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.

[1] Uncertainty remains on the origin of distal mass deposition maxima observed in many recent tephra fall deposits. In this study the link between ash aggregation and the formation of distal mass deposition maxima is investigated through reanalysis of tephra fallout from the Mount St. Helens 18 May 1980 (MSH80) eruption. In addition, we collate all the data needed to model distal ash sedimentation from the MSH80 eruption cloud. Four particle size subpopulations were present in distal fallout with modes at 2.2 F, 4.2 F, 5.9 F, and 8.3 F. Settling rates of the coarsest subpopulation closely matched predicted single-particle terminal fall velocities. Sedimentation of particles <100 mm was greatly enhanced, predominantly through aggregation of a particle subpopulation with modal diameter 5.9 ± 0.2 F (19 ± 3 mm). Mammatus on the MSH80 cloud provided a mechanism to transport very fine ash particles, with predicted atmospheric lifetimes of days to weeks, from the upper troposphere to the surface in a matter of hours. In this mechanism, ash particles initiate ice hydrometeor formation high in the troposphere. Subsequently, the volcanic cloud rapidly subsides as mammatus develop from increased particle loading and cloud base sublimation. Rapid fallout occurs as the cloud passes through the melting level in a process analogous to snowflake aggregation. Aggregates sediment en masse and form the distal mass deposition maxima observed in many recent volcanic ash fall deposits. This work provides a data resource that will facilitate tephra sedimentation modeling and allow model intercomparisons.

Deformed late Cenozoic tuffs in the central Coast Ranges of California have been correlated by means of trace-and minorelement chemistry of volcanic glass, supported by potassium-argon dates, petrographic data, and stratigraphy. Cluster analysis of the chemical data indicates that four orders of chemical variability exist in the trace-and minor-element composition of volcanic glass. The greatest differences are between tephra of silicic and intermediate composition. Considering silicic tephra alone, the greatest differences are observed between tephra erupted in different volcanic provinces. Differences between samples of silicic tephra erupted within the same volcanic field are smaller, while the smallest differences are observed between samples of tephra from individual eruptions. Five widespread tuffs and composite tephra units erupted during a period from approximately 1 to 6 million years ago have been recognized in the study area. These include the tuff in the type section of the Merced Formation, the Putah Tuff Member of the Tehama Formation, the Lawlor Tuff, the Pinole Tuff and the tuff in the Merced(?) Formation of Sonoma County. All except the first were erupted from local central Coast Range sources, probably in the Sonoma volcanic field; the tuff in the type section of the Merced Formation was derived from the southern Cascade Range, about 320 km north of the main study area. Tuff correlations indicate that Suisun Bay and Mount Diablo, in the eastern part of the main study area, were formed less than 4 million years ago and that drainage from the Great Valley of California to the ocean in the vicinity of the San Francisco Bay was established some time between 0.6 and 3.3 million years ago.

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