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.