U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Map 3344

Geologic Map of the Masters 7.5’ Quadrangle, 

Weld and Morgan Counties, Colorado

By Margaret E. Berry, Janet L. Slate, James B. Paces, Paul R. Hanson, and Theodore R. Brandt

Appendix 1

230Th/U ages Supporting Geologic Map of the Masters 

7.5’ Quadrangle, Weld and Morgan Counties, Colorado

By James B. Paces

Contents

Introduction................................................................................................................................................................................................................1

Samples......................................................................................................................................................................................................................1

Aliquots Prepared for U-Series Dating .................................................................................................................................................................2

Chemical Processing and Mass Spectrometry ...................................................................................................................................................2

Analytical Results .....................................................................................................................................................................................................3

Calculation of 230Th/U Ages ............................................................................................................................................................................3

Initial 234U/238U Activity Ratios.........................................................................................................................................................................6

Ages for Individual Samples ..........................................................................................................................................................................6

Conclusions..............................................................................................................................................................................................................10

Acknowledgments..................................................................................................................................................................................................10

References Cited.....................................................................................................................................................................................................10

Figures

1–1. Polished cross section of fossil bone sample Wp67 showing thin calcite-rich rind coating outer surface and dense, 

ivory-like interior................................................................................................................................................................................................1

1–2. Examples of polished slabs of alluvial clasts from sample Wp95 showing thin calcite-rich rinds coating outer 

surfaces (Wp95–A1 and Wp95–B), or rinds separated from their associated clasts (Wp95–E)................................................2

1–3. U-series isotope evolution plots showing U-Th isotopic compositions and 2-sigma (2σ) uncertainties for analyses 

listed in table 1–1 .....................................................................................................................................................................................5

1–4. Initial 234U/238U activity ratios for analyses of samples Wp67 and Wp95.........................................................................................7

1–5. Plot showing relative uranium concentrations across a section of hypothetical compact bone showing the concept 

of progressive uptake of U with time according to the Diffusion-Adsorption model (modified from figure 1 of Pike and 

others, 2002) .......................................................................................................................................................................................................7

1–6. Profiles of uranium concentration (A) and 230Th/U age and initial 234U/238U activity ratios (B) for subsamples of bone 

from sample Wp67...................................................................................................................................................................................8

1–7. Plot of 230Th/U ages for subsamples of clast rinds from Wp95.........................................................................................................9

Tables

1–1. U-Th concentrations, U-series isotopic compositions, and calculated 230Th/U ages and initial 234U/238U activity ratios 

for subsamples of Wp67 and Wp95 from sites EF and EIC, respectively, on the Masters 7.5’ Quadrangle, Weld and 

Morgan Counties, Colorado ...................................................................................................................................................................4

Conversion Factors

International System of Units

Multiply By To obtain

Length

millimeter (mm) 0.03937 inch (in.)

centimeter (cm) 0.3937 inch (in.)

meter (m) 3.281 foot (ft)

mile (mi) 0.6214 mile (mi)

Appendix 1. 230Th/U ages Supporting Geologic Map of the Masters 

7.5’ Quadrangle, Weld and Morgan Counties, Colorado

By James B. Paces

Appendix 1. 230Th/U ages Supporting Geologic Map 

of the Masters 7.5’ Quadrangle, Weld and Morgan 

Counties, Colorado

By James B. Paces

Introduction

Geologic mapping of surficial units in the Masters 

7.5ʹ Quadrangle (http://dx.doi.org/10.3133/sim3344) includes 

an evaluation of the history of sediment transport and deposition in relation to regional climate, hydrologic, and soilforming events in the high plains of northeastern Colorado. 

Numerical age estimates of depositional events were made on 

different materials using several methods, including optically 

stimulated luminescence, radiocarbon, and uranium-series 

disequilibrium. 

This report describes efforts made to date two samples 

collected from man-made excavations using uranium-series 

methods (specifically 230Th/U dating). One of the samples 

consists of a fossilized fragment of horn-core bone (presumed 

to be bison) and the other consists of thin rinds of secondary carbonate coating the alluvial clasts. Although both of 

these materials are amenable to uranium-series dating, neither 

directly dates the age of deposition due to the nature of postdepositional uranium accumulation. Nevertheless, 230Th/U 

ages described here provide useful constraints on minimum 

ages for the deposits in which these materials occur.

Samples

Sample Wp67 consists of a 4×8-cm fragment of bone 

collected from a deposit of fine-grained sediment associated 

with mainstream deposits of Broadway Alluvium (map unit 

Qba) from site EF (see map for sample location and stratigraphic context http://dx.doi.org/10.3133/sim3344). A section 

of compact bone 6–7 mm thick was cut and polished revealing 

a dense, cream-colored, ivory-like interior with sub-millimeter 

growth layering (figure 1–1). The outer surface is coated with 

a thin veneer of darker, harder calcite cement up to about 

0.5 mm thick. Although the compact bone contains internal 

laminations, tubular osteons characteristic of cortical bone are 

absent. To compare results using independent dating methods, a piece of the same bone sample was submitted to Aeon 

Laboratories for radiocarbon dating of the collagen residues. 

However, the sample was rejected by the laboratory because 

it had a low nitrogen content and a high carbon-nitrogen ratio. 

The low nitrogen concentration and associated high C:N ratio 

indicate that the sample lacked sufficient amounts of residual 

collagen proteins, which are required for routine radiocarbon 

dating of the organic constituents of bone.

Sample Wp95 consists of several well-rounded clasts 

associated with intermediate alluvium (map unit Qai) from 

site EIC (see map for sample location and stratigraphic context 

http://dx.doi.org/10.3133/sim3344). Clasts range from 20 

to 60 mm across and have thin (1–3 mm) calcite-rich rinds 

(figure 1–2). In some cases, rinds had become separated from 

their associated clasts; however, rind morphology clearly preserves inner and outer surfaces. Multiple clasts from the same 

soil horizon were collected in order to evaluate the consistency 

of their isotopic compositions and calculated ages.

Dated rinds consist predominantly of secondary calcite 

cement formed on the undersides (presumably) of gravel 

clasts. Textures are relatively simple consisting of lenses of 

dense, dark-tan to brown calcite with only minor amounts 

Figure 1–1. Polished cross section of fossil bone sample Wp67 

showing thin calcite-rich rind coating outer surface and dense, 

ivory-like interior. Numbered polygons show the distribution of submillimeter-thick subsamples obtained by sequential microdrilling.

Wp67 Bison horn core

11

1

2

3

4

5

6

7

8

9 10

Wp67 Bison horn core
2 Appendix 1. 230Th/U ages Supporting Geologic Map of the Masters 7.5’ Quadrange, Weld and Morgan Counties, Colorado

of detrital matrix included. Rind growth generally occurs by 

progressive accumulation of secondary cements that precipitate from water films present in soils (Sharp and others, 2003). 

Therefore, layers adjacent to the clast/rind interface are likely 

to yield the oldest dates of pedogenic cements that in turn provide minimum age constraints for deposition of the sediment. 

These inner-rind layers were targeted for dating in this study. 

Aliquots Prepared for U-Series Dating

Bone.—Subsamples of bone were obtained by mounting the polished slab in a manually operated horizontal milling 

stage and using a 1-mm-diameter carbide dental bur to excavate 

successive approximately 0.5-mm-thick layers starting from the 

outermost surface (figure 1–1). Powders were collected and the 

sample was cleaned between each depth interval using compressed air. Prior to subsampling bone, the thin (approximately 

0.5 mm) calcite-rich crust coating the outer surface was sampled 

(Wp67-1) by milling the surface until the underlying contact 

was reached.

Clast rinds.—Thin layers (typically about 1-mm or less) 

of calcite cement adjacent to the clast/rind interface were 

sampled by prying off fragments or milling material and 

collecting the resulting powder. Layers that were relatively 

free of obvious detrital material (sand and silt) were targeted 

to minimize contributions from detrital components that 

contain rock-derived 230Th not related to 230Th formed within 

authigenic cements (that is, to minimize the need to correct 

for initial 230Th not derived from in situ 234U decay). Layers 

with higher degrees of cementation typically have the greatest 

hardness and density, and they tend to be darker in color than 

less-well-cemented layers. Prior to collecting calcite subsamples, surfaces were cleaned by removing softer or dirtier 

material using a carbide dental bur. To gain confidence that 

ages derived from these materials have geological significance, samples of rinds from multiple clasts within the same 

soil horizon were obtained. Ideally, dates for inner clast rinds 

from the same horizon should be concordant if they formed 

during the same pedogenic episode.

Chemical Processing 

and Mass Spectrometry

Dating activities followed standard operating procedure USGS-DRIL-01, R0 Uranium-Thorium Disequilibrium 

Studies for chemical preparation and isotope analysis 

(http://esp.cr.usgs.gov/projects/uth/USGS_DRIL_01cR0_U_

series.pdf). This procedure addresses sample labeling and 

control, analysis of samples, maintenance of records, and 

reporting of results. These procedures are summarized below.

Aliquots of bone or clast rinds were weighed and spiked 

with known amounts of a high purity mixed-isotope tracer 

containing man-made spike isotopes (236U-233U-229Th). Samples 

were digested using nitric and hydrochloric acid in TeflonTM

PFA (perfluoroalkoxy copolymer resin) vials at atmospheric 

pressures and temperatures of 105° to 125 °C. The phosphatic 

and carbonate components were completely dissolved in 

this step; however, residue was common in rind samples and 

represents authigenic opal or detrital silicates including silt 

and clay. In order to avoid laboratory fraction of U and Th 

and produce 230Th/238U measurements that accurately reflect 

values inherent in the subsamples, residue was separated 

from the supernatant, digested using hydrofluoric acid, and 

recombined with the original solute resulting in analyses that 

represent total digestions. U and Th were separated from the 

acid digestates and purified by ion chromatography using 

BioradTM AG1×8 (200–400 mesh) resin. Resulting U salts were 

loaded on the evaporation side of rhenium double-filament 

assemblies. Th salts were loaded onto single rhenium filament 

assemblies as a sandwich between layers of graphite suspension. Isotope ratios were obtained on a Thermo Finnigan 

TritonTM thermal ionization mass spectrometer equipped with 

Figure 1–2. Examples of polished slabs of alluvial clasts from 

sample Wp95 showing thin calcite-rich rinds coating outer 

surfaces (Wp95–A1 and Wp95–B), or rinds separated from their 

associated clasts (Wp95–E). Sub-millimeter thick layers of calciterich material sampled for uranium-series analyses are shown in 

red-outlined areas.

Wp95-A1

Wp95 -E

Wp95-E1

Wp95-A1-1

Wp95-B1

Wp95-B2 Wp95-B3

Wp95-A1-2

Wp95-B
Analytical Results 3

a retarding potential quadrapole (RPQ) electrostatic filter. 

Ratios were determined using a single ETPTM discrete-dynode 

electron multiplier operating in peak-jumping mode.

Quality control was monitored by analyzing standards 

processed and run along with unknown samples. A NIST 

uranium-isotope solution was used as the primary standard, 

and several in-house materials of known age and isotopic 

composition were used as secondary checks. U isotopic compositions of NIST SRM 4321B U-isotope standard determined 

over the same time period yielded an average 234U/235U value 

of 0.0072862±0.25% (2×standard deviation for 122 analyses 

conducted between 2/11/2013 and 1/5/2015) which is within 

analytical uncertainty of the certified value of 0.0072939. 

Results for solutions of uranium ore from the Schwartzwalder 

mine yielded an average 234U/238U activity ratio (AR) of 

1.0003±0.0040 and an average 230Th/238U AR of 0.998±0.011 

(±2×standard deviation for 19 analyses conducted between 

6/26/2013 and 1/5/2015), both of which are within analytical uncertainty of the secular equilibrium values of 1.000 

expected for the 69.3-Ma ore (Ludwig and others, 1985). 

Results for an in-house late Pleistocene Acropora coral dating 

standard (age of 119.6±1.9 ka; Watanabe and Nakai, 2006) 

yielded an average age of 119.0±3.5 ka (±2×SD, N=11) and 

an average initial 234U/238U AR value of 1.152±0.006 (±2×SD, 

N=11), which is within uncertainty of accepted values for 

seawater (1.150±0.006; Delanghe and others, 2002).

All uncertainties presented in this report are given at 

the 95% confidence level (2-sigma, 2σ) and include errors 

arising from within-run analytical errors based on counting 

statistics, external errors based on reproducibility of standards, and errors propagated from uncertainties assigned to 

the assumed detrital component and the amount of detrital 

material present in a given sample (see section on Calculation 

of 230Th/U Ages).

Analytical Results

Seven analyses of bone from sample Wp67 and eight 

analyses of carbonate rinds on three separate clasts from sample Wp95 were analyzed for U and Th isotopes (table 1–1). 

All samples had U and Th concentrations that were amenable 

to U-series dating with elevated U concentrations (83 to 

116 μg/g for bone and 5.9 to 10.2 μg/g for carbonate rinds) 

and low Th concentrations (0.03 to 0.33 μg/g for bone and 

0.28 to 0.90 μg/g for rinds). Low 232Th/238U AR (0.00007 to 

0.00129 for bone and 0.0088 to 0.038 for clast rinds) and 

high 230Th/232Th AR (> 100 for bone and > 28 for clast rinds) 

reflect the elevated U/Th concentration ratios. For comparison, 

silicate rocks from which detrital components were derived 

commonly have Th/U concentration ratios of 3 to 6, which 

result in 232Th/238U AR values between about 1 and 2 with 230Th/232Th AR values of about 0.5 to 1.5. Because of their low 232Th concentrations, corrections for non-authigenic 230Th present at the time the U was incorporated in bone or rind-cements 

are negligible for most samples.

Calculation of 230Th/U Ages

Dating by the 230Th/U method is based on the 

in-growth of 230Th derived from in situ decay of 234U present 

in the sample since the time of its formation. Any 230Th that 

was present initially must be identified and excluded to avoid 

calculating erroneously old ages. Because of the very low 

solubility of Th in most near-surface water, purely authigenic 

cements commonly form with no initial 230Th, and measured 230Th/238U AR can be used to calculate ages. However, any 

silicate detritus present in the samples, including fine clay 

particles, will contain common Th (232Th) as well as small 

amounts of 230Th that must be mathematically subtracted from 

the measured 230Th in order to obtain only the 230Th that was 

contributed from in situ decay of U. Fortunately, measured 230Th/238U and 234U/238U AR values can be corrected for the 

presence of a non-authigenic component (that is, silicate 

sediment) using measured 232Th/238U AR values and the 

assumptions that this material is uniform and has a known 

composition (Ludwig and Titterington, 1994; Ludwig and 

Paces, 2002).

The isotope composition of detrital material was not 

measured directly from subsamples, which would require 

physical separation of clays, silt, or sand from the small bulk 

subsamples, or chemical separation through partial leaching to remove authigenic cements. Instead, detritus was 

assumed to have a Th/U concentration ratio similar to values 

estimated for average continental crust (Th/U = 4; Shaw and 

others, 1976; Taylor and McLennan, 1985; Rudnick and Gao, 

2003) and U-series isotopes in secular equilibrium (that is, 

234U/238U AR = 230Th/238U AR = 1.0). Because there is a greater 

likelihood that the true composition of the detritus deviates from 

a value of 4 (equivalent to 232Th/238U AR = 1.276), a larger uncertainty was arbitrarily assigned to the 232Th/238U AR value (±50%). 

The assumption that the U isotopes are in secular equilibrium is 

reasonable given the likelihood that parental rock is older than a 

million years. Therefore, uncertainties assigned to 230Th/238U AR 

and 234U/238U AR, were smaller (±25% and 10%, respectively). 

Uncertainties are propagated through the age calculation in 

such a way that errors for the corrected ratios are only slightly 

larger than analytical uncertainties if the measured 232Th/238U AR 

is negligible (say <0.1), but may be very large if substantial 

amounts of 232Th are present (that is, 232Th/238U AR > 0.2). Ages 

and initial ratios were calculated using routines in the program 

Isoplot/Ex (Ludwig, 2012) and decay constants for 234U and 230Th 

from Cheng and others (2013). Because of the small amounts 

of 232Th present in bone and clast-rind subsamples, differences 

between uncorrected and detritus-corrected ages are small, ranging from 0.01 to 3.51% for all analyses except for the “dirtier” 

Wp67-1 bone-rind (uncorrected age of 12.9 ka versus detrituscorrected age of 11.0 ka = 16.9%).

Values for detritus-corrected 230Th/238U and 234U/238U AR 

in the analyzed samples are given in table 1–1 and shown 

graphically on U-series isotope evolution plots (figure 1–3). 

Finite ages can be calculated using the first-derivative solution 

to the 230Th/U age equation for all analyses. This calculation 
Table 1–1. U-Th concentrations, U-series isotopic compositions, and calculated 230Th/U ages and initial 234U/238U activity ratios for subsamples of Wpand EIC, respectively, on the Masters 7.5’ Quadrangle, Weld and Morgan Counties, Colorado. [Note that ±2σ represents estimates of uncertainty given at the 95% confidence level; μg/g, micrograms per gram; σ, sigma.] Sample name Sample weight, g U concentration, in μg/g Th concentration, in μg/g Measured activity ratios1 Detritus-corrected activity ratios2 230Th

±2

232Th/238U AR ±2σ 230Th/238U AR ±2σ 234U/ 238U AR ±2σ 230Th/232Th AR 230Th/238U AR ±2σ 234U/238U AR ±2σ r48-084 EF-Wp67: Carbonate rind (subsample Wp67-1) and layers of bone (subsamples Wp67-2 through Wp67-11) across a section of horn-core (presumbably bison): Location: UTM 564654mE

Wp67-1 0.039 22.5 1.64 0.02397±0.00007 0.1357±0.0008 1.216±0.003 5.7 0.117±0.011 1.221±0.004 –0.55 1Wp67-2 0.039 83.0 0.325 0.00129±0.00001 0.1348±0.0011 1.213±0.003 104 0.1338±0.0012 1.214±0.003 –0.02 12.7Wp67-3 0.041 107 0.044 0.00013±0.00000 0.1521±0.0006 1.210±0.003 1150 0.1520±0.0006 1.210±0.003 0.00 14.5Wp67-5 0.016 116 0.025 0.00007±0.00001 0.1588±0.0007 1.215±0.003 2300 0.1588±0.0007 1.215±0.003 0.00 15.2Wp67-7 0.019 115 0.028 0.00008±0.00001 0.1601±0.0007 1.221±0.002 2020 0.1600±0.0007 1.221±0.002 0.00 15.2Wp67-9 0.020 116 0.032 0.00009±0.00001 0.1459±0.0006 1.217±0.003 1600 0.1458±0.0006 1.217±0.003 0.00 13.8Wp67-11 0.016 104 0.124 0.00039±0.00001 0.1258±0.0005 1.217±0.003 322 0.1255±0.0006 1.217±0.003 0.00 11.8

EIC-Wp95: Innermost carbonate rinds on alluvial clasts: Location: UTM 567342mE, 4458109mN, zone 13, NAD 275 Wp95-A1-1 0.041 5.92 0.280 0.01557±0.00009 1.041±0.007 1.123±0.007 66.9 1.040±0.008 1.125±0.007 –0.01 24

Wp95-A1-2 0.047 7.36 0.309 0.01384±0.00006 1.062±0.006 1.083±0.003 76.7 1.061±0.007 1.084±0.003 –0.01 33Wp95-A2-1 0.048 10.2 0.274 0.00881±0.00005 1.065±0.009 1.084±0.002 121 1.065±0.009 1.085±0.003 0.00 34Wp95-A3-1 0.049 7.12 0.549 0.02539±0.00010 1.049 ±0.007 1.091±0.003 41.3 1.048±0.009 1.094±0.004 –0.02 29Wp95-B1 0.043 7.9 0.901 0.03758±0.00011 1.058±0.006 1.074±0.002 28.2 1.057±0.01 1.077±0.004 –0.03 34Wp95-B2 0.041 7.6 0.509 0.02204±0.00008 1.049±0.006 1.077±0.002 47.6 1.048±0.007 1.079±0.003 –0.02 32Wp95-B3 0.046 9.58 0.672 0.02309±0.00007 1.070±0.005 1.070±0.002 46.3 1.069±0.007 1.072±0.003 –0.02 39Wp95-E1 0.059 9.91 0.461 0.01534±0.00005 1.038±0.005 1.052±0.002 67.7 1.037±0.006 1.054±0.003 –0.01 37

1 Measured activity ratios (AR) corrected for mass fractionation, spike contributions, procedural blank and normalized relative to an atomic ratio value for NIST SRM 4321B 

234U/238U=0.0000529. 2 Ratios corrected for an assumed Th-bearing detrital component having an atomic Th/U of 4 with the following activity ratios and 2σ errors: 232Th/238U=1.276±0.64; 234U/238U

230Th/238U=1.0±0.25. 3 230Th/U ages, initial 234U/238U ARs, and associated errors calculated using detritus-corrected activity ratios. 4 r48-08 = error correlation between 234U/238U AR and 230Th/238U AR. rage-go = error correlation between 230Th/U age and initial 234U/238U AR. 51927 North American Datum.
Analytical Results 5

Figure 1–3. U-series isotope evolution plots showing U-Th isotopic compositions (circles) and 2-sigma (2σ) uncertainties (error 

ellipses) for analyses of bone (Wp67; red-colored points) and calcite rinds (Wp95; teal-colored points) listed in table 1–1. All data are 

shown in (A) and at expanded scales for analyses of Wp67 in (B) and Wp95 in (C). In all plots, the thick curves represent 234U/238U and 230Th/238U compositional paths that evolve under closed conditions (no loss or gain of mass since the time of formation) for material 

with initial 234U/238U activity ratios ranging from 1.14 to 1.26. Straight sloping lines are isochrons given at intervals labeled on plots (in 

thousands of years). Isochrons older than about 500 ka become highly condensed, and define an infinite age line (red lines in A and C) 

representing the upper dating limit based on 230Th decay.

data-point error ellipses are 2σ

A1-1

A3-1

A2-1

A1-2

B1

B2

E1

1.24

1.22

1.20

1.18

1.16

1.14

1.26

C

50

100

150

200

250

300

350

400

500

Wp95

data-point error ellipses are 2σ

200

250

300

350

400

450

550

B3

A

B C

0.0 0.2 0.4 0.6 0.8 1.0 1.2

230Th / 238U activity ratio

0.08 0.10 0.12 0.14 0.16 0.18

230Th / 238U activity ratio 230Th / 238U activity ratio

0.98 1.00 1.02 1.04 1.06 1.08 1.10

1.04

1.08

1.12

1.16

1.20

1.24

1.28

234U / 238U activity ratio

1.20

1.21

1.22

1.23

234U / 238U activity ratio

234U / 238U activity ratio

1.04

1.06

1.08

1.10

1.12

1.14

1.16

10 15

1

11

2

9

7

5

3

1.230

1.225

1.220

1.215

1.210

Wp67

B
6 Appendix 1. 230Th/U ages Supporting Geologic Map of the Masters 7.5’ Quadrange, Weld and Morgan Counties, Colorado

uses the values for the detritus-corrected activity ratios, and 

yields valid solutions as long as the isotopic compositions fall 

to the left of the infinite-age line shown on isotope evolution 

plots (red lines on figures 1–3A and 1–3C). In those cases, 

analytical errors are used to directly estimate uncertainties 

for 230Th/U ages and initial 234U/238U AR. No points plot to 

the right of the infinite-age line, which indicates that obvious 

secondary U mobility has not occurred.

Initial 234U/238U Activity Ratios

Once a 230Th/U age (t) is determined for a given sample, 

it can be used to calculate the initial 234U/238U composition of 

the water from which the cement was precipitated knowing 

the present-day 234U/238U AR and the decay constant for 234U 

(l234 = 2.82203×10-6 yr-1):

234

238

234

238 1

1 234

U

U

AR

U

U

AR

e initial

present

t =

− 











+ −

In hydrogenic deposits formed from water having 234U/238U AR values greater than unity (that is, >1.0), initial 

ratios will be equal to or larger than the measured ratio, following the curved isotope-evolution paths in figure 1–3 to the 

point where they reach the Y-axis (dark curves). This is a very 

valuable check on ages because all samples precipitated from 

a single water source should have the same initial 234U/238U AR 

values regardless of their 230Th/U age. Samples that are 

affected by post-depositional removal of U will have anomalously large 230Th/238U AR values, which results in erroneously 

old ages and anomalously large initial 234U/238U AR values.

Samples of bone and clast rinds analyzed in this study 

have a limited range of initial 234U/238U AR values (figure 1–4). 

Samples from sample Wp67 with latest Pleistocene 

230Th/U ages have initial 234U/238U AR values of 1.225±0.008 

(±2×standard deviation) that overlap values of 1.219±0.034 

for the middle Pleistocene clast rinds from sample Wp95. 

The consistency of initial 234U/238U AR values is interpreted 

as evidence that both samples acquired their U from similar 

soil-water sources, and that despite their antiquity, pedogenic 

rinds from Wp95 retained their isotopic integrity and preserve 

reliable age information.

Ages for Individual Samples

Wp67 bone.—Subsamples of Wp67 bone (Wp67–2 through 

Wp67–11) have elevated U concentrations (83 to 116 μg/g) and 

very low Th concentrations (0.025 to 0.033 μg/g) resulting in 

U-series analyses that require very little or no corrections for 

initial 230Th. Subsamples of bone have apparent 230Th/U ages calculated from U-series isotope data that range from 11.82±0.06 ka 

to 15.26±0.08 ka (table 1–1). The calcite-rich crust coating the 

outer surface of the bone (Wp67–1) has a lower U concentration 

(22.5 μg/g) and higher Th concentration (1.64 μg/g) relative to 

bone samples and yields a slightly younger 230Th/U age with 

greater uncertainty (11.0±1.1 ka).

The date for the calcite crust represents a minimum depositional age for the bone as it was formed as part of the pedogenic process. U-series ages of bone also represent minimum 

ages as they reflect U that has been absorbed from infiltrating 

soil water after burial. Most vertebrates incorporate only small 

amounts of U in living bone tissue (typically <0.05 μg/g; 

Larivière and others, 2013). Therefore, the large amounts present in Wp67 were incorporated in the bone only after death 

and burial. The main challenge when dating bone by U-series 

methods is understanding when the U was added.

U is readily soluble in oxidizing near-surface water and 

interactions between those waters and the buried bone results 

in loss of much of the organic matter (collagen) and incorporation of U within the carbonated hydroxyapatite. Patterns of 

U uptake can take numerous forms depending on the degree 

of hydrologic saturation and water-migration history of the 

site, the diffusive characteristics of the bone, and the uranium 

concentrations in the diagenetic environment. Two simple 

conceptualizations of U behavior include early uptake where 

all U is acquired over a short time span (relative to the time of 

burial) and then held tightly in place, and linear uptake, where 

U uptake occurs slowly and continuously between burial 

and the present. True uptake histories can be more complex. 

Examination of profiles of U concentration and U-series ages 

across sections of compact (cortical) bone can offer insights 

into U uptake processes.

The diffusion-adsorption model (D–A model) was developed to explain profiles of U concentrations and U-series ages 

observed in bone samples (Millard and Hedges, 1996; Pike 

and others, 2002; Pike and Pettit, 2003). The model assumes 

that uranyl complexes (UO2

+2) in aqueous solutions are able 

to diffuse into porous compact bone and adsorb into the mineral fraction of bone, facilitated by its large internal surface 

areas. Oxidation of bone collagen may also contribute to this 

process by providing a mechanism for reduction of soluble 

hexavalent U(VI) to non-soluble tetravalent U(IV) within the 

bone structure.

The distribution of U (and resulting U-series ages) is 

dependent on a number of parameters inherent to a given bone 

including its diffusion coefficient, porosity, and the mineral/

water partition coefficient, as well as chemical and physical 

factors present in the burial environment (Pike and others, 

2002; Pike and Pettit, 2003). With time, U from migrating 

water is introduced into the bone starting from the outer and 

inner surfaces and progressing toward its center (figure 1–5). 

U uptake is non-linear and proceeds more rapidly during 

earlier stages and less rapidly during later stages. The U concentration profiles gradually flatten with time until the bone 

reaches a composition in equilibrium with U in the source 

water. The distribution of apparent U-series ages will result 

in similar U-shaped patterns, with apparent ages decreasing towards the center of the bone (Pike and others, 2002; 

Sambridge and others, 2012).
Analytical Results 7

Figure 1–4. Initial 234U/238U activity ratios for analyses of samples Wp67 and Wp95. Data 

are from table 1–1. Dark solid lines represent mean values and dashed lines represent 

values of ±2× standard deviations for each data set. The value for subsample Wp95–E1 

(open square) was not included in error-weighted mean calculations.

Figure 1–5. Plot showing relative uranium concentrations across a section of hypothetical 

compact bone showing the concept of progressive uptake of U with time according to the 

Diffusion-Adsorption model (modified from figure 1 of Pike and others, 2002). Curves show 

the development of U-shaped profiles that gradually flatten until equilibrium is reached with 

U in migrating groundwater.

1.12

1.14

1.16

1.18

1.20

1.22

1.24

1.26

1.28

Initial 234U/238U activity ratio 

Wp67

Wp95

EXPLANATION

Relative distance from center

Outer edge Inner edge

Increasing time

–1.0 –0.8 –0.6 –0.4 –0.2 0.0

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Relative uranium concentration

0.2 0.4 0.6 0.8 1.0
8 Appendix 1. 230Th/U ages Supporting Geologic Map of the Masters 7.5’ Quadrange, Weld and Morgan Counties, Colorado

Figure 1–6. Profiles of uranium concentration (A) and 230Th/U age and initial 234U/238U activity ratios (B) for subsamples of bone from 

sample Wp67. Data are from table 1–1. Two-sigma (2σ) analytical uncertainties are shown as error bars, which are mostly smaller than the 

symbols used for U concentrations and 230Th/U ages. Points are labeled with the subsample numbers shown in figure 1–1. Data for Wp67–1 

representing the carbonate rind coating the bone are not included.

10.0

11.0

12.0

13.0

14.0

15.0

16.0

–1 –0.5 0 0.5 1

Relative distance from center

Apparent closed-system

230Th/U age, ka

1.215

1.220

1.225

1.230

1.235

Initial 234U/238U activity

230

Th/U age

Initial 234

U/

238

U AR

2

3

5 7

9

11

80

85

90

95

100

105

110

115

120

–1 –0.5 0 0.5 1

Relative distance from center

Uranium concentration,

μg/g 

2

3

5 7 9

11

A

B

EXPLANATION

Profiles of U concentration measured for subsamples of 

Wp67 show a ∩-shaped pattern (figure 1–6A) rather than that 

expected for gradual, continuous uptake modeled in figure 1–5. 

This response implies that the bone has experienced a relatively recent loss of U after the initial uptake that lead to high 

concentrations in interior subsamples. Because U is mobile but 

Th is not, loss of U will eventually result in increased values of 230Th/238U AR that are not related to in situ ingrowth of 230Th. 

As values of 230Th/238U AR increase, points on U-series isotope 

evolution plots (figure 1–3) will move towards the right without 

substantially altering the 234U/238U AR. This behavior results in 

erroneously old ages and high apparent initial 234U/238U AR values. Therefore, U loss will result in M- or ∩-shaped concentration 

profiles and strongly U-shaped profiles of apparent closed-system 

230Th/U ages (see figure 3F and section 3.4 of Pike and others, 

2002). Pike and others (2002) showed that U loss can dramatically affect ages calculated for outer subsamples and eventually 

cause erroneously old ages for interior subsamples as well.

In contrast, measured 230Th/U ages for Wp67 subsamples 

also define a ∩-shaped profile (figure 1–6B). Furthermore, 

initial 234U/238U AR values are more-or-less uniform across the 

section and do not show systematic increases in samples with 
Analytical Results 9

the lowest U concentrations (figure 1–6B). Initial 234U/238U 

values for all but one bone sample are within analytical error 

of the value obtained for the calcite crust on the exterior of 

the bone. Therefore, data for Wp67 do not follow patterns 

expected for normal processes of U loss.

The available U-series isotope results for the single bone 

specimen analyzed from site EF do not allow a definitive age 

estimate. The date of 11.0±1.1 ka for the calcite crust (Wp67–1) 

represents a minimum age that is less subject to processes of 

secondary U mobility than the bone matrix itself. Two subsamples from the fragment interior (Wp67–5 and Wp67–7) have 

apparent closed-system 230Th/U ages that are within analytical uncertainty and yield an error-weighted average value of 

15.24±0.06 ka. The remaining subsamples of bone have ages 

between these two limits. Therefore, the best estimate for the 

minimum age of burial for sample Wp67 is between 11 and 

15 ka. Data for additional specimens are required to better 

define this estimate.

Wp95 clast rinds.—Subsamples of innermost rinds 

on clasts from Wp95 have U concentrations ranging from 

5.9 to 10.2 μg/g and uniformly low Th concentrations ranging from 0.27 to 0.90 μg/g (table 1–1). Consequently, measured 230Th/232Th AR values are high (28 to 121) and require only 

minor corrections for the presence of initial 230Th. Resulting 230Th/U ages range from 248±9 ka to 397±27 ka. Most clast-rind 

Figure 1–7. Plot of 230Th/U ages for subsamples of clast rinds from Wp95. Data are from table 1–1. Boxes represent the 2-sigma (2σ) 

age range based on closed-system evolution of U-series isotopes that are grouped by color to show two different mean values. Mean 

values are weighted by the analytical errors assigned in table 1–1. The value for MSWD (mean square of weighted deviates) represents 

a measure of how well the assigned analytical errors explain the observed scatter of values—if the MSWD is close to unity (1.0), the 

assigned analytical errors are considered the main cause for scatter. Additional sources of non-analytical error are likely if the MSWD 

value is much greater than unity (Ludwig, 2012).

analyses (6 of 8) yield initial 234U/238U AR values that span a 

narrow range and define a mean value of 1.212±0.011. Furthermore, ages and initial 234U/238U AR values of Wp95 clast rinds 

are poorly correlated (R2 value of 0.33). The fact that initial 234U/238U AR values consistently cluster around a narrow range 

that is analytically indistinguishable from values determined for 

late Pleistocene material from the same area (figure 1–4) is a 

strong indication that clast rinds were not subject to secondary 

U mobility, and that resulting data yield reliable 230Th/U ages.

Because pedogenesis is a dynamic process where new 

cement can be added at any time after alluvium was deposited, 

the oldest U-series dates observed for a given soil horizon are 

expected to better represent minimum ages for the depositional 

event. Ages for two of the rind subsamples are substantially 

younger than the rest (248 and 294 ka) and likely include soil 

components added well after the gravels were deposited. The 

remaining six subsamples have older ages that cluster in two 

groups (figure 1–7). Four subsamples from two separate clasts 

define an error-weighted-mean age of 334±9 ka whereas two 

subsamples from separate clasts define a somewhat older errorweighted-mean age of 382±16 ka. Therefore, available U-series 

data indicate with a high degree of confidence that alluvial 

clasts at site EIC were deposited prior to 334 ka. A depositional 

age somewhat older than 382 ka is considered likely based on 

the two oldest rind dates.

200

240

280

320

360

400

440

Box heights are 2σ

Weighted mean = 334 ±9 (2σ)

MSWD = 1.03, probability = 0.38

230Th /U age (ka)

A1-1

A1-2 A2-1

A3-1

B1

B2

B3

E1

Weighted mean = 382 ±16 (2σ)

MSWD = 2.1, probability = 0.15
10 Appendix 1. 230Th/U ages Supporting Geologic Map of the Masters 7.5’ Quadrange, Weld and Morgan Counties, Colorado

Conclusions

U-series ages determined on materials from two sites in 

the Masters 7.5ʹ Quadrangle are consistent with Quaternary 

geologic relations. Multiple subsamples of a single bone 

fragment and its associated calcite rind from mainstream 

deposits of Broadway Alluvium (map unit Qba) at site EF 

indicate that burial of the specimen occurred between 11 and 

15 ka. Although data for this specimen indicate a complex 

history of U uptake and subsequent leaching, the weighted 

average value of 15.24±0.06 ka is interpreted to more closely 

represent the minimum age. Multiple subsamples of innermost 

calcite rinds coating several clasts from a deposit of intermediate alluvium (map unit Qai) from site EIC indicate that the 

alluvium was deposited prior to 334±9 ka. Two of the oldest 

rind subsamples suggest the minimum age is likely closer to 

382±16 ka.

Acknowledgments

Comprehensive technical reviews of the data and 

age-interpretations included in this report were provided by 

Leonid A. Neymark and Wayne R. Premo. Any use of trade, 

firm, or product names is for descriptive purposes only and 

does not imply endorsement by the U.S. Government.

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Suggested citation:

Berry, M.E., Slate, J.L., Paces, J.B., Hanson, P.R., and Brandt, T.R., 

2015, Geologic Map of the Masters 7.5' Quadrangle, Weld and Morgan 

Counties, Colorado: U.S. Geological Survey Scientific Investigations 

Map 3344, 10 p. appendix, 1 sheet, 1:24,000, http://dx.doi.org/10.3133/

sim3344.