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Stratigraphy
The Fort Hays limestone is the lower member of the Niobrara formation (Moore, Frye, and Jewett, 1944, p. 152) of Cretaceous age. The Fort Hays member is recognized in the field primarily by its thick massive beds and small amount of shale. Chalky shale is the major constituent of the overlying Smoky Hill chalk. Underlying the Fort Hays limestone in most places is the Codell sandstone member of the Carlile shale. At some places the lenticular Codell sandstone is absent and the Blue Hill shale member of the Carlile is in contact with the Fort Hays limestone.
Thickness
The thickness of the Fort Hays limestone in Kansas is not uniform, the thinner parts lying to the north. According to A. R. Leonard (oral communication) the Fort Hays is 40 feet thick in the northern part of Jewell County. Landes and Keroher (1942, pp. 286, 289) report that the thickness in Phillips and Smith counties ranges from 40 to 50 feet. Landes (1930, p. 18) states that the thickness of the Fort Hays in Osborne County is about 60 feet. Two miles south of Natoma in sec. 20, T. 10 S., R. 15 W., an incomplete section of the Fort Hays measures 61.5 feet. In Rooks County, a 56-foot section of the Fort Hays was measured. Bass (1926, p. 25) states that the Fort Hays in Ellis County is 56 feet thick. Landes and Keroher (1939, p. 289) report a thickness of almost 75 feet for the Fort Hays in Trego County. Moss (1932, pp. 19-21) found 70 to 80 feet of the Fort Hays in Ness County. Latta (1944, p. 159) reports that the Fort Hays ranges from 55 feet in the northwest corner of Finney County to 80 feet in the northeast corner. The thickness in Hamilton County is reported by Bass (1926, p. 62) and McLaughlin (1943, p. 135) to be about 61 feet. Thus, the Fort Hays in Kansas is variable in thickness in the southwest, constant but thinner in the central part of the State, and thins rapidly in the northern part.
Topographic expression
The typical outcrop of the Fort Hays consists of cream-colored chalk bluffs overlying barren blue slopes of the Carlile shale. At many places along its belt of outcrop in northern Kansas the Fort Hays gives rise to a prominent east-facing escarpment and where the Saline, Smoky Hill, and Solomon Rivers cross this belt their valley sides are typified by prominent bluffs. In several localities, especially near the eastern boundary of the Fort Hays, flat-topped buttes and small plateaus occur. The overlying Smoky Hill chalk member is thinner-bedded, contains more shale than the Fort Hays, and weathers more readily, giving rise to gentler slopes. Exposures of the Fort Hays may be seen in many road cuts, especially in Phillips, Smith, Mitchell, Ellis, Osborne, and Rooks counties.
Slumping is common in the Fort Hays, and locally, massive chalk blocks are found as much as 50 feet below their normal stratigraphic position. In places, especially in southwestern Osborne County, small-scale faulting is widespread in the Fort Hays chalk. Twenhofel (1925, pp. 1060-1071) has suggested that the faulting results from adjustment of the Fort Hays to movement of the underlying shales. According to him the Cretaceous shales adjust themselves to irregularities of the Permian floor, or to sand lenses included in the Cretaceous. The more extensive the shales beneath the Fort Hays, the greater the adjustment, and the more intensive the faulting.
Means of recognition
The lower boundary of the Fort Hays is quite distinct. Either the Codell sandstone lentil of the Carlile or the noncalcareous Blue Hill shale member of the Carlile underlies the Fort Hays. The Codell sandstone is a friable, dirty-yellow to gray sandstone which is rarely calcareous. The Blue Hill shale is bluish black and noncalcareous.
Logan (1898, pp. 487, 493) found a paleontologic difference between the Fort Hays and the overlying beds of the Niobrara formation. Inoceramus deformis occurs in the Fort Hays and Inoceramus grandis in the Smoky Hill.
The contact between the Fort Hays and the Smoky Hill is gradational. Bass (1926, pp. 21-24) described in the Smoky Hill chalk a group of beds 13 1/2 feet thick which he called "group B." The base of these beds was 8.5 feet above the contact of the Smoky Hill and Fort Hays. The first bed in this group was a soft, chalky shale containing a yellow band of bentonitic clay a quarter of an inch thick. The thickness of the shale bed was 3 to 4 inches. "Group B" also contained two similar bands of bentonitic clay about 3 feet apart, the lower of which is 7 feet above the lowest band of bentonitic clay. Moss (1932, p. 20) states:
The contact between the Fort Hays limestone and the Smoky Hill chalk is transitional from predominating chalk beds to predominating chalky shale. The dividing point here used is the thin bentonite bed at the base of zone A of the Smoky Hill chalk. Although this is a few feet above any massive chalk beds, it is the only convenient and recognizable horizon at which to make a division.
According to Landes and Keroher (1939, p. 14) the top of the Fort Hays is at the top of the uppermost thick chalk beds several feet below the bentonitic seams in the Smoky Hill. However, since this horizon is not readily recognizable in all exposures, the two bentonite horizons lowest in the Smoky Hill chalk have been chosen by us to mark the boundary between the Fort Hays and the Smoky Hill. In Trego County, Landes and Keroher took the lowest bentonite stratum as the boundary between the Fort Hays and Smoky Hill. However, this bentonite stratum has not been mentioned in the reports on Hamilton, Osborne, and Phillips counties, and it is not known whether it occurs in these counties or in Rooks, Smith, or Jewell.
According to Landes (1930, pp. 17-18) the proportion of massive chalk to chalky shale in the Smoky Hill runs about one to one, whereas in the Fort Hays the ratio is at least six to one. Landes also mentions that the Fort Hays is not as thin-bedded as the Smoky Hill. The Smoky Hill member of the Niobrara formation contains more limonitic concretions and it is much thicker, ranging from 450 to 700 feet in Logan County (Moore, Frye, and Jewett, 1944). The chalky shale near the base of the Smoky Hill (Bass, 1926, pp. 25-26) weathers to a deep-yellow or orange color.
It is not known whether or not the bentonitic layer in the lowest part of the Smoky Hill chalk is always present, and there seems to be a gradation from the Fort Hays into the Smoky Hill.
Texture
Milner (1940, pp. 399-400) defines chalk as a rather pure, white, yellow, or gray friable fine-textured foraminiferal limestone. It may be incoherent or well consolidated. The Fort Hays is commonly poorly coherent fine-grained chalk or chalky limestone. The texture is uniform except for widely scattered megafossils and limonite concretions. The concretions are as much as one-half inch in diameter and 2 inches in length.
Color
The color of the unweathered chalk ranges from a dirty yellow through grayish white to pure white. The unweathered chalk just above the Codell sandstone is a dirty yellow because of the large amount of limonite present. Higher in the section the chalk is purer and whiter with only scattered yellow spots caused by limonite concretions.
The color of the weathered surface of the Fort Hays chalk is tan, yellowish white, or white. The predominant color of the exposed surface is yellowish white. The yellowish tint in the chalk is caused by limonite. Locally, the lower 10 feet of the Fort Hays has a blue-gray color probably because of a high shale content. Locally, on a small scale, the Fort Hays is stained black on the surface to a depth of about 0.03 mm by manganese dioxide, and has black dendritic patterning on joint surfaces.
The color of the shale partings in the Fort Hays is also variable. The color of the shale ranges from black, blue, or green to buff .
Bedding
The Fort Hays is made up of massive beds of chalk and limestone separated by thin shale partings. The thickness of the individual chalk beds ranges from 5 inches to more than 7 feet and averages about 2 1/2 feet. The shale partings range in thickness from 0.1 inch to 9 inches and average 2 inches. Some of the thin shale beds are chalky and grade into the chalky limestone. Many of the very thin shale partings are surface features only. In the few places where unweathered outcrops could be examined the overall thickness of many of the partings was no more than visible. In places, the shale partings are composed of two or more strata. In the western part of Osborne County, Landes (1930, p. 18) reports the presence of a shale bed containing mud cracks 25 feet above the base of the Fort Hays chalk.
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Kansas Geological Survey, Geology
Placed on web Oct. 6, 2008; originally published Feb. 1949.
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