Ordovician System

Type section and use of name: The St. Peter Sandstone was named by Owen (1847, p. 169-170) for the exposures along the river then called St. Peter (now the Minnesota River) in southern Minnesota. A St. Peter type section was later designated (Stauffer, 1934) as the exposures in the bluff at the junction of the Minnesota and Mississippi Rivers. This name has had wide use in the Midwest and in Kentucky.

Description: Generally the St. Peter Sandstone in lndiana is composed of fine to medium well-rounded and well-sorted frosted grains of quartz that are weakly cemented (Droste, Abdulkareem, and Patton, 1982). In some places secondary quartz overgrowth and siliceous intergranular cement have resulted in well-indurated rather than friable sandstone. In some southern Indiana localities the St. Peter has carbonate cement and thin interbeds of carbonate rocks, generally dolomite.

The distribution of the St. Peter in Indiana is restricted to the subsurface in the western half of the state except for the chaotic structure exposed in the Kentland structure in Newton County, Ind. (Gutschick, 1983). Generally the thickness of the St. Peter ranges from 0 foot at its depositional limit to about 140 feet (43 m). Sharp differences in thickness over a few tens of miles are the result of moderate relief developed along the unconformity on the subjacent Knox rocks and of facies changes from St. Peter rocks to Dutchtown or Joachim rocks within short distances. Within the continuous body of the St. Peter, abruptly increased thickening over short distances is also known but not well understood. For example, in a square mile (2.6 km2) (sec. 6, T. 31 N., R. 7 W., Jasper County, Ind.) the sandstone increases from 60 to 335 feet (18 to 102 m) in thickness as evidenced by two wells.

The lower part of the St. Peter Sandstone interfingers with the Dutchtown Formation. The upper St. Peter grades laterally into the Joachim Dolomite, which is Blackriverian in age. In only one well in northwestern Indiana is the St. Peter known to constitute the entire Ancell Group. In that well the St. Peter is overlain, probably unconformably, by the Pecatonica Formation of the Black River Group. Elsewhere in Indiana the St. Peter is overlain conformably by either the Dutchtown Formation or the Joachim Dolomite. The St. Peter overlies unconformably the Everton Dolomite in southwestern Indiana and the Shakopee Dolomite, the Oneota Dolomite, or the Potosi Dolomite elsewhere in the state.

Correlation: The St. Peter Sandstone of Indiana correlates generally with the St. Peter Sandstone of Illinois, Michigan, Ohio, and Kentucky. Its lower part is known to be Chazyan in age because of its conodonts that are closely similar to those from the Dutchtown Formation in its type locality in Missouri (Rexroad, Droste, and Ethington, 1982; Repetski, 1973) and from the lower part of the Wells Creek Dolomite in the subsurface of eastern Tennessee (Votaw and Repetski, 1982). The conodonts of this lower part of the St. Peter represent parts of faunas 5 and 6 of Sweet, Ethington, and Barnes (1971). The upper part of the formation is probably Blackriverian in age. Because the St. Peter is laterally equivalent to the Dutchtown Formation and the Joachim Dolomite, the articles here on these closely associated formations can be consulted for further details on correlation.