Zones of Shock Metamorphism at the Charlevoix Impact Structure, Quebec

Zones of shock metamorphism at the Charlevoix impact structure, Quebec

P. B. ROBERTSON Earth Physics Branch, Department of Energy, Mines and Resources, Ottawa, Canada

ABSTRACT complex crater with an upraised central peak (Mont des Eboule- ments) and a peripheral depression 35 km in diameter, from which The distribution of particular shock metamorphic effects has the land rises to the surrounding regional elevations at 30 km from been determined in the central uplift of the Charlevoix impact the center (Fig. 1). The structure's meteoritic origin has been estab- structure. Planar deformation features in quartz occur as much as lished from abundant shatter cones, microscopic shock metamor- 10 km from the central peak (Mont des Eboulements), whereas phic features in breccia dikes and basement gneisses, and a small equivalent shock features in K-feldspar are restricted to within 2 remnant of impact melt (Rondot, 1966, 1968, 1969, 1970, 1971, km of the crater center. Weak planar features in K-feldspar are nar- 1972a, 1972b; Robertson, 1968; Roy, 1974). Impact occurred on row 1 /urn) but can become broad (4 to 8 ¿im) deformation a shelf-facies sequence of Middle Ordovician limestones overlying twins in_more highly shocked samples. Common orientations are crystalline Grenville age rocks on the margin of the Precambrian (241), (241), and (110) in orthoclase, and (131) and (110) in mic- Shield (Fig. 2). The crater's age has been established from K-Ar rocline, and the relative abundance of specific orientations does not ages of 321 to 372 m.y. for the impact melt and breccia dikes change with shock level. Film perthite lamellae in K-feldspar break (Rondot, 1971). down to spindle microperthite within 6 km of the center, either as a Charlevoix is the only meteorite crater of the 22 confirmed in result of shock or as a function of original depth of burial. Canada (Robertson and Grieve, 1975) where basement rocks can Shock pressures were estimated for Charlevoix samples by equat- be sampled in detail from the center outward into the unshocked ing observed planar feature development with experimental data. regional terrain. From a study of shock effects mainly in the For example, type A shocked quartz develops above 7.5 GNm~2, type B above 10 GNm-2, type C above 14 GNm-2, and type D above 16 GNm-2. Maximum shock levels preserved on the central peak resulted from an estimated 22.5 GNm-2. Shock level contours at 5-GNm-2 intervals are broadly concentric with Mont des Eboulements. The transient cavity of the Charlevoix impact was reconstructed with a radius of 13.5 ± 2 km and a depth of 9.5 ± 1.5 km (rV2). The distribution of peak shock pressure levels in basement rocks beneath the transient cavity was calculated from theoretical pressure attenuation rates. Using a model for central uplift formation, these rocks were elevated to positions underlying the central uplift. The resulting configuration of shock pressure zones agrees with the surface expression of shock zones mapped from planar features and confirms the validity of current theories of cratering and shock pressure attenuation. Key words: extraterrestrial geology, meteor craters, shock metamorphism, K-feldspar, quartz.

INTRODUCTION

In this study, shock metamorphic effects observed in a large meteorite impact structure are related to concepts proposed by shock-wave propagation theory and crater modeling studies. Ob- servations and measurements were made on particular shock effects in basement rocks of the central uplift at the Charlevoix impact structure, Quebec. Based on experimental data, estimates were made of shock pressures attained in this material, and contours of equal shock pressure were defined on the present crater topography. The surface distribution of shock pressure contours is compared with a model for the configuration of shock zones underlying the central uplift of a complex crater.

Figure 1. Location (inset) and topography of the Charlevoix impact CHARLEVOIX structure, Quebec. Shading lightens with each 1,000-ft (305-m) increase in elevation from sea level at the St. Lawrence River (STL) to more than 3,000 The Charlevoix impact structure lies on the north shore of the St. ft (915 m) in the northwest. ME=Mont des Eboulements, BP=Baie-St- Lawrence River, approximately 100 km from Quebec City. It is a Paul, LM=La Malbaie, IC=Ile aux Coudres.

Geological Society of America Bulletin, v. 86, p. 1630-1638, 7 figs., December 1975, Doc. no. 51202.

1630

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mation features in quartz and potassium feldspars satisfy these re- quirements, whereas shatter cones, kink bands in biotite and pyroxene, deformation twinning in ilmenite, and planar features in apatite, although present at Charlevoix, do not satisfactorily meet both constraints.

MICROSCOPIC SHOCK EFFECTS IN QUARTZ

Natural

Planar deformation features in quartz are the most widely ap- plicable indicators of shock metamorphism in that they are easily recognized and they form throughout the low-to-moderate pressure range. Details of their appearance, orientation, abundance, and conditions of formation in naturally shocked quartz have been provided by Carter (1968), Robertson and others (1968), En- gelhardt and Bertsch (1969), and Stoffler (1972). Planar deformation features in quartz at Charlevoix are identical in form and orientation to those described from other impact sites (Fig. 3). All sets are of the "decorated" variety, containing minute inclusions (Fig. 3b). In their weakest development, at 10 km from Mont des Eboulements, they occur in only a small percentage of quartz grains with no grain having more than one set of planes. Approaching the central peak, there is a progressive increase in both the percentage of grains with features and the number of sets per grain. Within approximately 4 km of Mont des Eboulements, all quartz grains contain planar features, some grains displaying as many as 8 to 10 Figure 2. Generalized geology of the Charlevoix region (after Hargraves sets (Table 1). and Roy, 1974, Fig. 1). Unit 1 = granitic gneiss, 2 = anorthosite, 3 = charnockitic series, 4 = fine-grained clastic rocks (C?), 5 = limestone (Ord.). Robertson and others (1968) showed that the initial develop- Dots represent samples examined on the universal stage, crosses are addi- ment or abundance of specific orientations of natural quartz planar tional samples studied on the flat stage. ME = Mont des Eboulements. deformation features characterize successive levels of shock metamorphism. In their terminology, in order of increasing pres- quartz-bearing rocks of the central uplift, Robertson (1968) sure, type A quartz contain^ one set only, which is parallel to c showed that degree of shock metamorphism decreases concentri- (0001); sets parallel to w (1013) define type B; (2241) forms in type cally outward from Mont des Eboulements. This study enlarges on C quartz; II (1012) typifies class D development. Several less com- that work, the samples being taken from all the quartz-bearing or mon orientations are found at C and D levels. At Charlevoix, potassic feldspar—bearing units within the crater (Fig. 2). measurements were made on the universal stage of the orientation The particular shock effects considered in this study were chosen and number of planar features for generally 25 quartz grains per because (a) they are relatively abundant and widespread at Char- sample, and each quartz grain with planar features was classified as levoix, and (b) they have been produced in experiments in which type A to D (Table 1). As localized increased or decreased stress can pressures for their formation have been determined. Planar defor- be produced from reverberations in grain corners or at interfaces

Figure 3. Planar deformation features in shocked quartz, a. Type B development. Scale bar represents 0.25 mm, crossed nicols. b. Minute voids or gas-filled cavities, "decorations," along planar features. Scale bar represents 10 /xm; crossed nicols.

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SHOCK PRESSURES BASED ON PLANAR DEFORMATION FEATURES IN QUARTZ AND POTASSIUM FELDSPARS

Quartz planar features Feldspar planar features

Sample Distance from Avg. shock Mont des• % with Avg. sets per Z Z z z Or-orthoclase % with Avg. sets per pressure Eboulements (km) planar features grain with sets type A type B type C type D M-mlcrocline planar features grain with sets (GNm )

129 25 .1 quart2 not present Or 0 0 <5.0 130 24 .1 0 0 0 0 0 0 Or 0 0 <5.0 11 19 3 0 0 0 0 0 0 Or 0 0 <5.0 133 16 0 0 0 0 0 0 0 M 0 0 <5.0 8 13 4 0 0 0 0 0 0 Or 0 0 <5.0 64 10 3 8 1.0 8 0 0 0 M 0 0 5.0 150 7 7 20 1.0 20 0 0 0 6.0 26 6 7 20 1.0 20 0 0 0 6.0 39 6 4 35 1.0 35 0 0 0 7.0 61 7 2 36 1.0 36 0 0 0 7.0 110 7 9 45 1.0 45 0 0 0 7.5 84 7 2 48 1.0 48 0 0 0 Or 0 0 7.5 40 6 5 49 1.0 49 0 0 0 7.5 41 6 8 48 1.08 44 4 0 0 feldspar not present 7.5 9 9 0 56 1.23 47 10 0 0 8.0 52 5 0 62 1.08 58 4 0 0 Or 0 0 8.5 67 9 6 68 1.12 60 8 0 0 Or 0 0 8.5 139 6 3 68 1.18 60 8 0 0 M 0 0 8.5 29 5 5 72 1.06 68 4 0 0 9.0 22 3 8 92 1.17 80 12 0 0 9.0 70 8 3 80 1.58 44 31 5 0 9.5 81 4 3 80 1.60 44 36 0 0 9.5 4 8 4 64 1.97 20 44 0 0 10.0 86 7 4 76 1.95 28 44 4 0 10.0 56 8 4 83 1.97 35 48 0 0 10.0 87 6 4 100 1.56 52 42 0 0 10.5 137 5 5 80 2.20 20 60 0 0 10.5 5 3 7 82 2.15 24 58 0 0 10.5 24 3 5 92 1.78 36 56 0 0 10.5 71 4 8 92 2.00 36 56 0 0 10.5 73 3.6 100 1.88 44 56 0 0 10.5 82 5 2 88 1.95 16 64 8 0 11.0 77 3 3 96 2.33 36 52 4 4 Or 0 11.5 76 2 5 100 2.24 32 64 4 0 11.5 106 5 5 92 2.48 0 80 12 0 Or 0 12.0 136 6 3 96 2.83 20 72 4 0 12.0 7 8 1 100 2.54 0 100 0 0 12.5 6 3 8 100 2.96 12 88 0 0 12.5 104 2 7 100 2.72 4 88 8 0 Or 0 13.0 109 4 0 100 2.80 12 68 20 0 13.5 37 2 5 100 2.44 8 72 20 0 13.5 141 3 5 100 3.32 0 88 12 0 13.5 126 1 2 100 3.44 0 84 16 0 13.5 142 3 3 100 3.72 0 84 16 0 13.5 25 2 3 100 3.35 0 77 5 18 Or 2 1.0 14.0 118 2 6 100 4.88 0 56 28 16 14.5 102 2 1 100 2.84 0 56 4 40 16.0 36 0 9 100 4.58 0 58 6 36 16.0 53 2 2 100 4.68 2 58 6 34 16.0 74 1 7 100 3.76 0 44 8 48 Or 0 18.0 122 0 6 100 5.10 0 40 0 60 19.0 120 0 5 100 7.80 0 20 20 60 20.0 35 0 7 100 5.74 0 16 4 80 M 39 1.1 20.0 121 0 3 quartz not present M 44 1.8 20.0 97 1 5 100 4.80 0 28 36 36 M 56 1.7 21.0 91 0 1 quartz not present Or 57 1.6 21.0 100 2 4 quartz not present Or 66 1.6 21.5 95 0. 1 quartz not present Or 78 2.0 22.5 93 0. 1 quartz not present Or 83 1.8 22.5 51 0 0 100 5.50 0 16 6 78 Or 83 1.9 22.5 92 0 1 quartz not present Or 86 2.4 22.5 94 0. 05 quartz not present Or 91 1.5 22.5

* Generally 25 quartz grains measured per sample t Generally 35 feldspar graine measured per samp le r S i2 Gmr

with higher impedance grains (Rinehart, 1968), some grains can terpreted as a function of preferred orientation of the quartz crys- exhibit anomalously strong or weak shock levels by comparison tals with respect to the direction of shock-wave propagation. with the deformation of the sample as a whole. Thus within specific The initial formation of planar deformation features (basal samples, a range of types A through C or B through D can be orientations) occurs at or above the quartz Hugoniot elastic limit found. (HEL). The HEL of quartz has been found in experiments to be between 3.5 and 15.0 GNm-2, depending on crystallographic direc- Experimental tion (Wackerle, 1962; Fowles, 1967), rock type (Ahrens and Greg- son, 1964), propagation path length (Ahrens and Duvall, 1966), In a series of laboratory experiments on single crystals, Miiller and the final shock pressure attained (Ahrens and Liu, 1973). HEL and Defourneaux (1968) and Horz (1968) were able to partially is believed to be lowered also by lengthened shock pulses, particu- duplicate the types A through D shock_deformation sequence for larly in natural impacts where shock pressures endure 104 to 105 quartz. Planar features parallel to (1013J developed initially be- times as long as in experiments (Kleeman, 1971). The HEL for tween 10.5 and 11.9 GNm-2, and (1012) orientations formed quartz in the Charlevoix samples has therefore been estimated con- above 16.0 GNm"2, becoming as abundant as cosets at about 25.0 servatively at 7.5 ± 3.0 GNm-2, and this value is used to mark the GNm-2 (1 giga_Newton per square metre = 10 kb). Planar features development type A quartz. Values from the experiments of Horz -2 parallel to (2241) were produced at 17.0 GNm only. Basal fea- (1968) and Miiller and Defourneaux (1968) have been reduced ac- tures (0001) were not recorded by Miiller and Defourneaux, and cordingly for the natural impact because of lengthened shock pulse -2 they did not develop below 16.2 GNm in Horz's experiments, in duration, so that type B = 10.0 ± 3.0 GNm"2, type C = 14.0 ± 3.0 contrast with their apparent lower pressure development in natur- GNm-2, and type D = 16.0 ± 3.0 GNm-2. Shock pressures can be ally shocked quartz. This lack of (0001) in the experiments was in- extrapolated between these limits and above 16.0 GNm-2based on

Figure 4. Shock deformation of potassium feldspars, a. Continuous film perthite lamellae along the "murchisonite" parting (701), and abundant perthite blebs, typical of unshocked and weakly shocked orthoclase of the chamockitic gneisses. Length of bar=50 pm; interference contrast illumination, b. Film perthite lamellae along right-hand margin of grain grade into subparallel, spindle microperthite in the core of moderately shocked orthoclase. Larger perthite blebs are absent. Bar represents 50 /jm; interference contrast illumination, c. Weak development of fine planar deformation features restricted to corner of orthoclase grain. Scale bar=25 yum; interference contrast illumination, d. Two sets of twinlike planar features in orthoclase. Alternate lamellae are dark because of near-extinction position and abundant minute inclusions. Scale bar=50 /urn; crossed nicols, tilted on universal stage.

the number of sets of planar features per grain and on shock Orthoclase is the predominant potassium feldspar both in salic metamorphism of coexisting minerals. subunits and in the more mafic gneisses lacking quartz. Rocks with At Charlevoix, peak shock pressures were thus determined for intermediate microcline (mean triclinicity =0.12) are less common each quartz grain from the orientation and number of planar de- (Robertson, 1973). Both orthoclase and microcline generally con- formation sets, and a mean shock pressure was established for tain film perthite as thin lamellae parallel to the "murchisonite" each sample (Table 1). parting, approximately (701), and coarser bleb perthite restricted to the central portion of each grain (Fig. 4a). MICROSCOPIC SHOCK EFFECTS Within approximately 7 km of the central peak, perthitic inter- IN POTASSIUM FELDSPAR growth undergoes a progressive change. In grain centers, the film perthite is broken into a series of spindles by narrowing and pinch- Natural ing out at intervals along the lamellae. Continued breakdown re- sults in a core of braid microperthite generally subparallel to rem- Shock metamorphism of potassium feldspars from meteorite nant film perthite lamellae on the margins (Fig. 4b). The coarser craters is less fully documented than that of quartz. To fill at least perthite blebs (Fig. 4a), typical of the outlying feldspars, are absent part of this gap, a detailed study was undertaken of shock effects in from grains within 2.5 km of Mont des Eboulements. The observa- the K-feldspars in the chamockitic gneisses at Charlevoix. tion that this change in form of the perthite is restricted to outcrops

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Figure 4. (Continued). e. Set of twinlike planar features in orthoclase trends northeast. Darker alternate lamellae contain finer planar features trending northwest. Lighter alternate lamellae contain two sets of finer features trending north and east. Length of bar SO pm, plane-polarized illumination, f. Difference in relief between alternate lamellae in twinlike planar features in orthoclase is enhanced by interference contrast illumination. Bar represents 25 pun. g. Faint set of planar features, typical of weak development in microcline, trends east crossing original M-twins trending northeast and northwest. Scale bar = 50 /JITI; crossed nicols. h. Planar deformation features in microcline form twinlike sets (northeast and north-northeast), and a finer set (northwest) is confined to alternate albite twin lamellae (east). Scale bar=25 /urn; crossed nicols.

in the central peak displaying distinct shock effects implies that it fore, to nucleate along shock-induced dislocations producing the too is related to the impact crater formation. There are no reports, "discontinuous" type of braid or spindle microperthite. however, of this phenomenon as a shock-metamorphism product Alternatively, or in combination with the above process, the con- from other impact sites. Two possible theories for its origin are trasting forms of perthitic intergrowth could result from differences proposed. During primary crystallization, exsolution was mainly of in cooling history due to differences in original depth of burial. Ac- the "continuous" type producing albite film perthite lamellae in cording to the hypothesis developed below, rocks on Mont des structural continuity with the orthoclase lattice. Barth (1969, p. 29) Eboulements were upraised during formation of the central peak stated that diffusion of sodium atoms can be reactivated by an ex- from depths of approximately 10.5 to 11 km (see section below, ternal stress, and exsolution can be renewed at low temperatures. "Reconstruction of the Complex Crater"). Extrapolation of heat- At Charlevoix, external stress due to the shock wave and moderate flow values in this region of the Grenville province (A.S. Judge, postshock heating could have reactivated this process. Distortion 1974, personal commun.) indicates a temperature of 150° ± 10°C of the feldspar lattice disrupted the parallelism of a* between the at 11 km. This temperature elevation in itself may not have been orthoclase and albite phases. Sodium atoms found it easier, there- sufficient to account for differences in the types of exsolution.

from the central peak (Figs. 4c through 4g) and are initially similar to the planar features in quartz. A maximum of six sets in a single grain has been recorded. They first form as a few planes of short extent in domains near grain margins; but with stronger development, intersecting sets appear throughout and traverse grains. Commonly, sets with 2-/um spacing intersect sets in which planes are 5 fim apart. In orthoclase from outcrops closer to the crater center, the more strongly developed sets of planar deformation features are twinlike in appearance (Figs. 4d through 4f). The twinlike sets comprise alternating lamellae, 4 to 8 ¡u,m in width, differing slightly in extinction (7° to 8°, maximum 15°), refractive index, and alteration. The higher-relief, lower-index bands contain abundant, minute inclusions which occur along fine, closely spaced planar features confined to these bands (Fig. 4e). The alternate low-relief, high-index lamellae are generally featureless. Some of the higher- relief, twinlike shock features contain a yellowish-green, microcrys- talline aggregate which may be an alteration of diaplectic potassium-feldspar glass. Twinlike planar deformation features in microcline were not de- tected in outcrop but were observed (Fig. 4h) in breccia boulders in glacial float. Portions of some twin lamellae, both original M-twinning and shock-induced features, are almost isotropic and of lower refractive index than the microcline, indicating a breakdown in structural order toward a diaplectic glass state. Orientations of planar deformation features in K-feldspar of the charnockitic rocks were determined from stereographic plots (Fig. 5) compiled from universal-stage measurements on generally 35 grains per sample. In orthoclase, the planar features exhibit a wide scatter with major concentration^ near (241), (241), and (110), weaker groupings near (110), (111), (010), and (111), and a number of poorly defined zonal trends. Some data are obscured by the use of contoured diagrams. Where planar features are strongly developed, two, three, or four optically resolvable sets can form within 10° to y of one another. Twinlike sets are consistently parallel to (241 and (241). Narrow, decorated planar features within these lamellae are generally within the (102) zone (approx- imate), but^yith opposite sign to the twinlike lamellae. For example, broad (241) lamellae contain finely spaced planes with orientations near but not the same as (241) and vice versa (Fig. 5a). The pattern of orientations in microcline (Fig. 5b) resembles that for orthoclase with the strongest concentrations symmetrically op- posed to (010). Prominent orientations lie near (131) and (110). Less common are sets parallel to (120), (120), (010), and (310). As in orthoclase, two or three sets occur that have orientations within 5° to 15° of one another. In contrast to quartz, orientation measurements of planar deformation features in potassium feldspars at Charlevoix and also at Lac Couture (Robertson, 1973) reveal that the predominance of specific sets shows no systematic variation with increasing degree of impact metamorphism.

Experimental

Correlation of shock pressures with the formation of planar deformation features in potassium feldspars has not been so well Figure 5. Contoured stereograms of planar deformation features mea- defined experimentally as it has been for quartz. Very weak planar sured in (a) orthoclase and (b) microcline. Reference axes are optic direc- features in maximum microcline were produced above 15.0 -2 -2 tions X, Y, and Z. Miller indices are given for planes of the feldspar lattice GNm , and moderate to strong features above 20.0 GNm (open circles) and crystallographic zones (square brackets) near the con- (Robertson, 1975). These planar features formed parallel to toured maxima. specific crystallographic planes, although not generally those measured in the Charlevoix K-feldspars, but with no strong corre- However, higher thermal gradients may have existed during the lation between shock levels and the orientations developed. In precrater Grenville orogeny, or at the time of impact due to the agreement with the observations on naturally shocked microcline Taconian orogeny, providing a sufficient temperature differential and orthoclase, certain orientations transformed to deformation to account for cooling differences between near-surface rocks and twins at pressures greater than 20.0 GNm-2 (Robertson, 1975). those at depths of 11 km. Therefore, weak development of planar deformation features in the Planar deformation features occur in orthoclase and microcline Charlevoix potassium feldspars and transformation of the

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strongest sets to deformation twins are estimated to take place at 17.0 GNm"2 and 20.0 GNm"2, respectively. Mean shock pressure for each sample was estimated from the development of planar deformation features in K-feldspar (Table 1). Pressure estimates based on quartz and feldspar data from the same sample are in good agreement.

SHOCK ZONING

Estimated shock pressures were plotted on the outcrop map, and isobaric contours were drawn at 5.0 GNm"2 intervals (Fig. 6). Shock pressures in zones centered about Mont des Eboulements decrease outward from a maximum of 22.5 GNm"2 on the summit to 5.0 GNm"2 at the limit of weakest planar features in quartz at 8 to 9 km distance. The outer limit of detectable shatter cones, estimated as defining a shock pressure of 2.0 GNm-2, lies approximately 14 km from the central peak. With the exception of very weak kink bands in a few biotite grains, no distinctive effects of shock metamorphism are found beyond this limit. The concentric configuration of shock zones about Mont des Eboulements confirms the assumption that this marks the crater center. The zones are compressed along the St. Lawrence River margin, reflecting the distortion of the crater perimeter in this region. The asymmetry may be the result of activity along Logan's line of thrusts during the postcrater Acadian orogeny. On the other hand, it is possible that the asymmetry is an original feature caused by more rapid shock-wave attenuation within the granitic gneiss on the south side of the major northeast fault zone (Fig. 2) compared i 1 1 with attenuation in the charnockitic gneisses. Shock metamor- 0 5 10Km phism recorded in samples 7 and 29 (Fig. 6) is slightly anomalous Figure 6. Shock zoning in the central uplift basement rocks at the Char- (1.5 to 2.0 GNm"2 high) compared with overall levels where they levoix structure, based on planar feature development in quartz and potassium feldspar. Equal pressure contours at 5-GNm~2 intervals are centered crop out. The latter sample occurs in the highly faulted region near on Mont des Eboulements (cross) where deformation representing a max- the St. Lawrence River and is possibly from a block downfaulted imum of 22.5 GNm"2 is preserved. Planar features in K-feldspar are re- from a somewhat higher shock level. Sample 7 lies in a gorge where stricted to outcrops within the inner dashed contour. Samples 7 and 29 with faults have not been mapped, but it also may represent material slightly anomalous levels of deformation (see Table 1) are located by the removed from a higher shock level or an occurrence of localized upper and lower stars, respectively. high shock stress. Irregularities in the shock-pressure contours west of Mont des ellipsoidal volume of the country rocks. The shock wave attenuates Eboulements (Fig. 6) are well documented by the sample distribu- with distance, and the cavity ceases to grow at the point where the tion in this area (Fig. 2). The major indentation in the 5-, 10-, and strength of the rocks is no longer exceeded. The shock wave at- 15-GNm"2 contours and the adjoining bulges reflect valley and hill tenuates still further in the subcavity rocks and becomes an elastic outcrops, respectively. A decrease from approximately 10.0 to 7.5 wave when acoustic velocities are no longer exceeded. GNm"2over a 200-m drop in this region indicates that shock level Formulae relating various crater dimensions, such as depth, vol- decreases appreciably with depth, and that the dip of shock zones ume, and rim height, have been developed from surface observa- toward the center is shallow. tions on explosion craters and fresh-looking lunar and terrestrial meteorite craters (Baldwin, 1963; Pike, 1967). Such relationships, RECONSTRUCTION OF THE COMPLEX CRATER however, apply to the final or adjusted crater form rather than to the initial, transient excavation. From data obtained by diamond The Charlevoix structure belongs to the class of complex craters drilling and seismic reflection studies, Dence (1973) has suggested with a central uplift, a surrounding depression, and a gentle slope that the transient cavity of large, natural impact craters conforms 2 upward and outward to the undisturbed regional terrain. Dence closely to the paraboloid y = 2 px, where p is the crater depth (1968) considers that the same process of meteorite impact creates below the original ground surface and the radius of the cavity is both simple and complex craters, with size the major factor deter- given by r = (V2) p. mining the final form. The initial, short-lived crater (transient cav- The transient cavity (TC) at Charlevoix according to this for- ity) with upturned rim is unstable in large craters (more than 4-km mula is shown in cross section in Figure 7. The peripheral trough at diam in crystalline rocks). Material in the rim and crater walls Charlevoix and other complex craters corresponds to a point on slumps downward and inward, and shocked and fractured base- the outer side of the original rim near the crest. Material higher on ment rocks beneath the crater are forced upward and inward by the rim and on its inner slope was displaced downward and inward this action. along hemispherical surfaces during collapse, and came to rest Detailed discussion of the penetration mechanics of the meteorite near, but below, the intersection of the transient cavity with the and excavation of the transient cavity can be found in Gault and ground surface. The innermost limestone that crops out (LMS) at others (1968). The impact of a large meteorite at approximately 15 an average of 15.5 km from Mont des Eboulements (Fig. 7, top) km/sec (Whipple and Hughes, 1955) sets up a shock-wave front in was exposed by subsequent erosion of this rim material to approx- the target rocks (and meteorite) which propagates radially from imately the point BR (Fig. 7, bottom). Assuming — 1 km of erosion, the point of impact, engulfing and expanding, roughly spherical to the radius of the transient cavity is therefore estimated at 13.5 ± 2

km, a figure in close agreement with the estimate of Rondot (1971). jection of the transient cavity surface onto the horizontal plane of Using Dence's formula, the transient cavity depth is this, calculated smaller area is accommodated by formation of the elevated central as 9.5 ± 1.5 km. mound and by fracturing and jumbling of blocks. Irregularities in A pressure of 500 GNm~2 is taken for the point of impact (PI) the pressure distribution caused by this adjustment are not evident (Stoffler, 1971). At such a pressure, both impacting body and target at the scale of mapping. Comparisons with younger, less eroded rocks are vaporized. Based on observations of shock effects in the craters indicate that the overlying rim rubble and breccias (BR), basement rocks immediately underlying Brent, which is a simple impact melt, plus 300 to 400 m of the fractured basement has been crater (Dence, 1968), and in central uplift rocks from several com- eroded at Charlevoix, exposing the configuration of shock con- plex craters (Robertson and others, 1968), a value of 32.5 GNm~2 tours on the present crater surface (PC). In the upper diagram of is estimated for the base of the transient cavity. From nuclear ex- Figure 7, the shock zones underlying the central uplift are shown in plosion tests, Shoemaker (1963) calculated that shock pressure at- cross section. The gentle inward dip of the zones, particularly in the tenuates initially as an inverse function of the 3rd power of the outer regions, is in agreement with the slope postulated from the radius, and according to the 6th or 7th power below the level change in shock pressures with depth, derived from topographic where the target rocks are fused (approximately 60 GNm-2). The data. At its deepest, the 5.0-GNm"2 contour now lies approxi- configuration of shock pressure contours (IC) at 5-GNm"2 intervals mately 5 km beneath Mont des Eboulements. Also shown in this was thus calculated in the crater basement (Fig. 7, bottom). From diagram are the positions of limestone outcrops preserved in and the diagram, it is apparent that the progressively wider spacing be- around the peripheral valley and the small remnant of impact melt. tween calculated contours as pressure decreases is reflected in the spacing of corresponding pressure zones on the surface. The points DISCUSSION AND CONCLUSION of intersection of the 5.0-GNm~2 and 2.0-GNm~"2 ellipsoids with the transient cavity floor indicate that no unique evidence of shock This study has demonstrated that planar deformation features in metamorphism (that is, shatter cones or planar deformation fea- quartz and potassium feldspar can be used to establish the shock tures) would have been formed in the upturned rim. level of samples, at least in the 5.0- to 22.5-GNm"2 range, with Readjustment and formation of the central uplift elevated these good agreement where the shocked rocks contain both minerals. shock zones to their position in the original crater (OC) beneath a Observations of shock metamorphism of potassium feldspar alone layer of mixed breccias and melt rocks. The trajectories along provide pressures for initial formation of planar features and trans- which upward and inward movement occurred (Fig. 7, bottom) are formation to deformation twins, but specific shock levels do not modeled on structural data from the Gosses Bluff crater where up- appear to be reflected in the predominance of particular orienta- raised strata in the central uplift can be traced downward and out- tions of planar deformation features. A succeeding reference point ward to their preimpact horizons (Milton and others, 1972). Pro- for establishing shock levels in natural material is defined by the complete conversion of feldspars to diaplectic glass at 30.0 to 35.0 GNm-2 (Kleeman, 1971; Stoffler and Hornemann, 1972; Gibbons, 1975). Within the near future, it seems probable that pressure ranges will be delimited for the formation of unique and readily identifiable shock deformations in the majority of common rock- forming minerals, and mapping of zones of shock metamorphism can then take place in the manner of Barrovian regional metamorphic zones. Of perhaps greater significance is the compatibility of the observed configuration of shock zones with the crater-modeling concepts, especially with respect to transient cavity size and attenuation rates. The progressive increase in width of the mapped contour intervals confirms that the shock wave attenuates as a power of distance traveled. The attenuation rates must agree closely with those BR Breccia and Rubble values employed to reconstruct the subcavity contours of constant IC Isobaric Contours shock pressure. If, for example, attenuation had been according to IM Impact Melt LMS Limestone the sixth power of the radius throughout, the rim rocks should con- ME Mont des Eboulements tain both planar features and shatter cones, in contrast with the ob- OC Original Crater servations at Charlevoix and at young, simple craters, such as the OS Original Surface PC Present Crater New Quebec Crater, where the rim is perserved. In addition, the PI Point of Impact trajectories along which uplifting occurred would have to be con- PV Peripheral Valley siderably flatter than those established at Gosses Bluff in order to RP Regional Plateau TC Transient Cavity explain the surface configuration of the shock zones. The calculated vertical extent of the shock zones has yet to be Figure 7. Formation of shock zones at the Charlevoix crater. Lower confirmed. Five drill holes have been drilled in the central uplifts of diagram is a cross section of the transient cavity showing pressure- the East and West Clearwater Lake craters. It is hoped that shock attenuation limits (IC) in the underlying rocks. Formation of the central up- metamorphism data from this drill core can be used to define the lift raised these zones along the dashed arrows to their positions in the orig- subsurface distribution of shock pressures and to refine the model inal crater (OC). Erosion of the overlying breccias and 300 to 400 m of the for complex craters derived at Charlevoix. basement (including all material shocked to between 22.5 and 32.5 GNm-2) has exposed rocks from slightly deeper original levels (solid arrows), decreasing the radial extent of the 5.0-GNm~z contour, for example, ACKNOWLEDGMENTS by almost 2 km. Upper diagram is a cross section of the present crater, de- picting the extent of gently dipping shock zones underlying the central up- This study has benefited from many discussions with M. R. lift. Limestone from the transient cavity rim is preserved in the downfaulted Dence and R.A.F. Grieve of the Earth Physics Branch, and their peripheral valley, and a small outdrop of impact melt remains. criticisms of the manuscript are appreciated.

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