- Usage in publication:
-
- Foster Formation
- Modifications:
-
- Named
- Biostratigraphic dating
- Dominant lithology:
-
- Siltstone
- Shale
- Dolostone
- AAPG geologic province:
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- Michigan basin
Summary:
The Foster Formation is here named in the subsurface of the central Michigan basin. It consists of dark-gray dolomitic siltstone, black shale, and dark-gray dolostone with thinly interbedded sandstone, limestone, and anhydrite. Contorted bedding and bioturbation are common. Transitionally overlies the Cambrian Trempealeau Formation with no evidence of unconformity; gradationally underlies the Bruggers Formation (new name) with the contact placed arbitrarily below the lowest thick sandstone. Thickness is 1,578 ft (481 m) at type section. Age is Early and Middle Ordovician (Canadian, Whiterockian) based on conodonts.
Source: GNU records (USGS DDS-6; Reston GNULEX).
- Usage in publication:
-
- Foster Formation
- Modifications:
-
- Revised
- AAPG geologic province:
-
- Michigan basin
Summary:
Nomenclature used in this report assigns Foster Formation to the Prairie du Chien Group. Foster includes the "Brazos shale" at the top and underlies the St. Peter Sandstone. Foster overlies Umlor Formation, also of Prairie du Chien. [A 1987 abstract by C.T. Wheeler in AAPG Bull, v. 71, no. 9, p. 1112, also assigned Umlor and Foster Formations to the Prairie du Chien, along with overlying Bruggers and Goodwell Formations.]
Source: GNU records (USGS DDS-6; Reston GNULEX).
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 ([ ]).