OXYGEN AND CARBON ISOTOPE STRATIGRAPHY OF EARLY CAMBRIAN CARBONATES IN SOUTHEASTERN NEWFOUNDLAND AND ENGLAND

Carbonate rocks have been sampled through predominantly siliciclastic sediments above the Precambrian-Cambrian global stratotype level in southeastern Newfoundland to assess their potential for oxygen and carbon isotope stratigraphy. Comparable successions were sampled at Nuneaton and Comley in England. Greatly depleted delta-O-18 signals are attributed to widespread thermal alteration during deep burial and granitic intrusion, including within the stratotype region. Carbon isotope ratios appear to have been less affected and these are described from nine sections. A provisional, composite delta-C-13 curve is based on non-ferroan, pink nodular and bedded micrites. Several delta-C-13 excursions occur in the fossiliferous Bonavista Group and allow the position of the Tommotian-Atdabanian boundary to be identified. Chemostratigraphic correlation of the new Precambrian-Cambrian boundary stratotype may, however, prove difficult because of the lack of suitable. well-preserved carbonates. The search must begin for a comparable reference section allowing global correlation of the boundary level using chemostratigraphy as well as biostratigraphy.