The Port Askaig Formation, Dalradian Supergroup, Scotland

Arnaud, Emmanuelle
Fairchild, Ian
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Geological Society of London

The Port Askaig Formation is a thick glacigenic 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, though 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. Available chemostratigraphic data includes Chemical Index of Alteration, 13C and strontium isotope values. Palaeomagnetic analyses have shown to be subject to post-depositional Caledonian overprinting. There is also continued debate over the regional palaeogeographic 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 palaeogeographic and structural features of the Dalradian basin. Future efforts should focus on the chronological, structural and palaeogeographic constraints of this succession.

Port Askaig Formation, Neoproterozoic, Scotland, glacial sediments, cap carbonate