and central Midland Valley of Scotland. These successions are intruded by the early Stephanian, tholeiitic, (quartz-dolerite) ‘Midland Valley Sill’, or by Namurian/early Westphalian alkaline (olivine-dolerite or teschenite) sills, or by both alkaline and tholeiitic sill types. Patterns of reflectance variation in sequences invaded by the Midland Valley Sill are similar to those observed in Carboniferous successions in the north of England, where they are intruded by the ‘Great Whin Sill’, which is comagmatic with the Midland Valley Sill. The reflectance patterns all reveal extensive thermal aureoles that have modified organic matter for many tens of metres above and below the sill-sediment contacts. In contrast, no aureoles of similar scale appear to have developed, either above or below the alkaline sills. Reflectance values only rise to substantial levels in the immediate vicinity of the sediment-intrusion contacts. The most likely explanation for the radical contrast between the reflectance patterns produced by the two sill types, must be sought in the physical state of the sediments at the time of intrusion. The degree of sediment compaction, the volume of pore water in the sediments and the initial maturation level of the organic matter at the time the magma invaded the sediments will be the most significant factors, as opposed to the temperature levels of the magmas, which were probably similar ( T ≈ 1000 °C), but which, because of the physical condition of the sediments, produced little rise in temperature of the organic matter even very close to the alkaline sills. The data suggest that relationships between reflectance (thermal) gradients, stratigraphic level of sill intrusion and the timing of intrusion should be carefully examined to ensure successful geothermal evaluation of sedimentary basins in which the burial history has been interrupted by intrusive igneous activity. Geochemical/petrological appraisals of sedimentary basins for hydrocarbon prospects must also be influenced by these results.

Syn- and post-rift igneous activity in the Porcupine Seabight Basin and adjacent continental margin W of Ireland

Summary An igneous province in the Porcupine Seabight Basin and adjacent shelf region W of Ireland records repeated episodes of intrusive and extrusive igneous activity, from the mid-Jurassic to late Oligocene. The ‘Province’ maintains a regionally distinctive NE-SW and NNW-SSE curvilinear trend that is inherent to the basement. Mid-Jurassic pyroclastic airfall deposits were probably contemporaneous with mid-Kimmerian uplift. Voluminous early Cretaceous volcanism was coeval with an episode of rifting during the Valanginian to Barremian interval in the Porcupine and Goban Spur Basins. In particular, the 150 km long ‘Porcupine Median Volcanic Ridge’ (PMVR) is interpreted to be extrusive in the S, but progressively intrusive northwards; from an inferred fissure volcano to a laccolith. Aptian-Albian volcaniclastics are of unknown association, but coincide with the onset of oceanic spreading along the adjacent proto-Atlantic margin. Documented late Cretaceous intrusive and extrusive igneous activity was minor in scale although a period of marked inversion is recorded for the NE Porcupine Basin, which may relate to the emplacement of the areally extensive Brendan Igneous Centre. Intermittent intrusive and extrusive activity throughout the Palaeogene was broadly coincident with the full duration of the ‘Thulean’ Palaeocene igneous event. Further activity, variously dated from Eocene to late Oligocene, is partly attributed to a series of intrabasinal, parallel-aligned plugs of intrusive and extrusive origin situated in the upper Porcupine Basin and termed the ‘Slyne Fissures’. These fissures, whose basement-controlled lineation prolongates onshore as an extended group of sheet intrusions, appear to emanate from the Brendan Centre. Further dykes in County Kerry are also believed to extend towards the same centre. Within the Province a distinction is made between magmatism that is essentially syn-rift and linked to basin development (notably Valanginian-Barremian), and later post-rift activity (Aptian-Tertiary). The size, emplacement character and location of the PMVR may represent a sudden increase in axial volcanism that occurred during advanced stages of basinal rifting, and could be viewed as a prelude to, or transition from rifting to spreading prior to an abrupt failure. Temporal relationships can be proposed between active faulting associated with initial lithospheric stretching (late Kimmerian), late syn-rift igneous activity (Valanginian-early Barremian) and the initiation of subsequent lithospheric stretching (earliest Barremian) and, finally, the onset of seafloor spreading (Albian) on the continental margin W of Goban Spur. The later post-rift activity is described as ‘opportunistic’ and triggered by exterior tectonic events that utilized existing tensile trends within the Basin. Emplacement of the Brendan Centre was probably responsible for a phase of inversion with its concomitant influence on sedimentation patterns. Magmatism in the Eocene and Oligocene was associated with minor faulting, an unconformity and pronounced post-Eocene subsidence, together indicative of a final regional thermal episode.

Tectonic development of the Mid-Norway continental margin

The area reviewed covers the Mid-Norway continental margin between latitudes 62°N and 68°N. Main structural elements, as defined at the base Cretaceous level, are the Tröndelag Platform, underlying the inner shelf, the Möre and Vöring Basins, located beneath the outer shelf and slope, and the Möre Platform and the Outer Vöring Plateau, forming a base of slope trend of highs. Sediments contained in the Mid-Norway Basin range in age from Late Palaeozoic to Cenozoic. The basement was consolidated during the Caledonian orogenic cycle. Devonian and Early Carboniferous wrench movements along the axis of the Arctic-North Atlantic Caledonides are thought to have preceded the Namurian onset of crustal extension. Rifting processes were intermittently active for some 270 My until crustal separation between Greenland and Fennoscandia was achieved during the Early Eocene. During the evolution of the Norwegian-Greenland Sea rift system a stepwise concentration of tectonic activities to its axial zone (the area of subsequent continental separation) is observed. During the Late Palaeozoic to Mid-Jurassic a broad zone was affected by tensional faulting. During the Late Jurassic and Cretaceous the Tröndelag Platform was little affected by faulting whilst major rift systems in the Möre and Vöring Basins subsided rapidly and their shoulders became concomitantly upwarped. During the latest Cretaceous and Early Palaeogene terminal rifting phase only the western Möre and Vöring Basins were affected by intrusive and extrusive igneous activity. Following the Early Eocene crustal separation and the onset of sea floor spreading in the Norwegian-Greenland Sea, the Vöring segment of the Mid-Norway marginal basin subsided less rapidly than the Möre segment. During the Early and Mid Tertiary, minor compressional deformations affected the Vöring Basin and to a lesser degree the Möre Basin. Tensional forces dominated the Late Palaeozoic to Early Cenozoic evolution of the Mid-Norway Basin and effected strain mainly in the area where the crust was weakened by the previous lateral displacements. The lithosphere thinned progressively and the effects of the passively upwelling hot asthenospheric material became more pronounced. Massive dyke invasion of the thinned crust preceded its rupture.

GEOLOGIC TIME AS A PARAMETER IN ORGANIC METAMORPHISM AND VITRINITE REFLECTANCE AS AN ABSOLUTE PALEOGEOTHERMOMETER

Substantial published and unpublished data contradict accepted laws of organic metamorphism. Significant amounts of C15+ hydrocarbons and kerogen with significant generation potential exist in sediments which have been buried at high temperatures for long periods of geologic time. Graphite formation and greenschist metamorphism do not occur at model‐predicted burial times and temperatures. Maturation levels for sediments with long (up to 240 million years) burial times are much lower than expected, and conversely, higher than expected for sediments with short (2 million years) burial times. The threshold of intense petroleum generation is not detectable in Plio‐Miocene sediments at minimum temperatures of 160° C.Literature examination reveals that the only solid documentation of time as a controlling parameter in organic metamorphism is in the works of Karweil (1955), Lopatin (1971) and Connan (1974). Scrutiny of these authors' study areas reveals that all have had moderate to extreme paleogeothermal gradients from incipient and aborted rifting, extrusive and intrusive igneous activity, hydrothermal activity, orogenesis and/or uplift and erosion. Levels of organic metamorphism attributed to geologic time by the above authors are better explained by high paleogeothermal gradients resulting from major geologic activity.Mean vitrinite reflectance (RO) data have been compiled from different sedimentary basins currently at or near maximum geothermal gradients with sediment burial times ranging from 2–240 million years. The RO data from these basins show a strong correlation (r = 0.97) of increase in R, with increase in temperature. These same data show no correlation of increasing Ro with increasing burial times for any temperature interval. R, data from sedimentary basins affected by high‐temperature geothermal systems with heating times of only 1,000–10,000 years overlie R, data from these petroleum‐bearing sedimentary basins with burial (heating) times of 2–240 million years.Contrary to accepted thought, there is no evidence from the natural system that petroleum generation‐maturation reactions have first‐order reaction kinetics. By contrast, a substantial body of evidence from petroleum generation‐maturation laboratory studies carried out in closed water‐ wet systems show that these reactions are characterized by multiple‐order reaction kinetics.Geologically older sediments clearly tend to be more organically mature than younger sediments at the same burial temperatures. However, this correlation ismorea reflection that older sediments have had a greater chance of being affected by a major geologic event with an accompanying high heat flow. The major conclusions of this paper are that: (1) geologic time has no observable effect on organic metamorphism; (2) vitrinite reflectance can be used as an absolute paleogeothermorneter; and (3) the accepted models of organic metamorphism at the very least need substantial revision.

Aspects of Burial Diagenesis, Organic Maturation and Palaeothermal History of an Area in the South Viking Graben, North Sea

Summary A detailed study has been made of changes in interlayered clay minerals, sediment mineralogy, vitrinite reflectance, kerogen composition, and chemistry of the soluble organic extract through Well 16/22-2 in the South Viking Graben. The well sampled sediments range in age from Oligocene to, at total depth, Middle Jurassic (Dogger). Lithologies are typical of the South Viking Graben/Central Graben area with Upper Jurassic black pyritic shales, a thick Cretaceous chalk succession and prominent top Palaeocene tuff. Sandstones are common only in the Upper Jurassic Palaeocene and Eocene. Tuffs are sporadically present in the Middle Jurassic. Randomly interlayered illitesmectite is the dominant clay mineral through the Tertiary and Cretaceous and the change to an ordered structure which occurs just above the Jurassic/Cretaceous unconformity is interpreted as a diagenetic reaction. Estimates of illite-smectite composition indicate a reversal to smectite rich clays in the Lower Eocene-Top Palaeocene which is attributed to an insufficient supply of K + from detrital feldspar and mica due to the dominance of basic volcanic weathering products. The vitrinite reflectance profile for this well indicates progressive maturation of kerogen to marginal maturity in the Upper Jurassic. A large jump of 0.2% R o occurs at the Upper/Middle Jurassic boundary below which kerogen maturity is at the peak oil-generating stage. This maturation break is supported by an increase in spore colour index. Chemical parameters are strongly influenced by a change in kerogen type from mixed algal and humic to predominantly humic at this stratigraphic level. Very high heat flow due to localized intrusive igneous activity during the late Middle Jurassic is thought to have caused the increased maturity of the Middle Jurassic sediments.

Geochronology of hydrothermal uranium deposits and associated igneous rocks in the eastern source area of the Mount Belknap Volcanics, Marysvale, Utah

Radiometric ages of igneous rocks and hydrothermal uranium-fluorine veins in the eastern source area of the Mount Belknap Volcanics near Marysvale, Utah, show that igneous activity occurred over a time interval of about 9 million years and that hydrothermal uranium deposits were formed about midway through the interval, coincident with the last igneous activity in the Central mining area. Intrusive igneous activity in the Central mining area began about 28 m.y. ago with emplacement of the quartz monzonitic Central Intrusive of the Bullion Canyon Volcanics and was followed by a hypabyssal granitic stock, eruption of the Red Hills Tuff Member, and emplacement of dikes, all consisting of the Mount Belknap Volcanics, finally ending about 18 m.y. ago. Hydrothermal uranium mineralization occurred with the last event in this sequence and is genetically associated with emplacement of glassy rhyolite dikes between 18 and 19 m.y. ago. A 207 Pb/ 204 Pb- 235 U/ 204 Pb isochron age of 19.0 + or - 3.7 m.y. was obtained for whole-rock uranium-fluorine vein samples. Application of a technique using hydrothermal quartz in the veins as a natural external detector for fission tracks from adjacent pitchblende gave an apparent age of 16.5 + or - 4.3 m.y., consistent with the other geochronologic data.