Carbon-isotope stratigraphy of the uppermost Cambrian in eastern Laurentia: implications for global correlation

• Published in: Geological Magazine
• Main Author: AZMY, KAREM
• Format: Article in Journal/Newspaper
• Language: English
• Published: Cambridge University Press (CUP) 2018
• Subjects: Canada; Cow Head; Green Point; Broom Point; Martin Point; Newfoundland
• Online Access: http://dx.doi.org/10.1017/s001675681800002x; https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S001675681800002X

Abstract The δ 13 C profile from an interval of the Martin Point section in western Newfoundland (Canada) spans the upper Furongian (uppermost Cambrian). The interval (~90 m) is a part of the Green Point Formation of the Cow Head Group and consists of the Martin Point (lower) and the Broom Point (upper) members. It is formed of slope marine carbonates alternating with shales (rhythmites) and conglomeratic interbeds. The preservation of the investigated micritic carbonates was meticulously evaluated by multiple petrographic and geochemical screening tools. The δ 13 C and δ 18 O values (−0.5 ± 0.8 ‰VPDB and −7.1 ± 0.3 ‰VPDB, respectively) exhibit insignificant correlation ( R 2 = 0.002) and similarly the correlation of δ 13 C values with their Sr and Mn counterparts, which supports the preservation of at least near-primary δ 13 C signatures that can be utilized to construct a reliable high-resolution carbon-isotope profile for global correlations. The δ 13 C profile exhibits two main negative excursions, a lower broad excursion (~3 ‰) that reaches its maximum at ~70 m below the Martin Point / Broom Point members boundary and an upper narrow excursion (~2.5 ‰) immediately below the same boundary. The lower excursion can be correlated with the global latest Furongian HERB event (TOCE), which is also recognized in the C-isotope profile of the GSSP boundary section at Green Point whereas the upper excursion matches with that of the Cambrian‒Ordovician boundary in the same section. The peak of the HERB δ 13 C excursion is correlated with positive shifts on the Th/U and Ni profiles (redox and productivity proxies).