Lectures

The 1.2 billion year old Stoer Group, in the far north-west of Scotland, represents the oldest undeformed sedimentary succession in Britain. Within its hundreds of metres of mainly fluvial sandstones is a relatively thin (5-10 m) unit, the Stac Fada Member, in which green-grey shards of once molten rock are intimately mixed with the predominant red sandstone. For decades this was interpreted as a volcanic mudflow until, in 2009, Oxford geologist Ken Amor discovered shocked quartz grains and geochemical anomalies. These proved that the Stac Fada Member actually had been formed by the impact of a giant meteorite, the first such evidence discovered in the UK.

On my first visit to the outcrop of this remarkable rock in 2011 I discovered previously unrecognised evidence for high-speed ejection of unmelted rock fragments from the impact, and also deduced that the impact deposit had come from the east rather than the west as had been suggested previously. However, any crater of this age that lies to the east will now be concealed by rocks thrust westwards across much of northern Scotland along the Moine Thrust.

But even a buried impact crater can be detected by its gravity signature, with the shattered rock and sediment infill associated with the crater contrasting with the denser target rocks. Consulting the BGS gravity map of the UK revealed a huge gravity anomaly more than 50 km east of the Stac Fada Member outcrop, centred on the town of Laird. Could this 40 km wide anomaly really be the source crater?

This talk will describe the chance series of events that led to this discovery, and some of the ongoing work that has considerable implications for understanding the geology of Scotland and the nature of giant meteorite impacts.

Salt is not simply for fish 'n' chips or icy roads. It is a unique and perhaps underappreciated rock type, living in the shadows of its more glamorous carbonate and clastic neighbours. For example, not only is salt one of the economically most important rock types on earth, forming the seals to super-giant hydrocarbon accumulations, but it is also responsible for forming some of the most complex geological structures observed on the earth. In this talk I will celebrate salt, highlighting its unique physical properties, its role in the generation of complex geological structures, and its importance in terms of hydrocarbon exploration.