Abstract

Synopsis

On the Isle of Bute, the Carboniferous Period is represented mainly by volcanic rocks of the Clyde Plateau Volcanic Formation (Viséan, mid- to late Holkerian). Although previously considered to lie on rocks of the Upper Old Red Sandstone (Bute Conglomerate, mainly Famennian) or Kinnesswood Formation of the Inverclyde Group (mainly Tournaisian), exposed contacts in south Bute show the volcanic rocks to lie disconformably on the lower part of the Clyde Sandstone Formation (higher in that same group). Detailed mapping and measurement of stratigraphic sections at several locations in south Bute reveals the Courceyan to Chadian (Inverclyde) sedimentary succession to comprise the Kinnesswood, Ballagan, and the lower part of the Clyde Sandstone formations. In the Ascog area in the east-central part of Bute, the Birgidale Formation (Viséan; Holkerian) overlies these formations and is conformably overlain by volcanic rocks of the Clyde Plateau Volcanic Formation. Newly discovered exposures of the Ballagan Formation at Dunagoil Bay on the west side of south Bute extend the known distribution of the formation on the island. The relatively thin development of Inverclyde formations in south Bute probably reflects the palaeogeographical position of the area on the northeastern shoulder of the now-submerged North-East Arran Trough. The Inverclyde Group rocks are mostly of terrestrial origin but the Ballagan Formation denotes a short-lived shallow marine incursion. Overall, these rocks record a transition from semi-arid to humid climatic conditions, consonant with the suggestion, based on palaeomagnetic investigations, that at the time of deposition the area migrated northwards through arid to tropical palaeolatitudes.

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