Ordovician System

Type locality and history of name: According to reviews by Templeton and Willman (1963) and Buckley and Buehler (1904, p. 111), the name Plattin was originally proposed, but not published, by E. O. Ulrich for exposures along Plattin Creek, Jefferson County, Mo. After long use of the term Plattin Limestone or Formation in the upper Mississippi Valley area, the rocks of this unit were reclassified as a subgroup in the Platteville Group of Illinois (Templeton and Willman, 1963, p. 78). In Illinois they were underlain by the Pecatonica Formation of the Platteville Group. From Illinois the Plattin Subgroup was traced without subdivision into the Indiana subsurface by Droste, Abdulkareem, and Patton (1982). In Indiana the Plattin overlies the Pecatonica Formation and with the Pecatonica makes up the Black River Group.

Description: In gross aspect the Plattin is made up of the lithographic limestone that characteristically is associated with Black River rocks in many places in the eastern United States dolomite is an abundant component of the Plattin only in northwestern Indiana and northeastern Illinois. The Plattin ranges in thickness from less than 100 feet (30 m) in northwestern Indiana to more than 400 feet (120 m) in southwestern Indiana. Except for faulted blocks of the Plattin in the Kentland structure, Newton County, Ind., the Plattin is known only in the subsurface of Indiana (Gutschick, 1983).

The Plattin Formation is overlain with minor unconformity by the more coarsely bioclastic rock of the Trenton Limestone except for places in southern Indiana where rocks of the Kope Formation (Maquoketa Group) or of the Lexington Limestone lie above.

Correlation: The Plattin Formation in Indiana is correlated with the Plattin Subgroup of Illinois, the Plattin Formation of western Kentucky and the upper part of the High Bridge Group elsewhere in Kentucky, and the upper parts of the Black River Group of Michigan and of the Black River Limestone of Ohio. Some named subdivisions of these units in adjacent states are recognized in some wells in Indiana. For example, marker bed I of Stith (1979) in the Black River Limestone of Ohio is recognized in many wells in Indiana. These same rocks in Indiana correspond essentially to the uppermost member (Briton Member) of the Mifflin Formation, Plattin Subgroup, of northern Illinois. The Brickeys Member of the Mifflin Formation of Illinois has a characteristic lithology that can be identified in wells in Indiana in the lower part of the Plattin Formation. K-bentonite deposits called the Mud Cave and Pencil Cave Bentonite Beds in Kentucky and several K-bentonite units noted by Stith (1979) in Ohio are recognized in some wells in Indiana.