Magma and sediment-I Emplacement mechanism of late Carboniferous tholeiite sills in northern Britain

Abstract

Like other great tholeiite sheets of the world, the Midland Valley Sill and Whin Sill were emplaced in thick accumulations of sediments which, at the time of intrusion, were not much older nor had been orogenically deformed. However, the Carboniferous sediments of northern Britain had been deposited under the influence of continuous differential subsidence, so that they were already disposed in syn-sedimentary basins and swells when intrusion began. By drawing palinspastic contours of the sills relative to a late Westphalian marine band (approximating to the surface at the time of emplacement) it is established that the dolerites mainly follow bedding planes, dipping at angles of as much as 5 ° down to the bottoms of the basins. Isopachs of the sills indicate a further relationship, for the intrusions are thickest at the bottoms of the basins. These facts are not explained by any intrusion mechanism previously postulated, though there seems to be broad agreement as to the processes which have given rise to thinning and en echelon lensing at leading edges as well as to step-and-stair transgression of sediment bedding. It is proposed that emplacement must have been controlled in part by gravitational flow down dip from feeder dykes which extended to between 0.5 and 1.0 km below the contemporary surface. Thereafter hydrostatic equilibrium was attained first by accumulation at the bottom of the basins and then by further advance up-dip under pressure of head. The sheets are so similar in aspect to flood basalts that it must be assumed that the sediments offered only low frictional resistance to splitting and flow.

Dike propagation through layered rocks

“…In the Henry Mountains area, therefore some sills are found within a sedimentary formation and no particular lithology acts as a barrier to dike ascent. These observations are strongly reminiscent of those of Francis [1982] on the sills of northern Britain.…”

“…This does not appear to be so. Francis [1982], for example, found that no particular lithology determines the intrusion level in the Carboniferous strata of northern Britain.…”

“…Focussing on rigidity or density may not be sufficient to account for dike behavior, which depends on a buoyancy force developed over a finite nose length and hence a finite thickness of country rock. Discussing the observation by Du Toit [1920] and Mudge [1968] that the bases of thick mudstones are preferred horizons for dolerite sills in some regions, Francis [1982] emphasized that this does not apply to the sills of northern Britain. He proposed that none of the British Carboniferous mudstones were thick enough to have exerted that kind of control on magma emplacement.…”

Analysis of the geometry of diabase sills of the Serra Geral magmatism, by 2D seismic interpretation, in Guareí region, São Paulo, Paraná basin, Brazil

“…Among the many parameters that contribute and control the development of a sill, the work of Neumann et al (2003) compiles interpretations from various authors and lists the following factors: the density of the magma and the pressure applied in relation to the regional stress field (Anderson 1951); the thickness of the overburden package (Mudge 1968); the presence of wet sediments that interact with the magma, forming barriers and producing water vapor (Mudge 1968); mechanical discontinuities, such as the bedding of sedimentary rocks (Mudge 1968, Gretener 1969; the petrophysical properties contrast between different lithological units, inducing horizontal tension between the strata (Gretener 1969); the existence of fractures (Leaman 1975) or faults (Liss et al 2002); and the level of density inversion between the magma and the host rock (Francis 1982).…”

Electrical resistivity tomography applied to geologic, hydrogeologic, and engineering investigations at a former waste-disposal site

“…The landfill is situated within a quartz dolerite sill, which forms part of the late Carboniferous Midland Valley sill complex (Francis, 1982). Borehole records (i.e., BH60) from the site show a maximum sill thickness of 12.6 m. The sill has been partially excavated during quarrying operations, which in places extended to the base of the sill (e.g., BH05, BH06, BH07B and BH08).…”