MID-MIOCENE PALEOGEOGRAPHY OF THE SIERRA NEVADA–BASIN AND RANGE TRANSITION AT THE TYPE LOCALITY OF THE LOVEJOY BASALT

We present a new interpretation of the stratigraphy and geologic history for the type locality of the Lovejoy basalt, located at Red Clover Creek north of the town of Portola, California. The Lovejoy basalt is a distinctive, aphanitic olivine basalt that erupted from possible vents along the present-day Honey Lake fault of the Walker Lane shear zone, and flowed through a paleovalley across the northern end of the Sierra Nevada to the Sacramento Valley in northern California. The 150 km³ basalt was erupted in a tectonic setting transitional between Basin and Range, hot spot and subduction volcanism at approximately 16 Ma, coeval with basalts of the Columbia River, the Oregon Plateau, and the northern Nevada rift.

At the type localty, the Lovejoy basalt forms a >120 m thick sequence of at least eight lava flows that display a characteristic irregular jointing. The lower seven flows are aphanitic, while the uppermost flow contains conspicuous phenocrysts of plagioclase and olivine, and locally garnet xenocrysts. We interpret the unconformable contacts between the Lovejoy basalt and other Miocene strata at the type locality to be the result of complicated paleotopography created by fluvial erosion of the Lovejoy basalt. We observed no evidence of syndepositional or post-depositional faulting at the type locality. Erosion of the basalt created a steep sided, E-W trending paleocanyon with locally undercut walls. The canyon was subsequently filled by a series of polylithic, plag-pyx phyric, andesite-dominated lahars with interstratified monolithic block and ash flow deposits. This polylithic unit is interpreted to have overtopped the paleocanyon walls and spread out over the upper surface of the Lovejoy basalt as a flat sheet. This sheet is in turn overlain by one or more landslide megabreccia wedges derived from several distinct Oligocene to Early Miocene ignimbrites. The polylithic unit and landslide megabreccia are in turn overlain by a section of monolithic hornblende andesite block and ash flow deposits and lahars.

Our new interpretation of the mid-Miocene volcanic stratigraphy of the Red Clover Creek area reconciles the existing map and geochronologic observations, and demonstrates that significant topography existed during mid-Miocene time.