A late Miocene-early Pliocene chain of lakes fed by the Colorado River: Evidence from Sr, C, and O isotopes of the Bouse Formation and related units between Grand Canyon and the Gulf of California

Abstract

We report strontium isotopic results for the late Miocene Hualapai Limestone of the Lake Mead area (Arizona-Nevada) and the latest Miocene to early Pliocene Bouse Formation and related units of the lower Colorado River trough (Arizona-CaliforniaNevada), together with parallel oxygen and carbon isotopic analyses of Bouse samples, to constrain the lake-overfl ow model for integration of the Colorado River. Sr iso topic analyses on the basal 1-5 cm of marl, in particular along a transect over a range of altitude in the lowest-altitude basin that contains freshwater, brackish, and marine fossils, document the 87 Sr/ 86 Sr of fi rst-arriving Bouse waters. Results reinforce the similarity between the 87 Sr/ 86 Sr of Bouse Formation carbonates and present-day Colorado River water, and the systematic distinction of these values from Neogene marine Sr. Basal Bouse samples show that 87 Sr/ 86 Sr decreased from 0.7111 to values in the range 0.7107-0.7109 during early basin fi lling. 87 Sr/ 86 Sr values from a recently identifi ed marl in the Las Vegas area are within the range of Bouse Sr ratios. 87 Sr/ 86 Sr values from the Hualapai Limestone decrease upsection from 0.7195 to 0.7137, in the approach to a time soon after 6 Ma when Hualapai deposition ceased and the Colorado River became established through the Lake Mead area. Bouse Formation δ 18 O values range from -12.9‰ to +1.0‰ Vienna Pee Dee belemnite (VPDB), and δ 13 C between -6.5‰ and +3.4‰ VPDB. Negative δ 18 O values appear to require a continental origin for waters, and the trend to higher δ 18 O suggests evaporation in lake waters.Sr and stable isotopic results for sectioned barnacle shells and from bedding planes of the marine fi sh fossil Colpichthys regis demonstrate that these animals lived in saline freshwater, and that there is no evidence for incursions of marine water, either long-lived or brief in duration. Lack of correlation of Sr and O isotopic variations in the same samples also argue strongly against systematic replacement of Sr in Bouse carbonates after deposition. Our results reinforce the conclusion that the Bouse Formation was deposited in a descending series of basins connected by overfl ow of Colorado River water. The Hualapai Limestone records a separate and earlier lake that may have been progressively infl uenced by Colorado River water as the time of river integration approached.

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

“…This interpretation was based on: (1) strongly negative δ18O values and a wide range of δ13C; (2) offset of stable-isotope values between carbonate matrix and ostracodes in the marl unit, and a change to uniform values above the DCL; (3) cooccurrence of fresh-water, brackish, and marine organisms; and (4) similarity of isotopic values in fresh-and brackish-water ostracodes. These results are at odds with other studies that document co-variation of southern Bouse stable isotopes in mollusk shells and marl along a mixing trend from isotopically light (δ13C=−4; δ18O=−8) to seawater values (δ13C=+1; δ18O=0) (Roskowski et al, 2010;Crossey et al, 2015). Inconsistent results from different studies suggest a need to standardize sample preparation methods, which may influence interpretation of water chemistry and recognition of primary carbonate versus secondary cements and clay contamination.…”

“…However, subsequent studies have shown that 87Sr/86Sr ratios of Bouse carbonates are actually higher and more variable (0.7101-0.7123) than modern water of the lower Colorado River (0.7100-0.7101) (Roskowski et al, 2010;Crossey et al, 2015). Sr isotopes in Bouse carbonates thus record complex mixing of multiple poorly understood inputs.…”

“…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).…”

“…It is generally agreed that the northern Bouse Formation (Fig. 1) accumulated in a series of lakes that filled with water and sediment of the first-arriving Colorado River (Spencer and Patchett, 1997;House et al, 2008;Roskowski et al, The Bouse Formation has previously been subdivided into a basal carbonate unit, an interbedded unit of mudstone, sandstone, and siltstone, and an age-equivalent basin-margin association of tufa and conglomerate (Metzger, 1968;Buising, 1988Buising, , 1990.We refine this nomenclature and divide the southern Bouse into three informal members ( Fig. 3A): (1) basal carbonate consisting of travertine, bioclastic facies, and fine-grained marl (lime mudstone); (2) siliciclastic member which includes green claystone, red mudstone and siltstone, and Colorado River cross-bedded sandstone; and (3) upper bioclastic member.…”

Evidence for a marine incursion along the lower Colorado River corridor

“…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.…”