DRILL CORE DATA FROM THE WARNER RANGE AND SURPRISE VALLEY

The Warner Range/Surprise Valley fault system in northern California represents the northwesternmost expression of active Basin and Range extension. A thick, 3-km succession of volcanic, volcaniclastic, and sedimentary units (base not exposed) is present in the range, with units tilted WSW at 10 – 35 degrees, due to significant rotation and dip-slip motion along the east-dipping Surprise Valley (SV) normal fault. Drill core data from an exploratory geothermal well to the north of Lake City, CA, cuts the SV fault system and elucidates the details of deeper stratigraphic units not exposed in the range. The well intersected the footwall of the SV fault at ~390 m, where abundant hydrothermal alteration and fault zone deformation are present, suggesting that the fault dips as little as ~30 degrees, consistent with the westward tilt of units in the footwall. From 390 m to 1030 m depth, the core transects units in the footwall that consist of massive volcanic breccias (lahars), andesitic(?) lava flows, volcaniclastic sandstones and reworked ignimbrites, and pebbly volcaniclastic mudstones. This sequence is similar to the units exposed in the Warner Range and indicate little variation in geologic environment during deposition of more than 3.5 km of volcanic and volcaniclastic lithologies. A significant change occurs at 1030 m depth where mudstones, siltstones, arkosic sandstones, and clast-supported pebble and boulder conglomerate beds are present. The conglomerates contain well-rounded granitiic, volcanic, metasedimentary and sedimentary cobbles providing evidence for uplift and erosion of pre-Tertiary basement This clastic, non-volcanic sequence extends to a depth of 1300 meters, below which aphanitic and porphyritic basalt dikes or flows dominate, with an intervening 45 m section of arkosic sandstone. The drill core bottomed in basalt at ~1500 m, indicating that the total Tertiary section is >4.5 km thick. The depth to pre-Tertiary basement remains unknown, but the core extends the previously known record of volcanism and sedimentation further back into the Tertiary and has important implications for the paleogeography of the northern Sierra-Klamath Mountain-western Basin and Range region at the end of the Cretaceous and into the early Cenozoic.