The paradox of the global standard Late Ordovician–Early Silurian sea level curve: Evidence from conodont community analysis from both Canadian Arctic and Appalachian margins

Abstract

Relative sea level fluctuations in the Late Ordovician–Early Silurian were largely driven by Gondwana deglaciation, three possible short-lived ice readvances, and the closure of the Iapetus Ocean. This study establishes and compares sea level events on the Canadian Arctic and Appalachian margins of Laurentia based on the pattern of conodont communities, and demonstrates that sea level events were not synchronous on the two margins. Conodont data were compared for a) the Arctic margin from the Cape Phillips Formation (Richmondian, Upper Ordovician through lower Sheinwoodian, Lower Silurian), Cornwallis Island, Canadian Arctic Islands, and b) the Appalachian margin from the upper Ellis Bay, Becscie, Merrimack, Gun River, Jupiter and Chicotte formations (Richmondian, Upper Ordovician, Rhuddanian, Aeronian, and most of Telychian, Lower Silurian), Anticosti Island, Quebec. The pattern of Late Ordovician–Early Silurian conodont communities is established based on three cluster analyses on conodonts from Cornwallis Island (4967 specimens representing 54 species from 77 samples, Cape Phillips Formation) and Anticosti Island (1980 specimens representing 21 species from 25 samples, upper Ellis Bay Formation; 24,839 specimens representing 42 species from 123 samples, Becscie, Merrimack, Gun River, Jupiter and Chicotte formations). Overall, the sea level curves inferred from the distribution of the conodont communities from Cornwallis and Anticosti exhibit gradual and rapid oscillating patterns, respectively. Specifically, during the time interval of the latest Ordovician and the Early Silurian (Rhuddanian), the sea level behaved differently in these two regions: 1) during the latest Ordovician, conodont community changes show the different patterns in the two regions, which reflects a transgression on Cornwallis, but a regression on Anticosti; 2) during the earliest Rhuddanian, similar conodont communities arose in both regions, but they indicate a regression on Cornwallis and a transgression on Anticosti; 3) in the early Rhuddanian, these communities were replaced by a deeper water community on Cornwallis and a shallower water community on Anticosti, which suggest a transgression on Cornwallis and a regression on Anticosti; 4) a highstand drove out almost all conodonts from Anticosti by the end of Rhuddanian, which is not seen on Cornwallis; 5) a deep-water conodont community remained almost unchanged on Cornwallis from the late Rhuddanian to the early Aeronian, whereas a shallow water conodont community returned to Anticosti in the early Aeronian. However, the sea level dropped on both Cornwallis and Anticosti during the late Telychian, and the sea level curves show a similar pattern. The difference in the sea level pattern during the late Richmondian, Rhuddanian and Aeronian on both the northern and southern margins of Laurentia reflected by conodont communities is supported by the studies on Quaternary glaciation/deglaciation events, where the melting of an ice sheet was accompanied by sea level change but with variable regional isostatic effects that do not generate a uniform global eustatic signal.