Integrated Microfossil and OC-Sr Isotopic Evidence from the Late

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 brackish 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

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 River 5.51 ± 0.08 Ma Conant Creek Tuff (or possibly the 5.59 ± 0.05 Ma Wolverine corridor (Fig. 1; Spencer et al., 2013). The Bouse Formation was initially de- Creek tephra) is exposed ~60 m below NBF sediments in Cottonwood basin scribed in detail by Metzger (1968), Metzger and Loeltz (1973), and Metzger (Fig. 1; House et al., 2008; Pearthree and House, 2014). Preliminary dating et al. (1973) and can be broken out into three main units: (1) dense tufa de- of another volcanic ash higher in the section but still below NBF sediments posits found around basin edges that directly and disconformably overlie in Cottonwood basin produced a younger maximum age estimate of 5.35 ± coarse-grained Miocene fanglomerates or bedrock; (2) a fossiliferous basal 0.07 Ma (Crow et al., 2017). Thus, the early Colorado River was forming NBF limestone unit that varies from 1 to 30 m thick; and (3) an overlying siliciclastic- lakes in the Cottonwood basin no earlier than 5.6 Ma, and most likely Bouse dominated interbedded unit composed of clay, silt, and fine sand that can be a deposition there began after 5.3 Ma. few tens of meters thick to over 200 m thick. A newly named “upper limestone The age of the SBF is a topic of debate. Radiometric ages on basalt flows member” (Dorsey et al., 2018) or “upper bioclastic unit” (Gootee et al., 2016b) found in late Miocene gravels stratigraphically below the SBF show deposition caps the Bouse Formation. This capping unit corresponds to one of Metzger started after 9.2 ± 0.3 Ma (Reynolds et al., 1986; Buising and Beratan, 1993). et al.’s (1973) older alluvium bodies, “Unit A.” The origin of this capping unit A foraminifer-based biochronology implies that the basal carbonate unit was is beyond the scope of this paper, but it is thought to represent either a sec- being deposited before 6 Ma in marine or estuarine conditions (McDougall and ond, preferably marine flooding event (Dorsey et al., 2018) or an offlapping Miranda Martínez, 2014, 2016; Miranda-Martínez et al., 2017). The minimum mixture of stranded Bouse Formation sediments and local tributary sediments age of 6 Ma is derived from a single test of the planktic foraminifer Globorota- that prograded toward the basin axis as the water level of the southern Bouse lia lenguaensis, which has a global last appearance datum of 6 Ma (Wade et al., basin lowered (Gootee et al., 2016b). 2011). McDougall and Miranda Martínez (2014, 2016) hypothesized that marine

117° W 116° W 115° W 114° W 113° W 37° N 37° N

CANADA ARIZONA

GWT USA Las Vegas Colorado River 36° N MX NE 36° N CALIFORNI VADA Cottonwood basin (550 m)

A Py N Mohave basin (560 m) 35° N 35° N 100 km Figure 1. Regional map showing the relationship of Cottonwood, Chemehuevi Mohave, Chemehuevi, and Blythe basins along the lower Colorado Tk River corridor and in relation to North America (inset). Extents of A Chemehuevi basin (370 m) paleolakes that deposited the northern Bouse Formation are shown in darker blue. Extent of water body that deposited the southern Ay Bouse Formation is shown in light blue. Yellow dots are outcrop lo- Parker cations used in this study. Black bars with two-letter identifiers are paleodivides: Py—Pyramid, Tk—Topock, Ay—Aubrey, Ch—Chocolate Mountains. White stars are other locations mentioned in the text or figures: A—Amboy, BP—Buzzard Peak, BM—Big Maria quarry, C— 34° N 34° N BM Cibola, IM—Isla Montague, LS—Laguna Salada, SMG—Split Moun- tain Gorge. MX—Mexico, GWT—generalized location of Grand Wash Trough. Base figure is from the Global Multi-Resolution Topography Blythe basin (330 m) Salton Sea (GMRT) Synthesis (Ryan et al., 2009), http://www.geomapapp.org. Milpitas C Wash Hart Mine Wash, Marl Wash BP SMG Ch 33° N 33° N ARIZONA

USA Yuma MEXICO

LS USA MEXICO 32° N 32° N Pacific Ocean IM

Gulf of California

117° W 116° W 115° W 114° W 113° W

conditions persisted until the arrival of the early Colorado River water and southward progression by the advancing terminus of the early Colorado River sediments ca. 5.3 Ma, when the overlying siliciclastic interbedded unit was de- (e.g., House et al., 2008; Pearthree and House, 2014). This lacustrine interpre- posited in a saline lake environment. This hybrid marine to lake interpretation tation is based largely on carbonate d18O-d13C values and radiogenic carbonate will be referred to as a “partial marine model.” The 4.83 ± 0.01 Ma Lawlor Tuff 87Sr/86Sr ratios (Buising, 1988; Spencer and Patchett, 1997; Poulson and John, (Sarna-Wojcicki et al., 2011) provides the youngest age constraint on the SBF. 2003; Spencer et al., 2008; Crossey et al., 2015), by a freshwater faunal as- The Lawlor Tuff outcrops are within the basal carbonate unit but are high on semblage that lacks marine fossils (Reynolds, 2008; McDougall and Miranda the landscape at ~300 m asl (e.g., Spencer et al., 2013; Miller et al., 2014), which Martinez, 2014; Reynolds et al., 2016; Roeder and Smith, 2016), and by abrupt would imply that the SBF persisted until 4.8 Ma. This interpretation is sup- decreases in maximum paleowater surface elevations between basins (Spen- ported by the upstream age constraints for the NBF (e.g., House et al., 2008; cer et al., 2008; Pearthree and House, 2014). The fill-and-spill lacustrine inter- Crow et al., 2017), if the SBF was lacustrine. Dorsey et al. (2018) proposed that pretation is convincingly supported by catastrophic paleoflood deposits to the the Lawlor Tuff is within or just beneath their upper bioclastic member and, south of the Pyramid paleodivide (Fig. 1; House et al., 2008). Cloos (2014) and thus, postdates the deposition of the basal carbonate unit in the valley axis by Kimbrough et al. (2015) demonstrated that Laramide-aged detrital zircons from ~0.5 m.y. In Dorsey et al.’s (2018) interpretation, the Lawlor Tuff was deposited the upper Colorado River watershed were present in the first-arriving Colorado during a younger marine inundation of the Blythe basin. River sands deposited in the early Gulf of California, further supporting a “top- Similar discrepancies between radiometric age estimates and paleontolo- down” fill-and-spill model for lower Colorado River integration. gy-based age estimates for the onset of basin flooding exist at other locations The depositional environment of the SBF continues to be debated, with the in the Gulf of California. For example, Carreño and Smith (2007) and Helenes competing marine, estuarine, and lacustrine interpretations carrying import- et al. (2009) hypothesized that the earliest marine rocks in the central Gulf of ant consequences for the geomorphic and tectonic evolution of the surround- California (not shown on Fig.1) are middle Miocene in age, based on microfos- ing area (e.g., Lucchitta, 1979; Pearthree and House, 2014; Miller et al., 2014; sil content. Biostratigraphy coupled with radiometric dating of volcanic rocks Dorsey et al., 2018). Southern Bouse Formation carbonates share numerous on Isla Tiberón showed that marine deposition in the central Gulf of California similarities in their isotopic and trace-element compositions with the NBF (Ta- started after 6.2 ± 0.2 Ma (Bennett et al., 2015). The middle Miocene microfos- ble 1). Several tracers, such as their radiogenic 87Sr/86Sr ratios, are statistically sils were apparently reworked into younger marine strata (Bennett et al., 2015). indistinguishable (Table 1), which is consistent with similar origins. Previous Surface exposures of the Bouse Formation, regardless of the basin, are workers have argued that these similarities are entirely the result of postdepo- deeply incised and overlain by sands and gravels of the early Pliocene (ca. 4.5 Ma sitional alteration (e.g., Lucchitta et al., 2001). Efforts to determine the degree of to 3.5 Ma) Bullhead Alluvium (Howard et al., 2015). These deposits provide the diagenetic alteration of Bouse carbonates are ongoing (Ferguson et al., 2017). first unequivocal sedimentological evidence for a through-flowing early Colo- The partial marine model of McDougall and Miranda Martínez (2014, 2016), rado River (House et al., 2008; Pearthree and House, 2014; Howard et al., 2015). and as featured in Dorsey et al. (2018), relies heavily on foraminifer assemblages to hypothesize that the basal carbonate unit was deposited in a marine embayment or estuarine environment that was in existence for at least 0.7 m.y. Depositional Environment of the Bouse Formation: before the arrival of the early Colorado River. This model raises two critical Previous Interpretations concerns. Late Miocene marine 86Sr/86Sr ratios (0.709) have not been found in SBF carbonates (Spencer and Patchett, 1997; Spencer et al., 2013; Crossey The present consensus is that the NBF represents a series of freshwater et al., 2015), and a source of water capable of creating a Bouse estuary in the to slightly brackish-water lakes that were sequentially filled-and-spilled in a absence of the early Colorado River has not been identified.

TABLE 1. COMPARISON OF AVERAGED OXYGEN (δ18O) AND CARBON (δ13C) VALUES, 87Sr/86Sr RATIOS, AND TRACE-ELEMENT CONCENTRATIONS FROM NORTHERN AND SOUTHERN BOUSE FORMATION CARBONATES

18 13 δ O δ C MgCO3 Mn Fe Sr Ba Zn (‰,VPDB) (‰,VPDB) 87Sr/86Sr (mol %) (ppm) (ppm) (ppm) (ppm) (ppm) Northern Bouse Formation –7.9 –1.6 0.71084 1.17 300933 429136 4.2 Southern Bouse Formation –6.6 –1.7 0.71095 0.57 3941017325 341 3.0 p value 0.034 0.790 0.240 0.029 0.940 0.800 0.270 0.019 0.150 n 197 197 74 27 27 27 27 27 27 Note: Statistically signiﬁcant (p value < 0.05) results are highlighted in bold. Data were compiled from Spencer and Patchett (1997), Poulson and John (2003), Roskowski et al. (2010), Crossey et al. (2015), and Bright et al. (2016). VPDB—Vienna Pee Dee Belemnite.

Crossey et al.’s (2015) estuary model uses carbonate d18O-d13C values and spp.; Bright et al., 2016), and freshwater diatoms (e.g., Cocconeis diminuta, 86Sr/86Sr ratios to hypothesize that a downstream-propagating and progres- Pseudostaurosira brevistriata; Miller et al., 2014). The morphology of the abun- sively evolving mixture of radiogenic, strontium-rich, upstream water sources, dant basin-margin travertine (tufa) in the Blythe basin has recently drawn including the early Colorado River, entered the Blythe basin, where it may comparison with lacustrine travertine at Pyramid Lake, Nevada, (Crossey et have encountered seawater. Crossey et al. (2015) showed that this water could al., 2017). Abrupt downstream decreases in the maximum elevations of Bouse produce the radiogenic 87Sr/86Sr ratios (0.710–0.712) observed in the basal car- deposits across paleodivides between the Mohave, Chemehuevi, and Blythe bonate unit, even if 5% to 75% of the water in the Blythe basin was seawater. basins (Pearthree and House, 2014) lend regional support to the overspilling In this model, the early Colorado River was present to deliver that water. The lacustrine model. Physical evidence for a lake spill-over event at the Pyramid hypothesis that the Colorado River was present during deposition of the basal paleodam (Fig. 1), in the form of a southward-directed fluvial-boulder con- carbonate unit is a significant departure from the partial marine model dis- glomerate containing boulders sourced in the Pyramid hills some 7 km to the cussed previously (e.g., McDougall and Miranda Martínez, 2014, 2016; Dorsey north (House et al., 2008; Pearthree and House, 2014), provides an example of et al., 2018). Crossey et al.’s (2015) estuary model also highlights similarities in a paleodam overtopping in the lower Colorado River integration story. marl d18O-d13C values in the southernmost SBF and d18O-d13C values in modern The broad range of seemingly conflicting data sets leads to equally conflict- and late Holocene mollusc shells in the San Francisco Bay estuary (e.g., In- ing interpretational paradigms for the origin of the SBF. The marine and estuary gram et al., 1996a, 1996b). Crossey et al. (2015) concluded that SBF carbonate models argue that the freshwater fossil assemblage was simply reworked from compositions are “distinctly nonmarine” (p. 679), but because the strontium neighboring continental environments (e.g., Lucchitta et al., 2001; Miller et al., concentration of their first-arriving water mixture is unknown, they could not 2014). The lacustrine model argues that the marginal marine organisms were categorically eliminate the possibility of an intermittent marine influence. Their transported by birds (e.g., Figuerola and Green, 2002) or some other vector into modeling results do not negate a fully lacustrine interpretation, but they do a saline lake (Spencer and Patchett, 1997). Poulson and John (2003) reported show that a marine influence on SBF carbonate compositions is possible, al- higher carbonate d18O-d13C values and higher trace metal concentrations at the beit not required. southern end of Blythe basin. The number of southern Blythe samples (n = The various marine and estuary models require that the Blythe basin was 11) is higher than the number of northern Blythe samples (n = 3) in that partic- initially at sea level and was connected in some manner to the early Gulf of ular study, making it difficult to robustly compare data from the northern and California before experiencing ~330 m of post-Miocene uplift (Lucchitta, 1979; southern parts of the basin. Foraminifer species richness is also highest at the O’Connell et al., 2017), acknowledging that late Miocene global sea level may southern end the basin (McDougall, 2008; McDougall and Miranda Martínez, have fluctuated within ±30 m of the modern level (Miller et al., 2005; De Schep- 2014). Collectively, these data sets might suggest the presence of a Bouse estu- per et al., 2014). The presence of a variety of marine and marginal marine or- ary (e.g., Crossey et al., 2015), but other viable explanations are also available. ganisms, including abundant benthic (e.g., Ammonia beccarii) and rare plank- For example, Dumont (1998) documented an estuary-like salinity gradient in tic foraminifers (e.g., Globorotalia sp., Neogloboquadrina sp.; McDougall and the surface waters of the landlocked Caspian Sea. Salinities are lowest (< 1 Miranda Martínez, 2014), large numbers of the barnacle Amphibalanus subal- ppt) near where the Volga River enters the sea, and values gradually increase bidus (Zullo and Buising, 1989; Van Syoc, 1992; Pitombo, 2004), low numbers to ~13 ppt at the southern end of the sea. Salinity-driven zonation of pelagic of marine diatoms (e.g., Actinocyclus octonarius, Terpsinoë americana; Miller and benthic organisms within the sea is common (Dumont, 1998). Thus, faunal et al., 2014), and rare fossils of the marine fishColpichthys regis (Todd, 1976), distributions and isotopic gradients that might mimic those found in estuaries coupled with sedimentary structures that are interpreted as evidence for a tidal can also occur in lakes. Similarly, Blair (1978) and Blair and Bradbury (1979) influence, such as possible tidal rhythmites and sigmoidal bedding (O’Connell noted that the Hualapai Limestone of the Muddy Creek Formation, exposed et al., 2017), strengthen the marine and estuarine interpretations. in the Grand Wash Trough area (Fig. 1), contains a modestly diverse (n = 24 The lacustrine model of Spencer and Patchett (1997) and Spencer et al. species) and very Bouse-like mix of euryhaline coastal marine, estuarine, and (2008, 2013) relies heavily on strontium isotopes to hypothesize that the Blythe freshwater to brackish-water lacustrine diatoms. The marine and estuarine di- basin has been at or near its current elevation since the late Miocene, where it atoms include several species of Melosira and Amphora (Blair and Bradbury, was the last in a sequence of basins to be flooded by the southward advance 1979). The 12–6 Ma Hualapai Limestone is a spring-fed lacustrine deposit; it is of early Colorado River (Spencer et al., 2013; Pearthree and House, 2014). The not a coastal marine or estuarine deposit (Crossey et al., 2015). Thus, the small lacustrine model is supported by continental isotopic signatures (Spencer and numbers of marine and estuarine diatoms in the Bouse Formation at Amboy Patchett, 1997; Poulson and John, 2003; Roskowski et al., 2010; Crossey et al., (Fig. 1) could represent a similar lacustrine environment (Miller et al., 2014). 2015) and by the presence of a variety of continental fossils, such as freshwa- The marine and estuarine Bouse models can readily account for sedimentary ter clams (e.g., Sphaerium californica, Pisidium sp.) and snails (e.g., Amnicola structures in the Blythe basin that are interpreted to be marine in origin (i.e., longinqua, Fluminicola sp.; Reynolds and Berry, 2008), freshwater fish (e.g., tidal rhythmites, sigmoidal bedding; O’Connell et al., 2017). The lacustrine Gila cypha; Roeder and Smith, 2016), freshwater ostracodes (e.g., Candona model requires these sedimentary structures to be explainable by lacustrine

processes, and there are numerous examples where sedimentary structures 1.2‰ higher than calcite formed in the same water and at the same tem- typically considered to be marine have been recognized in lacustrine environ- perature (Kim et al., 2007). Biologic carbonates often incorporate a variable ments (e.g., Davis et al., 1972; Ainsworth et al., 2012; Fraser et al., 2012). “vital effect” that can increase their d18O values by typically 0‰ to 2‰ above inorganic calcite precipitated in the same water and at the same temperature (e.g., Killingly and Newman, 1982; Grossman, 1984; von Grafenstein et al., Rationale Behind an Integrated Microfossil and 1999). Because the range in d18O values in our study is very large, distinctions Stable Isotope Approach in d18O values less than 2‰ will be treated as arising from similar environments. All stable isotope data in this study were measured values (relative Many organisms are sensitive to a variety of ecological factors, such as to Vienna Pee Dee Belemnite [VPDB]) and were not corrected for mineralogy temperature and overall salinity, but many are also sensitive to the relation- or vital effects. ship between overall salinity and individual ionic concentrations or ionic ra- Frequent use of d18O-d13C cross-plots is made because they often provide tios. This has been demonstrated for continental ostracodes (Forester and important clues about the source of water and potential environments of Brouwers, 1985; Forester, 1986), diatoms (Blinn, 1993), brine flies (Bowen et deposition (e.g., Talbot, 1990; Gross et al., 2013) and because they facilitate al., 1985), molluscs (Sharpe and Forester, 2008), and other organisms (Plaziat, comparison with previous Bouse studies (e.g., Roskowski et al., 2010; McDou- 1993; Rasmussen, 1988; Herbst, 2001). The distribution of marine organisms, gall and Miranda Martínez, 2014; Crossey et al., 2015). Fully marine environ- on the other hand, is often presented in terms of salinity, temperature, and ments within the past 10 m.y. have typically produced carbonates that group substrate characteristics (e.g., Arnal et al., 1980; Chin et al., 2010; McGann et near the origin of a d18O-d13C cross-plot (Prokoph et al., 2008). Estuarine envi- al., 2013). Ionic ratios in seawater are less important because they are essen- ronments are typically characterized by a strong correlation between salinity tially stable. Numerous marginal marine organisms such as ostracodes (i.e., and isotope composition, and, thus, they typically generate carbonates that Cyprideis, Cytheromorpha), benthic foraminifers (i.e., Ammonia beccarii, El- are positively covariant, where lower-salinity environments near the head of phidium gunteri), and marine molluscs (i.e., Cardium, Potamides) are able to the estuary plot with lower d18O-d13C values, and more saline environments colonize continental lakes where the ratio of calcium to carbonate alkalinity near the mouth of the estuary plot with higher d18O-d13C values (e.g., Ingram (Ca/ALK; Forester, 1986) and the relative proportions of sodium and chloride et al., 1996a; Reinhardt et al., 2003; Sampei et al., 2005). The relationships be- converge on a “marine like” composition (e.g., Gasse et al., 1987; Anadón et tween salinity and carbonate d18O-d13C values can be fairly complex in lakes, al., 1986; Anadón, 1989, 1992; Plaziat, 1993; Wennrich et al., 2007). Some mar- and at times, they may even be counterintuitive (Li and Ku, 1997). It is not ginal marine ostracodes, like Cyprideis torosa and Cytheromorpha fuscata, uncommon to find examples where terminal but freshwater or mildly brack- are known to tolerate sulfate- and chloride-rich continental waters (Nielsen et ish lakes generate carbonate d18O-d13C values that overlap with or even ex- al., 1987; Mezquita et al., 1999). Similarly, continental organisms can persist in ceed those found in marine carbonates (e.g., Anadón et al., 2008; Sharpe and marginal marine and estuarine environments, but they are restricted to dilute Bright, 2014). Thus, lakes can produce carbonates that display a wide range environments where salinity typically remains below ~5–10 ppt (Forester and of d18O-d13C relationships (e.g., Talbot, 1990). The fundamental differences in Brouwers, 1985; Bulger et al., 1993). Here, the Bouse Formation microfossil the isotopic compositions of marine, estuarine, and lacustrine environments, assemblage is interpreted within the context of a well-established ecologi- coupled with the fundamental differences in the ecology of strictly marine, cal framework that is augmented by various types of isotopic data contained marine but continentally invasive, and fully continental organisms, provide within the microfossils themselves. the foundational framework for our interpretations. This study focuses more on d18O values because of the strong correlation between d18O and salinity in marine and estuarine environments (e.g., Rodri- guez et al., 2000; Dettman et al., 2004), and among d18O, salinity, inflow, and STUDY AREAS evaporation in lakes (e.g., Talbot, 1990; Leng and Marshall, 2004). Carbon isotopes will be discussed less frequently because numerous influential variables The principal study areas included five Bouse Formation outcrops that were (degassing, organic respiration, etc.) reduce the correlation with salinity. strategically located along a north-south transect from the Chemehuevi basin The discussion of d18O values of a variety of Bouse Formation carbonates to Marl Wash (Fig. 1). One outcrop of the lacustrine NBF in the Chemehuevi is grounded on assumption that carbonate produced in similar environments basin (34.4477°N, 114.4010°W; Fig. 1) was sampled at roughly meter-scale res- will have similar d18O values. Possible comparisons will be among aragonitic olution (Fig. 2). Four SBF outcrops were located at Parker, Arizona (34.1623°N, mollusc shells, and calcitic micrite, barnacle plates, ostracode valves, and 114.3021°W), in Milpitas Wash (33.2599°N, 114.7303°W), in Hart Mine Wash foraminifer tests. Although these different materials have slightly different (33.2907°N, 114.6352°W; Fig. 1), and in Marl Wash (33.2634°N, 114.6393°W; Fig. d18O values when precipitated in the same environment, the difference be- 1). The outcrops at Parker (Fig. 3), Milpitas Wash (Fig. 4), and Hart Mine Wash tween them is small. Low-temperature d18O values of aragonite are roughly (Fig. 5) were sampled at roughly 10–40 cm resolution. The SBF in Marl Wash

Cyp Lim Cand Dar B+S Sf45 Bf45 s 16 15

14 14 brown clay

Figure 2. Microfossil results from an outcrop of northern Bouse Formation in 10 NO SAMPLES the Chemehuevi basin. Cyp—Cyprideis, Lim—Limnocythere, Cand—Candona, sand Dar—Darwinula, B+S—fish bone and scale, Sf45—spiraled (benthic) foraminifers

Interbedded Unit in a concentrated aliquot of 45–120 µm residue (raw counts, not as per gram 8 sediment), Bf45—biserial foraminifers in a concentrated aliquot of 45–120 µm 12 residue (raw counts, not as per gram sediment), black diamonds with numbers— 13 individual samples that correspond to data archived in the Data Repository (see 11 text footnote 1). Stylized stratigraphic column is shown on far right. See Figure 6 10 1 for location.

9 green clay

Approximate thickness (m) 4 8

7 2 5 4 3 white marl

0 6 Basal Carbonate Unit 000.4 0 230 0 0.5 0 410 3 5 sand per gram sediment

(Fig. 6) was coarsely sampled (>meter scale) at prominent changes in lithology. the SBF was deposited in an estuary, and because the salinity and d18O val- Four additional spot collection sites targeted barnacle-rich sediments of the ues of water in an estuary are positively correlated (e.g., Ingram et al., 1996a; basal carbonate unit. Two of these sites were in the vicinity of Hart Mine Wash Dettman et al., 2004), then the microfossil assemblages and the carbonate (Fig. 1), and two sites were located in the Palo Verde Mountains, slightly to the d18O-d13C values should show a clear transition from lacustrine conditions in north of Milpitas Wash (Fig. 1). Modern barnacle shells from the Salton Sea the Chemehuevi basin, to mildly saline conditions at the head of the Bouse es- (Fig. 1) and from Isla Montague (Fig. 1) were also analyzed. Detailed descrip- tuary near Parker, to more marine-like conditions near the mouth of the Bouse A B view to the North tions of each of the Bouse sampling locations are available in the Supplemen- estuary at the southern end of the basin. A modern estuary analogy might brown claystone tal Materials.1 be found in the freshwater influence of the pre-dam-era Colorado River on sand 18 greyish white marl green claystone This sampling strategy was intended to quantify the microfossil content the d O values in Mulinia shells from the northern Gulf of California. There, and the d18O-d13C values in micrite and biologic calcite from a lacustrine en- the influence of the river can be measured up to 60 km away from its mouth vironment in the Chemehuevi basin and then compare those values against (Rodriguez et al., 2001). equivalent data sets from the contested environment in the Blythe basin. The greyish white marl outcrop of SBF at Parker, Arizona, is located near the area where the early Miocene sand & gravel

view to the North green clay contact Colorado River would have entered the Blythe basin (Fig. 1). The outcrops at METHODS Milpitas Wash, Hart Mine Wash, and Marl Wash are located near the south-

Miocene sand & gravel ern margin of the basin (Fig. 1), far from any major river input, and would Microfossil Analysis have been near the mouth of a possible Bouse estuary. If the SBF formed in 1Supplemental Materials. Detailed outcrop descrip- a normal marine environment, then the microfossil assemblages and the car- Sediment samples were disaggregated for microfossil analysis (n = 140) tions and supplemental tables. Please visit https:// 18 13 doi.org/10.1130/GES01610.S1 or the full-text article bonate d O-d C values from the various outcrops should be similar to each by soaking them in a weak sodium bicarbonate and sodium hexametaphos- on www.gsapubs.org to view the Supplemental other and recognizably marine in nature, and they should be clearly different phate solution for up to 1 wk. The most indurated samples were subjected Materials. from equivalent assemblages and isotopic values from the lacustrine NBF. If to a single freeze-thaw cycle. Repeated freeze-thaw cycling was avoided to

Cyp Lim Cand Sf120 B+S 30 3

28 green clay

2 Interbedded Unit

27 25 23 Figure 3. Microfossil results from an outcrop of southern Bouse Formation at Parker, Blythe basin. Cyp—Cyprideis, Lim—Limnocythere, Cand—Candona, 1 21 Sf120—spiraled (benthic) foraminifers, B+S—fish bone and scale, black dia- 19 monds with numbers—individual samples that correspond to data archived in the Data Repository (see text footnote 1). Stylized stratigraphic column is 17 Approximate thickness (m) shown on far right (x-bedded—cross-bedded). Note differences in scales on x 15 yellow marl axis for Cyprideis. See Figure 1 for location. 13

0 11 0100 0 0.6 00.3 030 00.2

per gram sediment Basal Carbonate Unit sand x-bedded 0.7 3 c

2 bioclasti limestone 0 1 050 0.6 00.3 030 00.2

per gram sediment gravel

reduce breaking foraminifer tests. Once disaggregated, each sediment slurry in Tables SI-1 to SI-5 (footnote 1). Microfossil data for Outcrop E were provided was washed over a 45 µm sieve. Microfossils in the >120 µm size fraction were in Bright et al. (2016). identified and counted. The foraminifers in a small concentrated aliquot of each of the 45–120 µm size fractions were counted. The foraminifers were not identified but were grouped by appearance. “Spiraled” foraminifers included Stable Isotope (d18O, d13C) Analysis predominantly benthic genera such as Rosalina and Ammonia, and “biserial” foraminifers included the benthic genus Bolivina and the planktic genus Strep- Previous studies of Bouse Formation sediments often reported bulk carbon- tochilus (McDougall and Miranda Martínez, 2014). Microfossil counts were ate d18O-d13C values (Poulson and John, 2003; Roskowski et al., 2010; Crossey normalized on a “per gram sediment” basis. Standardizing the microfossil et al., 2015). This study is the first effort to analyze the isotopic composition of content by sample mass allows for direct comparison of the actual abundance specific components within a bulk sediment sample. of each component in any sample. This is the first Bouse study that normalized Stable isotope (d18O, d13C) values were measured on fine-grained micrite microfossil content. Other Bouse studies have presented microfossil data as (<45 µm; n = 131), aliquots of ostracode valves (n = 238) and foraminifer tests either raw counts (Smith, 1970; McDougall and Miranda Martínez, 2014) or as (n = 21), individual barnacle fragments (n = 32), unidentified individual shell species percentage plots (Dorsey et al., 2018). Microfossil counts are available fragments (n = 4), and secondary calcite (n = 4).

Cyp Cyth Per Per-j Lim-j Het Cand Cand-j B+S Sf45 Bf45 9 37

7 36 Figure 4. Microfossil results from an outcrop of southern Bouse Formation at Milpitas Wash, Blythe basin. Ostracode valve and bone plus scale counts are expressed as square 6 root values in order to highlight low concentrations near the top of the outcrop. Cyp—Cyprideis, Cyth—Cytheromorpha ,

Interbedded Unit Per—adult Perissocytheridea, Per-j—juvenile Perissocyther- 35 green clay 5 idea, Lim-j—juvenile Limnocythere, Het—Heterocypris, Cand—adult Candona, Cand-j—juvenile Candona, B+S— fish bone plus scale, Sf45—spiraled foraminifers in a con- 4 34 centrated aliquot of 45–120 μm residue (raw counts, not as per gram sediment), Bf45—biserial foraminifers in a 33 concentrated aliquot of 45–120 µm residue (raw counts, not 3 as per gram sediment), black diamonds with numbers— individual samples that correspond to data archived in the Approximate thickness (m) Data Repository (see text footnote 1). Stylized stratigraphic 27 2 column shown on far right roughly corresponds to Strati- graphic Panel C1 in Homan (2014). See Figure 1 for location. 23 1 white marl Basal Carbonate Unit 0 13 0 1.25 0100.2 0 0.3 0 0.2 0 0.3 0 0.5 0 0.8 0 7 0210 0350 per gram sediment

Fresh sediment from each sample was disaggregated in reverse osmosis the “Streptochilus biofacies” of McDougall and Miranda Martinez (2016) and (RO) water and sieved at 45 µm to remove biogenic carbonate. Deflocculants Dorsey et al. (2018). The most abundant spiraled benthic foraminifers in this in- or other additives were not used. The <45 µm micrite slurries were dried over- terval were Rosalina columbiensis, with less common Ammonia beccarii (Mc- night in a 40 °C oven and then gently homogenized into powders with a por- Dougall and Miranda Martínez, 2014; Dorsey et al., 2018). The most abundant celain mortar and pestle. biserial foraminifer present was Streptochilus spp. (> 90%), but small numbers The best-preserved ostracode valves (Cyprideis sp., Cytheromorpha sp., of Bolivina spp. were also present (e.g., McDougall and Miranda Martínez, Candona spp., Heterocypris sp.; Bright et al., 2016) were picked out of the 2014). The “spiraled” and biserial foraminifer isotopic analyses were each >250 µm size fraction. Adult valves were preferentially chosen, but large ju- dominated by a single genus of foraminifer. The limited number of foramin- venile valves were used when suitable adult valves were not available. The ifer genera present in each analysis will reduce the influence of environmental ostracode valves were manually cleaned with a fine paintbrush. If secondary variability on the resulting d18O-d13C values. Foraminifers from the 45–120 µm carbonate growths or other resistant debris was present, it was removed with size fractions from each of the six samples were picked for analysis under a a fine needle or by quickly brushing the valves with dilute (0.5 N) hydrochloric binocular microscope using a two-step process. First, each residue was exam- acid followed by a thorough rinsing with RO water. Analysis of the larger os- ined under transmitted light, and only translucent foraminifer tests that were tracode species (Cyprideis, Candona, Heterocypris) used two valves per aliquot, free of visible encrustations or infilling sediments were selected. The picked and 3–6 valves per aliquot were required for the Cytheromorpha analyses. foraminifers were then subjected to a second selection process using reflected At Hart Mine Wash, six sediment samples near the top of the basal carbon- light, in which any stained or discolored tests were removed. A minimum sam- ate unit and at the base of the interbedded unit contained high concentrations ple mass of ~40 µg was required for analysis, but the number of foraminifers of spiraled and biserial foraminifers (Fig. 6). These sediments correspond to in each sample was not counted. Scanning electron microscope (SEM) images

Cyp Cyth Per Lim Het Cand Dar B+S Bf45 Sf45 87Sr/86Sr 10 re d clay 9

8 green clay

7 Interbedded Unit TM DCL 6 l

4 xidized mar

Approximate thickness (m) 2

1 upper white mar lo 0 00510 5 000.1 0 0.30 1 0.8 0 1.5 0030 200 150 0. 71 0. 71 05 14

8 87 86 7 Sr/ Sr micrite 6 fish bone Basal Carbonate Unit Cyprideis lower white marl 5

4 b.o.g.

Approximate thickness (m) 1 no disaggregation - equivalent sediments studied in Marl Wash (Fig. 6) bedded carbonate 0 00510 5 000.1 0 0.30 3 0.8 0 0.1 005 5 150 oxidized sand per gram sediment

Figure 5. Microfossil results and 87Sr/86Sr values in micrite, fish bone, and ostracode valves from multiple outcrops of southern Bouse Formation at Hart Mine Wash, Blythe basin. Cyp—Cyprideis plotted as square root values to highlight low abundances low in the section, Cyth—Cytheromorpha, Per-j— juvenile Perissocytheridea, Lim—Limnocythere, Het—Heterocypris, Cand—Candona, Dar—Darwinula, B+S—fish bone and scale per gram sediment, Sf45—spiraled (benthic) foraminifers in a concentrated aliquot of 45–120 μm residue (raw counts, not as per gram sediment), Bf45—biserial foraminifers in a concentrated aliquot of 45–120 mm residue (raw counts, not as per gram sediment), b.o.g.—barnacle oncoid grainstone, DCL—distinctive clay layer, TM—transitional marl. Note differences in scales on x axis for Het, Dar, Bf45, and B+S. See Figure 1 for location and Supplemental Materials (text footnote 1) for additional details about the sampling scheme.

Cyp Cyth Lim Het Cand Barn B+S Sf45 Bf45 colluvium 18 sand 8 Unit 7 gr c 16 4 Interbedded 5 6

9 14

Figure 6. Microfossil results for the southern Bouse Formation at Marl Wash, Blythe basin. Stylized stratigraphic column on far right roughly corresponds to Stratigraphic Panel A20 in Homan 12 white marl 10 (2014). Cyp—Cyprideis, Cyth—Cytheromorpha plotted as square root values to highlight low abundances throughout the section, Lim—Limnocythere plotted as square root values to highlight low abundances at the bottom of the section, Het—Heterocypris , 10 Cand—Candona, Barn—barnacle shell fragments, B+S—fish bone and scale, Sf45—spiraled foraminifers in a concentrated

b.o.g. aliquot of 45–120 μm residue (raw counts, not as per gram sedi- 11 ment), Bf45—biserial foraminifers in aliquot of 45–120 μm residue 8 (raw counts, not as per gram sediment), b.o.g.—barnacle oncoid 12 grainstone, grc—green and red claystone. Black diamonds with

Basal Carbonate Unit numbers—individual samples that correspond to data archived Approximate thickness (m) 13 in the Data Repository (see text footnote 1). See Figure 1 for lo- 6 cation and Supplemental Materials (footnote 1) for additional details about the sampling scheme.

14 bedded carbonate

4 s d

2 NO SAMPLES cross-bedde gravels and sand

0 15 0 27 04020 0.3 02040 0.5 0 14010 35 oxidized sand per gram sediment

of representative “spiraled” and biserial tests from Hart Mine Wash are pre- Two fragments of clean and pearlescent shell material and two fragments sented in Figure 7. of shell material that was clearly overgrown and contaminated with secondary Barnacle shell fragments were chosen for isotopic analysis based on their carbonate were selected from the >250 µm size fraction of the Parker basal car- appearance under a microscope using reflected light. Only smooth, pearles- bonate unit residues. In addition, four large pieces of purely secondary calcite cent material was selected. Barnacle fragments were soaked in 0.5 N HCl and were collected from the Hart Mine Wash basal carbonate unit residues. visually inspected until the majority of their mass was removed. The acid The carbonate d18O-d13C values were measured at the University of Arizo- leaching step ensured that any surface contaminants were removed. The re- na’s Environmental Isotope Laboratory using an automated KIEL-III carbonate maining fragments were thoroughly rinsed under RO water. Single barnacle preparation device coupled to a Finnigan MAT 252 gas-ratio mass spectrome- fragments were analyzed. Digital images of typical barnacle fragments are ter. The samples were reacted with dehydrated phosphoric acid under vacuum available in the Supplemental Material (footnote 1). at 70 °C. The isotope ratio measurement was calibrated based on repeated

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 1885 by guest on 26 September 2021 on 26 September 2021 by guest Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/14/4/1875/4265689/1875.pdf Research Paper = 50 μ m, except N and O, where scale bars = 25 μ m. (see Supplemental Materials for stratigraphic context [text footnote 1]). Note jagged edges on tests Cc and Dd suggesting they broke off of something larger. Scale bars ( Bolivina spp.) foraminifer tests, (Cc–Ff) “spiraled” benthic foraminifer test. Images P to Ff are from oxidized marl sample HM-38 near the top of the basal carbonate unit the base of interbedded unit (see Supplemental Materials for stratigraphic context [text footnote 1]). (P–W) Biserial ( Streptochilus spp.) foraminifer tests, (X–Bb) biserial foraminifer tests, (L–M) “spiraled” benthic foraminifer tests, (N–O) broken chambers on tests A and H, respectively. Images A to O are from transitional marl sample HM-55 at Figure 7. Scanning electron microscope (SEM) images of foraminifers from Hart Mine Wash. (A–H) Biserial ( Streptochilus spp.) foraminifer tests, (I–K) biserial ( Bolivina spp.) H P Cc U A I V B Q J Dd W C R K X D Ee L S Y E M Ff T Z F Aa O N G Bb

GEOSPHERE | Volume 14 | Number 4 Bright et al. | Freshwater plumes and brackish lakes 1886 Research Paper

measurements of standards NBS-18 and NBS-19, and acid fractionation fac- ported with respect to a value of 0.710250 for NBS-987. All 87Sr/86Sr ratios are tors were assumed to be identical for both aragonite and calcite. The stable available in Table SI-11 (footnote 1). isotope results are reported in standard delta (d) notation, where: d (‰) = 3 18 16 13 12 [(Rsample/Rstd) – 1] × 10 , and R = ratio of O: O or C: C. Rstd refers to the standard VPDB. Precision for d18O and d13C measurements were ±0.1‰ and ±0.08‰ RESULTS (1s), respectively. Stable isotope values are available in Supplemental Tables SI-6 to SI-10 (footnote 1). Additional stable isotope data for Outcrop E were Northern Bouse Formation provided in Bright et al. (2016). Sediments from the NBF contain the marginal marine but continentally invasive ostracode Cyprideis sp. in association with the continental ostracodes Strontium Isotope Ratio (87Sr/86Sr) Analysis Limnocythere sp., Candona sp., and Darwinula stevensoni (Fig. 2). Rare spiraled and biserial foraminifers were present at the bottom of the section (Fig. Previous studies of Bouse Formation sediments often reported the bulk 2). Digital photographs of several of the more common biserial foraminifers carbonate 87Sr/86Sr ratio (Spencer and Patchett, 1997; Roskowski et al., 2010; that are tentatively identified as Bolivina are presented in Figure 8. This is the Crossey et al., 2015). As with the stable isotope analyses, this study is the first first occurrence of foraminifers reported from the NBF, which is a significant effort to analyze the 87Sr/86Sr ratio of specific components within a bulk sedi- departure from previous studies (Reynolds, 2008; McDougall and Miranda ment sample. Martínez, 2014; Reynolds et al., 2016). To rule out the possibility of cross- Strontium isotope ratios (87Sr/86Sr) were measured on <45 µm micrite pow- contamination, raw sediments from three of the foraminifer-bearing samples ders (n = 5), >250 μm fish bone fragments (n = 9), and aliquots of ostracode were processed a second time but using different beakers and sieves. Similar (Cyprideis sp.) valves (n = 2) from the upper portion of the basal carbonate unit numbers and types of foraminifers were found. Ostracode abundances con- and from one sample near the bottom of the interbedded unit in Hart Mine spicuously decline and both Limnocythere sp. and the foraminifers disappear Wash (Fig. 5). The fish bones were sonicated in RO water for several minutes across an abrupt transition within the marl near the bottom of the outcrop and examined under a binocular microscope, and only the most pristine frag- (Fig. 2). ments were chosen for analysis. Two aliquots of adult Cyprideis sp. valves The micrite and ostracode valves displayed a wide range of d18O values were subjected to a brief 0.5 N hydrochloric acid bath to remove the outermost (-18‰ to +1‰) and d13C values (-4‰ to +3‰; Figs. 9 and 10A). The oxygen iso- 18 18 surfaces of the valves. The valves were rinsed in RO water and air dried, and tope values in micrite (d OSED) and ostracode valves (d OOST) were noticeably the most pristine valves were selected for analysis. higher at the bottom of the outcrop in association with the foraminifer-bearing Strontium isotope ratios were measured at Geochron Laboratories in sediments (Fig. 9). Chelmsford, Massachusetts. The samples were dissolved in 1 N hydrochloric acid, centrifuged, and evaporated. If organic material was present, the sample was treated with concentrated nitric acid and hydrogen Southern Bouse Formation peroxide. The samples were converted to nitrate with concentrated nitric acid and finally dissolved in 500 µL of 3.5 N nitric acid in preparation Sediments from the SBF contained an ostracode faunal assemblage that for strontium separation by ion exchange. Strontium was separated from is similar to, but more diverse than, the NBF. SEM images of SBF ostracodes other cations by using 50 µL columns filled with Eichrome Sr-Spec resin. are available in Bright et al. (2016), and additional images are presented in Fig- The samples, dissolved in 500 µL of 3.5 N nitric acid, were centrifuged and ure 11. A comparable Cyprideis sp., Limnocythere sp., Candona spp., and Dar- loaded onto the columns. The columns were rinsed with 1200 µL of 3.5 N winula stevensoni assemblage was observed to be variably present through- nitric acid. Strontium was then eluted with 800 µL of deionized water. A out the SBF (Figs. 3–6). Additional marginal marine but continentally invasive drop of 0.1 M phosphoric acid was added to each sample, and then they ostracodes Cytheromorpha spp. and Perissocytheridea sp. were also present, were dried down. For carbonate samples, this process was repeated twice but the Perissocytheridea sp. were rare and occurred primarily as juveniles. to improve separation of strontium from calcium, which interferes with One additional continental ostracode, Heterocypris sp., also appeared (Figs. the mass spectrometry. 4–6). No strictly marine ostracodes were found. We define “strictly marine os- Isotope ratio measurements were made on a GV IsoProbe T multicollector tracodes” as those genera that are not known to colonize continental lakes. mass spectrometer. Strontium was loaded on Re filaments in a solution of Representative genera include Caudities, Aurila, or Xestoleberis, all of which

tantalum chloride (TaCl5) and phosphoric acid, which were analyzed by dy- are common ostracodes in nearshore environments in the modern Gulf of namic multicollection. The data were corrected for 87Rb interference based on California (Swain, 1967), and all of which have been identified from fully ma- measured 85Rb abundance, fractionation corrected to 86Sr/88Sr = 0.1194, and re- rine sediments of the age-equivalent late Miocene and early Pliocene Imperial

bonate analyses of SBF carbonates (Poulson and John, 2003; Roskowski et al., 2010; Crossey et al., 2015). Bioclastic-rich and bedded limestones at the bottom of the basal carbonate unit exposed at Parker and at Hart Mine Wash yielded micrite and ostracode d18O and d13C values that are nearly identical, with values of about -6‰ and -2‰, respectively (Figs. 10B and 10D). Softer marls de- 18 posited above the bioclastic-rich sediments consistently yielded d OOST values 18 up to 12‰ higher than the associated d OSED values (Figs. 9 and 10B–10D). Several Cytheromorpha sp. samples from the top of the basal carbonate unit 18 13 18 13 had d OOST-d COST values that were nearly identical to d O-d C values from the bottom of the unit (Fig. 10D). Spiraled foraminifers at the top of the basal carbonate unit and at the bottom of the overlying interbedded unit in Hart Mine Wash had d18O and d13C values that were nearly identical to d18O-d13C values from the bottom of the basal carbonate unit (Fig. 10D), much like several of the Cytheromorpha sp. noted previously. The biserial foraminifers had d18O-d13C values similar to the benthic marginal marine ostracodes (Cyprideis sp. and Cytheromorpha sp.) with which they were associated (Figs. 9 and 10D). Notably, the Streptochilus d18O-d13C values at Hart Mine Wash were virtually identical to the Cyprideis sp. d18O-d13C values at Parker (Figs. 10B and 10D). The transition from the top of the basal carbonate unit to the overlying interbedded unit at both Hart Mine Wash and Milpitas Wash is associated with an increase in the abundance of continental ostracodes (i.e., Candona spp., Darwinula stevensoni; Figs. 4 and 5; Bright et al., 2016) and with an abrupt 18 ~8‰ decrease in d OOST values (Figs. 9, 10C, and 10D; Bright et al., 2016). The d18O values in the associated foraminifers did not change across this boundary 18 100 μm (Fig. 10D). An equivalent decease in d OOST values was not found at the Parker outcrop (Fig. 9). Clean shell fragments from the basal carbonate unit at Parker yielded d18O values (-9.0‰ and -8.7‰) that were fairly similar to the d18O values (-9.9‰ and

Figure 8. Digital photographic images of representative biserial foraminifers, tenta- -7.4‰) of shell fragments encrusted in secondary carbonate. Four large pieces tively identified as Bolivina spp., in sample CHEM-4 from the lower northern Bouse of purely secondary calcite from the bottom of the basal carbonate unit in Hart Formation marls in Chemehuevi basin. See Figure 2 for stratigraphic context. Tests Mine Wash yielded an average d18O value of -9.1‰ ± 0.3‰. are stained with iron oxide, similar to quartz grains and other objects in the associated >45 µm residues. Similar staining demonstrates that the foraminifer tests are Modern barnacle shells from the Salton Sea and from Isla Montague (Fig. autochthonous rather than sieve cross-contamination from other southern Bouse 1) yielded a comparatively narrow range of d18O values (-1‰ to +3‰) and d13C Formation samples. values (-2‰ to +2‰; Fig. 12). The fossil barnacle shells from the basal carbonate unit at SBF localities displayed a wider range of d18O (-12‰ to -5‰) and d13C (-6‰ to +1‰) values (Fig. 12). The isotopic compositions of the fossil bar- Formation in the Salton Trough (Jefferson, 2001; Reynolds et al., 2008). Fora- nacles tended to cluster by location (Fig. 12). minifers were variably abundant throughout the basal carbonate unit (Figs. Material-specific 87Sr/86Sr ratios from the oxidized marl at the top of the 3–6). In Hart Mine Wash, notable peaks in foraminifer abundances occurred basal carbonate unit and from the overlying transitional carbonate at the in the bioclastic-rich and bedded limestone horizon at the bottom of the basal base of the interbedded unit in Hart Mine Wash (Fig. 1) were often found to carbonate unit and again in the soft oxidized marls at the top of the basal car- be dissimilar (Fig. 5). The oxidized marl samples yielded 87Sr/86Sr ratios from bonate unit (Fig. 5). fish bone (0.71104 ± 0.00006; n = 8) and Cyprideis calcite (0.71096; n = 1) that The micrite and ostracode valves from the SBF displayed a wide range of were typically higher than 87Sr/86Sr ratios from the encasing micrite (0.71084 ± d18O values (-15‰ to -1‰) and d13C values (-7‰ to +2‰; Figs. 9 and 10B–10D), 0.00013; n = 4; Fig. 5). One sample of transitional marl yielded 87Sr/86Sr ratios in which are comparable to those measured in the NBF (Figs. 9 and 10A). Similar fish bone (n = 1), Cyprideis calcite (n = 1), and micrite (n = 1) of 0.71084, 0.71104, ranges of d18O and d13C values have been previously reported from bulk car- and 0.71097, respectively (Fig. 5).

Cyprideis 12 18 MM C δ O Cytheromorpha Candona Heterocypris 10 micrite MM - marginal marine C - continental 8

MM C δ18O 6 Hart Mine 4 MM C δ18O

2 MM C δ18O

-2 03100 0

0 14 0 1 -16 -8 0 0017 0.5 -16 -8 0 -4 (vpg) (‰, VPDB) (vpg) (‰, VPDB) approximate thickness (m) Parker Milpitas -6 0 0.430 -16-8 0 (vpg) (‰, VPDB) 0060 25 -16 -8 0 -8 Chemhuevi

-10 Oxidized marl Sand Upper white marl Brown clays -12 Lower white marl Red clays, siltstone, and fine sands Crossbedded sand Green claystone -14 Bioclastic-Grainstone Transitional marl Interbedded Unit Bedded carbonate Distinctive clay layer (DCL) Basal Carbonate Unit -16 0015 3 -16 -8 0 Miocene fan gravel (vpg) (‰, VPDB)

Figure 9. Compilation of d18O values in micrite and ostracode valves from one section of northern Bouse Formation in Chemehuevi basin and three sections of southern Bouse Formation in Blythe basin. See Figure 1 for locations. MM—abundance of marginal-marine (Cyprideis + Cytheromorpha + Perissocytheridea) ostracode valves (juvenile + adult) in valves per gram (vpg), C—total abundance of continental (Candona + Heterocypris + Limnocythere + Darwinula) ostracode valves (juvenile + adult) in valves per gram (vpg). Micrite and ostracode d18O values are similar in the shallow-water bedded limestone and lower marls because of enhanced mixing of river and lake water by waves. Micrite d18O values in the deeper-water marls are consistently several per mil lower than the ostracode valve d18O values because the epilimnic micrite formed in an isotopically stratified and poorly mixed seasonal freshwater cap derived from spring snowmelt flood discharge on the early Colorado River. The ostracode valves calcified in the benthos and represent the isotopic composition of deeper water. Stylized stratigraphic columns are aligned at the contact between the basal carbonate unit and the interbedded unit simply for convenience. No temporal correlations are implied. Dashed line in Chemehuevi panel correlates an abrupt decrease in ostracode d18O values in the Chemehuevi basin with similar abrupt decreases at Hart Mine Wash and Milpitas Wash. VPDB—Vienna Pee Dee Belemnite. Microfossil and isotopic data are available in the Data Repository (see text footnote 1).

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 1889 by guest on 26 September 2021 on 26 September 2021 by guest Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/14/4/1875/4265689/1875.pdf Research Paper clay layer. Ostracode and foraminifer and multiple genera ostracode results from the southern Bouse Formation, Hart Mine Wash. Results are grouped by stratigraphic horizon. DCL—distinctive from the southern Bouse Formation, Parker. (C) Micrite and multiple genera ostracode results from the southern Bouse Formation, Milpitas Wash. (D) Micrite (A) Micrite and multiple genera ostracode results from the northern Bouse Formation, Chemehuevi basin. (B) Micrite, Cyprideis , and shell fragment results Figure 10. Cross-plots of stable oxygen ( Milpita C southern Bouse Formation Chemehuev A northern Bouse Formation -2 -2 (‰, VPDB) δ (‰, VPDB) δ 18 0- 18 Ye Bioclastic limestone micrite Lower white marl micrit Upper white marl micrit Oxidized marl micrit 0- O O llow marl micrite Cypridei shell, with carbonate shell, clean barnacle fragment “ Streptochilus benthic foraminifer Candon Cypridei Candon Cypridei Heterocypri Candona Cytheromorpha Cypridei Heterocypri Cytheromorpha Cypridei s i a a s s s s s s s ” foraminifers

e 10 e e s 10 d 18 O values have not been corrected for vital effects. VPDB—Vienna Pee Dee Belemnite. See Figure 1 for locations. d 18 O) and carbon ( Basal Carbonate Unit

δ13C δ13C (‰, VPDB) (‰, VPDB) d -6 -4 -2 -6 -4 -2 2 4 13 2 4 C) isotope values in micrite and various forms of biologic calcite from the Bouse Formation. 5 5 Hart Mine W D southern Bouse Formation Parker B southern Bouse Formation -2 -2 “DCL” micrit Green claystone micrit Red silt and claystone micrite T Brown mudstone micrite (‰, VPDB) δ (‰, VPDB) δ ransitional marl-claystone micrit 0- 0- 18 18 O O “ Streptochilus benthic foraminifer Cytheromorpha Cytheromorpha Heterocypri Candona Cypridei Candona Cypridei e s s ash s ” foraminifers e s 10 10 e

Interbedded Unit

δ13C δ13C (‰, VPDB) (‰, VPDB) -6 -4 -2 2 4 -6 -4 -2 2 4 5 5

GEOSPHERE | Volume 14 | Number 4 Bright et al. | Freshwater plumes and brackish lakes 1890 Research Paper

A B C

D E F G

Figure 11. Scanning electron images of ostracode valves from the southern Bouse Formation. All images are external lateral views. (A) Candona sp. C, female right valve, sample HM73. (B) Candona sp. C, female left valve, sample HM73. (C) Candona sp. C, male left valve, sample HM73. (D) Candona sp. D, female right valve, sample HM73. (E) Candona sp. E, female right valve, sample EARP26. (F) Perissocytheridea sp., juvenile right valve, sample HM33. (G) Perissocytheridea sp., juvenile left valve, sample HM33. For stratigraphic information, refer to Figure 3 and the Supplemental Materials (text footnote 1) on Hart Mine Wash. Ostracode valves were coated with carbon and imaged on a Hitachi 3400N SEM. Original images were modified for contrast and clarity using Photoshop CS6. See Bright et al. (2016) for additional ostracode valve images.

DISCUSSION a single sample. For example, sample HM-38 (see Supplemental Materials 18 18 [footnote 1]) yielded a d OSED value of about -9‰, multiple Cyprideis d OOST Isotopic Resetting and Evidence for the Reworking of Microfossils values of about -2‰, multiple benthic foraminifer d18O values of about -6‰, and multiple planktic foraminifer d18O values of about -3‰. Similar items in Bouse carbonates have undoubtedly been subjected to postdepositional adjacent and distantly spaced samples showed a similar degree of isotopic influences since their deposition ca. 5 Ma. The question is whether or not the heterogeneity (Fig. 9). If significant isotopic resetting or recrystallization of the available isotopic information is a useful archive capable of informing us of various carbonate sources had occurred, then the repetitive and persistent dif- past environmental conditions. Several observations suggest that the isotopic ferences in d18O values between each carbonate type within any given sample information is useful, and they provide two compelling arguments against sig- would have been destroyed. Bouse carbonates are probably not pristine, but nificant isotopic resetting of (at least some) Bouse carbonates. First, Roskowski carefully selected items clearly retain a degree of internally consistent isotopic et al. (2010) analyzed the d18O and d13C values from a micromilled SBF barnacle heterogeneity that does not support claims of significant isotopic resetting or shell and found that the shell retained clear cyclic patterns in the d18O and the recrystallization (e.g., Lucchitta et al., 2001). d13C values, similar to that observed in modern (e.g., Surge et al., 2013) and The detailed nature of this data set allows us to address for the first time fossil (e.g., Wesselingh et al., 2006) shell materials. The barnacle d18O values the issue of reworking of fossils within the Bouse carbonates. Earlier studies ranged from -9‰ to -10.5‰ (Roskowski et al., 2010) and clearly do not reflect have suggested that the freshwater, continental fossils might have been re- a marine origin. The isotopic integrity of the shell would have been destroyed worked from the surrounding watershed into a marine or estuarine environ- if the shell had experienced significant postdepositional isotopic resetting or ment (Turak, 2000; Miller et al., 2014). In this study, two genera of marginal recrystallization. Second, this study documents a surprisingly high degree of marine ostracodes and two genera of continental ostracodes consistently oc- reproducible isotopic heterogeneity from a variety of carbonate sources within curred together and showed persistently similar d18O values (Figs. 9 and 13B).

δ18O (‰, VPDB) −20 −105

Figure 12. Cross-plot comparing d18O and d13C values from barnacle fragments (large colored circles and gray diamonds) and <45 μm micrite (colored −2 triangles) in the Blythe basin against barnacle fragments from the modern Salton Sea (red boxes), Isla Montague (orange stars), and marine barnacles from previously published sources (white circles). Published marine barnacle data were compiled from Smith et al. (1988), Craven et al. (2008), and Detjen et al. (2015). The d18O and d13C −4 values from pre-dam-era Mulinia specimens from Isla Montague and from other locations up to 65 km away from the mouth of the Colorado River (small black and gray circles, respectively; Rodri- guez et al., 2001; data provided by D. Dettman), as well as d18O and d13C values from late Pleistocene and Holocene (17–2 ka) planktic foraminifers from −6 the Guaymas basin (small purple circles; Keigwin, 2002; data provided by L. Keigwin), are plotted for C (‰, VPDB ) context. Symbols and respective colors for Blythe 13

δ basin barnacle values match locations outlined in the Supplemental Materials (text footnote 1). VPDB—Vienna Pee Dee Belemnite. No d18O values were corrected for mineralogical or vital effects. Late Pleistocene to Holocene (17-2 ka) planktic foraminifera, Guaymas Basin, Gulf of California Pre-dam era Mulinia, 20-65 km from mouth of the Colorado River, Gulf of California Pre-dam era Mulinia, Isla Montague, mouth of the Colorado River, Gulf of California modern barnacles, Isla Montague, mouth of the Colorado River, Gulf of California modern barnacles, Salton Sea marine barnacles

HM126 tidal bundle section (top) PV6 Roskowski et al. (2010) Milpitas HM125 tidal bundle section PV6 micrite Roskowski et al. (2010) Cibola HM124 tidal bundle section PVc Crossey et al. (2015) Cibola HM123 tidal bundle section (base) HM109 HM120 HM111 HM120 micrite HM113

A Micrite (this study) 4 B Ostracodes and foraminifers (this study) 4

2 2

δ18O δ18O (‰, VPDB) (‰, VPDB)

−20−10 5 −20−10 5

initial −2 −2 limestone initial horizon limestone horizon −4 −4

(‰, VPDB) −6 (‰, VPDB) −6 C C 13 13 δ δ

NBF - CHEM SBF - PKR SBF - MILP SBF - HMW NBF- CHEM - Candona SBF - HMW - Cyprideis NBF- CHEM - Cyprideis SBF - HMW - Cytheromorpha ~ 4.5 Ma Streptochilus globulosum SBF - PKR - Cyprideis SBF - HMW - Candona ~ 6 Ma Streptochilus latum SBF - MILP - Cyprideis SBF - HMW - Heterocypris ~ 10 Ma Streptochilus subglobigerum SBF - MILP - Cytheromorpha SBF - HMW - benthic foraminifers uncontested late Miocene and early Pliocene marine carbonate SBF - MILP - Candona SBF - HMW - Streptochilus spp. SBF - MILP - Heterocypris ~ 4.5 Ma Streptochilus globulosum ~ 6 Ma Streptochilus latum ~ 10 Ma Streptochilus subglobigerum uncontested late Miocene and early Pliocene marine carbonate

Figure 13. Cross-plots of composite stable oxygen (d18O) and carbon (d13C) isotope values from micrite, bulk sediment, and biologic calcite from outcrops of northern and southern Bouse Formation. (A) Comparison of northern Bouse Formation (NBF) and southern Bouse Formation (SBF) micrite (this study) and late Miocene and early Pliocene planktic foraminifers and nannofossils from the Pacific Ocean. (B) Comparison of multiple genera of northern and southern Bouse Formation ostracodes and foraminifers (this study) and late Miocene and early Pliocene planktic foraminifers and nannofossils from the Pacific Ocean. CHEM—Chemehuevi, PKR—Parker, MILP—Milpitas Wash, HMW—Hart Mine Wash. Pacific Ocean planktic foraminifer and nannofossil data (small white circles) were compiled from Douglas and Savin (1971, 1975, 1978), Keigwin (1979, 1982), Barrera et al. (1985), and Cannariato and Ravelo (1997). When written descriptions were given, the data were confined to “late Miocene” and “early Pliocene” samples, which approximates foraminifer-based age estimates for the southern Bouse Formation (McDougall and Miranda Martínez, 2014; Miranda-Martínez et al., 2017). When numerical ages were provided, the data were confined to the interval 5.8 Ma to 4.5 Ma. The western Pacific Ocean Streptochilus ages, isotopic values, and nomenclature are as in Resig and Kroopnick (1983) and Resig (1989). VPDB—Vienna Pee Dee Belemnite.

18 13 This indicates that both types of ostracodes were calcified in water with similar Potential Sources of the d OSED and d CSED Values in d18O values. The continental ostracodes do not have the low d18O values that the <45 µm Sediment Fraction would be expected if they calcified in meteoric water (e.g., streams) before 13 18 13 being reworked into a marine environment. The large offset in d COST values The <45 µm d OSED-d CSED values reflect autochthonous or detrital sources, between the continental and marginal marine ostracodes from the softer or perhaps a mixture of the two sources. Geologic mapping (Bishop, 1963; Jen- marls (Figs. 10B–10D) is consistent with a deeper-water environment (Homan, nings, 1967; Richard et al., 2000) has revealed that there are no detrital carbon- 2014) and seasonal differences in ostracode population dynamics. The conti- ate sources in the study area other than limited outcrops of Paleozoic marine nental ostracodes likely calcified during winter months, when the export of limestones in the Big Maria Mountains northwest of Blythe (Fig. 1) and in the organic matter (i.e., algae) from the epilimnion and the subsequent decay of Riverside Mountains near Parker (Fig. 1). The isotopic composition of these 12C-enriched organics in the benthos were minimal. In contrast, the marginal limestones is unknown, but the limited exposures suggest that they were not marine ostracodes likely calcified during the summer, when the epilimnion a significant source of detrital carbonate in this study, especially in the context was more productive, and the resulting benthic decay of 12C-enriched organic of the meters worth of carbonate sediments that typically comprise the basal matter was higher (e.g., Decrouy et al., 2011a, 2011b). Alternatively, the mar- carbonate unit. A second potential source of detrital carbonate is Colorado Pla- ginal marine ostracodes could have burrowed into the sediment before calcify- teau sediment delivered by the early Colorado River. This source is unlikely for ing, and so their valves might record the isotopic composition of 12C-enriched two reasons. First, river sediments were likely trapped in upstream basins as interstitial pore water rather than the composition of the overlying water body the river terminus worked its way south from the Grand Canyon area (Pearthree (e.g., Decrouy et al., 2011a, 2011b). The ostracode valves analyzed in this study and House, 2014). Sediment could not be transferred to the next basin down- do not support the notion that the SBF fossil assemblage is a mixture of conti- stream until sedimentation in the upstream basin had accumulated to the over- nental- and marine-derived fossils (e.g., Cronin et al., 2012). spill threshold. The first-arriving river water in any Bouse basin was probably This study does demonstrate that the abundant spiraled (benthic) foramin- relatively clear water, lacking abundant entrained sediment (e.g., Pearthree and ifers (i.e., Rosalina columbiensis, Ammonia beccarii) from the oxidized marls House, 2014). The basal carbonate unit was likely deposited in a clear-water en- 18 13 at the top of the basal carbonate unit and from the bottom of the interbedded vironment. Second, the SBF <45 µm d OSED-d CSED values can be compared to unit at Hart Mine Wash (Fig. 5) were likely reworked. Multiple samples of spi- the d18O-d13C values of potential detrital carbonate sources on and near the Col- raled foraminifer tests from these sediments consistently yielded displaced orado Plateau. For example, the averaged d18O and d13C values from a variety of d18O-d13C values that were dissimilar to the associated benthic ostracodes (Fig. carbonate bedrock types on the Colorado Plateau and nearby areas (i.e., Supai 10D), but that were virtually identical to values observed in micrite and ostrac- Group, Redwall Limestone, Leadville Limestone) are -5.3‰ ± 2.1‰ and -2.5‰ ± odes from the bioclastic and bedded limestone (Fig. 10D) at the bottom of the 1.8‰, respectively (n = 70; McKee, 1982; Muller and Mayo, 1986; Chidsey et 18 13 section. Both Rosalina and Ammonia are common in the bioclastic and bed- al., 2006). These values are distinctly different from the <45 µm d OSED-d CSED ded limestone horizon (Dorsey et al., 2018), and these sediments are a likely values from the NBF and SBF (Fig. 14A). The more terrigenous interbedded unit 18 13 13 source of the reworked tests. Several Cytheromorpha d O-d C values from sediments have d CSED values that are more similar to a potential detrital signal, 18 soft upper marl sediments displayed a similar, seemingly displaced, pattern but the interbedded unit d OSED values are several per mil lower than the de- (Fig. 10D). Cytheromorpha valves are the smallest and least massive ostrac- trital signal (Fig. 14A); thus, detrital carbonate cannot have been the dominant 18 13 18 ode valves analyzed in this data set. Their small size likely renders them more source of the interbedded unit d OSED-d CSED values. Furthermore, the d OSED- 13 susceptible to reworking, an explanation that is extended to the even smaller d CSED values from the interbedded unit are similar to those measured in as- foraminifer tests. In contrast, the biserial foraminifer tests from the oxidized sociated ostracode valves, which would not have incorporated a terrigenous marl have d18O-d13C values similar to the associated benthic ostracodes (Fig. carbonate signal (Figs. 10B–10D). Collectively, these observations show that the 18 13 18 13 10D), suggesting they were not reworked from older sediments. Biserial for- Bouse d OOST, d COST, d OSED, and d CSED values all recorded the isotopic com- aminifer tests from the oxidized marl showed nearly identical d18O-d13C val- position of the water body present during deposition. The influence of detrital ues as biserial foraminifer tests in the overlying interbedded unit (Fig. 10D). carbonate was minimal to nonexistent. Biserial foraminifers from the interbedded unit were likely reworked from the oxidized marls (Fig. 10D). Previous SBF studies have acknowledged that foraminifer tests have been reworked into the interbedded unit (McDougall Northern Bouse Formation—The Lacustrine Template and Miranda Martinez, 2016), but the “spiraled” foraminifer d18O-d13C values provide the first evidence that benthic foraminifer tests at the top of the basal The NBF exposed in the Chemehuevi basin contains a mix of continental carbonate unit were likely reworked as well. Foraminifer-based biostrati- (Limnocythere, Candona, Darwinula) and typically marginal marine but conti- graphic or biochronologic interpretations for the SBF should be interpreted nentally invasive (Cyprideis) ostracodes in association with rare foraminifers with caution. (Fig. 2).

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 1894 by guest on 26 September 2021 on 26 September 2021 by guest Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/14/4/1875/4265689/1875.pdf Research Paper A Detrital carbonate influence on <45 um micrite Bouse Formation, VPDB—Vienna Pee Dee Belemnite. See Figure 1 for locations. samples highlighting potential trends in orado Plateau data were compiled from McKee (1982), Muller and Mayo (1986), and Chidsey et al. (2006). (B) Previously reported southern Bouse Formation bulk carbonate (NBF) and southern Bouse Formation (SBF). (A) Comparison of a potential detrital carbonate signal from the Colorado Plateau area and <45 µm micrite from this study. Col - d Figure 14. Cross-plots of stable oxygen ( Ingram et al. (1996a). (D) Previously published southern Bouse Formation biologic carbonate. Data were compiled from Roskowski et al. (2010) and Crossey et al. (2015). No discussion. (C) Comparison of mollusc shell from the San Francisco Bay estuary and micrite and ostracode valves from this study. San Francisco Bay mollusc data are from 18 C Bouse Fm. versus San Francisco Bay estuar y O values have been corrected for mineralogical or vital effects. C—Cibola, M—Milpitas Wash, BM—Big Maria quarry, NBF—northern Bouse Formation, SBF—southern −2 0− SBF Hart Mine Wa SBF Milpitas micrite SBF Parker micrite San Francisco Bay estuary mollusc shel −2 0− NBF Chemehuevi basin micrit (‰, VPDB) δ (‰, VPDB) δ 18 18 O trend line for San Francisco Bay mollusc shell O fluvial-influenced Av SBF Basal Carbonate Unit - bioclastic horizon SBF Basal Carbonate Unit - soft marl horizon SBF Interbedded Unit NBF Chemehuevi basin (n = 70; error bars ± 1 σ ) San Fran. Ba y sh micrite erage, carbonate rocks, Colorado Plateau 10 10 e d

d 18 18 O) and carbon ( SBF Hart Mine Wa SBF Milpitas ostracode SBF Parker ostracode O- NBF Chemehuevi basin ostracode d 13 l C values. Data were compiled from Poulson and John (2003), Roskowski et al. (2010), and Crossey et al. (2015). See text for

δ13C δ13C

d (‰, VPDB) (‰, VPDB) 13 Chemehuevi −6 −4 marine-influenced C) isotope values from micrite, bulk sediment, and biologic calcite from outcrops of northern Bouse Formation −6 −4 −2 2 4 2 4 Cyprideis marg. marine San Fran. Ba y sh ostracode continental ostracodes ostracodes 5 5 D Previously published biologic carbonat B Previously published SBF bulk carbonat bivalve (Crossey et al., 2015) bivalve (Crossey et al., 2015) barnacle (Crossey et al., 2015) barnacle (Roskowski et al., 2010) −2 0− −2 0− Blythe basin bulk carbonate samples (Crossey et al., 2015) Other Blythe basin bulk carbonate samples (Roskowski et al., 2010) Marl Wa Blythe basin bulk carbonate samples (Poulson and John, 2003) (‰, VPDB) δ (‰, VPDB ) δ 18 18 trend line for Marl trend line for all other samples excluding Marl O R O 2 = 0.01 sh transect (Roskowski et al., 2010) “fish” marl s M Wa sh samples (n = 16) 10 10 C gastropod (Crossey et al., 2015) gastropod (Crossey et al., 2015) gastropod (Crossey et al., 2015) calcareous algae (Crossey et al., 2015) M C BM e e δ13C δ13C (‰, VPDB) Wa (‰, VPDB) −6 −4 −2 −6 −4 −2 R sh 2 4 2 4 2 = 0.69

5 5

GEOSPHERE | Volume 14 | Number 4 Bright et al. | Freshwater plumes and brackish lakes 1895 Research Paper

The marginal marine ostracode genus Cyprideis is common in western cording a through-flowing, fluvially dominated environment feeding the devel- U.S. lakes and wetlands (e.g., Stout, 1981; Jayko et al., 2008), as well as in oping paleolake Blythe to the south. nearshore marine and estuary environments (e.g., McGann et al., 2013). The This interpretation supports previous interpretations that the NBF in two species of Cyprideis commonly found in North America have broad sa- Chemehuevi basin was deposited in a lacustrine environment (Spencer and linity tolerances that range from ~200 to 40,000 mg L–1 (Forester et al., 2005). Patchett, 1997; Poulson and John, 2003; Reynolds, 2008; Crossey et al., 2015; Darwinula stevensoni specimens are rarely found in water where salinity ex- Reynolds et al., 2016). Now that the microfossil and stable isotope character- ceeds 1000 mg L–1 (Forester et al., 2005). However, Holmes et al. (2010) noted istics of a known fill-and-spill lacustrine setting in the Chemehuevi basin have that D. stevensoni can tolerate higher salinities (to 15,000 mg L–1), but only been characterized, the NBF lacustrine model can be compared against the when the salinity value is stable; otherwise, they generally prefer less saline microfossil and stable isotope results from the contested SBF. waters up to ~2000 mg L–1, similar to the upper limit reported by Forester et al. (2005). Although the Limnocythere sp. and Candona spp. are yet to be identified to species level, they do not belong to any of the common species of Southern Bouse Formation—Testing the Marine Model North American Limnocythere (i.e., Limnocythere staplini, Limnocythere sap- paensis) or candonid species (i.e., Fabaeformiscandona rawsoni) that might The defining criteria of the partial marine model are the age and the source indicate elevated salinity values above ~5000 mg L–1 (Forester et al., 2005). of the source of water that produced the SBF. The partial marine model hypoth- Cyprideis co-occurring with Limnocythere, Candona, and Darwinula in a esizes that the basal carbonate unit was being deposited in a marine basin by low-salinity lacustrine setting is not uncommon (e.g., Mischke and Wünner- at least 6 Ma, and at least 0.7 m.y. before the early Colorado River reached the mann, 2006). Finding foraminifers associated with freshwater or mildly brack- basin (McDougall and Miranda Martínez, 2014, 2016; Dorsey et al., 2018). Thus, ish species of Candona is less common, but it does occur (e.g., Willard et al., it would have been deposited in an environment that, from a faunal and isoto- 2000; Petit-Marie et al., 2010; Primavera et al., 2011). The absence of any known pic perspective, should have been fundamentally different from the younger salinity-tolerant candonids or limnocytherids and the initial presence of only Colorado River–fed lacustrine environments of the NBF. rare foraminifers suggest that salinity during the deposition of the NBF in the Numerous studies have suggested the SBF basal carbonate unit was de- Chemehuevi basin was probably on the order of 5000 mg L–1 or less. Although posited in a marine environment, based largely on the ecological constraints the salinity was relatively low, the Candona and Cyprideis valves at the bottom provided by its diverse foraminifer assemblage (McDougall and Miranda 18 13 of the section yielded marine-like d OOST-d COST values (Figs. 9 and 10A). These Martínez, 2014; Dorsey et al., 2018). This study reveals several important ob- high values likely reflect some combination of the initial evaporation of water servations that that make a marine origin for the SBF unlikely. First, micrite in Lake Mohave and additional evaporation of this water as it was impounded and ostracode valves from the limestone at the bottom of the formation, which in the Chemehuevi basin. The initial Cyprideis-Limnocythere ostracode fau- should represent the first flooding of the land surface by seawater (e.g., Mc- 18 nal assemblage associated with rare foraminifers and high d OSED and high Dougall and Miranda Martínez, 2014), lack a definitive marine isotopic signa- 18 d OOST values (Fig. 2) is interpreted to represent a nearly freshwater, terminal ture (Figs. 13A and 13B). Second, the ostracode fauna in the SBF is similar to, lacustrine environment that formed as the upstream Mohave basin (Fig. 1) first but more diverse than, the ostracode fauna of the lacustrine NBF described began to drain southward, by either groundwater sapping (e.g., Crossey et al., herein. No strictly marine ostracode genera were found in this study, much 2015) or threshold overspilling (e.g., Pearthree and House, 2014), or both. The like earlier studies (Smith, 1970; Metzger et al., 1973; Miller et al., 2014; Bright initial inflow to Chemehuevi basin may have been low compared to local evap- et al., 2016). Third, continental ostracodes (Candona, Heterocypris) yielding 18 18 18 oration rates, which would help elevate the initial d OSED and d OOST values. d OOST values that are virtually identical to their marginal marine ostracode The abrupt change in the microfossil assemblage coupled with an abrupt counterparts are variably present throughout the basal carbonate unit (Fig. 9). decrease in the isotopic composition of micrite and ostracode valves low in the Thus, the continental and marginal marine ostracodes were calcifying in water 18 18 13 18 13 Chemehuevi marl (Fig. 2) are interpreted to represent a south-directed over- with the same d O value. Fourth, d OSED, d CSED, d OOST, and d COST values topping event at the Aubrey paleodam (Fig. 1). This resulted in a transition from the SBF can be compared against the d18O-d13C values from a variety of from a closed-basin to a through-flowing basin configuration and thus, likely mid-Miocene to early Pliocene marine planktic foraminifers and nannofossils reflects the onset of truly freshwater conditions. An analogy can be drawn to from the Pacific Ocean (Figs. 13A and 13B). This comparison is grounded in the a similar event that occurred in Pliocene Villarroya Lake, Spain, (Anadón et assumption that the isotopic composition of the late Miocene and early Plio- al., 2008). There, marine-like d18O-d13C values in Candona valves and freshwa- cene seawater that inundated Blythe basin was comparable to late Miocene ter clam shells abruptly shifted to much lower values when the depositional and early Pliocene seawater in the Pacific Ocean. Small differences might be environment changed from a terminal-basin to an open-basin configuration expected, as illustrated in the modern Gulf of California, where the restricted (Anadón et al., 2008). The brown claystone at the top of the Chemehuevi se- environment and high evaporation rates cause surface-water d18O values to be 18 18 quence yielded the lowest d OSED values in the data set (Fig. 2), probably re- slightly higher than open-marine d O values (Keigwin, 2002). The late Miocene

and early Pliocene planktic foraminifers cluster near the origin of the graph, the local highlands surrounding Blythe basin. Crossey et al. (2015) provided (d18O = 0‰, d13C = 0‰), as expected for marine carbonates (Figs. 13A and 13B). a slightly different solution; they allowed water from the early Colorado River Multiple outcrops of the SBF yielded micrite and ostracode valves with iso- and other upstream sources to be present during the deposition of the basal topic compositions that are strongly dissimilar to the marine carbonate an- carbonate unit. Crossey et al. (2015) drew comparisons between the d18O-d13C alogue, but that are virtually indistinguishable from each other and that are values of marl in the southern Blythe basin and of mollusc shells from the San virtually indistinguishable from comparable material from the lacustrine NBF Francisco Bay estuary. The d18O-d13C composition of the San Francisco Bay es- (Figs. 13A and 13B). Most importantly, the most marine-like d18O-d13C values in tuary results from seawater that is diluted, in both salinity and isotopic compo- this study come from the valves of the continental freshwater to mildly brack- sition, by freshwater from the Sacramento and San Joaquin Rivers. The Sacra- ish water ostracodes Candona and Heterocypris from the deeper-water soft mento–San Joaquin River watershed covers nearly 150,000 km2, or nearly 40% marls of the SBF and from Candona valves in the lower marls in the Chemehu- of the state of California, and these rivers deliver an average annual inflow of evi basin (Figs. 10, 13A, and 13B). To our knowledge, there are no documented 600 m3 s–1 to the San Francisco Bay estuary (McGann et al., 2013, and refer- accounts of either continental ostracode genus living and calcifying in normal ences therein). During spring runoff season, these rivers drain voluminous seawater. Fifth, neither the benthic (“spiraled”) nor the biserial foraminifers amounts of snowpack with low isotopic values from the Sierra Nevada, the from the basal carbonate unit in Hart Mine Wash yielded convincingly marine Cascade Range, the Klamath Mountains, and the Coast Range. d18O-d13C values (Fig. 13B). Several studies have cited the foraminifer assem- The Bouse estuary models feature a positive and highly covariant trend blage as evidence that the basal carbonate unit was deposited in a marine in the d18O-d13C values from the basal carbonate unit (Crossey et al., 2015). embayment (McDougall, 2008; McDougall and Miranda Martínez, 2014, 2016; McDougall and Miranda Martínez (2014) featured data that encompass most of Dorsey et al., 2018), but the d18O-d13C values in the foraminifer tests themselves the Blythe basin, while Crossey et al. (2015) limited their discussion to marls re- are lower than expected for late Miocene and early Pliocene marine carbon- stricted to the southernmost portion of the basin, where planktic foraminifers ates (Fig. 13B) and suggest otherwise. Some of the biserial foraminifer tests are most abundant. Both data sets overlap considerably. The proposed Bouse from the upper basal carbonate unit and the lower interbedded unit show mild estuary trend is defined by bulk carbonated 18O-d13C values that range from carbonate encrustation under SEM (Fig. 7). The isotopic composition of this modestly low values (d18O ≈ -8‰, d13C ≈ -6‰) to values that approach those carbonate is not known. In a worst-case scenario the isotopic contribution of expected in normal marine carbonates (d18O ≈ +1‰, d13C ≈ +1‰; e.g., Crossey the encrusting carbonate might be enough to lower the d18O-d13C values of et al., 2015). The low values would presumably represent the most fluvially those samples. If so, then the original d18O-d13C values might be expected to dominated water in a Bouse estuary, whereas the high values would presum- plot closer to the origin of the graph, in the same area as co-occurring conti- ably represent the most marine-dominated water in the estuary. Crossey et al. nental ostracodes (Candona, Heterocypris) from the Blythe basin (Figs. 10C, (2015) showed that a similar trend with similar d18O-d13C values is seen in the 10D, and 13B), and in the same area as the Candona valves from the Cheme- San Francisco Bay estuary, based on the analysis of mollusc shells (Ingram et huevi basin (Figs. 10A and 13B). It would remain extremely improbable that al., 1996a, 1996b). planktic marine foraminifers would have similar d18O-d13C values as benthic Salinity and d18O values in the San Francisco Bay display a strong positive freshwater continental ostracodes (i.e., Candona), and especially benthic os- covariance (R2 value = 0.98), where dilute water (0.5 ppt) has low d18O values tracodes from the lacustrine Chemehuevi basin (Figs. 10A and 13B). Collec- (-10.5‰ relative to Vienna Standard Mean Ocean Water [VSMOW]), and saline tively, these observations demonstrate that the basal carbonate unit was not water (29.7 ppt) has high d18O values (-2.0‰ VSMOW; Ingram et al., 1996a). deposited in a marine embayment. Strong covariance between low/high salinity and low/high d18O-d13C values is common in nearshore marine and estuarine environments (e.g., Dettman et al., 2004; Sampei et al., 2005). Mollusc shells collected from nearest the mouth Southern Bouse Formation—Testing the Estuary Model of the San Francisco Bay estuary grew in water with an average salinity of ~28.2 ± 3.8 ppt (n = 17), and these shells yield the highest d18O-d13C values (ap- The Bouse estuary model has been expressed in two slightly different proximately -1‰ and -1‰, respectively; Ingram et al., 1996a). Mollusc shells contexts by McDougall and Miranda Martínez (2014) and Crossey et al. (2015). collected near the head of the San Francisco Bay estuary grew in water with McDougall and Miranda Martínez (2014) defined their use of the term estuary a lower average salinity of ~16.6 ± 7.1 ppt (n = 18), and these shells yielded as a “coastal body of water with free communication to the ocean and within lower d18O-d13C values (approximately -6‰ and -5‰, respectively; Ingram et which ocean water is diluted by freshwater derived from land” (p. 845, and al., 1996a). citing Valle-Levinson, 2010). They hypothesized that the basal carbonate unit The lowest d18O-d13C values in the inferred Bouse estuary trend (d18O ≈ -8‰, was being deposited at least 0.7 m.y. prior to the arrival of the early Colorado d13C ≈ -6‰) are comparable to the lowest values in the San Francisco Bay estu- River, and so any dilution of seawater in their Bouse estuary must, by neces- ary, yet the estuary component of the partial marine model (McDougall and Mi- sity, have resulted from water sourced in the small tributaries that drained randa Martínez, 2014) requires these values be generated in the absence of the

early Colorado River—a river that would have drained voluminous amounts of regis” (McDougall and Miranda Martínez, 2014) or as “southern Blythe fish” high-elevation snowpack from the Rocky Mountains. The partial marine model (Crossey et al., 2015) and are excluded from discussion. If these “fish” marls are requires that the lowest d18O-d13C values were generated by the isotopic dilu- included, then the proposed Bouse estuary-like trend becomes less impressive tion of seawater by small, local, and ephemeral tributaries that drained several (Fig. 14B). The trend becomes even less convincing if data from the northern low-elevation mountain ranges (<1000 m asl) adjacent to Blythe basin. There Blythe basin are also included in the discussion. Poulson and John (2003) pre- is no clear evidence, such as pre-Bouse terminal lakes in the Mohave or Blythe sented numerous basal carbonate unit d18O-d13C values from the northern part basins, that suggests that the study area was substantially wetter during the of the basin that do not conform to the proposed Bouse estuary trend. Their late Miocene and early Pliocene than it is today. It is highly unlikely that the Blythe basin data trend in a horizontal fashion, from low values similar to those volume and isotopic composition of local runoff could have been sufficient to measured in the southern Blythe “fish” marls d( 18O ≈ -12‰, d13C ≈ -1‰) to a isotopically dilute seawater in a Bouse estuary to the same degree as observed single analysis that yielded values close to those expected for normal marine in the modern San Francisco Bay estuary. A simple thought experiment using carbonate (d18O ≈ –2‰, d13C ≈ 0‰; Fig. 14B). local precipitation rates and the surface area of “paleolake Blythe” highlights Further analysis reveals that the proposed Bouse estuary trend (e.g., this improbability. Crossey et al., 2015) is dominated by data from a geographically limited por- The city of Blythe, California, is located just to the southeast of the Big tion of Blythe basin. Roskowski et al. (2010) analyzed 16 bulk samples of the Maria Mountains (Fig. 1) and receives roughly 0.1 m yr–1 of precipitation basal carbonate unit along an ~2-km-long section of “Marl” or “Cibola” wash (wrcc.dri.edu; station 040924). The evaporation rate for the local region is (Fig. 1). These samples yielded highly covariant d18O-d13C values (R2 value = ~1.4 m yr–1 (Spencer et al., 2013). There are no perennial streams in the Blythe 0.69) that define a positive trend from low to high, marine-like, values (Fig. basin today because the evaporation rate greatly exceeds the precipitation rate. 14B). However, the remaining bulk carbonate samples from Blythe basin do In this exercise, the drainage basin feeding the Blythe basin will be approxi- not behave in this fashion. If the Marl Wash data are excluded, then the R 2 mated by the maximum surface area of “paleolake Blythe” (Fig. 1), which is value drops to 0.01 (Fig. 14B). The dominance of the Marl Wash data is also roughly 10,400 km2 (Spencer et al., 2013). A maximum of 1 km3 yr–1 of water evident in the estuarine trend proposed by McDougall and Miranda Martínez could be generated if 100% of the precipitation falling on the Blythe basin is (2014). Furthermore, the few bulk sediment samples with the most marine-like converted to runoff and if evaporation and infiltration are completely (and unre- d18O-d13C values (d18O ≈ 0‰, d13C ≈ 0‰; Fig. 14B) seemingly require calcifica- alistically) excluded from consideration. In contrast, the Sacramento–San Joa- tion in 100% seawater, yet strontium isotope considerations suggest that the quin watershed delivers ~19 km3 yr–1 (estimated from McGann et al., 2013), i.e., amount of seawater present in Blythe basin could have been as little as 5% to nearly 20 times the estimated (and unreasonable) maximum volume of water 15% to as much as 60% to 75% (Crossey et al., 2015). Strontium isotope mod- that can be generated in the Blythe basin by precipitation alone. We also con- eling does not suggest that fully marine conditions were ever present (Crossey sider that discharge from the Sacramento–San Joaquin watershed includes a et al., 2015). Thus, previous discussions of estuarine isotopic trends in the SBF large seasonal contribution from high-elevation snowmelt (i.e., Sierra Nevada) cannot adequately account for the most marine-like d18O-d13C values in SBF that would have low d18O values (i.e., -12‰ VSMOW; Ingram et al., 1996a). In bulk carbonates, and they place a disproportional amount of interpretational contrast, the highest elevations around the Blythe basin are below 1000 m asl leverage on 16 analyses of bulk carbonate from one 2-km-long transect in Marl and would be incapable of generating runoff with similarly low, snowmelt- Wash (Fig. 1). derived, d18O values. Thus, it is inconceivable that precipitation falling on Blythe The d18O-d13C values in this study were compared to d18O-d13C values from basin during the late Miocene could realistically provide enough water of the mollusc shells from the San Francisco Bay estuary (Fig. 14C) following Crossey appropriate isotopic composition to create a Bouse estuary that could mimic the et al. (2015). The highest, most marine-influenced (highest salinity)d 18O-d13C 18 13 salinity and isotopic characteristics of the San Francisco Bay estuary. The estu- values from San Francisco Bay overlap with the Cyprideis d OOST-d COST values ary component of the partial-marine model (McDougall and Miranda Martínez, from the lowest marls in the Chemehuevi basin (Fig. 14C). Several data points 18 13 2014, 2016), and as featured in Dorsey et al. (2018), is untenable. Crossey et al.’s also overlap with the continental ostracode d OOST-d COST values from the soft (2015) estuary model resolves this issue by allowing the early Colorado River to marls at Hart Mine Wash and Milpitas Wash and several of the continental os- 18 13 be present during deposition of the basal carbonate unit. tracode d OOST-d COST values from the lacustrine Chemehuevi basin (Fig. 14C). Careful analysis of the proposed Bouse estuary trend reveals that McDou- The lowest, most fluvially influenced (lowest salinity)d 18O-d13C values from San gall and Miranda Martínez (2014) and Crossey et al. (2015) did not include the Francisco Bay are most similar to the marginal marine Cyprideis and Cyther- 18 13 18 13 d O-d C values from a suite of marl slabs containing the impressions and a omorpha d OOST-d COST values from the soft marls at Parker, Hart Mine Wash, 18 13 few bones of the marine fish Colpichthys regis (Roskowski et al., 2010). These and Milpitas Wash (Fig. 14C). The ostracode ecology and the d OOST-d COST marls yielded low, nonmarine d18O values of approximately -11‰ (Roskowski values in this study are, for all intents and purposes, inverted from what would et al., 2010), even though they preserve the impressions and bones of a ma- be expected in a normal estuary. The continental ostracodes should have been rine fish. These marls were identified in various earlier figures as “Colpichthys calcifying in the most dilute, fluvially influenced portion of theestuary (e.g.,

Elliott et al., 1966; Forester and Brouwers, 1985; Smith and Horne, 2002), and of the genera are presumably living in the bay alongside a diverse benthic the marginal marine ostracodes would presumably be calcifying their valves in foraminifer assemblage that has been cited as a potential analogue for the the most saline, marine-influenced portion of the estuary. This is not the case benthic foraminifer assemblage in the SBF (McDougall and Miranda Martinez, with the SBF (Fig. 14C). It is also unreasonable to suggest that the Candona 2014). The complete absence of similar marine ostracode genera in the SBF and Cyprideis valves in the lacustrine Chemehuevi basin (Figs. 10A and 13B) is convincing evidence that the SBF is not a comparable estuarine deposit. calcified in normal seawater. Although previous Bouse studies have provided thorough discussions of es- Several other observations make an estuarine origin for the SBF unlikely. tuary-like trends in the d18O-d13C values in SBF carbonates (McDougall and No evidence was found in this study for an isotopic gradient between the Miranda Martínez, 2014; Crossey et al., 2015), the combination of ostracode 18 13 northern (Parker, Fig. 1) and southern (Hart Mine Wash or Milpitas Wash, Fig. ecology and the d OOST-d COST values in the ostracode valves themselves is 1) margins of the Blythe basin (Figs. 13A and 13B), as might be expected in a incompatible with an estuarine environment. Bouse estuary (e.g., Ingram et al., 1996a; Sampei et al., 2005). No evidence for variable and evolving mixtures of marine and fluvial water masses that might be expected in an estuary was found either, presuming that sea level may have Southern Bouse Formation—Testing the Lacustrine Model varied or that the early Colorado River experienced century- or millennial-scale (or longer) climatically induced variations in discharge that would be detect- In contrast to the marine and estuary models, the lake model of Spencer able as antithetic fluctuations in marine-brackish-freshwater ostracode spe- et al. (2008, 2013) hypothesizes that the entirety of the SBF was deposited in cies (e.g., Mazzini et al., 1999) or detectible in the d18O values of the ostracode simply another Colorado River–fed lake and, thus, was deposited in an envi- valves (e.g., Cronin et al., 2005). ronment that, from a faunal and isotopic perspective, might be expected to Finally, the ostracode faunal assemblage of the San Francisco Bay estuary possess fundamental similarities with the upstream NBF. was summarized in McGann et al. (2013). They found 19 species, of which only two were continental, Physocypria globula and Ilyocypris gibba. Ilyocypris gibba was found at only two stations where salinities at the time of collection Similarities in Ostracode Faunas with the Northern Bouse Formation were 0 practical salinity units (psu) and 10 psu (McGann et al., 2013). Ilyocypris gibba have limited salinity tolerance and are considered restricted to oligoha- The ostracode fauna in the SBF is similar to, but more diverse than, that line (<5 ppt) environments (e.g., Wilkinson et al., 2005), and so its occurrence of the lacustrine NBF described above. The SBF fauna includes a mix of con- at 10 psu in the San Francisco Bay likely reflects transport into more brackish tinental ostracodes (Candona spp., Heterocypris sp., Darwinula stevensoni, water (McGann et al., 2013). No specific salinity measurements were provided Limnocythere sp.) and typically marginal marine ostracodes (Cyprideis sp., for the occurrences of Physocypria globula, but the overall salinity dynamics Cytheromorpha spp., rare Perissocytheridea sp.; Figs. 3–6).The similarities in of San Francisco Bay suggest that P. globula individuals occur where salinities the ostracode faunas argue for similar lacustrine paleoenvironments, although are typically less than 15 psu. Note that for the purposes of this study, “psu” the addition of Heterocypris sp., Cytheromorpha spp., and rare Perissocyther- and “ppt” can be considered equivalent (e.g., UNESCO, 1981). In contrast, idea (Figs. 4 and 5) in the SBF may imply slightly different water composition— the basal carbonate unit of the SBF contains a higher diversity of continental perhaps a composition slightly more similar to dilute seawater. Like the NBF, ostracodes, including Heterocypris, Darwinula, Limnocythere, and multiple the SBF also lacks any of the saline-tolerant limnocytherid or candonid species species of Candona (Bright et al., 2016; this study). Freshwater fish (Gila) and (e.g., F. rawsoni) that might suggest elevated salinity. The fairly persistent pres- a variety of continental molluscs (Pisidium, Sphaerium, Flumnicola, Amni- ence of Candona spp. during deposition of most of the basal carbonate unit cola, Physa[?]) are also variably present (Reynolds and Berry, 2008; Roeder (Figs. 3–6) suggests that salinity likely remained below 10,000 mg L–1, and may and Smith, 2016). Two marginal marine ostracodes, Cyprideis beaconensis have been lower (see NBF discussion). and Cytheromorpha grandwashensis, are present in San Francisco Bay, and Although the continental ostracode taxa in the SBF are found in association both Cyprideis and Cytheromorpha are present in the SBF. Critically though, with a variety of marine foraminifers, they are incompatible with a fully ma- the San Francisco Bay estuary contains 16 genera of marine ostracodes that rine or saline-estuarine environment (see previous discussion). A more com- as a group are completely absent in the SBF. The marine ostracode fauna parable environment might be found in Lake Pontchartrain, where a modestly in the San Francisco Bay estuary consists of Acuminocythere, Ambostracon, diverse benthic foraminifer fauna (Otvos, 1978) occurs alongside a very Bouse- Aurila, Cytherura, Eusarsiella, Kangarina, Loxoconcha, Palmoconcha, Para- like Cyprideis-Candona-Darwinula ostracode fauna in low-salinity (<10 ppt) doxostoma, Radimella, Robustaurila, Sagmatocythere, Sahnicythere, Serro- water (Willard et al., 2000). The only typically marginal marine ostracodes so cytherura, Spinileberis, and Xestoleberis (McGann et al., 2013). Some of the far recovered from the SBF (Cyprideis, Cytheromorpha, rare Perissocyther- marine ostracode valves appear to be reworked from fully marine environ- idea) are all genera that have also been reported from continental lakes (e.g., ments outside of the San Francisco Bay (McGann et al., 2013), but the majority Stout, 1981; Pérez et al., 2011; Pint et al., 2015). This is in sharp contrast to

the numerous strictly marine ostracode genera, such as Cushmandia, Puriana, 6). Similar sediments with similar isotopic values are not present at the bot- Aurila, and others, that appeared in the nearby continental Laguna Salada ba- tom of the NBF (Fig. 2). Nonetheless, the overall isotopic similarities with the sin (Fig. 1) when seawater apparently entered that basin during the early- to NBF (Figs. 13A and 13B), when coupled with the similar Cyprideis-Candona - mid-Holocene (Romero Mayén, 2008), or to the 19 species of marine ostrac- foraminifer association described from the bottom of the NBF section, is odes present in the San Francisco Bay (McGann et al., 2013). Thus, the ostrac- compelling evidence for a similar lacustrine origin for the Bouse Formation in ode fauna from the SBF is better explained by a lacustrine origin than a marine both regions. The persistent co-occurrence of marginal marine and continen- 18 or estuarine origin. tal ostracodes, coupled with the gradual increase in d OOST values in multiple In addition to a salinity-based interpretation, the more abundant marine ostracode genera throughout the thick Hart Mine Wash sedimentary sequence organisms in the SBF likely imply evolution of the overall ionic composition (Fig. 9), is more compatible with an evaporative terminal lake environment. and the Ca/ALK ratio of paleolake Blythe to a more sodium-, chloride-, and However, the low-salinity (mildly brackish) constraints imposed by the SBF os- 18 sulfate-rich, marine-like, composition (e.g., Anadón, 1992). Recall that a ma- tracode fauna is not compatible with the high, marine-like continental d OOST rine-like water composition was initially present in the lacustrine Chemehu- values encountered in this study (Fig. 13B). evi basin, as revealed by the early presence of Cyprideis sp. in association A lacustrine interpretation can reconcile this apparent conflict because, un- with rare foraminifers there (Fig. 2). Drawing on Spencer et al. (2008, 2013), like marine or estuarine systems, salinity and d18O values can be decoupled the more diverse marginal marine ostracode fauna in the SBF is interpreted in lakes. For example, Echo Lake, situated at nearly 3000 m asl in Colorado, as representing a gradual downstream evolution in lake waters along a chain has a low electrical conductivity that equates to a salinity of ~50 mg L–1, yet its of Colorado River–fed lakes (Fig. 1), with paleolake Blythe being the largest, water has a d18O value of about -4‰ VSMOW (Henderson and Shuman, 2009). the most distal, and therefore the most compositionally evolved and most This decoupling between salinity and d18O in some lakes readily explains how 18 marine-like lake in the chain (e.g., Wood and Talling, 1988). In contrast to Candona valves with high, marine-like d OOST values could be produced in a Spencer et al. (2008, 2013), the ostracode fauna implies that paleolake Blythe low-salinity, closed-basin environment at Villarroya Lake, Spain (Anadón et al., was not as saline as normal seawater and instead was only mildly brackish 2008), and in a freshwater alpine pond at Snowmass Village, Colorado (Sharpe (~5000–10,000 mg L–1). Higher-salinity phases of paleolake Blythe may have and Bright, 2014). A similar decoupling of salinity and d18O values in both the occurred at lower lake levels, but these would likely only be preserved in the Chemehuevi and Blythe basins effectively accounts for the marine-like Can- 18 subsurface closer to the depocenter of the basin. The ostracode fauna that dona d OOST values in both basins. has thus far been reported from the subsurface contains the same genera Bright et al. (2016) noted that the abundance of continental ostracodes in- found in the surface outcrops (Smith, 1970; Metzger et al., 1973), suggesting creases across the distinctive clay layer (e.g., Fig. 5) and interpreted that as that similar mildly brackish environments are preserved in the subsurface. A evidence for a decrease in overall salinity. Their study was limited to a single mildly brackish environmental interpretation echoes Taylor’s initial assess- outcrop in Hart Mine Wash. The larger data set featured in this study can now ment from nearly 50 yr ago (in Smith, 1970), which stated that the impover- place that faunal transition into a broader context. The abundance of continen- ished marine fauna of the SBF implies a water composition that was different tal ostracodes (e.g., Candona) does increase across the distinctive clay layer from normal seawater. (Figs. 4 and 5), but continental ostracodes, although rare, are variably present throughout the basal carbonate unit (Figs. 3–6). Ostracode abundances are noticeably low through several meters of sediments that were deposited prior to Similarities in d18O and d13C Values with the Northern Bouse Formation the distinctive clay layer (Figs. 4 and 5). These sediments are finely laminated, possibly indicating low-oxygen conditions in a stratified lake (e.g., Anadón et 18 13 18 Multiple outcrops of the SBF have yielded individual d OSED, d CSED, d OOST, al., 1998; Sáez and Cabrera, 2002). Low-oxygen conditions are also implied by 13 and d COST values that are virtually indistinguishable from comparable lacus- the high abundance of the biserial foraminifers Streptochilus/Bolivina in this trine NBF values (Figs. 13A and 13B). Additional similarities with the NBF can same interval (Figs. 4 and 5). Streptochilus/Bolivina spp. often dominate for- 18 18 be observed in the 8‰ to 10‰ difference between d OSED and d OOST values aminifer faunas in low-oxygen environments (e.g., Smart and Murray, 1994). at Parker, Milpitas Wash, and Hart Mine Wash (Fig. 9), and in an abrupt 8‰ The re-establishment of continental ostracodes above the distinctive clay 18 decrease in d OOST values across the basal carbonate–interbedded unit contact layer may be explained by a return to a more oxygenated benthos as much (the distinctive clay layer or “DCL” of Bright et al., 2016) at Milpitas Wash and as by a decrease in salinity. The fact that continental ostracodes are present 18 18 Hart Mine Wash (Fig. 9). Virtually identical differences in d OSED and d OOST val- throughout most of the basal carbonate unit suggests that any decrease in sa- 18 ues and a virtually identical abrupt decrease in d OOST values occur in the NBF linity across the distinctive clay layer was modest at best, and almost certainly in the Chemehuevi basin (Fig. 9). The isotopic stratigraphy of the SBF is slightly could not be characterized as a transition from a fully marine environment to more complex (Fig. 10), but much of the added complexity is accounted for by a river-fed environment (e.g., McDougall and Miranda Martínez, 2014, 2016; the bioclastic-rich and bedded limestones at the bottom of the SBF (Figs. 5 and Dorsey et al., 2018).

Isotopic Evidence for the First Benthic and Lacustrine Occurrence of A benthic Streptochilus interpretation carries important consequences for the Planktic Foraminifer Streptochilus the SBF debate. The presence of planktic foraminifers has long been offered as strong evidence for a marine environment (Smith, 1970; McDougall, 2008; The SBF Streptochilus fauna is dominated by the planktic foraminifer Strep- McDougall and Miranda Martínez, 2014). Streptochilus spp. comprise ~97% of tochilus latum (McDougall and Miranda Martínez, 2014), which may indicate the rare planktic foraminifers identified from the SBF (McDougall and Miranda water depths of ~100–150 m (Nikolaev et al., 1998). However, the associated Martínez, 2014; Bright, 2017). If the Bouse Streptochilus species are simply benthic foraminifers suggest inner neritic (<50 m) water depths (McDougall morphological variants of benthic Bolivina, regardless of their test morphol- and Miranda Martínez, 2014). Isotopic evidence presented in this study implies ogy, then their ecological constraints implying open-marine conditions would reworking of benthic foraminifers from nearshore sediments into deeper- be lost. Numerous Bolivina species have been reported from extant and paleowater sediments, but the data do not constrain if the deeper-water environ- lakes (Arnal, 1958; Resig, 1974; Patterson, 1987; Abu-Zied et al., 2007) and from ment was >100 m or <50 m deep. Miranda-Martínez et al. (2017) recently ac- dilute coastal lakes (Kane, 1967; Rao et al., 2000). knowledged that the Bouse Streptochilus might have been living in shallow water (presumably < 50 m) as benthic foraminifers. Rather than having d18O- d13C values that are similar to age-equivalent Streptochilus from normal sea- Evidence for a Lacustrine Population of Amphibalanus subalbidus water (Fig. 13B), the Bouse Streptochilus at Hart Mine Wash have d18O-d13C values that are similar to those measured in associated benthic Cyprideis valves The barnacle species found in the SBF is Amphibalanus subalbidus (Van (Figs. 10D and 13B) and that are virtually identical to those from Cyprideis Syoc, 1992; Pitombo, 2004). Although barnacles are typically considered ma- valves at Parker (Figs. 10D and 13B). Regardless of the water depth, the Bouse rine organisms, the salinity tolerance of A. subalbidus extends below 3000 mg Streptochilus specimens appear to have been calcifying alongside ostracodes L–1 (Poirrier and Partridge, 1979; Dineen and Hines, 1994), which approaches in the benthos. This interpretation would fully support the assertion made by the freshwater category as defined by Williams (1964). Several other barnacles Miranda Martínez et al. (2017) that perhaps the Bouse Streptochilus species in the genus Amphibalanus, such as Amphibalanus improvisus and Amphi- were benthic foraminifers (i.e., Bolivina). The broader context of this study balanus amphitrite, have also been found successfully living at freshwater or would suggest that the Bouse Streptochilus calcified in a mildly brackish lake near-freshwater salinities (Fyhn, 1976; Kennedy and DiCosimo, 1983). Barna- and in water with a d18O value that was below that of normal seawater. cles have been reported from unquestionably nonmarine environments, such Invoking a benthic lacustrine Streptochilus population is a controversial as pumping stations (Shatoury, 1958), desert oases (Pint et al., 2017), and lakes interpretation, but a combination of nomenclatural and genetic evidence sup- (Plaziat, 1991; Geraci et al., 2008). ports this conclusion. One of the more abundant benthic foraminifers in the Well-preserved barnacle fragments were analyzed from multiple expo- SBF is Bolivina subexcavata (McDougall and Miranda Martínez, 2014; Bright, sures of the basal carbonate unit in the Blythe basin, including four samples 2017). Bolivina subexcavata is synonymized with Bolivina variabilis (Hayward throughout a critical outcrop containing sigmoidal bedding featured in O’Con- et al., 2010). Darling et al. (2009) demonstrated that modern benthic Boliv- nell et al. (2017), which is often, but not always, associated with tidal envi- ina variabilis and modern planktic Streptochilus globigerus are actually ge- ronments (Tinterri, 2011). Sigmoidal bedding has been described from both a netically identical. Additional work by Kucera et al. (2017) has identified five littoral shoal or bar-form environment in the Green River Formation (Keighley phylogenetic clades within Bolivina, one of which is populated by exclusively et al., 2003), and from a littoral dune bed-form environment in Lake Bonneville benthic individuals as might be expected, one of which is populated by ex- (White, 1996), for example. None of the SBF barnacle fragments yielded d18O- clusively planktic individuals (“Streptochilus”), and three clades of which are d13C values that are consistent with calcification in normal seawater or with populated by both benthic and planktic individuals. Bolivina and Streptochi- calcification in a Colorado River estuary (Fig. 12). lus are the same foraminifer that can express two different test morphologies In this exercise, a marine analogy is delineated by the d18O-d13C values of (e.g., Smart and Thomas, 2007; Kucera et al., 2017), and thus, Streptochilus is late Pleistocene planktic foraminifers from the central Gulf of California, from a junior synonym of Bolivina (Darling et al., 2009). Bolivina variabilis might be fully marine barnacles from various sites around the globe, and from modern able to grow to adulthood and express both Bolivina and Streptochilus test barnacles from the intertidal zone of Isla Montague (Fig. 1). Modern barnacles morphologies in both planktic and benthic environments (Darling et al., 2009). from the saline lacustrine Salton Sea (Fig. 1) are included for context. An estu- Bolivina variabilis has been reported living in Lake Qarun in Egypt (Abu-Zied ary analogy is delineated by the d18O-d13C values in individual growth bands of et al., 2007), but to our knowledge there are no records of Streptochilus liv- pre-dam-era Mulinia clams from the modern northern Gulf of California (Fig. ing in lakes. A detailed assessment of Bolivina variabilis test morphologies in 1). The lowest Mulinia d18O-d13C values (Fig. 12) are from Isla Montague (Fig. 1) Lake Qarun might be an informative endeavor. The isotopic data presented and are thought to represent calcification during peak pre-dam-era Colorado here provide compelling evidence that the SBF contains the first benthic (e.g., River runoff, when voluminous spring snowmelt discharge on the river low- Miranda-Martínez et al., 2017) and lacustrine occurrence of Streptochilus. ered the d18O-d13C values in the northern Gulf of California (Rodriguez et al.,

18 18 2000). Nearly all of the SBF barnacles analyzed thus far yielded d O values material at Hart Mine Wash (Fig. 5). In this model, the high d OOST values and lower than even the most river-influenced estuary analog (Fig. 10). Accounting the higher 87Sr/86Sr ratios in the fish bone and Cyprideis calcite are interpreted for a roughly +1‰ offset between the Mulinia shells (aragonite) and the barna- as reflecting the average open-waterd 18O value and 87Sr/86Sr ratio of paleolake 18 87 86 cle plates (calcite) does not affect the outcome of this comparison in any ap- Blythe (Fig. 15). The much lower d OSED values and lower Sr/ Sr ratios in the preciable manner. Several SBF barnacle samples have isotopic compositions micrite represent more seasonally limited epilimnic calcium carbonate produc- more indicative of calcification in freshwater (Fig. 12). Thus, the SBF barnacles tion in paleolake Blythe during spring/summer floods on the early Colorado likely calcified in a lacustrine rather than a marine or estuarine environment. River (Fig. 15). This model is analogous to the freshwater plumes that are observed on Lake Turkana, Kenya (Yuretich and Cerling, 1983), and on Lake Van, Turkey A Brackish Lake–Freshwater Plume Model for (Reimer et al., 2009). At Lake Turkana, reduced surface-water total dissolved the Southern Bouse Formation solids (TDS) values are seen as far as 50 km from the mouth of the Omo River during flood season (Yuretich and Cerling, 1983; Halfman et al., 1989), indicat- A brackish lake and freshwater plume model (Fig. 15) can explain the iso- ing that density and salinity contrasts between the flood plume and the open topic complexity preserved in SBF carbonates (Figs. 13A and 13B), as well as lake water can be maintained for some distance and for a period of time. A 87 86 18 18 explaining the slight offset in Sr/ Sr ratios measured in micrite and biologic similar, but smaller, offset between d OOST and d OSED values also occurs in

Figure 15. Conceptual diagram of the “saline lake– N freshwater plume” model. View is to the north, mimicking a view from Hart Mine Wash toward Parker (see Fig. 1). Ostracodes calcified their valves Spring flood plume in a mildly brackish and evaporative lake with elevated d18O values (shown in black text within wa- δ18 87 86 (low O, Sr/ Sr ≈ 0.7108) ter column), a high 87Sr/86Sr ratio, and variable d13C values (not shown). The water-ostracode and water-micrite d18O relationship (VPDB) was calculated for water at 20 °C (benthos) and 30 °C (epilimnion), respectively, using Friedman and O’Neil (1977), and includes a +2‰ vital effect correction for the os- −7‰ VSMOW tracode calcite (Marco-Barba et al., 2012). VPDB— Vienna Pee Dee Belemnite, VSMOW—Vienna −4‰ VSMOW Standard Mean Ocean Water. Micrite forms during 87Sr/86Sr ≈ 0.7110 a spring/summer freshwater plume and has much lower d18O values and a lower 87Sr/86Sr ratio than −10‰ VPDB carbonate formed deeper in the lake. Ostracode d13C values (see Fig. 10) can depend on whether they were living in 12C-enriched bottom waters (low d13C values) or not (Decrouy et al., 2011a, 2011b). See text for discussion. See Figure 5 for the relationships among 87Sr/86Sr ratios in micrite, fish −2‰ VPDB bone, and ostracode calcite. Artwork is based on Young (2004).

oxidized marl benthic “Streptochilus” upper white marl ostracode basal bioclastic-marl benthic ostracode oxidized marl benthic foraminifer upper white marl benthic foraminifer basal bioclastic-marl benthic foraminifer oxidized marl benthic ostracode reworking of nearshore foraminifers into off-shore sediments

Lake Turkana (Halfman et al., 1989). Circulation patterns in Lake Turkana also custrine NBF in the Chemehuevi basin (Figs. 13B and 14D), over 100 km to the vary seasonally, such that Omo River water interacts with different parts of the north. The offset in d18O values between the aragonitic bivalve and snail shells lake depending on the time of year (Yuretich and Cerling, 1983). At Lake Van, (roughly +1‰) and the calcitic ostracode valves is on par with common Can- carbonate production occurs during the summer, when inflowing rivers pro- dona vital effects (+1‰ to +2‰; e.g., von Grafenstein et al., 1999). At the scale vide additional calcium to the lake (Reimer et al., 2009). The resulting carbon- of this data set, these materials can be directly compared. The southern Blythe ate is then distributed within the basin by eddies and currents (e.g., Stockhecke bivalves and snails calcified in a water mass that was isotopically similar to the et al., 2012). Similar interannual (or longer-scale) variability in the distribution water in paleolake Chemehuevi; a marine or estuarine origin for these shells of the proposed Colorado River plume might have promoted considerable is not required. complexity in the distribution of isotopic values in SBF carbonates (Figs. 14B and 14D). The plume model hypothesizes that the open water of paleolake Blythe was Reconciling Marine Sedimentology in a Lacustrine Environment mildly brackish with a high, evaporation-driven d18O value and a high, possibly more regional groundwater-derived 87Sr/86Sr ratio (Fig. 15; e.g., Crossey et al., The proposed tidal sedimentary structures recognized by O’Connell et al. 2015). The early Colorado River flood plume would have been composed of (2017), such as potential tidalites and sigmoidal bedding, seemingly require a very dilute and 16O-enriched snowmelt sourced in the Rocky Mountains. Pre- connection to the ocean, yet the broader isotopic and ecological constraints dam-era Anodonta shells from the modern Colorado River yield d18O values on the SBF presented here are incompatible with a marine or normal estua- approaching -18‰ (Dettman et al., 2004), for example. The 87Sr/86Sr ratio of the rine environment. Two possible explanations that could reconcile tidal features early Colorado River is not precisely known. However, the 87Sr/86Sr ratio in riv- with a nonmarine environment include deposition in a freshwater estuary (i.e., ers with large watersheds that incorporate a variety of bedrock lithologies can slight modification of Crossey et al., 2015) or deposition in a tidally influenced fluctuate on seasonal time scales (Rai and Singh, 2007; Santos et al., 2015). The Colorado River–fed lake situated well inland of the coast but at or near sea- 87Sr/86Sr ratio of the modern Colorado River (0.70945 ± 0.0082, n = 9) is slightly level (Fig. 16). This tidally influenced lake would have possessed a long res- lower than the 87Sr/86Sr ratio of modern regional groundwater (0.71120 ± idence time that would have allowed for a mild increase in salinity and for 0.00492, n = 44), and the 87Sr/86Sr ratio of modern Colorado River tributaries is the high d18O values measured in a variety of biologic carbonates, similar to even lower still (0.70874 ± 0.00074, n = 7; Crossey et al., 2015). The high 87Sr/86Sr what is observed at Lake Tanganyika (Dettman et al., 2005). Freshwater estuary ratios found in the 12–6 Ma Hualapai Limestone (0.7114–0.7190) provide con- environments with meter-scale tidal amplitudes are known to occur hundreds text for the 87Sr/86Sr ratio of regional groundwater in the western Grand Can- of kilometers inland from the ocean (e.g., Geyer and Chant, 2006; Gugliotta et yon area just prior to the deposition of the Bouse Formation (Crossey et al., al., 2017) and are populated by continental organisms (e.g., Sousa et al., 2005; 2015). Perhaps spring floods (Fig. 12) on the early Colorado River had a low Gugliotta et al., 2017). Examples of ancient freshwater rhythmite deposits en- 87Sr/86Sr ratio derived more from surface water in its watershed, while base- tirely lacking marine fossils have also been reported in the literature (Kvale and flow conditions had a higher 87Sr/86Sr ratio derived from an increase in the rela- Mastalerz, 1998). However, the freshwater portion of a normal estuary would tive proportion of groundwater in the river. Incomplete mixing of this seasonal be strongly influenced by river water (e.g., Ingram et al., 1996a) and might not flood plume in paleolake Blythe would have led to epilimnic carbonate having produce the high, nearly marine-like Candona d18O-d13C values measured in 18 87 86 a much lower d OSED value and a lower Sr/ Sr ratio than carbonate formed this study (Fig. 13B). Alternatively, the Lake Waihola-Waipori complex (Schal- deeper in the lake (e.g., Halfman et al., 1989; Sun et al., 2011). The low epilimnic lenberg et al., 2003; Schallenberg and Burns, 2003) and Pitt Lake, British Co- 18 d OSED values could have been enhanced by high summer temperatures and lombia (Ashley, 1977; Ashley and Moritz, 1979), are restricted freshwater lakes additional contributions from kinetic disequilibrium effects that are associated situated 12 and 30 km inland, respectively, yet they experience tidal ranges of with rapid calcification (e.g., Gabitov et al., 2012; Watkins et al., 2014). up to 0.5 m and up to 1–2 m, respectively (Hodgins and Quick, 1972; Ashley, Previously published bulk sediment samples and biologic carbonates from 1977; Schallenberg et al., 2003; Schallenberg and Burns, 2003). To our knowl- multiple locations around the Blythe basin yielded highly variable d18O-d13C edge the isotope systematics of the Lake Waihola-Waipori complex or Pitt Lake values (Figs. 14B and 14D) and can display clear location- and material-depen- have not been studied. Nonetheless, there are numerous modern analogues dent variability (Fig. 14D). Some of the heterogeneity in the bulk carbonate where tidal influences can be observed in freshwater lacustrine environments, results (Fig. 14B) might be explained by variability in the strength, mixing, and and nonanalogue environments may have existed in the past. spatial distribution of the early Colorado River plume (e.g., Yuretich and Cer- A freshwater estuary or tidally influenced lake model could potentially rec- ling, 1983; Fig. 15), although some of the variability appears to be related to the oncile many of the peculiarities of the SBF. Southern Bouse carbonates would material analyzed (Fig. 14D). Note that previously published d18O-d13C values have been deposited at or near sea-level but a considerable distance inland from bivalves and from shells of the intertidal marine gastropod Batillaria at and well away from seawater or a salt wedge, and thus, they would completely Milpitas Wash (Fig. 1) overlap with the Candona d18O-d13C values from the la- lack any marine geochemical indicators. In this model, the diverse foraminifer

Whipple Mts Figure 16. Cartoon illustration of a hypotheti- cal tidally influenced lake scenario for paleolake Blythe based on a scaled-up analogy to the Lake Blythe basin Waihola-Waipori complex in New Zealand. See TrigoTrigo Mt Mts s text for discussion. Paleolake Blythe is shown at or near sea level, with an outflowing stream connect- ing the lake to the early macrotidal Gulf of Califor- nia. Discharge on the outflowing river is impeded during high tides, causing the lake level to rise. Chocolate Mts Discharge resumes during low tides and lake level Paleolake Blythe falls. Paleolake Blythe is fed by the early Colorado River and has a continental water composition. tidal connection but Seawater rarely enters the lake, but the lake expe- seawater rarely enters basin riences tidally influenced changes in water level. Illustration is based on an unidentified photograph located at www.peency.com/ocean-mountain-river tidal lake situated Chocolate Mts -wallpaper-4394 at the time of writing. 60 km inland N early Gulf of California with extreme macrotidal diurnal periodicity

assemblage that is often cited as evidence for marine conditions (McDougall Yuma (Fig. 1). If the uppermost reaches of the early Gulf of California during and Miranda Martínez, 2014, 2016; Dorsey et al., 2018) would instead comprise the late Miocene was narrower and more constricted than the modern bathym- a large proportion of autochthonous euryhaline benthic foraminifers, such as etry, then the paleo–tidal range could have exceeded 10 m. A modern anal- Ammonia beccarii, which were living in relatively dilute conditions (e.g., Ot- ogy might be the narrow upper reaches of the Bay of Fundy, New Brunswick, vos, 1978; Boudreau et al., 2001). The autochthonous benthic foraminifers are where maximum tidal ranges are on the order of 15–16 m (Desplanque and mixed with small numbers of exotic planktic foraminifers that were reworked Mossman, 2001) and are currently one of the largest tidal ranges on Earth (Ar- inland by strong flood tidal currents or other processes (e.g., Arnal et al., 1980; cher, 2013). Nonanalogue paleo–tidal ranges in excess of 15 m in the early Gulf Ghosh et al., 2009). Foraminifer species richness is highest at the southern end of California could have conceivably existed in the past. of the Blythe basin (McDougall and Miranda Martínez, 2014), which could lend This study does not directly address questions about the original elevation support to this model. Perhaps small amounts of seawater periodically entered of the SBF. The late Miocene (and younger) tectonic history of the Blythe basin Paleolake Blythe during storm surges or during the highest tides (e.g., Crossey is a topic of active research (Beard et al., 2016; Karlstrom et al., 2017). Addi- et al., 2015), as sometimes happens at the Lake Waihola-Waipori complex tional information is clearly needed to test the viability of the tidally influenced (Schallenberg and Burns, 2003). If so, then they would have been short-lived lake model and to determine if the SBF can be reconciled with formation near events, and subsequent dilution or flushing (e.g., Schallenberg and Burns, sea level. This study does suggest that the early Colorado River was present 2003) would have limited or even prevented the preservation of a marine geo- during deposition of the SBF, similar to interpretations by Crossey et al. (2015). chemical signal in SBF carbonates. Thus, the SBF must be younger than 5.6 Ma and is likely younger than 5.3 Ma, Projecting a Gulf of California tidal influence nearly 60 km inland to the based on upstream chronological constraints for the arrival of the early Colo- Blythe basin is not unreasonable. The historical tidal range on the modern rado River in the Cottonwood basin (Fig. 1; House et al., 2008; Pearthree and Colorado River estuary is ~10 m, and tidal influences are observable 50 km House, 2014; Crow et al., 2017). 18 upstream (Thompson, 1968; Nelson et al., 2013; Zamora et al., 2013). This dis- The large and abrupt decreases in d OOST values in both the NBF and SBF tance is comparable to the distance between the southern Blythe basin and (Fig. 9) are more consistent with the overspilling chain-of-lakes model for

the integration of the lower Colorado River corridor (e.g., House et al., 2008; interpretation for the origin of the SBF. An alternative model invoking a fresh- Pearthree and House, 2014; Bright et al., 2016). A tidally influenced lake model water plume into a tidally influenced brackish lake that was located at or near (Fig. 16) for the SBF is supported by multiple lines of evidence (O’Connell et al., sea level should also be considered. Regardless of its original elevation, the 2017) and can also comprehensively integrate many of the seemingly irrecon- SBF was deposited in the last of a chain of mildly brackish to freshwater lakes cilable data sets generated thus far from the SBF. However, the abrupt decrease that record the southward advance of the terminus of the early Colorado River 18 in the d OOST values across the distinctive clay layer in Hart Mine Wash and as it migrated toward the early Gulf of California. Milpitas Wash (Fig. 9) seems better explained by a geologically instantaneous event associated with lake overspilling (e.g., Bright et al., 2016), which would not be consistent with direct tidal influence on the paleolake. The distinctive ACKNOWLEDGMENTS clay layer may be a xenoconformity (e.g., Carroll, 2017) and perhaps deserves Support for this project was provided by the U.S. Geological Survey, the University of Arizona’s additional study. Nonetheless, the detailed comparison with the lacustrine NBF Maxwell Short and Paul Martin Scholarships (Bright), and the Geological Society of America’s Lim- in Chemehuevi basin provides compelling, reproducible, and internally con- nogeology Division Kerry Kelts Research Award (Bright). Support was also provided by National Science Foundation (NSF) grant EAR-1545998 (Cohen). Thoughtful and constructive comments sistent evidence that the basal carbonate unit of the SBF was deposited in a from David Miller, Jon Spencer, and two anonymous reviewers improved this manuscript. We ap- mildly brackish lacustrine environment. A marine environment (e.g., Dorsey et preciate the numerous conversations with Sue Beard, Laurie Crossey, Becky Dorsey, Brian Gootee, al., 2018) is unable to account for the ecological complexities of the mixed mi- Steve Hasiotis, Mindy Homan, Kyle House, Karl Karlstrom, David Miller, Brennan O’Connell, Jon Spencer, and Scott Starratt that have sharpened our thinking about the Bouse Formation. We ap- crofossil assemblage or the isotopic constraints provided by the various types preciate conversations with Christopher Smart (University of Plymouth, UK) regarding Streptoch- of carbonates (inorganic and biologic) detailed in this study. No evidence for an ilus. We thank Jacob Favela at the University of Arizona’s LaserChron Scanning Electron Micro- estuarine environment was found either. The northern and southern Bouse For- scope (SEM) Facility for assistance with the SEM images. The University of Arizona’s LaserChron SEM Facility is supported by funds from NSF EAR-0929777. mations were deposited in mildly brackish (~5000 to <10,000 mg L–1) lakes with high water d18O values and compositions that mimicked dilute seawater. Both

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