CHRONOSTRATIGRAPHIC CORRELATION OF ......rich marine limestone intervals up to several hundred meters thick (cf. Love and Christiansen 1985), with 87 Sr/ 86 Sr ratios corresponding

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).


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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.


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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).


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


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).


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).


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).


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).


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.


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).


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 ......rich marine limestone intervals up to several hundred meters thick (cf. Love and Christiansen 1985), with 87 Sr/ 86 Sr ratios corresponding

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