SEQUENTIAL ARCHITECTURE AND CYCLICITY IN PERMIAN DESERT DEPOSITS, BRODICK BEDS, ARRAN, SCOTLAND

This study of ancient desert deposits (Permian Brodick Beds) tests the possibility that cyclicity in these interbedded aeolian, fluvial and lacustrine deposits was orbitally controlled. The 740 m thick successi on of desert deposits forms one sequence which can be divided into thr ee genetic units. A major deflation surface forms the basal sequence b oundary and represents a drop in base-level (groundwater table). On to p of the sequence boundary are rather uniform aeolian dune and sand-sh eet sediments (Unit 1) formed in an erg during low base-level with inc reasing rates of base-level rise. They are followed by interbedded aeo lian dune, aeolian sand-sheet and flood-reworked aeolian sediments (Un it 2) representing erg margin deposits formed during increasing rates of base-level rise. The uppermost part of the sequence is composed of gravel sheets (Unit 3) representing alluvial fan deposition during a h igh base-level with decreasing rates of base-level rise. The successio n is cyclic, and spectral analysis of sedimentary and geochemical para meters reveal cycles with a thickness of 25.7-29.3 m, 34.5 m, and 64.5 m, and fieldwork reveals cycles of 155 m and 675 m. These sedimentary cycles are thought to reflect climatic oscillations. The lack of deta iled time control makes it difficult to link these climatic fluctuatio ns to orbital cycles, although the detected cyclicity matches well wit h Milankovitch theory.