| Cottonwood Limestone | |
| Age: | (Gearyan stage)[1] |
| Period: | Asselian |
| Type: | Formation Member |
| Prilithology: | Limestone |
| Otherlithology: | Calcareous shale and siliceous to quartz nodules or infills of burrows |
| Namedfor: | Cottonwood Falls, Kansas |
| Namedby: | Prosser, C. S. |
| Year Ts: | 1894-5[2] [3] |
| Region: | Mid-continental |
| Unitof: | Beattie Limestone |
| Subunits: | Top Ledge, Bottom Ledge |
| Underlies: | Florena Shale member of the Beattie Limestone formation |
| Overlies: | Eskridge Shale |
| Thickness: | 3- often ~6feet |
| Extent: | Outcrops from southeast Nebraska to northeast-central Oklahoma |
Cottonwood Limestone, or simply the Cottonwood,[4] is a stratigraphic unit and a historic stone resource in east-central Kansas, northeast-central Oklahoma, and southeastern Nebraska in the Midwestern United States. It is the lowest member of the Beattie Limestone formation and commonly outcrops within the deep valleys and on top of the scenic residual ridges of the Flint Hills.
This important building stone, quarried in the Cottonwood River valley, had been used under the names "Cottonwood stone" or "Cottonwood Falls limestone" many years before the name Cottonwood Limestone appeared in scientific publications late in the 19th century. Similarly, "Manhattan stone" was the commercial name used for the same limestone when quarried in the vicinity of Manhattan, Kansas.[3] It was also called "Alma Limestone" when first quarried at Alma, Kansas.[5]
The Cottonwood Limestone is one of the most persistent and easily recognizable horizons in Kansas.[6] This limestone is usually light gray or buff in color, massively bedded, and has an average thickness of about six feet. The upper part of the member in particular is recognizable for being almost entirely made up of the small wheat-grain shaped foraminifera Fusulinida, with few other animal remains. The line of outcrop of the Cottonwood is generally marked by a line of massive rectangular blocks of light gray or bleached, bone-colored limestone, often supporting a line of shrubs. Because of its resistance to weathering, it often forms a prominent escarpment. The Cottonwood Limestone is noteworthy for the persistence of this lithologic character across its range.
Aside from shaping the landscapes of the communities within the Flint Hills, this limestone is most notable for its influence on the building architectures of the communities in its region. Near the surface, Cottonwood Limestone is a useful aquifer, while deeper subsurface Cottonwood Limestone is associated with oil and gas producing formations.
A regionally famous construction material, Cottonwood Limestone is yellowish, buff, to nearly white in color, even-textured, durable, and very suitable for cutting into dimensional stone for masonry. Moreover, its beds of usable stone are particularly massive for the region; blocks of stone three or more feet thick and several feet in length and width can be taken from a ledge.[7] While many other limestone formations are found in the region and have been widely used in local construction, most (excepting most notably the Fort Riley Limestone) weather poorly, lie with much thinner parting beds, or are generally much higher in the typical Permian silica or flint content and so are much more difficult to cut or sculpt.[4] As such, the lithologic characteristics of Cottonwood Limestone has made it one of the best building stones in the state and many past quarries, large and small, may be found on nearly every hill capped by this resource.[3]
Large, cut blocks of Cottonwood Limestone were famously used in historic, monumental buildings in the region, including churches, courthouses, and educational buildings; while rough stones of other limestones are more common in older, smaller homes and buildings. Its use in a building within its region can often be recognized by the use of large cut blocks, near-white or buff color, and showing abundant fossils or molds of “wheat-grain” shaped fusulinids coupled with general absence of spalling, splotchy black coloring from mold that find portions of the stone particularly favorable, and sculptured or protruding rough faces. Especially in older buildings close to a source, Cottonwood Limestone is sometimes paired with the Neva Limestone that is commonly exposed only 35 feet lower in the hills. In this combination, the softer, massive Cottonwood is used for carved trim while the Neva is used, cut or rough, for the general fill and facing,[8] the latter showing a whiter face with smoother glass-like fracture in contrast to the more buff-colored Cottonwood Limestone showing a rougher earthy fracture.
The Cottonwood is the main facing material for the campus buildings of Kansas State University.[3] The Topeka Performing Arts Center[9] as well as much of the Kansas State Capitol[7] and the Great Overland Station[10] in Topeka, Kansas are constructed with Cottonwood Limestone. The iconic Campanile in Lawrence, Kansas, is also built from the once-named “Manhattan stone”.[11] Cottonwood Limestone continues to be popular in constructions,[12] and was recently (2014) used to construct the West Side Stadium Center of the Bill Snyder Family Football Stadium.[13]
The massive, fine-grained, bright-white character of this limestone makes it a practical stone for large sculptures:
The Cottonwood is a reliable surface limestone aquifer in the region. The brush line seen on many hillsides at the base of this member indicates the moisture it provides. Many springs issue from beneath the massive ledge, especially the west-facing exposures—many of which serve as a source of water for livestock.[8] [17] In many places, springs or shallow wells from the Cottonwood provide large supplies of water for domestic home and farm use. This is especially true along westward-facing hillsides, as strata generally dip to the northwest.[3]
Natural gas has been produced in the Cottonwood Limestone in southwestern Kansas and northwestern Oklahoma. The member made an impact on gas production in the region in the 1960s and 1970s, particularly in the discovery of additional reserves in previously established and even abandoned fields.[18] [19] [20]
As the prominent member of the Beattie Limestone formation, Cottonwood Limestone can be traced on the outcrop from Nemaha County, Nebraska, southward to Osage County, Oklahoma, and followed in the subsurface westward to the vicinity of the Colorado-Kansas line as well as into northwestern Oklahoma.[21]
The surface outcropping of the Cottonwood Limestone is easily traced in the Flint Hills south of the Kansas River and east of Manhattan, Kansas. In Manhattan, it forms the scenic ridges surrounding the old downtown in the southeast of the city. From there, the line of outcrop is a tortuous one extending several miles north and west along the Kansas and Big Blue rivers and their tributaries.[3]
This limestone forms extensive outcrops in the Cottonwood River valley that shares its name; outcrops can be followed several miles up every tributary valley around Cottonwood Falls.
The Cottonwood Limestone is a marine limestone and calcareous shale, laterally persistent, bench-forming, and notably and consistently showing abundant fossils. An outstanding example of uniformity of thickness over considerable areas, the outcrop of the Cottonwood Limestone over a distance of 150 miles from Nebraska to central Kansas is nowhere less than 5.5 nor more than 6.5 feet. The notable consistency of these characteristics within its range serves to make it a distinctive marker bed. Even so, there are some changes in lithologic character that can be seen from north to south.
In the central and north range, the limestone is recognized to have distinct the upper and lower beds, long known as Top Ledge and Bottom Ledge,[22] which commonly part when weathered. This boundary between the upper and lower Cottonwood is evidenced by a sharp textural contrast, porosity difference, and changes in the opaque mineral content.[4] In this range, the whole member is fine-grained limestone,[23] but the Top Ledge has a yellower tint or a creamy gray with open grain while the Bottom Ledge is whiter with tight grain.[24] [25] With respect to fossil content, the upper weathering bed is characterized by the abundant Fusulinid assemblage, while the lower bed is composed of minute particles of various fragmented fossils, especially Osagia, echinoderms, and bryozoans.[23]
In northern east-central Kansas, Cottonwood Limestone forms particularly prominent outcroppings in the Flint Hills, especially in the Cottonwood River valley in Chase County, Neosho River valley in Morris County, and the Kansas River and Big Blue River valleys of Riley County. There, it often forms a very conspicuous hillside bench characterized by massive light-gray rounded limestone blocks. The presence of the Cottonwood Limestone in a hillside here is often marked by a persistent line of shrubs that can be seen for many miles. Wherever seen in these and nearby valleys, the Cottonwood Limestone member is almost uniformly six feet thick. It is everywhere massive and commonly seems to consist of only one or two beds. The limestone on a fresh fracture is yellowish gray to buff in color, weathering to light gray.[26] Fusulinids are common, especially in the upper part, and siliceous nodules project from weathered surfaces. Close observation removes any likelihood of confusing the Cottonwood Limestone with any other layer in this part of Kansas; its distinguishing characters are the massive ledge of uniform thickness, the silicified nodules, and the abundance of fusulinids, chiefly in the resistant upper part. The Cottonwood is not a flinty limestone such as those seen higher on the surrounding hilltops; however, nodules, concretions, and trace fossils (filled-in burrows) of partly silicified material that weather more slowly than the rest of the member can give the limestone an appearance of containing flint.[3]
While the abundance of fusulinids remains a characteristic of the top of the Cottonwood through to Osage County, Oklahoma,[27] south of Chase County, Kansas the member has lost its massive limestone characteristic and does not form the conspicuous outcrops seen farther north.[28] In Greenwood County, Kansas, the unit appears as light-gray limestone crowded with horizontal thin leaves of platy algae, attaining a maximum observed thickness of 8 feet.[27] The Cottonwood in southern Kansas and northern Oklahoma is an interbedded limestone and calcareous shale with a very diverse, abundant, and well-preserved fossil fauna, especially brachiopods, mollusks, and ammodiscid foraminifera (shelly facies). At the southern limit of its outcrop the Cottonwood Limestone is a medium crystalline limestone with thin, shelly layers. Mollusks, Osagia, and quartz silt are common (silty Osagia facies).[23]
Variations in the characteristic content and texture (facies) of the limestone reflect lateral changes in the environment from north to south.[4] [23]
The Cottonwood Limestone is easily accessed or viewed in several locations: