The Lower-Middle Ordovician transition in south-central Sweden: Phosphorite and iron ooid formation induced by volcanic ash

Abstract

Abstract The transition between the Lower and Middle Ordovician in south-central Sweden marks a change from rather stable sedimentological conditions to a period with several dramatic events. The transition is accompanied by a pronounced change in the macrofauna and there are indications of a eustatic regression leading to an hiatus of about 1 My, and traces of explosive volcanism have been detected. It is suggested that elements released by the devitrification of volcanic ash caused the formation of the chamosite ooids and iron impregnations that occur at the boundary. The volcanic ash may also indirectly have caused the formation of a thin, organic-rich phosphorite resting unconformably on the Lower Ordovician limestone. The phosphorite formation, and the enrichment of organic matter, is believed to have occurred when benthic communities were buried by the ash, and reducing conditions favoured precipitation of phosphate. The phosphate could also have been derived from volcanic gases.