Greenstone member
Greenstone member
Early Triassic, Permian, and Pennsylvanian
Greenschist facies metabasalt, basaltic andesite, agglomerate, and tuff. Association of marble with mafic volcanics near the Stanislaus River led Schweickert and others (1977) to suggest this unit there is part of a large fragment of basaltic seamount. Near the South Fork Cosumnes River north of Fiddletown and the Middle Fork American River, southwest of Foresthill, this unit includes small bodies of ultramafic rock. This unit also occurs in highly deformed lenses within phyllite (unit MRCS) north of Placerville. Together, these rocks north of Placerville form the unit named phyllite-greenschist by Schweickert (2015), who tentatively correlated it with the Don Pedro terrane to the south. However, a marble lens 5 km long in the phyllite north of the South Fork Feather River has yielded Permian to Pennsylvanian conodonts (Hietanen, 1981), so at least that part of the phyllite (and probably the interleaved metavolcanics) is substantially older than the Don Pedro terrane.