Structural Geology of Northern Salt River Range, Idaho-Wyoming Thrust Belt--Preliminary Report: ABSTRACT

Abstract

The northern Salt River Range is the structural culmination of the Absaroka-St. Johns thrust complex. The Stewart Peak quadrangle, located on the culmination, has been mapped to gain an understanding of the nature of the thrusts and folds in this part of the Idaho-Wyoming thrust belt. Rocks ranging in age from Middle Cambrian through Late Cretaceous are interleaved in a complex array of imbricate thrust faults and asymmetric folds. Major thrust faults in the Stewart Peak quadrangle include the Absaroka, Murphy, and Firetrail. Imbricate thrusts in the hanging wall of the Absaroka include the Star, Stewart, and four imbricate slices at the north-central margin of the quadrangle which may correlate with the St. Johns complex in the Snake River Range. The Grand Valley fault bounds the range near the west margin of the quadrangle where fanglomerates of probable Tertiary age (Pliocene?) are offset against Middle and Upper Cambrian strata. Several conclusions may be proposed regarding the northern Salt River Range. (1) Cataclasis occurs on a scale much greater than previously reported because deeper and more intensely deformed levels of the thrust belt are exposed relative to thrusts cropping out east and south in the Idaho-Wyoming salient. (2) Deformational intensity increases downward through the Paleozoic succession as the basal Absaroka decollement in the Cambrian Wolsey Shale is approached. (3) Stratigraphic thicknesses for units below the Mississippian Madison Group are tectonically thickened by ubiquitous small-scale thrust slivers (each with a few centimeters or more offset), stylolites, and small-scale folds. Stratigraphic correlations and isopach studies based on the present distribution of tectonically thicke ed Paleozoic units should not be made in this part of the thrust belt. (4) The Stewart Peak area represents a structural culmination in which the roots of the Absaroka thrust have been exposed, possibly from thrusting over a basement arch. In this regard, isopach trends of Cretaceous rocks east of the Darby-Hogsback thrust suggest that the Moxa arch may continue northwestward beneath the thrust belt in alignment with the Stewart Peak culmination. In addition, several structural discontinuities within the thrust belt northwest of LaBarge suggest the influence of a basement upwarp. The Stewart Peak culmination may therefore reflect a deeper structural level of exposure owing to thrusting over a basement arch above the regional level of decollement. This interpretation has important ramific tions regarding the structural control of potential oil and gas reservoirs beneath the Absaroka thrust. End_of_Article - Last_Page 833------------