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1 Supportive comment on: “Morphology and population of binary asteroid

2 impact craters” , by K. Miljković , G. S. Collins, S. Mannick and P. A.

3 Bland [Earth Planet. Sci. Lett. 363 (2013) 121 –132] – An updated

4 assessment

5

6 Martin Schmieder 1,2 , Mario Trieloff 3, Winfried H. Schwarz 3, Elmar Buchner 4 and Fred Jourdan 2

7 1School of Earth and Environment, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia

8 2Western Australian Argon Isotope Facility, Department of Applied Geology and JdL Centre, Curtin University, GPO Box

9 U1987, Perth, WA 6845, Australia

10 3Institut für Geowissenschaften, Universität Heidelberg, Im Neuenheimer Feld 234-236, D-69120 Heidelberg, Germany

11 4HNU-Neu-Ulm University, Edisonallee 5, D-89231 Neu-Ulm, Germany

12

13 In their recent paper, Miljković et al. (2013) assess the appar ent contradiction that the near-Earth asteroid population

14 consists of 15% binaries, while the terrestrial (and Martian) impact crater populations have only 2-4% of observable

15 doublet craters. The authors suggest that only a small fraction of sufficiently well separated binary asteroids yield

16 recognizable doublets. We generally agree with the conclusions by Miljković et al. (2013) and acknowledge the high

17 quality and relevance of the study. However, we would like to bring into focus additional geochronologic constraints

18 that are critical when evaluating terrestrial impact crater doublets. Miljković et al. (2013) appraised five potential

19 terrestrial doublets using the Earth Impact Database (EID; as of 2010). We hereby warn against the indiscriminate

20 usage of impact ages compiled in this database without an assessment based on solid isotopic and stratigraphic

21 constraints and comment on the geological, geochronological, and geochemical evidence for doublet impact craters

22 on Earth. 23 Geologic evidence

24 Firstly, the confirmation of macroscopic and microscopic shock effects in rocks and minerals recovered from

25 candidate terrestrial impact sites is a prerequisite to establish an impact origin of such structures (French and

26 Koeberl, 2010). Miljković et al. (2013) included the Crawford and Flaxman structures in South Australia in their

27 selection of ‘possible’ terrestrial crater doublets. These structures had been in troduced as impact structures in a 1999

28 conference abstract. Due to the lack of compelling evidence for impact, both structures were later classified as

29 ‘possible impact structures’ of uncertain age (Haines, 2005). Only proven impact structures should be part of

30 statistical considerations on the terrestrial impact rate.

31

32 Geochronological criteria

33 Secondly , we point out that some of the impact ages listed by Miljković et al. are outdated and incorrect. Precise and

34 accurate impact ages are crucial when it com es to ‘double impact’. All potential crater pairs of Miljković et al.

35 (2013) have apparent crater ages that comply with a theoretical double impact scenario; however, comparatively

36 large errors on the ages and, more importantly, a lack of unambiguous accuracy of some of the ages leave evidence

37 for double impact doubtful. The dating of terrestrial impact structures has undergone a dynamic evolution in recent

38 years, and more and more new and refined isotopic data are now available. Nevertheless, only a few of the ~188

39 known terrestrial impact structures have been dated with a satisfying uncertainty of ≤±2% by means of isotopic

40 and/or biostratigraphic methods (Jourdan et al., 2012). Isotopic approaches can yield results as precise as ~±0.5% in

41 their relative error on the age. Only synchronicity of two neighboring impact events within such narrow

42 uncertainties hardens the evidence for double impact.

43

44 Comments on the potential crater doublets listed in Miljković et al. (2013)

45 The ≥36 km Clearwater West and ~26 km Clearwater East impact structures in Canada are listed as a ‘very likely’

46 doublet in Miljković et al. (2013). The maximum age of both structures is stratigraphically constrained by the

47 shocked Ordovician limestones that overlie the Archean basement (e.g., Grieve, 2006). The 290 ± 20 Ma age for 48 both impact structures widely cited in the literature and the EID seems to be adopted from early, poorly robust, K-Ar

49 ages for Clearwater West published in a 1960s Geologic Survey of Canada report. Our group obtained two 40 Ar/ 39 Ar

50 plateau ages with a mean of 286 ± 2 Ma for Clearwater West, very similar to the age reported in Bottomley et al.

51 (1990). Age data for Clearwater East are still somewhat ambiguous. A mineral isochron Rb-Sr age for melt rocks

52 from Clearwater East yielded an age of 287 ± 26 Ma (Reimold et al., 1981), coeval within error with the ages for

53 Clearwater West. However, the Rb-Sr technique has frequently failed to provide accurate impact ages (e.g., Mark et

54 al., 2013). 40 Ar/ 39 Ar dating of melt rocks from Clearwater East produced a set of significantly older Ordovician

55 apparent ages around ~460-470 Ma (Bottomley et al., 1990). Similar ages were obtained by our group, suggesting

56 that Clearwater West and East are probably not a doublet. Currently, the idea of synchronicity of the two impact

57 events entirely relies on a single Rb-Sr age for Clearwater East and, probably, a general ‘agreement’ since the 1960s

58 based on the expectation that the two impact structures, closely spaced, must represent a doublet.

59 The ~25 km Kamensk and ~3 km Gusev impact structures, also a ‘very likely’ crater doublet in Miljković et al.

60 (2013), have been considered a doublet since the 1970s. Both craters are filled with sediments of the Globukaya

61 Formation of postulated Cretaceous-Paleogene boundary age, an impact-derived marine resurge breccia that overlies

62 the uppermost Cretaceous and is topped by Paleogene sands (Movshovich et al., 1991). Notwithstanding with the

63 previous biostratigraphic age estimates, 40 Ar/ 39 Ar dating of fresh impact glass from Kamensk by Izett et al. (1994)

64 yielded a fairly robust Eocene age of 50.36 ± 0.33 Ma (recalculated; Jourdan et al., 2012), contradicting a ~65 Ma

65 impact age (as given in Miljković et al. 2013). This also constrains a minimum age for the Globukaya Formation and

66 the smaller Gusev crater, and suggests sedimentary reworking processes in the Eocene. Despite some uncertainties

67 regarding the exact timing of the Kamensk-Gusev impact, stratigraphic correlation and geographic evidence support

68 a likely double impact scenario.

69 The ~24 km Ries and ~3.8 km Steinheim impact craters in Southern Germany, associated with the Central European

70 tektite strewn field, have been generally regarded as a typical crater doublet since the 1960s (Stöffler et al. 2002).

71 Miljković et al. (2013) conservatively labelled them a ‘likely’ crater doublet. Multiple dating campaigns have

72 established a robust Miocene 40 Ar/ 39 Ar age of 14.83 ± 0.15 Ma (Di Vincenzo and Skála, 2009; recalculated by

73 Jourdan et al., 2012) for the Ries crater. In contrast to a well-established isotopic age data set for the Ries, the nearby

74 Steinheim Basin has failed to yield any reliable isotopic dating results so far ( 40 Ar/ 39 Ar and (U-Th)/He dating by our 75 group; Buchner et al., 2011). No coherent Steinheim impact ejecta are preserved outside the crater that could be

76 correlated with the Ries ejecta blanket. As a result, the assumed synchronicity of the Ries and Steinheim impacts

77 completely relies on the biostratigraphy of the Miocene post-impact crater lake sediments at both sites. However,

78 assuming that both crater lakes formed shortly after impact, the oldest known freshwater deposits of the Ries contain

79 fossil mammals of the Neogene mammal zone MN6 (Langhian), whereas no evidence for such fossils could be

80 observed at Steinheim. Instead, the lowermost Steinheim lake deposits are representative of the younger mammal

81 zone MN7 (Serravallian; Heizmann and Hesse, 1995; Heizmann and Reiff, 2002). Therefore, one must consider that

82 the Ries crater might be slightly older than the Steinheim Basin. Until such issues are resolved, it is unsafe to treat

83 these craters as a ‘proven ’ doublet based on their proximity alone.

84 Miljković et al. (2013) also listed the ~12 km Serra da Cangalha and ~4.5 km Riachão impact structures as a

85 ‘possible’ doublet. For both impact structures precise and accurate ages are currently lacking, and only a

86 stratigraphic maximum age of ≤250 Ma can be assigned for the Serra da Cangalha impact that affected the ~250-

87 260 Ma Permian sandstones of the Pedra de Fogo Formation in the Parnaíba Basin of Brazil (Kenkmann et al.,

88 2011). This formation also forms the youngest impact-deformed target rocks at Riachão (Maziviero et al., 2012).

89 Due to deep erosion of both impact structures, no post-impact deposits are preserved that could constrain a

90 minimum impact age; nor are melt lithologies known from either of these structures to provide material for isotopic

91 dating. Based on the poor stratigraphic age constraints, evidence for a crater doublet is far from proven.

92

93 Additional candidate doublets?

94 A recent addition to the list of potential impact doublets on Earth are the Paleozoic marine Lockne and Målingen

95 impact structures in central Sweden. The ~1 km Målingen structure, ~16 km southwest of the ≥7.5 km Lockne

96 impact structure, was confirmed as of impact origin after the release of the paper by Miljković et al. (2013). The

97 Målingen impact affected Ordovician orthoceratid limestones and produced a sequence of impactites and a resurge

98 breccia overlain by the post-impact Dalby Limestone – a pre- to post-impact sequence essentially identical with that

99 at Lockne (Alwmark et al., 2013). No isotopic age data are currently available for these two impact structures, but

100 high-resolution biostratigraphic dating plots the Lockne event in the Lagenochitina dalbyensis chitinozoan chron

101 (Middle-Late Sandbian; Grahn and Nõlvak, 1993), with a dating precision of ~1%. Despite the high meteorite influx 102 during the Middle and Late Ordovician as a consequence of the L-chondrite parent asteroid breakup ~470 Ma ago

103 (Schmitz et al., 2007) and the obvious temporal and spatial clustering of impacts in Baltoscandia (also including

104 Granby and Tvären in Sweden, as well as Kärdla and the ‘Osmussaar breccia’ in Estonia), most of these impact

105 structures and deposits have slightly different biostratigraphic ages (Alwmark et al., 2010). However, neither of the

106 post-impact sequences at Lockne and Målingen seems to contain a separate impact ejecta layer that could indicate a

107 (slightly) younger impact event close-by, which may be further sedimentologic evidence for two synchronous

108 impacts within the same chitinozoan biozone. An Ordovician double impact event in Baltica at ~455 Ma is,

109 therefore, reasonably likely, and so far has more solid constraints than the list used by Miljković et al. (2013).

110 The ~4 km Suvasvesi North and South impact structures in Paleoproterozoic crystalline rocks of Finland are

111 generally perceived as a crater doublet bona fide (Werner et al., 2002). Although the Suvasvesi South impact

112 structure is currently n ot listed in the EID and thus not mentioned by Miljković et al. (2013), there is ample evidence

113 for impact (Donadini et al., 2006). The Suvasvesi structures are spaced at ~7 km from center to center and, thus,

114 probably overlap. Somewhat surprisingly, 40 Ar/ 39 Ar dating of impact melt rock chips recovered from the Suvasvesi

115 North drill core yielded a Cretaceous age of ~85 Ma (Schmieder et al., 2012), whereas a Proterozoic 40 Ar/ 39 Ar

116 (minimum) age of ≥700 Ma was obtained for a melt rock sample from Suvasvesi South (Buchner et al., 2009). We

117 propose that both craters were produced as a ‘false doublet’ by impacts far apart in time but, by pure coincidence,

118 extremely close in space.

119

120 Geochemical considerations

121 Finally, an additional variable not considered by Mil jković et al. (2013) is the geochemical signature that, if

122 detectable, the asteroid left in impact-produced lithologies. Whereas melt rocks from the Clearwater East impact

123 structure carry a distinct chondritic signature, no clear meteoritic signal was detected at Clearwater West (Palme et

124 al., 1978). Similarly, no resolvable enrichment of impactor-derived siderophiles was found in the Ries glasses

125 (Schmidt and Pernicka, 1994), in contrast to a strong exotic Ni-Co contamination in melt particles from the

126 Steinheim Basin (Buchner and Schmieder, 2010). Although seemingly unlikely, these two examples suggest that

127 different meteorite types might have been involved in the postulated double impact scenarios. Nevertheless, we

128 acknowledge that the theoretical binary impact model still works if one considers a different composition of the 129 primary and secondary asteroid; the impact of Asteroid 2008 TC 3 – Almahata Sitta in Sudan on October 07, 2008

130 revealed that even single meteorite falls can involve different types of impactor material (Bischoff et al., 2010).

131 Extraterrestrial chromite grains in the resurge deposits of the Ordovician Lockne impact structure suggested an L-

132 chondritic impactor (e.g., Schmitz et al., 2007), whereas a detailed geochemical characterization of the impact

133 deposits at its possible twin crater, Målingen, is still outstanding (Alwmark et al., 2013). No detailed information on

134 the individual composition of primary and secondary asteroids is currently available for the ~30 known near-Earth

135 binaries in space (Delbo et al., 2011).

136

137 Re-assessment and conclusions

138 In summary, from a mainly geochronological point of view, several of the suggested terrestrial impact crater

139 doublets are associated with ambiguous ages and require further study. This particularly concerns the Clearwater

140 and Ries-Steinheim crater pairs tr aditionally considered as classical doublets by ‘general agreement’ , whereas

141 current isotopic and stratigraphic evidence suggests they are not. As the exact timing of impact is critical, only a

142 combination of accurate and precise impact ages, along with geologic and geochemical evidence, can ultimately

143 resolve the existence of terrestrial doublet craters in the geologic record. So far, the number of definitely proven

144 doublets on Earth is 0. With somewhat loose criteria, we can consider the Kamensk-Gusev and Lockne-Målingen

145 crater pairs as ‘ very likely’ doublets. Finally, we wish to comment on a statement by Miljković et al. (2013 ) on their

146 p.124: “ […] even with such a large separation and poorly constrained ages, this crater pair is in such close

147 proximity that it is statistically unlikely for them to be formed by two single impacts ”. Again, the two closely spaced

148 Suvasvesi impact structures demonstrate that ‘false doublets’ can occur by a freak of nature. Considering a growing

149 number of impact structures known on our planet, the slightly revised population of possible crater doublets (based

150 on analytical evidence) on Earth – currently between 0 and 2 out of ~130 impact structures >5 km in diameter – is

151 on the order of ≤ 1.5%, with major uncertainties. This being a complementary comment, we emphasize and

152 independently confirm the very interesting results presented by Miljković et al. (2013) that binary asteroid impacts

153 are insufficiently represented by ‘true’ terrestrial crater doublets.

154 155 Acknowledgements : We kindly thank two anonymous reviewers for their constructive comments and the Editor

156 Christophe Sotin for careful handling of the manuscript. Lauri Pesonen was involved in previous and ongoing dating

157 of the Suvasvesi impacts. M.S., M.T., W.H.S. and E.B. thank the Klaus Tschira Stiftung gGmbH for financial

158 support.

159

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