Cyclical discharge variation recorded in alluvial sediments: An example from the Devonian of southwest Ireland

Cross-stratification is a common sedimentary structure in alluvial deposits and can often be recognized in core. The quantitative study of cross-strata can potentially yield information concerning the behaviour of alluvial systems through time. Such information is valuable to a reservoir geologist when assessing the architecture of an alluvial reservoir. Several alluvial systems in the Munster Basin (Devonian), southwest Ireland, display the characteristics of terminal fans generated in a 'hydrographically-closed' basin. Analysis of three formations reveals cyclical variations ('megacycles') in the dimensions of cosets and sets within channel sandbodies. The 'megacycles' are 100-200 m thick and record increases and subsequent decreases in the dimensions of sedimentary structures. The megacycles are also reflected by the density of channel deposits such that high densities are associated with relatively large channels and low densities are associated with relatively small channels. Cyclicity is also locally developed in the flood-plain deposits as alternations of red beds with calcretes (oxidizing conditions) and green/grey beds with pyrite nodules (reducing conditions). Established relationships between dune size and flow depth suggest that cyclical variations in the size of sedimentary structures reflect discharge fluctuations. This is supported by the close correlation between set thickness and coset thickness, the latter being closely related to sandbody thickness. The cyclical variations in channel sandbody density and flood-plain conditions are also thought to reflect discharge variation. Available biostratigraphic data and radiometric dates suggest that the cycles represent periods of several hundred thousand years. The periodicity and regularity of the cycles together with their relationship to discharge variation suggests that they may be related to astronomical forcing of the Earth's climate.