The Ordovician–Silurian boundary on Cornwallis and Truro islands, Arctic Canada: preliminary data

Abstract

Four sections of the lower part of the Cape Phillips Formation, two outcrops on northeastern Cornwallis Island and one outcrop and one drill core from Truro Island, Northwest Territories, Canada, provide significant new data on the Ordovician–Silurian boundary. They show evidence of continuous sedimentation through the boundary interval and a continuous sequence of graptolite zones, including the bohemicus and persculptus zones, which have not been previously found in Arctic Canada. Strata yield graptolites, including uncompressed specimens, and conodonts through most of the sections. The ordovicicus conodont Zone occurs within the pacificus to lower persculptus graptolite zones. The nathani conodont Zone contains a "transitional fauna," a mixing of species typical of the preceeding ordovicicus Zone and those generally regarded as Silurian indicators. This conodont zone ranges from the middle of the persculptus graptolite Zone into the lower acuminatus graptolite Zone and, thus, spans the Ordovician–Silurian boundary. The Ordovician–Silurian faunal turnover of the conodonts, therefore, also spans the Ordovician–Silurian boundary and is not coincident with the interval of major graptolite extinction, which occurs earlier, at the end of the pacificus Zone. The base of the kentuckyensis conodont Zone occurs in the acuminatus graptolite Zone. Sedimentologic evidence of the maximum eustatic sea-level drop can be seen within the bohemicus Zone (early Hirnantian) and possibly one or several smaller scale sea-level fluctuations through the underlying zones.

Late Ordovician to earliest Silurian graptolite and brachiopod biozonation from the Yangtze region, South China, with a global correlation

“…Based on its location between clearly corresponding P. pacificus and N. persculptus zones, we tentatively correlate the 'N. bohemicus' Zone of Melchin, McCracken & Oliff (1991) with the N. extraordinarius-N. ojsuensis Zone in the Yangtze region. Melchin (pers.…”

“…His Paraorthograptus pacificus Zone contains Amplexograptus latus (Elles & Wood) and Appendispinograptus supernus (Elles & Wood) from Huff ridge. In the Arctic islands, the P. pacificus Zone is succeeded immediately by an interval containing 'Neodiplograptus bohemicus (Marek)' and Normalograptus shanchongensis (Li) (Melchin, McCracken & Oliff, 1991). Following previous interpretations of the Yangtze region graptolites, Melchin, McCracken & Oliff (1991) referred this fauna to the N. bohemicus Zone.…”

“…In the Arctic islands, the P. pacificus Zone is succeeded immediately by an interval containing 'Neodiplograptus bohemicus (Marek)' and Normalograptus shanchongensis (Li) (Melchin, McCracken & Oliff, 1991). Following previous interpretations of the Yangtze region graptolites, Melchin, McCracken & Oliff (1991) referred this fauna to the N. bohemicus Zone. Štorch & Loydell (1996) suggested that these pre-Hirnantia Canadian specimens of N. bohemicus, like the type materials of N. bohemicus (Marek), may be synonymous with Normalograptus persculptus (Elles & Wood).…”

“…The interval above the N. persculptus Zone in the Canadian Arctic islands lacks A. ascensus and P. acuminatus. Melchin, McCracken & Oliff (1991) placed the base of the post-N. persculptus zone (their P. acuminatus Zone) at the FADs of N. madernii and N. lubricus. This N. madernii-N. lubricus fauna, which they recognized as a lower subzone of the P. acuminatus Zone, is succeeded by an upper subzone characterized by the appearance of Hirsutograptus sinitzini.…”

“…Correlation of the Ashgill to earliest Llandovery biozones between Yangtze and different continents. * was reported byMelchin, McCracken & Oliff (1991) as N. bohemicus Zone.…”