d13C CHEMOSTRATIGRAPHY OF THE MANITOULIN FORMATION (ORDOVICIAN-SILURIAN) AND UNDERLYING STRATA IN ONTARIO: A NEW INTERPRETATION OF AGE AND DEPOSITIONAL RELATIONSHIPS

The Manitoulin Formation, a widespread and lithologically distinctive unit in the Lake Huron region, has traditionally been considered to have been deposited during the initial Silurian transgression based on some fossil indications and the fact that the unit locally rests disconformably on poorly fossiliferous strata thought to be of Late Ordovician age. We have used many d¹³C samples from seven localities and two drill-cores to establish the d¹³C chemostratigraphy through the Manitoulin Formation and immediately underlying units. On Manitoulin Island and at Cabot Head, Bruce Peninsula there are baseline d¹³C values of 0‰ to –1‰ and no indication of a positive d¹³C excursion. This suggests that at those localities, the base of the Manitoulin Formation is either of very latest (post-HICE) Ordovician or earliest Silurian age and that it is separated from underlying Richmondian strata by a significant gap. At Hope Bay, in the Owen Sound area, and in the area south of Georgian Bay, the lower part of the Manitoulin Formation shows elevated d¹³C values of ~+2‰ to ~+3‰ compared to the baseline values of ~-1‰ in the higher part of the unit, indicating a positive d¹³C excursion. Because no such excursion is known from the early Llandovery elsewhere in the world, we interpret these values as representing the global Hirnantian isotopic excursion (HICE). Elevated δ¹³C values of up to +2‰ have also been obtained from the underlying Whirlpool Sandstone and the uppermost Queenston Formation suggesting that also these intervals are of Hirnantian age. The unfossiliferous Whirlpool Sandstone has traditionally been regarded as representing the earliest Silurian transgression. In our new interpretation, this very shallow-water unit was rather deposited during the general eustatic regression associated with the Hirnantian Gondwana glaciation. Furthermore, the marked disconformities above and below the Whirlpool Sandstone are interpreted as reflecting major regressions during two glacial maxima that are correlated with two similar stratigraphic gaps in shallow-water Hirnantian successions in the Upper Mississippi Valley and northern Europe.