Paleozoic EraThe early episodes in the geologic history of Seward County are obscured by the depth to which rocks formed have been buried beneath the deposits laid down in more recent geologic time. Paleozoic and younger rocks rest on the eroded surface of formations of pre-Cambrian age. The configuration of that surface, as well as the lithologic character of the rocks themselves, can be determined only after the results of deep test drilling over the county are known. From the logs of wells drilled in adjacent areas, it is thought that predominantly marine formations, ranging in age from the Cambrian to the Pennsylvanian (with the possible exception of Silurian and Devonian), lie over the pre-Cambrian floor and have an aggregate thickness of more than 6,000 feet. A deep test drilled near the southwest corner of Meade County in 1944 was drilled to the pre-Cambrian granite. Specific data on this well are not available.
As mentioned above, the production of gas comes from reservoirs in the Chase group of the Wolfcampian series which comprises part of the Permian system. Assuming that the gas produced in this county is from the same beds that supply gas to wells in Stevens County (Ver Wiebe, 1941, p. 98), there is basis for believing that a shallow extension of what is now the Gulf of Mexico covered Seward County in early Permian time. Both marine and shore facies are represented in the strata then laid down. Shortly after the time of deposition of the Chase group, the shallow sea withdrew toward the south and subsequent Permian deposition was of the non-marine type, including such lagoonal deposits as the salt and gypsum beds of the Sumner and Nippewalla groups (Leonardian series). It is thought that the Permian redbeds reached by the Survey test holes belong to the Guadalupian series, also of non-marine origin.
Mesozoic EraTriassic and Jurassic periods--With the cessation of Permian deposition in this area, there ensued a long interval in which erosion was preponderant over deposition. The occurrence of non-marine beds of Triassic (?) and Jurassic age in Morton County (McLaughlin, 1942, pp. 26-27) might justify the inference that formations of comparable age and character had been deposited in Seward County as well. If so, such deposits have since been removed completely by erosion or are so thin or discontinuous as to have escaped recognition in the test holes thus far drilled in the county.
Cretaceous period--Deposits laid down during the early part of the Cretaceous period underlie the northwestern corner of the county and adjacent areas. In nearby areas where they are better known they comprise two non-marine sandstones (Cheyenne sandstone and Dakota formation) separated by a dark marine shale (Kiowa shale). Of these, only the Dakota formation extends into Seward County. In many parts of Kansas these rocks are overlain by younger marine Cretaceous formations. Had these been deposited in this area, they subsequently must have been eroded completely inasmuch as the well logs studied give no indication of their present existence beneath the surface of Seward County.
Cenozoic EraTertiary period--No deposits laid down during the earlier epochs of the Tertiary period, the Paleocene, Eocene, Oligocene, and Miocene have been recognized in the county, either at the surface or in the subsurface. During this long geologic time interval, therefore, the processes of erosion seem to have had greater net effect than the processes of deposition.
During early Pliocene time, the area once again became one of deposition. In this interval the Laverne formation was deposited primarily by streams but also in lakes of relatively limited extent. Soon after deposition, the strata of the Laverne were involved in a deformative movement of the rocks of the area that caused the strata to be inclined locally as much as 15 degrees from the horizontal. The exact nature of the deformation is conjectural because the limited area of outcrop of the Laverne formation makes it difficult to determine the details of the structure produced or the source of the force responsible. The structure was caused at least in part by solution and collapse.
After the deposition of the Laverne formation there may have been a period of erosion. In middle Pliocene time the silt, sand, gravel, and caliche of the Ogallala formation were deposited over much of western Kansas, but these beds were removed from some areas of southwestern Kansas by a later period of erosion. The date of this period of erosion is not known because the exact age of the Rexroad (?) formation has not been determined. The nearest estimate that can be made at present is that the erosion took place between the close of the middle Pliocene and the early part of the Pleistocene. The erosion during this period may have removed most of the Ogallala formation between Point of Rocks in Morton County and Crooked Creek in Meade County.
In late Pliocene or early Pleistocene the deposits that comprise the Rexroad. (?) formation were laid down over all of Seward County and adjacent areas. These beds consist of sand and gravel at the base and silt, sand, clay, and caliche in the upper part. These beds are unconformably overlain by the lithologically similar deposits of the Meade formation and were deposited primarily by streams flowing eastward from the Rocky Mountains.
Quaternary period--Near the end of the Tertiary period or in the early part of the Quaternary period (after the deposition of the Rexroad (?) beds) there was folding and faulting in parts of southwestern Kansas. This period of erosion and deformation produced the major faulting in the Meade Basin in Meade County (Frye and Hibbard, 19~1), and is represented by the unconformity between the Meade and Rexroad formations. The period was followed by the deposition of the stream-laid silt, sand, and gravel comprising the Meade formation.
During late Pleistocene time, aggrading streams flowing at the upland level spread fine sand and silt over their flood plains. These sediments, which comprise a part of the Kingsdown silt, probably were laid down over all of Seward County. Subsequent erosion removed much of the water-laid part of the Kingsdown silt from the southern part of the county. Deposits of loess were laid down over part of Seward County in late Pleistocene or Recent time and comprise the uppermost part of the Kingsdown silt.
During late Pleistocene and Recent time the high terrace deposits of Cimarron River were laid down, the valley was deepened nearly to its present level, and the alluvium was deposited. More recently (since 1914) the channel of Cimarron River has widened greatly throughout its course in Seward County. The widening has destroyed much bottom land and has caused accelerated erosion in many of the small tributaries in this area.
Other events in Recent time included the deposition of dune sand over large areas along Cimarron River, the deposition of colluvium in Cimarron Valley, and the formation of sink holes in many places in the upland areas.
As mentioned in describing the topography, there seem to have been three generations of dunes formed primarily during Recent time, perhaps related to cyclical climatic changes. The three types of dunes seem to be autochthonous in origin—that is, their distribution and form are evidence that they were shaped from loose sands in their present immediate vicinities. There is no indication of the sand having been derived from the present valley of the Cimarron. The "windows" between dune areas (as in secs. 4, 5, 6, 8, 9,etc., T. 35 S., R. 34 W.) seem to contradict the possibility that the sand was moved by the wind from some more-or-less remote external region.
A fourth type of sand dune, unrelated to the upland types, is found in the Cimarron Valley and represents wind-concentrated sand of alluvial origin. Such dunes are small and are much more abundant in the northwestern half of the valley than they are in the southeastern half. In one place (secs. 3, 4, 9, and 10, T. 31 5., R. 34 W.) there is evidence that some dunes that originated in the valley have since migrated northeastward and upward onto the upland surface.
During much of Pliocene, Pleistocene, and Recent time, there was major or subsidence along Cimarron River in southeastern Seward and southern Meade Counties. It is believed that the course of Cimarron River from southeastern Seward County to the Ashland Basin in Clark County has been determined largely by solution and collapse. Much of the structure of the Laverne strata has been caused by subsidence in the area of outcrops of that formation. The overlying Rexroad (?) formation has also been strongly tilted in several places in southwestern Meade County. In both Meade and Seward Counties the Meade formation in several places dips toward Cimarron River. In southwestern Meade County a large block containing deposits of the Meade formation has dropped to stream level which is nearly 200 feet below its original position. As indicated by the above, the subsidence has been either continuous or recurrent since the deposition of the Laverne formation.
Kansas Geological Survey, Seward County Geohydrology|
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Web version Sept. 2001. Original publication date March. 1948.