THE STRUCTURE OF THE JEBEL WAQF AS SUWWAN IMPACT CRATER, JORDAN, AS REVEALED FROM FIELD ANALYSIS, REMOTE SENSING MAPPING, AND INTERPRETATION OF GEOPHYSICAL DATA. T. Kenkmann1, M. Khirfan2, W. U. Reimold1, E. Salameh2, K. Konsul3, and H. Khoury2, 1Museum für Naturkunde, Leibniz Institute at the Humboldt-University Berlin, Germany; 2Depart. of Geology, University of Jordan; 3Arab Union for Astronomy and Space Sciences (AUASS), Jordan; thomas.kenkmann@museum.hu-berlin.de

Introduction: A meteorite impact structure about 6 km in diameter has recently been discovered in the Middle East (Salameh et al., 2006; 2008a), at Jebel Waqf as Suwwan in Jordan (N31°02.9’ E36° 48.4’). (Fig. 1, 2). The circular structure has a central uplift that is well exposed and provides a view through the entire stratigraphic sequence from Lower Cretaceous to Eocene strata. It provides a laboratory to decipher mechanisms of central uplift formation.

The first detailed regional geological investigation of the structure was reported by Heimbach (1969). This publication resulted from regional mapping of Jordan by a German mission in the 1960s (Bender, 1968; 1975). The Arabic name of the structure, “Jebel Waqf as Suwwan”, is an appropriate term as it means “mountain of steeply inclined chert”. Heimbach (1969) favored a volcanic origin of the structure and proposed the existence of a magmatic body underneath the cen-ter of the structure. As he could not find evidence for volcanic activity he denoted the structure as a “crypto-volcanic” structure.

A new phase of research started when Salameh et al. (2006) found shatter cones in the uplifted center as unequivocal evidence for the presence of an impact crater. A more comprehensive description of the struc-ture including petrographic investigations and a first remote sensing analysis were presented by Salameh et al. (2008a). Salameh et al. (2006; 2008a) described ten locations with shatter cones. A joint field campaign in Spring 2008 with researchers of the University of Jor-dan and of the Museum für Naturkunde Berlin accom-plished detailed lithological and structural mapping and resulted in a number of abstracts (Khirfan et al., 2008; Kenkmann et al., 2008; Salameh et al., 2008b) and an upcoming GSA-Special Publication. Here we summarize our recent advances.

Geological framework: Strata in eastern central Jordan, in the region of Waqf as Suwwan, are horizon-tal or dip gently (1-2°) towards the northeast. The to-pography is dominated by plains, escarpments, and local monadnocks formed by isolated erosional rem-nants of bedrock. The oldest exposed rocks here are Lower Cretaceous sandstones of the so-called Kurnub Sandstone Group (Powell, 1989) that are exposed in the core of the Waqf as Suwwan structure. They are overlain by upper Cretaceous carbonates, chalks, and cherts. Large areas are underlain by chert of Eocene

age (Heimbach, 1969; Powell, 1989; Abdelhamid, 1997). In the Azraq area north of the crater these are unconformably overlain by Neogene to Quarternary basalts. Landsat and Radarsat imagery indicates pri-marily NW-SE lineaments in east central Jordan, and subordinate NE-SW, E-W, and N-S features. The NW-SE linear structures delineating the Azraq-Sirhan nor-mal fault system are believed to have controlled the development of both the Azraq and Sirhan basins (Ab-delhamid, 1997), in which the structure has been formed. Drainage systems follow the lineaments and are widespread on the plains. They have resulted in the ubiquitous cover of gravel deposits in low-lying plains, which are dominated by cherts. Aerial and satellite imagery reveal that the Waqf as Suwwan structure (Salameh et al., 2008a) is located within a NE-trending drainage system. Except during rare rainfalls the beds of the streams are usually dry and are denoted as wadi. The crater rim defines a local watershed with an outlet in the northern crater rim. All wadi systems ultimately drain towards the east into the desert of Saudi Arabia. The divide of the Jordan discharge system is situated about 30 km west of Waqf as Suwwan.

Crater structure: The rocks of Waqf as Suwwan belong to Cretaceous (Albian) to Paleogene (Eocene) strata. East of the structure is a prominent escarpment of flat-lying Middle Eocene strata with thick chert lay-ers capping massive marl and chalk deposits (Fig. 2). Escarpment aprons are usually formed from whitish-gray marls and chalks, that are commonly covered by alluvial fans. The stratigraphy of Waqf as Suwwan and the surrounding area is adopted from Heimbach (1969) but is adjusted to the currently-used Jordan stratigra-phy of Powell (1989) (Table 1). The given strata thick-nesses are based on our findings at Waqf as Suwwan. A more extensive and general introduction to the re-gional geology of eastern Jordan is given by Bender (1975).

Waqf as Suwwan has a prominent outer rim and a well exposed central uplift of ~1000 m diameter, which provides a section through the entire target stratigraphy. The impact occurred into sedimentary rocks of considerable competence contrast. The inner-most area of the central uplift exposes Lower Creta-ceous sandstones, the oldest strata of the crater. Lime-stones and marly limestones surround this core and are dismembered into competent blocks that are internally

folded. The limestone blocks, in turn, are encircled by a sequence of incompetent marls and chalks. These weak beds accommodated space incompatibilities dur-ing block deformation of the competent beds beneath and above. Thick chert beds form the prominent outer collar of the central uplift. Radial folding and faulting is the most conspicuous structural attribute of this se-quence. In the southwestern part of the collar normal layering dominates and fold axes plunge outward, whereas overturning of strata and fold axes is the rule in the northeastern part. This indicates a top-to-NE shearing component that is explained with an oblique impact scenario with an impact from the southwest. The inferred trajectory runs parallel to the SW-NE axis of symmetry of the central uplift defined by the expo-sure of strata. The moat between central uplift and crater rim is largely covered by alluvial wadi sedi-ments. The crater rim is composed of white chalks, marls, and massive chert beds of Eocene age, the youngest strata of the crater that also provide a maxi-mum age of the cratering event. Both antithetic and synthetic block slumping are common along the up-lifted crater rim.

Geophysical data: A geophysical survey of east-ern Jordan was undertaken in the early 1990s in search of potential oil reservoirs under the auspice of the Natural Resources Authority of Jordan. We have revis-ited some of these data. Based on a rectangular grid of 455 gravity data points that cover the major part of the crater a Bouguer gravity anomaly map was compiled. Superimposed on a trend to increasingly negative data toward the south a distinct positive excursion exist that exactly correlates with the morphological outline of the central uplift of Waqf as Suwwan. The residual gravity anomaly shows a weak positive anomaly of about 7 mgal in the center (Fig. 7), which is probably explained by the uplift of sandstones whose porosity is reduced by shock-induced compaction plus later seal-ing of tensile fractures. Reflection seismic data indi-cate that the uplift of dense basement material is of subordinate importance and may not account for the localized positive excursion. A circular residual grav-ity low of ~3.3 km diameter encircles the central high. This may correspond to the axis of the crater´s ring syncline. The circular low, in turn, is surrounded by a faint circular high at 4.7 km diameter of 1-2 mgal. The circular low is explained by the downshift of strata and the emplacement of brecciated material. The annular high corresponds to the uplift of strata near the rim as observed in the field. A north-south and east-west trending reflection seis-mic profile run through the center of Waqf as Suwwan (Fig. 8), being part of a regional seismic surveying of eastern Jordan. The crater is marked by a zone of dis-

turbance in which the horizontal reflectors that were most likely produced by the flat lying chert and lime-stone strata are missing or traceable only over short distances. Offsets of reflectors indicate inward dipping reverse faults near the morphological crater rim. Lo-cally the reflectors can be traced inside the crater and indicate strata uplift inside the rim (corresponding to the outer annular residual gravity high) as well as in-ward collapse in the ring syncline (corresponding to the circular gravity low). Numerous faults could be traced as well. The flanks of the central uplift seem to differ in steepness. The disturbance weakly affects a prominent reflector at 1.7 TWT. We interpret this lower reflector as the transition from basement to sedimentary cover rocks, occurring at a depth of ap-proximately 1000 m.

Block sizes within the central uplift: The deter-mination of block sizes and block size distributions at real craters is important to better constrain acoustic fluidization parameters for numerical computations of impact crater formation (Melosh, 1979; Melosh and Ivanov, 1999, Kenkmann et al., 2006). Block sizes could be determined in the Cretaceous limestone se-quence and in the uppermost Cretaceous chert se-quence of the central uplift by means of structural mapping (Fig. 6). In less competent sequences blocks could not be detected owing to limited exposures and a more pervasive and small-scale deformation. “Blocks” as described here are commonly internally deformed at the millimeter to decameter scale. Thus, bending, fold-ing and faulting within a block is the rule. A weak linear dependency of block size, b, on distance from the crater center, d was calculated. The block size in-creases with increasing radial range and follows the trend bls = 0.104 d + 25,8 for the limestone blocks and bch = 0.286 d - 53.2 for the chert blocks. The large scattering of data is expressed in the low stability indi-ces (R2) of 0.08 and 0.19, for limestone and chert blocks, respectively. Most of the limestone blocks oc-cur at distances of 200-300 m from the crater center, and the majority of the chert blocks are located 400-500 m away from the center. This indicates that the original stratigraphic position of the strata still controls their present location, although the layers are dismem-bered. Average block sizes in the limestone and chert sequences are 52.3 m and 75.5 m, respectively. While we favor the distance from the crater center being the main cause of the different block sizes, the unequal strength properties of chert and limestone give an al-ternative/ additional explanation for the observed size differences. The average block size determined within the central uplift falls in the range of sizes required for acoustic fluidization, e.g. Melosh and Ivanov (1999). Shock features Shatter cones are a common shock

feature found at Waqf as Suwwan. They occur along the entire periphery of the central uplift. Rare samples have been found near the crater center in sandstone beds. Shatter cones formed predominantly in micritic limestone, in limestone nodules and were locally also observed in chert layers. The Waqf as Suwwan target is predominantly composed of limestone lithologies, for which characteristic shock deformation features are lacking. Due to its fine grain size, microcrystalline chert is likewise not a carrier of shock features. The only stratigraphic unit with siliciclastic sediments is Lower Cretaceous sandstone. Salameh et al. (2006, 2008a) reported a single quartz grain from a sample of these strata that displays two sets of narrow spaced planar deformation features (PDFs) with typical inter-sections of the symmetrically equivalent lamellae and two sets of comparatively much wider spaced planar fractures (PFs). We found several quartz grains with so-called “feather textures”. Feather textures contain a planar fracture from which thinly spaced lamellae branch off at 40-60° angles, normally from one side. We found such feather textures crossing entire grains and low-angle grain boundaries. For a few PFs a dis-placement vector perpendicular to the grain surface could be demonstrated. The thin, straight, and parallel lamellae that branch off from these planar fractures typically have a spacing between 2 and 10 µm. They are much shorter than the main planar fractures from which they emanate. Preliminary analysis with the U-stage showed that {10-11} and {11-22} are dominant orientations of the feather textures. Feather textures have been reported from several proven impact struc-tures, such as Rock Elm (French et al., 2004; Poelchau and Kenkmann, 2009) and were also reproduced in Shock experiments (Poelchau and Kenkmann, 2009).

Heterogeneous target: Waqf as Suwwan was formed in flat-lying sedimentary target rocks with pro-nounced rheological variation. Considerable compe-tence contrasts exist between competent limestone and chert on the one hand and incompetent marl and chalk on the other. Marl, chalk and some claystone interbeds are important veneers between the competent lime-stone and chert layers and largely control the deforma-tion within the central uplift. Weak marl and chalk layers of the central uplift were apparently thick enough so that the competent beds deformed almost independently from each other. For instance, observed folds in the limestone and chert sequence became de-tached in the soft marl layer and do not extend from the limestones into the cherts (Figs. 3, 4). Buckle fold-

ing of the competent beds in addition to block faulting of the central uplift is the consequence. Wavelength, amplitude, and geometry of folds depend on the amount of shortening, the thickness of the competent and incompetent layers, and the viscosity contrast be-tween the materials. In many craters the most remark-able heterogeneity is marked by the transition from layered sedimentary rocks to competent crystalline basement rocks. There are, however, no indications that crystalline bedrock was involved in the cratering process at Waqf as Suwwan that would have affected the cratering flow field. The influence of the pre-impact fault distribution on the cratering process is unclear. The regionally dominant fault system (NW-SE) could not be traced along the crater, but could have contributed to the triangular shape of the central uplift. In contrast, the NE-SW trending fault system, commonly observed at Waqf as Suwwan, in particular in the southwestern and northeastern sectors of the crater, is of subordinate regional importance. This fault system may have played an important role during the proposed oblique impact.

Oblique impact: An ejecta blanket, the most sensi-tive indicator to decipher azimuth and angle of an oblique impact is not preserved at Waqf as Suwwan. The outline of the structure is circular, hence a low-angle impact of less than 10-15° which would result in an elliptical crater shape can be ruled out.

However, the central uplift shows some remarkable asymmetrical features that are indicative of an oblique impact (Scherler et al., 2006; Kenkmann and Poelchau, 2009). The central uplift as expressed in the outline of the chert collar has a bilateral symmetry with an axis trending SW-NE (Figs. 3, 4). This axis divides its tri-angular shape into two equal-sized right-angled trian-gles (Fig. 4). The southwestern and northeastern parts of the central uplift show a remarkable anisotropy. Whereas layering in the southwestern part is right-way-up, overturned folds and strata characterize the northeastern part (Figs. 4, 5). This and the dominance of SW-dipping strata in the central uplift (Fig. 5) sug-gests a top-to-the NE component of displacement which is explained by a downrange motion of rock during the crater excavation and modification stages. The presence of symmetrical folds with vertical axial planes and overturned fold axes in the downrange sec-tor of a central uplift is a new structural feature of complex impact craters and is interpreted as being in-dicative for oblique impact trajectories.

The subordinate SW-NE trending regional linea-ments could be responsible for the SW-NE-trending faults of the ring syncline (Fig. 4) and could poten-tially have influenced the bilateral symmetry of the central uplift, but we see no mechanism other than impact obliquity that could explain the systematic structural differences of the southwestern and north-eastern parts of the central uplift.

References: Abdelhamid, G.A.A., 1997, Geological interpretation of Landsat and

Radarsat images of east central Jordan: 1st International IRSS

Conduction, 11-12. Oct., Baghdad, Iraq. Bender, F., 1968, Geologie von Jordanien: Beitrag zur regionalen

Geologie der Erde, v. 7, 280 pp., Bornträger Publ., Berlin. Bender, F., 1975, Geology of the Arabian Peninsula, Jordan: USGS

Professional Paper, 560-I, p. 101-106. French, B.M., Cordua, W.S., and Plescia, J.B., 2004, The Rock Elm

meteorite impact structure, Wisconsin: Geology and shock-metamorphic effects in quartz: GSA Bulletin, v. 116, p. 200-218.

Heimbach, W., 1969, Vulkanogene Erscheinungen in der Kalktafel Zentraljordaniens: Beihefte Geol. Jahrbuch, v. 81, p. 149-160.

Kenkmann, T., and Poelchau, M.H., 2009, Low-angle collision with Earth: the elliptical impact crater Matt Wilson, NT, Australia: Ge-ology, v. 37, p. 459–462.

Kenkmann, T., Jahn, A., and Wünnemann, K., 2006, “Block size” in a complex impact crater inferred from the Upheaval Dome struc-ture, Utah: 37th LPSC, CD-ROM #1540.

Kenkmann, T., Khirfan, M., Salameh, E., Reimold, W.U., Omary, Y., Konsul, K., and Khoury, H., 2008, Structural Analysis and Re-mote Sensing of Jabal Waqf as Suwwan Meteorite Impact Crater: DMG..

Khirfan, M., Kenkmann, T., Salameh, E., Omary, Y., and Reimold, W.U., 2008, Remote Sensing and Structural Analysis of Jabal Waqf as Suwwan Meteorite Impact Crater: Large Met. Impacts Planet. Evolution IV, Parys, South Africa.

Melosh, H.J., 1979, Acoustic fluidization: A new geologic process? JGR, v. 84, p. 7513-7520.

Melosh, H.J., and Ivanov, B.A., 1999, Impact crater collapse: Annual Review of Earth and Planetary Sciences, v. 27, p. 385-415.

Powell, J.H., 1989, Stratigraphy and sedimentation of the Phanero-zoic rocks in central and south Jordan: Part A. Ram and Khreim

Groups: Jordan: NRA, Geology Division, Geol. Bull., v. 11, 72 pp.

Salameh, E., Khoury, H., and Schneider, W., 2006, Jebel Waqf as Suwwan, Jordan, a possible impact crater – a first approach: Zeitschrift deutschen Gesell. Geow., v. 157, p. 319-325.

Salameh, E., Khoury, H., Reimold, W.U., and Schneider, W., 2008a, First Large Meteorite Impact Structure Discovered in the Middle East: Jebel Waqf As Suwwan, Jordan: MAPS, v. 43, p. 1681-1690.

Salameh, E., Khoury, H., Reimold, W.U., Schneider, W., and Kenk-mann, T., 2008b, First Large Meteorite Impact Structure Discov-ered in the Middle East: Jebel Waqf As Suwwan, Jordan: Large Met. Impacts Planet. Evolution IV, Parys, South Africa. LPI No. 1423, paper id. 3106.

Scherler, D., Kenkmann, T., and Jahn, A., 2006, Structural record of an oblique impact: EPSL, v. 248, p. 28-38.

Table 1. Stratigraphy of Jebel Waqf as Suwwan impact crater