Rim uplift and crater shape in Meteor Crater: Effects of target heterogeneities and trajectory obliquity



[1] We have analyzed the rim structure of Meteor Crater, Arizona, in order to understand the mechanism of rim uplift in simple craters and the causes of the shape of polygonal impact craters. For this purpose, we systematically determined bedding orientation of the autochthonous crater wall and overturned flap and analyzed the kinematics of major radial faults. We found that rim uplift correlates with the crater shape and increases in the corners of the crater. The two main mechanisms of differential uplift are the formation of horizontal interthrust wedges, leading to the doubling of strata in the rim, and radial corner faults, or tear faults, that vertically displace bedrock. The development of Meteor Crater's quadrangular shape is caused by more effective crater excavation flow parallel to major joint sets. Additionally, we infer the impact direction with a newly developed technique, the two corners model, and review the arguments in favor of an oblique trajectory. While the data set is ambiguous, several indicators suggest an impact direction from the NNW. We conclude that oblique impacts should have an effect on early cratering and excavation flow, whereas target heterogeneities like joints start to play a prominent role in later stages when the stresses induced by the excavation flow are in the same order of strength as the material involved.

1. Introduction

[2] The Barringer Meteorite Crater, or Meteor Crater, is the prime example of a young, well-preserved and well-documented simple impact crater. As simple craters are among the most common morphological features on planetary surfaces in the solar system, understanding Meteor Crater is of major importance. While the outline of most simple craters is circular, the shape of Meteor Crater strongly deviates from a circle and resembles a quadrangle (Figure 1). The quadrangular shape is generally attributed to the occurrence of joint sets running through the diagonals of the crater, although, so far, little research has been performed on the details of the proposed process. Ground truth data are needed to better understand how the preimpact target structure can affect the shape and internal deformation of the crater wall and crater rim. This knowledge in turn can be used to enhance our understanding of the cratering process and the implications crater morphology and structure have on the configuration and composition of the surface and subsurface of other solid bodies in the solar system.

Figure 1.

(a) The distribution of the Canyon Diablo iron meteorite [after Barringer, 1909; also Kring, 2007]. Points in the map represent meteoritic iron, meteoritic iron oxide, and iron shale. Finds < 4.5 kg were mostly only a few grams in weight. (b) Geological map of Meteor Crater showing stratigraphic units of the crater wall and exposed ejecta blanket [from Shoemaker and Kieffer, 1974] and the presumed extent of the ejecta blanket based on drilling [Roddy et al., 1975], which shows slight discrepancies with the mapped ejecta in the north.

1.1. Meteor Crater

[3] Meteor Crater was formed ∼50,000 years ago in flat-lying sedimentary rocks of the Southern Colorado Plateau in Arizona by the iron Canyon Diablo meteorite. With a diameter of ∼1.2 km, Meteor Crater is a simple, bowl-shaped crater that is surprisingly well preserved by terrestrial standards. Shoemaker and Kieffer [1974] estimate 15–20 m of erosion in the rim crest, while the crater floor shows ∼30 m of postimpact sedimentation of lake sediments and alluvium. Talus covers the lower segments of the inner crater wall, leaving 80–100 m of the stratigraphic sequence of the Southern Colorado Plateau exposed (Figure 1). The lowest unit visible, the Permian Coconino sandstone, is only exposed in small areas of the crater wall, revealing white, finegrained quartzose sandstone layers. It is overlain by 3 m of Toroweap Formation, consisting mainly of white to yellowish-brown calcareous sandstones. The main unit visible in the crater wall is the 80 m thick Kaibab Formation, with the older gamma member (white-yellowish dolomite) and beta member (yellow, massive dolomite) overlain by yellow, well-bedded dolomite and sandstone interbeds of the alpha member. The top unit of this sequence is the 8.5 m thick Permo-Triassic Moenkopi Formation, which follows unconformably. It is subdivided into the reddish-brown, massive sandstone of the Wupatki Member and the reddish-brown, fissile siltstone of the Moqui Member, which both form a marked contrast to the underlying yellow Kaibab units and are easily traced in the crater wall. The inverted sequence of these rocks can be observed in the hinge zone and overturned flap and further outward in the ejecta blanket (Figure 1b). For a detailed review of the geology of Meteor Crater, see Kring [2007].

1.2. Previous Structural Research in Meteor Crater

[4] Structural aspects of Meteor Crater were first observed and published in the early 20th century by Barringer [1905], who described uplift of the crater rim and speculated that uplift is caused by crushed rock thrust into the crater walls. In their research on Meteor Crater, Shoemaker [1960, 1963] and Shoemaker and Kieffer [1974] also briefly focused on several structural aspects of the crater rim, noting (1) that joint sets coincide with the diagonals of the crater's roughly squareshaped outline, (2) that the joint sets additionally enable large-scale vertical faulting with “scissors-type displacement” in the crater wall, and (3) that coherent rock material has been emplaced in horizontal zones of weakness, resulting in thrusting and horizontal faulting in several areas of the crater rim. Roddy [1978] was the first to quantify structural features of the crater rim and showed how closely the crater diagonals are related to the orientation of joint sets. Until recently, no comprehensive quantitative structural data of the crater wall other than Roddy's concise data set of joints were available in peer-reviewed literature. Kumar and Kring [2008] measured over 1700 fractures. They were able to differentiate between impactinduced, outward dipping “conical fractures” and preimpact radial and concentric fractures. They conclude that a combination of faulting along joints oriented diagonally and “fracture-controlled motion” along the crater walls led to the crater's square shape. It should be stated that Kumar and Kring's study was coordinated with our study. While Kumar and Kring's focus was on the orientation and generation of fractures, our study investigated kinematic processes of cratering on the basis of bedding and GPS measurements.

[5] The effect of target structure, especially jointing, on the shape of craters has been discussed in several papers. Fulmer and Roberts [1963] concluded, from lunar observations and experiments with explosion cratering, that joint sets determine whether polygonal impact craters (PICs) are formed or not. They believe that PICs are not formed if an uppermost, unconsolidated target layer is at least 1/4 as thick as the (transient) crater depth or if the joint spacing is too large. Gault [1968] report squareshaped and hexagonal craters formed during hypervelocity impact experiments into “jointed” targets with a rather large spacing of 1/5 crater diameter but do not quantify the experimental outcome in detail (e.g., orientation of crater walls to joints). Eppler et al. [1983] propose a model that differentiates between the crater shapes of simple and complex craters and is based mainly on observations in Meteor Crater and publications of other authors. They suggest that in simple craters, excavation is more efficient parallel to joint sets, thus forming corners, while in complex craters, the dominant effect is slumping along joints during crater modification, resulting in walls that are parallel to the to the joint sets, not to the diagonals in simple craters. Interestingly, while tectonic regimes and structures have been inferred from PICs on the Moon and other planets [e.g., Elston et al., 1971; Öhman et al., 2006; Aittola et al., 2007], to the best of our knowledge there are only two terrestrial PICs in which joints have been correlated with crater shape: Söderfjärden Crater (an eroded, complex crater) and Meteor Crater.

[6] This paper provides new structural data obtained from the crater rim of Meteor Crater. On the basis of these new data the effects of target heterogeneities, as well as possible effects of oblique incidence, are analyzed and discussed.

1.3. Can the Impact Vector Be Derived in Simple Craters? A Working Hypothesis

[7] On the basis of the probability P of an impact occurring below a certain angle θ above the horizon given as P = sin2 θ [Shoemaker, 1962], one out of two impacts occurs below 45°, and one out three impacts occurs below 35°. Only highly oblique impacts of less than 10° incidence from the horizontal create elliptical craters. Therefore, the crater shape cannot normally be used as an indicator of an impact direction. The shape of the ejecta blanket, on the other hand, has proven its use in determining the impact vector in numerous studies if the impact angle is less than ∼35° from the horizontal [e.g., Gault and Wedekind, 1978; Herrick and Hessen, 2006; McDonald et al., 2008]. The offset position of the blanket in the downrange direction and the v-shaped uprange forbidden zone are two indicators of the presence of the nonradial aspects that exist during oblique cratering (Figure 2). This nonradial behavior of ejecta has been analyzed in hypervelocity impact experiments by Anderson et al. [2003] in which the vectors of ejected particles were imaged. A strong, nonradial, bilaterally symmetric signature can be seen in their data, which weakens as cratering progresses but is still present in images of the late stages. On Earth the ejecta blankets are rarely preserved, and thus, no craters have been found with an ejecta blanket capable of giving unambiguous indicators for an impact direction. While distal ejecta with its asymmetric features is lacking in terrestrial craters, the most proximal parts of the ejecta blanket, the overturned flap and hinge zone of the crater rim, are preserved in a number of craters on Earth. The possibility that these proximal parts may preserve nonradial symmetry was explored by Poelchau and Kenkmann [2008] at Wolfe Creek Crater. We refer to that paper for a comprehensive description of the so-called “two corners” model.

Figure 2.

Sketch map of Tooting Crater on Mars (derived from Thermal Emission Imaging System mosaic), which exhibits striations in its ejecta blanket that were mapped as possible indicators for ejecta trajectories (gray lines). Assumed strike (black bars) is orthogonal to the mapped ejecta trajectories and displays a nonradial, bilaterally symmetric pattern with two corners, which indicates an impact vector coming from the SW. Black arrows mark corners in the strike pattern as the strongest uprange indicators. (inset) Strike of folded bedding in the overturned flap is always orthogonal to the ejecta trajectories for originally horizontal target bedding.

[8] Poelchau and Kenkmann's [2008] two corners model is based on the observation that the proximal and distal ejecta blankets of oblique craters on the Moon and Mars display deviations from pure radial flow (Figure 2). When extrapolated backward into the crater, striations on the ejecta blankets do not meet in the crater center but focus along a line running from the uprange section of the crater to its center, suggesting nonradial, outward flow from a moving source of ejection, as also suggested from experiments [e.g., Anderson et al., 2003]. While these striations are a late-stage cratering phenomenon and on Mars may possibly indicate atmospheric interaction, the nonradial pattern the striations describe is believed to reflect early cratering asymmetries.

[9] It is expected that the bedding orientation of blocks in the ejecta blanket is too chaotic to be used to derive any deviation from radial ejection patterns. However, near the crater rim in the hinge zone and within the overturned flap, strata are often coherent over large distances. In these settings it can be assumed that strata strike is perpendicular to the excavation flow direction (for originally horizontal bedding) (Figure 2, inset). Thus, deviations from radial flow should lead to a measurable deviation in strike from a concentric direction. The expected pattern of strike should be bilaterally symmetric to the direction of impact and, on the basis of the analysis of Tooting Crater on Mars and Wolfe Creek, Australia, have two “corners” between the uprange and crossrange sector, in which an abrupt change in strike orientation occurs (Figure 2).

[10] In this paper we present new structural data of Meteor Crater and provide a kinematic model that is consistent with structural observations, in particular, the quadrangular crater shape. We quantify the characteristic structural asymmetries of this crater and address the question as to whether these asymmetries could be exclusively caused by target heterogeneities or if they are additionally related to an oblique impact scenario. Structural data are presented in section 4. Section 5 critically reviews arguments in favor of an oblique impact scenario and applies the two corners model [Poelchau and Kenkmann, 2008] to Meteor Crater in order to derive a possible impact vector. In section 4 phenomenological models are presented that bring rim uplift and shape into a causal context.

2. Methods

2.1. Measurement of Bedding Planes

[11] Field data were collected in the upper part of the crater rim, when possible in the area of the overturned flap, where the transition from uppermost autochthonous rock layers to overturned proximal ejecta beds occurs (Figure 3). The data set was subdivided into “autochthonous data” if normal bedding occurs (298 measurements) and “ejecta data” if strata are overturned (83 measurements). Data values consist of strike and dip of bedding planes along with their latitude, longitude, and altitude as measured with GPS. (Strike of a plane is defined as the crossing line between a theoretical, horizontal plane and the measured bedding plane. At the time of measurement, magnetic declination was at 11.2°E and was corrected afterward by rotating strike values clockwise.) As layering in the ejecta becomes more chaotic the further away it is from the crater center, care was taken to measure only overturned units that showed coherent behavior over tens of meters in length. Additionally, care was taken not to measure slumped blocks when measuring autochthonous bedding data.

Figure 3.

Field observations. (a) Displacement of bedding by a radial corner fault. To the right one can see the beginning of the interthrust wedge at Barringer Point. (b) Hinge zone and overturned flap, showing Kaibab alpha and Moenkopi beds. (c) Orthogonal joint sets seen on the surface of “House Block,” an ejected Kaibab block on the east rim. Photo size is roughly 2 × 3 m.

[12] We particularly focused our measurements on a single stratigraphic horizon, the contact bedding plane of the Kaibab and Moenkopi formations, which can be easily traced in the field (122 data values). Tracing this marker horizon allows differential rim uplift to be quantified. Because of differential uplift and erosion of the rim, parts of the highest strata and overturned flap are missing, particularly in the SE sector.

[13] The GPS altitude data of the Kaibab-Moenkopi contact proved to be unreliable. To compensate, we used a georeferenced topographic map of the crater (as presented in the current U.S. Geological Survey 7.5 min quadrangle for Meteor Crater) in combination with the much more precise latitude and longitude GPS data to derive the altitude of our measurements. For these values we estimate an error of ±5 m altitude.

[14] The Coconino-Kaibab contact was analyzed by combining the geological map compiled by Shoemaker [1960] with the topographic map of the crater for a more detailed overview of uplift. We estimate an error of ±10 m.

2.2. Conversion From a Geographic to an Azimuthal Reference Scheme: Concept of “Concentric Deviation”

[15] For analysis of the strike data, methods were applied that have been developed by Poelchau and Kenkmann [2008], and we refer the reader to that paper for a more comprehensive description. First, the position of each measurement was converted from its original geographic reference system to its azimuth on the basis of the crater center, giving angular values from 0 to 360° (90° equals east) (Figure 4). In a second step, the orientation of strike relative to the crater center was determined and was given an angular value that we refer to as “concentric deviation” (Figure 3). Strike that is tangential to a hypothetical circle around the crater center is defined as “concentric” and has a concentric deviation value of 0°. Strike that is rotated clockwise relative to the circle has positive concentric deviation values, while counterclockwise rotation produces negative values.

Figure 4.

Determination of the azimuth and concentric deviation from the strike and location of bedding. The azimuth is the position of a measurement relative to the crater center, ranging from 0 to 360°. Concentric deviation defines the orientation of strike relative to the crater center, with concentric strike lying tangential to a hypothetical circle (dashed curve) around the crater center. Concentric deviation ranges from –90 to +90°; positive values indicate clockwise rotation.

2.3. Statistical Treatment: “Overlapping Bins” Method

[16] When concentric deviation is plotted against its azimuth in an x-y plot (e.g., Figure 5), a large amount of scattering is displayed that requires smoothing for a better interpretation. We use the “overlapping bins” method, in which the arithmetic average of all concentric deviation values in a defined azimuthal range, or “bin” (e.g., 0–30°) is calculated. This bin is then rotated by 10°, and the average of all values in the bin is calculated again (e.g., 10–40°) and so on for each step, resulting in 36 smoothed concentric deviation values. For a better sense of spatial relationship, the data are also displayed in polar plots, showing the azimuth in “map view” and using the radial distance from the center of the plot to quantify the concentric deviation (Figure 5).

Figure 5.

Deviation of bedding planes from concentric strike plotted against its azimuth, displayed (left) in x-y plots and (right) in polar plots. Positive values quantify clockwise rotation of bedding strike (and vice versa). The curve displays data smoothed with the overlapping bins method (bin size 20°, step size 10°). The deviation from concentric strike can be most intuitively observed in the strike bars of the polar plots on the right, as seen in “map view.” Strike bar orientation is exaggerated by a factor of 2. The plots show that strike does not directly reflect the outline of the crater (compare Figure 4) but infers more complex internal structural features of faulting and folding.

3. Results

3.1. Strata Orientation, Joints, Faults, and Uplift in Meteor Crater

3.1.1. Bedding Data

[17] For the display in a stereographic projection (stereo plot), the normal, autochthonous bedding data set was split into crater corners and sides on the basis of its square shape (Figure 6). The data show four clusters of surface normals, or bedding plane poles, reflecting the four sides of the crater wall that accentuate the square shape of the crater. The north and south sides have more concentrated poles compared to the more scattered east and west sides. This indicates rotation in the east and west sides. The south side dips more gently on average (the poles of bedding planes are nearer to the center of the projection) than the east and west sides, while the north side has the steepest dip. The NE and NW corners are more concentrated, while the SE and SW corners show strong scattering and rotation.

Figure 6.

Stereographic projection (stereo plot) of 298 autochthonous bedding poles. White points represent corners; black points represent sides. Points represent surface normals (e.g., data from the south wall cluster is displayed in the north part of the plot). The sides of the crater are focused in four clusters that show differing amounts of scattering, indicating structural complexities beyond a simple, square-shaped deviation.

[18] Although we believe that the data displayed in the stereographic projection directly reflect faulting and differential uplift in the crater, variations of the pole distributions in the stereo plot, e.g., the relatively flat-lying south crater side, may in part be attributed to erosional effects. The steepest bedding is found in the hinge of the overturned flap (Figure 3b). When this is removed by erosion, flatter surfaces remain. Also, care must be taken in interpreting stereo plots as certain areas of the crater can be overrepresented or underrepresented during sampling. Nonetheless, bedding data reveal nonradial behavior that is more complex than simple, squareshaped deviation. The structural and tectonic factors that we believe control this complex, nonradial behavior are discussed in sections

3.1.2. Concentric Deviation

[19] Reflecting the results displayed in the stereo plot (Figure 6), the concentric deviation data of the autochthonous inner wall of Meteor Crater also show dispersions from radial symmetry, which appear to be stronger in the western half (Figure 5a). The ejecta bedding data (Figure 5b) were not sampled in a large enough quantity to be statistically meaningful but still show the same rough trends apparent in the autochthonous data.

[20] As opposed to stereo plots, concentric deviation diagrams allow a more detailed look into the bedding behavior relative to its azimuthal location in the crater wall. As can be seen in the polar plots, the orientation of strike does not directly reflect the square shape of the crater but shows more complex rotation. This indicates an influence of faulting and folding that has deformed the crater wall and has determined nonradial and non-square-shaped orientation not only of bedding in the crater wall but also of the ejected material on top of it. The relationship between strike behavior and structural features is discussed in more detail in section 3.1.5.

3.1.3. Joints and Radial Faults in the Crater Corners

[21] Joint sets form structurally weak zones along which vertical faults have propagated during crater formation. Two dominant joint orientations were recognized by Shoemaker [1960] and were later quantified by Roddy [1978]. Roddy's work was based largely on aerial photography and only to a small degree on field measurements. Nonetheless, his results suggest a strong correlation between the diagonals of the crater's square shape and the average orientation of joints (Figure 7). We were also able to observe small-scale orthogonal joints in rock units in the field (Figure 3c). For a more recent, detailed study on joint sets and fractures in Meteor Crater, see Kumar and Kring [2008].

Figure 7.

Field data from Roddy [1978] show a striking correlation between crater diagonals and the orientation of joint sets. The azimuths of the crater diagonals (36° and 304°) coincide with the average of joint sets measured in the crater and estimated in aerial photographs (30° and 304°). These joints have a spacing ranging between 0.5 and 10 m and have subdivided the target into small, square-shaped units.

[22] Vertical displacement of beds along faults in the crater wall is most strongly expressed in the four corners of the crater (Figures 3a, 8a, and 8b). Apparently, these faults have utilized the two main vertical zones of weakness that the joint sets form. Shoemaker and Kieffer [1974] introduced the term “tear faults” to describe these faults. This term is a misnomer in the context of Meteor Crater. In conventional tectonics, tear faults are vertically oriented fault planes that occur perpendicular to the direction of deformation and are caused by differential amounts of displacement. In such a tectonic situation, the displacement is mainly lateral or strike slip, with only minor vertical movement [Twiss and Moores, 1992]. At Meteor Crater we replace the term “tear faults” with “radial corner faults” as a purely descriptive term to express differential movement between blocks that occurred during rim uplift. The main component of movement displayed by these faults is vertical along with a rotational component or “scissors-type of displacement” [Shoemaker, 1960] that tips the blocks outward.

Figure 8.

Structural control of bedding behavior. (a) Simplified sketch of the main structural features, radial corner faults and interthrust wedges, in the crater wall. See Figure 1 for legend. (b) The geological contact of the two highest units in the crater wall was measured with GPS. The elevation is plotted against the azimuth, giving a simplified profile with the topography of the rim (top curve) and structural features. The trend lines of the data points are additionally based on field observations. Vertical exaggeration is ∼5X, and estimated error is ±5 m. Deviation of concentric strike plotted against its azimuth, displayed in (c) an x-y plot and (d) a polar plot with bars of average strike. Positive values quantify clockwise rotation of bedding strike (and vice versa). The curve displays data smoothed with the overlapping bins method (bin size 20°, step size 10°). The concentric deviation of bedding directly reflects the internal structure of the crater wall. The best example can be seen at 290°, where an interthrust wedge folds beds and creates an anticline at Barringer Point, causing the bedding strike to rotate. The rotation of strike bars in Figure 8d is exaggerated by a factor of 3.

[23] The kinematics of the radial corner faults is comparable to mode III shear fractures. The main component of motion in these shear fractures is a sliding motion parallel to the fracture edge. In comparison, mode II shear fractures have a sliding motion perpendicular to the fracture edge, and mode I fractures are extensional [Twiss and Moores, 1992].

[24] In Meteor Crater, radial corner faults displace rock beds by several decameters (Figures 3a and 8b). Maximum displacement of ∼45 m occurs along the SE radial corner fault, which cuts through all visible layers, including the ejecta, and exposes the largest segment of Coconino in the crater wall.

3.1.4. Interthrust Wedges

[25] Features that we have termed “interthrust wedges” can be observed in the inner crater walls. They are isolated, horizontal, lensoid bodies of coherent strata that are terminated by faults on all sides and lead to a repetition of strata in the wall. While the base of the wedges is usually stratiform, the hanging wall contact shows unconformities. The outward extent of the lensoid bodies is undetermined, but it is expected that they also have a wedge shape in cross section. Shoemaker [1960] and Shoemaker and Kieffer [1974] first described these features, which they termed “thrust faults,” in the north and west sectors of the crater. They can also be observed in the south wall and in the SW corner (Figure 8a).

[26] We prefer the term interthrust wedges to thrust faults, as thrust faults incompletely describe the situation observed. While thrusting plays a major role in this process, the process is actually the injection of a wedge of bedrock between two rock layers, therefore resulting in a lower, crater-outward directed thrust at the bottom of the wedge and an upper, crater-inward directed thrust bordering on top of the wedge.

[27] Kenkmann and Ivanov [2006] have shown that weak spallation by interference of shock and release waves near the target surface leads to decoupling of the uppermost target layers in the early cratering stage. We expect that the wedges exploit these horizontal zones of weakness in the layering of rock beds. Thin interbeds of clay-siltstone in the Moenkopi Formation and marl beds in the Kaibab Formation are soft layers and preferential sites for displacements and rock decoupling.

[28] The wedges have an effect on uplift. Where they occur, the top Kaibab and Moenkopi units are arched up and form anticlinal features, best represented by Barringer Point in the NW (Figures 8a and 8b). Interthrust wedges only appear to occur in Kaibab. This observation may be due to the fact that Kaibab is the main unit exposed in the crater walls. However, it seems plausible that interthrust wedges occur at a level where overburden is low, utilizing weaker marl interbeds, thus allowing layers to be bent upward in the formation process. A detailed model describing the injection and uplift is introduced in section 4.1. Whereas at Barringer Point we see a lensoid-type termination of the sides of the interthrust wedge bodies, near the museum complex at the northern crater wall the eastern limb of a box fold is exposed that terminates an interthrust wedge.

3.1.5. Differential Uplift

[29] The rim of the crater stands around 50 m on average above the surrounding surface. This is caused mainly by bedrock uplift and partially by the thickness of the overturned ejecta flap. Differences in rim elevation between the highest point of the rim crest in the SW corner at ∼1750 m and the lowest point in the NW at ∼1716 m are connected to the structural features described in sections 3.1.3 and 3.1.4, which control differential uplift of the bedrock in the crater wall. The orientation of the radial corner faults and interthrust wedges in the crater are sketched out in a simplified structural map in Figure 8a, showing a tendency for both features to be located in or near the four corners of the crater.

[30] As described in section 2.1, the elevation of both the Kaibab-Moenkopi (KM) contact and the lower Coconino-Kaibab (CK) contact were measured for a better control of differential uplift in the crater wall. Average height of the CK contact where exposed within the crater is ∼1645 m. Maximum elevation of the CK contact is higher in the corners of the crater (>1670 m) than along the sides (∼1640 m). The preimpact CK contact was located at ∼1590 m elevation on the basis of an average thickness of overlying strata of 90 m and an average surface elevation of 1680 m outside of the crater [e.g., Kring, 2007]. Therefore, actual uplift of the CK contact ranges from less than 40 m on the sides to a maximum of 80 ± 10 m in the corners.

[31] Results from the KM contact are plotted as a profile in Figure 8b together with the main structural elements and topography of the rim. It becomes apparent how directly bedding elevation is controlled by radial corner faults and interthrust wedges. For example, in the NW at 320°, movement of the radial corner fault (Figure 8b, point 5) has resulted in one of the lowest portions of the KM contact at 1690 m elevation, which is directly reflected as the lowest point in the topography of the rim crest (∼1716 m). Slightly to the SW, the interthrust wedge at Barringer Point (290°) has uplifted the KM contact to 1730 m altitude and has created a local topographical peak of 1744 m height. On the basis of a preimpact elevation of the KM contact at 1670 m, measurable differential uplift ranges from 15 to 65 m (Figures 8b and 9) and can be presumed to be over 70 m in eroded parts of the crater rim.

Figure 9.

Elevation of the Kaibab-Moenkopi contact plotted against its radial distance from the crater center on the basis of GPS measurement. A rough trend can be seen for higher uplift to occur farther away from the crater center.

[32] Differential uplift is also reflected in the concentric deviation data of the upper rim (Figures 8c and 8d). The interthrust wedge at Barringer Point created an anticline, folding strata outward on either side and thus rotating the strike of bedding planes around this feature (best observed in the strike bars of the polar plot in Figure 8d). In the SW a combination of an interthrust wedge and a radial corner fault has led to a “slope” with a NW component of dip, which can be seen in the clockwise rotation of strike in this area.

[33] In the x-y plot in Figure 9, a trend can be seen for the KM contact to be in a higher position the farther away it is from the crater center. This means that higher elevations are located in the corners of the quadrangular crater. The possibility that uplift, interthrust wedges and radial corner faults and their concentration in the corners of the crater are interlinked is debated in section 5.

3.2. Data Relevant to Determining an Impact Direction

[34] In this section we present and review different data sets that provide indications of an impact direction.

3.2.1. Deviation of Strike From Concentric Orientation

[35] The concept of the two corners model is described in section 1 and by Poelchau and Kenkmann [2008]. When compared to the two corners model rotated for a best fit, the concentric deviation data from the proximal ejecta and overturned flap of Meteor Crater show a rough fit that coincides with a direction of impact from the NNW at 330° (Figure 10). Mainly because of erosion of the rim, the quality of this data set is poorer than the autochthonous data set, and parts of the concentric deviation curve are insufficiently supported by outcrop measurements. We are apparently close to the limit of detection for any oblique signatures in this data set. Furthermore, target heterogeneity effects presented in section 3.1 are not accounted for in the phenomenological two corners model.

Figure 10.

Analysis of possible obliquity of Meteor Crater. Concentric deviation data of Meteor Crater ejecta are plotted against its azimuth. Data were smoothed with the overlapping bins method (bin size 40°, step size 10°) (solid curve). Data show a very rough correlation to the curve of the two corners model (dashed curve) with NNW orientation. The abrupt changes in the two corners model curve from negative to positive concentric deviation values represent the two corners (dashed circles) (compare also Figure 2); the star marks the uprange direction of the model, and the gray arrow marks the direction of impact.

3.2.2. Bedding Dip and Differential Uplift

[36] The stereographic projection (Figure 4) of autochthonous bedding data shows a bilaterally symmetric pattern with a symmetric axis running NNW-SSE. While the east and west crater walls display comparable dip values, the north crater wall shows steeper dip data than the south crater wall. The symmetry plane may correlate with the trajectory axis in an oblique impact scenario. Steeper rim inclination would be expected in the uprange sector. Also, differential uplift (Figure 8b) appears to be highest in the SE corner of the crater, where much of the KM contact has been eroded away. This may be an indicator for a downrange directed component of horizontal momentum from the impactor that could cause stronger uplift and deformation compared to the uprange sector.

3.2.3. Canyon Diablo Meteorite Distribution

[37] The distribution of the Canyon Diablo Meteorite around the crater was compiled by Barringer [1909] and is presented as a map in Figure 1a. Distal meteoritic material can be found predominantly in the SE and SW sectors to a distance of 6.5 km, including numerous specimens heavier than 5 kg. The NE sector has less distal material, reaching only to about 5 km and consisting of material under 5 kg, while distal meteorites in the NW can only be found as far as 3 km from the crater. It is notable that the north lacks any meteorites over 5 kg beyond a 1.5 km radius, while they can be found in the south, east, and west at 3 times that distance. Erosion certainly plays a role in the distribution of the smaller fragments. Grant and Schultz [1993] estimate 1 m average vertical erosion over the bulk of the ejecta located outside of the steep, proximal rim, at ranges of 0.25–0.5 crater radii, or 125–250 m from the rim. It is questionable, though, how strongly the distribution of the heavier fragments is affected by erosion.

[38] It is possible that the stronger concentration in the south reflects a direction of impact from the north, which would support the impact direction derived with the two corners model. On the other hand, the more proximal distribution of the Canyon Diablo meteorite shows a strong concentration of both heavy and light meteoritic mass in the NE sector of the crater up to a distance of 1 km (Figure 1a). This has led previous workers [e.g., Nininger, 1956; Rinehart, 1958] to propose a projectile coming from the SW. It should be stressed, though, that the distribution of the distal, not proximal, ejecta is the strongest indicator for impact trajectories on other planetary bodies.

3.2.4. Ejecta Blanket

[39] The ejecta blanket was drilled at several locations, and results were published by Roddy et al. [1975]. On the basis of the data, the “approximate end of the overturned flap in drill holes” was estimated [Roddy et al., 1975, p. 2629], which is interpreted as the lateral extent of the ejecta blanket, averaging at around 1.5 km distance from the crater center (Figure 1b). On the basis of this sketch, there is no observable offset pattern or forbidden zone that would infer an impact direction as described, for example, by Herrick and Hessen [2006] in ejecta blankets on Mars. The boreholes do reveal that on average, the blanket is thicker in the south than in the north, which would support an impactor coming from the NNW. Ejected Coconino is found mainly in the south part of the blanket and is missing in the west. Additionally, Barringer [1909] reports finding the deepest stratigraphic units of the Coconino sandstone in the SE ejecta, which would additionally support an impactor from the NNW, but this has not been confirmed by other workers.

3.2.5. Discussion of Impact Direction

[40] Our analysis of bedding orientation of the proximal ejecta with the two corners model suggests an impactor coming from the NNW, which is supported by nonradial behavior seen in differential uplift, bilateral symmetry in the stereographic projection of bedding, distal distribution of meteoritic material, and ejecta blanket thickness. The proximal distribution of meteoritic material may contradict the NNW trajectory, while the lateral extent of the ejecta blanket appears to give no indication of the impact direction. One must also consider the possibility that the impactor could have struck at a high angle relative to the horizon, excluding any measurable horizontal components capable of inferring the direction of impact. The bedding data set used for the determination of an impact direction may be too small to give a fit to the two corners model, even though our suggested direction coincides with other factors that lead to speculation on the impact direction. Additionally, there is always the problem of noise obscuring the patterns we are searching for, but as explored in section 3.1, we discovered that faulting and folding in the crater wall have a much stronger control on the deformation of bedding and strike orientation than the diluted, oblique signals we were originally searching for.

4. Synthesis of Data

[41] We discuss the indications our results have on rim uplift and crater shape and the effects that an oblique angle of impact and target heterogeneities have on the cratering process.

4.1. A Mechanism of Rim Uplift Formation

[42] The emplacement of interthrust wedges is a mechanism of rim thickening and uplift (Figure 11a). This is documented in Figure 8b and leads to anticlinal doming above interthrust wedges, e.g., at Barringer Point (Figure 11b). Certain ejected Kaibab boulders found on the crater rim (Figures 11c and 11d) provide an important observation that is relevant to understanding the formation of interthrust wedges. Within a single block two Kaibab beds are in 30° contact and have a brecciated unconformity plane which is most likely a fault plane. As no thrust ramps can be expected from the regional, preimpact geological context, we interpret these as ejected thrust ramps formed during the emplacement of the interthrust wedges.

Figure 11.

Field observations and a mechanism for the formation of “interthrust wedges.” (a) Four stages of the interthrust wedge formation. (top) After spallation induces horizontal zones of weakness, small gaps are formed during the excavation process while thrust ramps are formed in rock beds. (middle) Wedges of rock are thrust outward into the crater wall, warping up overlying beds. (bottom) Some of the thrust ramps formed are ejected during crater excavation, while others remain embedded in the crater wall as interthrust wedges. The ejected incipient thrust ramps are deposited as ejecta. (b) Barringer Point, one of the most prominent topographical peaks at Meteor Crater, is caused by an interthrust wedge that has folded the top beds in an anticline. (c) An incipient thrust ramp of an interthrust wedge found in Kaibab ejecta near the museum complex, showing obliquely terminating bedding planes and an inferred direction of movement. The upper rock unit is ∼3 m wide. (d) Whale Rock in the west of the crater, another possible ejected thrust ramp of an interthrust wedge.

[43] On the basis of these observations, we propose a model for the formation and excavation of interthrust wedges (Figure 11a). We assume that during the cratering process, spallation delaminates horizontal areas of weakness in upper stratigraphic units along bedding planes, e.g., clayey interbeds [Kenkmann and Ivanov, 2006]; then outward and upward directed excavation flow opens small gaps along these weakened zones (Figure 11a, top), which are exploited by the tips of incipient interthrust wedges (Figure 11a, middle). Further thrusting of these wedges into the gaps creates additional uplift, raising overlying units to a higher elevation than neighboring beds. As the transient crater continues to grow, the early stage interthrust wedges get incorporated into the excavation flow and are deposited as ejecta boulders (Figure 11a, bottom). Rim uplift by this mechanism appears to be specifically active near the radial corner faults.

4.2. A Model of Formation of Quadrangular Crater Shapes

[44] In an attempt to understand the mechanical processes behind the formation of the square shape of Meteor Crater, we suggest a simplified, qualitative model that compares surface stress to the excavation force exerted on rock units (Figure 12). In this model we divide the target rock into discrete cubes on the basis of the two joint sets plus horizontal layering planes. During cratering, the excavation flow field that ejects rock is directed outward and upward in the upper part of the crater. For simplification, we assume that the flow field is oriented radially from the crater center and upward at 45°. We can observe two situations in this model (Figure 12): (1) a situation where the flow field is directed parallel to one of the joint sets and (2) a situation which is directed at a 45° angle to both joint sets. The flow field exerts a force on the cube, which is proportional to the exposed surface of the cube, thus resulting in a force that is 1.41 times stronger in situation 2. The force is calculated as the surface area component that is orthogonal to the flow field. While the cube is cut in half along its diagonal for a better overview in Figure 12, situation 2, the surface area component orthogonal to the flow field remains the same for a complete cube. This force can be split into horizontal and vertical components that correspond to normal stress (σx, σy) and shear stress (τz), respectively. Vector addition (Figure 12) shows that the ratio of normal stress to shear stress in situation 1 is σy:τz = 1. In situation 2 the ratio is (σx + σy):τz = equation image because of the larger surface area exposed to the flow field and the circumstance that shear stress is exerted on two surfaces of the cube (marked in gray in Figure 12, situation 2), as opposed to one surface in situation 1. As less shear stress is resolved in situation 1, excavation should progress faster and farther, and the initial circular crater shape should start developing corners, resulting in the final square shape seen in Meteor Crater. Interestingly, the crater radius in the corners of a square is equation image times larger than a radius perpendicular to the sides. It should be noted that target anisotropies such as joints become important mainly in the final stages of crater excavation, when the stresses induced by the excavation flow are of the order of the strength of the target material. We discuss this further in the crater hinge model in section 4.3.

Figure 12.

A simplified model correlating joint sets and bedding planes with crater formation. Representations of blocks during crater excavation oriented parallel (situation 1) or at a 45° angle to excavation flow (situation 2). While the force of the excavation flow exerted on situation 2 is equation image times stronger than on situation 1 because of the exposed surface (dashed line), the shear stress exerted on the back of the cube (gray area) is twice as strong in situation 2 compared to situation 1. This results in more effective crater excavation parallel to the joint sets (situation 1), forming a square-shaped crater. See text for details.

4.3. Differential Rim Uplift in Quadrangular Craters

[45] Figure 8b shows that differential rim uplift is affected by the formation of interthrust wedges and is also controlled by the radial corner faults. There is a rough spatial relation of radial corner faults and interthrust wedges that suggests a connection between the two. Figure 13 illustrates how both mechanisms could be interconnected and controlled by the quadrangular crater shape. The ejection is more effective parallel to joints, and the crater grows faster within the corners of the crater, as demonstrated in section 4.2. As an example, the marker bed is originally uniform in Figure 13a and is uplifted farther in the corners of the crater in Figure 13b, as the excavation is equation image times more effective. If the marker bed remains coherent and intact, this flow would create marker bed highs in the corners. This fits perfectly to Figure 9, in which the correlation of uplift and radial distance of a marker bed is plotted. The model only works if the strata remain coherent. This describes only the late cratering stages, when the stresses induced by the excavation flow are of the same order of magnitude as the strength of the rocks involved. As the uplift is enhanced in and near the crater corners, these are also likely sites where gaps parallel to weak interbeds open that could subsequently be filled by interthrust wedges. The accentuation of the square crater shape is a process that is induced by the more effective excavation flow parallel to the joints. Once this process is initiated, this facilitates the formation of straight fold hinges of the overturned flap parallel to the crater walls, as can be observed at Meteor Crater (Figure 3a). In contrast, concentrically striking fold hinges require the tensile breakup of the ejected rock in the overturned limbs.

Figure 13.

The crater hinge model contrasts two scenarios: (a) a homogenous target resulting in a circular crater shape and (b) a target with perpendicular zones of weakness, resulting in a structurally complex, quadrangular crater with stronger rim uplift in the corners, as shown by the marker bed. The resulting geometry in Figure 13b must be compensated by the formation of radial corner faults and interthrust wedges.

5. Conclusions

[46] We present a comprehensive data set of bedding strike and dip combined with positional data based on GPS measurements for Meteor Crater. Application of a method devised by Poelchau and Kenkmann [2008] to infer an impact direction from bedding data, in combination with data on the spatial extent of the ejecta blanket and distribution of meteoritic material, did not lead to unambiguous results for an impact direction, in part because of a weak database of sampled ejecta. Although a projectile coming from the NNE is suggested from the data, the orientation of bedding is more strongly controlled by (1) preimpact joint sets resulting in radial corner faults and (2) horizontal weaknesses in the planes of rock strata that are exploited by anticlinal interthrust wedges; thus, any weaker signals of an impact direction appear to be concealed and superimposed by these features. The complete data set enables us to quantify differential uplift, folding, faulting, and rotation of specific areas of the crater wall and to display an azimuthal profile of the crater wall.

[47] Structures in ejected boulders were observed that were interpreted as ejected remnants of incipient ramp thrusts, leading to the formulation of a model describing the process of rim uplift by interthrust wedge formation. This model postulates interthrust wedge formation as an ongoing process of crater excavation with coherent rock material being thrust into the crater wall, only to be ejected shortly afterward. In our second model, a comparison of surface stress to the excavation force exerted on arbitrarily defined rock units parallel and at a 45° angle to orthogonal joints shows that 1.41 times less force is needed to eject rock units parallel to joint sets, suggesting a possible mechanical factor responsible for preferred ejection along the joint sets, which should result in a square-shaped crater with its diagonals parallel to the joints.

[48] Both models are confined to the late stages of crater excavation. We assume that the target rock in the models is sufficiently far away from the point of impact to form coherent units that are not dominated by impact-induced damage. This assumption also implies that for simple craters the effects of obliquity prevail during the early stages of crater excavation and give way to the effects of preexisting target features like joint sets as the shock wave diminishes and causes less homogenization of the target material. Target heterogeneities start to influence the cratering flow when the stresses induced by the flow are of the same order of magnitude as the strength of the target rock. In this case, planes of weakness, such as joints or bedding surfaces, become zones of deformation localization.

[49] The effect of target heterogeneity on the crater shape increases with time and progressing crater excavation, while the effect of obliquity decreases with time. This affects the ejecta patterns in different ways. Obliquity is seen most strongly in the early, distal ejecta, whereas target heterogeneity, as a factor that has an effect on the later stages of cratering, becomes visible in the late, proximal ejecta.


[50] We are very grateful to P. Senthil Kumar, who was kind enough to coordinate the publication of his structural data of Meteor Crater with ours. M.H.P. is grateful to the Barringer Family Fund for awarding a travel grant to present the ideas in this paper at the LMI 2008 and is grateful to the Dallas Family Fund for further aid. We are additionally grateful to the Barringer Family Fund for granting us permission to perform research on the crater. We appreciate the efforts John Spray and an anonymous reviewer have made to improve the content and style of this paper. Finally, we would like to thank the DFG for funding this project (KE 732–11/1). This is LPI contribution 1427.