LATEST ORDOVICIAN GLACIO-EUSTATIC FLUCTUATION IN THE WILLISTON AND HUDSON BAY BASINS OF MANITOBA, CANADA: CONODONT TURNOVER, ISOTOPIC CARBON EXCURSION, AND SUBAERIAL EXPOSURE

Latest Ordovician stratigraphic successions and biodiversity were influenced globally by glaciation in south polar Gondwana. In equatorial Laurentia, a regressive record includes shallow marine deposits and evidence of subaerial exposure. Within the intracratonic Williston and Hudson Bay basins, it was previously thought that latest Ordovician and possibly earliest Silurian deposits were absent due to this regression. The Ordovician-Silurian (OS) boundary was set at a mass extinction in which diverse Late Ordovician conodonts were abruptly replaced by an impoverished Early Silurian fauna including species of Ozarkodina. In some areas of North America, however, Ozarkodina appears within strata recording the Hirnantian isotopic carbon excursion (HICE) of latest Ordovician age.

The present study in Manitoba is based on data from the only known exposures of the OS boundary interval in the Williston Basin (two sites) and Hudson Bay Basin (one site), plus drill cores from near those sites. In both basins, a positive δ¹³C_carb excursion begins at the base of an argillaceous marker bed and reaches a peak (+2 to +3‰) within or a short distance above that bed. In all instances, the Ozarkodina fauna appears at or near the peak of the excursion. Similar patterns have been observed on Anticosti Island, Quebec (peak +4‰), in Nevada (+8‰), and in Illinois (+2.5‰), where the isotopic excursion has been identified as the HICE, and where representatives of Ozarkodina appear at the peak or in the upper part of the excursion where values are decreasing.

In Manitoba, the co-occurrence of the isotopic excursion and the appearance of Ozarkodina suggests that this excursion represents at least part of the HICE, indicating the presence of latest Ordovician strata. The relatively low peak values may indicate that only the upper, decreasing values of the HICE were recorded, corresponding to the transgression related to deglaciation. The base of the marker bed was possibly a surface of subaerial exposure during the time of glacio-eustatic regression. Exposure is suggested by a characteristic δ¹³C_carb pattern below the marker bed, and by lithologic features. In conclusion, the Late Ordovician stratigraphic record in the Williston and Hudson Bay basins may include a sequence of Hirnantian age, and therefore be more complete than previously thought.