Ash Shutbah: A possible impact structure in Saudi Arabia

Meteoritics & Planetary Science 49, Nr 10, 1902–1914 (2014)
doi: 10.1111/maps.12369
Ash Shutbah: A possible impact structure in Saudi Arabia
Edwin GNOS1*, Beda A. HOFMANN2, Martin SCHMIEDER3, Khalid AL-WAGDANI4,
Ayman MAHJOUB4, Abdulaziz A. AL-SOLAMI4, Siddiq N. HABIBULLAH4,
Albert MATTER5, and Carl ALWMARK6
1
Natural History Museum Geneva, Route de Malagnou 1, CP 6434, 1205 Geneva 6, Switzerland
2
Natural History Museum Bern, Bernastrasse 15, 3005 Bern, Switzerland
3
Western Australian Argon Isotope Facility, Department of Applied Geology and JdL Centre, Curtin University,
GPO Box U1987, Perth, Western Australia 6845, Australia
4
Saudi Geological Survey, P.O. Box 54141, Jeddah 21514, Kingdom of Saudi Arabia
5
Institute of Geological Sciences, University of Bern, Baltzerstrasse 1+3, 3012 Bern, Switzerland
6
Department of Geology, Lund University, S€
olvegatan 12, 223 62 Lund, Sweden
*
Corresponding author. E-mail: [email protected]
(Received 16 October 2013; revision accepted 31 July 2014)
Abstract–We have investigated the Ash Shutbah circular structure in central Saudi Arabia
(21°370 N 45°390 E) using satellite imagery, field mapping, thin-section petrography, and
X-ray diffraction of collected samples. The approximately 2.1 km sized structure located in
flat-lying Jurassic Tuwaiq Mountain Limestone has been nearly peneplained by erosional
processes. Satellite and structural data show a central area consisting of Dhruma Formation
sandstones with steep bedding and tight folds plunging radially outward. Open folding
occurs in displaced, younger Tuwaiq Mountain Limestone Formation blocks surrounding
the central area, but is absent outside the circular structure. An approximately 60 cm thick,
unique folded and disrupted orthoquartzitic sandstone marker bed occurring in the central
area of the structure is found 140 m deeper in undisturbed escarpment outcrops located a
few hundred meters west of the structure. With exception of a possible concave shatter cone
found in the orthoquartzite of the central area, other diagnostic shock features are lacking.
Some quartz-rich sandstones from the central area show pervasive fracturing of quartz
grains with common concussion fractures. This deformation was followed by an event of
quartz dissolution and calcite precipitation consistent with local sea- or groundwater
heating. The combination of central stratigraphic uplift of 140 m, concussion features in
discolored sandstone, outward-dipping concentric folds in the central area, deformation
restricted to the rocks of the ring structure, a complex circular structure of 2.1 km diameter
that appears broadly consistent with what one would expect from an impact structure in
sedimentary targets, and a possible shatter cone all point to an impact origin of the Ash
Shutbah structure. In fact, the Ash Shutbah structure appears to be a textbook example of
an eroded, complex impact crater located in flat-lying sedimentary rocks, where the
undisturbed stratigraphic section can be studied in escarpment outcrops in the vicinity of
the structure.
INTRODUCTION
Saudi Arabia covers an area of more than two
million square kilometers. In 2012, the international
Meteorite Impact Database (http://www.passc.net/
EarthImpactDatabase/) contained only the three small,
© The Meteoritical Society, 2014.
approximately 300-year-old Wabar impact craters
located in sand dunes of the Rub al Khali desert
(Fig. 1), the impact origin of which had been confirmed
on the ground (e.g., Philby 1933a, 1933b; Wynn 2002;
Prescott et al. 2004; Gnos et al. 2013). Eight additional
circular features (Fig. 1) may represent impact craters:
1902
Ash Shutbah: A possible impact structure in Saudi Arabia
1903
Fig. 1. Overview map of the Arabian Peninsula. Stars indicate
locations of the Ash Shutbah structure located south of Al
Haddar, and of other possible and proven impact crater sites
in Saudi Arabia.
The approximately 5 km Jabal Rayah (or Al Madafi),
the approximately 2.1 km Ash Shutbah, and the
approximately 17 km Wadi Na’am structures have been
proposed as possible impact craters on the basis of
remote sensing or field mapping studies (Janjou et al.
2004; Schmieder et al. 2009; Gnos et al. 2011; references
therein). The approximately 34 km Saqqar structure,
the approximately 19 km Jalamid structure, the
approximately
16 km
Hamidan
structure,
the
approximately 12 km Banat Baqar structure, and
the approximately 5 km Zaynan structure, which are all
complex subsurface features, have recently been
discussed as of possible impact origin based on seismic
and drill core evidence (Neville et al. 2014).
The purpose of a first field exploration in 2011 was
to verify on the ground whether the very prominent,
approximately 2.1 km sized, circular Ash Shutbah
structure located on the Jabal Tuwaiq plateau south of
Riyadh in an area devoid of volcanic rocks or salt
diapirism could represent an eroded impact crater.
THE ASH SHUTBAH CIRCULAR STRUCTURE
The Ash Shutbah circular structure centered at
21°370 N 45°390 E, approximately 50 km S of the village
of Al Haddar (Figs. 1 and 2), is located in flat-lying
limestones of the middle Jurassic Tuwaiq Mountain
Limestone Formation with a local thickness of 87 m
(Steinecke and Sanders 1958; Powers et al. 1966; Vaslet
Fig. 2. Landsat 7 Thematic Mapper image, bands 7, 4, 2 in
red, green, and blue. The image shows the Ash Shutbah
impact structure located in Tuwaiq Mountain Limestone
Formation near the edge of the Jabal Tuwaiq escarpment. The
escarpment to the west mainly consists of dark weathering
Dhruma Formation sandstones and limestones.
et al. 1985). Its western margin is located approximately
500 m from the approximately 200 m high Jabal
Tuwaiq escarpment, an erosional feature forming a
several hundred kilometer long, NE-trending, prominent
topographic feature (Figs. 2 and 3) in central Saudi
Arabia. The Ash Shutbah circular structure is located at
the upper end of Wadi Ash Shutbah and was named
accordingly (Schmieder et al. 2009; Gnos et al. 2011).
The Tuwaiq Mountain Limestone forms the western
part of Jabal Tuwaiq (Fig. 2; Table 1). Middle Jurassic
Dhruma Formation (Steinecke and Sanders 1958;
Powers et al. 1966; Vaslet et al. 1985) (Fig. 2) is
exposed in the escarpment below the limestone.
Whereas the Tuwaiq Mountain Limestone consists of
pure limestones, abundant sandstone beds containing
some dolomite characterize the Dhruma outcrops in this
area (Vaslet et al. 1985; Table 1). The base of the cliff is
formed by Lower Jurassic Marrat Formation
comprising sandstones, conglomeratic sandstones, and
pedogenic claystone (Table 1). This formation
uncomfortably overlies Permo-Triassic strata (Minjur
Formation; Steinecke and Sanders 1958; Powers et al.
1966; Vaslet et al. 1985; Table 1). The total thickness of
1904
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Fig. 3. N–S and W–E topographic lines across the Ash Shutbah structure. Data are from the Shuttle Radar Topographic
Mission. Note the very weak topographic expression of the structure (marked with boxes).
Table 1. Lithostratigraphy of the Jabal Rayah area based on Vaslet et al. (1985).
Age
Formation
Lower Kimmeridgian
or Tithonian
Early Kimmeridgian
Oxfordian
Arab
Middle to Late
Callovian
Bajocian to Lower
Callovian
Toarcian
Jubaila Limestone
Hanifa
Tuwaiq Mountain
Limestone
Dhruma
Marrat
Thickness
(m)
65
112
118
93.5
157
9
sediments below the Tuwaiq Mountain Limestone, and
overlying the crystalline basement, is approximately
400 m, consisting of 166 m Jurassic and 230 m of
Permo-Triassic (Vaslet et al. 1985). All these sediments
are shale- and sandstone-dominated and do not contain
significant evaporite layers (Vaslet et al. 1985).
The circular Ash Shutbah feature was mapped and
interpreted as a circular feature of carstic origin (Vaslet
et al. 1985). The structure was independently
rediscovered by two of us (B.A.H, M.S.) on satellite
imagery available on Google Earth. We then used
Lithology
White gypsum with white, beige, or gray bioclastic, peletoid, or
oolitic limestones and dolomites
Cream to brown carbonate sandstone and dolomitic limestone
Brown, yellow, and white sandy dolomite or carbonate sandstone
with local sandstone beds
Cream to white, bioclastic to oolitic limestone with corals
Tan, khaki to red-brown sandstones with sandy dolomite and
gypsiferous claystone layers
White or red sandstones and conglomeratic sandstones with
pedogenic claystone
satellite data to further characterize the structure and
correlate features on satellite imagery with field
observations.
Remote Sensing
Although the Ash Shutbah structure is only
approximately 2.1 km in diameter, a circular feature
with a central dark dot is easily recognizable on satellite
imagery (Fig. 2), where two branches of the upper Wadi
Ash Shutbah follow the outermost circular feature. The
Ash Shutbah: A possible impact structure in Saudi Arabia
1905
Fig. 4. Landsat 7 ETM+, bands 3, 2, 1 in red, green, and blue, merged with band 8 over Shuttle Radar Topographic Mission
(29 vertical exaggeration). Note the dark color in the lower part of the escarpment produced by debris from a 60 cm thick
orthoquartzite bed. The same marker bed occurs in the central part of the Ash Shutbah impact crater.
morphological expression, as revealed by Shuttle radar
data (SRTM), is very weak (Fig. 3). Nonetheless, due to
lithological contrast, a dark central area is clearly
distinguishable on satellite images. The compositional
variability of sedimentary units exposed in the Ash
Shutbah structure is particularly apparent in
multispectral band ratio images that are commonly used
for the “mapping” of arid areas (band ratio image 5/35/7-3/1 in red, green, and blue using the 5/7 “clay
mineral” and 3/1 “iron” band ratios; e.g., Gad and
Kusky 2006; Schmieder et al. 2013; Fig. S1). Whereas
the Tuwaiq Mountain Limestone Formation and the
overlying desert regolith and wadi gravels appear in
green, yellow, and pale bluish colors, the partially
siliciclastic underlying Dhruma Formation exposed near
the base of the Tuwaiq escarpment and in the central
domain of the Ash Shutbah structure appears in dark
crimson (Fig. S1). On a Landsat 7 ETM+ image (bands
3, 2, 1 in red, green, and blue, merged with Landsat
band 8 over SRTM), the situation is displayed in a 3-D
view in visible color, showing the dark central core
(Fig. 4). The dark beds forming the lower part of the
escarpment have the same spectral response as the rocks
occupying the central part of the Ash Shutbah feature.
In this view, it is also well visible that the structure has
been eroded to the plain level.
Field Observations
The Ash Shutbah circular structure was assessed from
the direction of Al Haddar village (Fig. 2), from where a
gravel road leads onto the Jabal Tuwaiq plateau.
Fieldwork was conducted from May 9 to 12, 2011. The
visit to the Ash Shutbah area confirmed that the original
morphological expression of the circular structure has
been strongly eroded by desert alteration and wind
erosion (peneplenation). However, the size of the structure
as mapped on the ground and the circular features
recognizable on satellite images are in good agreement.
The circular structure is located in undeformed, essentially
flat-lying limestone beds of the Tuwaiq Mountain
Limestone, with a regional dip of about 1° to the east
(Fig. 2). The relief between hilltops and depressions
within the circular feature is <15 m. Nonetheless, small
hills stand above the flat plateau and are easily
recognizable when approaching by car. The eastern limit
of the ring structure and the two branches of Wadi Ash
Shutbah forming circular features seem to lie outside of
the areas displaying a weak topography (Fig. 3). The
geological map shown in Fig. 5 has been produced using a
combination of data from compass, GPS and altimeter
measurements, and satellite imagery. The center of the
circular structure is located at 21°37.140 N/45°39.350 E. The
dark-colored central area, approximately 400 m wide
(Fig. 6a), consists of strongly folded, cream-colored
calcareous sandstone of the Dhruma Formation (Fig. 6b)
and is littered with black, desert-varnish–covered debris of
a massive, only 60 cm thick, orthoquartzite (quartzcemented sandstone) bed of the Dhruma Formation
(Fig. 6c). One possible concave shatter cone was found in
a small outcrop of this orthoquartzite (Fig. 6d). It was left
intact at its original position at 21°37.130 N/45°39.370 E for
future on-site inspection. In comparison to undeformed
cream-colored Dhruma Formation samples from the
escarpment, rocks from the central area of the ring
structure are locally discolored and white (Fig. 6e).
1906
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Fig. 5. Geological map of the Ash Shutbah area. Note the location at the edge of an escarpment. This permits us to locate the
marker bed outcropping in the central uplift (black line) in an undisturbed stratigraphic section in the escarpment 500 m to the
west of the structure. The position of the marker bed is indicated in the cross section. The silicified coral beds are not shown in
the profile because several horizons have been described in the Jabal Tuwaiq limestone by Vaslet et al. (1985).
Morphologically, the calcareous sandstones forming
the central part of the circular feature are more
intensely eroded than the surrounding ring of Tuwaiq
Mountain Limestone hills (Fig. 6a). Good outcrops
permitting a more detailed structural analysis of this
central part are lacking.
The outer annular ridges of the structure consist of
displaced packages of open folded but stratigraphically
undisturbed Tuwaiq Mountain Limestone containing
abundant, locally silicified, coral beds and occasionally
reddish marly beds. Silicified corals are stained with a
dark desert varnish. This is the reason why they can be
distinguished from the surrounding carbonate beds on
satellite images and mapped as a separate subunit
(Fig. 5). The structure shows shallow ring-shaped
depressions produced by erosion. Because several levels
of silicified corals have been reported in the Tuwaiq
Mountain Limestone (Vaslet et al. 1985), this
information has not been included in the profile (Fig. 5)
and was not used for stratigraphic correlation.
The unique, black-weathering and disrupted
orthoquartzitic Dhruma Formation marker bed present
in the central area has been located 140 m deeper in
outcrops along the escarpment, a few hundred meters
west of the circular structure (see cross section in
Fig. 5).
Ash Shutbah: A possible impact structure in Saudi Arabia
1907
Fig. 6. a) Panoramic view from the innermost Jabal Tuwaiq limestone hill located east of the structure over the central area
consisting of Bajocian to lower Callovian sandstones of the Dhruma Formation (dark) and dislocated Callovian Tuwaiq
Mountain Limestone blocks. Note the strong erosion (peneplenation) of the structure and the even stronger erosion of the dark
central area. b) Isoclinally folded calcareous Dhruma Formation sandstone beds, located in the outer part of the central area.
The fold axis plunges away from the center of the structure (see Fig. 7). c) Weathered, steeply dipping, folded, and disrupted,
approximately 60 cm thick orthoquartzite marker bed in Dhruma Formation of the central area. The contrasting dark color of
the central area is due to desert varnish on debris from the orthoquartzite marker bed littering the surface (see Fig. 6a).
d) Possible shatter cone imprint in orthoquartzite bed in the central area. e) Freshly broken sandstone bed in the central part of
the structure. The white color is due to the pervasive fracturing of sand grains in this rock.
Structures
Whereas
folded
and
boudinaged
Dhruma
Formation beds (Fig. 6c) stand upright at nearly 90°
in the central part of the Ash Shutbah structure, they
dip outward at shallower angles in the outer part of
the central area (Fig. 7). The beds show tight to
isoclinal folding (Fig. 6b), with fold axes plunging
radially outward (Fig. 7). The plunging is shallower in
a north-northwestern sector of the central area and
steeper in the southeastern part. As a whole, the
structures resemble a dome-shaped extrusion that
shows the characteristics of a sheath fold (e.g., Reber
2012). Rare cross-bedding indicates that the beds are
not overturned. In general, the dip of the bedding is
decreasing in the outer sections of the structure that
consists of displaced Jabal Tuwaiq Limestone
packages. Within these limestone blocks, open folds
are the most obvious structural feature. They are the
reason for the changing dip directions of the local
bedding (Fig. 7). Brecciation of the limestone has been
observed locally, but does not affect entire limestone
packages.
Samples and Methods
Thin sections of 3 Dhruma Formation sandstones
from the escarpment and 20 sandstone samples from the
Dhruma Formation collected in the central area (Fig. 5)
were analyzed for shock deformation features in
transmitted and reflected light using a polarizing
microscope. To test for the presence of coesite,
sandstone samples were crushed to <5 mm and treated
with dilute (10%) hydrochloric acid to remove calcite
and subsequently with dilute (8%) hydrofluoric acid for
8–12 h to partially remove quartz. In some cases, the
treatment was repeated. The washed residue was placed
on a Si-monocrystal holder and analyzed on a Philipps
1908
E. Gnos et al.
Fig. 7. Structural data collected over the inner part of the structure. Note the steeper orientation of the beds in the central area
and the outward plunging fold axes in the outer part of the central uplifted terrain. The plunging is shallower in the NW sector
than in the SE sector. Small white stars indicate sandstone sample locations.
Ash Shutbah: A possible impact structure in Saudi Arabia
PW1800 X-ray diffractometer (XRD) at low scanning
speed.
1909
DISCUSSION
Remote Sensing
PETROGRAPHY OF SANDSTONES FROM THE
ASH SHUTBAH STRUCTURE
Sandstones of the Dhruma Formation collected
from the undisturbed escarpment section outside the
Ash Shutbah structure are mature, with well-rounded
sand grains (Fig. 8a), and show various degrees of
cementation (porosity), with secondary quartz forming
oriented quartz overgrowths on detrital grains. Calcite
cement occurs only in minor quantities. No evidence of
quartz dissolution was observed. Individual sandstone
layers are well sorted. In contrast, discolored, white
sandstones from inside the Ash Shutbah structure
(Fig. 6e) show strong deformation of sand grains, with
few sand grains remaining unaffected (Fig. 8b). The
macroscopic discoloration is obviously due to the
pervasive cracking of quartz grains on a microscopic
scale (Fig. 8c), often expressed as concussion fractures
(Fig. 8d) as, for example, described from the Coconino
sandstone at Meteor Crater, Arizona (Kieffer 1971).
Concussion fractures are characterized by fan-shaped
cracking patterns at contact points due to collapse of
pore space during shock compaction. This leads to a
characteristic impinging of grains (Figs. 8d and 8e).
Despite a focussed search in the siliciclastic rocks,
planar features (PFs) or planar deformation features
(PDFs) in quartz have not been found in the studied
samples from Ash Shutbah.
The deformed Ash Shutbah sandstones consist of an
assemblage of shards (Figs. 8c and 8e) of partly fractured
sand grains with a matrix of fragments at all scales down
to submicroscopic sizes. Individual detrital grains of
quartz, but also of rare accessory minerals, are fragmented
into several pieces residing in the vicinity of each other.
Deformation of quartz is also indicated by strong
undulous extinction (Figs. 8c–e), especially of smaller and
strongly deformed grains, which is not observed in
undeformed reference sandstones from the Tuwaiq
escarpment. Some samples show cohesion without calcite
cementation (e.g., Fig. 8c), indicating the presence of a
small amount of quartz cement. Fractures in sandstone
grains are commonly filled by postdeformational calcite
cement consisting of large, several millimeter sized calcite
crystals completely filling the porosity in some samples.
Cementation with calcite is accompanied by strong
preferential dissolution of the smaller and more deformed
quartz particles, mainly along cracks (Fig. 8f).
XRD analysis of the residue obtained by partially
dissolving fragments of discolored and other sandstones
from the central area in dilute hydrofluoric acid yielded
no indication for the presence of coesite in 24 samples.
Satellite images show that the center of the 2.1 km
sized circular Ash Shutbah structure (Figs. 3 and S1) is
occupied by rocks that have the same spectral
reflectance as Dhruma Formation beds exposed in the
lower part of the approximately 200 m high escarpment
located west of the structure. Field observations showed
that a unique, only 60 cm thick orthoquartzite marker
bed of the Dhruma Formation, resistant to weathering,
has developed a dark desert varnish. The debris of this
resistant bed litters the central domain of the Ash
Shutbah structure and also covers the lower slopes of
the Jabal Tuwaiq escarpment outside the impact
structure. This is the reason why these areas appear
dark on satellite images (Figs. 3 and S1) and why the
central area shows a very distinct spectral color contrast
to the surrounding limestone.
Stratigraphy and Structures
Investigations in the field showed that the central
part of the ring structure consists of folded Dhruma
Formation sandstones and the outer parts of gently
deformed and displaced blocks of Tuwaiq Mountain
Limestone Formation. Whereas the Dhruma Formation
occurs in the escarpment outcrops in a normal
stratigraphic position below the Tuwaiq Mountain
Limestone Formation (Table 1), outcrops in the central
area of the ring structure are at the same level as
Tuwaiq Mountain Limestone Formation outcrops,
indicating disturbance of the normal stratigraphy. The
mapped size of the ring structure is in agreement with
the size estimated from remote sensing.
The Dhruma Formation sandstones of the central
area show steep bedding and radial orientation of
folds with outward-plunging fold axes (Fig. 7).
Whereas the orthoquartzitic marker bed is undisturbed
and flat-lying in outcrops of the escarpment
approximately 500 m west of the Ash Shutbah
structure, the bed located 140 m higher in the central
area is folded, boudinaged, and disrupted. In this bed,
a single possible shatter cone (Fig. 6d) was noted.
Considering the minor tilt of the flat-lying strata that
build up the Tuwaiq Mountain plateau, as well as
erosion following the formation of the circular
structure, this direct stratigraphic correlation indicates
a minimal structural-stratigraphic uplift of at least
approximately 140 m at the very center of the Ash
Shutbah structure.
Estimates of the sizes of dislocated Tuwaiq
Mountain Limestone blocks indicate approximately
1910
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Fig. 8. Thin-section photographs displaying microdeformation in sandstone samples. a) Undeformed Dhruma Formation
orthoquartzitic sandstone (1105-24) from the escarpment. Note the absence of fractures in sand grains. b) Thin-section
photograph of discolored sandstone bed (1105-19) from the central area. Quartz displays abundant irregular, subparallel, and
fan-shaped fractures with quartz shards filling the space between larger grains. c) Close-up of quartz grains in 1105-19
showing subparallel cracking. Small quartz shards fill pore space between larger grains. d) Radial concussion fractures in
quartz caused by indenting quartz grain to the right (in extinction). Pore space is filled with fine quartz shards (1105-19).
e) Quartz grain indenting another quartz grain, resulting in characteristic set of subparallel fractures (1105-19). The stronger
grain (light gray) contains only a few fissures. f) Fractured quartz grain in sample 1105-6 that was affected by dissolution and
precipitation of calcite cement following a fracturing event. The texture clearly shows that carbonate cementation occurred
only after fracturing.
250 9 250–1000 m in lateral extent. The vertical
thickness of the packages is difficult to estimate without
additional stratigraphic information.
The field observations confirm that the deformation
is restricted to the ring structure, that it is strongest in
the center of the structure, and that the bedding
becomes progressively steeper toward the center of the
structure, indicating doming.
Microstructures
At
the
microscale,
there
is
abundant
microfracturing of quartz grains resulting in local,
macroscopically observed discoloration of sandstones
from cream to white inside the central area. The
presence of characteristic concussion fractures in quartz
grains (with remnant porosity) suggests maximum shock
Ash Shutbah: A possible impact structure in Saudi Arabia
pressures of approximately 3 GPa (Grieve et al. 1996)
at the present erosion level. Similar concussion fractures
in quartz were earlier described from weakly shocked
sandstones from Meteor Crater, Arizona, USA (Kieffer
1971), but also occur in Middle Jurassic sandstones that
build up the central uplift of the approximately 3.8 km
Steinheim impact in Germany, which also lacks other
high-pressure shock features (Groschopf and Reiff 1969;
Buchner and Schmieder 2010). Comparable texture has
also been reported from weakly shock-lithified sand
from the smallest, 11 m Wabar crater in Saudi Arabia
(Gnos et al. 2013), while at the larger two Wabar
craters, abundant PDFs are observed in quartz, in
association with coesite and stishovite (e.g., Gnos et al.
2013).
Planar fractures and feather features in quartz (e.g.,
Poelchau and Kenkmann 2011), tentatively associated
with shock pressures exceeding approximately 5 GPa,
were not observed at Ash Shutbah. Thus, evidence for
impact-induced deformation seems to be restricted to
the lowest level of shock metamorphism (e.g., French
1998). The effect of apparently weak shock deformation
is somewhat similar to the low-grade shock features
observed at other complex terrestrial impact structures
in sedimentary targets, such as Steinheim (Buchner and
Schmieder 2010), the approximately 6 km Tin Bider
impact structure in Algeria (Lambert et al. 1981), the
approximately 8 km Upheaval Dome impact structure
in Utah, USA (Buchner and Kenkmann 2008), the
approximately 10 km Uneged Uul structure of likely
impact origin in Mongolia (Schmieder et al. 2013), the
approximately 12 km Marquez Dome in Texas, USA
(Buchanan et al. 1998), or the approximately 15 km
Yallalie structure in Western Australia (Dentith et al.
1999). All these structures lack clear evidence for higher
levels of shock metamorphism. As for these structures,
the shock wave generated during the Ash Shutbah
impact could have been attenuated and “buffered”
within the porous sedimentary target rocks, probably
under water-saturated conditions that would have
reduced the physical strength of the target rock (e.g.,
Kenkmann et al. 2011; Kowitz et al. 2013a, 2013b).
While most of the Jurassic sedimentary rocks on the
Arabian Platform were deposited in a shallow marine
environment (such as the coralliferous Tuwaiq
Mountain Limestone; e.g., Alsharhan and Magara
1994), the general paleogeographic conditions became
progressively continental during the Early Cretaceous,
with a marine transgression from the Late Cretaceous
until the Eocene, and following emergence and
denudation of the Arabian plate during the Neogene
(e.g., Powers et al. 1966). In both shallow marine and
continental setting cases, seawater and groundwater,
respectively, would have been involved in the impact
1911
process. Quartz dissolution and calcite precipitation in
sandstones from the central uplift are consistent with
impact shock and postshock heating, followed by seaor groundwater inflow. Because quartz solubility is
strongly enhanced by increased temperature (Rimstidt
1997), while calcite solubility decreases (Coto et al.
2012), the interaction of a volume of rock with
increased temperature with local sea- or groundwater
could explain the observed late-stage quartz dissolution
and calcite precipitation.
Evidence for an Impact Crater
The Ash Shutbah structure shows evidence of
intense and localized rock deformation at scales ranging
from decameters down to the microscopic scale, within
a geological setting otherwise devoid of postdepositional
deformation. Even though we have not found PFs or
PDFs in quartz or high-pressure phases, the steeply
inclined, folded, and uplifted sandstone beds, radially
outward plunging fold axes in Dhruma Formation
sandstones of the central area, and concussion fractures
found in the rocks at the center of the complex circular
Ash Shutbah structure are strong evidence for an
impact origin.
Alternative explanations are unsatisfactory. An
origin as a karst feature as suggested by Vaslet et al.
(1985) can be clearly excluded. Such an origin is
inconsistent with the apparent presence of a central
uplift and the observed rock deformations. In the case
of the approximately 8 km Upheaval Dome impact
structure in Utah, USA, an impact scenario was
discussed since Shoemaker and Herkenhoff (1983), but
the presence of a salt layer under ground led to another
interpretation that the structure represented a dissolved
salt dome (e.g., Jackson et al. 1998). This view persisted
until eventually shocked quartz was documented
(Buchner and Kenkmann 2008). In the case of Ash
Shutbah, salt tectonics can be definitely excluded. The
Cambrian salt deposits of the Arabian Peninsula only
occur farther to the east (e.g., Alsharhan and Kendall
1986). The Ash Shutbah structure is underlain by about
400 m of mainly clastic sediments overlying the
crystalline basement (Vaslet et al. 1985). Moreover,
the subsurface ascent of magma or mud diapirs and the
formation of structural domes that commonly show a
complex morphology once eroded are not compatible
with the small size and isolated occurrence of this exotic
feature on the Tuwaiq Mountain plateau and steeply
inclined strata inside the central area. Endogenic models
for the formation of structurally complex circular
structures on the Earth’s surface (e.g., Schmieder et al.
2013) seemingly cannot explain the formation of the
Ash Shutbah structure.
1912
E. Gnos et al.
Large-scale structural evidence consistent with
impact is the presence of a complex circular structure of
approximately 2.1 km diameter with a ground-truthed
central uplift, intense deformation including concussion
fractures in sandstone, restricted to the circular
structure, the unique orthoquartzitic marker bed present
in the central uplift and located 140 m below in
undisturbed outcrops of the escarpment approximately
500 m west of the Ash Shutbah structure, and the
finding of a possible shatter cone (Fig. 6d). Considering
the minor tilt of the flat-lying strata that build up the
Tuwaiq Mountain plateau and erosion following the
formation of the circular structure, this direct
stratigraphic correlation indicates a minimal structuralstratigraphic uplift of at least approximately 140 m at
the very center of the Ash Shutbah structure. Moreover,
the localized occurrence of tightly folded beds in
otherwise regionally flat-bedded and essentially
undeformed sedimentary rocks is only consistent with
the interpretation of the Ash Shutbah circular feature as
a strongly eroded complex impact crater. If we assume
that the plunging of the folds became steeper at a
higher, now eroded level of the structure or that the
steeper plunging folds became even overturned folds
comparable to those at the Waqf as Suwwan structure
(Kenkmann et al. 2010), the structural asymmetry
within the thrusts and folds of the eroded central uplift
observed and mapped in the field (Fig. 7) would
indicate an impactor that arrived from the NW
(compare Kenkmann and Poelchau 2009). The deep
level of erosion below the structural crater floor would
be the reason why impact melt lithologies and
impactites of the proximal ejecta blanket are missing.
Crater Size Estimate
A compilation by Grieve (1987) stated that the
transition from simple to complex impact craters occurs
at approximately 2 km diameter in a sedimentary target
(as exemplified by the BP structure in Libya) and at
approximately 4 km in crystalline rocks (as in the case
of Brent crater in Ontario, Canada). By using the
Holsapple (1993) equations, the transition from simple
to complex impact craters occurs at approximately
1.8 km. The equation for a complex impact crater:
structural uplift = 0.06*crater diameter1.1 (Grieve 1987)
yields a minimal structural uplift for the approximately
2.1 km crater of 136 m. Calculations with the Holsapple
(1993) scaling laws for impact craters, assuming that the
observed 2.1 km outer diameter of the eroded impact
structure also represents the approximate original
diameter of the fresh crater, yields for a stony or
metallic impactor arriving at an angle of 45° and speeds
of 5–15 km s1 a crater rim diameter of approximately
2.8 km with a central peak reaching approximately
25 m above the crater floor. However, based on the
facts that the Ash Shutbah circular structure is eroded
below the level of the original crater floor (no
impactites of the crater-filling breccia lens were
encountered in the field) and that the measurable
structural uplift of the orthoquartzite marker bed in the
central uplift is at least approximately 140 m, it is
possible that the original diameter was somewhat larger.
However, Ash Shutbah is morphologically comparable
to the 2 km diameter BP structure (French et al. 1974;
Koeberl et al. 2005).
Age of Formation
In the absence of any datable material (impactgenerated melt-bearing rocks or shock veins were not
detected), the age of the circular structure can only be
constrained stratigraphically to younger than the
Tuwaiq Mountain Limestone (post-Callovian; <165 Ma;
Powers et al. 1966; Vaslet et al. 1985).
CONCLUSIONS
Even though there is no unequivocal evidence, such
as PDFs or confirmed shatter cones, that the Ash
Shutbah structure is an impact crater (e.g., French and
Koeberl 2010), there are multiple indications that favor
an impact origin (1) central stratigraphic uplift of 140 m
consistent with estimates using the formula for impact
craters (Grieve 1987); (2) concussion fractures in
discolored sandstone of the central uplifted terrain, but
not in undisturbed sandstone of the outer escarpment;
(3) outward-dipping concentric folds in the central ring
structure; (4) intense deformation restricted to the rocks
of a ring structure; (5) a complex, rather than simple,
circular structure of 2.1 km diameter, consistent with
the type of impact crater that is expected in a
sedimentary target (Holsapple 1993); and (6) a possible
shatter cone.
The stratigraphic setting indicates a Callovian
maximum age for the circular structure. The observed
concussion fractures suggest low shock pressure
probably reflecting deep erosion and a shock wave
buffering effect within the porous and, at that time,
possibly “wet” sedimentary target. Strong quartz
dissolution and carbonate cementation observed in
sandstones of the central uplift with fractured quartz
are consistent with postimpact heating and water
saturation.
The location of the Ash Shutbah structure near the
edge of the 200 m high Jabal Tuwaiq escarpment
provides a unique situation where the undisturbed
stratigraphic sequence and impact-deformed beds can be
Ash Shutbah: A possible impact structure in Saudi Arabia
studied in close vicinity, comparable to a block diagram
(Fig. 4). The geometry and geological situation of the
Ash Shutbah circular structure indicate that it is
probably a strongly eroded complex impact crater,
comparable in size, shape, and structural features to the
approximately 2 km BP impact structure in “Nubian
Sandstone” of SE Libya (e.g., French et al. 1974;
Koeberl et al. 2005).
Acknowledgments—Saudi Geological Survey director
Dr. Zuhair Nawab, as well as Abdullah al Attas and
Saleh al Ghamdi are thanked for granting permission
for the project and for logistic support. Mutrif bin
Fahad Bin Namsham is thanked for providing
accommodation and guidance in Al Haddar. Philippe
Wagneur assembled the panoramic view. XRD analyses
were performed by Nicolas Greber. This study was
supported by the Swiss National Science Foundation
grant 200020-119937 and the Natural History Museum
of Geneva. T. Kenkmann and W. U. Reimold are
thanked for their helpful reviews.
Editorial Handling—Dr. W. Uwe Reimold
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SUPPORTING INFORMATION
Additional supporting information may be found in
the online version of this article:
Fig S1. Multispectral Landsat-7 ETM+ satellite
image (scene of path 166, row 045, acquired on 20
February 2005) of the Wadi Ash Shutbah area and the
Tuwaiq escarpment in central Saudi Arabia, with a closeup of the Ash Shutbah structure shown in the inset. Both
scenes are color composite images of band ratios 5/3, 5/7,
3/1 RGB pan-sharpened with band 8 (15 m ground
resolution), in which different lithologic units can be
distinguished. Whereas the Tuwaiq Mountain Limestone
and overlying gravels appear in green, yellow, and bluish
colors, the underlying siliciclastic Dhruma Formation
shows a crimson color at the base of the Tuwaiq
escarpment due to scree derived from the orthoquartzite
bed (crimson color along the slope at approximately
830 m above sea level) and is also exposed in the central
area of the Ash Shutbah structure (approximately 970 m
above sea level). Image credit: USGS and Global Land
Cover Facility.