Geoliday – Extended Edition

By John Donato, Principal Geophysicist

Geoliday in Isle of Man

One Footstep over the Iapetus Suture?

Recently my wife and I spent a week on the Isle of Man and I persuaded her to accompany me to some sites of geological interest.  There are many such sites on the island and we set off armed with the excellent Geologists’ Association Guide No 46, ‘The Geology of the Isle of Man’.  On one visit, the guide suggested to us that we should take the obligatory tea and cake from the café at Niarbyl Point, after which we wandered down to the foreshore to see the Niarbyl Fault.  In the photo, I am stepping over the line of this complex thrust fault.

Under my right foot (West) are Middle Silurian turbidite sediments of the Niarby Formation of the Dalby Group.  These sediments were derived from the Laurentian continent on the north-west side of the closing Iapetus Ocean.

My left foot (East) rests on the much older, Ordovician turbidites of the Creggan Mooar Formation of the Manx Group.  In contrast, these sediments were derived from a south-easterly source thought to lie within the small Avalonian continent on the south-east side of Iapetus.

The Niarby Fault has brought the two differing sequences into direct contact, possibly when the Laurentian and Avalonian plates docked along the Iapetus Suture during Late Silurian to Early Devonian times.
With my geological tongue firmly in cheek, I could be said to be stepping across two continents, and over the Iapetus Suture, with my left foot in Avalonia and my right foot in Laurentia!