Lake-level changes, sedimentation and faunas in a Middle Devonian basin-margin fish bed

Abstract

The classic Devonian fossil fish site at Tynet Burn, Fochabers, NE Scotland has been investigated following recent excavations. The fish assemblage consists of placoderms, acanthodians, an actinopterygian, porolepiforms, osteolepiforms and a dipnoan. The assemblage is typical of the 'Achanarras Fauna' of Eifelian age which is widespread in the Orcadian basin, representing a time of generally high lake levels. The most diverse fauna occurs in the Upper Nodule Bed which can be divided into top and bottom levels, where fish are mainly preserved in calcareous nodules, and a middle level with fish in laminated limestone. Three fluvial to lacustrine cycles are exposed in the Tynet Burn in 27 m of strata. Fish are abundant in the Upper Nodule Bed, and are also sparsely represented in lacustrine strata both above and below this level. The lacustrine laminites of the Upper Nodule Bed were deposited following a rapid rise in lake level. Subsequent fall in lake level resulted in deposition of a thin shallow water massive limestone, prior to exposure and desiccation of the lake floor. Disruption of laminites by sediment sliding and sand intrusion was followed by compaction and concretion formation. Chemotrophic bacteria later invaded open cracks and porosity and partially oxidized the fish remains; the bacterial oxidation event being responsible for the characteristic pink, red and purple colours of fossil fish from Tynet Burn. Subsequent lake level rise resulted in deposition of shales with abundant diagenetic concretions, rarely containing fish. With increasing clastic input alluvial sands prograded over the area from the south. The rapid deepening event associated with the base of the Upper Nodule Bed at Tynet is considered to be synchronous with similar events represented in other Achanarras-equivalent fish beds. However, elements of the Achanarras fish assemblage can be encountered in less persistent lacustrine intervals both above and below this level. Minor cyclicity, superimposed on the main Achanarras event, resulted in repetition of fluvial to lacustrine transitions. The main control on cyclicity is considered to have been climatic, but subsidence and sediment supply rates were also a significant influence.