Int J Earth Sci (Geol Rundsch) DOI 10.1007/s00531-013-0930-4 ORIGINAL PAPER The Waqf as Suwwan crater, Eastern Desert of Jordan: aspects of the deep structure of an oblique impact from reflection seismic and gravity data Till Heinrichs • Elias Salameh • Hani Khouri Received: 19 December 2011 / Accepted: 16 June 2013 Ó Springer-Verlag Berlin Heidelberg 2013 Abstract The deeply eroded Waqf as Suwwan ring impact obliquity based on vergency of folds exposed on the structure was recently discovered to be a large impact, the central uplift. first identified in the near east. Large-scale reflection seismic structure shows the impact situated high on the Keywords Waqf as Suwwan impact Á Oblique impact Á northeastern flank of the Jordan Uplift sloping into Wadi Rim uplift Á Radial transpressive ridge Á Gravity model Á Sirhan Basin. If exhumation is linked to the Arabia–Eurasia Jordan Uplift Á Wadi Sirhan Basin collision, a likely time window for the impact event may be latest Eocene to Late Oligocene. Impact into a shallow sea seems an optional scenario. Old reflection seismic lines Introduction: situation and state of investigation offer limited insight into the deep structure of the rim and of the impact crater part of the central uplift of the complex crater. An important structural clue is provided by a well-resolved Djebl Waqf as Suwwan (mountain of the upright chert; seismic horizon of a yet tentative correlation with a 36°480E, 31°030N; WaS in the following) is situated near Paleozoic black shale. The central gravity high is com- the northwestern watershed of the Wadi Sirhan catchment, patible with a mass surplus by the uplift of denser Paleo- also just W of and beneath the escarpment of the Eastern zoic basement below the central uplift. The gravity model Jordanian Chalk Plateau (Figs. 1, 2). The dense network of further indicates a ring of dense Paleozoic sediments rising wadis crossing the structure hints at substantial erosion. from below into the ring syncline. Seismics show pre- The structure shows as an outer ring of hills, ca 6 km in sumably radial synclines in the central uplift which are diameter, and a central uplift with an inner depression, ca interpreted by centripetal constrictional flow during crater 1 km across, either only some 50 m elevated above the collapse. Beneath the final crater’s outer boundary, a surroundings (Fig. 2). The outer ring exposes mid-Eocene shallow-dip normal fault zone, subtle seismic structure in chalks and cherts with variable shallow dips. The expo- uncollapsed footwall segments reveal an asymmetry of sures of the central uplift form an outward dipping strain. The asymmetry is attributed to the cratering flow by peripheral collar of Maastrichtian chalks and phosphatic an oblique impact directed toward NE. The finding pro- cherts underlain by Turonian to Santonian marls and vides independent support to an earlier suggestion of limestones surrounding an inner gentle depression where the oldest exposed strata are observed, e.g., sandy marls and friable sandstone of Albian/Cenomanian age. Between the central uplift and the outer ring, there is a moat-like T. Heinrichs (&) Applied Geology, Geoscience Center, Georg-August University, depression filled by up to 10 m of alluvium covering sparse 37077 Goettingen, Germany exposures predominantly of cherty chalks with irregular e-mail: [email protected] attitudes, probably lower-to-middle Eocene in age. Bio- stratigraphic ages, lithostratigraphy and thicknesses of the E. Salameh Á H. Khouri Faculty of Science, University of Jordan, Amman 11942, Jordan exposed strata are from Heimbach (1969, 1970). Despite e-mail: [email protected] the largely preserved stratigraphic succession, the central 123 Int J Earth Sci (Geol Rundsch) origin (Heimbach 1969). Recent discoveries of numerous occurrences of shattercones, well developed in the limestones and cherts of the central uplift, have turned the balance to an impact interpretation (Salameh et al. 2006, 2008). Supporting microscopic evidence has been found in samples from the central uplift, e.g., quartz with planar fractures (PF) and planar deformation features (PDF), though sparse (Salameh et al. 2006; Schmieder et al. 2011), and planar features with feather structures (Poelchau et al. 2009), summarily indicating shock pressures from about 5 to C10 GPa. Mesoscale structures in the exposed uplift were mapped in detail, and overturned folds geometries were interpreted as indicators of an oblique impact directed toward NE by Kenkmann et al. (2010). Based on geologic mapping and biostratigraphic work, WaS was assumed by Heimbach (1969) to be a bowl-shaped structure with the center uplifted by some 300 m. However, the subsurface structure has remained conjectural because extrapolations from intensely Fig. 1 Situation map of Waqf as Suwwan impact structure. Wadi and irregularly deformed outcropping strata were necessarily Sirhan catchment straddels Jordanian/Saudi Arabian border speculative. Based on hydrocode numeric modeling, Wu¨nnemann et al. (2011) presented a model of an originally 8-km-wide crater with a central uplift and substantial defor- mation dying away ca 1–1.5 km below ground, assuming 500 m depth of erosion since the impact event. They also modeled a 6.5 mgal (10-5m/s2) central gravity high by introducing an anomalously dense (2,750 kg/m3)rootless sandstone mass of 2 km diameter close to surface. In the present study, we try to retrieve additional structural information from existing geophysical data. In particular, we are going to discuss the original crater size, and an estimate of the age and depth of erosion in the context of regional tectonics, the deep target lithologies, the geometry of the central uplift and the related question of the enigmatic central gravity high, the deep structure of the ring syncline and the structure of the rim and its bearing on the obliquity of this complex impact. Target stratigraphy and regional geology Fig. 2 Djebl Waqf as Suwwan satellite image; note escarpment of Eastern Jordanian Chalk Plateau E of ring structure; bright center of uplift = Kurnub Sandstones to Wadi as Sir/Umm Gudran Limestone The impact, situated on the northeastern flank of the Jordan Fms, surrounded by dark brown collar = chert-rich Maastrichtian Uplift, is surrounded exclusively by exposed Paleogene Amman/alHisa Fms. Brown interfluves between wadis carry flint and Neogene sediments (Fig. 3, map; Bender, Deutsche fragment deflation residue with dark desert varnish. Red dashed = Geologische Mission in Jordanien 1968). The following is reflection seismic lines INOC-SJ89 205 and 214 an attempt to assemble the complete lithostratigraphy of the eventual impact target, including (a) the unexposed uplift of the structure shows very severe deformation on the Paleozoic below the Pre-Kurnub Unconformity (Pre- mesoscale, e.g., upright to overturned folds and fractures or Hercynian basement), (b) the unexposed part of the basal faults separating dekametric to hektometric megablocks of Cretaceous unit (basal Kurnub Group), (c) the Late Cre- the more competent rocks, with the marls or friable sand- taceous and Paleogene units exposed at the crater and stones intruded in between (Kenkmann et al. 2009, 2010). (d) the eroded top of the section (Fig. 3a, b). The two major In the absence of any preserved ejecta, WaS was long issues are (1) the depth to the Hercynian subcrop at the known as a ring structure of supposedly cryptovolcanic impact site and the nature of the topmost Paleozoic units, 123 Int J Earth Sci (Geol Rundsch) Fig. 3 Target stratigraphy assembled: a composite lithostratigraphic profiles in Figs. 7, 9. Colored capitals in right margin indicate column for Waqf as Suwwan: Paleozoic (bottom) is interpolated from position of seismic horizons A0 to C1 at their converted depths, cf. distant deep water- and oil wells (sources: Bender 1968; Abu Saad Fig. 4 and seismic interpretation. b Lithostratigraphic correlation and Andrews 1993; Hobler et al. 1994), Cretaceous/Palaeogene from Wadi Sirhan Basin wells onto Jordan Uplift up to exposures at carbonate platform deposits are from exposures at WaS (top). WaS: Dashed line is present erosion profile extending from exposure Nomenclature follows subsurface geology conventions of lithostrati- at WaS across wells—partly projected—where Wadi Shallalah Fm. is graphic units (Powell 1989; Andrews 1991, 1992). Late Cretaceous/ increasingly lost to erosion. Note: datum line at top, end B5 Palaeogene acc. to formal stratigraphic terms and codes A1-7, B1-5, sedimentation, is arbitrarily set at 500 m erosion depth at WaS (e.g., introduced for Heimbach’s informal lithostratigraphic units (Heim- the estimate by Wu¨nnemann et al. 2011). But erosion depth at WaS bach 1969, 1970; Bender 1968; Andrews 1992 and NRA mapping could be as low as 300 m, see text. c Generalized geologic map and division, also Kenkmann et al. 2009). The codes C4, tt1, tt2 refer to tectonic units, situation of profile Fig. 3b, deep wells, seismic sections Heimbach’s (1969) mapped units used in our study in setting up Fig. 4, gravity map of Fig. 5 123 Int J Earth Sci (Geol Rundsch) and (2) the restoration of the eroded top of the column, e.g., Carboniferous to the lower Cretaceous, resulting in 1–2 km late Paleogene and possibly younger pre-impact deposits. of late Silurian to Carboniferous strata removed (Konert The Hercynian subcrop in the WaS area was predicted et al. 2000). The unconformity is expected at around 400 m by Andrews (1991) to be gently eastward tilted Paleozoic asl near the impact as follows from the thicknesses of the strata near the Ordovician/Silurian boundary. A total of ca mapped Paleogene-Upper Cretaceous stratigraphic units by 3.0 km of Ordovician and Cambrian sediments is indicated Heimbach (1969), combined with a thickness estimate of at WaS by our interpolation between deep wells (Jafr-1, the barely exposed Kurnub Sandstone Group (KS, Albian/ 110 km SW of WaS; WS-3, 80 km SE; WG-2, 65 km to Cenomanian) attaining here 120 ± 30 m according to the N).