The terrestrial cratering record

ICARUS38, 212--229 (1979)

The Terrestrial Cratering Record I. Current Status of Observations 1 R. A. F. G R I E V E AND P. B. R O B E R T S O N Earth Physics Branch, Department of Energy, Mines and Resources, Ottawa K1A OY3 Canada Received August 11, 1978; revised October 18, 1978 The location, size, and principal characteristics of the currently known proven and probable terrestrial impact structures are tabulated. Of the 78 known probable structures, only 3 are Precambrian and the majority are (300 my in age. A survey of the variation in preservation with size and age indicates that, unless protected by sedimentary cover, a structure ( 2 0 km in diameter has a recognizable life of (600 my. The depth-diameter relationships of terrestrial structures are similar to lunar craters; however, it is believed that terrestrial craters were always shallower than their lunar counterparts. Complex structures formed in sedimentary targets are shallower than those in crystalline targets, and the transition from simple to complex crater morphology occurs in sedimentary strata at approximately one-half the diameter of the morphology transition in crystalline rocks. This is a reflection of target strength. Although observations indicate that crater size, target strength, and surface gravity are variables in the formation of complex craters, they do not permit an unequivocal choice between collapse and rebound processes for the formation of complex structures. It may be that both processes act together in the modification of crater morphology during the later stages of excavation. The major emphasis of recent shock metamorphic studies has been toward the development of models of cratering processes. An important contribution has been the identification, through meteoritic contamination in the melt rocks, of the type of bolide at a number of probable impact structures. This has also served to strengthen the link between the occurrence of shock metamorphic effects and their origin by hypervelocity meteorite impact. INTRODUCTION T h e c u r r e n t high degree of scientific i n t e r e s t in the process of h y p e r v e l o c i t y i m p a c t c r a t e r i n g was g e n e r a t e d b y t h e results of t h e l u n a r missions a n d has been m a i n t a i n e d t h r o u g h the M a r i n e r a n d Viking programs. I t is now generally a c k n o w l e d g e d t h a t i m p a c t c r a t e r i n g was a f u n d a m e n t a l process in the e v o l u t i o n of all t h e t e r r e s t r i a l p l a n e t s (Shoemaker, 1977). A m o n g t h e m a n y p r o b l e m s which an u n d e r s t a n d i n g of i m p a c t c r a t e r i n g can a d d r e s s are the e v o l u t i o n of p l a n e t a r y surfaces, the d e p t h l Contribution from the Earth Physics Branch, No. 746. 212

0019-1035/79/050212-18502.00/0 Copyright O 1979 b y Academic Press, Inc. All rightm of reproduction in any form reserved.

of origin of i m p a c t - d e r i v e d surface lithologies, a n d the e s t a b l i s h m e n t of a r e l a t i v e e v o l u t i o n a r y t i m e scale b e t w e e n t h e planets. A principal source of d a t a for e v a l u a t i n g c r a t e r i n g processes has been t h e i n t e n s i v e s t u d y of t e r r e s t r i a l i m p a c t structures, a n d m a j o r emphasis has r e c e n t l y been given to modeling t h e physical processes a c c o m p a n y i n g h y p e r v e l o c i t y i m p a c t ( R o d d y et al., 1977). This c o n t r i b u t i o n was p r o m p t e d b y t h e need for a c u r r e n t c o m p i l a t i o n of i m p a c t s t r u c t u r e s as a d a t a b a s e for the d e r i v a t i o n of a t e r r e s t r i a l c r a t e r p r o d u c t i o n rate, a n d t h u s a d a t a point in the e s t a b l i s h m e n t of c r a t e r counts as an i n t e r p l a n e t a r y d a t i n g

OBSERVATIONS ON TERRESTRIAL CRATERS

213

TABLE I PROVEN METEORITE IMPACT CRATERS : STRUCTURES WITH ASSOCIATED METEORITES a

Name

Latitude Longitude Number Diameter (largest)

Reference

(m) 35°02~N Barringer, Arizona, USA 22°37'S Boxhole, N. T., Australia 27°38'S Campo del Cielo, Argentina Dalgaranga, W. A., Australia 27°43'S 37°37'N Haviland, Kansas, USA 24°34'S Henbury, N. T., Australia 58°24'N Kaalij~rvi, Estonian SSR 52°29'N Morasko, Poland 31°48'N Odessa, Texas, USA Sikhote Alin, Primorye Terr. 46°07'N Siberia, USSR 46018'N Sobolev, Siberia, USSR 21°30PN Wabar, Saudi Arabia Wolf Creek, W. A., Australia 19°10~S

111°0VW 135°12'E 061 °42'W 117°05'E 099°05'W 133°10'E 022°40'E 016°54'E 102°30'W

1 1 20b 1 1 14 7 7 3

134040'W 137°52'E 050°28'E 127°47'E

122~ 1 2 1

1200 185 90 21 i1 150 110 100 168 26.5 51 97 850

Roddy et al., 1975 Krinov, 1966 Cassidy el al., 1965 Krinov, 1966 Krinov, 1966 Milton, 1968 Krinov, 1966 Freeburg, 1966 Evans, 1961 Krinov, 1966 Khryanina and Ivanov, 1977 Chao et al., 1961 Guppy and Matheson, 1950

a All proven meteorite craters are Recent in age and are simple, bowl-shaped structures. All have at least partly preserved rims and some have preserved ejecta. Total includes impact pits. technique. I n addition to providing a more complete listing of terrestrial i m p a c t structures t h a n the recent catalog of Classen (1977), the present compilation has been expanded to include an u p d a t e of the s u m m a r y of observations on terrestrial i m p a c t structures given b y Dence (1972). No m a j o r changes in the criteria given in Dence (1972) for the recognition of i m p a c t structures have been made. T h e occurrence of shock m e t a m o r p h i c effects remains the principal criterion, a n d the recognition of such effects has led to a d r a m a t i c increase in recent years in the n u m b e r of probable i m p a c t structures recognized in various p a r t s of the world. I n particular, Soviet scientists h a v e identified over 20 structures with shock m e t a morphic effects in the USSR, none of which were listed as probable i m p a c t structures in Dence (1972). A significant a d v a n c e in the general area of shock m e t a m o r p h i s m has been the identification of meteoritic c o n t a m i n a t i o n in i m p a c t melts a t some probable i m p a c t structures (Palme et al., 1978a). This has strengthened

the observational link between proven craters, with meteorite f r a g m e n t s and shock m e t a m o r p h i c effects, and large probable i m p a c t structures, with only shock m e t a morphic effects. CLASSIFICATION AND TABULATION Terrestrial i m p a c t craters h a v e been classified into : (1) proven meteorite craters, structures with associated meteorite fragments, and (2) probable meteorite craters, structures with shock m e t a m o r p h i c effects in the target rocks b u t lacking discrete meteorite fragments. T h e location and size of currently known sites in these two categories are listed in Tables I and I I , respectively. I n f o r m a t i o n on age, t a r g e t rock type, degree of preservation, morphology, and a single reference, corresponding where possible to the most comprehensive or recent easily accessible work, are also given for each structure. No t a b u l a t i o n has been m a d e of small i m p a c t pits, < 9 m in diameter, in which shock m e t a m o r p h i c effects are generally absent and the meteorite remains virtually

Aouelloul, Mauritania Araguainha Dome, Brazil Beyenchime-Salaatin, USSR Boltysh, Ukrainian SSR, USSR Bosumtwi, Ghana "B.P." structure, Libya Brent, Ontario, Canada Carswell, Saskatchewan, Canada Charlevoix, Quebec, Canada Clearwater L. East, Que., Canada Clearwater L. West, Que., Canada Conception Bay, Nfld., Canada Crooked Creek, Missouri, USA DeeaturviUe, Missouri, USA Deep Bay, Saskatchewan, Canada Dellen, Sweden Flynn Creek, Tennessee, USA Gosses Bluff, N. T., Australia Gow L., Saskatchewan, Canada Haughton Dome, N. W. T., Canada Holleford, Ontario, Canada Ile Rouleau, Quebec, Canada Ilintsy, USSR Janisj ~rvi, USSR Kaluga, USSR Kamensk, USSR Kara, USSR

Name

20015'N 16°46'S 71 °50'N 48045'N 06032'N 25"19'N 46005'N 58°27'N 47°32~N 56°05~N 56°13'N 47°27'N 37°50~N 37°54'N 56°24'N 61055'N 36°16'N 23°50'S 56°27'N 75°22'N 44°28'N 50°41'N 48°45'N 61 °58'N 54°30'N 48°20'N 69010'N 012°41'W 052°59'W 123°30'E 032°10'E 001°25'W 024°20'E 078°29'W 109°30'W 070°18'W 074°07'W 074°30rW 053°12tW 091°23'W 092°43'W 102°59'W 016°32'E 085"37'W 132°19'E 104°29'W 089°40'W 076°38'W 073°53'W 028°00rE 030°55'E 036°15'E 040°15'E 065 °00'E

0.37 40 8 25 10.5 2.8 3.8 37 46 22 32 ? 5.6 6 12 15 3.8 22 5 20 2 4 4.5 14 15 25 50

3.1 ± 0.3 <250 <65 100 -4- 5 1.3 -4- 0.2 <120 450 ± 30 485 ± 50 360 ± 25 290 -4- 20 290 ± 20 ~500 320 -~ 80 <300 100 ± 50 230 360 ± 20 130 ± 6 <200 15 550 ± 100 <300 495 ± 5 700 360 ± 10 65 57

Age (my) Sed Sed and Cry Sed Cry Cry Sed Cry Sed and Cry (Sed)Cry (Sed)Cry (Sed)Cry Cry Sed Sed (Cry) Cry Cry Sed Sed Cry Sed (Cry) Sed (Cry) Sed Cry Cry Sed and Cry Sed Sed

4 6 3 4 2 6 4 7 6 4 5 7 6 6 3 6 3 6 6 2 4 6 5 6 4 5 5

S Cr C C C C S C C C Cr ? C C C C C C C C S C S C C C C

Target Rock° Pres.~ Morph. c

II PROBABLE IMPACT CRATERS: STRUCTURES WITH SHOCK METAMORPHISM

Latitude Longitude Diameter (kin)

TABLE

Fudali and Cassidy, 1972 Dietz and French, 1973 Masaitis, 1975 Yurk et al., 1975 Littler et al., 1961 French eta/., 1974 Dence, 1968 Currie, 1969 Robertson, 1975 Dence el al., 1965 Dence et al., 1965 Engelhardt, 1975 Kiilsgaard et al., 1963 Offieldand Pohn, 1977 Dence et al., 1968 Svensson, 1968 Roddy, 1977a Milton et al., 1972 Thomas and Innes, 1977 Robertson and Grieve, 1978 Beals, 1960 Caty et al., 1976 Val'ter and Ryabenko, 1973 Masaitis et al., 1976a Masaitis, 1975 Masaitis, 1975 Masaitis, 1975

Reference

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Karla, RSFSR Kelly West, N. T., Australia Kentland, Indiana, USA Kjardla, Est. SSR Kursk, USSR Lac Couture, Quebec, Canada Lac La Moinerie, Quebec, Canada Lappaj~rvi, Finland Liverpool, N. T., Australia Logoisk, Bel. SSR Lonar, India Manicouagan, Quebec, Canada Manson, Iowa, USA Mien L., Sweden Middlesboro, Kentucky, USA Misarai, Lith. SSR Mishina Gora, USSR Mistastin, Labrador, Canada Monturaqui, Chile New Quebec Crater, New Que., Can. Nicholson L., N. W. T., Canada "Oasis," Libya Obolon, USSR Pilot L., N. W. T., Canada Popigai, USSR Puehe~h-Katunki, USSR Redwing Ck., N. Dakota, USA Ries, Germany

Name

57°45'N 19°30'S 40°45'N 57°00~N 51°40'N 60°08'N 57°26'N 63°09'N 12°24'S 54°12'N 19°58'N 51°23'N 42°35'N 56°25'N 36°37'N 54°00'N 58°40'N 55°53'N 23°56'S 61°17'N 62°40tN 24°35'N 49°30'N 60017'N 71°30~N 57°06'N 47°40tN 48°53~N

Latitude

048°00'E 132°50'E 087°24'W 022°42'E 036°00'E 075°18'W 066°36'W 023°42'E 134°03~E 027°48'E 076°31'E 068°42'W 094°31'W 014°52tE 083°44'W 023°54'E 028°00'E 063°18'W 068°17'W 073°40'W 102°4VW 024°24~E 032°55'E lll°01'W lll°00tE 043°35~E 102°30tW 010°37~E

18 2.5 13 4 5 8 8 14 1.6 17 1.83 70 32 5 6 5 2.5 28 0.46 3.2 12.5 11.5 15 6 100 80 9 24

Longitude Diameter (km) 10 550 300 500 ± 50 250 :t: 80 420 400 <600 150 ~ 70 100 ± 20 0.05 210 ± 4 <70 118 ± 2 300 500 =t= 80 <360 38 -4- 4 1 5 <450 <120 160 <:300 38 ± 9 183 :t: 3 200 14.8 :t: 0.7

Age (my)

TABLE II--Continued

Sed Sed Sed Sed? Sed and Cry Cry Cry Cry Sed Sed? Cry (Sed) Cry Sed and Cry Cry Sed Sed? Sed (Cry) Cry Cry Cry (Sed) Cry Sed Cry Cry Sed and Cry Sed and Cry Sed Sed and Cry

4 7 7 ? 5 6 7 6 3 ? 2 5 4 6 7 ? 5 6 2 3 6 6 5 6 3 4 4 2

Target Rock~ Pres)

C ? C ? C C C C S ? S Cr C C C ? C C S S C Cr C C? Cr C C Cr

Morph. ~

Masaitis et al., 1976b Tonkin, 1973 Laney and Van Schmus, 1978 Masaitis, pers. comm. Masaitis, 1975 Beals et al., 1967 Robertson and Grieve, 1975 Lehtinen, 1976 Guppy et al., 1971 Masaitis, pers. comm. Fredriksson et al., 1973 Murtaugh, 1977 Hoppin and Dryden, 1958 Svensson, 1969 Engiund and Roen, 1963 Masaitis, pers. comm. Masaitis, 1975 Grieve, 1975 Sanchez and Cassidy, 1966 Currie, 1966 Dence el al., 1968 French et al., 1974 Masaitis et al., 1976b Dence et al., 1968 Masaitis et al., 1975 Firsov, 1965 Brenan et al., 1975 Pohl et al., 1977

Reference

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23 2.5 5 23 6.4 12 2.5 13 52 30 25 3.4 24 140 1.3 1.9 8 140 8.5 14 2.7 1.4 10

Longitude Diameter (km)

45°49'N 000°50'E 49 °00'N 032°00'E 61 °23'N 022°25'E 51°4TN 098°33'W 39°02'N 083°24'W 046°52'W 08°05'S 42°42'N 072°42'E 30°36'N 102°55'W 61 °02'N 014°52'E 48°40'N 087°00'W 59°31'N 117°38'W 48°41'N 010°04'E 133°35'E 15°12'S 46°36'N 081°ll'W 44°06'N 109°36'E 22°55'N 010°24'W 55°06'N 024°36'E 027°30'E 27°00'S 46°44'N 080°44'W 36°23'N 087°40'W 49°46'N 095°11'W 47°25'N 035°23'E 49°00'N 061°00'E

Latitude

6 4 7 4 7 7 3 6 7 6 3 3 5 6 2 3 ? 7 5 7 4 ? 3

C S ? C C C S C C C C C C C S S ? C C Cr S S C

Target Rocko Pres. b Morph. c

160 4- 5 Cry 70 Cry 490 Cry 225 ± 40 Sed and Cry 300 Sed <300 Sed 12 Cry 100 Sed 365 4- 7 Sed and Cry 350 (Sed) Cry 95 4- 7 Sed and Cry 14.8 4- 0.7 Sed 150 4- 70 Sed (Cry) 1840 4- 150 Cry <30 Cry 2.5 4- 0.5 Cry 160 ± 30 Sed 1970 ± 100 Sed and Cry 37 4- 2 Cry 200 -4- 100 Sed 100 ± 50 Cry 120 ~- 20 ? 4.5 4- 0.5 (Sed) Cry

Age (my)

Lambert, 1977a Masaitis et al., 1976b Papunen, 1969 McCabe and Bannatyne, 1970 Bull et al., 1967 Dietz and French, 1973 Fel'dman and Granovsky, 1978 Wilshire et al., 1972 Svensson, 1971 Halls and Grieve, 1976 Carrigy, 1968 Reiff, 1977 Guppy et al., 1971 Guy-Bray, 1972 Shkerin, 1976 French et al., 1970 Masaltis, pets. comm. Manton, 1965 Denee and Popelar, 1972 Wilson and Stearns, 1968 Short, 1970 Masaitis, 1977 Florensky et al., 1977

Reference

Sed, sedimentary; Cry, crystalline; (), minor. b 1, Ejecta largely preserved; 2, ejecta partly preserved; 3, ejeeta removed, rim partly preserved; 4, rim largely eroded, crater-fill products preserved; 5, crater-fill products partly preserved ; 6, only remanents of crater-fill preserved, crater floor exposed ; 7, crater floor removed, substructure exposed. c S, Simple crater; C, complex structure with central uplift ; Cr, complex structure with multiring form.

Rochechouart, France Rotmistrovka, USSR S~ksj~rvi, Finland St. Martin, Manitoba, Canada Serpent Mound, Ohio, USA Serra da Canghala, Brazil Shunak, Kaz. SSR Sierra Madera, Texas, USA Siljan, Sweden Slate Is., Ontario, Canada Steen River, Alberta, Canada Steinheim, Germany Strangways, N. T., Australia Sudbury, Ontario, Canada Tabun-Khara-Obo, Mongolia Tenoumer, Mauritania Vepriaj, Lith. SSR Vredefort, S. Africa Wanapitei L., Ontario, Canada Wells Ck., Tennessee, USA West Hawk L., Manitoba, Canada Zeleny Gai, Ukr. SSR Zhamanshin, Aktyubinsk, USSR

Name

TABLE II--Continued

t~ ~V

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OBSERVATIONS ON TERRESTRIAL CRATERS intact (Krinov, 1966). A short, somewhat subjective, list of possible impact structures is given in Table III. These are structures with some physical evidence compatible with an impact origin but lacking definitive shock metamorphic features. The list of proven impact structures contains two additions over that given in Dence (1972): the Morasko craters in Poland and the Sobolev crater in the USSR (Table I). The world population of probable impact structures currently stands at 78 (Table II). This is an increase of 36 over the listing in Dence (1972) and is an indication of the rapid rate at which shock metamorphic structures have been recognized in recent years. The most recent catalog of terrestrial impact structures is by Classen (1977). It uses the same criteria for identification as this paper; however, it is incomplete and lists only 54 structures as having shock metamorphic effects. As well as numerous additions, there has been one deletion from the tabulation of probable impact structures in Dence (1972). The KSfels structure in Austria has been relegated to the possible category (Table III). Although KSfels contains fused rocks and mineral glasses (Storzer et al., 1971), the presence of shock metamorphic effects has not been satisfactorily proven, and a good case has been made for the structure and associated deformation being the result of a massive landslide (Erisemann et al., 1977). DISTRIBUTION AND AGE As noted by Classen (1977), known terrestrial structures of proven or probable impact origin are not evenly distributed over the Earth's land surface. By far the majority are in the Northern Hemisphere (Tables I and II). This is in part due to the larger areal extent of land in this hemisphere. Most of the probable impact structures are located on the North

217

American and European cratons, which are areas of relative geologic stability and thus conducive to the preservation of terrestrial craters. However, they are also areas in which various institutions and individual workers have undertaken active programs of research into impact craters. Proven terrestrial impact structures are all Pleistocene to Recent in age. This is not unexpected, as their small size (Table I) and the preservation of discrete meteorite fragments essentially require that they be relatively young in age. The youngest is Sikhote Alin, which occurred in 1947, and the oldest (and largest) is probably Barringer at a few tens of thousands of years. Probable impact structures range in age from Precambrian, the oldest being Vredefort at 1970 :t: 100 my (Table II), to Pleistocene. Less than 50% have welldefined ages, as determined by radiometric dating of their impact-melted lithologies. The majority have stratigraphic ages of variable reliability, and in some cases the age of the structure is only known to be younger than that of the surface on which it was formed. There is a bias towards the recognition of young probable impact structures, with 70% of the known shock metamorphic sites being ~300 my old (Fig. 1). Figure 1 gives no clear indication of any relationship between the size and age of recognized structures. Such a relationship might be expected as the probability of recognizing a terrestrial impact structure should vary directly with size and inversely with age (Zotkin et al., 1978). However, this assumes that the factors affecting the preservation of terrestrial craters, and thus the ease with which they can be recognized, are constant throughout the total world population. A survey of the literature indicates that this is not the case and that the presence or absence of a postcrater sedimentary cover is an important factor in

218

GRIEVE AND ROBERTSON TABLE III POSSIBLE IMPACT CRATERSa Name Aliou, Algeria Al Umchaimin, Iraq Amguid, Algeria Colonia, Brazil Darwin Crater, Tasmania, Australia Des Plaines, Illinois, USA Dumas, Sask., C a n a d a Dycus, Tennessee, USA Eagle Butte, Alberta, C a n a d a Elbow, Saskatchewan, C a n a d a El'gygytgyn, Chukotsk, USSR b Glasford, Illinois, USA Glover Bluff, Wisconsin, USA Gusev, USSR Hartney, Manitoba, C a n a d a Howell, Tennessee, USA Hummeln, Sweden Ilumetsy, Estonian, SSR (3) J e p t h a Knob, Kentucky, USA Kalkkop, South Africa Kilmichael, Mississippi, USA KSfels, Austria Labynkyr, Y a k u t SSR, USSR Lac Kakiattukallak, Que., Can. Lake Teague, W. A., Australia Meen L., N. W. T., C a n a d a Merewether, Labrador, C a n a d a Michlifen, Morocco (2) Murgab, Tadzhik SSR, USSR (2) Mr. Toondina, S. A., Australia Nyika Plateau Crater, Zambia-Malawi Ouarkziz, Algeria Patom, Irkutsk Prov., USSR Poplar Bay, Manitoba, C a n a d a Pretoria Salt Pan, South Africa Ramgarh, India Riachao Ring, Brazil Roter K a m m , South West Africa Sao Miguel do Tapuio, Brazil Sithylemenkat L., Alaska, USA Skeleton L., Ontario, C a n a d a Talemzane, Algeria Temimichat, M a u r i t a n i a T v a r e n Bay, Sweden Upheaval Dome, Utah, USA Veevers, W. A., Australia Versailles, K e n t u c k y USA Viewfield, Saskatchewan, C a n a d a Wetumpka, Alabama, USA Yenisei Ridge Crater, USSR

Latitude

Longitude

Diameter (km)

38°04'N 32°41'N 26°05'N 23 °52'S 42°15'S 42°02'N 49°55'N 36°22'N 49°42'N 50°58'N 67°30'N 40°22'N 43°55'N 48 °20'N 49°24'N 35°15'N 57°22'N 57°58'N 38°11'N 32°43'S 33°30'N 47°13'N 62°30'N 57°42'N 25°50'S 64°58'N 58°02'N 32°00'N 38°06'N 27°35'S 10°35'S 29°00'N 59°00'N 50°23'N 25°30'S 25°20'N 07°43'S 27°46'S 05°37'S 66°07'N 45°15'N 33 °18'N 24°15'N 58°46'N 38°26'N 22°58'S 38°02'N 49°35'N 32°32'N 59°00'N

002 °03'E 039°35'E 004°23'E 041 °24'W 145°36'E 087°56'W 102 °07'W 085o45'W l10°30'W 106°45'W 172°05'E 089°48'W 089°35'W 040°15'E 100°40'W 086°35'W 016°15'E 025 °25'E 085°07'W 024 °34'E 089°33'W 010°58'E 143°00 E 071 °40'W 120°55'E 087°40'W 064°02'W 003°00'E 074°20'E 135 °10'E 033 °43'E 007°30'E 116°25'E 095°47'W 028 °00'E 076°37'E 046 °39'W 016°18'E 041 °24'W 151 °23'W 079°26'W 004°06'E 009°39'W 017°25'E 109°54'W 125°22'E 084°45'W 103°04'W 086°14'W 093°09'E

5 3.2 0.4 3 1 8 4 -10 8 23 5 0.43 3 6 2.4 1.2 0.08 3.2 0.64 13 4 60 6 28 4 0.2 1.9 0.08 2 0.08 3.5 0.09 3 1 3 4 2.4 12 12.4 3.6 1.75 0.73 2 5 0.08 1.5 2.5 6.5 0.225

Age (my) < 100 -<3 -0.7 =t= 0.1 < 300 < 70 __ <40 <80 <40 440 65 150 4- 30 450 500 4- 100 <1 -< 200 0.008 < 600 <600 <0.01 -<1 < 120 <1 < 70 <1 100 =t= 50 <1 ---<0.01 450 4- 50 <3 <3 --<450 <450 200 100 <1

a Shock metamorphism not established. b Shock metamorphism recently discovered; is now considered probable impact structure (Gurov et al., 1978).

OBSERVATIONS ON TERRESTRIAL CRATERS I

I

I

219

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15 World population: Age-Size distribution probable impact structures

D~ km 1-4~ 4-16 ~ _

16-64~ N

!iiiiiiiiiiill

÷ .+÷+++, ~÷+÷÷+~ .+÷+÷÷~

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. . . . . . . . . . . . . .

200

300 m.y.

400

500

600

probableimpact structures, <600 my and >1 km in diameter.

the preservation and recognition of terrestrial craters. An a t t e m p t has been made to estimate the maximum preservation age for probable impact structures of a particular size. This was done b y plotting the observed degree of preservation against a preservation index, defined as (diameter in k m ) / ( a g e in my) (Fig. 2). Although it would be more correct to use true crater depth rather than diameter as a measure of size, when considering preservation or erosional level, it was found that the data on crater depths are too sparse to provide a good sample of the population. T h e various levels or degree of preservation follow those given in Dence (1972) and are listed in Table II, with level 7 corresponding to the exposure of the substructure of the crater and being the deepest level of erosion at which a probable impact structure is still recognizable. As indicated in Fig. 2 most probable impact structures have preservation indices greater than 0.03. M a n y of those structures with lower preservation indices are or were buried b y a postcrater cratonic cover. Others have only maximum age estimates

based on the age of the target rocks. Thus, it is considered t h a t a minimum preservation index of approximately 0.03 is required at preservation level 7 (Fig. 2) for the recognition of probable impact structures which have not been protected from erosion by sedimentary cover. This means that a 20-km terrestrial structure will be recognizable as a probable impact structure for a maximum of 600 my, whereas a 10-km structure, unless protected, will have a lifetime of only 300 my. This is a very general statement, for as indicated in Fig. 2 there is a considerable range in preservation index for a given preservation level within the group of unprotected structures. T h e range is due to large variations in the degree of preservation with age. These variations may be very local. For example, the Clearwater West and Mistastin structures are approximately the same size, 32 and 28 km, respectively, and are located only 500 k m a p a r t on opposite sides of the QuebecLabrador watershed in the Canadian Shield. T h e y differ in age b y approximately 250 my (Table II) but it is the younger structure, Mistastin at 38 -4- 4 my ( M a k e t

220

GRIEVE AND ROBERTSON I

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Preservation Index ( size, km/age, m.y.)

FIG. 2. Plot of preservation level, as defined in Table II, against preservation index (diameter, kin/age, my) of probable impact structures. All structures unprotected by cratonie cover and with radiometric or well-documented stratigraphic ages have preservation indices <0.03.

al., 1976), which is the more poorly preserved (Table TI). MORPHOLOGY There are two morphological or structural classes of impact craters : (i) simple craters, which have a bowl-like form with an uplifted and overturned rim; and (ii) complex craters, which have an uplifted central peak and a slumped or depressed rim. In the larger complex craters, the central peak may be replaced or augmented b y a concentric series of uplifts and depressions, giving the structure a multiring form. T h e observed transition from simple to complex structures takes place at a diameter of approximately 4 k m in structures formed in crystalline rocks. In sedimentary strata, the change in crater form occurs at a diameter of 2 to 3 km. Figure 3 is a plot of the a p p a r e n t diameter against a p p a r e n t and true depth for probable terrestrial impact structures,

for which some estimate of these dimensions are in the literature. Apparent d e p t h is from the top of the crater rim to the top of the crater-fill and true depth is from the rim to the base of the crater-fill, t h a t is to the autochthonous country rock floor of the crater. T o minimize the effect of erosion on crater dimensions, the structures plotted were restricted to those with preservation levels of 4 or b e t t e r (Table II) t h a t is, structures where at least a vestige of the original rim remains. T h e data plotted for structures of level 4 take account of the fact t h a t at this level the rim has largely been eroded. A correction has been applied for the a m o u n t of erosion as estimated in the literature or b y comparison with less eroded structures of a similar size and morphology. Much of the scatter for the larger structures in Fig. 3 may arise from the uncertainties due to erosion and the variable quality of the data on crater depths. Some depth estimates are well controlled b y drilling results, while

221

OBSERVATIONS ON T E R R E S T R I A L CRATERS

been modified to some degree by erosion. However, it is unlikely that pristine terrestrial craters ever conformed to the lunar depth-diameter relationship, as this would require erosion rates as high as 500 to 2000 m / m y for such young structures as Barringer and Lonar. It is believed that because of the higher surface gravity terrestrial craters were always shallower than their lunar counterparts. The transition from the simple to the complex form is indicated in Fig. 3 by the break in slope in the depth-diameter relationship. For complex structures with diameters in excess of approximately 4 km formed in crystalline rocks:

others are based on the interpretation of geophysical data or estimated from geological and structural observations. The depth-diameter relationships for simple craters with diameters less than 3.8 km formed in crystalline targets and diameters less than 1.2 km in sedimentary targets (Fig. 3) are described by the least-squares regression lines: Pt = 0.326D °'786,

(1)

P, = 0.159D °.8~9,

(2)

where Pt and Pa are the true and apparent depths, D is the rim diameter, and all measurements are in kilometers. Equation (2) can be compared with the depthdiameter measurements of simple craters on other planets, for example, Pa = 0-196D 1"°1°

Pt = 0.52D °'189.

However, the scatter is too large to derive a meaningful regression line between the apparent depth and diameter for structures in crystalline rocks. For complex structures greater than 2.5 km in diameter in sed-

(3)

for the Moon (Pike, 1974). The differences between (2) and (3) in part reflect the fact that all the terrestrial craters have I

I

I

I

I

I II

I

(4)

I

I

,

I

I

I

II

I

I

I

I

I

I

II

,

Lunar D t< ~P°/D rein" • . _ _ _ _ ~ 1 ~ ~ 1

t.0--

,

True ~.f].//~ ."

/

,

ii

,

1

tt?

_I

II

_]

v O..

"l

0.t--

IJ

///~Apparent depth Po

Structures in : c = crystalline targets e--x sedimentary targets

Z ~o L

DIAMETER, D (kin) I

I

I

I I IIIJ

I

I

I

I

I

I IIII

10

I

I

I

i

I I

100

FIG. 3. Depth-diameter relationships of proven and probable impact structures with preservation levels of less than 4. Note shallower depth of complex structures in sedimentary targets relative to structures in crystalline targets.

222

GRIEVE AND ROBERTSON

imentary rocks the equations are: Pt ~- 0.204D °'~7°,

(5)

Pa - 0.140D °'311.

(6)

The approximate parallelism of these expressions and that for lunar complex structures, P~ = 1.044D °.a°~ (Pike, 1974), suggests that the apparent depth-diameter expression for terrestrial complex structures in crystalline rocks should also approach P ~ ~ DO.3.

It may be inferred from Fig. 3 that with more data the terrestrial record would show the double branching in the depthdiameter curves observed in lunar and Mercurian craters (Malin and Dzurisin, 1978; Pike, 1974), where some craters with diameters that exceed that of the simple-complex transition retain the depthdiameter relationship of simple craters. The observed apparent depth of complex structures in sedimentary rocks is approximately 50% less than that of structures with the same diameter in crystalline rocks (Fig. 3). This, along with the transition to complex structures occurring at smaller diameters, is presumably a reflection of differences in rock strength. Among the various hypotheses advanced to account for the change in crater morphology from simple to complex are: cometary versus meteorite impact (Milton and Roddy, 1972), deep-seated centripetal collapse (Quaide et al., 1965; Gault etal., 1968), and elastic rebound (Baldwin, 1963; Pike, 1977). The cometary hypothesis, with the impact of iron and stone meteorites responsible for simple structures and low-density comets for complex structures, is the least tenable. The regular relationship between crater size and morphology (Fig. 3; Malin and Dzurisin, 1978; Pike, 1974) does not support this hypothesis. In addition, the conclusion that the simple Brent and complex Clearwater East structures were both formed by chondritic bodies (Palme et al., 1978a,b)

and the complex Rochechouart structure by an iron meteorite (Lambert, 1977b) cast further doubt on the cometarymeteorite hypothesis. A quantitative evaluation of gravitational collapse adjustments has been undertaken by Melosh (1977). In the collapse hypothesis, all impact structures have the same form during the excavation stage (Dence, 1973) and the final form depends on the type of collapse during the modification stage. In simple structures it is slope failure of the walls, and in complex structures it is floor failure of the volume of the target rocks containing the crater. The type of collapse is governed by the amount that some critical strength parameter is exceeded. This parameter is a function of excavated cavity depth, rock density, rock strength, and surface gravity. The observations that (i) the onset of complex structure formation is a function of crater size, (ii) terrestrial complex craters form in weaker sedimentary targets at smaller diameters than in crystalline rocks (Fig. 3), and (iii) the transition to complex structures in crystalline targets on Earth occurs at approximately one-sixth of the diameter of those on the Moon (Pike, 1977) support Melosh's suggestion that cavity depth, rock strength, and surface gravity are factors in cavity modification. A problem with the collapse model is that the observational data from the Moon require that lunar rocks have a strength of only 3 MPa (30 bars) during collapse of the cavity (Melosh, 1977). The data in Fig. 3 indicate approximately the same value for terrestrial craters in crystalline rocks and about half that for structures in sedimentary rocks. Melosh (1977) has suggested that such unusual conditions of low rock strength may be achieved by shearing of the shocked rocks following cavity excavation. The role of stress-wave interactions and elastic rebound in changing crater morphology has been recently modeled through

OBSERVATIONS ON TERRESTRIAL CRATERS

223

computer simulations (Ullrich et al., 1977). suite of shock metamorphic effects have Stratigraphic arguments from structures been largely identified and described for such as Gosses Bluff and Sierra Madera the common rock-forming minerals. These and observations on explosion craters have descriptions of shock metamorphic effects been cited in favor of the rebound hypoth- and stages of shock metamorphism, preesis (Pike, 1977; Roddy, 1977b). Observa- sented and summarized by Chao (1968), tions on the morphology of impact-melted Engelhardt and StSffler (1968), StSffler lithologies suggest that at complex struc- (1972, 197.4), and others, have been tures the modification of the excavated augmented or modified to a comparatively cavity may not be entirely a late-stage small extent by recent laboratory shock event in the cratering process (Grieve et al., experiments and further petrographic ob1977). The possible initiation of central servations from terrestrial impact craters. One example of the more recent data on uplift formation during the excavation process favors the rebound hypothesis, shock deformation is from the Al2SiO5 which should occur in time periods an system. Experiments on kyanite (Liu, order of magnitude shorter than gravita- 1974) and andalusite (Schneider and Horntional collapse (Ullrich et al., 1977). emann, 1975) reveal a breakdown to However, crater modification during the corundum and stishovite above 57.5 GPa highly dynamic conditions accompanying (1 GPa ~- 10 kbar), rather than a single excavation may well supply the conditions high-pressure phase. Although this breaknecessary to lower rock strength to the down has not been observed to date at impact structures, Robertson and Grieve required values for gravity collapse. It is apparent that the present stage of (1978) report the possible conversion of knowledge is such that neither the collapse sillimanite to mullite (3A1203.2SIO2) plus nor rebound hypotheses can be ruled out. cristobalite (SiO~) in diaplectic gneisses It is possible that both mechanisms act shocked to maskelynite grade from the together in complex crater formation. In Haughton structure. such a scenario central uplift formation Recent interest in the effects of shock would be initiated by rebound prior to on olivine and pyroxene stems from their rim collapse due to gravitational instability occurrence in shocked lunar samples rather and gravitational collapse would commence than from their terrestrial associations. during the highly dynamic phase of crater Studies of these minerals have virtually all excavation during which conditions of been from laboratory experiments, such reduced rock strength may be possible. as those on dunites (Reimold and StSffler, The fact that the rim rocks of impact 1978; Gibbons et al., 1975a; Jeanloz et al., craters are relatively unshocked, whereas 1977). Jeanloz et al. (1977) record the the rocks of the central uplift have been first occurrence of minor amounts of shocked to well in excess of their Hugoniot shock-produced olivine glass. Where the elastic limit (Robertson and Grieve, 1977) target materials have been basalts or single could conceivably result in differences in crystals, experiments on various pyroxenes the physical response of the target rocks. indicate little shock deformation up to This could permit the two modification pressures of approximately 55 GPa. Howprocesses to act virtually simultaneously in ever, with powdered pyroxene a substantial different parts of the structure. portion of the target material has been shock-vitrified by 50 to 65 GPa (Gibbons SHOCK METAMORPHIC STUDIES et al., 1975a,b). One group of minerals whose shock The range and variety of the unique modes of deformation which constitute the behavior remains poorly documented is

224

GRIEVE AND ROBERTSON

the carbonates, such as calcite and dolomite. With the discovery of the young Haughton structure, where carbonate clasts abound in the well-preserved, highly shocked ejecta deposits (Robertson and Grieve, 1978), this deficiency may soon be remedied. In recent years a number of probable impact structures have come to light through the discovery and recognition of shock features where no obvious crater-like structure had been noted. Shatter cones found at the Slate Islands (Halls and Grieve, 1976) and at Ile Rouleau (Caty et al., 1976) led to the realization that the islands were the central uplifts of otherwise submerged complex craters. Planar deformation features in quartz discovered in a few localities near Conception Bay, Newfoundland, indicate that an ancient impact site exists in this region, although the circular structure itself has apparently been obliterated (Engelhardt and Walzebuck, 1978). The emphasis on shock metamorphic research has largely shifted from the documenting of particular shock effects to applying such data to an understanding of crater-forming processes. Data from laboratory shock experiments on quartz combined with observations on the progressive shock damage of this mineral at probable impact structures have defined shock wave attenuation rates, which agree reasonably well with theoretical determinations for simple and complex structures (Robertson and Grieve, 1977; Engelhardt and Graup, 1977). However, the correlation of laboratory shock pressures with an equivalent degree of deformation in a natural event may not be directly applicable. Although studies are still in progress, it appears that the pressures required for the development of a particular deformation may be inversely proportional to the magnitude of the shock event, a function presumably of shock-pulse duration. For example, particular planar features in

quartz which are estimated to require pressures near 12 GPa in a large hypervelocity impact crater (Robertson, 1975) require 15 or more GPa, as measured directly by stress gauges, in a nuclear explosion (Borg, 1972) and approximately 20 GPa, as calculated from equation-ofstate data, in a laboratory-scale experiment (HSrz, 1968). The origin of shock metamorphic effects under shock pressures and high strain rates is now well established from laboratory and nuclear experiments. Debate over the origin of probable impact structures, where shock effects are present but meteoritic fragments do not exist, continues although somewhat abated. Those who favor a meteoritic origin argue that the presence of characteristic shock effects in both small hypervelocity craters, where meteorite fragments are preserved, and in the structurally continuous but larger probable impact structures is sufficient evidence for a common mode of origin. This argument has been greatly strengthened through recent intensive geochemical studies of the impact melt rocks from several probable impact sites. Siderophile trace element contents in the melt rocks have indicated contamination by a stony meteorite component at Clearwater East, Brent, and at the Ries structures (El Goresy and Chao, 1976; Grieve, 1978a,b; Morgan et al., 1977; Palme et al., 1978a,b). In addition, the Ni enrichment at the Rochechouart structure has been interpreted in terms of a Type IIA iron as the impacting body (Lambert, 1977b). Thus, although meteoritic material in its original form is not present at these probable impact structures, the existence and nature of the meteorite can still be diagnosed. This represents critical added evidence to support a meteoritic origin and reinforces the relationship between shock metamorphism and hypervelocity impact.

OBSERVATIONS ON TERRESTRIAL CRATERS CONCLUSION I t is a p p a r e n t t h a t t h e E a r t h has r e t a i n e d a n impressive record of cratering d u r i n g t h e Phanerozoic. T h e recent recognition of a large n u m b e r of probable i m p a c t s t r u c t u r e s in t h e U S S R bodes well for the discovery of a d d i t i o n a l i m p a c t structures, t h r o u g h the use of such t e c h n i q u e s as L a n d s a t i m a g e r y , in o t h e r less t h o r o u g h l y i n v e s t i g a t e d areas as S o u t h A m e r i c a a n d Africa. T h e P r e c a m b r i a n cratering record still remains sparse. H o w e v e r , the scientific c o m m u n i t y ' s increasing general awareness of the p e t r o g r a p h i c effects of s h o c k m a y well lead to the recognition of ancient, deeply eroded s t r u c t u r e s which no longer h a v e o b v i o u s c r a t e r forms. Less e m p h a s i s is n o w being placed u p o n the recognition of terrestrial craters a n d more on detailed observations. T h e s e o b s e r v a t i o n s h a v e s t r e n g t h e n e d the link between shock metamorphism and hyperv e l o c i t y m e t e o r i t e i m p a c t a n d are being used to c o n s t r a i n physical models of cratering processes. I n turn, these models p r o v i d e a f r a m e w o r k for t h e i n t e r p r e t a t i o n of the relative histories of the terrestrial planets, where i m p a c t c r a t e r i n g is n o w recognized as a f u n d a m e n t a l process occurring d u r i n g early p l a n e t a r y evolution. ACKNOWLEDGMENTS We would like to thank V. L. Masaitis for providing information on a number of structures in the USSR. This paper constitutes Contribution No. 29 of the Basaltic Volcanism Study Project, which is organized and administered by the Lunar and Planetary Institute, operated by the Universities Space Research Association under Contract NSR 09-051-001 with the National Aeronautics and Space Administration. REFERENCES BALDWIN, R. B. (1963). The Measure of the Moon. Univ. of Chicago Press, Chicago. BZALS, C. S. (1960). A probable meteorite crater of Precambrian age at Holleford, Ontario. Publ. Dominion Observ. Ottawa 24, No. 6. BEALS, C. S., DENCE, M. R., AND COHEN, A. J. (1967). Evidence for the impact origin of Lae

225

Couture. Publ. Dominion Observ. Ottawa 31, No. 10. BORG, I. Y. (1972). Some shock effects in granodiorite to 270 kilobars at the Piledriver site. In Flow and Fracture of Rocks (N. C. Heard, Ed.), pp. 293-312. Amer. Geophys. Union, Geophys. Monograph No. 16. BRENAN, R. L., PETERSON, B. L., AND SMITH, H. J. (1975). The origin of Red Wing Creek structure, McKenzie County, North Dakota. Wyom. Geol. Assoc. Earth Sc£ Bull. 8, No. 3. BULL, C., CORBATO,C. E., ANDZAHN,J. C. (1967). Gravity survey of the Serpent Mound area, southern Ohio. Ohio J. Sci. 67, 359-371. CARRIGY, M. (1968). Evidence of shock metamorphism in rocks from the Steen River structure, Alberta. In Shock Metamorphism of Natural Materia~ (B. M. French and N. Short, Eds.), pp. 367-378. Mono, Baltimore. CASSIDY, W. A., VILLAR, L. M., BUNCH, T. E., KOHMAN, T. P., AND MILTON, D. J. (1965). Meteorites and craters of Campo del Cielo, Argentina. Science 149, 1055-1064. CXTY, J.-L., CHOWN,E. H., ANDROY, D. W. (1976). A new astrobleme: Ile Rouleau structure, Lake Mistassini, Quebec. Canad. J. Earth Sci. 13, 824-831. CHAO, E. C. T. (1968). Pressure and temperature histories of impact metamorphosed rocks-Based on petrographic observations. N. Jahrb. Mineralogle 108, 209-246. CHAo, E. C. T., FAHEY, J. J., AND LITTLSR, J. (1961). Coesite from Wabar crater, near A1 Hadida, Arabia. Science 133, 882-883. CLASSEN, J. (1977). Catalogue of 230 certain, probable, possible and doubtful impact structures, Meteoritics 12, 61-78. CURRIE, K. L. (1966). Geology of the New Quebec crater. Geol. Surv. Canad. Bull. 150. CURRIE, K. L. (1969). Geological notes on the Carswell circular structure, Saskatchewan (74K). Geol. Surv. Canad. Pap. 57-32. DENCE, M. R. (1968). Shock zoning at Canadian craters: Petrography and structural implications. In Shock Metamorphism of Natural Materials (B. M. French and N. M. Short, Eds.), pp. 169-184. Mono, Baltimore. DENCE, M. R. (1972). The nature and significance of terrestrial impact structures. 2Jth Internat. Geol. Congr. Sect. 15, 77-89. DENCE, M. R. (1973). Dimensional analysis of impact structures. Meteoritics 8, 343-344. DENCE, M. R., AND POPELAR, J. (1972). Evidence for an impact origin for Lake Wanapitei, Ontario. In New Developments in Sudbury Geology (J. V.

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