Structural and stratigraphic analysis of Cryogenian rocks on Islay, Scotland

The Port Askaig Formation is a succession of Cryogenian glacial deposits found near the middle of the 25 km thick Dalradian Supergroup. On the Isle of Islay, Scotland, the Port Askaig Formation is approximately 750 mthick in the north and thins over approximately 30 km to a few tens of meters towards the southern end ofthe island. The thinning of this stratum was investigated by performing a literary review of previous works to construct six stratigraphic columns and three cross-sections across the eastern half of Islay. Two models were developed using these cross-sections and columns to explain the thinning of the Port Askaig Formation on Islay. The syndepositional model suggests that the Dalradian sediments were deposited during the late Neoproterozoic Era in an extensional basin over an array of listric normal faults with increasing tectonic activity resulting in lateral thickness changes over short distances. The synorogenic model suggeststhe Dalradian Basin was subjected to deformation in the Caledonian Orogeny during the Ordovician Period. The basin experienced both extensional and compressional stresses resulting in thickness changes throughfolding, faulting, and shearing of the Dalradian sediments. It can be concluded that the major control of thinning of the Port Askaig Formation is due to basin tectonics, followed by the folding of the Dalradian Basin by compressive stresses