The palaeogeography of the Middle Devonian Lower Eday Sandstone, Orkney

Abstract

Synopsis Sections through the topmost Rousay Group, and the Lower Eday Sandstone and Eday Flagstone Formations of the Eday Group can be correlated throughout eastern Orkney. The amalgamated sandstone unit of the Lower Eday Sandstone varies between 180 and 30 m in thickness. It was deposited in three interacting environments; sandy braided rivers, aeolian dunes and lake beaches. There were two river systems; a northern, SE-flowing alluvial fan, and a southern, NE-flowing alluvial plain. The rivers prograded eastwards across the lake basin. At times when the lake was full, beach ridges formed on the alluvial surfaces at various heights above the lake floor. Dominantly NNW winds reworked the sands, locally building up a thick dune field on the western part of the alluvial fan. The distribution of the beach ridges constrains the depth range of the Orcadian lake when it was full, showing it rarely exceeded 20 m depth during the Lower Eday Sandstone. Active extensional faulting during sedimentation had an important control on the palaeogeography, and on the subsidence pattern. This episode of extension gave rise to much of the present structure of the Orkneys.