Graptolite faunas from the Ballantrae ophiolite complex and their structural implications

Abstract

Synopsis Graptolites from the Bennane Head area of the Ballantrae complex indicate ages ranging from lowest Arenig (Lancefieldian, with Tetragraptus approximates) to middle Arenig (Chewtonian). The distribution of the graptolite faunas strongly suggests a tectonic repetition of the sequence of lavas and interbedded clastic sediments in which they are contained. This throws some doubt on previous assumptions of an anomalously thick volcanic succession within the complex, a feature which has been widely quoted in support of the various proposals for its origin. Instead it seems likely that volcanic sequences of different ages and origins have been structurally juxtaposed. The origins of the older, lower-middle Arenig volcanics are confused but a younger, middle-upper Arenig sequence seems likely to be the product of an island arc. A phase of major thrusting intervened between the two volcanic episodes.