Owens terrane - central Sierra Nevada Crest
Owens terrane - central Sierra Nevada Crest
Permian to Cambrian
Chiefly calc-silicate hornfels, quartzite, quartzofeldspathic hornfels, dark-gray siliceous argillite, and marble. Dominant protoliths are calcareous sandstone with local conglomerate, shale, limestone, and chert, with minor marl, and dolomite. Terrane interpreted as a deep-water, continental-margin assemblage with local gradation into shallow water. Occurs in the Log Cabin, Ritter Range, Mount Morrison, and adjacent roof pendants in the eastern Sierra Nevada. Unit modified from the Owens terrane of Nokleberg (1983), Rush Creek sequence of Green and others (1997), and Morrison block of Stevens and Green (1999). Terrane correlated with broadly similar fossiliferous continental-margin sedimentary strata in the White-Inyo Mountains to southeast and similar rocks in west-central Nevada (Stevens and Green, 1999). Schweickert and Lahren (1993, 2006) correlate this unit with rocks of the Antler orogenic belt (Roberts Mountains allochthon), but Stevens and Green (1999) point out important stratigraphic differences (e.g. lack of metavolcanic rocks in this unit and missing Carboniferous section in the Roberts Mountains section).
In the Mount Morrison roof pendant, the unit consists of two assemblages. The older is a Cambrian to Devonian, submarine fan to deep-water hemipelagic, continental-margin sequence of dark, fine-grained argillite, locally with thin marble interbeds; thick-bedded calcareous sandstone; and black radiolarian chert and argillite. The sequence contains conodonts indicating an age near the Cambrian-Ordovician boundary, Ordovician graptolites, (probably Middle) Devonian conodonts, and Late Devonian conodonts (Stevens and Green, 1999). The younger assemblage is a Mississippian to Permian continental-margin sequence of siliceous hornfels, chert, thick marble, and calc-silicate hornfels; transitioning from shallow water in the lower half to deep water hemipelagic and base of slope in the upper half. The sequence contains reworked Devonian and Mississippian conodonts near the base, Early Pennsylvanian crinoids, corals, brachiopods, and conodonts, Middle Pennsylvanian conodonts, and early Early Permian conodonts.
In other pendants, the lithologies and fossils are similar, although not every subunit is present. In the Bishop Creek pendant, only the Cambrian to Upper Devonian older assemblage, plus the Mississippian lowest part of the upper assemblage, is present. In the Gull Lake pendant, only the upper, Silurian(?) and Devonian part of the older assemblage is present. In the Log Cabin Mine roof pendant, the unit is chiefly older assemblage calcareous sandstone, shale, and chert; minor marl, limestone, and dolomite with Ordovician graptolites (Stevens and Greene, 1999), with only a sliver of younger assemblage mapped. North of Lundy Canyon, this unit is chiefly younger assemblage marble with probably Carboniferous crinoid stems (Chesterman and Gray, 1975).
The older sequence exhibits three generations of penetrative structures and isoclinal folds and faults. Younger sequence exhibits the younger two generations of penetrative structures with moderately appressed folds. Two sequences are generally in fault contact along the NNW-trending Laurel-Convict fault, along which the Laurel-Convict felsite is intruded with a U-Pb zircon age of 225 Ma, part of the Late Triassic and Early Jurassic Scheelite Intrusive Suite (Gsc).
In the northern Ritter Range pendant, the unit is named the Rush Creek sequence (Green and others, 1997). It is composed chiefly of calc-silicate hornfels, siliceous argillite, bedded chert, sandy marble, and calcareous quartzite. This sequence lacks diagnostic fossils but has been correlated with the Ordovician to Devonian part of the early sequence in the Mount Morrison pendant. A nearby lens of fossiliferous marble has yielded crinoids, brachiopods, corals and bryozoans suggestive of Late Mississippian through mid-Permian age (Green and others, 1997). It is correlated with the thick Pennsylvanian marble in the Mount Morrison pendant and thought to be a fault sliver offset from that pendant along the Gem Lake shear zone (Green and others, 1997).
In the Mount Pinchot 15’ Quadrangle, this unit is interbedded marble, calc-hornfels, biotite schist, and pelitic hornfels and quartzite. No recognizable fossils have been collected from this southern area, although similar rocks in Big Pine Canyon to the north (outcrops too small to show at scale) have Early Cambrian fossils (Moore and Foster, 1980).