Dougal A. Jerram

Silicic volcanism: An undervalued component of large igneous provinces and volcanic rifted margins

Silicic volcanic rocks are associated with most, if not all, continental ×ood basalt provinces and volcanic rifted margins, where they can form substantial parts of the eruptive stratigraphy and have eruptive volumes >10 4 km 3 . Poor preservation of silicic volcanic rocks following kilometer-scale uplift and denudation of the volcanic rifted margins, however, can result in only deeper level structural features being exposed (i.e., dike swarms, major intrusions, and deeply subsided intracaldera µlls; e.g., North Atlantic igneous province). The role of silicic magmatism in the evolution of a large igneous province and rifted margin may therefore be largely overlooked. There are silicic-dominated igneous provinces with eruptive volumes comparable to those of maµc large igneous provinces ( >10 6 km 3 ), but that have low proportions of basalt expressed at the surface. Some silicic large igneous provinces are associated with intraplate magmatism and continental breakup (e.g., Jurassic Chon Aike province of South America, Early Cretaceous eastern Australian margin), whereas others are tectonically and geochemically associated with backarc environments (e.g., Sierra Madre Occidental). Silicic volcanic rocks formed in these two environments are similar in terms of total eruptive volumes, dominant l ithologies, and rhyolite geochemistry, but show fundamental differences in tectonic setting and basalt geochemistry. Large-volume ignimbrites are the dominant silicic volcanic rock type of continental flood basalt and silicic large igneous provinces. Individual silicic eruptive units can have thicknesses, areal extents, and volumes that are comparable to, or exceed, in

Understanding crystal populations and their significance through the magma plumbing system

Abstract Crystals are rarely composed of a single crystal population that have grown solely from the batch of magma in which they are resident on emplacement, either by eruption or shallow intrusion. Close investigation of the majority of crystal populations reveal that they comprise up to four main components: phenocrysts, crystals co-genetic with their magmatic host; xenocrysts, crystals wholly, or in part, foreign to the magmatic host and magma system; antecrysts, crystals which are recycled one or several times before inclusion in the host magma but have an origin within the magmatic system; and microlites, which represent small co-genetic crystals which nucleate and grow rapidly on decompression and eruption. Textural analysis techniques are employed to quantify key aspects of the crystal population, including crystal shape, crystal size distributions, spatial distribution patterns and textural modification using dihedral angles. Santorini provides a case study of an active volcanic system where a combined textural analysis study has been developed, highlighting how the crystal population is being continuously modified by a series of replenishment and mixing events. Developing textural and microgeochemical techniques provides the next stage in the interrogation of crystal populations, linking textures to isotopic heterogeneities and providing fingerprints of where crystals are sourced and re-cycled.

Relating eolian bounding-surface geometries to the bed forms that generated them: Etjo Formation, Cretaceous, Namibia

The Cretaceous Etjo Formation is a 200-m-thick eolian and fluvial sandstone succession exposed in the Huab Basin of northwestern Namibia. Eolian sedimentation was terminated abruptly by the emplacement of flood basalts of the Etendeka igneous province across much of the basin at 132 ± 1 Ma. The lavas “drowned” the dunes and, in doing so, preserved bed forms with heights and wavelengths of up to 100 m and 1.3 km, respectively. Subsequent erosion has resulted in the exposure of these bed forms and provides a rare opportunity to accurately reconstruct the three-dimensional geometry of an ancient eolian system and to relate bed-form morphology to bounding-surface geometries. Some bed forms lacked a single active slipface but instead were characterized by a dome form over which smaller transverse dunes migrated. Other bed forms were characterized by an active slipface and a stoss slope over which smaller, superimposed bed forms migrated. One bed form records a transitional evolution between these two end-member types. Preserved bed sets exhibit a positive (but stoss-erosional) angle of downwind bed-form climb of 1°. Bed forms of the Etjo Formation provide an opportunity to directly relate bed-set thickness to original bed-form height via the angle of climb.

AEOLIAN AND ALLUVIAL DEPOSITION WITHIN THE MESOZOIC ETJO SANDSTONE FORMATION, NORTHWEST NAMIBIA

The Cretaceous Etjo Sandstone Formation is a mixed aeolian and fluvial unit exposed over 5000 km2 in the Huab Basin of Damaraland, northwest Namibia. The distribution of fluvial and aeolian facies reveals complex spatial and temporal variations in depositional style that may be related to four distinct phases of sedimentary evolution. The Krone Member forms the base of the formation and comprises coarse clastic material deposited in an and wadi system by flash floods. This is conformably overlain by a mixed aeolian-fluvial unit that is interpreted to represent a transition from ephemeral fluvial to aeolian sandsheet deposition, with the development of small barchan dunes as aeolian sediment supply increased. Repetitive cycles of alluvial plain to dune sedimentation are common and may indicate climate or subsidence controlled variations in the level of the water table relative to the level of the depositional surface. During deposition of the Main Aeolian Unit, sediment supply increased and large transverse draa bedforms migrated across the basin, rapidly filling available accommodation space. Preservation of isolated barchan dunes in the Upper Aeolian Unit indicates a rapid decrease in sediment supply and a return to restricted aeolian activity, following the emplacement across much of the region of Etendeka flood basalts related to continental break-up of West Gondwana at around 132 Ma. Synsedimentary faulting appears to have been a major control on the geometry and distribution of the Etjo Formation. Extrabasinal controls on depositional style include an initially increasing and subsequently decreasing external supply of aeolian sand to the basin, and a gradual transition from semi-arid to hyperacid climatic conditions.

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

The development of volcanic sequences at rifted margins: new insights from the structure and morphology of the Vøring Escarpment, mid‐Norwegian Margin

On the Voring Margin offshore mid-Norway, Paleogene continental breakup was characterized by the extrusion of large volumes of flood basalts erupted in different depositional environments. The transition from subaerial to submarine emplacement environment is marked by the formation of the Voring Escarpment which records the early encroachment of flood basalt into the basin and the buildup of a lava delta system. The increased availability of new and reprocessed high-quality seismic data allows a more detailed characterization of the along-strike and across-strike continuity and variability of the different volcanic seismic facies units. Detailed seismic interpretation shows that the ~350 km long NE-SW trending Voring Escarpment is a prominent feature along the Voring Margin with a height ranging between 200 and 1600 m. Structurally, the Voring Escarpment is segmented along strike into five segments (E1–E5) with different controlling factors leading to variation in accommodation space. Relative sea level change and magma supply are the major controlling factors for segments E2 and E4 which are characterized by a well-developed lava delta system and significant escarpment height. Tectonic movements along the Jan Mayen Fracture Zone resulted in second-order segmentation of the E1 segment into pseudoescarpments with a very thin lava delta system and limited escarpment height. Segments E3 and E5, situated along the flanks of Cretaceous/Paleocene highs, are controlled by the structural highs, which were possibly reactivated during breakup time. Our mapping results provide crucial information about the paleogeography and yield important information regarding the paleo–water depth and depocenter locations prior to and during the breakup of the Voring Margin.

Effects of igneous intrusions on the petroleum system: a review

Igneous intrusions feature in many sedimentary basins where hydrocarbon exploration and production is continuing. Owing to distinct geophysical property contrasts with siliciclastic host rocks (e.g., higher Vp, density and resistivity than host rocks), intrusions can be easily delineated within data sets including seismic and CSEM profiles, provided igneous bodies are larger than the detection limit of the geophysical methods. On the other hand, igneous bodies affect geophysical imaging in volcanic basins. Recent analyses of 3D seismic data, supported by field observations and lab-based experiments, have provided valuable insights into the prevailing geometries of intrusions, i.e. (1) layerdiscordant dykes, (2) layer-parallel sills and (3) saucer-shaped intrusions. Where emplaced, intrusive bodies affect all five principal components of a given petroleum system: (1) charge, (2) migration, (3) reservoir, (4) trap and (5) seal. Magmatic activity may positively or adversely affect any of these individual components, for instance by locally enhancing maturation within regionally immature source rocks, typically 30-250% of the intrusion thickness, or by causing compartmentalization of source and reservoir rocks. Site-specific evaluations, including the timing and duration of the magmatic event are needed to evaluate the overall effect of intrusions on a given sedimentary basin’s petroleum system, and these are highlighted by case studies from different volcanic basins.

The petrophysical and petrographical properties of hyaloclastite deposits: Implications for petroleum exploration

Offshore sequences of volcaniclastic rocks (such as hyaloclastite deposits) are poorly understood in terms of their rock properties and their response to compaction and burial. As petroleum exploration targets offshore volcanic rifted margins worldwide, understanding of volcanic rock properties becomes important both in terms of drilling and how the rocks may behave as seals, reservoirs, or permeability pathways. The Hawaiian Scientific Drilling Project phase II in 2001 obtained a 3 km-(2-mi)-long core of volcanic and volcaniclastic rocks that records the emergence of the largest of the Hawaiian islands. Core recovery of 2945 m (9662 ft) resulted in an unparalleled data set of volcanic and volcaniclastic rocks. Detailed logging, optical petrology, and major element analysis of two sections at depths 1831–1870 and 2530–2597 m (6007–6135 and 8300–8520 ft) are compared to recovered petrophysical logs (gamma ray, resistivity, and P-wave velocity). This study concludes deviation in petrophysical properties does not seem to correlate to changes in grain size or clast sorting, but instead correlates with alteration type (zeolite component) and bulk mineralogy (total olivine phenocryst percentage component). These data sets are important in helping to calibrate well-log responses through hyaloclastite intervals in areas of active petroleum exploration such as the North Atlantic (e.g., Faroe-Shetland Basin, United Kingdom, and Faroe Islands, the Norwegian margin and South Atlantic margins bordering Brazil and Angola).

Facies architecture of the Etjo Sandstone Formation and its interaction with the Basal Etendeka Flood Basalts of northwest Namibia: implications for offshore prospectivity

Abstract The Basal Etendeka Flood Basalt stratigraphy in the Huab Basin of northwest Namibia comprises a series of lava flows interleaved with aeolian sandstone bodies of the Etjo Sandstone Formation. The sandstone units are characterized by three main types: (1) the major erg — a mixed aeolian and fluvial facies up to 150 m thick; (2) minor ergs — aeolian facies which occur directly above the first volcanic units and are up to 60 m thick; (3) isolated bodies — multidune, single dune and lava topography infills. A variety of bypass surfaces identified by sand-filled cracks and sediment-lava breccias occur on lava top surfaces. Preserved ripples and pahoehoe lava imprints indicate that the aeolian sand dunes were actively migrating during basalt emplacement. Observations recorded in the Basal Etendeka Flood Basalts which may be of relevance to offshore hydrocarbon exploration include: a major-minor erg relationship resulting in large sandbodies up to 60 m thick which occur directly after the first volcanic units; the occurrence of sand-filled fissures up to 36 m in depth which would greatly influence connectivity in an offshore setting; the identification of bypass surfaces as marker horizons which may laterally correlate with isolated sandbodies.

Plumbing systems of shallow level intrusive complexes

We have come a long way from simple straw and balloon models of magma plumbing systems to a more detailed picture of shallow level intrusive complexes. In this chapter, the sub-volcanic plumbing systems is considered in terms of how we can define the types and styles of magma networks from the deep to the shallow subsurface. We look at the plumbing system from large igneous provinces, through rifted systems to polygenetic volcanoes, with a view to characterising some of the key conceptual models. There is a focus on how ancient magmatic centres can help us better understand magmatic plumbing. New innovative ways to consider and quantify magma plumbing are also highlighted including 3D seismic, and using the crystal cargo to help fingerprint key magma plumbing events. Conclusions are drawn to our understanding of the 3D plumbing system and how these recent advances can be helpful when exploring the other chapters of this contribution.