Cordilleran Section - 109th Annual Meeting (20-22 May 2013)

Paper No. 5
Presentation Time: 1:30 PM-5:30 PM

UPDATED FIELD MAPPING IN THE LAVA HILLS, SAN BERNARDINO COUNTY, CALIFORNIA


PRICE, Jason B.1, HARVEY, Janet C.1, HAMON, Jennifer L.1 and STOCK, Joann M.2, (1)Division of Geological and Planetary Sciences, California Institute of Technology, MC 100-23, Pasadena, CA 91125, (2)Seismological Laboratory, California Institute of Technology, 1200 E California Blvd, MC 252-21, Pasadena, CA 91125, jprice@caltech.edu

We present an updated field map for ~6 km2of the Lava Hills, San Bernardino County, California (Siberia 7.5’ quad) centered at 34°39.3’ N, 115°58.0’ W. Cretaceous(?) monzogranite and Tertiary extrusive rocks were mapped at 1:10,000. Per Miller (1993), the volcanic section is a moderately southwest-dipping homocline of tuffaceous rocks divided in half by a prominent ridge of andesitic lava. The homocline rests nonconformably upon the granite. It is capped by locally-sourced, angularly unconformable dacite porphyry lava (22.4 ± 0.6 Ma, K/Ar on biotite; Miller, 1993). The volcanic package is unconformably overlain by Quaternary-Neogene cobble conglomerate. At the SW end of the map area, a high-angle, NW-striking fault with NE-side-down puts youngest tilted lapilli- and ash-tuffs against granite. This fault is cut by ENE-striking faults exhibiting left-lateral separation. Another NW-striking fault of unknown slip-sense cuts along the median lava ridge and may have been a conduit for the magma. No faults with definitive dextral motion were observed in the map area.

The Tertiary section comprises (bottom to top): 1. Friable, oxidized, coarse-grained arkose derived from underlying granite; ~6 m thick, 2. Fractured, incoherently-folded lacustrine limestone and minor planar-bedded arkose intercalated with ash- and lapilli-tuff and tuff-breccia; ~50 m thick, 3. Lithic lapilli- and ash-tuff with minor aphyric lava and volcanic breccia; ~90 m thick, 4. Purple hornblende andesite porphyry lava (phapl) with autobreccia; max ~30 m thick, 5. Dark green aphanitic basaltic andesite lava (gabal) that cuts phapl as a dike and also forms capping lava flows on older tuff; max ~50 m thick. Together, phapl and gabal form the prominent median ridge of the map area, 6. Sedimentary basin ~300 m long x 150 m wide x 40 m deep with lacustrine arkoses and water-lain tuffs that formed in a recessive part of the phapl-gabal lavas, 7. Varicolored unit of brick-red, white, brown, and grey lapilli- and ash-tuffs with layers <2 m thick containing minor arkose; ~85 m thick, 8. Tan pumiceous lapilli-tuff with laterally coextensive varicolored ash-tuff; ~275 m thick, 9. An angular unconformity marks the bottom of a volcanic cap rock sequence comprising a local diamictite overlain by reddish-grey dacite tuff-breccia and porphyritic lava; ~40 m thick.