Composition and provenance of the Silurian to Devonian sandstone sequences of the southern Midland Valley

Abstract

Synopsis A provenance study of rocks from the Silurian inliers and Siluro-Devonian Lanark Group sandstones of the southern Midland Valley has shown that they are petrographically and compositionally similar, and were derived from the same source terrane. The clast content of these rocks indicates that before widespread Lower Devonian calc-alkaline volcanism, this source terrane included volcanic and hypabyssal igneous rocks that appear to have been associated with a wacke sandstone-dominated sedimentary sequence and a granitic igneous suite. The possibility that the source rocks for these sequences are within the Midland Valley Terrane, but now not exposed, is discussed. Minor differences in sandstone composition between the Carmichael, Eastfield and North Esk inliers are interpreted as reflecting either slight differences in the source or deposition within individual sub-basins. Comparable regional variations within the Swanshaw Sandstone Formation suggest that sub-basin architecture continued to influence sediment dispersal patterns during the deposition of the basal Lanark Group. The onset of Lower Devonian calc-alkaline volcanism coincided with a major change in sandstone composition within the Lanark Group and accompanied the replacement of the relatively small Silurian sub-basins by a single larger basin.

Zircon age constraints on the provenance of Llandovery to Wenlock sandstones from the Midland Valley terrane of the Scottish Caledonides

“…This Lower Silurian sub-basin architecture continued into the Upper Silurian and controlled the deposition of the redbed fluviatile sandstones and alluvial fan conglomerates which form the lower part of the overlying (and partly Devonian) Lanark Group. Provenance studies, in particular Phillips et al (2004), have shown that strata of the Silurian inliers and Siluro-Devonian Lanark Group sedimentary rocks were derived from the same source, which included volcanic and hypabyssal igneous rocks, a wacke sandstone-dominated sedimentary sequence, granitic plutonic rocks and older metamorphic rocks (also see McGiven 1967;Bluck 1983;Heinz & Loeschke 1988;Syba 1989). Minor differences in sandstone composition between the inliers reflect the influence of the sub-basin architecture on sediment dispersal patterns.…”

“…Apart from thin calcalkaline, rhyodacitic metabentonite beds (Batchelor & Clarkson 1993) (Phillips et al 1998), none has been recognised unequivocally within the sandstones of the Lanark Basin or older Silurian sub-basins (Phillips et al 2004).…”

“…This clast assemblage is consistent with these sedimentary rocks having been derived from a recycled orogenic provenance, comprising an eroded volcanic or igneous complex with an associated sedimentary cover, founded upon an older metamorphic basement (cf. Phillips et al 2004). Minor differences in sandstone composition between the clastic sequences within the individual inliers are thought to record localised changes in the composition of the source terrane, and/or sediment dispersal patterns within individual sub-basins (Phillips et al 2004).…”

Upper Llandovery–Wenlock (Silurian) palynology of the Pentland Hills inliers, Midland Valley of Scotland

“…The tectonic evolution of the Midland Valley of Scotland during the Lower Palaeozoic and its setting in Caledonide terrane models have been the focus of much recent debate (Phillips et al 1998(Phillips et al , 2004Smith 1995;McKerrow et al 1991;Williams & Harper 1988). Central to this debate are the palaeontological, sedimentological and structural data from the Silurian rocks of the inliers along the southern margin of the Midland Valley ( Fig.…”

The Southern Uplands Terrane in Scotland – a notional controversy revisited

“…Waldron et al 2008Waldron et al , 2014 Midland Valley conglomerates. These were derived from the south but from a provenance that is not represented within the currently exposed Southern Uplands Terrane, a circumstance established by a wide range of evidence and discussed by, amongst others, Bluck (1983Bluck ( , 1984, Syba (1989), Stone & Evans (2001) and Phillips et al (2004). The complications and contradictions arising from these provenance mis-matches between terranes as currently positioned have been widely exploredsee Bluck (2013) for a recent comprehensive review -but remain unresolved.…”