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 massi- ve 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 con- tains microfauna (foraminifers and osrracods), and in or- der of decreasing importance bivalves, eastropods, bra- chiopods, ammonoids, cnidarians and nautiloids (Fig. 3; complete list in Appendix). Samples studied for cono- donts (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 lo- cation 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 I 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.