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The Rochechouart crater: Shock zoning study
Earth and Planetary Science Letters. 35 0 9 7 7 ) 2 5 8 - 2 6 8 © Elsevier Scientific Publishing C o m p a n y , A m s t e r d a m - Printed in The Netherlands
258
THE ROCHECHOUART CRATER: SHOCK ZONING STUDY P. L A M B E R T
Bureau de Recherches Gbologiques et Minibres, Orleans (France)
Revised version received March 18, 1977
Rochechouart crater (France) occurs in crystalline rocks of the northwestern French Massif Central. The crater is deeply eroded and the present ground level is exactly (-+50 m) tangent to the crater floor. No morphologic evidence of the crater has been preserved. A complete range o f shock effects is known on the scale o f rocks and minerals, which permitted a s t u d y of shock zoning. Shock level was determined in thin section from petrographic analysis of each specimen. A systematic sampling was done on all the structure. Shock zones were determined at t h e same time in the fall-back unit and in the target. Correlations between rock types or shock level in allochthonous a n d autocht h o n o u s materials were observed. They imply restrictions for the late excavation stage: part of the material lying above the crater floor limit was never ejected, b u t only mixed with small relative displacement. Shock level is relatively higher in allochthonous breccias than in the target. The distribution of shock effects is very complicated at all scales, even at the scale of the whole structure. However, shock level is relatively high around the center o f the structure defined from the breccia geometry. The most probable impact point is located about 4 k m west o f Rochechouart. The breccia unit is extremely thin (less than 60 m). The crater floor is extremely flat; its elevation does not vary more than -+50 m over the whole structure (about 300 km2). The lack of circular s y m m e t r y in particular in t h e distribution of impact melts could suggest a pronounced anisotropic structure o f the target and/or an inclinated trajectory o f the projectile. The original crater size is most probably between 20 and 25 k m in diameter, determined from t h e actual extent of differents rock types or shock effects. A rapid post-crater readjustment is proposed to explain the flat floor. There was no important vertical displacement since the flat shape o f the crater floor was attained.
Le crat~re de Rochechouart (France) affecte les roches critallophylliennes du NW du Massif Central franqais. I1 a perdu t o u t e expression morphologique car il est tr6s ~rod& Seul le fond du crat6re est pr6serv& La g a m m e des effets de choc est complete h l'6chelle de la roche c o m m e ~ l'6chelle du min6ral. La zondographie est basge sur la r6partition de ces effets de choc. L'intensit~ du m ~ t a m o r p h i s m e de choc est d6termin6e en lame mince pour chaque ~chantinon. Un pr61~vement syst6matique a gt~ fair sur l'ensemble de la structure. La zon6ographie a 6td envisag6e ~ la fois dans la pattie allochtone et dans les zones en place. Les comparaisons sont envisag6es: on observe en particulier des corr6lations locales entre certains facies de roche, ou certains niveaux de choc, entre mat6riel d6plac~ et mat6riel autochtone. Ceci implique certaines restrictions au m6canisme responsable de la fin de rexcavation. Une partie du mat6riel reposant au-dessus du fond du crat~re n ' a donc jamais dt6 ejectS, mais seulement mdlang6, avec des ddplacements relatifs faibles. Les br~ches allochtones sont plus intens~ment choqu~es que celles autochtones. La distribution des effets de choc est tr~s compliqu6e ~ toutes ~chelles, m~me £ ceUe de l'ensemble ~le la structure. Cependant l'intensit6 du choc est relativement plus forte au centre de la structure (d6fini d'apr6s la gdometrie de l'unit6 brdchique). Le point d'impact le plus probable se situe fi 4 k m fi l'ouest de Rochechouart. La formation br6chique est e x t r 6 m e m e n t fine ( < ~ 60 m d'6paisseur). Le plancher du crat6re est e x t r ~ m e m e n t plat: ses variations d'altitude ne d6passent pas -+50 m sur l'ensemble de la structure (couvrant environ 300 km2). Le m a n q u e de sym6trie circulaire, en particulier pour la distribution des produits de fusion sugg~re une anisotropie marqu6e de la cible et/ou une trajectoire inclin6e pour le projectile. La taille, fi l'origine, du crat6re est discut6e d'apr6s la rdpartition de diff6rents types de roches et celle des effets de choc. Les valeurs les plus probables se situent entre 20 et 25 k m de diam6tre. Un r~ajustement rapide du crat~re est supposg pour expliquer le fond plat. I1 n ' y a pas eu d ' i m p o r t a n t s d6crochements verticaux de-
puis.
259 1. Introduction
The Rochechouart impact structure is located about 40 km west of Limoges (France) !n the northwestern part of the French Massif Central. The target area is composed of Hercynian gneisses and granites. The nearest occurrences of sedimentary rocks are about 2 0 - 2 5 km west of Rochechouart. There is no morphologic or topographic evidence of the crater. The present topography is very flat, as a result of quaternary glacial abrasion. Relief (magnitude 30--150 m) is due to small rivers cutting this platform. Good outcrops are rare because of an intense vegetation. The geologic map of the Rochechouart area is published elsewhere [1 ]. The breccia unit covers about 300 km 2. It consists of allochthonous and autochthonous breccias and breccia veins in the target. In the inner zone (13 km diameter), the breccia unit is continuous and subhorizontal. The thickness is less than 60 m, in most cases only some meters. Outside of this zone scattered patches of breccias occur. Breccia veins from 1 cm to some meters wide are common in the crystalline basement rocks. The breccias are sometimes faulted (Fig. 1), but vertical displacement is less than 2 0 - 3 0 m. The most probable age of impact is 165 Myr from K-At measurements [2]. Impact melt and lithic clasts in the breccias are very similar in composition to the target rocks (gneisses, granites) [3]. In particular there is no trace of sedimentary rocks whereas Mesozoic marine sediments (Triassic and Lower Jurassic) which form the margin of the Aquitaine Basin are known 2 0 - 2 5 km west of Rochechouart. The breccias are not layered. There is no graded bedding. They have not been transported. This suggests that impact took place on a very shallow sea or on a water-free area, presumably on the western limit of a N-S horst [4] during the Middle Jurassic. A structural study of the Rochechouart area confirmed the importance of post- (or late) Hercynian faulting oriented N-S on the western part of the structure [5]. In particular N-S dikes of microgranite are related to this phase [5]. This tectonism was still active after breccia deposition [1 ] but movements were slight (see above). Rochechouart breccias have been studied by geologists since the beginning of 19th century [1 ], an impact origin was first suggested in 1967 [6] and
definitely confirmed in 1969 [7] by the evidence of shock metamorphic features. Fall-back breccias of Rochechouart are contaminated by the projectile [3,8] interpreted to be an iron IlA meteorite [9].
2. The zoneographic study of Rochechouart crater This study is based on the evidence of shock effects observed at the rock and mineral scale. All these shock features have been previously described in detail [6,7,1,2] for the Rochechouart structure. The different rock types were systematically sampled and subdivision was deduced from petrographic study in the laboratory (1000 thin sections were studied). In this structure I recognize five types o f transformation of the target rocks from morphological and textural changes. They are significant of increasing shock level at the rock scale and are independent of the physico-chemical characteristics of the cohesive rocks of the target. Type A are autochthonous fractured rocks of the basement, including highly fractured rocks [1 ], and shatter cones. Type B designates autochthonous monomict breccias. Compared to type A rocks, the rock fragments are rotated relative to each other and a matrix of finer clasts is produced. Type C are allochthonous breccias without glass. Clasts of different rock types of the basement are mixed together and the matrix consists of fine-grained clastic material. Type D corresponds to suevites which contain melt inclusions in addition to the clastic material observed in type C. Type E represents impact melts defined as breccia with a glassy matrix. All kinds of shock effects, from the weakest to the strongest, are known in Rochechouart minerals [1,2]. These shock effects are classified in five stages for which P-T conditions are roughly established, as deduced from Hugoniot data of several minerals [ 10, 11 ] an d nuclear explosion data [ 12,13 ] (see Table 1). This classification is adapted from that of progressive shock metamorphism introduced by StiSffler [14]. I have subdivided his stage I into stage Ia and
260 TABLE 1 Classification (adapted from St6ffler [14]) of different shock effects in Rocheehouart minerals used for the shock zoning study of the crater. Approximate pressures are indicated from extrapolation of Hugoniot datas on minerals [10,11] and from nuclear explosion investigation [ 12,13 ] Stage No. 0
Approximate pressure range (kbars) <100
Ia
80-150
Ib
150-350
II III
350-450 >450
Shock effects in minerals
<" /"'9 ?,
fracturing and/or kinking
#
,'~
"..
isotropic quartz and feldspars (diaplectic glass) fused minerals
Ib because weak shock features are the most common in Rochechouart. The limit between I a a n d Ib corresponds to the first evidence of decrease in birefringence in feldspars, and/or development of planar elements in quartz. In stage Ia, only rocks with beginning of development of planar elements in quartz have been plotted. This subdivision is quickly determined from thin section examination. The Dence [16] subdivision of the corresponding range into four classes related to planar-element orientation in quartz was not used because of the time involved for a statistical analysis of 1000 samples.
3. Results
3.1. Extent o f breccia unit and crater floor limit The extent of the breccias is indicated in Fig. 1. The limit of the crater floor cannot be deduced became allochthonous and autochthonous breccias are mapped as one unit. The contact between these two units cannot be determined in the field. The limit of the crater floor is only clearly exposed when aUochthonous breccias are superimposed on undisturbed or shattered basement.
.
~
'
"
'.
~
:::
JF'IIPr"::
:,
beginning of development of planar elements in quartz extensive development of planar dements in quartz (many sets per grain, many elements per set); loss of birefringence of quartz and feldspars (diaplectic minerals)
t
•~
,o
.2
.
4 ~m
"."
i .:.1
Fig. 1. Distribution of shock effects in faU-baekbreccias. Dotted line represents the limit of the breccia unit, solid lines are faults; 1 = Machat, 2 = Babaudus, 3 = Chassenon, 4 = Mountoume. Rocks consisting of more than 25% of fused material are in dicated by large shaded circles, those comprising between 25 and 5% by large, black triangles. Intermediate circles (with dash) correspond to the beginning of fusion and occurrence of diapleetic glasses (isotropized minerals). Small black triangles indicate the occurrence of numerous sets of planar elements in quartz and diapleetic minerals (minerals with unusually low refractive index); small black dots represent few occurrences of planar dements in quartz. On the other hand, there is sometimes a spatial connection between target and allochthonous material. This is the case for the location of some uncommon rock types of the basement such as granophyric microgranite or granoblastic gneiss, and the location of allochthonous polymict breccia showing basement clasts of such rock types. It was found that type C, D and E breccias may occur on top of undisturbed basement, weakly shocked basement (type A), strongly shocked basement (showing diaplectic minerals and diaplectic glasses), and on weakly shocked B breccias, or on strongly shocked B breccias (showing diaplectic minerals and diaplectic glasses). The contact between the breccias and the basement is very fiat throughout the whole unit. In particular the elevation of the contact between autochthonous breccias and the basement is the same as the one between allochthonous breccias and the basement. When the contact of the crater floor is visible, its
261
shape is very complicated and is influenced by structural characteristics of the target [ 1,2].
3.2. Relative importance of each type of shock effect at the rock scale Shattered rocks very commonly underlay the breccia unit [2]. Shatter cones are restricted to the center of the structure (determined from the geometry of the breccia) and are rare compared to highly fractured rocks, but when they are present they occur together. In fact the fracture network [I ] corresponds to the crossing of shatter cone surfaces. Shatter cones are known in each rock type of the Rochechouart basement, but nearly 50% of those recovered are developed in microgranite whereas it represents only about 1% of the exposed target rocks. Structural analysis of the shatter cones and their orientation is not yet finished and will be presented later. Table 2 indicates the relative abundance of each breccia type from the sampled material. Autochthonous (B) and type C breccias are more common in the field than the remaining breccias (types D, E). A volume estimation was done by extrapolation of each breccia extent and by taking into consideration the local value of the breccia thickness (Table 2). This model is limited because the eventual vertical stratigraphy of the different breccia types is neglected. There is also a large possible error in the extrapola-
tion of the lateral extent of each breccia type from this ponctual analysis. Since geophysical prospection (for the lateral extent) and drilling (for the vertical succession) have not been done, this model remains the only possibility. Comparison between direct estimation and volume estimation (see Table 2) shows that there are significant changes for B, C and D breccias because monomict breccias (B) occur over a wide but thin area. Considering the average glass content of suevites and impact melts of Rochechouart, the total proportion of fused to displaced materials is 11% (see Table 2). Except for a few cases in suevites (D), the glass in Rochechouart melts (E) and suevites (D) is entirely recrystaUized to K-feldspars and quartz [3]. Compared to the rock source, glasses are enriched in K and depleted in Na [8]. Fig. 2 shows the approximate glass content of the different specimens analyzed. Most suevites contain very little glass; their average content is less than 13%. 100% glass in Rochechouart impact melts is very rare; the average is about 70%. In a few cases an intermediate stage between D and E breccias was observed where glasses form a part of the matrix and part of the clasts. The average glass content of these rocks is about 40%.
3. 3. Distribution o f the different breccia deposits in the allochthonous blanket Except when the breccia unit is thin enough to be sure that there is only one type lying on the base-
TABLE 2 Proportion of each breccia type sampled in the Rochechouart structure. Volume estimation is made by extrapolation of e a c h breccia type e x t e n t and the local value of t h e breccia t h i c k n e s s ( s e e text) Breccia type *
Percent with respect to whole breccia unit: from n u m b e r o f analyses from volume estimation Percent with respect to aUochthonous breccias from n u m b e r o f analyses from volume estimation Percent glass with respect to allochthonous breecias
B
C
D (average glass content = 13%)
50 19
35 43.5
10 30
70 53.5
20 37.2
10 9.3
4.8
6.5
-
E (average glass c o n t e n t = 70%)
5 8.5
* B = a u t o c h t h o n o u s m o n o m i c t breccias, C = allochthonous polymict breccias w i t h o u t glass, D = suevites, E = impact melts.
262 20,
mber of analyses
15,
2O
lS.
E
10,
S,
O
I'1'1'1'1'1 I I I 10 20 30 40 50 60 70 80 90 100
!i
0
,~1 I't'l'l'['
~ '1
20 30 40 50 60 70 80 90 100
glass ( 7. Fig. 2. Approximate glass contents of suevites (type D) and impact melts (type E) from estimation in thin section.
ment, the stratigraphy of the aUochthonous breccias of Rochechouart is not dear. Contact between the different kinds of allochthonous breccias has not been observed in the field. Moreover the blanket is too thin to look for continuity between the different localities where the studies were done. The whole breccia unit as well as the aUochthonous unit nearly shows a centro-symmetric distribution in the inner zone o f the structure (see section 1). The low values along N-S and NW-SE directions are due to the E-W Vienne river valley cutting the north part o f the structure, and the NW-SE Graine river valley cutting the northwestern part of the structure. C and D breccias are following the same trend. This is not the case for E breccias which are clearly concentrated in a NE-SW band (Table 3).
3.4. Layered unit of Chassenon In the old quarry at Chassenon one occurrence of a layered rock resembling an ash deposit (see Fig. 3)
Fig. 3. Layered rock at the top of the suevite at Chassenon.
263 TABLE 3 Distribution of breccias in the inner zone of the structure and distribution of shock features in the target Maximum extent (km)
B + C + D + E breccias (entire breccia unit) C + D + E breccias (entire allochthonous unit) C breccia D breccia E breccia High shock level in autochthonous unit (occurrence of diaplectic glass) Weak shock level in autochthonous unit (occurrence of planar elements in quartz)
NE-SW
E-W
NW-SE
N-S
12 - 1 3 10 -10.5 1.0-10.5 9.5- 9 10
12 - 1 3 10 -11 8 -11 8.5 2 - 7
8.5-11 6 - 9.5 7 - 7.5 7.5- 8 2 - 6.5
10 - 1 0 10 -10.5 7 -10.5 8 . 5 - 8.5 6 - 7
7
2 -4
2 -4
4 -6
8
8
7
8
was f o u n d at the t o p o f the suevite. This u n i t is only c o m p o s e d o f thin mineral debris, s o m e t i m e s associated with thin debris o f b a s e m e n t rocks (particle size b e t w e e n 10 and 150 ~ m ) . S h o c k effects such as kink bands in micas or planar elements in q u a r t z are comm o n in this unit. The c o n t a c t b e t w e e n suevite and this unit is n o t sharp. There is a m i x e d z o n e a b o u t I 0 c m wide where b a s e m e n t clasts or glass debris are associated w i t h very thin particles.
heterogeneous. Particularly in the center there exist some a l l o c h t h o n o u s breccias w i t h o u t s h o c k effect evidence at the mineral scale, whereas in the same area i m p a c t melts were sampled. The distribution o f
N~
°
/
~
~ o
....~ :: ....
~"
.'
"
o
.c: ..
3.5. Shock zoning based on shock effects o f minerals For this study I refer to the highest s h o c k effects * observed in the thin section to determine the shock stage o f the specimen. Fig. 1 shows the result o f this study for the allocht h o n o u s breccia unit. The highest shock levels are distributed over the whole unit but t h e y are more f r e q u e n t in the center o f the structure ( d e f i n e d f r o m the breccia g e o m e t r y ) . In particular i m p a c t melts w i t h bubbles are recovered o n l y in this area (the interior o f the bubbles is more or less filled b y glass i m p l y i n g the presence o f a silicate vapor phase). There is no regular decrease f r o m the center towards the periphery. There are some e x t e r n a l maxima, for instance, in the M o n t o u m e area ( s o u t h o f the structure, see Fig. 1) or near Machat ( n o r t h o f the structure, Fig. 1). On a local scale (100 m) the distribution is also * To be significant they have to represent at least 2 - 5 % of registered shock effects of the minerals in the sample.
"""'"
o,..
o..
..::." .:
~./~ -. ~!" ~/~"~~!i ~ ...... ~
:'O..6"i~ ~......,
.....'.. ".'
~ ~'..
c
~ o~.~: ~
t.~ o~'.
',~ i n , : ~
~:~.,~. # ~ , o
~
~
~--~,~
• .r.. ".
~o-~
7 ~'~ "~"r .~' f;"o ~
.:!.;~ o....oi ' ~,...~ ,~ ,..,..~ ~;. o:; ~ o° ~..-. ~: . .. ..., •;..
e
0
9
o
o
.......
• ;
o
'
0
o
o :~
'"'~ .......:
°o: . , . ~ . . c ~
o
.""
......~
:0"
~.:~
.
."'"
'~~. []
...~..:- .'~
oo o,~
..~
2, 4 km e
o
o
..
.... ,.~ o
.......
~
Q~ ~, ~
¢
°~'~
Fig. 4. Distribution of shock effects in the target (basement plus autochthonous breceias). Dotted line limits the breccia unit, solid lines are faults; 1 = Machat, 2 = Babaudus, 3 = Chassenon, 4 = Montoume, 5 = Rochechouart. Large shaded circles indicate that at least a part of the minerals of the rock are isotropized (diaplectic glasses). Large black triangles indicate quartz with numerous sets of planar elements and unusually low refractive index for quartz and feldspar (diaplectic minerals); small triangles correspond to few occurrences of planar elements in quartz. Small open circles represent rocks with highly fractured and/or kinked minerals. Large open arrows indicate rocks with evidence of shock effects at the rock scale (brecciation, shattering) but not a t t h e mineral scale.
264 shock effects in one hand specimen is also heterogeneous, even in autochthonous material. However, the range of variation is larger in ejected masses than in the target. Fig. 4 shows the distribution o f shock levels in the basement (type A and B) breccias. The highest values are most common in the center. Even in this nondisplaced material the distribution is heterogeneous; in particular, there are some external maxima such as in the Machat area (north o f the structure, see Fig. 4). There is some indication o f a concentric s y m m e t r y in the distribution of shock levels, except for the highest values (see Fig. 4 and Table 3) which are only located in a NE-SW band. Comparison o f Figs. 1 and 4 shows that the shock level is relatively higher in allochthonous than in autochthonous material. There is a spatial correlation of the highest values between the autochthonous and allochthonous material, not only near the center of the breccia unit but also at the periphery. In the Machat area for instance (north of the structure), a local increase o f shock level in the target corresponds to a local increase of shock level in the allochthonous breccias (Figs. 1,4). 4. Discussion The most probable impact point is deduced from the center o f highest shock level from Figs. 1 and 4. This point is located 4 km west o f Rochechouart and is very close to the center of the allochthonous breccia unit and also o f the whole breccia unit. From this point the maximum extent o f the different shock levels at the mineral and rock scale in the target and in the allochthonus unit are indicated (Fig. 5). The maximum size o f recovered shock effects is 23 km diameter. Circular s y m m e t r y is still present for the whole breccia unit. As the breccias are very thin it is implied that the deep erosion has not changed their original circular distribution. Consequently it is suggested that there was no important faulting o f the actual crater floor geometry (maximum vertical amplitude o f such movement is necvssary less than the breccia thickness: < 6 0 m). Because of the very fiat crater limit and occurrence o f breccias near the probable impact point, it is suggested that there was no prominent uplift at the center o f the crater.
/'
0
6
10
Fig. 5. Scheme of shock zoning at Rochechouart crater: maximum extent of shock effects in the fall-back breccias and in the target. Dashed line limits the breccia unit, solid lines are faults. Circle 1." basement - shatter cones in rocks exhibiting planar elements in quartz; fall-back breccias - evidence of vaporized material from recovery of glass covering the interior of bubbles in impact melts. Circle 2: basement - occurrence of isotropic minerals (diaplectic glass). Circle 3) basement shatter cones or planar elements in quartz. Circle 4: fall-back breccias - glass (suevite and impact melt), planar elements in quartz. Orcle 5: fall-back breccias - maximum extent. Circle 6:20 km diameter - the most probable size of the original crater (see text). Q'rcle 7: basement - brecciation, fractures related to the impact. Circle 8: possible crater size from the melt extent (see text). The flat crater floor is difficult to understand if there was no important movement after the crater excavation. A readjustment seems also necessary to explain the same elevation o f the crater floor and the contact between sedimentary rocks and basement outside o f the structure. In this case, the previous remarks provide serious restrictions for the possible geometry of such supposed post-crater adjustment. They are only consistent with a regular uplifting o f the whole crater floor, which is strongest in the center and decreases towards the periphery. This would drive the original crater floor, paraboloid in shape, to a new one, very fiat, everywhere at the same elevation with less than 50 m difference. Such i m p o r t a n t readjustment must have occurred soon after the crater
Possible Roehechouart crater diameter (km) from previous ratio considering (see Fig. 5): shatter cone extent = 9.5 km breccia extent = 19 km melt extent = 11 km
Ratio of crater diameter to shatter cone extent Ratio of crater diameter to breccia extent Ratio of crater diameter to melt extent
Estimated crater diameter (km) Approximate diameter of shatter cone extent (km) Approximate diameter of breccia extent (km) Approximate diameter of melt extent (kin)
12
1.17
28 [22] 24 [21 ]
14
1.46
35 [23] 24 [21]
Charlevoix (Qu&)
8.5
0.90
18 [24] 20 [24]
14
1.50
30 [23] 20 [241
Carswell (Sask.)
23
1.2
10 [251
12 [25]
Nicholson Lake (N.W.T.)
Estimation of the diameter of the Rochechouart crater from estimated data of several deeply eroded Canadian craters
TABLE 4
>14.5
1.33
60 [26]
>80 [26]
Manicouagan (Qu6.)
- 1 0 [27]
17
-22
1.54- 2.0
13
20 [27]
Mistastin Lake (Labr.)
266
formation because neither breccias nor glasses are displaced and extensively faulted. Because of such readjustment and the actual thickness of the allochthonous unit, any stratigraphic correlation has probably no sense in these remnants. The numerous breccia dikes in the Rochechouart basement prove that there was extensive faulting due to the impact. But this study, as does the regional study from ERTS [5], shows that there are no radial or concentric faults in the Rochechouart target. Circular symmetry of faults associated with impact craters seems to be only typical for those occurring on a quite undisturbed target such as a sedimentary blanket (e.g. Wells Creek [17] or Flynn Creek [18]). When the target is deeply fractured (as is the crystalline basement of Rochechouart), tectonism related to impact assumes the pattern of the previous regional system. The heterogeneous distribution of shock levels in the target is related to the texture and anisotropy of the target at all scales (mineral interfaces, layering in rocks, contacts of different rock type, fractures, regional faults). In allochthonous breccias this distribution is complicated by mixing. This study shows the necessity of a large statistical investigation. It appears that it is not necessary to look for more shock stages in such a kind of study. Only statistical analysis of each different shock effect occurring in a specimen would provide information. But, since textural effects in artificially shocked rocks are not well understood, it is actually not possible to determine the "true" shock level with more accuracy. The spatial correlation of rock type and shock level on all sides of the crater floor suggests that at least part of the allochthonous material of Rochechouart never moved far from its original location, and was probably not ejected. This implies that, if most of the excavation occurred as ballistic ejection, at least the termination of crater growth was characterised by weak movements. The 11% of fused material in the Rochechouart breccias is probably far from the original proportion of fused to displaced material. The particular setting of an impact melt sheet at the bottom of the crater probably involves an unduly high estimation of its volume in the case of Rochechouart. The restricted outcrop of stratified ash on the top of suevite at Chassenon reveals an ambiguous prob-
lem. Because of evidence of shock effects in the thin debris and because of its location, this ash could represent a late deposit of thin particles produced by the impact phenomenon. As in the case of a volcanic event, these fine-grained products could have escaped the main mixing phenomenon of large and small debris and could have needed a longer time to deposit. In the case of a well-preserved crater such as Ries, suevites constitute the top of the fall-out unit [19]. They also occur at the top of the fall-back unit as shown by drilling results at Ries [20] or at Brent [16]. However, such an ash deposit as in Chassenon was never observed. Related or not to the impact phenomenon, this fine debris was deposited after the crater readjustment. The correlation between impact melts and high shock in the underlying basement is reasonable; this boundary represents the original cavity floor. Differential erosion might explain the oriented distribution, but does not seem to be supported by any local or regional argument. The Vienne and Graine river valley responsible for the small ellipticity of B, C, D breccia distribution would explain the melt one, if melt was covering B, C and D breccias. This is not the case. A non-representative sampling is still possible but seems unlikely considering the number of analyses, in particular in the target. More probably, the orientation is primary and related to an ellipsoidal shock wave geometry. This geometry could be due to a large anisotropy of the target, or a low-angle trajectory for the projectile. There is no direct evidence of the original crater size at Rochechouart. Two types of estimation may be made. The first comes from comparison with other old terrestrial craters, and these estimations are summarized in Table 4. The results have only a comparative sense as they are based on estimated data. The second way is to compare general morphologic characteristics of the impact crater. Crater size from the extent o f the allochthonous unit. Because we are at the crater floor it is rather unlikely that fall-out breccias still survive because of erosion. Therefore it can be deduced that the actual allochthonous breccia extent determines the minimum crater size (13.5 km diameter, see Fig. 5). This estimation is pessimistic as erosion tends to reduce the fallback breccias extent at the periphery, because of the
267 prominent rim. East Clear Water Lake crater (Quebec), on crystalline basement, is approximately at the same erosion level as Rochechouart [25]. In his model, Dence presumed an original crater 16 km in diameter [25]. In any case, values obtained from comparison with Carswell and Charlevoix for shatter cone extent are too low.
Crater size from the limit o f disturbed rocks. Recent geophysical studies o f Wanapitei [28] and Ries [29] indicate that the disturbed zone (faulting and weak shock features) may be quite thick under the impact point in the target. It decreases radially near the periphery, and is non-existent at the surface. Consequently, whatever the erosion level o f the crater, this zone is restricted in the target to an area less than the original crater diameter. This area is 23 km in diameter at Rochechouart (see Fig. 5). Comparison o f the Rochechouart zoning to the Ries deep drilling results. At Ries, a young crater 24 k m in diameter where the morphology is still preserved [19], three recent drilling studies indicate that there is no impact melt, even near the center. The nearest is located 3.8 k m from it [ 2 0 - 2 9 ] . Three explanations are possible:((1) there is no impact melt layer at the b o t t o m o f the Ries crater; (2) there are scattered patches o f melt, but the three drill holes missed them; and (3) there is a continuous melt sheet at the b o t t o m of Ries crater o f less than 3.8 km radius. As compared to other large impact craters, the first hypothesis is very improbable as we do not know of other cases. The second is possible, in particular if we compare Ries to the actual geometry o f Rochechouart impact melts. However, the actual lack o f connection between the occurrences of melt at Rochechouart is not conclusive of an original scattering because o f the very deep level o f erosion of the crater. The isolated occurrences o f melt at Mistastin are considered to represent remnants of a large sheet o f impact melt which once lined the crater cavity [19]. The third possibility seems the most probable as it is the regular scheme for large craters [30]. Tlfis hypothesis is also favored b y St6ffler [31] from recent volumetric calculations o f melt in Ries suevites. In such a case Rochechouart could be at least 35--40 km diameter, as impact melts occur in an area 11 km in diameter.
5. Conclusion Tile Rochechouart crater, produced on Hercynian basement during Middle Jurassic times by an iron meteorite was very probably more than 16 km in diameter. The most probable values are between 20 and 25 km. The shock wave geometry was not spherical but probably elhpsoidal. I propose that there was a rapid uplift of the b o t t o m o f the crater after its formation. However, this uplift never produced a prominent hill at the center but a flattening o f the crater floor. Since this rapid readjustment there have been no important vertical tectonic movements. The crater is now deeply eroded and the actual ground level is tangent to the crater floor. As the crater floor is very flat, the breccia unit covers a large area but is extremely thin. However, all the breccias types and all shock effects at the scale of rocks and minerals are known in this restricted volume. Moreover, this exceptional location permits the investigation o f shock zoning in the target and in the allochthonous unit. The distribution of shock effects is very complicated and heterogeneous at all scales, even in autochthonous material. But it is possible to do a regional interpretation from statistical measurements.
References 1 P. Lambert, Etude g6ologique de la structure impactitique de Rochechouart (Limousin, France) et son contexte, Bull. BRGM 2, Sect. 1 (1974) 153. 2 P. Lambert, La structure d'impact de m6t6orite g~ante de Rochechouart, Th~se Doc. Spec., Universit~ de Paris-Sud, Paris (1974) 148 pp. 3 P. Lambert, Rochechouart impact crater, statistical geochemical investigations and meteoritic contamination, in: Proceedings of the Planetary Cratering Symposium (in press). 4 J. Delfau, Un ~16ment majeur de la pal6og6ographie du sud de la France au Jurassique moyen et sup~rieur. Le haut fond occitan. C.R.J. Soc. C~ol. Fr. 2 (1973) 58. 5 P. Lambert, La structure impactitique de Rochechouart (Limousin) et son contexte structural r6gional, par interpretation de "photo satellite" image ERTS, Bull. BRGM 2, Sect. 1 (1974) 177. 6 F. Kraut, Sur l'origine des clivages du quartz darts les br~ches "volcaniques" de la r~gion de Rochechouart, C.R. Acad. Sci. Fr. 264 (1967) 2609. 7 F. Kraut, Uber ein neues Impaktit-Vorkommen in Gebeite von Rochechouart, Chassenon (Departements Haute-Vienne und Charentes, Frankreich), Geol. Bavarica 61 (1969) 428.
268 8 P. Lambert, Nickel enrichment of impact melt rocks from Rochechouart; preliminary results and possibility of meteoritic contamination, Meteoritics l0 0975) 433. 9 M.J. Jansens, J. Hertogen, H. Takahashi, E. Anders and P. Lambert, Rochechouart meteorite crater: identifcation of projectile, J. Geophys. Res. (in press). 10 J. Wackerle, Shock wave compression of quartz, J. AppL Phys. 33 (1962) 922. 11 T.J. Ahrens and J.T. Rosenberg, Shock metamorphism, experiments on quartz and plagioclase, in: Shock Metamorphism of Natural Materials, B.M. French and N.M. Short, eds. (Mono, Baltimore, Md., 1968) 59. 12 N.M. Short, Progressive shock metamorphism of quartzite ejecta from Sedan nuclear explosion crater, Goddard Space Flight Center, Greenbelt, Md., Preprint X, 622, 69, 537 (1969). 13 J. Faure, Recherches sur les effets g6ologiques d'explosions nuel6aires souterraines dam un massif de granite saharien, Rap. CEA R, 4257 (1972) 273 pp. 14 D. St6ffier, Progressive metamorphism and classification of shocked and brecciated crystalline rocks at impact craters, J. Geophys. Res. 76 (1971) 5541. 15 O.B. James, Shock and thermal metamorphism of basalt by nuclear explosion, Nevada test site, Science 166 (1969) 1615. 16 M.R. Dence, Shock zoning at Canadian craters: petrography and structural implications, in: Shock Metamorphism of Natural Materials, B.M. French and N.M. Short, eds. (Mono, Baltimore, Md., 1968) 169. 17 R.G. Stearns, C.W. Wilson, H.A. Tiedemann, J.T. Wilcox and P.S. Marsh, The Wells Creek structure, Tennessee, in: Shock Metamorphism of Natural Materials, B.M. French and N.M. Short, eds. (Mono, Baltimore, Md., 1968) 323. 18 D.J. Roddy, The Flynn Creek crater, Tennessee, in: Shock Metamorphism of Natural Materials, B.M. French and N.M. Short, eds. (Men 0, Baltimore, Md., 1968) 29119 W. Von Engelhardt and D~ St6ffler, Ries meteorite crater, Germany, Excursion B-4, Fortsehr. Mineral. Beih 52 (1974) 103.
20 D. St6ffler, Ries deep drilling results; implication for the structure of excavated masses, Meteoritics l0 (1975) 495. 21 D.W. Roy, Back rotation of shatter cones from the Charlevoix astrobleme. P. Que., Canada, Meteoritics 10 (1975) 481. 22 J. Rondot, Impactite of the Charlevoix structure, Quebec, Canada, J. Geophys. Res. 76 (1971) 5414. 23 M.R. Dence, The nature and significance of terrestrial impact structures, 24th Int. Geol. Congr., Sect. 15 (1972) 77. 24 M. Pagel, Cadre g6ologique des gisements d'uranium darts la structure Carsweil (Saskatchewan, Canada). "Etude des phases fluides'. Th6se Dec. Spec., Universit6 de Nancy, Nancy (1975) 158 pp. 25 M.R. Dence, M.J.S. Innes and P.B. Robertson, Recent geological and geophysical studies of Canadian craters, in: Shock Metamorphism of Natural Materials, B.M. French and N.M. Short, eds. (Mono, Baltimore, Md., 1968) 339. 26 R.J. Floran, C.H. Simonds, R.A.F. Grieve, W.C. Phinney, J.S. Warner, M.J. Rhodes, B.M. Jahn and M.R. Dence, Petrology, structure and origin of the Manicouagan melt sheet, Quebec, Canada: a preliminary report, Geophys. Res. Lett. 3 (1976) 49. 27 R.A.F. Grieve, Petrology and chemistry of the impact melt at Mistastin Lake crater, Labrador, Geol. Soc. Am. Bull. 86 (1975) 1617. 28 M.R. Dence and J. Popelar, Evidence for an impact origin for lake Wanapitei, Ontario, Geol. Assoc. Can., Spec. Paper 10 (1972) 117. 29 CoUectif-Bayer Geologisches Landesamt, ed., Geol. Bavarica 72 (1974). 30 M.R. Dence, Impact melts, J. Geophys. Res. 76 (1971) 5552. 31 D. St6ffler, Ries deep drilling: fallback breccia profile and structure of the crater basement, in Abstracts Contributed to the Symposium on Planetary Cratering Mechanics (Lunar Science Institute, Houston, Texas, 1976) 136.
258
THE ROCHECHOUART CRATER: SHOCK ZONING STUDY P. L A M B E R T
Bureau de Recherches Gbologiques et Minibres, Orleans (France)
Revised version received March 18, 1977
Rochechouart crater (France) occurs in crystalline rocks of the northwestern French Massif Central. The crater is deeply eroded and the present ground level is exactly (-+50 m) tangent to the crater floor. No morphologic evidence of the crater has been preserved. A complete range o f shock effects is known on the scale o f rocks and minerals, which permitted a s t u d y of shock zoning. Shock level was determined in thin section from petrographic analysis of each specimen. A systematic sampling was done on all the structure. Shock zones were determined at t h e same time in the fall-back unit and in the target. Correlations between rock types or shock level in allochthonous a n d autocht h o n o u s materials were observed. They imply restrictions for the late excavation stage: part of the material lying above the crater floor limit was never ejected, b u t only mixed with small relative displacement. Shock level is relatively higher in allochthonous breccias than in the target. The distribution of shock effects is very complicated at all scales, even at the scale of the whole structure. However, shock level is relatively high around the center o f the structure defined from the breccia geometry. The most probable impact point is located about 4 k m west o f Rochechouart. The breccia unit is extremely thin (less than 60 m). The crater floor is extremely flat; its elevation does not vary more than -+50 m over the whole structure (about 300 km2). The lack of circular s y m m e t r y in particular in t h e distribution of impact melts could suggest a pronounced anisotropic structure o f the target and/or an inclinated trajectory o f the projectile. The original crater size is most probably between 20 and 25 k m in diameter, determined from t h e actual extent of differents rock types or shock effects. A rapid post-crater readjustment is proposed to explain the flat floor. There was no important vertical displacement since the flat shape o f the crater floor was attained.
Le crat~re de Rochechouart (France) affecte les roches critallophylliennes du NW du Massif Central franqais. I1 a perdu t o u t e expression morphologique car il est tr6s ~rod& Seul le fond du crat6re est pr6serv& La g a m m e des effets de choc est complete h l'6chelle de la roche c o m m e ~ l'6chelle du min6ral. La zondographie est basge sur la r6partition de ces effets de choc. L'intensit~ du m ~ t a m o r p h i s m e de choc est d6termin6e en lame mince pour chaque ~chantinon. Un pr61~vement syst6matique a gt~ fair sur l'ensemble de la structure. La zon6ographie a 6td envisag6e ~ la fois dans la pattie allochtone et dans les zones en place. Les comparaisons sont envisag6es: on observe en particulier des corr6lations locales entre certains facies de roche, ou certains niveaux de choc, entre mat6riel d6plac~ et mat6riel autochtone. Ceci implique certaines restrictions au m6canisme responsable de la fin de rexcavation. Une partie du mat6riel reposant au-dessus du fond du crat~re n ' a donc jamais dt6 ejectS, mais seulement mdlang6, avec des ddplacements relatifs faibles. Les br~ches allochtones sont plus intens~ment choqu~es que celles autochtones. La distribution des effets de choc est tr~s compliqu6e ~ toutes ~chelles, m~me £ ceUe de l'ensemble ~le la structure. Cependant l'intensit6 du choc est relativement plus forte au centre de la structure (d6fini d'apr6s la gdometrie de l'unit6 brdchique). Le point d'impact le plus probable se situe fi 4 k m fi l'ouest de Rochechouart. La formation br6chique est e x t r 6 m e m e n t fine ( < ~ 60 m d'6paisseur). Le plancher du crat6re est e x t r ~ m e m e n t plat: ses variations d'altitude ne d6passent pas -+50 m sur l'ensemble de la structure (couvrant environ 300 km2). Le m a n q u e de sym6trie circulaire, en particulier pour la distribution des produits de fusion sugg~re une anisotropie marqu6e de la cible et/ou une trajectoire inclin6e pour le projectile. La taille, fi l'origine, du crat6re est discut6e d'apr6s la rdpartition de diff6rents types de roches et celle des effets de choc. Les valeurs les plus probables se situent entre 20 et 25 k m de diam6tre. Un r~ajustement rapide du crat~re est supposg pour expliquer le fond plat. I1 n ' y a pas eu d ' i m p o r t a n t s d6crochements verticaux de-
puis.
259 1. Introduction
The Rochechouart impact structure is located about 40 km west of Limoges (France) !n the northwestern part of the French Massif Central. The target area is composed of Hercynian gneisses and granites. The nearest occurrences of sedimentary rocks are about 2 0 - 2 5 km west of Rochechouart. There is no morphologic or topographic evidence of the crater. The present topography is very flat, as a result of quaternary glacial abrasion. Relief (magnitude 30--150 m) is due to small rivers cutting this platform. Good outcrops are rare because of an intense vegetation. The geologic map of the Rochechouart area is published elsewhere [1 ]. The breccia unit covers about 300 km 2. It consists of allochthonous and autochthonous breccias and breccia veins in the target. In the inner zone (13 km diameter), the breccia unit is continuous and subhorizontal. The thickness is less than 60 m, in most cases only some meters. Outside of this zone scattered patches of breccias occur. Breccia veins from 1 cm to some meters wide are common in the crystalline basement rocks. The breccias are sometimes faulted (Fig. 1), but vertical displacement is less than 2 0 - 3 0 m. The most probable age of impact is 165 Myr from K-At measurements [2]. Impact melt and lithic clasts in the breccias are very similar in composition to the target rocks (gneisses, granites) [3]. In particular there is no trace of sedimentary rocks whereas Mesozoic marine sediments (Triassic and Lower Jurassic) which form the margin of the Aquitaine Basin are known 2 0 - 2 5 km west of Rochechouart. The breccias are not layered. There is no graded bedding. They have not been transported. This suggests that impact took place on a very shallow sea or on a water-free area, presumably on the western limit of a N-S horst [4] during the Middle Jurassic. A structural study of the Rochechouart area confirmed the importance of post- (or late) Hercynian faulting oriented N-S on the western part of the structure [5]. In particular N-S dikes of microgranite are related to this phase [5]. This tectonism was still active after breccia deposition [1 ] but movements were slight (see above). Rochechouart breccias have been studied by geologists since the beginning of 19th century [1 ], an impact origin was first suggested in 1967 [6] and
definitely confirmed in 1969 [7] by the evidence of shock metamorphic features. Fall-back breccias of Rochechouart are contaminated by the projectile [3,8] interpreted to be an iron IlA meteorite [9].
2. The zoneographic study of Rochechouart crater This study is based on the evidence of shock effects observed at the rock and mineral scale. All these shock features have been previously described in detail [6,7,1,2] for the Rochechouart structure. The different rock types were systematically sampled and subdivision was deduced from petrographic study in the laboratory (1000 thin sections were studied). In this structure I recognize five types o f transformation of the target rocks from morphological and textural changes. They are significant of increasing shock level at the rock scale and are independent of the physico-chemical characteristics of the cohesive rocks of the target. Type A are autochthonous fractured rocks of the basement, including highly fractured rocks [1 ], and shatter cones. Type B designates autochthonous monomict breccias. Compared to type A rocks, the rock fragments are rotated relative to each other and a matrix of finer clasts is produced. Type C are allochthonous breccias without glass. Clasts of different rock types of the basement are mixed together and the matrix consists of fine-grained clastic material. Type D corresponds to suevites which contain melt inclusions in addition to the clastic material observed in type C. Type E represents impact melts defined as breccia with a glassy matrix. All kinds of shock effects, from the weakest to the strongest, are known in Rochechouart minerals [1,2]. These shock effects are classified in five stages for which P-T conditions are roughly established, as deduced from Hugoniot data of several minerals [ 10, 11 ] an d nuclear explosion data [ 12,13 ] (see Table 1). This classification is adapted from that of progressive shock metamorphism introduced by StiSffler [14]. I have subdivided his stage I into stage Ia and
260 TABLE 1 Classification (adapted from St6ffler [14]) of different shock effects in Rocheehouart minerals used for the shock zoning study of the crater. Approximate pressures are indicated from extrapolation of Hugoniot datas on minerals [10,11] and from nuclear explosion investigation [ 12,13 ] Stage No. 0
Approximate pressure range (kbars) <100
Ia
80-150
Ib
150-350
II III
350-450 >450
Shock effects in minerals
<" /"'9 ?,
fracturing and/or kinking
#
,'~
"..
isotropic quartz and feldspars (diaplectic glass) fused minerals
Ib because weak shock features are the most common in Rochechouart. The limit between I a a n d Ib corresponds to the first evidence of decrease in birefringence in feldspars, and/or development of planar elements in quartz. In stage Ia, only rocks with beginning of development of planar elements in quartz have been plotted. This subdivision is quickly determined from thin section examination. The Dence [16] subdivision of the corresponding range into four classes related to planar-element orientation in quartz was not used because of the time involved for a statistical analysis of 1000 samples.
3. Results
3.1. Extent o f breccia unit and crater floor limit The extent of the breccias is indicated in Fig. 1. The limit of the crater floor cannot be deduced became allochthonous and autochthonous breccias are mapped as one unit. The contact between these two units cannot be determined in the field. The limit of the crater floor is only clearly exposed when aUochthonous breccias are superimposed on undisturbed or shattered basement.
.
~
'
"
'.
~
:::
JF'IIPr"::
:,
beginning of development of planar elements in quartz extensive development of planar dements in quartz (many sets per grain, many elements per set); loss of birefringence of quartz and feldspars (diaplectic minerals)
t
•~
,o
.2
.
4 ~m
"."
i .:.1
Fig. 1. Distribution of shock effects in faU-baekbreccias. Dotted line represents the limit of the breccia unit, solid lines are faults; 1 = Machat, 2 = Babaudus, 3 = Chassenon, 4 = Mountoume. Rocks consisting of more than 25% of fused material are in dicated by large shaded circles, those comprising between 25 and 5% by large, black triangles. Intermediate circles (with dash) correspond to the beginning of fusion and occurrence of diapleetic glasses (isotropized minerals). Small black triangles indicate the occurrence of numerous sets of planar elements in quartz and diapleetic minerals (minerals with unusually low refractive index); small black dots represent few occurrences of planar dements in quartz. On the other hand, there is sometimes a spatial connection between target and allochthonous material. This is the case for the location of some uncommon rock types of the basement such as granophyric microgranite or granoblastic gneiss, and the location of allochthonous polymict breccia showing basement clasts of such rock types. It was found that type C, D and E breccias may occur on top of undisturbed basement, weakly shocked basement (type A), strongly shocked basement (showing diaplectic minerals and diaplectic glasses), and on weakly shocked B breccias, or on strongly shocked B breccias (showing diaplectic minerals and diaplectic glasses). The contact between the breccias and the basement is very fiat throughout the whole unit. In particular the elevation of the contact between autochthonous breccias and the basement is the same as the one between allochthonous breccias and the basement. When the contact of the crater floor is visible, its
261
shape is very complicated and is influenced by structural characteristics of the target [ 1,2].
3.2. Relative importance of each type of shock effect at the rock scale Shattered rocks very commonly underlay the breccia unit [2]. Shatter cones are restricted to the center of the structure (determined from the geometry of the breccia) and are rare compared to highly fractured rocks, but when they are present they occur together. In fact the fracture network [I ] corresponds to the crossing of shatter cone surfaces. Shatter cones are known in each rock type of the Rochechouart basement, but nearly 50% of those recovered are developed in microgranite whereas it represents only about 1% of the exposed target rocks. Structural analysis of the shatter cones and their orientation is not yet finished and will be presented later. Table 2 indicates the relative abundance of each breccia type from the sampled material. Autochthonous (B) and type C breccias are more common in the field than the remaining breccias (types D, E). A volume estimation was done by extrapolation of each breccia extent and by taking into consideration the local value of the breccia thickness (Table 2). This model is limited because the eventual vertical stratigraphy of the different breccia types is neglected. There is also a large possible error in the extrapola-
tion of the lateral extent of each breccia type from this ponctual analysis. Since geophysical prospection (for the lateral extent) and drilling (for the vertical succession) have not been done, this model remains the only possibility. Comparison between direct estimation and volume estimation (see Table 2) shows that there are significant changes for B, C and D breccias because monomict breccias (B) occur over a wide but thin area. Considering the average glass content of suevites and impact melts of Rochechouart, the total proportion of fused to displaced materials is 11% (see Table 2). Except for a few cases in suevites (D), the glass in Rochechouart melts (E) and suevites (D) is entirely recrystaUized to K-feldspars and quartz [3]. Compared to the rock source, glasses are enriched in K and depleted in Na [8]. Fig. 2 shows the approximate glass content of the different specimens analyzed. Most suevites contain very little glass; their average content is less than 13%. 100% glass in Rochechouart impact melts is very rare; the average is about 70%. In a few cases an intermediate stage between D and E breccias was observed where glasses form a part of the matrix and part of the clasts. The average glass content of these rocks is about 40%.
3. 3. Distribution o f the different breccia deposits in the allochthonous blanket Except when the breccia unit is thin enough to be sure that there is only one type lying on the base-
TABLE 2 Proportion of each breccia type sampled in the Rochechouart structure. Volume estimation is made by extrapolation of e a c h breccia type e x t e n t and the local value of t h e breccia t h i c k n e s s ( s e e text) Breccia type *
Percent with respect to whole breccia unit: from n u m b e r o f analyses from volume estimation Percent with respect to aUochthonous breccias from n u m b e r o f analyses from volume estimation Percent glass with respect to allochthonous breecias
B
C
D (average glass content = 13%)
50 19
35 43.5
10 30
70 53.5
20 37.2
10 9.3
4.8
6.5
-
E (average glass c o n t e n t = 70%)
5 8.5
* B = a u t o c h t h o n o u s m o n o m i c t breccias, C = allochthonous polymict breccias w i t h o u t glass, D = suevites, E = impact melts.
262 20,
mber of analyses
15,
2O
lS.
E
10,
S,
O
I'1'1'1'1'1 I I I 10 20 30 40 50 60 70 80 90 100
!i
0
,~1 I't'l'l'['
~ '1
20 30 40 50 60 70 80 90 100
glass ( 7. Fig. 2. Approximate glass contents of suevites (type D) and impact melts (type E) from estimation in thin section.
ment, the stratigraphy of the aUochthonous breccias of Rochechouart is not dear. Contact between the different kinds of allochthonous breccias has not been observed in the field. Moreover the blanket is too thin to look for continuity between the different localities where the studies were done. The whole breccia unit as well as the aUochthonous unit nearly shows a centro-symmetric distribution in the inner zone o f the structure (see section 1). The low values along N-S and NW-SE directions are due to the E-W Vienne river valley cutting the north part o f the structure, and the NW-SE Graine river valley cutting the northwestern part of the structure. C and D breccias are following the same trend. This is not the case for E breccias which are clearly concentrated in a NE-SW band (Table 3).
3.4. Layered unit of Chassenon In the old quarry at Chassenon one occurrence of a layered rock resembling an ash deposit (see Fig. 3)
Fig. 3. Layered rock at the top of the suevite at Chassenon.
263 TABLE 3 Distribution of breccias in the inner zone of the structure and distribution of shock features in the target Maximum extent (km)
B + C + D + E breccias (entire breccia unit) C + D + E breccias (entire allochthonous unit) C breccia D breccia E breccia High shock level in autochthonous unit (occurrence of diaplectic glass) Weak shock level in autochthonous unit (occurrence of planar elements in quartz)
NE-SW
E-W
NW-SE
N-S
12 - 1 3 10 -10.5 1.0-10.5 9.5- 9 10
12 - 1 3 10 -11 8 -11 8.5 2 - 7
8.5-11 6 - 9.5 7 - 7.5 7.5- 8 2 - 6.5
10 - 1 0 10 -10.5 7 -10.5 8 . 5 - 8.5 6 - 7
7
2 -4
2 -4
4 -6
8
8
7
8
was f o u n d at the t o p o f the suevite. This u n i t is only c o m p o s e d o f thin mineral debris, s o m e t i m e s associated with thin debris o f b a s e m e n t rocks (particle size b e t w e e n 10 and 150 ~ m ) . S h o c k effects such as kink bands in micas or planar elements in q u a r t z are comm o n in this unit. The c o n t a c t b e t w e e n suevite and this unit is n o t sharp. There is a m i x e d z o n e a b o u t I 0 c m wide where b a s e m e n t clasts or glass debris are associated w i t h very thin particles.
heterogeneous. Particularly in the center there exist some a l l o c h t h o n o u s breccias w i t h o u t s h o c k effect evidence at the mineral scale, whereas in the same area i m p a c t melts were sampled. The distribution o f
N~
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....~ :: ....
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.'
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o
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3.5. Shock zoning based on shock effects o f minerals For this study I refer to the highest s h o c k effects * observed in the thin section to determine the shock stage o f the specimen. Fig. 1 shows the result o f this study for the allocht h o n o u s breccia unit. The highest shock levels are distributed over the whole unit but t h e y are more f r e q u e n t in the center o f the structure ( d e f i n e d f r o m the breccia g e o m e t r y ) . In particular i m p a c t melts w i t h bubbles are recovered o n l y in this area (the interior o f the bubbles is more or less filled b y glass i m p l y i n g the presence o f a silicate vapor phase). There is no regular decrease f r o m the center towards the periphery. There are some e x t e r n a l maxima, for instance, in the M o n t o u m e area ( s o u t h o f the structure, see Fig. 1) or near Machat ( n o r t h o f the structure, Fig. 1). On a local scale (100 m) the distribution is also * To be significant they have to represent at least 2 - 5 % of registered shock effects of the minerals in the sample.
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Fig. 4. Distribution of shock effects in the target (basement plus autochthonous breceias). Dotted line limits the breccia unit, solid lines are faults; 1 = Machat, 2 = Babaudus, 3 = Chassenon, 4 = Montoume, 5 = Rochechouart. Large shaded circles indicate that at least a part of the minerals of the rock are isotropized (diaplectic glasses). Large black triangles indicate quartz with numerous sets of planar elements and unusually low refractive index for quartz and feldspar (diaplectic minerals); small triangles correspond to few occurrences of planar elements in quartz. Small open circles represent rocks with highly fractured and/or kinked minerals. Large open arrows indicate rocks with evidence of shock effects at the rock scale (brecciation, shattering) but not a t t h e mineral scale.
264 shock effects in one hand specimen is also heterogeneous, even in autochthonous material. However, the range of variation is larger in ejected masses than in the target. Fig. 4 shows the distribution o f shock levels in the basement (type A and B) breccias. The highest values are most common in the center. Even in this nondisplaced material the distribution is heterogeneous; in particular, there are some external maxima such as in the Machat area (north o f the structure, see Fig. 4). There is some indication o f a concentric s y m m e t r y in the distribution of shock levels, except for the highest values (see Fig. 4 and Table 3) which are only located in a NE-SW band. Comparison o f Figs. 1 and 4 shows that the shock level is relatively higher in allochthonous than in autochthonous material. There is a spatial correlation of the highest values between the autochthonous and allochthonous material, not only near the center of the breccia unit but also at the periphery. In the Machat area for instance (north of the structure), a local increase o f shock level in the target corresponds to a local increase of shock level in the allochthonous breccias (Figs. 1,4). 4. Discussion The most probable impact point is deduced from the center o f highest shock level from Figs. 1 and 4. This point is located 4 km west o f Rochechouart and is very close to the center of the allochthonous breccia unit and also o f the whole breccia unit. From this point the maximum extent o f the different shock levels at the mineral and rock scale in the target and in the allochthonus unit are indicated (Fig. 5). The maximum size o f recovered shock effects is 23 km diameter. Circular s y m m e t r y is still present for the whole breccia unit. As the breccias are very thin it is implied that the deep erosion has not changed their original circular distribution. Consequently it is suggested that there was no important faulting o f the actual crater floor geometry (maximum vertical amplitude o f such movement is necvssary less than the breccia thickness: < 6 0 m). Because of the very fiat crater limit and occurrence o f breccias near the probable impact point, it is suggested that there was no prominent uplift at the center o f the crater.
/'
0
6
10
Fig. 5. Scheme of shock zoning at Rochechouart crater: maximum extent of shock effects in the fall-back breccias and in the target. Dashed line limits the breccia unit, solid lines are faults. Circle 1." basement - shatter cones in rocks exhibiting planar elements in quartz; fall-back breccias - evidence of vaporized material from recovery of glass covering the interior of bubbles in impact melts. Circle 2: basement - occurrence of isotropic minerals (diaplectic glass). Circle 3) basement shatter cones or planar elements in quartz. Circle 4: fall-back breccias - glass (suevite and impact melt), planar elements in quartz. Orcle 5: fall-back breccias - maximum extent. Circle 6:20 km diameter - the most probable size of the original crater (see text). Q'rcle 7: basement - brecciation, fractures related to the impact. Circle 8: possible crater size from the melt extent (see text). The flat crater floor is difficult to understand if there was no important movement after the crater excavation. A readjustment seems also necessary to explain the same elevation o f the crater floor and the contact between sedimentary rocks and basement outside o f the structure. In this case, the previous remarks provide serious restrictions for the possible geometry of such supposed post-crater adjustment. They are only consistent with a regular uplifting o f the whole crater floor, which is strongest in the center and decreases towards the periphery. This would drive the original crater floor, paraboloid in shape, to a new one, very fiat, everywhere at the same elevation with less than 50 m difference. Such i m p o r t a n t readjustment must have occurred soon after the crater
Possible Roehechouart crater diameter (km) from previous ratio considering (see Fig. 5): shatter cone extent = 9.5 km breccia extent = 19 km melt extent = 11 km
Ratio of crater diameter to shatter cone extent Ratio of crater diameter to breccia extent Ratio of crater diameter to melt extent
Estimated crater diameter (km) Approximate diameter of shatter cone extent (km) Approximate diameter of breccia extent (km) Approximate diameter of melt extent (kin)
12
1.17
28 [22] 24 [21 ]
14
1.46
35 [23] 24 [21]
Charlevoix (Qu&)
8.5
0.90
18 [24] 20 [24]
14
1.50
30 [23] 20 [241
Carswell (Sask.)
23
1.2
10 [251
12 [25]
Nicholson Lake (N.W.T.)
Estimation of the diameter of the Rochechouart crater from estimated data of several deeply eroded Canadian craters
TABLE 4
>14.5
1.33
60 [26]
>80 [26]
Manicouagan (Qu6.)
- 1 0 [27]
17
-22
1.54- 2.0
13
20 [27]
Mistastin Lake (Labr.)
266
formation because neither breccias nor glasses are displaced and extensively faulted. Because of such readjustment and the actual thickness of the allochthonous unit, any stratigraphic correlation has probably no sense in these remnants. The numerous breccia dikes in the Rochechouart basement prove that there was extensive faulting due to the impact. But this study, as does the regional study from ERTS [5], shows that there are no radial or concentric faults in the Rochechouart target. Circular symmetry of faults associated with impact craters seems to be only typical for those occurring on a quite undisturbed target such as a sedimentary blanket (e.g. Wells Creek [17] or Flynn Creek [18]). When the target is deeply fractured (as is the crystalline basement of Rochechouart), tectonism related to impact assumes the pattern of the previous regional system. The heterogeneous distribution of shock levels in the target is related to the texture and anisotropy of the target at all scales (mineral interfaces, layering in rocks, contacts of different rock type, fractures, regional faults). In allochthonous breccias this distribution is complicated by mixing. This study shows the necessity of a large statistical investigation. It appears that it is not necessary to look for more shock stages in such a kind of study. Only statistical analysis of each different shock effect occurring in a specimen would provide information. But, since textural effects in artificially shocked rocks are not well understood, it is actually not possible to determine the "true" shock level with more accuracy. The spatial correlation of rock type and shock level on all sides of the crater floor suggests that at least part of the allochthonous material of Rochechouart never moved far from its original location, and was probably not ejected. This implies that, if most of the excavation occurred as ballistic ejection, at least the termination of crater growth was characterised by weak movements. The 11% of fused material in the Rochechouart breccias is probably far from the original proportion of fused to displaced material. The particular setting of an impact melt sheet at the bottom of the crater probably involves an unduly high estimation of its volume in the case of Rochechouart. The restricted outcrop of stratified ash on the top of suevite at Chassenon reveals an ambiguous prob-
lem. Because of evidence of shock effects in the thin debris and because of its location, this ash could represent a late deposit of thin particles produced by the impact phenomenon. As in the case of a volcanic event, these fine-grained products could have escaped the main mixing phenomenon of large and small debris and could have needed a longer time to deposit. In the case of a well-preserved crater such as Ries, suevites constitute the top of the fall-out unit [19]. They also occur at the top of the fall-back unit as shown by drilling results at Ries [20] or at Brent [16]. However, such an ash deposit as in Chassenon was never observed. Related or not to the impact phenomenon, this fine debris was deposited after the crater readjustment. The correlation between impact melts and high shock in the underlying basement is reasonable; this boundary represents the original cavity floor. Differential erosion might explain the oriented distribution, but does not seem to be supported by any local or regional argument. The Vienne and Graine river valley responsible for the small ellipticity of B, C, D breccia distribution would explain the melt one, if melt was covering B, C and D breccias. This is not the case. A non-representative sampling is still possible but seems unlikely considering the number of analyses, in particular in the target. More probably, the orientation is primary and related to an ellipsoidal shock wave geometry. This geometry could be due to a large anisotropy of the target, or a low-angle trajectory for the projectile. There is no direct evidence of the original crater size at Rochechouart. Two types of estimation may be made. The first comes from comparison with other old terrestrial craters, and these estimations are summarized in Table 4. The results have only a comparative sense as they are based on estimated data. The second way is to compare general morphologic characteristics of the impact crater. Crater size from the extent o f the allochthonous unit. Because we are at the crater floor it is rather unlikely that fall-out breccias still survive because of erosion. Therefore it can be deduced that the actual allochthonous breccia extent determines the minimum crater size (13.5 km diameter, see Fig. 5). This estimation is pessimistic as erosion tends to reduce the fallback breccias extent at the periphery, because of the
267 prominent rim. East Clear Water Lake crater (Quebec), on crystalline basement, is approximately at the same erosion level as Rochechouart [25]. In his model, Dence presumed an original crater 16 km in diameter [25]. In any case, values obtained from comparison with Carswell and Charlevoix for shatter cone extent are too low.
Crater size from the limit o f disturbed rocks. Recent geophysical studies o f Wanapitei [28] and Ries [29] indicate that the disturbed zone (faulting and weak shock features) may be quite thick under the impact point in the target. It decreases radially near the periphery, and is non-existent at the surface. Consequently, whatever the erosion level o f the crater, this zone is restricted in the target to an area less than the original crater diameter. This area is 23 km in diameter at Rochechouart (see Fig. 5). Comparison o f the Rochechouart zoning to the Ries deep drilling results. At Ries, a young crater 24 k m in diameter where the morphology is still preserved [19], three recent drilling studies indicate that there is no impact melt, even near the center. The nearest is located 3.8 k m from it [ 2 0 - 2 9 ] . Three explanations are possible:((1) there is no impact melt layer at the b o t t o m o f the Ries crater; (2) there are scattered patches o f melt, but the three drill holes missed them; and (3) there is a continuous melt sheet at the b o t t o m of Ries crater o f less than 3.8 km radius. As compared to other large impact craters, the first hypothesis is very improbable as we do not know of other cases. The second is possible, in particular if we compare Ries to the actual geometry o f Rochechouart impact melts. However, the actual lack o f connection between the occurrences of melt at Rochechouart is not conclusive of an original scattering because o f the very deep level o f erosion of the crater. The isolated occurrences o f melt at Mistastin are considered to represent remnants of a large sheet o f impact melt which once lined the crater cavity [19]. The third possibility seems the most probable as it is the regular scheme for large craters [30]. Tlfis hypothesis is also favored b y St6ffler [31] from recent volumetric calculations o f melt in Ries suevites. In such a case Rochechouart could be at least 35--40 km diameter, as impact melts occur in an area 11 km in diameter.
5. Conclusion Tile Rochechouart crater, produced on Hercynian basement during Middle Jurassic times by an iron meteorite was very probably more than 16 km in diameter. The most probable values are between 20 and 25 km. The shock wave geometry was not spherical but probably elhpsoidal. I propose that there was a rapid uplift of the b o t t o m o f the crater after its formation. However, this uplift never produced a prominent hill at the center but a flattening o f the crater floor. Since this rapid readjustment there have been no important vertical tectonic movements. The crater is now deeply eroded and the actual ground level is tangent to the crater floor. As the crater floor is very flat, the breccia unit covers a large area but is extremely thin. However, all the breccias types and all shock effects at the scale of rocks and minerals are known in this restricted volume. Moreover, this exceptional location permits the investigation o f shock zoning in the target and in the allochthonous unit. The distribution of shock effects is very complicated and heterogeneous at all scales, even in autochthonous material. But it is possible to do a regional interpretation from statistical measurements.
References 1 P. Lambert, Etude g6ologique de la structure impactitique de Rochechouart (Limousin, France) et son contexte, Bull. BRGM 2, Sect. 1 (1974) 153. 2 P. Lambert, La structure d'impact de m6t6orite g~ante de Rochechouart, Th~se Doc. Spec., Universit~ de Paris-Sud, Paris (1974) 148 pp. 3 P. Lambert, Rochechouart impact crater, statistical geochemical investigations and meteoritic contamination, in: Proceedings of the Planetary Cratering Symposium (in press). 4 J. Delfau, Un ~16ment majeur de la pal6og6ographie du sud de la France au Jurassique moyen et sup~rieur. Le haut fond occitan. C.R.J. Soc. C~ol. Fr. 2 (1973) 58. 5 P. Lambert, La structure impactitique de Rochechouart (Limousin) et son contexte structural r6gional, par interpretation de "photo satellite" image ERTS, Bull. BRGM 2, Sect. 1 (1974) 177. 6 F. Kraut, Sur l'origine des clivages du quartz darts les br~ches "volcaniques" de la r~gion de Rochechouart, C.R. Acad. Sci. Fr. 264 (1967) 2609. 7 F. Kraut, Uber ein neues Impaktit-Vorkommen in Gebeite von Rochechouart, Chassenon (Departements Haute-Vienne und Charentes, Frankreich), Geol. Bavarica 61 (1969) 428.
268 8 P. Lambert, Nickel enrichment of impact melt rocks from Rochechouart; preliminary results and possibility of meteoritic contamination, Meteoritics l0 0975) 433. 9 M.J. Jansens, J. Hertogen, H. Takahashi, E. Anders and P. Lambert, Rochechouart meteorite crater: identifcation of projectile, J. Geophys. Res. (in press). 10 J. Wackerle, Shock wave compression of quartz, J. AppL Phys. 33 (1962) 922. 11 T.J. Ahrens and J.T. Rosenberg, Shock metamorphism, experiments on quartz and plagioclase, in: Shock Metamorphism of Natural Materials, B.M. French and N.M. Short, eds. (Mono, Baltimore, Md., 1968) 59. 12 N.M. Short, Progressive shock metamorphism of quartzite ejecta from Sedan nuclear explosion crater, Goddard Space Flight Center, Greenbelt, Md., Preprint X, 622, 69, 537 (1969). 13 J. Faure, Recherches sur les effets g6ologiques d'explosions nuel6aires souterraines dam un massif de granite saharien, Rap. CEA R, 4257 (1972) 273 pp. 14 D. St6ffier, Progressive metamorphism and classification of shocked and brecciated crystalline rocks at impact craters, J. Geophys. Res. 76 (1971) 5541. 15 O.B. James, Shock and thermal metamorphism of basalt by nuclear explosion, Nevada test site, Science 166 (1969) 1615. 16 M.R. Dence, Shock zoning at Canadian craters: petrography and structural implications, in: Shock Metamorphism of Natural Materials, B.M. French and N.M. Short, eds. (Mono, Baltimore, Md., 1968) 169. 17 R.G. Stearns, C.W. Wilson, H.A. Tiedemann, J.T. Wilcox and P.S. Marsh, The Wells Creek structure, Tennessee, in: Shock Metamorphism of Natural Materials, B.M. French and N.M. Short, eds. (Mono, Baltimore, Md., 1968) 323. 18 D.J. Roddy, The Flynn Creek crater, Tennessee, in: Shock Metamorphism of Natural Materials, B.M. French and N.M. Short, eds. (Men 0, Baltimore, Md., 1968) 29119 W. Von Engelhardt and D~ St6ffler, Ries meteorite crater, Germany, Excursion B-4, Fortsehr. Mineral. Beih 52 (1974) 103.
20 D. St6ffler, Ries deep drilling results; implication for the structure of excavated masses, Meteoritics l0 (1975) 495. 21 D.W. Roy, Back rotation of shatter cones from the Charlevoix astrobleme. P. Que., Canada, Meteoritics 10 (1975) 481. 22 J. Rondot, Impactite of the Charlevoix structure, Quebec, Canada, J. Geophys. Res. 76 (1971) 5414. 23 M.R. Dence, The nature and significance of terrestrial impact structures, 24th Int. Geol. Congr., Sect. 15 (1972) 77. 24 M. Pagel, Cadre g6ologique des gisements d'uranium darts la structure Carsweil (Saskatchewan, Canada). "Etude des phases fluides'. Th6se Dec. Spec., Universit6 de Nancy, Nancy (1975) 158 pp. 25 M.R. Dence, M.J.S. Innes and P.B. Robertson, Recent geological and geophysical studies of Canadian craters, in: Shock Metamorphism of Natural Materials, B.M. French and N.M. Short, eds. (Mono, Baltimore, Md., 1968) 339. 26 R.J. Floran, C.H. Simonds, R.A.F. Grieve, W.C. Phinney, J.S. Warner, M.J. Rhodes, B.M. Jahn and M.R. Dence, Petrology, structure and origin of the Manicouagan melt sheet, Quebec, Canada: a preliminary report, Geophys. Res. Lett. 3 (1976) 49. 27 R.A.F. Grieve, Petrology and chemistry of the impact melt at Mistastin Lake crater, Labrador, Geol. Soc. Am. Bull. 86 (1975) 1617. 28 M.R. Dence and J. Popelar, Evidence for an impact origin for lake Wanapitei, Ontario, Geol. Assoc. Can., Spec. Paper 10 (1972) 117. 29 CoUectif-Bayer Geologisches Landesamt, ed., Geol. Bavarica 72 (1974). 30 M.R. Dence, Impact melts, J. Geophys. Res. 76 (1971) 5552. 31 D. St6ffler, Ries deep drilling: fallback breccia profile and structure of the crater basement, in Abstracts Contributed to the Symposium on Planetary Cratering Mechanics (Lunar Science Institute, Houston, Texas, 1976) 136.