GEOLOGIC MAP OF THE POLLOCK MOUNTAIN 7.5-MINUTE QUADRANGLE, WEST-CENTRAL IDAHO

The Pollock Mountain 7.5-minute quadrangle located in the Salmon River Mountains of western Idaho, contains metamorphic assemblages thought to record Mesozoic island arc accretion onto the western margin of Laurentia. Miocene Columbia River Basalts cap prominent peaks (Pollock Mountain, Ant Butte) in the southeastern portion of the quadrangle. Normal faults with ~400 m of displacement are observable by vertical offset of the base of the Columbia River Basalt. Underlying the basalt in the eastern portion of the quadrangle, the Pollock Mountain thrust sheet contains the Mesozoic Pollock Mountain amphibolite and intercalated orthogneisses, and is bound to the west by the Pollock Mountain thrust fault. The Pollock Mountain thrust fault separates northeast-southwest striking, southeast dipping midcrustal rocks in the hanging wall from north-south striking rocks of the Riggins Group in the footwall. The Riggins Group is internally dissected by numerous thrust faults and is bound to the west by the Rapid River thrust fault that places the Riggins Group onto the Lucile Formation which contains graphitic phyllite and minor marble lenses. Underlying the Lucile Formation are the rocks of the Seven Devils Group. Marbles previously assigned to the Martin Bridge Limestone are interbedded with greenstones and are mapped as Triassic marbles and greenstones of the Doyle Creek Formation, respectively, which also contains lower greenschist facies metamorphosed clastic sequences. The Doyle Creek Formation overlies greenstones and monomictic volcaniclastic breccias of the Triassic Wild Sheep Creek Formation. The Permian Hunsaker Creek Formation is exposed in the northwestern-most corner of the quadrangle and contains quartz-bearing, polymictic volcaniclastic breccias that are interbedded with greenstones. The Seven Devils Group is intruded by the Echols Mountain pluton (123 Ma) in the southwest portion of the quadrangle which is cut along its eastern boundary by the Rapid River thrust fault. The deformation fabric observed in the Seven Devils is absent in the Echols Mountain pluton, which suggests that greenschist metamorphism and deformation in the Seven Devils Group concluded pre-123 Ma; however, thrust faults to the east cut the Echols Mountain pluton suggesting thrust fault deformation continued post-123 Ma.