Logtown Ridge Formation
Logtown Ridge Formation
early Late to late Middle Jurassic, Oxfordian to Callovian
Chiefly interbedded massive volcanic breccia metamorphosed to greenstone. Local augite porphyry andesite, pillow lava, and volcanic breccia. Local massive sills or flows of plagioclase-augite porphyry greenstone. Also includes volcanogenic gritstone and pebbly mudstone with lesser silty slate and tuff. Units form a nearly vertical homocline with tops toward the east, structurally repeated multiple times. Interfingered with overlying Mariposa Formation (CMMS). The western part contains more non-volcanic detritus, suggesting a western source of limestone, amphibolite, chert, and granite. Chert clasts in the western part of the unit contain Late Permian radiolarians derived from chert blocks at the base of the underlying Brandon Formation (unit CMBS). Formed in an island arc setting.