Dish structures in Eocene volcanic ash layers, Denmark

Abstract

ABSTRACTWell developed dish structures occur in two out of 179 numbered ash layers in a 60 m thick section of Lower Eocene marine diatomite in northern Denmark. One of the two dish‐structured ash layers also contains Type A pillar structures of Lowe (1975). The ash layers are well graded and the grain sizes fall in the range of fine sand to coarse silt. The ash was transported by wind from volcanoes located in the Skagerrak and settled grain‐by‐grain on the sea floor. The ash layers were thus laid down by rapid sedimentation compared to the relatively slow deposition of the diatomite. The two ash layers showing dish structures are among the coarsest grained and thickest of the 179 layers. Other layers, which are equally thick but significantly finer grained or equally coarse‐grained but thinner, show no water escape structures.The rapid deposition of the coarse‐grained ash on the fine‐grained, waterlogged diatom ooze may have started consolidation processes in the ooze, thereby inducing a water flow into the ash layer. This led to liquefaction and localized fluidization of the ash layer resulting in the formation of dish and pillar structures.The present study supports the interpretation of dish and pillar structures as secondary structures, formed by water escape. This occurrence represents the first description of these structures from beds not deposited by currents and indicates that dish structures are not diagnostic of any type of sediment gravity flow.