The impact of wildfire on an Early Carboniferous coastal environment, North Mayo, Ireland

Abstract

At Creevagh Head, near Ballycastle, North Mayo, Ireland, a 28-m sequence of Lower Carboniferous (Holkerian-Arundian) carbonates and clastics is exposed in low sea-cliffs. These rocks have been described in terms of five facies and were deposited in a coastal marine setting. Horizontally laminated heterolithic siltstones were formed in a tidal flat environment (Facies 1). Lithologically similar deposits infilling a broad (150 m wide), shallow (2 m deep) erosional surface represent the sediments of a major estuarine channel (Facies 2). Units of inclined heterolithic stratification (IHS), 2–4 m thick, displaying arcuate structures in plan, record the migration of meandering estuarine creeks which dissected the tidal flat (Facies 3). Beds of carbonate mudstone bearing abundant Volvocacean green algae and organic rich siltstones probably formed in oligohaline coastal lagoons (Facies 4). Finally, bioclastic sandstones with in situ corals formed in stenohaline waters at the mouth of tidally dominated estuaries (Facies 5). Vast quantities of charcoal present in one of the IHS units (the Charcoal Unit) are interpreted as the product of a single, catastrophic wildfire at this time. The fire appears to have destroyed vegetation cover on the hinterland destabilising local sedimentary, hydrological and biochemical systems. Comparison of the Charcoal Unit with IHS units immediately underlying it, suggest that run-off and sediment discharge were increased greatly downstream following the fire, replacing tide-dominated sedimentation with a river-dominated regime. Abundant fish skeletal remains in the Charcoal Unit indicate that this event coincided with massive mortalities amongst marine fish populations. This ecological catastrophe may have been linked to increased turbidity and nutrient flushing into the estuary, causing eutrophication and short-lived anoxia.