Type of Culminating Activity
Master of Science in Geology
Clyde J. Northrup, Ph.D.
Structural, stratigraphic, and U-Pb geochronologic studies of the Inskip Formation of the north-central East Range place new constraints on the depositional age of the Inskip Formation and the timing of contractional deformation in the East Range, including Golconda thrusting. The Inskip Formation was initially considered Permian in age and to rest unconformably on the Valmy (or Leach) Formation (Ferguson et al., 1951). Later mapping by Whitbread (1978, 1994) divided the formation into the Upper and Lower Inskip Formation, both of which were inferred to be Mississippian in age. Laterreinterpretation of Whitebread’s (1994) work and new paleontological data revised the age of the Inskip Formation as Permian through the Devonian, and interpreted the Upper and Lower Inskip Formation as facies of the Havallah sequence (Ketner et al., 2000, 2008). The current study retains the use of Upper and Lower Inskip Formation for discussion and their use as distinct tectono-stratigraphic units; however, the new data from the study have led to a significantly revised interpretation of what the units represent.
The Lower Inskip Formation is reinterpreted to be structurally above the ValmyFormation (Silberling, 1962; Hargett, 2002) and unconformably overlain by the Upper Inskip Formation. The Lower Inskip Formation contains arkosic sandstone, conglomerate, phyllite, siliceous argillite, quartzite, minor shale and limestone, and some greenstones. Fossils from the Lower Inskip Formation have yielded ages ranging from Pennsylvanian to the Devonian (Ketner et al., 2000; Hargett, 2002). This study assigns the Lower Inskip Formation to the Havallah sequence. In contrast, Siberling et al. (1962) only assigned the northern most section of the Inskip Formation to the Havallah sequence and separated it from the rest of the Inskip Formation by a thrust fault, and Ketner et al. (2000) assigned the entire Inskip Formation to the Havallah sequence. The Upper Inskip Formation lies unconformably over the Lower Inskip Formation and contains amphibolite/greenstone, phyllite, and quartzite, with lesser amounts of limestone and some felsic volcanic deposits. Only one fossil has been identified in the Upper Inskip Formation and it yielded an age of Permian (Ketner et al., 2000). However, new U-Pb zircon geochronology of a tuff (sample 08JW534) collected in the Upper Inskip Formation yielded an age of Early Triassic (249.08 +/- 0.14 Ma). Due to the new radiometric age control and similarity of lithostratigraphy, the Upper Inskip Formation is correlated with the Limerick Member of the Koipato Group.
The Golconda Thrust places upper Paleozoic Havallah sequence basinal rocks on top of lower Paleozoic rocks of the Roberts Mountains allochthon and its overlap sequence, while the Koipato Group rests unconformably on the Havallah sequence (Silberling et al., 1962). With the new age data and assignment of the Lower Inskip Formation to the Havallah sequence and the Upper Inskip Formation to the Limerick Member, the same relationship exists in the north-central East Range as in northern Tobin Range where the Sonoma orogeny was originally recognized (Silberling et al., 1962). Therefore, the contact between the Lower Inskip Formation and the Valmy Formation is interpreted to be a portion of the Golconda Thrust, which is considered the basal thrust of the Sonoma orogeny.
At least five phases of deformation occurred in the Upper and Lower Inskip Formation of the East Range. The first phase of deformation (D1) formed the angular unconformity between the Inskip Formation and the Rochester Member. The second phase (D2) included thrusting of Early Triassic rocks and upper Paleozoic rocks of the Havallah basin over the lower Paleozoic rocks of the Roberts Mountains allochthon. D3 formed the penetrative foliation, which is prevalent throughout the area. The fourth phase of deformation (D4) resulted in map scale folding, including the large northeast-southwest trending antiform across the north-central East Range. The fifth phase (D5) produced SE-vergent folding throughout the area and a crenulation cleavage in the less competent phyllite and along some of the western limbs of these folds. The age of the entire deformational sequence is constrained to have occurred between the last depositional event (the Upper Inskip Formation), which has an age of Early Triassic (249.08 +/- 0.14 Ma), and the intrusion of relatively nondeformed late Middle Jurassic sills (161.80 +/-0.04 Ma). The timing and structural style of deformation exhibited in (D5) appears very similar to that resulting from the Fencemaker fold and thrust belt (Oldow et al., 1984; Speed et al., 1989; Wyld, 2002). Deformation in the East Range and the Sonoma Range are interpreted here as expressions of Mesozoic contraction, kinematically linking the Luning-Fencemaker and the Golconda thrust systems.
Wilkins, Joshua David, "Structural and Stratigraphic Age Constraints of the Inskip Formation, East Range, Nevada: Implications for Mesozoic Tectonics of Western North America" (2010). Boise State University Theses and Dissertations. 165.