Coastal plain sedimentation in the late Devonian of Southern Ireland; hummocky cross-stratification in fluvial deposits?

Abstract

The late Devonian Toe Head Formation crops out extensively in southwest Ireland and has been considered previously as the topmost Old Red Sandstone (terrestrial) formation in a conformable non-marine to marine sequence. The most characteristic lithofacies, the flat laminated and inclined parallel laminated sandstones, displays a range of structures from flat to gently inclined laminae and includes sets which meet the four criteria of Harms et al. (1982) for hummocky cross-strata. Associated facies include desiccated mudrocks, palaeosols and ripple cross-laminated sandstones which lack the flaser-linsen dominated bedsets characteristic of the overlying marine strata. Only non-marine fossils are recorded and numerous palynological preparations lack the marine indicators which appear at the top of the formation. Palaeocurrents show considerable spread with indications of overall easterly transport of sand. There is no preferred vertical sequence of lithofacies. The balance of evidence strongly favours a non-marine environment with the implication that hummocky cross-strata, as presently defined, are not good environmental or even process indicators. The fine to very fine sand sizes and possibly the amount of suspended load were important controls on the resultant structures. A depositional model of essentially fluvial deposition on a low-gradient coastal plain is proposed.