Age of the Antrim Shale in the Michigan basin is changed to Late Devonian only based on conodonts, and is divided into four members, from base to top: Norwood, Paxton, Lachine, and upper members. The name Norwood Member formalizes the poorly defined "Norwood shale" of Ehlers (1938), which is retained because of long-standing usage. Norwood is the lowest 8 m of the Antrim, consisting of black fissile pyritic shale with thin fossiliferous limestone beds, calcareous concretions, and greenish-gray bioturbated shale. Conformably overlies the Squaw Bay Limestone [See Note Below]; conformably underlies the Paxton Member of the Antrim. Age is Late Devonian (early Frasnian) based on conodonts.
[NOTE: In accordance with Department of Interior's Secretarial Order 3404, discontinuing use of derogatory term squaw, the Squaw Bay Limestone is in process of being formally renamed Birdsong Bay Limestone (RESERVED). The geographic feature Squaw Bay, from which the geologic name was taken (Warthin and Cooper, 1935), was renamed Birdsong Bay by U.S. Board on Geographic Names, September 2022, USGS GNIS FeatureID 638662.]

Source: GNU records (USGS DDS-6; Reston GNULEX).