Journal of fhe Geological Society, London, Vol. 145, 1988, pp. 473-478, 5 figs. Printed in Northern Ireland

Holarctic correlation of non-marine Palaeocene-Eocene boundary strata using mammals

D. DASHZEVEG Geological Institute of the Mongolian Academy of Science, Ulan Bator, People’s Republic of Mongolia

Abstract: A new biostratigraphic correlation is suggested for the Palaeocene-Eocene boundary strata in the Holarctic. Great significance is attached to the Thanetian hiatus which has critical importance in the placing of the Palaeocene-Eocene boundary in Western Europe. Intercontinental correlation of the Palaeocene-Eocene boundary in non-marine strata by means of mammals reveals the duration of theThanetian hiatus in the Pans Basin. It corresponds almost entirely tothe Clarkforkian Land Mammal Age of North America and possibly to the Zhigden and Naran Members of the Naran-Bulak Formation of Mongolia, which suggests a Palaeocene age for the Clark’s Fork fauna. The Gray Bull and Bumban faunas appear to be practically contemporaneous and are correlated with the Sparnacian fauna of West European Meudon. The Palaeocene-Eocene boundary in North America is thus traced beneath the Gray Bull zone (Wasatchian Land Mammal Age), while in Central Asia it is believed to underlie the Bumban Member of the Naran-Bulak sections where there is evidence of the simul- taneous appearance of penssodactyls,primates (Ornomyidae), rodents, condylarths of thegenus Hyopsodus and others.

The problem of the Palaeocene-Eocene boundary has long Key sections and mammals of the Upper Palaeocene been and remainsa subject for debate. A more precise and Lower Eocene of Mongolia positioning of this boundary has critical importance for the In Mongolia,Palaeocene and Lower Eocene deposits are correlation of major events of the earlier Tertiary history of confined to Gobiintermontane depressions (Fig. 1). the Earth. The advantage of the Palaeogene succession of Characteristic sections are known in the Nemegt, Ulan-Nur Western Europe lies in its almost equal proportions of and Bugintsav Basins,and ample collections of mammal marine and continentaldeposits thatintercalate; often fossils have been made (Fig. 2). stratigraphiccorrelation by directobservation is possible. However, the position of the Palaeocene-Eocene boundary itself is ambiguous in Western Europe since deposits with Nemegt Basin diverse facies are difficult tocorrelate and since different Upper Palaeocene and Lower Eocene sequences are groups of organisms (marine and continental) suggest developed in Naran-Bulak. The Naran-BulakFormation conflicting solutions. consists of alternating sandstones and clays, with intercala- Because of the absence of marinedeposits the tions of light grey carbonates; light grey and red colours exclusively continental deposits of the Palaeocene to Eocene predominate. Thestrata are horizontaland cross-bedding sections in Central Asia can offer no solution to the disputed occurs. With a total thickness of 80m,the Naran-Bulak placement of the Palaeocene-Eocene boundary in Western Formation is divided, from the bottom to the top, into the Europe.Hence, the stratigraphy theof Mongolian Zhigden, Naran, Bumban and Aguyt Members (Dashzeveg Palaeogene has only a relative significance, based solely on 1982a). data of the terrestrial fauna and flora. On the other hand, The mostcomplete section of the ZhigdenMember the Central Asian and North American sections have been crops out at the western edge of Tsagan-Khushu where it is correlated to those of Western Europe through comparison almost 27m thick;eastward, the member thins and of land mammal faunas (Gingerich 1976; Rose 1981). The disappears near Ulan-Bulak. It is represented largely by value of land mammals for correlations on a global scale is monotonousred sandstones with intercalations of fine widely recognized. Thus, along with marine organisms, the gravel. Theupper part of the Zhigden sections in the Palaeocene-Eocene boundary can be clarified on the basis southern and northern Naranregion produced surface fossils of mammalian palaeontology. of Archaeolambdaplanicanina, Prodinoceras martyr and Until recently it was considered that the domination of Arctostylops sp. theGashato andNaran-Bulak faunas of Mongolia by TheNaran Member is fairly widespread and is endemic forms made their correlation with coeval faunas in represented by grey sandy clay deposits overlying, with an Western Europeand NorthAmerica difficult (Szalay & erosional contact, the Zhigden Member. The member shows McKenna 1971). As a result of the studies of new mammal two horizons: the lower one is alluvial, andthe upper collections from key sections in Naran-Bulak, it has become lacustrine. Both horizons can betraced in thenorthern possible to placethese faunas accurately within the outcrops of Naran-Bulak, but the upper one is represented internationalsequences, as well as to consider the most completely in the Tsagan-Khushu section. The Palaeocene-Eoceneboundary on a global biostratigraphic maximum thickness of themember is 30m.The alluvial scale (Dashzeveg 1982a). horizon has produced a good mammalian assemblage with

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Fig. 1. (A) Situation of the Nemegt region in south central Mongolia. (B) The Nemegt region and situation of the principal Palaeogene localities. 1, Ulan-Bulak; 2, Naran-Bulak; 3, Tsagan-Khushu. Black circles indiate hilltops; open circles, towns.

uintatheres (Prodinoceras martyr; see Dashzeveg 1982b) and is represented by a 20m thickness of red clays. In the predominating. Abundant fossil mammals are recorded in Tsagan-Khushu outcrops, mammalian fossils are associated the northern outcropsof Naran-Bulak; Prodinoceras martyr, with the lower red clays just above the contact with the Pachyaena nemegetica and Archaeolambda planicanina Naran Member. Mammals described from this part of the occur. Various levels of the alluvial horizon yield rare fossils section include Burnbanius rarus, Oedolius perexiguus, of Eurymylus laticeps, Pseudictops lophiodon and Naranius infrequens, Tsaganius ambiguus, Hyopsodus Arctostylops macrodon. The lacustrinehorizon is charac- orientalis, Altanius orlovi, Hyracotherium gabuniai and terized by the mammal fauna that has beenfound in the Homogalax namadicus (Dashzeveg 1977, 1979a, 1979b; northern Naran-Bulak area. Typicalforms are Dissacus Dashzeveg & McKenna 1977; Russell & Dashzeveg 1986). indigenus, Oxyaena sp., ‘Sinopa’ sp., Eurymylus laticeps, The top part of the Naran-Bulak Formation, the Aguyt Pseudictops lophiodon, Prionessus lucifer and Arctostylops Member, is composed of grey sandy clay rocks, with macrodon. secondary beds of carbonate concretions. The only fossil TheBumban Member is developed only along the recorded from this member is Gomphos sp. southern edge of the basin where it forms a red southerly trending range. It conformably overlies the Naran Member Ulan-Nur Basin NEHEGT BASIN Palaeocene and Lower Eocene sequences are found in the LGUYT vicinity of Khashat (Gashato).The KhashatFormation is represented largely by variegated sandy clayswith .. intercalations of carbonate concretions. The overall BUGIN TSAV l thickness is 40m.The formation is divided intothree members (Fig. 2). Asfor the basal one,the Khashat horizon, it is not included here in the Palaeocene, since it might be of anolder, possibly Upper Cretaceous age (Devyatkin 1981). hhato Member I lies onthe gravelly clays of basal Fauna conglomerates and comprises alternating red and brownish sandy clays with intercalations of calcareous mark.The mammalian Gashato fauna was found in three levels of the j 17m thick member.Member I1 (11 m) is continuous throughout its section, represented by brownish clays with Fig. 2. Biostratigraphic correlation between Gashato (Ulan-Nur discrete lenses of gravels and sands. A gravel lens was found Basin), Naran-Bulak (Nemegt Basin) and Khaichin-Ula I1 to contain Gomphos elkema. Member 111 overlies Member (Bugintsav Basin). 1, Prionessus lucifer; 2, Naranius infrequens; 3, Bumbanius rarus; 4, Oedolius perexiguus; 5, Tsaganius ambiguus; I1 with a distinct erosional contact. It shows 12 m of gravels 6, Hyopsodus orientalis; 7, Dissacus indigenus; 8, Pachyaena and grey clays. The gravels contained Gomphos elkema nemegetica; 9, Hapalodectus sp.; 10, Eurymylus laticeps; 11, (Zhegallo & Shevyreva 1976). Zagmys insolitus; 12, Rhombomylus sp.; 13, Gomphos elkema; 13a, A basalt unit at the top of Member I1 of the Khashat Gomphos sp.; 14, Pseudictops lophiodon; 15, Mixodontia nov.; 16, Formation produced an absolute age of 51 f 2 My (Gabunia Rodentia nov.; 17, Oxyaena sp.; 18, ‘Sinopa’ sp.; 19, Altanius et al. 1975). A second dating of Khashat basalts yielded a orlovi; 20, Archaeolambda planicanina; 21, Archaeolambda sp.; 22, younger age of 37 f 1 Ma, so it is evident that the absolute Archaeolambda trofimovi; 23, Coryphodon tsaganensis; 24, age needs further verification (Devyatkin 1981). Prodinoceras martyr; 25, Prodinoceras sp.; 26, Arctostylops In recentyears, fossils of Gomphos elkema have been macrodon; 27, Arctostylops iturus; 28, Arctostylops sp.; 29, found by theauthor in several different levels of the Ernanodon sp.; 30,Hyracotherium gabuniai; 31, Homogalax Bumban Member in the vicinity of Naran-Bulak. The early namadicus. a, fossiliferous lenses; b, hiatus; c, fossil locality; d, Eocene age of G. elkema is supported by the fossils basalt. associated with itat Naran-Bulak, such as Hyopsodus

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E UR O P E ASIAEUROPE NORTH AMERICA

Fig. 3. Correlation of Palaeocene-Eocene boundary strataof Europe, Asia and North America. Correlation of the P and NP zones taken from Cavelier& Pomerol(l986).

orientalis and Homogalax namadicus. Thesedata strongly pantodonts Archaeolambda trojimovi Flerov & Dashzeveg, suggest a Lower Eocene age for the Khashat Members I1 1974 and by Prodinoceras cf. martyr. These finds permit the and 111. Bugin Member to be referred to the Upper Palaeocene and to be correlated with theNaran Member (of the Bugintsau Basin Naran-Bulak Formation) of the Nemegt Basin. Immediately above the Naran Member in the Naran-Bulak section is the Palaeocene and Eocene deposits crop out on the southern Bumban Member, which correlates well with Khashat’s edge of this basin, some 10 km south of theUpper Member 11. The commonest element is Gomphos elkema, Cretaceous Bugintsav fossil locality, wherethey form a which is the most typical of theupper part of the chain of cliffs that stretch from east to west for 20 km. Naran-Bulak section. The Aguyt Member and Member 111 TheUpper PalaeoceneNaran-Bulak Formation was in the above occurrences can be approximately correlated so described here at a time when it was not yet distinguished in far only by their position in the section and similar its stratotype section in the Nemegt Basin (Shuvalov et al. thickness, as well as by the shared presence of Gomphos. 1974). The present paper describes Upper Palaeogene sequences in the Khaichin ranges, referred to as the Bugin Biostratigraphic significance of the Naran-Bulak Member (Fig. 2). Its typical sections crop out in the cliffs of members’ boundaries the Khaichin-Ula I1 locality. The member is composed of rhythmically alternating sandstones, clays and gravelstones The change in the type of sedimentation that delimits the of grey, light grey and often greenish-grey colour that members is substantiated by changes in the dominant forms overlie with an erosionalcontact theUpper Cretaceous of the mammalian fauna; the latter may be due to evolving Nemegt clays. The thickness of the member is 20m. In the ecosystems in time and space. course of field work,the Soviet-Mongol palaeontological Three large assemblages of mammal fossils canbe expeditionfound unintatheres (Prodinoceras) and pan- distinguished in the stratotype sections of the Naran-Bulak todonts (Archaeolambdatrofimovi, Flerov & Dashzeveg Formation (Fig. 4). One (Phase I) encompasses the upper 1974) in the middle part of the member. part of the Zhigden Member and the alluvial horizons of the Naran Member. The second (Phase 11) is associated with the lacustrine part of theNaran Member, while the third Correlation of Upper Palaeocene and Lower Eocene assemblage (Phase 111) corresponds to the basal part of the sequences of Mongolia Bumban Member.Each is considered to represent a Theimportant faunas of the Naran-Bulak section in community with its own ecologic dominants and a unique Mongolia allow us to regard it as an occurrence of Asia-wide ecological association. The only exceptions are some significance. Atpresent, the faunas and stratigraphy of ecologic eurybiontics that could inhabitvarious environ- Naran-Bulak have been studied more thoroughly than those ments. Among such is Dissacus, recorded in a variety of of any other Lower Palaeogene occurrence of Central Asia communities. giving, in consequence, abundant data. Though the Zhigden and Naran Members are separated As mentioned above,the Naran-BulakFormation has by a hiatus, it is short enough so that Prodinoceras martyr, four distinct members, while the Khashathas three.The Archaeolarnbda planicanina and Arctostylops sp., found in Zhigden Member of Naran-Bulak is unique. So far,no theupper part of theZhigden, are also known in the analogues of this member in the Khashat section have been overlying alluvial sequence of the Naran Member. recognized. The other members of Naran-Bulak correlate in The first tangible change in ecological dominants is fixed faunaand stratigraphic position with the Khashatsection. at the alluvial-lacustrine interface within the Naran Member Of these, the Naran Member andKhashat’s Member I share (Fig. 4). The upper part of the alluvial sequence features most elements, and are dominated by endemic Asian forms occasional Pseudictops lophiodon, Eurymylus laticeps and (Fig. 2). Arctostylopsmacrodon, which may be attributedto The similar fauna of Khaichin-Ula I1 (lowermost part) intermittentspreading of riparianbiotopes. The Naran- that has been found in the Bugintsav Basin is known by the Bumbanboundary is due to a difference in ecosystem, as

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Fig. 5. The Meudon section near Paris, France, with the Thanetian hiatus indicated between the Palaeocene and Eocene (from Gignoux 1952). L, Lutetian Calcaire grossier; Sp, Sparnacian I Argiles plastiques; C, Meudon Conglomerate with fossil mammals; Th, Thanetian hiatus; M,, Upper Montian Meudon mark; M1, 1 Lower Montian pisolitic limestones; Cr, Upper Cretaceous. I the Eocene, but even based on the same marine organisms there is a certain divergence in views on the position of this boundary(see Hay & Mohler 1967; Costa et al. 1978; Hardenbol & Berggren 1978). I If correlation by marine organisms is more or less unambiguous, the relationshipbetween marine and l continentalsequences is much more complicated. The Eocene Sparnacianmammalian assemblage (Coryphodon and Hyracotherium) requires the placement of the boundary between thetop of theThanetian and the base of the Fig. 4. Time range of dominant species and the relationships of Sparnacian (Gingerich 1976). However, this boundary does lithostratigraphic and biochronologic units. The numerical not agree with the change in the marine fauna. At present, importance of the taxa is indicatedl by the heaviness, or the Palaeocene-Eoceneboundary in Western European completeness, of the line. continentaldeposits is established beneaththe Meudon. Conglomerate of the Paris Basin (Gingerich 1976; Gingerich this level displays a distinct change in ecological dominants. & Rose 1977). This is also the opinion of many Western Anotherchange occurs in the sections of the Bumban researchers,but Berggren et al. (1978), and Hardenbol & Memberwhere the first examples of the perissodactyls Berggren (1978) considered that the base of the Sparnacian Hyracotherium and Homogalax appear, as well as (Meudon Conglomerate) is olderthan the base of the condylarths (Hyopsodus) and primates (Altanius). The time Eocene. of burial of the Bumban fauna coincides with the onset of a new sedimentary cycle and arestructuring of Palaeocene North America ecosystems which introduced theEocene. The faunistic The lower limit of the Eocene was established below the evidence hasbeen recognized throughoutthe Holarctic. Gray Bullzone (Wasatchian). The oldergeneration of This was the turningpoint in the biotic history of the Americanpalaeontologists gave much importance to Palaeocene-Eocene boundary. perissodactyls for zonal division of the continental Palaeogenesequences in North America (Granger 1914). The position of the Palaeocene-Eocene boundary in The lower part of the Wasatchian (the Gray Bull zone of the West European and North American the Lower Eocene) includes the Homogalax zone,the stratigraphic scales middle part (Lysite) the Heptodon zone, and the upper part (LostCabin) contains the Lambdotherium zone(Van Houten 1945). The lowermost horizon of the GrayBull, Western Europe with a 100 m thick section in the Clark’s Fork Basin, is An important step in the study of the Palaeocene-Eocene identified as the Sandcouleezone. It is the first in North boundary was made at the Paris conference on the Ilerdian America featureto Hyracotherium (Gingerich 1980; Stage in 1974. The majority at the conference pronounced Gingerich et al. 1980). themselvesin favour of theboundary at the base of the American palaeontologists have obtained much new data Ilerdian marine stage (Russell et al. 1982). on the age and correlation of the Clark’s Fork fauna that The detailedbiozones erectedon variousgroups of was previously believed to represent the Upper Palaeocene marine organisms, particularly planktonic foraminifera and in North America (Wood et al. 1941). calcareous nannoplankton, have considerably improved and Because the Clarkforkian fauna and that of the Meudon clarified the stratigraphy of theWestern European Conglomerate from Parishave a number of common and Palaeogene. The Palaeocene-Eoceneboundary based on characteristic mammalian forms it has been suggested that foraminfera is placed between zones P4and P5 PB,i.e. the two can be correlated, and that theClarkforkian belongs slightly above the base of the Ilerdian(Berggren 1972). to the lowermost part of the Lower Eocene (Gingerich & Pomerol (1973) suggested relating the P5 zone to the base of Rose 1977). However, many of the genera that are common

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in the two faunas (Plesiadapk, Phenacolemur, Paramys and lowermost part of the Naran-Bulak section indicates, instead Coryphodon) are forms that occur bothabove and below of the time of their origin, the time of their appearance in the Palaeocene-Eocene boundary and cannot, therefore, be the area. TheZhigden Member is the first in Central Asia to used for unequivocal delimitation of the Palaeocene. show the immigrants Arctostylops and Prodinoceras of the Rose (1980, 1981) has divided the ClarkforkianLand Late PalaeoceneClarkforkian fauna as biostratigraphic Mammal Age into zones on the primate Plesiadapis. The markers (Fig. 4). zonal species Plesiadapis gingerichi hasbeen found in the Phase I1 from the upper part of the Naran Member, the upper part of the Tiffanian and in the lower 100 m of the second community, has vast biostratigraphic significance on Clarkforkian,where it occupies an intermediate position the inter-regionalscale, as similar faunas are known from between the P. simonsi and P. cookei zones. P. gingerichi is Gashato and Nomogen in Mongolia and China (Matthew et SO close to P. tricuspidens from Cernay, France, that thetwo al. 1929; Chow et al. 1976). Finally, dominants of the third may represent geographical subspecies. P. cookei from community (Phase 111, from the Bumban Member) spread higher in the Clarkforkian and P. russelli from the early through the Holarctic. Hyracotherium, Homogalax, Hyop- Eocene Meudon Conglomerate may share an ancestor in P. sodus and Hapalodectes, in well-studied Wasatchian gingerichiltricuspidens. Thus, there is reason to believe that sections, are registered for the first time in the Gray Bull the P. gingerichi zonecorresponds tothe uppermost fauna, whereas they are practically unknown in the older Palaeocene, while the zones of P. cookei and Phenacodus- Clarkforkian fauna. The appearance of these taxa in North Ectocion from the Clarkforkian fauna belong to the Lower America is regarded as resulting from their migration into Eocene.Hence, the Palaeocene-Eoceneboundary in the thearea (Gingerich & Rose 1977; Rose 1981), but from North Americancontinental sequences has been placed where is still unclear. Primitive perissodactyls within the Clarkforkian by Rose. (Hyracotherium and Homogalax), some condylarths (Hyopsodus and Hapalodectes) may have migrated to the Biostratigraphic reconstructions based on the New World from Asia at the end of the Clarkforkian epoch. author’s material It is essential for dating the Naran-Bulak section that the Conclusions age of the NorthAmerican Clarkforkian fauna is known (1) A palaeontologically substantiatedPalaeocene-Eocene which, as shown above, can becorrelated with it. boundaryhas been established for the first time for Sedimentation took place on both continents and of the land Mongolia in the stratotype sections of Naran-Bulak in the mammal faunas that developed contemporaneously many of Nemegt Basin. Atthe presentstage of research, this the genera were related phylogenetically. boundary may beconsidered as situated in astandard Study of the transitionalPalaeocene-Eocene strat- sequence for Central Asia’s Palaeocene-Eocene. igraphic sequences of Mongolia and theirfaunas, as (2) Intercontinentalcorrelation based onthe mam- compared to the succession of faunas in Western Europe malian faunas helps to add precision to the stratigraphy of and North America, has greatly underestimated the role of the continentalPalaeocene-Eocene boundary strata of the Thanetian hiatus for the Palaeocene-Eocene boundary Western Europe.The far-reachingcorrelations of Mon- in West Europe. The Thanetian hiatus is well-documented golian andNorth American data have supportedthe in the Pans Basin (Fig. 5) (Gignoux 1952; Pomerol 1973). It presence of asedimentary hiatus within the Paris Basin encompasses all the zones of theNorth American deposits that corresponds tothe uppermost part of the Clarkforkian and also the timeinterval (Phase I) of the Thanetian age. Therefore, and in contrast to earlier ideas, Zhigden and lower NaranMember of the Mongolian theUpper Palaeocenestratigraphy and fauna of North Naran-Bulak Formation (Dashzeveg 1982~). America and Central Asia have proved more complete than The pre-Clarkforkian Tiffanian fauna of North America that of Western Europe. can be correlated with the reliably dated Cernay fauna of the West EuropeanThanetian (Russell 1964; Gingerich Theauthor thanks E. V. Devyatkinand D. E. Russellfor their 1976). The post-Clarkforkian Gray Bull and Bumban faunas usefulremarks and corrections regarding the manuscript of this appear to bealmost contemporaneous, and to correlatewith paper. the Sparnacian fauna of the Meudon Conglomerate in the Pans Basin. Thus, chronologically, Clarkforkian faunas are intermediate with respect to Western European faunas of References theUpper Palaeocene(Cernay) and Lower Eocene BERGGREN,W. A. 1972. A Cenzoic time-scale: some implications for regional (MeudonConglomerate, Dormaal). This suggests that the geology and paleobiogeography. Lethaia, 5, 195-215. Clarkforkianfaunas are of Late Palaeoceneage (Fig. 3). -, MCKENNA,M. C., HARDENBOL,J. & OBRADOVICH,J. D. 1978. Revised Paleogene polarity time scale. Journal of Geology, 86, 67-81. The Palaeocene-Eocene boundary in North America should CAVELIER,C. & POMEROL, C.1986. Stratigraphy of the Paleogene. Bulletin de be placed below the Gray Bull zone (Wasatchian) while in la SocitW g6ologique de France, 8(U), 2, 225-65. Central Asia it is beneaththe BumbanMember in the CHOW,M,, QI, T. & YONG,L. 1976. Paleocenestratigraphy and fauna1 Naran-Bulak sections that display almost synchronous entry characters of mammalian fossils of Nomogen Commune, Si-Zi-Wang-Qi, Nei Mongol. Vertebratae PalAsiatica, 14, 228-33 (in Chinese,English of condylarths (Hyopsodus), primates (Altanius) and summary). perissodactyls (Hyracotherium and Homogalax). COSTA, L., DENISON,C. & DOWNIE,C. 1978. ThePaleocene-Eocene The oldest remains of Arctostylops and Prodinoceras are boundary in the Anglo-Paris Basin. Journal of the GeologicalSociely, known from thelate Tiffanain andthe lowermost London, l35,261-4. DASHZEVEG,D. 1977. On the firstoccurrence of Hyopsodur Leidy, 1870 Clarkforkian(North America). Migration of thesegroups (Mammalia,Condylarthra) in MongolianPeople’s Republic. proceeded from North America to Asia (Gingerich & Rose Transactions of the Joint Soviet-Mongolian Paleontological Expedition, 1977). Consequently, the appearance of these genera in the 4,7-l3 (in Russian, English summary).

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Received 5 February 1987; revised typescript accepted 25 September 1987.

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