Geologic Formations and their Water-bearing Properties
Cenozoic RocksNeogene Systems
Rocks believed to be equivalent in age to a part of the 0gallala Formation underlie younger sediments in much of the broad depressional area now occupied by the Arkansas River valley. These older sediments are best preserved in the deeper parts of the depression to the north and northwest of Wichita and are thought to be the remnants of much more extensive Neogene valley fill deposits. The presence of pre-Pleistocene sediments in the fill of the depressional area in McPherson County was recognized by Lohman and Frye (1940), Frye and Hibbard (1941), Hibbard (1948), and Williams and Lohman (1949). The name "Delmore Formation" was proposed by Williams and Lohman (1949) for lithologically distinct sediments in McPherson County where they crop out at the surface. They did not recognize pre-Pleistocene sediments in northern Sedgwick County and considered all the unconsolidated sediments in the area to be of Pleistocene or Recent age and assigned them to either their all-inclusive McPherson Formation or to the Recent alluvium. Frye, Leonard, and Swineford (1956) correlated the sediments of McPherson County with the Ash Hollow Member of the Ogallala Formation on the basis of the contained fossils, recognizing that they were in part derived from sources different than the Ogallala of western Kansas. More recent work in Sedgwick and adjacent counties, including extensive test drilling and a restudy of formation samples from earlier work, has more clearly defined the nature and development of the extensive depressional area and the sediments filling it.
Character--The Ogallala (?) Formation is composed of calcareous, gray to pink-tan silt and clay, fine to coarse sand, and fine to coarse gravel. These deposits strongly reflect two different sources of the sediments. Calcareous carbonate is common in the silt and clay beds in the form of nodules, root tubes, thin layers, and small crevice fillings. The sand and gravel beds are poorly sorted and vary in thickness within short distances owning to the uneven bedrock floor and the interbedded silt and clay layers. The sand and gravel is composed dominantly of well-worn grains and pebbles of quartz, feldspar, some mica, and other dark minerals. The sand and gravel that has a definite aspect of Rocky Mountain derivation is interbedded in parts of the Arkansas Valley with sand and gravel derived from Cretaceous and Permian rocks to the north and east. The sand and gravel derived from the Rocky Mountains generally is pink. The locally derived deposits are finer and are composed almost entirely of tan and gray quartz and contain pebbles of ironstone, chert, and shale. The Ogallala (?) sand and gravel is finer, contains more silt, and is less permeable than that of overlying younger beds. Northward from the Arkansas River the percentage of granitic gravels in the Formation becomes less, and in most of Harvey and McPherson counties granitic gravels are rare or entirely absent.
Silt and clay beds in the Ogallala (?) Formation similarly show a difference in character and distribution that is quite marked. In the Arkansas Valley the fine clastics are predominantly sandy silt, containing some clay, that is pink-tan to buff colored. Northward from the Arkansas River the silt and clay beds are less sandy, and are predominantly light to dark gray. In Harvey and McPherson counties fine clastics compose a larger part of the Formation, and this is probably a reflection of the type of material available in the source area of the depositing streams.
Origin--The rocks comprising the Ogallala (?) Formation were deposited by streams having two distinct sources of sediment. The stream judged to be the master stream entered Sedgwick County near the northwest corner and followed the depression in the bedrock surface that lies to the south and west of the present Arkansas River (see Pl. 3). The course of this stream to the west of Sedgwick County is not known with certainty, but it probably was near the present course of the Arkansas River with its headwater region extending into western Kansas near the present Pawnee and Walnut River valleys. Such a stream would have provided an outlet to the east and south for some of the heavily laden Neogene streams carrying sediment eastward from the Rocky Mountains. This stream was joined in northern Sedgwick County by one or more streams draining the broad depressional area in Harvey and McPherson counties.
These northern streams and their tributaries were actively eroding the Cretaceous and Permian rocks in the uplands to the north, east, and west and depositing sediments in the lower parts of the area. The sediments deposited strongly reflect their source area, and the sand and gravel derived from the Kiowa and Dakota formations that crop out to the north of the area. Sharks' teeth and fragments of Inoceramus derived from the Cretaceous rocks can be found in the sand and gravel obtained from well and test-hole cuttings. The silt and clay derived from the Cretaceous and Permian rocks bear a marked resemblance to the black and gray shale of the Kiowa, Dakota, and Wellington formations in the surrounding areas. The absence of granitic gravels indicate that the streams did not carry drainage from the west and that Pliocene streams crossing northern Kansas found outlets to the east or north.
Streams transporting sediments derived from the west and those derived locally joined in northern Sedgwick County, where two lithologically distinct types of sediment can be seen to interfinger. This contrast in lithology can easily be seen in the cuttings from the wells and test holes.
Distribution and thickness--The Ogallala (?) Formation does not crop out in Sedgwick County but is deeply buried under younger sediments in the broad depressional area of the Arkansas Valley to a point south-west of Wichita. The nearest known outcrop of the Formation is about 20 miles north in southwestern Marion County, but the distinctive lithology of the sediments in this area can be traced in the subsurface into northern Sedgwick County. As the Arkansas River is approached, sediments derived from the Rocky Mountains make up the bulk of the Formation, and it is difficult to distinguish them from overlying younger sediments. Owing to the lithologic similarity of the Ogallala (?) to overlying younger sediments in the northwestern part of the County, the maximum thickness of the Formation cannot be determined accurately, but it is thought to be about 150 feet. The Formation thins to a featheredge to the south and along the west edge of the depressional area. It has been removed by later erosion along the Little Arkansas River and the Arkansas River southward from Wichita.
Age and correlation--Much of the available evidence for assigning the sediments described above to the Ogallala ( ? ) Formation is indirect but nevertheless quite strong. The silt and clay beds of the Ogallala (?) in Sedgwick County are fossiliferous, but because they are deeply buried, only fragmentary material has thus far been recovered from wells and test holes, and accurate dating of the sediments is not possible. Fossils of middle Pliocene age collected from the Ogallala Formation in McPherson County have been reported by Frye and Hibbard (1941) and Hibbard (1952). The distinctive lithology and stratigraphic position of the sediments from which these fossils were collected has been traced into Sedgwick County in the sub-surface and this evidence strongly suggests the sediments are a part of the same formation.
Other evidence pointing to the Pliocene age is the position of this formation in relation to younger sediments of known age. In the depressional area occupied by the Arkansas River valley, the Ogallala (?), where present, forms the lowest part of the fill and is overlain in part of the area by Early Pleistocene sediments of late Kansan age. The age of the latter sediments was determined by the presence of the Pearlette ash bed which was penetrated by several wells and test holes in the County (see logs 26-2W-15add, 26-2W-26aaa, and 29-1W-9aaa). In southeastern Kingman County, deposits of late Nebraskan or early Aftonian age (Hibbard, 1956, Taylor, 1960, and Lane, 1960) form the highest topographic elements in the area. These deposits are remnants of sediments in the filled valley of a stream that flowed toward and was presumably a tributary to a stream flowing near the present course of the Arkansas River but at a level near the present uplands. Sand and gravel underlying loess near the upland level in central Sedgwick County are believed to be correlative with the deposits in Kingman County (section E-E', Pl. 2). If the earliest Pleistocene streams in the area flowed at a level near the present uplands, it follows that the large depressional area in Sedgwick and adjacent counties to the north and northwest was filled with sediments prior to Pleistocene time which have been in part removed by Pleistocene stream erosion. From the known record of Pliocene events in Kansas, a Pliocene (0gallala) age would best fit the pre-Pleistocene unconsolidated sediments in Sedgwick County.
Water supply--The material comprising the Ogallala (?) Formation in Sedgwick County is finer grained and more poorly sorted than the sediments overlying it. However, the Formation is hydraulically connected with the more permeable overlying beds and supplies a part of the water obtained from many municipal, industrial, and irrigation wells drilled through it. Most wells of large yield penetrate the complete section of unconsolidated rocks and are screened through most of the permeable zones, making it difficult to determine by methods presently available the water supply available from any one formation. However, water samples obtained by special sampling techniques indicate the water from the Ogallala is only moderately hard, and that it is suitable for most uses. Locally the water in the basal part of the Formation contains undesirable quantities of dissolved salt, but the known areas of contaminated water are of small areal extent and are thought to be confined to the lowest elevations on the bedrock surface.
The Pleistocene Series in Kansas is divided into four glacial stages and three interglacial Events during each of the stages (Table 2). Events during each of the stages of continental glaciation and deglaciation followed a similar pattern. The cycle in the zone marginal to the glaciated area is characterized by downcutting in stream valleys and some local deposition of sediment during the southward advance of the glacial ice, then deposition of coarse clastics that became finer grained as the glacial front retreated northward, and finally the development of soil over large areas during the interglacial period that followed.
Deposits representing all the glacial stages of the Pleistocene are present in the County, but these cannot be clearly differentiated in parts of the area. Sediments of the Holdrege and Fullerton formations (Late Nebraskan age) were not recognized in the Arkansas Valley, and the Grand Island and Sappa formations are buried under younger sediments. These formations are considered under a single heading as Lower Pleistocene, undifferentiated, in the following section of the report and on the geologic map (Pl. 1), although they are discussed separately and, where possible, are differentiated on geologic sections (Pl. 2) and well logs.
Sediments of Late Pleistocene age are better exposed and thus more readily distinguished in the field and are discussed separately except for the Wisconsinan terrace deposits and Recent alluvium which are combined as a single unit.
Lower Pleistocene Subseries
Lower Pleistocene Deposits, undifferentiated
Two distinct periods of erosion and deposition are represented by sediments of late Nebraskan, late Kansan, and Yarmouthian age in Sedgwick County. The sediments of late Nebraskan age, probably equivalent to a part of the Holdrege Formation, unconformably overlie Permian rocks near the upland level on the west and south of the divide between the Arkansas and Ninnescah rivers. Equivalent deposits were not recognized in the Arkansas Valley, although they were probably deposited there and removed by later erosion. Sediments of late Kansan and Yarmouthian age underlie younger deposits in the Arkansas Valley, except where removed by erosion near the Arkansas River, and unconformably overlie upper Paleozoic (Permian) and Neogene rocks. Equivalent deposits were not recognized in other parts of the County and, if once deposited, they have been removed by later erosion.
Character--The material comprising the Lower Pleistocene deposits is stream-laid silt, clay, sand, and gravel, and, locally, a thin bed of volcanic ash. The silt and clay beds are generally sandy and vary in color from shades of light tan to light gray. Caliche pebbles are common throughout the silt and clay beds but are best developed in the upper beds. The beds of sand and gravel are composed predominantly of quartz but contain much feldspar, some mica, and other darker minerals. The sand and gravel exhibit some minor differences in lithology, which probably reflect the source areas from which they were derived. The sand and gravel of the Nebraskan deposits lie on Permian rocks and contain many fragments of Permian shale, some iron-stone derived from Cretaceous rocks, and a high percentage of quartz. The degree of weathering of the feldspars suggests that the gravels were reworked from older gravels, probably in the 0gallala Formation to the west. The sand and gravel in the deposits of Kansan age in the Arkansas Valley also show differences in lithology that suggest different source areas. The stream that entered the County from the direction of Harvey County deposited granitic gravels that show a more advanced degree of weathering than those deposited by the stream entering the County from the northwest. The gravels from the north contain much less feldspar and are grayish-pink in color, while those from the northwest are bright pink.
The gravels from the two source areas are mixed below the junction of the two streams and lose their distinctive appearance, but their characteristics suggest that the gravels from the north were reworked from older gravels while those from the northwest were at least in part derived directly from the Rocky Mountains. The sand and gravel in the Lower Pleistocene deposits are coarser and better sorted than those of the Ogallala (?) Formation but less coarse and less well sorted than those of the Upper Pleistocene deposits with which they are associated.
Origin--The Lower Pleistocene deposits are the result of stream deposition during the Nebraskan and Kansan stages. Throughout much of south-central Kansas the early Pleistocene drainage pattern was much different than that of today, but the master stream then, as now, was probably in the broad depressional area now occupied by the Arkansas Valley. During Nebraskan time several major tributaries with eastward gradients less than those of later Pleistocene streams drained the area to the west and at least one of these streams entered west-central Sedgwick County near the present North Fork of the Ninnescah Valley. This stream followed a southeasterly course paralleling the present North Fork of Ninnescah and the Ninnescah valleys and probably joined the master stream near the southern boundary of the County. During late Nebraskan time this stream and others draining south-central Kansas were unable to transport the heavy loads of coarse clastics they formerly carried. To the west of Sedgwick County the streams filled their bedrock valleys and coarse alluvial material was spread over former divides to form an almost coalescing sheet of alluvium over the entire area. These deposits may have once extended over most of Sedgwick County as far east as the Arkansas Valley and were subsequently removed by erosion. Present streams in the County, having steeper gradients than the Nebraskan streams, have incised their valleys much below the level of the Nebraskan streams, and sediments deposited by the early streams are found only near the present upland level. No sediments associated with the Nebraskan Stage have been recognized in the Arkansas Valley. Alluviation of the valley must have been widespread, with later erosion completely removing the sediments.
Stream deposits associated with the Kansan Stage of glaciation were recognized only in a part of the Arkansas Valley where they are buried by younger rocks. During early Kansan time the stream flowing near the present course of the Arkansas River deeply entrenched its valley and removed a large volume of older sediments. This stream was joined in northern Sedgwick County by a large tributary carrying the ancestral Smoky Hill River southward through western McPherson and Harvey counties. This stream may have extended well into west-central Kansas near the course of the present Smoky Hill River with a major tributary flowing from the Saline Valley through the abandoned Wilson Valley (Frye, Leonard, and Hibbard, 1943). Perhaps the Ogallala Formation in western Kansas could have provided sediment to this stream. The drainage pattern to the west of the County during this period is not clear but appears to have been initially a widespread distributary system carrying a large volume of water and sediment from the mountains. Tributaries to the master stream in Sedgwick County may have integrated a part of this drainage. During late Kansan time alluviation was widespread as streams became incompetent to carry their heavy load of sediment. West of the County and south of the great bend of the Arkansas River a thick sheet of coarse sediment was deposited, and this forms the present high plains surface in much of the area. This sheet deposit may have coalesced with the Kansan deposits now buried in the Arkansas Valley, but, if so, it has been removed by later erosion as far west as central Kingman County (Lane, 1960).
Distribution and thickness--Deposits of Early Pleistocene age are found in the Arkansas Valley and near the upland level on the west and south sides of the bedrock divide between the Arkansas and Ninnescah River valleys (Pl. 2, sections E-E' and F-F'). The sediments associated with the Nebraskan glaciation were recognized only near the upland level flanking the Ninnescah Valley where they underlie late Pleistocene loess. These deposits are remnants of a filled high-level valley system that has been greatly dissected by later stream erosion. Exposures are poor but are found in road cuts and stream channels. The areal extent of the rocks was determined principally from subsurface data obtained by extensive test drilling. Sediments of similar age were probably deposited over a large part of the Arkansas Valley but were removed by later erosion. The Nebraskan deposits range in thickness from a featheredge to about 19 feet in the area near test hole 27-2W-29bbb.
Deposits associated with the Kansan glaciation were recognized only in a part of the Arkansas Valley where they underlie younger sediments. The areal extent of these sediments cannot be definitely ascertained owing to the similarity of lithology of underlying and overlying deposits. They were probably deposited over the entire valley, but the thickest deposits were west of the present Arkansas River where valley cutting by Kansan streams was greatest. The Arkansas River now flows along the east edge of the valley from the north edge of Wichita southward through the County and has removed any older sediments that once may have been present in this area. The topographic position of the Kansan deposits rises with respect to the present level of Arkansas River southward through the County but not sufficiently to be exposed at the surface. To the south of the County, near the state border, the Kansan sediments are perched well above river level and underlie the highest elements of the topography. The maximum thickness of the Kansan deposits is not known, but it is estimated to be about 28 feet in the northwestern corner of the County.
Age and correlation--A Nebraskan age for the older part of the Lower Pleistocene deposits is based primarily on their topographic position near the upland surface, their history of deposition, their relation to adjacent younger sediments, and their equivalence to Nebraskan sediments in nearby areas that have been dated by fossils and stratigraphic evidence.
The Kansan age of a part of the sediments filling the depressional area of the Arkansas Valley is based primarily on the presence of the late Kansan Pearlette ash bed which was penetrated by several test holes drilled in the County and in adjacent areas (logs 26-2W-15add, 26-2W-26aaa, and 29-1W-9aaa). The ash bed was also found in cuttings from several test holes in Harvey and McPherson counties that were restudied during the investigation for this report. The silt and clay beds in the Kansan sediments are commonly fossiliferous, but they do not crop out in the County, and only fragmentary fossil material, unsuitable for identification, was recovered from drill cuttings. Deposits of Kansan age were not recognized outside the Arkansas Valley and if deposited elsewhere in the County, have been removed by erosion.
Water supply--The early Pleistocene sand and gravel deposits of Nebraskan age are relatively thin, and in most of the area where present, they are covered by late Pleistocene loess that probably reduces the rate of recharge. However, near the upland level where the deposits are thickest, they are saturated and yield water to many stock and domestic wells. The town of Goddard is supplied by wells screened in these deposits, which yielded about 50 gpm (gallons per minute) when test pumped. The water is moderately hard but suitable for most uses.
The deposits of Kansan age in the Arkansas Valley are overlain and in part underlain by water-bearing sediments, and wells are seldom screened in only one water-bearing bed. The complete saturated section responds to long-term pumping as if it were a single unit, and the water supply of individual formations is difficult to determine. However, based on the lithology of the Kansan sediments, it is estimated that they would yield 500 to 1,000 gpm to wells, depending on the thickness of sand and gravel beds.
Upper Pleistocene Subseries
The Upper Pleistocene in Kansas includes all sediments younger than those of the Yarmouth interglacial period, and in Sedgwick County includes Illinoisan terrace deposits, Wisconsinan terrace deposits, Recent alluvium, undifferentiated loess deposits, colluvium or slope deposits, and dune sand. The Wisconsinan terrace deposits and Recent alluvium are difficult to distinguish from each other and are combined as a single unit in the section that follows.
Illinoisan Terrace Deposits
Character--Deposits of silt, sand, and gravel of Illinoisan age underlie an extensive terrace surface west of the Arkansas River and locally along the north side of the Ninnescah River valley westward to the mouth of Spring Creek. A part of the fill in an abandoned valley crossing the extreme southwestern corner of the County is believed to be of similar age. The terrace deposits consist of fine to coarse sand and fine to coarse gravel that grades into sandy silt in the upper part. The sand and gravel is composed predominantly of quartz but contains much feldspar and other minerals typical of igneous rocks derived from the Rocky Mountains. Thin beds of gray to tan silt interbedded with the sand and gravel are common but are local in extent. The sand and gravel is generally more coarse and better sorted than the older underlying sand and gravel deposits. Clay balls up to one foot in diameter are common in the sand and gravel beds. The silt in the upper part of the terrace deposits is sandy and is tan to reddish-tan in color.
Origin--The Illinoisan terrace deposits present in the County are remnants of extensive valley fill deposited by streams flowing near the present stream courses. In early Illinoisan time the Arkansas River was probably established near its present course as a through stream from the mountains, and its principle tributaries were actively extending their drainage area by headward erosion. The former tributary carrying the Smoky Hill River southward into Sedgwick County was captured by a tributary of the Kansas River, and its former course through McPherson and Harvey counties was abandoned. Valley deepening by the Illinoisan streams evidently was not pronounced in the Arkansas River valley because the easily eroded Kansan sediments underlie the terrace deposits, and they appear to have remained intact. The stream shifted laterally over a wide area, depositing a sheet of coarse alluvium that originally may have been as much as 15 miles wide. The reason for the widespread deposition is thought to have been a reduction in the gradient of the stream as it flowed through the broad depressional area in Sedgwick and Harvey counties. The present slope of the terrace surface is less than that of the present river and the topographic position of the surface with respect to the river rises perceptibly downstream. In the Ninnescah River valley the Illinoisan terrace deposits rest on Permian rocks, and if older sediments were present on the valley floor, they were removed by erosion before the deposition of the Illinoisan or were not recognized.
Distribution and thickness--The Illinoisan terrace deposits along the Arkansas River underlie a broad terrace west of the river and may be present under younger deposits adjacent to the river. The eastern edge of the terrace is marked by a distinct scarp and the terrace surface abuts against the bedrock walls of the valley on the west. Along the western edge of the terrace late Pleistocene loess overlaps the valley wall onto the terrace making the contact indistinct. Sand and gravel underlying loess along the east valley wall in the vicinity of Derby is believed to be a part of the Illinoisan terrace deposits. Southwest of the town of Maize the terrace deposits reach a maximum width of about 7 miles.
The Illinoisan terrace deposits in the Arkansas valley range in thickness from 0 against the valley wall to a maximum of about 75 feet near test hole 26-2W-15aad.
Illinoisan terrace deposits underlie the surface of a terrace north of the Ninnescah River that is accordant with that in the Arkansas Valley. The terrace is graded toward the river, is less distinct than that in the Arkansas Valley, but is continuous to a point about 3 miles west of Clearwater. Remnants of this terrace are also present adjacent to the mouth of Clearwater Creek. The terrace deposits along the Ninnescah River range in thickness from 0 along the valley wall to about 56 feet near the town of Clearwater.
A small abandoned valley crosses the extreme southwestern corner of the County in T 29 S, R 4 W (Pl. 1 and 3). The basal part of the fill in this valley is believed to be equivalent in age to the Illinoisan terrace deposits. These basal deposits attain a maximum thickness of about 50 feet and are overlain by recent slope deposits.
Age and correlation--An Illinoisan age for the terrace deposits is based on the relative position of the sediments in the Arkansas Valley where they unconformably overlie sediments of late Kansan age containing the Pearlette ash bed and are in part topographically higher than late Pleistocene sediments adjacent to them. The terrace surface is drained by Cowskin Creek which has cut a narrow valley in the surface. In the lower reaches of Cowskin Creek valley a terrace surface well below the Illinoisan surface is present and is accordant with terrace deposits adjacent to the Arkansas River that are judged to be Wisconsinan in age from the contained fossils. The Illinoisan deposits contain both vertebrate and invertebrate fossils. Surface exposures are rare, and the fossils are known only from fragmentary material obtained from drill cuttings.
Water supply--The Illinoisan terrace deposits supply water to many stock and domestic wells in the County and to some municipal, irrigation, and industrial wells. As in most of the Arkansas Valley, wells of high yield located on the terrace surface commonly penetrate the entire section of unconsolidated rocks, and the water supply of any individual unit of the section is difficult to determine. However, well yields of 500 gpm are readily available in most of the area in the Arkansas Valley underlain by the terrace deposits, and yields of 1,000 gpm or more might be obtained at favorable locations. In the Ninnescah Valley the deposits are thinner, the saturated thickness is less, and well yields are correspondingly smaller. Many stock and domestic wells in the Ninnescah Valley obtain water from these deposits, and the town of Clearwater is supplied by four wells, each capable of yielding about 270 gpm. In the small abandoned valley in the southwestern corner of the County only a few domestic and stock wells obtain water from the buried Illinoisan deposits, but it is estimated that well yields of 50 to 100 gpm might be available in the thicker parts of the valley fill.
Wisconsinan Terrace Deposits and Recent Alluvium, Undifferentiated
Character--The Wisconsinan terrace deposits and Recent alluvium are combined as a single unit in this report owing to their lithologic similarity, their hydraulic continuity, and the difficulty of separating the deposits in the field. A narrow deposit of alluvium is present in most of the small tributary valleys in the County, but it is not shown on the ecologic map (Pl. 1). The deposits adjacent to the Arkansas River consist of fine to coarse sand and fine to very coarse gravel containing only minor amounts of silt and clay. The sand and gravel grade upward into clayey silt. The sand and gravel are mostly quartz fragments but the gravel contains much pink feldspar and other minerals typical of the Rocky Mountains. Clay balls are common in the sand and gravel, some attaining a maximum diameter of more than 12 inches. The clayey silt forming the upper part of the deposits is gray to light grayish-tan in color, is sandy,and contains much caliche in the lower part. The caliche occurs as thin bands, root tubes, and small crevice fillings, and is believed to have been deposited in the zone of water-table fluctuation. These deposits are well exposed in many gravel pits in the area.
The terrace deposits and Recent alluvium in the Ninnescah Valley contain much silt and fine sand and only minor amounts of fine to medium gravel. Gravel is most common in these deposits in and near the present river channel.
Origin--The Wisconsinan terrace deposits and Recent alluvium are the result of deposition by streams following a period of erosion associated with the Wisconsinan glaciation. In the Arkansas Valley much of the Illinoisan deposits were removed and an inner valley was cut as much as 70 feet below the surface of these deposits. Lateral planation was an important feature of this erosional period, for the Wisconsinan terrace deposits and Recent alluvium now cover an area as much as 9 miles wide in the County. Following this period of erosion the stream transported into the area a large volume of coarse clastics derived from older sediments to the west and from the Rocky Mountains. The stream became incompetent to carry its load, and widespread deposition took place.
In the Ninnescah Valley and smaller tributary valleys in the County a similar but less extensive period of erosion and deposition took place. The stream regimen in these areas was controlled to a large extent by the climatic changes in the area, and the deposits in the valley were derived entirely from sediment available to the streams within their drainage basins.
Distribution and thickness--The Wisconsinan terrace deposits and Recent alluvium in the Arkansas Valley underlie a broad, flat surface adjacent to the present river channel. The slope of this surface is the same as that of the river, and north and west of the mouth of the Little Arkansas River the channel is about 5 feet below it. Downstream from the junction of Arkansas and Little Arkansas rivers the channel is more deeply entrenched and is about 10 feet below the terrace surface where the river leaves the County. The deposits attain a maximum width of about 9 miles normal to the river in the north-central part of the County but narrow to about 4 miles along the south county line. The deposits average about 45 feet in thickness.
In the Ninnescah Valley the deposits are more restricted but attain a maximum width of about 2 miles. Above the junction of the North Fork and the South Fork of the Ninnescah River the deposits become more restricted in areal extent. They range in thickness from a featheredge to a maximum of about 45 feet. In the smaller tributary valleys in the County, the Wisconsinan terrace deposits and the alluvium are confined to a very narrow strip adjacent to the stream channels and are not shown on the geologic map (Pl. 1). However, these deposits attain a thickness of as much as 40 feet in some small stream valleys.
Age and correlation--A Wisconsinan age for most of the sediments included in the Wisconsinan terrace deposits and Recent alluvium is based on the relative abundance of large vertebrate remains recognized as latest Pleistocene in age. Remains of horses, bison, and elephants are commonly recovered during screening operations at the many gravel pits operating in the County. Smaller forms are probably present in the deposits, also, but are not recovered by the screening process. The deposits are very poorly exposed and fossils are seldom found except in excavations. In much of the area bordering the Arkansas River where the surface has not been modified by overflows from the present river channel, complex patterns of meander scars mark the terrace surface. These features are seldom visible when observed from the ground but they show up well on aerial photographs. Some of these surface markings are as far as 7 miles from the present river channel and are believed to be traces of the channel during the waning phases of Wisconsinan deposition.
Water supply--The Wisconsinan terrace deposits and Recent alluvium are the most widely used source of ground water in the County and yield water to many stock, domestic, municipal, industrial, and irrigation wells. The deposits in the Arkansas Valley are the most permeable in the County and are readily recharged by precipitation. Wells of moderate depth penetrating the entire thickness of the deposits are capable of large yields. In most of the area underlain by the deposits the water table is less than 10 feet below the land surface, and centrifugal pumps operated at the surface produce 500 gpm or more. Large-diameter wells with pumping equipment not limited by suction lift can yield 1,500 to 2,000 gpm. The chemical quality of the water varies with the location in the County. At normal and low stages the Arkansas River water is of poor quality and in parts of the area is freely interchanged with ground water. Generally the quality of the water improves with distance from the river, although a high iron content is common and often trouble-some. In the Ninnescah Valley the deposits are much less permeable, but moderately large yields are possible at favorably located well sites.
Undifferentiated Loess Deposits
Origin--Wind-deposited silt (loess) forms the surficial material over a large part of the County but it is most extensive on the valley slopes and uplands bordering the Arkansas Valley. The loess probably represents two major periods of deposition and may be correlative with the Loveland and Peoria formations of late Illinoisan and Wisconsinan age that are widespread in north-central and western Kansas. Exposures of the loess are few and its thickness and character are known primarily from test-hole data. No attempt was made to differentiate the loess, because fossil soils that are well developed and have been used for correlation in northwestern Kansas and fossil mollusks that have been studied at many localities in the State were not recognized in the test-hole cuttings in Sedgwick County. However, caliche zones found at several horizons in the loess may indicate that deposition was not continuous and that surface stability may have prevailed, allowing some leaching. The loess is thought to have been derived from the flood plain of the Arkansas River during late Pleistocene time and carried by the wind to the site of deposition. The late Pleistocene streams in the Arkansas Valley probably carried much more water than the present river, and seasonal floods would have left widespread silt deposits on the flood plain. Strong gusty winds during part of the year were probably prevalent then as now and would have easily picked up and transported the silt.
Distribution and thickness--Loess was probably deposited over most of the County, but it has been removed by erosion along much of the Ninnescah Valley and the smaller tributary valleys remote from the Arkansas Valley. The loess is thickest on the slopes and uplands bordering the valley and thins rapidly away from it. The greatest thickness penetrated was 74 feet in test hole 27-3-12aaa. East of the Arkansas River about 60 feet of silt overlying sand and gravel was penetrated by test hole 29-2E-18ccc. Most of the contacts of the loess on older rocks shown oil the geologic map (Pl. 1) are indicated by a dashed line and represent the point where the thickness of the loess is believed to be about 4 feet or more.
Age and correlation--A late Illinoisan through Wisconsinan age is assumed for the loess in Sedgwick County as it is believed to be equivalent to the Loveland and Peoria formations in adjacent parts of the State. The lower part of the loess along the west side of the Arkansas Valley is interfingered with the Illinoisan terrace deposits, which are overlapped by the upper part of the loess. The loess is rarely exposed and in only a few places could the leached zone immediately below the surface be examined in the field. No fossils were found.
Water supply--The loess is very fine-grained, well sorted, and has a low permeability and, therefore, would not ordinarily be considered as an aquifer. However, in parts of the County where the loess is thick, the lower part is saturated and may contribute some water to wells from the sandy zones or to wells completed in the weathered bedrock immediately below it. Many stock and domestic wells in the upland areas of the County are drilled through the loess into bedrock and supply adequate water for farm use, but the contribution of water from the loess to these wells is not known.
Undifferentiated Colluvial Deposits
Colluvial deposits as described in this report are those sediments transported and deposited by slope processes, primarily sheet wash and soil creep. They are most commonly found near the base of long gentle slopes developed on relatively weak, easily eroded rocks. Colluvial deposits are present in Sedgwick County overlying the Ninnescah Shale and Wellington Formation in the Ninnescah River valley and in the small abandoned valley in the southwestern corner. The colluvium consists mostly of silt but normally includes a heterogeneous mixture of silt, bedrock fragments, and sand and gravel, if these materials are present upslope from the deposit. The deposits normally grade imperceptibly downward into the terrace deposits, effectively masking the outer limit of the terrace. Deposition of the colluvium may have started as early as Illinoisan time, and it continues to the present time. The deposits generally are thin and probably do not exceed a maximum of 30 feet in thickness. In much of the area the deposits are above the water table and thus yield no water to wells. However, where the deposits are thick and contain local accumulations of sand and gravel, wells yielding a few gallons per minute might be obtained.
Fine to medium sand containing some silt has been accumulated by wind action into low rounded dunes at a few locations in the County. Two small dune areas are located northwest and southeast of the town of Bentley, but these are not shown on Plate 1. A few small areas of dune accumulation were formerly found northwest and south of Wichita adjacent to the channel of the Arkansas River. Expansion of the city and agricultural activity have now obliterated these features. Where present, the dunes form low rounded mounds only a few feet thick and support field crops in most years. The dune sand readily absorbs precipitation, but its areal extent is so small that it is not considered significant as a recharge medium in the County.
Kansas Geological Survey, Geohydrology of Sedgwick County|
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Web version April 1998. Original publication date Dec. 1965.