FLUVIAL PROCESSES AND CYCLICITY IN TERMINAL FAN DEPOSITS - AN EXAMPLE FROM THE LATE DEVONIAN OF SOUTHWEST IRELAND

The Gun Point Formation is a sandstone-dominated sequence, approximately 1.5-2.5 km thick, which forms part of a major (> 6 km) Late Devonian alluvial basin fill succession in southwest Ireland. It is interpreted as the deposits of a southerly flowing terminal fan system which operated under a semi-arid climate. The proximal zone was characterized by low-sinuosity gravelly feeder channels which passed downstream into a medial zone dominated by a sandy braidplain. Further south, in the distal zone, phases of high-energy flood activity were centred on transient, low-sinuosity ephemeral channels and were separated by more quiescent periods of shallow sheetflooding, subaerial exposure and non-deposition. Sedimentary parameters such as set thickness, coset thickness, channel sand body density and maximum sand body thickness all exhibit regular cyclic fluctuations with a periodicity of 160-180 m. The cycles are interpreted as evidence of regular discharge variations which may have been a response to the 412 ka orbital eccentricity cycle. A second periodicity of approximately 700 m has also been detected and is thought to reflect long-term variations in basin subsidence.