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

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.