ABSTRACT

Biostratigraphic data based on macrofossils and conodonts from wells and outcrops in the Williston and Hudson Bay basins permit new interpretations of stratigraphic units within the Ordovician-Silurian boundary interval. On the basis of conodont data, the Ordovician -Silurian boundary has been restricted to an 8 cm interval that essentially coincides with the top of the upper t-marker in the Stonewall Formation in Saskatchewan and within a 2 m interval associated with the t-marker in other outcrops and wells in Manitoba. Correlations in the Hudson Bay region are less precise because of sparse data. A major disconformity is documented within the Stonewall Formation of the Williston Basin with the associated hiatus ranging from latest Ordovician (Gamachian) to probably earliest Silurian (part of the Rhuddanian). Cyclic sedimentation related to relative sea-level changes is suggested by the repetition of carbonate lithologies separated by marker beds that probably represent disconformities of uncertain duration. The late Rhuddanian brachiopod Virgiana decussata (Whiteaves) occurs at the base of the Interlake Group in the Williston Basin and in the Severn River Formation of the Hudson Bay Basin, indicating the probable connection of the two basins at that time. In northern Manitoba, conodont data clearly indicate that the type section of the Port Nelson Formation is entirely Late Ordovician (Richmondian) and is correlated with the Stonewall Formation in the Williston Basin. The name Port Nelson Formation is advocated to replace the name Red Head Rapids.