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 pres- sure 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 ef- fects in basement rocks of the central uplift at the Charlevoix im- pact 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 topog- raphy. The surface distribution of shock pressure contours is com- pared 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 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/12/1630/3433405/i0016-7606-86-12-1630.pdf by guest on 30 September 2021 ZONES OF SHOCK METAMORPHISM, CHARLEVOIX IMPACT STRUCTURE, QUEBEC 1631 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 pres- sure 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 deforma- tion 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 = char- nockitic 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. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/12/1630/3433405/i0016-7606-86-12-1630.pdf by guest on 30 September 2021 1632 P. B. ROBERTSON 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