Biostratigraphy and palaeoenvironments of the Ballagan Formation (lower Carboniferous) in Ayrshire

Abstract

Synopsis A local palynostratigraphical subdivision for the Ballagan Formation in Ayrshire is established, based on the Heads of Ayr coastal section. The lower part of the formation is characterized by a low-diversity CM Biozone Colatisporites-dominated assemblage, while the upper part is characterized by high-diversity assemblages containing Acanthotriletes cf. macrogaleatus and Radiizonates mirabilis, and assemblages dominated by Leiosphaeridia algal palynomorphs. Sedimentological analysis of the lower part of the formation suggests deposition in an arid coastal floodplain environment. Up sequence, there is a gradual reduction in sandstones, suggesting reduced fluvial influence. The Ballagan Formation yields ostracodes indicative of marginal marine and brackish environments, which supports the sedimentological interpretation. The lower part of the formation contains species that are typical of the Tournaisian Beyrichiopsis glyptopleuroides-Eriella Biozone, providing a stratigraphical tie with the CM Biozone assemblage. The upper part of the formation yields ostracodes that elsewhere have been reported from the Viséan, but Lycospora pusilla, the index taxon for the Viséan Pu palynomorph Biozone, was not recovered. Two age interpretations can be offered for this part of the section: either late Tournaisian or early Viséan. A late Tournaisian–early Viséan age for the Ballagan Formation would imply that L. pusilla is so rare in the lower parts of its range that dense sampling failed to recover it. By contrast, a Tournaisian age for the entire Ballagan Formation at the coast implies recalibration of some ostracode ranges. Above the Ballagan Formation, mid-Viséan palynomorph assemblages from the Lawmuir Formation allow the Heads of Ayr volcanic vent to be dated in the range late Tournaisian-mid-Viséan, implying correlation with the Clyde Plateau Volcanic Formation.