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Rocks of the Stuart Fork Formation, previously considered part of the central metamorphic belt of the Klamath Mountains, California, are correlative with unnamed rocks of the adjacent western Paleozoic and Triassic belt. Originally designated by Hershey in 1901 as the Abrams Formation of Precambrian (?) age, the type Stuart Fork rocks in the Minersville Quadrangle represent western Paleozoic and Triassic belt rocks exposed in a large window through an overlying thrust plate of central metamorphic belt units (Salmon and Grouse Ridge Formations). The Stuart Fork window or fenster, which occupies the core of a major north-south antiformal fold nearly 20 mi. long, lies 7-10 mi. east of the western belt.
Correlation of Stuart Fork rocks with those of the western belt is based on lithologic similarities (rhythmically bedded metacherts, slates and phyllites, basic metavolcanic rocks, and marbles, in decreasing order of abundance), mutual tectonic separation from overlying Salmon hornblende schists of higher metamorphic grade, and similar metamorphic and structural histories. It is probable that additional mapping of Stuart Fork rocks in the northern Cecilville and Coffee Creek Quadrangles will show them to be continuous with the western Paleozoic and Triassic terrane. Fossils collected in the western belt form limestone southeast of Cecilville have not yet been identified, but limestones in the same belt to the south have yielded Pennsylvania (?) to Triassic faunas. The age of Salmon an Grouse Ridge rocks, also considered to be Precambrian (?) by Hershey, which structurally overlie Stuart Fork--western belt rocks, is still unknown.
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