Newly Discovered Inlier of Ordovician Phi Kappa Formation, South-Central Idaho

ABSTRACT An inlier of Ordovician Phi Kappa Formation at Meridian Creek, Custer County, Idaho, provides new information on the distribution and character of this formation. The exposure, which trends 2.3 km along the drainage and ranges up to 0.5 km in width, is isolated from other Phi Kappa outcrops by Tertiary Challis Volcanics. Although the rocks here are similar in age and general lithology to the Phi Kappa exposed 7.1 km northwest in the East Pass Creek window and 5.2 km south on the North Fork of the Lost River, there are significant differences. The Ordovician at Meridian Creek has a minimum thickness of 210 m. About 150 m is graptolite-bearing, Middle Ordovician (Caradocian) shale and calcareous siltstone, whereas the remainder consists of thick (30 m) units of dolomitic limestone and radiolarian-bearing chert. The carbonate unit, which consists of medium to thick beds of fine to medium-grained, dolomitic limestone with clasts of mudstone up to 1 cm long, is coarser grained and thicker than carbonates in Ordovician rocks in nearby areas. The limestones seem to be turbidites, but this interpretation is difficult to reconcile with potential carbonate sources. The uppermost Phi Kappa at Meridian Creek is intensely deformed. Similar relations are present in adjacent areas where the Mississippian Copper Basin Formation is thrust over the Ordovician. Although Mississippian rocks are absent along Meridian Creek, it seems a Copper Basin thrust plate once extended across this area. Regional considerations also suggest that the Ordovician rocks at Meridian Creek are allochthonous.