Ordovician and Silurian Phi Kappa and Trail Creek formations, Pioneer Mountains, central Idaho: Stratigraphic and structural revisions, and new data on graptolite faunas

Abstract

Recent geologic mapping in the northern Pioneer Mountains combined with the identification of graptolites from 116 new collections indicate that the Ordovician and Silurian Phi Kappa and Trail Creek Formations occur in a series of thrust-bounded slices within a broad zone of imbricate thrust faulting. Though confirming a deformational style first reported in a 1963 study by Michael Churkin, our data suggest that the complexity and regional extent of the thrust zone were not previously recognized. Most previously published sections of the Phi Kappa and Trail Creek Formations were measured across unrecognized thrust faults and therefore include not only structural repetitions of graptolitic Ordovician and Silurian rocks but also other tectonically juxtaposed lithostratigraphic units of diverse ages as well. Because of this discovery, the need to reconsider the stratigraphic validity of these formations and their lithology, nomenclature, structural distribution, facies relations, and graptolite faunas has arisen. The Phi Kappa Formation in most thrust slices has internal stratigraphic continuity despite the intensity of deformation to which it was subjected. As revised herein, the Phi Kappa Formation is restricted to a structurally repeated succession of predominantly black, carbonaceous, graptolitic argillite and shale. Some limy, light-gray-weathering shale occurs in the middle part of the section, and fine-grained locally pebbly quartzite is present at the base. The basal quartzite is here named the Basin Gulch Quartzite Member of the Phi Kappa. The Phi Kappa redefined on a lithologic basis represents the span of Ordovician time from W. B. N. Berry's graptolite zones 2-4 through 15 and also includes approximately 17 m of lithologically identical shale of Early and Middle Silurian age at the top. The lower contact of the formation as revised is tectonic. The Phi Kappa is gradationally overlain by the Trail Creek Formation as restricted herein. Most of the coarser clastic rocks reported in previously measured sections of the Phi Kappa, as well as the sequence along Phi Kappa Creek from which the name originates, are excluded from the Phi Kappa as revised and are reassigned to two structural plates of Mississippian Copper Basin Formation; other strata now excluded from the formation are reassigned to the Trail Creek Formation and to an unnamed Silurian and Devonian unit. As redefined, the Phi Kappa Formation is only about 240 m thick, compared with the 3,860 m originally estimated, and it occupies only about 25 percent of the outcrop area previously mapped in 1930 by H. G. Westgate and C. P. Ross. Despite this drastic reduction in LOCATION AND ACCESS The Ordovician and Silurian rocks considered here are in the northern Pioneer Mountains, mainly in the northeast part of the Rock Roll Canyon 7 1 /2-minute quadrangle, but extending into adjacent parts of the Phi Kappa Mountain and Meridian Peak 7 1 /2-minute quadrangles (pi. 1; fig. 1). The mapped area is about 16 km northeast of Ketchum, a town on U.S. Route 93 in the...

Geology and mineral resources of the Hailey 1° x 2° quadrangle and the western part of the Idaho Falls 1° x 2° quadrangle, Idaho

“…Our observation is that an Antler fabric is present in many, but not all, outcrops of Devonian and older strata west of the Trail Creek fault but is not present east of the fault. The presence of an Antler fabric in central Idaho has been long debated (Dover, 1980), and the nature of such a fabric requires further study.…”

“…Based on the presence of a Mississippian-age flysch trough (Copper Basin Formation) containing clasts derived from the Milligen Formation, and Nilsen (1977) suggested that the Milligen Formation was thrust eastward to form a highland during the Late Devonian to Early Mississippian Antler orogeny. The thrust fault that accommodated this shortening cannot, however, be conclusively located in south-central Idaho (Dover, 1980). If it is present in the Hailey 1 x2 quadrangle, it may be concealed beneath the outcrop belt of upper Paleozoic Wood River Formation in the Boulder Mountains (Roberts and Thomasson, 1964), or it may have been reactivated to become the Cretaceous Pioneer thrust fault (Wust, 1986) or the early Eocene(?)…”

“…Milligen strata are in thrust contact with the Lower and Upper Mississippian Copper Basin Formation to E2 the east and are structurally overlain by Middle Pennsylvanian to Lower Permian Wood River Formation and Eocene Challis Volcanic Group to the west. This fault-bound package of Milligen strata was named the Pioneer allochthon by Dover (1980).…”

“…The Milligen Formation, along with black argillite, chert, and siltstone of the Lower Ordovician to Middle Silurian Phi Kappa and Middle Silurian Trail Creek Formations and an unnamed Silurian and Devonian unit, is interpreted as part of the Roberts Mountains allochthon, a sequence of outer continental margin strata thrust eastward over the continental margin during the Early Mississippian Antler orogeny (Roberts and Thomasson, 1964;Dover, 1980). The sandstone turbidite facies of the Mississippian Copper Basin Formation that is exposed east of the Pioneer Mountains core complex is inferred to have been derived from the exposed Roberts Mountains allochthon ("Antler highlands") to the west (Nilsen, 1977; Poole and Sandberg, 1977).…”

“…Paleozoic strata are weakly metamorphosed, folded by north-to northwest-trending, east-vergent folds, and cut by east-vergent thrust faults of Mesozoic age (Dover, 1980. These Mesozoic structures are offset by low-angle extensional faults related to development of the Eocene Pioneer Mountains core complex.…”