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Maquoketa Group
The Maquoketa Group is an assemblage of several geologic formations. It is Upper Ordovician in age and named for the Maquoketa River in Iowa. It exists in Missouri, Wisconsin, Iowa, Illinois, and Indiana. It is equivalent to the all but the basal formations of the Cincinnati Group in Ohio. Illinois and Indiana are the only states where the Maquoketa is considered a group. In other states it is a formation. The Maquoketa was deposited in a shallow intercontinental sea. Most of the clastic sediments are derived from the Appalachians, which were being raised at the end of the Ordovician. To a lesser degree some of the sediments were derived from the Ozark highlands to the southwest.
==Scales Formation==
In Illinois, the basal formation is called the Scales Formation and consists mainly of dark brown and gray dolomitic shale. In north central Illinois, the bottom half is dominantly a yellowish gray coarse crystalline dolostone (rock form of dolomite which is related to calcite). It does not contain many fossils, although large trilobites and large cephalopods are known from Elmer-Lason Quarry. It is typically 75 to 100 feet thick. It is the only formation of the Maquoketa Group that is not exposed in northeastern Illinois.
The type section for the Scales Formation is in Jo Daviess County, Illinois. In Illinois the Formation is divided into two members, the lower Elgin Member (mostly dark gray to brown and black shale with dolostone) and the upper Clermont Member (upper gray pure shale) both with type sections in Fayette County, Iowa. However, in Iowa the Scales, Elgin, and Clermont are all members of the Maquoketa Formation (the Maquoketa is not considered a group in Iowa). Over the years, this has slowly led to the Scales being used in Eastern Iowa for the dark shale and the Elgin used in Western Iowa to refer to the dolostone that grades laterally into the Scales. The difference in terms between Iowa and Illinois has made the present nomenclature all but impractical. There is an active effort to revise the nomenclature. Present studies suggest that the Maquoketa should remain a group and not a formation for practical purposes.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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