Limestone concretion growth documented by trace-fossil relations

A concretionary limestone bed and subjacent black laminated shales examined within the upper part of the Jurassic Posidonienschiefer in the Holzmaden area of southern Germany contain trace fossils that are derived exclusively from an overlying bioturbated green claystone bed that reflects a short episode of oxygenation. The character and systematic distribution of trace-fossil assemblages, which include Chondrites, Thalassinoides, and Rhizocorallium, provide a record of depth and timing of syngenetic concretionary limestone growth that is more precise than growth histories provided by more traditional physical and geochemical criteria. Ichnologic evidence indicates that concretion growth began at isolated nuclei at or just below the sediment-water interface just prior to or contemporaneous with the onset of the oxygenation episode. Concretion growth progressed downward and laterally from these nuclei during and after minor compaction but was essentially complete before the termination of the oxygenation event at a depth less than 1 m below the sea floor. The temporal association of this concretion growth with the oxygenation episode implies a genetic relation that warrants further investigation of the significance of bottom-water oxygenation fluctuations for the formation of other concretionary limestones in the stratigraphic record.