Invertebrate Trace Fossils from Triassic Continental Successions of San Juan Province, Argentina

Asociación Paleontológica Argentina. Publicación Especial 7 ISSN 0328-347X VII International Symposium on Mesozoic Terrestrial Ecosystems: 127-131. Buenos Aires, 30-6-2001

Invertebrate trace fossils from Triassic continental successions of San Juan Province, Argentina

J 2 3 Ricardo N. MELCHOR , Jorge F. GENISE and Mariano VERDE

Abstract. This paper is the first report on invertebrate trace fossil assemblages from Marayes-El Carrizal and Ischigualasto-Villa Unión Basins (Triassic),San Juan Province, northwestern Argentina. The four an- alyzed lithostratigraphic units consist of three different fluvial systems (Carrizal, Ischigualasto, and Los Colorados Formations) and a lacustrine delta (Los Rastros Formation). The ichnofossil content of the Late Triassic Carrizal Formation (fluvial channels and shallow floodplain lakes) includes Taenidium barretti (Bradshaw), and Skolithos isp. The Los Rastros Formation (Middle Triassic) yielded a subaqueous ich- noassemblage that consist of Helminthopsis d. H. abeli Ksiazkiewicz, Skolithos d. S. verticalis (Hall), and Palaeophvcus isp. Floodplain facies of the Ischigualasto Formation (Late Triassic) contain Taenidium, Skolithos and laterally spreading root systems. Most trace fossils of the Los Colorados Formation (Late Triassic) come from sandy sheetflood facies -with two interbedded paleosols- and consist of Palaeophycus tubularis Hall, d. Skolithos isp. Taenidium barretti, and Cochlichnus isp, All alluvial successions contain Taenidium and Skolithos, and lack evidence for a fully terrestrial ichnofauna in spite of the presence of abundant paleosols.

Key words. Argentina. San Juan. Triassic. Continental. Invertebrate ichnofossils, Taenidium. Skolithos.

Introduction Ischigualasto, and Los Colorados Formations at Ischigualasto Provincial Park) of San Juan Province Invertebrate trace fossils from Triassic continental (figure 1). sequences of Argentina are poorly known. Vertebrate ichnofossils (mainly footprints) have received more Carrizal Formation (Late Triassic) attention since the first description of large quadrupedal footprints from the Los Rastros A low diversity ichnofossil assemblage was Formation earlier in the previous century. Many re- found in the middle section of the Carrizal cent studies mention the occurrence of "bioturba- Formation (Late Triassic), at Marayes river, near tion" and/or "root traces" in different basins but no Marayes town. Fine-grained carbonaceous sand- detailed account was given (see review by Melchor et stone, gravelly sandstone alternating with dark al., in press). One exception is a preliminary report mudstones and thin coal seams are the most com- (Melchor, 1998) on invertebrate and vertebrate ich- mon sedimentary facies of this formation (Bossi, nocoenoses from the Ischichuca Formation (Middle 1976). The local paleoenvironment is interpreted as to Late Triassic of Ischigualasto- Villa Unión Basin) at an intermediate to high sinuosity river system with its type locality (Quebrada Ischichuca Chica, La Rioja significant development of lacustrine and swamp fa- Province). cies. Recorded ichnofossils include Taenidium barretti This paper reports on the results of a detailed sur- (Bradshaw), Skolithos isp, and d. Palaeophycus isp., vey of invertebrate trace fossils at selected localities which occur at different stratigraphic horizons, usu- from Marayes-El Carrizal Basin (Carrizal Formation) ally associated with channel fill facies. Specimens of and Ischigualasto- Villa Unión Basin (Los Rastros, Taenidium barretti are unlined, straight to slightly curved, subhorizontal, sharp walled, 0.2-0.6 cm wide 'Consejo Nacional de Investigaciones Científicas y Tecnológicas, meniscate burrows preserved as endichnia and con- Universidad Nacional de la Pampa, Av. Uruguay 151, 6300 Santa Rosa, La Pampa, Argentina. vex epichnia (maximum length is 8 cm) (figure 2.A). 'Consejo Nacional de Investigaciones Científicas y Tecnológicas, Arcuate, well-defined menisci are composed of sedi- Departamento de Icnología. Museo Paleontológico Egidio ment similar to the host although successive packets Feruglio. Av. Fontana 1409100 Trelew, Chubut, Argentina. are slightly different in grainsize. Burrows often in- 'Departamento de Paleontología, Instituto de Geología y tersect; two specimens can be traced laterally into a Paleontología, Facultad de Ciencias, Iguá 4225, 11400 Montevideo, Uruguay. highly inclined burrow. Taenidium barretti appears ©Asociación Paleontológica Argentina 0328-347X/01$00.00+50 128 R.N. Melchor, J.F. Genise and M. Verde Ischigualasto Formation (Late Triassicl / '';' .. ·ISCHIGUALASTO \~ ~ - _'::::::::sediments punc- -<':-.., o tuated by numerous paleosols and channel sand- ~:::::-~¡.2 ":-:-:'.1 sandstones SAN JUAN \ I with interbedded paleosols, thick channel sandstone I 0.5 1 bodies, and thin (less than m thick) crevasse-splay I deposits (Martínez, 1994). Previous workers I \ , (Martínez, 1994; Milana and Alcober, 1994) recog- \ nized the presence of "root traces", burrows and/or , bioturbation at many horizons, as well as probable borings in fossil bone. Paleosols commonly exhibit vertic features (e.g. mukkara structure and deep homogeneous profiles) Figure 1. Location map showing Triassic basins of central north- and slender, shallow penetrating root traces (howev- west Argentina (shaded areas) and visited localities (asterisks). er, they are not abundant at any horizon). A drab-col- Modified frorn López Gamundí et al. (1994). ored massive paleosol with tabular root systems, frequently in fine-grained micaceous sandstone beds abundant rhynchosaur remains and coprolites (Y. with parallel or ripple cross-Iamination usually asso- Contreras, personal communication) were also recog- ciated with Skolithos isp. The material assigned to nized. Skolithos includes chiefly vertical, unlined, straight to A Taenidium-Skolithos association was distin- curved burrows with massive fill (coarser than host guished in crevasse-splay/massive floodplain de- rock) and elliptical to circular cross-section, Burrow posits. Thin «0.5 m thick) ripple cross-laminated, diameters are slightly larger than those of co-occur- brownish gray, fine-grained sandstone beds interca- ring Taenidium (0.3-1.4 cm), maximum recorded lated in massive siltstones and sandstones contain length 4 cm. Highly bioturbated beds were also subhorizontal to highly inclined meniscate burrows. recognized in the same section. They are 0.5-1 cm wide, up to 7 cm long, subhorizon- tal to inclined, unlined burrows, with arcuate menis- Los Rastros Formation (Middle Triassic) ci similar to host rock (marked by minor grainsize contrast), here assigned to Taenidium barretti. Scarce This is a lacustrine unit that records repeated subvertical Skolithos burrows with massive fill seems progradation of deltaic sandstone bodies into an to be restricted to interbedded green to red siltstones open lake (e.g., Milana and Alcober, 1994). Martínez and sandstones lacking pedogenic features (figures (1994) pointed out the presence of bioturbation in 2.B-C). However, a couple of featureless burrows beds of this formation at La Chilca river locality.The were recorded from a paleosol. few ichnofossils identified during this study occur both on the heterolithic interval and on top of wave rippled sandstone beds of intermediate thickness « 1 Los Colorados Formation (Late Triassic) m). Three morphologies were distinguished: (1) Helminthopsis d. H. abeli Ksiazkiewicz. Burrows This is a red bed sequence interpreted as the prod- about 0.5 cm in diameter, irregularly winding and uct of an ephemeral fluvial system. Sandy sheetflood meandering with massive fill, preserved as convex deposits with interbedded highly bioturbated het- epichnia on top of wave rippled siltstone and sand- erolithic intervals prevail in the lower part of the unit stone beds. (2) Skolithos d. S. verticalis (Hall). (Milana and Alcober, 1994). Our study was conduct- Subvertical to inclined, 0.4-0.6 cm wide, 3-4 cm long, ed at the base of the impressive cliffs called cylindrical burrows with massive sandstone fill in "Barrancas Coloradas" near Salado river, focusing on thin-bedded siltstone and mudstone beds showing a 30 m thick section located about 100 m above the parallel, wavy, and flaser bedding. (3) Palaeophycus transition with the underlying Ischigualasto isp. Subhorizontal, slightly curved, 1 cm in diameter, Formation. At the study area, red laminated silt- subcylindrical, thinly-lined burrows preserved as en- stones and mudstones interbedded with thin «0.10 dichnia with massive fill similar to the host sand- m) fine-grained sandstone beds and centimeter thick stone. Helminthopsis and subhorizontal burrows ap- gypsum laminae are capped by a -10 m thick multi- pear in mouth bar facies, and Skolithos is rare in he- storey (?) channel belt (-500 m wide) filled with ma- terolithic delta front facies. ssive and parallel bedded meter-scale fine-grained

A.P.A. Publicación Especial 7, 2001 Triassic invertebrate ichnology of San Juan, Argentina 129

D

Figure 2. Representative ichnotaxa from the analyzed triassic successions. A, Taenidillm barretti (Bradshaw) from the Carrizal Formation. B, Skolithos isp. in cross-laminated siltstone facies. C, Skolithos isp. from crevasse-splay sandstones. B and C are from the Ischigualasto Formation. D, Palaeophycus tubularis Hall and probable Skolithos isp. E, small Taenidium barretti (Bradshaw) in flood- plain sandstone facies. D and E from the Los Colorados Formation. Scale bar = 1 cm.

A.P.A. Publicación Especial 7, 2001 130 R.N.Melchor,J.F.Geniseand M.Verde sandstone bodíes. There are two paleosols (one of models for thís kind of environrnents (cf. Buatois and them with vertic features) in the mentioned fine het- Mángano, 1996).On the other hand, Skolithos is as- erolithic interval; which is interpreted as deposited in signed to a deeper water (as suggested by the sedi- a floodplain setting with temporary ponds. mentary facies), resident, and presumably suspen- Contreras and Braceo (1998) described poorly pre- sion feeder ichnofauna. Melchor (1998)described an served tetrapod footprints from one sandstone block ethologically comparable ichnofauna from similar of the channel facies at this locality. sedimentary facies of the same basin (though from Invertebrate ichnofossils are preserved in parallel the Ischichuca Formation at La Rioja Province), laminated, fine-grained, micaceous sandstone and whose main differences with this assemblage are siltstone beds of the overbank interval. The most higher ichnodiversity (seven recorded ichnogenera) common ichnotaxa are Palaeophycus tubularis Hall and absence of vertical dwelling burrows. and d. Skolithos isp., although Taenidium barretti and The ichnocoenosis from crevasse-splay deposits Cochlichnus isp. also occur. Specimens of Palaeophycus of the Ischigualasto Formation reflects the coloniza- tubularis are straight, smooth, thinly lined, horizontal tion of a wet/moist substrate by deposit-feeder, bur- burrows with massive fill (0.2cm-l.Ocm in diameter; rowing organisms that constructed meniscate back- up to 20 cm long) and elliptical cross section (figure filled structures and the (subsequent?) excavation of 2.D).Bifurcations are absent or rare, occasional cross- dominantly vertical domiciles in a slightly drier sub- cuttings can occur. Taenidium barretti consist of strate. The Los Colorados Formation contains a simi- straight to slightly curved, unwalled, 0.3-0.5cm in di- lar ichnoassemblage. Many examples of similar trace ameter, meniscate burrows. Menisci are tightly fossil assemblages have been reported in the litera- packed forming a non-compartmentalized backfill ture. In particular, Sarkar and Chaudhuri (1992)de- (figure 2.E). Tentatively, we assign vertical, unlined scribed a comparable association of the same age in burrows with structureless fill to Skolithos (some of India. them are aligned, see figure 2.0), which occur associ- All the alluvial successions analyzed in this study ated with Taenidium. A single sinusoidal trail pre- contain unlined, meniscate backfilled burrows served as concave epichnia (furrow) along with (Taenidium barretti) and dominantly vertical burrows Palaeophycus and Taenidium is identified as (Skolithos) excavated in soft, granular, not desiccated Cochlichnus isp. The degree of bioturbation could be substrates (this assertion is supported by smooth moderate although it is usually low. burrow surfaces, lacking any ornamentation). Conversely, there is almost no evidence of a fully ter- Concluding remarks restrial ichnofauna (except for the presence of few featureless Skolithos burrows in paleosols from the Invertebrate trace fossils are not abundant in the Ischigualasto Formation), although paleosols are analyzed Triassic successions, which include a lacus- abundant and generally well developed. trine delta and different types of fluvial systems. The When evaluating the reasons for the low preser- invertebrate ichnofauna in channel-fill facies of the vation of ichnofossils in the numerous stacked pale- Carrizal Formation comprises predominantly hori- osols from the Ischigualasto and Los Colorados zontal, feeding, backfilled burrows (Taenidium) asso- Formations at least two factors should be consid- ciated with vertical semi-permanent domiciles ered. (1) The most common producers of trace fos- (Skolithos). There is no indication of emergence in the sils in paleosols, such as ants, termites, dung-beetles analyzed interval although a subaqueous origin for and bees, whose constructed nests are the most these traces is unlikely. The presence of a low-diver- preservable ones (Genise and Bown, 1994) would sity association including Taenidium, Skolithos and have appeared later in the Mesozoic Era. Possible highly bioturbated beds has been recognized as char- soil inhabitants of the Triassic would have produced acteristic of fluvial channel facies (see review in excavated structures having low potential of preser- Buatois and Mángano, 1996).Taenidium and Skolithos vation, especially in the absence of diagenetic associated with other ichnotaxa were also recognized processes such as ferricretization and silcretization in other continental settings, including shallow-la- that would enhance their preservation. There are custrine wave-influenced deposits (e.g., Poiré and possible records of insect nests from the Triassic Larriestra, 1993). Chinle Formation (i.e., Hasiotis and Dubiel, 1995), The lacustrine delta assemblage of the Los Rastros although these findings require further documenta- Formation reflects a fully subaqueous, low diversity, tion. (2) Taphonomic overprint: trace fossils could mostly surface grazing ichnofauna attributed to de- result blurred or destroyed by intense pedogenic posit feeder, non-specialized organisms that exploit- homogenization in the soil profile; particularly in ed the substrate temporarily available in relatively vertisols, which are common in the studied succes- shallow water. These features agree with existing sions.

A.PA PublicaciónEspecial7,2001 Triassic invertebrate ichnology of San Juan, Argentina 131 Acknowledgments Melchor, RN., Genise, J.F. and Poiré, D.G. in press. Icnología de los depósitos continentales triásicos. In: A.E. Artabe, E.M. This research was founded by a grant of Fundación Antorchas Morel and A.B. Zamuner (eds.), E/ sistema Triásico en to J.F. Genise and S. Peralta. V. Contreras, M. González, and S. Argentina, Fundación Museo de La Plata "Francisco Pascasio Peralta helped with various aspects during fieldwork. Moreno", La Plata, pp. 101-135. Milana, J.P. and Alcober, O. 1994. Modelo tectosedimentario de la cuenca triásica de Ischigualasto (San Juan, Argentina). Revista References de /a Asociación Geo/ógica Argentina 49: 217-235. Poiré, D.G. and Larriestra, C.N. 1993. Trazas fósiles del Cretácico Bossi, G. 1976. Geología de la Cuenca de Marayes-EI Carrizal continental de la sierra de Los Cóndores, Embalse Río Tercero, (provincia de San Juan, República Argentina). 6° Congreso provincia de Córdoba, Argentina. 1o Reunión Argentina de Geológico Argentino (Bahía Blanca 1975), Actas 1: 23-38. lcnologia (Santa Rosa), Resúmenes: 19. Buatois, L. and Mángano, M. 1996. Icnología de ambientes conti- Sarkar, S. and Chaudhuri, A. 1992. Trace fossils in Middle to Late nentales: problemas y perspectivas. Asociación Paleontológica Triassic redbeds, Pranhita-Godavari Valley, South India. Ichnos Argentina, Publicación Especial 4: 5-30. 2: 7-19. Contreras, V. and Braceo, A. 1998. Aportes a la icnología de la cuenca triásica de Ischígualasto- Villa Unión, oeste de Argentina. 3° Reunión Argentina lcnologia y P Reunión de lcnologiu Mercosur (Mar del Plata), Resúmenes: 10. Genise, J.F. and Bown, T. 1994. New Miocene scarabeid and hy- menopterous nests and Early Miocene (Santacrucian) pale- oenvironments, Patagonian Argentina. Ichnos 3: 107-117. Hasiotis, S.T. and Dubiel, RF. 1995. Termite (Insecta: Isoptera) nest ichnofossils from the Upper Triassic Chinle Formation, Petrified Forest National Park, Arizona. Ichnos 4: 119-130. López Gamundí, O.R, Espejo, 1.S., Conaghan, P. J. and Powell, C.McA. 1994. Southern South America. In: J.J. Veevers and C.McA. Powell (eds.), Permian-Triassic Pangean basins and foldbelts along the Panthalassan margin of Gondwanaland, Ceologicat Society of America, Memoir 184: 281-329. Martínez, R 1994. [Estratigrafía de/ sector Agua de /a Peña-Río de /a Chi/ca, Formación Ischigua/asto (Triásico Superior), Cuenca de Ischigualasto, San Juan. Licenciate thesis. Universidad Nacional de San Juan, San Juan, 128 p. Unpublished]. Melchor, RN. 1998. Icnología de los depósitos fluviales y lacustres de la Fm. Ischichuca (Triásico), Cuenca Ischigualasto Villa Unión, Quebrada Ischichuca Chica, La Rioja, Argentina. 3° Reunión Argentina lcnologia y 1o Reunión de icnoiogta Mercosur (Mar del Plata), Resúmenes: 19-20. Accepted: November Ié'", 2000.

A.P.A. Publicación Especial 7, 2001