CORRELATION OF DISTAL ~13-10 MA COUGAR POINT TUFF UNITS IN THE BIG DEVILS TABLE-SALMON FALLS CREEK AREA, KNOLL BASIN, NE NEVADA, USING MAJOR MINERAL ASSEMBLAGES

Knoll basin is a Miocene extensional basin adjacent to the Bruneau-Jarbidge volcanic center, which erupted at least nine 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 and typically contains augite as the only ferromagnesian mineral. Our new mapping in the NW portion of Knoll Basin, the Big Devils Table area, reveals the presence of six welded units of the CPT. These units are interbedded with clastic sediment of the Humboldt Formation and are informally referred to as units 1 through 6.

The goal of our research was to correlate units 1-6 to known eruptions of the CPT using the identification of major minerals. Major minerals were extracted from samples by crushing, sieving, and heavy liquid separation. Mineral identification was done using a petrographic microscope and a SEM with an EDS attachment. Our preliminary results indicate that two of the nine known eruptions of the CPT, CPT XII and CPT III, are not represented in the six units because they do not contain minerals diagnostic of those units. The stratigraphically highest and youngest unit, Unit 1, correlates to CPT XIII because of the occurrence of augite as the only ferromagnesian mineral. Units 2 and 3 contain the same mineral assemblage as Unit 1 and are also correlated to CPT XIII, indicating these units have been repeated by normal faulting. This correlation is corroborated by the occurrence of Paleozoic sedimentary and Mesozoic granitic lithic fragments present in units 1, 2, 3, and their absence in other units. The sample from unit 4 did not have enough ferromagnesian grains to conduct an accurate analysis. Units 5 and 6 both contain augite and pigeonite indicating they correlate to two of the following older CPT units: XI, X, IX, VII, and V. Augite in units 5 and 6 show increasing magnesium and decreasing iron content, typical of older CPT units.

In conclusion, testing for diagnostic mineral assemblages is useful for correlating CPT XIII between isolated outcrops. In turn, these correlations allow for recognition of faulting in areas with abrupt sedimentary facies changes and similar looking tuffs.