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

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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 \~ ~ - _'::::::::<VILLA UNiÓN This formation is of fluvial origin with thick suc- ::•••::::.•..::::::~BASI N <C ..., cessions of floodplain fine-grained sediments punc- -<':-.., o tuated by numerous paleosols and channel sand- ~:::::-~¡.2 ":-:-:'.1 <C stones. At the surveyed localities ("Cancha de "'<..•.... ..J Bochas" and "VallePintado") there is a dominance of " tuffaceous siltstones and fine-grained 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.
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