CORRELATION OF DISTAL ~13-10 MA COUGAR POINT TUFF UNITS IN KNOLL BASIN, NORTHEASTERN NEVADA, USING MAJOR MINERAL ASSEMBLAGES

Knoll basin is a Miocene extensional basin adjacent to the Bruneau-Jarbidge volcanic center, which erupted at least 10 large-volume ash-flow tuffs known collectively as the ~13-10 Ma Cougar Point Tuff (CPT). The welded CPT XIII unit has been identified by tephrochronology in the eastern part of the basin where it marks the southeastern limit of the unit. Our new mapping reveals the presence of at least two stratigraphically-separated, heretofore unidentified, welded units of the CPT along the western margin of the basin, and another unidentified unit on the east side.

Welded units of the CPT look similar and hence preclude correlation based on hand-sample characteristics. However, previous studies of the CPT indicate that some units contain unique major-mineral assemblages. For example, the CPT XIII unit contains fayalite and augite. We utilized a petrographic microscope and SEM-EDS analysis to assess major minerals in the CPT XIII and unidentified units in Knoll basin to determine if they contain mineral assemblages that would permit correlation to specific units. Our analysis reveals that the upper tuff on the west side of the basin contains the diagnostic assemblage fayalite and augite indicating it is correlative with CPT XIII. The older, lower tuff on the west side contains pigeonite and augite indicating that it is not CPT XIII. On the east side of the basin, our analyses indicate that two samples of the known CPT XIII contain augite, but no fayalite. The unidentified CPT tuff on the east-side of the basin appears to be stratigraphically lower than the CPT XIII and contains augite, but no fayalite suggesting that this unit is also CPT XIII and has been repeated by faulting. The presence of fayalite in CPT XIII on west side, and its absence on the east side, of the basin is surprising. Because east-side samples are more distal to the eruptive center than those on the west, we postulate that the absence of fayalite in the east may be an artefact of mechanical sorting during emplacement, due to its markedly higher density than augite.

In summary, testing for diagnostic mineral assemblages is useful for correlating CPT XIII between isolated outcrops, and can aid in assessing repetition of units due to faulting. Future studies will include more detailed sampling of welded tuffs to confirm the presence or absence of sorting of major phases.