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Compendium
  
Cabot Head Member

(CBR)

Cataract Formation,

Silurian System


Type locality and use of name: The Cabot Head was originally named by Grabau (1913) "the Cabots Head beds" for exposures at Cabots Head on Manitoulin Island in northern Lake Huron, Ontario. This unit later became the Cabots Head Shale Member and was assigned, along with the Manitoulin Limestone Member and the Whirlpool Sandstone Member, to the Cataract Formation (Schuchert, 1913). In the Manitoulin Island area the Cabot Head now has formational rank and some usage includes in its upper part as members the Dyer Bay, the Wingfield, and the St. Edmund (Liberty and Bolton, 1971), though these units are considered to be interfingering formations in other usage. In that area the Cabot Head in places is in a facies relationship with the underlying Manitoulin Dolomite.

Description: As recognized by Rexroad (1980) in its northeastern Indiana extent, the Cabot Head Member of the Cataract Formation consists of varying amounts of shale, dolomitic shale, and shaly to argillaceous dolomite ranging from intimate intermixtures to dominance by one lithology. The dolomite is generally fine grained, but lenses of coarser fossiliferous rock are present in some places. Chert is known in a few sections. Although color varies, the rocks generally have a greenish east. The member ranges from zero to slightly more than 60 feet (17 m) in thickness,

The Cabot Head Member and the Manitoulin Dolomite Member, which gradationally underlies the Cabot Head, are traced into the Michigan Basin part of northeastern Indiana from Michigan and are recognized only north of Allen County. In the southern extent of the Cataract the two are so thin and intergradational that they cannot be distinguished. The Cabot Head is overlain unconformably by the Stroh Member of the Cataract, a unit of similar lithology. The basal bed of the Stroh, however, is commonly a tannish, very fine grained laminated dolomite that serves as a marker (Rexroad, 1980).

Correlation: The Cabot Head of Indiana is continuous with the Cabot Head of Michigan. It does not include rocks as young as the Dyer Bay and the Wingfield of the Manitoulin Island area, northern Lake Huron. In Indiana it is in a facies relationship with the upper part of the Brassfield Limestone but generally pinches out east of the area of recognition of the Sexton Creek Limestone, although it is time correlative with part of the Sexton Creek. The Cabot Head contains conodonts of the Distomodus kentuckyensis Assemblage Zone and in terms of British terminology is approximately middle Llandoverian in age (Rexroad, 1980) (Alexandrian in the North American standard).






 
 
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