| Madison Limestone | |
|---|---|
| Stratigraphic range:Mississippian | |
 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] |
TheMadison Limestone is a thick sequence of mostlycarbonate rocks ofMississippian age in theRocky Mountain andGreat Plains areas of the westernUnited States. The rocks serve as an importantaquifer as well as anoil reservoir in places. The Madison and its equivalent strata extend from theBlack Hills of westernSouth Dakota to westernMontana and easternIdaho, and from theCanada–United States border to westernColorado and theGrand Canyon ofArizona.
The Madison is formally known as the MadisonGroup. In Montana, where its thickness reaches 1,700 feet (520 m), the group is subdivided into the Mission Canyon Formation and Lodgepole Formation. Equivalents of the Madison are named the Pahasapa Limestone in the Black Hills,Leadville Limestone (Colorado), Guernsey Limestone (Wyoming), andRedwall Limestone in the Grand Canyon. The upper part of the Madison Group, the Charles Formation in the subsurface of North Dakota and northern Montana, is not strictly an equivalent of the Madison Limestone as usually defined.[3]
Most of the Madison Limestones were deposited during Early to Middle Mississippian time (Tournaisian toVisean stages), about 359 to 326 million years ago. Older North American usage lists the Madison as being laid down during the Kinderhookian, Osagian, and Meramecian stages.
Neither a type locality nor derivation of the name was designated when the termMadison Limestone was first used by Peale (1893),[4] but since the original work focused on the area ofThree Forks, Montana, it is likely that the name relates to outcrops along theMadison River, Montana. A reference section has been designated on the north side of Gibson Reservoir in SE/4 sec. 36, T. 22 N., R. 10 W., Patricks Basin quad, Teton Co., Montana.[5]
Limestones anddolomites dominate the Madison. Because the rock is highly soluble, it often develops caves andkarst topography.Lewis and Clark Caverns, Montana, is an example of a cave developed in the Madison. The rocks were deposited in a generally shallow marine setting, indicated by the richly fossiliferous rocks of the Madison. In theWilliston Basin, water was shallow enough foroolite shoals to develop; they later became reservoirs for oil.[6] The gray cliffs along theMissouri River in theGates of the Mountains, Montana are formed by Madison Limestone.[7]
The following formations are recognized inMontana,Wyoming andManitoba, from top to base:
| Sub-unit | Age | Lithology | Max. Thickness | Reference |
|---|---|---|---|---|
| Charles Formation | Mississippian | dolomitic limestone | 244 m (800 ft) | [8] |
| Mission Canyon Formation | Osagian | white bioclastic limestone, ooliticcalcarenite, occasionallydolomitized;anhydrite in theWilliston Basin | 183 m (600 ft) | [9] |
| Lodgepole Formation | Kinderhookian | limemudstones,shale,chert, containsbitumen | 245 m (800 ft) | [10] |
The following subdivisions (offormation rank) are recognized inSaskatchewan, from top to base:
| Sub-unit | Age | Lithology | Max. Thickness | Reference |
|---|---|---|---|---|
| Poplar Beds | Meramecian | limestone, argillaceousdolomite,evaporite | 152 m (500 ft) | [11] |
| Ratcliffe Beds | Osagian | densedolomite,mudstone with threeanhydrite beds | 80 m (260 ft) | [12] |
| Midale Beds | Osagian | oolitic topisolitic and skeletalgrainstone topackstone with vuggy porosity,dolomite, porouswackestone | 45 m (150 ft) | [13] |
| Frobisher Evaporite | Osagian | supratidalanhydrite | 9 m (30 ft) | [13] |
| Kisbey Sandstone | Osagian | poroussiltydolomite and calcareoussandstone | 10 m (30 ft) | [14] |
| Alida Beds | Osagian | oolitic to pisoliticgrainstone andpackstone, silty anddolomitic limestone, crinoidal limestones | 63 m (210 ft) | [15] |
| Tilston Beds | Kinderhookian | oolitic, pisolitic andcrinoidal grainstone and packstone;cherty ordolomitic limestone, silty limestone,anhydrite | 80 m (260 ft) | [16] |
| Souris Valley Beds | Kinderhookian | argillaceous limestone, calcareousshale,chert | 176 m (580 ft) | [17] |