Review and analysis of the age and origin of the Pliocene Bouse Formation, lower Colorado River Valley, southwestern USA

Abstract

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.

Punctuated Sediment Discharge During Early Pliocene Birth of the Colorado River: Evidence from Regional Stratigraphy, Sedimentology, and Paleontology

“…Deposition of the southern Bouse Formation spanned about 1.5Myr, from ~6.3 to 4.8Ma. This chronology differs from previous interpretations that the Bouse Formation accumulated in b50,000 years (Spencer et al, 2013) during the time window defined by analytical uncertainty of plagioclase 40Ar-39Ar ages in the Lawlor Tuff of northern California (4.834 ± 0.011 Ma; Sarna-Wojcicki et al, 2011). Our data do not support such a short duration for Bouse deposition.…”

“…20B; e.g., Spencer and Patchett, 1997). The inland-lake hypothesis has gained support fromstudies of carbonate geochemistry, Sr isotopes, C and O isotopes, stratigraphy, and hydrologicmodels (e.g., Poulson and John, 2003;House et al, 2008;Roskowski et al, 2010;Spencer et al, 2008Spencer et al, , 2013Pearthree and House, 2014;Bright et al, 2016).We suggest that some of these datasets should be reevaluated in light of new constraints presented in this paper. The marine-embayment hypothesis (this study) is supported by multiple lines of evidence including micro-and macro-paleontology, Sr, O, and C isotopes, chemical mixing models, process sedimentology, and Fourier transform analysis of tidal rhythmites (Table 3).…”

“…Previous studies inferred that the Colorado River post-dates the 4.83-Ma Lawlor Tuff, because the tuff is overlain by limestone that was assumed to correlate to Bouse basal carbonate (e.g., Spencer et al, 2013;Harvey, 2014). Until recently there was no reason to question this assumption because the known Bouse Formation included only a thin basal carbonate, an interbedded siliciclastic unit, and age-equivalent travertine (e.g., Metzger, 1968;Buising, 1990).…”

Evidence for a marine incursion along the lower Colorado River corridor

“…Except for a disputed outcrop near the Laguna Diversion and Imperial Dams (sample locality J219 of Winterer, 1975; see also Olmsted et al, 1973, fi g. 15 therein), the Bouse Formation is identifi ed only in the subsurface in the Yuma area of Arizona McDougall , 2008a;Spencer et al, 2013). Numerous core holes and wells that encountered the Bouse Formation in the subsurface of the Blythe basin were summarized previously Olmsted et al, 1973;P.J.…”

“…The lacustrine origin suggests deposition of the Bouse Formation in a series of lakes that fi lled and spilled from one basin to the next until system was integrated and the Colorado River was a through-fl owing river (House et al, 2005a(House et al, , 2005bSpencer et al, 2005Spencer et al, , 2008. Arguments in favor of a lacustrine origin of the Bouse Formation are based on isotopic analyses (Spencer and Patchett, 1997;Buising, 1990;Poulson and John, 2003;Roskowski et al, 2010;Spencer et al, 2013), step-like maximum elevations of the Bouse paleolakes suggesting that no uplift or southward tilting has occurred since deposition of the unit (Spencer et al, 2008(Spencer et al, , 2013, and sedimentological evidence of fl oodwater infl ux derived from northern sources immediately preceding Bouse Formation deposition in the Mohave and Cottonwood Valleys Spencer et al, 2013). An alternative interpretation suggests that the Bouse Formation accumulated in lakes along the northern part of the lower Colorado River corridor, but in the Blythe basin, the formation was initially deposited in a marine environment at the northern end of the proto-Gulf of California (Buising, 1988(Buising, , 1990.…”