26176 1st pages / 1 of 5
Regional structure and kinematic history of the Sevier fold-and-thrust belt,
central Utah: Reply
James C. Coogan†
Department of Geology, Western State College of Colorado, Gunnison, Colorado 81231 USA
Peter G. DeCelles‡
Department of Geosciences, University of Arizona, Tucson, Arizona 85721 USA
Christie-Blick and Anders highlight worthwhile aspects of the debate over the magnitude
of Cenozoic crustal extension associated with
the Sevier Desert detachment of central Utah.
We welcome this opportunity to expand the necessarily limited treatment of the Sevier Desert
detachment in DeCelles and Coogan (2006) with
an evaluation of the most recent components
of the “mounting evidence” cited by ChristieBlick and Anders that the Sevier Desert detachment does not exist. For brevity, we limited
our objections in DeCelles and Coogan (2006)
to the principal geologically based unconformity interpretations for the lower boundary of
the Cenozoic Sevier Desert basin espoused by
Hamilton (1994), Anders and Christie-Blick
(1994), Anders et al. (1995), Wills and Anders
(1999), and Anders et al. (2001). We did not take
the opportunity to expand our objections to the
details of geophysically based hybrid interpretations for the boundary (salt tectonics, Mesozoic
thrust faults, and unconformities) most completely presented by Wills et al. (2005) that form
the principal bases for the current discussion by
Christie-Blick and Anders.
Fortunately, the key geophysical objections
raised by Christie-Blick and Anders are efficiently addressed with publicly available geophysical and well data along a single transect
of the west-central margin of the Sevier Desert
basin (Fig. 1). The analysis presented along this
transect is identical to that with which we corroborated our interpretation along the DeCelles
and Coogan (2006) transect located 35 km north
of Figure 1. A depth conversion of a published,
unmigrated, 12-fold vibroseis reflection seismic
profile forms the geophysical backbone of the
transect (Pan Canadian 3 of McDonald, 1976).
This profile is also reproduced and interpreted
by Wills et al. (2005, their Fig. 11), and thus
provides a common basis for comparing inter†
E-mail: jcoogan@western.edu
E-mail: decelles@geo.arizona.edu
‡
pretation methods and results. The seismic data
are complimented by a complete Bouguer gravity anomaly profile extracted from the United
States Gravity Data Repository System (PanAmerican Center for Earth and Environmental
Studies, 2006). Importantly, the profile incorporates three deep industry wells that lie in the
plane of the profile (Cominco American Beaver
2), or within 4 km of the profile (Chevron Black
Rock 1–29 and ARCO Pavant Butte 1) to constrain the velocity, density, and depth structure
along the transect. Sonic and density logs, as
well as some dipmeter and sample logs for these
wells, are available through the Utah Division of
Oil, Gas, and Mining Web site.
SIGNIFICANCE OF WELL
CORRELATIONS
The Cominco well in Figure 1 provides
a critical constraint for locating the Canyon
Range and Pavant thrust sheets in the hanging
wall of the Sevier Desert detachment and for
correlating them to seismic reflections that truncate downward against an underlying reflection
zone that McDonald (1976) first correlated to
a major detachment fault. This apparent hanging wall cutoff geometry provides the main
constraint for Sevier Desert detachment extension estimates because equivalent Sevier Desert
detachment footwall cutoffs for both thrusts lie
~45 km to the east of the well near fault exposures in the Canyon and Pavant Ranges (Fig. 1).
The truncation of these thrust sheet reflections
is a principal feature of the western margin of
the Sevier Desert basin that we and other workers have correlated through the regional seismic
network for 50 km north and 10 km south of the
Cominco well (McDonald, 1976; Allmendinger
et al., 1983; Von Tish et al., 1985; Mitchell and
McDonald, 1987; Coogan and DeCelles, 1996).
This correlation has only been strengthened in
recent years by the release of additional industry
strike profiles diligently secured by Wills et al.
(2005), but incorrectly correlated by them to the
Cominco well.
Figure 1 illustrates the difference between our
2557 m measured depth for the Canyon Range
thrust in the Cominco well and the 1222 m
interpreted depth for the thrust from Wills et al.
(2005). Wills et al. (2005) follow Mitchell and
McDonald’s (1986) geophysical log interpretation for repeats of the Lower Cambrian Pioche
Shale and Prospect Mountain Quartzite below
Proterozoic quartzites and argillites starting at
1222 m depth. Our correlation of the Cominco
log suite closely follows the combined gamma
ray log and sample analysis by John E. Welsh
(reported in Hintze and Davis, 2003), who interpreted the 1222 m depth to lie ~20 m above the
contact between the Proterozoic Caddy Canyon Quartzite and the older Blackrock Canyon
Limestone. The combined sample and geophysical logs for the 1996 Chevron Black Rock
well (Fig. 1) closely match the thicknesses and
sequence of Welsh’s (Hintze and Davis, 2003)
Proterozoic assignments for the Cominco well.
Both wells display a complete Proterozoic stratigraphy down to a common Canyon Range
hanging wall décollement level in the Pocatello
Formation—the same décollement level
exposed in the southernmost Canyon Range,
55 km to the east. We contend that Wills et al.
(2005) misidentified the lower Caddy Canyon
and Blackrock Canyon phyllites and limestones
as the micaceous shale- and limestone-bearing
Pioche Formation of the Pavant hanging wall.
We further insist that this aggressive reinterpretation of the well data is sufficiently countered
by the previous work of Welsh (Hintze and
Davis, 2003), which, unfortunately, was not
addressed and cited by Wills et al. (2005) as an
alternative but authoritative stratigraphic assignment for this critical well interval.
A larger stratigraphic oversight by Wills et al.
(2005) is their correlation of the Pavant thrust to
the 2557 m depth in the Cominco well, where
we place the Canyon Range thrust at the base of
GSA Bulletin; May/June 2007; v. 119; no. 5/6; p. 000–000; doi: 10.1130/B26176.1; 1 figure.
For permission to copy, contact editing@geosociety.org
© 2007 Geological Society of America
1
26176 1st pages / 2 of 5
112o 45’
Sevier Desert
Basin
16
8
Arco
Cricket Mtns
Wills et al. (2005)
basin boundary
response
112o 30’
Canyon Range thrust
Clear Lake fault
Profile
Chevron
Pan Canadian 3
Cominco
Pavant thrust
0
Rng
39o15’
N
Pavant
24
Sevier
De
break sert
away
Complete Bouguer Anomaly (mgal)
Coogan and DeCelles
39o
10 km
Calculated
Observed
-8
Cricket Mountains
Chevron
Black Rk Fed 1-29
Cominco American
Beaver Fed 2
253o
Sevier Desert Basin
073o
Arco
Pavant Butte 1
Hypothetical
Clear Lake Location
Clear Lake
fault scarps
Beaver River
2080 kg/m3
2350 m/s
T3
0
TQ
TQ
T3
2315 kg/m3
2350 m/s
T3
Neoproterozoic Lower Cambrian
sedimentary rocks
2650 kg/m3
4900 m/s
CRTW
T2
22 o
dip
T2
CRT
Elevation (km)
T1
5
TQ
T3
T1
2535 kg/m3
4280 m/s
2400 kg/m3
3667 m/s
T1
2510 kg/m3
4700 m/s
T2
T1
Eastern SDR
PVT hw
2780 kg/m3
5750 m/s
Precambrian basement Paleozoic sedimentary
rocks
2780 kg/m3
5750 m/s
V.E. = 1.25
5 km
Western SDR
Figure 1. Combined seismic depth profile and gravity model along seismic line Pan Canadian 3. Solid lines outline velocity and density fields
used for the seismic depth conversion and gravity model. Dashed lines on profile show the locations of interpreted faults within constant
velocity/density domains. T1, T2, and T3—Tertiary strata. TQ—Late Tertiary and Quaternary deposits. Well locations are indicated by
synthetic seismograms. CRT—depth of the Canyon Range thrust in the Cominco well. CRTW—depth of Canyon Range thrust interpreted
by Wills et al. (2005). PVT hw—Pavant hanging wall strata at base of Cominco well. Dashed gravity profile is calculated complete Bouguer
anomaly for basin boundary interpretation of Wills et al. (2005). Eastern SDR and Western SDR are segments of the Sevier Desert reflection described in the text.
the Proterozoic Pocatello Formation and above
lower Paleozoic carbonates (Fig. 1). More than
the overlying hanging wall correlation, the palinspastic setting of the footwall carbonate section
precludes the Wills et al. (2005) contention that
it lies below the Pavant thrust. Wills et al. (2005)
and Welsh (Hintze and Davis, 2003) agree with
us that this 1411-m-thick carbonate and shale
succession correlates in thickness and lithology
to the complete stratigraphic succession from
the Ordovician-Cambrian Notch Peak Dolomite
through the Pioche Formation, a section that is
exposed only in the hanging wall of the Canyon
Range thrust. This sequence contains distinctive shale units (Whirlwind Formation, Wheeler
Shale, and Chisholm Formation) that are absent
eastward in the Pavant Range, where they correlate to the thinner section of Ajax Dolomite
through Ophir Formation exposed in the frontal
Pavant thrust hanging wall. The only palinspastically valid interpretation for the Cominco Cambrian sequence is that it lies in the trailing part of
the Pavant hanging wall, where it carries a more
2
basinal miogeoclinal section than is found near
the leading edge of the same thrust sheet.
The miscorrelation of thrust sheet stratigraphy by Wills et al. (2005) demonstrates a disregard for the necessary application of structural
balancing principles in the seismic interpretation of deformed sedimentary terrains. More
troublesome are current comments by ChristieBlick and Anders, in which they apparently fail
to comprehend that balanced cross sections are
precisely geometric and kinematic validation
tools (Elliott, 1983). Cross-section restoration
to a reasonable original geometry is a validation
step that demonstrates that an interpretation is
geometrically possible. Sequential restorations,
such as those presented in DeCelles and Coogan
(2006), add a further kinematic validation to an
interpretation. The Pavant thrust footwall correlation proposed by Wills et al. (2005) for the lower
part of the Cominco well cannot be restored with
the stratigraphy of the frontal Pavant hanging
wall to a reasonable pre-thrust basin geometry.
In short, it is geometrically invalid.
On a larger scale, the Cominco miscorrelation invalidates the entire Wills et al. (2005)
correlation of pre-Tertiary structural features
throughout the western Sevier Desert basin seismic grid, which is only tied to the regional thrust
sheets at this single well. The regional tectonic
implication of the miscorrelation is apparent by
observing that the Pavant thrust must lie below
the Prospect Mountain Quartzite penetrated at
the 4021 m total depth of the well, rather than
directly above the prominently arched reflection
group where we correlate the Canyon Range
thrust (Fig. 1). The miscorrelation permits Wills
et al. (2005) to correlate the Pavant and overlying Canyon Range thrusts above the arched
reflections to an eastward erosional truncation
at the base of the basin. Instead, the necessary
position of the Pavant thrust below the well
bore indicates that reflections from the Pavant
hanging wall arch eastward and truncate downward against the reflection zone associated with
the Sevier Desert detachment. These hanging
wall truncation locations form the basis for the
Geological Society of America Bulletin, Month/Month 2007
26176 1st pages / 3 of 5
Discussion and Reply
extension estimates relative to equivalent footwall cutoff positions along the west flank of the
Pavant Range.
GEOPHYSICAL INTERPRETATION OF
THE WESTERN BASIN MARGIN
The contention by Christie-Blick and Anders
and Wills et al. (2005) that there is little evidence for Tertiary detachment-related rotation in the western Sevier Desert detachment
hanging wall is based on their poor geophysical discrimination of reflection groups through
the regional reflection seismic grid. A coherent
sequence of 8°–25 o eastward dipping reflections that lies above Mesozoic thrust sheets is
correlated along the entire western basin margin
into the Pan Canadian 3 profile as unit T1 immediately east of the Cominco well in Figure 1.
These strata are generally correlative with strata
originally designated as Eocene-Oligocene by
McDonald (1976), “older Tertiary” by Mitchell
and McDonald (1987), and later correlated by
Von Tish et al. (1985) to a dated 26–28 Ma Oligocene interval in the key Gulf Gronning well
in the northwestern basin. We have long recognized that the lack of an acoustic log for the
Gulf well permits arguments against the tie of
the Oligocene dates to the 8°–25 o eastward dipping reflections (Coogan and DeCelles, 1996).
Recognizing that density logs across the basin
delineate a >100 kg/m3 density contrast between
Proterozoic-Paleozoic and older Tertiary sedimentary rocks (Fig. 1), we used density and
gravity data to corroborate the seismic correlations of Von Tish et al. (1985) for Consortium
for Continental Reflection Profiling (COCORP)
Utah Line 1 and Coogan and DeCelles (1996)
for Geophysical Service Incorporated (GSI)
Line 15 in the northern basin prior to completing
the regional transect presented in DeCelles and
Coogan (2006). Figure 1 presents a combined
depth and density model for the Pan Canadian
3 profile that is comparable to our modeling of
the northern basin and confirms that the 8°–25 o
eastward dipping reflections have physical characteristics of the lower Tertiary, rather than Proterozoic or Paleozoic, sedimentary rocks found
throughout the basin.
The model was constructed by correlating the
principal velocity interval boundaries on the seismic time section using synthetic seismograms
for the three wells along the profile (Fig. 1), as
well as by using ties along the regional seismic
grid to previously modeled profiles. Simple vertical incidence depth conversion of the seismic
time image in LithoTect software incorporated
sonic log velocities from the three wells along
the profile. Densities were then constrained for
each velocity interval from density logs from the
three profile wells. Velocity and density logs for
comparable stratigraphic intervals and depths
for all wells in the basin further constrained permissible variations along the model (Planke and
Smith, 1991). Complete Bouguer gravity data
were extracted from the United States Gravity
Data Repository System (Pan-American Center
for Earth and Environmental Studies, 2006) with
complete Bouguer anomaly values calculated
relative to a third-order polynomial regional
surface. The density/depth profile was modeled
with GM-SYS software as a 2.5-dimensional
model with the profile oriented 50° to the strike
of the dominant anomaly trend that is parallel to
the strike of the Cricket Mountains.
The gravity model provides an additional
level of constraint to the seismic time interpretation and the resulting borehole-constrained
depth model. The close fit of the model to the
observed gravity anomaly corroborates the seismic depth interpretation of a ~1,200 m thick
body of intermediate density (unit T1 in Fig. 1)
that corresponds to the reflection package that
lies directly east of the Cricket Mountain block
with 22° eastward apparent dip (Fig. 1). The
2525 kg/m3 density modeled for this body lies
in the middle of the range for older Tertiary
strata drilled in the deepest wells through the
basin (Planke and Smith, 1991) and is consistent with interpretation of the body as the Oligocene and older sedimentary sequence dated
or interpreted elsewhere in the basin (Von Tish
et al., 1985; Mitchell and McDonald, 1987). An
interpretation of Pan Canadian 3 by Wills et al.
(2005, their Fig. 11) provides a fair comparison
of the gravity model fit to contrasting interpretations. Wills et al. (2005) interpret all but the
upper ~100 m of the reflection package as tilted
Proterozoic through Cambrian rocks of the Canyon Range and Pavant thrust sheets. Changing
the model to the Wills et al. (2005) position
for the western basin boundary by extending
the 2650 kg/m3 average density for the Canyon
Range thrust sheet eastward results in a 3 mgal
overestimation of the anomaly across these dipping reflections, an approximate 10% error relative to the entire anomaly range across this high
gradient basin boundary (Fig. 1). The magnitude of the computed gravity error between our
basin boundary location and that of Wills et al.
(2005) does not change in simpler two-dimensional modeling.
The gravity corroboration of eastward tilted
older Tertiary strata along the western basin
margin is consistent with rotation of these
strata above a detachment fault, but is in direct
conflict with assertions by Christie-Blick and
Anders and Wills et al. (2005) that the margin
is principally marked by onlap of subhorizontal
strata. Review of the Wills et al. (2005) seismic
interpretations indicates that the steeply dipping
reflection package is present below the onlap
package along the entire western basin seismic
grid, including the area south of the Cominco
well. We contend that Wills et al. (2005) misinterpreted these reflections as lying within Mesozoic thrust sheets below the basin (Figs. 11, 17,
and A5 of Wills et al., 2005), and elsewhere split
the middle of the reflection group with the basin
boundary (Figs. 16, A1, and A3 of Wills et al.,
2005), or segregated the east-dipping reflections
into stacks of low relief (~200 ms) clinoforms
bounded by subhorizontal surfaces (Figs. 8 and
A4 of Wills et al., 2005). In the latter case, Wills
et al. (2005) failed to address zones of likely
interference between steeply dipping primary
reflections and subhorizontal coherent noise, an
interpretation pitfall previously highlighted by
Coogan and DeCelles (1998).
Wills et al. (2005) correctly note that thrust
sheet reflections, including our pick for the
Canyon Range thrust, are truncated by the basal
unconformity of the western basin boundary
south of the Cominco well, a geometry previously described by Planke and Smith (1991).
Christie-Blick and Anders and Wills et al. (2005)
emphasize the obvious point that these reflections cannot be used to interpret a hanging wall
cutoff geometry for the Sevier Desert detachment in this area, but these observations do not
conflict with our cutoff constraints adjacent to
and north of the Cominco well. Instead, southward erosional truncation of the Canyon Range
through Pavant thrust sheets matches the footwall erosional truncation geometry exposed east
of the Sevier Desert detachment breakaway zone
identified adjacent to the Canyon Range near
Oak City, Utah (Otton, 1995), and the Pavant
Range near Fillmore, Utah (Royse, 1993). The
exposed Canyon Range and Pavant thrust sheets
and thrust surfaces are truncated southward by
successively younger upper Cretaceous through
Eocene strata into successively deeper structural levels of the thrust sheets from the northern Canyon Range through the southern Pavant
Range (Hintze and Davis, 2002; Hintze et al.,
2003). Thus, the southward erosional truncation
of thrust-related reflections in the buried Sevier
Desert detachment hanging wall is consistent
with the outcrop truncation geometry in the
Sevier Desert detachment footwall, with the preextensional erosion level of the hanging wall and
footwall matching across the detachment fault.
We agree with Christie-Blick and Anders that
the incomplete coverage and uneven quality of
Sevier Desert basin reflection seismic data allow
some latitude for interpretational differences,
but we do not agree that this truism indicates that
the Wills et al. (2005) interpretation, described
in Christie-Blick and Anders as “based on the
Geological Society of America Bulletin, Month/Month 2007
3
26176 1st pages / 4 of 5
Coogan and DeCelles
tracing of reflections through the entire grid of
available profiles,” is valid. Aside from the miscorrelation of well ties, the Wills et al. (2005)
seismic interpretation overlooks many best
practices of geophysical interpretation including
corroboration with other available geophysical
data, discrimination of primary reflections from
coherent noise, and validation of time interpretations in the depth domain. Christie-Blick and
Anders’ contention that the rooted Sevier Desert
detachment interpretation is based on the fortuitous alignment of reflections from Mesozoic
thrusts in the west (Western SDR in Fig. 1) and
an unconformity in the east (Eastern SDR in
Fig. 1) may be one result of their working exclusively in the time domain. The largest break in
the continuity of the Sevier Desert detachment
reflection is precisely in the area of largest lateral velocity contrast across the axis of the basin,
where the thickest section of low velocity Tertiary rocks lie adjacent to the thickest section of
high velocity Proterozoic and Paleozoic rocks
across the western basin margin (Fig. 1). Wills et
al. (2005) consistently interpret east-side-down
high-angle normal faults cutting the Sevier Desert reflection along the deep basin axis throughout the time data (Figs. 9, 10, 16, 17, A1, A3, A4,
and A6 of Wills et al., 2005), precisely where
the velocity “push-down” effect under the basin
exhibits the greatest contrast with the velocity
“pull-up” effect under Mesozoic thrust sheets
across the basin margin, and where necessarily
complex wave paths predictably disrupt reflections. Although unmigrated and further limited
by our simple depth conversion methodology,
Figure 1 sufficiently illustrates that an east-sidedown normal fault is difficult to reconcile with
the depth domain image across the basin axis.
LIMITATIONS OF CORE AND SAMPLE
ANALYSES
The original experimental basis for ChristieBlick and Anders’ dismissal of a supradetachment origin of the Sevier Desert basin is the
analysis of drill cuttings and core presented by
Anders and Christie-Blick (1994) and expanded
by Anders et al. (2001). Both studies suffer from
unavoidably coarse sampling precision and
questionable rock-mechanical assumptions that
limit their relevance for proving or disproving
the existence of the Sevier Desert detachment.
Allmendinger and Royse (1995) and Anders et
al. (1995) noted the imprecision in correlating
millimeter-scale cuttings to measured depths
in boreholes, but assumptions by Anders and
Christie-Blick (1994) and Anders et al. (2001)
concerning the mechanical stratigraphy of the
basin and the resulting grain-scale response to
faulting require comment. In examining drill
4
cuttings from Tertiary clastic rocks near the
lower basin contact, Anders and Christie-Blick
(1994) and Anders et al. (2001) assume that that
elevated microfracture densities in individual
quartz grains are diagnostic of fault zones. This
assumption is mainly based on outcrop studies of
faulted sandstone and clast-supported conglomeratic sandstone by Brock and Engelder (1977)
and Anders and Wiltschko (1994). Thus, the
lack of an increase in microfracture density from
cuttings reported by Anders and Christie-Blick
(1994) at the basin boundary might only indicate
the absence of faulting in sandstone or mechanically similar quartz-bearing rock. Gamma ray,
sonic, and density log responses in all logs that
sample the deep basin document thin (0.5–2 m)
clay-bearing intervals (shale) along and immediately above the contact. Where geophysical
logs of the boundary are absent in the Argonaut
Energy well, samples above the detachment
consist of granule-sized quartzite, carbonate,
and volcanic clasts in a clay matrix (Mitchell,
1979; Anders et al., 2001). If displacement at
the base of the Tertiary section is preferentially
accommodated along shaley intervals, it is likely
that fault rock samples would not recognizably
survive disaggregation associated with drilling and subsequent transport through a 2–3 km
borehole mud column. In addition, strain may be
partitioned between the clay matrix and quartz
clasts of coarser matrix-supported lithologies
with little microfracturing of quartz grains.
Anders and Christie-Blick (1994) and Anders
et al. (2001) also note the absence of discrete or
distributed ductile or incipient ductile deformation in a carbonate core from ~13 m beneath the
basin boundary. Ironically, this core observation
is consistent with the Anders et al. (2001) field
and petrographic study of carbonates above the
Cave Creek detachment, in which they constrained discernable deformation only within
15 m of the fault. Observations of sub-basin
metamorphic grade by Anders and ChristieBlick (1994) and Anders et al. (2001) conflict
with those of Welsh (reported in Hintze and
Davis, 2003), who describes samples below the
Sevier Desert reflection zone as “Cambrian phyllite and marble” (p. 276) in the ARCO Pavant
Butte 1 well and as “marbleized Cambrian
limestone and dolomite, sheared, graphitic, with
fractured quartzite vein” (p. 259) in the Argonaut Energy 1 well. Fault zones are distinctive
precisely because they are discrete zones of
deformation with finite widths. The conflicting
interpretations from observations above and
below the basin boundary are likely the result
of the unavoidably incomplete sampling of discrete brittle and ductile deformation zones that
is inherent in the use of industry data that are not
necessarily suited to fault detection.
STRATIGRAPHIC AND
THERMOCHRONOLOGIC
CONSTRAINTS
Christie-Blick and Anders acknowledge the
necessity of pre-Miocene displacement along
the Sevier Desert detachment in supradetachment interpretations for basin genesis (McDonald, 1976; Mitchell and McDonald, 1987; Von
Tish et al., 1985; Coogan and DeCelles, 1996).
We find a Paleogene age for detachment initiation to be consistent with the regional timing
and style of initial extension as well as with
thermochronologic constraints. Constenius
(1996) identified synextensional Oligocene
strata associated with the extensional collapse
of thrust sheets through a broad region of the
Cordilleran fold-thrust belt, including the Great
Salt Lake basin to the north, and the Juab, Sanpete, and Sevier valleys immediately east of the
Sevier Desert basin. Interpretations that initiate
the Sevier Desert detachment along the former
Pavant thrust plane (Mitchell and McDonald,
1987; Allmendinger et al., 1986; DeCelles and
Coogan, 2006) are consistent with the geometry
and timing of this regional event. The pre-Oligocene erosional level of Sevier Desert detachment hanging wall is independently constrained
by the provenance of synorogenic conglomerates derived from thrust sheets at the end of the
Sevier orogeny (DeCelles and Coogan, 2006).
From this erosional datum, a significant increment of Paleogene extensional exhumation of
the Sevier Desert detachment footwall is a necessary precursor for eventual Miocene uplift
through the apatite annealing temperatures of
more shallow parts of the detachment in the Canyon Range (Stockli et al., 2001), and through the
zircon annealing zone for the deeper part of the
detachment beneath the axis of the Sevier Desert basin (Allmendinger and Royse, 1995).
TOWARD AN INTEGRATED
INTERPRETATION FOR THE SEVIER
DESERT BASIN
Considering the strength of the geophysical
and well evidence for the existence of the Sevier
Desert detachment, Christie-Blick and Anders’
appeal for scientific drilling in the Sevier Desert basin seems unnecessary for confirming the
presence of an already well-constrained fault.
However, the current discussion of the basin
geometry and genesis highlights the Sevier
Desert basin’s suitability for integrated geological and geophysical characterization of the
processes associated with high- and low-angle
faulting, basin subsidence, and footwall uplift
in regional continental extension. A scientific
drilling program could be a cornerstone of such
Geological Society of America Bulletin, Month/Month 2007
26176 1st pages / 5 of 5
Discussion and Reply
integrated interpretation if designed toward characterizing phenomena such as the in situ stress,
fluid pressure, heat flow, density, and velocity
distributions, along with fault zone deformation
mechanisms through a vertical sample of the
basin fill, intrabasin faults, and the basin-bounding detachment fault and footwall. Such a program should also provide biostratigraphic and
thermochronologic constraints for reconstructing basin subsidence and footwall uplift.
A scientific drill site for the Sevier Desert
basin should take advantage of existing borehole and seismic data to provide pre-drill geological and geophysical context and to constrain
depth prognoses for stratigraphic and structural
targets. A drill site located in the hanging wall
of the Clear Lake fault zone of the southwestern
basin (Fig. 1) meets these criteria and provides
an additional opportunity for sampling multiple
fault zones in a seismically active part of the
basin. The Clear Lake fault zone is marked by
post-17 ka and Holocene fault scarps (Oviatt,
1989) that correlate downward to two seismically imaged normal faults that appear to terminate at the Sevier Desert detachment reflection
(Fig. 1; Crone and Harding, 1984). The apparent
geometric linkage between the Clear Lake faults
and the Sevier Desert detachment permits speculation that displacement along the Clear Lake
fault zone is rooted to an active western segment
of the Sevier Desert detachment. Thus, a Clear
Lake drill site may present a rare opportunity to
characterize deformation processes along active
high- and low-angle continental extensional
faults. Full penetration of the hanging wall and
footwall of the Sevier Desert detachment at
a Clear Lake drill site would require a ~4 km
deep borehole. Intermediate objectives based on
the Clear Lake hanging wall location illustrated
in Figure 1 would include the Clear Lake faults
between 1.1 and 1.8 km depth and the Sevier
Desert detachment at 3.4 km depth.
The technical and funding challenges of
a deep, integrated scientific drilling project
require cooperation across a broad spectrum of
geoscience disciplines. The borehole studies of
Anders and Christie-Blick (1994) and Anders
et al. (2001), with all of their inherent limitations, provide a foundation for designing more
appropriate drilling, logging, and sampling
protocols and analysis techniques for identifying and characterizing fault zones in the Sevier
Desert basin. A more complete analysis of the
existing geophysical and geological data set,
including seismic reprocessing with migration,
depth, and gravity modeling, and integration of
proprietary biostratigraphic and geochronologic
data, should be a prerequisite for any scientific
drilling proposal. We hope to work with Christie-Blick and Anders in securing the release and
cooperative analysis of such data as a necessary
first step in advocating scientific drilling in the
Sevier Desert basin.
REFERENCES CITED
Allmendinger, R.W., Sharp, J.W., Von Tish, D., Serpa, L.,
Brown, L., Kaufman, S., and Oliver, J., 1983, Cenozoic
and Mesozoic structure of the eastern Basin and Range
province, Utah, from COCORP seismic-reflection
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