Marine sedimentary member
Marine sedimentary member
Triassic, Permian, and Carboniferous
Primarily phyllite and metachert with interlayered limestone and marble. Extends almost the entire length of the western Sierran Nevada, including a narrow band north of Placerville, a broader band roughly between Placerville and Mariposa, and extending as scattered pendants south to the Kern River area.
In the northern band, the unit consists of phyllite and metachert with lesser limestone interbeds. One 5-km long, 50-100 m-thick limestone interbed has yielded Pennsylvanian to Permian conodonts (Hietanen, 1981). Phyllite is quartz with muscovite, biotite, and(or) chlorite. Pebbly layers crop out locally, with pebbles of metachert, quartzite, and calcite. Metachert is recrystallized and lacks preserved fossils. The primary cleavage in this area is tightly folded, resulting in a secondary transecting cleavage. This unit is interleaved with and locally overlain by metavolcanic rocks (unit MRCG). Schweickert (2015) correlated this band with his Don Pedro terrane to the south, but the Paleozoic age of the limestone bed precludes that correlation.
In the central band, the unit is well-foliated argillite, phyllite, schist, and metachert, with marble bodies ranging from small blocks up to interleaved bodies up to 10 km in length (unit MRCC), and localized lenses of serpentinite and talc schist.  Includes the phyllite and chert of Hites Cove and part of the phyllite of Briceburg of Bateman and others (1985).  A small limestone bed in metachert at the south end of the belt near the Merced River yielded Early Triassic conodonts (Bateman and others, 1985). Another limestone nearby was reported to have Pennsylvanian or Permian foraminifers (Clark, 1964), but those collected specimens have been lost and the exact location of sample location is unknown, so that age is not certain and the limestone body could be an olistolith. The central band includes far fewer interleaved volcanic rocks, primarily limited to mafic metavolcanics associated with the large limestone bodies. Schweickert (2015) suggests that all the limestone bodies in this unit are olistoliths, but Bateman and others (1985) indicate that the limestone bed containing the Early Triassic conodonts was interbedded with chert and was not an olistolith. It seems likely, therefore, that at least the southeastern part of this band was deposited in the Early Triassic and incorporates Paleozoic olistoliths. Farther west, the unit locally contains Early Triassic conodonts collected from limestone blocks that are probably olistoliths, perhaps derived from limestone to the east (Bateman and others, 1985). Bateman and others (1985) further suggest that small masses of metagabbro within this unit are hypabyssal equivalents to nearby ~187 Ma volcanic rocks (unit MRDGS), so the unit there would be older than that, but younger than the Early Triassic age of the olistoliths.
In the southern roof pendants, this unit is higher grade, predominantly quartz-biotite schist and quartzite with lesser interbedded marble and metavolcanic rock (amphibolite). In the southernmost roof pendants in this unit, south of the Kaweah River, the metasedimentary rocks are interleaved with and overlain by siliciclastic and volcaniclastic strata assigned by Saleeby (2011) to the Late Triassic to Early Jurassic Kings sequence, but included by us in this unit. Saleeby and others (1987) report a Late Permian Tethyan fauna from an olistolith in the Yokohl Valley pendant. A hornblende diorite dike in the northwest part of this unit in the same pendant has yielded a zircon U-Pb age of 168 Ma (Saleeby and Sharp, 1980). The middle Jurassic age suggests that this part of the pendant could instead be part of the Don Pedro terrane.
This unit is bounded to the east by the Calaveras-Shoo Fly thrust and Feather River terrane and to the west by the Sonora, Melones, Gillis Hill, and Dogwood Peak faults. We differ from the formally accepted age for this unit based on younger fossils found within its accepted extent (Bateman and others, 1985).