ORDER WITHIN THE CHAOTIC: FRANCISCAN COMPLEX FIELD RELATIONS SHOW KM-SCALE OVERTURNED FOLDS, AN OLISTOSTROME DEPOSITED on INTRA-FRANCISCAN SERPENTINITE, AND MORE
Coherent sandstone in the field area dips beneath the Nicasio Reservoir terrane, is moderately foliated and contains minor clasts of serpentinite and trace sodic amphibole. Sandstone-matrix mélange in the study area is minimally strained, indicating a pre-tectonic sedimentary origin as a submarine slide mass, and contains basalt blocks up to 0.7 km and chert blocks up to 0.1 km in exposed diameter. Additionally, the present study revealed serpentinite clasts within breccia and sandstone near the base of the mélange, indicating an overturned depositional contact on serpentinite. Two olistostromes were distinguished in the study area based on field relationships and differences in sandstone composition. Sandstone olistostrome in the westernmost portion of the study area comprises mostly volcanic lithics with little feldspar and monocrystalline quartz, is more weathered, and exhibits more pressure solution between grains than sandstone olistostrome to the east; the eastern olistostrome contains more sedimentary lithics and a larger proportion of quartz and feldspar than its western counterpart. Contact relationships with the serpentinite, sandstone bedding, and serpentinite foliation indicate that the serpentinite and adjacent units are folded into multi-kilometer scale isoclinal overturned folds trending NW-SE. By contrast, regional scale contact relationships show that the Nicasio Reservoir terrane appears to be folded into a multi-kilometer scale open synform trending NW-SE. Multi-meter scale tight folds trend NE-SW; their relationship to larger scale folds is unclear.