A

A'

CORRELATION OF MAP UNITS

SURFICIAL DEPOSITS

Eolian 

deposits 

Qsh

Alluvial and 

colluvial 

deposits 

Qfc

Qaa

Qbs

Qlv

BEDROCK UNIT

Kp

Alluvial deposits

Qa1

Qa2

Qa3

?

Qba QUATERNARY

Pleistocene

Holocene 

Upper

Cretaceous CRETACEOUS

Qes

Shoreline

deposits

af

Artificial

fill

Qai

Qao

SCALE 1: 24 000

1 1/ 2 0 1 MILE

1000 1000 0 2000 3000 4000 5000 6000 7000 FEET

1 0 .5 1 KILOMETER

Geologic Map of the Masters 7.5’ Quadrangle, Weld and Morgan Counties, Colorado

By

Margaret E. Berry,1

 Janet L. Slate,1 James B. Paces,1 Paul R. Hanson,2 and Theodore R. Brandt1

2015

APPROXIMATE MEAN

DECLINATION, 2015

8 ̊

TRUE NORTH

MAGNETIC NORTH

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1-888-ASK-USGS

This report is available at:

http://pubs.er.usgs.gov/publication/sim3344

Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. 

Government.

Although this information product, for the most part, is in the public domain, it also contains copyrighted materials as noted 

in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.

This database, identified as SIM 3344, has been approved for release and publication by the U.S. Geological Survey (USGS). 

Although this database has been subjected to rigorous review and is substantially complete, the USGS reserves the right to 

revise the data pursuant to further analysis and review. Furthermore, it is released on condition that neither the USGS nor 

the U.S. Government may be held liable for any damages resulting from its authorized or unauthorized use.

Although these data have been processed successfully on a computer system at the U.S. Geological Survey (USGS), no 

warranty expressed or implied is made regarding the display or utility of the data on any other system or for general or 

scientific purposes, nor shall the act of distribution constitute any such warranty. The USGS or the U.S. Government shall 

not be held liable for improper or incorrect use of the data described and/or contained herein.

Publishing support provided by: 

Denver Publishing Service Center

Edit and digital layout by J.A. Herrick

Manuscript approved for publication September 4, 2015

Base from U.S. Geological Survey Masters Quadrangle, 1951; photoinspected 1975.

Polyconic projection. 1927 North American Datum (NAD 27)

10,000-foot grid based on Colorado coordinate system, north zone

1000-meter Universal Transverse Mercator grid ticks, zone 13

CONTOUR INTERVAL 10 FEET

NATIONAL GEODETIC VERTICAL DATUM OF 1929

COLORADO

QUADRANGLE LOCATION

ISSN 2329-132X (online)

http://dx.doi.org/10.3133/sim3344

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.

1

U.S. Geological Survey

2

University of Nebraska–Lincoln, School of Natural Resources

The geology was mapped by M.E. Berry from 2011–2013. J.L. Slate 

assisted with the OSL and 14C sampling, description, and stratigraphic 

interpretation. J.B. Paces processed and analyzed samples for U-series 

dating, interpreted results, and wrote the appendix on the ages. P.R. 

Hanson processed and analyzed samples for OSL dating and interpreted 

results. T.R. Brandt prepared the digital topographic base map, digital 

compilation, and GIS database of the geologic map.

EF

ER

EIC

Figure 2. Shaded relief was derived from U.S. Geological Survey National Elevation Dataset (NED) with 10-meter resolution elevation data (accessed on June 17, 

2014 at http://ned.usgs.gov/).

Fort Morgan Fort Morgan 

Dune Field Dune Field

Greeley Greeley 

Dune Field Dune Field

South Platte RiverSouth Platte RiverSouth Platte RiverSouth Platte River

Bijou CreekBijou CreekBijou CreekBijou Creek

Riverside

Res.

Jackson

Res.

Empire

Res.

Fort Morgan Fort Morgan 

Dune Field Dune Field

Kio

wa

Cree k

 Kiowa Cree k

Kio

wa

Cree k

 Kio

wa Cree k

Fort Morgan

Wildcat Creek

 Wildcat Creek

Wildcat Creek

 Wildcat Creek

104 ̊00'00“

5 10 MILES

5 10 KILOMETERS

0

0

104 ̊15'00“ 103 ̊45'00“

40 ̊22'30“

40 ̊15'00“

Masters Quadrangle

• •

•

•

•

•

■

104 ̊07 ́30 ̋ 104 ̊00 ́00 ̋

40 ̊22 ́30 ̋

40 ̊15 ́00 ̋

104 ̊22 ́30 ̋ 104 ̊15 ́00 ̋ 103 ̊52 ́30 ̋ 103 ̊45 ́00 ̋ 103 ̊37 ́30 ̋

40 ̊07 ́30 ̋

40 ̊30 ́00 ̋

Jackson

Reservoir

Kiowa

Creek

Wildcat Creek

Bijou Creek

Riverside

 Reservoir

South Platte River

Fort Morgan

POINT OF ROCKS GREASEWOOD 

 LAKE

SUNKEN LAKE JUDSON HILLS PEACE VALLEY

 SCHOOL

DEAD HORSE

 SPRINGS

DEARFIELD MASTERS WELDONA FORT MORGAN BRUSH WEST

ROGGEN WIGGINS VALLERY LAMB ROUND TOP WELD COUNTY MORGAN COUNTY

ER

KC

TK–R

H–R

BC

AK

OMAR

Figure 1. Index map showing map area (in yellow) and nearby 7.5′ quadrangles. Geochronology sample sites 

shown where green and red symbols represent the following: squares indicate 230Th/U sample sites, solid circles 

indicate optically stimulated luminescence sample sites, and red cross within circles indicate radiocarbon sample 

sites; site names (in red) link to sample field numbers (see tables 1–3).

34

52

76

39

144

76

Orchard

ORCHARD

■

EF

EIC

Empire Reservoir

A

Bt

Bk

Photo right. View of late Pleistocene eolian sand (Qes dark stipple) with A/Bt/Bk soil profile of 

Osgood series, exposed in a gravel pit near the southern edge of Masters 7.5′ quadrangle west of 

Empire Reservoir. The deposit, dated at a nearby location within the gravel pit (site ER), yielded an 

OSL age estimate of 26.4±2.5 ka (UNL–3467, 5 percent moisture, table 1). Knife is 26 cm long. 

Photograph by M.E. Berry, July 2012.

Photo above. View of old alluvium (Qao) in a quarry exposure near southern edge of Masters 7.5′ 

quadrangle west of Empire Reservoir (near geochronology sample site ER). Knife is 23 cm long. 

Photograph by M.E. Berry, July 2010. 

Qlv

Qbs

Qa2

Qa1

Qaa

af

DESCRIPTION OF MAP UNITS

[Calibrated radiocarbon ages are expressed as “cal ka B.P.,” which stands for calibrated thousand 

years before present (0 yr B.P. = 1950 A.D.). Uncertainties are given at the 95 percent (2σ) 

confidence level. Calibrated ages are reported as the midpoint of the calibrated range. In cases 

where calibration produced more than one age range with a probability of 5 percent or more, ages 

are based on the mean of the ranges weighted by their probabilities and are presented without 

uncertainties. 

Soil-horizon designations and other descriptive soil terminology used in this report follow criteria 

outlined in Soil Survey Division Staff (1993), Birkeland (1999), and Schoeneberger and others 

(2012). Most colors are field dry colors and based on Munsell soil color charts (Munsell Color, 

1975). The term “consistence” is the resistance to crushing of soil or surficial material in the hand, 

as described by Soil Survey Division Staff (1993). In descriptions of clast lithology, the term 

“granite” refers to phaneritic igneous or meta-igneous rock types that are felsic to intermediate in 

composition]

SURFICIAL DEPOSITS

Artificial fill

Artificial fill (latest Holocene)—Includes abutments where roadway crosses South 

Platte River, raised track of railroad, some artificial levees and banks, and dam 

embankments at Empire and Riverside Reservoirs. Artificial fill is present along 

most canals and ditches but only mapped locally. Chiefly consists of gravel, 

sand, silt, and rock and concrete fragments 

Shoreline deposits

Shoreline deposits (latest Holocene)—Mostly pale brown, brown, or yellowish 

brown, moderately to well sorted, fine to medium grained sand reworked from 

eolian sand deposits by shoreline processes at Riverside and Empire Reservoirs. 

Deposits post-date reservoir construction. Periodically inundated when water 

levels in reservoirs are high. Estimated thickness less than 2 m

Alluvial deposits

Active channel and floodplain alluvium (late Holocene)—Equivalent in part to 

post-Piney Creek alluvium of Scott (1963). Very pale brown, light brownish 

gray, light yellowish brown, or light pinkish brown. Mostly fine to coarse sand, 

pebbly sand, silty sand, and sandy pebble and minor small cobble gravel, 

interstratified with thin (<1–5 cm) layers or lenses of very pale brown silt and 

dark grayish brown, organic-rich clayey silt and clay. Poorly to well-sorted, and 

weakly to moderately stratified. Sand beds commonly cross-stratified. Filled 

burrows common. Clasts mostly granite, gneiss, pegmatite, quartz, feldspar, and 

sandstone; most clasts subrounded to rounded. Also contains minor amounts of 

lignite fragments thought to be eroded mostly from Upper Cretaceous Laramie 

Formation cropping out along tributaries south of the map area (Braddock and 

Cole, 1978). Locally, grains stained with reddish yellow iron oxide and black 

manganese oxide at water level. Uppermost 25–50 cm of sediment on floodplain 

typically light grayish brown, crudely stratified, poorly sorted sandy silt, silty 

sand, and granule- to fine-pebbly sand. Less active parts of floodplain have thin, 

weakly developed surface soil (A/C profile). Estimated thickness 3–5 m

Young alluvium 1 (late Holocene)—Equivalent in part to post-Piney Creek alluvium 

of Scott (1963). Forms low terrace approximately 1.5 m above active floodplain. 

Surface occasionally flooded. Alluvium very pale brown, light brownish gray, 

light yellowish brown, or light pinkish brown, fine to coarse sand, pebbly sand, 

and sandy pebble and minor small cobble gravel. Interstratified with thin (<1–5 

cm) layers or lenses of very pale brown silt and dark grayish brown, organic-rich 

clay. Poorly to well-sorted, weakly to moderately stratified. Sand beds 

commonly cross-stratified. Loose to slightly hard dry consistence. Locally, 

grains stained with reddish yellow iron oxide and black manganese oxide. Some 

of the finer-grained beds show signs of soft sediment deformation. Filled 

burrows common. Clasts mostly granite, gneiss, quartz, feldspar, pegmatite, 

chert, and sandstone; most clasts subrounded to rounded. Locally contains minor 

amounts of lignite fragments. Uppermost sediment packet in alluvial sequence 

typically light grayish brown or pale brown, crudely stratified, poorly sorted 

sandy silt, silty sand, and granule- to fine-pebbly sand with lenses of silt and 

dark grayish brown clay. Surface soil typically poorly drained A/C profile. 

Estimated thickness 3–5 m

Young alluvium 2 (late Holocene)—Equivalent in part to post-Piney Creek alluvium

of Scott (1963). Forms low terrace approximately 3 m above active floodplain. 

Surface occasionally flooded in some places and rarely flooded in others. 

Alluvium very pale brown, light brownish gray, light yellowish brown, or light 

pinkish brown, fine to coarse sand, pebbly sand, and sandy pebble and minor 

small cobble gravel. Interstratified with thin (<1–5 cm) layers or lenses of very 

pale brown silt and dark grayish brown, organic-rich clay. Poorly to well-sorted, 

weakly to moderately stratified. Sand beds commonly cross-stratified. Loose to 

slightly hard dry consistence. Locally, grains stained with reddish yellow iron 

oxide and black manganese oxide. Some beds show signs of soft sediment 

deformation. Filled burrows common. Clasts mostly granite, gneiss, quartz, 

feldspar, pegmatite, chert, and sandstone; most clasts subrounded to rounded. 

Locally contains minor amounts of lignite fragments. Uppermost sediment 

packet in alluvial sequence typically light grayish brown or pale brown, crudely 

stratified, poorly sorted sandy silt, silty sand, and granule to fine pebbly sand 

with lenses of silt and dark grayish brown clay. Surface soil in Masters 

quadrangle variable, ranging from A/C profiles to profiles with weakly 

developed textural B horizons (U.S. Department of Agriculture Natural 

Resources Conservation Service, 2009a, 2009b). Estimated thickness 3–5 m

Young alluvium 3 (Holocene and latest Pleistocene?)—Piney Creek Alluvium of 

Gardner (1967) and Scott (1978). Forms terrace 3–6 m above active floodplain; 

Gardner (1967) and McFaul and others (1994) considered this terrace correlative 

to Kuner terrace of Bryan and Ray (1940). Alluvium pale yellow, light yellowish 

brown, light olive brown, or grayish brown. Poorly to well-sorted, weakly to 

moderately stratified, fine sand, silt, clayey silt, pebbly sand, sandy pebble 

gravel, and silt with coarse sand lenses and granule to fine pebble stringers. Soft 

to slightly hard dry consistence. Clasts mostly granite, gneiss, quartz, feldspar, 

pegmatite, chert, and sandstone; most clasts subrounded. Locally, weakly 

developed buried soil separates deposits within alluvial sequence. Soil Survey 

(USDA Natural Resources Conservation Service, 2009b) shows surface soil as 

typically having thin argillic B horizon and weakly developed Bk horizon with 

stage I–II carbonate morphology.

Radiocarbon (14C) and optically stimulated luminescence (OSL) age 

estimates from a section exposed in a stream cut along a small unnamed 

tributary in Weldona 7.5′ quadrangle (site AK, near section AK of Gardner, 

1967) indicate that basal alluvium was deposited in latest Pleistocene or early 

Holocene (tables 1 and 2; Berry and others, 2013). Probable Succinea snail 

shells collected at depths of approximately 4.7 m and approximately 3.6 m 

yielded 14C ages of 11.90±0.28 cal ka B.P. (Aeon–994, table 2) and 11.95±0.24 

cal ka B.P. (Aeon–995, table 2), respectively. Sediment collected at a depth of 

approximately 3.5 m, from a location about 10 m upstream from where the snail 

shells were collected, gave an OSL age estimate of 9.1±0.9 ka (UNL–3503, 5 

percent moisture, table 1).

A latest Pleistocene to early Holocene age for basal alluvium is also 

supported by an age of 11.68±0.36 cal ka B.P. (10.11±0.09 14C ka B.P., 

AA–11084A) obtained by Haynes and others (1998) for humic acids in charcoal 

from Kuner terrace alluvium (of Bryan and Ray, 1940) roughly 55 km west of 

the map area. Timing of uppermost alluvium deposition and surface stabilization 

is constrained by a date from McFaul and others (1994) of 5.84±0.18 cal ka B.P. 

(5.12±0.08 14C ka B.P., Beta–42564) for soil humate from a buried A horizon 

developed on Qa3 terrace alluvium about 1.3 km northeast of the former Masters 

Post Office site (south of river near western edge of quadrangle). Degree of 

development of the buried soil indicates that the Qa3 surface may have been 

stabilized for a few thousand years prior to being buried locally by a thin layer of 

eolian sediment (not shown on map) around 5.8 cal ka (McFaul and others, 

1994). Stabilization of the Qa3 surface by mid-Holocene time is supported by 

degree of development of soils mapped on the terrace surface by USDA Natural 

Resources Conservation Service (2009b). Estimated thickness 2–5 m

Sidestream deposits of Broadway Alluvium (late Pleistocene)—Mapped as 

informal Bijou Flats tongue of Broadway Alluvium by Gardner (1967) and upper 

member of Broadway Alluvium by Scott (1978). Overlies and presumably 

interfingers with mainstream deposits of Broadway Alluvium (Qba) south of 

South Platte River mostly east of the Weld County–Morgan County line. 

Consists mainly of sheetflood deposits thought to have been deposited primarily 

by large-magnitude floods along Bijou and Kiowa Creeks (Gardner, 1967; Scott, 

1978, 1982; Berry and others, 2015), which join the river downstream of Masters 

quadrangle. Distribution of deposits suggests that influx of large amounts of 

sidestream alluvium during flood events deflected the South Platte River to the 

north side of its valley and may have episodically dammed the river for short 

periods of time (Scott, 1982). Accumulation of the sidestream alluvium built up 

surface to a level that now stands approximately 21 m above the active 

floodplain in Masters quadrangle; terrace surface increases to maximum height 

of approximately 27 m above active floodplain several kilometers downstream, 

near Bijou Creek confluence (Weldona 7.5′ quadrangle). Deposits identified as 

far downstream as Prewitt Reservoir (roughly 65 km east) and Atwood, Colorado 

(roughly 80 km east) by Gardner (1967) and Scott (1978), respectively. 

Alluvium light yellowish brown, light olive brown, pale yellow, or light 

gray, poorly to well-sorted, moderately stratified, coarse to very fine sand and 

silty sand in beds generally 20–60 cm thick. Sand beds commonly separated by 

thin (1–5 mm) mats of organic debris and clay, or packets (5–10 cm thick) of 

thinly laminated dark-grayish brown or black organic-rich clay interbedded with 

layers of pale yellow or light-gray silt and fine sand. Contains abundant detrital 

lignite thought to be mostly eroded from Upper Cretaceous Laramie Formation 

(see discussion below). Graded beds that fine upward from poorly sorted coarse 

sand and granules to mostly fine sand are common. Also common are sand beds 

that are cross-stratified, finely laminated, crudely stratified, or massive. Sand 

grains mostly subangular; lesser amounts subrounded. Coarse sand grains and 

granules of quartz and feldspar common. Few scattered fine pebbles of tuff, 

shale, and granitic rock types. Soft to hard dry consistence. Iron oxide nodules, 

masses, or pore linings present locally, particularly in sandy clay layers. 

Typically forms vertical exposures, especially along South Platte River bluff 

(Berry and others, 2015). Contact with mainstream alluvium typically sharp, 

most notably marked by difference in color and coherence. Soil profile 

characterized by thin argillic B horizon with moderate prismatic structure, and 

Bk horizon with stage II filamentous carbonate morphology. Soil profile buried 

by younger eolian sand in many places. 

OSL and 14C age estimates indicate a late Pleistocene age for Qbs. A dated 

section at the mouth of Kiowa Creek (site KC, in Orchard 7.5′ quadrangle) 

yielded OSL age estimates (5 percent moisture) of 12.0±1.1 ka (UNL–3462) at 

approximately 1.7 m depth, 16.8±1.7 ka (UNL–3466) at approximately 2.6 m 

depth, and 15.2±1.5 ka (UNL–3463) at approximately 3.6 m depth (table 1). 

These age estimates are in good agreement with a 14C age of 14.53±0.56 cal ka 

B.P. (Aeon–1064, table 2) we obtained for probable Succinea snail shells 

collected at a depth of approximately 3.7 m from a section of Qbs exposed in the 

South Platte River bluff about 4.8 km downstream of Kiowa Creek (site H–R, in 

Orchard 7.5′ quadrangle, at section H of Gardner, 1967). The ages also are in 

good agreement with OSL age estimates (5 percent moisture) for a section 

exposed in the north cutbank of Bijou Creek (site BC, in Weldona 7.5′ 

quadrangle about 13.5 km upstream of the river confluence): 12.4±1.1 ka 

(UNL–3498) sampled at approximately 1.1 m depth; and 14.6±1.2 ka 

(UNL–3499) sampled at approximately 1.9 m depth (table 1).

Organic material collected from a thin (2–5 mm) interbed of organic debris 

and clay at approximately 2.5 m depth, just above OSL sediment sample 

UNL–3466, yielded a 14C age estimate of 43.90±5.69 cal ka B.P. (Aeon–950, 

table 2), which is likely an infinite age (J.S. Pigati, U.S. Geological Survey 

(USGS), oral commun., 2012). Gardner (1967) also dated organic material from 

Qbs that yielded an anomalously old date. Pollen analysis of his dated sample 

indicated a plant assemblage of Late Cretaceous age, leading Gardner (1967) to 

interpret the sample as lignite from the Laramie Formation, which crops out 

along Kiowa and Bijou Creeks south of the map area. We suspect our sample 

(Aeon–950) also contained lignite from the Laramie Formation. Probable lignite 

debris is common to abundant in other exposures of Qbs along Bijou Creek and 

the South Platte River bluff, as also noted by Scott (1978) for areas downstream. 

Estimated thickness up to 6 m in Masters quadrangle, 6–10 m where exposed 

along river bluff in Orchard quadrangle, and possibly as much as 18 m near the 

southern border of Orchard quadrangle around Bijou and Kiowa Creeks 

(Gardner, 1967)

Mainstream deposits of Broadway Alluvium (late Pleistocene)—Mapped as 

Broadway Alluvium by Gardner (1967) and lower member of Broadway 

Alluvium by Scott (1978). Forms terrace approximately 12–15 m above active 

floodplain; terrace surface largely covered by eolian sand (Qes) but well 

exposed in Green City Flats area. Underlies and interpreted to interfinger with 

sidestream Broadway Alluvium (Qbs) from Kiowa and Bijou Creeks south of 

South Platte River mostly east of the Weld County–Morgan County line. Unit 

crops out poorly due to cohesionless nature of the sediment when dry. High 

potassium feldspar content gives unit a distinctive pinkish hue, particularly in 

exposures along South Platte River bluff in Orchard and Weldona quadrangles. 

Alluvium pink, pinkish gray, or light reddish brown, moderately well- to 

poorly sorted, fine to coarse sand, pebbly sand, silty sand, and sandy granule to 

pebble gravel. Commonly cross-stratified. Locally has interbeds of pinkish gray 

to very pale brown silt or laminations of dark grayish brown clay. Sand grains 

mostly subrounded. Gravel typically rounded to subrounded clasts of granite, 

gneiss, pegmatite, feldspar, quartz, chert, and sandstone. Largest clasts 

commonly 3–5 cm diameter, but cobbles and small boulders (up to about 32 cm 

in diameter) present locally. Accumulations of reddish-yellow to yellowish-red 

iron oxide and black manganese oxide common as coats on grains, masses in 

matrix, and linings that accentuate bedding. Surface soil profile typically has 

thin argillic B horizon with weak to moderate prismatic structure, and a thin Bk 

horizon with stage II carbonate morphology. 

At a site roughly 1.4 km east of Masters, Colo. (on north side of river near 

western edge of quadrangle), unit includes 70 cm of crumbly clay, pinkish silt, 

platy silty fine sand, burrowed sandy silt with gray and yellowish green color 

banding, over white silt with common shell fragments and small articulated clam 

shells. Hard dry consistence. Interpreted as quiet water deposits that may have 

been deposited at a time when the river was temporarily dammed by sidestream 

flood deposits (Qbs). Underlying sands brightly stained reddish yellow, 

yellowish red, or dark red due to iron oxide accumulation.

An OSL age estimate (5 percent moisture) of 9.4±1.0 ka (UNL–3504, table 

1) was obtained for mainstream Broadway Alluvium where it underlies Qbs at a 

locality about 0.8 km upstream of Kiowa Creek confluence (site TK–R, in 

Orchard 7.5′ quadrangle). This age estimate is anomalously young and 

stratigraphically inconsistent with all ages obtained for overlying Qbs (see 

discussion above), and therefore is not considered a realistic estimate for age of 

the sediment. Another OSL age estimate for mainstream Broadway Alluvium 

(8.0±0.7 ka, UNL–3502, 5 percent moisture, table 1), collected downstream in 

the Weldona 7.5′ quadrangle (near section AK of Gardner, 1967) also is 

anomalously young and stratigraphically inconsistent with ages obtained for the 

overlying deposit (Qa3), and therefore also not considered a realistic estimate. 

Reasons for the anomalously young OSL age estimates are unknown, but could 

be due to a number of factors associated with sampling and dating, or exposure 

of sediment to sunlight by burrowing animals. Results of a dose recovery test 

performed on UNL–3504, however, rule out most systematic problems with 

luminescence behavior of the sediments. 

Uranium-series age analyses were done on a fragment of horn-core bone 

(possibly bison) and a thin coating of pedogenic carbonate on the outer edge of the 

bone, which was encased in fine-grained sediment from the uppermost meter of 

mainstream Broadway Alluvium at site EF. Results indicate that the best estimate 

for minimum age of burial of the bone fragment is between 11 and 15 ka (table 

3; see discussion in appendix; 

http://pubs.usgs.gov/sim/3344/sim3344_appendix_1.pdf). The oldest ages are for 

two subsamples from the fragment interior that produced apparent closed-system 

230Th/U ages within analytical uncertainty of one another (Wp67–5 and Wp67–7, 

table 3; figure 1-1 in appendix). The error-weighted average value of these two 

oldest ages, 15.24±0.06 ka, may most closely estimate the minimum age of the 

deposit that contained the bone. However, because data for the specimen indicate 

a complex history of uranium uptake and subsequent leaching (see discussion in 

appendix), data for additional specimens are needed to better constrain the 

minimum age of the deposit. 

Broadway Alluvium is considered coeval with the Pinedale glaciation 

(Bryan and Ray, 1940; Hunt, 1954; Scott, 1960, 1975; Madole, 1991), which 

spanned from >31 ka to about 15–13 ka (Nelson and others, 1979; Madole, 1986; 

Schildgen and others, 2002; Benson and others, 2004, 2007; Licciardi and Pierce, 

2008; Madole and others, 2010; Young and others, 2011). Fluvial sediment load 

may have been greatest during and shortly after deglaciation (Church and Ryder, 

1972; Madole, 1991; Schildgen and others, 2002; Lindsey and others, 2005), a 

process that started either about 17 ka (Licciardi and others, 2004; Benson and 

others, 2005; Schaefer and others, 2006) or about 16–15 ka (Young and others, 

2011), and largely was completed between about 15 and 13 ka (Benson and 

others, 2007; Young and others, 2011; and references therein). Correspondingly, 

Clovis artifacts present in upper part of terrace alluvium near Kersey, Colo. 

(roughly 26 km west of the Masters quadrangle), combined with other 

archaeological data, indicate that aggradation of Broadway Alluvium was still in 

progress between about 13.3 and 12.9 cal ka (11.5–11 14C ka) but completed and 

the surface stabilized by about 11.5 cal ka (10 14C ka) at the latest (Holliday, 

1987). The 230Th/U age of 11.0±1.1 ka (Wp67–1, table 3) for pedogenic carbonate 

coating the outer edge of bone fragment encased in uppermost Broadway 

Alluvium at site EF further supports surface stabilization and the onset of soil 

formation sometime prior to 11 ka. Estimated thickness 12 m to as much as 30 m

Louviers Alluvium (middle Pleistocene)—Unit shown in cross section only. Pebble 

to cobble gravel, sand, and finer grained alluvium filling deep paleochannels 

(Bjorklund and Brown, 1957; Colorado Division of Water Resources, 2013); 

inferred to be Louviers Alluvium by Gardner (1967) and Scott (1978). Some of 

these paleochannels locally may contain deposits of older alluvium. Unit buried 

by deposits of Broadway Alluvium (Qba and Qbs). Terrace deposits on north 

side of South Platte River previously were mapped as Louviers Alluvium by 

Gardner (1967) in adjacent Orchard 7.5′ quadrangle, but are mapped here as 

mainstream deposits of Broadway Alluvium (Qba). Gardner (1967) similarly 

recognized these deposits as late Pleistocene in age based on soil development 

and fossil evidence, but mapped them as Louviers Alluvium because he inferred 

the age of Louviers Alluvium to be only about 50–10.5 ka.

The Louviers Alluvium is considered coeval with the Bull Lake glaciation 

(Scott, 1975; Madole, 1991). Timing of Bull Lake glaciation is less well 

constrained than that of Pinedale, but ages that have been obtained for Bull Lake 

deposits suggest the glaciation spanned from about 190 to ≤130 ka (see 

discussions in Madole, 1991; Schildgen and others, 2002; Pierce, 2003; Sharp 

and others, 2003; Kellogg and others, 2008; Licciardi and Pierce, 2008). Fluvial 

sediment load may have been greatest during and shortly after deglaciation 

(Church and Ryder, 1972; Madole, 1991; Schildgen and others, 2002; Lindsey 

and others, 2005); therefore, some of the youngest alluvium may post-date the 

Bull Lake glaciation by a few thousand years. Thickness is uncertain because 

unit is mostly buried by younger deposits along the South Platte River corridor 

between Greeley and the Colorado–Nebraska State Line (for example, see 

Colton, 1978; Scott, 1978), but subsurface lithologic data from test holes and 

water wells (Bjorklund and Brown, 1957; Colorado Division of Water Resources, 

2013) indicate that in buried paleochannels, unit could be up to 30 m thick in 

Masters quadrangle and up to 45 m thick in adjacent Orchard quadrangle

Intermediate alluvium (middle Pleistocene)—Mostly covered by eolian sand but 

exposed locally along Empire intake canal and in small gravel pit near Empire 

Reservoir. Yellowish or light reddish brown, poorly to moderately sorted, weakly 

to moderately stratified, moderately cemented, sandy pebble and cobble gravel, 

pebbly sand, sand, and silty sand in inferred terrace remnant approximately 24 m 

above active floodplain. Pebbles and cobbles mostly quartzite, gneiss, granite, 

pegmatite, and sandstone; includes bluish-gray quartzite that probably originated 

from outcrops in Coal Creek Canyon (Lindsey and others, 2005). Diameter of 

largest clasts commonly 10–11 cm but ranges up to 21 cm. Clasts subrounded to 

well rounded. Yellowish-red and reddish-yellow iron oxide stains common to 

abundant on clasts and in matrix. Clay-rich argillic B horizon present locally but 

poorly preserved. Pedogenic carbonate horizon has stage III morphology; coats 

(rinds) of calcium carbonate on clasts commonly thin. 

Uranium-series age analyses were done on innermost calcium carbonate 

rinds subsampled from multiple clasts, which were collected at approximately 

100–120 cm depth below the ground surface from intermediate alluvium (Qai) 

cropping out beneath eolian sand along the Empire Intake Canal (site EIC). The 

analyses produced reliable results (see discussion in appendix, 

http://pubs.usgs.gov/sim/3344/sim3344_appendix_1.pdf), and oldest 230Th/U 

ages that primarily cluster in two groups (figure 1-7 in appendix); one cluster 

made up of four subsamples from two separate clasts (Wp95–A1–2, 

Wp95–A2–1, Wp95–B1, and Wp95–B2, table 3) has an error-weighted-mean age 

of 334±9 ka; the other cluster, made up of two subsamples from separate clasts 

(Wp95–B3 and Wp95–E1, table 3) has an error-weighted-mean age of 382±16 

ka. Therefore, the data indicate with a high degree of confidence that the alluvial 

clasts were deposited prior to 334 ka; based on the two oldest ages, the best 

estimate for the minimum age of the deposit is somewhat older than 382 ka. 

These results support correlating intermediate alluvium at site EIC with older 

deposits of Slocum Alluvium, which have a proposed age range of 320–390 ka 

(Kellogg and others, 2008). Thickness is poorly constrained but may be 13–18 m

Old alluvium (early Pleistocene)—Mostly covered by eolian sand but exposed in 

gravel pit near south edge of quadrangle. Light reddish or yellowish brown, 

moderately to well-cemented, poorly to moderately sorted, sandy pebble and 

cobble gravel, pebbly sand, sand, and silty sand in inferred terrace remnant 

approximately 60 m above active floodplain. Weakly to moderately stratified; 

sand beds commonly cross-stratified. Pebbles, cobbles and minor small boulders 

(up to 30 cm diameter) generally rounded to well rounded. Clasts mostly 

quartzite, gneiss, granite, pegmatite, and sandstone; includes bluish-gray 

quartzite that probably originated from outcrops in Coal Creek Canyon (Lindsey 

and others, 2005). Dense accumulations of red, yellowish-red, and 

reddish-yellow iron oxide and lesser amounts of black manganese oxide 

prevalent in matrix, on grains, and along bedding planes. Secondary silica may 

also be present based on the strength of cementation. Calcium carbonate 

accumulations common along fractures and as thin coats on clasts. 

Gardner (1967, p. 57) interpreted deposit as Rocky Flats Alluvium, which 

has an estimated age of at least 1.4–1.6 Ma (Birkeland and others, 1996) or about 

1.5 Ma (Dethier and others, 2001) to about 2 Ma (Birkeland and others, 2003; 

Riihimaki and others, 2006; see discussion in Kellogg and others, 2008), 

although younger ages have also been proposed (Riihimaki and others, 2006; 

Dühnforth and others, 2012). A fossil tooth (Stegomastodon elegans) from a site 

downstream of Fort Morgan (NW1⁄4SW1⁄4, sec. 7, T. 5 N., R. 55 W.), collected 

from a deposit also approximately 60 m above the active floodplain and mapped 

as Rocky Flats Alluvium by Scott (1978, 1982), supports an early Pleistocene age 

for the deposit (Scott, 1982). Thickness poorly constrained but may be as much 

as 9 m (Scott, 1978)

Alluvial and colluvial deposits

Fan alluvium and hillslope colluvium (Holocene)—Mapped mostly along south 

side of South Platte River, where hillslope processes are actively degrading river 

bluff. Includes individual and coalescing alluvial fans at mouths of headwarderoding gullies and colluvium mantles on slopes. Very pale brown, crudely 

stratified, poorly sorted pebbly sand, and moderately sorted sand with silt lenses 

and medium- to coarse-grained sand stringers. Most clasts <3 cm diameter and 

subangular to subrounded. Many clasts potassium feldspar in composition. 

Surface soil weakly developed A/C profile. Estimated thickness up to 3 m

Eolian deposits 

Eolian sand (Holocene and late Pleistocene)—Forms simple and compound 

parabolic dunes and low-relief sand sheets. Pale brown, brown, or yellowish 

brown, moderately to well-sorted, mostly fine to medium grained sand. Locally 

contains minor amounts of coarse to very coarse sand and a few scattered, 

subangular to subrounded, granules and fine pebbles of mostly granitic and 

gneissic rock types. Loose to slightly hard dry consistence. Sand deposits of 

more than one age commonly separated by buried soils. Locally may include 

sheetwash deposits of reworked eolian sand, and marsh or pond deposits in 

interdune and other low-lying wetland areas. Deposits of Qes may be more 

extensive than mapped due to the difficulty in distinguishing thin eolian sand 

cover from sandy terrace alluvium in a region that has been farmed and 

cultivated for more than a century.

Unstippled parts of Qes are shown in county soil surveys as having mostly 

Valent, Dwyer, and Valentine soils, characterized by weakly developed A/C or 

A/AC/C soil profiles (USDA Natural Resources Conservation Service, 2009a, 

2009b), typical of relatively young (late and middle Holocene) eolian sand 

deposits (Madole, 1995; Muhs and others, 1996; Madole and others, 2005). Light 

stippled parts of Qes are shown in county soil surveys as having mostly Bijou, 

Bresser, and Truckton soils, characterized by argillic (Bt) horizon development in 

sandy, mostly alluvial soil parent material (USDA Natural Resources 

Conservation Service, 2009a, 2009b). Field investigation revealed argillic 

horizon to be part of a buried soil developed in late Pleistocene stream and 

sheetflood alluvium and later covered by a veneer of younger eolian sand. 

Therefore, as in unstippled areas, eolian sand deposits in light stippled areas 

probably are Holocene in age, but generally thin (<1 m), such that underlying 

buried soil controls soil classification and series designation. In contrast, dark 

stippled parts of Qes are shown in county soil surveys as having mostly Vona 

soils (north of river) and Osgood soils (south of river), characterized by A/Bt/Bk 

(stage I–II+ carbonate morphology) soil profiles developed in eolian sand (U.S. 

Department of Agriculture Natural Resources Conservation Service, 2009a, 

2009b), development typical of late Pleistocene eolian sand deposits (Madole, 

1995; Muhs and others, 1996; Madole and others, 2005). North of river dark 

stipple also includes lesser areas of Terry soils, shallow (<1 m) to bedrock soils 

similar to Vona soils but developed primarily in residuum formed from sandstone 

(U.S. Department of Agriculture Natural Resources Conservation Service, 2009a, 

2009b) presumably of the upper transition member of Pierre Shale (Braddock 

and Cole, 1978; Scott, 1978); the latter areas are inferred to have thin eolian sand 

cover that may be late Pleistocene in age based on soil development and 

therefore are included in Qes dark stipple; locally may contain small outcrops of 

Pierre Shale (Kp). Holocene sand deposits too small to show separately present 

locally within dark-stippled areas. 

Widespread deposition of eolian sand during the late Holocene is indicated 

by radiocarbon ages reported by Madole (1994, 1995) and Madole and others 

(2005) for buried soils that provide maximum-limiting ages for uppermost sand 

deposits in adjacent or nearby quadrangles: 1.14±0.16 cal ka B.P. (1.23±0.08 14C 

ka B.P., Beta–70543) at Milliron Draw, Orchard 7.5′ quadrangle; 0.88±0.21 cal 

ka B.P. (0.96±0.11 14C ka B.P., Beta–84821) at south edge of Orchard 7.5′ 

quadrangle near Bijou Creek; 0.64±0.11 cal ka B.P. (0.68±0.08 14C ka B.P., 

Beta–70542) near Bijou Reservoir, Weldona 7.5′ quadrangle; and, 0.82±0.16 cal 

ka B.P. (0.89±0.10 14C ka B.P., Beta–62192) in Greasewood Lake 7.5′ 

quadrangle. Radiocarbon ages for soil carbonate rhizoliths collected from a 

quarry exposure north of Orchard are in good agreement, indicating a 

maximum-limiting age for uppermost eolian sand of about 1.44±0.13 cal ka B.P. 

(1.56±0.07 14C ka B.P., CAMS–8234, Muhs and others, 1996). Ages from 

elsewhere in the region also support widespread deposition of eolian sand during 

the late Holocene (Clarke and Rendell, 2003, and references cited therein). 

Eolian sand deposits of probable middle Holocene age are locally exposed, 

mostly underlying late Holocene sand deposits. At a site near Empire Reservoir 

(ER), approximately 1 m below the surface, a middle Holocene sand deposit 

covered by a thin veneer of late Holocene sand has an OSL age estimate of 

7.7±0.8 ka (UNL–3468, 5 percent moisture, table 1). At Milliron Draw, Orchard 

7.5′ quadrangle, the age of a buried sand deposit is constrained by ages on buried 

soils that provide a maximum-limiting age of about 6.33±0.16 cal ka B.P. 

(5.51±0.09 14C ka B.P., Beta–72203) and a minimum-limiting age of about 

1.14±0.16 cal ka B.P. (1.23±0.08 14C ka B.P., Beta–70543) for the deposit 

(Madole, 1995; Madole and others, 2005). Radiocarbon ages for soil carbonate 

rhizoliths collected from a quarry exposure north of Orchard suggest a 

minimum-limiting age for buried eolian sand of about 3.90±0.20 cal ka B.P. 

(3.60±0.07 14C ka B.P., CAMS–6378, Muhs and others, 1996) at that site. 

Episodes of eolian sand deposition during the middle Holocene are also 

documented at other sites within the region (Clarke and Rendell, 2003, and 

references cited therein). 

Sand deposits of probable late Pleistocene age (marked by dark stipple) are 

extensive in the Masters quadrangle as well as regionally (Muhs and others, 

1996; Madole and others, 2005). At site ER a deposit of late Pleistocene sand has 

an OSL age estimate of 26.4±2.5 ka at approximately 0.7 m depth (UNL–3467, 5 

percent moisture, table 1). This age is in good agreement with radiocarbon ages 

Muhs and others (1996) obtained for soil carbonate nodules from an underlying 

buried soil at a site roughly 2 km north of site ER that provide 

maximum-limiting ages for the sand of 31.21±0.23 cal ka B.P. (27.30±0.17 14C 

ka B.P., CAMS–11339), 31.36±0.48 cal ka B.P. (27.42±0.30 14C ka B.P., 

CAMS–16612), and 30.58±0.51 cal ka B.P. (26.41±0.24 14C ka B.P., 

CAMS–16604). Eolian sand deposition during the late Pleistocene likely was 

intermittent, and may have resulted in late Pleistocene deposits of various ages 

within the quadrangle; some may be as young as 12 or 13 ka (Madole and others, 

2005).

Main source of eolian sand is South Platte River for sands in the Fort 

Morgan dune field, made up of a series of dune fields south of the river, and 

Laramie Formation for sands in the Greeley dune field, located north of the river 

(Muhs and others, 1996; Aleinikoff and Muhs, 2010). Trends of dunes within the 

quadrangle (averaging about N. 34° W. south of river and about N. 27° W. north 

of river) are consistent with regional average trends reported for the dune fields 

by Muhs (1985) and Muhs and others (1996); combined with dune distribution 

with respect to primary sand source, dune orientation indicates sand-transporting 

winds that formed the dunes primarily came from the northwest, as do dominant 

winds at present (Muhs and others, 1996; Madole and others, 2005). Currently, 

dunes are mostly stabilized by vegetation. Thickness up to 20 m in compound 

parabolic dunes both north and south of river, but thins to less than half a meter locally

BEDROCK UNIT 

Pierre Shale (Upper Cretaceous)—Unit shown in cross section only. Underlies 

Quaternary deposits in map area (Bjorklund and Brown, 1957; Colorado 

Division of Water Resources, 2013). Where unit crops out in nearby quadrangles 

composed of upper transition member of Pierre Shale: dark-gray calcareous 

marine silty shale interbedded with light-gray shaly sandstone and sandy shale. 

Sandstone beds and concretionary limestone layers up to a meter or more thick. 

Concretions common. Upper Cretaceous marine fossils sparse to abundant. 

Thickness of formation close to 1,800 m in map area (Gardner, 1967; Scott, 1978) 

EXPLANATION OF MAP SYMBOLS

Contact—Dashed where approximately located

Dune Crests—Only crests clearly visible on National Agriculture Imagery Program 

(NAIP) orthoimagery are shown

Areas where eolian sand Qes probably is Holocene in age but generally thin

Areas where eolian sand Qes may be late Pleistocene in age

Intermittent wetlands and ponds—Low-lying areas prone to wetness and ponding 

within eolian sand deposits (Qes). Includes areas that in some years have 

relatively large ponds, but that appear to lack standing water in 2013 NAIP 

orthoimagery

Water—Includes water in ponds, reservoirs, and channels of South Platte River wide 

enough to be mapped at 1:24,000 scale. Water bodies change in position and size 

seasonally and annually. Location and dimension of ponds and river channels 

shown on this map are based on 2013 NAIP orthoimagery. NAIP imagery taken 

in 2011 and 2009 shows a greater volume of water in both ponds and South 

Platte River relative to 2013 imagery

Optically stimulated luminescence (OSL) sample location—With site name that 

links to sample field number (see table 1)

Uranium-Thorium (230Th/U) sample location—With site name that links to sample 

field number (see table 3) 

INTRODUCTION

The Masters 7.5′ quadrangle is located along the South Platte River corridor on the semiarid 

plains of eastern Colorado and contains surficial deposits that record alluvial, eolian, and hillslope 

processes that have operated in concert with environmental changes from the Pleistocene to the 

present. The South Platte River, originating high in the Colorado Front Range, has played a major 

role in shaping the surficial geology of the quadrangle, which is situated downstream of where the 

last of the major headwater tributaries (St. Vrain, Big Thompson, and Cache la Poudre) join the 

river. Recurrent glaciation (and deglaciation) of basin headwaters has affected river discharge and 

sediment supply far downstream, influencing deposition of alluvium and terrace formation in the 

Masters quadrangle. Kiowa and Bijou Creeks are unglaciated tributaries originating in the Colorado 

Piedmont east of the Front Range and joining the South Platte River just downstream of the 

Masters quadrangle. They have played a major role by periodically delivering large volumes of 

sediment to the river during flood events, which could have temporarily dammed the river. 

Along the reach of river in the Masters quadrangle, the river has incised into Upper 

Cretaceous Pierre Shale, which, although buried by alluvial deposits here, is locally exposed 

downstream along the South Platte River bluff near the Bijou Creek confluence, in some of the 

larger draws, and along Wildcat Creek (Scott, 1978). Eolian sand deposits of the Greeley (north of 

river) and Fort Morgan (south of river) dune fields cover much of the quadrangle and record past 

episodes of sand mobilization during times of prolonged drought (Muhs and others, 1996). With the 

onset of irrigation and damming during historical times, the South Platte River has changed from a 

broad, shallow, and sandy braided river with highly seasonal discharge to a much narrower, deeper 

river with braided-meandering transition morphology and more uniform discharge (Nadler and 

Schumm, 1981; Harvey and others, 1985).

METHODS

The digital geologic map of the Masters quadrangle was completed using a combination of 

field investigation, geochronologic research, and interpretation of the following: previous mapping 

of nearby quadrangles by M.E. Gardner during the 1960s (Gardner, 1967); stereoscopic pairs of 

National Aerial Photography Program (NAPP) color-infrared (1988, 1:40,000 scale) and historical 

black and white (1948, 1:39,230 scale; 1949, 1:16,620 scale) aerial photographs; National 

Agriculture Imagery Program (NAIP) orthoimagery (U.S. Department of Agriculture (USDA) Farm 

Service Agency Aerial Photography Field Office, 2009, 2011, 2013); topographic and digital 

elevation (DEM) data; digital soil survey data (USDA Natural Resources Conservation Service, 

2009a, 2009b); satellite imagery viewed with Google Earth; Federal Emergency Management 

Agency (1989) flood maps (1:19,200 approximate scale); and subsurface lithologic data from test 

holes and water wells (Bjorklund and Brown, 1957; Colorado Division of Water Resources, 2013). 

Field mapping and research were carried out between 2011 and 2013. Digital mapping using 

ArcMap software was done initially on NAIP orthoimagery taken in 2009 and later refined using 

imagery taken in 2011 and 2013; location and dimensions of the South Platte River and other water 

bodies shown on this map are based on the 2013 imagery. 

The topographic base is somewhat out of date particularly with regard to locations of channel 

banks of the South Platte River, low terrace scarps on the valley floor, shorelines along reservoirs, 

and dune crests in areas where center-pivot irrigation or other agricultural practices have been 

introduced. Therefore, geologic mapping of young fluvial features, shoreline deposits, and dune 

crests also is based on the NAIP imagery. Dunes outside of cultivated areas are mostly stabilized by 

sandsage prairie vegetation (Chapman and others, 2006), and dune form is no longer distinct in 

many places due to degradation by hillslope processes and the grazing of livestock; dune crests (for 

simple parabolic dunes and for individual dunes within compound parabolic dunes) were only 

mapped where clearly visible in the NAIP imagery and therefore were only mapped for a portion of 

the dunes within the dune fields. All field mapping and all imagery used for mapping predate the 

large-magnitude flood on the South Platte River that took place during mid-September, 2013. 

Geologic mapping was aided by optically stimulated luminescence (OSL), radiocarbon (14C), 

and uranium-series (230Th/U) age determinations (tables 1–3). Sediment samples for OSL dating 

were processed and analyzed at the University of Nebraska–Lincoln Luminescence Geochronology 

Laboratory using the single aliquot regenerative (SAR) method (Murray and Wintle, 2000). 

Equivalent dose (De) was measured on a Risø DA–20 TL/OSL reader and values were calculated 

using the central age model of Galbraith and others (1999). Dose rates and age estimates were 

calculated using both field moisture of the sample and a fixed estimate of 5 percent moisture (table 

1). Moisture content of some of the samples (collected in November, 2011) was very low and 

unlikely to approximate an average condition; the fixed estimate of 5 percent (an intermediate value 

within the range of values for samples collected in November, 2011 and April–May, 2012) may 

better represent moisture history of the samples for the purpose of calculating estimated ages from 

the dose rates. Radiocarbon ages, including those obtained in our study (table 2) and previously 

published ages cited from the literature, were calibrated using the IntCal13 dataset and CALIB 7.0 

(Stuiver and Reimer, 1993; Reimer and others, 2013) for better comparison to ages generated by 

other dating methods. Uranium-series age analyses were done at the U.S. Geological Survey 

Denver radiogenic isotope laboratory using procedures outlined in “USGS–DRIL–01, R0 

Uranium-Thorium Disequilibrium Studies” (accessible at 

http://esp.cr.usgs.gov/projects/uth/USGS_DRIL_01cR0_U_series.pdf). Supplemental details about 

the samples analyzed and results obtained are provided in the appendix to the geologic map (J.B. 

Paces, 230Th/U ages supporting geologic map of the Masters 7.5′ quadrangle, Weld and Morgan 

Counties, Colorado, http://pubs.usgs.gov/sim/3344/sim3344_appendix_1.pdf). The 230Th/U 

analyses provide constraints on the minimum ages of deposits from which samples were collected 

(table 3); the oldest 230Th/U ages are interpreted as being closest temporally to the minimum age of 

the depositional event.

ACKNOWLEDGMENTS

This project was supported by the U.S. Geological Survey’s National Cooperative Geologic 

Mapping Program. The National Association of Geoscience Teachers (NAGT)–U.S. Geological 

Survey Cooperative Summer Field Training Program also provided support through a student 

summer intern. We thank the many landowners who graciously granted access to their property. We 

also thank the many other individuals who contributed to the project: Emily M. Taylor (USGS) for 

accessing and compiling test hole and water well locations and subsurface lithologic data for use in 

ArcMap; Sarah R. Survis (NAGT Intern) for assisting with field work; Shannon A. Mahan (USGS) 

for her loan of a portable gamma spectrometer to measure OSL dose rates in the field; and Jeffrey 

S. Pigati (USGS) for helpful discussions of radiocarbon dating methods and samples, identification 

of probable Succinea snail shells, and submission of our radiocarbon samples for analysis. Daniel 

R. Muhs (USGS) and Ralph R. Shroba (USGS) provided thorough reviews that greatly improved 

this Scientific Investigations Map. Leonid A. Neymark (USGS) and Wayne R. Premo (USGS) 

provided comprehensive technical reviews of the appendix on 230Th/U ages 

(http://pubs.usgs.gov/sim/3344/sim3344_appendix_1.pdf).

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Qa3

Qba

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Kp

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EF

Qes

Qes

Kp Kp

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Qa2

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Qes Qes

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Qes

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North South

4700

4600

4500

4400

A A'

4400

4500

4600

4700

FEET

4300 4300

VERTICAL EXAGGERATION x10

Contact between bedrock (Kp) and overlying Quaternary deposits based on drill data given in Bjorklund and Brown (1957) and downloaded from the Colorado Division of Water Resources (2013) 

FEET

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South Platte River

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Carbonate rind (subsample Wp67–1) and layers of bone (subsamples Wp67–2 through Wp67–11). Location of EF–Wp67: UTM 564654E., 4462502N.4

Wp67–1 22.5 1.64 5.7 0.117±0.011 1.221±0.004 11±1.1 1.228±0.004

Wp67–2 83.0 0.325 104 0.1338±0.0012 1.214±0.003 12.70±0.13 1.221±0.003

Wp67–3 107 0.044 1,150 0.1520±0.0006 1.210±0.003 14.58±0.07 1.219±0.003

Wp67–5 116 0.025 2,300 0.1588±0.0007 1.215±0.003 15.21±0.08 1.224±0.003

Wp67–7 115 0.028 2,020 0.1600±0.0007 1.221±0.002 15.26±0.08 1.231±0.003

Wp67–9 116 0.032 1,600 0.1458±0.0006 1.217±0.003 13.87±0.07 1.226±0.003

Wp67–11 104 0.124 322 0.1255±0.0006 1.217±0.003 11.82±0.06 1.225±0.003

Innermost carbonate rinds on alluvial clasts. Location of EIC–Wp95: UTM 567342E., 4458109N.4

Wp95–A1–1 5.92 0.280 66.9 1.040±0.008 1.125±0.007 248±9 1.252±0.010

Wp95–A1–2 7.36 0.309 76.7 1.061±0.007 1.084±0.003 337±15 1.217±0.009

Wp95–A2–1 10.20 0.274 121 1.065±0.009 1.085±0.003 341±20 1.222±0.012

Wp95–A3–1 7.12 0.549 41.3 1.048±0.009 1.094±0.004 294±13 1.216±0.008

Wp95–B1 7.90 0.901 28.2 1.057±0.01 1.077±0.004 345±24 1.205±0.012

Wp95–B2 7.60 0.509 47.6 1.048±0.007 1.079±0.003 324±15 1.197±0.008

Wp95–B3 9.58 0.672 46.3 1.069±0.007 1.072±0.003 397±27 1.221±0.014

Wp95–E1 9.91 0.461 67.7 1.037±0.006 1.054±0.003 372±21 1.153±0.008

EF–Wp67

EIC–Wp95

Lab

sample

name 

U concentration,

in μg/g

Th concentration,

in μg/g 

230Th/U age

±2σ (ka)3

Initial 234U/238U AR

±2σ3

1

Measured activity ratio (AR) corrected for mass fractionation, spike contributions, procedural blank and normalized relative to an atomic ratio value for NIST SRM 4321B U-isotope standard of 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=1.0±0.1; and 230Th/238U=1.0±0.25. 3 230Th/U ages, initial 234U/238U activity ratios (ARs), and associated errors are calculated using detritus-corrected activity ratio values. 4

Zone 13, NAD 27. 

Field

number 

Measured

230Th/ 232Th/

AR1 230Th/238U AR±2σ 234U/238U AR±2σ

Detritus-corrected activity ratios2

Table 3. U-Th concentrations, U-series isotope compositions, and calculated 230Th/U ages and initial 234U/238U activity ratios for subsamples of EF–Wp67 and EIC–Wp95.

[Abbreviated version of table 1-1, available in the online appendix (J.B. Paces, 230Th/U ages supporting geologic map of the Masters 7.5′ quadrangle, Weld and Morgan Counties, Colorado 

http://pubs.usgs.gov/sim/3344/sim3344_appendix_1.pdf). Note that ±2σ represents estimates of uncertainty given at the 95% confidence level; μg/g, micrograms per gram; σ, sigma.]

Laboratory

number

7.5′

quad

UTM

Easting1

UTM

Northing1

Material

dated

Approx.

depth

(m) 

P5

Aeon–994 Weldona 592473 4463764 Probable Succinea

(single shell) 4.7 -6.8 0.98

Aeon–995 Weldona 592473 4463764 3.6 -4.6 0.98

Aeon–1064 Orchard 581980 4466741 3.7 -3.8 1.00

Aeon–950 Orchard 577915 4464174 mostly lignite? 2.5 -23.2 1.00

AK–8

12.36±0.15

39.80±3.10

H–R–3,4,5,9

AK–1

Field

number

KC–245

10.22±0.07

10.24±0.06

δ13C

(‰)2

Table 2. Radiocarbon sample information, 14C ages, and calibrated ages.

[UTM, Universal Transverse Mercator; δ13C, delta Carbon-13; ‰, per mil; 14C ka B.P., Carbon-14 thousand years before present; cal ka B.P., calibrated 

thousand years before present; P, probability; Approx., approximate; m, meter] 

1

1927 North American Datum (NAD 27), zone 13. 

2

Relative difference between 13C/12C ratio of carbon extracted from sub-sample and that of VPDB international standard. 3

Conventional radiocarbon age, normalized to -25‰, based on 5,568-year half-life; uncertainty ±1 σ.

4

Calibrated age calculated using CALIB 7.0, IntCal13 dataset (Stuiver and Reimer, 1993; Reimer and others, 2013); 0 yr B.P.=1950 A.D.; uncertainty ±2 σ. Calibrated age reported as midpoint of calibrated range. 5

Probability of calibrated age falling within reported range as calculated by CALIB.

Probable Succinea

(single shell) 

Probable Succinea

(four shells from 

~15 cm thick bed) 

14C age

(14C ka B.P.)3

Calibrated age

(cal ka B.P.)4

11.90±0.28

11.95±0.24

14.53±0.56

43.90±5.69

In situ8 In situ8

UNL–3462 Orchard 577915 4464174 1.7 5.3 20.0 3.8 67.6±3.3 22/30 2.9 5.77±0.32 11.7±1.0 5.0 5.64±0.35 12.0±1.1

UNL–3467 Masters 568185 4455813 0.7 2.9 11.4 3.4 109.7±5.6 20/37 5.0 4.15±0.27 26.4±2.5 5.0 4.15±0.27 26.4±2.5

UNL–3468 Masters 568198 4455836 1.0 2.4 8.1 3.7 31.4±2.2 29/37 1.9 4.22±0.25 7.4±0.7 5.0 4.07±0.28 7.7±0.8

UNL–3498 Weldona 585123 4459026 1.1 4.8 19.5 3.7 67.0±2.8 31/48 7.7 5.23±0.39 12.8±1.2 5.0 5.39±0.34 12.4±1.1

UNL–3499 Weldona 585123 4459026 1.9 4.3 17.6 4.1 79.1±2.9 36/54 6.7 5.33±0.38 14.8±1.3 5.0 5.43±0.35 14.6±1.2

UNL lab9 UNL lab9

UNL–3502 Weldona 592468 4463758 5.1 5.7 23.3 2.7 40.2±1.4 43/54 7.0 4.92±0.35 8.2±0.7 5.0 5.03±0.31 8.0±0.7

UNL–3503 Weldona 592478 4463772 3.5 3.5 9.9 2.6 32.5±1.8 27/35 5.9 3.52±0.24 9.2±0.9 5.0 3.56±0.23 9.1±0.9

UNL–3504 Orchard 577216 4463702 8.0 1.5 6.7 3.9 35.8±2.0 48/54 3.3 3.88±0.26 9.2±0.9 5.0 3.81±0.28 9.4±1.0

7.5′

quad

Field

number

UNL lab

number

UTM

Easting2

UTM

Northing2

Approx.

depth

(m)

U

(ppm)

Th

(ppm)

K2

O

(wt. %)

De

(Gy)

±1 Std. err.3

Aliquots

(n)4

In situ

H2

O field

(%)5

Dose rate

(Gy/ka)6

OSL age

ka±1 σ6

In situ

H2

O est.

(%)5

Dose rate

(Gy/ka)7

OSL age

ka±1 σ7

Table 1. Optically stimulated luminescence (OSL) age estimates, with equivalent dose (De ) and dose rate data.

[UNL Lab, University of Nebraska–Lincoln Luminescence Geochronology Laboratory; UTM, Universal Transverse Mercator; Approx., approximate; m, meter; U, Uranium; ppm, parts per million; Th, Thorium; 

K2O, Potassium oxide; wt.%, weight percent; Gy, gray1; Std. err., standard error; n, number; H2O, water; ka, thousand years; σ, sigma] 

1

Gray is an SI unit of absorbed dose of ionizing radiation (Taylor and Thompson, 2008).

2

1927 North American Datum (NAD 27), zone 13.

3

Determined using the Central Age Model of Galbraith and others (1999).

4

Accepted disks/all disks. 

5

Assumes 100% error in estimated moisture content. Field, moisture content of sample, measured at UNL lab; Est.,estimate of moisture content, which may better represent moisture history of sample. 6

Calculated using field moisture content of sample.

7

Calculated using 5% as estimated moisture content.

8

Measured in field with a portable gamma spectrometer.

9

Measured at UNL Lab using inductively coupled plasma-mass spectrometry.

UNL–3466 Orchard 577915 4464174 2.6 3.3 16.3 4.3 87.8±5.4 24/37 2.1 5.41±0.30 16.2±1.5 5.0 5.23±0.34 16.8±1.7

OSL–1–KC

OSL–6–ER

OSL–7–ER

OSL–BC–14

OSL–BC–13

OSL–AK–10

OSL–AK–11

OSL–TK–R–12

OSL–4–KC

OSL–5–KC UNL–3463 Orchard 577915 4464174 3.6 3.7 15.8 4.3 80.6±4.8 22/32 2.4 5.45±0.31 14.8±1.4 5.0 5.29±0.35 15.2±1.5

DIVISION OF QUATERNARY TIME USED IN THIS REPORT1

Holocene

1

Ages for time divisions are from Walker and others (2012) and Cohen and others (2013; updated). Ma, million years; 

ka, thousand years. 

2

Calabrian and Gelasian Ages.

Quaternary

Period Epoch

0–11.7 ka

late 11.7–126 ka

middle 126–781 ka

early2 781 ka–2.58 Ma

Age

Pleistocene

CONVERSION FACTORS

Multiply By To obtain

meter (m) foot (ft) 3.281

millimeter (mm) 0.03937 inch (in.)

kilometer (km) 0.6214 mile (mi)

centimeter (cm) inch (in.) 0.3937

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Map 3344