New field observations are combined with strontium isotope and trace e
lement analyses in a stratigraphical investigation of the Durness Grou
p at Balnakeil Bay, NW Scotland. The karst fissures and breccias used
by Palmer et al. (1980) as evidence of a major unconformity between Lo
wer Cambrian (Sailmhor (III) Formation) and overlying Ordovician strat
a (Sangomore (IV) Formation), are here interpreted as being of Tertiar
y or Recent age. Similar features are visible at outcrops along the en
tire coastline in association with modern cemented beach-sand. For the
first time in the Palaeozoic, strontium isotope stratigraphy is used
to date rocks whose age has remained unresolved by conventional bio- a
nd litho- stratigraphy. All formations at Balnakeil Bay display consis
tent Lower Ordovician, Middle-Upper Cambrian Sr-87/Sr-86 ratios when c
ompared with established data for this time interval (Donnelly et al.
1990; Keto and Jacobsen 1987). The Sr-87/Sr-86 ratios of Lower Toyonia
n archaeocyaths from Labrador (supported by currently unpublished data
for the Tommotian and Atdabanian of Siberia) suggest that Lower Cambr
ian Sr-87/Sr-86 seawater ratios were significantly lower than those in
the late Middle and Upper Cambrian. The increase in Sr-87/86 Sr ratio
s may correlate with the Toyonian archaeocyath extinctions and mark th
e beginning of a global regressional event (which reached its maximum
during the Late Cambrian). As there is no such excursion present withi
n the Sr isotope curve for Balnakeil Bay, this implies that the upper
Eilean Dubh (II) Formation is of Middle Cambrian age or possibly young
er.