(JBD & RHS)
Detroit River Formation,
Type and reference sections: The Milan Center Dolomite Member was named for the middle Detroit River dolomite that is exposed in the Woodburn Quarry of May Stone and Sand, lnc., and that was cut in Indiana Geological Survey drill hole 188, eastern Allen County, Ind. (NE¼ sec. 23, T. 31 N., R. 14 E.). The name was taken from Milan Center Township, in which the Woodburn Quarry is located (Doheny, Droste, and Shaver, 1975, p. 28-30). A principal reference section consists of the pertinent rocks cored in the Northern Indiana Public Service Co. August Taelman No. 1 well, LaPorte County, Ind. (NE¼ sec. 16, T. 36 N., R. 1 \V.).
Description: The Milan Center is dominantly a rich brown granular, finely vuggy dolomite that is a replacement of pelletoidal, oolitic, and bioclastic rocks that have been termed grainstones, packstones, and boundstones. In areas of greatest thickness, a gray-yellow cherty dolomite is at the top of the member.
The Tioga Bentonite Bed, consisting of an inch to a few inches (0.03 to 0.15 m) of metabentonite, is found in the Milan Center in a part of its distribution.
The Milan Center rests conformably on the Grover Ditch Member of the same formation and is generally overlain conformably by the Cranberry Marsh Member. In two places, however, it is overlain unconformably by the Traverse Formation (Middle Devonian) in an overlapping manner. One place is along the southward eroded limit of Detroit River rocks, and the other is in part of Noble County where the Cranberry Marsh Member apparently was not deposited.
The greater distribution of the Milan Center is in northwestern Indiana, where it thickens from an erosional zero in the Lake County and Pulaski County areas to more than 40 feet (12 m) in St. Joseph County, but over most of northeastern Indiana the thickness ranges from zero (nondepositional?) to a few feet (Doheny, Droste, and Shaver, 1975, pl. 2D). This eastward decrease in thickness is facies controlled; as the Milan Center thins, the underlying Grover Ditch thickens .
Correlation: Index fossils have not been recovered from the Milan Center, but because the Milan Center is mostly a facies of the Grover Ditch and because the Tioga bentonite occurs in the topmost Milan Center rocks in part of northern Indiana, much of the Grover Ditch correlation can be applied to the Milan Center. The Milan Center member is Eifelian in age (early Erian, North American standard). The greatest age for the Grover Ditch (possibly Emsian) does not apply, however, to the Milan Center.