Taphonomy of trace fossils at omission surfaces (Middle Triassic, East Germany)

In shallowing-upward cycles of the East German (Thuringian) Muschelkalk (Middle Triassic), sedimentological (probably storm) events interrupted longer periods of low net-sedimentation. The ichnofauna changed with the diagenesis of the substrate; it does not represent an ecological succession. The presence of both firmground and hardground traces in the same samples indicates superimposed Trypanites and Glossifungites ichnofacies. The ichnofabric of burrows was `frozen' by cementation during omission, the amount of erosion controlling the preservation of the upper tiers (borings). In general, both burrowing and boring has been intense but patchy, and recruitment of tracemakers had been almost monospecific. A hostile environment, perhaps due to high salinity and/or erosion, may thus be assumed. The type of preservation was governed by different reactions to sulphide production in different chemical microenvironments during early diagenesis. In absence of significant amounts of iron, H2S escaped into burrows or borings from the adjacent anaerobic sediment. Depending on local physicochemistry, this resulted in calcite or pyrite precipitation. Well-aerated parts of burrows facilitated the formation of celestite after reoxidation of sulphide. On goethite-encrusted hardgrounds, pyrite rings were formed around numerous borings, trapping the active H2S immediately after its formation. These haloes were previously mistaken as excrements.