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Compendium
  
Point Pleasant Member

Lexington Limestone,

Ordovician System


Type locality and use of name: The name Point Pleasant Beds was originally given by Orton (1873) to 50 feet (15 m) of blue limestone interbedded with shale along the Ohio River at Cincinnati. The base of these beds is not exposed, but they were considered by Orton to be the lowermost unit of the Cincinnati Group. These rocks were traced into north-central Kentucky by Foerste (1906). The unit has had a complex history in terms of its varying rank assignment, but in Kentucky along the Ohio River across from Cincinnati and Switzerland County in Indiana, it is considered as the Point Pleasant Tongue of the Clays Ferry Formation (Swadley and others, 1975). Closely corresponding rocks in Indiana, of which only about 50 feet (17.5 m) are exposed at Patriot in Switzerland County, were included in the Lexington Limestone by Gray, Brown, and Lineback (1966). These and similar rocks in the immediate subsurface area of eastern Switzerland County are here named the Point Pleasant Member of the Lexington Limestone. In Indiana the designated reference section is the pertinent rocks cored from the bedrock surface downward in Indiana Geological Survey drill hole (SDH) 133 on the Robbins farm in sec. 1, T. 2 N., R. 1 W., in Switzerland County. About 90 feet (27 m) of Point Pleasant rocks were penetrated. The exact thickness is unknown because an upper weathered zone of a few feet was not cored.

Description, distribution, and correlation: The Point Pleasant Member of the Lexington Limestone is the upper of two named subdivisions of the Lexington in Indiana. (See the article on the Curdsville Member.) The Point Pleasant is medium-gray fossiliferous limestone that contains thin (1.5 feet or 0.5 m, average thickness) interbeds of calcareous shale. In SDH 133 the ratio of limestone to shale is about 2 to 1. This unit is present in Indiana only in eastern Switzerland County, where it ranges from zero to 90 feet (27 m) in thickness. To the north in Dearborn County and to the west in western Switzerland County, the Point Pleasant is replaced through what is believed to be a facies change into an unnamed limestone unit of the Lexington that is a clean fossiliferous limestone of a thickness about equal to that of the Point Pleasant. The Point Pleasant Member bears lithologic affinity both to other Lexington rocks, which in part underlie and in part are laterally equivalent to the Point Pleasant, and to the overlying Kope Formation of the Maquoketa Group. The Point Pleasant correlates with part of the Kope Formation in Indiana and, as noted above, with rocks designated by the same name in Kentucky, which are there assigned to the Clays Ferry Formation rather than to the Lexington Limestone. The Point Pleasant also correlates with the formation of the same name in southwestern Ohio.






 
 
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