THE IMPORTANCE OF TERTIARY VOLCANICS IN UNDERSTANDING THE GEOLOGIC HISTORY OF THE NORTHERN SIERRA NEVADA MOUNTAINS, CALIFORNIA

The in-progress Portola 30x60’ compilation map in the northern Sierra Nevada features Tertiary volcanics sourced from both local and distant regions. These deposits span the early Pliocene to Oligocene, with periods of greater activity between ~ 10-14 Ma, and ~ 21-30 Ma. Although geochronological data is limited, geologic map relations highlight their importance in constraining the timing of tectonic events that have altered the landscape.

Sierra Valley, a large basin in the Portola quadrangle, formed as extensional faulting initiated presumably during the Miocene. Large, inactive volcanic complexes, possibly coeval with the arrival of extensional faulting, surround the valley and range in age from ~ 10-13 Ma. Sierra Valley is filled by lacustrine and alluvial sediments, and well logs in deeper parts of the valley record deposits that indicate episodic volcanic activity. While these tuffs and flows have not been dated or correlated, they likely originate from the volcanoes around Sierra Valley. It’s possible for the deepest tuffs to correlate to Oligocene/early Miocene rhyolitic tuffs found west of the valley. Some of these exposed tuffs are similar to an ash-flow tuff in Nevada, which would require much lower relief in the region, and correlating them across the Portola quadrangle could provide more constraint to the timing of extensional faulting.

The Bonta and Penman Formations, mapped extensively throughout the Portola area, consist mostly of reworked volcanics thought to originate from the complexes around Sierra Valley. However, these units are not uniform in age or lithology, and other sources cannot be ruled out. Extrusive domes west of the Mohawk Valley Fault Zone may be related to these units or possibly the source of rhyolitic tuffs. Previously mapped as Pliocene or Quaternary flows, several of these domes have been sampled for radiometric dating to help clarify their relation to other volcanic deposits in the region.

The Lovejoy Basalt, a unit of significance throughout northern California, was first described at Lovejoy Creek ~ 20 km east of Quincy. Its origin is further north near Susanville, and its path follows paleo drainages southwest across the Sacramento Valley. Chemically the Lovejoy is similar to the Columbia River Basalt Group and overlaps in age (16 Ma), though how they are related is speculative.