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National Geologic Map Database

Geologic Unit: Gammon

Usage:

Gammon Member of Pierre Shale (CO*,MT*,NE*,SD*,WY*)
Gammon Member of Cody Shale (MT*,WY*)
Gammon Ferruginous Member of Pierre Shale (MT*,SD*,WY*)
Gammon Shale (MT*,SD*-notably in subsurface of Williston basin)
Gammon Shale of Montana Group (MT*-notably in Porcupine Dome area)


Subunits:

FORMATION STATUS: Shannon Sandstone Member (MT*,SD*).
MEMBER STATUS (alphabetical): /Groat Sandstone Bed (MT*,SD*,WY*), Shannon Sandstone Bed (WY*), Sussex Sandstone Bed (WY*).


Geologic age:

Late Cretaceous (Campanian)*


Type section, locality, area and/or origin of name:

Named from exposures along Gammon Creek, in T. 57 N., R. 67 and 68 W., Crook Co., WY (Rubey, 1938).
Reference locality (of Pierre Shale): at Red Bird, in sec. 13, T. 38 N., R. 62 W., and in sec. 18, T. 38 N., R. 61 W., Niobrara Co., WY (Gill and Cobban, 1966).


AAPG geologic province:

Powder River basin*
Williston basin*
Denver basin*


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 does not conform with nomenclatural guidelines (CSN, 1933; ACSN, 1961, 1970; NACSN, 1983, 2005). This may be explained within brackets ([ ]).