Equilibrium and response to climatic and tectonic forcing: A study of alluvial sequences in the Devonian Munster Basin, Ireland

The Upper Palaeozoic Munster Basin is characterised by thick (> 6 km), Late Devonian alluvial successions (the Old Red Sandstone) that are well exposed throughout southwest Ireland. These represent the deposits of large-scale, complex terminal fan systems which operated under a semi-arid climate. A coarse biostratigraphic subdivision of these sequences has been combined with established radiometric evidence for Devonian stage durations in order to estimate a sediment accumulation rate for this part of the Munster Basin (0.38-0.46 mm a ^-1). This rate has then been used to estimate the duration of cyclic variations in a range of sedimentary parameters that have been identified throughout both proximal and distal basin-fill sequences. 'Spot'-data derived from variables, such as coset thickness, maximum bed thickness and sand-stone content, were used to generate time series which have been subjected to routine Fourier analysis. The application of the estimated sediment accumulation rate to the three most prominent cycle thicknesses (31-175 m) and the assessment of relative frequencies has been used to establish the approximate cycle periodicities, all of which appear to lie within the Milankovitch Band. It is suggested that these features represent the stratigraphic expression of climatic perturbations related to orbital forcing (particularly eccentricity variation). It is further suggested that the recognition of these cycles throughout the basin-fill may, in future, form the basis of high resolution relative timescale and allow the correlation of proximal and distal sections through the basin-fill at scales comparable to the high frequency cyclicities (c. 40 m). The stratigraphic expression of these cycles and their interpretation as evidence of climatic rather than tectonic variation are considered to be a function of their relationship to the equilibrium time constant (T _eq) of the Munster Basin. The establishment of T _eq requires an assessment of diffusivity which is largely controlled by the rate of water supply to the depositional systems. The latter is assessed by palaeohydrological reconstructions of the scale and discharge characteristics of the major fluvial distributary systems within the Munster Basin. Two methods have been applied. Method I uses a series of established empirical relationships to estimate palaeohydrological variables and Method II uses the thickness of sets and cosets of cross-strata within alluvial sandbodies to assess discharge and channel depth. The resulting estimates of water flux allow the assessment of diffusivity which suggests a T _eq for the Munster Basin of the order of 2 × 10 ⁶ a. The Milankovitch Band (10 ⁵ a) periodicities (T) of the cycles observed in the basin-fill are therefore considered to represent 'rapid' variations (T«T _eq) in water/sediment flux associated with relatively high frequency climatic perturbations. However, these cycles are super-imposed on kilometre-scale, sand-prone 'wedges' that dominate the basin-fill architecture. These are considered to record the basinward progradation and subsequent retreat of the terminal fan systems. The 1.5-2 km thicknesses of these units represent time intervals greater than T _eq for the Munster Basin. These features are therefore considered to record 'slow' variations (T>T _eq) in base level related to changes in tectonic subsidence rate and/or sediment flux.