Abstract The Port Askaig Formation (Fm.) is a thick glaciogenic succession within the Dalradian Supergroup that consists of over 700 m of variably dolomitic diamictite, conglomerate, sandstone mudstone and minor dolomite, and is bounded by mixed siliciclastic–carbonate successions of the Islay (Lossit) and Bonahaven formations. These strata are exposed in the metamorphic Caledonides of Scotland, although excellent preservation of sedimentary structures can be found at several sites. An extensional setting for this succession has been proposed based on stratigraphic and structural arguments. The available chemostratigraphic data include the Chemical Index of Alteration, δ 13 C and Sr-isotope values. Palaeomagnetic analyses have been shown to be subject to post-depositional Caledonian overprinting. There is also continued debate over the regional palaeogeographical reconstructions of the Scottish promontory for this time period. The succession is chronologically poorly constrained with U–Pb analyses of stratigraphically much higher or lower deposits. The thick succession is thought to record glacially influenced marine sedimentation and reworking of unstable sediments in a tectonically active setting with evidence of ice-margin fluctuations. Alternative palaeoenvironmental interpretations that focus on glacial terrestrial processes and emphasize climatic influence instead of tectonic activity have also been proposed. The overlying carbonate is a lithologically diverse coastal complex and so does not fit the Neoproterozoic norm. Research has to date focused on the stratigraphic and sedimentological aspects of this succession, as well as some of the broader palaeogeographical and structural features of the Dalradian basin. Future efforts should focus on the chronological, structural and palaeogeographical constraints of this succession.

Abstract Of the three major Neoproterozoic supracrustal units in the Scottish and Irish Highlands (the Torridonian, Moine and Dalradian Supergroups), only the latter contains evidence of Neoproterozoic glaciations. The Dalradian is siliciclastic-dominated and constitutes much of the Scottish–Irish Highlands between the Great Glen and Highland Boundary Fault Zones, and their correlatives in Ireland. At the time of writing, three stratigraphically distinct glacial intervals in the Dalradian have been documented in the literature. The oldest is the Port Askaig Formation (Fm.) at the base of the Argyll Group (see Arnaud & Fairchild 2011 ). It ranges from several tens to many hundreds of metres in thickness and occurs in numerous localities in Scotland and the north of Ireland. A second glacial is recorded in the middle part of the Argyll Group (Easdale Subgroup) and consists of localised sedimentary breccias as well as pelites and schists containing dropstone/lonestone units inferred to be ice-rafted debris; these rocks are patchily preserved and typically a few metres or less in thickness. It is sharply overlain by a variably developed carbonate unit that is marked by a 1–7-m-thick, light-coloured, basal dolostone or dolomitic limestone interpreted as a cap carbonate. This succession is best preserved in Donegal, Ireland, as the Stralinchy–Reelan glacial and Cranford cap-carbonate sequence. A correlative cap carbonate, the Whiteness Limestone, has been identified in the Shetland Islands. The third and youngest glacial is represented by locally preserved dropstone and polymict diamictite beds ranging in thickness from several to a few tens of metres in thickness in the lower Southern Highland Group. These include the MacDuff and Loch na Cille Boulder Beds in, respectively, NE and SW Scotland, and the Inishowen Beds in Donegal, Ireland.