| Americus Limestone | |
| Type: | Formation member |
| Period: | Late Carboniferous |
| Prilithology: | Limestone, shale |
| Otherlithology: | stromatolite limestone, lime-sand mudstone/grainstone, flint |
| Namedfor: | Americus, Kansas |
| Namedby: | M. Z. Kirk, University Geological Survey of Kansas |
| Year Ts: | 1896 |
| Region: | Kansas |
| Country: | United States |
| Unitof: | Foraker Formation of the lower Council Grove Group |
| Underlies: | Hughes Creek Shale of the Foraker Formation |
| Overlies: | Hamlin Shale member of the Janesville Shale |
The Americus Limestone is a member of the Foraker Limestone Formation in eastern Kansas, where it is quarried as a distinctive ornamental stone. In outcrop, it is typically recognized as two relatively thin but persistent beds of hard limestone separated by shale that forms the lowest prominent bench of the many benches of the Flint Hills. The recognizable facie of the member in excavated or eroded exposures is two thin limestone beds separated a bed of shale and adjacent shales above and below having a particular gray or bluish color darker than higher limestones. A third, lower, highly variable algal limestone is often present and included as the base of the member. The unit is not particularly massive, the limestone pair totaling NaNabbr=offNaNabbr=off in places, more in other locations but less to the North, and up to nearly to 9abbr=offNaNabbr=off at the type location of Americus, Kansas. The addition of the lower algal limestone as a base for the unit increases the thickness to over 18abbr=offNaNabbr=off. Initially thought to be the lowest of the Permian rock of Kansas and as such classified as the lowest unit of the Council Grove Group, the unit is now dated within the uppermost Late Carboniferous.[1]
Limestone couplet: The common recognition of the Americus Limestone is a persistent, wide-ranging pair of durable limestone beds separated by shale. These two limestone beds may be referred to as upper and lower Americus Limestone beds. However, this terminology can lend to confusion with a third limestone bed that sometimes can be found below this pair, and, where found, is included within the Americus member.
"Lower Americus Limestone": 0to below the rather consistent limestone pair is a less consistent third limestone. The variability of this bed is employed to reconstruct the range of environments represented by the outcrop. The base of this limestone, where present, is consistently formed of Collenia stromatolites. The stromatolite base of this limestone can overlie orange lime-sand mudstone to grainstone recording the advancement a shoreline through the area creating the open shallow sea environment where the upper limestones would form.[2] This bed is typically not illustrated on Group and Series scale charts.
Relatively, the paired limestones record broad environmental events across a wide shallow sea while the lowest limestone records a transgressive shoreline.
This limestone has been confused for the Houchen Creek limestone bed of the Hamlin Shale Member of the Janesville Shale. Both are limestones found below the upper paired limestones, and both have large masses of tidal Collenia stromatolite; however, the Houchen Creek's stromatolite mounds are more widely spaced and are directly blanketed by shale rather than the skeletal limestone that encases the Americus stromatolites.[3]
The limestone beds are quarried for construction material. The tougher uppermost limestone bed in particular is sold commercially as Tuxedo Gray or "Flint Hills Gray", and is popular in eastern Kansas for its abundant visible fossils, gray tone that contrasts with the buff tone of Cottonwood Limestone, and the ability of the stone to take a polish that accentuates both the fossils and darker gray color.[2] [4] [5]
As much of the Americus environment was shallow seawater with tidal currents, the formation is known for abundant, fragmented, and sorted remains of fusilinids, crinoids, brachiopods, and stromatolites.
While the upper two limestone are consistent and reflect more open marine conditions, this study reports that the lower limestone records several different transient near-shore tidal and subtidal environments.
The fauna of the Hughes Creek Member is very similar to the underlying Americus Limestone Member.