Madison Group

Madison Limestone
Stratigraphic range: Mississippian PreꞒ Ꞓ O S D C P T J K Pg N
Thrust segment of the Madison Limestone, Sun River canyon, Montana
Type	Geological formation
Underlies	Big Snowy Group
Overlies	Bakken Formation (Three Forks Group)
Thickness	up to 2,100 feet (640 m)[1]
Lithology
Primary	Limestone
Other	Shale
Location
Region	South Dakota, Montana, Idaho, Colorado, Arizona, Alberta, Saskatchewan, Manitoba, Utah
Country	United States  Canada
Type section
Named for	Madison Range
Named by	A.C. Peale, 1893[2]

The Madison Limestone is a thick sequence of mostly carbonate rocks of Mississippian age in the Rocky Mountain and Great Plains areas of the western United States. The rocks serve as an important aquifer as well as an oil reservoir in places. The Madison and its equivalent strata extend from the Black Hills of western South Dakota to western Montana and eastern Idaho, and from the Canada–United States border to western Colorado and the Grand Canyon of Arizona.