Mountfield Quarry ASSI
|Site No||ASSI 311|
|Keywords||Omagh Thrust Fault|
Mountfield Quarry exposes a section through the Omagh Thrust Fault, a major structural feature in Northern Ireland’s geology. The site is important as it allows study of the structural relationship between the youngest rocks of the deformed Dalradian sequence and the Tyrone Volcanic Group.
Mountfield Quarry offers unique access to the plane of the Omagh Thrust, one of the most important structural discontinuities in Northern Ireland. It is of international geological importance since it provides evidence of the nature of late-Caledonian to Variscan modification of the boundary between the Central Highlands (Grampian) Terrane and the Midland Valley Terrane.
The fault occurred at a late stage of the Caledonian Orogeny, or mountain building event, some 440 million years ago. It marks the southern boundary of the Sperrin Mountains Dalradian belt and has moved the older Dalradian strata over the younger Ordovician Tyrone Volcanic Group. The fault then represents the boundary between the Grampian and Midland Valley Terranes in Northern Ireland. The site allows study of the structural relationship between the youngest rocks of the deformed Dalradian sequence and the Tyrone Volcanic Group.
The fault takes the form of a pale green to silver-grey mylonitic horizon within a wider (>3m) mylonite zone. The Dalradian rocks are found in the hanging wall of the fault i.e. above the fault plane and are part of the Mullaghcarn Formation within the Southern Highlands Group. Undeformed rocks from the Mullaghcarn Formation are not exposed in the quarry but small, relatively undeformed fragments of grey quartz semipelite and micaceous quartz psammite found within the mylonitic clay matrix, give an indication of the original lithologies. The rocks of the Ordovician Tyrone Volcanic Group in the footwall i.e. below the fault plane are green to greenish black metabasalt and tuff.