The grabens of Canyonlands National Park are a young and active system of sub-parallel, arcuate grabens, whose evolution is the result of salt movement in the subsurface and a slight regional tilt of the faulted strata. We present results of ground-penetrating radar (GPR) surveys in combination with field observations and analysis of high-resolution airborne imagery. GPR data show intense faulting of the Quaternary sediments at the flat graben floors, implying a more complex fault structure than visible at the surface. Direct measurements of heave and throw at several locations to infer fault dips at depth, combined with observations of primary joint surfaces in the upper 100 m, suggest a highly dilatant fault geometry. Sinkholes observed in the field as well as in airborne imagery give insights in local dilatancy and show where water and sediments are transported underground. Based on correlations of paleosols observed in outcrops and GPR profiles, we argue that either the grabens in Canyonlands National Park are older than previously assumed or that sedimentation rates were much higher in the Pleistocene.
Understanding the structure of dilatant fractures in normal fault zones is important for many applications in geoscience. Reservoirs for hydrocarbons, geothermal energy and freshwater often contain dilatant fractures (e.g., Ehrenberg and Nadeau, 2005; Wennberg et al., 2008; Jafari and Babadagli, 2011). A better understanding of the internal structures of such fault zones is required to extrapolate fault geometries in typical reservoir depths below the industrial seismic resolution. This is especially complicated when failure structures of different generations and with different mechanical properties interact with each other.
In this paper we present a case study focusing on the spectacular grabens of the Needles fault zone in Canyonlands National Park, Utah, USA (field photographs Fig. 1a–c), a fault array that reactivates two preexisting major joint sets. The area is well known as a sandstone reservoir analogue, and sedimentary rocks and soil are well exposed due to the semi-arid climate. Estimates of the beginning of graben formation vary from 60–80 ka (based on the dating of sediments in deep open fissures; Biggar and Adams, 1987) to 1.4 Ma, which is the estimated time of incision of the Colorado River (Schultz-Ela and Walsh, 2002). Present-day motions are known from interpretation of interferometric synthetic aperture radar (InSAR) data (Furuya et al., 2007). Reheis et al. (2005) were able to date Quaternary strata deposited during formation of the grabens. These studies show that the grabens of the Canyonlands are an active fault zone with syntectonic sedimentation. The goal of this study is to better understand the coupling between deformation, erosion and deposition in such an active system, as well as to gain insights into the internal structure of dilatant fracture systems. To this end, we combine detailed field analyses with ground-penetrating radar and remote-sensing data.
The Needles fault zone is located in Canyonlands National Park on the
western rim of the Paradox Basin in southeastern Utah, USA (Fig. 2). This zone of extensive faulting is
characterized by an arcuate array of grabens, also known as
Geology of SE Utah and major structural units. Inlet indicates location of Fig. 3. The study area (grabens in the Canyonland National Park) is located at the northern end of the Monument Upward, east of the Colorado River. Geological and structural data compiled from Huntoon et al. (1982), Condon (1997), Hintze et al. (2000) and Gutiérrez (2004).
The regional tectonic influence of the Monument Upwarp (Fig. 2), a regional fold structure developed during the
Laramide Orogeny in early Tertiary (Goldstrand,
1994; Condon, 1997; Walsh and Schultz-Ela, 2003), caused differential local
tilting of the Needles fault zone, ranging from 2
The stratigraphy of the study area consists of more than 300 m of Pennsylvanian evaporites at the base, mainly halite, gypsum and anhydrite (e.g., Cartwright et al., 1995). These units are now located at depths between 300 and 500 m, and they crop out only within the Colorado River incision (e.g., Mertens, 2006). These are overlain by sandstones, intercalated shales and limestones of the Pennsylvanian, as well as the prominently jointed Permian Cedar Mesa sandstones on top (Lewis and Campbell, 1965; Huntoon, 1982; McGill et al., 2000). This entire overlying sediment package reaches up to 500 m in thickness (e.g., McGill and Stromquist, 1979; Trudgill and Cartwright, 1994; Schultz and Moore, 1996; Cartwright and Mansfield, 1998; Moore and Schultz, 1999; Fossen et al., 2010).
The grabens are filled with unconsolidated Quaternary soft sediments. According to the geological map of Billingsley et al. (2002) most of Devil's Pocket and Cyclone Canyon as well as northern Devil's Lane are dominated by Holocene and Pleistocene alluvial deposits. The southern section of Devil's Lane is mostly filled with Holocene alluvial fan deposits. Isolated depressions permitted ponding of Holocene silt and mud in Devil's Lane and Cyclone Canyon with sediment thicknesses of up to 2 m. Faulting significantly influences the distribution of local deposition centers and the drainage system (Trudgill, 2002). All grabens contain rockfall debris (Fig. 1b).
Incision of the Colorado River, in combination with the local tilt, is seen
by most researchers as the cause for the formation of the grabens, either due to
salt movement or gravitational gliding. Huntoon (1982) proposed that salt
movement occurs as a reaction to the unloading effect of the incising
Colorado expressed as the Meander Anticline, a fold structure following the
Colorado River, with the stream flowing in the fold axis striking roughly
NNE. Furuya et al. (2007) showed by using InSAR measurements that the uplift of the Meander
Anticline as well as the graben formation is ongoing, which is expressed
most prominently by NW-oriented extensional movements of up to 3 mm a
To investigate sediment thicknesses and their distribution,
Grosfils et al. (2003) and Abrahamson
(2005) used seismic refraction surveys in northern Devil's Lane and Cyclone
Canyon, respectively. Grosfils et al. (2003)
additionally collected gravity data. They
estimate maximum thicknesses of Pleistocene and Holocene sediments of more
than 90 m in Devil's Lane and 60–75 m in Cyclone Canyon. The resulting
total throw (i.e., sediment thickness plus graben wall height) in both
grabens exceeds earlier estimates (e.g., Cartwright et
al., 1995) by a factor of up to 1.5 (e.g.,
In the northern section, the upper 100 m of outcropping hard rock
are cut by two characteristic regular joint sets
(e.g., McGill and Stromquist, 1979). One joint set
strikes NNE, the second one SE. Both joint sets change their orientation
slightly towards the south: the NNE set following the change of the graben
orientation, while the second joint set stays roughly normal to the first one and
to the faults with a deviation of up to 30
Detailed mapping of faults and joints as well as a remote-sensing analysis of topography and structures was carried out as a basis for geophysical data collection and field mapping. We used high-resolution airborne orthoimagery (25 cm per pixel; Utah Automated Geographic Reference Center, 2009) for the northern section and aerial images with 1 m resolution (National Agriculture Imagery Program, 2009) for the southern parts. This imagery is also used in the figures of this paper. Maps throughout this paper, including the supplementary map, were created using ArcGIS® software by Esri. (ArcGIS® and ArcMap™ are the intellectual property of Esri and are used herein under license. Copyright© Esri. All rights reserved.)
In addition to remote sensing and classical field mapping with GPS, compass, high-resolution photographs and a laser range finder, we used a ground-penetrating radar (GPR) system by GSSI (Salem, USA) to image subsurface features. The survey equipment consisted of the SIR 3000 field computer with GPS tracker, 100 and 400 MHz antennas, and a survey wheel. We aimed at reaching penetration depths up to 10 m or more with the lower-frequency antenna. Data with a surface-near resolution better than 0.1 m are routinely gathered by the 400 MHz system. Data processing was performed with the ReflexW software package (Sandmeier, 2011) and included static corrections, background removal, gain adjustments and frequency filtering. Topographic correction (Neal, 2004) was applied to those profiles with significant topographic variations only.
Wave travel times were converted into depths based on literature velocity
values and hyperbola analyses where possible. Values between 0.1 and
0.15 m ns
Vertical resolution of the data depends on the wave velocity as well.
Maximum achievable vertical resolution is a quarter of the wavelength
(wavelength equals wave velocity divided by frequency:
A total of 66 GPR profiles were collected with a cumulative length of more than 7 km, distributed over Devil's Pocket, Devil's Lane and Cyclone Canyon. The different antenna types were used to achieve either higher penetration depth or better resolution. Profiles normal to fault strike were investigated with both antennas as we expect them to show more structures than strike-parallel profiles. For minimum invasion on the sensitive biological soil crust (Rosentreter et al., 2007) we restricted our surveys to existing tracks and trails in agreement with the National Park Service.
Modern airborne orthorectified photos with very high resolution (up to 25 cm per pixel) allow a detailed remote mapping of joints and faults in the Needles fault zone. To analyze orientations of the joint generations and the fault system, we mapped more than 20 000 joints and 800 faults. Figure 4 shows all faults, divided into four sections with similar orientations. For each of the sections we plotted rose diagrams of fault and joint orientations, and for each section an airborne photograph is given with interpreted faults (white lines) and joints (red lines) as an illustration of our interpretations. Within each section the orientations are quite constant and one joint set is always parallel to the main faults. A detailed map showing joints and faults overlain on aerial photographs is included in the Supplement of this paper.
Fault and joint orientations in the Needles fault zone derived from airborne image mapping. Four sections group areas of similar orientations. One joint set is always parallel to the major faults. Small red rectangles depict locations of associated detail photographs showing the interpretations of faults (white lines) and joints (red lines). Numbers 1–4 on the photographs correspond to the sections. Gray arrows indicate the local direction of graben opening.
We focused on Devil's Pocket, Devil's Lane and Cyclone Canyon sites because
in these grabens the walls are less weathered and jointing is most distinct,
but we also included an excursion to Cross Canyon in the south and Lens Canyon in
the west. Besides the GPR data that are described later, we also collected
basic geometric information of these grabens using a laser range finder and
visual observations. Three major findings are of importance:
Along the four-wheel-drive track leading from Devil's Lane southwards along the Bobby
Jo Camp to Bobby's Hole we observed a number of localized depressions at
graben boundaries and faults, interpreted as sinkholes in agreement with work
by Biggar and Adams (1987). The sinkholes can span
from a few meters to tens of meters in length and several meters in depth.
Numerous sinkholes observed in airborne imagery are not located at graben
boundaries but coincide with faults within or across grabens. Figure 5a shows an example of an airborne image of such
a sinkhole located at the eastern graben wall in a graben east of Cow Canyon
(see arrow “1” in Fig. 3). A channel-like structure
protruding towards the graben wall indicates sediment transport towards the
depression at the graben wall. The corresponding field photograph (Fig. 5b) shows the geometry and extent of this
sinkhole. Apparently, water as well as transported sediment disappears
underground at these locations. Due to their depth of several meters they
often act as a trap for the abundant tumbleweed. It is unclear, however,
whether there is an interconnected system of open fractures that allows a
transport of sediments over larger distances, or whether the opening rate of these
fissures is larger than sediment input, such that they do not fill up. Figure 5c shows a sinkhole that is not located at a
graben boundary but at a graben-crossing fault (for location see arrow “2”
in Fig. 3). The corresponding field photograph (d)
shows the dimension of the sinkhole and the void where rainwater and
sediments are transported underground. According to national park staff,
this sinkhole opened up in 2011 during heavy rainfall, which washed away the
sediment cover. At three locations we were able to exactly measure heave and throw of
offset blocks that remained visible within the grabens. While throw can be
determined precisely by horizontal marker beds, heave is affected by an
unknown amount of erosion, and thus measured values are considered
overestimates. One measurement was done in northern Devil's Pocket (Fig. 6a). The throw at the western graben wall could be
measured quite precisely (38.7 m) using the laser range finder. The heave is
estimated to be To test hypotheses on dilatant faulting, we looked for slickensides or
tool marks at non-weathered fault/joint surfaces.
Moore and Schultz (1999) described slickenlines at
calcite-coated joint surfaces, but we did not find any examples of that.
Calcite coatings on joint surfaces that subsequently accumulate displacement
due to underlying faulting are formed by rainwater flow over the surfaces in
streams, thus producing irregular trails of calcite. Further it is possible
to form tool marks on soft calcite coatings by reworking of rockfall debris
even in extensional faulting regimes. The majority of our observations point
to extensional fracturing, without frictional sliding (Fig. 7).
Three locations where heave and throw were measured and fault dips
at depth were estimated.
Widely unweathered surfaces of joints that accommodated offset due
to underlying faulting show no slickenlines or tool marks at
A number of profiles selected for their descriptive value are shown and interpreted in the following paragraphs. Especially the profiles crossing from a graben to a horst, thus being crosscuts of a graben-bounding fault, show the most interesting results. The given examples illustrate sediments dipping towards the graben wall, sediments dipping towards the graben center, changing deposition rates within a graben and graben-internal faulting. Dip angles of the faults cannot be determined exactly since depth migration of the GPR data was not possible due to amplification of artifacts during migration. The location of all described profiles is illustrated in Fig. 3a with numbers according to the subheadings.
The profiles were acquired with both the 400 and 100 MHz antenna and stitched from two segments each (see white suture lines). They cross Devil's Lane from the western graben wall over 135 m to ESE as illustrated in Fig. 8. Radargrams and interpretations are depicted in Fig. 9 and Fig. 10 for the 400 MHz profile and the 100 MHz profile, respectively.
Aerial photograph of the WNW–ESE (red line) profiles crossing
through Devil's Lane.
Radargram
Radargram
Both antennas image sediments dipping towards the graben-bounding fault in the form of growth strata over a length of 30–40 m, but they differ in the depth at which they can be observed. Whereas layers may be interpreted to become horizontal at a depth of around 2.5 m in the 400 MHz profile, the 100 MHz antenna shows dipping structures down to 5 m depth. The discrepancy is assumed to be the result of different resolutions. In the airborne imagery (a) this area coincides with darker colors, indicating more humidity and a slight surface depression, which is also observable in the field.
More localized areas of dipping strata are observed in three other positions. Dips from the west towards the 60 m mark can be observed in the 400 MHz profile as well as dips from the east towards the 60 m mark, at depths of up to approximately 1.5 m. These are not visible in the 100 MHz profile, although here some dipping occurs from 60 m westwards at depths between 2 and 5 m. Some west-dipping layers in the upper meter are found in the 400 MHz profile between 85 and 90 m but not at all using 100 MHz. Finally, some east-dipping strata occur east of 115 m (400 MHz) down to 1 m and east of 125 m (100 MHz) between 2 and 3.5 m depth. In general, both antennas show very similar features at the same positions, although they differ in the depths at which the observations can be made.
In addition to dipping sediments we interpret a number of faults in these profiles (Figs. 9b and 10b) which cannot be seen at the surface. These faults mostly correlate between both profiles, although some smaller faults seen in the 400 MHz profile could not be resolved in the 100 MHz profile. The average spacing between the faults is about 10 to 15 m. Many faults reach up close to the surface, but there are also some that show no more offset at depths of several meters.
This 100 MHz profile in northern Cyclone Canyon starts at the western graben wall and follows the trail to the SE (see Fig. 11a). It shows a sediment package of about 10 m thickness dipping from the surface down to 10–20 m depth over a length of 100 m along the profile. This dip is only an apparent dip, as the actual structure is unknown (e.g., alluvial fan). The overlying horizontal layers gain thickness along the profile and finally reach down to 10 m.
Photo plate showing the traces of GPR profiles in
In this profile there are also faults to be found as illustrated by dashed lines and named in Fig. 12b. We interpret three major faults and four associated minor faults. Fault 1 is clearly visible due to terminating reflectors and a change in reflector intensity at the 16 m mark. This fault is interpreted to be the graben-bounding fault of the northern graben segment. Faults 2 and 3 are located more distal from the graben wall and both show small associated faults to their west. Again, terminating reflectors and changes in reflector intensity indicate the existence of these faults. The strike direction of faults 2 and 3 is uncertain, but we assume them to be parallel to fault 1, following the general regional trend of fault strikes. The three major faults are indicated as interpreted with white dashed lines in the airborne image (Fig. 11a).
NW–SE profile in northern Cyclone Canyon.
Evidence for changes in either deposition or deformation rate were found in a profile in central Cyclone Canyon, starting at the western graben wall and then changing to a NNE direction as illustrated in Fig. 11b. This 85 m long profile was shot using the 400 MHz antenna to achieve a high resolution in the upper 4 m of sediments. Figure 13a shows the profile without interpretation, and Fig. 13b indicates the prominent sedimentary features as we interpret them. Three distinct horizontal beds can be seen here – the first one being within the upper 0.2–0.4 cm, the second one at a depth of about 2.2 m and the third one at about 3.4 m depth.
SW–NE profile crossing the central Cyclone Canyon.
The distance between the two deeper horizons is about 1.2 m, and they are separated by material producing diffuse and wavy reflectors. The vertical distance between the two upper horizons is about 2 m, and the sediments in between show several cycles of foreset beds and onlaps, typical in deltaic sequences. Apparent dips along the profile are to the E/NNE, although the last 25 m of the profile also reveal layers with different dips, onlapping on the NNE-dipping sediments. No clear sign of faulting was observed here.
This 400 MHz profile was taken along a WNW–ESE path crossing Devil's Kitchen as shown in Fig. 11c. It starts right behind the western graben-bounding fault of the northern section of Devil's Pocket graben and then leads to the eastern graben wall, again not crossing the bounding fault. Both bounding faults are indicated by white dashed lines in Fig. 11c. Due to the high resolution, the profile provides useful information only down to 5–6 m. Figure 14a depicts the raw profile after processing, while in Fig. 14b we present our interpretation. Two major faults are visible by terminating reflectors and changing reflector intensity. Fault 1 is located at around 40 m and shows at least three fault strands as illustrated by dashed lines in Fig. 14b. Caused by the lack of GPR information, the lower extent of the fault cannot be determined. Nevertheless, all fault strands appear to have their upper fault tip at a depth of about 0.5 m. Fault 2 (at 115 m) is less sharply defined and characterized by reflectors dipping diffusely towards the interpreted slip plane. Its upper termination is again assumed to be at a depth of 0.5 m. The assumed fault traces for both faults are also marked in the airborne photograph (Fig. 11c) by black dashed lines.
Devil's Kitchen WNW–ESE profile.
Sedimentary structures vary strongly throughout the profile. The westernmost section has sediments dipping towards fault 1 at a depth of 0.5–2 m, while in the uppermost 0.5 m west-dipping layers overlie the faulted segment. These west-dipping layers extend to a depth of about 3 m on the eastern side of fault 1 and show lateral extents from 0.2 to 0.3 m. Layers in the upper 0.3–0.4 m of the entire profile are horizontal. Sediments at fault 2 dip towards it from both sides but in a rather small lateral extent of about 5–10 m each. At the easternmost section of the profile, layers dip to the west at depths of 1–2.5 m and a lateral extent of roughly 25 m. Parts not affected by the described dipping layers are characterized by horizontal reflectors through the entire sediment layer down to 5 m.
Combined with the field and remote-sensing data (observations, mapping, airborne imagery and digital elevation models), the information derived from the GPR profiles supports and enhances the knowledge of how the grabens form and which processes are involved.
The key evidence for dilatant faulting is the occurrence of sinkholes that we observed along graben-bounding faults in the more eastern grabens (cf. Fig. 3). Tensile opening gaps allow water and sediments to be transported underground (Fig. 5). Analysis of airborne imagery showed several sinkholes located within grabens at locations where a fault is crossing. These sinkholes are comparable to pit craters described by Ferrill et al. (2004, 2011).
Also, during our fieldwork we never observed slickensides at surfaces of unweathered joints that accommodate offset and extension due to reactivation by underlying normal faults. Slickenlines should be abundant at discrete slip surfaces in an area with as many faults as the Needles fault zone. The absence of such striations indicates dilatant faulting by reactivation of the preexisting vertical joints.
Layers distinctly dipping towards the graben wall, partly in the form of growth strata, require a depression into which they can dip. The GPR profile from Devil's Lane (Fig. 9) shows an example of this. At both ends of the profile, layers dip towards the respective graben wall with growth strata at the western graben wall. The profile from Devil's Pocket (Fig. 14) partially shows this situation. Layers to the east of fault 1 dip towards the western graben wall, and at fault 2 dip occurs from east and west.
The combination of opening of preexisting joints during faulting and the
considerable vertical displacement in the grabens requires the presence of
non-vertical faults at depth. The measurements of heave and throw in Devil's
Pocket, Devil's Lane and the graben east of Cow Canyon provide direct
evidence of fault dip at depth according to geometric considerations
illustrated in Fig. 15a. The fault dip
Other authors have found field evidence for such a vertical change of the fault
dips in the grabens of the Canyonlands by investigating crosscut grabens
along Y Canyon, Cross Canyon and Lower Red Lake Canyon. Based on
observations in these crosscuts McGill and
Stromquist (1979) proposed that faults are vertical over about 100 m
followed by dips of about 75
A similar fault geometry is reproduced in the experiments of Holland et al. (2006) and van Gent et al. (2010). They used hemihydrate powder to investigate the evolution of normal faults in brittle materials and observed a tensile fracturing in the upper fault sections, a mixed-mode fracture in the middle and a shallow dipping shear movement in the lowest part of the fault zone. Hence, the surface expression of these faults is a vertical cliff with an open void at the graben-bounding fault and some vertical offset. Kettermann and Urai (2015) studied the lateral geometry of brittle faults in analogue models using hemihydrate powder. They described a rough and patchy geometry with relays and large blocks rotating into the fault as a result of linkage of fault segments. Similar features are described in basalts of Hawaii by Holland et al. (2006).
This `pull-apart' model in mechanically layered stratigraphy causing opening in the more competent layers is well known from outcrops (e.g., Peacock and Sanderson, 1992; Peacock, 2002; Ferrill and Morris, 2003; Crider and Peacock, 2004; Ferrill et al., 2014) and shown in DEMs as well (e.g., Schöpfer et al., 2007a, b, c; Abe et al., 2011). We suggest that in the Canyonlands this effect is additionally controlled by the abundant vertical joint sets that are defining the present-day graben walls.
Analogue modeling of the interaction of preexisting joints and normal faults
in brittle rocks was performed by Kettermann (2012). The basic
deformation box, setup and scaling relations were the same as in the work of
Holland et al. (2011). Additionally, joints were
introduced to the system by hanging sheets of paper into the box during
filling and carefully removing them before the deformation. As the
hemihydrate is cohesive, open joints are formed and can then affect the
faulting. The maximum depth of the joints as well as the joint spacing was
chosen to represent the situation in the Canyonlands. In a series of
experiments the angle between joint strike and fault strike was varied in a
range observed in the field. Figure 16 shows a top
view of an experiment with an 8
Top view image showing an analogue experiment of
Kettermann (2012) with 8
A fault–joint relationship was similarly observed in volcanic structures in
Iceland (Gudmundsson
and Bäckström, 1991; Angelier et al., 1997). Although several times
larger than the Canyonlands grabens, the Sveinagja graben in basalts of
northeast Iceland (Gudmundsson and
Bäckström, 1991) resembles a similar situation. A 34 km long graben
structure shows large open gaps along the bounding faults. Heave and throw
values indicate comparable fault dips of about 60
It is, however, possible that joints subparallel to a basement fault form as initial stage of deformation (e.g., Destro, 1995; Kattenhorn et al., 2000; Balsamo et al., 2008). Later the main faults from underneath can localize along these early formed joints and accommodate offset as observed by Grant and Kattenhorn (2004). These joint-related faults can look quite similar to the faults described in this paper; however the joints are always slightly inclined towards the faults.
In general, as both faults with and without preexisting vertical joints can form vertical extension fractures at low overburden stresses, the main difference is the lateral geometry. Without preexisting joints the heterogeneity of the rocks defines the position of individual fault segments in an early stage that link (hard link or soft link) with increasing displacement and finally form typical faults with relays and fault blocks. With preexisting joints the fault geometry is widely controlled by the joint pattern, as is the case in parts of Iceland or the Canyonlands. Figure 17 illustrates the effects of different prerequisites of a faulted region.
Conceptual models illustrating different fault shapes according to
the influence of preexisting joints. With no preexisting joints
Fault zones are usually complex in geometry and strain partitioning (McClay, 1990; Childs et al., 1996a, b; Walsh et al., 1999, 2003; Mansfield and Cartwright, 2001; Peacock, 2001, 2002; Wilkins and Gross, 2002; Soliva et al., 2008). For the Canyonlands grabens, Baker (1933) first described a graben-internal deformation by a number of graben-parallel faults in the downthrown blocks where they were exposed, although he did not define the location of his observations precisely. Walsh and Schultz-Ela (2003) showed field evidence from a cross section in Y Canyon, indicating at least one graben-internal fault and successfully compared this observation with numerical models. They also note incremental multiple faulting from aerial photographs.
This graben-internal faulting is consistent with our observations from Cross Canyon as illustrated in Fig. 18. The black lines depict the location of the main graben-bounding faults of this graben (for location see cross in Fig. 3), and the white lines show two conjugate faults affecting the graben floor. A number of GPR profiles in all three investigated grabens – i.e., Devil's Pocket (Fig. 14), Devil's Lane (Fig. 8) and Cyclone Canyon (Fig. 13) – reveal faulting in the grabens that is not visible at the surface. The faults in Devil's Pocket and northern Cyclone Canyon additionally show more than one fault strand. Our observations show that even flat and apparently undisturbed graben floors can contain complex fault systems. As these are not visible at the surface, the faults are presumably not active any more. The amount of graben-internal faults might be slightly overestimated by GPR profiles since these faults propagate upwards from the brittle basement through several tens of meters of sediments in which the faults refract and form branches. The GPR profile in Devil's Lane for example shows several faults that do not reach to greater depth as well as some blind faults that are restricted in depth.
Graben in Cross Canyon. Black lines show main graben-bounding faults. White lines show two conjugate faults cutting through the graben floor. Dashed line illustrates marker horizon. Example for intra-graben faulting.
Combining all the information presented, we propose a model for the formation
of a typical Canyonlands graben as shown in Fig. 19. Faults dip at
60–80
Schematic model of a typical Canyonlands graben from the northern section. Joints of the Cedar Mesa sandstones are reactivated by faulting. Shallower fault dips at depth cause open gaps at surface and secondary faulting within the grabens. Rockfall debris and alluvial sediments fill up open gaps and graben floors.
The GPR profile of central Cyclone Canyon revealed detailed sedimentary structures to a depth of about 4 m (Fig. 13). We noticed three distinct horizons of horizontal reflectors and in between delta-like sequences of foreset beds and onlaps. The interpretation of lacustrine delta sediments is supported by typical layering of the dipping sediments, visible by changing reflector intensities, which implies sedimentation in water due to sudden events such as flash-floods. The geological map of Billingsley et al. (2002) shows that ponded deposits are common in Cyclone Canyon, and the authors state that these ponds were capable of sustaining water for several months to years. The thickness of the ponded deposits of up to 2 m fits well to the observed thickness in the radar profiles.
Especially the three horizontal layers that we interpret as paleosols fit very well to a graben fill east of Virginia Park, described and dated by Reheis et al. (2005). The location is marked with a black star in Fig. 3b. Thanks to the incision of a stream, they were able to visually study a soil profile down to 6 m depth and apply optically stimulated luminescence (OSL) dating. They described three paleosols: one close to the surface, one at 2 m depth and a third one at about 3 m depth. Dating right above the second paleosol results in an age of about 15–16 ka, and sediments above the third paleosol were dated to 27–28 ka. Figure 20 shows the GPR profile of the central Cyclone Canyon with the stratigraphic column and OSL ages of Reheis et al. (2005) as an overlay. The depths of all paleosols match, with deviation of only a few centimeters.
400 MHz profile of central Cyclone Canyon with overlay of the profile and OSL ages of Reheis et al. (2005). Three horizontal layers in the GPR profile fit well to paleosols from outcrop profile with an error of less than 0.5 m.
Based on the proposed age of the grabens (Biggar and Adams, 1987;
Schultz and Moore, 1996; Trudgill, 2002), the sediment thickness in Cyclone
Canyon (Grosfils et al., 2003) and the known age of
these paleosols (Reheis et
al., 2005), we propose three possible scenarios of graben formation and
sedimentation:
The horizons in GPR and outcrop are not the same. Still we would
interpret the observed horizons as paleosols. Assuming a faster deposition
rate in Cyclone Canyon due to its preferred role as a sediment trap would
then suggest long-range climatic changes within the recorded 4 m of
the profile and distinct differences in deposition over short distances. The horizons in GPR and outcrop are the same. An explanation for the
small discrepancy in the depth of the paleosols would be a faster
sedimentation between the first and second paleosol. This is likely since
Cyclone Canyon is a quite deep graben compared to the one east of Virginia
Park and thus a better sediment trap. The delta-like foreset beds observed
between the upper two paleosols indicate a high deposition rate in the
presence of water, i.e., extensive flash floods as they occur from time to
time, depositing in ponds. Additionally, slight differences in depth can
result from imperfect time–depth conversion. Applying the OSL ages from
Reheis et al. (2005) to
the depths in Cyclone Canyon allows an estimation of sedimentation rates.
The first OSL age of roughly 16 ka at a depth of 2.2 m leads
to a sedimentation rate of 0.14 mm a In the third case the paleosols are the same, a sediment package of 50 m is given and we assume that the rate of deposition has not changed
dramatically in the past. That would point towards an earlier initiation of
graben formation than previously estimated by termoluminescence dating of
sediments found in sinkholes (Campbell, 1987), although their
results may not apply for all grabens. Using a deposition rate of 0.15 mm a
To resolve the remaining problems, more data are needed from different
grabens and locations. Especially case (1) can only be validated or rejected
by drilling in Cyclone Canyon and actually comparing and dating the
sediments directly. However, it seems legitimate to assume an intermediate
situation of cases (2) and (3). Some authors
(Trudgill and Cartwright, 1994;
Schultz-Ela and Walsh, 2002) propose graben initiation at 500 ka or more,
following the timing of the Colorado River incision. Higher deposition rates
are also possible. A mixture of both would be a good explanation for the
intermediate age of 260–300 ka proposed in our model.
Having applied ground-penetrating radar surveys combined with field
observations in the Needles fault zone of Canyonlands National Park,
Utah, USA, we draw the following conclusions:
Our GPR profiles revealed faults in the subsurface that were overprinted
and not visible at the surface. We can therefore conclude that the fault
zone of the Canyonlands grabens is more complex than obvious. Since this
area is an analogue for reservoirs, knowledge of the fault zone complexity
can be used to enhance seismic interpretation and fault zone permeability
estimates. Dilatant faulting is evident in many places, even if the surface
expression is overprinted by sedimentation. Measurements of heave and throw
at some faults additionally provide evidence for vertical fractures at the
surface in combination with joint reactivation and inclined faults at depth. We propose a model of graben formation based on our observations,
including fault localization at tips of vertical joints, shallow fault dips
at depth, graben-internal faulting and reactive diapirism. Paleosols in the subsurface of Cyclone Canyon correlate with the climate
reconstruction of Reheis
et al. (2005). Comparing these paleosols, their ages and the underlying
sediment thicknesses imply either a graben age older than expected from
previous dating or higher sedimentation rates in Cyclone Canyon.
We like to thank Marc Miller and Vicky Webster from the National Park Service for their kind support in the preparation of the field study. Further, we thank Andrea Billi and Fabrizio Balsamo for their helpful reviews.Edited by: F. Rossetti