Structural and climatic controls on facies distribution in a mixed fluvial and aeolian reservoir: the Triassic Sherwood Sandstone in the Irish Sea

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Abstract
Interpretation of nearly 5000 km of seismic data, and facies analysis of approximately 900 m of core from 11 wells, has produced structural and sedimentological models of the Triassic Sherwood Sandstone in the East Irish Sea Basin. Sedimentary facies recorded include moderate to low-sinuosity braided fluvial channel, aeolian dune and sandsheet, unconfined fluvial sheetflood and non-evaporitic playa deposits. The establishment of facies associations and correlation of these with wireline logs has enabled extrapolation of sedimentary models into uncored intervals and between wells. The identification of differing seismic character associated with the various facies has made possible the mapping of these diagnostic seismic packages using a seismo-stratigraphic approach. The resulting distribution of seismically derived facies accords with proposed structural and sedimentological models, predicts the position and course of fluvial channels operating in the basin during late Sherwood Sandstone time, and shows that these are largely controlled by fault activity. Detailed facies analysis of sandflat sequences reveals a systematic variation in the proportion of sheetflood and playa to aeolian deposits. It is suggested that this corresponds to a climatic control with sheetflood- and playa-dominated sequences representing relatively wet episodes with a high water table, and aeolian-dominated sequences representing relatively dry episodes with a lowered water table. Correlation of these climatic events may introduce a chronostratigraphic element to the analysis of these continental ‘red-bed’ sequences.