Heu. .Ifus. Arge~ztinoClenc. Not., n.s. G(21: 227-232,2004 Buer~osAir-es, ISSN 1514.5158
First fission-track age for the dinosaur-bearing Neuqukn Group (Upper Cretaceous),Neuqukn Basin, Argentina ffugo CC)RBEI,L.A!', E'erriando E. NOVASl.3, Sebast,ihn APESTEGU~A!." &&tor A. LEWZA".G
I Musoo Argcritiiio de Ciencias Xaturales ,cBernardino Ilix~adavia. and CONICET, Av. A. Gallardo 470. 1405 Huenos Aires, Argontina. [email protected]~m.ai', [email protected] paiooninja@~~ahoo.coinn~i.6Servicio Geoldgico Minero Argontino and CONICET Av Julio A. ltocv 651 I'iso 10, 1322 Riienos Aires, Argentina. %[email protected]
Abstract,: The first radiometric age for the Ncuqu6ii Croup, based on fission-track analysis made on ail ash- flow tuff at the base or Lhe IIuincul Fomacion. is reoorted iil this moor. The radiome!ric am offers a more
age of88 i; 3.9 My This value suggost Lhat he i!nincul Forination was deposited botwerii ail ago range ii.orri 84.1 to 91.9 My, chat is froin the biise .if the Turonian to the top of tho Sanlonian.
Key words: Upper Cretaceous, Neuqu6n Husiii, Patagonia, geochi.onoiogy, tctrapods.
Because of its areal extension, thickness and ForrnaLion at Cerro 10s Overos iNeuqi16n fossil tetrapod abundance (see Leanza el al., 2004), Province), but this rock level still awaits to be the Neuqudn Group is one of the most important radimetrically dated. Here we oCfer for the first Cretaceous contincntal units in Soutli America. time a fission-track age for tile Neuqn6n Group. Its beds, widely exposed in northwestern Patagonia, have been explored since the end of GEOLOGICAL AND PALEOXTOLOGICAI, 19th century particularly in the search for SETTINGS vertebrate remains. The Neuqn6n Group contains a wide variety of different groups of tetrapods, Tile Neuqu6n basin is localed in west central mainly reptiles, which currently represent the Argentina and eastern Chile between 34" and 41" most complet,eand chroiiologicaliy comprelieiisive S. In Argentina, their outcrops are exposed in the sequence of Cretaceous continental fossil provinces of NeuquBn (from which it takes the asselnhlages in she Southern contiiients. Due to name), Mendoza, Rio Negro, and La Pampa its ahuridance in dinosaur remains, Kejdel(1917) [Digregorio & Uliana, 1980; Legarrcta & llliana originally termed these beds as -Estratos con 1991; Gulisano & Guti6rrez Pieiriiling, 1995).The Dznosaurios.~~However, the age of the entire Ncuquen Group (Stipanicic et al., 1968) in this Neuqueu Group, as well as each of its constituent region lies between the Bajada del Agrio and subunits, is Far fkom settled and mostly relies on Malargue Groups, and is bounded by two major the limiting ages of the under- and overlying regional surfaces recognized as seismic reileclor marine formations, as well as on sequence horizons: the iVain Miranican (Aibian/Ceno- stratigraphy analysis (Legarreta & Ijliana, 1998; rnanian boundary) and the Hiiar~traiquican(Mid- Letinza, 1999; Leanza & Hugo, 2001, Ilugo & Campanian) unconibrmit,ies (see 12amos; 1981, Leanza, 2001). Macellari, 1988, Vergani el al., 1995, Leanza, Although magnetostratigraphic data were 1999). It consists of continental red beds up to obtained for the upper part of the Neuquen 1,300 m in thickness (Cazau & IJliana, 1973), Group (Din~iset al., 2000), rocks suitable for ii~cludingconglomerates, sandstones, and dating by radionletric methods in the Cretaceous mudrocks, dominantly deposited under alluvial contincntal strata of the Neuqubn basin were processes (Legarreta & Uliana, 1998). It is almost unknown. The exception is the tuff level generally accepted that the Neuqu6n Group is reported by Ramos (1981) ftom the Anacleto Ceriomanian to Early Campaniari in age 228 Revista del Mweo Argentina de Ciencias Naturales, n, s. 6 121, 2004
39'41' I
em zr.s REFERENCES Anaclelo Fm.
Pionis Fm. NEOGENE Huincul Fm. aCandeieros Fm. PALEOGENE
Fig. 1. Geological map showing location of the studied arca (down left) and the three places in which the ash-flowtuff was studied: 11 Agua de las Vacas, 21 La Piedra Sola, and 3) Cerro Policia hill.
(Legarreta & Uliana, 1998: Leariza & Iiugo, 2001, area around the Limay River, in the border Hugo & Lcariza, 2001). between Neuqubn and Rio Negro Provinces. So Current subdivision of' the Neuquen Group far, two dinosaur taxa from the Huincul Formation embraces, from base to top, three fining upward have been named: the titanosaurian sauropod cycles of continental deposition (Cazau & Uliana, Argentinosaurus huinculensis (Bonaparte & 1973): Rio Limay, Rio Neuquen and Rio Colora- Coria, 1993) and the abelisauroid theropod do Subgroups. The Rio Limay Subgroup is made Iloizelesia aguadagrandensis (Coria & Salgado, up by three formational units: Candeleros, 20001, but there are also abundant remains of Huincul, and Cerro Lisandro. carcharodontosaurid and ahelisauroid theropods Dinosaur remains discovered in this unit were (Coria & Curric, 1997; Novas & Bandyopadhyay, found in different fbssii spots located in a wide 1999; de Valais et al., 2002), rebbachisaurid and Corbella et a1 Fcrst fzsszon-track age for the Neuquen G,oup 229
Mexia, Ilugo & 1,eanza (2001) later demonstrated that this white level is stratigraphically situated iii the lower part of the Hoincul Fnrmat,ion. For the purposes of this paper, the tufSlevel lias been examined in three localities extending at least 10 km around the southwestern extrerriity of Meseta de Reuteria rcgio~l(Fig. I), as follows: 1) Agua de las Vacas (S 39' 35' - W 68" 37'; Fig. 3B), 2) La Piedra Soia (S 39" 36' - W 68" 43'; Fig. 30, and 3) the Cerro Policia hill (S 3!3Q 40' - W 68" 29', Fig. 3A). The tuff level, which is interpreted as an ash- flow (Ross & Smith, 1960; Llamhias, 2003) was collected 16 m above the base of the E-Iuincul Formation (see Fig. 2), and il may reach up to 2.80 m in thiclmess. It is composed by reworked materials, including abundai~tglass siiards strongly altered in zeolites (heulandite - Conglomerates clinoptilolite series), crystailoclasts of alkali feldspars, scarce fermmagnesian minerals* tiny 30 m 0Sandstones crystals of Fe-Ti oxides, rutile and zircon. The internal structure of the tuff is locally massive, but 20 m 0Siltstones more comrnorily shows marked contorted lamination suggescirlg at least local rernohilization. Calcareous The basal decimeters have abundant endichnial 10 m concretions burrows up to 2 cni in dianseter (Fig. 3D). Interbedded colored clastic leinses similar to the MudrocKs over or undcrlyingsedimerrts arc common. The top 0 m of the white level is commonly eroded and covered Paieosoii~ by microconglomerates. VERTICAL Selected zircon crystals oftlse white tufflevel, SCALE luffs outcropping at Agua de Las Vacas site, were dated by fission-track method at Gcotrack Iristitute of Mdhourne, Australia (see Table 1).Fissiois-track ages were determinad from twenty grains chosen Fig. 2. Stratigraphic column at Cerro Policia hill, from a field zircon crystal coilcentrate. The final showing the position (arrow) of the radiometrically analyzed tuff ir~thc basal portion of the Huincul zircon concentrate comprises a suite of colorless, Formation. elongate euhedrai grains. The sample shows a significant spread of ages, with the followiilg lour age groupings (error quoted at the n- 1.0 level): 271.0 ? 32.0 My (3 gains); 157.4 i 17.7 .My (2 grains); 88.0 i 3.9 My (14 grains); 50.5 I5.8 My (1 grain). The youngest group is represented by titanosaurian sauropods (Calvo & Salgado, 1995; only a single grain and this is regarded as Calvo, 1999; Salgado et al., 1991; Simon, 2001). anon~alous.The 88.0 i: 3.9 My population Basal euiguanodontian ornithopods originally dominates the sample (fourteen out of twenty rcporled frorn the IIuincul Forrnation (Coria, grains) and is regarded as the best age estirriated 1999) are now referred to the overlying Cerro for the priniary volcanism, which, according to the Lisandro Formation (R. Coria, pers. con~m.). stratigraphic chart of the Interriaiioiial Commission on Stratigraphy (Gradstein & Ogg, Characteristics and radiometric age of the 2004), belongs to an age near tile Turonian - tuff bed Coriiacian boundary The older ages represent a Pioneering work of Wichmann (1934) led t,o detrital input from surroundirig basement rocks, the identification of a white tuff bed at the Cerro presumably the Jurassic rhyolites fiorn the Marifil Policia (Ria Negro Province), close to the Formation and the Perrno-Triassic grariitoids from southern coast of the Lago Ezequiel Rarnos the Curac6 Complex (see Hugo & Leanza, 2001). 230 12cuistn dei Mzisi,oArgentino de Cien.(:~asNc~liirnles, n. s. 6 (a,20M
Fig. 3. Different aspects of the ash-[low tuff level in the Huiiicul Formatioi~.A, Cerro Policia hill showing the white tuff level (arrow). B, the white tufllevcl (1.20 m thick) as expossed at the Agua de las Vacas site. C, tile white tuSY'level (2,80 m thick) at the La Piedra Sola locality. U, abundant endichnial burrows, up to 2 cin in diameter, prescut in the basal decimeters of tlie while tuff at the Cerro Policia site.
'Table 1. Analytical values for the group ofselected 14 zircon crystals. Pooled age= 88 ? 3.9 My
Niiniber of spoiitaiieous tracks in Ns grid squares Ss = 2457 Number of induced tracks in Na grid squares Xi = 1197 Spontaneous track density = NsIiNu lama of basic unit) r, = 7.596E+06 Induced track density = h'l/(Na ^ area of basic unit! r, = 3.701F:+06 Ratio (KslKi) = Cranium contciit of each graiii (=C coriteiit of standard glass " r,/r) U (ppn,) = Fission-track age , calculated using il~eequation B1 F. 'T. Age (My) = Area of ione microscope eyepiocc rid square Area of basic unit = 6.293);-07cm-2 ~"parailietet. , uscd to vssoss vuriatiori ofsinglc grain agos within thc sample cZ13.155 with 13 frecdorn Probability of ohraininz obscrved c' valua for the relcviilit numbel. ol'dcgrre of freedom, if nil grains helong ti] a single population P(c2! = 43.6% 'A variation in singlc grain ages Age dispcrsioli = 2.449% l'oolcd rmio, total spontnrieous tracks divided bit total induced tracks for all grains Ss/Ni = 2.053 ? 0.072 Moan of (NsINi: for individiial gains Mevri radio = 2.105 i 0.085 Agos calcuiaied using a zeta or 87.7 t 0.8 for U3 glass r ,,,,,,,,~,,x:,, ND = 1548
CONCLIJSIONS authors (i.e.,Cnlvo, 1991, 19991,whereas others, on the basis of stratigraphic corlsiderations, Ttie age of the Candeicros Yorrnation was interpreted the age of this unit as Early considered on the basis of its tetrapod content Cenomaniail (i.e., Vcrgani el al., 1995; Legarreta as Albian - Cenoinariian or even Alhian by sorric & Uliana, 3998; Leanza, 1999). The overlying Cor.hella el al.: First fission-lracli age for the Neuy~~PnGroup 231
IIuincul Formation was tentatively ascrilicd to Cenomaiiian of i\rge:riitiria; new evidoiice on thc the Late Ccnornarlian, anti the Ccrro 1,isandro origiii of thc 1)iplodocidae. Gnca I. 1:13-;13. Formation to tile Latest Cenoniarliar~- Early Cazirii, L. B. & M. A. Ulianri. 1973. GI Creticico Sizpe- rior coiitinc!iilal de la Cueiica Kcuqui~in.VCo:oi~g,-e- Turorliall (i.e., Learlza & IIugo, 2001; Leanza et so Geoldgico Arge,~tino,3 : 131.163, al., 20041. According to the present results, Lhc Colla, : It.~ 1'199. Mirlcrialcs dr ornit6podos do loi iiive- Tiuincui and Ccrro Lisandro Formatio~isu,ould les iiif'eriori.s del Criipo Kei;cj~i&:,iirnegl~iii~n~ra 36 he younger than previously thought, as (4) sr:ppl.: 9R. radiometric dat,a ilupiies that the deposition or Cori'i ., R. & I? Curric. 1997, A ne\v theronod from the the first, iiriit prcsurrlahly start.cd around thc Itio Limay iiormatioz;. Jorcri~aio/ Vertebi-irtc Early Turonian. Nevertheless, it might have P
Special Publication (London) 12:429-450. origin, geologic relations and identification. - 1998. Anazo~nyof hinterland depositional Geological Survey Professior~alPaper 366 : 1-81, sequences: 1Jpper Cretaceous fluvial strata, Salgado, L., Calvo J. & R. Coria. 1991. Estudio prelimi- Neuquen Easin, west central Argentina, In: nar de dinosaurios saorbpodos do la Formacirin Rio Shanley, K., W and McCabe, l? d. (eds.): Relative Limay (CretQcieo presenuniano, provincia do1 role ofoustaey, climate, and tectonism in continen- Neuquen). Amegh,i~ziana,28 (1-2):134. tal rocks. Society o/'Econo,nic Paleo,~tologistsand Simbn, M. 2001. A giant sauropod from the Upper Mineralogists, SEPM, Special Piiblication, 59:83- Cretaceous of El Chochn, Neuqubn, Argentina. 92. Anzeghiniana 38(4) suppI.:lgR. Llambias, E. J. 2003. Geologiv de 10s cuerpos igneos. Stiuanicic, P N.. I? Rodrieo. 0. L. Baulies & C. G. Asociuciijn Geolijgica Ar~eiitina,Serie 8, Didicti- cay Complementaria - NQ27 : 1-182. Macellari, C. E. 1988. Cretaceous pvleogeography and - depositional cycles in western South America. geitina; 23(2):367-388. Journal on South American Earth Science, 1373- Vergani, G. D., A. J Tankard, 13. J. Belotti, & H. J. 418. Welsink. 1995. Tcctonic evolution and Novas, F 6.& S. Ban~iyopadhyay2001 Abelisaurid paleogeogmphy of the Neuqubn Wasin, Argentina. pedal unguals Roin the Late Cretaceous or India. In: A.J. Tankard, R. Sudrez and H. J Welsink (eds.): 7Unternational Symposium on Mosozoie Potroleurn basins of South America. American Torresirial Ecosystoma, Asociaciijn Paleo,ztijlogicn Associolior~of Petroleum Geologists, Mcrnoir Arger~tir~a,Publicaci6n Especial 7:145-149. 6'2:383-402. Rarnos, V A. 1981,Deseripci6n g-eol6gica dela Hoja 33c, Wichmann, It. 1934. Contribuci6n a1 eonocirniento I,ns Chihuidos Norle, provincia do1 Neuqubn. Ser- gcol6gibo de 10s Territarios del Neuqubn y del Rio uieio Wmioiial Minelo Geol6~ie0,Boletin !82. Negro. Direceibn deMinas y Cleulogia,Boletin 39:l- Ross, C. S. & R.L. Smith. 1960. Ash-flow luffs: their 27.
Recibido: 2-XI-2004 Aeeptado: 13-XII-2004