NEW ISOTOPIC DATA SOLVING AN OLD BIOSTRATIGRAPHIC PROBLEM - THE AGE OF THE UPPER ORDOVICIAN BRACHIOPOD HOLORHYNCHUS-GIGANTEUS

Recent work has shown that there is a pronounced positive late Ordovic ian excursion in both delta(13)C and delta(18)O which appears to be gl obal and identifies a clear chemostratigraphic interval that is confin ed to part of the Hirnantian Stage. Pre-Hirnantian (Rawtheyan) brachio pod carbonate values of delta(13)C are typically in the range -1 to +1 parts per thousand PDB and lower Hirnantian values are typically in t he range of +4 to +7 parts per thousand PDB. This contrast in isotopic values has been used to assess the age of the distinctive late Ordovi cian brachiopod Holorhynchus giganteus. The age of the Holorhynchus as sociation is important in terms of its relationship to the major late Ordovician extinction that was initiated al the start of the Hirnantia n. The species, which traditionally was regarded as uppermost Ordovici an (Hirnantian) in age has been shown on stratigraphic evidence to be pre-Hirnantian al many locations. If this is so it predates the first phase of extinction and is not associated with the Hirnantia fauna, wh ich characterizes the interval between the two main episodes of late O rdovician extinction. However, al its type locality in the Asker distr ict of Norway the stratigraphic evidence has pointed to it being Hirna ntian in age. To resolve this ambiguity, analyses have been made on Ho lorhynchus at three locations, (1) in the east Baltic region where Hol orhynchus occurs below demonstrable Hirnantian rocks, (2) in the Boda Limestone of central Sweden where Holorhynchus occurs high on a carbon ate mud mound where its age is unclear and (3) in the type area of the species in Norway. At all three locations the delta(13)C values are l ow, 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 rathe r than subsidence during the latest Ordovician. This study emphasizes the value of chemostratigraphy in high-resolution stratigraphy.