DISENTANGLING MIOCENE VOLCANIC UNITS FROM THEIR JURASSIC-CRETACEOUS BASEMENT AT BASALT HILL, SAN LUIS RESERVOIR, MERCED COUNTY, CALIFORNIA

The Basalt Hill Complex (BH Complex) includes hypabyssal basalts and basaltic lava flows (olivine-pyroxene-iddingsite basalt with typical trachytic and felty textures), as well as pyroclastic rocks (lahars?) where basaltic lava fragments are mixed with blueschist (glaucophane-muscovite-quartz schist of fine crystallinity), of Franciscan Complex origin, and sedimentary rocks of the Great Valley Sequence (GV Sequence). The GV Sequence within the field area is of the Upper Cretaceous Chico Group and consists of deposits of gradational marine shales, lithic sandstones and coarse conglomerates (consisting of turbidite and channel sequences). The stratigraphic and structural relations of the BH Complex to the underlying “basement” of the Jurassic Franciscan Complex and the Cretaceous GV Sequence (GV Sequence) are complicated by faulting. Our mapping and previous work suggests that the GV Sequence was in fault contact with the Franciscan Complex (Mesozoic Coast Ranges Thrust) and was later cut by the Neogene Ortigalita Fault, which in turn controlled the development of a steep-sided valleys where GV Sequence and Franciscan rocks were exposed (now marked by thin paleo-soils and stream deposits rich in quartz). It was over this paleo-topography that the basaltic flows and pyroclastic rocks of the BH Complex accumulated. The core of the BH complex was probably formed by a fissure eruption that evolved into a “typical” cinder cone. Unlike “typical” cinder cones, the BH cone sat atop a landscape of steep valleys, which provided the right conditions for the development of lahars or avalanche deposits. The pyroclastic deposits are unique in that they contain a noticeable amount of Franciscan Complex (e.g., glaucophane schist) and GV Sequence rock fragments (e.g., Sierran granite and metamorphic rocks). It is because of these fragments that we favor a rock avalanche origin to the bulk of the entangled pyroclastic deposits.