Caldera-forming eruptions are amongst the most hazardous events in the Earth’s history, and understanding their formation is essential to forecasting activity at active calderas worldwide. In this study we present new field and geochronological evidence for a Paleocene-Eocene caldera from Skye, NW Scotland. Magma exploited a regional thrust fault as a conduit, and then ponded against intrusive igneous rocks emplaced against a regional extensional fault. Replenishment of silicic magma reservoirs with basaltic magma triggered eruptions. The eruptions typically deposited extremely coarse ignimbrites, demonstrating catastrophic collapse of the caldera, which occurred via an inner ring fault and a complexly faulted marginal zone. Collapse was followed by remobilisation of silicic magma and caldera resurgence. The magma consumed dolostone country rocks, causing significant release of CO2 and contributed to the Paleocene-Eocene Thermal Maximum. Our results demonstrate how tectonics localise magma and caldera development, and how this can cause cataclysmic volcanic and climatic hazards.
The Sgurr of Eigg Pitchstone on the Isle of Eigg, NW Scotland, is a crystal-rich, trachydacitic, partially vitrophyric rock, which has previously been interpreted both as a lava and as a sill. We interpret this rock as a chemically zoned, rheomorphic, lava-like ignimbrite that formed during a sustained pyroclastic eruption. The Sgurr of Eigg Pitchstone can be subdivided into discrete emplacement units distinguished by the following features: (1) their present-day weathering characteristics; (2) the orientation, spacing and morphology of the columnar joints; (3) sharp, undulating boundaries with marked topographic breaks. The absence of weathered surfaces, palaeosols, pyroclastic fall deposits or sedimentary rocks at emplacement unit boundaries suggests deposition from a single eruption. The emplacement units, some of which display upper and basal vitrophyres, represent distinct depositional packages that record several rapidly emplaced ignimbrites, which welded, cooled and devitrified as a simple, essentially single, cooling unit, during eruption from a sustained, low pyroclastic column. The Sgurr of Eigg Pitchstone displays a pervasive base-parallel flow banding, which is folded into intrafolial recumbent isoclinal folds. The flow banding and folds indicate that rheomorphism occurred throughout deposition. The Sgurr of Eigg Pitchstone is interpreted as an erosional remnant of an extensive ignimbrite sheet, the first such unit recorded within the North Atlantic Igneous Province.
Evidence for meteorite impacts in the geological record may include the presence of shocked minerals, spherule layers, and geochemical anomalies. However, it is highly unusual to find unmelted crystals from the actual impactor within an ejecta layer. Here we detail the first recorded occurrence of vanadium-rich osbornite (TiVN) on Earth, from two sites on Skye, northwest Scotland, which are interpreted as part of a meteoritic ejecta layer. TiVN has only previously been reported as dust from comet Wild 2, but on Skye it has been identified as an unmelted phase. Both ejecta layer sites also contain niobium-rich osbornite (TiNbN), which has not previously been reported. An extraterrestrial origin for these deposits is strongly supported by the presence of reidite (a high-pressure zircon polymorph), which is only found naturally at sites of meteorite impact. Barringerite [(Fe,Ni)2P], baddeleyite (ZrO2), alabandite (MnS), and carbon-bearing native iron spherules, together with planar deformation features and diaplectic glass in quartz, further support this thesis. We demonstrate through field relationships and Ar-Ar dating that the meteorite strike occurred during the mid-Paleocene. This is the first recorded mid-Paleocene impact event in the region and is coincident with the onset of magmatism in the British Palaeogene Igneous Province (BPIP). The Skye ejecta layer deposits provoke important questions regarding their lateral extent at the base of the BPIP and the possibility of their presence elsewhere beneath the much larger North Atlantic Igneous Province.
Abstract Research on the British Paleocene Igneous Province (BPIP) has historically focused on the emplacement, chemistry and chronology of its elaborate central intrusive complexes and lava fields. However, the BPIP has also been dramatically shaped by numerous erosion, sedimentation and volcano-tectonic events, the significance of which becomes ever clearer as localities in the BPIP are re-investigated and our understanding of volcano-sedimentary processes advances. The resultant deposits provide important palaeo-environmental, palaeo-geographical and stratigraphical information, and highlight the wide range of processes and events that occur in ancient volcanic settings such as the BPIP. In this paper we review the sedimentary and volcano-tectonic processes that can be distinguished in the BPIP, and conceptualize them within a generalized framework model. We identify, and describe, the sedimentary responses to four broadly chronological stages in the history of the BPIP volcanoes: (1) the development of the lava fields, (2) early intrusion-induced uplift, (3) caldera collapse and (4) post-volcano denudation and exhumation of central complexes. We highlight and illustrate the range of sedimentary processes that were active in the BPIP. These operated on and helped shape a dynamic landscape of uplands and lowlands, of alluvial fans, braided rivers, lakes and swamps, and of volcanoes torn apart by catastrophic mass wasting events and/or caldera collapse.
Abstract The movement of magma through the shallow crust and the impact of subsurface sill complexes on the hydrocarbon systems of prospective sedimentary basins has long been an area of interest and debate. Based on 3D seismic reflection and well data, we present a regional analysis of the emplacement and magmatic plumbing system of the Palaeogene Faroe‐Shetland Sill Complex (FSSC), which is intruded into the Mesozoic and Cenozoic sequences of the Faroe‐Shetland Basin (FSB). Identification of magma flow directions through detailed seismic interpretation of approximately 100 sills indicates that the main magma input zones into the FSB were controlled primarily by the NE–SW basin structure that compartmentalise the FSB into its constituent sub‐basins. An analysis of well data shows that potentially up to 88% of sills in the FSSC are <40 m in thickness, and thus below the vertical resolution limit of seismic data at depths at which most sills occur. This resolution limitation suggests that caution needs to be exercised when interpreting magmatic systems from seismic data alone, as a large amount of intrusive material could potentially be missed. The interaction of the FSSC with the petroleum systems of the FSB is not well understood. Given the close association between the FSSC and potential petroleum migration routes into some of the oil/gas fields (e.g. Tormore), the role the intrusions may have played in compartmentalisation of basin fill needs to be taken fully into account to further unlock the future petroleum potential of the FSB.
Abstract The mechanisms by which coatings develop on weathered grain surfaces, and their potential impact on rates of fluid-mineral interaction, have been investigated by examining feldspars from a 1.1 ky old soil in the Glen Feshie chronosequence, Scottish highlands. Using the focused ion beam technique, electron-transparent foils for characterization by transmission electron microscopy were cut from selected parts of grain surfaces. Some parts were bare whereas others had accumulations, a few micrometres thick, of weathering products, often mixed with mineral and microbial debris. Feldspar exposed at bare grain surfaces is crystalline throughout and so there is no evidence for the presence of the amorphous ‘leached layers’ that typically form in acid-dissolution experiments and have been described from some natural weathering contexts. The weathering products comprise sub-urn thick crystallites of an Fe-K aluminosilicate, probably smectite, that have grown within an amorphous and probably organic-rich matrix. There is also evidence for crystallization of clays having been mediated by fungal hyphae. Coatings formed within Glen Feshie soils after ∼1.1 ky are insufficiently continuous or impermeable to slow rates of fluid-feldspar reactions, but provide valuable insights into the complex weathering microenvironments on debris and microbe-covered mineral surfaces.
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