Snowball Earth on Islay
These rocks record major global climate fluctuations some 650 million years ago: glacial deposits indicating a possible worldwide ice age (Snowball Earth) are sandwiched between carbonate rocks (with some spectacular stromatolites) which were deposited in warmer climates.
There are four key localities on the eastern part of Islay that record the transition into and emergence from a ‘Snowball Earth. At Beannan Dubh, near Ballgrant, the Lossit Limestone Formation with its oolites and stromatolites is evidence of a warm temperate climate consistent with the probable location of Islay in tropical latitudes and is overlain (probably unconformably) by a variety of glacial deposits (mainly diamictite) of the Port Askaig Tillite Formation. A dramatic brecciated and karstic erosion surface is exposed overlain and infilled by an enigmatic sequence of iron-rich siltstones and sandstones before the main sequence of diamictites. At Port Askaig itself and along the coast to the south around Fionn Phort a thick seqence of diamictite is exposed containing numerous large granite ‘stones’. The glacial sequence is followed by a shallow marine carbonate and clastic sequence – purported to be a ‘cap carbonate’. At Caol Isla, just to the north of Port Askaig, the lower part of the overlying Bonahaven Dolomite is exposed with siltstones and carbonates containing mudcracks and ripple marks and further north at Bunnahabhain, a thick sequence of dolomitic rocks is exposed with stromatolites and some desiccation features indicating the return to the climate conditions that would be expected at these tropical latitudes.
There are also dramatic exposures of the stromatolitic horizons within the Bonahaven Dolomite – including extensive beds of exhumed three-dimensional bioherms – on the remote North Coast of Islay and the most complete and best-exposed occurrence of the Port Askaig Tillite occurs on the uninhabited Garvellach Islands in the southern Firth of Lorne some 40 km to the north.
A significant amount of geochemical sampling has been undertaken through this sequence – using mainly carbon and strontium isotopes as a palaeo-temperature proxy. Carbonates in the Lossit Limestone immediately below the Port Askaig Tillite record a gradual decrease in d13C (the so-called ‘Islay Anomaly’ – which has world-wide correlative value) and the Bonahaven Dolomite displays negative d13C throughout with a pronounced rapid recovery to positive values at the top. Recent work has concentrated on attempting to ascertain the relative effects of metamorphic fluid flow and diagenesis on the isotope signature.
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