Usage:
Shell Rock Formation of Cedar Valley Group (IA)
Shell Rock Formation (IA)
Shell Rock Limestone (IA)
Subunits:
(alphabetical) all in IA: Mason City Dolomite Member, Nora Member, Rock Grove Member.
Geologic age:
early Late Devonian (Frasnian)
Type section, locality, area and/or origin of name:
Typical development along Shell Rock River between Rockford and Nora Springs and on northward, also about Mason City and to the north and northwest. Named from Shell Rock River (Thomas, 1924).
Reference section (subsurface): FM-4 core, in sec. 31, T. 96 N., R. 18 W., Floyd Co., IA (Witzke and others, 1988).
AAPG geologic province:
Iowa shelf
For more information, please contact Nancy Stamm, Geologic Names Committee Secretary.
Asterisk (*) indicates published by U.S. Geological Survey authors.
"No current usage" (†) implies that a name has been abandoned or has fallen into disuse. Former usage and, if known, replacement name given in parentheses ( ).
Slash (/) indicates name conflicts with nomenclatural guidelines (CSN, 1933; ACSN, 1961, 1970; NACSN, 1983, 2005, 2021). May be explained within brackets ([ ]).