NEOGENE SEDIMENTARY RECORD OF PALEOGEOGRAPHY ALONG THE EASTERN EDGE OF THE SIERRA NEVADA FROM 11 TO 3 MA: VERDI BASIN, WESTERN NEVADA

The Verdi basin sedimentary section was deposited on a heterogenous and tectonically active basement. The sediments overlie intermediate volcanics of Miocene age at both the southern (Carson Range) and northern (Peavine Mountain) edges of the basin. Sediments lie on crystalline basement in several places in the basin center. Coarse fanglomerates at the base of the section record uplift of granitic basement on local faults at the onset of basin history at about 11 Ma. Facies patterns and paleocurrent indicators suggest that the original north-south extent of the basin was not much greater than that of the preserved section today.

Lahars of intermediate composition and basaltic andesite flows are locally interbedded with sediments at the base of the section, constraining its age. There are no volcanic units higher in the section, but air-fall tuffs and pumice-rich pebble conglomerates record continuing regional and local volcanic activity.

Paleocurrent data and distribution of facies record a progressive eastward shift of the depocenter with time, but no intrabasin faulting. Coarse fanglomerate and fluvial deposits at the base of the section grade upward into deltaic sediments and then into a thick lacustrine section. Sandy fluvial and deltaic facies overlie the lacustrine section in most places, indicating that the basin had filled by 3 to 2.5 Ma. Mammal fossils and air-fall tephras date the middle and upper section. Dramatic incision, rapid burial, abrupt change to coarse fluvial conglomerate, and a shift in paleocurrent directions signal a late phase of tectonism that buried the earlier basin sediments at about 2.5 Ma.