IMPLICATIONS OF ERUPTIVE, EROSIVE, AND DEPOSITIONAL PROCESSES PRIOR TO A SUPER ERUPTION IN THE SOUTHERN BLACK MOUNTAINS
Stratigraphically below the unconformity in the Secret Pass Canyon area of the southern BM lies a sedimentary package, averaging ~100m thick, consisting of brown, immature, volcanic sediments of trachytic composition. Bulk elemental analysis, lithics, and framework mineralogy identify the underlying pre-PST trachyte lavas (18.9 Ma) (Lang et al., 2008) as the likely source. Interbedded within the upper part of the package are basaltic trachyandesite lava flows. The basaltic trachyandesite lava confirms the introduction of increasingly mafic magmas prior to the super eruption (Flansburg et al., 2014). Sediment/lava interactions, including baked sediment at contacts, pillowed lava, and fragments of sandstone engulfed by lava, demonstrate that sedimentation was coeval with the lower pre-PST trachyte lava as well as the basaltic trachyandesite. Rhyolitic volcaniclastic sediments immediately above the unconformity, together with overlying silicic lavas and pyroclastic flows, contrast strongly with the brown sandstone package (Williams et al., 2014). This sequence thus reveals a rapid transition from copious intermediate to hotter mafic magmatism, and then a shift to much more silicic volcanism. Furthermore, the presence of a sedimentary package and lava units underlying an unconformity represents a transition from depositional basin to an erosive surface in this area. The location and age of the unconformity suggest a changing environment intimately connected to the eruption of the PST.