Alexandrian Series

Silurian System

Type section, history of name, and use in Indiana: The name Sexton Creek Limestone was applied by Savage (1909) to Silurian strata above the Edgewood Limestone and below Helderbergian (Lower Devonian) rocks for exposures at the mouth of Sexton Creek in Alexander County in southwestern Illinois. In the same report the Edgewood was defined as lying between the Girardeau Limestone (Lower Silurian) and the Sexton Creek. The Sexton Creek was later restricted (Ulrich, 1911) to approximately its present stratigraphic limits, and the overlying rocks now referred to the St. Clair Limestone were eliminated from the formation. The boundary between the Edgewood and the Sexton Creek has been variously defined. At present the brachiopod Platymerella manniensis is used in the type area in Illinois to characterize the basal beds of the Sexton Creek. As currently defined, the Sexton Creek is the precise equivalent of the Brassfield Limestone and is physically continuous with it, and so the junior name Sexton Creek serves only to designate conveniently a distinctive facies of the Brassfield. The name was first used in Indiana in 1974 (Becker). The Schweizer Member is present as a basal unit of the Sexton Creek in parts of northwestern Indiana (Rexroad and Droste, 1982)

Description: Throughout most of the extent of the Sexton Creek Limestone in Indiana, exclusive of the Schweizer Member, the formation is an impure very cherty dolomite or limestone, but the chert commonly does not extend all the way to the top or the bottom of the formation. In places chert constitutes 60 to 70 percent of the rock. Almost everywhere in Indiana the carbonate component is light to medium brown, although there is some gray banding and mottling. It is fine or medium grained and commonly granular but includes coarse lenses and beds, some of which are bioclastic. Irregular gray or brown shale partings are erratically present. The Schweizer is much more argillaceous and contains little or no chert.

The Sexton Creek is present in the subsurface in the western half to two-thirds of Indiana and averages between 40 and 50 feet (12 and 15 m) in thickness. It is more than 100 feet (30 m) thick in the northwest comer of the state, but in that area it is highly variable in thickness because of relief on the Ordovician erosion surface on which it was deposited. In southeastern Indiana at the facies boundary with the Brassfield Limestone it is as thin as 12 feet (3.7 m). The Sexton Creek unconformably overlies Ordovician rocks of the Maquoketa Group. It is overlain unconformably by the St. Clair Limestone in southwestern Indiana and by the Salamonie Dolomite elsewhere in the state, except in Lake County where the relationship is possibly conformable.

Correlation: The Sexton Creek Limestone is recognized under that name in southeastern Missouri and southwestern Illinois. It is in a facies relationship with the Birds Member of the Wilhelmi Formation and with the Elwood and Kankakee Formations of northeastern Illinois. It is also in a facies relationship with the Manitoulin Dolomite Member and the Cabot Head Member of the Cataract Formation of northeastern Indiana and with the Brassfield Limestone of east-central and southeastern Indiana and in north-central Kentucky. In terms of conodont zones the Schweizer Member of the Sexton Creek belongs in the Panderodus simplex Assemblage Zone, and the rest of the Sexton Creek is in the Distomodus kentuckyensis Assemblage Zone (Rexroad and Droste, 1982). The latter approximates the middle Llandoverian part of the British standard Silurian sequence, but the Sexton Creek also includes rocks of late Llandoverian age where it merges with the Brassfield of southwestern Indiana.