Paper No. 6
Presentation Time: 10:15 AM
REVISED STRUCTURE OF THE WESTERNMOST SAN JOSE ANTICLINE, SOUTHERN CALIFORNIA
The Quaternary, west-plunging San Jose Anticline (SJA) is shown as a nearly symmetrical and essentially featureless fold exposing the four members of the Puente Formation: Sycamore Canyon (SC; youngest), Yorba (Y), Soquel (SQ), and La Vida (LV). Reevaluation of the geology of the BKK Landfill, which spans the fold axis and south limb, suggests the SJA is highly asymmetrical and south verging. Five interpretations follow, based on sandstone petrography, fault trenching, analysis of core, and synthesis of previous work. 1) The south limb incorporates a secondary, steeply-dipping monoclinal to overturned fold in the lower and middle Y adjacent to a subparallel south-adjacent crowded syncline. The synclinal section, previously mapped largely as Y is now interpreted as SC. This section, possibly several hundred feet thick, is difficult to accommodate within the verging fold at depth without placing overturned Y over shallow dipping SC. 2) The transition at the surface is marked by the south-dipping Blue Streak-Nogales fault (BS-NF), interpreted as a back-thrust antithetical to a system of low-angle, north-dipping or flat, bedding sub-parallel thrusts in the SC section. The BS-NF and the bedding sub-parallel faults appear to be expressions of synclinal crowding, and may represent secondary structures above a north-dipping, blind extension of the San Jose fault. 3) To the north, the Miranda Spring fault (MSF) lies along a similar trend. This fault is north-dipping reverse based on apparent offset of the SQ-LV contact, and forms a groundwater barrier marked by springs. East of the site it has geomorphic definition. To the west it merges with the monoclinal fold and may be imbricate with a deeper blind structure. 4) North of the MSF the continuity of the Y, SQ, and LV sequence is interrupted by northeast trending high- and low-angle faults cutting the north ridgeline. Poorly constrained to the southwest, at the ridgeline these faults affect groundwater, and one, a low-angle fault, separates two chemically distinct groundwater systems. In trend they are similar to the Walnut Creek fault to the west but the sense of motion appears to be mainly reverse. 5) These structures (1 thru 4) apparently die-out near the western boundary of the site; tectonic strain then appears to be expressed in similar structures to the southwest.