High-resolution isotope stratigraphy of the Lower Ordovician St. George Group of western Newfoundland, Canada: implications for global correlation

The Lower Ordovician St. George Group of western Newfoundland consists mainly of shallow-marine-platform carbonates (similar to 500 m thick). It is formed, from bottom to top, of the Watts Bight, Boat Harbour, Catoche, and Aguathuna formations. The top boundary of the group is marked by the regional St. George Unconformity. Outcrops and a few cores from western Newfoundland were sampled at high resolution and the extracted micritic materials were investigated for their petrographic and geochemical criteria to evaluate their degree of preservation. The delta C-13 and delta O-18 values of well-preserved micrite microsamples range from -4.2 parts per thousand to 0 parts per thousand (VPDB) and from -11.3 parts per thousand to -2.9 parts per thousand (VPDB), respectively. The delta C-13(carb) profile of the St. George Group carbonates reveals several negative shifts, which vary between similar to 2 parts per thousand and 3 parts per thousand and are generally associated with unconformities-disconformities or thin shale interbeds, thus reflecting the effect of or link with significant sea-level changes. The St. George Unconformity is associated with a negative d13Ccarb shift (similar to 2 parts per thousand) on the profile and correlated with major lowstand (around the end of Arenig) on the local sea-level reconstruction and also on those from the Baltic region and central Australia, thus suggesting that the St. George Group Unconformity might have likely had an eustatic component that contributed to the development-enhancement of the paleomargin. Other similar delta C-13(carb) shifts have been recorded on the St. George profile, but it is hard to evaluate their global extension due to the low resolution of the documented global Lower Ordovician (Tremadoc - middle Arenig) delta C-13(carb) profile.