What is the biostratigraphic value of the ichnofossil Oldhamia for the Cambrian: a review
Département des Sciences de la Terre et de l’Environnement, Université Libre de Bruxelles, Belgium
Department of Geology and Soil Science, Ghent University, Belgium
Abstract
We have undertaken an exhaustive review of the literature to check if the ichnogenus Oldhamia could be used as a biostratigraphical tool in the Cambrian. On the 19 occurrences observed worldwide and positioned in the recent global chronostratigraphy of the Cambrian, only 16 have a time range sufficiently precise to constrain the stratigraphic interval during which the organisms producing Oldhamia lived in the Cambrian. They are clearly distributed in two age groups: a “younger” group of 14 occurrences shows a very well constrained time range from the base of Stage 3 to the three quarter of Stage 5 and an “older” group of 2 occurrences shows a poorly constrained time range which seems restricted to the Fortunian. With this contrasting situation we propose two alternative interpretations: in the first we accept all the results and in the second, more restrictive, we reject the age of the “older” group as scientifically insufficiently substantiated. Oldhamia can only be used as a stratigraphic tool in the second interpretation: it appears in the upper part of Stage 2, shows an acme in Stage 3 and 4 and rapidly disappears during Stage 5. This rapid disappearance could be interpreted as the arrival of the Cambrian agronomic revolution in the deep marine setting. The overview also shows that this ichnogenus has a cosmopolitan distribution.