Igneous Rocks and Other Animals

It’s strange how things happen in research. Six years ago I had absolutely no interest in igneous geology, being mentally scarred from 1st-year undergraduate geochemistry lectures and microscope-based petrography practicals. No offence to the teaching staff or the lecture materials, it’s just it wasn’t my thing. I was into glamorous, field-based stuff like structural geology and sedimentology…

An XPL image showing euhedral plagioclase laths and pyroxene present largely wiithin intergranular areas. Field of view 10 mm. Image courtesy of the Imperial College rock library. 1st-year undergraduate memories coming flooding back. Absolutely thrilling.

Then, during my time working in the oil industry for Norsk Hydro (now Statoil) in Bergen, Noway, I was more-or-less forced to map some reservoir sections below igneous intrusions in the Møre Basin, offshore mid-Norway; it was grim, with the sill-complexes massively hampering seismic imaging of the underlying succession. At that point, I’d more or less had it with igneous rocks; even the seismically imaged ones were a pain in the ass. I then stumbled across some interesting research being done by Joe Cartwright and students on the application of these data to understanding magma emplacement, the geometry of sills and associated deformation, and the genesis and internal structure of volcanic vents. They showed igneous rocks are very, very conducive to being imaged by seismic reflection data; these rocks are very dense and acoustically very fast, meaning they are very reflective when encased in less dense, acoustically slower sedimentary rocks. In essence, they had begun to show how the ascent of magma through the Earth’s crust, or at least the ‘fossilised’ products of this process (e.g. sill and dyke networks), could be studied using seismic reflection data. They showed it was possible to probe the structural geology and distribution of sub-volcanic plumbing systems, and to investigate the internal architecture of the associated eruptive product (e.g. vents and volcanoes) themselves.

Seismic profile from the NE Rockall Basin, offshore Ireland, showing a suite of large, highly reflective igneous sills underlying a lava flow and a ‘forced fold’, the latter forming to accommodate emplacement of an underlying saucer-shaped sill (Sill 40). See Hansen & Cartwright (2006). See also the rather fruity ‘Discussion’ by Thomson (2006) and ‘Reply’ by Hansen & Cartwright (2007).

Having been re-inspired, I started playing around with some of the seismic data we had in-house. However, there were more sills and volcanoes than hours in the day. And then I met Dr Craig Magee (@DrCraigMagee), an unassuming young man from the sprawling metropolis of Solihull. Craig, a field-based structural geologist, with strong igneous geology leanings, applied for and got a ‘open’ Post-Doctoral Researcher Associate (PDRA) position I advertised, having recently completed a PhD at the University of Birmingham. He was interested in learning new stuff, in particular the use of seismic reflection data to better understand igneous systems, and I was growing increasingly interested by all the super high-amplitude, strata discordant reflections I was observing within many of the sedimentary basins I was working in. In short, it was match made in heaven and, as they say, the rest is history…

Left: time-structure map showing middle Eocene (ca. 42 Ma) volcanic build-ups in the Bight Basin Igneous Complex (BBIC), Ceduna sub-basin, offshore southern Australia. Right: map showing the spatial relationship between sills and volcanic build-ups. See Jackson (2012) for details.

Along with collaborators Dr Nick Schofield and Dr Simon Holford (@simonholford), and a veritable (insert collective noun here) of MSc and Undergraduate students, Craig and I have worked on many, many basins around the world. We’ve studied sill-induced forced folding in the Irish Rockall Basin and the NW Shelf of Australia, the impact of igneous intrusions on petroleum system development, and the interaction between magma and normal faults, amongst many other things.

Highly reflective igneous laccolith (left) and sills (right) in the Bight Basin Igneous Complex, Ceduna sub-basin, offshore southern Australia. The laccolith is overlain by a large forced fold that formed to accommodate magma emplacement (see Jackson et al., 2013).

In the talk ‘Hot Rocks Under Our Feet; Imaging Igneous Geology with Seismic Reflection Data‘ I will demonstrate that seismic reflection data are an awesome, super-exciting tool for studying the large-scale geometry of igneous bodies, and inferring the mechanics and controls on magma emplacement in the upper crust. I will also show how igneous rocks negatively and positively impact petroleum system development in sedimentary basins. I’ll try and convince people that seismic reflection images of igneous rocks should be shown in undergraduate ‘igneous geology’ lectures, and that the structures allowing and forming due to the emplacement of magma are as exciting as the magmatic products themselves. In short, I will show how I fell back in love with igneous geology…

Featured image: igneous sills (black) emplaced in layered clastic sedimentary rocks. Image courtesy of John Howell (University of Aberdeen)

Author: Christopher Aiden-Lee Jackson

I am Professor of Basin Analysis @imperialcollege. I ❤️ 🏃🏿, 🚴🏿 and @basinsIC (⛏). I obsess about the tectono-stratigraphic development of sedimentary basins. Why? Because I'm hopeless at everything else.

5 thoughts on “Igneous Rocks and Other Animals”

  1. Hi Chris, is seismic still not used in undergrad igneous lectures? It would add a lot of scale and shape context to rather abstract diagrams or isolated outcrops. Spotting sills on 3D seismic off Australia was fun, new geometries and effects for me since university.

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