The palaeoenvironment and deformation of Beaconites -like burrows in the Old Red Sandstone at Gortnabinna, SW Ireland

Abstract

The Gortnabinna area contains a number of well-preserved, deformed burrows within the fluviatile Old Red Sandstone of S Ireland. Detailed logging of the local succession enables subdivision into four facies; Facies 1 represents sheet-flood deposits, Facies 2 ephemeral stream channels, Facies 3 marginal sheet-flood deposits and Facies 4 low-energy settling of fines. The burrows occur in Facies 1 and 4. The sediments are now folded into a large, upright, westward plunging anticline. The fold has a well-developed axial planar cleavage and is extended along strike by listric normal faults. The burrows in Facies 1 were originally circular and orthogonal to bedding and reveal a N–S layer parallel shortening strain of 2:1 followed by a shear strain of 1.48 during folding. These burrows have a bimodal distribution of diameters with peaks in the ranges 50–80 and 110–170 mm. The burrows of Facies 4 are sub-parallel to the bedding plane and are generally curved, with a maximum width of 180 mm and lengths of up to 2.3 m. These burrows display passive infill following abandonment. The burrows are morphologically similar though distinct from Beaconites barratti, which has been described from elsewhere in the Old Red Sandstone, and are tentatively interpreted as lungfish burrows.