Named as one of four new formations of the newly named Riggins Group. Type section is an exposure along Salmon River north of Riggins from 0.1 to 1.7 mi north of Goff Bridge, Idaho Co, ID, Idaho Mountains province. Named for Lightning Creek, T25N, Rs1 and 2E, Idaho Co. Overlies Fiddle Creek Schist (new) and underlies Squaw Creek Schist (new) [See Below], both of Riggins Group. Mappable southeastward to area about 4 mi east of Riggins where metamorphic grade becomes higher than at type. Upper contact with Squaw Creek questionable south of Salmon River where both formations are high-grade metamorphic rocks. Is about 8,000 ft thick on northeast limb of Riggins syncline. Composed of metavolcanic schists whose composition included mafic to silicic basalt and spilite, andesite and keratophyre, dacite, rhyodacite and quartz keratophyre. The middle- to low-grade rocks are fine-grained greenschist. Agglomerates and fine-grained tuffs were present in the original sequence. Metamorphic grade increases eastward and those rocks are gray quartz-plagioclase schist and gneiss whose dominant mafic minerals are biotite and hornblende. Many of the layers are conspicuously laminated. Was originally a sequence of massive volcanic rocks and of sediments derived directly from them. Is older than Idaho batholith. Shown as Paleozoic or Mesozoic age on geologic map.
[NOTE: The Squaw Creek Schist is in the process of being formally renamed, in accordance with the Department of the Interior's Secretarial Order 3404, issued on November 19, 2021, that identifies the term “squaw” as derogatory.]

Source: GNU records (USGS DDS-6; Denver GNULEX).

Some medium- and high-grade metamorphosed volcanic rocks mapped east of the Salmon River area are mapped as metamorphic units whose relative age and correlation are unknown; these rocks are tentatively correlated with the Seven Devils Group; they are shown as Permian and Triassic age; they are also mapped as present in two plates--the Rapid River and North Fork plates--of the Wallowa terrane. These metamorphosed rocks were assigned to the Riggins Group and mapped as one of its formations--Squaw Creek Schist [See Below], Lightning Creek Schist, Berg Creek Amphibolite, or Fiddle Creek Schist, depending on locality. There is no evidence found during this study to indicate that rocks of the Riggins and the Wallowa are different island arcs. Also, all identifiable map characteristics useful for distinguishing the formations have been obliterated by multiple episodes of metamorphism and deformation. The mapped units cannot be correlated with or assigned to individual formations with confidence. Therefore, the name Riggins and its component formations are not recognized in this study.
[NOTE: The Squaw Creek Schist is in the process of being formally renamed, in accordance with the Department of the Interior's Secretarial Order 3404, issued on November 19, 2021, that identifies the term “squaw” as derogatory.]

Source: GNU records (USGS DDS-6; Denver GNULEX).