Research Paper GEOSPHERE Freshwater plumes and brackish lakes: Integrated microfossil and O-C-Sr isotopic evidence from the late Miocene and early Pliocene GEOSPHERE; v. 14, no. 4, p. XXX–XXX Bouse Formation (California-Arizona) supports a lake overflow https://doi.org/10.1130/GES01610.1 model for the integration of the lower Colorado River corridor 16 figures; 1 table; 1 set of supplemental files Jordon Bright1, Andrew S. Cohen1, David L. Dettman1, and Philip A. Pearthree2 CORRESPONDENCE: [email protected] 1Department of Geosciences, University of Arizona, 1040 E. 4th Street, Tucson, Arizona 85721, USA 2Arizona Geological Survey, 1955 East 6th Street, Tucson, Arizona 85704, USA CITATION: Bright, J., Cohen, A.S., Dettman, D.L., and Pearthree, P.A., 2018, Freshwater plumes and brack- ish lakes: Integrated microfossil and O-C-Sr isotopic evidence from the late Miocene and early Pliocene Bouse Formation (California-Arizona) supports a lake overflow model for the integration of the lower Colo- rado River corridor: Geosphere, v. 14, no. 4, p. 1875– ABSTRACT for the microfossil and isotopic complexities observed in this southern Bouse 1911, https://doi.org/10.1130/GES01610.1. Formation data set. A freshwater plume model is entirely consistent with fill- Uncertainty over the depositional environment of the late Miocene and and-spill models for the downward integration of the early Colorado River. early Pliocene Bouse Formation hinders our understanding the evolution of Science Editor: Raymond M. Russo Associate Editor: Nancy Riggs the lower Colorado River corridor. Competing marine and lacustrine models for the origin of the southern Bouse Formation remain extremely difficult to INTRODUCTION Received 31 August 2017 reconcile after nearly 60 yr of study. This paper compares new microfossil Revision received 18 February 2018 data, inorganic and biologic carbonate d18O and d13C values (relative to Vienna The evolution of the Colorado River is a crucial link in understanding Accepted 27 April 2018 Pee Dee Belemnite), and carbonate and fish bone87 Sr/86Sr ratios from north- the late Cenozoic tectonic evolution of the southwestern United States (e.g., Published online 8 June 2018 ern and southern outcrops of the Bouse Formation. The lacustrine northern Blackwelder, 1934). Large portions of the Colorado River watershed were at Bouse Formation and the contested southern Bouse Formation share a core sea level ~65 m.y. ago, but since then, the Rocky Mountains and the Colorado Cyprideis (mixed marginal marine), Limnocythere (continental), and Candona Plateau have been uplifted ~3 and 1.5 km, respectively (Pederson et al., 2002; (continental) ostracode assemblage, indicating similar environmental con- Karlstrom et al., 2012). Late Cenozoic extension in the western United States ditions. Micrite and ostracode valves from both areas yield nearly identical lowered the Basin and Range Province, and strike-slip displacement along the d18O and d13C values, suggesting similar origins. Ostracode valves from both San Andreas fault opened the Gulf of California (e.g., Karig and Jensky, 1972; areas document a large and abrupt shift from high d18O values (–2‰) to low Dickinson, 2002). The late Cenozoic saw a significant reorganization of water- values (–10‰), consistent with fill-and-spill lacustrine origins. Tests of the sheds in the southwestern United States (e.g., Cather et al., 2012) in response planktic foraminifer Streptochilus from a southern outcrop yielded d18O and to a change in regional base level. Exotic Colorado River gravels appear be- OLD 13 18 13 G d C values that are nearly identical to benthic ostracode d O and d C values. yond the western end of the modern Grand Canyon after 6 Ma (Lucchitta, Recognition of benthic Streptochilus weakens a categorically marine interpre- 1972; Spencer et al., 2001), providing clear evidence that an early Colorado tation for the southern Bouse Formation. Barnacle shell fragments at a key River had reached the southwestern United States. The river then integrated outcrop of the southern Bouse Formation that preserves sigmoidal bedding a southward course across the chaotic landscape of the southeastern Basin OPEN ACCESS with possible spring-to-neap tidal bundling yielded low d18O values (–8‰ ± and Range Province (Faulds et al., 2008) before eventually reaching the Gulf 1‰) that are incompatible with calcification in seawater. The 87Sr/86Sr ratios of California ca. 5 Ma (Dorsey et al., 2007, 2011; Crow et al., 2017). Understand- from co- occurring fish bones (0.71104) and ostracode valves (0.71100) and the ing the evolution of the Colorado River system is of critical importance when surrounding micrite (0.71086) reveal an isotopically complex lacustrine dep- considering the timing and rates of uplift, faulting, and subsidence within its ositional environment for the southern Bouse Formation. A model invoking watershed (Lucchitta, 1979; Karlstrom et al., 2007); it is the common thread freshwater plumes from the early Colorado River into either a terminal or a tid- that ties much of the late Cenozoic tectonic history of the southwestern United This paper is published under the terms of the ally influenced, mildly brackish lake followed by an abrupt transition to a fresh- States together. How the Colorado River system evolved is the topic of in- CC-BY-NC license. water lake provides a comprehensive and internally consistent explanation tense and long-standing debate (Ranney, 2014), particularly the sequence of © 2018 The Authors GEOSPHERE | Volume 14 | Number 4 Bright et al. | Freshwater plumes and brackish lakes Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/14/4/1875/4265689/1875.pdf 1875 by guest on 26 September 2021 Research Paper events that finally led to the lower Colorado River reaching the Gulf of Califor- Several of the southernmost exposures of the basal limestone unit in the nia (e.g., Spencer and Patchett, 1997; McDougall and Miranda Martínez, 2014; Blythe basin have been described in detail by Homan (2014) and O’Connell Pearthree and House, 2014; Dorsey et al., 2018). A highly contested record of (2017), who documented a variety of carbonate sediments, including lime- the processes that integrated the lower Colorado River corridor is preserved stone, calcarenite, and several marl variations. We prefer to use a broader as the Bouse Formation. “basal carbonate unit” designation for these sediments, following Homan In this paper, we present microfossil assemblages, d18O-d13C values from (2014). Dorsey et al. (2018) referred to these sediments as the “basal carbonate inorganic calcite and biologic calcite (ostracodes, barnacles, foraminifers), and member.” The thickness and details of the Bouse Formation vary from out- 87Sr/86Sr ratios from inorganic calcite, ostracode valves, and fish bone. The data crop to outcrop, with the most conspicuous differences being that the capping presented were collected at one outcrop of Bouse Formation in the Cheme- bioclastic unit and an enigmatic mixture of marginal marine and freshwater huevi basin (i.e., northern Bouse Formation, hereafter “NBF”; Fig. 1), and at fossils (e.g., Reynolds and Berry, 2008; McDougall and Miranda Martínez, 2014) four Bouse Formation locations in the Blythe basin (i.e., southern Bouse For- have been found only in the Blythe basin. Otherwise, the overall physical char- mation, hereafter “SBF”; Fig. 1). This sampling strategy took advantage of the acteristics of the Bouse sedimentary package are noticeably similar through- uncontested lacustrine origin of the NBF to provide a template against which out lower Colorado River corridor. to compare the contested marine, estuarine, or lacustrine origin for the SBF. Outcrops of the NBF are discontinuously exposed in the Cottonwood, Mo- The nearly 100 km between the northernmost and southernmost SBF outcrops have, and Chemehuevi basins (Fig. 1; Spencer et al., 2008) at elevations as allowed us to test for an isotopic gradient along a possible Bouse estuary (e.g., high as 550–560 m above sea level (m asl) in the Mohave and Cottonwood Smith, 1970; Crossey et al., 2015). This multi-indicator approach combined a basins before stepping down across the Topock bedrock paleodivide (Fig. 1) to robust sampling scheme embedded within an improved understanding of a maximum elevation of ~370 m asl in the Chemehuevi basin (Fig. 1; Pearthree Bouse Formation stratigraphy (e.g., Homan, 2014; Gootee et al., 2016a; O’Con- and House, 2014). The SBF is confined to the Blythe basin and has a maximum nell, 2017) to further test the marine, estuary, and lacustrine models for the elevation of ~330 m asl (Fig. 1; Spencer et al., 2008, 2013). Sediments described origin of the SBF. as Bouse Formation have been encountered in the subsurface of the Blythe basin to depths of at least 200 m below sea level (Metzger and Loeltz, 1973; Metzger et al., 1973), and the Bouse Formation may also be present several BACKGROUND hundred meters below the surface near Yuma, Arizona, (Fig. 1), where it over- lies older Miocene marine rocks (Olmsted et al., 1973; McDougall, 2008). Geology and Geography of the Bouse Formation The late Miocene and early Pliocene Bouse Formation (House et al., 2008; Age of the Bouse Formation Sarna-Wojcicki et al., 2011; Harvey, 2014; McDougall and Miranda Martínez, 2014, 2016; Crow et al., 2017) is discontinuously exposed in four sequential Several volcanic ashes provide a maximum age estimate for the NBF. The basins located along a nearly 300-km-long stretch of the lower Colorado