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Reply to comment on: ‘Planar lamellar substructures in quartz’
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Earth and Planetary ScienceLetters 125 (1994)479-481
Reply to comment on: 'Planar lamellar substructures in quartz' John B. Lyons, Charles B. Officer Earth Sciences Department, Dartmouth College, Fairchild Science Center, Hanover, NH 03755, USA
Received 18 February 1994; revision accepted 17 May 1994
Reimold points out, correctly, our technical error in listing maskelynite (isotropized feldspar) in Table 1, which deals only with quartz and its shocked products. We should point out another oversight, not picked up in proof reading. The captions in Plate 1, bottom, are reversed; Toba is on the lower left, and Finland on the lower right. We disagree with Reimold's charge that our main intention was to convey "a false impression; namely that studies of shock metamorphism are usually mainly restricted to observations on quartz", and that the paper "serves to discredit the optical criteria for the recognition of bona fide shock produced 'lamellae' in quartz." On the contrary, the purpose of the first half of our paper, of Table 1, the 2 figures and the 2 plates was to review, and not to discredit, the optical criteria (grain characteristics and orientation plots) useful for identifying shock features in quartz. We do go on, then, to point out that caution and a critical attitude are essential in attempting to apply these criteria to a given situation, because there are several sources of potential error in interpretation. Unfortunately, the useful criteria developed for the Canadian craters are not present at all impact sites, for reasons that are poorly understood, but are possibly related to such factors as prior or subsequent ther-
[uc]
real history, erosion level, etc. There is an obvious lack of consistency among natural and experimental phenomena, such as why orientation plots of quartz from nuclear underground tests (Fig. 2c) closely resemble plots from tectonically deformed quartz (Fig. la), but neither resemble orientation plots of the Canadian craters. We deliberately refrained from discussing other physical or mineralogical criteria of shock effects, beyond those in Table 1, in order to abbreviate the paper. Readers of this journal really do not need to be reminded by Reimold that "deformation effects are, wherever possible, studied in various minerals". If, as is contended by Reimold, PDFs produced in experimental studies are "identical and characteristic", it is curious that the ultimate experiment (i.e. a nuclear blast) does not reproduce the effects of a bolide impact. It is also well known that, in experimental studies of shocked quartzites [1], there are pronounced changes in both orientations and abundances of PDFs, depending upon ambient temperature, as well as unexplained anomalies such as the absence of the omega orientation so commonly observed in naturally shocked quartz. One therefore cannot expect that all observations on naturally shocked quartz, even at a given pressure, will yield consistently uniform results. This is also obvious from the fact that in fallout ejecta only a small proportion of quartz grains exhibit PDFs, which indi-
0012-821X/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0012-821X(94)00113-D
480
J.B. Lyons, C.B. Officer/Earth and Planetary Science Letters 125 (1994) 479-481
cates that stress distributions in shocked rocks are highly anisotropic on the scale of the individual grains. We have used the example of Meteor Crater to point out sampling problems, even in locations where the evidence of a bolide impact is overwhelming. Our example cited 2 different investigations of quartz lamellae, one involving 54 sets and the other 1756 sets, but both yielding different lamellae orientations and neither finding the common omega orientation. It is pointed out by Reimold that Kieffer et al. [2] subsequently found rare omega orientations in their few Meteor Crater samples, which simply confirms our conclusion that there are, indeed, inconsistencies in lamellar orientations and abundances in the fallout ejecta at a given impact site. The situation is even more enigmatic at sites such as at Vredefort, where Reimold [3] recognized planar features in quartz "similar to those found in tectonic settings", and Reimold and Horz [4] also reported histograms of planar microdeformation features which look "tectonic". On the other hand, Grieve et al. [5] believed they were suggestive of an impact origin. Despite reassurances that "deformation effects are, wherever possible, studied in various minerals, and ... full consideration of the regional and local geological situation are the rule, not the exception "it is ironic that Reimold himself totally ignores these guidelines, because they are extremely inconvenient to his discussion of the controversial Chicxulub site, where the best geological information currently available [6,7] - Yucatan No. 6 well log - - shows andesite and limestone, locally interbedded, extending downward through the Upper Cretaceous into the Turonian. No one, to our knowledge, has presented evidence negating the geological facts of the drill core. The mineralogical evidence consists of photomicrographs of rare quartz grains showing PDFs, with no orientation data, which he accepts as shock-generated, but which we believe are more likely tectonic. As a clincher, he quotes the recent work of Kamo et al. [8] who report shocked zircon at the well-known K - T site in the Raton Basin of Colorado. That paper, and Reimold, then indulge in a fine example of circu-
lar reasoning when they ascribe the source of the zircon to Chicxulub, for which there is absolutely no evidence, whatever the age. Afficionados of shock events and locations should, perhaps, be reminded of the formerly much-publicised and quoted K - T connection between Manson, Iowa and the Raton Basin, a tie-in which has now been thoroughly demolished on the basis of new age dating. It is useful to recall that shocked quartz grains have also been reported [9,10] in parts of the stratigraphic column where there is no known likelihood of an impact event, or where the only evidence for such an event consists of the PDFs themselves. A new, but undocumented assertion by Reimold is that, contrary to Table 1, "relative abundances of specific orientations" of PDFs is relatively unimportant. Despite the clear evidence presented in our paper that there are, indeed, PDFs of endogenous terrestrial origin, we also learn that "to date nobody has been able to demonstrate that PDFs have ever been formed by internal (volcanic or tectonic) processes", and that our prior proposal that the Beloc, Haiti deposits (irrelevant to this discussion) are probably volcanic is absurd. Toba is a clear example of volcanic PDFs, and Hanson and Toft [11] have recently reported PDFs in quartz from Paleocene ash layers in Denmark; they have also commented on the fact that there are, in the literature, many examples of shocked quartz in clastic sedimentary rocks of different geological ages and of no known impact source. An examination of our plate 1B for Finnish quartzites clearly shows Reimold's error in asserting that all tectonic lamellae are non-planar. All of his negative pronouncements have been made without discussion or consideration of the relevant data, so we assume they should fall under the heading of predisposition, rather than of science. The long and irrelevant digression into the significance of lechatelierite and thetomorphic quartz implies, perhaps not surprisingly, that he missed the entire point of our discussion of the K6fels, Austria landslide. It was crystal clear that we did not support a bolide-impact origin for this unusual deposit, but were summarizing some of
J.B. Lyons, C.B. Officer/Earth and Planetary Science Letters 125 (1994) 479-481
the several criteria, including PDFs in quartz, advanced by proponents of that theory. Of direct relevance to our paper, as recently as 1988, PDFs observed in quartz from the landslide [12] were used to support the bolide theory for K6fels. We illustrated (Plates 3c, 3d) that the quartz lamellae were both discontinuous and lacked 'planarity' (to borrow Reimold's term) and therefore could not be 'shocked' quartz. Hence, the identification of PDFs reported in the literature may, on occasion, be misleading and erroneous. So, despite the complaint that our paper "unjustifiably belittles the value of optical recognition of bona fide shock-produced PDFs", we believe that misinterpretations are not at all unlikely, that a critical attitude is necessary, and that other criteria, such as TEM, may be necessary to resolve uncertainties in interpretation. We stand by those conclusions.
References [1] A.R. Huffman, Shock deformation and volcanism across the Cretaceous-Tertiary transition, Ph D. Thesis, Dep. of Geophysics, Texas A. & M. Univ., 346 pp., 1990. [2] S.W. Kieffer, P.R. Phakey and J.M. Christie, Shock processes in porous quartzite: transmission electron microscope observations and theory, Contrib. Mineral. Petrol. 59, 41-93, 1976.
481
[3] W.U. Reimold, Shock experiments with preheated Witwatersrand quartzite and the Vredefort controversy, Lunar Planet. Sci. Conf. Abstr. 19, 970-971, 1988. [4] W.U. Reimold and F. Horz, Textures of experimentally shocked (5.1-35.5 GPa) Witwatersrand quartzite, Lunar Planet. Sci. Conf. Abstr. 17, 704-704, 1986. [5] R.A.F. Grieve, J.M. Corderre, P.B. Robertson and J. Alexopoulos, Microscopic planar deformation features in quartz at the Vredefort structure: anomalous but still suggestive of an impact origin, Tectonophysics 171, 185200, 1990. [6l C.B. Officer, C.L. Drake, J.L. Pindell and A.A. Meyerhoff, Cretaceous-Tertiary events and the Caribbean caper, GSA Today 2, 69-75, 1992. [7] A.A. Meyerhoff, J.B. Lyons, and C.B. Officer, Chicxulub structure: a volcanic sequence of Late Cretaceous age, Geology 22, 3-4, 1994. [8] S.L. Kamo, T.E. Krough and B.H. Bohor, U - P b dating of single shocked zircon: a method for locating the target site and determining the time of impact, Geol. Soc. Am., Abstr. Progr. 25, A-319, 1993. [9] G.A. Izett, The Cretaceous/Tertiary boundary interval, Raton Basin, Colorado and New Mexico, and its content of shock-metamorphosed minerals; Evidence relevant to the K / T boundary impact-extinction theory, Geol. Soc. Am. Spec. Pap. 249, 1-100, 1990. [10] D.M. Bice, C.R. Newton, S. McCauley, F.W. Reiners and C.A. McRoberts, Shocked quartz at the Triassic-Jurassic boundary in Italy, Science 255, 443-446, 1992. [11] H.J. Hanson and P. Toft, Criteria for meteoritic impact at the K-T boundary, in: Global Boundary Events; An Interdisciplinary Conference, Kielce, Poland, IUGPIUGS, C-2, 1993. [12] A. Gratz and G. Kurat, Good news from Koefels, Austria: abundant lamellae in quartz, Bet. Geol. B.-A., 15, 9, 1988.
Earth and Planetary ScienceLetters 125 (1994)479-481
Reply to comment on: 'Planar lamellar substructures in quartz' John B. Lyons, Charles B. Officer Earth Sciences Department, Dartmouth College, Fairchild Science Center, Hanover, NH 03755, USA
Received 18 February 1994; revision accepted 17 May 1994
Reimold points out, correctly, our technical error in listing maskelynite (isotropized feldspar) in Table 1, which deals only with quartz and its shocked products. We should point out another oversight, not picked up in proof reading. The captions in Plate 1, bottom, are reversed; Toba is on the lower left, and Finland on the lower right. We disagree with Reimold's charge that our main intention was to convey "a false impression; namely that studies of shock metamorphism are usually mainly restricted to observations on quartz", and that the paper "serves to discredit the optical criteria for the recognition of bona fide shock produced 'lamellae' in quartz." On the contrary, the purpose of the first half of our paper, of Table 1, the 2 figures and the 2 plates was to review, and not to discredit, the optical criteria (grain characteristics and orientation plots) useful for identifying shock features in quartz. We do go on, then, to point out that caution and a critical attitude are essential in attempting to apply these criteria to a given situation, because there are several sources of potential error in interpretation. Unfortunately, the useful criteria developed for the Canadian craters are not present at all impact sites, for reasons that are poorly understood, but are possibly related to such factors as prior or subsequent ther-
[uc]
real history, erosion level, etc. There is an obvious lack of consistency among natural and experimental phenomena, such as why orientation plots of quartz from nuclear underground tests (Fig. 2c) closely resemble plots from tectonically deformed quartz (Fig. la), but neither resemble orientation plots of the Canadian craters. We deliberately refrained from discussing other physical or mineralogical criteria of shock effects, beyond those in Table 1, in order to abbreviate the paper. Readers of this journal really do not need to be reminded by Reimold that "deformation effects are, wherever possible, studied in various minerals". If, as is contended by Reimold, PDFs produced in experimental studies are "identical and characteristic", it is curious that the ultimate experiment (i.e. a nuclear blast) does not reproduce the effects of a bolide impact. It is also well known that, in experimental studies of shocked quartzites [1], there are pronounced changes in both orientations and abundances of PDFs, depending upon ambient temperature, as well as unexplained anomalies such as the absence of the omega orientation so commonly observed in naturally shocked quartz. One therefore cannot expect that all observations on naturally shocked quartz, even at a given pressure, will yield consistently uniform results. This is also obvious from the fact that in fallout ejecta only a small proportion of quartz grains exhibit PDFs, which indi-
0012-821X/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0012-821X(94)00113-D
480
J.B. Lyons, C.B. Officer/Earth and Planetary Science Letters 125 (1994) 479-481
cates that stress distributions in shocked rocks are highly anisotropic on the scale of the individual grains. We have used the example of Meteor Crater to point out sampling problems, even in locations where the evidence of a bolide impact is overwhelming. Our example cited 2 different investigations of quartz lamellae, one involving 54 sets and the other 1756 sets, but both yielding different lamellae orientations and neither finding the common omega orientation. It is pointed out by Reimold that Kieffer et al. [2] subsequently found rare omega orientations in their few Meteor Crater samples, which simply confirms our conclusion that there are, indeed, inconsistencies in lamellar orientations and abundances in the fallout ejecta at a given impact site. The situation is even more enigmatic at sites such as at Vredefort, where Reimold [3] recognized planar features in quartz "similar to those found in tectonic settings", and Reimold and Horz [4] also reported histograms of planar microdeformation features which look "tectonic". On the other hand, Grieve et al. [5] believed they were suggestive of an impact origin. Despite reassurances that "deformation effects are, wherever possible, studied in various minerals, and ... full consideration of the regional and local geological situation are the rule, not the exception "it is ironic that Reimold himself totally ignores these guidelines, because they are extremely inconvenient to his discussion of the controversial Chicxulub site, where the best geological information currently available [6,7] - Yucatan No. 6 well log - - shows andesite and limestone, locally interbedded, extending downward through the Upper Cretaceous into the Turonian. No one, to our knowledge, has presented evidence negating the geological facts of the drill core. The mineralogical evidence consists of photomicrographs of rare quartz grains showing PDFs, with no orientation data, which he accepts as shock-generated, but which we believe are more likely tectonic. As a clincher, he quotes the recent work of Kamo et al. [8] who report shocked zircon at the well-known K - T site in the Raton Basin of Colorado. That paper, and Reimold, then indulge in a fine example of circu-
lar reasoning when they ascribe the source of the zircon to Chicxulub, for which there is absolutely no evidence, whatever the age. Afficionados of shock events and locations should, perhaps, be reminded of the formerly much-publicised and quoted K - T connection between Manson, Iowa and the Raton Basin, a tie-in which has now been thoroughly demolished on the basis of new age dating. It is useful to recall that shocked quartz grains have also been reported [9,10] in parts of the stratigraphic column where there is no known likelihood of an impact event, or where the only evidence for such an event consists of the PDFs themselves. A new, but undocumented assertion by Reimold is that, contrary to Table 1, "relative abundances of specific orientations" of PDFs is relatively unimportant. Despite the clear evidence presented in our paper that there are, indeed, PDFs of endogenous terrestrial origin, we also learn that "to date nobody has been able to demonstrate that PDFs have ever been formed by internal (volcanic or tectonic) processes", and that our prior proposal that the Beloc, Haiti deposits (irrelevant to this discussion) are probably volcanic is absurd. Toba is a clear example of volcanic PDFs, and Hanson and Toft [11] have recently reported PDFs in quartz from Paleocene ash layers in Denmark; they have also commented on the fact that there are, in the literature, many examples of shocked quartz in clastic sedimentary rocks of different geological ages and of no known impact source. An examination of our plate 1B for Finnish quartzites clearly shows Reimold's error in asserting that all tectonic lamellae are non-planar. All of his negative pronouncements have been made without discussion or consideration of the relevant data, so we assume they should fall under the heading of predisposition, rather than of science. The long and irrelevant digression into the significance of lechatelierite and thetomorphic quartz implies, perhaps not surprisingly, that he missed the entire point of our discussion of the K6fels, Austria landslide. It was crystal clear that we did not support a bolide-impact origin for this unusual deposit, but were summarizing some of
J.B. Lyons, C.B. Officer/Earth and Planetary Science Letters 125 (1994) 479-481
the several criteria, including PDFs in quartz, advanced by proponents of that theory. Of direct relevance to our paper, as recently as 1988, PDFs observed in quartz from the landslide [12] were used to support the bolide theory for K6fels. We illustrated (Plates 3c, 3d) that the quartz lamellae were both discontinuous and lacked 'planarity' (to borrow Reimold's term) and therefore could not be 'shocked' quartz. Hence, the identification of PDFs reported in the literature may, on occasion, be misleading and erroneous. So, despite the complaint that our paper "unjustifiably belittles the value of optical recognition of bona fide shock-produced PDFs", we believe that misinterpretations are not at all unlikely, that a critical attitude is necessary, and that other criteria, such as TEM, may be necessary to resolve uncertainties in interpretation. We stand by those conclusions.
References [1] A.R. Huffman, Shock deformation and volcanism across the Cretaceous-Tertiary transition, Ph D. Thesis, Dep. of Geophysics, Texas A. & M. Univ., 346 pp., 1990. [2] S.W. Kieffer, P.R. Phakey and J.M. Christie, Shock processes in porous quartzite: transmission electron microscope observations and theory, Contrib. Mineral. Petrol. 59, 41-93, 1976.
481
[3] W.U. Reimold, Shock experiments with preheated Witwatersrand quartzite and the Vredefort controversy, Lunar Planet. Sci. Conf. Abstr. 19, 970-971, 1988. [4] W.U. Reimold and F. Horz, Textures of experimentally shocked (5.1-35.5 GPa) Witwatersrand quartzite, Lunar Planet. Sci. Conf. Abstr. 17, 704-704, 1986. [5] R.A.F. Grieve, J.M. Corderre, P.B. Robertson and J. Alexopoulos, Microscopic planar deformation features in quartz at the Vredefort structure: anomalous but still suggestive of an impact origin, Tectonophysics 171, 185200, 1990. [6l C.B. Officer, C.L. Drake, J.L. Pindell and A.A. Meyerhoff, Cretaceous-Tertiary events and the Caribbean caper, GSA Today 2, 69-75, 1992. [7] A.A. Meyerhoff, J.B. Lyons, and C.B. Officer, Chicxulub structure: a volcanic sequence of Late Cretaceous age, Geology 22, 3-4, 1994. [8] S.L. Kamo, T.E. Krough and B.H. Bohor, U - P b dating of single shocked zircon: a method for locating the target site and determining the time of impact, Geol. Soc. Am., Abstr. Progr. 25, A-319, 1993. [9] G.A. Izett, The Cretaceous/Tertiary boundary interval, Raton Basin, Colorado and New Mexico, and its content of shock-metamorphosed minerals; Evidence relevant to the K / T boundary impact-extinction theory, Geol. Soc. Am. Spec. Pap. 249, 1-100, 1990. [10] D.M. Bice, C.R. Newton, S. McCauley, F.W. Reiners and C.A. McRoberts, Shocked quartz at the Triassic-Jurassic boundary in Italy, Science 255, 443-446, 1992. [11] H.J. Hanson and P. Toft, Criteria for meteoritic impact at the K-T boundary, in: Global Boundary Events; An Interdisciplinary Conference, Kielce, Poland, IUGPIUGS, C-2, 1993. [12] A. Gratz and G. Kurat, Good news from Koefels, Austria: abundant lamellae in quartz, Bet. Geol. B.-A., 15, 9, 1988.