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Geologic Setting
Summary of Stratigraphy
[Note: The classification and nomenclature of rock units used in this report are those of the State Geological Survey of Kansas and differ somewhat from those of the U.S. Geological Survey.]
Subsurface Rocks
The subsurface rocks In Decatur County do not yield potable water to wells. However, a brief summary of their occurrence is presented because of their significance to the occurrence of oil in northwestern Kansas.
Structural setting--Decatur County is located on the northwestern flank of the Cambridge arch, the principal structural element in northwestern Kansas. This uplift of granitic, gneissic, and schistose basement rocks, which are about 3,800 feet below land surface, is reflected nearly to the surface through that thickness of sedimentary rocks of Paleozoic and Mesozoic ages (Merriam and Hambleton, 1956; Merriam, 1963; Scott and McElroy, 1964). The Precambrian arched surface plunges southeastward and slopes both northeastward and southwestward. The Jennings anticline on the west side of the arch in Decatur County is a subsidiary structure that plunges southward (Merriam, 1963).
Paleozoic rocks--Rocks of Pennsylvanian and Permian age, which overlie Precambrian rocks in much of Decatur County, are known from oil-well borings. Rocks of Mississippian age thin on the flanks of the Cambridge arch and are absent over the crest. Older Paleozoic rocks are upturned, truncated, and overstepped on the flanks. The Paleozoic rocks are about 1,700 feet thick and yield some oil to wells in Decatur County.
Mesozoic rocks--In addition to the thick rocks of Late Cretaceous age that crop out sparsely in Decatur County (discussed below), other Mesozoic rocks of Jurassic and Early Cretaceous age underlie the county. The thickness of all the Mesozoic rocks ranges from about 1,600 to about 2,200 feet (Merriam, 1963).
Oil production from subsurface rocks--Through 1965 about 5,500,000 barrels of oil had been produced from 14 small oil fields having from 1 to 18 wells each. Oil production has been from Pennsylvanian rocks from 109 wells ranging in depth from 3,156 to 3,863 feet. No oil has yet been produced from rocks of other ages, but exploration continues in the county (Beene and Oros, 1967). [For current information on oil and gas production, see the Survey's oil and gas page for Decatur County.]
Outcropping Rocks
The areal distribution of rocks exposed in Decatur County is shown on plate 1. The rocks are sedimentary in origin and range in age from Cretaceous to Recent. A generalized section of the rock units and their water-bearing properties is given in table 2. The stratigraphic relation of the rock units is illustrated by geologic sections on plate 2.
Table 2--Generalized section of outcropping rocks and their water-bearing properties.
| System | Series | Stage | Formation | Maximum thickness, feet |
Physical character | Water supply |
|---|---|---|---|---|---|---|
| Quaternary | Pleistocene | Recent and Wisconsinan |
Alluvium | 90 | Stream-deposited silt, sand, and gravel. Thick, coarse deposits in major valleys, and finer deposits in smaller valleys. | Yields moderate to large quantities of water to wells along major valleys, and lesser amounts along smaller valleys. |
| Wisconsinan and Illinoisan |
Peoria and Loveland Formations |
52 | Silt, mostly eolian, sandy in lower part. Mantles most of the uplands and masks much of the valley walls. | Yields no water to wells. | ||
| Illinoisan | Crete Formation |
62 | Stream-deposited silt, sand, and gravel in a terrace position (older alluvium) along the major valleys. | Yields small to large quantities of water to wells where deposits occur along major valleys. | ||
| Tertiary | Pliocene | Ogallala Formation |
240 | Fluviatile deposits of sand, gravel, silt, and clay. Mostly unconsolidated, but cemented locally to various degrees. | Yields small to moderate quantities of water to wells in most of the county. | |
| Cretaceous | Upper Cretaceous |
Pierre Shale |
600 | Fissile dark-gray clayey shale. Limonite stains common along fractures. Selenite crystals characteristic of outcrops. | Yields no water to wells. | |
| Niobrara Chalk |
600 | Chalk and chalky shale, thin-bedded and platy. Light gray to dark gray where fresh; weathers to orange, brown, or yellow. | Yields no water to wells. |
The oldest rocks that crop out in Decatur County are chalk and shale beds of Late Cretaceous age. Two small outcrops of Cretaceous rocks were noted in Decatur County by the author; elsewhere, Cretaceous rocks are overlain by younger deposits. The Cretaceous rocks are relatively impervious and retard or prevent the downward percolation of water, thereby serving as an impervious floor below the overlying permeable fluviatile deposits. The rocks underlying the Cretaceous chalk and shale beds are not known to contain potable water in northwestern Kansas.
The Ogallala Formation of Tertiary age (Pliocene Series) unconformably overlies the Cretaceous rocks in the upland areas of Decatur County. Erosion during the Pleistocene Epoch has removed it along the valleys of the larger streams, but elsewhere in the county, the Ogallala Formation is present. Owing to its widespread occurrence and generally porous texture, the Ogallala is an important source of ground water.
Unconsolidated deposits of both fluviatile and eolian origin represent the Pleistocene Series in Decatur County. Fluviatile deposits occur along the principal streams and are shown on plate 1 as alluvium (Wisconsinan and Recent age) and as the Crete Formation (Illinoisan age). The Crete Formation underlies the terraces along the larger stream valleys. Eolian deposits (Loveland Formation of Illinoisan age and the Peoria Formation of Wisconsinan age) mantle the uplands and valley slopes and in places overlie the terrace deposits along the stream valleys.
Geologic Formations
Cretaceous System--Upper Cretaceous Series
Niobrara Chalk
The Niobrara Chalk of Late Cretaceous age is the oldest rock formation that crops out in Decatur County. Only one small outcrop of the Niobrara was noted in Decatur County--along the road and ditches about 0.4 mile cast of the SW cor. sec. 12, T. 1 S., R. 26 W., on the north side of Sappa Creek valley near the northeastern corner of the county. The total thickness of the Niobrara Chalk in Decatur County is about 600 feet, but only a small part of the formation is exposed. The outcropping rock consists chiefly of brown and orange-brown silicified chalk beds separated by thin chalky shale partings. The Niobrara is relatively impervious and is not known to yield water to wells in Decatur County.
Pierre Shale
The Pierre Shale of Late Cretaceous age conformably overlies the Niobrara Chalk. A small outcrop of the Pierre Shale occurs in the southwestern part of the county along the north side of South Fork Sappa Creek valley in secs. 2 and 3, T. 4 S., R. 30 W. Because it is relatively soft and easily eroded, the Pierre Shale does not crop out as good exposures. However, it is locally near the surface under the slopes of the larger valleys, particularly along Beaver Creek valley and along North Fork Sappa Creek valley in the western part of the county. The Pierre Shale consists of dark-gray thin-bedded clayey shale, commonly with limonite stains along fractures. Selenite crystals characterize weathered exposures. It ranges in thickness from a thin edge in the central part of the county to about 600 feet in the northwestern part. It is relatively impervious and is not known to yield water to wells in Decatur County.
Tertiary System--Pliocene Series
Ogallala Formation
The Ogallala Formation of Pliocene age is divided into three members in Kansas which are, in ascending order, the Valentine, Ash Hollow, and Kimball. No attempt was made to divide the Ogallala Formation in Decatur County, and it is treated as a single unit in this report.
Character--The Ogallala Formation was deposited on an erosional surface of Upper Cretaceous rocks by eastward-trending streams whose source of sediment was igneous rocks of the Rocky Mountains and sedimentary rocks of eastern Colorado. During early stages of deposition of the Ogallala, through-flowing streams which headed in the Rockies occupied broad, shallow valleys across what now constitutes the High Plains. As deposition continued, valleys became filled, divides were covered, topographic relief was reduced, and the depositional zones of individual streams overlapped and coalesced. Thus, the Ogallala Formation consists of a heterogeneous complex of predominantly clastic deposits, ranging in texture from clay to very coarse gravels and pebbles. The lithology changes abruptly both vertically and laterally, and only rarely can an individual bed be traced for any appreciable distance. Although the sediments are largely unconsolidated, cementation of beds occurs to some extent. Calcium carbonate is a common constituent, both as interstitial material and as stringers and nodules of caliche. Silica also is present as a cementing material in beds of opaline sandstone or as chert.
Sand is the principal material within the Ogallala and is present at all horizons. Although beds of uniform grain size occur, the sand in most beds ranges from fine to coarse and commonly is mixed with gravel, silt, or clay. Beds of silt, sandy silt, and clayey silt are present throughout the Ogallala and are mostly pink, tan, gray, and greenish gray. Where the fine sediments contain a large amount of calcium carbonate, they are light gray or white.
The topographic expression of the formation includes flat uplands, gentle erosional slopes, and nearly vertical cliffs. Typical outcrops are cemented to various degrees and are ash gray in color. Because the cemented beds are more resistant to erosion, hard ledges and knobby, irregular benches are characteristic.
Distribution and thickness--The Ogallala Formation occurs extensively in Decatur County, overlying the Cretaceous rocks in most of the county. Erosion during the Pleistocene Epoch has removed the Ogallala along the principal valleys, but elsewhere in the county, although generally mantled with eolian silts, the Ogallala underlies the uplands and valley slopes and serves as the principal aquifer from which domestic and stock supplies are obtained.
The Ogallala Formation rests on a subaerially eroded surface developed on chalk and shale beds of Late Cretaceous age. This surface has a relief of several hundred feet in Decatur County and slopes generally eastward at about 10 to 15 feet per mile. Logs of test holes show that the thickness of the Ogallala in Decatur County ranges from a thin edge along the outer margins of the stream valleys to more than 200 feet in the interstream areas.
Quaternary System--Pleistocene Series
Deposits of Pleistocene age, although relatively thin, are the surficial deposits in much of Decatur County. They include the Crete Formation of Illinoisan age (terrace deposits), the Loveland Formation of Illinoisan age and the Peoria Formation of Wisconsinan age (loess deposits), and alluvium of late Wisconsinan and Recent age.
The classification of Pleistocene deposits by the State Geological Survey of Kansas is based on the classification of glacial deposits in the midcontinent region (Frye and Leonard, 1952; Bayne and O'Connor, 1968). Correlations between the glaciated and nonglaciated areas have been made on the basis of continuous loesses, molluscan fauna, buried soils, and petrologically distinctive volcanic ash (Condra and others, 1947; Frye and others, 1948).
Crete Formation
Character--Deposits classified as the Crete Formation in this report occur along the major valleys as older alluvial deposits in a position topographically higher than the modern flood plain. The deposits consist chiefly of sand, gravel, and silt that were deposited by streams during earlier aggradational cycles. Only the upper part of the deposits is exposed in the county, and the deposits have not been dated on the basis of fossils. That the deposits are largely the Crete Formation of Illinoisan age is indicated by their topographic position above the modern flood plain, the relative youthfulness of the terrace, and the lithologic and stratigraphic similarity to deposits that are classified as Illinoisan in age in adjacent and nearby areas. It is possible that fluviatile deposits of Kansan age occur in places in the basal part of the deposits as they do locally in central and north-central Kansas, but the terrace is believed to be Illinoisan in age. Loess deposits blanket and obscure the terrace locally. Where loess is present, the terrace deposits appear to grade into the Loveland Formation, further indicating an Illinoisan age for the terrace deposits.
The upper part of the terrace deposits is composed of tan or gray silt that is similar in many respects to the upland loess. The soil profile generally is well developed, and at places there are one or more buried soil zones. Columnar structure is common, and shells of fossil gastropods are found locally. In general, the deposits are coarser at depth, and sandy silt, sand, and gravel are found in the basal part.
Distribution and thickness--The Crete Formation occurs as terrace deposits along the major valleys in the county. The deposits are much more common along the north sides of the valleys than along the south sides. Along the north side of Beaver Creek valley in the northwestern part of the county and along the north side of the North Fork Solomon River valley in the southeastern corner, the Crete is exposed nearly continuously. The Crete occurs along much of Sappa Creek valley northeastward from the city of Oberlin. However, much of the Crete Formation has been removed by stream erosion, and in places only remnants remain. The width of the terrace is generally from a quarter to half a mile. Loess and slope wash have obscured the deposits to the extent that it is difficult to delineate the outer edge of the terrace.
The Crete Formation ranges in thickness from a thin edge along the margins of the deposits to more than 60 feet in test hole 1-26W-8ddd. Along the principal valleys it is generally from 50 to 60 feet thick. In general, the thickness increases eastward across the county.
Loveland and Peoria formations
Character--The Loveland Formation is a reddish-tan silt, mostly eolian, which commonly grades into sandy silt or sand in the lower part. The buried Sangamon Soil marks the top of the Loveland and separates it from the overlying Peoria Formation. The Peoria is a tan to gray massive eolian silt that blankets much of the upland areas. The deposits classified as Loveland and Peoria in this report are above the water table and yield no water to wells.
Distribution and thickness--Loess of late Pleistocene age covers a considerable part of Decatur County with a relatively thin mantle ranging in thickness from a thin edge to as much as 52 feet in test hole 1-31W-1aaa. In general, the thickness of the loess is greater in the northwestern part of the county. The loess caps the rolling hills and flat uplands and tends to mask the valley slopes and subdue the topography. Although the Sangamon Soil occurs between the Loveland Formation of Illinoisan age and the Peoria Formation of Wisconsinan age, the formations could not be readily separated, and are mapped as one unit on plate 1.
Alluvium
Character--Alluvium of late Wisconsinan and Recent age occurs as relatively narrow deposits beneath the flood plains of the principal streams in the county. The alluvium consists of the deposits beneath the narrow stream channels and the adjacent low terraces commonly inundated by flood waters. The deposits, largely silt and sand, become coarser with depth. Beds of very coarse sand and gravel are common in the middle and lower parts of the alluvium. The upper part consists predominantly of silt and sandy silt deposited during floods.
Distribution and thickness--The width and thickness of the alluvium are greatest in the principal valleys. Along Beaver Creek and Sappa Creek valleys, the alluvium ranges from about half a mile to a mile in width; along Prairie Dog Creek and South Fork Solomon River valleys, the alluvium ranges from about half to three-quarters of a mile in width. It ranges in thickness from a thin edge along the margins of the deposits to as much as 90 feet in irrigation well 1-27W-34bab. In the principal valleys the alluvium is generally about 50 to 70 feet thick in the deepest parts of the valley fill.
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Kansas Geological Survey, Geology
Placed on web Oct. 8, 2008; originally published Dec. 1969.
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