Carbon-isotope stratigraphy of the Middle–Upper Cambrian in eastern Laurentia: Implications for global correlation

Abstract

The examined interval of the March Point and Petit Jardin formations (total thickness ~130 m) spans the Middle−Upper Cambrian (lower Port au Port Group) at the Belle Isle Strait, western Newfoundland. The deposits consist of dolomitized shallow-water carbonates ranging from microbial mudstones to oolitic grainstones. A multi-technique evaluation proved the petrographic and geochemical preservation of the investigated micritic carbonates. The δ13C and δ18O values (−1.2 ± 1.1‰ VPDB and −6.7 ± 1.3‰ VPDB, respectively) exhibit an insignificant correlation (R2 = 0.16) and similarly the δ13C values with their Mn/Sr ratio (R2 = 0.01). This supports the preservation of at least near-primary δ13C signatures that can be utilized to construct a high-resolution δ13C profile for reliable global correlations. The δ13C profile exhibits two major negative excursions, the lower excursion (~4‰) immediately below the lower boundary of the March Point Formation (Hawke Bay/March Point boundary) and the upper excursion (~2‰) almost at the lower boundary of the Petit Jardin Formation where these excursions are likely associated with short-term sealevel changes. The trilobite biozonation scheme suggests the correlation of the lower excursion with the DICE (Drumian Carbon Isotope Excursion) event and the upper excursion with the base of the SPICE (Steptoean Positive Carbon Isotope Excursion) counterpart and both events have been globally documented on different paleocontinents.