Cretaceous outliers in south-west Ireland and their implications for Cretaceous palaeogeography

Abstract

An investigation of the Gweestin Valley, near Killarney, Co. Kerry, has revealed the presence of datable deposits of Chalk at an elevation of about 300 to 400 ft o.d. The Chalk forms the matrix in a breccia of Namurian shale fragments, the breccia apparently occurring as a pocket in surrounding unbrecciated Namurian shales. The faunal content indicates a Senonian age for at least some of the Chalk, whereas the lithology and texture of the breccia and a knowledge of the local Carboniferous succession and structure strongly suggest that the deposit originated as a rapid submarine intra-Cretaceous karstic subsidence. Evidence is presented bearing upon the size of the existing pocket, the elevation of the local Chalk sea bed, and the sequence of events leading to the collapse. The Chalk–shale deposit is termed the Ballydeenlea outlier. Two other karstic-subsidence outliers occur nearby at Kilkneedan and Ballyhar; though neither contains chalk sediment, they are thought to be genetically related to the Ballydeenlea outlier. The existence of the Chalk sea floor not far above 400 ft o.d. in the centre of the Dingle Bay synclinorium suggests, from one point of view, that the contemporary physiography of mid-Kerry is essentially an exhumed pre-Senonian landscape, or, from an alternative point of view, that the Old Red Sandstone ranges of Iveragh and Slieve Mish have been uplifted by at least 2500 ft during Tertiary times. The evidence, which is inconclusive, is presented in review of these postulates. The Gweestin valley deposits are briefly compared with other British karstic subsidence phenomena.