H. B. Willman and T. C. Buschbach
The Plattin Subgroup (Ulrich, 1904, p. 111; Weller and St. Clair, 1928, p. 109-110) is named for Plattin Creek, Jefferson County, Missouri, near the mouth of which it is partially exposed.
The Plattin Subgroup is equivalent to the McGregor Formation in the Upper Mississippi Valley above Dubuque, Iowa, except district where the McGregor fined to exclude the Quimbys (Agnew and Heyl, 1946).
As originally defined, the Plattin included strata equivalent to the Decorah at the top but not the Pecatonica below. Weller and St. Clair removed the Decorah, and, as thus defined, the Plattin was long considered equivalent to the Platteville in northern Illinois. Because the Plattin did not include the Pecatonica Formation of the Platteville Group (the Pecatonica is not present in the Plattin Creek area), the Plattin was reclassified as a subgroup (Templeton and Willman, 1963, p. 78).
The Plattin Subgroup includes the Mifflin (at the base), Grand Detour, Nachusa, and Quimbys Mill Formations. The Plattin Formation, as defined in Missouri, consists of the strata from the oolite in the Brickeys Member of the Mifflin Formation to the base of the Decorah Formation.
The Plattin is a distinctive unit, particularly in the northern outcrop area and in other regions where it is dominantly fine-grained and lithographic limestone, separated by a minor unconformity from the dolomitic Pecatonica Formation below.
AGNEW, A. F., and A. V. HEYL, JR., 1946, Quimbys Mill, new member of Platteville Formation, Upper Mississippi Valley: American Association of Petroleum Geologists Bulletin, v. 30, p. 1585-1587.
TEMPLETON, J. S., and H. B. WILLMAN, 1963, Champlainian Series (Middle Ordovician) in Illinois: Illinois State Geological Survey Bulletin 89, 260 p.
ULRICH, E. O., 1904, in E. R. Buckley and H. A. Buehler, Quarrying industry of Missouri: Missouri Bureau of Geology And Mines, v. 2, 371 p.
WELLER, STUART, and STUART ST. CLAIR, 1928, Geology of Ste. Genevieve County, Missouri: Missouri Bureau of Geology and Mines, v. 22, 352 p.
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