Pg. 107-110, 235-236. Castlewood Member of Spechts Ferry Formation of Decorah subgroup [informal] of Galena Group. Present in Missouri, Illinois, Iowa, Minnesota, and Wisconsin. Consists of dark-brown shale, bentonite [Deike K-bentonite Bed], and argillaceous limestone. In type area, includes chert at base. Thickness about 7 feet at Caledonia, Minnesota. Underlies Glencoe Member (new); overlies Quimbys Mill Formation of Platteville Group. Castlewood of this report is equivalent only to basal Carimona Limestone in type area, but where upper unit is absent, which is common south of Decorah, Iowa, the Carimona is equivalent to the Castlewood. Age is Middle Ordovician (Champlainian).
Type section: exposure in south bluff of Meramec River along St. Louis-San Francisco RR, 0.25 mi [northeast] of Mincke Siding, near center E/2 SE/4 SE/4 sec. 21, T. 44 N., R. 4 E., Manchester 7.5-min quadrangle, St. Louis Co., eastern MO. Named from village of Castlewood, St. Louis Co., eastern MO, 1.5 mi northeast of type section.
["Subgroup" not recognized as a formal stratigraphic rank term (CSN, 1933; ACSN, 1961, 1970; NACSN, 1983, 2005, 2021). Considered informal and should not be capitalized.]

Source: US geologic names lexicon (USGS Bull. 1350, p. 128-129); supplemental information from GNU records (USGS DDS-6; Reston GNULEX).

Although a massive bed 3 to 7 feet thick in its type area in Missouri, Castlewood is only a few inches to 1.5 feet thick in northwestern Illinois. Consists mostly of limestone overlying a thin bentonite that is generally less than 1 inch thick [Deike K-bentonite Bed], which in turn overlies a dark-brown shale up to 3 inches thick. Separated from underlying Plattin subgroup [informal] by widespread unconformity. Fossils are not abundant, but PIONODEMA SUBAEQUATA and other Trentonian fossils are present. Was previously called LINGULA ELDERI bed in Minnesota, where it is equivalent to Carimona Limestone.
["Subgroup" not recognized as a formal stratigraphic rank term (CSN, 1933; ACSN, 1961, 1970; NACSN, 1983, 2005, 2021). Considered informal and should not be capitalized.]

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

Pg. 43. Castlewood Member of Spechts Ferry Formation. Revised in Missouri, Illinois, Iowa, and Wisconsin, to include Deicke Bentonite Bed near or at base. Unconformably overlies Quimbys Mill Formation. Age is Middle Ordovician. [See entry under Deicke.]

Source: Inferred from US geologic names lexicon (USGS Bull. 1564, p. 44).

Castlewood Member here removed from the Trentonian Decorah and reallocated to the Blackriveran Plattin Limestone. Bentonite bed at its base, the type Deicke, correlates with the Pencil Cave Metabentonite of KY. Name Deicke, therefore, is "suppressed." [Pencil Cave is considered an informal name by the GNU.] The Carimona is not equivalent to the Castlewood as previously thought and the bentonite bed at its base is not the type Deicke. Carimona is younger than Castlewood.

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

Castlewood Limestone Member of Spechts Ferry Formation of Decorah Group. [Author follows usage of Thompson (1991).] Underlies Glencoe Shale Member. Contains Deike K-bentonite Bed of Willman and Kolata (1978) near its base. Age is Middle Ordovician (Mohawkian).

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

Castlewood Limestone Member of Spechts Ferry Formation of Decorah Group. Recognized in St. Louis region, Missouri and Illinois. Lower part includes Deicke K-bentonite Bed. Underlies Glencoe Shale Member. Age is Late Ordovician (Mohawkian; late Turinian).
Use discontinued by the State Geological Surveys of Illinois, Iowa, and Minnesota. Rocks reallocated to: Quimbys Mill Formation of Platteville Group in Illinois (Kolata, 2005, Illinois Geol. Survey Map, no. 14); Carimona Member of Decorah Shale of Galena Group in Iowa (Witzke and Ludvigson, 2005, Iowa Geol. Survey Gdbk., no. 24, pt. 1 Contrib. Papers, p. 3-21); Carimona Member of Decorah Formation of Galena Group in Minnesota (Mossler, 2008, Minnesota Geol. Survey Rpt. Inv., no. 65). See also Kolata and others (1996) entry under Decorah.

Source: N.R. Stamm, compiler, US geologic names lexicon, Geolex.