Sequential architecture and cyclicity in Permian desert deposits, Brodick Beds, Arran, Scotland

Abstract

This study of ancient desert deposits (Permian Brodick Beds) tests the possibility that cyclicityin these interbedded aeolian, fluvial and lacustrine deposits was orbitally controlled. The 740 m thicksuccession of desert deposits forms one sequence which can be divided into three genetic units. A majordeflation surface forms the basal sequence boundary and represents a drop in base-level (groundwatertable). On top of the sequence boundary are rather uniform aeolian dune and sand-sheet sediments (Unit1) formed in an erg during low base-level with increasing rates of base-level rise. They are followed byinterbedded aeolian dune, aeolian sand-sheet and flood-reworked aeolian sediments (Unit 2) representingerg margin deposits formed during increasing rates of base-level rise. The uppermost part of the sequenceis composed of gravel sheets (Unit 3) representing alluvial fan deposition during a high base-level withdecreasing rates of base-level rise. The succession is cyclic, and spectral analysis of sedimentary andgeochemical parameters reveal cycles with a thickness of 25.7–29.3 m, 34.5 m, and 64.5 m, and fieldworkreveals cycles of 155 m and 675 m. These sedimentary cycles are thought to reflect climatic oscillations. The lack of detailed time control makes it diffcult to link these climatic fluctuations to orbital cycles, although the detected cyclicity matches well with Milankovitch theory.