NEW INSIGHTS ON THE MANHATTAN CALDERA AND CORRELATIVE 25.1 MA OUTFLOW TUFF OF GABBS VALLEY UNIT 1, WESTERN NEVADA VOLCANIC FIELD

The Manhattan caldera (Shawe, 1995, 1998, 1999) lies in the southern Toquima Range and intracaldera tuff of the Round Rock Formation (RRF) has not previously been correlated with any outflow tuffs. From new field studies, the lower member of the RRF is here correlated with the 25.1 Ma lower unit (Tgv1) of the three-part “tuffs of Gabbs Valley” (TGV) of Ekren and Byers (1986). Tgv1 is a crystal-poor, very distinctive, and exceptionally pumice- and lithic-rich ash-flow tuff that crops out from the Paradise Range to Carson City, NV, and likely in the southern Toiyabe Range (Brem et al., 1991) and southern Shoshone Mountains (Whitebread et al., 1988). Correlation is based on distinctive megascopic textures, consistency of the vertical sequence of diverse textures between outflow and intracaldera tuff, phenocryst content, pumice characteristics, and lithic content and lithology, including large granitic clasts in Tgv1 that match clasts in intracaldera tuff, derived from the Round Mountain and Belmont plutons on the margins of the caldera. Correlation is supported by the roughly linear WNW-trending distribution of Tgv1 to the west of the caldera that overlaps with the overall distribution trend of outflow tuffs erupted from the Toquima caldera complex.

In the Monte Cristo Mountains, Ekren and Byers included six regional tuffs in Tgv1 with ages from 31.5–27.3 Ma. Because the name TGV includes multiple unrelated tuffs that correlate with named regional tuffs (Law, 2023), it is suggested that the name TGV be abandoned. The informal name “tuff of Manhattan” is here given to Tgv1, the outflow tuff of the Manhattan caldera.

Shawe mapped multiple megabreccia units in the RRF and interpreted them as erupted material from within the caldera (Shawe and Snyder, 1988). Megabreccia clasts in the middle member of the RRF are mainly ash-flow tuff and andesite lava that Shawe inferred as genetically related to the Manhattan caldera magma system. Field observations from this study indicate that the ash-flow tuff clasts consist of various pre-25.1 Ma tuffs: primarily lower tuff of Mt. Jefferson, as well as tuff of Round Mountain, tuff of Campbell Creek, and Nine Hill Tuff. The middle member megabreccia is here interpreted as a caldera wall collapse deposit that formed during caldera floor subsidence following eruption of the outflow tuff of Manhattan.