Type locality and use of name: The Manitoulin Dolomite Member of the Cataract Formation was named by Williams (1913) because of its importance on Manitoulin Island in the northern part of Lake Huron, Ontario. There it underlies the Cabot Head Shale, although in places the two are also in a facies relationship. Both of these units are considered as formations in that area (Liberty, 1968), but in Indiana both are thin and gradational and are given member rank in the Cataract Formation in the area in the northern part of the state where they can be separated (Rexroad, 1980).
Description: The dolomite of the Manitoulin is variable. It is rather pure and slightly vuggy to very argillaceous and contains many thin shale partings it is also fine grained to very coarse grained and fossiliferous and light tan to brown, gray brown, and gray. The Manitoulin is less than 20 feet (6 m) thick in Indiana except possibly in northern Steuben County, where it may be somewhat thicker (Rexroad, 1980).
The Manitoulin Dolomite Member and the Cabot Head Member are recognized only north of Allen County where they are marginal units in the Michigan Basin. In the more southern extent of the Cataract Formation in Indiana the two members are so thin and intergradational that they cannot be distinguished. The Manitoulin rests unconformably on rocks of the Maquoketa Group (Upper Ordovician) and is gradationally overlain by the Cabot Head.
Correlation: The Manitoulin of Indiana is continuous with the Manitoulin Dolomite (Cataract Group) of Michigan. To the south it is in a facies relationship with the Brassfield Limestone and to the west with the Sexton Creek Limestone. It contains conodonts of the Distomodus kentuckyensis Assemblage Zone and is approximately middle Llandoverian in age (Alexandrian in the North American standard) (Rexroad, 1980).