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Geology
Summary of Stratigraphy
The rocks exposed in Ford County are of Quaternary, Tertiary, and Cretaceous age. The areal distribution of the formations is shown in plate 1. The oldest formation cropping out in the county is the Dakota formation. It is best exposed along Sawlog Creek in the northern part of the county and on the north side of Arkansas River in the vicinity of Ford. The overlying Cretaceous formations, represented in this area by the Graneros shale and the Greenhorn limestone, are best exposed in the northern part of the county where tributary streams have cut beneath the overlying Tertiary deposits. The uppermost beds of the Cretaceous are absent in Ford County. The Ogallala formation of Tertiary age, which rests unconformably on the Cretaceous rocks, mantles a large part of the county, but south of the river it is largely covered by younger deposits. The Pleistocene water-laid deposits of the Kingsdown formation constitute the surface material of a broad area south of the Arkansas valley. The plains surface is mantled both north and south of the River by deposits of loess of variable thickness, ranging in age from Pleistocene to Recent, which form the upper part of the Kingsdown silt. Dune sand occupies a belt of varying width bordering the south side of Arkansas River and also mantles other isolated tracts. The soils, alluvium, drifting dune sand, and terrace deposits are the most recent deposits in the area.
The character and groundwater supply of the geologic formations in Ford County are described briefly in the following generalized section (table 1) and in more detail under "Water-bearing formations."
Table 1--Generalized section of the geologic formations of Ford County, Kansas.
| System | Series | Subdivision | Thickness (feet) | Character | Water Supply | |
|---|---|---|---|---|---|---|
| Quaternary | Pleistocene and Recent | Alluvium and Terrace deposits | 0-60 | Sand, gravel, and silt, comprising stream deposits in the Arkansas valley and in the valleys of many smaller streams. Coarse gravels occur as terrace deposits bordering the present flood plain of Arkansas River at levels 15 to 25 feet above the flood plain. | The alluvium yields large supplies of water to wells in the Arkansas valley and lesser amounts in the smaller stream valleys; supplies many irrigation wells in the Arkansas valley. Some waters from the alluvium are very hard, containing from 238 to 1,413 parts per million of hardness. Terrace deposits are dry, except locally where they may occur below the water table. | |
| unconformable on older formations | ||||||
| Dune sand | 0-70 | Fine eolian sand. Except where reopened by recent blowouts, the dunes are well stabilized by vegetation. | Probably does not supply water directly to wells, but constitutes favorable catchment area for ground-water recharge to adjacent and underlying formations. | |||
| unconformable on older formations | ||||||
| Kingsdown silt | 0-135 | Predominantly light buff, even-bedded soft silt and clay containing small, scattered lime nodules; contains unstratified loess in its upper part which grades gradually upward into loess of Pleistocene and Recent age. Contains light-colored sand and gravel at the base that may be correlative with the Meade formation in Meade County. Greatest development of this formation occurs south of Arkansas river. | Yields little or no water in its upper part, but sand and gravel deposits near the base may furnish some water to wells where the water table lies above them. Most of the Kingsdown silt is dry and relatively impermeable. | |||
| disconformity | ||||||
| Tertiary | Middle and upper Pliocene | Ogallala formation | Rexroad member | 20-250 | Alternating beds of gray to greenish clay, buff-colored sandy silt, and rusty sand and gravel. Gravel contains many large-sized pebbles and water-worn fragments of caliche. Has not been recognized north of Arkansas River. (May contain some beds in upper part that are equivalent to the Meade formation.) | Yields good supplies of water to most of the wells located on the uplands south of Arkansas River, including most of the irrigation wells. |
| disconformity | ||||||
| Gravel, sands, silts, "caliche" and structureless silt and silty sand with hard and soft layers of sandstone and conglomerate, much of which is cross-bedded and cemented with lime. Gravel and coarse sand are abundant in the basal part, and lime-cemented beds common in the upper part. | The principal source of water supply in many parts of the county. Yields adequate supplies of water of good quality to domestic, stock, municipal, and industrial wells. Supplies water to many irrigation wells, particularly to deep wells in the Arkansas valley, that tap the so-called "second water." | |||||
| unconformable on older formations | ||||||
| Cretaceous | Greenhorn limestone | Pfeifer shale member | 20 +/- | Chalky shale with beds of thin chalky limestone, discoidal concretions, and thin beds of bentonite. "Fencepost" limestone at top. | Very few wells obtain water supplies from the Greenhorn limestone in the county. Only very limited supplies of comparatively hard water may be expected from wells penetrating this formation. In general, the water is hard, ranging in hardness from about 350 to 600 parts per million. | |
| Jetmore chalk member | 20 +/- | Alternating beds of chalky shale and chalky limestone, "Shell" limestone at top. | ||||
| Hartland shale member | 80 | Chalky shale with a few thin beds of chalky limestone and bentonite. | ||||
| Lincoln limestone member | Yellowish chalky shale with hard, thin-bedded, finely-laminated, crystalline limestone at top and bottom, and a few thin beds of chalky limestone. | |||||
| Graneros shale | 43-45 | Dark bluish-black, fissile, noncalcareous clay shale with numerous thin lenses of sandy shale, sandstone, and interbedded ironstone concretions. Outcrops are strewn with selenite crystals. | Practically barren of water; no wells are known to derive water supplies from this formation in the county. Any water that might be encountered probably would be highly mineralized and very bitter to the taste. | |||
| Dakota formation | 56-235 | Fine-grained, gray to white to yellow-brown sandstone, irregularly-bedded, and varicolored clay and sandy shale. Only the top of the formation is exposed in the county. | Yields moderate supplies of water of good quality to wells in the northeastern part of the county. Several irrigation wells in the extreme southwestern corner of the county tap the Dakota, but obtain most of their water from the overlying Ogallala formati | |||
| Kiowa shale | 44 + | Black to bluish-black to gray to yellowish-gray argillaceous shales with a few thin beds of yellowish or pinkish limestone. Not exposed in county. | Not known to yield water to wells in Ford County. | |||
| Cheyenne sandstone | 70 + | Light gray to yellow, fine to coarse-grained, quartz sandstone with interbedded bluish-gray, silty and sandy shale | Not known to yield water to wells in Ford County. | |||
| unconformity | ||||||
Geologic History
[Parts of the following discussion are taken from: Darton, N.H., Geology and underground waters of the Arkansas valley in eastern Colorado: U.S. Geol. Survey, Prof. Paper 52, pp. 45, 48, 1906.]
Although the oldest formation exposed at the surface in Ford County is the Dakota formation, it is known from the records of several deep tests for oil and gas in the county that the exposed. mocks are underlain by older sedimentary rocks of Paleozoic age, which in turn rest upon crystalline rocks of pre-Cambrian age. [Logs and information regarding the several deep test wells for oil and gas in Ford County were furnished by Raymond Keroher of the Kansas Geological Survey.]
Paleozoic Era
Cambrian and Ordovician periods
During early Cambrian time Ford County was a land surface along with a large part of west-central United States. In middle Cambrian time there began the development of an interior sea with a resultant change to marine conditions. Submergence of the land continued with similar shore lines through part of Ordovician time with extensive deposition of lime sediments that were later indurated to form limestones and dolomites. During this interval was deposited the sandy cherty dolomite that forms the Arbuckle limestone or "Siliceous lime." Several of the deep tests in Ford County were drilled into this formation, the top being encountered at depths ranging from about 5,600 to 6,000 feet, although its exact thickness is not known.
Limestones and dolomites that have been correlated with the Viola and Simpson formations of Ordovician age are also known to be present, the top of the Viola being encountered at a depth of about 5,400 feet.
Silurian and Devonian periods
There is little or no evidence that rocks of Silurian and Devonian age are present under Ford County, either they were never deposited in this area or they were removed by erosion prior to the deposition of the overlying Mississippian strata.
Carboniferous period
During early Mississippian time there was extensive deposition of marine dolomitic limestone and some shale. According to the logs of several deep tests, Mississippian strata are present under Ford County between the depths of about 4,900 and 5,400 feet. In later Mississippian time there was an uplift, during which time the surface of the early Mississippian strata was subjected to erosion. Following this there was a return to marine conditions at which time the so-called Chester "lime" of late Mississippian age was deposited on the weathered surface of older Mississippian strata.
A long period of erosion intervened between the deposition of the youngest Mississippian rocks and the oldest Pennsylvanian rocks next above. Alternate subsidence below and emergence above sea level were repeated many times during the Pennsylvanian, giving rise to both marine and continental deposits consisting of sandstone, shale, coal, and limestone. This sequence of deposition was interrupted at times when the land surface was elevated and subjected to erosion. Based on the logs of the several deep tests in Ford County, the thickness of the underlying Pennsylvanian strata averages about 1,500 feet, the top being encountered at a depth of about 3,400 feet.
Permian period
The transition from rocks that are regarded as Pennsylvanian to those that have been classed as Permian is apparently unbroken. Marine conditions during early Permian time were somewhat similar to those existing during late Pennsylvanian time, and alternate successions of limestones, dolomites, and shales were deposited. Following this there was an interval when beds of continental origin were deposited alternately with beds of marine origin. Gradually continental deposition became the dominant mode of origin for late Permian sediments. Most of the deposition took place in shallow water, so that there must have been subsidence that kept pace with deposition during this interval. Presumably an arid climate prevailed and evaporation took place in shallow basins giving rise to extensive deposits of salt and anhydrite interbedded with deposits of gypsum correlated with the Guadalupe series. According to the logs of the several deep tests in Ford County approximately 3,000 feet of Permian sediments are known to underlie the area, including both lower and upper Permian rocks. Permian redbeds have been reported as shallow as 400 feet in one test.
Mesozoic Era
Deposition evidently was terminated by an uplift that brought the region above water at the close of the Paleozoic. Probably this condition extended through the latter part, if not all, of Triassic time and through Jurassic time, during which there was no deposition and probably considerable erosion. Rocks representing the Triassic and Jurassic are not known to occur in Ford County.
Following an early Cretaceous uplift there was at first a land surface followed by a shallow-water body in Comanche time during which were deposited the sandstone and shale of the Cheyenne sandstone. The deposits were laid down either by streams or in a shallow sea or perhaps they were deposited in part on a beach, suggesting that the place of deposition was not far above or far from a shore line (Twenhofel, 1924, pp. 19-21).
Following this there was a change from continental to marine conditions as a result of submergence of the land surface during which time the Kiowa shale was deposited. The Cheyenne sandstone and the Kiowa shale both have been encountered in test drilling in Ford County and are known to underlie at least part of the county.
In Late Cretaceous time there was a return to conditions similar to those under which the Cheyenne sandstone and sands and clays of the Dakota formation were laid down. The Dakota formation is a fresh-water deposit that was laid down on beaches and near the shore during an uplift in which the sea retreated far to the south. The Dakota formation crops out at several places in the northern and northeastern parts of the county, and is present at varying depths elsewhere in the county.
Following the deposition of the Dakota formation there was a rapid change in the conditions of sedimentation to those under which several thousand feet of clay, lime, and chalk were deposited, begin-fling with the Graneros shale and including the Greenhorn limestone, the Carlile shale, the Niobrara formation, and the Pierre shale. This marks the beginning of very extensive later Cretaceous submergence, in which marine conditions prevailed over a large area for a long time. Sedimentation was interrupted from time to time by emergence of the land to a point at or near sea level.
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Kansas Geological Survey, Ford County Geohydrology
Web version April 2002. Original publication date Dec. 1942.
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