Icarus 208 (2010) 608–620
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Icarus
journal homepage: www.elsevier.com/locate/icarus
Erosional modification and gully formation at Meteor Crater, Arizona: Insights
into crater degradation processes on Mars
P. Senthil Kumar a,b,*, James W. Head b, David A. Kring c
a
National Geophysical Research Institute, Council of Scientific and Industrial Research, Uppal Road, Hyderabad 500 007, India
Department of Geological Sciences, Brown University, Providence, RI 02912, USA
c
Lunar and Planetary Institute, Universities Space Research Association, Houston, TX 77058-1113, USA
b
a r t i c l e
i n f o
Article history:
Received 4 August 2009
Revised 20 January 2010
Accepted 24 March 2010
Available online 1 April 2010
Keywords:
Earth
Mars, Surface
Mars, Climate
a b s t r a c t
Hydrogeological modification of Meteor Crater produced a spectacular set of gullies throughout the interior wall in response to rainwater precipitation, snow melting, and possible groundwater discharge. The
crater wall has an exceptionally well-developed centripetal drainage pattern consisting of individual
alcoves, channels, and fans. Some of the gullies originate from the rim crest and others from the middle
crater wall where a lithologic transition occurs; broad gullies occur along the crater corner radial faults.
Deeply incised alcoves are well developed on the soft Coconino Sandstone exposed on the middle crater
wall, beneath overlying dolomite. In general, the gully locations are along crater wall radial fractures and
faults, which are favorable locales of erosion due to preferential rock breakup from faulting, and groundwater flow/discharge; these structural discontinuities are also the locales where the surface runoff from
rain precipitation and snow melting can preferentially flow, causing erosion and crater degradation.
Channels are well developed on the talus deposits and alluvial fans on the periphery of the crater floor.
Caves exposed on the lower crater level point to percolation of surface runoff and selective discharge
through fractures on the crater wall. In addition, lake sediments on the crater floor provide significant
evidence of a past pluvial climate, when the water table was higher, and groundwater may have seeped
from springs on the crater wall. Although these hydrological processes continue at Meteor Crater today,
conditions at the crater are much more arid than they were soon after impact, reflecting a climatic shift.
This climate shift and the hydrological modifications observed at Meteor Crater provide insights for landscape sculpturing on Mars during various parts of its history.
Ó 2010 Elsevier Inc. All rights reserved.
1. Introduction
Impact craters are one of the most conspicuous geologic features on planetary surfaces. They have been traditionally examined
to understand the geologic process of impact cratering to study exposed substrate characteristics, and as a relative chronometer of
planetary surface age. They are also subject, however, to post-impact erosion that produces features that can be used to deduce
planetary surface processes, the environmental conditions that
govern them, and any climate changes that may have modified
those processes. For example, orbital and robotic observations of
craters on Mars have provided insights to general degradation processes of impact craters on Mars both early in its history (Grant
and Schultz, 1993a; Craddock et al., 1997; Craddock and Howard,
* Corresponding author at: Room No. 122, Main Building, National Geophysical
Research Institute, Council of Scientific and Industrial Research, Post Bag No. 724,
Uppal Road, Hyderabad 500 606, Andhra Pradesh, India. Fax: +91 40 27171564.
E-mail addresses: [email protected], [email protected] (P.S. Kumar).
0019-1035/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved.
doi:10.1016/j.icarus.2010.03.032
2002; Forsberg-Taylor et al., 2004; Moore and Howard, 2005; Irwin
et al., 2005; Golombek et al., 2006; Grant et al., 2006, 2008) and in
more recent history (e.g., Malin and Edgett, 2000, 2001). The report
of young martian gullies by Malin and Edgett (2000) initiated new
studies and interpretations, involving water-related formation
mechanisms (e.g., melt water from surface and near-surface ice
deposits, and groundwater) and non-water related mechanisms
like eolian and dry granular flows (Malin and Edgett, 2001; Costard
et al., 2002; Hecht, 2002; Gilmore and Phillips, 2002; Treiman,
2003; Heldmann and Mellon, 2004; Balme et al., 2006; Bridges
and Lackner, 2006; Dickson et al., 2007; Heldmann et al., 2007;
Head et al., 2008; Dickson and Head, 2009). To provide a benchmark for those types of interpretations, studies of similar processes
are needed at craters on Earth where they can be tested with in situ
observations. One of the classic impact sites on Earth is Barringer
Meteorite Crater (aka Meteor Crater), which lends itself perfectly
to this type of study, as its interior wall depicts erosional features
produced by a combination of fluvial and eolian sedimentary processes including mass wasting.
P.S. Kumar et al. / Icarus 208 (2010) 608–620
Meteor Crater was formed 50,000 years ago when an iron
meteoroid struck Phanerozoic sedimentary rocks on the southern
Colorado Plateau. The basic geologic and geophysical characteristics associated with the formation of the crater are already well
known (see reviews by Shoemaker and Kieffer (1974) and Kring
(2007)). The ejecta surrounding the crater has previously been
studied as an analog of ejecta degradation around some martian
craters (see Grant and Schultz, 1993ab) and the erosion of the inte-
609
rior crater walls has also been used to infer post-impact modification processes (Shoemaker and Kieffer, 1974; Grant, 1999). In this
study, we expand observations of the crater interior and gullies
that help to further characterize Meteor Crater, and that may provide insights to gully formation on Mars. In particular, we examine
bedrock exposures on erosional scarps, talus deposits, and alluvial
deposits to document the distribution of gullies on the entire interior walls, and assess the involvement of crater wall materials and
structures in the formation of gullies. In addition, we review paleoclimate records and present day meteorological data to understand
the link between atmospheric processes and crater degradation.
2. Geological setting
Fig. 1. An aerial view of Meteor Crater showing the spectacular development of
gullies. Source: United States Geological Survey.
Meteor Crater (Fig. 1) is a bowl-shaped topographic low (a simple impact crater) of 180 m depth and 1200 m diameter (Barringer, 1905, 1910; Shoemaker, 1960; Shoemaker and Kieffer,
1974; see Supplementary data). The rim crest rises 30 m to
60 m above the surrounding plains. As seen along the upper crater wall, the rim is produced by up to 50 m of impact ejecta and an
uplifted sequence of sedimentary bedrock. The Coconino Sandstone is partly exposed on the present day middle to lower crater
wall; it consists of white, granular, cross-bedded quartzose sandstone (>97% quartz). The thickness of this sandstone unit varies
from 210 m to 240 m around the crater. It is overlain by an approximately 3-m-thick Toroweap Formation, which consists of a whiteto yellowish brown, medium- to coarse-grained, calcareous sandstone inter-bedded with thin dolomite beds. It is overlain by
80-m-thick Kaibab Formation, which consists of three members:
Alpha (yellowish, vuggy, well bedded dolomite inter-bedded with
a few white sandstone beds), Beta (yellowish, massive dolomite),
and Gamma (white to yellowish massive dolomite), from the top
to bottom, respectively. The Moenkopi Formation unconformably
Fig. 2. Centripetal drainage pattern in Meteor Crater. The drainage systems are composed of alcove, channel and fan. Note some drainage originates from the rim crest, while
some is from the mid-crater wall where a lithologic transition occurs from harder dolomite to softer sandstone. On the upper crater wall, the gullies incise the bedrock along
fractures and faults, whereas in the lower wall, the gullies cut across the soft sediments such as allogenic breccia, mixed breccia and talus sediments. The four crater corners
show a spectacular development of gullies and are controlled by the radial faults. Note the cave occurs at the base of Kaibab Formation and is responsible for water discharge
and formation of a gully system from the middle crater wall (see Fig. 10).
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Fig. 3. Gullies on the interior wall of Meteor Crater. Note that the gullies are composed of alcove, channel and fan. The gullies originate both from the upper and middle walls,
and a few along the faults. Crater wall pictures: (a) eastern wall, (b) southern wall, and (c) northern wall.
overlies the Kaibab Formation. It consists of two members: a 6m-think, upper, Moqui Member (fissile dark-brown siltstones)
and a 3-m-thick, lower, Wupatki Member comprised of pale, reddish brown, cross-bedded, massive sandstones. Below the present
day crater floor, there is 30-m-thick lens of lake sediments, a
200-m-thick breccia and an underlying >750-m-thick sequence
of Paleozoic sedimentary rocks. Crystalline basement rocks (e.g.,
granites) occur at a depth of 1070 m below the surface. Additional details of these lithologies can be found elsewhere (Chapter
2 of Kring (2007), and references therein).
Ejecta deposits are very well preserved out to a radial distance
>1 km from the crater rim (Shoemaker, 1960; Roddy et al., 1975);
the blanket consists of ejected debris derived from the rock types
exposed on the interior crater wall. Debris also occurs within the
crater in the form of complex breccia deposits that Shoemaker
(1963) mapped as three units (see Supplementary data): authigenic breccia, allogenic breccia and mixed debris. The authigenic
breccia is basically the fault gouge materials that occur along crater
wall faults. The allogenic breccia forms a thick lens of impact debris at the bottom of the crater and coats the lower to middle portions of the crater walls. It is composed of shocked rock
fragments and meteoritic materials and is interpreted to be the
remnants of the breccia that coated the transient crater wall before
slumping downward during the modification stage of crater formation. The allogenic breccia is covered with a mixed breccia of rock
fragments derived from the crater wall and meteorite fragments.
This unit is interpreted to be fallback ejecta materials. The fall-back
breccia presumably coated the entire crater, but is only preserved
where it was buried by talus that eroded from the upper crater
walls (Shoemaker and Kieffer, 1974). The breccia deposits are overlain by talus that was later dissected, producing a second generation of alluvium. The formation of the original talus slopes and
the subsequent dissection implies two generations of wetter climatic conditions than that seen today. Rare debris flows down
the crater walls still continue, although they may be influenced
by human activities. In September 1906, for example, a heavy rain
caused mass wasting that obliterated a trail near a radial fault and
produced trains of boulders that flowed out onto the crater floor
(Fairchild, 1907).
3. Observations
3.1. Gully features
Meteor Crater contains a spectacular centripetal drainage pattern on its interior wall (Figs. 1 and 2). The drainage system is composed of an alcove, a channel, and a fan (Fig. 3). The alcoves occur
on the bedrock exposures on the upper to middle crater walls. In
general, the alcoves originate from the rim crest, while most others
originate from the middle wall, below the contact of Kaibab dolomite and Coconino sandstone (Figs. 1–4). The mid-wall alcoves
originating from the Coconino Sandstone are more prominent than
those in the overlying Kaibab Formation: the Coconino alcoves are
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Fig. 4. (a–c) Mid-wall gullies originating from the Coconino Sandstone. (d) Channels originating from the Coconino Sandstone just below the Kaibab Formation. Note that the
incision of alcoves and channels is deeper in the Coconino Sandstone.
deeper and wider, while the Kaibab alcoves are shallower and narrower (Fig. 5). The prominent alcoves (10–80 m wide) preferentially form in the Coconino Sandstone, which are less resistant to
weathering and erosion compared to the hard Kaibab dolomites
(Fig. 4). Some of the alcoves cut through the talus, mixed breccia,
and allogenic breccias that coat the crater wall (Fig. 6). A few
exceptionally large gullies are exposed on the crater corners (Figs. 5
and 7). These gullies have very broad alcoves (width > 120 m),
originating from the rim crest and the Kaibab Formation that are
unique within the crater. Interestingly, they are also associated
with large faults in the crater corners. These gullies show prominent channels and fans. In general, gullies can be classified in
two types: large gullies that incise bedrock and small gullies that
incise talus and alluvial deposits (Fig. 7). The large gullies are older,
but have the imprint of young fluvial gullies that are similar to
those developed on talus and alluvial deposits (Fig. 7).
Channels originate from the head alcoves and pass through the
talus deposits and debris aprons; at places, they show avulsion
within the fans. Widths of the channels vary from <1 m to 5 m.
Channel beds are characterized by sub-angular boulders, gravels,
pebbles and sand-sized sediments, derived largely from the alcove
regions. The fans are composed mostly of sands and silts, and form
a part of alluvium along the periphery of crater floor sediments.
Most of the channels disappear within the crater floor periphery.
It is possible that water from surface runoff might have infiltrated
through the marginal sediments. The slopes of the upper crater
walls at Meteor Crater average 50° and include near-vertical cliffs
(Kring, 2007). Slopes of the lower crater walls are much gentler
(20°), in part because of the presence of primary impact breccia
deposits and secondary alluvium produced from the erosion of
the upper crater walls. Gully incisions are greatest in these relatively low-slope regions of the crater that are coated with unconsolidated breccias and alluvium. Channels that cut through these
low-slope regions will sometimes have high rock levees, which
are characteristic of rock flows (or debris flows) in arid terrains.
Thus, a portion of the gully-forming activity involved rock flow
and levee construction, and were not purely channel-cutting
events.
3.2. Association of gullies with fractures and faults
The lateral distribution of gullies is controlled by impact-related
structures (fractures and faults). The crater has suffered from impact-related structural uplift, fractures, thrust faults and radial
faults (Shoemaker, 1960; Kumar and Kring, 2008; Poelchau et al.,
2009). The gullies occur preferentially along the fractures and
faults (Fig. 8). The rim uplift is highly variable; the western and
eastern rims have higher uplift relative to other parts of the rim,
and the southern rim is the lowest. The crater is dissected by three
distinct groups of fractures: radial, concentric and conical (Kumar
and Kring, 2008). The radial fractures have more or less steep dips,
and strikes approximately perpendicular to the bedding planes,
although some of them are oriented obliquely. Traces of these fractures are generally vertical on the crater wall, and are often exposed on gully floors. The concentric fractures strike generally
parallel to the bedding planes and dip away from the dip directions
of the bedding planes; the amount of dip is highly variable. The
conical fractures are similar to the concentric fractures in their
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3.3. Association of gullies with weak lithologies
The vertical distribution of gullies varies as a function of lithology. The gullies preferentially occur in association with weaker
lithologies (Figs. 4 and 5). One set of gullies begins in the upper crater wall, which is composed of an ejecta blanket underlain by siltstone and sandstone of the Moenkopi Formation. Another set of
gullies begins in lower crater wall exposures of the Coconino Sandstone. These rock units are softer sedimentary rock materials and
more susceptible to degradation than the cliff-forming dolomites
in the middle crater wall. Thus, the distribution of gullies is a function of both the relative strengths of the crater wall lithologies and
fracture–fault systems in the crater walls.
3.4. Cave
There is a 5-m-long and <1-m-wide rectilinear cave exposed
at the base of the Kaibab Formation above the contact of Toroweap
Formation (Shoemaker and Kieffer, 1974), near the southeastern
corner of crater (Figs. 2 and 10). The cave is concordant with the
bedrock stratification. It has a few parallel openings (more than
four); width of the individual openings vary from 20 cm to
>1 m. Cross-sections of the cave openings are elliptical. Like the
gullies, the cave is associated with prominent fractures (Fig. 10).
The cave roof is also collapsed because of the lithologic fracture
network. A gully is formed around the cave with a 30-m-wide alcove and a channel emerging from the cave openings. The channel
from these caves deeply incises the talus deposits and forms an
alluvial fan near the crater floor. Large quantities of groundwater
must have been discharged from these caves leading to the formation of the alcove, channel and fan. The age of the cave is unknown;
it may have formed after crater excavation. Alternatively, an
underground cavern could have existed before the impact, but
opened on the crater wall because of impact excavation. For example, we observed several caves in the Kaibab Formation outside the
crater, where Canyon Diablo now exists, and others (Hill et al.,
2008) have reported similar caves in the Redwall Formation along
the Grand Canyon section of the Colorado River. The caves are
important dwelling locations for animals and microorganisms.
3.5. Eolian modification
Fig. 5. Alcove width of gullies in the Coconino Sandstone and Kaibab Formation.
Alcoves above 10 m width are only shown. Measurements are based on satellite
imageries.
strikes, but dip in the direction of the bedding planes with highly
varying dip values. Traces of the concentric and conical fractures
are generally horizontal on the crater walls, and are at high angles
to the gullies, suggesting a less significant influence in gully formation. Faults in the crater wall are more widely spaced than the fractures, although the geometry of the faults is similar to that of the
radial fractures. The fault traces on the crater wall are approximately vertical and are exposed along gullies. The faults with the
greatest vertical displacements occur in the corners of the crater;
broad gully systems are associated with these corner faults
(Fig. 7). The field relationships clearly establish an association of
gullies with radial fractures and faults (Fig. 8). In addition, most
of the gullies in the Coconino Sandstone originate from the base
of fractures/faults in the overlying Kaibab Formation (Fig. 8). Because the gullies are structurally controlled, they have preferred
orientations similar to those of radial fractures and faults (Fig. 9).
The current environment around Meteor Crater is dominated by
semi-arid condition. This results in wind activity being a major crater-modifying process. Eolian deposits are widespread around the
crater (e.g., Grant and Schultz, 1993b; Ramsey, 2002). Notably,
the Coconino Sandstone in the southern ejecta blanket is being
eroded and swept up into dunes on the southern crater rim. Sands
swept farther away are deposited on the upper interior wall
(Fig. 11) for more than 50 m. These leeward-side eolian deposits
contain virtually no gullies. These are active dunes, however, so
they are not incised by gullies. Rather, they are more likely to fill
in the pre-existing gullies.
4. Discussion of degradation at Meteor Crater
4.1. Crater wall collapse and modification
Crater wall collapse occurs immediately after the excavation
stage of the cratering process and provides a final shape to an impact crater (Shoemaker, 1960; Dence, 1968). During the collapse, a
large volume of debris slides down the crater wall, producing a lens
of debris on the crater floor. This process is analogous to mass
wasting or landslides. In Meteor Crater, crater wall collapse occurred preferentially along the concentric fractures (Kumar and
P.S. Kumar et al. / Icarus 208 (2010) 608–620
613
Fig. 6. Gully formation on crater wall soft sediments: (a) northern crater wall showing the middle wall gullies cutting across the talus and breccia deposits, (b) gully wall
section showing the allogenic breccia (c), mixed breccia (d) and post-impact talus deposits (e). The youngest alluvium covers the gully floor. The allogenic breccia coats the
transient crater wall. The mixed breccia is also known as fallback debris. The post-impact talus formed from eroded sediments derived from the crater rim and upper crater
walls.
Kring, 2008). Some of the debris entrained in the breccia lens was
immense. As reviewed recently (Kring, 2007), exploration drilling
of the crater floor in the early 1900s located a slab of rock 25–
125 m thick with a surface area of 10,000–20,000 m2 in the breccia
lens in the northeast corner of the crater floor. Also, the crater corners suffered greater uplift due to large displacement along the radial faults (Shoemaker, 1960; Kumar and Kring, 2008; Poelchau
et al., 2009). Large-scale gullies exposed on the crater corners
may have partly formed by mass wasting associated with this
late-stage crater forming process (Fig. 7). The mass-wasting process must have generated highly variable slopes that influenced
the geometry of the old gully systems that were formed immediately after the impact and subsequently modified by fluvial processes (Grant, 1999).
4.2. Role of present and past climate
To evaluate how climate affected the availability of water to
erode the crater walls and create gullies, we begin by examining
present environmental conditions around Meteor Crater and then
assess geologic records of past climate. Arid conditions currently
prevail at the crater, limiting the amount of rain and snow. Over
the past century, mean total annual rainfall averages 20 cm and
mean annual snowfall averages 29 cm in and around the crater
(Fig. 12). Although it sometimes (rarely) exceeds 40 cm of annual
rainfall and 100 cm of snowfall, these values are among the lowest
in North America. Therefore, present day precipitation is probably
not adequate to produce the prominent gullies. Rather, its effects
are probably limited to the production of small gullies, modifications of pre-existing gullies, and, during the heaviest storms, a
small number of debris slides in the areas affected by humans
(Fairchild, 1907). Currently, there are no springs emanating from
the crater wall, because the water table is below the observable
floor of the crater. Hence, groundwater discharge is also an unlikely
process to produce the gullies today. The crater floor is dry, except
for minor ponding during a rainstorm. The meteorological data and
our observations suggest that most of the erosion and gullies have
an older origin.
The impact occurred approximately 50,000 years ago (Sutton,
1985; Phillips et al., 1991; Nishiizumi et al., 1991), during an interstadial (or relatively warm period) within the Wisconsin glaciation
(Kring, 1997, 2007). Thirty meters of lake sediments cover allogenic breccias on the crater floor (Fig. 13), indicating the crater was
flooded after the impact. As there are no streams that breach the
crater rim from the outside, the crater must have been flooded
from the inside, either because precipitation was greater at the
time, the groundwater table was at a shallower level, or both.
The contact between the breccia and lacustrine sediments is sharp,
with no intervening alluvium, so the lake may have formed immediately after the impact (Shoemaker, 1960; Shoemaker and Kieffer,
1974; Roddy, 1978). Interestingly, the lower 1.5 m of the lacustrine
sediments contains blocks of lechatelierite, a highly vesicular form
of shock-melted sandstone. These low-density blocks were able to
float while water flooded the crater floor. Eventually the blocks became waterlogged and sank. If the source of water is arguably from
the springs, the water table may have been at least 30 m higher in
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Fig. 7. (a–c) Pictures showing gully landforms formed at or near the crater corners where radial faults exist. These landforms may have formed partly as a result of mass
wasting immediately after the impact. The channels in the gullies are also shown. The channels that incise the bedrock are relatively older than those that cut across the talus
and alluvium. (d) A USGS aerial photograph showing the location of gullies shown in a–c.
the Coconino Sandstone than it is today (Shoemaker, 1960; Shoemaker and Kieffer, 1974; Roddy, 1978). The lake harbored a variety
of fresh-water species. The types of fossil molluscs in the sediments suggest a fluctuating perennial water environment (Reger
and Batchhelder, 1971). The system evolved over time. The types
of ostracodes in the sediments indicate a saline lacustrine environment changed to a fresh cold water environment and finally into a
marsh (Forester, 1987). Eventually, however, the climate became
more arid, approaching the present day conditions. As a result,
the water table fell and the lake sediments dried, and were covered
by 1.8 m of playa sediments.
4.3. Hydrologic properties of target rocks
The rock types around the crater (for example, the Moenkopi
and Coconino Sandstones) are excellent groundwater reservoir
rocks. For example, the porosity of the Coconino Sandstone where
unaffected by the impact event is 20%, although it can vary from
values of <10% to 25% (Ahrens and Gregson, 1964; Shipman et al.,
1971; Kieffer, 1971; Kring, 2007). Where unaffected by the impact
event, the porosity of the Kaibab Formation also averages 20%,
although values range between 13% and 25% (Watkins and Walters, 1966). The permeabilities of the pre-impact lithologies are
not known, but they must have been greatly enhanced by the impact event. Additionally, the regionally occurring fracture–fault
networks (pre-impact weakness planes in the target rocks) can
be efficient zones of groundwater recharge, into and through
which surface water can percolate. For example, the sedimentary
rocks exposed around Meteor Crater have a dense network of
pre-impact tectonic fracture and fault systems (Shoemaker, 1960;
Roddy, 1978). Our structural analysis indicates that there are three
prominent sets of fractures with NW–SE, NE–SW, and ENE–WSW
orientations, and two less dominant fractures with NNE–SSW
and ESE–WNW orientations (Kumar and Kring, 2008). These fractures are sub-vertical. Spacing of these fractures varies from a
few centimeters to a few meters. Surface fracture width varies
from <1 mm to several centimeters. Most of them are tensional
fractures filled with soils (and vegetation), where significant
amounts of surface water could have percolated during the wetter
period. In addition, there are normal faults a few kilometers long
and that occur sub-parallel to the NNW–SSE to NW–SE fracture
systems in the area. Interestingly, these fault and fracture systems
also control the regional fluvial drainage development on this Plateau (Shoemaker, 1963), acting as pathways for surface water
migration to greater depth, recharging the regional groundwater
system. The pathways through the Kaibab Formation are so efficient at dewatering the unit that there are no perennial streams
on the Kaibab Plateau surrounding the Grand Canyon (Huntoon,
2000).
4.4. Precipitation, groundwater and springs
As discussed earlier, the pluvial climate at the time of impact
enhanced precipitation and surface runoff. The groundwater table
was likely much higher at the time of impact than it is today (Shoemaker, 1960; Roddy, 1978). It appears to have been within the
Coconino Sandstone and exposed in the uplifted crater walls, producing artesian flooding of the crater (see, also, Kring, 2007). This
flooding, post-impact groundwater springs, and additional precipitation produced the crater lake described above. In addition, there
may have been a significant amount of top-down surface runoff
from the rim crest to the floor because of precipitation of rain.
The surface runoff washed the crater wall soft sediments into the
growing lake. Later, it incised the bedrock. The channels of this
flow are now obliterated or buried beneath the younger alluvium.
As the groundwater table fell (because of decreasing precipitation),
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Fig. 8. (a–d) Fracture and fault controlled gullies. Note that the channels on the Coconino Sandstone originate from the fractures in the Kaibab Formation. Where more than
one fracture zones interact, it can form a discharge zone, where the groundwater can potentially be discharged.
Fig. 9. Bi-directional rose diagrams showing the orientations of the radial faults,
radial fractures and upper wall gullies. Note that all of these have preferred
orientations; the orientation of gullies is more or less similar to those of faults and
fractures, but there are some differences due to slope variation.
the talus slopes produced by debris from the upper crater walls
stabilized, producing the alluvium aprons that extend up to, and
in some cases above, the contact between the Coconino Sandstone
and Kaibab Formation on the crater walls. Throughout these peri-
ods, additional mass wastage would have been fostered by fractures and faults that fed water from the top of the surrounding
plain to the crater floor, most often at the base of the Kaibab units
as described above. Eventually this latter mechanism appears to
have become the dominant mode of gully formation, because the
gullies observed today are linked to exposed fractures and faults.
On the interior wall of the crater, rock discontinuities such as
impact structures (faults and fractures) enhance the material removal. For example, the radial faults consist of fault breccias (also
called authigenic breccia) composed of non-cohesive angular fragments of the host rocks (dolomites and sandstones). The surface
runoff has easily removed these non-cohesive breccia materials
producing the gully systems. Also, the prominent radial fractures
are characterized by tightly spaced fractures, which are susceptible
to easy degradation. The fractures and faults show widening of
fracture apertures because of percolation of surface runoff, as these
structures are highly permeable (e.g., Micarelli et al., 2006; Leckenby et al., 2005). In a core recovered from the south rim, the rock
was cut by a set of horizontal fractures that were spaced at an average of 2–3 in. (Haines, 1966). Measured permeabilities within the
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Fig. 10. (a–c) Cave formation at the contact of Kaibab and Toroweap Formations. Note that groundwater discharged from the caves produced channels on the talus deposits.
Also, note that the cave locations are controlled by fractures. See Fig. 2 for location of the cave.
ejecta are highly variable, ranging from 2.5 to 17.3 millidarcies.
Grant and Schultz (1993b) report highly variable hydraulic conductivity of ejecta deposits (1.5–9.0 m/d) depending upon the
proportion of gravel, sand, silt and clay. In the underlying (and
overturned) Moenkopi Formation, permeability is usually
<0.4 millidarcies, although one sample had a permeability of
1.3 millidarcies. The permeability of underlying (and overturned)
Kaibab is generally higher, ranging from <0.4 to values as high as
80.7 millidarcies. The dolomite sections of the Kaibab are not very
permeable, but the sandstone strata are very permeable (Watkins
and Walters, 1966). Because of enhanced permeabilities of the crater wall rocks, the infiltrated water has emerged at discharge
points at lower stratigraphic and structural levels (Fig. 8d).
The radially arranged gully system (Fig. 2) may have formed
along with the lake sediments (Fig. 13), when wet climatic conditions prevailed. Although the mass-wasting processes might have
displaced sediments initially, concomitant with this, fluvial processes eroded the materials from the crater walls and deposited
them as Pleistocene alluvium along the periphery of the crater
floor, inter-fingering with the lacustrine sediments (Fig. 13), during
the same period of enhanced precipitation and groundwater volume. The Pleistocene alluvium may have been derived by erosion
from the alcoves in the upper to middle crater wall. Later, the alluvium was dissected, producing gullies on the lower crater wall,
implying a second period of enhanced precipitation (Shoemaker
and Kieffer, 1974) (Fig. 7). The younger, post-Pleistocene alluvium
that was swept downward by the gully-forming event is now being
dissected and overlapped by recent playa deposits. Arid conditions
throughout the region began to dominate the environment about
11,000 years ago (see Kring, 1997). The basal playa sediments
may have been produced at that time. The uppermost 30 cm of
the playa sediments, however, were deposited above a thin ash
layer that formed approximately 900 years ago. This geologic evidence seems to suggest a tentative age of the gullies of around
50–11 ka. The present climate system only modifies the existing
gully systems. These modifications are represented, in part, by a
few debris flows that produced rock channels with rock levees
on the lower slopes of the crater walls.
4.5. Summary
Observations at Meteor Crater suggest that (a) gullies can form
on the interior walls of terrestrial impact craters, (b) such gullies
have alcove-channel-fan morphology, (c) gullies can form at different elevation (depth) levels on the crater interior wall, (d) the location of some gullies are determined by less-resistant bedrock and
structural discontinuities, (e) crater wall sediments, such as talus,
alluvium and impact breccia, enhance gully erosion, (f) most of
these gullies formed by fluvial sedimentary process, (g) the source
of water was from rainfall, snowfall and springs, (h) the rates of
formation of gullies were largely determined by climate, lithology
and structures, (i) gully erosion and debris transport form the major source of lake sediments, (j) eolian processes affect the crater
interiors but do not produce gully landforms, (k) climate change
plays a decisive role in gully evolution, particularly when they incise alluvial fans, (l) caves are present in the crater environment
and may have been formed from groundwater flow, and (m) crater
floor sediments contain geologic records of climate change. It is
worth mentioning here that, on the sediments comprising the
exterior crater ejecta, a radial drainage pattern is also developed
and that this drained surface water from the rim crest to outside
the crater (Grant and Schultz, 1993a). The drainage system might
have formed simultaneously with the gullies on the interior wall,
in response to the changing climate system.
5. Implications for Mars
5.1. Present day crater degradation on Mars
Present day Mars is a hyperarid, extremely cold desert environment, similar to the upland stable zone in the Antarctic Dry Valleys
(Marchant and Head, 2007). Although eolian activity dominates in
this environment, formation of young gullies on impact crater
walls has been interpreted as evidence of recent water activity.
For example, Malin and Edgett (2000, 2001) described a class of
features that they termed gullies, consisting of an alcove, a channel
P.S. Kumar et al. / Icarus 208 (2010) 608–620
617
Fig. 11. Eolian deposits covering the crater rim and wall of Meteor Crater: (a) eolian sand deposits cover the southern crater rim, see the fragments of the Coconino
Sandstone; the crater wall is to the left, and the approximate wind direction is also shown. Look direction is approximately east. (b) The SSE crater upper wall is covered with
eolian sand deposits. The crater interior wall forms the leeward side of the eolian deposits. Note that no gullies are exposed on the upper wall. (c) A close-up view of the
leeward side of the eolian deposits, and (d) a schematic map showing the prevailing wind direction and the leeward eolian deposits on the rim and upper crater wall.
Fig. 12. Time-series of annual total precipitation of rainfall and snowfall in
Winslow, Arizona (30 km east of Meteor Crater). Source: The Western Regional
Climate Center.
and a fan. Restricted to middle and high latitude locations (Balme
et al., 2006; Bridges and Lackner, 2006; Dickson et al., 2007; Heldmann et al., 2007; Dickson and Head, 2009), these features were
interpreted by some to have originated through processes related
to groundwater discharge (e.g., Malin and Edgett, 2000; Marquez
et al., 2005) or by others to surface runoff through melting of
near-surface ice (Costard et al., 2002; Head et al., 2008) that may
have been transported from the poles to mid-latitudes during recent high obliquity periods (Head et al., 2003; Christensen,
2003). Detailed analysis of the conditions under which H2O could
flow as a liquid in the current and relatively recent Mars environment shows a range of conditions under which gully-forming
activity is possible (e.g., Mellon and Phillips, 2001; Hecht, 2002;
Stewart and Nimmo, 2002; Heldmann et al., 2005; Williams
et al., 2008, 2009). However, alternative interpretations exist. Some
gullies, for example, have been interpreted to be the products of
dry granular flows associated with eolian activities (Treiman,
2003) and landsliding (Bart, 2007). Despite meter-scale resolution
in some orbital images, a consensus about the origins of the gullies
remains elusive.
Terrestrial analogs to martian environments have provided insight into the processes operating on Mars. For example, the nature
of perennial saline springs forming channels on Axel Heiberg Island
in the Canadian High Arctic has been used to support the argument
that liquid water can reach the surface in the thick permafrost regions without melting by volcanic heating (Andersen et al., 2002).
Low-arctic gullies led some to interpret near-surface thaw conditions as being linked to gully formation (Soare et al., 2008). Field
studies in the Antarctic Dry Valleys have provided ample evidence
618
P.S. Kumar et al. / Icarus 208 (2010) 608–620
Fig. 13. A schematic geologic cross-section of Meteor Crater showing the various rock units, structures and crater floor sediments. The geologic section is based on Shoemaker
and Kieffer (1974). Note that the concentric, radial, and conical fractures were produced during the impact event and re-activated pre-existing tectonic fractures (see Kumar
and Kring, 2008).
for top-down melting of annual and perennial snow and ice (Marchant and Head, 2007). Mudflow processes occurring in Atacama
Desert environment have been proposed as a martian analog
(Heldmann et al., 2008) for recently active bright gully deposits
on Mars (Malin et al., 2006). These terrestrial analogs contributed
to the understandings of various gully formation conditions. However, the gullies that occur within impact craters on Earth have not
been discussed in detail, although gullies at other landforms have
been investigated (e.g., Beavis, 2000; Panin et al., 2009; Brayshaw
and Hassan, 2009). Gullies in Meteor Crater are widespread around
the crater wall, are linked to lithologic and structural aspects of the
crater, formed over a long period (50 ka) of changing climate conditions ranging from wetter (and warmer?) earlier to arid today,
involved precipitation, a rising and lowering water table, and
involved a crater floor lake that changed character with time.
Although the environmental conditions of gully formation at
Meteor Crater are dissimilar to those on Mars, some of their characteristics are useful for understanding the crater degradation on
Mars.
As fractures and faults are abundant in a crater environment
(e.g., Kumar, 2005; Kumar and Kring, 2008), genetic models of gullies on Mars should consider the involvement of these structures,
where fractured bedrock and fluvial processes are involved in the
gully formation or enhancement. For example, Mars Exploration
Rovers clearly show the occurrence of dense fractures on impact
crater walls and evidence of erosion along the fractures (Grotzinger
et al., 2005; Grant et al., 2008; Squyres et al., 2009). Also, the
majority of fractures in some martian craters are linked to the
pre-existing tectonic fractures surrounding the craters (Watters,
2006). Similar to the Meteor Crater example, either top-to-down
surface runoff or groundwater discharge can efficiently exploit
these weakness zones to form the gullies there. Secondly, many
gullies at Meteor Crater are associated with softer rocks (sandstones) rather than competent rocks (dolomites). Similarly, martian craters expose softer sedimentary rocks (which are
susceptible to degradation), where gully landforms may likely to
occur. Also, as in Meteor Crater, the greatest volume of gully incision is in the softer deposits that coat the crater wall, including an
allogenic breccia, mixed (fall-back) breccia, and talus eroded from
the crater rim and upper crater walls. We also note that (i)
although craters may be filled in with secondary sediments, crater
walls may also be covered by allogenic and fall-back breccias; (ii)
based on observations at Meteor Crater, gullies on Mars may be entrenched in five types of softer sediments: eolian deposits, crater
wall alluvium, fall-back breccia, allogenic breccia, and authigenic
breccia. These soft sediments can be analogs to the martian crater
wall mantling deposits, on which gullies are abundant.
Eolian processes dominate present day Mars, and are the major
contributor to the crater modification on Mars. For example, the
Mars Exploration Rovers unequivocally documents the wind-related modification of small craters, comparable in size to Meteor
Crater (Grant et al., 2006, 2008). In these craters, the wind removes
the ejecta materials and deposits them onto the crater floor. Similarly, the present day crater modification at Meteor Crater is dominated by eolian activity, and the crater interior is a locale for
eolian sedimentation. Interestingly, observations from Victoria
Crater on Mars show that eolian processes exploit structural weakness in the crater wall rocks to form mass wasting and to enhance
the evolution of alcoves (Grant et al., 2008; Squyres et al., 2009).
Treiman (2003) suggests that eolian processes lead to the formation of gullies on Mars and that these gullies can form on the leeward side of eolian dune deposits by dry granular flow. At Meteor
Crater, however, eolian deposits are not incised by gullies. Rather,
eolian deposits are more likely to fill pre-existing gullies. This does
not necessarily undermine the model of dry granular flow, but may
be a product of different environmental conditions between Meteor Crater and Mars.
5.2. Past crater degradation on Mars
The gullies at Meteor Crater provide very important insight into
the nature of crater wall and rim degradation during the Noachian
period in Mars history, when surface runoff and shallow groundwater processes are much more likely to have been operating during this possible ‘‘warmer and wetter” period when valley
networks eroded and transported material across the surface of
Mars (e.g., Fassett and Head, 2008a,b; Grant and Schultz, 1993b;
Grant, 1999). Specifically, they show that surface runoff and
groundwater flow operating in a cratered terrain environment:
(a) is likely to be structurally controlled; (b) can produce abundant
wall gullies that assist in the erosion of the walls and rim crest and
the infilling of the crater floor; (c) can produce localized springs
that cause enhanced erosion; (d) can form caves; (e) can produce
crater floor lakes that have no external channels feeding them,
and whose levels change with changing water table position and
climatic conditions; (f) can produce lakes that are habitats for a
range of life, depending on changing environmental conditions;
and (g) can produce lakes that leave residual evaporite-like depos-
P.S. Kumar et al. / Icarus 208 (2010) 608–620
its when the climate transition to a more arid condition. Therefore,
the processes seen at Meteor Crater (gully formation and lakes)
should be considered as a terrestrial analog example that can be
studied to learn about possible impact crater degradation processes and rates on early Mars. Application of this Meteor Crater
gully and structure analysis can provide very important insight
into the nature of impact crater erosion and to the origin and processes responsible for recent gullies and ancient crater
degradation.
6. Conclusions
Meteor Crater is a classic example of a young simple crater emplaced in layered sedimentary rocks. Since its formation, the crater
has undergone degradation in response to changing climatic conditions. In its earlier history (from the time of crater formation
50,000 year ago to 11,000 years ago), pluvial climate conditions
dominated the crater area, there was abundant precipitation of
rainfall that flowed in and around the crater, and that enriched
groundwater systems. As a result, abundant surface runoff and
numerous springs discharged into the crater interior wall and this
led to the formation of radially distributed gully landforms; the
flowing water eroded the soft sediments such as talus, allogenic
breccia, authigenic breccia and mixed breccia, and eventually the
crater wall bedrock. The impact-related fractures and faults present in the crater wall are likely to have played an important role
in groundwater flow and discharge on the crater wall, causing preferential erosion of fragmented rocks along the faults. The flow may
have also been responsible for formation of a lake on the crater
floor that harbored a variety of flora and fauna. Later (from
11,000 years ago to the present), the climate regime changed to
an arid environment in which there was much less rainfall and
snow precipitation, similar to the present day climate. Because of
these changes, surface runoff became less abundant and seasonal,
the lake dried up, and the groundwater level lowered; this climatic
shift caused a substantial decrease of crater wall erosion. The present day rainfall and snow precipitation are of the order of a few
centimeters per annum, and gully activity become correspondingly
much less. Therefore, Meteor Crater provides insights into the degradation history of an impact crater in relation to changing climatic
conditions. These observations can provide a basis for application
to crater degradation processes in the recent and past history of
Mars.
Acknowledgments
We thank Drew N. Barringer, Barringer Crater Company, and
Brad Andes, Meteor Crater Enterprises, for granting permission to
conduct the field studies in Meteor Crater. P.S.K. thanks the Department of Science and Technology, Government of India for awarding
the BOYSCOST Fellowship for conducting collaborative research at
Brown University; T. Seshunarayana for encouragement and support; V.P. Dimri, Director, NGRI for permission to publish this paper. Thanks to Jay Dickson, Caleb Fasset, Joe Levy, Gareth
Morgan, Sam Schon, Misha Kreslavsky, Lionel Wilson and the participants of the Martian Gullies Workshop for scientific discussions; Jay Dickson provided a thoughtful review of an earlier
version of the manuscript. We acknowledge Anne Côté, Nancy
Christy and Bill Collins for support. The help from the staff (including Greg and Mandy) of Meteorite Crater Enterprises is fondly
appreciated. LPI Contribution No. 1395. Thanks are extended to
the National Aeronautics and Space Administration (NASA) for support under a Mars Data Analysis Program Grant to J.W.H.
(NNXO7AN95G) and a Mars Fundamental Research Program Grant
to D.A.K. (NNX07AK42G).
619
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.icarus.2010.03.032.
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