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The Naturita Formation is a Cretaceous Period sedimentary geologic formation, found in western Colorado and eastern Utah. It was named for a rock outcrop of the formation located near Naturita, Colorado.〔Young, R.G. 1965. Type section of the Naturita Formation. Bulletin of the American Association of Petroleum Geologists 49:1512-1516.〕 ==Geology== The formation overlies the Cedar Mountain Formation, and underlies the Mancos Shale formation, thus occupying the position of sedimentary strata that have historically been called the Dakota Formation. That term is no longer used for Cretaceous strata that were deposited on the western side of the Cretaceous Seaway.〔Witzke, B.J., and Ludvigson, G.A. 1994. The Dakota Formation in Iowa and its type area. In Shurr, G.W., Ludvigson, G.A., and Hammond, R.H. (eds). Perspectives on the eastern margin of the Cretaceous Western Interior Basin. Geological Society of America, Special Paper 287:43 78.〕〔Young, R.G. 1960. Dakota Group of Colorado Plateau. Bulletin of the American Association of Petroleum Geologists 44:156-194.〕 In most areas, the Naturita Formation is composed of a lower unit of conglomeratic sandstone, a middle part of lignitic mudstones and coal, and fine- to medium-grained sandstones in the upper part. The Naturita is not uniform in thickness and in many places is very thin or missing so that the Mancos Formation is in direct contact with the Cedar Mountain Formation. Where missing, a lag of conglomerate may be present to indicate winnowing of sediments, which occurred by advancing Cretaceous seas. In other places, deposition of Naturita sediments did not occur, and these areas may have been quiet lagoons. Coastal coal swamps also formed in low areas as the encroaching sea raised the base level of rivers and the water table. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Naturita Formation」の詳細全文を読む スポンサード リンク
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