A Late Arenig (early Yapeenian) graptolite fauna and the coeval tectonic development of the Ballantrae Complex ophiolite, SW Scotland

Abstract

Synopsis A Late Arenig graptolite fauna is known from one isolated occurrence within the Ballantrae Complex ophiolite; a borehole core taken near North Ballaird farm. Reassessment of that fauna shows it to be of early Yapeenian (Yal) (just possibly late Castlemainian; Ca4) age and confirms that it is significantly younger than all other dated components of the complex. Of particular importance are the records of Arienigraptus gracilis (Ruedemann), Isograptus australis Cooper and Yutagraptus ? v-deflexus (Harris). The graptolitic sequence conformably underlies a microconglomerate bed in which the clasts are coated by algal encrustation. Some of those clasts are of serpentinite, which was therefore clearly available for erosion into shallow water during the early Yapeenian stage. It seems most likely that the serpentinite was structurally elevated during obduction of the ophiolite, a process driven by arc-continent collision. On a broad, regional scale this can be associated with the collisional event initiating the Grampian Orogeny, for the timing of which the early Yapeenian age of the North Ballaird graptolite fauna provides an additional control.