Restudy of some Ordovician–Silurian boundary graptolites from Anticosti Island, Canada, and their biostratigraphic significance

Abstract

The Ellis Bay Formation on Anticosti Island has long been recognized for the biostratigraphic importance of its latest Ordovician conodont, palynomorph, and shelly fossil assemblages. However, a sparse record of graptolites has made it difficult to correlate these assemblages with the graptolite biozonation. Restudy of some previously described and examination of newly collected normalograptid specimens from the Ellis Bay and lower Becscie formations, however, shows that biostratigraphically important taxa are present and these species provide important constraints on the age of the strata. Normalograptus parvulus and N. minor have been identified in the upper half of the Ellis Bay Formation, suggesting that these strata are late Hirnantian in age (N. persculptus Biozone). Normalograptus imperfectus and Normalograptus sp. aff. N. acceptus occur in the basal beds of the Becscie Formation, indicating that these strata are earliest Silurian. These graptolite data support the hypothesis that the positive carbon isotope excursion seen in the uppermost strata of the Ellis Bay Formation is isochronous with that seen in the Hirnantian strata at Dob's Linn, Scotland, and does not span much of the lower to mid‐Hirnantian, as is the case in Arctic Canada and Nevada, USA. Anticosti Island, Hirnantian, graptolites, Ordovician, Silurian.

Terminal Ordovician carbon isotope stratigraphy and glacioeustatic sea-level change across Anticosti Island (Quebec, Canada)

“…Comparing these data with the global δ 13 C database for the Upper Ordovician, we conclude that the base of the Ellis Bay Formation should be placed in the Katian Stage, perhaps near the K2-K3 boundary (Bergström et al, 2009(Bergström et al, , 2010Ainsaar et al, 2010). This interpretation is at odds with interpretations of Anticosti Island stratigraphy that place the base of the Hirnantian Stage at the bottom of the Ellis Bay Formation based on the brachiopod (Copper , 2001;Jin and Copper, 2008), graptolite (Melchin, 2008), and chitinozoan (Achab et al, 2010) biostratigraphy.…”

“…The Ellis Bay-Becscie contact has been suggested to represent the Ordovician-Silurian boundary (Cocks and Copper, 1981;Nowlan, 1982;Long and Copper, 1987a;Soufi ane and Achab, 2000;Copper, 2006;Melchin, 2008), but the Ordovician-Silurian boundary has also been placed within the lower several meters of the Becscie Formation (McCracken and Barnes, 1981;Petryk, 1981;Copper, 1999;Desrochers et al, 2010). As described already, in the west, the basal Becscie Formation records a quick return in δ 13 C carb to its pre-excursion 0‰ baseline following the Hirnantian positive excursion.…”

“…However, there has not been widespread agreement on the interpretation of the graptolite record (Delabroye and Vecoli, 2010). Melchin (2008) suggested that a small δ 13 C carb peak in the Velleda and Grindstone Members at the western exposures of the Ellis Bay Formation (Long, 1993) may correspond to the initial rise in the δ 13 C record of the Hirnantian excursion, and the δ 13 C carb peak in the Laframboise Member may correlate with the second rise in the Hirnantian excursion, a viewpoint also expressed by Desrochers et al (2010) and Achab et al (2010). In this framework, the entire Ellis Bay Formation represents the most expanded section of Hirnantian stratigraphy in the world, with tens of meters of strata having δ 13 C carb of 0‰ separating two Hirnantian δ 13 C carb maxima.…”

“…However, as reviewed in Kaljo et al (2008), Hindella and Eospirigerina have their fi rst appearances in Baltic sections in the Katian Stage and therefore may not be diagnostic of the Hirnantian Stage. Melchin (2008) recently identifi ed graptolites indicative of the Hirnantian Normalograptus persculptus zone in the Lousy Cove Member. If these identifi cations are correct, (1) the base of the Hirnantian is below the Lousy Cove Member, and (2) the positive carbon isotope excursion in the Laframboise Member begins in the N. persculptus zone, and not in the N. extraordinarius zone (Melchin and Holmden, 2006;Melchin, 2008), as argued by Baltic chemostratigraphers (Brenchley et al, 2003;Kaljo et al, 2008).…”

“…Melchin (2008) recently identifi ed graptolites indicative of the Hirnantian Normalograptus persculptus zone in the Lousy Cove Member. If these identifi cations are correct, (1) the base of the Hirnantian is below the Lousy Cove Member, and (2) the positive carbon isotope excursion in the Laframboise Member begins in the N. persculptus zone, and not in the N. extraordinarius zone (Melchin and Holmden, 2006;Melchin, 2008), as argued by Baltic chemostratigraphers (Brenchley et al, 2003;Kaljo et al, 2008). However, there has not been widespread agreement on the interpretation of the graptolite record (Delabroye and Vecoli, 2010).…”