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40
Ar/
39
Ar age of the Kaiparowits Formation, southern Utah, and
correlation of contemporaneous Campanian strata and vertebrate
faunas along the margin of the Western Interior Basin
Eric M. Roberts
a,
*, Alan L. Deino
b
, Marjorie A. Chan
a
a
Department of Geology and Geophysics, University of Utah, Salt Lake City, UT 84112, USA
b
Berkeley Geochronology Center, Berkeley, CA 94709, USA
Received 10 June 2004; accepted in revised form 11 January 2005
Available online 16 March 2005
Abstract
Laser-fusion
40
Ar/
39
Ar analysis of four bentonite horizons produces the first absolute ages for the 860-m-thick Kaiparowits
Formation and resolves previous age uncertainty caused by ambiguous biostratigraphy. A late Campanian (Judithian) age of ca.
76.1e74.0 Ma is determined, resulting in a high-resolution temporal framework for the richly fossiliferous formation. Detailed
stratigraphic correlation reveals that the Kaiparowits Formation is contemporaneous with many of the most important vertebrate
fossil-bearing formations in the Western Interior Basin, and with other well-studied strata across Utah and southeastern Wyoming,
including portions of the Book Cliffs sequence. The Judithian age determination and correlations for the Kaiparowits Formation
presented here provide a new chronological basis for addressing questions relating to mammal biostratigraphy, vertebrate evolution,
biodiversity and paleobiogeography (e.g., dinosaur provincialism) in the Cretaceous Western Interior Basin.
Ó2005 Elsevier Ltd. All rights reserved.
Keywords: Cretaceous; Geochronology; Stratigraphy; North America; Vertebrata
1. Introduction
Foreland basins commonly preserve thick, richly
fossiliferous, nonmarine sedimentary sequences that are
important for reconstructing the tectonic and evolution-
ary histories of ancient terrestrial ecosystems (Hunt, 1991;
Rogers, 1993; Badgley and Behrensmeyer, 1995). The
Western Interior Basin (WIB), extending from Alberta to
Mexico, is among the most well-studied foreland basins in
the world, particularly with regard to vertebrate evolu-
tion. However, temporal correlation of strata and faunas
in the WIB remains problematic. A paucity of fossil
vertebrates and limited radiometric age control of non-
marine strata in the central WIB in Utah (e.g., Kaiparowits
Basin, Book Cliffs) has, until now, hampered stratigraphic
and faunal correlation with more well-studied northern
and southern regions (Sampson et al., 2002).
Recent fossil discoveries in the Kaiparowits Formation
are beginning to improve our understanding of Late
Cretaceous vertebrate faunas from Utah (Weishampel
and Jensen, 1979; DeCourten and Russell, 1985; Eaton
and Cifelli, 1988; Cifelli, 1990a,b; Hutchison et al., 1997;
Eaton, 2002). The Kaiparowits Formation is particularly
important because it is one of the few richly fossiliferous
Late Cretaceous formations in the central WIB, and is
thus crucial for linking fossil-rich northern and southern
strata, and for testing paleogeographic hypotheses on
evolution and provinciality among terrestrial vertebrate
* Corresponding author. Department of Geosciences, Idaho State
University, Pocatello, ID 83209, USA.
E-mail address: robeeric@isu.edu (E.M. Roberts).
0195-6671/$ - see front matter Ó2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.cretres.2005.01.002
www.elsevier.com/locate/CretRes
Cretaceous Research 26 (2005) 307e318
faunas (e.g., Horner et al., 1992; Lehman, 1997; Weil,
1999). Here we report the first radioisotopic ages for the
Kaiparowits Formation. Detailed
40
Ar/
39
Ar analysis of
four altered ash beds (bentonites) results in a high-
resolution chronostratigraphy and permits detailed cor-
relation between other strata and vertebrate faunas.
2. Geologic setting
The Kaiparowits Formation is exposed in the
Kaiparowits Basin of southern Utah, primarily within
the boundaries of the Grand Staircase-Escalante Na-
tional Monument (Fig. 1). Because of its rich and
relatively unexplored fossil resources (e.g., dinosaurs),
the Kaiparowits Formation represents an important
area in the United States for new fossil discoveries. The
formation is easily recognized by its distinctive, bad-
land-forming blue-gray sandstones and mudstones,
which are in stark contrast to the typical tan sandstones
of the underlying Wahweap and Straight Cliffs for-
mations and the overlying, maroon conglomerates of
the Canaan Peak Formation. At ca. 860 m thick, the
Kaiparowits Formation comprises nearly half of the 2-
km-thick Upper Cretaceous sequence in the Kaipar-
owits Basin. The section is informally subdivided into
three units (lower, middle, upper), based on distinct
changes in alluvial architecture (Fig. 2).
Kaiparowits strata were deposited as part of a single
prograding clastic wedge, derived from a source area
located ca. 300e650 km to the southwest in the Sevier
fold and thrust belt in southeastern Nevada and eastern
California (Eaton, 1991; Goldstrand, 1992). The fine-
grained nature of the sediment and nonmarine fauna
suggest deposition upon a low-relief, inland alluvial
plain setting. Sedimentological analysis suggests a rela-
tively warm, humid paleoclimate dominated by fluvial
and paludal environments (Eaton, 1991; Little, 1995;
Roberts et al., 2003). The relative isolation of the
formation and the lack of correlative or bounding
marine units with reliable index fossils have hampered
previous age assessments. Preliminary investigation of
the palynology and vertebrate paleontology of the
formation suggested a Maastrichtian age for these strata
(e.g., Lohrengel, 1969; DeCourten and Russell, 1985).
However, more recent work on mammals in the
formation (e.g., Eaton and Cifelli, 1988; Cifelli,
1990a,b; Eaton, 2002) and re-examination of palyno-
morph data (Eaton, 1991) suggest a Campanian age.
3. Bentonites
Eight discrete bentonites were identified within the
composite 860-m-thick succession (Fig. 2). The benton-
ites are primarily composed of smectitic clay, which
produces a distinctive, easily recognizable popcorn-style
weathering. Individual phenocrysts of euhedral biotite,
plagioclase, and sanidine are localized along basal
contacts, and they are typically between 200e700 mm.
Bentonite beds range in thickness from 10 to 50 cm, are
commonly pale olive (10Y 6/2) to yellowish gray (5Y 7/2)
in color and have sharp to wavy basal contacts. Upper
contacts are commonly gradational. The abundance of
Fig. 1. General locality map for the Kaiparowits Formation and primary study locations.
308 E.M. Roberts et al. / Cretaceous Research 26 (2005) 307e318
paludal environments in the formation likely resulted in
enhanced preservation of these ashes.
The relative thinness of individual beds and the
smallemedium size of most phenocrysts recovered from
the bentonites suggest that ashes were derived from
a fairly distant source area, likely within or to the west
of the Sevier thrust belt. The source area is difficult to
determine due to post-depositional erosion; however
several late Campanian volcanic centers have been
identified. The most well known of these volcanic
centers is the Elkhorn Mountain Volcanics in central
Montana, considered to have produced one of the
largest ash flow tuff fields in the world (Smith, 1960;
Smedes, 1966). Campanian bentonites across the WIB,
from Alberta, Montana, Wyoming, Nebraska, and the
Dakotas have been linked to the Elkhorn Mountain
Volcanics (Spivey, 1940; Gill and Cobban, 1973;
Thomas et al., 1990; Rogers et al., 1993; Roberts and
Hendrix, 2000). The numerous interpretations for an
eastern source area in the Elkhorn Mountain Volcanics
suggest a dominantly eastward wind-transport direction
in the WIB during the Late Cretaceous. Other possible
contemporaneous sources have been identified to the
south in Arizona, eastern California, and northern
Mexico (Hayes, 1970; Drewes, 1978; Dickinson et al.,
1989).
4.
40
Ar/
39
Ar dating
We applied the
40
Ar/
39
Ar laser-fusion dating tech-
nique to four of the most phenocryst-rich bentonites
from the formation (KDR-5; KBC-109; KBC-144;
Fig. 2. Composite measured section of the Kaiparowits Formation in the Grand Staircase-Escalante National Monument. Location and
40
Ar/
39
Ar
ages of the four bentonites dated in this study are shown to the right.
309E.M. Roberts et al. / Cretaceous Research 26 (2005) 307e318
KBO-37), employing a tightly focused, continuous laser
beam to fuse individual sanidine phenocrysts for
isotopic age analysis. The lowest bentonite, KDR-5,
was collected from ca. 80 m above the base of the section
in the Death Ridge area, ca. 20 km south of Utah
Highway 12 (Figs. 1, 2;Table 1). The other three
bentonites were collected in the Blues area, along Utah
Highway 12 (Fig. 1), at ca. 420 m (KBC-109), ca. 490 m
(KBC-144), and ca. 790 m (KBO-37) above the base of
the formation (Figs. 1, 2;Table 1).
4.1. Methods
Approximately 20 kg of each bentonite were disag-
gregated using warm tap water and repeated agitation,
and then sieved through 20, 40, 60 and 100 mesh screens.
Feldspars were separated from the coarse fractions with
a Franz isodynamic separator, and sanidine was isolated
from plagioclase by density separations using heavy
liquids.
Sanidine crystal concentrates were loaded into wells
in an aluminum disk in preparation for irradiation. The
arrangement consisted of 12 wells (0.80$deep !0.130$
diameter) in a 0.260$diameter circle, with four stand-
ards at the cardinal points and unknowns in the
remaining positions. After additional protective pack-
aging, the samples were irradiated for 200 h in the Cd-
lined, in-core CLICIT facility of the Oregon State
University TRIGA reactor. Sanidine from the Fish
Canyon Tuff of Colorado was used as a mineral
standard, with a reference age of 28.02 Ma (Renne et al.,
1998).
40
Ar/
39
Ar extractions were performed at the Berkeley
Geochronology Center, using a focused CO
2
laser to
fuse and rapidly liberate trapped argon from individual
sanidine crystals. Gasses were scrubbed with SAES
getters for several minutes to remove impurities (CO,
CO
2
,N
2
,O
2
, and H
2
), followed immediately by
measurement of the purified Noble gasses for five argon
isotopes on a MAP 215-50 mass spectrometer for
approximately 30 min. From 10 to 40 grains were
analyzed per sample, the greatest number of analyses
being necessary where contamination with older feld-
spars was heaviest.
The neutron fluence parameter, J, appropriate to
each unknown was predicted from a planar, multiple-
regression model of standard Js against known disk
positions. A conservative, arbitrary uncertainty of 0.2%
is assigned to J, though residuals of the fit would suggest
0.1% or better. Weighted-mean ages of the Kaiparowits
sanidine populations were calculated after elimination
of analyses that failed one of multiple criteria: (1)
radiogenic
40
Ar content less than 96% of total
40
Ar; (2)
Ca/K ratio greater than one (indicating plagioclase); (3)
very low
39
Ar yield indicating incomplete fusion or non-
feldspar composition (i.e., quartz); (4) obviously too-old
age (O78 Ma); and (5) a final outlier detection scheme
based on deviation from the median age (1.4 normalized
median absolute deviations from the median). Weight-
ed-mean ages were calculated from the remaining 15e27
analyses per sample.
4.2. Results
Summary
40
Ar/
39
Ar analytical data for the four
bentonite samples are presented in Table 2 (full
analytical data to be made available in Roberts’ PhD
dissertation). Age-probability spectra are shown in
Fig. 3. All samples yield gaussian-like unimodal age
distributions. The most significant variable controlling
Table 1
Location and characteristics of dated bentonite horizons within the Kaiparowits Formation
Sample Location Stratigraphy Sedimentology
KDR-5 Death Ridge area; UTM 0434999/4156941
Death Ridge Quadrangle
w80 m above base of Kaiparowits
Formation
w50 cm thick; pale yellow-green; sharp
basal contact
KBC-109 Blues area; UTM 0424535/4165074 Upper
Valley Quadrangle
w420 m above base of Kaiparowits
Formation
w10 cm thick; olive green; wavy basal
contact
KBC-144 Blues area; UTM 0425382/4165351 Upper
Valley Quadrangle
w490 m above base of Kaiparowits
Formation
w30 cm thick; dark green; sharp basal
contact
KBO-33 Blues area; UTM 0424773/4167799 Upper
Valley Quadrangle
w790 m above base (w70 m below
top) of Kaiparowits Formation
w50 cm thick; pale yellow-green wavy
basal contact
Table 2
Summary of the analytical data for
40
Ar/
39
Ar analysis of four bentonites from the Kaiparowits Formation
Sample Lab
ID#
J
(!10
ÿ3
)1s
Ca/K
1s
39
Ar Mol
!10
ÿ14
40
Ar*/
39
Ar
1s
Age (Ma)
1s
(with GJ)
1s
MSWD Prob. n/n
total
KBC-109 22644 52.60 0.10 0.0124 0.0002 5.1 0.8072 0.0005 75.02 0.05 0.15 1.02 0.43 23/30
KBC-144 22647 52.81 0.10 0.01183 0.00016 6.1 0.8040 0.0004 75.02 0.04 0.15 0.94 0.55 27/34
KBO-37 22625 52.85 0.10 0.0050 0.0006 2.4 0.7946 0.0012 74.21 0.11 0.18 1.66 0.06 15/40
KDR-5 22646 52.74 0.10 0.01068 0.00004 38.3 0.8154 0.0003 75.96 0.02 0.14 0.36 1.00 21/23
310 E.M. Roberts et al. / Cretaceous Research 26 (2005) 307e318
the spread of the age population is predominately grain
size; improvement in analytical precision tracks the
increase in grain size from KBO-37 with the smallest
sanidine (!0.180 mm) to KDR-5 with the largest (0.55e
0.71 mm). Relative grain size is approximated by the
39
Ar abundance released during total fusion (Fig. 3;
Table 2) if potassium contents of these sanidines are
approximately equal (an assumption supported by the
narrow range of Ca/K ratios observed).
The stratigraphically lowest bentonite (KDR-5)
dated in this study yielded an age of 75.96 G0.14 Ma
(Fig. 3;Table 2). Within the middle unit, two bentonites
within 70 m of each other (KBC-109, KBC-144) gave
identical ages of 75.02 G0.15 Ma (Fig. 3;Table 2). The
highest bentonite (KBO-37), in the upper unit, yielded
an age of 74.21 G0.18 Ma (Fig. 2;Table 2).
5. Campanian/Maastrichtian boundary
Correlations of the middle Campanianeearly Maas-
trichtian in the WIB are presented based on a synthesis
of ammonite zones with radiometric determinations,
magnetostratigraphy, and North American land mam-
mal ‘‘ages’’ (Lillegraven and McKenna, 1986; Kennedy
et al., 1992; Obradovich, 1993; Gradstein et al., 1994).
The placement of the Campanian/Maastrichtian bound-
ary in the WIB of North America is subject to
considerable debate (e.g., Obradovich and Cobban,
1975; Bergstresser and Frerichs, 1982; Berggren et al.,
1985; Lillegraven and McKenna, 1986; Eaton, 1987;
Lillegraven, 1991; Kennedy et al., 1992; Obradovich,
1993; Gradstein et al., 1994). The magnetobiochrono-
logic Campanian/Maastrichtian boundary of ca.
71.3 Ma, accepted by Kennedy et al. (1992), Obradovich
(1993), Gradstein et al. (1994), and others is followed in
this study.
6. Age of Kaiparowits Formation
Calculation of the average rock accumulation rate for
strata bracketed between the basal sample, KDR-5, and
the highest sample, KBO-37, permits an estimate of the
total duration of the Kaiparowits Formation. These
samples bracket ca. 710 m of strata. By dividing this
Fig. 3. Age-probability spectra of single-crystal, total-fusion
40
Ar/
39
Ar analyses of sanidine from four Kaiparowits bentonites. The relative
probabilities displayed in the lower panel are generated by summing the assumed gaussian errors of the individual analyses for a given sample.
Dashed curves incorporate all analyses; solid curve represents age-probability distributions generated after trimming outliers from the data set.
Weighted-mean ages and 1sstandard error of the mean are displayed by the diamond and error bars associated with each distribution. The inner tics
on the error bar represent the uncertainty excluding the error in the neutron fluence parameter, J, while the outer tics incorporate a 0.2% error in J.
The middle panel displays ordered individual ages and 1serrors, while the upper panel shows a rough proxy for grain size, the total moles of
39
Ar
released upon fusion of a grain.
311E.M. Roberts et al. / Cretaceous Research 26 (2005) 307e318
thickness by the time duration (1.75 Ma) calculated
between samples KDR-5 and KBO-37, an average rock
accumulation rate of ca. 41 cm/ka is obtained. Rock
accumulation rates of 39 cm/ka and 42 cm/ka were
calculated, respectively, for the lower part of the
formation between KDR-5 and KBC-109/KBC-144,
and for the upper part of the formation, between KBC-
109/KBC-144 and KBO-37. The similarity between each
of these calculated rates demonstrates that sedimenta-
tion rates remained relatively constant throughout the
formation (Fig. 4).
Utilizing an average rock accumulation rate of
41 cm/ka, the ca. 860-m-thick Kaiparowits Formation
accumulated for ca. 2.1 Ma, from ca. 76.1e74.0 Ma. This
estimate demonstrates that the formation is late Campa-
nian in age, spanning the upper part of the Judithian
land mammal ‘‘age’’ (Lillegraven and McKenna, 1986).
7. Stratigraphic correlations
The
40
Ar/
39
Ar ages reported here are the first
radiometric dates for the Kaiparowits Formation. This
study permits high-resolution temporal correlation of
coeval strata and contemporaneous terrestrial vertebrate
faunas across the WIB. Chronstratigraphic, magneto-
stratigraphic, biostratigraphic and lithostratigraphic
data were utilized to construct the detailed correlations
presented here (Lillegraven and McKenna, 1986;
Obradovich, 1993; Gradstein et al., 1994). Radiometric
ages provide the most accurate framework for large-
scale regional correlations in terrestrial settings and
have been utilized in correlations wherever possible
(Goodwin and Deino, 1989; Eberth and Hamblin, 1993;
Rogers et al., 1993; Rogers, 1994; Fassett and Steiner,
1997; McDowell et al., 2004).
7.1. Regional correlations
The Kaiparowits Formation correlates with portions
of the well-studied Book Cliffs in central Utah.
Correlative strata (in the Book Cliffs) west of Green
River include the Upper Castlegate Sandstone, the
Bluecastle Tongue of the Castlegate Sandstone, and
the basal Price River Formation (Fouch et al., 1983;
Lawton, 1986; Willis, 2000)(Figs. 5, 6). To the east of
Green River, the Neslen Formation, the Bluecastle
Tongue of the Castlegate Sandstone, and the basal
Farrer Formation are correlative with the Kaiparowits
Formation (Fouch et al., 1983; Lawton, 1986; Willis,
2000)(Figs. 5, 6). Both the ‘‘beds on Tarantula Mesa’’,
in the Henry Basin of central Utah, and the upper
Ericson Sandstone in the Rock Springs Uplift of
southwestern Wyoming are also contemporaneous
(Peterson and Ryder, 1975; Eaton, 1990; Martinsen
et al., 1999)(Figs. 5, 6).
7.2. Faunal correlations
The Kaiparowits Formation was deposited during an
important period of Late Cretaceous dinosaur evolution,
during the zenith of dinosaur diversity (Sloan, 1976;
Dodson, 1983; Clemens, 1986; Dodson and Tatarinov,
1990). Many of the most fossiliferous, vertebrate-bearing
formations in the WIB are closely contemporaneous with
Fig. 4. Stratigraphic height above the base of formation vs. age of the
Kaiparowits Formation. A strong linear correlation (R
2
Z0.98)
suggests a roughly constant rock accumulation rate (ca. 41 cm/ka).
Fig. 5. Map of Utah showing location of major Late Cretaceous age
exposures in Utah and NW Wyoming. Symbols: A, Markagunt and
Paunsaugunt plateaus; B, Kaiparowits Plateau; C, Henry Mountains;
D, Book Cliffs, west of Green River; E, Book Cliffs, east of Green
River; F, Rock Springs Uplift. Black line through localities represents
the stratigraphic transect (AeF) presented in Fig. 6.
312 E.M. Roberts et al. / Cretaceous Research 26 (2005) 307e318
the Kaiparowits Formation. Principally, to the north, the
fossil-rich Dinosaur Park Formation and the most
fossiliferous, upper portions of the Judith River and
Two Medicine formations are coeval (Figs. 7, 8);
temporally constrained by multiple
40
Ar/
39
Ar ages
(Goodwin and Deino, 1989; Eberth and Hamblin, 1993;
Rogers et al., 1993; Rogers, 1994). To the southeast, the
Fruitland and lower Kirtland formations (Fassett and
Steiner, 1997) in New Mexico are partially correlative
with the Kaiparowits Formation (Figs. 7, 8). However,
the primary fossil bearing units of the Fruitland and
Kirtland formations are mostly younger and do not
correlate as well with fossil-bearing portions of the
Kaiparowits Formation, which are concentrated in the
middle and lower units (Hunt and Lucas, 1992; Roberts,
unpublished data) (Fig. 8). Further south, the upper shale
member of the Aguja Formation (Texas) is considered to
be partially correlative with the Kaiparowits Formation
based on biostratigraphic and chronostratigraphic data
from bounding units and correlative marine strata (Rowe
et al., 1992; Cifelli, 1994; Lehman, 1997; Sankey, 2001;
McDowell et al., 2004)(Figs. 7, 8). Dating of the Aguja
Formation is currently too poor to reveal the precise
temporal relationships of the Aguja and Kaiparowits
faunas.
8. Discussion
The Kaiparowits Formation contains one of the
richest and most diverse vertebrate faunas for all Late
Cretaceous terrestrial sequences in the WIB. The
formation occupies a relatively central position in the
basin, and thus provides a crucial faunal data set for
addressing recent hypotheses relating to Mesozoic
mammal biostratigraphy, and vertebrate evolution,
biodiversity, and provinciality (Lillegraven and McKen-
na, 1986; Horner et al., 1992; Lehman, 1997; Weil,
1999).
8.1. Mammal biostratigraphy
Dated bentonite horizons provide temporal con-
straints for this important fauna, and bracket mammal
localities in the Kaiparowits Formation, corroborating
the Judithian land mammal ‘‘age’’ reported by Eaton
Fig. 6. Correlation chart showing age relations of late Campanian strata in Utah and SW Wyoming (see transect line in Fig. 5). Time scale from
Gradstein et al. (1994). Western Interior ammonite zones from Obradovich (1993). The following sources of data are denoted by the numbers at the
top of the columns: 1, Lawton et al. (2003);2,Eaton (1991); 3, Roberts et al. (this study); 4, Eaton (1990);5,Fouch et al. (1983);6,Lawton (1986);7,
Willis (2000);8,Martinsen et al. (1999).
313E.M. Roberts et al. / Cretaceous Research 26 (2005) 307e318
and Ciffeli (1988), Cifelli (1990a,c) and Eaton (2002).
Eaton (2002) suggested that the Kaiparowits mammal
fauna may be slightly older than the type Judith River
Formation because of several taxa found in the
Kaiparowits Formation that are more closely related
to Aquilan than Judithian faunas. However, based on
the ages reported in this study, the Kaiparowits
Formation is shown to be entirely contemporaneous
with the upper portion of the Judith River Formation
(Fig. 8).
The ages reported here also have bearing on the
recently constructed Kirtlandian land vertebrate ‘‘age’’
of Sullivan and Lucas (2003). The Kirtlandian was
created in order to fill a reported ‘‘biochronologic gap’’
between the Judithian and ‘‘Edmontonian’’ land mam-
mal ‘‘ages’’ (74.9e72.0 Ma). It is characterized by the
vertebrate fossil assemblages of the Fruitland and
Kirtland formations, and is reported to correlate to
the lower part of the Bearpaw Formation in Montana
and Alberta and the Kaiparowits Formation (Sullivan
and Lucas, 2003). Based on the
40
Ar/
39
Ar ages reported
herein for the Kaiparowits Formation, and an evalua-
tion of the Kaiparowits fauna, a Judithian age
assignment is considered to be more parsimonious than
a Kirtlandian assignment.
Evidence in support of this includes the distribution
of diagnostic mammal taxa throughout the formation,
including the presence of Dakotamys magnus and
Gypsonictops lewisi, taxa considered to be unique to
the Judithian (Lillegraven and McKenna, 1986), from
high in the section (Fig. 2, ca. 640-m level; Eaton, 2002).
Additionally, the principle index fossil for the Kirtlan-
dian, Pentaceratops sternbergii, has not yet been
identified from the Kaiparowits Formation, while
multiple specimens of a new, undescribed ceratopsian
dinosaur occur within it (Getty et al., 2003; Smith et al.,
2004). Given the paleoenvironmental similarities and
close spatial relationships (!300 km apart) between the
two formations, some overlap in the distribution of
these two taxa is expected. The lack of apparent overlap
suggests that the Kirtland-Fruitland and Kaiparowits
faunas were probably not entirely contemporaneous.
Although less likely, the absence of both taxa from the
two formations could also be due to sampling bias or
paleoenvironmental differences. Several other taxa
considered as potential index fossils for the Kirtlandian
land vertebrate ‘‘age’’ have been identified in the
Kaiparowits Formation (e.g., Parasaurolophus,Krito-
saurus;Sullivan and Lucas, 2003); however, they were
recovered from low in the section (200e350 m level) and
Fig. 7. Map showing the paleogeographic relationships of Campanian strata (in black) in the Western Interior Basin. Symbols: A, Dinosaur
Provincial Park; B, Judith River Type Area; C, Two Medicine River; D, Kaiparowits Basin; E, San Juan Basin; F, Big Bend N.P. Modified from
Lehman (1997).
314 E.M. Roberts et al. / Cretaceous Research 26 (2005) 307e318
are bracketed by mammals and ash beds indicating
a Judithian age, no younger than 75 Ma.
A review of the distribution of major fossil localities
in the Kirtland-Fruitland and Kaiparowits formations
also reveals little temporal overlap between formations
(Fig. 8). Principle vertebrate localities of the Kirtland-
Fruitland formations (Hunter Wash and Willow Wash
local faunas) are located in the Fossil Forest, Hunter
Wash, and De-na-zin members (Hunt and Lucas, 1992),
dated between 74.5 and 73.0 Ma (Fassett and Steiner,
1997; Sullivan and Lucas, 2003). The majority of fossil
localities documented in the Kaiparowits Formation are
from the upper part of the lower unit and the middle
unit (Eaton and Cifelli, 1987; Eaton, 1991; Hutchison
et al., 1997; Roberts et al., 2003), ranging from ca. 75.9
to 74.8 Ma (see Fig. 4). The few fossil localities in the
upper unit of the Kaiparowits Formation contain
Dakotamys magnus and Gypsonictops lewisi (Cifelli,
1990c; Eaton, 2002), bracketing them as Judithian.
Thus, there appears to be limited evidence supporting
a Kirtlandian age assignment for the Kaiparowits
Formation. Additionally, the presence of well-docu-
mented Judithian mammals as high as 640 m in the
Kaiparowits Formation suggests that the upper bound-
ary of the Judithian land mammal ‘‘age’’ is no older
than ca. 74.6 Ma (Fig. 4).
9. Conclusions
This study was aimed at resolving the disputed age of
the Kaiparowits Formation (see Eaton, 1991).
40
Ar/
39
Ar
dating of four bentonite horizons distributed through-
out the 860-m-thick section produces the first absolute
ages for the formation. New data indicate a late
Campanian (Judithian) age, between ca. 76.1 and
74.0 Ma. These findings are consistent with the mammal
biostratigraphy of Eaton (2002), and provide geochro-
nologic evidence supporting an upper bracket, no older
than ca. 74.6 Ma, for the Judithian land mammal ‘‘age’’.
Fig. 8. Correlation chart showing the age relations of important late Campanian (Judithian) dinosaur-bearing formations in the Western Interior
Basin. Time scale from Gradstein et al. (1994); Western Interior ammonite zones from Obradovich (1993). The following sources of data are denoted
by the numbers at the top of the columns: 1, Eberth and Hamblin (1993); 2, D. Eberth (pers. comm. 2004); 3, Goodwin and Deino (1989);4,Rogers
et al. (1993);5,Rogers (1994);6,Rogers (1998);7,Rogers and Kidwell (2000);8,Eaton (1991); 9, Roberts et al. (this study); 10, Hunt and Lucas
(1992); 11, Fassett and Steiner (1997); 12, Sullivan and Lucas (2003); 13, Lehman (1997); 14, Rowe et al. (1992); 15, Sankey (2001); 16, McDowell
et al. (2004).
315E.M. Roberts et al. / Cretaceous Research 26 (2005) 307e318
The results of this study allow for improved
correlation of contemporaneous strata and vertebrate
faunas in Utah and the WIB, and lay the groundwork
for future geochronologic refinement of strata in the
Kaiparowits Basin. Rapid deposition (ca. 41 cm/ka) and
distribution of dated ash beds throughout the richly
fossiliferous Kaiparowits Formation permits the evalu-
ation of recent hypotheses relating to the diversity,
latitudinal distribution, and evolution of Late Creta-
ceous vertebrate faunas in the WIB (e.g., Horner et al.,
1992; Lehman, 1997; Weil, 1999).
Bentonites documented in this study are also
recognized as laterally extensive marker beds, providing
a means of correlating stratigraphic sections and fossil
localities throughout this important and otherwise
monotonous nonmarine record. This permits correla-
tion of sections and fossil localities across the O800 km
2
of outcrop area. In addition, the rapid accumulation of
strata, coupled with preliminary magnetostratigraphic
investigations by Imhof and Albright (2003), highlight
the potential for high-resolution magnetostratigraphy in
the formation, particularly within the upper unit.
Acknowledgements
Fieldwork and funding for this project were support-
ed by the BLM-Grand Staircase Escalante National
Monument and a grant from the Monument to EMR
and MAC. We thank Doug Powell for facilitating this
research and for his help in obtaining collection and
research permits. Additional funding awarded to EMR
by the AAPG, the Rocky Mountain Section of the
SEPM, and RMAG also supported this research. We
thank Leif Tapanila and Ray Rogers for constructive
discussions that contributed to the development of this
paper. This manuscript was also improved by the
comments provided by two anonymous reviewers.
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