RI-33 Sedimentology of the Middle Ordovician Platteville Formation Southeastern Minnesota

Abstract

The Platteville Formation, a thin Middle Ordovician unit, is subdi vided into several members in Minnesota. The basal Pecatonica and upper Carimona Members are present throughout all the Minnesota/western Wisconsin outcrop belt. The medial McGregor, which is recognized in the southern part of the Minnesota outcrop, is replaced by the Mifflin, Hidden Falls, and Magnolia Members in the Twin Cities area. Distribution of the Mifflin is restricted to the Twin Cities structural basin; the other two units extend slightly beyond the Twin Cities basin. Their distribution may have been influenced by proximity to a paleoshoreline north of the present outcrop. The Platteville Formation was deposited in a transgressing sea as part of the Middle Ordovician-Silurian Tippecanoe sequence. It is interpreted to have been deposited on a shallow marine shelf in deeper water than the shoreward facies represented by the underlying St. Peter Sandstone and Glenwood Formation. Fossils of stenohaline organisms are present throughout the formation. The formation has no exposure indicators, stromatolites, or intraclasts. Features indicative of shallower water, such as ferruginous ooids and coquina layers, are present in the Twin Cities/western Wisconsin area. Hardgrounds--surfaces of submarine nondeposition indicative of low rates of sedimentation--characterize the Pecatonica and lower McGregor Members, but are less common in the upper part of the formation. The Carimona Member is characterized by limestone beds separated by shale beds and partings that thicken toward the west and north. This increased detrital sedimentation during deposition of the Carimona Member and the succeeding Decorah Shale may indicate uplift along the Transcontinental Arch. The proportion of dolomi te to calci te in the carbonate rocks of the Platteville increases toward the north and northeast. In southern Minnesota the formation is principally limestone; in the Twin Cities Metropolitan Area and western Wisconsin, it has a high proportion of dolomite. The formation does not have any diagenetic fabrics indicative of subaerial exposure, and deposition of the overlying Decorah Shale appears to have followed deposition of the Platteville without interruption. The pervasive dolomitization is probably a result of burial diagenesis.