The Clogrenan Formation is a lithostratigraphic unit of Carboniferous age located in the southeast of Ireland. It is primarily identified within the Barrow Valley region, particularly in County Carlow and parts of County Kilkenny. The formation is named after the townland of Clogrenan, where significant quarrying operations have historically exposed the strata.
Geological Context and Age
The Clogrenan Formation dates to the late Viséan stage (specifically the Brigantian sub-stage) of the Mississippian subsystem. In the regional stratigraphic sequence, it typically overlies the Ballyadams Formation. It represents a period of marine deposition during a time when much of Ireland was covered by a warm, shallow tropical sea.
Lithology
The formation consists predominantly of pale-grey to medium-grey limestones. The rock is characterized as being fine- to coarse-grained and is often massive or thick-bedded. A defining feature of the Clogrenan Formation is the frequent occurrence of chert, which appears as nodules or irregular bands within the limestone beds. In some sections, the formation exhibits paleosols (ancient soil horizons) and evidence of subaerial exposure, indicating fluctuating sea levels during its deposition.
Paleontology
The Clogrenan Formation is fossiliferous, containing a variety of marine invertebrates typical of the Carboniferous period. Common fossils include:
- Brachiopods: Diverse assemblages of shelled organisms.
- Corals: Both solitary and colonial forms, such as Lithostrotion.
- Crinoids: Skeletal fragments of "sea lilies."
- Microfossils: Foraminifera and algae used for precise biostratigraphic dating.
Economic Significance
The limestone of the Clogrenan Formation has significant economic value. It is extensively quarried for use as crushed stone, aggregate for construction, and as a source of high-calcium lime for agricultural and industrial purposes. The Clogrenan Quarry is one of the most prominent sites associated with the extraction of this geological unit.