Shoo Fly Complex, Sierra City mélange
Shoo Fly Complex, Sierra City mélange
earliest Late to latest Early Devonian
Composed chiefly of disrupted blocks of serpentinite, gabbro, pillowed and massive basalt, chert, sandstone, pebbly mudstone, limestone breccia, and bioclastic limestone in a sheared matrix of slate, sandstone, and chert. A wide variety of sedimentary clasts occur in the mélange, including subarkose and quartz-rich sandstone, conglomeratic mudstone with volcanic clasts, pebbly mudstone, and chert. Blocks of limestone in the mélange have yielded Late Ordovician and Middle to Late Ordovician conodonts and Late Ordovician corals and brachiopods (Harwood, 1992). A chert block has yielded Ordovician to Silurian radiolarians (Varga and Moores, 1982). A tuff in the mélange matrix has yielded a zircon U-Pb age of ~423 Ma (Late Silurian; Saleeby and others, 1987). A sandstone block in the mélange has a detrital zircon MDA of ~395 Ma (Grove and others, 2008) which provides a maximum age for the formation of the mélange. The mélange is unconformably overlain by the Late Devonian Grizzly and Sierra Buttes Formations (part of unit NSSB) and intruded by ~358-369 Ma hypabyssal volcanic rocks (Powerman and others, 2020), so formed prior to deposition and emplacement of those rocks. The Sierra City mélange is truncated southward by the downcutting unconformity and is not present south of Bowman Lake.
This unit is interpreted as part of a subduction complex (Girty and Pardini, 1987) that formed in the Devonian, perhaps at the same subduction zone related to the unconformably overlying and intruding Sierra Buttes arc (units NSSB and Gsb). The mélange occurs east of, and structurally overlies, the coherent units of the Shoo Fly Complex (unit NSSA).