A silica deposit of possible Tertiary age in the Carboniferous Limestone near Birr, County Offaly, Ireland

Abstract

Summary. Stony clays, comprising weathered bryzoan-rich chert and silica clay, are found adjacent to Carboniferous Limestone on the west and a weathered dolomitized zone on the east near Birr, County Offaly, Ireland. A gravity survey together with a stratigraphy obtained from boreholes and other test borings suggests a depth of about 40 m and a surface width or diameter of approximately 60 m for the deposit. Indirect evidence points to a possible Tertiary age. The interpreted origin of the stony clays is by silification of Carboniferous Limestone and preservation by solution subsidence along a possible fault zone.