The lower Pliocene Bouse Formation in the lower Colorado River Valley (southwestern USA) consists of basal marl and dense tufa overlain by siltstone and fi ne sandstone. It is locally overlain by and interbedded with sands derived from the Colorado River. We briefl y review 87 Sr/ 86 Sr analyses of Bouse carbonates and shells and carbonate and gypsum of similar age east of Las Vegas that indicate that all of these strata are isotopically similar to modern Colorado River water. We also review and add new data that are consistent with a step in Bouse Formation maximum elevations from 330 m south of Topock Gorge to 555 m to the north. New geochemical data from glass shards in a volcanic ash bed within the Bouse Formation, and from an ash bed within similar deposits in Bristol Basin west of the Colorado River Valley, indicate correlation of the two ash beds and coeval submergence of both areas. The tuff bed is identifi ed as the 4.83 Ma Lawlor Tuff derived from the San Francisco Bay region. We conclude, as have some others, that the Bouse Formation was deposited in lakes produced by fi rst-arriving Colorado River water that entered closed basins inherited from Basin and Range extension, and estimate that fi rst arrival of river water occurred ca. Ma. If this interpretation is correct, addition of BristolBasin to the Blythe Basin inundation area means that river discharge was suffi cient to fi ll and spill a lake with an area of ~10,000 km 2 . For spillover to occur, evaporation rates must have been signifi cantly less in early Pliocene time than modern rates of ~2-4 m/yr, and/or Colorado River discharge was signifi cantly greater than the current ~15 km 3 /yr. In this lacustrine interpretation, evaporation rates were suffi cient to concentrate salts to levels that were hospitable to some marine organisms presumably introduced by birds.

We applied multiple geochemical tracers ( 87 Sr/ 86 Sr, [Sr], d 13 C, and d 18 O) to waters and carbonates of the lower Colorado River system to evaluate its paleohydrology over the past 12 Ma. Modern springs in Grand Canyon reflect mixing of deeply derived (endogenic) fluids with meteoric (epigenic) recharge. Travertine (<1 Ma) and speleothems (2-4 Ma) yield 87 Sr/ 86 Sr and d 13 C and d 18 O values that overlap with associated water values, providing justification for use of carbonates as a proxy for the waters from which they were deposited. The Hualapai Limestone (12-6 Ma) and Bouse Formation (5.6-4.8 Ma) record paleohydrology immediately prior to and during integration of the Colorado River. The Hualapai Limestone was deposited from 12 Ma (new ash age) to 6 Ma; carbonates thicken eastward to ~210 m toward the Grand Wash fault, suggesting that deposition was synchronous with fault slip. A fanningdip geometry is suggested by correlation of ashes between subbasins using tephrochronology. New detrital-zircon ages are consistent with the "Muddy Creek constraint," which posits that Grand Wash Trough was internally drained prior to 6 Ma, with limited or no Colorado Plateau detritus, and that Grand Wash basin was sedimentologically distinct from Gregg and Temple basins until after 6 Ma. New isotopic data from Hualapai Limestone of Grand Wash basin show values and ranges of 87 Sr/ 86 Sr, d 13 C, and d 18 O that are similar to Grand Canyon springs and travertines, suggesting a long-lived springfed lake/marsh system sourced from western Colorado Plateau groundwater. Progressive up-section decrease in 87 Sr/ 86 Sr and d 13 C and increase in d 18 O in the uppermost 50 m of the Hualapai Limestone indicate an increase in meteoric water relative to endogenic inputs, which we interpret to record progressively increased input of high-elevation Colorado Plateau groundwater from ca. 8 to 6 Ma. Grand Wash, Hualapai, Gregg, and Temple basins, although potentially connected by groundwater, were hydrochemically distinct basins before ca. 6 Ma. The 87 Sr/ 86 Sr, d 13 C, and d 18 O chemostratigraphic trends are compatible with a model for downward integration of Hualapai basins by groundwater sapping and lake spillover.The Bouse Limestone (5.6-4.8 Ma) was also deposited in several hydrochemically distinct basins separated by bedrock divides. Northern Bouse basins (Cottonwood, Mojave, Havasu) have carbonate chemistry that is nonmarine. The 87 Sr/ 86 Sr data suggest that water in these basins was derived from mixing of high-87 Sr/ 86 Sr Lake Hualapai waters with lower-87 Sr/ 86 Sr, first-arriving "Colorado River" waters. Covariation trends of d 13 C and d 18 O suggest that newly integrated Grand Wash, Gregg, and Temple basin waters were integrated downward to the Cottonwood and Mojave basins at ca. 5-6 Ma. Southern, potentially younger Bouse basins are distinct hydrochemically from each other, which suggests incomplete mixing during continued downward integration of internally drained basins. Bouse carbonates display a southward trend ...

The Lawlor Tuff is a widespread dacitic tephra layer produced by Plinian eruptions and ash flows derived from the Sonoma Volcanics, a volcanic area north of San Francisco Bay in the central Coast Ranges of California, USA. The younger, chemically similar Huichica tuff, the tuff of Napa, and the tuff of Monticello Road sequentially overlie the Lawlor Tuff, and were erupted from the same volcanic field. We obtain new laser-fusion and incremental-heating 40 Ar/ 39 Ar isochron and plateau ages of 4.834 ± 0.011, 4.76 ± 0.03, ≤4.70 ± 0.03, and 4.50 ± 0.02 Ma (1 sigma), respectively, for these layers. The ages are concordant with their stratigraphic positions and are significantly older than those determined previously by the K-Ar method on the same tuffs in previous studies.Based on offsets of the ash-flow phase of the Lawlor Tuff by strands of the eastern San Andreas fault system within the northeastern San Francisco Bay area, total offset east of the Rodgers Creek-Healdsburg fault is estimated to be in the range of 36 to 56 km, with corresponding displacement rates between 8.4 and 11.6 mm/yr over the past ~4.83 Ma.We identify these tuffs by their chemical, petrographic, and magnetic characteristics over a large area in California and western Nevada, and at a number of new localities. They are thus unique chronostratigraphic markers that allow correlation of marine and terrestrial sedimentary and volcanic strata of early Pliocene age for their region of fallout. The tuff of Monticello Road is identified only near its eruptive source.

Tpd Diatomite and diatomaceous mudstone-The diatom flora in these strata (fossil locality 4) suggest a shallow, eutrophic, neutral to slightly alkalic, stagnant lacustrine setting (S. Staratt, written commun. 2006) Tplg Lignite and lignitic mudstone Tpwd Silicified (petrified) wood Tplm Limestone-Brackish or fresh water depositional setting, locally fossiliferous, present as lenses in the upper parts of basaltic andesite flows and breccias Tprb Breccia-Composed dominantly of angular, tectonically slickened clasts of rhyodacitic rocks with variable textures, interpreted to represent a fault scarp-derived breccia. Breccia includes clasts up to 3m in a comminuted matrix derived from rhyodacitic intrusives, flows and tuffs. Breccia locally includes: Tprg Gravel lenses-Intercalated in breccia, up to ~5m thick, unsorted to poorly sorted, unorganized to weakly segregated and cross-bedded. Gravel is composed of rounded to subangular pebbles to cobbles derived from Franciscan and related Mesozoic sources and from older Tertiary volcanics deposited in alluvial fans, debris flows and talus settings LATE TERTIARY VOLCANIC ROCKS Sonoma Volcanics (Pliocene and Miocene)-Rhyolitic to dacitic ash-flow and air fall tuff, andesitic water-lain tuff, and rhyolitic to basaltic flows and flow breccia. Regionally, the volcanic section becomes increasingly silicic up-section, and youngs from southwest to northeast, across the Rodgers Creek-Healdsburg and Maacama faults (McLaughlin and others, 2005; Fox and others, 1985). The Sonoma Volcanics consist of the following units. Tsd Dacitic flows-Mapped near the northeast corner of the Santa Rosa quadrangle and in the southeast half of the quadrangle along the Rodgers Creek Fault Zone. The dacite lacks macroscopic quartz or K-feldspar, commonly contains plagioclase phenocrysts, and rare hornblende Tsr Rhyolitic and rhyodacitic flows and intrusive rocks-Porphyritic to aphanitic, with phenocrysts of quartz and plagioclase. Includes the rhyodacitic rocks of Zamaroni Quarry (7.26±0.04 Ma), the rhyodacitic rocks of Cook Peak (7.94±0.02 Ma), and perlitic to banded rhyolitic to rhyodacitic flow rocks and obsidian in Annadel State Park (4.5±0.01 Ma) Tst Rhyolitic to dacitic and minor andesitic pumiceous tuff-Mostly ash-flows and minor air fall. This unit includes named and unnamed tephra layers of different ages in the Sonoma Volcanics (see Tephra data Tables 2.2 and 2.3, figure 2.2, and discussion in pamphlet) Tstw Crystal-rich rhyolitic to rhyodacitic welded tuff-Welded zones locally at tops of tuff layers that are overlain by flows of andesite or basalt Tstb Andesitic to rhyodacitic breccia (Pliocene)-Mapped locally in the northeast part of Santa Rosa 7.5' quadrangle, between the 4.83 Ma Lawlor Tuff and overlying basaltic andesite. Breccia consists of angular boulders and blocks of basalt, andesite, and vitric, porphyritic rhyodacite, in a lithic rhyo-dacitic pumiceous tuff matrix. The breccia may be associated with syn-volcanic faulting and (or) proximal pyroclastic venting Tsb Andesite, basaltic ...

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Miocene strata in the Middlegate area, Churchill County, Nevada, consist of the Middlegate Formation unconformably overlain by the Monarch Mill Formation. The Middlegate Formation is about 110m thick and consists of a lower member of fluvial and lacustrine tuffaceous sandstone and conglomerate and an upper member of lacustrine diatomaceous siltstone. Megabreccia landslide deposits are common in the Middlegate Formation. Tephrochronologic studies indicate that the Middlegate Formation is about 15.2 Ma. The Monarch Mill Formation consists of lacustrine, fluvial, and alluvial-fan deposits that are at least 700 m thick in the central part of the Middlegate basin. Coarse, near-source, alluvial-fan deposits of the Monarch Mill Formation interfinger with finergrained fluvial and lacustrine deposits from source areas to the north and east and southeast of the Middlegate basin. Local megabreccia deposits are present in the Monarch Mill Formation in the southern and eastern parts of the basin. Tephrochronologic studies indicate that the Monarch Mill Formation ranges from about 14.7 to 9.8 Ma.The Middlegate Formation is mostly a quiet-water lake deposit that appears to have been deposited in an environment distinctly different from the present-day environment that is dominated by alluvial-fan deposits. It does, however, contain megabreccia deposits, indicative of nearby escarpments that produced far-traveled landslide deposits. The Monarch Mills Formation represents a distinct change in the paleogeographic setting in the Middlegate area. It contains coarse near-source deposits that indicate local high-relief, probably related to the development of faults and topographic relief that led to the presentday structural setting of the Middlegate area. This change occurred at about 14 Ma.