The Triassic/Jurassic Boundary in the Andes of Argentina

Home , Plagiostoma (bivalve), Psiloceras

Rivista Italiana di Paleontologia e Stratigrafia volume 110 no.1 pp.69-76 ,;; | ^0,,,

THE TRIASSIC/JURASSIC BOUNDARY IN THE ANDES OF ARGENTINA

ALBERTO C. RICCARDI', SUSANA E. DAMBORENEA', MIGUEL O. MANCENIDO' 8. MARIA P IGLESIA LLANOS,

Received September 19,2002; accepted July 24, 2003

Key words: Triassic{urassic boundary, Argentina, Ammonoids, thropteris sp. La sezione è stata anche campionata per quanto riguarda Bivalves, Brachiopods, Biostratigraphy. conodonti e radiolari, finora con risultati negativi. Uno studio paleo- magnetico è in corso. Abstract. The Arroyo Malo Formation at Alumbre Creek, on the northern bank of the Atuel River, west central Argentina, comprises a c. m thick continuous marine succession across the Triassic-Jurassic 300 lntroduction System boundary consisting of massive and laminated pelites indicative of a slope depositional environment. Late Triassic invertebrates, includ- Until quite recently all evidences indicated that in ing an.rmonoids, nautiloids, bivalves, gastropods, brachiopods and corals are restricted to the lower 150 m. Beds between 125-135 m from the bot- Argentina the Triassic was only represented by continen- tom yield Cboristoceras cf. marshi Hauer, a species found in the Marshi/ tal strata (Stipanicic 1983 and references therein). Previ- Crickmayi Zone of Europe and North Anrerica, together with loose ous records (Groeber 1924,1929) of marine Triassic were fragments of Psiloceras cf. pressum Hillebrandt, coeval with the lower based on some bivalves and brachiopods found in west- to middle part of the Hettangian Planorbis Zone. About 80 m higher central Argentina, which were subsequently dated as Early are beds yielding Psiloceras cÍ. rectocostatum Hillebrandt, a species that gives name to an Andean biozone partially coeval with theJohnstoni and Jurassic (Leanza 1948; Frenguelli 1948). Until 1986 the Plicatulum Subzones, upper Planorbis Zone. Other fossils recorded in oldest recorded Mesozoic marine strata - dated as Sine- the Rhaetian strata of this section are foraminifers, ostracods and plant murian - were located on the northern bank of the Atuel remains identified as Zuberia cl. zuberi (Szaj.) Freng. and Clathropteris River, within a succession studied by different authors sp. The section was also sampled for conodonts and radiolarians, thus since Bodenbender 1892 (see also Burckhardt 1900, 1903; far with negative results. A palaeomagnetic study is underway. Gerth t9z5; Groeber 1947; Stipamcic 1969; Volkheimer Rìassunto. La Formazione Arroyo Malo ad Alumbre creek, sul- 1970,1978; Hillebrandt 1989).In otherareas of west cen- la sponda settentrionale del fiume Atuel, Argentina centro-occiden- tral Argentina the base of the marine Mesozoic was dated tale, comprende una successione marina continua spessa circa 300 m as late Early Pliensbachian. attraverso il limite Triassico-Giurassico, ed è costituita da peliti massive e laminate indicative di un ambiente deposizionale di scarpata. Gli In 1986 a field trip aimed at locating the oldest ma- invertebrati del Triassico superiore, che includono ammonoidi, nauti- rine beds in the rio Atuel area, resulted in the first record loidi, bivalvi, gasteropodi, brachiopodi e coralli sono limitati ai primi of Hettangian ammonites for west-central Argentina (Ric- 150 nr. Gli strati fra i 125-135 m dalla base hanno dato Choristoceras cardi er al. 1988, 1991). Furthermore, a succession c. 300 cf . marsbi Hauer, una specie trovata nella Zona a Marshi/Crickmayi di m thick below the oldest ammonites prompted Riccardi Europa e Nord America, insieme con frammenti sparsi di Psiloceras cf . pressum Hillebrandt, coeva con la parte mediana della Zona a Planor- et al. (1988) to suggest the possible occurrence of marine bis dell'Hettangiano. Circa 80 m piir in alto ci sono strati contenenti Triassic in those 'unfossiliferous'beds. From these levels Psiloceras cÍ. rectocostdtum Hillebrandt, una specie che dà il nome ad Ballent (1994) recorded some microfossils, and indicated una biozona andina parzialmente coeva con le Sottozone a Johnsto- its possible significance future research on the Trias- ni e Plicatulum, Zona a Planorbis superiore. Altri fossili documentati for negli strati del Retico di questa sezione sono foraminiferi, ostracodi e sic-Jurassic boundary. resti di piante identificati come Zuberia cf. zuberi (Szaj.) Freng. e C/a- Undoubtedly Tiiassic marine invertebrates and plant

Inverrebrate Palaeontology Department, La Plata Natural Sciences Museum, Paseo del Bosque s/n, 1900 La Plata (Argentina). E-mai1: riccardi@)museo.fcnym.unlp.edu.ar Laboratorio de Paleomagnetismo "Daniel Valencio", Departamento de Ciencias Geológicas, Universidad de Buenos Aires, 1428 Buenos Aires (Argentina). 70 A. C. Riccardi, S. E. Datnborenea, M. O. Mancenido tx M. P lplesia Llanos

and corals, as well as plant frasments. At 230 m the first Hettangian ammonites occur. This succession, included i<) sè-, lA-i in the Arroyo Malo Form:rtion, is conforrnably covered by 5-20 m thick cross bedded and imbricated polymictic 34'45' conglomerates grading upwards into coarse massive sand- Cerro Sosneado stones, the El Freno Forn-ration. Above follows, conform- ably, a coarsenine-upward succession of c. 800 m of well- stratified pelites and sandstones, the El Cholo Formation, bearing Hettangian and Siner.nurian ammonites.

Palaeontology

As mentioned, the Arroyo Malo Formation contains microfauna (foraminifers and osrracods), and in or- der of decreasing importance bivalves, eastropods, brachiopods, ammonoids, cnidarians and nautiloids (Fig. 3; complete list in Appendix). Samples studied for conodonts (G. L. Albanesi) and radiolarians (E,. Carter) were reported as barren. A report on the microfossils was given by Ballent Fig. 1 -Loc;.rtionmap. (1994). She recorded the presence of poorly preserved, infilled and often broken, small foraminifera, consisting of coiled Haplophragmoides-like asclurinared resrs and nodosariids. A few ostracods included healdiids such as remains were found in the same area during successive O grn o con ch ella and cytheraceans. Bivalves field trips carried out during 1994-1999 (Riccardi et al. are the most numerous and diverse eroup 1997a-b; Riccardi & Islesia Llanos 1999). Late Triassic represented in the tiassic part of the Arroyo Malo For- invertebrates are restricted to the lower 150 m (Riccardi mation, with at least a dozen species belonging to almosr et al. 1997a-b, Damborenea & Mancenido 1998; Riccardi as many families (see stratigraphic distribution in Fig.2). & Iglesia Llanos 1999). Despite intensive sampling, the material is very scarce in As there are very few sections world-wide where the the succession, and preservation is generally poor, though marine Triassic-Jurassic transition is exposed, the investi- it differs according to lithology. The dark shales contain gation of this ney/ section could add significantly to the scattered complere (articulated or disarticulated) shells study and correlation of the system boundary. Therefore, of relatively larue specin.rens. The best specimens are pre- it is currently being the subject of a multidisciplinary re- ser-ved :rs external moulds in nodular local shell concen- search project aimed at that end. trations. These contain a very diverse fauna, but some size sorting appears to have taken place, since most shell pieces irre less than 1 cm across. The few complete spec- Stratigraphy imens are small-sized indìviduals of all species. The pre- servàtion of delicate ornamentation details, the presence In the northern bank of the Atuel River the marine of articulated bivalves, and the lack of abraded fragments Triassic-Jurassic transition is exposed on rhe risht bank of indicate short transport, probably only minor local win- Alumbre, a small tributary of arroyo Malo, about 5 nowing allowine size-selection. ^r(oyo km north of its junction with the Atuel River (Fig. 1). No bivalves have been found in beds from the earli- The section is exposed on rhe west limb of a north- est Hettrnsian (Fig. 2). After the boundary beds, the first south striking anticline. The lower part of the section con- bivalves appear in beds with ammonites of the "Waehn- sists of 286 m of massive and laminated pelites and tur- eroceras-Schlotheìntia" Zone, separated by about 150 m bidites with massive pebbly mudsrones, intraforn-rational of sediments from the last Triassic bivalve. The only bi- breccias, clast supported conglomerates with normal gra- valve species which appears both below and above the dation and cross-bedded sandstone lenses (Fie. 2). Synsedi- boundary is Praechlamys cf . z,aloniensls (Defrance) (Fig. mentary folding and slump srructures are quite comn.ìon. 3t), which w:rs found near the level with Choristoceras cf . The sedimentological features indicate a shallou,ine up- tnarsbi and extends to the Late Sinemurian "Epopbiocer- wards sequence related to a conrinental slope fan delta. as Zone" (Damborene a 20A2a) . This species has a similar Between 50 and 150 m there are severirl fossiliferous str:rtieraphical distribution across the boundary in other levels containing rather poorly preserved representatives parts of the world (McRoberts et al. 1997; Ivimey-Cook of Triassic ammonoids, nautiloids, brachiopods, bivalves et al. 1999). All other bivalve species are resrricted eirher Tt'iassic/Jurassic boundary in the Antles of Argentína 71

Fig. 2 - Stratieraphic section with location of samples (nodified So::.e;È * se from Riccardi et al. 19971 ìE ÈH à:gX é ;Rl",Só&:- and dìstribution of bivalve -oìÈò*;Nó iGiN.o-Òi^ b 't species. Stippled portion in- à o ÈÈ*eSiEtÈtr" .l dicates the absence of bios- m Samples tr;rtì graphically useful fossils, :HF$SÈFgfgÈ 3 f! ÈeóSS&Èa*Èa*8 d in rvhich the Triassic-Jurassic 1390 Waehneroceras A boundarl' should be located. E 1 937 a TL 1 936 1 935 o 1 389 '1388 _c O a-ia---. '1384,'1934 1 385 ]lt I IJJ 1 933 É I l

Psiloceras cf . rectocostatum

E ,::l |.L (\. o (g 2185

I 150 At\43 1782 AM2 ? Choristoceras c'f . marshi Ò 2061, 1775 1774 AM1 1779 2057 ,2184 h 2056 r-!>--- 9 a U) ooaaa f _-_----_1 É. i------l F------l..---1 o 2060,2182 ------l o 1 780 -----l 1 774 :------l '' '- 21Ra 1777 - -- 1776 :-_:-:-_------

t In situ sample o Float sample EF El Freno Fm.

l-lsandstone f6ì congtomerate E- sittstone f.-- Jv1spl lD Nodule

to Triassic or Jurassic deposits. At the generic level C,zs- Plagiostoma? sp., Pinna sp., Astartidae gen. et sp. indet., sianella, Mìnetrigonia, Palaeocardita and Septocardìa do not and Pleuromya sp. extend after the Rhaetian worldu'ide. The earliest Jurassic Ammonoids in the Triassic part of the section are bivalve f,runas at Arroyo Malo contain inste,'rd Palmoxyto- quite rare and so far only part of a whorl and several ex- ma c[. rygnipes (Young and Bird), Cdmptonectes? c[. subu- ternal moulds have been found. They are referred rc Chor- Iatus (Munsrer), Eopecten cl. oelatus (Goldfuss) and Prae- istoceror sp. (Fig. 3b) ,rnd Choristocertrs cf . ntarshi Hauer chlamys oaloniensìs (Defrance) (see Damborenea 2002; (Fig. 3r), the htter indicative of the Marshi/Crickmayi Fig. 3u), and ,rlso Grypbaea sp., undeten.nined oysters, Zone,Late Rhaetian. The first record of in situ Hettang- 72 A. C. Riccardi, S. E. Danborenea, M. O. Mancenido t< M. P lglesia LLanos ian ammonoids is from 80 m highea where are present lent present latitudes at Curepto and Los Molles (Chile) the first specimens comparable with material from Chile (Fuenzalida-Villegas 1 93 Z; Thiele-Carta gena 19 67 ; Cecioni included by Hillebrandt (2000) rn Psiloceras cf . callipbyl- & \flestermann 1968), which belong to the "outer belt". loides Pompeckj and Psiloceras rectocostatum Hillebrandt, Goodwin (1997) analysed a similar case in the North- of the Rectocostatum Andean Zone, which is partially ern Hemisphere: Norian faunas from Sonora (Mexico) equivalent to the Johnstoni and Plicatulum Subzones, are very close to those from Nevada and Oregon (USA) Planorbis Zone. Between the levels with Choristoceras cf. despite latitudinal differences in present day location. marsbi and P cf. rectocostatum, no other ammonoids have He proposed that the situation could be explained ei- been found thus far. These levels should be equivalent to ther sugeesting important terrane displacements or as the lower part of the PlanorbisZone, i.e. Primocostatum a consequence of independent response to similar envi- and TilmanniZones of the Andean zonation. Significantly ronmental conditions. It is interesting to note that those enough, loose material from the same levels where Cboris- Northern Hemisphere faunas (described among others toceras cf.marshi was found, is comparablewrth Psiloceras by Newton 1986; Newton et al. 1987; Tamura & McRob- erugaturn Buckman and P pressum Hillebrandt, species in- errs 1993; Stanley et al. 1994; Stanley &. Gonzilez-León dicative of the lower to middle part of the Planorbis Zone 1995; Damborenea & Gonzilez-León 1998) are also very (see Bloos & Page 2000; Hillebrandt 2000). On top of the close to those from the South American "inner belt". levels with Psiloceras cf.rectocostatum follows a fossilifer- The Triassic bivalve fauna contains both shallow in- ous succession with Early-Late Hettangian and Early Sine- faunal and epifaunal suspension feeders. Cassianella was murian ammonites (Riccardi et al. 1988, 1991). probably a reclining suspension feeder (Fùrsich Ec Sfendt Brachiopods are likewise rare, apparently absent 1,977;Laws 1982; Newton et al. 1987); Liostrea was a per- from the key interval spanning the system boundary. Nev- manently cemented oyster; Cblamys and Otapiria were ertheless, it is worth reporting a significant faunal change epibyssate shells; and Minetrigonia, "Astarte", Palaeocar- from the faunule bearing Zugmayerella? cf . hoerneri Sandy dita, Septocardia and Tutcheria were all shallow burrowers (Riccardi er al. 1,997a; Damborenea & Mancenido 1998; (Newton et al. 1l8z). They indicate a shallow subtidal, Riccardi et al. 2000b; Mancefrido 2002;Fig.3e-g) occurring well-oxygenated environment of normal salinity. about 30 m below the level wirh Choristoceras cf. marshi, Though most of the bivalve genera present at Ar- and the next assemblage present ctrca 215 m above it, royo Malo are cosmopolitan in distribution, at species level characterized by Furcirbyncbia cf. trecbmanni MacFarlan, the fauna has clear east-Pacific affinities (from Oregon to 1992 (Mancenido 1994; Riccardi et al. 2000a-b; Manceni- Chile, references already mentioned), and is very different do,2002; Fig.3h-i). f rom con temporary Tethyan faunas (Damborene a 2002b). Representatives of the spiriferinid genus Zugmay- From the 12 Triassic bivalve species, 6 are comparable to erella are widely distributed in Norian-Rhaetian deposits species from Perú and Chile, one is cosmopolitan and the world-wide, and their alleged persistence in EarlyJurassic others were not determined at species level. Vith regard to beds is in need of further confirmatory evidence (Riccardi brachiopods, the material assigned to Zugmayerel/a shows et al. 1997; Carter et al. 1994). The incoming of the rhyn- affinities with East Pacific and Maorian faunas, as pointed chonellid genus Furcirhyncbia in the Hettangian is a re- out by Manceiido (2002). markable feature in such distant geographical areas as the Alps, New Zealand and the Andes (Mancenido 2000). Magnetostratigraphy

Palaeoecology and palaeobiogeography Although magnetostratigraphic studies have been carried out in the Lower Jurassic of the area, the low- Two main faunal types may be recognized fron.r a est levels studied are above the Triassic-Jurassic transi- general survey of Late Triassic bivalve faunas from South tion and well within the Hettangian. Samples from these America. The geographical distribution of these as two levels carried a reversed polarity indicating that the beds "belts" roughly parallel to the palaeoshore, each along a assigned to the "PsilocerasZone", and considered equiv- wide latitudinal range, suggests a strong palaeoecologi- alent to the boundary between the Planorbis and Liasi- cal control. The Arroyo Malo bivalve fauna belongs to cus Standard Zones (Iglesia Llanos & Riccardi 20OO), are the "inner belt". It is similar to lower latitude faunas at least equivalent to the uppermost Planorbis Zone and from the following areas of Chile and Perú: Domeyko most probably to the Liasicus Zone, as indicated by Hil- (Hayami et al. 1977; Chong tr Hillebrandt 1985), Junín- lebrandt (2000), who placed the ammonite fauna in the Cerro de Pasco (Jaworski 1922; Steinmann 1929; Cox Bayoensis Zone of the Andean zonation. 1949; Boit 1966; Maeda et al. 1983;, Huairas (Jaworski New magnetostratigraphic information is expected 1922) and Acrotambo (Kórner 1937). This "inner belt" in the near future, as study of samples comins from levels spans nearly 30" in latitude. On the other hand, this fau- where the tiassic-Jurassic transition is most probably lo- na has no element in common with those from equiva- cated is in progress (by M.P Ielesia Llanos). Ti'iassic/.f urassic boundatl, in the Andes of Argentina 73

ffi, c -@;Ò,q

Fig.3 -t)Choristocer,tscl.n,trshi Heuer,MLP2tì536,x1;b)Cboristoteras sp., l!{t-P28537,x1;c)Psilocerascf.pressunt Hillebrandt,Hettans- irn, NILP 30!Ì23, xl; d) Psiloteras cl. re.tocostdtuni Hillebrandt, Hettrngian, MLP 1082+, xl: e-g) Zugntq,erelLt? sp. cf . koemeri Stndy, Rhlretiln'MLP27763,\'entral,Iater;1]i1nditnterior\'icn,s'x|lh.i)Furcirb1,nchiacÍ.trec|lntanni vjens, MLP 10826, 12; j-k) PtLteocdrdita ci. perutiana Cox, Rhreti;rn, lcfr valve u'ith rn epizoic cor.rl, MLP 2777a,anrerior and lateral vicrvs, x I ; l-nt) Liostrett sp., Rh,re ti,rn, MlÌ 27766, right r':rlvc, intern,rl nrould rnd l;1rex cast, x2; n-o) "Astdrte" cf. izcae Jrrworski, Rha- ctien, MLP 27767,lttcx crìst, dorsrrl rncl lef t r';rlvc vieu-s. x3; p'1 Saptocardia? sp., Rh.teti.rn. Ieft fr.rsnrent, MLP 27762,xl.5; q) ".rìr'c llì- netrigortia? ntulticost,tt"t (Kórner), Rlraetirrn, liìtc\ cirst of right velve fr;rgment, MLP 27765. 13; r-s) CasslareLla cf . peruana Kòrner, Rha- etirn,]\{LI)27766,intcrninrou|dso1trr'oleftr..r]ves,x3;t),Praechlant),sC|.N4lonie/1s r';rfve intcrn,rl ntould, MI-P 30825. r2l u) PuechLtnl's cl.taloniensis (Defrance), Hettiìnuiìn, ri{Ìht velve inrernal mould, MI-I'250lO-b, x2. All iigured specinrcns rrre houscd in thc Invertebrate Plheontologv Dep.rrrnrent collection, La Plata Niltur!ìl Sciences Museum (MLP).

The Triassic-Jurassic boundary Conclusions

In the Arroyo M:ilo sectiorl levels with Choristoceras The Arroyo Malo section is a new addition to the c[. marshì and Psiloceras cf. rectocostittutlt L'.re separared by a ferv marine sections depicting the Triassic-Jurassic bound- c. 80 m thick succession, n'here thus far no i'rnlmonites have ar,Y. been found. As in the Andean zonation (Hillebrandt 2OOO) The boundary is located in an apparent lithological- the Psiloceras rectocostatutn Zone overlies the Prinrocosrarum ly continuous succession, where no evident hiatus exists. and Tilnranni Zones irnd these two zones are equivalent to The record of ammonites appears to be rather poor, but the lou,ern.rost Jurassic, it is thus possible that n.rost of the the section is very well exposed and offers good prospects intervening unfossiliferous stratx nlxv belong in the Prinro- for findine more fossil material, especially within levels costatum and Tilmanni Zones as xttested bv loose fragr.nents thlt probably represent the lowermost Hettangian. Fur- of P cf. pressum, a species of the PrinrocostatLlur Zone. lf ther biostratigraphic studies are planned, both for macro this assumption is correct tl.re Tri:rssic-Jur,rssic boundary and n-ricrofauna. should be close to the levels where the Choristoceras speci- This research project, in prosress, also includes a mens were found. It is expected that nen, fieldwork could detailed sedin.rentolouical study by S. Lanés, which is near yield evidence in support of tl.ris assumption. compìetion. A stable isotope study is also planned. 74 A. C. Riccardi, S. E. Damborenea, M. O. Mancenido A M. P lglesia Llanos

Acknou,ledgemen ts. $fle thank the following colle,rgues who par- was financed ri,ith grants from the Consejo N;rcionll de Investigaciones ticipated in some of the field trips and collected p,rrt of the studied nr,r- Científicas v Técnicas (CONICET) :rnd the Agenci:r Nacional de Pro- terial: S.C. Ballent (L;r Plata Museum), H. Vizín, S. Lrnés, R. Scasso moción Científic;r y Tecnológica, Argentin;r. This is a contribution to (Buenos Aires University) and G. Gonzílez Bonorino (Salta Univer- IGCP Project 458 "Triassic-Jurassic boundarv events" sity). Ve are grateful to G. Albanesi and E. Carter for the processìng The authors rvould also like to thank the reviervers G. Bloos of samples in search of conodonts and radiolarìans respect;\'el!. rnd to and K. Page for their critical reviervs and suggestions that improved PN. Stipanicic for the identification of the plant fossils. This project the nranuscript.

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Appendix List of Late Triassic fossils from arroyo El Alumbre, Atuel river region, southern Mendoza.

Cephalopoda Nautiloidea indet. Choristoceras cÎ. marshi Hauer, 1865 (Fig. 3.a) (also figured in Riccardi and Iglesia Llanos 1999, p.298-299, frg.2a) Choristoceras sp. (Fig. 3.b) (also figured in Riccardi and Iglesia Llanos 1999, fig. 2b)

Bivalvia Cassianella cf . peruana Kórner 1937 (Fig. 3r-s) (also figured in Riccardi er al. 1997a., fig. 3.a) Oapiria? sp. (figured in Riccardi er aL.1997a, fig. 3.1) Praecbkmys cÎ. valoniensis (Defrance in Caumont, 1825) (Fig. 3t) Limidae gen. et sp. indet. Liostrea sp. (Fig. 3l-m) Minetrigonia? muhicostata (Kcirner, 1937) (Fig.3q) (also figured in Riccardi et a1.,7997a, fig. 3.5) Astarte cf. incae Jzworski 1922 (Fig. 3n-o) (also figured in Riccardi et al. 1997a, fig. 3.7a-b). Palaeocardita cf. peru,Liana Cox, 1949 (Fig. 3i-k) (also figured in Riccardi et a.l. 1997a, fig. 3.2a.-b) Tutcheria aÍÍ. densestriata (Kòrner, 1932) Septocardiaperuoiana? (Cox,1949) (alsofiguredinRiccardi eral. 1997a, fig.3.6) Septocardia? sp. (Fig. 3p)

Bivalvia gen. et sp. indet. 1 Bivalvia gen. et sp. indet. 2

Gastropoda cf. Guidonia ? sp. Omphaloptycba cf. jautorskii Haas, 1953 Gastropoda gen. et sp. indet.

Brachiopoda Zugmayerella? sp. cf. koerneri Sandy 1994 (Fig. 3e-g) (figured in Riccardi er al. 7997a., fig. 3.3a-c; Manceflido 2002, fig. 6.1b-d)

Foraminiferida (Ballent, 1994) H ap I opbragm o ides-like Nodosariids

Ostracoda (Ballent, 1994) Ogmoconcbella sp. Cytheraceans

Plants (Stipanicic in Riccardi et al. 1.997a) ?Zuberia cI. zuberi (Szajnocha, 1889) Frenguelli, 1943 (figured in Riccardi et al.1.997a, fig. 3.8) Clatbropteris sp.