STRATIGRAPHY AND STRUCTURE OF DEVONIAN FLUVIAL SEDIMENTS WESTERN BEARA PENINSULA, SOUTH-WEST IRELAND

A stratigraphic and structural summary of 5 km of Upper Devonian strat a of south-west Beara, part of the Munster Basin, south-west Cork is p resented. Five formations are recognized on the basis of lithofacies g eometries and associations. The lowest Caherkeen Formation comprises r apidly alternating sheet-like, plane-bedded, low-angle, cross-bedded s andstones and laminated, rippled or desiccated mudrocks. The incoming of thin lenticular sandstone packages, commonly with high-angle cross- bedding, distinguishes the overlying Eagle Hill Formation. The succeed ing Reen Point Formation comprises thicker bedsets of plane-bedded, lo w-angle and high-angle cross-bedded sandstones, separated by thick, ma ssive mudrocks. Intraformational breccias and calcareous nodular siltr ocks are distinctive features. The Tholane Formation is characterized by thick massive red-green mudrocks and a general absence of coarser g rained lithologies. This sequence was deposited in a terminal fluvial fan in a basin characterized by decreasing subsidence rates and gradie nt with time. The overlying Toe Head Formation represents the deposits of a fluvial coastal plain. It has siltrocks that are predominantly g reen, often with preserved fish and plant debris. There is a relative absence of high-angled cross-bedded sandstones and an abundance of fla t-laminated and inclined parallel-laminated sandstones. The structure of the Beara Peninsula comprises a WSW plunging anticlinorium with a s ingle cleavage formed during the Variscan orogeny. Evidence from local ly transecting cleavages and from mapping indicates dextral transpress ion. Development of the three main fault trends was contemporaneous wi th the folding. The well-known copper mineralization of the Allihies a rea is associated with east-west fault trends, a pattern observed else where in the western Munster Basin. Basement involvement during both b asin development and deformation is likely, but is difficult to test.