Abstract

This 250-word extract was created in the absence of an abstract.

This field study attempts to explain, in as much detail as possible, the depositional processes, environment and history of the Port Askaig Tillite in the Dalradian succession.

In the 750 m thick Tillite sequence, abundant and presumably far-travelled granite stones (up to l-5m in diameter) and sedimentary fragments (the largest of which measures 320 x 64 x 45 m) are contained in 47 mixtites (till-like beds with thicknesses from 50 cm to 65 m), which are separated by siltstone, sandstone, conglomerate and dolomite interbeds (ranging from a few centimetres to 200m in thickness).

The Tillite lies at the same horizon, between formations which are rich in carbonates and contain stromatolites, for 700 km from north-east Scotland to western Ireland. Successions at five outcrops (Garvellachs, Port Askaig, Mull of Oa, Fanad, Schichallion) are described and a type section of the Tillite, containing five members, is erected in the area round Port Askaig. These members are correlated between the outcrops described and certain individual mixtites are correlated for 160 km between the Garvellachs, Islay and Fanad.

Several sedimentary features are described: (of the mixtites)their sharp lower contacts, internal bedding, soft sediment folds, sandstone downfolds and the tectonic nature of the pebble fabric; (of the interbeds) their very variable palaeo-currents, the beach conglomerates, wave-cut erosion surfaces, varves and the outsize stones and drop-in structures produced by ice-rafting. Polygonal sandstone wedges-inferred to be of periglacial origin-are described for the first time in a pre-Pleistocene formation and are contrasted with post-compactional sandstone dykes.

After discussing