Summary of Stratigraphy
[Note: The stratigraphic classification used in this report is that of the State Geological Survey of Kansas and differs somewhat from that of the United States Geological Survey.]
The areal distribution of rocks exposed in Mitchell County is shown on Plate 1. The rocks are sedimentary in origin and range in age from Cretaceous to Recent (Moore and others, 1951). A generalized section of the geologic rock units is given in Table 1. The Dakota Formation of Cretaceous age is the oldest stratigraphic unit exposed in Mitchell County. It underlies the entire county and crops out in the eastern part. Overlying the Dakota Formation is a conformable sequence of Cretaceous marine rocks classified in ascending order as the Graneros Shale, Greenhorn Limestone, Carlile Shale, and Niobrara Formation. The Sanborn Group, of late Pleistocene age, is represented by wind-deposited loess mantling the uplands, by colluvial deposits of unsorted Cretaceous rock fragments and eolian silt lapping against the valley walls, and by high terrace deposits, of which only a few isolated remnants remain. Adjacent to the river channel and its tributaries is a narrow belt of alluvium. The alluvium is of Recent age, and this material is still being deposited by the streams. Adjacent to the alluvium of Solomon River and its main tributaries are terrace deposits of late Wisconsinan age laid down by these streams during an earlier aggradational cycle. These terrace deposits are classified as a part of the Sanborn Group, but owing to differences in lithology and water-bearing characteristics, they have been mapped and discussed separately in this study.
Table 1--Outcropping geologic formations of Mitchell County and their water-bearing properties
|Physical character||Water supply|
|0-30||Unconsolidated sand, gravel, silt, and clay. Crossbedded and lenticular, stratified in upper part. Underlies recent flood plain and occurs in stream channels.||Yields moderate quantities of water to wells along Solomon Valley; smaller supplies along tributary valleys.|
|0-60||Unconsolidated sand and gravel grading upward into stratified clay and silt. Lower part lenticular and crossbedded. Humic layers common in upper part. Underlie flat terraces in valleys of larger streams.||Yield moderate quantities of water to wells along Solomon Valley; smaller supplies along tributary valleys.|
|0-35||Massive eolian silt and sandy silt, calcareous, tan and gray. Blankets much of upland.||Yields no water to wells.|
|0-10||Reddish-tan silt. Part eolian and part slope-wash consisting of eroded Cretaceous bedrock.||Yields little or no water to wells.|
|0-20||Remnants of alluvial deposits consisting of sand, gravel, and silt, forming high terraces along Solomon Valley and its major tributaries.||Yields no water to wells.|
|0-45||Massive beds of cream-colored chalky limestone separated by thin partings of chalky shale. Forms prominent buttes.||Yields no water to wells.|
|0-300||Gray to black clayey shale containing large septarian concretions near top and thin silty sandstone (Codell Sandstone zone) at top. Lower part consists of alternating beds of chalky shale and thin chalky limestone; limestone beds contain discoidal calcareous concretions.||Yields no water to wells.|
|0-85||Alternating beds of calcareous shale and chalky limestone. Thin beds of hard crystalline limestone at base. Contains thin bentonite beds.||Yields meager to small supplies of water of variable quality from place to place.|
|0-25||Shale, noncaleareous, fissile, blue gray. Locally contains red-brown sandstone lenses.||Yields little or no water to wells.|
|350||Clay, shale, siltstone, and sandstone; interbedded and varicolored. Contains lignite and "ironstone." Sandstone is lenticular, cross-bedded, soft, and brown to orange.||Yields small to moderate supplies of water of variable quality from place to place in eastern part of county, becoming too mineralized for use in the central and western parts.|
Geologic History and Geomorphology
The oldest rocks exposed in Mitchell County are the sandstones and clay shales of the Dakota Formation of Cretaceous age. The history of geologic events that preceded the deposition of the Dakota Formation is deduced partly from logs of oil and gas test wells in Mitchell County and surrounding areas and partly from surface exposures of rocks that, although deeply buried in Mitchell County, crop out farther east.
The oldest rocks beneath Mitchell County are the Precambrian crystalline rocks. These rocks have not been reached in any known test wells drilled in Mitchell County but have been penetrated in adjacent areas and are the basement rocks upon which later rocks were deposited. Landes (1927), in a study of the Precambrian rocks in Kansas from samples of deep-well cuttings, determined that the Precambrian rocks consist mainly of granite or granite gneiss and schist. The Precambrian rocks were subjected to long exposure and erosion, which reduced the land surface to a relatively level plain.
Upper Cambrian sandstone and dolomite are the oldest rocks of Paleozoic age in this area of Kansas (Lee and others, 1948). Throughout the Paleozoic, this area was repeatedly inundated by epicontinental seas and subjected to subaerial erosion, resulting in deposition of sandstone, limestone, dolomite, and shale separated by many hiatuses.
A period of folding that began in Mississippian time and continued with diminishing movement through Pennsylvanian into Permian time (Lee and others, 1948) produced the Nemaha Anticline, which trends slightly east of north and extends through Kansas along a line running through Sumner County and Nemaha County. Upward folding along the Nemaha Anticline was accompanied by a downwarping along its flanks. This downwarping created a large syncline, known as the Salina Basin, on the west flank of the anticline and paralleling the northern flank of the Central Kansas Uplift, which was still active at that time (Lee and others, 1948). Pre-Pennsylvanian erosion removed nearly all the Paleozoic rocks from the anticline and beveled part of the rocks on its flanks. In Mitchell County, within the Salina Basin, an estimated total of 1,200 feet of Cambrian, Ordovician, Silurian, Devonian and Mississippian rocks remained at the beginning of Pennsylvanian time.
During most of Pennsylvanian and early Permian times, the floor of the Salina Basin stood close to sea level, and cyclical deposits of alternating beds of limestone and shale were laid down in shallow and fluctuating seas (Moore, 1936). A middle and late Permian interval during which a widespread emergence produced shallow basins and low plains is recorded by "redbed" deposits of sandstone, shale, siltstone, and evaporites. Chemical precipitates of salt and gypsum were deposited as products of evaporation in shallow bodies of water resulting from an~ and climate during middle and late Permian time. These salt deposits thin to the east and north and are not present in the subsurface in the eastern and northern parts of Mitchell County (Bass, 1926a, p. 90). A continental environment predominated during the latter part of the Permian, and by the close of the period the shallow seas had withdrawn completely.
Triassic and Jurassic rocks are not present in Mitchell County. If deposits of Triassic and Jurassic age, as well as late Permian, were deposited, they were later removed. During the Triassic and Jurassic periods and into early Cretaceous time, this area of Kansas was subjected to subaerial erosion.
The Cheyenne Sandstone, of continental and littoral origin, is the oldest Cretaceous deposit in Kansas and was laid down across the beveled edges of Permian rocks as the Cretaceous sea advanced northward. The Cheyenne Sandstone, however, does not reach as far north as Mitchell County (Frye and Brazil, 1943). The oldest Cretaceous rocks in Mitchell County are believed to be the marine shales of the Kiowa Shale. Overlying the Kiowa Shale are clay shales and sandstones of the Dakota Formation, which represent a return of continental and littoral deposition. The Cretaceous sea again advanced northward, and marine conditions prevailed during deposition of the thick sequence of Cretaceous shale, limestone, and chalk of the Graneros Shale, Greenhorn Limestone, Carlile Shale, and Niobrara Formation, that forms much of the bedrock in Mitchell County. At the close of Cretaceous time the sea had withdrawn from the area, and the environment has been continental since that time.
Tertiary Period--In early Tertiary time the Rocky Mountain province to the west was uplifted extensively. Eastward-flowing streams that crossed the Great Plains during most of Tertiary time removed considerable quantities of Cretaceous rocks in western and central Kansas. During the Pliocene Epoch a reversal from stream erosion to stream deposition occurred. Streams from the Rocky Mountains that crossed western and central Kansas began to aggrade their channels and deposited large quantities of alluvial material in their valleys. As the stream valleys became filled, the streams spread across the bedrock divides, shifted laterally, and developed an extensive alluvial plain of sand, gravel, clay, and silt (Ogallala Formation) that merged westward with the erosional surface in the Rocky Mountain region (Frye, 1945). Low stream gradients and choking of the channels and lateral shifting of streams near the end of deposition of the Ogallala may have resulted in the formation of many small water-table lakes and abandoned channel segments. The "Algal limestone," a distinctive hard bed of limestone, marks the top of the Ogallala Formation. The origin of the "Algal limestone" is controversial. Elias (1931) postulated a lacustrine origin for the capping limestone. Later workers advanced the hypothesis of subaerial origin as a caliche zone. Smith (1940) discussed the two hypothesis regarding the origin of the "Algal limestone," and more recently Frye, Leonard, and Swineford (1956) critically discussed the origin of the bed, postulating a mode of origin by development of a mature to senile lime-accumulating soil, which was later modified by solution.
As much as 350 feet of the Ogallala Formation was deposited in parts of western Kansas, but these deposits thin rapidly toward the east. In Mitchell County, the only known exposure of the Ogallala is a small outcrop of "Algal limestone" about 1 foot thick on an upland stream divide in the SE NE sec. 1, T. 9 S., R. 6 W., on the Mitchell-Ottawa County line. Other thin "Algal limestone" deposits in Mitchell County similar to this one may be concealed between the Carlile Shale and the overlying loess cover.
Quaternary Period--The events that formed the present topographic features in Mitchell County began with the close of "Algal limestone" deposition and the beginning of the Pleistocene Epoch. Climatic changes that resulted in the formation of the great ice sheets characterize the Pleistocene Epoch. Although none of the continental ice sheets that advanced toward the central United States reached Mitchell County, the development of the present landscape of the county was influenced greatly by Pleistocene glaciation and the associated climatic fluctuations that prevailed during Pleistocene time.
The Pleistocene in Kansas is divided into four glacial stages, each one followed by an interglacial stage. The Nebraskan Stage (glacial) was followed by the Aftonian Stage (interglacial), the Kansan Stage (glacial) was followed by the Yarmouthian Stage (interglacial), the Illinoian Stage (glacial) was followed by the Sangamonian Stage (interglacial), and the Wisconsinan Stage (glacial) was followed by the Recent Stage, which may be regarded as an interglacial stage.
No deposits of Nebraskan or Kansan age have been identified in Mitchell County. Thus, the sequence of events that took place during Nebraskan and Aftonian time and Kansan and Yarmouthian time is deduced from early Pleistocene deposits in other areas. Shortly before the beginning of the Pleistocene Epoch there was either uplift of the land or a climatic change that caused streams to entrench their channels through former deposits and, in those areas where the Ogallala Formation was thin, to cut into the underlying bedrock. The major streams of this area during early Pleistocene time were probably flowing at about the same locations as are the present streams. These streams were probably not trunk streams, however, but headed not far west of Mitchell County. Thus, they would not have cut into the thick Tertiary deposits of sand and silt farther west until later in the Pleistocene. During the Nebraskan Stage the principal activity of these streams probably consisted of cutting down their channels, and only a minor amount of alluvial material seemingly was deposited, later to be removed by erosion.
Deposits of late Kansan age at Kirwin in Phillips County (Frye and Leonard, 1954) attest that by Kansan time Solomon River had become a trunk stream and the approximate pattern of modern drainage was established. During early Kansan time, stream activity in Mitchell County was probably much the same as during early Nebraskan time, the principal activity being downcutting of the drainage system. During late Kansan time the streams became overloaded and began to aggrade their channels. Deposits of sand and gravel forming high terraces along North Fork Solomon River near Cedar in Smith County and near Portis in Osborne County have been classified as late Kansan in age (Leonard, 1952, p. 38). No deposits of Kansan age have been recognized along North Fork or Solomon River downstream from Portis, near the Osborne-Smith county line.
During the early part of the Illinoian stage, stream activity was renewed and streams again degraded and widened their valleys. A depositional phase of stream activity during Illinoian time consisted of aggrading the earlier channels with coarse chalk fragments, sand, gravel, and silt. In Mitchell County these poorly sorted materials form early Illinoian channel deposits classified as the Crete Formation. Almost all these deposits subsequently have been eroded away, and most of those remaining are being quarried for road-construction material. Much of the Crete Formation is of local origin, but arkosic sand and gravels that are present in the deposits indicate that Solomon River during Illinoian time was a trunk stream carrying eroded Tertiary deposits from the west.
Following Illinoian fluviatile deposition and probably in part contemporary with it, eolian silt classed as the Loveland Formation was deposited over, much of the upland of Mitchell County. Aggraded valleys and their resultant flood plains were probably the source of the wind-deposited silt making up the Loveland loess. Late Sangamonian time represented a period of stability at which time a well-developed soil (Sangamon) formed on the Loveland loess. The topography of the area at this time was more subdued than the present topography, as Solomon River had not made its maximum incision.
During Wisconsinan time streams again deepened their valleys and most of the earlier Illinoian channel deposits were removed. Solomon River cut its channel in Mitchell County an estimated 100 feet below the base of the Crete Formation of Illinoian age. This period of cutting was followed by rapid alluviation during late Wisconsinan time, and alluviation probably extended into early Recent time. During this time Solomon River filled its channel with deposits of sand, gravel, and silt (late Wisconsinan terrace deposits). In the deeper parts of the valley fill, these alluvial deposits range from 40 to 60 feet in thickness and constitute the most important ground-water reservoir in Mitchell County.
The blanket of Peoria loess that mantles the upland and overlies the Loveland loess has been dated as early Wisconsinan in age (Frye and Leonard, 1952). The Peoria probably was picked up by the wind from the flood plains of Platte and Republican Rivers to the north and spread in a broad sheet across northwestern and north-central Kansas. The Peoria loess overlies the Loveland of late Illinoian age in the upland but is not present on the late Wisconsinan terrace deposits of the Solomon Valley. That the deposits of the Peoria are mostly eolian in origin is attested by their fine texture, lack of bedding, molluscan fauna, and stratigraphic position above an extensive well-developed soil, and by the fact that they blanket the upland, valley slopes, and high terraces alike.
In some areas of Kansas, cut-and-fill cycles of both early and late Wisconsinan age are present and in places, very distinct (Frye and Leonard, 1952). In north-central Kansas early Wisconsinan alluvial deposits are rarely exposed, if at all, however, and if present they generally constitute the lower part of the alluvial valley fill below the Kirwin Terrace surface. Inferred early Wisconsinan deposits in the lower part of valley fills have been penetrated by test drilling in parts of northern Kansas (Frye and Leonard, 1952, p. 125; Leonard, 1952, p. 50). Cross section E-E' (Pl. 2) illustrates what seems to be a separate and older cut-and-fill cycle within the Wisconsinan stage and may be early Wisconsinan alluvial deposits. From the topography of the area it seems that the north wall of Solomon Valley runs along a line about half a mile north of Solomon Rapids. Data.from test holes, however, show that at this point the valley wall stands more than a mile farther north. Against the north side of the, valley wall, as shown in cross section E-E', is an old meander loop that has been cut off and abandoned. The depth of the incision is well below the late Illinoian terrace deposits, but is about 30 feet above the deeper incision in the main part of the Solomon Valley cut. Logs of test holes (6-8-35bbb, 6-8-35bcc, and 6-8-35ccc) show that the fill contains sand and gravel in the lower part and silt in the upper part. A buried soil horizon in the upper part of the silt was recognized in the test holes. Surface expression of the old channel is masked by eolian silt that covers the older alluvial deposits (Pl. 1). The old meander, which seemingly swings back into Solomon Valley about 3 miles west of Beloit, may be responsible, in part, for the high yield of the irrigation well (7-8-1adc1) on the Gerald Smith farm.
In early Recent time, Solomon River was again rejuvenated and began to cut into the late Wisconsinan alluvial fill. Since this cutting cycle began, Solomon River has entrenched its channel well into the older alluvial deposits, cutting both vertically and laterally. Segments of intermediate minor terrace scarps on the flood plain record former flood-plain levels as the meander belt of the stream shifted laterally back and forth across the flood plain. Slip-off slope terraces facing the undercut amphitheaters in expanded meander curves are common along the edges of the narrow flood plain.
Solomon River continues to broaden its flood plain, removing the older terrace deposits. As the meander belt swings back and forth across the valley, terrace deposits are eroded away on the outer bank of meander curves by means of bank caving; beds of lateral accretion are deposited on the inner side of meander curves, building and extending the alluvial flood plain. Deposition during flood stages continues to spread a thin veneer of fine sediments (vertical accretion) over the modern flood plain. This process of erosion and deposition still goes on as Solomon River continues to deepen its channel and widen its flood plain. That the meander belt of the river is active is exemplified by old meander scars, ox-bow lakes, and cutoffs on the modern Solomon River flood plain.
Kansas Geological Survey, Geology
Placed on web June 16, 2009; originally published April 1959.
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