Classical stratigraphic studies of the Niagaran of the Michigan basin have all been made in the zone of outcrop, which nearly encircles the southern peninsula of Michigan at the periphery of the basin. Here there is little difficulty establishing equivalency of units, for distinctive faunal and lithologic zones can be traced from Manitoulin Island, Ontario, through the northern peninsula of Michigan and southward into eastern Wisconsin. This has led to the generally accepted stratigraphic interpretation of a succession of Niagaran seas, each with a distinctive environment and the deposits of each separated from those of the preceding and succeeding seas by significant unconformities.

With recent drilling information, however, it has become increasingly apparent that the characteristic outcrop zones quickly disappear in the subsurface of Michigan. A detailed examination has been made by the writer of the field relationships of these units on Manitoulin Island which has excellent exposures and which is used as the reference area of this paper. This study has failed to confirm any widespread unconformities above the base of the Niagaran.

It is here proposed that the entire Niagaran sequence of the northern rim of the Michigan basin belongs to one large transgressing littoral-lagoon-barrier reef association. Evidence for this hypothesis is seen in the results of insoluble residue analyses made from samples collected at typical Manitoulin Island sections. An unconformity is indicated at the base of the Dyer Bay carbonate unit, which is interpreted as a littoral accumulation of fine carbonate sand deposited at the shoreward edge of the advancing lagoon. The overlying Wingfield and St. Edmund siltstones and calcisiltites reflect the calm, shallow bottom conditions of the lagoon proper, while the Fossil Hill calcarenites and calcirudites were deposited in the higher energy environment of the back-reef detrital zone. The Am bel comprises several facies of the reef core. Overlying the Amabel are detrital beds, formerly mapped as Fossil Hill or its correlatives, which are interpreted as fore-reef detritus, Detritus also occurs in passes through the Amabel reef-core facies.