We've Moved!
Visit SDSU’s new digital collections website at https://digitalcollections.sdsu.edu
Description
The Sierra City mélange comprises the structurally highest and easternmost tectonostratigraphic unit of the Shoo Fly Complex, northern Sierra Nevada, California. A north- to northwest-trending, steeply east- dipping thrust fault separates the mélange from the underlying Culbertson Lake allochthon. To the east, the contact between the mélange and the Upper Devonian Sierra Buttes Formation is an angular unconformity. Inclusions within the mélange are comprised of slabs and lenses of rhythmically bedded, radiolarian chert, volcanic conglomeratic mudstone, pebbly mudstone, siltstone, and sandstone. The inclusions are embedded in a fine-grained, highly deformed, argillaceous matrix. The presence of radiolarian chert with probable gravity flow-deposited clastic inclusions suggests deposition may have occurred in relatively deep water. Sandstone inclusions consist of two distinct and separate types, quartz-rich and plagioclase-rich. Quartz-rich varieties are characterized by a lack of feldspar and a predominance of sedimentary and metamorphic rock fragments. Plagioclase-rich sandstones are distinguished by an abundance of feldspar and volcanic rock fragments. In addition, a Precambrian provenance age for detrital zircons was obtained for quartz-rich sandstones, whereas a 506 ± 22 m.y. old provenance age was obtained for a plagioclase-rich sandstone. These data, the petrologic characteristics, and the point-count results plotted on provenance discrimination diagrams suggest that the quartz-rich sandstones are composed of detritus that was derived from metamorphsed sedimentary rocks and plutonic rock. In contrast, plagioclase-rich sandstones are composed of detritus derived from a magmatic arc source. The Sierra City mélange has undergone at least four phases of deformation. The oldest deformation (D1) probably occurred during post-Cambrian to pre-Late Devonian time, and produced the chaotic fabric of the mélange. Features such as brecciated zones within and along the margins of chert slabs, pinch and swell structures in chert beds and sandstone inclusions, a faint foliation in the matrix that conforms to the shape of the inclusion, and extensional fractures in sandstone inclusions define D1. The second deformation (D2) produced a northwest-trending cleavage (S2cl), and a northwest-trending flattening plane (S2e) and gently southeast-plunging lineation (L2) in clasts in the mudstone units. The third deformation (D3) produced a northeast-trending cleavage (S3). The second and third deformations probably occurred between Early and Late Jurassic time during the Nevadan orogeny. The fourth deformation produced a widely spaced, east-west-trending cleavage, whose age may be Cretaceous, but is poorly constrained. Biostratigraphic, paleomagnetic, and petrologic evidence suggest that the Sierra City mélange formed during post-Late Silurian to pre-Middle Devonian time, and may have been significantly influenced by the Alexander terrane now in southeastern Alaska. The Alexander terrane may have been the source of terrigenous detritus in the Sierra City mélange; however, this does not preclude the North American continent as a source of the terrigenous detritus.
