Lower Mississippian Carbonate Stratigraphy of Southeastern Idaho and Northeastern Utah

G. D. Webster, L. E. Davis, D. W. Wickwire

Lower Mississippian carbonates of southeastern Idaho and northeastern Utah have been referred to the Madison Group or Lodgepole Limestone since the early part of this century. Although time equivalent to the Lodgepole and basal part of the Mission Canyon Limestones of central Montana, the southeastern Idaho and northeastern Utah strata are lithologically distinct as a result of deposition in a more distal shelf edge to basin margin environment. Furthermore, the strata may be subdivided into three members that are traceable throughout the region.

The basal member, informally known as the Chinese Wall, consists of thin-bedded wackestones and mudstones with a few grainstones. Graded beds are common throughout the unit, as are chert nodules and discontinuous nodular chert bands. Where dipping less than 30° it weathers as a sheer cliff 16 to 84 m thick. Conodont faunas from the unit belong to the Lower and Upper Siphonodella crenulata Zone, of middle to late Kinderhookian age. The middle member is thin-bedded wackestones and mudstones with fewer graded beds and rarely contains chert nodules. The middle member weathers as a slope 47-90 m thick and is of late Kinderhookian age, S. isosticha-Upper S. crenulata Zone. The upper member consists of grainstones with a few wackestones, thin to medium bedded, moderately common graded b ds, chert nodules present in the upper half, weathers as a cliff or cliff capped with a receding slope, and the thickness ranges from 30 to 58 m. Conodont faunas belong to the S. isosticha-Upper S. crenulata Zone in the lower part and the Gnathodus typicus Zone in the upper part.

New stratigraphic names are being proposed for each of the units discussed.

AAPG Search and Discovery Article #91040©1987 AAPG Rocky Mountain Section Meeting, Boise, Idaho, September 13-16, 1987.