CONODONTS AS A TOOL IN ESTABLISHING THE AGE AND STRATIGRAPHIC POSITION OF UPPER ORDOVICIAN PETROLEUM SOURCE ROCK INTERVALS ACROSS HUDSON BAY PLATFORM, CANADA

Upper Ordovician strata are widely distributed in the Hudson Bay, Hudson Strait and Foxe basins across the Hudson Platform (~ 1 200 000 km²); they are divided into Bad Cache Rapid and Churchill River groups and Red Head Rapids Formation in the Hudson Bay Basin, and Amadjuak, Akpatok and Forster Bay formations in the Hudson Strait and Foxe basins. Petroleum source rock intervals have been discovered in these successions in all three basins; however, their age and stratigraphic positions have been long debated. For several decades it had been considered that there was only one organic-rich interval at the same stratigraphic position across the entire platform, i.e. between Bad Cache Rapid and Churchill River groups in the Hudson Bay Basin and between Amadjuak and Akpatok formations in the Hudson Strait and Foxe basins, with an uniform age of mid-Maysvillian, Late Ordovician (Heywood & Sanford, 1976; Sanford & Grant, 2000).

Conodonts recovered from the organic-rich intervals in the Upper Ordovician succession on Southampton, southern Baffin and Akpatok islands provide a reliable age assessment for the petroleum source rocks in the Hudson Bay, Hudson Strait and Foxe basins. They are characterised by Amorphognathus ordovicicus and Rhipidognathus symmetricus in the lower Red Head Rapids Formation on Southampton Island (Zhang, 2011) and lower Foster Bay Formation on Akpatok Island (Zhang, 2018), and by Phragmodus undatus and Periodon grandis as well as possible A. superbus in the lower Amadjuak Formation on southern Baffin Island (McCracken, 2000; Zhang, in preparation). These data suggest that the organic-rich intervals in the Hudson Bay and Hudson Strait basins are correlated to the middle-upper A. ordovicicus Zone, or a local R. symmetricus Zone, with an age of late Richmondian, or late Katian, and the horizon in the Foxe Basin to the A. superbus Zone, with an age of early Edenian, or early Katian. This new insight is also supported by the associated graptolite faunas (Zhang & Riva, 2018).

This study has clarified the age, stratigraphic position, thickness and regional distribution of the Upper Ordovician organic-rich intervals that are important petroleum source rock targets in the possible future hydrocarbon exploration.