Reconnaissance geologic map of the Hyampom 15' quadrangle, Trinity County, California

Abstract

The Hyampom 15' quadrangle lies west of the Hayfork 15' quadrangle in the southern part of the Klamath Mountains geologic province of northern California. It spans parts of four generally northwest-trending tectono- stratigraphic terranes of the Klamath Mountains, the Eastern Hayfork, Western Hayfork, Rattlesnake Creek, and Western Jurassic terranes, as well as, in the southwest corner of the quadrangle, a small part of the Pickett Peak terrane of the Coast Range province. Remnants of the Cretaceous Great Valley overlap sequence that once covered much of the pre-Cretaceous bedrock of the quadrangle are now found only as a few small patches in the northeast corner of the quadrangle. Fluvial and lacustrine deposits of the mid-Tertiary Weaverville Formation crop out in the vicinity of the village of Hyampom. The Eastern Hayfork terrane is a broken formation and m-lange of volcanic and sedimentary rocks that include blocks of chert and limestone. The chert has not been sampled; however, chert from the same terrane in the Hayfork quadrangle contains radiolarians of Permian and Triassic ages, but none clearly of Jurassic age. Limestone at two localities contains late Paleozoic foraminifers. Some of the limestone from the Eastern Klamath terrane in the Hayfork quadrangle contains faunas of Tethyan affinity. The Western Hayfork terrane is part of an andesitic volcanic arc that was accreted to the western edge of the Eastern Hayfork terrane. It consists mainly of metavolcaniclastic andesitic agglomerate and tuff, as well as argillite and chert, and it includes the dioritic Ironside Mountain batholith that intruded during Middle Jurassic time (about 170 Ma). This intrusive body provides the principal constraint on the age of the terrane. The Rattlesnake Creek terrane is a melange consisting mostly of highly dismembered ophiolite. It includes slabs of serpentinized ultramafic rock, basaltic volcanic rocks, radiolarian chert of Triassic and Jurassic ages, limestone containing Late Triassic conodonts and Permian or Triassic foraminifers, and small exotic(?) plutons. The plutons probably are similar to ones to the southeast beyond the quadrangle boundary that yielded isotopic ages ranging from 193 Ma to 207 Ma. The Rattlesnake Creek terrane contains several areas of well- bedded sedimentary rocks (rcs) that somewhat resemble the Galice(?) Formation and may be inliers of the Western Jurassic terrane. The Western Jurassic terrane in the Hyampom quadrangle appears to consist only of a narrow tectonic sliver of slaty to semischistose detrital sedimentary rocks of the Late Jurassic Galice(?) Formation. The isotopic age of metamorphism of the rocks is about 150 Ma, which probably indicates when the terrane was accreted to the Rattlesnake Creek terrane. The Pickett Peak terrane, which is the most westerly of the succession of terranes in the Hyampom quadrangle, is the accreted eastern margin of the Coast Ranges province. It mainly consists of semischistose and schistose metagraywacke of the South Fork Mountain Schist and locally contains the blueschist-facies mineral lawsonite. Isotopic analysis indicates a metamorphic age of 120 to 115 Ma. During the Cretaceous period, much of the southern fringe of the Klamath Mountains was onlapped by sedimentary strata of the Great Valley sequence. However, much of the onlapping Cretaceous strata has since been eroded away, and in the Hyampom quadrangle only a few small remnants are found in the northeast corner near Big Bar. Near the west edge of the quadrangle, in the vicinity of the village of Hyampom, weakly consolidated fluvial and lacustrine rocks and coaly deposits of Oligocene and (or) Miocene age are present. These rocks are similar to the Weaverville Formation that occurs in separate sedimentary basins to the east in the Weaverville and Hayfork 15? quadrangles. This map of the Hyampom 15' quadrangle is a digital version of U.S. Geological Survey Miscellaneous Field Study.