3
S.Lynn Peyton, Barbara Carrapa, 2013, An overview of low-temperature
thermochronology in the Rocky Mountain and Its application to petroleum
system analysis, in C. Knight and J. Cuzella, eds., Application of structural
methods to Rocky Mountain hydrocarbon exploration and development:
AAPG Studies in Geology 65, p. 37–70.
An Overview of Low-temperature
Thermochronology in the Rocky
Mountains and Its Application to
Petroleum System Analysis
S. Lynn Peyton
Coal Creek Resources Inc., 1590 S. Arbutus Pl., Lakewood, Colorado, 80228, U.S.A.
(e-mail: slpeyton@coalcreekresources.com)
Barbara Carrapa
Department of Geosciences, University of Arizona, 1040 E. 4th St., Tucson, Arizona, 85721, U.S.A.
(e-mail: bcarrapa@email.arizona.edu)
ABSTRACT
A synthesis of low-temperature thermochronologic results throughout the Laramide foreland illustrates that samples from wellbores in Laramide basins record either (1) detrital Laramide or older cooling ages in the upper ~1 km (0.62 mi) of the wellbore, with younger ages at
greater depths as temperatures increase; or (2) Neogene cooling ages. Surface samples from
Laramide ranges typically record either Laramide or older cooling ages. It is apparent that
for any particular area the complexity of the cooling history, and hence the tectonic history
interpreted from the cooling history, increases as the number of studies or the area covered by
a study increases. Most Laramide ranges probably experienced a complex tectono-thermal
evolution. Deriving a regional timing sequence for the evolution of the Laramide basins
and ranges is still elusive, although a compilation of low-temperature thermochronology
data from ranges in the Laramide foreland suggests a younging of the ranges to the south
and southwest.
Studies of subsurface samples from Laramide basins have, in some cases, been integrated
with and used to constrain results from basin burial -history modeling. Current exploration
for unconventional shale-oil or shale-gas plays in the Rocky Mountains has renewed interest in thermal and burial history modeling as an aid in evaluating thermal maturity and understanding petroleum systems. We suggest that low-temperature thermochronometers are
underutilized tools that can provide additional constraints to burial -history modeling and
source rock evaluation in the Rocky Mountain region.
Copyright ©2013 by The American Association of Petroleum Geologists.
DOI:10.1306/13381689St653578
37
10711_ch03_ptg01_hr_037-070.indd 37
6/5/13 7:59 AM
38 Peyton and Carrapa
INTRODUCTION
Low-temperature thermochronologic techniques have
many uses relevant to petroleum exploration. Ageelevation profiles can provide the timing and rate
of cooling; inverse and forward modeling of thermochronometer ages can help to constrain the thermal history of an area; ages can be used to constrain
basin models; the base of a partial annealing zone
(PAZ) or partial retention zone (PRZ) can be used as
a structural marker for mapping; and detrital thermochronology can illuminate sediment provenance and
unroofing history. These techniques and methods are
discussed in detail in Chapter 2. We strongly recommend that readers unfamiliar with thermochronologic
techniques and terminology read Chapter 2 (Peyton
and Carrapa, 2013).
In the last decade, industry focus on unconventional
petroleum plays has led to the emergence of shale
formations such as the Niobrara Formation, Mowry
Shale, Mancos Shale, and Bakken Formation, to name a
few, as important exploration targets within the Rocky
Mountain region. Exploration success in these shale
plays is often dependent upon a clear ­understanding of
source-rock maturation and thermal history. Vitrinitereflectance (%Ro) and Rock-Eval pyrolysis studies have
typically been used to evaluate the thermal maturity
of source rocks, and used as input into basin modeling in the Rocky Mountain region (e.g., Hagen and
Surdam, 1984; Dickinson, 1986; Nuccio, 1994a; Roberts
et al., 2008). We suggest that low-temperature thermochronology should be used to provide additional
constraints on the thermal evolution of these source
rocks because it has the potential to provide information on the maximum temperature reached, the time
when that maximum temperature was reached, and
the timing, rate, and amount of cooling. Note however
that source rocks are probably too fine grained to yield
usable apatite or zircon, and samples should be taken
from coarser grained rocks in the stratigraphic section.
To date, a few papers have examined the thermal
and petroleum history of the Sevier fold-thrust belt
using fission-track thermochronology (Burtner and
Nigrini, 1994; Osadetz et al., 2004), but there are no
published studies that comprehensively document the
use of low-temperature thermochronology for petroleum exploration in the Laramide foreland (Figure 1)
or that integrate results with burial-history modeling
and other thermal indicators (such as %Ro and fluid
inclusions) to understand the thermal evolution and
maturity of resource plays in this area. Thus, lowtemperature thermochronology techniques, such as
apatite fission-track dating (AFT), zircon fission-track
dating (ZFT), apatite (U-Th)/He dating (AHe), and
zircon (U-Th)/He dating (ZHe), are underutilized
10711_ch03_ptg01_hr_037-070.indd 38
tools in the exploration for oil and gas in the region.
AFT and AHe dating have been used elsewhere
throughout the world as tools for petroleum exploration, and in particular for basin modeling studies
(e.g., Duddy et al., 1991; Duddy et al., 1994; Duddy
et al., 1998; Crowhurst et al., 2002; Osadetz et al., 2002;
Osadetz et al., 2004; Parnell et al., 2005).
Although AHe and ZHe dating have been successfully used to elucidate shallow crustal processes
in many parts of the North American cordillera (e.g.,
House et al., 1997; Stockli et al., 2000; Cecil et al.,
2006; Flowers et al., 2007; Flowers et al., 2008), there
are only two published studies within the Laramide
foreland (Figure 1) (Crowley et al., 2002; Peyton et al.,
2012). In contrast, there have been many AFT and
ZFT studies in the Laramide foreland, because the
earlier development of these techniques resulted
in their widespread use (Figure 1) (Giegengack et
al., 1986; Naeser, 1986; Shuster, 1986; Naeser, 1989;
­C erveny, 1990; Kelley and Blackwell, 1990; Naeser,
1992; ­C erveny and Steidtmann, 1993; Omar et al.,
1994; Chen and Lubin, 1997; Kelley and Chapin, 1997;
Pazzaglia and Kelley, 1998; Kelley, 2002; ­Naeser et al.,
2002; ­Kelley and Chapin, 2004; Kelley, 2005).
The goals of this chapter are to review and summarize all low-temperature thermochronology studies
of the Laramide Rocky Mountain region to date, and
then to illustrate how these techniques may be applied
to petroleum exploration using examples from elsewhere. After a brief overview of the geologic history
of the Rocky Mountain region, we review the vitrinitereflectance technique and discuss the reliability and relevance of old thermochronologic data. We divide our
summary of low-temperature thermochronologic work
in the Laramide Rocky Mountains into two sections.
First, we look at low-temperature thermochronologic
studies of the Laramide basins, which date subsurface
samples from petroleum wellbores (Figure 1). Most of
these samples reached maximum burial after the Laramide Orogeny but before Neogene exhumation, and
provide information about Neogene cooling and basin exhumation. Using AFT ages from the Green River
­Basin (Naeser, 1986, 1989; Naeser et al., 1989b) we illustrate how forward modeling of AFT ages and vitrinite
reflectance data can be used to evaluate published results and interpretations. Second, we discuss studies
that have dated samples from the Laramide ranges
(Figure 1). Samples are largely from the Precambrian
crystalline cores of the ranges, which probably reached
maximum burial during the Cretaceous, followed by
exhumation and cooling during the Laramide Orogeny, resulting in Late Cretaceous and Paleogene rather
than Neogene cooling ages. The high topographic relief
of many of the ranges led workers to analyze samples
from elevation transects. In areas with less topographic
6/5/13 7:59 AM
An Overview of Low-temperature 39
112
108
Crazy
Mountains
Basin
104
SOUTH DAKOTA
d
BT a a
TR
BH
a,b
e
OC
Wind River
a
Basin
a WR a,c,l,p
g
f
c
front
LR
Greater Green
River Basin
thru
st
NEBRASKA
a
GM
HU
c
a
c
c,k
LARAMIDE ROCKY
MOUNTAINS
UM
MB
PR
COLORADO
c,k
c
m w
Denver
Basin
Uinta Basin
r
n
GR
WRU
Piceance
Basin
r
q
r FR
q,r,s,t,u
SR
112
WM
AHe subsurface samples
SC
AFT subsurface samples
Precambrian crystalline
basement
TA
NEW MEXICO
t
COLORADO
PLATEAU
112
t,u
v
AFT surface samples
100
s,u
o
UU
AHe surface samples
0
BL
i
c
c,l,p
c,h
Powder
River Basin
g
40
40
Bighorn
Basin
44
Peyton et al. (2012)
Crowley et al. (2002)
Cerveny (1990)
Omar et al. (1994)
Roberts & Burbank (1993)
Naeser (1986,1989);
Naeser et al. (1989)
g: Beland (2002)
h: Strecker (1996)
i: Naeser (1992)
k: Kelley (2005)
l: Cerveny & Steidtmann (1993)
m: Painter (p.c., 2012)
IDAHO
n: Kelley & Blackwell (1990)
UTAH
o: Kelley (2002)
p: Shuster, 1986
q: Bryant & Naeser (1980)
r: Naeser et al. (2002)
s: Kelley & Chapin (1997)
Sevier
t: Pazzaglia & Kelley (1998)
u: Kelley & Chapin (2004)
v: Lindsey et al. (1986)
w: Constenius & Kelley
(p.c. 2012)
WYOMING
4
44
4
44
LP
a:
b:
c:
d:
e:
f:
MONTANA
a,c
a
c,d
CB
MR
t
N
SN
200 km
ARIZONA
108
104
Figure 1. Regional map showing approximate locations of AFT (red) and AHe (blue) studies, along with basins and ranges
discussed in this chapter. See individual papers for more specific sample location information. Gray shading depicts region of
Sevier foreland basin deformed during the Laramide Orogeny, referred to as the Laramide Rocky Mountains. Stippled areas
show crystalline basement outcrop. Thick dashed line is outline of the Colorado Plateau. Thin dashed line represents the
Cheyenne Belt. Abbreviations as follows: BL, Black Hills; BH, Bighorn Range; BT, Beartooth Range; CB, Cheyenne Belt; FR,
Front Range; GM, Granite Mountains; GR, Gore Range; HU, Hartville Uplift; LP, Lima Peaks; LR, Laramie Range; MB, Medicine
Bow Mountains; MR, Madison Range; OC, Owl Creek Mountains; PR, Park Range; SC, Sangre de Cristo Range; SN, Sierra
Nacimiento; SR, Sawatch Range; TA, Taos Range; TR, Teton Range; UM, Uinta Mountains; UU, Uncompahgre Uplift; WM, Wet
Mountains; WR, Wind River Range; WRU, White River Uplift; p.c., personal communication. 100 km (62 mi).
10711_ch03_ptg01_hr_037-070.indd 39
6/5/13 7:59 AM
40 Peyton and Carrapa
relief, studies have used a wide areal distribution of
samples to interpret the structure of the Precambrian
basement.
The Colorado Plateau (Figure 1) also experienced
deformation during the Laramide Orogeny, resulting
in a series of monoclines and flexures with much less
structural relief than the foreland ranges. Although
the Colorado Plateau has been studied using both AFT
and AHe dating (Naeser et al., 1989a; Kelley et al.,
2001; Naeser et al., 2001; Flowers et al., 2007; Flowers
et al., 2008), this chapter focuses on results from the
Laramide foreland (Figure 1).
Finally, we discuss the workflow used by many
workers for integrating low-temperature thermochronology results into petroleum studies, and summarize some studies from other parts of western
North America, such as the Canadian thrust belt
(Osadetz et al., 2004), and the Sevier thrust belt of
Idaho and Wyoming (Burtner and Nigrini, 1994).
BACKGROUND
Geologic History
During the Late Jurassic, subduction along the western margin of North America consolidated into a
continuous orogen and magmatic arc, with deformation propagating eastward to form the Sevier retroarc
thrust belt and its associated foreland ­basin (DeCelles,
2004). In the United States section of the orogen, Late
Cretaceous flattening of the subducting Farallon slab
caused a cratonward sweep of magmatism coeval
with propagation of deformation into the Sevier foreland basin (Coney and ­R eynolds, 1977; Dickinson
and Snyder, 1978; Constenius, 1996; Saleeby, 2003).
Although thin-skinned thrusting continued in the
Sevier fold-thrust belt, deformation in the foreland,
known as the Laramide Orogeny, had a thick-skinned,
basement-involved style. These basement-involved
thrusts dissected the foreland basin into a series of
smaller basins and ranges bound by an anastamosing system of faults (Brown, 1988; Erslev, 1993),
herein referred to as the Laramide Rocky Mountains
(Figure 1). This thick-skinned deformation persisted
until Eocene time (as did Sevier thrusting), when
the subducting slab rolled back and magmatism migrated westward (Coney and Reynolds, 1977; Dickinson and Snyder, 1978; Constenius, 1996). Shortening
in each Laramide range was on the order of several
kilometers, and was accommodated on major rangebounding thrusts that dip ~30° beneath the ranges
and extend at least into the midcrust (Smithson et al.,
1978). ­Total shortening across the foreland was ~45 km
10711_ch03_ptg01_hr_037-070.indd 40
(~27.9 mi), or ~12–15% (Brown, 1988; Stone, 1993). In
comparison, shortening across the ­U tah-Wyoming
part of the Sevier thrust belt was at least 100 km
(62.1 mi), or 50% (Royse et al., 1975; Coogan, 1992;
Yonkee, 1992). Foreland basin ­sedimentary rocks were
eroded from the uplifting basement blocks during the
Laramide Orogeny and were deposited in adjacent
basins (Dickinson et al., 1988). Basin-bounding, proximal conglomerates represent the erosion of more resistant Paleozoic rocks from the ranges (e.g., DeCelles
et al., 1987; Perry, Jr. et al., 1988; DeCelles et al., 1991;
Hoy and Ridgway, 1998).
Dickinson et al. (1988) studied Laramide ­b asin
fill and proposed nearly-synchronous onset of
­L aramide deformation (~75 to 65 Ma) across the
foreland that terminated earlier in the northern
foreland (~50 Ma) than the southern (~35 Ma) foreland. Gries (1983) suggested that north-south trending Laramide structures become younger from west
(Campanian) to east (Paleocene), that northwest
trending structures are Paleocene in age, and that
east-west trending structures formed during Eocene
time, and supported her hypothesis with ages from
basin fill. DeCelles (2004) recognized an eastward
sweep of deformation beginning in Campanian
time and ending in Eocene time, based on proximal,
syntectonic sedimentary rocks associated with several Laramide uplifts, and suggested the Laramide
­region may simply be the frontal part of the Cordilleran orogenic wedge.
Post-Laramide burial is evidenced by remnants
of Oligocene through Pliocene sedimentary rocks
that are preserved at high elevations in some ranges.
Rocks of early Miocene age that crop out on the crest
of the Bighorn Range at ~3 km (~1.86 mi) elevation
led McKenna and Love (1972) to reconstruct cross
sections of Eocene, Oligocene, and Miocene basin
fill across the Bighorn and Powder River basins.
These cross sections indicated that the thickness
of sedimentary rocks in the basins during Miocene
time may have been more than 1 km (0.6 mi) greater
than present. Love (1960) stated that by the end of
Oligocene time only the upper ~0.3 to 1.3 km (~0.1
to 0.8 mi) of the highest peaks in the major Laramide ranges were exposed above the aggradational
plain. These high-elevation outcrops of Oligocene
and ­M iocene sedimentary rocks suggest that after
the Laramide Orogeny, the Rocky Mountains were
buried to high levels and have since been exhumed
(Mackin, 1947; Love, 1960; McKenna and Love,
1972; Gregory and Chase, 1994; McMillan et al.,
2006; Riihimaki et al., 2007). The cause, timing, and
amount of Cenozoic burial and exhumation are still
controversial.
6/5/13 7:59 AM
An Overview of Low-temperature 41
Overview of Vitrinite Reflectance (%Ro)
Vitrinite is an organic component of coal. Vitrinite
reflectance (%Ro) is the percentage of incident light
that is reflected off a polished particle of vitrinite, and
is dependent upon the maximum paleotemperature
of the sample and the time spent at that temperature
(e.g., Senftle and Landis, 1991). %Ro tends to increase
with increasing burial in a sedimentary basin and is
commonly used to evaluate organic maturity in sedimentary rocks (e.g., Tissot and Welte, 1978). Burnham
and Sweeney (1989) and Sweeney and Burnham (1990)
presented a kinetic model for the evolution of %R o
with temperature and time and showed that temperature has a larger influence on %Ro than time. Duddy
et al. (1991) showed that the annealing kinetics of fission tracks in apatite (Laslett et al., 1987) are similar to
the kinetics of %Ro (Burnham and Sweeney, 1989) and
that a given degree of annealing in apatite will always
be associated with the same value of %Ro. For example,
a %Ro value of 0.7 is associated with total annealing of
all fission tracks in Durango fluorapatite (Duddy et al.,
1994). Uncertainties associated with measured %Ro
values are typically about ±0.1% in shale, and ±0.05%
in coal (B. Claxton, personal communication, 2012).
AFT and %Ro techniques complement each other because %Ro values can be used to determine maximum
paleotemperatures when temperatures were greater
than the annealing temperature of fission tracks, while
AFT data provide information on the timing and rate
of cooling from the maximum paleotemperature that
%Ro cannot. Both techniques provide information on
the thermal history of a sample.
Evaluation of Existing Thermochronologic Studies
Many of the studies discussed in this chapter are more
than 20 years old, over which time thermochronologic
techniques have evolved significantly. Early fissiontrack researchers did not measure track lengths or
recognize the influence of kinetic parameters such
as apatite composition on annealing properties (e.g.,
Bryant and Naeser, 1980). For several years researchers used electron microprobes to analyze apatite composition for a few grains from a few samples, and
applied these results to all samples in the study (e.g.,
Kelley and Blackwell, 1990). The diameter of a fissiontrack etch pit (Dpar) was recognized as an indicator
of annealing kinetics by Donelick (1993). Some studies estimated Dpar qualitatively, rejecting grains with
wide etch-pit diameters (e.g., Kelley, 2005), but now
Dpar is measured routinely and used to determine annealing kinetics. Apatite which anneals rapidly (e.g.,
10711_ch03_ptg01_hr_037-070.indd 41
fluorapatite) typically has Dpar values <~2 mm. Higher
Dpar values are associated with annealing-resistant apatite (e.g., chlorapatite).
Early AFT papers did not report details of the methodology used to etch the fission tracks. The concentration and type of acid, and the time allowed for etching,
may be quite variable and influence the number of fission tracks visible for counting (Murrell et al., 2009). In
recent years, etching has tended to follow a standard
recipe (20 s with 5.5 M nitric acid at 21°C). Older studies did not correct track lengths for their orientation
with respect to the crystallographic c-axis, making
track-length measurements less reliable.
Although AHe dating is a much newer technique, it
has also experienced a rapid evolution in recent years.
The recognition that grain size and radiation damage
can affect AHe ages (Reiners and Farley, 2001; Shuster
et al., 2006; Flowers et al., 2009) has led to different
approaches to interpret AHe ages (e.g., Flowers et al.,
2008). However, unless there were problems with the
laboratory, the fundamental measurements of parent
and daughter nuclides, and the resulting AHe age,
are likely sound. The biggest issue with older studies is that very few grains or aliquots (typically two)
were measured per sample, making it difficult to
recognize the influence of radiation damage or grain
size. In ­areas where radiation damage is suspected to
be a problem, such as cratonic basement, at least several grains or aliquots should be dated per sample.
If He implantation is suspected, the only recourse at
present is to discard the aliquot or the entire sample.
We anticipate that in the future this will be mitigated
by routinely abrading grains (either mechanically or
chemically) to remove the outer part of the crystal
­affected by He ­implantation before dating.
In summary, because AFT dating techniques have
developed over the last 30+ years, AFT ages from
more recent studies can be considered to be more
accurate and reliable than those from older studies.
With AHe dating, measurement techniques have not
changed significantly, but our understanding of how
to interpret AHe ages has evolved. All of the studies discussed in this chapter, along with the methods
each study used, have been summarized in Table 1
to allow the reader to evaluate the relevance of the
­results from each paper. We have also included a
summary of each paper’s conclusions, and our opinion of the data quality (Table 1). We emphasize that
the authors were using the techniques available to
them at the time, and that our comments in no way
reflect on the quality of their work. The improvement
in the reliability and accuracy of AFT ages through
time results from advancements in the ­understanding
of the technique.
6/5/13 7:59 AM
42 Peyton and Carrapa
Table 1. Summary of previous low-temperature thermochronologic studies in the Laramide Rocky Mountain region.
Study
Laramide Basins
Beland (2002)
Method
Location
Sample
Type
Lithologies
AFT Track
Lengths?
AFT Track Lengths
Corrected to
c-axis?
AFT,
AHe
Casper Arch thrust
and Maverick Springs
thrust, Green River
Basin, Wyoming
cuttings
crystalline
basement,
sedimentary
rocks
Yes
Measured parallel
to c-axis
Chen and Lubin
(1997)
ZFT
Bighorn Basin,
Wyoming
surface
sedimentary
rocks
NA
NA
Kelley (2002)
AFT,
ZFT
Denver Basin,
Colorado
core
sedimentary
rocks
Not
documented
Not documented
Kelley and
Blackwell (1990)
AFT,
ZFT
Piceance Basin,
Colorado
core
sandstone
Yes
No
Naeser (1986,
1989), Naeser
et al. (1989)
AFT,
ZFT
Green River Basin,
Wyoming
core
sandstone
No - due to
low track
density and
low apatite
yield
NA
Naeser (1992)
AFT
Powder River Basin,
Wyoming
core
sandstone
Yes
Measured parallel
to c-axis
10711_ch03_ptg01_hr_037-070.indd 42
6/5/13 7:59 AM
An Overview of Low-temperature 43
AFT Annealing
Properties:
Composition or
Dpar?
AFT Etching
Technique (for
apatite to be
dated)
AHe
Max #
Aliquots
Forward
or Inverse
Modeling?
Summary/Conclusions
Comments
electron
microprobe, four
samples
Not
documented
14
(single
& multicrystal)
Forward
modeling
Miocene AFT and AHe ages
shallow, bimodal AFT and AHe
ages deep (>2.5 km [>1.5 mi]).
Burial by thrusting during the
late Miocene?
Results are probably
reliable but are
inconsistent with other
studies and difficult to
explain geologically. More
work needed in these
areas.
NA
NA
NA
No
ZFT ages not reset by burial.
Stratigraphic ages refined using
ZFT ages.
Insufficient number of
grains dated per sample
for a thorough provenance
study (maximum of 14).
No
25 s in 5 M
nitric acid
NA
No
Used Gaussian peak-fitting
methods to distinguish age
populations. ZFT and AFT
ages document a predictable
unroofing sequence for the
Front Range.
AFT and ZFT ages not
documented in the paper.
Don’t know if sufficient
apatite grains were
measured for a detrital
study. Apatite annealing
kinetics not discussed.
electron
microprobe, 4
samples
20 s in 5 M
nitric acid
NA
inverse
Rapid cooling during last
10 Ma due to downcutting of
Colorado River. Background
heat flow between 10 Ma and
present was similar to today.
Maximum paleotemperatures
of samples < 240 °C.
Counted a max of 20
grains/sample - modern
studies would count
more grains. Ages and
conclusions probably
reliable, but only
measured composition on
four samples.
No
Not
documented
NA
No
Latest phase of rapid cooling of
20°C or more was initiated ~4
to 2 Ma. No annealing of ZFT
since deposition.
Ages from this study
should be used with
caution. It is difficult to
interpret ages with no
track lengths or kinetic
data. Conclusions are
tested in this paper using
forward modeling.
Discarded
apatite with
anomalouslywide etch pits
Not
documented
NA
Yes. No
details
provided
Samples cooled from above
annealing temp. at ~12 Ma.
AFT data indicate higher
paleotemperatures than %Ro
data. Discrepancy may be due
to fluid flow.
Ages probably reliable for
fluorapatite. Present-day
temperatures estimated
from a geothermal
gradient and probably not
accurate. Cooling at 12 Ma
determined from plotting
ages from a large area
(~90 3 40 km [~55.9 3
24.85 mi]) on the same
age-elevation profile.
(continues)
10711_ch03_ptg01_hr_037-070.indd 43
6/5/13 7:59 AM
44 Peyton and Carrapa
(continued)
Study
Method
Location
Sample
Type
Lithologies
AFT Track
Lengths?
AFT Track Lengths
Corrected to
c-axis?
Laramide Ranges
Bryant and Naeser
(1982)
AFT
Front Range, Sawatch
Range, Colorado
surface
crystalline
basement
No
NA
Cerveny (1990)
AFT,
ZFT
Multiple ranges—
Wyoming, Colorado,
South Dakota and
southern Montana
surface,
core
crystalline
basement
Yes
Measured parallel
to c-axis
Cerveny and
Steidtmann (1993)
AFT,
ZFT
Wind River Range,
Wyoming
surface,
core
crystalline
basement
Yes
Measured parallel
to c-axis
Kelley and
Chapin (1997)
AFT
Front Range,
Colorado
surface
crystalline
basement
Yes
Not documented
Kelley and
Chapin (2004)
AFT
Front Range,
Colorado
surface
crystalline
basement
Yes
Yes
Kelley (2005)
AFT
Laramie, Park,
Sierra Madre and
Medicine Bow
Ranges, Wyoming
and Colorado
surface
crystalline
basement
Yes
Yes
Naeser et al.
(2002)
AFT,
ZFT
White River Uplift,
Gore Range, Front
Range, Colorado
surface
crystalline
basement,
sandstone
Yes
Not documented
Omar et al. (1994),
Giegengack et al.
(1986)
AFT
Beartooth Range,
Wyoming and
Montana
surface,
cuttings
crystalline
basement,
sedimentary
rocks
Yes
No
10711_ch03_ptg01_hr_037-070.indd 44
6/5/13 7:59 AM
An Overview of Low-temperature 45
AFT Annealing
Properties:
Composition or
Dpar?
AFT Etching
Technique (for
apatite to be
dated)
AHe
Max #
Aliquots
Forward
or Inverse
Modeling?
Summary/Conclusions
Comments
No
Not
documented
NA
No
Using base of fossil PAZ,
determined ~6.5 km (~4 mi) of
uplift of Front Range between
Cretaceous and present day.
AFT ages from Sawatch Range
suggest Eocene uplift during
Laramide Orogeny, and
Miocene uplift along eastern
edge of range due to Rio
Grande rifting.
Ages should be used with
caution. No track length or
kinetic data.
No
30–45 s in 7%
nitric acid
NA
No
Broad study covering many
ranges. Wind River Range
has complex cooling history.
Cooling began in Wind River
Range by ~85 Ma. Most
rapid cooling in Wind River,
Beartooth and Park Ranges
between ~62 and 57 Ma.
Cooling ~40 Ma at toe of Wind
River thrust.
Ages probably reliable, but
no measure of annealing
properties.
No
30-45 s in 7%
nitric acid
NA
See summary above for
Cerveny (1990)
Ages probably reliable, but
no measure of annealing
properties.
Not documented
Not
documented
NA
No
Mapped base of PAZ. More
exhumation in center of Front
Range than at edges.
No table with ages and
track length data. Many
details of methodology not
documented.
Electron
microprobe,
6 samples
25 s in 5 M
nitric acid
NA
inverse
AFT ages used to map structure
of Precambrian basement
and amount of denudation.
Laramide cooling 75-45 Ma.
Reliable ages.
Qualitative Dpar
only: unusually
large etch pits
not observed
25 s in 5 M
nitric acid
NA
No
AFT ages used to map structure
of Precambrian basement
and amount of denudation.
Proterozoic structures
reactivated by Laramide
tectonics in some areas, but not
others.
Ages probably reliable, but
no measure of annealing
properties.
Dpar and
microprobe
40 s in 7% nitric
acid at 24 °C
NA
Yes.
Inverse?
ZFT ages have not been reset
since the Proterozoic. AFT ages
from the central and eastern
Front Range and White River
Uplift record Laramide cooling.
The Gore Range and western
Front Range record ~30 Ma
cooling related to the Rio
Grande Rift.
Ages probably reliable,
but orientation of tracks to
c-axis not documented.
No
Not
documented
NA
inverse
No annealing kinetic data.
Rapid uplift of ~4 to 8 km (~2.4
to 4.9 mi) began ~61 Ma. Second Track lengths not corrected
for orientation.
episode of ~4 km (~2.4 mi) of
uplift began between 15 and 5 Ma.
(continues)
10711_ch03_ptg01_hr_037-070.indd 45
6/5/13 7:59 AM
46 Peyton and Carrapa
(continued)
Study
Method
Location
Sample
Type
Lithologies
AFT Track
Lengths?
AFT Track Lengths
Corrected to
c-axis?
Pazzaglia and
Kelley (1998)
AFT
Southern Rocky
Mountains, Colorado
and New Mexico
surface
crystalline
basement
—
see comments
Peyton et al. (2013)
AHe,
AFT
Beartooth, Wind
River, Laramie and
Beartooth Ranges,
Wyoming and
Montana
surface,
cuttings
crystalline
basement
Yes
Yes
Roberts and
Burbank (1993)
AFT
Teton Range,
Wyoming
surface
crystalline
basement
Yes
No
Shuster (1986)
AFT
Wind River Range,
Green River Basin
surface,
core
crystalline
basement
No
NA
Steidtmann et al.
(1989)
AFT
Wind River Range,
Wyoming
surface
crystalline
basement
—
see comments
Strecker (1996)
AFT,
ZFT
Black Hills, South
Dakota
surface
crystalline
basement
Yes
No?
Crowley et al.
(2002)
AHe
Bighorn Range,
Montana and
Wyoming
surface
crystalline
basement
NA
NA
10711_ch03_ptg01_hr_037-070.indd 46
6/5/13 7:59 AM
An Overview of Low-temperature 47
AFT Annealing
Properties:
Composition or
Dpar?
AFT Etching
Technique (for
apatite to be
dated)
AHe
Max #
Aliquots
Forward
or Inverse
Modeling?
—
—
NA
Dpar
20 s in 5.5 M
nitric acid at
21°C
No
Summary/Conclusions
Comments
No
Integrated geomorphology
and AFT data to test different
models for evolution of
topography. Data support
significant Laramide crustal
thickening followed by low
rates of exhumation and uplift.
Used a compilation of
previously-published AFT
data.
10
(single &
multicrystal)
forward
and inverse
Radiation damage affecting
AHe ages. Inverted AHe age effective U pairs to find best-fit
thermal history. Exhumation
likely started earlier in the
Bighorn Range (before ~71 Ma)
than in the Beartooth Range
(before ~58 Ma).
He implantation
affecting some AHe
ages. Important to date
a sufficient number of
apatite grains to recognize
radiation damage and He
implantation. Authors did
not consider temperature
change due to elevation or
climate change.
Not
documented
NA
No
Most AFT ages record
Laramide cooling between 85
and 65 Ma. Second episode of
exhumation during Miocene.
Less exhumation in south
relative to north.
No annealing kinetic
data. Tracks lengths not
corrected for orientation.
No
Not
documented
NA
No
Uplift of Wind River Range
began no later than 78 Ma and
continued through late Eocene.
Ages should be used with
caution. No track length
or kinetic data. Cerveny
(1990) dated the same
samples and measured
track lengths.
—
—
NA
No
Recognized differential
exhumation of fault blocks
from core of Wind River Range
using AFT data.
Used data from Shuster
(1996) - see comments
above.
No. Recognized
that granitic
samples had
lower AFT
ages than
metasedimentary
samples,
probably due to
apatite chemistry.
15-18 s in
5% nitric
acid at room
temperature
NA
No
Exhumation and cooling
~66-52 Ma. Increase in cooling
rate ~58 Ma. AFT ages define
an anticline, with older AFT
ages at edge of range.
No annealing kinetic
data, but divided samples
into suites based on
rock type, which were
interpreted separately.
Track lengths not corrected
for orientation - interpret
lengths with caution.
NA
NA
2 (multicrystal)
No
Samples were from a fossil AHe
PRZ. Either insufficient burial
during Cretaceous to reset ages,
low geothermal gradient, or
diffusion properties of He in
apatite different to Durango
apatite.
Insufficient aliquots
measured per sample to
recognize large scatter
due to radiation damage.
Peyton et al. (2012) later
showed that radiation
damage had increased
the temperature of the
PAZ, and that geothermal
gradients may have been
very low.
10711_ch03_ptg01_hr_037-070.indd 47
6/5/13 7:59 AM
48 Peyton and Carrapa
LARAMIDE BASINS
Subsurface well samples have been used to study
the thermal evolution of Laramide basins. Unlike the
ranges, which reached their maximum burial immediately before the onset of the Laramide Orogeny, the
basins reached maximum burial during the Neogene,
and have subsequently experienced some amount of
erosional exhumation (Love, 1960; Dickinson, 1986;
Nuccio, 1994b; Roberts et al., 2008). Most basinal thermochronologic studies have documented the timing
of post-Laramide cooling and maximum burial of the
basins using AFT dating (Naeser, 1986, 1989; Kelley
and Blackwell, 1990; Naeser, 1992). All detrital ZFT
ages reported from Laramide basins are coincident
with, or older than, their stratigraphic age, implying
that there was insufficient burial and heating during
the Cenozoic to reset the ZFT ages, that is, temperatures were less than ~240°C (Naeser, 1986; Chen and
Lubin, 1997).
Green River Basin
Naeser (1986; 1989) and Naeser et al. (1989b) presented
results from AFT and ZFT analysis of samples from the
Wagon Wheel #1 well (Section 5, Township 30 N, Range
108 W, Sublette County, Wyoming) on the Pinedale Anticline in the northern Green River Basin (approximate
location on Figure 1). AFT length measurements were
not possible due to low track density and low apatite
yield. Annealing kinetics such as Dpar or apatite composition were not measured. The shallowest sample
dated was from a depth of 2237 m (7340 ft). Interpretation of the AFT ages led the authors to suggest that the
most recent phase of cooling of at least 20°C initiated
~4 to 2 Ma during Pliocene time (Naeser, 1986, 1989;
Naeser et al., 1989b). This interpretation was based
on AFT ages between ~4 and 2 Ma that were found
between temperatures of ~95°C and 120°C in the wellbore. Naeser (1989) noted that this cooling is consistent
with other evidence for widespread erosion and uplift
in the Green River Basin starting in the late Pliocene,
although she did not estimate an erosion amount from
this cooling. All ZFT ages suggested that well samples
did not reach temperatures high enough to anneal
fission tracks in zircon.
Dickinson (1986) presented results of burial history modeling using %Ro data and the Lopatin model
of Waples (1980) for the northern Green River Basin, including the Wagon Wheel #1 well. Dickinson
(1986) required his burial models to have ~20–30°C
of cooling between 3 Ma and present to be consistent with Naeser’s (1986) conclusions. He noted that
10711_ch03_ptg01_hr_037-070.indd 48
a component of this cooling was from a surface temperature decrease of ~10°C between the Pliocene and
present caused by surface uplift of the region (Dorr et
al., 1977; Dickinson, 1986). However, Dickinson (1986)
used uncorrected present-day borehole temperatures
from well logs, which may be several degrees cooler
than actual temperatures, and thus his model may
include more cooling than necessary. By projecting
the %Ro data to a value of 0.2% (the normal assumed
Ro value for newly deposited sediments), Dickinson
(1986) suggested that 1520 m (4,986 ft) of section has
been eroded from the surface at the Wagon Wheel #1
well location, although stratigraphic data indicate
much less erosion (~470 m [~1541 ft]). He proposed
that a 2°C/km increase in the geothermal gradient
between Eocene and Pliocene time could result in
this disparity, ­although he acknowledged that there
is no direct evidence for this increase.
Using modern modeling tools (e.g., HeFTy,
Ketcham, 2005; Ketcham et al., 2007) it is possible to
reevaluate the interpretation of these young AFT ages.
Because these samples reside within the present-day
PAZ the ages should be interpreted with caution. Inverse modeling of AFT data is not possible without
track-length data, so forward modeling was used
to test the validity of Naeser ’s (1986) proposed interpretation. Data relevant to the thermal history of
the Wagon Wheel #1 well, such as %Ro, depositional
age (Dickinson, 1986) and AFT data (Naeser, 1986,
1989) were compiled (Table 2). Measured values of
%Ro (Dickinson, 1986) suggest that burial paleotemperatures were high enough to anneal fluorapatite
samples (%Ro > 0.7 for all samples, Table 2) (Duddy
et al., 1994) unless they were resistant to annealing.
The ranges of measured AFT ages and individual
grain ages within each sample (Table 2) suggest that
either not all detrital grains were totally reset by
burial, and therefore many grains must be resistant
to annealing, or that some %Ro values were less than
0.7 due to the large uncertainty (up to +/– 0.1%) in Ro
values. The latter case is unlikely because all but the
shallowest two samples have %Ro > 0.8. Naeser (1986)
dated between six and 11 apatite grains per sample,
which is an insufficient number to recognize multiple detrital populations (Galbraith, 2005). Frequency
histograms of individual-grain ZFT ages indicated
that most detrital zircon grains (and hence we assume
detrital apatite grains) cooled (1) between ~100 Ma
and time of deposition (i.e., during the Laramide and
Sevier orogenies), (2) ~300 Ma (during the Ancestral
Rockies Orogeny), or (3) ~600 Ma (during Proterozoic
rifting) (Naeser, 1986, 1989).
The present-day corrected temperature within
the borehole (Naeser, 1989) and approximate
6/5/13 7:59 AM
10711_ch03_ptg01_hr_037-070.indd 49
73
81
2465
2731
3098
3361
3370
4022
4557
4904
WWH-9
NN1
WWH-13
WWH-16
WWH-24
WWH-26
WWH-21
NN2
128
121
110
96
96
90
81
74
68
Temperature
Corrected °C
Rock Springs
Rock Springs
Ericson
Lance
Lance
Lance
Lance
Lance
Tertiary
Formation
82.5
79
74
71
71
70.5
70
69
63
Depositional
Age (Ma)
AFT data from Naeser (1986, 1989). Depositional ages and %Ro data from Dickinson (1986). 100 m (328 ft).
122
114
102
84
84
67
61
2237
WWH-5
Temperature
Uncorrected °C
Depth
(m)
Sample
Name
Table 2. Compilation of data from the Wagon Wheel #1 well in the Green River Basin.
1.63
1.49
1.13
1.03
0.96
0.95
0.85
0.78
0.72
%Ro
0
2.6
2.4
3.8
8
14.3
36.5
36.3
20.9
AFT Age
(Ma)
0
2.3
1.7
2.7
7.3
5.6
8.9
8.4
4.6
AFT Error
(Ma)
6
9
11
6
6
9
8
10
8
# Grains
NA
0-10
0-36
0-17
0-19
0-35
NA
6-81
9-67
Grain Age
Range (Ma)
An Overview of Low-temperature 49
6/5/13 7:59 AM
50 Peyton and Carrapa
depositional age (Dickinson, 1986) are known for each
sample ­(Table 2). Assumptions for the forward model
included (1) an average surface temperature of 15°C
during the Cretaceous (Amiot et al., 2004), (2) heating
of rocks due to burial between the time of their deposition and 50 Ma, (3) no heating or cooling between 50
and 4 Ma (an unrealistic but simplifying assumption),
(4) paleogeothermal gradients from 50 Ma to present
the same as the present-day geothermal gradient, and
(5) detrital grains remained at surface temperatures after initial cooling until deposition in these rock layers.
We constructed a time-temperature model in HeFTy
for sample WWH-9 of Naeser (1986), and adjusted
the maximum temperature of the model between
50 and 4 Ma to provide a match to the %R o data
­(Figure 2A). HeFTy uses the kinetics of Sweeney and
Burnham (1990) to calculate a %R o from a model
time-temperature path. This model time-temperature
path was then extrapolated to other samples using
the present-day geothermal gradient between 50 and
0 Ma, and time of deposition adjusted for each sample.
%Ro values were calculated for each time-temperature
path and compared to measured values (Figure 2B).
For all samples but one, calculated %R o values are
within 0.06% of measured %R o ­v alues, leading us
to conclude that the calculated values provide a
good match to the measured values ­( Figure 2B).
With no AFT kinetic data available, we calculated
AFT ages for Dpar values between 1.65 mm (default
value of HeFTy) and 5 mm, and calculated AFT ages
for apatite that cooled immediately before deposition (i.e., volcanic origin, [Figure 3A]), at ­115–100
Ma (Figure 3B), and at 315–300 Ma (Figure 3C)
to represent different source terranes.
Model results (Figures 2 and 3) show that Naeser’s
(1986, 1989) conclusion of at least 20°C of cooling between 4 Ma and present in the Wagon Wheel #1 well
is reasonable if the apatites dated were resistant to annealing, as suggested by the %Ro data. Model results
also show that AFT ages of detrital grains that have
experienced the same thermal history since deposition can vary greatly depending upon the annealing
properties and provenance of the apatite, and illustrate the importance of measuring kinetic data such
as composition or Dpar, track lengths, and a sufficient
number of grains to recognize detrital populations.
Inclusion of %Ro data is critical in this case for estimation of maximum burial temperature and recognition of the presence of annealing-resistant apatite.
We have not tested other possible time-temperature
paths here, but recognize that there are almost certainly multiple forward models that will fit the data.
It is possible, for example, that temperatures between
50 and 4 Ma could be adjusted in the forward model
(e.g., to represent less burial/lower temperatures in
the Eocene, and more burial/higher temperatures
between the Oligocene and Pliocene), and still
produce the same results. The key is to find a timetemperature path that gives both a good mathematical fit to the thermochronologic data and that
corresponds to a plausible geological model.
Figure 2. (A) Time-temperature paths for three possible
Powder River Basin
apatite grains from sample WWH-9, a volcanic grain which
cooled as it was deposited (solid line), a detrital grain
which cooled 115–100 Ma (long dashed line), and a
detrital grain which cooled 315–300 Ma (short dashed
line). (B) Actual measured %Ro data (open circles)
(Dickinson, 1986) versus calculated %Ro from forward
­thermal modeling (black triangles). 1 km (0.6 mi).
10711_ch03_ptg01_hr_037-070.indd 50
Naeser (1992) calculated AFT ages for 13 core
samples from 11 wells in the southwestern Powder
River Basin (Figure 1). These Cretaceous-age samples were from sandstones in the Lewis Shale, the
Parkman Sandstone Member of the Mesaverde
6/5/13 7:59 AM
An Overview of Low-temperature 51
2000
Dpar = 1.65 mm
=
3
mm
r
D pa
3000
r
4
=
mm
D pa
5
Grain age = Formation age
pa
r
=
3500
D
Depth (m)
2500
mm
A
4000
4500
AFT age (Naeser, 1989)
5000
0
10
20
30
40
50
60
70
80
90
100
AFT age (Ma)
2000
Dpar = 1.65 mm
B
2500
D par
=3
mm
Depth (m)
3000
D par
=4
mm
3500
D
pa
r
=
5
mm
100 Ma detrital grains
4000
4500
AFT age (Naeser, 1989)
5000
0
10
20
30
40
50
60
70
80
90
100
AFT age (Ma)
2000
Dpar = 1.65
mm
C
2500
D par =
3 mm
Depth (m)
3000
D par =
3500
4 mm
300 Ma detrital grains
4000
4500
AFT age (Naeser, 1989)
5000
0
10
20
30
40
50
60
70
80
90
100
AFT age (Ma)
Figure 3. Results of forward modeling for apatite grains
which (A) cooled as they were deposited (i.e., volcanic
grains), (B) cooled 115–100 Ma, and C) cooled 315–300 Ma.
Solid and dashed lines show ages predicted for
different Dpar values. Black circles are AFT ages of Naeser
(1986, 1989), error bars are 2s. Horizontal gray bars are
the range of individual grain AFT ages for each sample
(Naeser, 1986). 1000 m (3280 ft).
10711_ch03_ptg01_hr_037-070.indd 51
Formation, the Shannon Sandstone bed of the
Steele Member of the Cody Shale, and the Frontier
Formation. Lacking downhole temperature data,
Naeser (1992) calculated present-day sample temperatures using a mean geothermal gradient from
geothermal gradient maps. Kinetic data were not
measured directly, but instead were estimated qualitatively from the width of etched tracks; grains
with anomalously wide tracks (and hence high annealing temperatures) were discarded. Between
four and thirteen grains were counted per sample.
Track lengths were measured for 6 of the 13 samples, but other samples had insufficient tracks for
counting. AFT ages were younger than depositional
ages at two standard deviations, indicating that all
sandstones had experienced significant annealing.
For samples from similar temperatures and depths,
Naeser (1992) created composite track-length distributions and concluded that much of the complexity in the six track-length distributions was due
to the small number of tracks measured. Naeser
(1992) noted that all samples between present-day
temperatures of ~70°C and 110°C have AFT ages of
~12 Ma, and concluded that all samples were heated
to high enough temperatures to almost totally anneal before cooling began ~12 Ma. Naeser (1992)
observed that the time and rate of cooling could
not be determined precisely because (1) it is possible that not all grains were totally annealed before
cooling, (2) analytical uncertainties in the AFT ages
were large, (3) some Cl-rich grains may have been
included, and (4) the deeper samples were from the
present-day PAZ.
In contrast to the northern Green River Basin,
the southwestern Powder River Basin has %R o values that are anomalously low (~0.6% near the basin
axis) compared to other organic geochemical data
(Merewether and Claypool, 1980) and the maximum
temperatures suggested from AFT ages (Nuccio,
1990; Naeser, 1992). AFT data suggest cooling of at
least 35°C starting at ~12 Ma (Naeser, 1992), but geological data imply that no more than ~630 m (~2066 ft)
of post-mid-Miocene rock has been eroded (Nuccio,
1990). Naeser (1992) suggested that either geothermal gradients were higher in the past, or that hightemperature fluids moved through the sandstones
and influenced apatite annealing. These two different possibilities have very different implications for
source-rock maturity and petroleum generation in
the Powder River Basin. The low %Ro values suggest
that it is unlikely that geothermal gradients were
higher in the past, because %R o would also have
been affected. Thus, hot fluids may have annealed
apatite grains in porous and permeable sandstones
6/5/13 7:59 AM
52 Peyton and Carrapa
but left impermeable vitrinite-bearing layers unaffected. Another possible explanation is that %R o
values may have been underestimated (Price and
Barker, 1985).
Age (Ma)
0
100
200
300
400
500
600
700
0
Casper Arch AFT
1000
Bighorn Basin
Wind River Basin
Beland (2002) studied two boreholes that drilled
through Precambrian basement carried on thrusts at
the northwestern (American Quasar 9-24 Tribal well,
Section 9, Township 42 N, Range 105 W, Fremont
County, Wyoming) and eastern (W.A. Moncrief #16-1
Tepee Flats well, Section 16, Township 37 N, Range
86 W, Natrona County, Wyoming) boundaries of the
Wind River Basin (these wells penetrated the Maverick
Springs thrust and Casper Arch thrust, respectively).
AFT ages and track lengths were measured in both
wells, and AHe ages in the Maverick Springs well
only. Apatite composition was measured for four samples using an electron microprobe. Beland (2002) only
measured the lengths of confined fission tracks that
were parallel to the crystal c-axis.
Interestingly, both wells do not follow the typical trend of thermochronometer age decreasing with
depth. Results from the Maverick Springs well show
young (Miocene) AFT and AHe ages at depths less
than ~2.5 km, and bimodal ages (AFT <15 and >300 Ma;
AHe <10 and >175 Ma) below ~2.5 km (~1.5 mi)
(Figure 4). AFT ages from the Casper Arch well are
10711_ch03_ptg01_hr_037-070.indd 52
Maverick Springs AHe
2000
Depth (m)
Chen and Lubin (1997) used the ZFT technique to date
sedimentary rock samples from the Upper Jurassic
Morrison and Lower Cretaceous Cloverly formations from surface outcrop in the Bighorn Basin. They
­divided their zircon samples into those from volcanic
ash beds (bentonites), and those from detrital samples.
With one exception, ZFT ages of volcanic origin were
consistent with their corresponding stratigraphic ages,
indicating that samples had not reached sufficiently
high temperatures since deposition to partially anneal
the fission tracks in the zircons. Using detrital ZFT age
spectra, Chen and Lubin (1997) concluded that the
amount of ash input into the Bighorn Basin intensified ~175 Ma and that this increase lasted at least until
115 Ma, through Morrison and Cloverly deposition.
Using the youngest detrital age in the population, they
refined stratigraphic ages of the detrital samples. ZFT
age spectra indicated that the Morrison and Cloverly
formations received sediment from multiple source
­areas of different ages.
Maverick Springs AFT
3000
4000
5000
6000
Figure 4. Age-elevation plot showing AFT (green) and AHe
(blue) ages for a well from the Maverick Springs Dome
(squares) at the northern boundary of the Wind River Basin,
and a well from the Casper Arch (triangles) at the eastern
boundary of the Wind River Basin. From Beland (2002).
Error bars are 2s. 1000 m (3280 ft).
similar, with young ages (<7 Ma) above ~3.5 km
(~2.1 mi) depth, and older ages (>300 Ma) below
~4 km (~2.4 mi) (Figure 4). Using forward modeling
to match AFT ages and track length distributions,
Beland (2002) suggested the most plausible explanation for these results was two episodes of thrusting, the first during the Laramide Orogeny which
was followed by basin filling, and the second during
­Miocene time (<15 Ma), followed by erosion. The second episode of thrusting likely reactivated existing
Laramide thrusts and created new thrust imbricates.
The older AFT and AHe ages at depth indicate that
these samples were not buried deeply enough by
Phanerozoic sedimentary rocks for ages to be completely reset.
All young, shallow AFT ages in Beland’s (2002)
study are from apatite grains that have Fl-rich (i.e.,
­Cl-poor) compositions, and thus anneal at lower temperatures than the Cl-rich apatite with Paleozoic ages
from deeper in the wells (Green et al., 1986). Bimodal
AFT ages (<15 Ma and >300 Ma) from the lower three
samples in the Maverick Springs well could be due
to contamination from shallow in the well, since the
young grains are also Fl-rich (Beland, 2002). However,
6/5/13 7:59 AM
An Overview of Low-temperature 53
as Beland (2002) noted, the deeper samples may contain different apatite compositions that have different
AFT annealing temperatures. If the young AFT ages
in the deep samples were due to different annealing
properties, then it might not be necessary to invoke
Miocene thrusting to explain the young ages at shallow depths. However, a similar bimodal pattern is
seen in the AHe ages from the Maverick Springs well,
even though the closure temperature of the AHe system is unaffected by apatite composition (Warnock
et al., 1997). It is possible that old AHe ages were a
result of He implantation, which has recently been
suggested as a cause of anomalously old AHe ages in
Precambrian basement rocks in the Laramide foreland
(Reiners et al., 2008; Peyton et al., 2012). Beland (2002)
argued that the younger, deep AFT ages are unlikely
to be simply an effect of annealing temperature, because the Cl-rich apatites with Paleozoic ages were at
temperatures where total annealing should have occurred. Thus, he concluded that the deep, Paleozoicage samples had not been exposed to their present-day
high temperatures for very long, consistent with burial by thrusting during the late Miocene. Similarly,
even if the shallower, young ages were caused by hot
fluid flow, it is still difficult to explain why the deep
samples were not annealed, given their present-day
temperatures.
Beland (2002) cited several examples from the
Rocky Mountain foreland where Neogene shortening has been documented, and suggested that extensive erosion across the region may have resulted in
few ­examples being preserved or identified. Such late
thrusting at the edges of the Laramide basins would
have significantly impacted the thermal maturity and
migration history of any potential subthrust ­petroleum
play, although it is unlikely it would have affected the
basin centers. Other low-temperature thermochronology studies that have analyzed samples from wells
which penetrate Precambrian basement on basinbounding thrusts have not found inverted cooling age
profiles with young AFT or AHe ages at shallower
depths than older ages (Cerveny and ­S teidtmann,
1993; Omar et al., 1994; Peyton et al., 2012). ­Further
work at the boundaries of the Wind River ­B asin is
warranted.
Colorado) (Figure 1). Seventeen samples were from
sandstone core in the Upper Cretaceous Mesaverde
Group, while two were from sandstone cuttings from
the Wasatch Formation. Seven of the core samples
contained sufficient apatite to measure track lengths.
Kelley and Blackwell (1990) used an electron microprobe to analyze the compositions of apatite from four
samples but found no strong correlations between Cl
content and AFT age. Track lengths were not corrected
for their orientation with respect to the apatite c-axis.
AFT ages ranged from 42.8 ± 8.2 Ma for the shallowest sample, to 1.0 ± 0.2 Ma for the deepest sample (Figure 5). All AFT ages were younger than the
stratigraphic ages of the formations, indicating that
all samples resided within a PAZ at some time. The
uppermost three samples represent a fossil PAZ
­(Figure 5) with a marked change in slope at a presentday estimated temperature of ~66°C (1390 m depth
[4560 ft]) representing the base of the fossil PAZ.
­I nversion modeling of AFT ages and track lengths
showed that cooling of 10–15°C/Ma occurred in
the last 5 Ma (Kelley and Blackwell, 1990). ZFT ages
were not reset, indicating that temperatures never
Piceance Basin
Kelley and Blackwell (1990) determined AFT and ZFT
ages of 19 samples from three wells <55 m (180.4 ft)
apart at the Multi-Well Experiment site in the eastcentral Piceance Basin, northwestern Colorado (Section 34, Township 6 S, Range 94 W, Garfield County,
10711_ch03_ptg01_hr_037-070.indd 53
Figure 5. AFT results from the Piceance Basin (Kelley
and Blackwell, 1990). 1000 m (3280 ft).
6/5/13 7:59 AM
54 Peyton and Carrapa
reached 240 ± 25°C for any length of time. Kelley
and Blackwell (1990) compared their fission-track
results to results of burial history modeling. %Ro and
burial data indicate that maximum paleotemperature
at the bottom of the wellbore was ~150°C to 200°C
at ~10 Ma, and that 1500 m (4,921 ft) of sedimentary
rocks were eroded since 10 Ma by downcutting of
the Colorado River (Bostic, 1983; Bostic and Freeman,
1984), consistent with the AFT and ZFT results.
Denver Basin
Kelley (2002) measured AFT and ZFT ages of detrital
grains from synorogenic sediments from two wells in
the Denver Basin (Section 17, Township 8S, Range 63W,
Elbert County, Colorado, and Section 9, Township 7S,
Range 67W, Douglas County, Colorado) ­(Figure 1).
These shallow (<1 km depth [<0.6 mi]) samples were
not buried deeply enough for AFT ages to have been
reset during the Neogene. Age populations represent
a combination of cooling ages of the source terrane, in
this case the Front Range, and a volcanic component
derived from ash blown from the west. Detrital material varied from recycled Mesozoic and Paleozoic sedimentary grains to grains derived from the Precambrian
basement. In both wells, the percentage of metamict zircon grains increased up section, indicating an ­increase
through time in the contribution of grains from the
Proterozoic basement as it was unroofed. ­Kelley (2002)
also used the base of the pre-Laramide fossil PAZ as
a stratigraphic marker within basement rocks. Grains
eroded from basement above the fossil PAZ typically
have an AFT age >100 Ma, whereas grains from below
the PAZ have AFT ages between 70 and 50 Ma. ­Kelley
(2002) documented a predictable AFT age sequence
representing unroofing of the southern ­Colorado Front
Range during the Laramide Orogeny.
LARAMIDE RANGES
Surface samples of Precambrian basement from Laramide ranges record either AFT and AHe exhumation
ages of ~80 to 40 Ma related to the Laramide Orogeny
(Dickinson and Snyder, 1978), or older ages of a preLaramide fossil PAZ or PRZ. Unlike ­low-temperature
thermochronologic results from the Laramide ­basins,
which can be used to calibrate models of basin
­e volution and petroleum maturation, results from
the ranges generally provide information on the
structure and evolution of the crystalline basement
uplifts, and possibly on the thermal history of sedimentary rocks beneath the range-bounding thrusts.
10711_ch03_ptg01_hr_037-070.indd 54
The spatial distribution of the base of the fossil PAZ
has been used to document faulting, folding, and differential exhumation of basement. AFT ages are often
younger in the cores of Laramide ranges, and become
older toward the edges, indicating higher exhumation in the core and a domal structure of the range
(e.g., Roberts and Burbank, 1993; Strecker, 1996;
Kelley and Chapin, 1997; Kelley, 2005). Neogene AFT
ages from the mountains of central Colorado and
northern New Mexico are related to extension of the
Rio Grande rift (Bryant and Naeser, 1980; Lindsey
et al., 1986; Pazzaglia and Kelley, 1998; Naeser et al.,
2002). ZFT ages reported from Laramide ranges are
all Proterozoic, indicating that basement rocks have
not experienced temperatures >~240°C since Proterozoic time (Cerveny, 1990; Cerveny and Steidtmann,
1993; Naeser et al., 2002).
AHe ages from ranges within the Rocky Mountain
foreland often demonstrate significant scatter (tens to
hundreds of Ma) and are sometimes older than corresponding AFT ages (Crowley et al., 2002; Peyton et
al., 2012). As discussed in Chapter 2 of this volume,
radiation damage of apatite, along with He implantation from sources external to the apatite, are likely
causing many of these issues. While the ­effects of radiation damage on AHe ages are predictable and provide
constraints on the thermal history (Flowers et al., 2007;
Flowers, 2009; Flowers et al., 2009; Peyton et al., 2012),
He implantation is unpredictable and renders AHe ages
geologically meaningless (Reiners et al., 2008; Orme,
2011; Peyton et al., 2012). To document AHe age scatter and allow for the potential identification of radiation
damage, several apatite grains should be dated for each
crystalline bedrock sample. Recognizing AHe age scatter due to ­radiation damage or He implantation may be
especially difficult in sedimentary rocks because there
may also be variation in AHe grain ages from different source terranes. Grain abrasion to remove the outer
20 mm of the apatite crystal (Kohn et al., 2008) is one
possible way to address He implantation, but is not
yet a routine and tractable part of AHe dating. Careful
analysis of plots of AHe age versus effective U concentration (eU = [U] + 0.235 [U]), and AHe age versus grain
size, along with comparison with AFT ages, may allow
the identification of some samples that exhibit AHe age
scatter due to He implantation rather than radiation
damage (Flowers and Kelley, 2011; Peyton et al., 2012).
Bighorn Range
All published results from the Bighorn Range are presented in Figure 6. Crowley et al. (2002) studied the
Bighorn Range using AHe dating of surface samples.
6/5/13 7:59 AM
An Overview of Low-temperature 55
A
4000
Surface samples
3000
10
Subsurface samples
1000
20
30
0
40
-1000
BIGHORN RANGE
AHe Peyton et al. (2012)
AFT Peyton et al. (2012)
AHe Crowley et al. (2002)
AFT Cerveny (1990)
-2000
-3000
50
0
100
200
300
Temperature °C
Elevation (m)
2000
60
70
400
Age (Ma)
B
-250
250
-250
250
1250
750
Depth below pC unconformity (m)
Depth below pC unconformity (m)
750
1750
2250
2750
3250
3750
1250
1750
2250
2750
3250
3750
4250
4250
4750
5250
AHe Peyton et al. (2012)
AFT Peyton et al. (2012)
AHe Crowley et al. (2002)
AFT Cerveny (1990)
4750
5250
0
20
40
60
80
Age (Ma)
0
100
200
Age (Ma)
300
400
Profiles of AHe age versus elevation from these surface samples have low slopes, which Crowley et al.
(2002) suggested represent a fossil pre-Laramide PRZ.
­Crowley et al. (2002) acknowledged that these ages
are problematic because burial depth during the Cretaceous should have been great enough to reset AHe
ages. They suggested that either burial was not as great
10711_ch03_ptg01_hr_037-070.indd 55
100
120
140
Figure 6. Age-elevation plot of
thermochronology results from
the Bighorn Range (Peyton et al.,
2012). Reprinted by permission of
the American Journal of Science.
50 m (164 ft).
as originally thought, that geothermal gradients were
unusually low, or that closure temperature of the AHe
system was higher than expected based on He diffusion kinetic data available at the time. The effect of radiation damage on AHe ages and closure temperatures
was not recognized until more recently (Shuster et al.,
2006; Flowers et al., 2009). Later work by Peyton et al.
6/5/13 7:59 AM
56 Peyton and Carrapa
(2012) using subsurface samples from the Gulf Granite
Ridge #1-9-2D well (Section 9, Township 53N, Range
84W, Sheridan County, Wyoming), along with surface
samples from Cloud Peak, confirmed that radiation
damage of apatite had effectively increased the closure
temperature of the AHe system, resulting in preservation of a fossil PRZ in Bighorn surface samples. Peyton
et al. (2012) also found a low present-day geothermal
gradient of 14°C/km in the borehole and suggested
that gradients could have been this low in the past.
AFT results from the Bighorn Range indicate that
almost all surface sample ages belong to a fossil PAZ,
and that insufficient exhumation has occurred to expose rocks that record Laramide AFT cooling ages
(Cerveny, 1990; Peyton et al., 2012). Cerveny (1990)
used track-length data to suggest that exhumation
and cooling of the Bighorn Range began ~75 Ma.
Peyton et al. (2012) interpreted the change in slope
on an AFT age-elevation profile, which represents the
onset of rapid exhumation from Cloud Peak, to be
between ~99 and 57 Ma. They could not refine the onset
of cooling and/or exhumation further using these AFT
ages due to lack of track-length data, large errors, and
few data points. Inverse modeling of AHe ages using a
radiation-damage diffusion model showed that exhumation and/or cooling of the Bighorn Range started
before (and possibly significantly before) ~71 Ma
(Peyton et al., 2012). Peyton et al. (2012) also suggested
from AHe inversion results that the Bighorn Range
cooled by ~80°C during the Laramide Orogeny, corresponding to as much as ~5.7 km (~3.5 mi) of exhumation if the paleogeothermal gradient was similar to the
present-day gradient of 14°C/km. This amount compared favorably with the ~5.5 km (~3.4 mi) of exhumation estimated from sequential restorations of cross
sections (Hoy and Ridgway, 1997).
Peyton et al. (2012) did not consider temperature
changes due to climate or elevation change in their
calculations. Using oxygen isotopic data from vertebrates, Amiot et al. (2004) showed that mean annual temperatures during the Late Cretaceous were
similar to present-day for low latitudes (<30°N), were
between ~1°C and 8°C warmer between 30°N and
60°N, and were significantly warmer (up to ~25°C)
at high latitudes (>60°N). For northern Wyoming at a
present-day latitude of 45°N, cooling of the mean annual temperature at sea level due to climate change
from the Late Cretaceous to present was likely offset by warming due to the southward movement of
the North American plate by at least 5° of latitude
(Besse and Courtillot, 2002; Amiot et al., 2004). The
elevation of Wyoming has changed substantially
from the Late Cretaceous, when the entire state was
near sea level. Present-day elevations range from
10711_ch03_ptg01_hr_037-070.indd 56
~1 km (~0.6 mi) in the northeast corner of the state
to ~4 km (~2.4 mi) at the highest peaks. Basin floor
­elevations are generally ~1.2 to 2.3 km (0.7 to 1.4 mi).
Using a temperature lapse rate of 6 ± 2°C/km, a surface sample from 2 km (1.2 mi) elevation in the Bighorn Range probably cooled ~12°C due to elevation
change alone. Peyton et al. (2012) estimated 80°C of
cooling by inverting AHe data from a wellbore sample at 1.3 km (0.8 mi) elevation, 0.76 km (0.4 mi) below the surface. Although the surface temperature at
the well location cooled ~12°C between the Late Cretaceous and present, the effects of changing surface
temperature decrease with depth and as a result the
cooling estimates of Peyton et al. (2012) are probably
overestimated by <10°C. Reducing the amount of
cooling by 10°C would reduce the estimate of exhumation by ~0.7 km (~0.4 mi), or 12%, to 5 km (3.1 mi)
(using the present-day geothermal gradient of
14°C/km from Peyton et al., 2012).
Beartooth Range
Omar et al. (1994) calculated AFT ages of samples from
the Amoco Beartooth #1 well (Section 19, Township 8S,
Range 20E, Carbon County, Montana) in the northeast
corner of the Beartooth Range (Figure 1), along with
surface samples collected along the Beartooth Highway (U.S. Highway 212). Cerveny (1990) also dated
surface samples from along the same section of highway using the AFT technique. Peyton et al. (2012)
determined AHe ages from the Amoco Beartooth #1
well, but selected surface samples closer to the well
­location. All published results are shown in Figure 7.
The AFT results of both Cerveny (1990) and Omar
et al. (1994) show that the highest samples in the Beartooth Range (>3.1 km [>1.9 mi] elevation) are in a fossil
PAZ, with AFT ages ranging from ~350 to 100 Ma. Below ~3.1 km (~1.9 mi) elevation, AFT ages range from
~68 to 48 Ma and have narrow, unimodal track length
distributions with mean track lengths between 14.8 and
11.8 mm, resulting from rapid exhumation during the
Laramide Orogeny (Cerveny, 1990; Omar et al., 1994).
Peyton et al. (2012) inverted AHe age-eU pairs for
a sample from the Amoco Beartooth #1 well using a
radiation damage diffusion model and extrapolated
the best-fit solution to other sample elevations using the present-day geothermal gradient. They found
that the AHe age-eU distribution predicted by the
best-fit thermal history matched the age-eU distributions at other sample locations that showed an age-eU
correlation, confirming the influence of radiation damage on AHe ages. AHe ages from an intrusive sample
were older than the zircon U/Pb crystallization age,
6/5/13 7:59 AM
An Overview of Low-temperature 57
Figure 7. Age-elevation plot of
thermochronology results from
the Beartooth Range (Peyton
et al., 2012). Inset shows same
data but with an expanded time
scale for more detail. All error bars
are 2s. Reprinted with ­permission
of the American Journal of ­Science.
1000 m (3280 ft).
s­ uggesting that He implantation was affecting some
samples from the Beartooth Range (Peyton et al., 2012).
Using AFT ages, Omar et al. (1994) proposed that
uplift of the northeast corner of the Beartooth Range
began at ~61 Ma. Cerveny (1990) concluded from AFT
ages that a major uplift event began in the Beartooth
Range at ~68 Ma and increased in rate at ~57 Ma.
Peyton et al. (2012) concluded that Laramide exhumation and cooling in the Beartooth Range started before
~58 Ma, and that the range cooled by ~98°C, corresponding to ~5 km (~3.1 mi) of exhumation (assuming a
paleogeothermal gradient of 20°C/km), although
again they did not consider decreases in temperature
10711_ch03_ptg01_hr_037-070.indd 57
due to climate change or increasing elevation. The
surface sample used by Peyton et al. (2012) to estimate ~98°C of cooling was from an elevation of 2.1 km
(1.3 mi). Therefore, ~12°C of this cooling was likely due
to elevation change, reducing estimates of exhumation
by ~0.5 km (~0.3 mi).
Wind River Range
The Wind River Range (Figure 1) has been studied using both AFT dating (Shuster, 1986; Steidtmann et al.,
1989; Cerveny, 1990; Cerveny and Steidtmann,
6/5/13 7:59 AM
58 Peyton and Carrapa
1993; Peyton et al., 2012) and AHe dating (Peyton
et al., 2012). Cerveny and Steidtmann (1993) dated
samples from several traverses throughout the range,
and interpreted a complex cooling history, with the
earliest cooling initiating between ~85 and 75 Ma at
the northeast and southwest flanks of the range, and
rapid cooling in the core of the range between 62 and
57 Ma. AFT ages from Peyton et al. (2012) show similar cooling ages to those of Cerveny and Steidtmann
(1993) in the core of the range (Gannett and Fremont
Peaks). Inverse modeling of the AFT ages and track
lengths from Gannett Peak showed rapid cooling and
exhumation between ~60 Ma and 50 Ma (M. Fan, personal communication, 2012). AHe ages from Gannett
Peak and Fremont Peak are consistently older than
corresponding AFT ages by an average of 23 Ma and
14 Ma respectively, and do not increase significantly
with increasing elevation as would be expected in
a fossil PRZ. Peyton et al. (2012) interpreted these
anomalously old AHe ages to be the result of He implantation rather than radiation damage. Steidtmann
et al. (1989) used AFT dating to recognize differential
exhumation of fault blocks in the core of the range.
They suggested that these blocks moved relative to
one another after Laramide cooling. Using sedimentological evidence they concluded that the core of the
range was reactivated during late Oligocene time.
Shuster (1986), Cerveny and Steidtmann (1993), and
Peyton et al. (2012) dated subsurface samples from the
Air Force well (Section 2, Township 32N, Range 107W,
Sublette County, Wyoming) on the west side of the range
(Figure 8). AFT ages from Cerveny and Steidtmann (1993)
preserve a fossil PAZ above ~1100 m (~3608 ft) ­elevation,
and have a steep slope below ~1100 m (~3608 ft)
elevation, indicating rapid cooling at ~42 Ma.
Individual grain AHe ages for the well range from
8 Ma at an elevation of –623 m (–2043 ft), to 82 Ma at an
elevation of 1626 m (5334 ft), with the exception of one
aliquot with an age of 163 Ma (Figure 8). Samples with
multiple single-grain aliquots from the Air Force well
show AHe age scatter of between ~20 and 40 Ma. At
least one sample from the Air Force well showed a good
correlation between AHe age and eU, indicating that
radiation damage affected the AHe ages. Inverse modeling of data from this sample (elevation 1626 m [5,334
ft]) suggested that the Wind River Range cooled at least
69°C during the Laramide Orogeny, corresponding to
~3 km to 4 km (1.8-~2.4 mi) of exhumation using the
present-day geothermal gradient (19°C/km from
Cerveny, 1990), and that exhumation started before
~66 Ma (Peyton et al., 2012). Similar to Bighorn and
Beartooth Range, Peyton et al. (2012) probably overestimate the amount of cooling due to exhumation by not
considering surface-temperature effects.
Ranges of Colorado, Southern Wyoming,
and Northern New Mexico
Figure 8. Age-elevation plot of thermochronology results
from the Air Force well in the Wind River Range (Peyton
et al., 2012). Reprinted by permission of the American
Journal of Science.
10711_ch03_ptg01_hr_037-070.indd 58
Kelley and Chapin (1997; 2004), and Pazzaglia and
Kelley (1998) measured AFT ages from the southern Front Range, Wet Mountains, the Taos Range
in the Sangre De Cristo Mountains, and the Sierra
Nacimiento (Figure 1). In the Front Range they recognized the base of a pre-Laramide, fossil PAZ that separates samples with AFT ages > 90 Ma and short mean
track lengths (10–12 mm), from those with AFT ages of
40 to 75 Ma and longer mean track lengths (12–14 mm).
Farther south, the northern parts of both the Sierra
Nacimiento and the Wet Mountains preserve
Laramide exhumation ages, whereas their southern
parts preserve Rio Grande Rift-related Neogene exhumation ages. Pazzaglia and Kelley (1998) suggested
that these age changes coincide with, and were perhaps influenced by, boundaries between Proterozoic
basement provinces. AFT ages from the Taos Range
6/5/13 7:59 AM
An Overview of Low-temperature 59
vary between 18 Ma and 34 Ma and reflect cooling
during early Rio Grande rift deformation.
Exhumation and cooling due to Rio Grande rifting was also documented along the flanks of the Blue
River half graben between the Front Range and the
Gore Range of central Colorado (Figure 1) (Naeser
et al., 2002). AFT ages on the eastern side of the Gore
Range vary from ~5 Ma at the base of the range to
~27 Ma at Mount Powell on the crest of the range.
Modeling of the AFT data showed that samples cooled
from temperatures > 130°C at ~30 Ma to surface temperatures (0°C to 5°C) by 5 Ma (Naeser et al., 2002). AFT
ages from the western Front Range are similar to those
from the western Gore Range, and modeling showed
rapid cooling beginning ~25 Ma to 15 Ma (Naeser
et al., 2002). Naeser et al. (2002) also dated samples
from the White River Uplift (Figure 1), which gave
AFT ages between 64 Ma and 34 Ma, with long mean
track lengths between 13.5 mm and 14.8 mm. They interpreted these ages to result from exhumation and
cooling during the Laramide Orogeny.
AFT ages from the central Front Range below ~3500 m
(11,482 ft) vary between ~75 Ma and 50 Ma, with long
track lengths indicative of rapid cooling during the
Laramide Orogeny (Bryant and Naeser, 1980; Kelley
and Chapin, 1997; Naeser et al., 2002). Ages are youngest towards the center of the range. At elevations above
~3500 m (~11,482 ft), AFT ages vary between ~458 Ma
and 90 Ma, and the samples, which have shortened
track lengths, are interpreted to represent a fossil PAZ
(Kelley and Chapin, 1997; Naeser et al., 2002). The elevation of the base of this PAZ indicates that the Front
Range has a broad domal structure that has been dissected by reverse faults (Kelley and Chapin, 1997).
­Estimates of rock uplift for the central Front Range
vary from 6 km (3.7 mi) at the eastern edge of the range
to 7.5 km (4.6 mi) at the summit of Mount Evans;
estimates for the amount of rock eroded vary from
3.5–5.2 km (2.1 to 3.2 mi) (Bryant and Naeser, 1980;
Kelley and Chapin, 1997).
Kelley (2005) mapped the base of the PAZ in the
Laramie, Medicine Bow, and Park Ranges (Figure 1),
and determined if Proterozoic basement structures
were reactivated during the Laramide Orogeny.
Similar cooling ages and histories on each side of the
Cheyenne belt in the Laramie Range (Figure 1) indicate
that this was not a zone of weakness during the Laramide Orogeny. Structural elevation of the AFT PAZ in
the Laramie Range indicates a broad basement domal
structure, similar to that observed in the Front Range,
Black Hills, and Bighorn Range. In the Medicine Bow
Mountains, however, Kelley (2005) incorporated data
from Cerveny (1990) and showed that the Cheyenne belt was likely reactivated during the Laramide
10711_ch03_ptg01_hr_037-070.indd 59
Orogeny. AFT ages north of the Cheyenne belt shear
zone in the Medicine Bow Mountains represent rapid
Laramide cooling, whereas ages south of the shear
zone are from a PAZ, indicating different amounts of
exhumation on either side. In the Park Range, the Fish
Creek–Soda Creek shear zone was not strongly reactivated during the Laramide Orogeny (Kelley, 2005).
Cerveny (1990) found that samples from the east side of
the Park Range had long (>14 mm) mean track lengths
and AFT ages between 61 Ma and 60 Ma, leading him
to conclude that a cooling event occurred between
61 Ma and 60 Ma.
Three AFT ages and corresponding track length
data from Laramie Peak in the Laramie Range led Cerveny (1990) to conclude that cooling occurred ~65 Ma.
AHe ages from Laramie Peak showed scatter of hundreds of Ma (Peyton et al., 2012), with all AHe ages
older than the three AFT ages for the range. The large
dispersion of these anomalously old AHe ages was
interpreted to be due to radiation damage of apatite
(Peyton et al., 2012). Peyton et al. (2012) also dated
samples from a wellbore at the northern end of the
Laramie Range (Texaco Government Rocky Mountain
#1 well, Section 12, Township 32N, Range 76W, Converse County, Wyoming). Using forward and inverse
modeling of these AHe ages, Peyton et al. (2012) concluded that a Laramide-age thrust was present in the
wellbore, and consequently that samples above and
below the thrust had experienced different thermal
histories.
Teton Range
The present-day Teton Range formed by normal faulting that began during the late Miocene (~9 Ma). Offset
of Pliocene tuffs across the Teton fault shows that the
hanging wall east of the Teton normal fault subsided
~5 km (~3.1 mi), and the footwall was uplifted to ~2 km
(~1.2 mi) above the present surface (Roberts and
Burbank, 1993, and references therein). The majority
of AFT ages from three profiles across the Teton Range
vary between 85 Ma and 65 Ma, indicating that the
range was also exhumed during the Laramide Orogeny (Roberts and Burbank, 1993). Unlike other nearby
ranges, where the majority of exhumation was due to
the Laramide Orogeny, the Tetons experienced major
exhumation twice, first during the Laramide Orogeny
with the formation of the ancestral Teton-Gros Ventre Uplift, and later during Miocene time (Love et al.,
1972; Dorr et al., 1977). AFT track lengths record this
complex exhumation history, with lengths decreasing and distributions broadening with decreasing
present-day elevation. Most samples currently at the
6/5/13 7:59 AM
60 Peyton and Carrapa
surface resided in the PAZ after Laramide exhumation, and were exhumed to the surface during the
Miocene (Roberts and Burbank, 1993). None of the
ages record Miocene exhumation, indicating that all
of the samples were within or above the PAZ at the
end of Laramide exhumation. No ages represent the
pre-Laramide fossil PAZ, which has been eroded.
Samples from the northern end of the range record
younger AFT ages (67–26 Ma) than those from the
south (83–67 Ma), leading Roberts and Burbank (1993)
to conclude that the north end of the range resided in
the PAZ for a longer time than the southern end, and
cooled more recently to temperatures below the PAZ.
They also concluded that there was less exhumation
in the southern part of the range relative to the north,
resulting in a tilting of the Precambrian-Cambrian
unconformity ~3° to the south.
Black Hills
The Blacks Hills of South Dakota are the easternmost
basement-cored range formed during the Laramide
Orogeny (Figure 1). Unlike other ranges discussed
here, no range-bounding reverse fault is mapped at
the Black Hills. Paleozoic and Mesozoic rocks dip
concentrically away from the Precambrian core of
the range. Both Cerveny (1990) and Strecker (1996)
determined AFT ages of surface samples from the
Black Hills, but to date, no AHe studies have been
reported. Strecker (1996) dated 19 samples from Precambrian granite and metasedimentary rocks across
the range. AFT ages ranged from 53.5 Ma ± 2.4 Ma
to 113.5 Ma ± 11.4 Ma. Multiple populations of track
lengths indicated that none of the samples were totally annealed before cooling. Strecker (1996) did
not measure apatite composition but did recognize
that all granitic samples had lower AFT ages than
metasedimentary samples, and suggested that this
was due to compositional differences resulting in
different annealing temperatures. Spatially, the AFT
ages define a fold, with older ages at the edges of the
uplift. An age-elevation plot of the widely spaced
samples showed no indication of AFT age increasing
with present-day elevation, with older ages at lower
elevations near the edge of the range. However, by
flattening the dip of the fold limb about the hinge
of the anticline and projecting the ages to the new
corrected elevations, the age-elevation plot shows a
much clearer trend of increasing age with increasing sample elevation (Strecker, 1996). Strecker ’s
(1996) method for correcting elevations is similar
to that used by Crowley et al. (2002) in the Bighorn
Range, who calculated elevation relative to the
Precambrian-Cambrian unconformity. Interpretation
10711_ch03_ptg01_hr_037-070.indd 60
of this age versus corrected elevation plot led
Strecker (1996) to conclude that exhumation and
cooling began in the Black Hills in earliest Paleocene
time (~66 Ma)and continued until ~52 Ma, with an
increase in cooling rate at ~58 Ma. Estimates of exhumation ranged from 3.8 km (2.3 mi) ± 1.2 km (0.7 mi)
to 5.0 km (3.1 mi) ± 1.5 km (0.9 mi), with denudation
rates most likely ~0.2 km/Ma to 0.4 km/Ma.
Cerveny (1990) dated three samples from Precambrian granite in the Black Hills. The oldest age
of 262 Ma ± 28 Ma was found on the outer edge of
the batholith. Ages of 79.4 Ma ± 9 Ma and 71.4 Ma ±
5.7 Ma were found structurally lower in the batholith. Similar to Strecker’s (1996) data, the three ages
of Cerveny (1990) increase with structural elevation
but not with actual elevation. A bimodal track length
distribution and short mean track length (12.1 mm ±
2.4 mm) indicate that the oldest age was from a fossil
PAZ. Mean track length distributions close to 14 mm
for the younger two samples were interpreted to indicate that exhumation and uplift occurred at ~75 Ma
(Cerveny, 1990).
It is interesting that Cerveny (1990) concludes that
uplift was occurring at ~75 Ma, whereas Strecker
(1996) concludes that cooling began ~66 Ma in the
same area. With only three data points it is possible
that Cerveny (1990) misinterprets his two youngest
ages as indicating rapid cooling, whereas with more
data it might have been apparent that they actually
belong to a fossil PAZ, although the long mean track
lengths suggest otherwise.
Uinta Mountains
One sample collected from the Uinta Mountain Group
in the eastern Uinta Mountains provided an AFT age
of 52.28 Ma ± 4.97 Ma, but had insufficient tracks to
provide a reliable mean track length (C. Painter, personal communication, 2012). Unfortunately, due to
the quartzitic nature of the Uinta Mountain Group,
additional surface samples did not yield any apatite
grains.
Two samples collected from the McMoran-Freeport
43-2 Middle Mountain well (Section 2, Township 2N,
Range 25E, Daggett County, Utah) at depths of 1372 m
(4500 ft) and 2286 m (7500 ft) yielded AFT ages of
38.9 Ma ± 2.6 Ma and 30.3 Ma ± 2.2 Ma respectively
(K. Constenius and S. Kelley, personal communication, 2012). Each of these samples was a composite
of well cuttings over a depth range of ~150 m (500 ft)
from metamorphic rocks of the Archean Owiyukuts
Complex. Sample depths represent the central depth
of the composite depth range. Track lengths were not
available. Two grains with large etch pits were noted
6/5/13 7:59 AM
An Overview of Low-temperature 61
for the sample from 1372 m (4501 ft) (K. Constenius
and S. Kelley, personal communication, 2012). Bottomhole temperatures from well logs were corrected using
the methods of Waples and Ramly (2001), Waples et al.
(2004a), and Waples et al. (2004b) and extrapolated to
sample depths. Results indicate that present-day temperatures of these samples are ~45°C ± 10°C at 1372
m depth, and ~60°C ± 10°C at 2286 m (7500 ft). These
temperatures suggest that the shallowest sample was
cooler than the present-day AFT PAZ (~60–120°C for
fluorapatite, Green et al., 1989), and may represent
cooling during the late Eocene (~39 Ma); however,
without track-length and kinetic data it is difficult to
evaluate if fission tracks have been partially annealed
in the deeper sample.
Summary of Laramide Cooling Ages
To summarize the results reviewed previously, we
have mapped Laramide cooling and exhumation ages
from ranges across the entire region (Figure 9). When
available, the Laramide cooling ages determined by inverse modeling of either AFT or AHe data are shown.
Figure 9. Map showing
112
108
CMB
104
57 48 Ma (AFT)
SOUTH DAKOTA
80 40 Ma (AHe model)
57 54 Ma (AFT surface)
(Omar et al., 1994;
85
Ma
(AHe
model)
100
Peyton et al., 2012)
(Peyton et al., 2012)
BT
MONTANA
MR
WYOMING
LP
77 60 Ma
83 67* Ma
62 Ma
TR
(Roberts & Burbank, 1993)
(Cerveny, 1990;
Strecker, 1996)
BH
(Cerveny, 1990)
(Peyton et al., 2012)
(Cerveny, 1990)
WR
UTAH
thru
st
39 30* Ma
LR
79 60 Ma
NEBRASKA
71 Ma
(Cerveny, 1990)
MB
76 46 Ma
COLORADO
(Kelley, 2005
Cerveny, 1990)PR
52* Ma
70 45 Ma (AFT model)
(Kelley & Chapin, 2004)
(Painter, p.c. 2012)
AHe surface samples (bold symbol
outlines indicate results from
inverse modeling)
AHe subsurface samples
DB
WRU
GR
PB
40
UB
(Kelley, 2005)
(Kelley, 2005
Cerveny, 1990)
(Constenius & Kelley, p.c. 2012)
UM
40
HU
82 65 Ma
GRB
Sevier
?
GM
front
IDAHO
70 60 Ma (AHe forward model)
71 Ma
CB
(Peyton et al., 2012
Cerveny & Steidtmann, 1993
Fan, p.c. 2012)
WRB
BL
PRB
OC
60 50 Ma (AFT surface model)
39 44 Ma (AFT subsurface)
70 60 Ma (AHe model)
44
44
BB
FR
75 50 Ma
(Kelley & Chapin, 2004
Kelley & Chapin, 1997)
63 34 Ma
(Naeser et al., 2002)
SR
AFT ages from surface samples
(track lengths >12 mm)
* no or variable
112track lengths
AFT subsurface samples
UU
WM
Precambrian crystalline basement
Youngest Laramide cooling ages:
80 Ma +
60 50 Ma
80 70 Ma
50 40 Ma
70 60 Ma
< 40 Ma
SC
TA
Younger ages related to Rio Grande rifting
COLORADO
PLATEAU
AHe ages related to He implantation or PRZ
0
112
10711_ch03_ptg01_hr_037-070.indd 61
100
N
SN
52 33 Ma
200 km
ARIZONA
NEW MEXICO
(Pazzaglia & Kelley, 2004)
108
104
AFT and AHe cooling
ages from multiple studies of Laramide ranges.
Colors represent youngest cooling ages. Thick
dashed line is outline of
the Colorado Plateau.
Thin dashed line represents the Cheyenne Belt.
See text for description of
how dates were selected.
Abbreviations as follows:
BB, Bighorn Basin; BL,
Black Hills; BH, Bighorn
Range; BT, Beartooth
Range; CB, Cheyenne
Belt; CMB, Crazy Mountains Basin; DB, Denver
Basin; FR, Front Range;
GM, Granite Mountains;
GR, Gore Range; GRB,
Green River Basin; HU,
Hartville Uplift; LP, Lima
Peaks; LR, Laramie
Range; MB, Medicine
Bow Mountains; MR,
Madison Range; OC,
Owl Creek Mountains;
PB, Piceance Basin;
PR, Park Range; PRB,
Powder River Basin; SC,
Sangre de Cristo Range;
SN, Sierra Nacimiento;
SR, Sawatch Range; TA,
Taos Range; TR, Teton
Range; UB, Uinta Basin;
UM, Uinta Mountains;
UU, Uncompahgre Uplift;
WM, Wet Mountains; WR,
Wind River Range; WRB,
Wind River Basin; WRU,
White River Uplift; p.c.,
personal communication.
100 km (62 mi).
6/5/13 7:59 AM
62 Peyton and Carrapa
If no inversion results are available, we include cooling
ages interpreted from age-elevation profiles. If they
are available and significant, we present results for
multiple age-elevation profiles in a range. For example, age-elevation profiles in the Wind River Range are
available from high-elevation samples (e.g., Gannett
Peak and Fremont Peak) and subsurface samples (e.g.,
Air Force well). Figure 9 shows both the older cooling
ages from higher in the range, and younger cooling
ages from the subsurface. We exclude AFT ages that
have mean track lengths <12 mm because they are not
indicative of rapid cooling. In areas with no age-elevation profiles, we use cooling ages interpreted from individual AFT ages and track length distributions (e.g.,
for the Owl Creek and Granite mountains we used
only single AFT ages with track lengths >12 mm).
We have also included a new, as yet unpublished AFT
surface age from the Uinta Mountains (C. Painter,
personal communication, 2012), as well as subsurface
AFT ages from the McMoran-Freeport 43-2A Middle
Mountain well at the eastern end of the Uinta Mountains (K. Constenius and S. Kelley, personal communication, 2012). We do not include cooling ages from
the Laramide basins, or ages that are interpreted to be
due to cooling from events unrelated to the Laramide
Orogeny (e.g., Miocene uplift due to the Rio Grande
Rift or basin-and-range extension at the Teton Range).
Symbol colors in Figure 9 represent the youngest recorded cooling age.
Examination of Figure 9 shows an apparent younging
of both oldest and youngest cooling ages to the south
and southwest, with the youngest ages from the Uinta
Mountains, White River Uplift, and Sierra Nacimiento,
and the oldest ages from the Bighorn, Beartooth, and
Teton Ranges. It is interesting to note that the youngest
ages (<40 Ma) are from samples that are near the edge
of the Colorado Plateau (Figure 9). These youngest ages
generally agree with a study of basin-fill sediments by
Dickinson et al. (1988), who suggested that Laramide
deformation terminated at ~50 Ma in the northern foreland and at ~35 Ma in the southern foreland.
Care should be taken when interpreting such a diverse data set. The absence of old ages may indicate
either that an area has been exhumed and older ages
eroded, or that cooling started at a later time than in
areas with older ages. The presence of a fossil PAZ
or PRZ in an age-elevation profile can help with this
distinction. If a fossil PAZ or PRZ is absent, then erosion of older ages is a possibility. Similarly, the depth
and present-day temperature of a subsurface sample
should also be considered: Young ages from a wellbore sample may be due to the sample residing within
the present-day PRZ or PAZ (i.e., the age is partially
reset), and not due to its cooling history.
10711_ch03_ptg01_hr_037-070.indd 62
APPLICATION TO MATURATION STUDIES
Combining basin modeling (burial-history or geohistory modeling) with low-temperature thermochronology and %Ro maturity data can provide information
on if and when source rocks reached high enough
temperatures for a sufficient amount of time to generate oil and gas. Low-temperature thermochronometers may also reveal the timing of tectonically driven
exhumation and fluid flow by recording associated
cooling. The success of exploration for conventional
petroleum plays is critically dependent upon the timing of both petroleum generation and trap formation.
In unconventional reservoirs, the shale may be the
source, seal, and reservoir, but fracturing, and hence
timing of tectonics, is often critical for successful
production.
Workflows and examples of reconstructing the
thermal history of a sedimentary basin by integrating
basin modeling, low-temperature thermochronology, %R o, and sometimes fluid inclusion data have
been discussed by Duddy et al. (1994), Duddy et al.
(1998), Crowhurst et al. (2002), Green et al. (2004),
and Parnell et al. (2005). Burial-history models are
typically created for samples from a wellbore, using
stratigraphic thicknesses and depositional ages, porosity-depth relationships, lithology and mineralogy
(to determine thermal conductivity), the present-day
geothermal gradient calculated from present-day
corrected borehole temperatures, and estimates of
paleosurface temperatures and heat-flow history. In
many studies that integrate basin modeling, lowtemperature thermochronology, and %Ro data, an initial estimate of the burial history is first determined
for a sample by assuming no cooling or exhumation,
a constant geothermal gradient, and constant ­basal
heat flow. This initial estimate is often referred to
as the “default thermal history” (e.g., Duddy et al.,
1994; Green et al., 2004; Parnell et al., 2005). %R o
values can then be calculated for this default thermal history and compared to actual %Ro values for
the sample. If predicted and calculated %R o values
match, then the sample is currently at its maximum
temperature and has not experienced any cooling.
However, if reliable measured %Ro values are higher
than those predicted by the default thermal model,
then the sample experienced higher paleotemperatures in the past than its present-day temperature.
Similarly, low-temperature-thermochronology ages
can be calculated from the default thermal history
and compared to actual values. If measured ages are
younger than predicted ages, then the sample was
likely at higher temperatures in the past than predicted by the default model.
6/5/13 7:59 AM
An Overview of Low-temperature 63
Estimated
erosion
Present-day surface
Depth
l
uria
al b
ition
Add
Aquifer with hot fluids
d
se
ea
cr
tion
he
at
w
flo
flow
flow
fluid
fluid
tion
ura
nt
ura
In
gd
Lon
rt d
adie
Sho
y gr
t-da
sen
10711_ch03_ptg01_hr_037-070.indd 63
Paleo-surface temp.
Pre
The maximum paleotemperature experienced by
a sample can be estimated from %Ro data using the
­kinetics of Burnham and Sweeney (1989) and Sweeney
and Burnham (1990). A range of possible maximum
paleotemperatures can also be estimated from AFT
age- and track-length data using an inversion program
such as HeFTy, which calculates a best-fit thermal
history and associated confidence limits (Ketcham,
2005; Ketcham et al., 2007). If individual grain AHe
data for a sample show a correlation between grain
age and grain size (Reiners and Farley, 2001), or
grain age and eU content (i.e., radiation damage,
Flowers et al., 2009), then AHe data can also be inverted to find the best-fit thermal history (Flowers
et al., 2007; Flowers, 2009).
Plotting the maximum paleotemperature against
depth in a wellbore gives an estimate of paleogeothermal gradient (Bray et al., 1992). If the paleogeothermal gradient is parallel to the present-day
geothermal gradient, but offset to higher temperatures, then samples were likely buried deeper in the
past but geothermal gradients and heat flow were
similar to present day (Figure 10). On the other hand,
if the paleogeothermal gradient is higher than present
day, then heat flow was higher in the past (Bray et al.,
1992). Extrapolating these paleogeothermal gradients
to an assumed paleosurface temperature provides an
estimate of the amount of material eroded from the
basin (Figure 10). Nonlinear paleogeothermal gradients can result from large changes in thermal conductivity (e.g., from a low-conductivity shaly section to
a higher-conductivity carbonate-evaporite sequence),
but in many cases indicate the passage of hot fluids.
Although the movement of high-temperature fluids
necessary to create a noticeable change in geothermal
gradient will be fundamentally vertical, the fluids
may also have a lateral component to their flow, and
thus may not affect all parts of a sedimentary section
equally. Duddy et al. (1994) suggested that hot fluid
flow in a formation would result in a characteristic
shape of the paleotemperature-depth profile, with
higher paleogeothermal gradients above the formation that experienced the hot fluid flow than below
it. Using the maximum paleotemperatures and paleogeothermal gradient, the default thermal history is
adjusted to create a basin thermal history that more
closely matches the %R o and thermochronometer
data. Similarly, the time-temperature histories from
inversion of thermochronometer data can be used iteratively to refine the basin model.
Using AFT and %Ro data, Green et al. (2004) followed the above methodology to determine a thermal history with two episodes of cooling from peak
paleotemperatures for a well from the Otway Basin in
Paleotemperature (oC)
Figure 10. Paleotemperature plotted versus depth.
Dotted black line is present-day geothermal gradient.
Solid line is paleogeothermal gradient if sample was
buried deeper in the past but the paleogeothermal
gradient and heat flow were similar to present day.
Gray solid line is the projection of this paleogeothermal
gradient to the assumed paleosurface temperature,
which gives an estimate of the amount of material
eroded. Long dashed line is paleogeothermal gradient
if heat flow was higher in the past. Solid red line is
paleogeothermal gradient after a long duration of
fluid flow. Dashed red line is paleogeothermal gradient
after a short duration of fluid flow. Modified from
Duddy et al. (1994) and Duddy et al. (1998).
6/5/13 7:59 AM
64 Peyton and Carrapa
southeastern Australia. With insufficient data to identify a variation of AHe age with grain size, they used
forward modeling of AHe data to refine the timing
of the youngest cooling episode. Parnell et al. (2005)
integrated burial history modeling, AFT, %R o and
fluid ­inclusion data to recognize a hot fluid event in
Cenozoic sandstones on the UK Atlantic margin that
was confined to fractures and did not reset AFT ages.
­Crowhurst et al. (2002) refined basin thermal-history
models for the Taranaki Basin of New Zealand using AHe and %R o data using a paleotemperature
­approach, and then forward modeled AHe ages for
four possible Neogene cooling scenarios. A model with
two discrete episodes of cooling between 8.5 and 2 Ma
provided the best fit to the measured AHe ages. In
that case, AHe dating provided additional control on
the nature of the cooling history due to its sensitivity
to lower temperatures than AFT dating. The relative
­timing of maximum burial, and hence ­hydrocarbon
generation, and the formation of trapping structures
are critical to predicting which structures may contain petroleum in the Taranaki Basin (Armstrong et al.,
1996; Crowhurst et al., 2002).
Osadetz et al. (2002) used burial history modeling
and AFT data to interpret the thermal history of the
Canadian Williston Basin. They concluded that although maximum burial of basement was attained
during late Cretaceous–early Tertiary time, maximum
paleotemperatures were attained during the late Paleozoic. Burial depths of <2 km (<1.2 mi) in the late Paleozoic imply that heat flow was significantly elevated,
and that lower Paleozoic source rocks may have generated oil at that time. Armstrong (2005) reviewed and
summarized this paper and its results, so it will not be
discussed further here.
As mentioned earlier, there have been no published
studies from the Laramide foreland (Figure 1) that
used low-temperature thermochronology to determine source-rock maturation and basin history.
Dickinson (1986) incorporated the conclusions of
Naeser (1986) into his basin model, but did not iteratively adjust his model to match AFT ages. Two studies
have been published from the adjacent Sevier foldthrust belt, one from the Idaho-Wyoming part of the
thrust belt (Burtner and Nigrini, 1994), the other from the
Canadian fold-thrust belt in southern Alberta (Osadetz
et al., 2004).
Burtner and Nigrini (1994) determined AFT ages
for surface and subsurface samples from the IdahoWyoming part of the Sevier fold-thrust belt. Almost all AFT ages were significantly younger than
depositional ages and were interpreted as cooling
ages. They used forward and inverse modeling of
AFT ages and tracks to calculate time-temperature
10711_ch03_ptg01_hr_037-070.indd 64
histories for two samples. After computing a burial
history using BasinMod (Platte River Associates,
http://www.platte.com), basal heat flow was varied until the time-temperature histories matched
those calculated from the AFT data. After varying the amount of Cretaceous strata eroded and the
heat flow model, Burtner and Nigrini (1994) could
not match deep AFT and %Ro data. They concluded
that movement on the Crawford thrust ~80–90 Ma
was coeval with a major decrease in the geothermal
gradient, and suggested that this decrease was due
to gravity-driven fluid flow. Increased heat flow ~110–
90 Ma in Lower Cretaceous Gannett Group rocks was
attributed to movement of hot fluids prior to movement on the Crawford thrust.
Osadetz et al. (2004) sampled rocks for AFT dating from both the hanging wall and footwall of the
Late Cretaceous–Paleocene Lewis thrust, and the
hanging wall and footwall of the Eocene–Oligocene
Flathead normal fault in southern Alberta. Using
only samples with a sufficient number of measured tracks, they modeled thermal histories using
MonteTrax (Gallagher, 1995). Major cooling events
from AFT modeling could be related to times of major displacement on the Lewis and Flathead faults.
Thermal-history models from samples at high levels
of the Lewis thrust showed one cooling event that
began ~75 Ma, coincident with displacement on the
Lewis thrust. Samples from the footwall of the Flathead normal fault (both hanging wall and footwall of
the Lewis thrust) also recorded a cooling event that
initiated in the Eocene due to extension on the Flathead fault. AFT ages from Oligocene sediments on
the hanging wall of the Flathead fault represent cooling ages of the sediment source terrains, and were
not reset by burial. By integrating AFT modeling results with %Ro data, Osadetz et al. (2004) concluded
that the geothermal gradient during Lewis thrusting
(~8.6°C/km to 12°C/km) was lower than present
day (~17°C/km), which they attributed to advective cooling by topographically driven groundwater
flow. They also concluded that petroleum generation
below the Lewis thrust was either due to tectonic
burial by the overriding Lewis thrust, or deep burial
by rapidly deposited sedimentary rocks that are no
longer preserved.
CONCLUSION
Results from AFT and AHe dating of samples from the
Rocky Mountain foreland show that to study the timing of the Laramide Orogeny, the best places to collect
samples are either from the ranges, where AFT and
6/5/13 7:59 AM
An Overview of Low-temperature 65
AHe ages reflect either Laramide cooling or an older
PRZ or PAZ, or the upper kilometer of sedimentary
rock from wellbores in the basins, which represent a
detrital unroofing sequence if thermochronometers
have not been reset due to burial. Detrital thermochronology from preserved surface sedimentary rocks
may also provide information on older exhumation
events that are no longer preserved in the ranges today (e.g., Pennsylvanian sedimentary rocks in the
Laramide Rocky Mountains may record cooling ages
associated with the Ancestral Rockies Orogeny, even
though the present-day ranges record Laramide cooling). This application has not been exploited yet in
the western United States. The best way to investigate
Neogene exhumation of the Rocky Mountains is using
samples from deep boreholes in the basins where ages
were reset due to burial during and after the Laramide
Orogeny.
Reviewing the results of more than 30 years of
low-temperature thermochronology in the Rocky
Mountains, a regional temporal evolution of the
Laramide Orogeny is elusive, but new AFT data
(C. Painter, K. Constenius and S. Kelley, personal
communication, 2012) combined with existing ages
seem to indicate a younging of cooling ages to the
south and southwest. Where multiple elevation
transects have been dated from a single range, it is apparent that cooling and exhumation histories within
ranges are complex (Cerveny, 1990; Cerveny and
Steidtmann, 1993; Roberts and Burbank, 1993; Kelley
and Chapin, 2004; Kelley, 2005; Peyton et al., 2012).
For example, it is perhaps unrealistic to conclude
that the Wind River Range experienced rapid cooling
at 42 Ma (Cerveny, 1990; Cerveny and Steidtmann,
1993), and better to conclude (as Cerveny and Steidtmann, 1993, did) that the west side of the range in the
vicinity of the Air Force well experienced rapid cooling at that time, and that this scenario may not apply
to the entire range.
Studies from the Beartooth Range (Cerveny, 1990;
Omar et al., 1994; Peyton et al., 2012), Wind River
Range (Cerveny, 1990; Cerveny and Steidtmann, 1993;
Peyton et al., 2012), Park Range (Cerveny, 1990) and
Black Hills (Strecker, 1996) all concluded that (possibly
rapid) cooling occurred ~62 Ma to 57 Ma, suggesting
a regionwide cause. The earliest cooling documented
to date in the Laramide foreland is ~85 to 75 Ma in the
Wind River Range (Cerveny and Steidtmann, 1993),
Bighorn Range (Cerveny, 1990; Peyton et al., 2012),
and Teton Range (Roberts and Burbank, 1993). Significant scatter of AHe ages, likely due to radiation damage and He implantation (Peyton et al., 2012), along
with often-large error bars on many AFT ages (± ~5–
10 Ma 1s) and variation in AFT ages between different
10711_ch03_ptg01_hr_037-070.indd 65
workers (e.g., Cerveny, 1990; Strecker, 1996), makes
resolving detailed timing of exhumation of one range
versus another difficult, and leaves room for more
work in the region.
Our understanding of the diffusion of He in apatite is still developing, particularly in rocks that have
spent a long time (relative to their age) in the PAZ
(e.g., the Precambrian crystalline basement exposed
in the Laramide ranges). Although well established in
other areas, AHe dating is still somewhat of a nascent
technique in the Rocky Mountain region. Recognition of the influence of radiation damage on AHe ages
from cratonic regions such as the Rocky Mountains
has advanced the understanding and utility of these
ages significantly in recent years (Flowers et al., 2007;
Flowers, 2009; Flowers and Kelley, 2011; Peyton et al.,
2012). The development of a tractable way to remove
the effects of He implantation will expand their usefulness further.
Low-temperature thermochronometers such as
AFT and AHe dating have many potential uses in understanding the structural and thermal evolution of
petroleum-producing regions. Age-elevation profiles
from ranges adjacent to sedimentary basins can provide the timing and rate of exhumation of the source
area of the sedimentary rocks. If the cooling and exhumation are directly related to tectonics, then they also
constrain the timing of deformation, which may have
implications for hydrocarbon migration and trapping. Age-elevation profiles from wells within a sedimentary basin provide information on the timing and
amount of burial and exhumation of the basin. Dating
a suite of samples from as large a depth range as possible in a single wellbore reduces overall uncertainty,
since all samples must conform to a single basin thermal-history model.
If shallow sedimentary rocks have not been thermally reset, their thermochronometer ages are detrital
cooling ages and may help constrain the provenance
and maximum possible depositional age of the rock,
as well as the unroofing history of the source areas.
Thermochronometer ages can provide specific information on timing and amount of cooling, and can thus
be used to constrain basin thermal-history models.
Integrating a proposed burial-history model derived
from stratigraphic and sedimentary data with (1) thermochronometer data, (2) constraints on total heating
from %Ro, and (3) constraints on temperatures at specific times in the past from fluid inclusions, allows one
to arrive, iteratively, at a highly constrained thermalhistory model. This new model can in turn improve
our understanding of the level and timing of sourcerock maturation, and thus improve our exploration
success.
6/5/13 7:59 AM
66 Peyton and Carrapa
Low-temperature thermochronology is an underutilized petroleum exploration tool in the Rocky
Mountains. We have illustrated that it has been used
successfully throughout the world to constrain sourcerock maturation history and burial history of sedimentary basins, as well as tectonic history and sediment
provenance. With the current high interest in unconventional shale reservoirs throughout the Rocky
Mountain region, we recommend that low-temperature
thermochronology be added to the explorationist’s
tool box.
ACKNOWLEDGeMENTS
We thank Shari Kelley, Jim Steidtmann, and Doug
Waples for their constructive reviews. Connie Knight
and Rich Bottjer provided many helpful suggestions
for improving the manuscript. Majie Fan lent her expertise in paleoaltimetry and paleotemperatures. Clay
Painter generously shared his AFT age from the Uinta
Mountains. We thank Kurt Constenius and Shari
Kelley for sharing their data from the McMoranFreeport 43-2 Middle Mountain well. Doug Waples
also provided assistance with well-log temperature
corrections. We are grateful to Cirque Resources for
providing a quiet place to write.
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