Ash-flow tuffs and paleovalleys in northeastern Nevada: Implications for Eocene paleogeography and extension in the Sevier hinterland, northern Great Basin

Abstract

Northeastern Nevada is generally interpreted as an area of large-magnitude Eocene extension possibly due to gravitational collapse of crust thickened during the Sevier orogeny. The extensional interpretation is based in part on the presence of widespread Eocene conglomerates and lacustrine basins, as well as on thermochronology-based evidence of major Eocene cooling and uplift of the Ruby Mountains-East Humboldt Range core complex.The distribution of 45-40 Ma ash-fl ow tuffs and interbedded coarse conglomerates and lacustrine deposits, however, indicates they were predominantly deposited in a system of east-draining paleovalleys incised into a plateau or moderate-relief upland. A large, contiguous sedimentary basin probably was never present. Paleovalleys were as much as 10 km wide and 500 m to possibly as much as 1.6 km deep, based on the thickness of intravalley deposits. Ash-fl ow tuffs are widely distributed near source calderas but are almost entirely confi ned to the paleovalleys as little as 20 km from their source. Basal, mostly pre-volcanic conglomerates contain clasts up to 6 m in diameter. The clasts are well rounded, indicating signifi cant fl uvial transport, not derivation from nearby fault scarps. Lacustrine deposits also are restricted to paleovalleys and accumulated during two periods that are interpreted to coincide with episodes of minor, northwest-directed extension, one before 41 Ma and possibly as old as 46 Ma, and another between 40 and 38 Ma. Extension formed small displacement, northeast-striking, mostly down-to-thenorthwest faults that temporarily dammed the paleovalleys to form lakes. Lakes probably also formed where volcanic rocks or landslides dammed paleovalleys, a common process both in the Eocene and historically in the western United States. The absence of major Eocene extension suggests that gravitational collapse of overthickened crust, even assisted by thermal weakening of lithosphere by intense magmatism, was not suffi cient to generate major extension.Absolute elevation of the high plateau is uncertain, but it was high enough to have paleovalleys as much as 1.6 km deep. Based on published paleofl ora data, interfl uves could have been at elevations of ~4 km.The Eocene paleovalleys in northeastern Nevada most likely drained eastward to remnants of the Uinta basin. An approximately north-south paleodivide through northeastern Nevada separated these east-draining paleovalleys from paleovalleys that drained westward to the Pacifi c Ocean.

The effect of drainage reorganization on paleoaltimetry studies: An example from the Paleogene Laramide foreland

“…Results presented here support the thesis that basin development and lake hydrology in the Laramide foreland were characterized by large-scale changes in Cordilleran drainage patterns tapping distal source areas (DeCelles, 1994). Though drainage from the hinterland seems to have been the dominant pattern for much of the Paleogene (Fouch et al, 1983;Gierlowski-Kordesch et al, 2008;Henry, 2008), tectonically mediated drainage rearrangements within the foreland in some cases profoundly influenced the developing Laramide basins and their O isotope records. Unrecognized, such drainage rearrangements might easily confound studies of isotope paleoaltimetry.…”

The Great Basin Altiplano during the middle Cenozoic ignimbrite flareup: insights from volcanic rocks

“…For example, in the northeastern Great Basin area, the Camilleri et al (1997) model states that the crust thinning was from 70 to 50 km by extensional unroofing between the Cretaceous and Eocene-Oligocene, whereas Constenius (1996) concluded that extensional basins formed from the middle Eocene to early Miocene. However, on the basis of his study of interbedded ash-flow tuffs and sedimentary deposits in northeastern Nevada, Henry (2008) concluded that Eocene extension was minor. The well-documented extreme extension found locally in core complexes in the eastern Great Basin (e.g.…”

Eruptive history, geochronology, and post-eruption structural evolution of the late Eocene Hall Creek Caldera, Toiyabe Range, Nevada

“…In contrast, recent work in the Eocene Caetano caldera at the north end of the Toiyabe Range ( fig. 2) also documented large-magnitude (approximately 100 percent strain) extension, but concluded that it took place in the middle Miocene (around 17-16 to 12-10 Ma), long after formation of the 34 Ma Caetano caldera (Colgan and others;2011, Colgan and others;2008, John and others;2008).…”

“…Many of the ash-flow tuffs erupted from the calderas shown in figure 1 dispersed widely across the landscape by flowing along paleovalleys carved into the pre-Tertiary bedrock in the late Mesozoic or earlier in the Cenozoic (for example, Henry, 2008). Their distribution thus informs knowledge of regional topography prior to formation of the modern Basin and Range Province, when the region was thought to be a broad, high orogenic plateau, much of which drained to the Pacific Ocean (for example, DeCelles, 2004;Cassel and others, 2009;Henry and John, 2013).…”

“…18). This cycle of continued partial erosion and redeposition of ash-flow tuffs is a common feature of middle Tertiary paleovalley sequences in the Great Basin (for example, Henry andothers 2008, 2012;Cassel and others, 2009). …”