Stratigraphic Details of Lower Mississippian Rocks of Northeastern Utah and Southwestern Montana

Abstract

This paper is based on a detailed stratigraphic restudy of the lower part of the Madison group at the type section in Montana; and a section, measured bed by bed, of the Madison limestone near Logan, Utah, is compared with the type section. A new formation which underlies the Madison in northeastern Utah is described and compared with the Sappington sandstone which underlies the Madison at the type section. Both on a lithologic and faunal basis the Sappington and the new unit appear to be contemporaneous formations. The Sappington appears to be Kinderhookian since the typical Upper Devonian Cyrtospirifer fauna was not found above, or the Lower Mississippian Syringothyris fauna below, a thin black shale at the base of the Sappington. The Lodgepole limestone, lowermost formation of the Madison group, overlies the Sappington. It consists of a remarkable sequence of dark gray, hard, very fine-crystalline limestones rhythmically interbedded with softer light gray to yellowish gray, argillaceous limestone or shale. The fauna of the Lodgepole indicates that it too is entirely Kinderhookian. In northeastern Utah Devonian fossils were found at the top of a prominent limestone ledge the base of which was previously believed to mark the base of the Madison. This ledge is now assigned to the Jefferson formation of the Devonian. The new formation, made up of a series of shales, sandy shales, and nodular limestones, unconformably overlies the ledge now attributed to the Jefferson and carries the same Syringothyris fauna as the Sappington sandstone of Montana. The base of the Madison limestone is accordingly drawn at the base of 30 feet of brownish black fissile shale which overlies this new formation. Above this basal shale unit the Madison consists of dark gray, fine-crystalline limestone rhythmically interbedded with shaly limestone beds. The similarity of the lithology, End_Page 1697------------------------------ especially the rhythmic nature of the bedding, combined with the faunal similarities, indicates that the Madison in this area is entirely Lodgepole in age. There are apparently no beds equivalent to upper Madison, that is, the Mission Canyon limestone of Montana, in the Utah section.