Abstract

A sharp but conformable boundary with the overlying March Formation is present at the top of a section of the Nepean Formation near Ottawa, here proposed as the type section. The basal March unit can be recognized by its consistent and distinctly higher carbonate content (calcite and dolomite) than the underlying Nepean sandstones. The March and upper Nepean strata in this section contain a lower Arenigian conodont fauna, which can be correlated with similar faunas elsewhere in North America, and in Europe, Asia, and Australia. The Nepean–March contact is younger near Ottawa than around Brockville, indicating that the lower Ordovician succession represents a marine transgression, which progressed northward with time.

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