Abstract

Mississippian strata in the Pioneer and White Knob Mountains of south-central Idaho contain the transition, now telescoped by Cretaceous thrust faults, from a faulted foreland basin associated with the Antler orogeny on the west to a carbonate platform on the east. We revise stratigraphic nomenclature for Lower and Upper Mississippian siliciclastic rocks west of the Copper Basin thrust fault, and assign these rocks to the Copper Basin Group which is composed of three formations. From bottom to top, these are the Little Copper Formation, Drummond Mine Limestone, and Argosy Creek Formation (new name). The Argosy Creek Formation contains four members. From the base upward, they are the Scorpion Mountain, Muldoon Canyon, Brockie Lake Conglomerate, and Iron Bog Creek members. The Green Lake Limestone bed occurs within the Muldoon Canyon Member.

The Copper Basin Group contains three tectonically-driven stratigraphic sequences. Sequence 1 (lower Kinderhookian) includes the basal part of the Copper Basin Group, and is composed of a prograding submarine fan system (Little Copper Formation) interpreted to represent a lowstand systems tract, overlain by a transgressive/highstand systems tract of the Drummond Mine Limestone, which contains distal calciclastic turbidite deposits plus proximal carbonate-bank debris. Sequence 2 (upper Kinderhookian to? lower Osagean) includes the Scorpion Mountain and Muldoon Canyon members of the lower Argosy Creek Formation. It records a second fan delta to submarine fan system (lowstand to transgressive systems tract) that prograded northward from uplifted fault blocks in the southernmost Pioneer Mountains. Sequence 3 (upper Osagean?, Meramecian and Chesterian) includes the upper part of the Argosy Creek Formation (Brockie Lake Conglomerate and Iron Bog Creek members). It overlies sequence 2 with angular unconformity and represents an upward-fining fan delta to shallow marine succession (transgressive and highstand systems tracts).

Kinderhookian strata are 4272 m thick, several times thicker than Lower Mississippian Antler flysch deposits in eastern Nevada. After backstripping, these strata indicate extremely rapid tectonic subsidence (300 to 800 m/m.y.), interpreted to have been caused by thrust loading to the west plus episodic normal faulting that provided locally deep basins. The low-angle angular unconformity, possibly spanning much of Osagean time, that separates fine grained middle to outer submarine fan turbidite deposits of the Muldoon Canyon Member from overlying cobble to boulder, fan delta conglomerate of the Brockie Lake Conglomerate Member, suggests intrabasinal deformation. The upper part of the Argosy Creek Formation (the Meramecian and Chesterian Brockie Lake Conglomerate and Iron Bog Creek members which are over 1700 m thick) demonstrates less pronounced tectonic subsidence (100-120 m/m.y.). These strata were deposited in a slowly subsiding basin in which eustatic sea level changes are evident.

The Lower Mississippian McGowan Creek Formation, exposed in the White Knob Mountains east of the Copper Basin thrust fault, represents distal turbidite deposits, broadly correlative with sequences 1 and 2. The overlying Upper Mississippian White Knob Limestone (correlative with sequence 3) represents carbonate platform deposition at the eastern margin of the Antler turbidite basin.