Nick Schofield

Sill morphology and comparison of brittle and non-brittle emplacement mechanisms

Magmatic sheet intrusions contribute significantly to the upper crustal magma transport network. The emplacement mechanism of the magmatic sheets controls the final geometry of the intrusions and the characteristics of host rock deformation. Previous observations have highlighted the preponderance of brittle structures, associated with shallow-level sheet intrusions. However, recent studies have suggested that non-brittle host rock behaviour also occurs, particularly related to the formation of magma fingers during shallow-level sill intrusion. Here, we examine both brittle and non-brittle intrusion mechanisms and expand upon them with field observations from a series of widespread and variable magmatic systems. Non-brittle emplacement appears primarily associated with viscous flow of the host rock during intrusion and is therefore intimately linked to the contemporaneous host rock rheology as well as magma dynamics. Purely brittle and non-brittle emplacement processes are found to be end members with many intrusions containing evidence of both behaviours. Deriving the host rock characteristics is therefore important for discerning potential diagnostic intrusion indicators and intrusion geometries both within the field and in modelling. Incorporation of variable host material behaviours in numerical and analogue modelling, tuned using direct field observations, may consequently further our understanding of the controls on shallow-level intrusion.

Magma fingers and host rock fluidization in the emplacement of sills

The Golden Valley Sill in South Africa, like many similar structures imaged in three-dimensional seismic data, is saucer shaped with a transgressive rim. This rim exhibits magma fingers tens to hundreds of meters wide radiating from a central axis and forming the outer dipping rim. Localized fluidization of the host rock is observed close to the margins where the fingers occur, suggesting a causal link between fluidization of the country rocks and finger formation. We relate the large-scale formation of fingers to host rock fluidization caused by flash boiling of pore fluids following the tensional failure of the roof of the inner dish at a maximum radius. Once fluidization has occurred by this method, the mechanical heterogeneity controlling the propagation of a concordant sill is broken and the intrusion can transgress. This work emphasizes the importance of understanding the nature of magma intrusion on host rocks, depending on their lithology, and in particular the ability of host rocks to behave in either a brittle or nonbrittle manner during magma intrusion.

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.

Seismic imaging of 'broken bridges': linking seismic to outcrop-scale investigations of intrusive magma lobes

Three-dimensional seismic datasets have provided unrivalled insights into magma flow within sub-volcanic systems. One of the key revelations is that sills appear to be constructed of a series of discrete magma lobes that form during the emplacement of magma into host-rock. We focus on a large sill, within the Faroe–Shetland Basin, North Atlantic, that is well imaged on seismic data, and identify the presence of ‘broken bridges’ within the sill, developed between elongate magma lobes, and reveal for the first time in three dimensions the development of broken bridges. Critically, by relating the imaged structures to key outcrop-scale examples we confirm that bridge and broken-bridge structures are oriented perpendicular to the magma flow direction. This work thus demonstrates a key link that can be made between seismic-scale investigation of intrusions and sub-seismic (outcrop-scale) processes, highlighting the seemingly scale-invariant nature of the magmatic emplacement process.

The potential role of igneous intrusions on hydrocarbon migration, West of Shetland

Numerous challenges for petroleum exploration exist within basins containing sequences of intrusive and extrusive rocks, ranging from seismic imaging to drilling. One poorly understood element in dealing with volcanic-affected basins is assessing the impact magmatism has on the elements of the petroleum system. Within this study we attempt to evaluate the potential impact that the extensive sequence of igneous intrusions of the Faroe–Shetland Basin may have on hydrocarbon migration. Using available well data combined with regional 3D seismic surveys, we show that geometrical relationships between sills location and overlying hydrocarbons shows, together with several cases of gas-charged open fractures in the sills, point toward the recognition of igneous intrusions as a factor in hydrocarbon migration through sill intrusions acting as both barriers or conduits to hydrocarbon migration. We also provide a series of general conceptual models dealing with hydrocarbon migration and igneous compartmentalization within sedimentary basins, which can be applied not just to the Faroe–Shetland Basin, but to other sedimentary basins world-wide if it is found (via well data or other methods) that the intrusions are interacting with a petroleum system.

The influence of normal fault geometry on igneous sill emplacement and morphology

Magma flow within the subsurface is heavily influenced by the pre-existing structure of the upper crust. During continental rifting, normal faults modify the geometry of igneous networks by providing preferential pathways for the intrusion of magma. However, the way in which magma intrudes into fault planes is poorly understood. Here, we quantitatively document the relationship between fault architecture and intrusion distribution and geometry using three-dimensional seismic reflection data from the Exmouth Sub-basin, offshore northwest Australia. Inclined segments of saucer-shaped sills intrude several faults along convex-into-the-hangingwall fault-plane corrugations. We suggest that stress field perturbations associated with the fault-plane corrugations provide suitable conditions for fault reactivation as magma conduits. Pre-existing faults also modify sill geometries through the offset of stratigraphic horizons that may be preferentially intruded, potentially resulting in the formation of a new sill or the development of minor intrusive steps. This work emphasizes the importance of the pre-existing structural template in controlling the growth and final geometry of intrusive networks.

Development of intra-basaltic lava-field drainage systems within the Faroe–Shetland Basin

The Faroe–Shetland Basin, located on the NW continental shelf of the UK, represents arguably the last frontier area of hydrocarbon exploration of the UK territorial waters. The basin contains an aerially extensive lava sequence, which forms part of the Palaeogene-aged North Atlantic Igneous Province (NAIP). In 2004, a major oil and gas discovery was made within the Palaeocene–Eocene lavas in the form of the Rosebank Field. Unusually, the reservoir intervals are a series of intra-basaltic fluvial clastic sequences separated by basalt lava flows, hyaloclastites and volcaniclastic sediments, giving rise to a new hydrocarbon play concept. The discovery has led to an increased emphasis on trying to understand the detailed stratigraphy of the offshore lava fields within the Faroe–Shetland Basin, as well as the nature of the regional intra-basaltic drainage systems and sediments that form the reservoir intervals in Rosebank. In this paper we show a regional integrated three-dimensional (3D) seismic and well analysis of the lava sequences within the area of the Corona Ridge, exploring the age relationships and spatial distribution of the sequence T40 and T45 Palaeocene lava field. In particular, we note the occurrence of an intra-basaltic (sequence T40, Colsay Member) drainage network that appears to represent a major drainage pathway within the Faroe–Shetland Basin lava fields. Few published studies have dealt with drainage system development on lava but our preliminary analysis appears to indicate that lava flow-field morphology plays a dominant role in controlling the development and evolution of a drainage system.

Basin-scale architecture of deeply emplaced sill complexes: Jameson Land, East Greenland

Igneous sills are common components in rifted sedimentary basins globally. Much work has focused on intrusions emplaced at relatively shallow palaeodepths (0 – 1.5 km). However, owing to constraints of seismic reflection imaging and limited field exposures, intrusions emplaced at deeper palaeodepths (>1.5 km) within sedimentary basins are not as well understood in regard to their emplacement mechanisms and host-rock interactions. Results from a world-class, seismic-scale outcrop of intruded Jurassic sedimentary rocks in East Greenland are presented here. Igneous intrusions and their host rocks have been studied in the field and utilizing a 22 km long ‘virtual outcrop’ acquired using helicopter-mounted lidar. The results suggest that the geometries of the deeply emplaced sills (c. 3 km) are dominantly controlled by host-rock lithology, sedimentology and cementation state. Sills favour mudstones and even exploit centimetre-scale mudstone-draped dune-foresets in otherwise homogeneous sandstones. Sills in poorly cemented intervals show clear ductile structures, in contrast to sills in cemented units, which show only brittle emplacement structures. The studied host rock is remarkably undeformed despite intrusion. Volumetric expansion caused by the intrusions is almost exclusively accommodated by vertical jack-up of the overburden, on a 1:1 ratio, implying that intrusions may play a significant role in uplift of a basin if emplaced at deep basinal levels. Supplementary materials: Uninterpreted versions of Figures 7, 8 and 11 are available at http://doi.org/10.6084/m9.figshare.c.3281882

Frontier exploration and the North Atlantic Igneous Province: new insights from a 2.6 km offshore volcanic sequence in the NE Faroe–Shetland Basin

The Lagavulin exploration well 217/15-1Z penetrated a c. 2.6 km thick volcanic sequence dominated by extrusive basaltic rocks spanning the Paleocene–Eocene boundary in the NE Faroe–Shetland Basin. The well comprises one of the thickest drilled sequences through the North Atlantic Igneous Province. Integrated analysis of drill cuttings and wireline-log data reveals key volcanic lithofacies: (1) tabular lava flows; (2) compound lava flows; (3) hyaloclastite; (4) volcaniclastic rocks. The volcanic facies reveal two major sub-aqueous to subaerial sequences consistent with lava delta progradation. These sequences are separated by a volcanic hiatus represented by extensive reddened soils, which preceded the re-submergence of the area. Emergence followed by submergence of the first lava delta is interpreted to record an intra-T40 transient uplift event near the Paleocene–Eocene boundary. Basalts from the lower c. 1.3 km have low TiO2 (<1.5 wt%) and low Zr/Y (2–3), with olivine-phyric picrites towards the base (Mg# 70–82; olivine Fo85–91). The hiatus correlates precisely with a change to high-TiO2 (2.5–3.2 wt%), high-Zr/Y (>4) compositions, which dominate the upper sequence. The associated change in lava geochemistry, transient uplift and volcanic hiatus appears consistent with a transient pulse of hot buoyant plume material passing beneath the area. Supplementary material: All raw geochemical data and supplementary analyses are available at http://www.geolsoc.org.uk/SUP18888.

3D seismic imaging of the shallow plumbing system beneath the Ben Nevis Monogenetic Volcanic Field: Faroe-Shetland Basin

Seismic reflection data allow for the 3D imaging of monogenetic edifices and their corresponding plumbing systems. This is a powerful tool in understanding how monogenetic volcanoes are fed and how pre-existing crustal structures can act as the primary influence on their spatial and temporal distribution. This study examines the structure and lithology of host-rock as an influence on edifice alignment and provides insight into the structure of shallow, sub-volcanic monogenetic plumbing systems. The anticlinal Ben Nevis Structure, located in the northerly extent of the Faroe–Shetland Basin, NE Atlantic Margin, was uplifted during the Late Cretaceous and Early Paleocene by the emplacement of a laccolith and a series of branching sills fed by a central conduit. Seismic data reveal that multiple intrusions migrated up the flanks of the Ben Nevis Structure after its formation, c. 58.4 Ma (Kettla-equivalent), and fed a series of scoria cones and submarine volcanic cones. These monogenetic edifices are distributed around the crest of the Ben Nevis Structure. The edifices are fed from a complex network of sills and transgressive sheets, involving lateral magma migration of tens of kilometres before extrusion at the surface. This work highlights the importance of underlying basin structures in influencing the sites and development of subaerial monogenetic fields, and the importance of lateral magma flow within volcanic systems. Supplementary materials: The results of the statistical alignment analysis of the monogenetic edifices are available at https://doi.org/10.6084/m9.figshare.c.3662809.