Mid-Miocene paleogeography of the Sierra Nevada-Basin and Rangetransition at the type locality of the Lovejoy Basalt

Abstract

We present a new interpretation of the stratigraphy and geologichistory for the type locality of the Lovejoy Basalt, located at RedClover Creek north of the town of Portola, California. The LovejoyBasalt is a distinctive, aphanitic olivine basalt that erupted frompossible vents along the present-day Honey Lake fault of the WalkerLane shear zone, and flowed through a paleovalley across the northernend of the Sierra Nevada to the Sacramento Valley in NorthernCalifornia. The 150 km (super 3) basalt was erupted in a tectonicsetting transitional between Basin and Range, hot spot and subductionvolcanism at approximately 16 Ma, coeval with basalts of the ColumbiaRiver, the Oregon Plateau, and the northern Nevada Rift. At the typelocality, the Lovejoy Basalt forms a >120 m thick sequence of atleast eight lava flows that display a characteristic irregularjointing. The lower seven flows are aphanitic, while the uppermostflow contains conspicuous phenocrysts of plagioclase and olivine, andlocally garnet xenocrysts. We interpret the unconformable contactsbetween the Lovejoy Basalt and other Miocene strata at the typelocality to be the result of complicated paleotopography created byfluvial erosion of the Lovejoy Basalt. We observed no evidence ofsyndepositional or post-depositional faulting at the type locality.Erosion of the basalt created a steep sided, E-W trending paleocanyonwith locally undercut walls. The canyon was subsequently filled by aseries of polylithic, plag-pyx phyric, andesite-dominated lahars withinterstratified monolithic block and ash flow deposits. Thispolylithic unit is interpreted to have overtopped the paleocanyonwalls and spread out over the upper surface of the Lovejoy Basalt as aflat sheet. This sheet is in turn overlain by one or more landslidemegabreccia wedges derived from several distinct Oligocene to earlyMiocene ignimbrites. The polylithic unit and landslide megabreccia arein turn overlain by a section of monolithic hornblende andesite blockand ash flow deposits and lahars. Our new interpretation of themid-Miocene volcanic stratigraphy of the Red Clover Creek areareconciles the existing map and geochronologic observations, anddemonstrates that significant topography existed during mid-Miocenetime.

29 samples from this reference