GEOCHEMISTRY OF MIOCENE TUFFS AND TIMING OF FAULTING IN MIDWAY VALLEY, NEVADA

Midway Valley, Nevada, located in the mid-Miocene southern Nevada volcanic field, is underlain by tuffaceous rocks poorly represented in surface exposures. Whole-rock concentrations of Ba, Ca, Ce, Fe, K, La, Mn, Nb, Rb, Sr, Ti, Y, and Zr have been used for tephrostratigraphic correlation of lithostratigraphic units in the area, and the geochemistry of fracture-filling tuffaceous sediment samples further constrains the timing of faulting based on correlations to in situ deposits. Concentrations of Ti and Zr are indicative of magma chemistry and most useful because they are not affected by post-depositional crystallization and alteration processes. All 110 samples were collected from boreholes UE-25 RF #30 and UE-25 RF #32, located near the middle of the valley and intercepting some of the thickest tuff deposits formed before the 11.6-Ma Rainier Mesa Tuff (Tmr) and after the 12.7-Ma Tiva Canyon Tuff (Tpc). Continuously cored sections include the Tmr, pre-Rainier Mesa Tuff bedded tuff (Tmbt1), rhyolite of Comb Peak (Tpk), post-Tiva Canyon Tuff bedded tuff (Tpbt5), and the upper parts of the Tpc. Distinctive compositions or ranges in compositions distinguish thick ignimbrites (Tmr, Tpk, and Tpc) and the bedded tuffs that consist of interstratified ignimbrites, fallout tephra, and redeposited materials (Tmbt1, Tpbt5, and beds near the base of Tmr and Tpk). Many changes in geochemistry result from differences in depositional features and paleosols; some features (including paleosols) do not affect the geochemical trends. These changes in stratigraphic features and compositions document eruption histories of several magmatic sources.

Several boreholes located on the hanging wall side of normal faults intercept fractures (some as much as 2 m wide) filled with tuffaceous sediments. Such deposits originate from dry grain flows derived from the collapse of fracture walls and from fluvial events that poured into fractures. Geochemistry of hanging-wall fracture-filling materials in UE-25 RF #14 and UE-25 RF #17 is consistent with the lower part of Tmbt1, thereby constraining timing of fracture opening and filling. Vertical changes in Ti and Zr concentrations in the fracture-filling materials indicate several apparent compositional cycles that are consistent with intermittent partial filling of fractures.