Carbon-isotope stratigraphy of the East Isthmus Bay (Port au Port and St. George groups) and Broom Point (Cow Head Group) sections, western Newfoundland, Canada

Scorrer, Sebastian (2018) Carbon-isotope stratigraphy of the East Isthmus Bay (Port au Port and St. George groups) and Broom Point (Cow Head Group) sections, western Newfoundland, Canada. Masters thesis, Memorial University of Newfoundland.

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Abstract

The carbon-isotope stratigraphy of two carbonate sections spanning the Upper Cambrian to lowermost Ordovician from western Newfoundland were investigated for their potential for regional and global correlation with coeval carbonate sections. The East Isthmus Bay (EI) section consists of carbonates from the uppermost Cambrian (Furongian) Berry Head Formation (~ 160 metres thick), and the lowermost Ordovician (Tremadocian) Watts Bight Formation (~ 30 metres thick) that accumulated in a shallow-marine setting on the Laurentian platform/passive margin in western Newfoundland, Canada. Petrographic investigation shows that the sampled carbonates have retained at least their near-primary sedimentary fabrics, though there is minor recrystallisation in the lower section. The Broom Point (BP) section encompasses the Broom Point North (~ 20 metres sampled) and Broom Point South (~ 120 metres sampled) sections. The sections cover the Shallow Bay (Furongian) and Green Point (Tremadocian) formations, these carbonates formed further down the slope in slightly deeper water. Petrographic examination of the carbonates implies the retention of near-primary sedimentary fabrics. Correlation between the Sr and Mn concentrations of carbonates from both sections suggests the Broom Point carbonates have been more impacted by diagenesis than their East Isthmus Bay counterparts due to the higher correlation (R²BP = 0.4, R²EI = 0.18, respectively). The East Isthmus Bay δ¹³C values show insignificant correlation with their Sr (R²EI = 0.04), Mn (R²EI = 0.001), and Fe (R²EI = 0.02) counterparts, implying preservation of at least near-primary carbon-isotope compositions. On the other hand, the Broom Point section shows considerable correlation of Sr and Mn with their δ¹³C counterparts (R²BP = 0.54 and R²BP = 0.29, respectively), implying considerable alteration of the primary geochemical signatures. Though a poor correlation is seen between δ¹³C and Fe (R²BP = 0.14) in the Broom Point carbonates and insignificant correlation is seen between the δ¹⁸O and δ¹³C values in both the investigated sections (R²EI = 0.0003, R²BP = 0.06). The petrographic and geochemical examination suggests that a reliable carbon-isotope profile can be reconstructed from the δ¹³C signatures of the East Isthmus Bay section. On the contrary the geochemical analyses do not support the preservation of near-primary δ¹³C signatures from the Broom Point section, emphasising the importance of using geochemical analyses alongside petrographic examination to inspect the degree of preservation of δ¹³C signatures of carbonate rocks. The East Isthmus Bay section is largely fossil poor but the δ¹³C profile shows a pattern with distinct variations that can be matched with those of the Laurentian Lawson Cove Auxiliary Boundary Stratigraphic Section and Point (ASSP) section, Utah, USA. Therefore, a conclusive conodont biozonal match was possible to reconstruct by matching the δ¹³C profile with its counterpart from the Lawson Cove ASSP section. At the base of the East Isthmus Bay section, the δ¹³C profile exhibits a broad excursion (the top of the Herllnmaria-Red Tops Boundary, HERB Event), which can be matched with the base of the Eoconodontus Zone (mid-Furongian), followed by an enrichment trend through the Cordylodus intermedius Zone (top Furongian). A positive anomaly (Hirsutodontus simplex spike or HSS) is recorded in the upper Cordylodus intermedius Zone (the top Cambrian), and a prominent positive peak characteristic for the upper Cordylodus lindstromi Zone is recorded from the top of the investigated section. The δ¹³C values of the Newfoundland carbonates are generally ~ 1 ‰ VPDB lower than the Lawson Cove values, which is likely attributed to a relative higher productivity and/or organic burial in the Utah region. The investigated carbon-isotope stratigraphy revealed four main excursions: (1) an upper negative excursion (~ 2.5 ‰ VPDB), thought to be associated with the Cambrian−Ordovician boundary, (2) a lower broad negative excursion (~ 2 ‰ VPDB) which can be correlated to the latest Furongian HERB Event. (3) A positive excursion of ~ 1 ‰ VPDB seen at the base of the Cordylodus proavus conodont Zone and finally (4) a distinct gradual rise in positive δ¹³C values known as the Hirsutodontus simplex Spike (or HSS). These four excursions can be seen on various sections worldwide spanning the uppermost Cambrian to lowermost Ordovician.

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