HIGH-RESOLUTION CHEMOSTRATIGRAPHY OF THE FURONGIAN SERIES STAGE 10 BELOW THE CAMBRIAN-ORDOVICIAN GSSP

Chemostratigraphy is an important tool for correlating global stratigraphic sections. Preserved/near primary carbon isotope signatures in marine carbonates can provide high-resolution profiles for sedimentary sequences eliminating the need for distinguishing fossils from different depositional environments and those lacking fossil material.

The Global Stratotype Section and Point (GSSP) of the Cambrian-Ordovician (C-O) boundary is located at Green Point in the Green Point Formation of the Cow Head Group in western Newfoundland (Canada). To reconstruct a continuous and high-resolution chemostratigraphy from the C-O GSSP to the Furongian Series Stage 10, we adopted the δ¹³C results of the Green Point Formation covering the C-O GSSP interval (Azmy et al., 2014).

The Green Point Formation consists of alternating dark gray to black fissile shale and thin ribbon limestone rhythmites, with scarce fossils. The carbonate samples are micrite and were determined to be in primary to near-primary condition based on multiple screening tests. Cathodoluminescence screening revealed dull to red luminescence within the samples indicative of differential preservation. This was confirmed by the stable isotope and trace element analyses. The δ¹³C and δ¹⁸O values range from -6.44‰ to +0.33‰ (VPDB) and from -8.63‰ to -5.67‰ (VPDB), respectively, with poor correlation (R²=0.07). Mn and Sr contents exhibit no significant correlation (R²=0.005) and Mn/Sr ratios range from 0.63 to 9.82 with no correlation to δ¹³C values (R²=0.002).

Carbon isotope compositions in the Green Point Formation below the C-O GSSP have fluctuated but remained essentially invariantly negative. However, the δ¹³C values reveal a significant negative carbon isotope excursion, with nadir of -6.44‰ at the base of the Eoconodontus conodont zone, which is the proposed GSSP for the base of Cambrian Furongian Series Stage 10. It correlates well with the HEllnmaria-Red Tops Boundary (HERB) carbon isotope excursion and the Top of Cambrian carbon isotope Excursion (TOCE) in the United States, Australia, north China and Argentina. This is achieved without an adequate record of conodonts. Therefore, high-resolution chemostratigraphic trends of carbon isotope compositions may facilitate the correlation of intercontinental and intracontinental sequences.