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Arabian Journal of Geosciences ISSN 1866-7511 Arab J GeosciDOI 10.1007/s12517-014-1427-6

Some observations facing the interpretationof Waqf as Suwwan structure, SE desert ofJordan, as an impact crater

Abdulkader M. Abed, Belal S. Amireh &Khetam Al Zghoul

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ORIGINAL PAPER

Some observations facing the interpretation of Waqf as Suwwanstructure, SE desert of Jordan, as an impact crater

Abdulkader M. Abed & Belal S. Amireh &

Khetam Al Zghoul

Received: 11 December 2013 /Accepted: 10 April 2014# Saudi Society for Geosciences 2014

Abstract Twenty-seven samples from Waqf as Suwwanstructure in the southeastern desert of Jordan were analyzedfor their petrography, mineralogy, and geochemistry. All threetypes of analysis failed to show any evidence supporting thestructure as an impact crater, but they cannot be used as anevidence discrediting the structure as an impact crater. This isbecause of the presence of shatter cones and rare shockmetamorphic features. Shatter cones are well established inthe Waqf as Suwwan structures and are accepted internation-ally as a criterion for impact craters. Shock metamorphicfeatures reported in previous works are extremely rare andatypical. Planar features (PFs) were reported in one singlequartz grain taken from a 30-m-thick Kurnub sandstone hori-zon. They are also reported from one single loose chert nod-ule, despite the extremely abundant bedded and loose chertwithin the structure. The PFs in the chert nodule arequestioned because they are possibly due to diagenesis.More important is the complete absence of any type of brec-cias, lithic, melt, or suevitic and ejecta within and around thestructure. The "deep erosion of the crater" used by advocatescannot stand for rigorous discussion on the amount of erosionin the desert environment in southeastern Jordan. Given theexact age of the cratering event is not yet known, and anapproximate long-term rate of erosion of 1 m/Ma brecciasand ejecta should be present in and/or around the structure.

Keywords Waqf as Suwwan . SE desert . Jordan . Breccias .

Ejecta . Rate of erosion

Introduction

Waqf as Suwwan is a domal structure, located in the south-eastern desert of Jordan, exposing rock units as old as theEarly Cretaceous within a Late Eocene marine sequence, thusserving as an "inlayer" or "stratigraphic window." Heimbach(1969) interpreted the structure as due to volcanic plug, whileAbdelhamid (2001) described principal ring faults around thestructure which had possibly acted as a preferable place for theemplacement of an igneous body. The age of the formation ofthe Waqf as Suwwan structure is not exactly known, but itshould be post-Eocene because it is dislocating the LateEocene Shallala Formation.

During 2005, Salameh et al. (2006) came out with theinterpretation of the Waqf as Suwwan domal structure as animpact crater. Their evidence included shatter cones, brecci-ated chert nodules, melted chert nodules, burnt limestonefragments, abundant ejecta, and planar features (PFs) in asingle quartz grain. Apart from the shatter cones and the singlePF, all other criteria are either not present within or around thecrater, e.g., the ejecta, melted chert nodules, and burnt lime-stone fragments, or can be explained by indigenous(terrestrial) geological processes rather than extraterrestrial,e.g., brecciated chert nodules which are widespread in theJordanian bedded cherts.

Since then, several papers were published on the Waqf asSuwwan structure, thus strengthening its status as an impactcrater (Salameh et al. 2008; Kenkmann et al. 2009; Khirfan2010; Reimold 2010; Kenkmann et al. 2010; Schmieder et al.2011). Schmieder et al. (2011) described some PFs and planardeformation features (PDFs) in quartz crystals within one brec-ciated chert nodule. The dendritic–suborthogonal fractures inthe chert groundmass and recrystallization veins are interpretedas possibly due to impact deformation or recrystallization andthus cannot be used as definite shock indicators (Schmiederet al. 2011). Also, a single quartz grain from a sandstone sample

A. M. Abed (*) : B. S. AmirehDepartment of Geology, The University of Jordan, Amman 11942,Jordane-mail: aabed@ju.edu.jo

K. Al ZghoulDepartment of Earth and Environmental Sciences, The HashemiteUniversity, Zarqa, Jordan

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showed possible PFs (Salameh et al. 2006, 2008).Furthermore, no traces of the impactor, the melt material,and the high-temperature/pressure minerals or the presenceof suevitic breccias, lithic breccias, or ejecta in or around thesite was reported. More recently, three boreholes were drilledwithin the structure; one of them down 140 m failed to findany traces of lithic breccia, melt breccia, glass, or part of thebolide itself (Khoury et al. 2013; Salameh et al. 2013). Itshould be emphasized that impact craters, much older thanthe Waqf as Suwwan structure and in a more humid areaswhere weathering and erosion should be higher, still preserveabundant melt, glass, and breccia in and around the structure(e.g., Dressler et al. 2004; Meibom et al. 2005; Coney, et al.2007; Kloberdanz 2010; Pierazzo and Artemieva 2012).

In short, the only solid criteria, advanced in more than 10papers by advocates, for considering Waqf as Suwwan as animpact crater is the presence of shatter cones and the very rarePFs and PDFs. It should be emphasized that shatter cones areaccepted by workers in the field to be sure criteria for the recog-nition of impact craters (Koeberl 2002; French andKoeberl 2010).This is simply because no terrestrial explanation for their forma-tion is advocated including the present authors of this work.

The aim of this work is not to discredit the Waqf as Suwwanstructure as an impact crater but to raise and discuss somemajorobservations facing this interpretation: (a) the complete absenceof any type of breccia or ejecta within or around the site, (b) theextreme rarity of the planar features, and (c) the possibility of adiagenetic origin of the planar features in chert.

Geological setting

Waqf as Suwwan structure is located in the southeastern desertof Jordan, some 150 km SE of the capital Amman at coordi-nates 31° 03′N and 36° 48′E (Fig. 1).Waqf as Suwwan area isa chert (Suwwan in Arabic) plateau made of the Late EoceneShallala Formation and extends till the Saudi Arabian bordersin the SE. The western limits of the plateau are cliffy (Waqf inArabic) with the Eocene Rijam Formation making the low-lying area further west.

TheWaqf as Suwwan structure is around 6 km in diameter andconsists of three conspicuous parts: the central uplift, the inter-mediate, relatively flat zone, and the outer rim (Figs. 1 and 2).

The central uplift is essentially an anticlinal structure albeitbeing complex with faulting. The oldest rocks cropping out atthe core of the central uplift are the uppermost KurnubGroup ofthe Early Cretaceous (Table 1). They are about 30-m thick andconsist of sandstone, limestone, dolomite, and glauconite. Thisunit is overlain by the Late Cretaceous (Cenomanian–Turonian)Ajlun Group which is made of limestone/dolomite alternatingwith marl. The outer part of the central uplift consists of thelower formations of the Belqa Group: Ghudran, Amman/Al-Hisa, and Muwaqqar Formations, Coniacian–Paleocene

(Table 1 and Figs. 1 and 2). The Ghudran Formation has aprofound chert horizon, the Amman/Al-Hisa is essentiallymade of bedded chert with minor carbonates and phosphorite,while the Muwaqqar Formation is almost completely made ofsoft marly material. Figure 2 is a cross section describing thelithology of the formations from the core to the flanks of thecentral uplift in southeastern direction.

The intermediate area is relatively flat and much wider thanboth the central uplift and the outer rim. Not much can be seenwithin this area. However, several localities west of the centraluplift, slightly higher than their surroundings, show tight foldingin alternating thinly bedded chert and chalk from the EoceneRijam Formation (Fig. 3). The latter formation is also croppingalong the flanks of a 2.5-m-deep wadi draining the intermediatearea (Fig. 2). Thus, it seems reasonable to conclude that theintermediate area is made of the Rijam Formation overlying theMuwaqqar Formation of the outer flanks of the central uplift,i.e., normal stratigraphic succession. It might be worth mention-ing that despite the extensive search, no breccia bodiesrepresenting the impactites have been seen within this area.

The outer rim consists of the Rijam and Shallala Formations,the last formations of the Belqa Group. The twomembers of theShallalah Formation, the soft, white chalky ThlaithuwatMember below and the dark, cherty Gharandal Member above,are conspicuous throughout the outer rim. Strata of both mem-bers dip outwards especially in the western rim.

Methodology

Field work was carried out to investigate the stratigraphy andfield relationships and to help collect representative samplesof material present in Waqf as Suwwan structure. Twenty-seven samples were collected from all bed rock lithologies,from recent and sub-recent wadi terrace deposits within theintermediate zone, and from a 50-cm deep pit in a mud flat(Gaa') towards the northern outer rim.

The samples were thin sectioned and studied by a polarizedmicroscope for their petrography. Part of each sample was pul-verized to pass 200 meshes by means of a Teema Mill. Themineralogy of the samples was made using an X-ray diffractom-eter (XRD) on the random powder. All the samples were runusing a scanning rate of 2°/min from 2° to 65° 2θ, range 4×103,and a chart speed 2 cm/min on a Philips X'pert MPD housed inthe Geology Department, The University of Jordan. Around 20 gof the powder of each sample was sent to ACME Laboratories,Vancouver, Canada for chemical analysis. Major elements wereanalyzed by LiBO2/Li2B4O7 fusion ICP-ES analysis. Some sam-ples were run in duplicates to ensure reproducibility. Trace ele-ments were analyzed by Neutron Activation Analysis (NAA) atBecquerel Laboratories, Canada in the following procedure.

The samples are weighed and transferred to a plastic cap-sule at Becquerel. They are then taken to the McMaster

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Nuclear Reactor where they are exposed to neutron flux of 8×1012 neutrons/cm2/s and stored for about 5 days. This irradi-ation causes most of the elements in the sample to become

radioactive and to begin to emit radiation in the form ofpenetrating gamma rays. The gamma radiation of the decayingnuclei can be analyzed for energy and number of gamma rays

Fig. 1 Upper: Location map ofthe Waqf as Suwwan structure.Lower: Geological map of thestructure showing its three parts,the central uplift, the intermediatezone, and the outer rim. Theformations cropping out withinthe structure and sample locations(1, 2, 3…) are also shown

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at each energy using a gamma ray spectrometer systemconsisting of a high-resolution co-axial germanium detector(typically 15 % efficiency) coupled to a multi-channel analyz-er. The interactions of the gamma rays with the detector lead todiscrete voltage pulses, which are proportional in height to theincident gamma ray energies and thus the intensity of theanalyzed element. Blanks, controls, and standards are insertedthroughout the batch. The chemical analysis is shown inTable 2.

Results

Mineralogy and petrography

All 27 samples were studied by means of XRD for their miner-alogy and thin sections for their petrography. No one sampleshows the presence of the high-temperature and/or high-pressureminerals, such as the two high-pressure forms of SiO2, coesiteand stishovite. The same result was reported also by previous

Fig. 2 A generalized cross section from the core of the central uplift to the outer rim showing the formations (not to scale)

Table 1 Nomenclature of the rock units involved in the Waqf as Suwwan structure (Powell 1989)

Formation MemberGroupAgeShallala (WSC)

Belqa

EoceneTertiary Rijam (URC)

Muwaqqar (MCM)PaleoceneQatrana

PhosphoriteAl-Hisa (AHP)Paleocene -Maastrichtian

Late

Cretaceous

Bahiyya CoquinaSultani Phosphorite

Amman (ASL)Campanian –Santonian

Dhiban ChalkGhudranConiacian Tafila

Mujib chalkWadi Es Sir

Ajlun

TuronianShueib

HummarCenomanian Fuheis

Na'urKurnub (Hathira) Sandstone GroupAptian -

AlbianEarly

Cretaceous

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workers advocating the Waqf as Suwwan as an impact crater(e.g., Kenkmann et al. 2010; Schmieder et al. 2011).

Petrographically, the samples studied consist of carbonates,chert, and minor sandstone samples. All samples investigatedshow standard particles and microfacies typical of similar rockselsewhere in Jordan with no signs of metamorphism due toimpacting, even advanced recrystallization has not been noticed.Figure 4a, b shows an ostracod packstone from the base of theAjlun Group in the central uplift and a planktonic foraminiferalwackestone from the chalk of the Shallala Formation of thewestern outer rim. A loose sandstone sample is shown inFig. 4c from the base of the Kurnub Group at the core of thecentral uplift, and a bedded chert is presented in Fig. 4d (RijamFormation from the intermediate zone of the structure).Furthermore, no signs of amorphous or glassy materials are seenin the studied thin sections. Also, no PFs or PDFs are seen in allthin sections studied especially the sandstone horizons of theEarly Cretaceous Kurnub Group at the base of the central uplift.See the discussion below. Consequently, it can be concluded thatthe succession at Waqf as Suwwan structure consists of normalsedimentary rocks bearing no noticeable signs of the high pres-sures and temperatures associated with meteorite impacting.

Geochemistry

Twenty-seven samples were analyzed for their major and cer-tain trace elements. The analysis and lithology of the samplesare shown in Table 2. As mentioned in "Methodology," thesamples analyzed represent a wide range of material: bedrock,

wadi terraces within the intermediate zone of the structure, andmud flat few tens of centimeters deep towards the outer rim ofthe structure. However, no sample was taken from lithic brec-cias and melt breccias (suevitic breccias) simply because theyare not present within or around theWaqf as Suwwan structure.

The elements that are usually used in this respect are theplatinum group elements (PGEs), Ir, and some othersiderophile elements such as Co, Ni, and Cr. These elementsand others were used successfully by Alvarez et al. (1980) inidentifying the K/T boundary. Enrichment of Ir, in particular,≥1–2 ppb can be used as a motif for pursuing other moresubtle evidence for impactites. Furthermore, and in order todemonstrate the level of enrichment or the changes in thechemistry of impacted material, it is generally advised tocompare the elemental analysis of the suspected impact struc-ture material with country rocks not affected by impacting.

For this purpose of comparison, average two chert samples(WQS 21 and 24) are compared (Table 3) with similar rocks,in age and lithology, fromWadi Mujib in central Jordan, some100 km to the west of Waqf as Suwwan (Kraishan 1988). Thereason for selecting this far away area is that the Waqf asSuwwan structure is "an inlayer" or "a stratigraphic window"of Cretaceous strata within a younger Eocene sequence. Thegeneral southeastern dip of strata necessitates going to centralor western Jordan to find comparable formations (Bender1974; Powell and Moh'd 2011). The comparison of the majorelements is shown in Fig. 5a where the two sets of samplesexhibit an essentially similar trend of variation in the distribu-tion of the major elements. The small differences in

Fig. 3 A field photo showing thetight folding in the RijamFormationwithin the intermediatezone. Width of the view is around4 m

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Tab

le2

Major

andsometraceelem

entanalysisof

theanalyzed

samples

12

67

910

1113

1415

1617

1819

LDL

ClS

stSst

Lst

Lst

St.S

edGaa'

Gaa'

Sst

Lst

Lst

GlL

stLst

Lst

Lst

SiO2%

0.01

66.15

83.95

0.48

0.23

19.41

28.02

35.84

50.91

1.56

0.95

12.62

0.32

1.66

0.30

AI 2O3

0.01

7.77

0.24

0.10

0.04

1.52

2.04

3.64

13.18

0.19

0.12

5.35

0.07

0.50

0.09

Fe2O3

0.04

3.22

0.21

0.11

0.05

1.05

1.19

1.84

8.88

0.26

0.18

2.10

0.15

0.41

0.08

MgO

0.01

0.58

0.08

0.17

0.21

3.01

1.31

1.87

3.33

0.27

0.22

1.04

0.17

0.26

0.17

CaO

0.01

7.31

7.76

55.88

55.53

38.77

35.42

28.44

1.02

54.13

54.55

40.74

55.36

53.85

55.77

Na 2O

0.01

0.49

0.12

<0.01

0.01

0.09

0.16

0.24

1.99

0.01

0.01

0.41

0.01

0.02

<0.01

K2O

0.01

0.27

0.06

0.02

0.03

0.31

0.42

0.66

4.03

0.09

0.05

0.99

0.02

0.14

0.03

TiO

20.01

0.62

0.01

<0.01

<0.01

0.15

0.22

0.34

1.02

0.03

<0.01

0.28

<0.01

0.03

<0.01

P 2O5

0.01

0.07

1.50

0.02

0.03

0.45

0.74

0.55

0.07

0.08

0.09

0.11

0.02

0.01

0.02

LOI

5.1

13.4

6.0

43.2

43.8

35.0

30.2

26.3

15.3

43.2

43.7

36.3

43.8

43.1

43.5

Sum

99.96

99.96

99.98

99.99

99.93

99.95

99.94

99.89

99.99

99.98

99.97

99.98

99.97

99.99

Nip

pm20

<20

54<20

<20

<20

2535

52<20

<20

<20

<20

<20

<20

Co

15

<1

<1

<1

45

613

<1

<1

2<1

<1

<1

Cr

557

68<5

<5

2544

6291

6<5

29<5

85

Zn

50<50

52<50

<50

<50

5457

58<50

<50

<50

<50

<50

<50

Sr2

225

143

95169

253

325

324

178

234

170

130

117

144

103

Rb

15<15

<15

<15

<15

<15

1519

72<15

<15

23<15

<15

<15

Irppb

5<5

<5

<5

<5

<5

<5

<5

<5

<5

<5

<5

<5

<5

<5

2021

2324

2526

2728

2930

3132

33Lst

Chert

Clk

Chert

Chert

ChClk

All

All

All

All

DlL

stDlL

stCllchh

SiO2%

0.40

91.02

0.09

93.73

77.16

53.95

33.86

10.99

18.79

4.78

5.71

33.44

50.27

AI 2O3

0.04

0.36

0.03

0.28

0.51

0.36

0.73

0.42

0.89

0.62

1.86

1.09

0.97

Fe2O3

0.12

0.21

0.06

0.12

0.29

0.19

0.34

0.18

0.43

0.34

2.27

1.18

1.25

MgO

0.24

0.10

0.18

0.08

0.21

0.22

0.68

0.43

1.17

0.68

13.33

8.48

7.40

CaO

55.30

3.79

56.79

1.64

10.74

24.41

34.25

35.24

34.15

37.76

29.07

24.43

16.50

Na 2O

0.11

0.21

0.02

0.36

0.58

0.05

0.12

0.65

3.56

0.10

0.33

0.09

0.24

K2O

0.04

0.12

0.01

0.16

0.30

0.06

0.15

0.25

0.23

0.07

0.49

0.28

0.28

TiO

2<0.01

0.02

<0.01

0.01

0.03

0.02

0.04

0.03

0.06

0.04

0.10

0.06

0.07

P 2O5

0.29

1.01

0.21

0.32

0.38

1.21

0.26

0.13

0.42

0.18

0.05

0.29

0.36

LOI

43.3

3.1

42.5

3.2

9.5

19.3

29.3

23.8

35.4

28.6

43.0

30.4

22.4

Sum

99.92

99.98

99.97

99.98

99.98

99.98

99.97

72.44

95.33

73.27

96.33

99.83

99.83

Nip

pm<20

26<20

3028

3127

2942

2429

2724

Co

<1

<1

<1

21

12

12

<1

21

2

Cr

2151

3343

6292

3026

4844

130

6677

Zn

110

<50

<50

<50

<50

<50

<50

<50

<50

<50

<50

<50

<50

Sr182

101

397

112

210

415

190

962

367

320

181

193

161

Rb

<15

<15

<15

<15

<15

<15

<15

<15

<15

<15

<15

<15

<15

Irppb

<5

<5

<5

<5

<5

<5

<5

<5

<5

<5

<5

<5

<5

LDLlower

limits

ofdetection,Sstsandstone,L

stlim

estone,G

aa'm

udflatsediments,C

hlchalk,Gglauconite,C

lclay,Allalluvial(w

adi)deposits,D

ldolom

ite

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abundance of the elements can be explained by variations inthe local depositional environment and/or diagenesis.

Also, CaO is plotted against loss on ignition (LOI) for allsamples (Fig. 5b). Figure 5b clearly shows a significant pos-itive correlation (r=0.93) that the LOI is dominated by the

CO2 present in the carbonates with slight deviations, probablyrelated to the structural water in the clay minerals. This simplymeans that the carbonate minerals had not been affected by thehigh temperatures associated with the impacting event, thussupporting the conclusions arrived in petrography and

Fig. 4 Rock types in the Waqf aSuwwan structure do not appearto be affected by impact cratering.a Ostracod packstone from thebase of the AjlunGroup just southof the core of the central uplift.Note that ostracod shells and shellfragments are not deformed(XPL). b Foraminiferalwackestone from the chalk of theShallala Formation, western outerrim. Again, the microfossils arenot deformed (PPL). c A loosesandstone sample from the baseof the Kurnub Group, treated withepoxy and thin sectioned (XPL).d Brecciated bedded chert fromthe Rijam Formation in theintermediate zone (autoclasts)(XPL)

Table 3 Major and some traceelement analysis of two beddedchert samples and one silicifiedlimestone sample from Waqf asSuwwan (this study) and compa-rable rocks from Wadi Mujib incentral Jordan (Kraishan 1988)

Waqf as Suwwan samples Wadi Mujib samples

WQS 21 WQS 24 WQS 10 14c 22 26Cher Cher Silicif. Lmst Chert Chert Silicif. Lmst

SiO2% 91.02 93.73 28.02 94.01 92.34 25.84

AI2O3 0.36 0.28 2.04 0.21 0.1 0.08

Fe2O3 0.21 0.12 1.19 0.3 0.08 0.04

MgO 0.10 0.08 1.31 2 0.16 5.4

CaO 3.79 1.64 35.42 1.970 2.35 28.33

Na2O 0.21 0.36 0.16 0.06 0.1 0.1

K2O 0.12 0.16 0.42 0.03 0.01 0.01

TiO2 0.02 0.01 0.22 nd nd nd

P2O5 1.01 0.32 0.74 0.25 007 0.19

LOI 3.1 3.2 30.2 0.67 2.7 42A6

Sum 99.98 99.98 99.95 100.4 99.16 99.37

Ni 26 30 25 185 140 155

Co <1 2 5 75 80 60

Cr 51 43 44 150 60 100

Zn <50 <50 54 107 104 73

Sr 101 112 325 12 18 76

Rb <15 <15 15 45 4 1

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mineralogy above. Furthermore, the clay mineral chemistrylooks unaffected by the impact event as demonstrated byrelationships of some of the major elements representing theclay minerals as shown in Fig. 5c, d. The Al2O3 is plottedagainst K2O with a significant correlation coefficient (r=0.76)in Fig. 5c, while Al2O3 is plotted against Fe2O3 (r=0.98) inFig. 5d, indicating that the clay minerals in the studied sam-ples are intact. The slight deviations from the fit line can beexplained by the fact that parts of these elements are notpresent within the clay minerals but, for example, as discretehematite independent of the clay mineral structure. Moreover,the Early Cretaceous Kurnub sandstone beds, at the base ofthe inner core, are friable and are similar to the friable Kurnubmaterial elsewhere in the country. Consequently, the sand-stones, carbonates, and chert of theWaqf as Suwwan structureare comparable with normal sedimentary rocks (e.g.,Krauscopf and Bird 1995; White 1997).

Iridium, in all samples, is below the lower limit of detection(LDL in Table 2) of less than 5 ppb of the NAA method usedat Becquerel Laboratories, Canada. The <5-ppb Ir content ofthe analyzed samples from Waqf as Suwwan is a bet higher

that the ≥1–2-ppb Ir content accepted generally to indicate anextraterrestrial origin of the material analyzed (e.g., Koeberl2002; French and Koeberl 2010). Unfortunately, we wereunable to go to a lower limit of detection because of budgetshortages in the project.

On the other hand, the siderophile elements mostly used asa possible indicator for an extraterrestrial origin of the ana-lyzed material are Co, Ni, and Cr. These elements are sup-posed to be enriched in the impactites, and their concentrationshould be higher than normal terrestrial sedimentary rock ofsimilar lithology. These element and others, such as Zn, arecompared, in Table 3, with the analysis of similar rocks in theMujib area. Two chert samples and one silicified carbonatesare compared in both areas. Nickel and Co are several foldshigher in the Mujib area compared with Waqf as Suwwanstructure, while Cr and Zn are two to three times as much, thusindicating no enrichment of these elements in the Waqf asSuwwan materials. It should be mentioned that the authors arewell aware of the shortcoming of the use of the data in Table 3to indicate that Waqf as Suwwan is not an impact crater.However, we take it as a passive evidence.

Fig. 5 Some geochemical relationships between the analyzed elementsin the studied samples showing that the samples are still normal sedimen-tary rocks not affected especially by the high temperatures associated withthe impact. a Comparison of the major elements of two bedded chertsfrom the Waqf as Suwwan structure with two sample, similar in age andlithology, from the Mujib area. b A plot between the CaO% and the loss

on ignition (LOI)% for all the samples analyzed, indicating that most LOIis represented by CO2 in carbonates. c A plot between Al2O3% andK2O% for the analyzed sample. d A plot between the Al2O3 and theFe2O3% for the analyzed samples. Both c and d are taken to indicate thatthe detrital clay minerals have no changes due to impacting

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Discussion

The above three lines of observation, mineralogy, petrography,and geochemistry, cannot firmly indicate that Waqf as Suwwanis not an impact crater but raise problems in the interpretation.However, more problematic observations are discussed below.

Breccia and ejecta

When a normally bedded sedimentary succession, as it is thecase in Waqf as Suwwan area, is subjected to a bolide impact,the succession, under the high pressure of the impact, is brec-ciated, thrown in the air, and redeposited as lithic brecciawithin the crater or ejecta (outside the crater itself). Theamounts of breccia and ejecta are proportional to the size andvelocity of the impactor. The bolide itself is also fragmented.With the high temperatures during the impacting event, moltenmaterial can be produced within the bolide and the resultantbreccia (suevitic breccia). This description is documented in somany examples in the literature throughout the geologicalhistory (e.g., Dressler et al. 2004; Meibom et al. 2005; Coneyet al. 2007; Kloberdanz 2010; Pierazzo and Artemieva 2012).

At the Waqf as Suwwan structure, no breccia deposits ofany kind, lithic breccia, melt breccias (suevitic breccia), orejecta, within this structure or around it, nor any traces of thebolide have been seen in this study or reported in previousworks (e.g., Salameh et al. 2008; Khirfan 2010; Kenkmannet al. 2010). Furthermore, the intermediate zone between thecentral uplift and the outer rim of the structure was drilleddown to a depth of 140 m, and no signs of any type of brecciadeposits were reported (Khoury et al. 2013; Salameh et al.2013). Advocates of an impact crater for the Waqf as Suwwanstructure explain the non-presence of such large deposits ofbreccias and ejecta as due to "deep erosion of the structure"(e.g., Schmieder et al. 2011; Salameh et al. 2013).

Deep erosion that completely removes all the material exca-vated from the approximately 6-km diameter crater to a depth ofaround 330 m in a desert environment like theWaqf as Suwwanarea needs explanation. It is true that Jordan was not aridthroughout its recent geological history, and more humid condi-tions were reported during the Pleistocene (Huckriede andWiesemann 1968; Abed and Yaghan 2000; Abed et al. 2008).But that does not change the fact that the youngest marine strata,the late Eocene Shallala Formation, is still preserved in the largerWaqf as Suwwan area and SE of it (Powell 1989; Abdelhamid2001). Shallala Formation is the last marine formation depositedfrom the Neo-Tythes Ocean before migrating from the easternMediterranean region (Bender 1974; Brew et al. 2001; Sharlandet al. 2001; Abed 2013). Consequently, its top was truncated byerosion in measured outcrops throughout the country (Andrews1992; Al-Bashish and Abu Azzam 2008). The maximum re-ported thickness in the outcrops varies between 120 and 130 m,and in the Waqf as Suwwan area, it is 90-m thick. That implies

an approximate long-term rate of erosion of 20–30 m in 35 Maor less depending on the age of the impact event or approxi-mately a rate of erosion of 1 m/1 million year. Simple calcula-tions can show that the excavated material of a 6-km-diametercrater to a depth of 330 mwould produce a layer of breccias andejecta in and around the carter that exceeds calculated amount oferosion depending on the age of the event. The complete non-presence of this layer or remnants of it would seriously questionthe notion of "a deep erosion of the crater," used by advocates(e.g., Salameh et al. 2013), to explain its absence.

It should not be said that the impacted succession consistsof soft sedimentary rocks that can be easily eroded. This isbecause the Waqf as Suwwan succession has three hard,bedded chert horizon (Fig. 2) that can withstand chemicaland physical weathering and erosion. On the other hand, thereare many examples of much older impact craters, situated inmore humid areas and consequently more active weatheringand erosion, which still have abundant breccia preserved, forexample, the Dhala impact structure in India with age between2.5 and 1.6 Ga (Pati et al. 2006), Gardnos impact structure insouthern Norway with an age between 900 and 350 Ma withlithic and melt breccia (Gilmour et al. 2003), and theChicxulub impact structure at the K/T boundary in Mexico(Dressler et al. 2004). Consequently, the complete non-presence of any type of breccias or ejecta in and around theWaqf as Suwwan structure raises a major problem in theinterpretation of the structure as an impact crater.

PFs and PDFs

The high pressure of a bolide impact on rocks usually pro-duces shock waves through the area, and shock metamorphicfeatures are produced within the impactites. Most importantamong these features are the PFs and PDFs which are sureevidence for impact cratering (e.g., Koeberl 2002; French andKoeberl 2010 among many others). Provided that they arecorrectly interpreted, PFs and PDFs in impactites are acceptedby workers as firm evidence for impact cratering. Despite themore than 10 works on Waqf as Suwwan as an impact craterby advocates mentioned in the "Introduction," a single exam-ple of PFs was found in one single quartz grain from the looseKurnub sandstones at the base of the central uplift (Salamehet al. 2006, p. 324). It should be mentioned that 30 m of theKurnub Group, with several sandstone horizons, is exposed atthe base of the central uplift (Fig. 2), and our petrographicinvestigation, also, revealed no PFs or PDFs.

On the other hand, Schmieder et al. (2011) had investigatedthe shock metamorphism feature in three chert nodules, thefirst is a loose (not in situ) nodule from the central uplift area,the second also loose from the intermediate zone, while thethird is in situ taken from the northern outer rim. According tothem, the former nodule showed shock metamorphic featuresPFs and PDFs, while the latter two nodules were negative. It

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should be mentioned, first, that exposed, in situ bedded chertwith nodules (Fig. 6a) is abundant in the Waqf as Suwwanstructure forming significant parts of the Ghudran, Amman,Al-Hisa, Rijam, and Shallala Formations (Table 1 and Fig. 2).Also, loose chert nodules with suborthogonal fracture patternare frequent in the whole structure (Fig. 6b). Again, and likethe single quartz grain PFs in sandstones, the presence of PFsand PDFs in on single loose nodule indicates the rarity ofshock metamorphism features in the structure. Furthermore,similar chert nodules with suborthogonal fracture pattern arepresent in other parts of the country, e.g., in Jebal Khuzaymasome 70 km south of the Waqf as Suwwan structure (Fig. 6c).

Chert nodules with suborthogonal fracture pattern wereaccepted by Salameh et al. (2006) as a shock feature, whileSchmieder et al. (2011) are of the idea that this "suborthogonalfracture pattern was probably generated by shear deformationnot directly related to shock, i.e., likely impact-induced."Autobrecciated chert nodules with suborthogonal fracture pat-tern can be formed through the early diagenetic dehydrationprocess from opal A to microquartz (Laschet 1984; Kraishan1988; Abed and Kraishan 1991; Kolodny and Garrison 1994).

The fact that such nodules are present elsewhere in Jordan,away from the Waqf as Suwwan structure, would cast doubton the interpretation of such nodules as due to shock meta-morphism or related to them and would support our interpre-tation that they are diagenetic in nature and not due to shock orinduced metamorphism.

Chert, especially brecciated chert, is a rock with a complexdiagenetic history. Senonian brecciated, bedded cherts fromthe Negev and Jordan exhibit at least two diagenetic stages:replacement of the preexisting carbonates leading to the for-mation of chert autoclasts, followed by precipitation of thefine silica matrix leading to the silicification of the whole rock(e.g., Steinitz 1977; Kraishan 1988; Kolodny and Garrison1994). Sometimes, some features of the preexisting sedimentswere preserved or at least not completely obliterated by silic-ification. For example, Kraishan (1988) reported fine, micro-scopic cross-lamination in bedded chert of the Mujib area,while Steinitz (1977) documented relic evaporates in theSenonian bedded cherts of the Negev. One sample from thecentral uplift showed a calcite vein exhibiting perfect twotrends of calcite twinning (Fig. 7a) that does resemble PFs if

Fig. 6 Chert nodules withsuborthogonal fracture pattern areabundant in Waqf as Suwwanstructure and elsewhere in Jordan.a An in situ nodule withsuborthogonal fracture patternfrom the bedded chert of theShallala Formation, outer rim(pin=14 cm). b A loose chertfragment from the lower slope ofthe western outer rim (diameter=10 cm). c A similar nodule fromJebal Khuzaymah, some 70 kmsouth of Waqf as Suwwan,diameter=8 cm

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present in quartz. The vein does look like the silica veinsdescribed by Schmieder et al. (2011) in the chert nodule whichhe studied. What if such calcite vein was silicified keepingsome relics of the calcite twinning? Figure 7b shows a quartzcrystal postdating the calcite formation in the same vein(arrows). A close up on that grain (Fig. 7c) shows two visibleplanar features. Another quartz crystal in the same vein(Fig. 7d) exhibits two visible planar features continuing intothe calcite crystal (arrow at the upper right). It is thus believedthat the PFs and PDFs in chert can be produced throughdiagenesis and should be interpreted with care.Consequently, and in addition to the rarity of the shock meta-morphic features in chert in the Waqf as Suwwan structure,these feature could be due to diagenesis.

This leaves the interpretation of Waqf as Suwwan as inimpact crater with one evidence, that is, the shatter cones. Theshatter cones are accepted, worldwide, as due to extraterres-trial impacting, and no terrestrial (geological) interpretation oftheir formation is present up till now (e.g., Koeberl 2002;French and Koeberl 2010). However, and despite the presenceof the shatter cones, the observations, discussed above, needexplanation before fully accepting Waqf as Suwwan structureas an impact crater.

Summary

Waqf as Suwwan structure has been under investigation as animpact crater since 2006, with more than 10 works published.

In spite of that and with our knowledge of the presence ofshatter cones as an accepted criterion for the recognition ofimpact craters, major problems are still standing and needaddressing.

Despite the careful selection of the samples for this work,as well as previous works, mineralogical analysis failed toshow high-pressure and/or temperature minerals such ascoesite and stishovite within and around the structure. Also,geochemical investigation analysis showed that Ir is low in-deed, and the siderophile elements Ni, Co, and Cr in thestructure are several folds below comparable country rocks.

Petrographic investigation also failed to find shock meta-morphic features indicative of impact cratering. Furthermore,previous works indicate the extreme rarity of such feature. Asingle quartz grain showed atypical PFs from a 30-m-thickKurnub sandstones of the lower Cretaceous at the base of thecentral uplift. Again, questionable PFs and PDFs were report-ed in a single, loose brecciated chert sample from an ocean ofbedded and loose chert present within the structure from theCretaceous-Eocene Belqa Group. Our petrography investiga-tion shows that the PFs and PDFs in the chert may be ex-plained as due to diagenesis (Fig. 7).

Moreover and possibly more important is the completeabsence (non-presence) of any type of breccias, lithic, melted(suevitic), and ejecta in and around the Waqf as Suwwanstructure. "Deep erosion of the crater" used by advocatescannot stand for rigorous discussion on the amount of erosionin the desert environment in southeastern Jordan. Given theexact age of the cratering event is not yet known, and an

Fig. 7 Fracture patterns inquartz. a Distinct twinning in acalcite vein (XPL). bQuartz grainpostdating the formation of calcite(upper right, calcite grain; cal =calcite, qz = quartz; XPL). cCloseup on b showing visible twotrends of fractures within thequartz grain, partly traced toindicate the two trends (XPL). dAnother quartz grain in the samesample showing two clear trendsof fractures within a quartz grain.Note continuity of the fracturesbetween the calcite and quartz,partly traced to indicate the twotrends (arrow on the upper right;XPL)

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approximate long-term rate of erosion of 1 m/Ma breccias andejecta should be present in and/or around the structure.

Acknowledgments The authors would like to thank the two anony-mous reviewers of the Arabian Journal of Geosciences for their com-ments and suggestions which have improved the quality of the manu-script. This work was partially supported by the Deanship of ScientificResearch of the University of Jordan. Thanks are also due to M.Hassouneh for his help in the petrographic investigation of the samples.

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