Lovejoy Basalt
Lovejoy Basalt
middle Miocene
Tholeiitic flood basalt consisting of a series of voluminous lava flows erupted from Thompson Peak, in the northern Sierra, at ~15.4 Ma (Garrison and others, 2008). Most flow units are glassy and aphyric, but the uppermost flow unit occurs sporadically and contains conspicuous plagioclase phenocrysts (Coe and others, 2005; Garrison and others, 2008). Paleomagnetic remanence directions generally form a tight cluster of reversed polarity distinct from the mean field direction indicating eruption within a relatively short time period (Coe and others, 2005). This is the most widespread and far-travelled lava flow documented within California, and is referred to as “California’s only flood basalt” (Garrison and others, 2008); it flowed ≥ 240 km westward across the Great Valley to Putnam Peak in the Coast Ranges (Wagner and others, 2000), and is traceable in the Sacramento Valley subsurface via wells and negative aeromagnetic anomalies (van den Berge, 1968; Langenheim and others, 2014). It is coeval with ancestral Cascade arc volcanic rocks (map unit aca) but is compositionally distinct, with major- and trace-element characteristics of the coeval Columbia River Basalt Group to the north in northern Nevada, Oregon, and Washington (Garrison and others, 2008).