SEQUENCE, AGE, AND SOURCE OF SILICIC FALLOUT TUFFS IN MIDDLE TO LATE MIOCENE BASINS OF THE NORTHERN BASIN AND RANGE PROVINCE

The latest Cenozoic (<6 Ma) ash beds in the western United States have been intensively studied for several decades. The more widespread of these ash beds are well-documented event horizons that are of great va lue in studies of the timing and pace of geological, climatological, a nd biological events throughout the region, Because explosive volcanis m was not restricted to latest Neogene time in this region, many older ash beds are likely to prove as useful as younger beds as event horiz ons, once they are located, characterized, and dated, As a first step in developing a useful chronology of older Cenozoic ash beds in the we stern United States, we have sampled and analyzed silicic fallout tuff s in middle to late Miocene sedimentary basins across the northern Bas in and Range province. The northern Basin and Range basins, ideally si tuated in the vicinity of major coeval silicic volcanic centers, conta in numerous relatively unaltered, silicic fallout tuffs, We have corre lated tuffs between all sampled sections on the basis of glass shard c omposition, The composite stratigraphic sequence established by the co rrelations contains more than 200 individual tuffs, including 59 widel y distributed tuffs termed correlative tuffs, The tuffs vary widely in composition, but most are in one of two compositional groups: gray me taluminous vitric tuffs (Gm tuffs) or white metaluminous vitric tuffs (Wm tuffs), Distribution patterns, compositional characteristics, and correlation with ash-flow tuffs show that the source for most Gm tuffs was the Snake River Plain volcanic province along the northern edge o f the northern Basin and Range, and the source for most Wm tuffs was t he southwestern Nevada volcanic field in the southern part of the nort hern Basin and Range, The northern Basin and Range tuffs range in age from ca, 16-6 Ma, The ages of individual tuffs are determined variousl y by direct isotopic dating, by correlation to previously dated fallou t and ash-now tuffs, or by interpolation age estimation, Ages for most tuffs are known to within 0.25 m.y. (1 sigma) or less and for many tu ffs to within 0.1 m.y. or less, The sequence and ages of tuffs establi shed in this study provide insights into the evolution of the northern Basin and Range basins and patterns of explosive volcanism in coeval volcanic centers, and contribute to the development of a high-resoluti on stratigraphy and chronology of coeval sedimentary deposits througho ut the western United States.