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Comment on ‘Planar lamellar substructures in quartz’ by J.B. Lyons, C.B. Officer, P.E. Borella and R. Lahodynsky
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Earth and Planetary ScienceLetters 125 (1994)473-477
Comment on 'Planar lamellar substructures in quartz' by J.B. Lyons, C.B. Officer, P.E. Borella and R. Lahodynsky Wolf Uwe Reimold Economic Geology Research Unit, University of the Witwatersrand, P.O. Wits 2050, Johannesburg, 2001 South Africa
Received 29 November 1993; revision accepted 17 May 1994
Lyons et al. [1] draw attention to the fact that apparently several types of subplanar to planar 'lamellae' occur in naturally deformed quartz, some of which may be of shock (impact) origin and others of internal, high-strain rate-related, origin. Similar ideas have been expressed earlier, for example, by Reimold [2] or Brandl and Reimold [3]. Lyons et al. [1] suggest thorough transmission electron microscopic (TEM) analysis as a suitable method for further detailed characterization of these different types of microdeformations. This is undoubtedly worthy of support, as also discussed in the past [2]. It should be emphasized that, with the exception of a few early studies [e.g., 4], such detailed T E M investigations have only recently been undertaken [e.g., 5-7]. However, in the need for clarification and correction, several aspects of Lyons et al.'s paper need to be discussed further. Lyons et al. [1] set out to introduce a 'Proposed scheme for identification of shock-generated features in quartz'. Shatter cones, clearly a macroscopic deformation phenomenon, are listed, together with otherwise microscopic features. Their list (their Table 1) of characteristic shock metamorphic effects generated in quartz is also
[ul
incomplete (e.g., the stage of partial to complete isotropization was neglected). They do, however, list maskelynite (i.e., diaplectic feldspar glass) as a deformation effect in quartz, which is erroneous. This short table also conveys a false impression; namely, that studies of shock metamorphism are usually mainly restricted to observations on quartz, with perhaps some in feldspar. On the contrary, a shock metamorphic study is normally carried out on all minerals encountered in a possibly shocked rock or geological area (cf., for example, fig. 16 of [14] and fig. 3.6.4. of [24]). The next major part of Lyons et al.'s paper conveys the impression that it serves to discredit the optical criteria for the recognition of bona fide shock-produced 'lamellae' in quartz (they state, e.g., "Deductions of physical conditions of deformation from optical morphology, spacing, multiplicity and orientation information alone remain somewhat conjectural: full characterization by transmission electron microscopy (TEM) appears to be necessary" - - their abstract) (The term 'shock lamellae' has been abandoned, by the way, in favor of the term 'Planar Deformation Features', abbreviated PDF; see [8].) It is valid to stress the limitations of a direct comparison between deformation effects in naturally shockmetamorphosed rocks and experimentally produced shock metamorphism. However, the authors fail to acknowledge that the impact commu-
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nity is fully aware of this fact (as born out of numerous publications, e.g., [2] and references therein) and generally makes use of more evidence than just that based on possible shock metamorphic effects in quartz: deformation effects are, whenever possible, studied in various minerals and both macroscopic and microscopic evidence of shock, normally together with full consideration of the regional and local geological situation, are the rule, not the exception. Lyons et al. then focus on the differences between shock metamorphic effects generated in single-crystal shock experiments and those produced in polycrystalline (and in addition, in polycrystalline a n d polymineralic!) targets, which are well documented (e.g., patterns of crystallographic orientations or the number of sets of PDFs at specific shock pressure levels; compare [9-12]). It would have been helpful to admit that the optical criteria of PDFs formed in either type of shock experiment are identical and characteristic, and in no way could these shock features be misidentified as anything else but shock-produced PDFs on the basis of their appearance. The criteria for the recognition of PDFs, as set out by Alexopoulos et al. [13] [see also 14-16], have always been sufficient to allow recognition of PDFs in experimentally shocked materials. On page 436, Lyons et al. discuss a 'high level of uncertainty in any interpretation of origin or inferred pressure' in cases where 'there are few or no orientation data' or only a 'few quartz grains with interesting lamellar structures are observed'. This statement diminishes the value of recognition of bona fide PDFs by standard optical methods. Undoubtedly, the aspect raised by Lyons et al. is potentially important, particularly where deeply eroded structures are concerned or only little exposure can be studied. Nevertheless, it has been possible to identify - - in favorable circumstances - - impact structures purely on the basis of recognition of shocked quartz. An example is the Kalkkop impact crater in South Africa [17], where little other, corroborating evidence, apart from the recognition of a few quartz grains with PDFs, was available. An impact origin for Kalkkop was later independently confirmed by R e - O s isotopic analysis [18]. It may be prudent,
however, to attempt to corroborate any identification of shocked quartz through findings of other characteristic shock metamorphic effects (such as diaplectic glasses, high-pressure polymorphs, the melting of minerals and mineral assemblages) or other macroscopic (the occurrence of impact breccias), chemical or geological evidence. In addition, quantitative measurements of density, refractive index and unit cell parameters, and studies of optical and X-ray mosaicism may assist in identification of shocked silicates and their transformation phases, as well as in the determination of shock pressure levels. In old and large, deeply eroded or tectonically overprinted structures the recognition of PDFs may be difficult, perhaps even impossible. This is especially true in cases where thermal overprinting has taken place. A case in point is the Vredefort Structure, where subplanar and planar microdeformations in quartz have been controversial for a long time [2]. Even stern impact supporters [19] attested their anomalous nature. Only the most recent TEM characterization of 'planar features' from Vredefort identified their true nature and likely shock (impact) origin [20]. Lyons et al. [1] proceed to discuss a 'problem of sampling and the scale of observation'. They refer to Meteor Crater studies that allegedly failed to provide evidence for {1013} and {1012} 'lamellar substructures' in quartz, that was, however, shocked strongly enough to be partially converted to the high-pressure polymorphs coesite and stishovite. It is implied that statistics of crystallographic orientations of PDFs may be inaccurate and unreliable as shock (impact) indicators. It needs to be pointed out to the authors that Kieffer et al. [4] did indeed observe 'lamellae' parallel to {1013} and {1012} [4, p. 70, bottom; fig. 12, p. 64] in shocked Coconino Sandstone from Meteor Crater. While I agree that a sufficiently large sample of PDF orientations is always desirable, I must object to the conclusion that "statistical reliability can be assumed for only a few of the known or proposed bolide impact sites, so interpretation and speculation dominate most discussions of enigmatic structures". This not only, unjustifiably, belittles the value of optical recognition of bona fide shock-produced PDFs,
W.U. Reimold / Earth and Planetary Science Letters 125 (1994) 473-477
but also of all other mineralogical, chemical or geological evidence usually presented as evidence of impact origin. H e r e it is also of interest that P D F orientation studies on shocked quartz from the Pretoria Saltpan impact crater, a structure of very similar dimensions to Meteor Crater, yielded shock-characteristic P D F orientations, such as w or ~-, but these orientations are not dominant in the overall distribution pattern, with a number of minor maxima between 0 and 80 ° to the c axis [20]. Small impact structures, formed in, relatively, less energetic events, generally display little evidence for shock metamorphism in rim or basement rocks and P D F statistics may be different from those determined for shocked rocks from larger structures. Distribution of shock pressure in time (shock pulse duration) and space, fabrics inherent to the target rocks (H6rz [21] has documented different P D F statistics for quartz shocked parallel to either [0001] or [1010]), target rock stratigraphy, as well as target temperature [2,12,22] are only some of the parameters that may strongly influence P D F orientation statistics. It is the identification of bona fide PDFs at o~, ~-, ~ and ~-, z orientations that is the important criterion for the recognition of shock deformation not the relative abundances of specific orientations. In the light of the fact that, to date, nobody has been able demonstrate that PDFs have ever been formed by internal (volcanic or tectonic) processes (contrary to the absurd and unfounded claims by Lyons and Officer [29] - see as well the relevant discussion in [30]), the statement by Meyerhoff, Lyons and Officer [31] that "certain forms and angular distributions (of P D F orientations, such as the one shown by [32]) were characteristic of tectonic or volcanic lamellae" is preposterous. Nobody with experience in shock metamorphism would doubt that the deformation lamellae from Chicxulub (such as those shown in [33]) are bona fide PDFs of impact origin. Besides, additional evidence in favor of impact at Chicxulub was provided by [34] in the form of shocked zircon - - again never produced from a volcanic or tectonic source. Lyons et al. [1] illustrate their argument that it is not valid for Preston's [23] Finnish quartzite -
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samples to state that 'tectonic lamellae do not always occur in straight intersecting sets' by reproducing a single photograph from Preston's paper. The conscientious reader of Preston's publication, who also studies the other photographs of deformation bands and lamellae in this socalled glassy quartzite, is in for a surprise: nobody with a knowledge of naturally or experimentally produced PDFs would claim that Preston's 'features' could be mistaken for PDFs. While rejecting Lyons et al.'s approach, I do not want to give the impression that 1 wholeheartedly defend Alexopoulos et al.'s criteria [13]. Quite the contrary, the application of these criteria to the microdeformations in Vredefort quartz failed [2], apparently because of the effects of post-impact recrystallization. Clearly, every occurrence of planar and subplanar 'features' needs to be judged on its own merits. The indiscriminate application of a generalized scheme, such as that of Alexopoulos et al., ought to be avoided. It is always important to collect a comprehensive data set, including description and orientation data for any 'features', as well as fabric-related information and regional tectonic data. Lyons et al.'s point that not only tectonic lamellae, but also 'shock lamellae' formation is subject to potential interference by local stress field(s) and mineralogical influences is well taken - - and has been noted by many impact workers in the past. Unfortunately, reports of so-called impact structures that do not follow this critical attitude still appear occasionally [e.g., 25]. To criticize individual optical criteria for PDFs, as in Lyons et al.'s discussion of the Finnish and the Tapeats lamellae, is not appropriate because: (1) Alexopoulos et al. did not define their criteria very restrictively - - they did allow for some variability of optical characteristics; and (2) more importantly, it is the ouerall appearance of planar deformation features, not their consistence with one or two of Alexopoulos et al.'s criteria, that may lead to the recognition of characteristic shock effects. (Incidentally, what is the meaning of the term 'well-shocked' quartz?) Lyons et al. conclude (p. 438) that "multiple intersecting lamellae are not unique to shocked quartz, nor is a lamellar spacing in the range 5
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~ m " . However, this conclusion is clearly unjustified, as it is based on a case of non-planar deformation lamellae, and the foremost criterion for the recognition of P D F s is that of 'planarity'. All their examples of tectonic m i c r o d e f o r m a t i o n s in quartz lack this important criterion, which renders m u c h of their discussion obsolete. W h a t is more, no w e l l - d o c u m e n t e d occurrences of tectonically p r o d u c e d multiple sets of P D F s with orientation statistics similar to those p r o d u c e d in shock experiments and observed in impact structures have ever, and also not by Lyons et al., b e e n documented. Two misconceptions have crept into their section on d e f o r m a t i o n associated with the K6fels landslide, which is not accepted as being of impact origin [26]: (1) T h e t o m o r p h i c quartz and lechatelierite are offered as evidence for the shock-triggered origin of this landslide. However, the existence of ' t h e t o m o r p h i c quartz' (a term sometimes used as a synonym for 'diaplectic quartz glass', see, e.g., [27, pp. 194 and 736]) has not b e e n confirmed for the K6fels case. Lechatelierite is defined as 'naturally fused a m o r p h o u s silica' [e.g., 27, p. 402], a silica glass f o r m e d at high temperatures. This definition does not contain a genetic link to shock processes, and other natural processes, such as frictional melting, could lead to the formation of such glass. Indeed, an origin by frictional melting is widely accepted for the occurrences of aphanitic material (pseudotachylite?) at K6fels [6,26]. (2) T h e possibility that pseudotachylite could be indicative of shock p r o d u c e d by bolide impact is far fetched and should not be spread in the literature, in the light of the n u m e r o u s occurrences worldwide of pseudotachylite of unambiguous tectonic origin. A recent T E M study of planar ' f e a t u r e s ' in K6fels quartz did not yield any evidence in favor of impact-triggered deformation [6]. It is, therefore, not valid to use the case of a structure of doubtful origin to cast doubt on o t h e r proven cases. Unfortunately, the whole article by Lyons et al. [1] suggests that recognition of P D F s of shock (impact) origin by normal optical m e t h o d s is not possible. I doubt that anybody in the planetological c o m m u n i t y has been impressed by Lyons et
al.'s one-sided approach, because the cautious impact worker (always wary of opening a new can of worms, such as the V r e d e f o r t controversy or the K / T b o u n d a r y debate) will look for other, i n d e p e n d e n t confirmation of a possible impact origin; for example, in the form of other characteristic shock m e t a m o r p h i c effects, of fragmental or melt breccias that could be explained as impact formations, shatter cones (that most, but not all, workers accept as unequivocal impact indicators), or geochemical evidence. Finally, I agree with Lyons et al. that where only unsatisfactory optical evidence is available, T E M techniques may prove very helpful in further constraining the nature of planar deformation effects in silicates, as has b e e n d e m o n s t r a t e d just recently [5-7].
Acknowledgement C o m m e n t s by Christian Koeberl and by an a n o n y m o u s referee improved the manuscript.
References [1] J.B. Lyons, C.B. Officer, P.E. Borella and R. Lahodynsky, Planar lamellar substructures in quartz, Earth Planet. Sci. Lett. 119, 431-440, 1993. [2] W.U. Reimold, The controversial microdeformations in quartz from the Vredefort structure, South Africa, S. Afr. J. Geol. 93, 645-663, 1990. [3] G. Brandl and W.U. Reimold, The structural setting and deformation associated with pseudotachylite occurrences in the Palala Shear Belt and Sand River Gneiss, Northern Transvaal, Tectonophysics 171,201-220, 1990. [4] S.W. Kieffer, P.P. Phakey and J.M. Christie, Shock processes in porous quartzite: Transmission electron microscope observations and theory, Contrib. Mineral. Petrol. 59, 41-93, 1976. [5] O. Goltrant, P. Cordier and J.-C. Doukhan, Planar deformation features in shocked quartz: a transmission electron microscopy investigation, Earth Planet. Sci. Lett. 106, 103 115, 1991. [6] H. Leroux and J.-C. Doukhan, Dynamic deformation of quartz in the landslide of K6fels, Austria, Eur. J. Mineral. 5, 893-902, 1993. [7] H. Leroux, W.U. Reimold and J.-C. Doukhan, A T.E.M. investigation of shock metamorphism in quartz from the Vredefort dome, South Africa, Tectonophysics 230, 223239.
W..U. Reimold / Earth and Planetary Science Letters 125 (1994) 473-477 [8] R.A.F. Grieve, V.L. Sharpton and D. St6ffier, Shocked minerals and the K / T controversy, LOS Trans. Am. Geophys. Union 71, 1792, 1990. [9] C.B. Officer and N.L. Carter, A review of the structure, petrology and dynamic deformation characteristics of some enigmatic terrestrial structures, Earth Sci. Rev. 30, 1-49, 1991. [10] A.R. Huffman, Shock deformation and volcanism across the Cretaceous-Tertiary transition, Ph.D. Thesis, Texas A&M Univ., College Station, Tex., 1990. [11] W.U. Reimold and F. Horz, Textures of experimentally shocked (5.1-35.5 GPa) Witwatersrand quartzite, Lunar Planet. Sci. 17, 703-704, 1986. [12] W.U. Reimold, Shock experiments with preheated Witwatersrand quartzite and the Vredefort microdeformation controversy, Lunar Planet. Sci. 19, 970-971, 1988. [13] J.S. Alexopoulos, R.A.F. Grieve and P.B. Robertson, Microscopic lamellar deformation features in quartz: discriminative characteristics of shock generated varieties, Geology 16, 796-799, 1988. [14] D. St6ffier, Deformation and transformation of rock forming minerals by natural and experimental shock processes. I. Behavior of minerals under shock compression, Fortschr. Mineral. 49, 50-113, 1972. [15] W. Von Engelhardt and W. Bertsch, Shock induced planar deformation structures in quartz from the Ries crater, Germany, Contrib. Mineral. Petrol. 20, 203-234, 1969. [16] B.M. French and N.M. Short, eds., Shock Metamorphism of Natural Materials, 644 pp., Mono Book, Baltimore, 1968. [17] W.U. Reimold, F.G. Le Roux, C. Koeberl and S.B. Shirey, Kalkkop Crater, Eastern Cape - - A new impact crater in South Africa, Lunar Planet. Sci. 24, 1197-1198, 1993. [18] C. Koeberl, W.U. Reimold, S.B. Shirey and F.G. Le Roux, Kalkkop Crater, Cape Province, South Africa: Confirmation of impact origin using osmium isotope systematics, Geochim. Cosmochim. Acta 58, 1229-1234. [19] R.A.F. Grieve, J.M. Coderre, P.B. Robertson and J. Alexopoulos, Microscopic planar deformation features in quartz of the Vredefort structure: Anomalous but still suggestive of an impact origin, Tectonophysics 171, 185200, 1990. [20] D. Brandt, Structural and petrological investigations of the Pretoria Saltpan impact crater and surrounding area. M.Sc. Thesis, Univ. Witwatersrand (unpubl.), 1994. [21] F. H6rz, Statistical measurements of deformation structures and refractive indices in experimentally shock
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loaded quartz, in: Shock Metamorphism of Natural Materials, B.M. French and N.M. Short, eds., pp. 243-253, Mono Book, Baltimore, 1968. [22] F. Langenhorst, A. Deutsch, D. St6ffier and U. Hornemann, Effect of temperature on shock metamorphism of single-crystal quartz, Nature 356, 507-509, 1992. [23] J. Preston, Quartz lamellae in some Finnish quartzites, Bull. Comm. Geol. Finl. 180, 65-78, 1958. [24] D. St6ffier, A. Bischoff, V. Buchwald and A.E. Rubin, Shock effects in meteorites, in: Meteorites and the Early Solar System, J.F. Kerridge and M.S. Matthews, eds., pp. 165-202, Univ. Arizona Press, 1988. [25] J.-F. Becq-Giraudon, O. Rouzeau, E. Goachet and S. Solages, Impact hyperveloce d'une meteorite geante a l'origine de la depression circulaire d'Aorounga au Tchad (Afrique), C.R. Acad. Sci. Paris Set. II 315, 83-88, 1992. [26] R.A.F. Grieve, Terrestrial impact: The record in the rocks, Meteoritics 26, 175-194, 1991. [27] M. Gary, R. McAfee Jr. and C.L. Wolf, eds., Glossary of Geology, 805 pp. + Bibliography, Am. Geol. Inst., Washington, 1972. [28] H. Heuberger, L. Masch, E. Preuss and A. Schrocker, Quaternary landslides and rock fusion in Central Nepal and in the Tyrolean Alps, Mt. Res. Develop. 4, 345-362, 1984. [29] J.B. Lyons and C.B. Officer, Mineralogy and petrology of the Haiti Cretaceous/Tertiary section, Earth Planet. Sci. Lett. 109, 205-224, 1992. [30] D.A. Kring, The Chicxulub impact event and possible causes of K / T boundary extinctions, in: Proc. 1st Annu. Symp. of Fossils of Arizona, D. Boaz and M. Dornan, eds., pp. 63-79, Mesa Southwest Museum and Southwest Paleontolog. Soc., 1993. [31] A.A. Meyerhoff, J.B. Lyons and C.B. Officer, Chicxulub structure: A volcanic structure of Late Cretaceous age, Geology 22, 3-4, 1994. [32] V.L. Sharpton, G.B. Dalrymple, L.E. Marin, G. Ryder, B.C. Schuraytz and J. Urrutia-Fucugauchi, New links between the Chicxulub impact structure and the Cretaceous/Tertiary boundary, Nature 359, 819-821, 1992. [33] A.R. Hildebrand, G.T. Penfield, D.A. Kring, M. Pilkington, A. Camargo, S.B. Jacobson and W.V. Boynton, Chicxulub Crater: A possible Cretaceous/Tertiary boundary impact crater on the Yucatan Peninsula, Mexico, Geology 19, 867-871, 1991. [34] T.E. Krogh, S.L. Kamo, V.L. Sharpton, L.E. Marin and A.R. Hildebrand, U - P b ages of single shocked zircons linking distal K / T ejecta to the Chicxulub crater, Nature 366, 731-734.
Earth and Planetary ScienceLetters 125 (1994)473-477
Comment on 'Planar lamellar substructures in quartz' by J.B. Lyons, C.B. Officer, P.E. Borella and R. Lahodynsky Wolf Uwe Reimold Economic Geology Research Unit, University of the Witwatersrand, P.O. Wits 2050, Johannesburg, 2001 South Africa
Received 29 November 1993; revision accepted 17 May 1994
Lyons et al. [1] draw attention to the fact that apparently several types of subplanar to planar 'lamellae' occur in naturally deformed quartz, some of which may be of shock (impact) origin and others of internal, high-strain rate-related, origin. Similar ideas have been expressed earlier, for example, by Reimold [2] or Brandl and Reimold [3]. Lyons et al. [1] suggest thorough transmission electron microscopic (TEM) analysis as a suitable method for further detailed characterization of these different types of microdeformations. This is undoubtedly worthy of support, as also discussed in the past [2]. It should be emphasized that, with the exception of a few early studies [e.g., 4], such detailed T E M investigations have only recently been undertaken [e.g., 5-7]. However, in the need for clarification and correction, several aspects of Lyons et al.'s paper need to be discussed further. Lyons et al. [1] set out to introduce a 'Proposed scheme for identification of shock-generated features in quartz'. Shatter cones, clearly a macroscopic deformation phenomenon, are listed, together with otherwise microscopic features. Their list (their Table 1) of characteristic shock metamorphic effects generated in quartz is also
[ul
incomplete (e.g., the stage of partial to complete isotropization was neglected). They do, however, list maskelynite (i.e., diaplectic feldspar glass) as a deformation effect in quartz, which is erroneous. This short table also conveys a false impression; namely, that studies of shock metamorphism are usually mainly restricted to observations on quartz, with perhaps some in feldspar. On the contrary, a shock metamorphic study is normally carried out on all minerals encountered in a possibly shocked rock or geological area (cf., for example, fig. 16 of [14] and fig. 3.6.4. of [24]). The next major part of Lyons et al.'s paper conveys the impression that it serves to discredit the optical criteria for the recognition of bona fide shock-produced 'lamellae' in quartz (they state, e.g., "Deductions of physical conditions of deformation from optical morphology, spacing, multiplicity and orientation information alone remain somewhat conjectural: full characterization by transmission electron microscopy (TEM) appears to be necessary" - - their abstract) (The term 'shock lamellae' has been abandoned, by the way, in favor of the term 'Planar Deformation Features', abbreviated PDF; see [8].) It is valid to stress the limitations of a direct comparison between deformation effects in naturally shockmetamorphosed rocks and experimentally produced shock metamorphism. However, the authors fail to acknowledge that the impact commu-
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nity is fully aware of this fact (as born out of numerous publications, e.g., [2] and references therein) and generally makes use of more evidence than just that based on possible shock metamorphic effects in quartz: deformation effects are, whenever possible, studied in various minerals and both macroscopic and microscopic evidence of shock, normally together with full consideration of the regional and local geological situation, are the rule, not the exception. Lyons et al. then focus on the differences between shock metamorphic effects generated in single-crystal shock experiments and those produced in polycrystalline (and in addition, in polycrystalline a n d polymineralic!) targets, which are well documented (e.g., patterns of crystallographic orientations or the number of sets of PDFs at specific shock pressure levels; compare [9-12]). It would have been helpful to admit that the optical criteria of PDFs formed in either type of shock experiment are identical and characteristic, and in no way could these shock features be misidentified as anything else but shock-produced PDFs on the basis of their appearance. The criteria for the recognition of PDFs, as set out by Alexopoulos et al. [13] [see also 14-16], have always been sufficient to allow recognition of PDFs in experimentally shocked materials. On page 436, Lyons et al. discuss a 'high level of uncertainty in any interpretation of origin or inferred pressure' in cases where 'there are few or no orientation data' or only a 'few quartz grains with interesting lamellar structures are observed'. This statement diminishes the value of recognition of bona fide PDFs by standard optical methods. Undoubtedly, the aspect raised by Lyons et al. is potentially important, particularly where deeply eroded structures are concerned or only little exposure can be studied. Nevertheless, it has been possible to identify - - in favorable circumstances - - impact structures purely on the basis of recognition of shocked quartz. An example is the Kalkkop impact crater in South Africa [17], where little other, corroborating evidence, apart from the recognition of a few quartz grains with PDFs, was available. An impact origin for Kalkkop was later independently confirmed by R e - O s isotopic analysis [18]. It may be prudent,
however, to attempt to corroborate any identification of shocked quartz through findings of other characteristic shock metamorphic effects (such as diaplectic glasses, high-pressure polymorphs, the melting of minerals and mineral assemblages) or other macroscopic (the occurrence of impact breccias), chemical or geological evidence. In addition, quantitative measurements of density, refractive index and unit cell parameters, and studies of optical and X-ray mosaicism may assist in identification of shocked silicates and their transformation phases, as well as in the determination of shock pressure levels. In old and large, deeply eroded or tectonically overprinted structures the recognition of PDFs may be difficult, perhaps even impossible. This is especially true in cases where thermal overprinting has taken place. A case in point is the Vredefort Structure, where subplanar and planar microdeformations in quartz have been controversial for a long time [2]. Even stern impact supporters [19] attested their anomalous nature. Only the most recent TEM characterization of 'planar features' from Vredefort identified their true nature and likely shock (impact) origin [20]. Lyons et al. [1] proceed to discuss a 'problem of sampling and the scale of observation'. They refer to Meteor Crater studies that allegedly failed to provide evidence for {1013} and {1012} 'lamellar substructures' in quartz, that was, however, shocked strongly enough to be partially converted to the high-pressure polymorphs coesite and stishovite. It is implied that statistics of crystallographic orientations of PDFs may be inaccurate and unreliable as shock (impact) indicators. It needs to be pointed out to the authors that Kieffer et al. [4] did indeed observe 'lamellae' parallel to {1013} and {1012} [4, p. 70, bottom; fig. 12, p. 64] in shocked Coconino Sandstone from Meteor Crater. While I agree that a sufficiently large sample of PDF orientations is always desirable, I must object to the conclusion that "statistical reliability can be assumed for only a few of the known or proposed bolide impact sites, so interpretation and speculation dominate most discussions of enigmatic structures". This not only, unjustifiably, belittles the value of optical recognition of bona fide shock-produced PDFs,
W.U. Reimold / Earth and Planetary Science Letters 125 (1994) 473-477
but also of all other mineralogical, chemical or geological evidence usually presented as evidence of impact origin. H e r e it is also of interest that P D F orientation studies on shocked quartz from the Pretoria Saltpan impact crater, a structure of very similar dimensions to Meteor Crater, yielded shock-characteristic P D F orientations, such as w or ~-, but these orientations are not dominant in the overall distribution pattern, with a number of minor maxima between 0 and 80 ° to the c axis [20]. Small impact structures, formed in, relatively, less energetic events, generally display little evidence for shock metamorphism in rim or basement rocks and P D F statistics may be different from those determined for shocked rocks from larger structures. Distribution of shock pressure in time (shock pulse duration) and space, fabrics inherent to the target rocks (H6rz [21] has documented different P D F statistics for quartz shocked parallel to either [0001] or [1010]), target rock stratigraphy, as well as target temperature [2,12,22] are only some of the parameters that may strongly influence P D F orientation statistics. It is the identification of bona fide PDFs at o~, ~-, ~ and ~-, z orientations that is the important criterion for the recognition of shock deformation not the relative abundances of specific orientations. In the light of the fact that, to date, nobody has been able demonstrate that PDFs have ever been formed by internal (volcanic or tectonic) processes (contrary to the absurd and unfounded claims by Lyons and Officer [29] - see as well the relevant discussion in [30]), the statement by Meyerhoff, Lyons and Officer [31] that "certain forms and angular distributions (of P D F orientations, such as the one shown by [32]) were characteristic of tectonic or volcanic lamellae" is preposterous. Nobody with experience in shock metamorphism would doubt that the deformation lamellae from Chicxulub (such as those shown in [33]) are bona fide PDFs of impact origin. Besides, additional evidence in favor of impact at Chicxulub was provided by [34] in the form of shocked zircon - - again never produced from a volcanic or tectonic source. Lyons et al. [1] illustrate their argument that it is not valid for Preston's [23] Finnish quartzite -
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samples to state that 'tectonic lamellae do not always occur in straight intersecting sets' by reproducing a single photograph from Preston's paper. The conscientious reader of Preston's publication, who also studies the other photographs of deformation bands and lamellae in this socalled glassy quartzite, is in for a surprise: nobody with a knowledge of naturally or experimentally produced PDFs would claim that Preston's 'features' could be mistaken for PDFs. While rejecting Lyons et al.'s approach, I do not want to give the impression that 1 wholeheartedly defend Alexopoulos et al.'s criteria [13]. Quite the contrary, the application of these criteria to the microdeformations in Vredefort quartz failed [2], apparently because of the effects of post-impact recrystallization. Clearly, every occurrence of planar and subplanar 'features' needs to be judged on its own merits. The indiscriminate application of a generalized scheme, such as that of Alexopoulos et al., ought to be avoided. It is always important to collect a comprehensive data set, including description and orientation data for any 'features', as well as fabric-related information and regional tectonic data. Lyons et al.'s point that not only tectonic lamellae, but also 'shock lamellae' formation is subject to potential interference by local stress field(s) and mineralogical influences is well taken - - and has been noted by many impact workers in the past. Unfortunately, reports of so-called impact structures that do not follow this critical attitude still appear occasionally [e.g., 25]. To criticize individual optical criteria for PDFs, as in Lyons et al.'s discussion of the Finnish and the Tapeats lamellae, is not appropriate because: (1) Alexopoulos et al. did not define their criteria very restrictively - - they did allow for some variability of optical characteristics; and (2) more importantly, it is the ouerall appearance of planar deformation features, not their consistence with one or two of Alexopoulos et al.'s criteria, that may lead to the recognition of characteristic shock effects. (Incidentally, what is the meaning of the term 'well-shocked' quartz?) Lyons et al. conclude (p. 438) that "multiple intersecting lamellae are not unique to shocked quartz, nor is a lamellar spacing in the range 5
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~ m " . However, this conclusion is clearly unjustified, as it is based on a case of non-planar deformation lamellae, and the foremost criterion for the recognition of P D F s is that of 'planarity'. All their examples of tectonic m i c r o d e f o r m a t i o n s in quartz lack this important criterion, which renders m u c h of their discussion obsolete. W h a t is more, no w e l l - d o c u m e n t e d occurrences of tectonically p r o d u c e d multiple sets of P D F s with orientation statistics similar to those p r o d u c e d in shock experiments and observed in impact structures have ever, and also not by Lyons et al., b e e n documented. Two misconceptions have crept into their section on d e f o r m a t i o n associated with the K6fels landslide, which is not accepted as being of impact origin [26]: (1) T h e t o m o r p h i c quartz and lechatelierite are offered as evidence for the shock-triggered origin of this landslide. However, the existence of ' t h e t o m o r p h i c quartz' (a term sometimes used as a synonym for 'diaplectic quartz glass', see, e.g., [27, pp. 194 and 736]) has not b e e n confirmed for the K6fels case. Lechatelierite is defined as 'naturally fused a m o r p h o u s silica' [e.g., 27, p. 402], a silica glass f o r m e d at high temperatures. This definition does not contain a genetic link to shock processes, and other natural processes, such as frictional melting, could lead to the formation of such glass. Indeed, an origin by frictional melting is widely accepted for the occurrences of aphanitic material (pseudotachylite?) at K6fels [6,26]. (2) T h e possibility that pseudotachylite could be indicative of shock p r o d u c e d by bolide impact is far fetched and should not be spread in the literature, in the light of the n u m e r o u s occurrences worldwide of pseudotachylite of unambiguous tectonic origin. A recent T E M study of planar ' f e a t u r e s ' in K6fels quartz did not yield any evidence in favor of impact-triggered deformation [6]. It is, therefore, not valid to use the case of a structure of doubtful origin to cast doubt on o t h e r proven cases. Unfortunately, the whole article by Lyons et al. [1] suggests that recognition of P D F s of shock (impact) origin by normal optical m e t h o d s is not possible. I doubt that anybody in the planetological c o m m u n i t y has been impressed by Lyons et
al.'s one-sided approach, because the cautious impact worker (always wary of opening a new can of worms, such as the V r e d e f o r t controversy or the K / T b o u n d a r y debate) will look for other, i n d e p e n d e n t confirmation of a possible impact origin; for example, in the form of other characteristic shock m e t a m o r p h i c effects, of fragmental or melt breccias that could be explained as impact formations, shatter cones (that most, but not all, workers accept as unequivocal impact indicators), or geochemical evidence. Finally, I agree with Lyons et al. that where only unsatisfactory optical evidence is available, T E M techniques may prove very helpful in further constraining the nature of planar deformation effects in silicates, as has b e e n d e m o n s t r a t e d just recently [5-7].
Acknowledgement C o m m e n t s by Christian Koeberl and by an a n o n y m o u s referee improved the manuscript.
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