New methods for ichnofabric analysis and correlation with orbital cycles exemplified by the Baden-Sooss section (Middle Miocene, Vienna Basin, Lower Austria)

A two step cluster analysis based on log-likelihood measures for categorial variables using ‘Schwarz’s Bayesian Criterion’ for grouping allows the automatic detection of ichnofabric categories from a large data set. Preferred successions of these ichnofabrics were tested by ‘Embedded Markov Chains’. This leads to the following ichnofacies interpretation: Alternating periods of higher/lower accumulation rates with higher/lower inputs of particulate food and higher/lower oxygen contents in pore waters led to sequential colonization of the substrate. The trace fossils Phycosiphon and Nereites represent opportunistic colonization of oxygenated sediments rich in particulate organic matter (POM) by deposit-feeding animals, quickly after an increased sediment input. A further stage of colonization caused by the decrease of POM induced by consumption and oxidation forced the animals to search for food on sediment surfaces and from the water column. The open burrows Thalassinoides, Chondrites, Trichichnus and Zoophycos indicate stable-bottom conditions in periods of low accumulation rates. Zoophycos, Phycosiphon, Nereites and Teichichnus suggest the Zoophycos ichnofacies for the lower section of the core; a transition to the distal part of the Cruziana ichnofacies is suggested for the upper section of the core with the appearance of Thalassinoides. The changes between stable and unstable bottom conditions significantly correlate with periods in magnetic susceptibility and calcium carbonate content, both forced by orbital cycles.