New Mississippian Formations and Faunal Zones in Chesterfield Range, Portneuf Quadrangle, Southeast Idaho

ABSTRACT Mississippian rocks in the Chesterfield Range, southeast Idaho, consist of the Lodgepole Limestone overlain by a 1,900-foot limestone-sandstone sequence here designated the Chesterfield Range Group. The lower terrigenous part, about 965 feet thick, is the Little Flat Formation; it has a basal siltstone member, a middle sandstone member, and an upper sandy limestone member. The upper carbonate part of the Chesterfield Range Group, here named the Monroe Canyon Limestone, is about 925 feet thick and contains a lower massive member, a medial medium-bedded member, and an upper cherty member. The Chesterfield Range Group is of Late Mississippian age. Three coral zones and four brachiopod zones are recognized. The coral sequence is composed of a lower Eksvasophyllum Zone succeeded by the Faberophyllum Zone and, at the top, the Caninia Zone. The lowest brachiopod zone of Quadratia hirsutiformis is succeeded by the Echinoconchus cf. E. alternatus Zone, the Striatifera brazeriana Zone, and the Spirifer brazerianus Zone. Correlations with Mississippian sequences elsewhere in the northern Rocky Mountains emphasize relationships with the Dry Lake and Old Laketown Canyon sections in northern Utah and with the Deep Creek Mountains section in southeastern Idaho. Relationships with the faunal zones recently established for the Mississippian in western Canada are indicated.