Lithological and structural controls on the emplacement and morphology of sills in sedimentary basins

Abstract

Three-dimensional seismic data from the NW European Atlantic margin provide detailed data on the relationships between sills, dykes, laccoliths and pre-existing basin structure. The data show that sills are predominantly concave-upwards in shape, being complete or partial versions of radially or bilaterally symmetrical forms that possess flat inner saucers connected to a flat outer rim by a steeply inclined sheet. Sills can be sourced from the steep climbing portions of deeper sills, and can be shown to exploit pre-existing faults to feed shallower sills and laccoliths. Magma flow patterns, as revealed by opacity rendering, suggest that sills propagate upwards and outwards away from the magma feeder. As an individual sill can consist of several leaves emplaced at different stratigraphic levels, and as a sill or dyke can provide magma to other sills and laccoliths, the data suggest that neutral buoyancy concepts may not provide a complete explanation for the mechanism and level of sill emplacement. Instead, the data suggest that the presence of ductile horizons such as overpressured shales may permit sill formation below the neutrally buoyant level. Emplacement below the level of neutral buoyancy would permit sill inflation, and the associated fracturing of the country rock would permit magma to exploit these pathways in order to ascend to shallower levels and feed shallower intrusions.

Mechanisms of overburden deformation associated with the emplacement of the Tulipan sill, mid-Norwegian margin

“…Such a structure is in good agreement with the two-stage model of shallow magma emplacement, whereby an initial flat sill intrudes along a stratigraphic weakness followed by an upward, outward transgressive emplacement of inclined sheets (e.g., Malthe-Sørenssen et al, 2004;Hansen and Cartwright, 2006;Polteau et al, 2008;Thomson and Schofield, 2008;Galland, 2012). The radial offsets visible along the inclined sheets are interpreted as radial flow indicators, i.e., intrusive step and bridge structures indicating upward and outward magma flow during the emplacement into a brittle host rock (e.g., Thomson and Schofield, 2008;Hutton, 2009;Schofield et al, 2010;Magee et al, 2016), also in agreement with the two-stage model of saucer-shaped sill emplacement.…”

“…The 3D volume visualization techniques (e.g., opacity rendering; Supplemental Figures 1, 2, and 3 can be accessed through the following links: s1.eps, s2.eps, and s3.eps) and different surface and volume attributes (e.g., rootmean-square amplitude, coherence, and spectral decomposition) were used to extract additional characteristics of the interpreted reflections in the sedimentary overburden of the Tulipan sill . The interpretation of sills in the seismic data was performed according to their reflection properties, with laterally discontinuous, strata-discordant, high-amplitude anomalies considered to represent intrusions (Smallwood and Maresh, 2002;Polteau et al, 2008;Thomson and Schofield, 2008;Jackson et al, 2013;Magee et al, 2014;Planke et al, 2015). At the location of the Tulipan sill, constrained by Polteau et al (2008), the seismic data display a high-amplitude reflection (Figures 5 and 4).…”

“…When the stresses generated in response to elastic bending exceed the strength of the overburden rocks, failure occurs and characteristic fracture patterns develop. These fracture pattern can be classified into four main types depending on their location in the overburden (Figure 1b): (1) circumferential, tensile fractures at the surface coincident with the area of the largest convex SK24 Interpretation / August 2017 curvature above the sill edges (e.g., Pollard and Johnson, 1973;Bunger and Cruden, 2011;Galland et al, 2016), (2) radial tensile fractures (mode I) and/or normal faults (mode II) due to an extensional regime (i.e., outer-arc stretching) in the central part of the dome structure Magee et al, 2013;Galland et al, 2016), (3) dilational fractures near the peripheral hinge of the dome, close to the intrusion tips, which progress toward the surface (Pollard and Johnson, 1973;Menand, 2008;Thomson and Schofield, 2008;Galland and Scheibert, 2013;Agirrezabala, 2015), and (4) shear fractures resulting from differential uplift at the dome edge (de Saint-Blanquat et al, 2006;Hansen and Cartwright, 2006;Wilson et al, 2016).…”

“…The intrusion hosting sedimentary infill is composed of mainly claystone, with interbedded sand- The simple elastic model envisages the formation of a dome structure (forced fold) above an intrusion (e.g., Pollard and Johnson, 1973;Galland and Scheibert, 2013). (b) Four fracture types related to elastic overburden deformation: (1) circumferential, tensile fractures coincident with the area of the largest convex curvature of the dome (e.g., Pollard and Johnson, 1973;Bunger and Cruden, 2011;Galland et al, 2016), (2) radial tensile fractures (mode I) and/or normal faults (mode II) due to outer-arc stretching (Hansen and Magee et al, 2013;Galland et al, 2016), (3) dilational fractures progressing toward the surface (Pollard and Johnson, 1973;Menand, 2008;Thomson and Schofield, 2008;Galland and Scheibert, 2013;Agirrezabala, 2015), and (4) shear fractures resulting from differential uplift (de Saint-Blanquat et al, 2006;Hansen and Cartwright, 2006;Wilson et al, 2016). (c) Model of volume reduction, i.e., porosity decrease, affected by the aureole of the intrusion (e.g., Hansen and Cartwright, 2006;Jackson et al, 2013).…”

Contrasting mechanisms for crustal sulphur contamination of mafic magma: evidence from dyke and sill complexes from the British Palaeogene Igneous Province

“…At shallow depths, over-pressured shale units can act as 'ductile horizons', permitting sill formation below the expected level of neutral buoyancy, leading to sill inflation and fracturing of the country rock. This has been documented in the North Atlantic Igneous Province in the Judd Basin, where Palaeogene magmas have intruded through a thick sequence of Jurassic-Palaeocene sediments, forming saucer-shaped sills (Thomson & Schofield 2008). It is possible that a similar mechanism of intrusion was in operation during formation of the Trotternish Sill Complex on Skye (Fig.…”