Volcanic deposits, intermediate
Volcanic deposits, intermediate
middle Pleistocene
Consists of undifferentiated volcanic rocks dispersed across the northern and eastern Sierra, described here from north to south.
Mafic lava flows of low volume occur on the broad western slope of the Sierra, northwest and southwest of Lake Tahoe. These mafic lavas include basalt, olivine basalt, trachybasalt, and latite, and may be correlative with units of the Tahoe-Truckee volcanic field (map unit ttv) to the east.
A small basaltic trachyandesite lava flow is well exposed in the Middle Fork Stanislaus River canyon, south of Hwy 108, near Pigeon Flat. The lava is known as the Columns of the Giants because of prominent columnar joints exhibited by lower parts of the flow. This lava has an Ar-Ar age of 177 ± 7 ka (Farmer and others, 2013) and represents the youngest dated mafic volcanic rock in the region (Busby and others, 2018b).
Varied pyroclastic flow and fall deposits, some reworked, are interbedded with tuffaceous sediments east of Long Valley caldera. These include older distal pyroclastic deposits erupted from the Glass Mountain rhyolite volcanic complex (map unit lvg) and may also include younger material from other (mostly or entirely Long Valley) eruptive sources.