Summary of Stratigraphy
[Note: The geologic classification and nomenclature of this report follow the usage of the State Geological Survey of Kansas and differ somewhat from those of the U. S. Geological Survey.]
The rocks that crop out in Cloud County are sedimentary and range in age from Cretaceous (Gulfian) to Recent. Their areal distribution is shown on Plate 1, and their stratigraphic relation is shown on Plate 3. A generalized section of the geologic formations is given in Table 1.
Table 1--Generalized section of geologic formations in Cloud County. The stratigraphic nomenclature is that of the State Geological Survey of Kansas.
|System||Series||Group||Stage||Stratigraphic unit||Thickness||Character||Water supply|
|Quaternary||Pleistocene||Recent||Dune sand||0-50||Sand, fine to medium, quartz, and some silt.||Generally above water table, but yields small quantities of water to a few wells.|
|Alluvium||0-130||Clay, silt, sand, and gravel, unconsolidated.||Yields large quantities of water to wells.|
|Sanborn||Wisconsinan||Terrace deposits||0-125||Clay, silt, sand, and gravel, stream deposited; coarser materials generally in lower part of deposits.||Yield large quantities of water to wells.|
|Peoria Formation||0-10||Silt, eolian, mantling upland and older terrace deposits along major streams.||Above water table and yields no water to wells.|
|Illinoian||Loveland Formation||0-10||Silt, eolian, mantling upland and in places older terrace deposits along major streams.||Above water table and yields no water to wells.|
|Crete and Loveland
|0-75||Silt and clay, waterlaid, containing minor amounts of sand and gravel; generally more gravel near base.||Yield small to moderate quantities of water to wells.|
|Crete Formation||0-30||Sand, gravel, and silt in terrace position along some major streams. Gravel is principally limestone.||Lies generally above water table, but yields small quantities of water to wells where below water table.|
|Meade||Kansan||Sappa Formation||0-60||Sand and gravel, locally derived, overlain by silt and clay. Occurs in deeper parts of Republican River valley.||Water generally of poor quality; high chloride content in part of county.|
|Grand Island Formation|
|Tertiary||Pliocene||Ogallala Formation||0-1||Deposits of "Algal limestone" of very small areal extent.||Yields no water to wells.|
|Cretaceous||Gulfian||Colorado Group||Carlile Shale||Fairport Shale member||0-30||Shale, thin bedded, calcareous, capping high upland in west-central part of county.||Lies above water table and yields no water to wells.|
|Pfeifer Shale member||80-90||Limestones and shales, thin bedded, chalky; thin streaks of bentonite.||Yields small quantities of hard water to a few wells.|
|Jetmore Chalk member|
|Hartland Shale member|
|Lincoln Limestone member|
|Graneros Shale||20-40||Clay and fissile shale, noncalcareous, black and olive drab.||Yields no water to wells in county.|
|Janssen Clay member||25-400±||Clay, shale, siltstone, and sandstone; some lignite.||Yields moderate to large quantities of water of good quality to wells in most of county. High chloride content locally.|
|300-400||Clay, gray shale, and a few beds of limestone and chert.||Yields no water to wells in county.|
The oldest rocks exposed in Cloud County are nonmarine Cretaceous rocks of the Dakota Formation. Overlying the Dakota Formation without apparent disconformity are Cretaceous marine deposits of the Graneros Shale, Greenhorn Limestone, and Carlile Shale, successively. In western Cloud County, in an upland position near the divide between the major streams, Tertiary (Pliocene) deposits classified as the Ogallala Formation crop out in two small areas. Eolian deposits included in the Sanborn Group (Pleistocene) are widely distributed in the upland and along valley walls. Along the major streams these eolian silts are underlain by stream-deposited silts and -sands, which are in a "high terrace" position with respect to the valleys and are classified as Illinoian in age in this report. Along Solomon and Republican Rivers and many of their principal tributaries are terrace deposits of Wisconsinan age. These deposits are locally important sources of ground water. Alluvium along Solomon River forms only a narrow belt adjacent to the stream channel, but the alluvium bordering Republican River has considerable width.
In places in the valleys of Republican River and Buffalo Creek where the deposits extend to the greatest depth, locally derived gravel, sand, and silt classified as Kansan in age underlie the Wisconsinan terrace deposits and alluvium. In local areas along Republican River the prevailing southerly winds have formed dunes, which lie on both Wisconsinan terrace deposits and alluvium.
Pre-Pleistocene Geologic History
The oldest rocks exposed in Cloud County are the clay shales and sandstones of the Dakota Formation, which is thought to be late middle Cretaceous in age. The history of geologic events that preceded their deposition is known partly from deep tests for oil in the county and partly from surface exposures of rocks, deeply buried in Cloud County, that crop out east of the area. The Precambrian crystalline rocks are the oldest rocks beneath Cloud County and are the basement rocks upon which later sedimentary rocks have been laid down. These rocks have not been penetrated in test wells in Cloud County but have been penetrated in surrounding areas. The Precambrian rocks were subjected to a long period of erosion; the seas then covered the rocks intermittently, depositing limestone and shale during parts of Cambrian, Ordovician, Silurian, Devonian, and Mississippian time. The thickness of these deposits totals nearly 2,000 feet. just before but more especially after Mississippian deposition, the rocks were gently folded along a line extending from Nemaha County southward through Sumner County. This fold is named the Nemaha Anticline; its axis passes about 50 miles east of Cloud County. Upward folding along the anticline was accompanied by downwarping on the flanks. This downwarping on the west flank created a large syncline known as the Salina Basin. The post-Mississippian folding elevated the area above sea level, starting a long period of erosion during which nearly all the sediments were removed from the highest part of the anticline. In Cloud County part of the sequence was beveled, all the Mississippian rocks being removed in northeastern Cloud County. Subsequently, the area again was covered by the sea, and Pennsylvanian and Permian rocks were deposited. This deposition was followed by another long period of erosion, which lasted through Triassic, Jurassic, and early Cretaceous time. During this period upper Permian deposits were removed, if they had ever been deposited, and the surface rock in Cloud County at the beginning of Cretaceous deposition was the Wellington Formation.
Cretaceous deposition in Cloud County began with the marine Kiowa Shale, which was followed successively by the nonmarine Dakota Formation and the marine Graneros Shale, Greenhorn Limestone, and Carlile Shale. By the end of Cretaceous time the sea withdrew, and the environment has been continental since that time.
During most of Tertiary time the area was eroded, and great quantities of Cretaceous rocks were removed, leaving a beveled eastward-sloping surface. About 500 feet of Cretaceous rocks remain in western Cloud County, but in parts of eastern Cloud County only about 100 feet of Cretaceous rocks remain. During Pliocene time streams from the Rocky Mountains area deposited as much as 3-00 feet of sand, gravel, and silt over western Kansas. Eastward these deposits (Ogallala Formation) thin, and in Cloud County only the uppermost rocks are present. Probably they were never more than a few feet thick and consisted of "Algal limestone" containing grains of sand.
Pleistocene Geologic History
The events that shaped the present topographic features began after the deposition of the "Algal limestone" and the beginning of Pleistocene erosion. During early Pleistocene time streams formed on the sloping plain and began to cut into the underlying rocks.
The Pleistocene is divided into four glacial and four interglacial stages. The Nebraskan was the first glacial stage and was followed chronologically by the Aftonian (interglacial) Stage, the Kansan (glacial) Stage, the Yarmouthian (interglacial) Stage, the Illinoian (glacial) Stage, the Sangamonian (interglacial) Stage, the Wisconsinan (glacial) Stage, and the Recent, which in Kansas usage is regarded as an interglacial stage.
No deposits of Nebraskan age were recognized in Cloud County. Streams in the county during Nebraskan time possibly beaded not far to the west of Cloud County, and did not carry heavy loads of silt and sand from the Tertiary deposits farther west. Therefore, during the Nebraskan Stage the streams probably were incising their channels, and minor deposits have been removed by later erosion.
During Aftonian and early Kansan time, the streams in Cloud County continued to incise their channels, and the valleys probably were relatively deep and averaged about 2 miles wide (Pl. 3). Buffalo Creek or Marsh Creek probably was the principal stream, and that part of the present Republican River extending from the mouth of Buffalo Creek to Republic City was a tributary to Buffalo Creek. The ancestral Republican River did not flow through Cloud County during early Kansan time. It entered Kansas near its present location and flowed southeastward to about Republic City, thence northeastward across Republic County, and re-entered Nebraska near Chester, Nebraska (Lohman in Fishel, 1948, p. 29). The ancestral Republican River carried a heavy load of sand and gravel, a part of which was deposited in the channel.
The absence in the Republican Valley in Cloud County of arkosic deposits like those in the ancestral valley, which are known to be Kansan in age, indicates that the present drainage pattern in Cloud County was not established until very late Yarmouthian or early Illinoian time. Basal deposits in the Republican Valley in Cloud County principally are locally derived materials because the drainage system headed not far west and did not arise in arkosic materials. The deposits are not thick, probably because the gradient of the streams was relatively steep.
During Yarmouthian or early Illinoian time, the streams in Cloud County again incised their channels and partly removed the Kansan deposits. In late Illinoian time, the streams widened their channels and deposited sediments. Illinoian deposits in the Solomon Valley consist mostly of locally derived sand and gravel and minor amounts of silt, but the presence of some arkosic gravel indicates that Solomon River was a trunk stream carrying material eroded from Tertiary deposits farther west. Illinoian deposits in the Republican Valley are thick water-laid silts containing imbedded gravel; in most places a thin bed of gravel lies at the base of the Illinoian deposits.
After, and in part contemporary with, Illinoian fluvatile deposition, eolian silts were deposited over a part of the upland of Cloud County. These late Illinoian and Sangamonian deposits are overlain by younger eolian silts of early Wisconsinan age, which lie upon Illinoian terrace deposits but not upon the Wisconsinan terrace deposits.
Beginning in the Wisconsinan stage, the early Illinoian deposits were partly removed, and stream channels were deepened to a point below the deepest Illinoian deposits and were later partly backfilled with deposits of sand, gravel, and silt.
During Recent time the streams have remained in their present channels. In early Recent time the Wisconsinan deposits were removed in places, the channel being cut below the Wisconsinan incision. Since this cutting cycle the streams have deposited material in the channels until at present the surface of the alluvial deposits is only about 12 feet lower than the surface of the Wisconsinan deposits.
Locally in the Republican valley prevailing winds have built up dunes on the alluvium and Wisconsinan terrace deposits. These dunes are the youngest deposits in the valleys, except for the alluvium in the active channels of the streams.
Kansas Geological Survey, Geology
Placed on web June 29, 2009; originally published May 1959.
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