New isotopic data solving an old biostratigraphic problem: the age of the upper Ordovician brachiopod Holorhynchus giganteus

Abstract

Recent work has shown that there is a pronounced positive late Ordovician excursion in both δ 13 C and δ 18 O which appears to be global and identifies a clear chemostratigraphic interval that is confined to part of the Hirnantian Stage. Pre-Hirnantian (Rawtheyan) brachiopod carbonate values of δ 13 C are typically in the range – 1 to + 1‰ PDB and lower Hirnantian values are typically in the range of +4 to +7%o PDB. This contrast in isotopic values has been used to assess the age of the distinctive late Ordovician brachiopod Holorhynchus giganteus. The age of the Holorhynchus association is important in terms of its relationship to the major late Ordovician extinction that was initiated at the start of the Hirnantian. The species, which traditionally was regarded as uppermost Ordovician (Hirnantian) in age has been shown on stratigraphic evidence to be pre-Hirnantian at many locations. If this is so it predates the first phase of extinction and is not associated with the Hirnantia fauna, which characterizes the interval between the two main episodes of late Ordovician extinction. However, at its type locality in the Asker district of Norway the stratigraphic evidence has pointed to it being Hirnantian in age. To resolve this ambiguity, analyses have been made on Holorhynchus at three locations, (1) in the east Baltic region where Holorhynchus occurs below demonstrable Hirnantian rocks, (2) in the Boda Limestone of central Sweden where Holorhynchus occurs high on a carbonate mud mound where its age is unclear and (3) in the type area of the species in Norway. At all three locations the δ 13 C values are low, indicating a pre-Hirnantian, pre-extinction, age for Holorhynchus. The re-assessment of the age of the Asker sequence in Norway implies the presence of a cryptic unconformity there and suggests uplift rather than subsidence during the latest Ordovician. This study emphasizes the value of chemostratigraphy in high-resolution stratigraphy.