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Intra-arc sedimentation and tectonism: Late Paleozoic evolution of the eastern Klamath terrane, California

¹ Department of Geology, Stanford University, Stanford, California 94305

Rocks of the eastern Klamath terrane (Redding section) in northern California record deposition, magmatism, and tectonism in an island-arc setting. Volcanism and sedimentation persisted for more than 220 m.y., spanning Devonian to Middle Jurassic time, and yielding thick successions of water-laid volcanic debris and lavas, with interstratified carbonate and clastic sequences. This study focuses on the upper Paleozoic rocks within the section.

The oldest rocks studied, within the Upper Devonian(?) and Mississippian Brag-don Formation, contain epiclastic debris that was deposited in a submarine-fan setting. Sandstone compositions indicate that quartzose sedimentary and metasedimentary rocks formed the source area for epiclastic debris and that volcanism persisted throughout sedimentation. Transitional shallow-water facies in the upper part of the Bragdon interfinger with the overlying Baird Formation, which contains abundant volcanic debris. Volcanogenic shelf and strandline deposits characterize the lower part of the formation, and volcaniclastic turbidites characterize the upper part. The Baird Formation is locally interfingered with the overlying Lower Permian McCloud Limestone, which records two stages of Early Permian carbonate platform development: early Wolfcampian and late Wolfcampian to early Leonardian. Only the younger of the two platforms is present within the study area, and it contains more distal facies than those preserved to the south. The strata contain abundant fusulinids and corals which provide age constraints for carbonate-platform development and form the basis for regional biogeographic correlation.

The time-transgressive Upper Permian Bollibokka Group (Nosoni and Dekkas Formations) succeeds the McCloud Limestone, comprising thick sequences of volcanogenic debris, intercalated lava, and, in the upper part, lenses of shallow-water limestone. The Upper Permian strata reflect volcanism and rapid subsidence within the are. All upper Paleozoic units were intruded by small gabbroic bodies and diabase dikes which may be partly cogenetic with Upper Permian basalts in the upper Dekkas Formation.

The upper Paleozoic succession records local pulses of magmatism and variable subsidence rates within the eastern Klamath arc. Mississippian epiclastic rocks reflect uplift of the volcanic-arc basement, which contained rocks similar to those now exposed in the adjacent Yreka terrane or Shoo Fly Complex. Pulses in volcanism were restricted to mid-Carboniferous and Late Permian time. Diminished volcanism during the Early Permian allowed development of carbonate platforms. Rapid fluctuations of sedimentation and sub-sidence rates; pronounced lateral facies and age variations; accumulation of thick, volcanic-debris aprons during episodic volcanism; and sporadic persistence of volcanism during apparent lulls characterized deposition within this succession and are common processes in modern volcanic arcs.

The stratigraphic record is composed predominantly of volcanogenic rocks and lesser carbonates, reflecting events and processes within the arc itself. Evidence for nearby basins or landmasses is sparse. The Mississippian epiclastic rocks, however, which were derived from the arc basement, reflect an ultimate sialic crustal source. Comparison of volcanic-arc evolution with the late Paleozoic evolution of continental margin and basinal rocks to the east indicates possible paleogeo-graphic ties between the eastern Klamath volcanic arc and rocks of the Golconda allochthon or the North American continent.

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