Scouting Trip – Carboniferous

The Early Carboniferous Limestone, circa 343-339 Million years ago, was deposited during a Global Highstand in sea-level in shallow, warm seas between the Variscan and Laurussian plates at which time the UK was situated circa 10°S of the paleo-equator. South Wales was located in a shallow embayment of the much deeper Tethys Ocean located far to the southeast of the proto-U.K.

Warm tropical seas in a tropical climate would result in precipitation of large volumes of calcium carbonate (CaCo3) to produce limestone. This environment would also encourage the growth of numerous shelly organisms such as cockle-like creatures and corals. A very good analogue of the Early Carboniferous environment of the UK is the Bahamas today.

A modern-day envioronment analogue for the Lower Carboniferous Limestone

Limestone is formed by the precipitation of calcium carbonate either to form the shells and hard casing of organisms or, as crystals directly from the water. The shells of the organisms can either be preserved in situ, for example as a preserved reef, or can be broken down by waves and currents into pieces that subsequently get re-deposited as Limestone.

Calcium carbonate is soluble to carbonic acid contained in rainwater and more generally in the soil as rain percolates through the Limestone it slowly dissolves the rock fabric. Often Limestone landscapes are characterised by weird shapes known as Klints & Grikes of a Karstic landscape the result of water flowing across and through the rock. Stalactites and Stalagmites (Speleothems) grown when the calcium carbonate contained in the freshwater gets re-precipitated.

Typical Karst features affecting Limestone

The Carboniferous Limestone is exposed at the far western end of the area operated by South Wales GeoTours and is represented by the High Tor Limestone Formation. Outcrops are limited but can be seen at either Porthcawl or at Ogmore. Both these localities are close to car parks and are easily accessible on foot. Generally these outcrops are above Mean High Tide level providing access at almost any time.

Carboniferous Limestone exposed at coast near Porthcawl

The exposures were visited on two separate occasions, at Easter 2018 and Winter 2019. On both occasions I took along my Canon 5D together with the USM 24-70ml lens and the tripod to gather images to include in the design of this website and presentation materials. I made sure that I visited these outcrops when the weather was dry and bright to maximise clarity of the photos taken.

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About swgt62

Kevin Bate holds a B.Sc. in Geology from Aberystwyth University and an M.Sc. in Petroleum Geology from the University of Aberdeen. He has 30 years plus experience as a geologist in the Oil & Gas industry Kevin is a Fellow of the Geological Society of London (FGS) and is a member of the American Association of Petroleum Geologists (AAPG) Kevin has recently been certificated for the WERS2 First Aid course for outdoor instructors together with gaining a Level 3 Award in Education and Training (AOFAQ) Kevin established South Wales GeoTours as an independent business providing fee-based, guided one-day geological tours to tourists and amateur geologists alike in the South Wales area

1 thought on “Scouting Trip – Carboniferous

  1. “Calcium carbonate is soluble to hydrochloric acid” – yes, but you won’t find much of that “contained in rainwater” (as opposed to geologist’s droppers) – carbonic acid is the usually quoted culprit, and even then mostly generated in the soil

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