Palaeoecology of a condensed section (carboniferous leitrim group, north-west ireland)

An integrated study of sedimentology, palaeoecology and geochemistry is used to elucidate the environmental significance of sediment starvation and winnowing developed during a Carboniferous marine-flooding event in northwest Ireland. These data show that the distinctive beds of the Derrens Limestone Member (basal Carraun Shale Formation, Leitrim Group), known throughout north-west Ireland, reflect a regionally significant early Brigantian sea-level rise. Slow sedimentation accompanied multiple phases of physical reworking and faunal inhabitation. The resulting shell-rich limestones have a high gamma-ray response when compared to formations above and below. Elevated radioactive emissions are not restricted to one bed but occur throughout a 1m-thick succession of limestone beds with interbedded shales. The limestone bed with highest faunal diversity shows the most obvious features of physical reworking. The condensed nature of the succession is evidenced by the presence of postmortem boring and encrustation of a cephalopod fauna due to taphonomic recycling; the preservation in three dimensions of the cephalopods; the occurrence of locally derived re-exhumed blocks of colonial tabulate corals (Michelinia); winnowed shelly lags; and carbonate hiatus nodules. These indicate slow deposition during the turn-around between transgression and highstand, producing an unusual complex condensed suite of sediments that provide information on sea-floor conditions in northwest Ireland during the Brigantian (Carboniferous). © Royal Irish Academy.