Geology of the Franciscan Complex Central Belt Between Redwood Valley and Potter Valley, Mendocino County, California

Between Redwood Valley and Potter Valley a range of hills rises 1000' above the valley floors. Three units have been mapped and described in these hills, which were previously mapped as undifferentiated rocks of the Central belt of the Franciscan Complex. Lower elevations on the east, south and west are underlain by Franciscan Complex Central belt melange (CBM). The melange matrix of sheared and folded shale and sandstone contains disconnected blocks of harder graywacke, greenstone, chert, and minor blueschist. The central higher part of the hills is underlain by two map units, a mafic igneous unit (MIU) and a tan sandstone unit (TSU), which together cover 40% of the 30 sq. km. mapped. Three km north of the study area a well-exposed faulted contact dipping 45 degrees NE separates CBM on the southwest from MIU. The MIU is a continuous igneous body of varied texture. Phaneritic outcrops are altered olivine gabbronorite with ophitic to subophitic textures. Aphanitic rocks have an intersertal texture, are occasionally vesicular, and occur as pillows, flow-banded fragmental deposits and mesocopically featureless outcrops. X-ray fluorescence spectroscopy (XRF) data on igneous rocks from MIU and CBM yield Ti/Zr ratios within the MORB field (Pearce and Cann, 1973). A few samples fall outside this field with greater zirconium concentrations. Most samples from both sets plot in the ocean floor basalt field of the Ti-Zr-Y ternary diagram, with some individuals plotting in the calc-alkaline and within-plate basalt fields. The tan sandstone unit is light tan, friable, massive or bedded, with Q/Q+F+L = 32 ± 8% (2 standard deviations) and F/Q+F+L = 37 ± 11% (terminology after Dickinson, 1970). CBM graywackes are dark gray or greenish, well indurated, with Q/Q+F+L = 54 ± 9% and F/Q+F+L = 21 ± 8%. CBM graywacke outcrops are usually too small to map, unlike the TSU. CBM exposures range from highly fractured to unfractured. Textures range from featureless to foliated. Original sedimentary structures are not common although contorted and disrupted pods and bands of sandstone occur with shale within the Central belt melange matrix and may reflect original layering. Two chert bodies enclosed by CBM yielded Pliensbachian - Toarcian to Kimmeridgian - Berriasian radiolarians. Two chert bodies surrounded by MIU and TSU yielded upper Jurassic to Albian radiolarians (Isozaki, 1988). On the basis of similar lithology and radiolarian chert ages, the MIU and TSU with associated chert resemble The Geysers and the Marin Headlands terranes. Study area rocks do not show the regular imbricate structure preserved at Marin Headlands. Accretion at a convergent margin may have caused the imbricate structure of the Marin Headlands terrane while subduction of ocean floor rocks of the study area caused fragmentation of individual units and inclusion of the fragments in the Central belt melange.