The Early Carboniferous volcanic outliers of Little Cumbrae and south Bute: implications for westward attenuation of the Clyde Lava Plateau

Abstract

Little Cumbrae and south Bute are the closest outliers of Early Carboniferous Clyde Plateau volcanic rocks to those forming the plateau itself, and the only ones with the potential to reveal how the volcanic succession of the plateau becomes attenuated westward beneath the Firth of Clyde. Closest links are to the Renfrewshire Hills Block of the plateau proper, where the Clyde Plateau Volcanic Formation attains its greatest thickness and widest extent.Basal lavas in both outliers can be correlated to the lower part of the Strathgryfe Member of the Clyde Plateau Volcanic Formation (Viséan, Holkerian), which rests disconformably on the Clyde Sandstone Formation (Tournaisian, Chadian). Relative to the Renfrewshire Hills Block, this implies intervening overlap of three older members of the Clyde Plateau Formation and overstep of a varied sedimentary foundation. Higher lavas and pyroclastic accumulations in south Bute are correlative to the Misty Law Trachytic Member and upper part of the Strathgryfe Member. Thin lavas that cap the Renfrewshire Hills Block on its eastern flank probably never accumulated as far west as the present Firth of Clyde.Southwestward from the Renfrewshire Hills, the Clyde Plateau lavas lose about two thirds of their 1000-m thickness in 25–30 km – a rate of thinning that implies they may extend no farther than the Sound of Bute or northeastern Arran.

Structural controls in the western offshore Midland Valley of Scotland: implications for Late Palaeozoic regional tectonics

“…The small tilted fault blocks are capped by lavas assigned to the Strathgryfe Member of the Clyde Plateau Formation, which lie disconformably on the lower member of the Clyde Sandstone Formation (Figs 2, 6). The Inverclyde Group is much condensed relative to the sequence in adjacent Great Cumbrae, implying rapid thinning on the south Bute shoulder, as suggested by Young & Caldwell (2012) (see also Caldwell & Young, 2011; Fig. 1, and cf.…”

“…5), remapping has revealed the presence of two open folds with gentle westerly plunge – the Boat House Anticline and the Hawk's Neb Syncline – referred rather arbitrarily to the E–W fold-set on the basis of their northeasterly axial traces (Figs 5, 8). The Hawk's Neb Syncline includes rocks as young as Clyde Plateau volcanic products correlative with the Misty Law Trachytic Member of the Renfrewshire Hills Block (Caldwell & Young, 2011, fig. 10; Young & Caldwell, 2011 b , pp.…”

“…23–5). The folding in south Bute may post-date the remaining volcanic rocks there held to correlate to part of the Upper Strathgryfe lavas of the Renfrewshire Hills, but this cannot be demonstrated unequivocally in the field (Caldwell & Young, 2011, fig. 10; Young & Caldwell, 2011 b , figs 2, 7).…”

“…1). Within this tract, (a) a group of faults parallel to the HBF played an important role in positioning the Clyde Plateau volcanic outliers of Little Cumbrae and south Bute (Caldwell & Young, 2011); (b) a group of NW-trending faults, first recognized in south Bute (Boué, 1820; Bryce, 1872), includes those that define the North-East Arran Trough (McLean & Deegan, 1978) to the SW and is present on both Cumbrae islands to the NE; and (c) a group of faults, N to NNE in orientation, sharply separates the upper Firth from the adjoining mainland (Caldwell & Young, 2012, fig. 11).…”

“…This fault originally may have juxtaposed the geologically discrete south Bute promontory against the adjoining part of the island further N. Thin wavy lines in the lavas (green) denote the scarps of successive lava flows. All lavas are believed to correlate to the Strathgryfe and Misty Law Trachytic members of the Clyde Plateau Formation in the Renfrewshire Hills (Caldwell & Young, 2011). …”