SUBAQUEOUS TUFFS IN THE EARLY MIOCENE VOLCANIC SEQUENCE, BLACK MOUNTAINS, AZ, AND SACRAMENTO MOUNTAINS, CA
Sequences in both ranges include pale, fine-ash-sized layers and coarse-ash to lapilli-sized layers whose field characteristics suggest that they may be tuffs (both are matrix supported and have aphanitic matrices). The pale-colored, fine-grained layers are most prominent in the Black Mountains sequence, where they are massive in hand sample but laminated on cm-dm scale and up to 15 m in total thickness. The fine-grained rocks are interbedded with the coarser rocks, which have grains that we interpret as phenocrysts. Both of these lithologies are interbedded with 2-3 cm sandstone layers and in some areas with carbonates (McCosby et al 2015).
Petrography and geochemistry support the interpretation that both the fine and coarser lithologies are tuffs. They have very sparse (fine-grained rocks) to abundant (coarser rocks) phenocrysts of feldspar, biotite, and accessory minerals set in altered, aphanitic matrices. Elemental compositions of the coarser rocks are dacites and trachydacites. Fine-grained rocks indicate they were rhyolites, considerably more silicic than the lavas that preceded PST (70-72 wt% SiO2, ~14% Al2O3, 3-4% Fe2O3(t)+MgO, 7-9% Na2O+K2O). Compositions of the coarser rocks partially overlap with the lavas but extend into the rhyolite range (65-72 wt% SiO2, 12-15% Al2O3, 2-6% Fe2O3(t)+MgO, 6-10% Na2O+K2O).
We interpret the sequence of tuffs to be fallout deposited at least in part in a lacustrine environment. The tuffs document numerous small, explosive, silicic eruptions that mark a departure from the thick, less silicic lavas that characterize the pre-PST sequences that both underlie and overlie the tuff-sedimentary rock sequence.