GEOLOGY OF THE KESSLER CREEK QUADRANGLE, WEST-CENTRAL IDAHO: EDMAP PROGRESS REPORT

The Kessler Creek 7.5’ quadrangle in west-central Idaho [116^°30’00’’–116^°22’30’’x45^°30’00’’–45^°22’30’’] hosts accreted island-arc volcanogenic, carbonate, and siliciclastic rocks along the eastern margin of the Wallowa terrane/Blue Mtns. province. In the northwest, the Seven Devils Group [Hunsaker Creek Fm.] is intruded by ca. 226 Ma biotite-hornblende granodiorite [U-Pb zircon; Kauffman et al. 2014] of the Cold Spring pluton. In the east, Miocene flood basalts [Columbia River Basalt Gp.] overlie and are locally juxtaposed with undated marine metasedimentary units tentatively assigned to the Late Triassic Martin Bridge and Lucile Slate Formations. Mafic to intermediate composition volcanic flows [basaltic-andesite] and volcaniclastic rocks [epiclastic conglomerate + breccia] exposed in the southeast correlate with the Middle to Late Triassic Wild Sheep Creek Formation [Seven Devils Gp.]. Consistent with quadrangles to the north [Kirkwood Creek] and south [Heavens Gate], pre-Cenozoic units in the Kessler Creek quadrangle record widespread penetrative deformation and metamorphism largely in the greenschist facies. Mesoscopic structures include a northerly-striking, subhorizontal to vertical synmetamorphic foliation [S1: defined by chlorite mats, flattened lithic clasts, slaty cleavage] which commonly contains a down-dip to steeply plunging mineral or volcanic clast stretching lineation [L1]. Linear-planar tectonite fabrics are [1] folded by at least two generations of postmetamorphic macroscopic structures, [2] cut by shallow to moderately inclined, northerly-striking intraformational dip-slip faults, and [3] carried by the east- to southeast-dipping regional-scale Heavens Gate fault. At this latitude, the Heavens Gate fault marks the western limit of high strain ductile deformation related to Jura-Cretaceous suturing of the Wallowa terrane with western Laurentia. This project is funded by the EDMAP component of the USGS National Cooperative Geologic Mapping Program.