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National Geologic Map Database

Geologic Unit: Muddy

Usage:

Muddy Sandstone (MT*,ND*,SD*,WY*)
Muddy Sandstone of Dakota Group (CO,WY)
Muddy Formation (WY)
Muddy Sandstone Member of Thermopolis Shale (WY*)


Subunits:

FORMATION STATUS (alphabetical): Fort Collins Member (CO), Horsetooth Member (CO*).


Geologic age:

late Early Cretaceous (Albian)*


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

Origin of name from appearance, rather than a geographic feature; "…in drilling, the soft shales above and below [the sand] are so mixed with the white sand grains that the mixture has a muddy appearance…" (US geologic names lexicon, USGS Bull. 896).
Reference locality: exposure near Greybull, in N/2 sec. 36, T. 53 N., R. 93 W., Bighorn Co., WY (D.L. Eicher, 1960, Yale Univ., Peabody Mus. Nat. History Bull. 15, p. 5 (fig. 2), 11, 14-16, 25-26, strat. sections).


AAPG geologic province:

Williston basin*
Denver basin
Powder River basin*
Wind River basin*
Montana folded belt*
Green River 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 ([ ]).