Stratigraphy of Copper Basin Group, Pioneer Mountains, South-Central Idaho

Abstract

A continuous sequence of 18,000 ft of Lower(?) Mississippian to Middle(?) Pennsylvanian rocks in the central Pioneer Mountains, south-central Idaho, comprises the Copper Basin Formation. We propose raising this formation to group rank and provide within the type area a type section that is divided into six formations. Five of these formations are proposed as new stratigraphic units, and a reference section is provided for the previously defined Milligen Formation. The six formations in the Copper Basin Group, from oldest to youngest, are: (1) the Lower(?) to Upper Mississippian Milligen Formation consisting of 3,700+ ft of dark-gray, thin- to medium-bedded argillite with some interbeds of dark quartzite and granule-size conglomerate; (2) the Upper Mississippian Drummond Mine Limestone consisting of 2,620 ft of dark-gray, thin- to thick-bedded micritic limestone with interbeds of argillite and fine-grained sandstone; (3) the Upper Mississippian Scorpion Mountain Formation consisting of 3,625 ft of medium-gray, thick- to very thick-bedded, chert-quartzite-argillite pebble conglomerate with interbedded quartzite; (4) the Upper Mississippian(?) to Lower Pennsylvanian(?) Muldoon Canyon Formation consisting of 4,150 (± 800) ft of dark-gray argil ite with minor interbeds of quartzite, conglomerate, and limestone--including the proposed Green Lake Limestone Member near the top; (5) the Lower(?) to Middle(?) Pennsylvanian Brockie Lake Conglomerate consisting of 2,150 (± 250) ft of very light-gray, thick- to very thick-bedded, quartzite-chert cobble conglomerate and interbedded light-colored quartzite; and (6) the Middle(?) Pennsylvanian Iron Bog Creek Formation consisting of 1,500+ ft of medium-gray, thin- to medium-bedded argillite and shale with a few interbeds of conglomerate and quartzite. The Copper Basin Formation was previously considered Early Mississippian to Early Permian in age and was interpreted as an intertonguing complex between supposedly equivalent rocks on the west in the Big Wood River valley and on the east in the White Knob Mountains and Lost River Range. Our studies suggest a different interpretation, probably involving major faults, to explain the present relations of this thick, predominantly terrigenous sequence in the Pioneer Mountains.