Carboniferous Limestone - Depositional Basins

Depositional Basins

Within Great Britain the suite of rocks known traditionally as the Carboniferous Limestone Series were deposited as marine sediments in three distinct ‘provinces’ separated by contemporary landmasses. One of these landmasses was the Wales-London-Brabant Massif, an east-west aligned belt of land stretching through central Wales and the English Midlands to East Anglia and on in to Belgium. The limestones deposited to its south form a distinct South Wales-Mendip province which extends from Pembrokeshire in the west through southern Carmarthenshire, Glamorgan and south Powys to Monmouthshire, Gloucestershire and north Somerset. These rocks continue eastwards at depth beneath Oxfordshire. The Carboniferous Limestone sequence of South Wales and the Bristol area is currently (2012) subdivided thus:

• Pembroke Limestone Group (uppermost/youngest)
  • Oystermouth Formation
  • Oxwich Head Limestone Formation
    • Penderyn Oolite Member
    • Honeycombed Sandstone Member
  • Dowlais Limestone Formation
  • Abercriban Oolite Subgroup & Clydach Valley Subgroup
• Avon Group
  • Cwmyniscoy Mudstone Formation
  • Castell Coch Limestone Formation (lowermost/oldest)

The limestone found north of the Wales-London-Brabant Massif and south of the emergent Southern Uplands block is identified as a separate northern province. It is characterised by the presence of numerous ‘blocks’ and ‘basins’ each with its own particular depositional style.

To the north of the Southern Uplands are the limestones of the Scottish Midland Valley stretching from Ayrshire and Arran in the west to Fife, Lothian and Berwickshire in the east. Though of Carboniferous age, the limestones of this Scottish province are not assigned to the Carboniferous Limestone Supergroup.

The Carboniferous Limestone is widespread throughout Ireland.