Chronostratigraphic Correlation of Lacustrine Deposits Using 87Sr/86Sr Ratios, Eocene Green River Formation, Wyoming, U.S.A

Journal of Sedimentary Research, 2017, v. 87, 406–423 Research Article DOI: http://dx.doi.org/10.2110/jsr.2017.27

CHRONOSTRATIGRAPHIC CORRELATION OF LACUSTRINE DEPOSITS USING 87SR/86SR RATIOS, EOCENE GREEN RIVER FORMATION, WYOMING, U.S.A.

M’BARK BADDOUH,* ALAN R. CARROLL, STEPHEN R. MEYERS, BRIAN L. BEARD, AND CLARK M. JOHNSON Department of Geoscience University of Wisconsin–Madison, Madison, Wisconsin 53706, U.S.A.

ABSTRACT: The reconstruction of detailed, basin-scale depositional histories from sedimentary rocks fundamentally depends on the availability of reliable time markers. Unlike marine strata, lacustrine strata typically lack rapidly evolving, cosmopolitan fauna or ﬂora that might serve this purpose. Depending on their geologic context, lacustrine strata may also lack tephras that could provide isochronous markers or radioisotopic age. Variations in 87Sr/86Sr ratios could potentially provide an alternative means of chronostratigraphic correlation for carbonate-rich lake deposits, based on the hypothesis that Sr isotopes are well mixed in a lake and do not experience signiﬁcant fractionation. To test this hypothesis we measured 87Sr/86Sr ratios in 114 samples from two drill cores of the upper Wilkins Peak Member from the Green River Formation that are located ~ 23 km apart. These cores can be independently correlated using distinctive tephras and organic-carbon rich mudstone horizons. Measured 87Sr/86Sr ratios range from 0.71154 to 0.71504, and vary inversely with lake-water depth, as interpreted by sedimentary lithofacies characteristics. Lower ratios of 87Sr/86Sr are found in lithofacies deposited during lake highstands, which are marked by laminated dark-gray mudstone and elevated organic-carbon enrichment (as measured by Fischer Assay analysis). Higher 87Sr/86Sr ratios occur in lithofacies deposited during lake lowstands, which are marked by organic-lean gray-green mudstone. 87Sr/86Sr in approximately time-equivalent samples from the two cores show a strong positive correlation (r ¼ 0.68), despite the likely presence of small temporal mismatches between approximately correlative samples. We conclude that lake-water was consistently well mixed with respect to Sr across distances of at least 23 km. These results suggest that 87Sr/86Sr can serve as a powerful tool to aid high- resolution chronostratigraphic correlation of lake deposits.

INTRODUCTION manifest dramatic lateral lithofacies variations that render lithology effectively useless for chronostratigraphic reconstruction (e.g., Bohacs et Lake deposits occur on all of the continents, and offer rich archives of al. 2000; Norsted et al. 2015). tectonic, magmatic, climatic, and biologic evolution. They also contain A variety of approaches have been employed in an effort to overcome important economic resources of coal, oil, oil-shale, soda ash, evaporites these problems, such as radioisotopic dating of tephras (e.g., Steenbrink et and other valuable resources (e.g., Dyni 2006; Johnson et al. 2011). al. 1999; Smith et al. 2008; Smith et al. 2010; Machlus et al. 2015), Accurately reading these archives and fully exploiting the economic magnetic-reversal stratigraphy (e.g., Olsen et al. 1996; Magyar et al. 2007; potential of lacustrine strata requires reconstruction of the processes by van Vugt et al. 1998; Barberà et al. 2001), astrochronology (Van Houten which they were deposited. This in turn requires the ability to accurately 1962; Olsen 1986; Aswasereelert et al. 2013), and seismic stratigraphy correlate sedimentary lithofacies that were deposited synchronously at different locations. Unlike marine strata, lacustrine deposits usually lack (e.g., Scholz and Rosendahl 1988; Liro and Pardus 1990). Each of these rapidly evolving, cosmopolitan index fossils that might facilitate approaches carries intrinsic advantages and limitations. Tephrochronology chronostratigraphic correlation. Lake-levels and sediment sources also provides radiogenic ages but is limited to strata containing dateable tuffs, may fluctuate very rapidly compared to many marine systems (e.g., Oviatt which in some cases have been altered by contact with lake-waters (cf. 1997; Pietras and Carroll 2006), adding to the challenge of correlating Smith et al. 2003). Magnetostratigraphy and astrochronology offer synchronous lithofacies tracts. As a result of these difficulties, lake basin excellent temporal resolution but are limited in application to suitable evolution must often be inferred largely from lithostratigraphic correlation lithofacies and do not by themselves provide unique, numerical (e.g., Fouch 1975; Roehler 1993; Yang et al. 2010; Burton et al. 2014). In geochronological ages. Seismic stratigraphy can help to decipher large- underfilled lake basins (cf. Carroll and Bohacs 1999), lithostratigraphy scale temporal patterns of basin fill but generally lacks the resolution may nearly replicate chronostratigraphy due to the dominant influence of required to capture fine details of a rapidly evolving lake. basin-wide wet–dry cycles (e.g., Culbertson 1961; Tänavsuu-Milkeviciene 87Sr/86Sr ratios have long been used to correlate coeval marine deposits, and Sarg 2012). At the other extreme, overfilled basins characteristically based on the observation that the oceanic reservoir is well mixed, its 87Sr/86Sr ratio has changed continuously through the Phanerozoic, and Sr * Present Address: Department of Atmospheric, Oceanic and Earth Sciences, isotopes are not significantly fractionated by meteoric processes or during George Mason University, Fairfax, Virginia 22030, U.S.A. the precipitation of carbonate and phosphate minerals (e.g., DePaolo and

Published Online: April 2017 Copyright Ó 2017, SEPM (Society for Sedimentary Geology) 1527-1404/17/087-406/$03.00 JSR CHRONOSTRATIGRAPHIC CORRELATION OF LACUSTRINE DEPOSITS USING 87SR/86SR RATIOS 407

Ingram 1985; Veizer 1989; Capo et al. 1998). The same approach might Cenozoic incision of the Green River, a tributary of the Colorado River. potentially be used to provide highly detailed chronostratigraphies of However, the Sr isotopic compositions of smaller tributary streams that carbonate-rich lacustrine strata, if the lacustrine reservoir was similarly drain local catchments in basin-bounding uplifts likely mirror those of their well mixed. Results reported for late Quaternary pluvial lake systems Eocene precursors (Doebbert et al. 2014). suggest that this may indeed be the case. For example, lacustrine carbonate The Bridger Basin is bounded on the west by the Cordilleran Fold and lithofacies of Bonneville basin in Utah have 87Sr/86Sr ratios that principally Thrust Belt (CFTB) (Fig. 1), which contains thick, structurally repeated reflect the elevation of the paleolake surface at the time they were intervals of Cambrian through Cretaceous marine strata. These include Sr- deposited, despite being geographically separated by tens to hundreds of rich marine limestone intervals up to several hundred meters thick (cf. km (Hart et al. 2004). Over the past ~ 15,000 years, lakewater 87Sr/86Sr Love and Christiansen 1985), with 87Sr/86Sr ratios corresponding to marine increased from ~ 0.71125 to ~ 0.71387 as lake-level fell. An opposite values at the time of deposition (generally in the range of ~ 0.707–0.7095; relationship between paleolake-level and 87Sr/86Sr is preserved in Burke 1982). Modern rivers draining the CFTB have previously reported lacustrine tufa of the Lahontan basin in Nevada; 87Sr/86Sr in tufa carbonate 87Sr/86Sr ratios of 0.70869 to 0.70917 (Doebbert et al. 2014). The lithofacies decreased from ~ 0.70786 to ~ 0.7056 as lake-level declined Paleocene–Eocene Flagstaff Formation, which consists of lacustrine over the past ~ 21,000 years (Benson and Peterman 1996). These lithofacies deposited south of the Uinta uplift, has 87Sr/86Sr ratios that Quaternary examples primarily record the terminal desiccation of pluvial are indistinguishable from modern streams arising in the adjacent CFTB lakes, which left behind relict geomorphic features that can be used to (Gierlowski-Kordesch et al. 2008). Cenozoic sedimentary rocks as far directly measure past lake surface elevations. The potential utility of north as the Alberta foreland basin record similar 87Sr/86Sr ratios (Fan et al. 87Sr/86Sr for chronostratigraphic correlation of older strata that embody 2011). Eocene volcanic rocks to the north and northwest of the Bridger longer, more complex histories has not yet been tested. Ironically, the lack Basin have lower 87Sr/86Sr ratios, in the range of 0.7050 to 0.7060 of detailed, independent chronostratigraphy in many such deposits itself (Doebbert et al. 2014), but it appears that these rocks did not contribute poses an impediment to such tests. significant drainage to Lake Gosiute until after deposition of the WPM The Wilkins Peak Member (WPM) of the Green River Formation (GRF) (Carroll et al. 2008; Doebbert et al. 2010; Chetel et al. 2011). In contrast, offers an ideal opportunity to test the efficacy of 87Sr/86Sr for lacustrine highly radiogenic Precambrian crystalline rocks are commonly exposed in chronostratigraphic correlation for several reasons. Previous studies have basement-cored uplifts to the north (Wind River) and south (Uinta) of the demonstrated that its stratigraphy is dominated by repeated wet–dry cycles Bridger Basin (Fig. 1). The original sedimentary cover of these uplifts was (e.g., Eugster and Hardie 1975; Smoot, 1983; Carroll and Bohacs 1999; generally eroded before the deposition of the WPM (Love et al. 1963; Bohacs et al. 2000; Pietras and Carroll 2006). These cycles at least Carroll et al. 2006; Fan 2009). Doebbert et al. (2014) reported that modern partially reflect Milankovitch forcing and thus should be expressed rivers draining those areas have 87Sr/86Sr ratios of 0.71566 to 0.74318. The synchronously across the entire basin (e.g., Fischer and Roberts 1991; Bridger Basin is bounded to the east by folded Cretaceous strata of the Machlus et al. 2008; Meyers 2008; Aswasereelert et al. 2013). However, Rock Springs Arch, which separates upper WPM lacustrine and alluvial other factors such as shorter-term climate change or geomorphic drainage strata in the Bridger Basin from time-equivalent alluvial deposits of the instability appear to have influenced lake-levels over shorter time periods Cathedral Bluffs Member of the Wasatch Formation to the east (Figs. 1, 2; (Pietras et al. 2003; Pietras and Carroll 2006). Organic-carbon rich Sullivan 1985; Roehler 1993; Smith et al. 2008; Chetel et al. 2011). The mudstone (oil-shale) beds provide particularly distinctive markers of lake Rock Springs Arch appears to have acted as a partial topographic barrier deepening episodes (Carroll and Bohacs 2001) and have in some cases during the Eocene. Siliciclastic detritus derived from Precambrian-cored been correlated across distances of . 100 km (Pietras and Carroll 2006; uplifts farther east was mostly baffled in the Sand Wash, Washakie, and Smith et al. 2015). The WPM also contains distinctive tuff horizons that Great Divide basins, while radiogenic river water spilled westward into the aid in regional chronostratigraphic correlation. Recent 40Ar/39Ar and U-Pb Bridger Basin (Smith et al. 2014; Smith et al. 2015). dating has established the WPM as one of the best-dated intervals of pre- The WPM lacustrine strata consist of repetitive lithofacies successions Quaternary sedimentary rock anywhere (Smith et al. 2003; Smith et al. that record episodic expansion and contraction of Eocene Lake Gosiute 2008; Smith et al. 2010; Machlus et al. 2015). Several previous studies across a low-relief basin floor (Eugster and Hardie 1975; Smoot 1983; have argued that carbonate lithofacies in Green River Formation mudstone Roehler 1993; Pietras and Carroll 2006). Individual successions (or do preserve a faithful record of 87Sr/86Sr in Eocene lakewater (Rhodes et ‘‘cycles’’) commonly begin with interbedded carbonate-rich mudstone, al. 2002; Doebbert et al. 2014; Baddouh et al. 2016). calcareous sandstone, and intraclast conglomerate, deposited during Herein we present 87Sr/86Sr and X-ray diffraction (XRD) data for WPM shoreline transgression. Scour marks, desiccation cracks, mudstone samples collected from two drill cores located ~ 23 km apart (Fig. 1). We intraclasts, wave ripples, and wavy bedding are common. These littoral combined previously reported data from 49 samples from the White lithofacies grade upward into sub-littoral to profundal lithofacies consisting Mountain #1 (WM) drill core, that were previously reported by Baddouh et of dark gray to brown, thinly laminated, kerogen-rich mudstone (oil-shale). al. (2016), with newly reported data from 65 samples from the Blacks Fork Primary bedded trona and halite are closely associated with the profundal #1 (BF) drill core. lithofacies near the basin depocenter, and diagenetic, displacive shortite crystals commonly crosscut and disrupt primary sedimentary fabrics across GEOLOGICAL BACKGROUND much of the Bridger Basin (Jagniecki and Lowenstein 2015). Profundal lithofacies typically grade upward into gray-green carbonate-rich mudstone The WPM was deposited by Eocene Lake Gosiute in the Bridger Basin, and siltstone lithofacies that record gradual regression of the lake. Wavy southwestern Wyoming, between ~ 51.5 and ~ 50.0 Ma (Smith et al. lamination, mudcracks, brecciation, and shortite crystals are common, and 2010; Machlus et al. 2015) (Fig. 1). The Eocene watershed surrounding the are interpreted to record deposition in littoral to palustrine environments. lake was underlain by bedrock lithologies of widely varying age, Sr The WPM lacustrine lithofacies are punctuated by up to nine discrete, concentration, and Sr isotope composition (e.g., Beard and Johnson 2000; regionally correlatable intervals of dominantly alluvial, siliclastic lithofa- Fan et al. 2011; Bataille and Bowen 2012). The broad lithologic and cies (labeled A through I in Figure 2; Culbertson 1961; Smoot 1983). Each structural configuration of this area has changed relatively little since the of these represents a complex, composite bedset that may include Eocene. Neogene normal faults locally produced high topographic relief, sandstone, siltstone, and mudstone, with minor interbeds of lacustrine but the overall magnitude of extension is modest (cf. Love et al. 1963; carbonate or evaporite. These lithofacies have been interpreted to record Snoke 1997). Regional drainage has been substantially altered by the late episodic westward bypass of arkosic detritus around or through the Rock 408 M. BADDOUH ET AL. JSR

FIG. 1.—Geologic map of the Greater Green River Basin showing Eocene lacustrine basins and adjacent Paleozoic and Precambrian uplifts. BF, US ERDA Blacks Fork #1 core (41.3565068 N, 109.5249528 W); WM, US ERDA White Mountain #1 core (41.5508568 N, 109.4185228 W). A–A0 north–south stratigraphic cross section shown in Figure 2 (map modiﬁed from Smith el al. 2008).

FIG. 2.—North–south cross section across the Greater Green River Basin, illustrating the stratigraphic associations between the members of the Green River Formation. The BF and WM cores are about 23 km apart. This study focuses on the upper WPM and lower Laney Member, represented by white box (modiﬁed from Pietras and Carroll 2006). JSR CHRONOSTRATIGRAPHIC CORRELATION OF LACUSTRINE DEPOSITS USING 87SR/86SR RATIOS 409

FIG. 3.—Core slab photograph of the US ERDA Blacks Fork #1 core (depth interval 445–470 feet (136–143 meter); U.S. Geological Survey, Core Research Center Library #E216). 410 .BDOHE AL. ET BADDOUH M.

FIG. 4.—Core slab photograph of the US ERDA White Mountain #1 core (depth interval 400–421.4 feet (122–128.4 meter); U.S. Geological Survey, Core Research Center Library #U847). JSR JSR CHRONOSTRATIGRAPHIC CORRELATION OF LACUSTRINE DEPOSITS USING 87SR/86SR RATIOS 411

occur in the study interval and provide robust isochronous markers (Smith et al. 2003; Smith et al. 2008). The study cores can be independently correlated based on organic- carbon rich mudstone beds (oil-shale) (Fig. 5). These intervals were identified visually in core for this study (Figs. 3, 4, 5) and can also be identified on the basis of previously reported Fischer Assay analyses (Goodfellow and Atwood 1974; Heistand and Humphries 1976; U.S. Geological Survey Oil-Shale Assessment Team 2011). Fischer Assay samples were obtained from continuous half- or quarter-drill cores, by dividing the split core into discrete segments 30–130 cm in length and then pulverizing and homogenizing each segment. This averaging approach provides a complete quantitative assay of the cored interval, but it also limits the spatial precision of the resultant data. X-ray diffraction (XRD) and Sr isotope analyses were based on smaller, more discontinuous samples than the samples used for Fischer Assay analysis. Mudstone lithofacies were sampled at ~ 30 cm intervals, by microdrilling an area measuring ~ 0.5 cm horizontally by ~ 2cm vertically. A total of 114 samples were collected, comprising 65 samples from the BF core and 49 samples from the WM core. Mineralogy was determined using powdered samples that were placed in glass capillary tubes and analyzed using a Rigaku Rapid II diffractometer with a curved two-dimensional imaging plate (2D IP). The Mo Ka X-ray tube operates at 50 kV and 50 mA (rated at 2.5 kW). The combination of the 2D IP and the high intensity X-ray source gives increased diffraction X-ray intensity. Materials Data, Inc. (MDI) DataScan4, and JADE software were used for mineral phase identification and quantitative analysis. JADE software allows the user to identify the peak of each mineral based on its 2h angle and d value by using multiple databases that provide information about each mineral. After identifying each phase the software quantifies the percentage of each mineral in each sample based on their peak height and spectrum shape. Carbonate Sr isotope ratios, Rb and Sr concentrations, and percent of carbonate were measured on 100 milligram aliquots of powdered material obtained from splits of the BF and WM drill cores (Fig. 1). Analytical methods are identical to those reported in Baddouh et al. (2016). Importantly, samples were leached with ammonium acetate leaching method before dissolution in acetic acid, to avoid Rb and Sr contribution FIG. 5.—Thin-section photomicrographs of typical WPM carbonate-rich mud- from the siliclastic material (which constitutes from ~ 20 to ~ 90% of the stone lithofacies. Upper: organic-carbon-rich mudstone from the Apache Lane mass of individual samples). Acetic acid carbonate solutions were spiked outcrop section. Lower: organic-carbon-lean, partially dolomitized mudstone from 87 84 the Kanda outcrop section. See Pietras and Carroll (2006) for locations. with a mixed Rb– Sr tracer to determine Rb and Sr concentrations by isotope dilution mass spectrometry (IDMS) as well as to measure 87Sr/86Sr ratios. Strontium isotope ratios were analyzed using Ta filaments and Springs Arch (Sullivan 1985; Smith et al. 2014), at time intervals H3PO4 using a three-jump multi-dynamic analysis on a VG Instruments corresponding to ~ 100 ky and ~ 400 ky eccentricity (Aswasereelert et al. Sector 54 multi collector thermal ionization mass spectrometer. Sr isotope 2013). Smith et al. (2014) further argued that they were deposited during ratios were corrected from instrumental mass bias to an 86Sr/88Sr of 0.1194 eccentricity minima and may correspond with Eocene hyperthermal events. using an exponential mass fractionation law. The reported 87Sr/86Sr ratio is This interpretation contrasts with that of Lourens et al. (2005) and Laurin based on the average of 120 ratios with an 88Sr ion intensity of 3 3 10–11 et al. (2016), who argue instead that Eocene hyperthermals occurred during A. Reported errors are the internal 2-standard errors (2-SE) which is eccentricity maxima. slightly less than the long-term external error, which is defined as two standard deviations of the mean (2-SD) based on analysis of the NIST SAMPLING AND METHODS SRM-987 Sr isotope standard (0.710262 6 0.000016; 2-SD; n ¼ 66) that was analyzed during the course of this study. In addition to the NIST SRM- For this study, we visually described lithofacies in the same stratigraphic 987 standard analyses the EN-1 modern marine carbonate standard was interval of the WPM, in two drill cores separated by ~ 23 km (Figs. 1, 3, analyzed 12 times, including three analyses that had been spiked with our 4). The BF core is located closer to the basin depocenter as compared to mixed Rb–Sr spike (average 87Sr/86Sr ¼ 0.709194 6 0.000034; 2-SD; n ¼ the WM core. The study interval lies at the top of the WPM, bounded 12; Sr concentration 1233 ppm). Rubidium was analyzed on Ta filament ‘‘ ’’ 87 85 below by an alluvial siliciclastic interval (the I-bed ; Culbertson 1961) with H3PO4, and Rb/ Rb analyses were determined using a static multi- and above by the Laney Member of the Green River Formation. This collector analysis. Based on 20 replicate analyses of NIST SRM-984 Rb, interval was selected in part because deposition appears to have been measured 87Rb/85Rb ratios are estimated to be precise to 6 0.7%. The relatively uniform across the basin, simplifying lithostratigraphic correla- percent carbonate, Rb and Sr concentration, 87Rb/86Sr and the 87Sr/86Sr tion (Pietras and Carroll 2006; Smith et al. 2008; Aswasereelert et al. ratio and its 2-SE for all the carbonate fractions are reported in Table S3 for 2013). Two distinctive volcanic tuffs, the Layered Tuff and the 6th Tuff, the BF core and Baddouh et al. (2016) for the WM core. 412 .BDOHE AL. ET BADDOUH M.

IG F . 6.—Stratigraphic correlation between BF and WM cores. Gray shading marks intervals of laminated, dark-gray mudstone. Fisher Assay data are from U.S. Geological Survey Oil-Shale Assessment Team (2011). U/ JSR Pb ages are from Machlus et al. (2015). Small black rectangles on the right side of the ﬁgure indicate 20 ka increments, using a linear sedimentation model based on the two U–Pb ages. JSR CHRONOSTRATIGRAPHIC CORRELATION OF LACUSTRINE DEPOSITS USING 87SR/86SR RATIOS 413

TABLE 1.—Mineralogy Summary of Blacks Fork #1 and White Mountain #1 Cores.

Blacks Fork #1 Core (n ¼ 65) White Mountain #1 Core (n ¼ 48)

Percentages Min Max Mean Std. Dev. Min Max Mean Std. Dev.

Carbonate (total) 12.3 73.6 45.9 12.2 11.9 87.3 50.2 17.4 Calcite 0 57.9 6.6 10.8 0 86.6 25.4 23.9 Dolomite 5.6 73.6 39.3 12.5 0 54.6 24.6 13.8 Shortite 0 22.9 2.1 5.1 0 9.1 0.5 1.7 Siderite 0 0.8 0 0.1 0 6.8 0.1 1 Quartz 0 48.5 18.4 9.3 1.3 25.4 10.7 4.8 Feldspar (total) 4.3 86.5 25.4 14.7 2.1 75.9 22.6 13.1 Orthoclase 4.3 63.5 16.3 9.4 2.1 67.4 18.5 11 Albite 0 23 9.2 5.3 0 8.5 4.2 2.1 Quartz/Feldspar (total) 0.0 2.7 0.8 0.5 0.1 4.0 0.6 0.6 Albite/Orthoclase 0.0 5.2 0.9 1.0 0.0 0.9 0.3 0.2 Clay (total) 2.5 27.2 7.9 3.5 0 52.3 15.5 13.6 Illite 2.5 19.5 7.7 2.9 0 52.1 14.6 12.5 Montmorillonite 0 7.7 0.2 1 0 8.4 0.9 2.1 Hematite 0 2.3 0.2 0.5 0 2.4 0.4 0.6

The silicate fraction was also analyzed after acetate leaching and to the relatively coarse spatial resolution of Fischer Assay samples. In some carbonate extraction. Silicate fraction analyses included eight samples from instances elevated oil-yields may also correspond to originally laminated the BF core (newly reported in this study) and seven samples from the WM lithofacies, in which primary depositional fabric has been obscured by core (previously reported in Baddouh et al. 2016). The methods for these secondary growth of evaporite crystals (see for example the interval from analyses are reported in Baddouh et al. (2016). ~ 420–422 ft. in the WM core; Figs. 4, 6). Statistical analyses of the geochemical data sets utilize the Astrochron package for R (Meyers 2014; R Core Team 2015). Assessment of the Mineralogy significance of the correlation between variables employs the phase- randomized surrogate approach of Ebisuzaki (1997), as modified by All of the samples consist of calcitic to dolomitic mudstone, with Baddouh et al. (2016) to allow comparison of data sets with different varying amounts of organic carbon (Fig. 5). X-ray diffraction (XRD) sampling grids (see Baddouh et al. (2016) for more details). analyses show that on average the samples from both cores to contain ~ 46 to 50% carbonate, with the remaining fraction consisting mostly of quartz, RESULTS feldspar, and clays (Table 1; Fig. 7) (see Tables S1 and S2 for detailed information, see Supplemental Materials). The variability among different Sedimentology and Lithostratigraphy lithofacies in each core is more pronounced than the differences between The interval between the ‘‘I-bed’’ and the Laney Member is ~ 32.3 m the average values in the two cores. This is consistent with the thick in the BF core and ~ 27.7 m thick in the WM core, a difference that interpretation that repeated lake deepening and shallowing episodes is consistent with the closer proximity of the BF core to the basin exerted a dominant, basin-wide influence on sedimentary lithofacies. On depocenter (Figs. 1, 2, 6; Roehler 1992; Wiig et al. 1995). The WM core average, the BF core contains more dolomite, in agreement with a previous contains more evidence for intermittent subaerial exposure, including scour study that documented an increase in dolomite nearer the basin depocenter marks, mudcracks, mudstone intraclasts, and burrows. The BF core (Mason 2012). The mean values for % dolomite and dolomite/calcite ratio contains more frequent intervals of displacive evaporite crystals. overlap at 1 standard deviation, reflecting the high degree of variation at Sedimentary lithofacies successions in both of the study cores are each site. Shortite is slightly more abundant in the BF core, but the dominated by the alternation of laminated, dark-gray mudstone, versus difference falls within one standard deviation. lighter gray-green mudstone. Fischer Assay oil-yields (U.S. Geological The silicate mineralogy of these samples is highly variable in each Survey Oil-shale Assessment Team 2011) generally follow the same core (Table 1; Fig. 7) (see Supplementary Tables S1 and S2 for detailed patterns, although the correspondence between oil-yields and dark gray, information). Mean % quartz, % albite, quartz/feldspar ratio, and Na/K laminated lithofacies is imperfect. In part, the mismatches are attributable feldsparratioarehigherinBFthaninWMsamples.Clay(mostly

FIG. 7.—Beanplot of quantitative percentage XRD mineralogy for WM and BF core data. The plot represents the distribution of mineralogy in each core as a density shape and horizontal lines indicate average distribution (Kampstra 2008). 414 M. BADDOUH ET AL. JSR

TABLE 2.—Blacks Fork #1 Core and White Mountain #1 Core Geochemistry Summary.

Note White Mountain data were from Baddouh et al. (2016).

Blacks Fork Core# 1 (n ¼ 65) White Mountain Core #1 (n ¼ 55)

Min Max Mean Std. Dev. Min Max Mean Std. Dev.

Sample Mass (g) 0.0047 0.0055 0.0052 0.0002 0.0048 0.0057 0.0052 0.0002 % Carbonate* 11.03 58.06 32.62 7.73 13.1 86.55 41.09 14.17 Rb (ppm) 0.3418 4.2707 1.0354 0.7144 0.0411 2.31 0.683 0.4722 Sr (ppm) 477 5248 1483 577 575 3211 1505 477 87Rb/86Sr 0.00044 0.00693 0.00205 0.00108 0.00013 0.0071 0.00145 0.00138 87Sr/86Sr measured 0.71156 0.71482 0.71252 0.0007 0.71154 0.71504 0.71271 0.00066 2-s 0.00001 0.00001 0.00001 0.000001 0.00001 0.00001 0.00001 0.000001

* measured by acid digestion. illite), on the other hand, is more abundant on average in the WM core although 87Sr/86Sr does correlate weakly with % calcite (r ¼ 0.36) and % (Fig. 7). dolomite (r ¼ –0.33) in the BF core. The silicate fractions of the study samples contain more Rb and have Geochemistry higher 87Rb/86Sr ratios than the carbonate fractions (Table 3, Fig. 9C, D). 87Rb/86Sr in the carbonate fractions correlates negatively with % carbonate Carbonate-mineral content (excluding shortite) as measured by acid digestion is highly variable in each core, ranging from 11.0% to 86.6% in both cores (r ¼ –0.71, in BF and r ¼ –0.69, in WM; Fig. 8D) and (Table 2, see Supplementary Table S3 for detailed information). Average positively with Rb concentration (r ¼ 0.75, in BF and r ¼ 0.86, in WM; Fig. carbonate mineral values are lower than those measured via XRD in our 8F), which suggests possible contamination from more radiogenic silicate analyzed intervals; the WM core average is ~ 9% lower, and the BF core minerals. The potential magnitude of such contamination is very small, average is ~ 14% lower. These results could indicate incomplete however. 87Rb/86Sr ratios in the carbonate fractions are all very low (on the dissolution of carbonate, particularly dolomite. Alternatively, some order of 10–3; Tables 2, 3; Fig. 8F) and no signiﬁcant correlation is evident carbonate minerals may have been removed during ammonium acetate between 87Sr/86Sr and either % clay or 87Rb/86Sr (Fig. 10A, B). Doebbert leaching, or XRD analysis may have overestimated the proportion of et al. (2014) suggest a 87Rb/86Sr ratio of . 0.02 as a basis for excluding 87 86 carbonate minerals. These differences do not affect measured Sr/ Sr or samples from interpretation; all of the samples in the present study have Sr concentration (Fig. 8A, B). 87Sr/86Sr is also not closely correlated with ratios , 0.008 (Figs. 8F, 10B). Therefore, we conclude that the ammonium Sr concentration (Fig. 8C). 87 86 acetate leaching was successful in minimizing radiogenic Sr derived from Rb/ Sr and Rb concentration are higher on average in the BF core 87 86 than in the WM core, but average Sr concentration and 87Sr/86Sr are similar silicates. The Rb/ Sr ratios as well as Rb concentration of the silicate in both cores (Table 2; Fig. 8D, E). All of these values display wide ranges fraction from the BF core tend to be lower as compared to the WM core of variation in each core. 87Rb/86Sr is positively correlated with Rb silicate fraction (Table 3, Fig. 9C, D), and this is considered to reﬂect the concentration in both cores (Fig. 8F). 87Sr/86Sr also appears to be largely mineralogical differences where the WM core has more illite and less albite independent of carbonate mineralogy as measured by XRD (Fig. 9A, B), as compared to the BF core (Fig. 7).

TABLE 3.—Measured Rb-Sr isotope data and percent carbonate of carbonate and silicate fractions from the Blacks Fork #1 and White Mountain #1 cores. Note White Mountain data reported in Baddouh et al. (2016).

Carbonate Fraction Silicate Fraction

Field Name % Carbonate Rb (ppm) Sr (ppm) 87Rb/86Sr 87Sr/86Sr Rb (ppm) Sr (ppm) 87Rb/86Sr 87Sr/86Sr

Blacks Fork BF-446-2.5 44.53 0.7030 1965 0.001036 0.714034 10 752 0.03928 0.71468 BF-447-1.5 11.09 3.6178 1815 0.005850 0.713371 17 29 1.70427 0.74308 BF-464-1 37.27 1.0170 1375 0.002141 0.712217 10 266 0.10914 0.71387 BF-465-11.5 27.79 2.4900 1823 0.003953 0.712747 11 115 0.26441 0.71563 BF-468-2 35.88 2.5396 1914 0.003845 0.712701 10 84 0.36240 0.71744 BF-476-4.5 39.88 0.6617 1403 0.001365 0.711983 9 435 0.06314 0.71340 BF-479-1.5 39.90 0.6948 1547 0.001300 0.711845 6 501 0.03654 0.71251 BF-491-10 29.74 1.0961 1475 0.002154 0.712435 9 366 0.07049 0.71368 White Mountain WM-371-3 13.10 1.4764 3211 0.00133 0.71291 85.60 44.2 5.6029 0.71794 WM-381-35 23.50 1.9708 1461 0.00390 0.71208 93.79 156.6 1.7340 0.71408 WM-382-0 27.10 1.1261 1249 0.00261 0.71247 56.98 72.2 2.2853 0.71973 WM-418-0 17.39 1.2364 3118 0.00115 0.71378 51.05 66.3 2.2321 0.72208 WM-425-9 24.42 1.4316 914 0.00453 0.71317 73.21 2133.7 0.0993 0.71269 WM-435-9 36.89 0.4564 1843 0.00072 0.71168 48.69 163.5 0.8623 0.71741 WM-445-10 44.82 0.5195 1634 0.00092 0.71382 71.03 408.8 0.5030 0.71477 JSR CHRONOSTRATIGRAPHIC CORRELATION OF LACUSTRINE DEPOSITS USING 87SR/86SR RATIOS 415

87 86 87 86 87 86 FIG. 8.—Cross-plots of Sr/ Sr, Rb/ Sr, Sr concentration, and Rb concentration vs. % carbonate from both BF and WM cores, as well as cross-plots of Sr/ Sr, 87Rb/86Sr vs. Sr concentration and Rb concentration, respectively. Pearson correlation coefﬁcients and their associated p values are listed, as determined with the ‘‘surrogateCor’’ function in Astrochron (see Baddouh et al. 2016). 416 M. BADDOUH ET AL. JSR

87 86 FIG. 9.—Cross-plots of carbonate fraction Sr isotope ratios vs. A) calcite, B) dolomite, and silicate fraction Rb concentration versus C) the percent carbonate and Rb/ Sr versus D) the percent carbonate in the sample for the from both BF and WM cores. Note different vertical-axis scales for all the plots. Pearson correlation coefﬁcients and their associated p values are listed, as determined with the ‘‘surrogateCor’’ function in Astrochron (see Baddouh et al. 2016).

87Sr/86Sr and Wilkins Peak Member Stratigraphy 87Sr/86Sr ratios, but these are likely attributable in part to the different sampling strategies associated with the two datasets. The BF core 87Sr/86Sr Carbonate-mineral 87Sr/86Sr ratios generally correlate with sedimentary ratios show a strong negative correlation with oil-yield (r ¼ –0.54) (Fig. lithofacies, particularly in the WM core (Fig. 6). Lower 87Sr/86Sr ratios 11A, B), and this correlation is similar to that observed in the WM core generally correlate with laminated sedimentary lithofacies and higher Fischer Assay oil-yield, and higher 87Sr/86Sr ratios occur in non-laminated (Baddouh et al. 2016). 87 86 lithofacies and lower oil-yield. This relationship is somewhat less strongly The two Sr/ Sr records cannot be directly compared based on core developed in parts of the BF core (i.e., the interval between ~ 480 and 520 depths alone because the study-interval thickness differs between the ft.; Fig. 6). Slight offsets do occur between peak oil-yield and the lowest two cores—implying different sedimentation histories. However, if both JSR CHRONOSTRATIGRAPHIC CORRELATION OF LACUSTRINE DEPOSITS USING 87SR/86SR RATIOS 417

87 86 FIG. 10.—Cross-plots of Sr isotope ratios vs. A) clay and B) Rb/ Sr from both BF and WM cores. Pearson correlation coefﬁcients and their associated p values are listed, as determined with the ‘‘surrogateCor’’ function in Astrochron (see Baddouh et al. 2016).

cores are transformed to time using the U-Pb ages of the 6th and DISCUSSION Layered Tuffs (Machlus et al. 2015), the temporal trends in 87Sr/86Sr The utility of 87Sr/86Sr for reconstructing the paleohydrology and ratios match very closely (Fig. 12A). Using the two tuff ages noted chronostratigraphy of lake deposits depends mainly on the availability of above (Machlus et al. 2015), the calculated average net rock accumulation rates for the interval as a whole are 95 mm/kyr in the minerals that faithfully preserve an accurate record of lakewater isotopic BF core and 82 mm/kyr in the WM core. The differential thickness composition. A number of previous studies of the detailed sedimentology, 87 86 patterns evident in Figure 6 suggest that changes in the relative petrography, mineralogy, stable isotope geochemistry, and Sr/ Sr of the accumulation rates between the two cores may have occurred. The Wilkins Peak Member have established that most of the carbonate minerals correspondence between the two 87Sr/86Sr records can be improved it contains formed either through primary precipitation from lakewater or using a more complex time model that infers three different average during early diagenesis (Bradley and Eugster 1969; Eugster and Surdam sedimentation rates for discrete segments of the WM core (Fig. 12B). 1973; Eugster and Hardie 1975; Smoot 1983; Pietras and Carroll 2006; The stepwise transition points between these segments were arbitrarily Doebbert et al. 2014; Murphy et al. 2014). An additional contribution of chosen to obtain an optimal match to the sedimentary lithofacies detrital carbonate from Phanerozoic marine units exposed in the catchment succession (in particular the occurrence of oil-shale beds) in the BF of the lake or from airborne dust cannot be excluded. However, given the core. Based on the more complex age model, time-equivalent 87Sr/86Sr relatively high 87Sr/86Sr ratios reported here and by Doebbert et al. (2014) ratios from the two cores show a strong and signiﬁcant positive it is unlikely that extrabasinal detrital carbonate could represent a correlation (r ¼ 0.68, p , 0.001; Fig. 13). signiﬁcant constituent. Some detrital carbonate did originate in the basin, 418 M. BADDOUH ET AL. JSR

87 86 FIG. 11.—Cross-plots of A) BF core Sr/ Sr ratios and resampled oil-yield values, using the ‘‘surrogateCor’’ function in Astrochron (see Baddouh et al. 2016). B) First difference of the BF core 87Sr/86Sr ratios and resampled oil-yield values. The use of ﬁrst differences permits evaluation of relative changes between sequential stratigraphic samples, rather than absolute values (see Baddouh et al. 2016). Pearson correlation coefﬁcients and their associated p values are listed, as determined with the ‘‘surrogateCor’’ function in Astrochron (see Baddouh et al. 2016).

from erosion of up-dip lacustrine and lake-plain deposits (Smoot 1983; diagenetic fluids. The deposits of underfilled lake basins, which are Pietras and Carroll 2006; Murphy et al. 2014). especially sensitive archives of climate change, commonly lack metazoan Murphy et al. (2014) directly addressed the question of primary versus fossils altogether (cf. Bohacs et al. 2000). Primary micritic carbonate, in diagenetic origin of Green River Formation calcite and dolomite, through a contrast, is relatively ubiquitous across a wide range of lake deposits, thus detailed petrographic, geochemical, and stable isotope study that included opening many more opportunities for constructing high-resolution chemo- the same interval of the BF core for which 87Sr/86Sr ratios are reported here stratigraphies. (although not for identical samples). They concluded that this interval Carbonate mudstone lithofacies sampled in this study preserve contains a heterogeneous, laminated mixture of both primary and relatively large 87Sr/86Sr fluctuations, at scales as fine as ~ 300 lm diagenetic phases. A primary origin was interpreted for microcrystalline scale. It is therefore clear that pervasive resetting of Sr isotope (, 15 lm), subhedral to anhedral dolomite contained in laminae , 300 compositions by a single diagenetic fluid has not occurred. Mass-balance lm thick. A diagenetic origin was inferred for ~ 300 lm laminae modeling by Doebbert et al. (2014) suggested relatively short Sr containing euhedral calcite and Fe-rich dolomite crystals up to ~ 70 lm. residence times in Eocene Lake Gosiute, on the order of 103–104 years These diagenetic phases were inferred to represent overgrowth cements or less. This result confirms that primary lacustrine carbonate could have that formed around detrital cores. Shortite in the WPM clearly formed preserved a high-resolution record of changing primary water sources to during diagenesis based on its crosscutting of primary sedimentary the lake. Doebbert et al. (2014) also noted that biogenic carbonate, structures. Jagniecki et al. (2013) reported that shortite forms only at burial biogenic apatite, and micritic carbonate from the same horizon have temperatures . 558C, based on experiments with pirssonite as a precursor. 87Sr/86Sr ratios that are essentially indistinguishable from each other, Shortite was, at most, a minor constituent of the samples in the present relative to the large magnitude of observed changes with depth. They study, and because it is water soluble it was likely partially or wholly therefore interpreted these ratios to reflect contemporary lakewater removed during acetate leaching and subsequent rinsing. We therefore infer isotopic composition. that diagenetic shortite had a minimal impact on the 87Sr/86Sr ratios We infer that 87Sr/86Sr ratios reported here represent a combination of reported here. primary lakewater compositions and early diagenetic fluids. The higher Ideally, 87Sr/86Sr records would be based on materials for which average percentages of dolomite, quartz, and albite in the BF core (which is diagenetic modification can be confidently excluded, such as unaltered closer to the basin center) suggests increased precipitation of diagenetic aragonitic molluscan shells (e.g., Fan et al. 2011). However, such pristine phases in lake muds during lake contraction, as lake-water became more material is often rare or absent from lacustrine carbonate facies. The saline. If so, then primary 87Sr/86Sr ratios may have been partially reset as preservation of unaltered fossils requires a favorable conjunction of the lake shrank. The potential depth range of such penecontemporaneous paleoenvironmental, taphonomic, and diagenetic factors. For example, diagenesis is unknown, although modern analogs may provide some freshwater lacustrine mollusks are often preserved in geologically young approximate constraints. Early diagenetic dolomite has been documented coquina beds, but such beds by nature tend to be relatively permeable to in association with a number of modern lakes, where it appears to form at JSR CHRONOSTRATIGRAPHIC CORRELATION OF LACUSTRINE DEPOSITS USING 87SR/86SR RATIOS 419

87 86 FIG. 12.—Stratigraphic correlation between BF (blue) and WM (red) Sr/ Sr, based on nominal ages of Layered and 6th tuffs (Machlus et al. 2015). Note the adjusted sedimentation rate for the WM core, while the BF core sedimentation rate is constant. 420 M. BADDOUH ET AL. JSR

The effective use of Sr isotope chemostratigraphy also requires that the lacustrine reservoir be isotopically homogeneous and that lake 87Sr/86Sr ratios change substantially through time. The first of these requirements has been previously demonstrated for late Quaternary deposits of the Bonneville paleolake system (Hart et al. 2004), but this study represents the first time that it has been clearly demonstrated for older, more complex strata. When corrected for differences in sediment accumulation rate, the Sr isotope stratigraphies of the upper WPM at two localities are nearly indistinguishable, despite being separated by a distance of ~ 23 km. This study does not assess the potential for isotopic heterogeneity over longer distances, and given the relatively shallow bathymetry inferred during deposition of the WPM it is possible that lake-water 87Sr/86Sr ratios were not entirely uniform. Further study is needed to evaluate this possibility. Lateral lithologic variations add to the challenge of establishing larger-scale patterns of lake-water isotopic composition, due to the increased difficulty of establishing independent chronostratigraphic correlations over longer distances. The limited WPM interval examined in this study was chosen in part because it appears to offer an example of relatively simple ‘‘layer-cake’’ stratigraphy. Lateral thickness and lithofacies changes are nonetheless evident even in this apparently simple interval (Fig. 6). The large magnitude of variation in WPM 87Sr/86Sr values make them particularly effective for chronostratigraphic correlation. To put this statement in perspective, consider the comparison between the WPM 87 86 FIG. 13.—Cross-plot of BF and WM Sr isotope ratios, following application of the and marine carbonate strata. The range of WPM Sr/ Sr variation over three-step time model for the WM core. The Pearson correlation coefficient and the ~ 120,000-year period represented by this study is nearly 50% greater associated p value are assessed using the ‘‘surrogateCor’’ function in Astrochron (see that the range of marine carbonates deposited over the entire Phanerozoic Baddouh et al. 2016). (cf. Burke et al. 1982; DePaolo and Ingram 1985; McArthur et al. 2012). More significantly, WPM 87Sr/86Sr can change by . 0.001 over an interval representing ~ 10,000 years, providing a level of age resolution that is at depths ranging from the sediment–water interface surface to 60–70 cm least an order of magnitude better than that attainable from any marine below it (Callender 1968; Muller¨ et al. 1972; Owen et al. 1973; Dean and deposits. Gorham 1976; Rosen and Coshell 1992). If early diagenesis was confined A variety of possible mechanisms could be invoked to explain why lake- to the upper few centimeters of WPM lake mud, then its impact on the use water 87Sr/86Sr changed systematically with lake-level during deposition of 87 86 of Sr/ Sr ratios for chemostratigraphic correlation would have been the WPM, such as differential weathering of parent minerals during climate minimal. Diagenesis down to 70 cm below sediment–water interface fluctuations, or cation exchange with clay minerals reworked from the lake surface, on the other hand, could substantially alter the 87Sr/86Sr record plain during lowstand incision (cf. Rhodes et al. 2002). However, changes reported in this study, which has a nominal sample resolution of 30 cm. in 87Sr/86Sr in late Pleistocene to Holocene pluvial lakes have generally WPM 87Sr/86Sr ratios generally correlate closely with sedimentary been ascribed either to changing geographic sources of runoff (e.g., lithofacies and oil-yield records of lake-level fluctuations. This supports Benson and Peterman 1996; Joordens et al. 2011; Placzek et al. 2011), or the argument that these ratios reflect lake-water at or near the time of else changes in the balance of surface runoff versus groundwater influx deposition. (e.g., Grove et al. 2003; Ojiambo et al. 2003; Hart et al. 2004; Sun et al. The lowermost sample in the WM core stands out as a prominent 2011). Similar factors were most likely responsible for the very large 87 86 exception to the general inverse relationship between lake depth and changes in WPM Sr/ Sr. Baddouh et al. (2016) proposed that lake-level 87Sr/86Sr; it comes from an oil-shale interval deposited during lake changes were governed by ESNO-driven changes in runoff from highstand but has the highest 87Sr/86Sr of any samples in this study (Figs. Cordilleran Fold and Thrust Belt, and that these changes were 6, 12A, B). This sample was taken from just above the arkosic ‘‘I bed’’ astronomically modulated by insolation cycles. Lake highstands were alluvial interval. Pietras (2003) proposed that the lake-waters had interpreted to be driven by increased influx of Pacific moisture into the anomalously high 87Sr/86Sr during the first major transgression above western U.S., which caused larger amounts of less radiogenic runoff to each of the alluvial siliclastic intervals in the WPM, due to the influence of enter Eocene Lake Gosiute. During lowstands, more radiogenic runoff from the east contributed larger relative share of the lake’s hydrologic highly radiogenic siliclastic detritus derived from Precambrian basement budget (Fig. 14). rocks. Alternatively, the underlying sandstone bed may have served as a conduit for relatively radiogenic diagenetic fluids. Based on sedimentary lithofacies and oil-yield data, the WM core CONCLUSIONS interval examined in this study appears to record 11 lake deepening events, This study indicates that 87Sr/86Sr ratios in calcitic and dolomitic and the equivalent interval of the BF core records 13 deepening events lacustrine mudstone lithofacies of the WPM represent a variable (Fig. 6). This difference could reflect heterogeneity of the original contribution of primary and early diagenetic mineral phases. Diagenetic 87 86 depositional environments, localized scour, or both. Sr/ Sr ratios influence appears to have been more pronounced nearer the Bridger basin corresponding to the ‘‘missing’’ cycles in the WM core are similar to the depocenter, based on greater average percentages of dolomite, silica, and corresponding intervals in the BF core, which may argue for lateral albite, and therefore may have been related to increasing evaporative lithofacies change rather than scour. Additional studies at finer sample concentration of Eocene Lake Gosiute. However, pervasive diagenetic resolution are needed to resolve such uncertainties. resetting of 87Sr/86Sr ratios does not appear to have occurred. In the JSR CHRONOSTRATIGRAPHIC CORRELATION OF LACUSTRINE DEPOSITS USING 87SR/86SR RATIOS 421

FIG. 14.—Schematic interpretive model relat- ing lake-level fluctuations and lakewater 87Sr/86Sr to changes in runoff from the Cordillera Fold and Thrust Belt west of Eocene Lake Gosiute (based on Baddouh et al. 2016). uppermost WPM interval examined in this study 87Sr/86Sr varies between the original manuscript. Financial support was provided by the Donors of the 0.71154 and 0.71504, a range that is ~ 50% greater than the entire range of Petroleum Research Fund of the American Chemical Society, the Center for Oil- Phanerozoic seawater. Fluctuations in 87Sr/86Sr ratios are inversely related shale Technology and Research (COSTAR), the Geoscience department at the to sedimentary lithofacies evidence for lake-water depth—low ratios University of Wisconsin–Madison, NSF-EAR 1151438 (SRM), and NSF-ATM correspond to lake highstands, and high ratios correlate to lake lowstands. 0081852 (CMJ). Core samples were made available by the U.S. Geological This correspondence suggests that at the resolution of the present Survey Core Repository, Denver, Colorado. investigation, measured 87Sr/86Sr ratios are closely related to lake-water 87Sr/86Sr at or near the time of deposition. REFERENCES No systematic difference in 87Sr/86Sr is apparent between lake ASWASEREELERT, W., MEYERS, S.R., CARROLL, A.R., PETERS, S.E., SMITH, M.E., AND FEIGL, lithofacies deposited at the same time ~ 23 km apart. Eocene Lake K.L., 2013, Basin scale cyclostratigraphy of the Green River Formation, Wyoming: Gosiute therefore appears to have been well mixed with respect to Geological Society of America, Bulletin, v. 125, p. 216–228. 87Sr/86Sr, at this spatial scale. 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