Ordovician System

Type locality and reference section: The Joachim Dolomite (Winslow, 1894, p. 311, 352; McQueen, 1937, p. 12) was named for exposures along Joachim Creek in Jefferson County, Mo. A more complete reference section (Templeton and Willman, 1963) was designated in the river bluffs between Cape Girardeau and Dutchtown, Cape Girardeau County, Mo.

Description: The Joachim Dolomite, exceeding 200 feet (60 m) in thickness in Indiana, has the greatest subsurface distribution of the three formations of the Ancell Group (Droste, Abdulkareem, and Patton, 1982). The only Indiana exposure is in the Kentland structure, Newton County, where the major faulting is along the contact of the St. Peter Sandstone and the Joachim Dolomite (Gutschick, 1983). The Joachim is the lateral equivalent of the upper part of the St. Peter Sandstone, and except in one well in extreme northwestern Indiana where the St. Peter constitutes the entire Ancell Group the Joachim Dolomite is the upper formation of the Ancell Group.

Where fully developed the Joachim consists of three divisions. The lowest unit consists typically of very fine grained to fine-grained argillaceous dark to light-colored dolomite and limestone. Thin beds of silty to sandy dolomite and dolomite fine- to medium-grained sandstone provide evidence for the St. Peter-Joachim facies relationship. Where the Joachim overlies the Dutchtown Formation, the darker Joachim rocks grade downward into lighter colored Dutchtown rocks. This part of the Joachim ranges from 0 to 70 feet (0 to 21 m) in thickness.

The middle unit of varicolored limestone and dolomite is the thickest part of the Joachim in Indiana. It is purer than the lower and upper units in that it contains fewer sandy, silty, and argillaceous components. The middle Joachim ranges from 0 to 130 feet (0 to 40 m) in thickness. In local Indiana areas where erosional highs exist on the post-Knox unconformity, the upper part of the Joachim may be the only representative of the Ancell Group between rocks of the Black River Group above and rocks of the Knox Supergroup below. These upper rocks contain light-colored to dark silty to very argillaceous and very fine grained to fine-grained dolomite and limestone and inter bedded greenish to dark-gray to black shales. Thin beds of bimodal sandstone are also present. These shaly, silty, and sandy interbeds in the dolomite and limestone of the upper Joachim have led others to designate the upper unit of the Ancell Group as the Glenwood Shale. Because carbonate components dominate in the upper Joachim in Indiana, the term Glenwood Shale has been abandoned. The upper part of the Joachim ranges from 0 to 80 (0 to 24 m) in thickness.

Correlation: As noted above, the Joachim in Indiana is a lateral equivalent of upper St. Peter rocks in Indiana. Further, it correlates with the Joachim Dolomite of Illinois and Kentucky and with the lowest part of the Black River Limestone of Ohio (as described by Stith, 1979), and it is stratigraphically equivalent to the upper part of the St. Peter Sandstone and of the Glenwood Formation in Illinois and Michigan (Droste and Shaver, 1983: Shaver and others, 1985).