Pj. Brenchley et al., NEW ISOTOPIC DATA SOLVING AN OLD BIOSTRATIGRAPHIC PROBLEM - THE AGE OF THE UPPER ORDOVICIAN BRACHIOPOD HOLORHYNCHUS-GIGANTEUS, Journal of the Geological Society, 154, 1997, pp. 335-342
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.