Redox Conditions Across the Cambrian-Ordovician Boundary: Elemental and Isotopic Signatures Retained in the GSSP Carbonates — Expansion of the High-Resolution Chemostratigraphic Profile Set of the GSSP Enhanced Global Correlation Tool

Abstract

Abstract

The scope of the current Cambrian-Ordovician chemostratigraphic GSSP boundary section at Green Point (western Newfoundland, Canada) was expanded for future accurate chronostratigraphic correlation. Lime mudstone samples (rhythmites) were collected at high resolution from outcrops. The sequence (∼ 45 m-thick) consists of slope carbonates with alternating shale and siltstone interbeds, and it spans the boundary located between the Martin Point and Broom Point members of the Green Point Formation (Cow Head Group), which is believed to be a source rock for potential hydrocarbon reservoirs (e.g., St. George Group dolomites) in the area. Samples were extracted from micritic rhythmites by microdrilling and subsequently screened using petrographic and geochemical criteria to evaluate their degree of preservation. Although the δ¹³C_org profile (-29.7 to -25.6 ‰ VPDB) shows insignificant variations, the TOC values (0.1 to 4.1 %) exhibit a generally upward decreasing trend. A negative δ ¹³C_carb excursion marks a geochemical anomaly that correlates with an increase in the N contents (0 to 2.9 %) of organic matter and the δ ¹⁵N_org values (-0.6 to +6.0 ‰), which suggests a change in the oceanic redox conditions. Although the Mo (0.01 to 1.28 ppm) and U (0.1 to 3.0 ppm) contents lack correlation, the δ ⁹⁸Mo values (-0.30 to 2.34 ‰) shows enrichment in correlation with the δ ¹³C_carb excursion but the δ ²³⁸U counterparts (-0.97 to 0.00 ‰) exhibits depletion, thus suggesting an enhancement in the reducing conditions, which is consistent with δ ¹⁵N_org results and the associated sealevel rise. The unexpected enrichment in the δ ⁹⁸Mo values is suggested to reflect an expansion in the dysoxic conditions rather than strongly euxinic or anoxic environments. The rhythmic occurrence of organic-rich shale with thin carbonate interbeds influenced the δ ⁹⁸Mo and δ ²³⁸U signatures of carbonates and resulted in trends opposite to those normally expected from shales.

AAPG Datapages/Search and Discovery Article #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016