Alexandrian and Niagaran Series

Silurian System

Type locality and use of name: The Cataract Formation was named by Schuchert (1913) for a part of what was commonly called the Medina Sandstone at Niagara Fails. The unit has been raised to group rank and includes in Ontario along the northern part of the Niagara Escarpment the Whirlpool, Manitoulin, and Cabot Head Formations (for example, Liberty and Bolton, 1971). The Whirlpool pinches out westward, and in Michigan only the Manitoulin Dolomite and the Cabot Head Shale are included in the Cataract Group. The Cataract is recognized in Indiana only in the northeastern part of the state, which is a marginal part of the Michigan Basin, and there the Manitoulin and Cabot Head rocks are relatively thin and intergradational. Therefore, the Cataract has been reduced to formational rank, and the other named units are considered to be members (Rexroad, 1980).

Because of the lithologic similarity of the Cabot Head and the overlying unit, the Stroh Member, the latter is also included in the Cataract at member rank even though the two are unconformable. The three members can be recognized only north of Adams County, and so the Cataract Formation is undivided as to member in its southernmost extent.

Description: Where it is undivided, the Cataract is generally a dolomite that is more impure and argillaceous in its middle and upper parts, a lithology that is generally like that of the Manitoulin farther north. The dolomites of the Manitoulin are variable above them the Cabot Head is a mixture of shale and argillaceous dolomite that generally has a greenish east. Similarly, the overlying Stroh consists of argillaceous dolomite and shale. The dolomite tends to be gray or tannish gray but commonly has a greenish cast, and the shale is commonly green. The thickest Cataract, slightly more than 100 feet (30 m), is in the extreme northeast corner of the state, and the thinnest, somewhat less than 10 feet (3 m), is along the axis of the Madison Trend (Rexroad, 1980), a north-southward-trending, structurally high feature in eastern northern Indiana.

Correlation: The Manitoulin and the Cabot Head of Indiana are continuous with the same named units in Michigan, and the Stroh part is continuous with the Clinton Group (undifferentiated) of Michigan. The upper Cataract (Stroh Member) to the south and the west is in a facies relationship with the basal beds of the Salamonie Dolomite. The middle Cataract member (Cabot Head) pinches out to the south and the west, although in part it is in a facies relationship with the underlying lower member (Manitoulin), which in turn is in a facies relationship with the Brassfield Limestone to the south and the Sexton Creek Limestone to the west. The lower and middle Cataract rocks (Manitoulin and Cabot Head) contain conodonts of the Distomodus kentuckyensis Assemblage Zone of generally middle Llandoverian age, and the upper member (Stroh) has conodonts representative of the upper part of the Pterospathodus amorphognathoides-KockelelIa ranuliformis Assemblage Zone and is of early Wenlockian age.