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Meteoritics amp Planetary Science 39 Nr 9 1495ndash1508 (2004)Abstract available online at httpmeteoriticsorg

1495 copy Meteoritical Society 2004 Printed in USA

Shocked rocks and impact glasses from theElrsquogygytgyn impact structure Russia

Eugene P GUROV1 and Christian KOEBERL2

1Institute of Geological Sciences National Academy of Sciences of the Ukraine 55b Oles Gontchar Street Kiev 01054 Ukraine2Department of Geological Sciences University of Vienna Althanstrasse 14 A-1090 Vienna Austria

Corresponding author E-mail christiankoeberlunivieacat

(Received 18 July 2003 revision accepted 4 May 2004)

AbstractndashThe Elrsquogygytgyn impact structure is about 18 km in diameter and is located in the centralpart of Chukotka arctic Russia The crater was formed in volcanic rock strata of Cretaceous agewhich include lava and tuffs of rhyolites dacites and andesites A mid-Pliocene age of the crater waspreviously determined by fission track (345 plusmn 015 Ma) and 40Ar39Ar dating (358 plusmn 004 Ma) Theejecta layer around the crater is completely eroded Shock-metamorphosed volcanic rocks impactmelt rocks and bomb-shaped impact glasses occur in lacustrine terraces but have been redepositedafter the impact event

Clasts of volcanic rocks which range in composition from rhyolite to dacite represent all stagesof shock metamorphism including selective melting and formation of homogeneous impact meltFour stages of shocked volcanic rocks were identified stage I (le35 GPa lava and tuff contain weaklyto strongly shocked quartz and feldspar clasts with abundant PFs and PDFs coesite and stishoviteoccur as well) stage II (35ndash45 GPa quartz and feldspar are converted to diaplectic glass coesite butno stishovite) stage III (45ndash55 GPa partly melted volcanic rocks common diaplectic quartz glassfeldspar is melted) and stage IV (gt55 GPa melt rocks and glasses) Two main types of impact meltrocks occur in the crater 1) impact melt rocks and impact melt breccias (containing abundantfragments of shocked volcanic rocks) that were probably derived from (now eroded) impact meltflows on the crater walls and 2) aerodynamically shaped impact melt glass ldquobombsrdquo composed ofhomogeneous glass The composition of the glasses is almost identical to that of rhyolites from theuppermost part of the target Cobalt Ni and Ir abundances in the impact glasses and melt rocks arenot or only slightly enriched compared to the volcanic target rocks only the Cr abundances show adistinct enrichment which points toward an achondritic projectile However the present data do notallow one to unambiguously identify a meteoritic component in the Elrsquogygytgyn impact melt rocks

INTRODUCTION

The Elrsquogygytgyn impact crater is situated in the centralmountainous area of the Chukotka Peninsula Russiacentered at 67deg30primeN and 172deg34primeE (Fig 1) The Elrsquogygytgyndepression was discovered in 1933 from an aerogeologicalstudy of this region (Obruchev 1957) in comparison withlunar craters this author suggested a volcanic origin forElrsquogygytgyn The possible impact origin of the crater was firstproposed by Nekrasov and Raudonis (1963) Howeverbecause these authors did not find coesite in eight thinsections of volcanic rocks from the crater rim they concludedthat the structure has a tectonic origin Nevertheless Zotkinand Tsvetkov (1970) and Engelhardt (1974) included theElrsquogygytgyn depression in their lists of probable impact

structures Using these data Dietz and McHone (1976)studied LANDSAT images of the site and concluded that it isprobably the largest Quaternary impact crater on EarthShortly thereafter Dietz (1977) suggested that theElrsquogygytgyn crater could be a possible source of theAustralasian tektites

The impact origin of the Elrsquogygytgyn structure wasconfirmed by Gurov and co-authors from the recognition ofshock metamorphic effects in the volcanic rocks from thecrater (Gurov et al 1978 1979a b) Subsequent petrologicalgeochemical and geophysical studies of the Elrsquogygytgynimpact crater and its rocks have been carried out by Feldmanet al (1981) Dabija and Feldman (1982) and Kapustkina etal (1985)

The age of the Elrsquogygytgyn impact crater was determined

1496 E P Gurov and C Koeberl

to be 452 plusmn 011 Ma by fission track dating (Storzer andWagner 1979) 345 plusmn 015 Ma by later fission track dating(Komarov et al 1983) and 350 plusmn 050 Ma by K-Ar dating(Gurov et al 1979b) The most recent 40Ar39Ar date yieldedan age of the impact glasses of 358 plusmn 004 Ma (Layer 2000)which is in good agreement with the earlier data All thesedates exclude a relation between the Australasian tektites andthe Elrsquogygytgyn crater

During the last years the Elrsquogygytgyn crater andespecially its post-impact sedimentary record have beenintensely investigated in a joint program of the Alfred-Wegener Institute Potsdam Germany the University ofMassachusetts Amherst USA and the North-EastInterdisciplinary Scientific Research Institute MagadanRussia A 127 m-long core of the lake sediments was studiedfor paleoclimatic information of this continental region of theArctic (Brigham-Grette et al 1998 Nowaczyk et al 2002)The dynamics and morphology of ice covering theElrsquogygytgyn Lake were studied from radar satellite data byNolan et al (2003) who also speculated on the location of apossible central uplift that is buried by the lake sediments

The purpose of the present paper is to summarize for theinternational literature some geological and petrographicobservations on the Elrsquogygytgyn structure made by one of us(E Gurov) over several decades supplemented by some newpetrographic and geochemical data

GEOLOGIC BACKGROUND

The Elrsquogygytgyn crater is a flat-floored circular basinwith a rim-rim diameter of about 18 km (Fig 2) that issituated in the central part and southeastern slope of theAcademician Obruchev Ridge in central Chukotka The craterfloor about 14 km in diameter is occupied by the nearlycircular Elrsquogygytgyn Lake which is 12 km in diameter and upto 170 m deep in its center A complex system of lacustrine

terraces surrounds the lake The highest terrace is about 80 mabove the lake level and the lowest one is 1ndash3 m high Thirty-eight streams run into the lake and only the Enmivaam Riverflows out of the lake and determines the water level in it Acentral peak is not exposed on the recent surface of the craterfloor Gravity measurements by Dabidja and Feldman (1982)indicate the presence of a central peak about 2 km indiameter underneath post-impact sediments centered relativeto the crater outline In contrast Nolan et al (2003) suggestfrom radar images that the central uplift might be centeredrelative to the lake outline (ie asymmetric relative to theactual crater) New seismic data (F Niessen personalcommunication to C Koeberl 2004) seem to agree better withthe earlier gravity data

The crater is surrounded by an uplifted rim that has anasymmetrical cross section with steep inner walls and gentleouter slopes The average height of the rim about 180 mabove the lake level and 140 m above the surrounding areawas calculated from 24 radial morphological profiles of thecrater The initial height of the rim before erosion wasestimated to be about 230 m (Gurov and Yamnichenko 1995)A low concentric outer rim was discovered by morphologicalinvestigations of the structure The height of the second rim isup to 14 m at a distance of 175 crater radii

Fig 1 Location of the Elrsquogygytgyn impact crater on the ChukotkaPeninsula northeast Russia

Fig 2 Satellite (Seasat SAR) image of the Elrsquogygytgyn impactcrater The crater floor is occupied by the Elrsquogygytgyn Lake about12 km in diameter The Enmivaam River (dark in the image markedwith arrow) crosses the crater rim in its southeastern part North isshown with an arrow Image courtesy J Garvin (NASA GoddardSpace Flight Center)

Shocked rocks and impact glasses from Elrsquogygytgyn 1497

A complex system of faults surrounds the crater (Fig 3)Short radial faults predominate while concentric arcuatefaults and some other fault types are subordinate around theimpact structure The lengths of the faults range from 05 kmto 10 km In some cases they are expressed on the surface astrenches that are one to several m wide Fault zones arecomposed of poorly consolidated detrital material with fine-grained matrix without any indications of hydrothermalalteration

The areal distribution of faults around the Elrsquogygytgyncrater was studied using aerial photographs The highest faultdensity occurs within the inner walls of the structure at adistance of 09ndash10 crater radii from the center The arealdensity of the fault system decreases outward from the craterrim to a distance of about 27 crater radii where it reaches theregional level (Gurov and Gurova 1983)

Megablock breccia zones occur at the northern andwestern inner slopes of the crater rim within an area of about4 km2 The megabreccia shows a gradual transition tointensely faulted crater rim and is buried underneathlacustrine sediments Block sizes in the megabreccia rangefrom tens to hundreds of m They are cemented by detritalmaterial These blocks are poorly exposed in relief but arevisible in aerial photographs The blocks of the megabrecciaare composed of rhyolites ignimbrites and rhyolite tuffs Noshock metamorphic effects have been noted in the rocks of thecrater rim

The target rocks of the crater comprise the volcanic strataof the Pykarvaam and Milguveem series of Albian toCenomanian age (Kotlar et al 1981 Feldman et al 1981)Laser 40Ar39Ar dating of the volcanic rocks of the craterbasement show ages from 832 to 893 Ma (Layer 2000) Thevolcanic rocks include lava tuffs and ignimbrites of rhyolitesand dacites rarely andesites and andesitic tuffs (Fig 4) Thesedeposits dip gently with 6deg to 10deg to the east-southeast Ageneralized sequence of the strata was composed using theseparate sections of the volcanic rock complex exposed at thecrater and partly on the outer slopes of the rim (Gurov andGurova 1991) From top to bottom the exposed rocks areignimbrites (250 m) tuffs and lava of rhyolites (200 m) tuffsand lava of andesites (70 m) ash tuffs and welded tuffs ofrhyolites and dacites (ge100 m)

Thus the thickness of the whole exposed section is morethan 600 m This section of volcanic rock strata corresponds tothe western northern and northeastern sections of the craterarea while dacite and andesite lava and tuff predominate in itssoutheastern part A basalt plateau about 075 km2 in size and110 m thick occurs on the surface of the rhyolites andignimbrites in the northeastern part of the crater rim butshocked basalts are very rare in the crater The composition ofthe main types of volcanic rocks of the crater basement ispresented in Table l The average target rock compositiongiven in Table 1 was calculated from the relative thicknessesof the target rocks described in the previous paragraph

Fig 3 Fault system around the Elrsquogygytgyn impact crater (after fieldwork by E Gurov and E Gurova)

Fig 4 Schematic geological map of the Elrsquogygytgyn impact craterafter Gurov and Gurova (1991) with additional information (fieldwork by E Gurov and E Gurova) North is up The locations of someof the more important samples on the terraces from which impactrocks were studied are indicated by solid circles


Ignimbrites and tuffs of rhyolites and dacites are the mostabundant rock types of the crater basement The compositionof the ignimbrites is given in Table 2 Mineral clasts andfragments of those rocks are quartz orthoclase (Or80Ab20 toOr60Ab40) and plagioclase (An20 to An30) The refractiveindices of unshocked orthoclase are nγ = 1526ndash1529 nα =1520ndash1523 and birefringence of 0006ndash0007 and ofplagioclase are nγ = 1548ndash1553 nα = 1542ndash1547 andbirefringence of 0006 Mafic minerals are biotite and rarelyamphibole The fine-grained matrix of lavas and tuffs consistsof quartz and feldspars Andesites and andesite tuffs containfragments and clasts of andesine (An45 to An40)clinopyroxene and amphibole

SAMPLES AND ANALYTICAL METHODS

Samples

The samples for this study were collected by E Gurovand E Gurova during expeditions to the Elrsquogygytgyn craterfrom 1977 to 1980 All types of impact rocks are redepositedon the terraces of the Elrsquogygytgyn Lake The samples ofunshocked volcanic rocks of the crater target were collected atthe crater rim the inner walls and the outer slopes of thestructure

Apart from samples described by Gurov and Gurova(1991) the following samples were studied specifically forthis paper

bull El-669-8 (impact melt rock) Dark gray vesiculardevitrified glass with abundant inclusions of mineralsand glass clasts Mineral inclusions are predominantly

shocked quartz and lechatelierite Greenish-gray brownand black glass clasts up to 4 cm in diameter in sharpcontact with the melt

bull El-669-10 (impact melt rock) Contains abundant lithicand mineral clasts and glass fragments The impact glassshows a fluidal texture with vesicles up to 1 mm indiameter Clasts le2 mm in diameter are stronglyshocked volcanic rocks and glasses Mineral inclusionsare shocked quartz and lechatelierite

bull El-669-5 (partly melted rhyolite) Gray fluidal vesicularheterogeneous glassy matrix with relics of quartzphenocrysts Quartz is converted into transparentdiaplectic glass with coesite veinlets Remnants of blackhomogenous impact melt occur on clast surfaces

bull El-985-91 (glass bomb) 11 times 21 mm in size Dark fluidaltransparent glass with bands of brown glass Raremineral clasts are lechatelierite and shocked quartz

bull El-689 (glass bomb) Aerodynamically shaped bombThe glass is semitransparent and dark colored It containsvery rare lechatelierite clasts up to 03 mm in diameterRare vesicles are up to 1 mm in diameter

bull El-699 (glass bomb) This sample is very similar to theprevious one Glass is semitransparent and shows fluidaltexture Inclusions comprise shocked quartz andlechatelierite Vesicles occur in the rim parts of the bomb

Experimental Methods

Petrographic and mineralogical investigations of impactrocks were carried out by optical microscopy Shockmetamorphic effects of siliceous volcanic rocks were studied(Gurov and Gurova 1979 1991) and the orientation of planardeformation features (PDFs) in quartz was determined using afour-axis universal stage Refractive indices of quartz andfeldspars were measured with immersion liquids to estimateshock pressures using the data presented by Stoumlffler andLangenhorst (1994) The presence of high pressure phases ofsilica and some other minerals was determined by X-raydiffractometry To detect stishovite and coesite fractions ofshocked quartz were dissolved in hydrofluoric acid

The chemical composition of impact rocks and basementrocks was determined by wet chemical methods at theInstitute of Geological Sciences of the Ukraine Splits ofunaltered impact rocks were carefully selected under abinocular microscope The composition of minerals wasanalyzed by electron microprobe analysis using a Camebaxinstrument at the Institute of Geological Sciences of theUkraine (cf Gurov and Gurova 1991)

Concentrations of major elements V Cu Y and Nb onthe new samples were measured on powdered samples bystandard X-ray fluorescence (XRF) procedures (see Reimoldet al [1994] for details on procedures precision andaccuracy) All other trace elements were analyzed byinstrumental neutron activation analysis (INAA see Koeberl[1993] for details) For most samples Sr and Zr

Table 1 Major element composition of volcanic rocks from the Elrsquogygytgyn impact cratera

Ignimbrite (7)

Rhyolitetuff and lava (16)

Andesite-dacitetuff and lava(5)

Target averageb

SiO2 7016 7251 6300 7072TiO2 033 030 056 034Al2O3 1447 1279 1657 1390Fe2O3 188 131 340 176FeO 105 076 137 086MnO 008 005 012 006MgO 073 049 159 072CaO 225 116 373 201Na2O 303 221 329 257K2O 392 536 308 448P2O5 008 007 014 010CO2 048 068 091 062H2O 027 041 056 039

Total 9873 9810 9832 9853aWet chemical analyses Data in wt Numbers in parentheses give the

number of samples analyzedbTarget average composition was calculated in accordance with the relative

thickness of the main volcanic rock types in the strata of the basement rocksof the crater


concentrations were determined by both XRF and INAA andfor some of the low abundance samples Ni data were alsoobtained by XRF Iridium was measured with γ-γmultiparameter coincidence spectrometry (see Koeberl andHuber [2000] for details)

SHOCK METAMORPHISM OF VOLCANIC ROCKS OF THE ELrsquoGYGYTGYN IMPACT CRATER

Impact rocks are exposed at the Elrsquogygytgyn crater at thepresent-day erosional level only in redeposited state Impactmelt rocks glasses and shock-metamorphosed volcanic rocksoccur in deposits of lacustrine terraces inside the crater basinand rarely in alluvial deposits of some streams on the outerslopes of the crater rim (Fig 5) The source of these depositswas the ejecta on the crater rim and its slopes The sizes ofimpact rock clasts are mainly to 5ndash10 cm and only rare lumpsof impact melt rocks are up to ~1 m in diameter

Shock-metamorphosed volcanic rocks are tuffsignimbrites and lava of rhyolites and dacites rarely andesitesand andesite tuffs Clasts of impact rocks have predominantlyirregular shapes while partly impact-melted volcanic rockshave aerodynamically shaped forms Remnants of vesicularimpact melt on the surface of some rock clasts indicate thattheir source rocks were suevites and impact melt breccia Allrocks are fresh and only limited weathering is observed in afew rare cases

The classification of the shock-metamorphosedcrystalline rocks of Stoumlffler (1971) was used for thedescription of the shocked volcanic rocks of the Elrsquogygytgynstructure while considering some peculiarities of porphyric

rocks and tuffs Shock-metamorphic effects are more distinctin phenocrysts in porphyric rocks and mineral clasts in tuffswhile shock metamorphism of the fine-grained matrix is lessnoticeable until selective melting occurs (Gurov and Gurova1979 1991)

In the following sections we describe the characteristics ofthe rocks representing the various shock stages that we studied

Stage I (Shock Pressures up to 35 GPa)

At shock pressures le28 GPa weakly to moderatelyshocked rhyolite clasts are somewhat fractured at the

Table 2 Chemical composition of ignimbrites of the Elrsquogygytgyn impact cratera616-b 649 781 925 945 963-a 974 Average

SiO2 6820 7146 6778 7001 7050 7065 7250 7038TiO2 041 014 046 029 034 037 031 036Al2O3 1304 1372 1549 1468 1480 1568 1385 1447Fe2O3tot 245 150 275 177 133 211 123 185FeO 125 072 072 143 115 093 115 105MnO 014 014 007 003 009 lt001 009 008MgO 096 061 106 078 078 032 062 073CaO 361 254 273 191 204 126 168 225Na2O 340 298 306 310 317 240 315 303K2O 368 441 360 411 388 374 400 392P2O5 012 005 011 006 008 lt001 012 008CO2 088 044 024 088 014 046 034 048H2O 054 020 012 lt001 055 016 035 027LOI 105 048 162 113 100 156 058 129

Total 9974 9956 9960 9961 9952 9964 9956 ndashaWet chemical analyses data are in wt Samples 616-b lava-like porphyric rock with fragments of plagioclase orthoclase quartz biotite and amphibole in

fine-grained fluidal glass Lenticular inclusions of brown fluidal tuff and lava occur in the rock 649 porphyric rock with fragments and clasts of quartzplagioclase orthoclase biotite and amphibole in fine-grained recrystallized matrix that partly preserves fluidal structure Flattened particles of tuff and lavado not have sharp contacts with matrix 781 as 649 945 lava-like porphyric rock with fluidal fine-grained matrix and fragments of quartz feldspars biotiteand rarely amphibole Rare lenticular inclusions of tuff and lava of the same composition are present in the rock 963-a as 945 974 lava-like rock withxenoliths of brown flattened particles of tuff and phenocrysts of plagioclase orthoclase quartz and biotite Matrix is recrystallized in fine-grained matter butpreserves fluidal structure

Fig 5 Accumulation of impact glasses (dark gray to black) and clastsof shock metamorphosed volcanic rocks (gray to light gray) on alacustrine terrace at the southeastern shore of Lake Elrsquogygytgyn (site908) Aerodynamical shapes of some glass bodies and partly meltedvolcanic rocks are visible


macroscopic scale it is difficult in some cases to distinguishthem from unshocked rocks The quartz contains planarfractures (PFs) and PDFs (Fig 6) The predominant PForientation is parallel to 1011 rarely to 1014 2241 and1013 while the PDF orientations are parallel to 10131014 1011 1122 1122 5161 and 2241 Therefractive indices of 2241 quartz are slightly lowered to ne= 1548ndash1550 and no = 1540ndash1541 and the birefringence is0008ndash0009 corresponding to shock pressures of up to 28GPa according to Stoumlffler (1974) and Stoumlffler and Langenhorst(1994) Coesite and stishovite were not found The refractiveindices of orthoclase are lowered to nβ = 1516ndash1521 and nα= 1511ndash1515 Polysynthetic twins are preserved inplagioclase with refractive indices of nβ = 1527ndash1533 and nα= 1522ndash1527 Kink bands are abundant in biotite

Significant changes of volcanic rocks occur at highershock pressures Open fractures (up to 1 mm wide) occur inthe tuff clasts The fracture volume increases from 15 to7 vol derived from counting in thin sections The refractiveindices of quartz fragments and clasts are strongly reduced to1475ndash1494 and birefringence is about 0002ndash0003corresponding to shock pressures in the range of 28 to 35 GPa(Stoumlffler 1974 Stoumlffler and Langenhorst 1994) PDFs areabundant in quartz (Fig 7) with up to 12 sets of PDFsoccurring in some phenocrysts The predominant orientationis 1012 while 1014 1013 1122 2131 4041and 1010 are less common The presence of stishovite andcoesite was determined by X-ray powder diffractometry ofheavily shocked opalescent quartz from two samples ofrhyolite tuff A fraction of pure stishovite (main X-rayreflections at 0296 and 0153 nm) was obtained after quartzwas dissolved in hydrofluoric acid (Gurov et al 1979a)Potassium feldspar contains up to two systems of PDFs andhas lowered refractive indices nβ = 1512ndash1514 nα = 1509ndash

1511 and the birefringence is ~0003 Oligoclase containsseveral sets of PDFs its refractive indices are lowered to nβ =1525ndash1530 and nα = 1520ndash1526 but polysynthetic twinsare still preserved Biotite grains are partly replaced byopaque minerals which according to X-ray powder patternscomprise magnetite

Stage II (Shock Pressures from 35 GPa to 45 GPa)

Samples of tuffs have open fractures up to 1 mm wideQuartz and potassium feldspar fragments and clasts aretransformed into diaplectic glasses Diaplectic quartz glass istransparent (Fig 8) its refractive indices range from 1462 to1468 X-ray diffraction patterns with only one weakdiffusion line at d = 0335 nm confirm the amorphous stateThin semi-translucent zones and veinlets often intersectgrains of diaplectic quartz glass (Fig 9) The zones are ofcomplex structure and are composed of coesite segregationsin central parts of veinlets while the outer parts are composedof secondary quartz Individual coesite grains (nz = 1594 nx= 1590 birefringence of 0004) within the segregations arefrom 1 to 25 microm in diameter After treatment withhydrofluoric acid the remnant showed the X-ray powderpattern of pure coesite (main reflections d = 0344 03100171 nm) No stishovite was observed in diaplectic quartzglass from the Elrsquogygytgyn crater Grains of potassiumfeldspar in tuffs and lavas preserve their initial shapes but areconverted into transparent diaplectic glass with rare sphericalvesicles up to 005 mm in diameter (Fig 10) While quartzand potassium feldspar are converted into diaplectic glassesplagioclase in the same sample mainly preserves theproperties of the crystalline phase Relics of polysynthetictwins occur in this mineral The refractive indices ofplagioclase are lowered to nβ = 1524ndash1528 and nα = 1520ndash1524 with birefringence of 0004 Full isotropization ofplagioclase was observed in several tuff samples where clastsare converted into translucent isotropic glass with rare voids

Fig 6 Clast of shocked quartz in rhyolite Irregular rough fracturesand multiple PDFs occur in quartz (sample 1032-7 20 mm widecrossed polarizers)

Fig 7 Multiple PDFs in shocked quartz from rhyolite tuff (sample699-A 09 mm wide crossed polarizers)


The refractive indices of the glass range from 1513 plusmn 0001 to1517 plusmn 0002 The composition of these maskelynites isabout An27 to An36 using a diagram compiled by Stoumlffler(1974) as reference Biotite is replaced by ore minerals Thematrix of the porphyric rocks and tuffs is isotropic

Rare clasts of strongly shocked andesites occur in theterrace deposits They contain phenocrysts of plagioclaseorthopyroxene and amphibole Plagioclase phenocrystspreserve their initial shapes but are converted into transparentdiaplectic glass Phenocrysts of pyroxene are intensely twinned

Stage III (Shock Pressures from ~45 to ~55 GPa)

Partly melted volcanic rocks belong to this stage They

are widespread in terrace deposits inside the Elrsquogygytgyncrater a few also occur in nearby areas just outside the craterrim Volcanic rocks shocked at this stage are vesicularpumice-like rocks of low density some of which float inwater The clasts often have aerodynamic forms Transparentdiaplectic quartz glass with refractive indices from 1462 to1467 contains veinlets and irregular segregations of coesitewhich is more abundant in outer parts of the grain Potassiumfeldspar and plagioclase are converted into irregular to lens-shape vesicular colorless melt glasses The refractive indicesrange from 1488 to 1490 for potassium feldspar glass andfrom 1505 to 1515 for plagioclase glasses The contacts offeldspar glasses with fused matrix range from sharp togradual The matrix of porphyric rocks and tuffs comprisesinhomogeneous fluidal vesicular glass with refractive indicesfrom 1485 to 1515 Abundant spherical inclusions with arefractive index of ~1590 are probably coesite as the X-raypatterns of these glasses contain a weak reflection at d =0310 nm

Stage IV

At shock pressures exceeding ~55 GPa impact meltrocks and impact glasses are formed These rocks aredescribed in the next section

IMPACT MELT ROCKS AND GLASSES OF THE ELrsquoGYGYTGYN CRATER

Impact melt rocks and impact melt breccias occur interrace deposits within the crater basin but were not foundoutside of the crater These rocks were probably derived fromflows and pools of impact melt on the crater rim and wall

Fig 8 Strongly shocked rhyolite tuff Grain of transparent diaplecticquartz glass with the veinlets of coesite and secondary quartz Thematrix is melted and converted into inhomogeneous fluidal glass(sample 908-7B 19 mm wide parallel polarizers)

Fig 9 Segregations of coesite and secondary quartz in diaplecticquartz glass Coesite forms grains with high relief in central parts ofclustered veinlets (black arrows) while their peripheral zones arefine-grained aggregates of secondary quartz (sample 908-7B065 mm wide parallel polarizers)

Fig 10 Two grains of diaplectic potassium feldspar glass (center andupper right) in strongly shocked tuff The minerals in matrix preservetheir initial shapes but are mainly diaplectic glasses The fluidaltextures of matrix testify about the beginning of shock melting(sample 908-11B 28 mm wide parallel polarizers)


similar to the distribution of impact melt flows in some lunarcraters (Hawke and Head 1977) Fragments of impact meltrocks have irregular angular forms and range in size fromseveral cm to 1 m Black and dark gray impact melt rocksconsist of vesicular glass with abundant clasts of shockedvolcanic rocks minerals and glasses Fragments of darkgreen brown and gray glasses are in sharp contact with therock matrix Clasts of volcanic rocks predominantly show

high stages of shock metamorphism The matrix of the impactmelt rocks consists of partly devitrified vesicular glass withrefractive indices ranging from 1500 to 1515 for fresh glassand up to 1520 for devitrified glass Microlites oforthopyroxene (Fig 11) and in some cases of albite occur inglass (Gurov and Gurova 1991) The compositions of theimpact melt rocks (Table 3) are very similar to the averagecomposition of the target rocks

The inner surface of vesicles in impact melt rocks are insome cases covered with thin (le02 mm) layers of transparent(colorless) glass Transparent clusters of this glass up to 2 mmin size occur on the surface of the voids The formation of thisglass by condensation of silicate vapor has been suggested byGurov and Gurova (1991) as no evidence of hydrothermalactivity has been seen in the shocked rocks of the crater

Abundant glass bombs occur in the terrace deposits ofthe Elrsquogygytgyn crater together with shock-metamorphosedrocks and massive impact melt rocks While impact meltrocks occur only inside the crater basin glass bombs arealso found outside the crater in terrace deposits of somestreams The bombs have aerodynamical forms includingdrops ropes cakes and cylinders and are rarely of irregularshape (Fig 12) The bomb surfaces are rough and withoutluster and rounding during late-stage transportation bywater is rare Bomb diameters commonly range from 1 to15 cm Their weight varies mostly from 3 to 400 g but somelarge bodies up to 2 kg were also found The density of theglass ranges from 240 plusmn 005 to 250 plusmn 005 gcm3 The

Fig 11 Impact melt glass with glassy matrix and prismaticorthopyroxene microlites The areas of opaque glass are extensivelydevitrified The fragments (bottom left) are vesicular lechatelieriteand shock-metamorphosed volcanic rock (sample 985-60 22 mmwide parallel polarizers)

Table 3 Chemical composition of impact melt rocks of the Elrsquogygytgyn impact cratera651-b 651-5 669-10 669-20 678-I 900-11 900-12 900-13 Average

SiO2 6768 6792 7050 6965 7040 6964 6962 6842 6923TiO2 030 039 031 034 026 031 031 031 032Al2O3 1563 1529 1456 1508 1507 1552 1503 1543 1520Fe2O3 081 079 102 133 133 141 141 121 116FeO 215 253 172 165 158 197 179 215 194MnO 007 008 010 007 010 ndash 012 012 008MgO 119 109 077 095 061 068 078 091 087CaO 287 240 188 240 211 228 218 327 242Na2O 300 314 249 310 275 228 218 327 278K2O 383 383 490 329 442 388 417 362 399P2O5 016 016 003 009 005 006 006 008 009SO3 tr tr 002 tr 001 tr tr tr trCO2 nd nd 046 038 024 044 044 044 040H2Ominus 030 022 025 030 022 012 008 010 019LOI 229 258 062 104 103 028 069 066 114

Total 10028 10042 9963 9967 10018 9887 9886 9999 9981aWet chemical analyses data in wt tr = traces (lt001 wt) nd = not determined Samples 651-b impact melt rock is vesicular devitrified glass with

rare zones of black fresh glass Devitrified glass contains microlites of pyroxene and spherulitic aggregates of prismatic microlites of plagioclase 651-5 as651-b clasts of ballen quartz 669-10 vesicular and porous impact melt breccia composed of gray devitrified glass and xenoliths of colorless highly vesicularglass and diaplectic quartz glass Breccia matrix is glass with abundant microlites of pyroxene and feldspar 669-20 as 669-10 matrix is vesicular devitrifiedglass 678-I impact melt breccia with numerous xenoliths of vesicular glass and highly shocked volcanic rocks Matrix is highly recrystallized glass withspherulitic segregations of feldspars 900-11 impact melt rock with microlites of pyroxene in glass matrix Dark brown bands and spots of devitrificationunderlie fluidal structure of glass 900-12 fluidal impact melt rock with lithic fragments of vesicular glasses shock-metamorphosed rocks and mineral clastsMelt rock has fluidal texture and contains microlites of pyroxene and brown devitrification zones 900-13 strongly devitrified impact melt breccia withxenoliths of glass and diaplectic quartz glass The fluidal vesicular matrix consists of pyroxene and feldspar microlites cemented by gray-brown devitrifiedglass Diaplectic quartz glass contains coesite inclusions


density of vesicular glass is lower at about 235 gcm3 Thecolor of glass is black rarely dark gray in hand samples andcolorless to pale yellow in thin section Bands of brownglass enhance its fluidal structure (Fig 13) Gas bubbles upto a few mm in diameter occur within the glass The

refractive indices vary mostly from 1505 to 1515 but reach1540 in brown glass The glass is fresh and itsdevitrification is rare The mineral inclusions in glass areshocked quartz (Fig 14) as well as diaplectic quartz glassand lechatelierite (Fig 15) Rare inclusions in the form ofisolated spherical grains or clusters are magnetitetitanomagnetite and ilmenite Microprobe analyses oftitanomagnetite grains show TiO2 contents from 54 to118 wt and Al2O3 contents up to 66 wt

Deep open cracks ranging from 1 to 5 mm in width and1 to 7 mm in depth are the main characteristics of the bombsurface morphology (Fig 12) The cracks formed during thecontraction of the melt by cooling and solidification duringtheir transport in the atmosphere It is assumed that thevolume of open cracks corresponds to a decrease in meltvolume from the beginning of glass formation to its

Fig 12 Form of aerodynamically shaped glass (bombs) All bombshave shiny black surfaces a) teardrop-shaped bomb 7 times 12 times 12 cm(sample 689-1) b) elongated glass bomb 4 times 6 times 11 cm with striatedrelief and deep transversal cracks (sample 689-12) c) fragment (6 times10 times 125 cm) of disk-shaped glass bomb with deep radial cracks(sample 985-98)

Fig 13 Fluidal structure of inhomogeneous glass (sample 69035 mm wide parallel polarizers) Distorted layers of brown glassshow flow during melting

Fig 14 Shocked quartz with multiple PDFs in vesicular glass(sample 918 21 mm wide parallel polarizers)


solidification The volumes of open cracks of 25 bombs from5 to 12 cm in diameter were determined as the differencebetween bomb volumes with filled and with open emptycracks For this purpose the cracks were filled with paraffinThe open cracks equal from 045 to 367 of the whole bombvolume Bombs with a crack volume from 10 to 15predominate A decrease of 10ndash15 vol in melt volumecorresponds to a temperature drop of about 300ndash450 degC fromthe beginning of the formation of the glass crust to itssolidification

Traces of ablation and secondary melting are visible onthe surface of eight bombs out of 160 that were studied Thesebombs have drop shapes and are from 40 times 50 times 69 cm to 73times 103 times 110 cm in size The ablation is indicated by striatedrelief on the frontal and lateral surface of the bombs Openbubbles that are cut by ablation to about 3 mm in diameteroccur on the lateral surface of those bombs Secondarymelting is visible on the inner walls of some open cracksPositive surface relief protected some areas of the bombsurface from ablation A striated relief appears some mmbehind the steps on the bomb surface (Fig 16) The maximalthickness of ablation and secondary melting zones approaches1 mm According to OrsquoKeefe (1978) a thickness of an ablatedglass zone of about 2 mm corresponds to an entrance velocityof the bomb into the dense atmosphere of about 5 kms Theoccurrence of bombs at a distance of about two crater radiifrom the crater center indicates subvertical trajectories Thefollowing history of those bombsrsquo formation is indicated 1)ejection of melt from the crater 2) formation ofaerodynamically shaped bombs 3) solidification of thesurfaces of the bombs during flight along upward anddownward trajectories 4) ablation and secondary melting ofthe bomb surface during re-entrance into the denser parts ofthe atmosphere 5) falling of bombs on the surface Theabsence of local rock fragments attached to the surface of the

bomb indicates that their surfaces were already solid whenthey hit the surface

Glass bombs from eight different areas of the Elrsquogygytgynimpact crater show only a limited range in chemicalcomposition (Tables 4 and 5) The SiO2 content varies from6860 to 7148 Na2O from 266 to 316 and K2O from 330 to437 wt (Gurov and Gurova 1991 Feldman et al 1982) Thecomposition of the glass bombs is similar to the calculatedaverage composition of the target (Table 1) but the closestagreement is to the composition of the rhyolite ignimbrites ofthe uppermost horizon of volcanic strata in the crater area(Table 2) We assume that these rhyolite ignimbrites were themain source material for melt bomb formation during theearliest stage of impact melting of the target One maincharacteristic of the glass composition is the low Fe2O3FeOratio due to reduction in high temperature melts

In addition to the major element data report here westudied three glass bombs two impact melt rocks and onesample of selectively melted rhyolite for their trace elementcomposition (Table 4) Their major element composition(newly measured by XRF) is similar to that of target volcanicrocks (cf Table 1) The trace element contents show a limitedvariation A chondrite-normalized REE diagram for selectedsamples of impact glass is presented in Fig 17 It shows thatthe melt rock samples melt bombs and the rhyolite havebasically identical composition no evidence for a post-formational hydrothermal alteration is seen Compared to theone rhyolite sample the siderophile element content of themelt bombs is not very high only the Cr content is significantlyhigher while the Co and Ni abundances show hardly anydifference The iridium content in these samples ranges from24 ppt in the glass bomb to 74 and 111 ppt in impact melt rockswhich are not high values compared to those that have been

Fig 15 Transparent lechatelierite inclusion in brown fluidal glassThe lechatelierite (center) has sharp contacts with the surroundingglass (sample 652-25 17 mm wide parallel polarizers)

Fig 16 Striated relief of glass bomb surface Traces of ablation areabsent behind the step (dark area in the lower part of image) andappear at a distance of about 2 mm from it (sample 985-89 105 mmwide) This image illustrates secondary melting of the bomb surfaceduring transportation in the atmosphere


Table 4 Composition of selected impactites from the Elrsquogygytgyn impact crateraEL 669-5 rhyolite (pm) EL 669-8 IMR EL 669-10 IMR EL 689 glass bomb EL 699 glass bomb EL 985 glass bomb

SiO2 7155 6899 6956 7191 7162 7050TiO2 040 039 038 036 042 038Al2O3 1338 1475 1532 1423 1521 1541Fe2O3 311 313 292 294 300 323MnO 008 009 009 009 009 011MgO 123 084 109 103 120 117CaO 256 269 247 232 268 268Na2O 293 331 298 274 217 272K2O 366 356 401 391 394 400P2O5 008 011 012 008 008 008LOI 145 124 136 020 006 008

Total 10043 9910 10030 9981 10047 10036

Sc 758 814 131 845 815 958V 33 33 34 ndash ndash 36Cr 81 312 461 206 451 332Co 453 471 441 516 589 504Ni 29 8 12 21 16 11Cu lt2 lt2 lt2 lt2 lt2 lt2Zn 61 71 68 120 173 52Ga 10 10 20 26 22 25As 707 806 511 208 291 341Se 016 032 026 026 044 024Br 06 05 06 07 04 04Rb 150 148 155 165 145 155Sr 273 324 260 195 225 242Y 33 27 31 lt10 lt10 34Zr 210 205 200 221 175 230Nb 11 11 12 lt8 lt8 13Sb 065 123 094 085 064 145Cs 597 893 757 909 711 954Ba 780 800 850 870 840 830La 375 425 455 408 371 408Ce 708 771 883 751 669 749Nd 341 348 439 349 316 359Sm 568 604 804 596 525 635Eu 109 098 098 102 093 103Gd 62 55 65 51 49 52Tb 091 08 112 085 076 088Tm 047 043 057 042 041 053Yb 293 299 391 299 278 325Lu 045 049 063 047 044 049Hf 587 585 629 581 496 582Ta 076 086 081 088 074 076W 13 09 12 07 09 09Ir (ppt) nd 111 plusmn 21 73 plusmn 17 24 plusmn 10 nd ndAu (ppb) 06 09 03 05 02 04Th 164 173 162 173 147 159U 481 469 408 425 375 426

KU 6341 6326 8190 7667 8756 7825ZrHf 358 350 318 380 353 395LaTh 229 246 281 236 252 257HfTa 772 680 777 660 670 766ThU 341 369 397 407 392 373LaNYbN 865 961 786 922 902 848EuEu 056 052 041 057 056 055

aMajor elements in wt trace elements in ppm except as noted all Fe as Fe2O3 IMR = impact melt rock (pm) = partially melted for a detailed sampledescription see text


found in impact melts of other structures However a value ofabout 100 ppt Ir is somewhat elevated but confirmation as towhether this indicates a meteoritic component will requireseveral target rock analyses as well At present an achondriticprojectile is the most likely interpretation of the trace elementdata as most basaltic achondrites have Ni Co and lowplatinum group element contents but high Cr contents

Glass bombs of the Elrsquogygytgyn crater are similar inform and texture to the bombs found at some other impactstructures for example at the Ries and Zhamanshin cratersThe Elrsquogygytgyn bombs are similar to the Ries glassesmainly to ldquohomogeneous glass bodiesrdquo and partly to ldquonormalglass bombsrdquo as described by Staumlhle (1972)

Some properties of the Elrsquogygytgyn crater bombs

Table 5 Chemical composition of impact melt glasses of the Elrsquogygytgyn impact cratera652 689-2 689-3 699-1 699-2 699-34 908-9 985-8 985-92

SiO2 6990 6860 7044 6860 6999 6984 6946 6950 7035TiO2 029 038 036 035 038 039 042 047 041Al2O3 1441 1534 1508 1524 1531 1539 1576 1584 1466Fe2O3 128 120 080 123 061 067 054 107 075FeO 256 250 230 308 251 237 262 237 248MnO 006 008 007 012 008 008 008 011 007MgO 069 135 110 127 140 140 140 177 134CaO 270 297 261 275 296 314 290 313 257Na2O 316 296 304 288 270 272 266 282 288K2O 437 385 380 360 357 347 330 366 388P2O5 004 009 006 008 006 007 007 012 007CO2 ndash 004 007 ndash 007 017 021 ndash ndashH2O 006 028 tr 028 tr tr 004 010 026LOI 027 043 020 046 012 tr 017 016 026

Total 9980 10028 9998 9995 9976 9971 9963 10012 9967aWet chemical analyses data in wt Samples 652 black shiny glass in thin section colorless transparent glass with rare inclusions of diaplectic quartz glass

689-2 black shiny glass colorless and transparent in thin section 689-3 as 689-2 699-1 black glass body with deep cracks Colorless fresh glass with lightbrown bands that underlie its fluidal structure Rare inclusions are colorless vesicular lechatelierite 699-2 colorless fluidal glass with abundant inclusions ofvesicular lechatelierite and diaplectic quartz glass 699-34 as 699-2 908-9 fluidal glass with xenoliths of lechatelierite and vesicular heterogeneous glasses985-8 colorless fluidal glass with bands of brown glass Rare inclusions are vesicular lechatelierite 985-92 vesicular transparent glass

Fig 17 Chondrite-normalized abundances of REE in impact melt rocks and a rhyolite of the Elrsquogygytgyn crater The normalization factorsare after Taylor and McLennan (1985)


including shape traces of ablation and secondary meltingdensity and refractive indices appear similar to thecharacteristics of some tektites (eg philippinites Chao1963) However the Elrsquogygytgyn bombs formed in a smallerimpact event and were not ejected as far as tektites It can bespeculated that the impact angle of the Elrsquogygytgyn projectilewas steeper than that of the tektite-forming events resultingin more localized distributions

SUMMARY AND CONCLUSIONS

The 18 km-diameter Elrsquogygytgyn crater is one of thebest-preserved young impact structures The crater wasformed in a series of Cretaceous volcanic rocks composed oflavas and tuffs of rhyolites dacites and andesites Such atarget composition is unique among terrestrial impactstructures and provides the only possibility for the study ofnatural shock metamorphic effects in siliceous volcanic rocks

All impactites found at Elrsquogygytgyn so far have beenredeposited thus none of the samples investigated here werecollected in situ Shock-metamorphosed volcanic rocks occurin the lacustrine terraces inside the crater rim and in fluviatileterraces on the outer slopes of the rim The remnants ofimpact melt glass on the surface of some fragments ofshocked rocks indicate that they are derived from suevites andimpact melt breccias Stishovite and coesite were found inmoderately to strongly shocked rocks Stishovite occurs inshocked quartz while coesite is more abundant in diaplecticquartz glass Selectively melted volcanics with shock-meltedglass matrix and grains of diaplectic quartz glass are commonin terrace deposits The classification system of shock-metamorphosed crystalline rocks of Stoumlffler (1971) andStoumlffler and Langenhorst (1994) was used to classify theshocked siliceous volcanic rocks based on properties ofquartz and feldspar and their glasses in impact rocks of thecrater (Gurov and Gurova 1979) Four stages of shockmetamorphism of the acid volcanic rocks are recognized

bull Stage I Tuff and lava contain weakly to strongly shockedclasts of quartz and feldspar Stishovite and coesite occurin quartz Shock metamorphism of the fine-grainedmatrix is difficult to identify

bull Stage II Quartz and feldspars in lava and tuff areconverted into diaplectic glasses Coesite occurs indiaplectic quartz glass Biotite is replaced by opaquesThe groundmass of the tuffs is isotropic

bull Stage III In rocks shocked to this stage quartz isconverted into diaplectic glass often with coesite whilefeldspar clasts are melted and have an irregular or lens-like form most of them preserve sharp contacts with thesurrounding matrix which is composed of heterogeneousvesicular glass

bull Stage IV At pressures exceeding 55 GPa impact meltrocks and glasses form by the complete melting ofvolcanic rocks

Flows of impact melt on the crater walls were possiblythe sources of these rocks before redeposition Impact meltrocks are composed of dark glass matrix and numerous clastsof shocked rocks minerals and glasses The fragments ofimpact glasses and shocked volcanic rocks have sharpcontacts with the impact melt due to rapid cooling of therocks The composition of the impact melt rocks and meltbreccias is similar to the composition of acid volcanic rocksfound at the crater

Impact glasses occur not only inside the crater but arealso common on the outer slopes of the structure Theaerodynamical shape of glass bombs indicates their formationduring transportation in the atmosphere Ablation of somebomb surface is evidence for solidification and remeltingwhile traveling on very steep trajectories The composition ofimpact glasses is very similar to the composition of volcanicrocks of the upper 250 m-thick layer of the crater basementthe closest similarity is observed for ignimbrites

Previous studies of siderophile element contents in impactmelt rocks and impact melt glasses have shown only minorenrichment in Cr Ni and Co compared to target rocks this wasalso confirmed in the present study for Ir the maximum contentof which was found to be on the order of 100 ppt The mostpronounced enrichment is for Cr which could indicate anachondritic projectile However no unambiguous conclusionsregarding the presence and nature of a meteoritic component inthe Elrsquogygytgyn impactites can be made so far

AcknowledgmentsndashWe are grateful to Dr E Gurova foroptical measurements of impact rocks and to R Rakitskayafor X-ray diffraction analyses This work was supported bythe Austrian Science Foundation (project Y58-GEO to CKoeberl) and by the International Exchange Programs of theAustrian Academy of Science and the University of Vienna(to E Gurov and C Koeberl) We are grateful to P Claeys ADeutsch and R Schmitt for detailed and helpful reviews ofthe manuscript

Editorial HandlingmdashDr Alexander Deutsch

REFERENCES

Brigham-Grette J Glushkova O Minyuk P Melles M Overdun PP Zielke A Nowacyzk N Nolan M Stone D and Layer P1998 Preliminary lake coring results from Elrsquogygytgyn cratereastern Siberia (abstract) Eos Transactions AmericanGeophysical Union 79F477

Chao E T C 1963 The petrographic and chemical characteristics oftektites In Tektites edited by OrsquoKeefe J A Chicago Universityof Chicago Press pp 51ndash94

Dabija A I and Feldman V I 1982 Geophysical characteristics ofsome astroblemes of USSR Meteoritika 4091ndash101 In Russian

Dietz R S 1977 Elrsquogygytgyn crater Siberia Probable source ofAustralasian tektite field (and bediasites from Popigai)Meteoritics 12145ndash157

Dietz R S and McHone J F 1976 Elrsquogygytgyn Probably worldrsquoslargest meteorite crater Geology 4391ndash392


Engelhardt W V 1972 Shock-produced rock glasses from the Riescrater Contributions to Mineralogy and Petrology 36265ndash292

Engelhardt W V 1974 Meteoritenkrater Naturwissenschaften 61413ndash422

Feldman V I Granovsky L B Kapustkina I G Karoteeva N NSazonova L V and Dabija A I 1981 Meteorite craterElrsquogygytgyn In Impactites edited by Marakhushev A AMoscow Moscow State University Press pp 70ndash92 In Russian

Gurov E P and Gurova E P 1979 Stages of shock metamorphismof volcanic rocks of acid composition on example of theElrsquogygytgyn crater (Chukotka) Doklady Academii Nauk 2491197ndash1201 In Russian

Gurov E P and Gurova E P 1982 Composition of impact meltrocks of the Elrsquogygytgyn crater and content of Ni and Cr inthem In Kosmicheskoe veschtestvo na Zemle edited bySobotovich E V Kiev Naukova Dumka Press pp 120ndash123 InRussian

Gurov E P and Gurova E P 1983 Regularities of fault spreadingaround meteorite craters (an example of the Elrsquogygytgyn crater)Doklady Academii Nauk 275958ndash961 In Russian

Gurov E P and Gurova E P 1991 Geological structure and rockcomposition of impact structures Kiev Naukova Dumka Press160 p In Russian

Gurov E P and Yamnichenko A Y 1995 Morphology of rim ofcomplex terrestrial craters (abstract) 26th Lunar and PlanetaryScience Conference pp 533ndash534

Gurov E P Valter A A Gurova E P and Serebrennikov A I 1978Impact meteorite crater Elrsquogygytgyn in Chukotka DokladyAcademii Nauk 2401407ndash1410 In Russian

Gurov E P Gurova E P and Rakitskaya R B 1979a Stishoviteand coesite in shock-metamorphosed rocks of the Elrsquogygytgyncrater in Chukotka Doklady Academii Nauk 248213ndash216 InRussian

Gurov E P Valter A A Gurova E P and Kotlovskaya F I 1979bElrsquogygytgyn impact crater Chukotka Shock metamorphism ofvolcanic rocks (abstract) 10th Lunar and Planetary ScienceConference pp 479ndash481

Hawke B R and Head J W 1977 Impact melt on lunar craters rimsIn Impact and explosion cratering edited by Roddy D J PepinR O and Merrill R B New York Pergamon Press pp 815ndash841

Kapustkina I G Kolesov G M and Feldman V I 1985Contamination of the Elrsquogygytgyn crater impactites withmeteoritic matter Doklady Academii Nauk 280755ndash758 InRussian

Koeberl C 1990 The geochemistry of tektites An overviewTectonophysics 171405ndash422

Koeberl C 1993 Instrumental neutron activation analysis ofgeochemical and cosmochemical samples A fast and reliablemethod for small sample analysis Journal of Radioanalyticaland Nuclear Chemistry 16847ndash60

Koeberl C and Huber H 2000 Optimization of the multiparameterγ-γ coincidence spectrometry for the determination of iridium in

geological materials Journal of Radioanalytical and NuclearChemistry 244655ndash660

Komarov A N Koltsova T V Gurova E P and Gurov E P 1983Determination of the Elrsquogygytgyn crater impactitesrsquo age by thefission track method Doklady Academii Nauk 1011ndash13 InRussian

Kotlar I N Beliy V F and Milov A P 1981 Petrochemistry ofmagmatic formations of the Ochotsk-Chukotsk volcanogenicbelt Moscow Nauka Press 221 p In Russian

Layer P W 2000 Argon-40argon-39 age of the Elrsquogygytgyn eventChukotka Russia Meteoritics amp Planetary Science 35591ndash599

Nekrasov I A and Raudonis P A 1963 Meteorite craters Priroda1102ndash104 In Russian

Nolan M Liston G Prokein P Brigham-Grette J Sharpton V Land Huntzinger R 2003 Analysis of lake ice dynamics andmorphology on Lake Elrsquogygytgyn NE Siberia using syntheticaperture radar (SAR) and Landsat Journal of GeophysicalResearch 1083-1ndash3-12 doi 1010292001JD000934

Nowaczyk N R Minyuk P Melles M Brigham-Grette JGlushkova O Nolan M Lozhkin A V Stetsenko T VAndersen P M and Forman S L 2002 Magnetostratigraphicresults from impact crater Lake Elrsquogygytgyn northeasternSiberia A 300 kyr long high-resolution terrestrial palaeoclimaticrecord from the Arctic Geophysical Journal International 150109ndash126

Obruchev S V 1957 Across the tundra and mountains of ChukotkaMoscow State Press of Geography 198 p In Russian

OrsquoKeefe J A 1976 Tektites and their origin Amsterdam Elsevier254 p

Reimold W U Koeberl C and Bishop J 1994 Roter Kamm impactcrater Namibia Geochemistry of basement rocks and brecciasGeochimica et Cosmochimica Acta 582689ndash2710

Staumlhle V 1972 Impact glasses from the suevites of the NoumlrdlingerRies Earth and Planetary Science Letters 17275ndash293

Stoumlffler D 1971 Progressive metamorphism and classification ofshocked and brecciated crystalline rocks Journal of GeophysicalResearch 765541ndash5551

Stoumlffler D 1974 Deformation and transformation of rock-formingminerals by natural and experimental shock processes IIPhysical properties of shocked minerals Fortschritte derMineralogie 51256ndash289

Stoumlffler D and Langenhorst F 1994 Shock metamorphism of quartzin nature and experiment I Basic observations and theoryMeteoritics 29155ndash181

Storzer D and Wagner G A 1979 Fission track dating ofElrsquogygytgyn Popigai and Zhamanshin craters No source forAustralasian or North American tektites (abstract) Meteoritics14541ndash542

Taylor S R and McLennan S M 1985 The continental crust Itscomposition and evolution Oxford Blackwell ScientificPublications 312 p

Zotkin I T and Tsvetkov V I 1970 Search of meteorite craters onthe Earth Astronomitcheskiy Vestnik 455ndash65 In Russian


to be 452 plusmn 011 Ma by fission track dating (Storzer andWagner 1979) 345 plusmn 015 Ma by later fission track dating(Komarov et al 1983) and 350 plusmn 050 Ma by K-Ar dating(Gurov et al 1979b) The most recent 40Ar39Ar date yieldedan age of the impact glasses of 358 plusmn 004 Ma (Layer 2000)which is in good agreement with the earlier data All thesedates exclude a relation between the Australasian tektites andthe Elrsquogygytgyn crater

During the last years the Elrsquogygytgyn crater andespecially its post-impact sedimentary record have beenintensely investigated in a joint program of the Alfred-Wegener Institute Potsdam Germany the University ofMassachusetts Amherst USA and the North-EastInterdisciplinary Scientific Research Institute MagadanRussia A 127 m-long core of the lake sediments was studiedfor paleoclimatic information of this continental region of theArctic (Brigham-Grette et al 1998 Nowaczyk et al 2002)The dynamics and morphology of ice covering theElrsquogygytgyn Lake were studied from radar satellite data byNolan et al (2003) who also speculated on the location of apossible central uplift that is buried by the lake sediments

The purpose of the present paper is to summarize for theinternational literature some geological and petrographicobservations on the Elrsquogygytgyn structure made by one of us(E Gurov) over several decades supplemented by some newpetrographic and geochemical data

GEOLOGIC BACKGROUND

The Elrsquogygytgyn crater is a flat-floored circular basinwith a rim-rim diameter of about 18 km (Fig 2) that issituated in the central part and southeastern slope of theAcademician Obruchev Ridge in central Chukotka The craterfloor about 14 km in diameter is occupied by the nearlycircular Elrsquogygytgyn Lake which is 12 km in diameter and upto 170 m deep in its center A complex system of lacustrine

terraces surrounds the lake The highest terrace is about 80 mabove the lake level and the lowest one is 1ndash3 m high Thirty-eight streams run into the lake and only the Enmivaam Riverflows out of the lake and determines the water level in it Acentral peak is not exposed on the recent surface of the craterfloor Gravity measurements by Dabidja and Feldman (1982)indicate the presence of a central peak about 2 km indiameter underneath post-impact sediments centered relativeto the crater outline In contrast Nolan et al (2003) suggestfrom radar images that the central uplift might be centeredrelative to the lake outline (ie asymmetric relative to theactual crater) New seismic data (F Niessen personalcommunication to C Koeberl 2004) seem to agree better withthe earlier gravity data

The crater is surrounded by an uplifted rim that has anasymmetrical cross section with steep inner walls and gentleouter slopes The average height of the rim about 180 mabove the lake level and 140 m above the surrounding areawas calculated from 24 radial morphological profiles of thecrater The initial height of the rim before erosion wasestimated to be about 230 m (Gurov and Yamnichenko 1995)A low concentric outer rim was discovered by morphologicalinvestigations of the structure The height of the second rim isup to 14 m at a distance of 175 crater radii

Fig 1 Location of the Elrsquogygytgyn impact crater on the ChukotkaPeninsula northeast Russia

Fig 2 Satellite (Seasat SAR) image of the Elrsquogygytgyn impactcrater The crater floor is occupied by the Elrsquogygytgyn Lake about12 km in diameter The Enmivaam River (dark in the image markedwith arrow) crosses the crater rim in its southeastern part North isshown with an arrow Image courtesy J Garvin (NASA GoddardSpace Flight Center)












Samples















Ignimbrite (7)



Target averageb


































Stage IV



































Total 10043 9910 10030 9981 10047 10036



























REFERENCES



















































Samples















Ignimbrite (7)



Target averageb


































Stage IV



































Total 10043 9910 10030 9981 10047 10036



























REFERENCES





































































Stage IV



































Total 10043 9910 10030 9981 10047 10036



























REFERENCES




































































Total 10043 9910 10030 9981 10047 10036



























REFERENCES































































REFERENCES








































and Tsvetkov (1970) and Engelhardt (1974) included the ...€¦ · and Tsvetkov (1970) and...

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Transcript of and Tsvetkov (1970) and Engelhardt (1974) included the ...€¦ · and Tsvetkov (1970) and...