Screwauger Breccia is named for exposures on the southwest slope of Duncan Peak and in the adjacent upper reaches of Screwauger Canyon (sec.9,10,15,16, T15N R13E, Duncan Peak 7.5' quad), Placer Co, CA. Is a chaotic (type I melange) unit composed primarily of broken beds of quartzite in a black pelite matrix. Mappable blocks of gray ribbon chert, fine- to coarse-grained massive orthoquartzite, and quartz-feldspar granule conglomerate occur sporadically in the unit. Total thickness is unknown, but maximum exposed thickness is at least 300 m. Overlies the Antoine Canyon Formation (new) [locally?]; underlies the Big Valley Bluff Formation (new). Age is Ordovician(?) to Devonian(?) based on regional stratigraphic relationships.

Source: GNU records (USGS DDS-6; Menlo GNULEX).