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 fundamentallydepends on the availability of reliable time markers. Unlike marine strata, lacustrine strata typically lack rapidlyevolving, cosmopolitan fauna or flora that might serve this purpose. Depending on their geologic context, lacustrinestrata may also lack tephras that could provide isochronous markers or radioisotopic age. Variations in 87Sr/86Srratios could potentially provide an alternative means of chronostratigraphic correlation for carbonate-rich lakedeposits, based on the hypothesis that Sr isotopes are well mixed in a lake and do not experience significantfractionation. To test this hypothesis we measured 87Sr/86Sr ratios in 114 samples from two drill cores of the upperWilkins Peak Member from the Green River Formation that are located ~ 23 km apart. These cores can beindependently 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 interpretedby sedimentary lithofacies characteristics. Lower ratios of 87Sr/86Sr are found in lithofacies deposited during lakehighstands, which are marked by laminated dark-gray mudstone and elevated organic-carbon enrichment (asmeasured 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 thetwo cores show a strong positive correlation (r ¼ 0.68), despite the likely presence of small temporal mismatchesbetween approximately correlative samples. We conclude that lake-water was consistently well mixed with respect toSr 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
Lake deposits occur on all of the continents, and offer rich archives of
tectonic, magmatic, climatic, and biologic evolution. They also contain
important economic resources of coal, oil, oil-shale, soda ash, evaporites
and other valuable resources (e.g., Dyni 2006; Johnson et al. 2011).
Accurately reading these archives and fully exploiting the economic
potential of lacustrine strata requires reconstruction of the processes by
which they were deposited. This in turn requires the ability to accurately
correlate sedimentary lithofacies that were deposited synchronously at
different locations. Unlike marine strata, lacustrine deposits usually lack
rapidly evolving, cosmopolitan index fossils that might facilitate
chronostratigraphic correlation. Lake-levels and sediment sources also
may fluctuate very rapidly compared to many marine systems (e.g., Oviatt
1997; Pietras and Carroll 2006), adding to the challenge of correlating
synchronous lithofacies tracts. As a result of these difficulties, lake basin
evolution must often be inferred largely from lithostratigraphic correlation
(e.g., Fouch 1975; Roehler 1993; Yang et al. 2010; Burton et al. 2014). In
underfilled lake basins (cf. Carroll and Bohacs 1999), lithostratigraphy
may nearly replicate chronostratigraphy due to the dominant influence of
basin-wide wet–dry cycles (e.g., Culbertson 1961; Tanavsuu-Milkeviciene
and Sarg 2012). At the other extreme, overfilled basins characteristically
manifest dramatic lateral lithofacies variations that render lithology
effectively useless for chronostratigraphic reconstruction (e.g., Bohacs et
al. 2000; Norsted et al. 2015).
A variety of approaches have been employed in an effort to overcome
these problems, such as radioisotopic dating of tephras (e.g., Steenbrink et
al. 1999; Smith et al. 2008; Smith et al. 2010; Machlus et al. 2015),
magnetic-reversal stratigraphy (e.g., Olsen et al. 1996; Magyar et al. 2007;
van Vugt et al. 1998; Barbera et al. 2001), astrochronology (Van Houten
1962; Olsen 1986; Aswasereelert et al. 2013), and seismic stratigraphy
(e.g., Scholz and Rosendahl 1988; Liro and Pardus 1990). Each of these
approaches carries intrinsic advantages and limitations. Tephrochronology
provides radiogenic ages but is limited to strata containing dateable tuffs,
which in some cases have been altered by contact with lake-waters (cf.
Smith et al. 2003). Magnetostratigraphy and astrochronology offer
excellent temporal resolution but are limited in application to suitable
lithofacies and do not by themselves provide unique, numerical
geochronological ages. Seismic stratigraphy can help to decipher large-
scale temporal patterns of basin fill but generally lacks the resolution
required to capture fine details of a rapidly evolving lake.87Sr/86Sr ratios have long been used to correlate coeval marine deposits,
based on the observation that the oceanic reservoir is well mixed, its87Sr/86Sr ratio has changed continuously through the Phanerozoic, and Sr
isotopes are not significantly fractionated by meteoric processes or during
the precipitation of carbonate and phosphate minerals (e.g., DePaolo and* Present Address: Department of Atmospheric, Oceanic and Earth Sciences,
George Mason University, Fairfax, Virginia 22030, U.S.A.
Published Online: April 2017Copyright � 2017, SEPM (Society for Sedimentary Geology) 1527-1404/17/087-406/$03.00
Ingram 1985; Veizer 1989; Capo et al. 1998). The same approach might
potentially be used to provide highly detailed chronostratigraphies of
carbonate-rich lacustrine strata, if the lacustrine reservoir was similarly
well mixed. Results reported for late Quaternary pluvial lake systems
suggest that this may indeed be the case. For example, lacustrine carbonate
lithofacies of Bonneville basin in Utah have 87Sr/86Sr ratios that principally
reflect the elevation of the paleolake surface at the time they were
deposited, despite being geographically separated by tens to hundreds of
km (Hart et al. 2004). Over the past ~ 15,000 years, lakewater 87Sr/86Sr
increased from ~ 0.71125 to ~ 0.71387 as lake-level fell. An opposite
relationship between paleolake-level and 87Sr/86Sr is preserved in
lacustrine tufa of the Lahontan basin in Nevada; 87Sr/86Sr in tufa carbonate
lithofacies decreased from ~ 0.70786 to ~ 0.7056 as lake-level declined
over the past ~ 21,000 years (Benson and Peterman 1996). These
Quaternary examples primarily record the terminal desiccation of pluvial
lakes, which left behind relict geomorphic features that can be used to
directly measure past lake surface elevations. The potential utility of87Sr/86Sr for chronostratigraphic correlation of older strata that embody
longer, more complex histories has not yet been tested. Ironically, the lack
of detailed, independent chronostratigraphy in many such deposits itself
poses an impediment to such tests.
The Wilkins Peak Member (WPM) of the Green River Formation (GRF)
offers an ideal opportunity to test the efficacy of 87Sr/86Sr for lacustrine
chronostratigraphic correlation for several reasons. Previous studies have
demonstrated that its stratigraphy is dominated by repeated wet–dry cycles
(e.g., Eugster and Hardie 1975; Smoot, 1983; Carroll and Bohacs 1999;
Bohacs et al. 2000; Pietras and Carroll 2006). These cycles at least
partially reflect Milankovitch forcing and thus should be expressed
synchronously across the entire basin (e.g., Fischer and Roberts 1991;
Machlus et al. 2008; Meyers 2008; Aswasereelert et al. 2013). However,
other factors such as shorter-term climate change or geomorphic drainage
instability appear to have influenced lake-levels over shorter time periods
(Pietras et al. 2003; Pietras and Carroll 2006). Organic-carbon rich
mudstone (oil-shale) beds provide particularly distinctive markers of lake
deepening episodes (Carroll and Bohacs 2001) and have in some cases
been correlated across distances of . 100 km (Pietras and Carroll 2006;
Smith et al. 2015). The WPM also contains distinctive tuff horizons that
aid in regional chronostratigraphic correlation. Recent 40Ar/39Ar and U-Pb
dating has established the WPM as one of the best-dated intervals of pre-
Quaternary sedimentary rock anywhere (Smith et al. 2003; Smith et al.
2008; Smith et al. 2010; Machlus et al. 2015). Several previous studies
have argued that carbonate lithofacies in Green River Formation mudstone
do preserve a faithful record of 87Sr/86Sr in Eocene lakewater (Rhodes et
al. 2002; Doebbert et al. 2014; Baddouh et al. 2016).
Herein we present 87Sr/86Sr and X-ray diffraction (XRD) data for WPM
samples collected from two drill cores located ~ 23 km apart (Fig. 1). We
combined previously reported data from 49 samples from the White
Mountain #1 (WM) drill core, that were previously reported by Baddouh et
al. (2016), with newly reported data from 65 samples from the Blacks Fork
#1 (BF) drill core.
GEOLOGICAL BACKGROUND
The WPM was deposited by Eocene Lake Gosiute in the Bridger Basin,
southwestern Wyoming, between ~ 51.5 and ~ 50.0 Ma (Smith et al.
2010; Machlus et al. 2015) (Fig. 1). The Eocene watershed surrounding the
lake was underlain by bedrock lithologies of widely varying age, Sr
concentration, and Sr isotope composition (e.g., Beard and Johnson 2000;
Fan et al. 2011; Bataille and Bowen 2012). The broad lithologic and
structural configuration of this area has changed relatively little since the
Eocene. Neogene normal faults locally produced high topographic relief,
but the overall magnitude of extension is modest (cf. Love et al. 1963;
Snoke 1997). Regional drainage has been substantially altered by the late
Cenozoic incision of the Green River, a tributary of the Colorado River.
However, the Sr isotopic compositions of smaller tributary streams that
drain local catchments in basin-bounding uplifts likely mirror those of their
Eocene precursors (Doebbert et al. 2014).
The Bridger Basin is bounded on the west by the Cordilleran Fold and
Thrust Belt (CFTB) (Fig. 1), which contains thick, structurally repeated
intervals of Cambrian through Cretaceous marine strata. These include Sr-
rich marine limestone intervals up to several hundred meters thick (cf.
Love and Christiansen 1985), with 87Sr/86Sr ratios corresponding to marine
values at the time of deposition (generally in the range of ~ 0.707–0.7095;
Burke 1982). Modern rivers draining the CFTB have previously reported87Sr/86Sr ratios of 0.70869 to 0.70917 (Doebbert et al. 2014). The
Paleocene–Eocene Flagstaff Formation, which consists of lacustrine
lithofacies deposited south of the Uinta uplift, has 87Sr/86Sr ratios that
are indistinguishable from modern streams arising in the adjacent CFTB
(Gierlowski-Kordesch et al. 2008). Cenozoic sedimentary rocks as far
north as the Alberta foreland basin record similar 87Sr/86Sr ratios (Fan et al.
2011). Eocene volcanic rocks to the north and northwest of the Bridger
Basin have lower 87Sr/86Sr ratios, in the range of 0.7050 to 0.7060
(Doebbert et al. 2014), but it appears that these rocks did not contribute
significant drainage to Lake Gosiute until after deposition of the WPM
(Carroll et al. 2008; Doebbert et al. 2010; Chetel et al. 2011). In contrast,
highly radiogenic Precambrian crystalline rocks are commonly exposed in
basement-cored uplifts to the north (Wind River) and south (Uinta) of the
Bridger Basin (Fig. 1). The original sedimentary cover of these uplifts was
generally eroded before the deposition of the WPM (Love et al. 1963;
Carroll et al. 2006; Fan 2009). Doebbert et al. (2014) reported that modern
rivers draining those areas have 87Sr/86Sr ratios of 0.71566 to 0.74318. The
Bridger Basin is bounded to the east by folded Cretaceous strata of the
Rock Springs Arch, which separates upper WPM lacustrine and alluvial
strata in the Bridger Basin from time-equivalent alluvial deposits of the
Cathedral Bluffs Member of the Wasatch Formation to the east (Figs. 1, 2;
Sullivan 1985; Roehler 1993; Smith et al. 2008; Chetel et al. 2011). The
Rock Springs Arch appears to have acted as a partial topographic barrier
during the Eocene. Siliciclastic detritus derived from Precambrian-cored
uplifts farther east was mostly baffled in the Sand Wash, Washakie, and
Great Divide basins, while radiogenic river water spilled westward into the
Bridger Basin (Smith et al. 2014; Smith et al. 2015).
The WPM lacustrine strata consist of repetitive lithofacies successions
that record episodic expansion and contraction of Eocene Lake Gosiute
across a low-relief basin floor (Eugster and Hardie 1975; Smoot 1983;
Roehler 1993; Pietras and Carroll 2006). Individual successions (or
‘‘cycles’’) commonly begin with interbedded carbonate-rich mudstone,
calcareous sandstone, and intraclast conglomerate, deposited during
shoreline transgression. Scour marks, desiccation cracks, mudstone
intraclasts, wave ripples, and wavy bedding are common. These littoral
lithofacies grade upward into sub-littoral to profundal lithofacies consisting
of dark gray to brown, thinly laminated, kerogen-rich mudstone (oil-shale).
Primary bedded trona and halite are closely associated with the profundal
lithofacies near the basin depocenter, and diagenetic, displacive shortite
crystals commonly crosscut and disrupt primary sedimentary fabrics across
much of the Bridger Basin (Jagniecki and Lowenstein 2015). Profundal
lithofacies typically grade upward into gray-green carbonate-rich mudstone
and siltstone lithofacies that record gradual regression of the lake. Wavy
lamination, mudcracks, brecciation, and shortite crystals are common, and
are interpreted to record deposition in littoral to palustrine environments.
The WPM lacustrine lithofacies are punctuated by up to nine discrete,
regionally correlatable intervals of dominantly alluvial, siliclastic lithofa-
cies (labeled A through I in Figure 2; Culbertson 1961; Smoot 1983). Each
of these represents a complex, composite bedset that may include
sandstone, siltstone, and mudstone, with minor interbeds of lacustrine
carbonate or evaporite. These lithofacies have been interpreted to record
episodic westward bypass of arkosic detritus around or through the Rock
CHRONOSTRATIGRAPHIC CORRELATION OF LACUSTRINE DEPOSITS USING 87SR/86SR RATIOSJ S R 407
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).
M. BADDOUH ET AL.408 J S R
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).
CHRONOSTRATIGRAPHIC CORRELATION OF LACUSTRINE DEPOSITS USING 87SR/86SR RATIOSJ S R 409
FIG
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M. BADDOUH ET AL.410 J S R
Springs Arch (Sullivan 1985; Smith et al. 2014), at time intervals
corresponding to ~ 100 ky and ~ 400 ky eccentricity (Aswasereelert et al.
2013). Smith et al. (2014) further argued that they were deposited during
eccentricity minima and may correspond with Eocene hyperthermal events.
This interpretation contrasts with that of Lourens et al. (2005) and Laurin
et al. (2016), who argue instead that Eocene hyperthermals occurred during
eccentricity maxima.
SAMPLING AND METHODS
For this study, we visually described lithofacies in the same stratigraphic
interval of the WPM, in two drill cores separated by ~ 23 km (Figs. 1, 3,
4). The BF core is located closer to the basin depocenter as compared to
the WM core. The study interval lies at the top of the WPM, bounded
below by an alluvial siliciclastic interval (the ‘‘I-bed’’; Culbertson 1961)
and above by the Laney Member of the Green River Formation. This
interval was selected in part because deposition appears to have been
relatively uniform across the basin, simplifying lithostratigraphic correla-
tion (Pietras and Carroll 2006; Smith et al. 2008; Aswasereelert et al.
2013). Two distinctive volcanic tuffs, the Layered Tuff and the 6th Tuff,
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 ~ 2 cm
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
from the siliclastic material (which constitutes from ~ 20 to ~ 90% of the
mass of individual samples). Acetic acid carbonate solutions were spiked
with a mixed 87Rb–84Sr 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
H3PO4 using a three-jump multi-dynamic analysis on a VG Instruments
Sector 54 multi collector thermal ionization mass spectrometer. Sr isotope
ratios were corrected from instrumental mass bias to an 86Sr/88Sr of 0.1194
using an exponential mass fractionation law. The reported 87Sr/86Sr ratio is
based on the average of 120 ratios with an 88Sr ion intensity of 3 3 10–11
A. Reported errors are the internal 2-standard errors (2-SE) which is
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
SRM-987 Sr isotope standard (0.710262 6 0.000016; 2-SD; n¼ 66) that
was analyzed during the course of this study. In addition to the NIST SRM-
987 standard analyses the EN-1 modern marine carbonate standard was
analyzed 12 times, including three analyses that had been spiked with our
mixed Rb–Sr spike (average 87Sr/86Sr¼ 0.709194 6 0.000034; 2-SD; n¼12; Sr concentration 1233 ppm). Rubidium was analyzed on Ta filament
with H3PO4, and 87Rb/85Rb analyses were determined using a static multi-
collector analysis. Based on 20 replicate analyses of NIST SRM-984 Rb,
measured 87Rb/85Rb ratios are estimated to be precise to 6 0.7%. The
percent carbonate, Rb and Sr concentration, 87Rb/86Sr and the 87Sr/86Sr
ratio and its 2-SE for all the carbonate fractions are reported in Table S3 for
the BF core and Baddouh et al. (2016) for the WM core.
FIG. 5.—Thin-section photomicrographs of typical WPM carbonate-rich mud-
stone lithofacies. Upper: organic-carbon-rich mudstone from the Apache Lane
outcrop section. Lower: organic-carbon-lean, partially dolomitized mudstone from
the Kanda outcrop section. See Pietras and Carroll (2006) for locations.
CHRONOSTRATIGRAPHIC CORRELATION OF LACUSTRINE DEPOSITS USING 87SR/86SR RATIOSJ S R 411
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The silicate fraction was also analyzed after acetate leaching and
carbonate extraction. Silicate fraction analyses included eight samples from
the BF core (newly reported in this study) and seven samples from the WM
core (previously reported in Baddouh et al. 2016). The methods for these
analyses are reported in Baddouh et al. (2016).
Statistical analyses of the geochemical data sets utilize the Astrochron
package for R (Meyers 2014; R Core Team 2015). Assessment of the
significance of the correlation between variables employs the phase-
randomized surrogate approach of Ebisuzaki (1997), as modified by
Baddouh et al. (2016) to allow comparison of data sets with different
sampling grids (see Baddouh et al. (2016) for more details).
RESULTS
Sedimentology and Lithostratigraphy
The interval between the ‘‘I-bed’’ and the Laney Member is ~ 32.3 m
thick in the BF core and ~ 27.7 m thick in the WM core, a difference that
is consistent with the closer proximity of the BF core to the basin
depocenter (Figs. 1, 2, 6; Roehler 1992; Wiig et al. 1995). The WM core
contains more evidence for intermittent subaerial exposure, including scour
marks, mudcracks, mudstone intraclasts, and burrows. The BF core
contains more frequent intervals of displacive evaporite crystals.
Sedimentary lithofacies successions in both of the study cores are
dominated by the alternation of laminated, dark-gray mudstone, versus
lighter gray-green mudstone. Fischer Assay oil-yields (U.S. Geological
Survey Oil-shale Assessment Team 2011) generally follow the same
patterns, although the correspondence between oil-yields and dark gray,
laminated lithofacies is imperfect. In part, the mismatches are attributable
to the relatively coarse spatial resolution of Fischer Assay samples. In some
instances elevated oil-yields may also correspond to originally laminated
lithofacies, in which primary depositional fabric has been obscured by
secondary growth of evaporite crystals (see for example the interval from
~ 420–422 ft. in the WM core; Figs. 4, 6).
Mineralogy
All of the samples consist of calcitic to dolomitic mudstone, with
varying amounts of organic carbon (Fig. 5). X-ray diffraction (XRD)
analyses show that on average the samples from both cores to contain ~ 46
to 50% carbonate, with the remaining fraction consisting mostly of quartz,
feldspar, and clays (Table 1; Fig. 7) (see Tables S1 and S2 for detailed
information, see Supplemental Materials). The variability among different
lithofacies in each core is more pronounced than the differences between
the average values in the two cores. This is consistent with the
interpretation that repeated lake deepening and shallowing episodes
exerted a dominant, basin-wide influence on sedimentary lithofacies. On
average, the BF core contains more dolomite, in agreement with a previous
study that documented an increase in dolomite nearer the basin depocenter
(Mason 2012). The mean values for % dolomite and dolomite/calcite ratio
overlap at 1 standard deviation, reflecting the high degree of variation at
each site. Shortite is slightly more abundant in the BF core, but the
difference falls within one standard deviation.
The silicate mineralogy of these samples is highly variable in each
core (Table 1; Fig. 7) (see Supplementary Tables S1 and S2 for detailed
information). Mean % quartz, % albite, quartz/feldspar ratio, and Na/K
feldspar ratio are higher in BF than in WM samples. Clay (mostly
TABLE 1.—Mineralogy Summary of Blacks Fork #1 and White Mountain #1 Cores.
Percentages
Blacks Fork #1 Core (n ¼ 65) White Mountain #1 Core (n ¼ 48)
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
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).
CHRONOSTRATIGRAPHIC CORRELATION OF LACUSTRINE DEPOSITS USING 87SR/86SR RATIOSJ S R 413
illite), on the other hand, is more abundant on average in the WM core
(Fig. 7).
Geochemistry
Carbonate-mineral content (excluding shortite) as measured by acid
digestion is highly variable in each core, ranging from 11.0% to 86.6%
(Table 2, see Supplementary Table S3 for detailed information). Average
carbonate mineral values are lower than those measured via XRD in our
analyzed intervals; the WM core average is ~ 9% lower, and the BF core
average is ~ 14% lower. These results could indicate incomplete
dissolution of carbonate, particularly dolomite. Alternatively, some
carbonate minerals may have been removed during ammonium acetate
leaching, or XRD analysis may have overestimated the proportion of
carbonate minerals. These differences do not affect measured 87Sr/86Sr or
Sr concentration (Fig. 8A, B). 87Sr/86Sr is also not closely correlated with
Sr concentration (Fig. 8C).87Rb/86Sr and Rb concentration are higher on average in the BF core
than in the WM core, but average Sr concentration and 87Sr/86Sr are similar
in both cores (Table 2; Fig. 8D, E). All of these values display wide ranges
of variation in each core. 87Rb/86Sr is positively correlated with Rb
concentration in both cores (Fig. 8F). 87Sr/86Sr also appears to be largely
independent of carbonate mineralogy as measured by XRD (Fig. 9A, B),
although 87Sr/86Sr does correlate weakly with % calcite (r ¼ 0.36) and %
dolomite (r ¼ –0.33) in the BF core.
The silicate fractions of the study samples contain more Rb and have
higher 87Rb/86Sr ratios than the carbonate fractions (Table 3, Fig. 9C, D).87Rb/86Sr in the carbonate fractions correlates negatively with % carbonate
in both cores (r ¼ –0.71, in BF and r ¼ –0.69, in WM; Fig. 8D) and
positively with Rb concentration (r¼0.75, in BF and r¼0.86, in WM; Fig.
8F), which suggests possible contamination from more radiogenic silicate
minerals. The potential magnitude of such contamination is very small,
however. 87Rb/86Sr ratios in the carbonate fractions are all very low (on the
order of 10–3; Tables 2, 3; Fig. 8F) and no significant correlation is evident
between 87Sr/86Sr and either % clay or 87Rb/86Sr (Fig. 10A, B). Doebbert
et al. (2014) suggest a 87Rb/86Sr ratio of . 0.02 as a basis for excluding
samples from interpretation; all of the samples in the present study have
ratios , 0.008 (Figs. 8F, 10B). Therefore, we conclude that the ammonium
acetate leaching was successful in minimizing radiogenic Sr derived from
silicates. The 87Rb/86Sr ratios as well as Rb concentration of the silicate
fraction from the BF core tend to be lower as compared to the WM core
silicate fraction (Table 3, Fig. 9C, D), and this is considered to reflect the
mineralogical differences where the WM core has more illite and less albite
as compared to the BF core (Fig. 7).
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 47787Rb/86Sr 0.00044 0.00693 0.00205 0.00108 0.00013 0.0071 0.00145 0.0013887Sr/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.
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).
Field Name
Carbonate Fraction Silicate Fraction
% 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
M. BADDOUH ET AL.414 J S R
FIG. 8.—Cross-plots of 87Sr/86Sr, 87Rb/86Sr, Sr concentration, and Rb concentration vs. % carbonate from both BF and WM cores, as well as cross-plots of 87Sr/86Sr,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).
CHRONOSTRATIGRAPHIC CORRELATION OF LACUSTRINE DEPOSITS USING 87SR/86SR RATIOSJ S R 415
87Sr/86Sr and Wilkins Peak Member Stratigraphy
Carbonate-mineral 87Sr/86Sr ratios generally correlate with sedimentary
lithofacies, particularly in the WM core (Fig. 6). Lower 87Sr/86Sr ratios
generally correlate with laminated sedimentary lithofacies and higher
Fischer Assay oil-yield, and higher 87Sr/86Sr ratios occur in non-laminated
lithofacies and lower oil-yield. This relationship is somewhat less strongly
developed in parts of the BF core (i.e., the interval between ~ 480 and 520
ft.; Fig. 6). Slight offsets do occur between peak oil-yield and the lowest
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
ratios show a strong negative correlation with oil-yield (r ¼ –0.54) (Fig.
11A, B), and this correlation is similar to that observed in the WM core
(Baddouh et al. 2016).
The two 87Sr/86Sr records cannot be directly compared based on core
depths alone because the study-interval thickness differs between the
two cores—implying different sedimentation histories. However, if both
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 87Rb/86Sr
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).
M. BADDOUH ET AL.416 J S R
cores are transformed to time using the U-Pb ages of the 6th and
Layered Tuffs (Machlus et al. 2015), the temporal trends in 87Sr/86Sr
ratios match very closely (Fig. 12A). Using the two tuff ages noted
above (Machlus et al. 2015), the calculated average net rock
accumulation rates for the interval as a whole are 95 mm/kyr in the
BF core and 82 mm/kyr in the WM core. The differential thickness
patterns evident in Figure 6 suggest that changes in the relative
accumulation rates between the two cores may have occurred. The
correspondence between the two 87Sr/86Sr records can be improved
using a more complex time model that infers three different average
sedimentation rates for discrete segments of the WM core (Fig. 12B).
The stepwise transition points between these segments were arbitrarily
chosen to obtain an optimal match to the sedimentary lithofacies
succession (in particular the occurrence of oil-shale beds) in the BF
core. Based on the more complex age model, time-equivalent 87Sr/86Sr
ratios from the two cores show a strong and significant positive
correlation (r ¼ 0.68, p , 0.001; Fig. 13).
DISCUSSION
The utility of 87Sr/86Sr for reconstructing the paleohydrology and
chronostratigraphy of lake deposits depends mainly on the availability of
minerals that faithfully preserve an accurate record of lakewater isotopic
composition. A number of previous studies of the detailed sedimentology,
petrography, mineralogy, stable isotope geochemistry, and 87Sr/86Sr of the
Wilkins Peak Member have established that most of the carbonate minerals
it contains formed either through primary precipitation from lakewater or
during early diagenesis (Bradley and Eugster 1969; Eugster and Surdam
1973; Eugster and Hardie 1975; Smoot 1983; Pietras and Carroll 2006;
Doebbert et al. 2014; Murphy et al. 2014). An additional contribution of
detrital carbonate from Phanerozoic marine units exposed in the catchment
of the lake or from airborne dust cannot be excluded. However, given the
relatively high 87Sr/86Sr ratios reported here and by Doebbert et al. (2014)
it is unlikely that extrabasinal detrital carbonate could represent a
significant constituent. Some detrital carbonate did originate in the basin,
FIG. 10.—Cross-plots of Sr isotope ratios vs. A) clay and B) 87Rb/86Sr 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).
CHRONOSTRATIGRAPHIC CORRELATION OF LACUSTRINE DEPOSITS USING 87SR/86SR RATIOSJ S R 417
from erosion of up-dip lacustrine and lake-plain deposits (Smoot 1983;
Pietras and Carroll 2006; Murphy et al. 2014).
Murphy et al. (2014) directly addressed the question of primary versus
diagenetic origin of Green River Formation calcite and dolomite, through a
detailed petrographic, geochemical, and stable isotope study that included
the same interval of the BF core for which 87Sr/86Sr ratios are reported here
(although not for identical samples). They concluded that this interval
contains a heterogeneous, laminated mixture of both primary and
diagenetic phases. A primary origin was interpreted for microcrystalline
(, 15 lm), subhedral to anhedral dolomite contained in laminae , 300
lm thick. A diagenetic origin was inferred for ~ 300 lm laminae
containing euhedral calcite and Fe-rich dolomite crystals up to ~ 70 lm.
These diagenetic phases were inferred to represent overgrowth cements
that formed around detrital cores. Shortite in the WPM clearly formed
during diagenesis based on its crosscutting of primary sedimentary
structures. Jagniecki et al. (2013) reported that shortite forms only at burial
temperatures . 558C, based on experiments with pirssonite as a precursor.
Shortite was, at most, a minor constituent of the samples in the present
study, and because it is water soluble it was likely partially or wholly
removed during acetate leaching and subsequent rinsing. We therefore infer
that diagenetic shortite had a minimal impact on the 87Sr/86Sr ratios
reported here.
Ideally, 87Sr/86Sr records would be based on materials for which
diagenetic modification can be confidently excluded, such as unaltered
aragonitic molluscan shells (e.g., Fan et al. 2011). However, such pristine
material is often rare or absent from lacustrine carbonate facies. The
preservation of unaltered fossils requires a favorable conjunction of
paleoenvironmental, taphonomic, and diagenetic factors. For example,
freshwater lacustrine mollusks are often preserved in geologically young
coquina beds, but such beds by nature tend to be relatively permeable to
diagenetic fluids. The deposits of underfilled lake basins, which are
especially sensitive archives of climate change, commonly lack metazoan
fossils altogether (cf. Bohacs et al. 2000). Primary micritic carbonate, in
contrast, is relatively ubiquitous across a wide range of lake deposits, thus
opening many more opportunities for constructing high-resolution chemo-
stratigraphies.
Carbonate mudstone lithofacies sampled in this study preserve
relatively large 87Sr/86Sr fluctuations, at scales as fine as ~ 300 lm
scale. It is therefore clear that pervasive resetting of Sr isotope
compositions by a single diagenetic fluid has not occurred. Mass-balance
modeling by Doebbert et al. (2014) suggested relatively short Sr
residence times in Eocene Lake Gosiute, on the order of 103–104 years
or less. This result confirms that primary lacustrine carbonate could have
preserved a high-resolution record of changing primary water sources to
the lake. Doebbert et al. (2014) also noted that biogenic carbonate,
biogenic apatite, and micritic carbonate from the same horizon have87Sr/86Sr ratios that are essentially indistinguishable from each other,
relative to the large magnitude of observed changes with depth. They
therefore interpreted these ratios to reflect contemporary lakewater
isotopic composition.
We infer that 87Sr/86Sr ratios reported here represent a combination of
primary lakewater compositions and early diagenetic fluids. The higher
average percentages of dolomite, quartz, and albite in the BF core (which is
closer to the basin center) suggests increased precipitation of diagenetic
phases in lake muds during lake contraction, as lake-water became more
saline. If so, then primary 87Sr/86Sr ratios may have been partially reset as
the lake shrank. The potential depth range of such penecontemporaneous
diagenesis is unknown, although modern analogs may provide some
approximate constraints. Early diagenetic dolomite has been documented
in association with a number of modern lakes, where it appears to form at
FIG. 11.—Cross-plots of A) BF core 87Sr/86Sr 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).
M. BADDOUH ET AL.418 J S R
FIG. 12.—Stratigraphic correlation between BF (blue) and WM (red) 87Sr/86Sr, 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.
CHRONOSTRATIGRAPHIC CORRELATION OF LACUSTRINE DEPOSITS USING 87SR/86SR RATIOSJ S R 419
depths ranging from the sediment–water interface surface to 60–70 cm
below it (Callender 1968; Muller et al. 1972; Owen et al. 1973; Dean and
Gorham 1976; Rosen and Coshell 1992). If early diagenesis was confined
to the upper few centimeters of WPM lake mud, then its impact on the use
of 87Sr/86Sr ratios for chemostratigraphic correlation would have been
minimal. Diagenesis down to 70 cm below sediment–water interface
surface, on the other hand, could substantially alter the 87Sr/86Sr record
reported in this study, which has a nominal sample resolution of 30 cm.
WPM 87Sr/86Sr ratios generally correlate closely with sedimentary
lithofacies and oil-yield records of lake-level fluctuations. This supports
the argument that these ratios reflect lake-water at or near the time of
deposition.
The lowermost sample in the WM core stands out as a prominent
exception to the general inverse relationship between lake depth and87Sr/86Sr; it comes from an oil-shale interval deposited during lake
highstand but has the highest 87Sr/86Sr of any samples in this study (Figs.
6, 12A, B). This sample was taken from just above the arkosic ‘‘I bed’’
alluvial interval. Pietras (2003) proposed that the lake-waters had
anomalously high 87Sr/86Sr during the first major transgression above
each of the alluvial siliclastic intervals in the WPM, due to the influence of
highly radiogenic siliclastic detritus derived from Precambrian basement
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
interval examined in this study appears to record 11 lake deepening events,
and the equivalent interval of the BF core records 13 deepening events
(Fig. 6). This difference could reflect heterogeneity of the original
depositional environments, localized scour, or both. 87Sr/86Sr ratios
corresponding to the ‘‘missing’’ cycles in the WM core are similar to the
corresponding intervals in the BF core, which may argue for lateral
lithofacies change rather than scour. Additional studies at finer sample
resolution are needed to resolve such uncertainties.
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-water87Sr/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 chronostrati-
graphic 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
and marine carbonate strata. The range of WPM 87Sr/86Sr variation over
the ~ 120,000-year period represented by this study is nearly 50% greater
that the range of marine carbonates deposited over the entire Phanerozoic
(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
least an order of magnitude better than that attainable from any marine
deposits.
A variety of possible mechanisms could be invoked to explain why lake-
water 87Sr/86Sr changed systematically with lake-level during deposition of
the WPM, such as differential weathering of parent minerals during climate
fluctuations, or cation exchange with clay minerals reworked from the lake
plain during lowstand incision (cf. Rhodes et al. 2002). However, changes
in 87Sr/86Sr in late Pleistocene to Holocene pluvial lakes have generally
been ascribed either to changing geographic sources of runoff (e.g.,
Benson and Peterman 1996; Joordens et al. 2011; Placzek et al. 2011), or
else changes in the balance of surface runoff versus groundwater influx
(e.g., Grove et al. 2003; Ojiambo et al. 2003; Hart et al. 2004; Sun et al.
2011). Similar factors were most likely responsible for the very large
changes in WPM 87Sr/86Sr. Baddouh et al. (2016) proposed that lake-level
changes were governed by ESNO-driven changes in runoff from
Cordilleran Fold and Thrust Belt, and that these changes were
astronomically modulated by insolation cycles. Lake highstands were
interpreted to be driven by increased influx of Pacific moisture into the
western U.S., which caused larger amounts of less radiogenic runoff to
enter Eocene Lake Gosiute. During lowstands, more radiogenic runoff
from the east contributed larger relative share of the lake’s hydrologic
budget (Fig. 14).
CONCLUSIONS
This study indicates that 87Sr/86Sr ratios in calcitic and dolomitic
lacustrine mudstone lithofacies of the WPM represent a variable
contribution of primary and early diagenetic mineral phases. Diagenetic
influence appears to have been more pronounced nearer the Bridger basin
depocenter, based on greater average percentages of dolomite, silica, and
albite, and therefore may have been related to increasing evaporative
concentration of Eocene Lake Gosiute. However, pervasive diagenetic
resetting of 87Sr/86Sr ratios does not appear to have occurred. In the
FIG. 13.—Cross-plot of BF and WM Sr isotope ratios, following application of the
three-step time model for the WM core. The Pearson correlation coefficient and
associated p value are assessed using the ‘‘surrogateCor’’ function in Astrochron (see
Baddouh et al. 2016).
M. BADDOUH ET AL.420 J S R
uppermost WPM interval examined in this study 87Sr/86Sr varies between
0.71154 and 0.71504, a range that is ~ 50% greater than the entire range of
Phanerozoic seawater. Fluctuations in 87Sr/86Sr ratios are inversely related
to sedimentary lithofacies evidence for lake-water depth—low ratios
correspond to lake highstands, and high ratios correlate to lake lowstands.
This correspondence suggests that at the resolution of the present
investigation, measured 87Sr/86Sr ratios are closely related to lake-water87Sr/86Sr at or near the time of deposition.
No systematic difference in 87Sr/86Sr is apparent between lake
lithofacies deposited at the same time ~ 23 km apart. Eocene Lake
Gosiute therefore appears to have been well mixed with respect to87Sr/86Sr, at this spatial scale. This result is based on a relatively small area
of Eocene Lake Gosiute and does not exclude the possibility that Sr
isotopic compositions were more heterogeneous across the entire lake.
Moreover, this study demonstrates for the first time that 87Sr/86Sr ratios can
serve as a uniquely useful chronostratigraphic correlation tool in
carbonate-rich lacustrine mudstone, provided that the lake experienced
substantial 87Sr/86Sr fluctuations, was geochemically well mixed and
appropriate measures are taken to limit the potential contribution of
radiogenic Sr contained in clay minerals. Finally, in addition to correlation
tool described in this study, 87Sr/86Sr ratios offer extra information about
water provenance and atmospheric moisture sources (Baddouh et al. 2016)
that is important to studying paleohydrology and lake-level evolution
history.
SUPPLEMENTAL MATERIAL
Three tables are available from JSR’s Data Archive: http://sepm.org/
pages.aspx?pageid¼229.
ACKNOWLEDGMENTS
We are grateful to B.J. Linzmeier, A.C. Doebbert, T.K. Lowenstein, J.T.
Murphy, J.T. Pietras, K.M. Dyer-Pietras, M.K. Rhodes Carson, and M.E Smith,
for discussions and other assistance that benefitted this study. We also thank
E.A. Jagniecki, K.C. Benison, and L.A. Melim for their constructive reviews of
the original manuscript. Financial support was provided by the Donors of the
Petroleum Research Fund of the American Chemical Society, the Center for Oil-
shale Technology and Research (COSTAR), the Geoscience department at the
University of Wisconsin–Madison, NSF-EAR 1151438 (SRM), and NSF-ATM
0081852 (CMJ). Core samples were made available by the U.S. Geological
Survey Core Repository, Denver, Colorado.
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Received 6 June 2016; accepted 19 February 2017.
CHRONOSTRATIGRAPHIC CORRELATION OF LACUSTRINE DEPOSITS USING 87SR/86SR RATIOSJ S R 423