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Page 1 of 6 DSAR-2.6 Site and Environs Geology Rev 0 Safety Classification: Usage Level:
Safety Information Change No.: EC 69283 Reason for Change: This section is being updated to reflect the permanent cessation of operations of Fort Calhoun Station. The contents of this section have been changed to remove any information which is not applicable during decommissiong.
Preparer: J. Reimers Fort Calhoun Station
DSAR-2.6 Information Use Page 2 of 6 Geology Rev. 0 Table of Contents 2.6 Geology ................................................................................................................... 3 2.6.1 Physiography .............................................................................................. 3 2.6.2 Regional Geology ....................................................................................... 3 2.6.3 Regional Structure ...................................................................................... 3 2.6.4 Areal Structure ............................................................................................ 4 2.6.5 Geologic History.......................................................................................... 4 2.6.6 Glacial History of the Missouri Valley .......................................................... 5 2.6.7 Site Geology ............................................................................................... 5 2.6.8 Conclusions ................................................................................................ 6
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2.6 Geology 2.6.1 Physiography Topographically, the site is part of the Missouri River bottom land, which is a nearly level plain about 15 miles wide at Blair, 8 miles wide at the site and narrowing to 3 miles wide in the vicinity of Omaha-Council Bluffs. The elevation of this plain averages about 1,000 feet above mean sea level at the site.
2.6.2 Regional Geology The principal geologic features of the region include a 100 to 200 foot thickness of soil deposited by a glacial action, streams, and wind, underlain by sedimentary bedrock of 500 to 2,500 feet in thickness consisting primarily of limestone, shale, and sandstone. There are no bedrock outcrops on the site, but there are exposures within three and five miles from the site at two limestone quarries.
2.6.3 Regional Structure With few exceptions, the bedrock formations of eastern Nebraska and western Iowa are nearly flat with a gradual westward dip. This regional attitude is modified by several well defined faults, basins, and arches.
However, none of these structural features occurs in the vicinity of the site.
The dominant structural features of the Nebraska-Iowa region include the Thurman-Wilson Fault which extends from 12 miles southeast of Lincoln, Nebraska, northeast for approximately 150 miles, almost to Des Moines, Iowa. It lies approximately 55 miles southeast of the site; the upthrown side is on the northwest.
The Forest City Basin which is approximately 100 miles southeast of the Omaha area lies along the downthrown east flank of the Nemaha Ridge, a very sharp buried uplift extending from near Lincoln, Nebraska, south southwest across Kansas and into northern Oklahoma. The depth to the Precambrian crystalline granite core is about 400 to 500 feet at the Nebraska-Kansas line, near the Missouri Border. The Salina Basin and its northern extension, the Central Nebraska Basin are approximately 150 to 200 miles southwest of Omaha. In northern Kansas, the Abilene Arch lies between the Salina Basin and Nemaha Uplift. In southeast South Dakota, approximately 125 miles north of the site, is the Sioux Uplift, and southwest of the Sioux Uplift, in Western Nebraska, are the Siouxana Arch and the Cambridge Arch.
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2.6.4 Areal Structure The structures nearest the site are the Nehawka-Richfield Arch and the La Platte Fault. The arch extends from near Omaha south about 20 to 30 miles into Sarpy and Cass Counties, Nebraska. Deep well records indicate that a limb of this uplift may extend northward into the Omaha area and possibly farther.
The closest known regional fault is the La Platte Fault which cuts across the Nehawka-Richfield Arch in the lower Platte River Valley. It apparently crosses the Thurman-Wilson Fault some 60 miles south of the site. This nearly vertical fault has a maximum upthrow of about 50 feet on the west.
There is no record of movement of the fault in historic times, nor any indication of activity in recent geologic time.
2.6.5 Geologic History The early Paleozoic Era in western Iowa and eastern Nebraska was marked by long periods of marine inundation which resulted in deposition of thick beds of limestone, dolomite, shale, and sandstone. In later Paleozoic time, shorter and more frequent periods of alternating marine and continental deposition occurred resulting in the production of thin coal beds and layers of shale, limestone, and some sandstones.
The major tectonic features of the mid-continent region began to develop late in the Paleozoic Era, and probably most of the important structural features of the Nebraska Iowa Missouri River Valley area had already developed or were developing by the end of the Permian period. From the end of the Permian to the beginning of the Cretaceous, there is no depositional record to correlate with structural movements. The undisturbed beds of the Cretaceous Dakota formation indicate the lack of tectonic activity since that time.
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During the Pleistocene period, when the interior of the continent was covered by continental glaciers, Nebraska was occupied by only the first two of four major ice sheets. The first of these resulted in a general leveling and deposition of till. This was followed by alluvial deposition during the inter-glacial stage, and deposition of additional till and outwash by subsequent glaciation. Ice damage to streams during this period resulted further in a deposition of lacustrine sands and gravels. Nebraska and western Iowa were not covered by later ice sheets, but during the retreat and advance of these ice sheets nearby, windblown deposits of fine sand and silt accumulated to thicknesses of as much as 100 feet. These deposits, known as loess, form the steep sided hills and bluffs of eastern Nebraska and western Iowa. Recent erosion and alluvial deposition have altered the landscape to its present form.
2.6.6 Glacial History of the Missouri Valley At the beginning of the Pleistocene period, the Missouri River Valley and its main tributaries were established in their approximate present positions.
Subsequently under successive glacial movements, the valleys were filled and re-opened several times. During this period, the Peorian loess was deposited on the terraces and adjacent uplands. It is probable that only the upper part of the alluvium in the Missouri River Valley is actually of recent age and that deeper deposits are mostly of Pleistocene age.
2.6.7 Site Geology Unconsolidated sediments at the plant site generally range from 65 to 75 feet in thickness. The soils are typically interstratified and cross-bedded. The beds change facies or grade laterally so rapidly that no bed lithologic correlation is possible from boring to boring. The boring data indicate that the upper 20 to 50 feet of soil are predominately silty sands, and the lower beds consist of fine sands with occasional interbedded lenses of gravel.
The bedrock beneath the site lies between 65 and 75 feet below the surface.
The maximum relief of the bedrock surface in the site area is in the order of 13 feet. Some borings indicate a thin layer of clay on top of bedrock, others a soft to hard shale up to a thickness of seven feet. The bedrock consists of various types of limestone formations.
Site geological and other relevant data are presented in Appendices B, C and D. The details of the solution cavities found in the bedrock are given in Appendix C. A description of the corrective action and of the criteria used for driving and seating the pipe piles is given in Section 5.7 and Appendix D.
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2.6.8 Conclusions The bedrock beneath the site provides suitable support for the plant structures. There is no geologic feature of the site or surrounding area which adversely affects the use of the site for a nuclear power plant.
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