Biostratigraphy of Upper Mississippian and Associated Carboniferous Rocks in South-Central Idaho

Abstract

Carboniferous rocks above the Milligen Formation in the central Lost River Range of south-central Idaho range in age from Late Mississippian (middle Meramecian or middle Visean) through Pennsylvanian (Atokan or Westphalian), as determined by small calcareous foraminifers, corals, and brachiopods. The lower part of the post-Milligen sequence, composed largely of limestone, is Mississippian and is divided into four formations first described by Huh in the Lemhi and Lost River Ranges. These rocks are overlain by an unnamed Pennsylvanian sandstone unit which, in turn, is overlain by a thick sequence of unnamed Pennsylvanian limestone. The composite section in the Lost River Range has a minimum exposed thickness of 7,000 ft. Two partial sections in the Hawley Mountains are des gnated reference sections for the four formations of Mississippian age in the Lost River Range--the Middle Canyon Formation at the base, the Scott Peak Formation, the South Creek Formation, and the Surrett Canyon Formation at the top. Foraminiferal Zones 13(?) through 21 are recognized in the Carboniferous rocks. Zone 19, the Homoceras ammonoid time equivalent, is described herein for the first time from North America. Representatives of megafaunal Zones Pre-E through Post-K are also recognized in the Mississippian and the directly overlying rocks of the Carboniferous sequence. Microfaunas and megafaunas indicate that the Meramec-Chester boundary is near the middle of the Scott Peak Formation, that the Visean-Namurian boundary is near the base of the Surrett Canyon Formation, and that the Mississippian-Pennsylvanian boundary is about at the base of the unnamed sandstone unit. Carbonate lithologies and associated faunas and microfaunas indicate deposition in a marine environment in which there was a general change from quiet water having locally restricted circulation during Middle Canyon Formation deposition to turbulent water that was freely circulating during deposition of the upper strata. Sand in the basal Pennsylvanian rocks may indicate a distant orogenic pulse very near the Mississippian-Pennsylvanian time boundary.