AMEGHINIANA (Rev. Asoc. Paleontol. Argent.) - 42 (2): 347-362. Buenos Aires, 30-06-2005 ISSN 0002-7014 Miospores and chlorococcalean algae from the Los Rastros Formation, Middle to Upper Triassic of central-western Argentina Eduardo G. OTTONE, Adriana C. MANCUSO and Magdalena RESANO Abstract. Lacustrine strata of the Los Rastros Formation (Middle to Upper Triassic) at Río Gualo section (La Rioja province), yield a distinctive palynological assemblage of miospores and chlorococcalean algae. The miospore association is characterized by a relative abundance of corystosperm pollen grains with sub- ordinate inaperturates, diploxylonoid disaccates, spores, monocolpates, monosaccates and striate pollen grains. The phytoplankton are mostly represented by Botryococcus but also by Plaesiodictyon, a form prob- ably related to the Hydrodictyaceae. Geological data and variations in phytoplankton content indicate that the lacustrine system probably evolved from a stretcht of freshwater with eutrophic conditions, into a body with oligotrophic conditions through the middle and upper part of the Río Gualo section. The genus Variapollenites is emended in order to amplify its original diagnosis. Resumen. MIOSPORAS Y ALGAS CHLOROCOCCALES DE LA FORMACIÓN LOS RASTROS, TRIÁSICO MEDIO A SUPERIOR DEL CENTRO-OESTE DE ARGENTINA. El estudio de los niveles lacustres de la Formación Los Rastros (Triásico Medio a Superior) en la sección de Río Gualo (provincia de La Rioja), incluye una interesante palinoflora compuesta por miosporas y algas Chlorococcales. Entre las miosporas abundan los granos de polen de Corystospermales, con presencia subordinada de inaperturados, disacados diploxilonoides, esporas, monocolpados, monosacados y polen estriado. En el fitoplancton se destaca Botryococcus, pero también se observa Plaesiodictyon, que es una forma probablemente relacionada con las Hydrodictyaceae. Datos ge- ológicos y variaciones en el contenido fitoplanctológico señalan que el sistema lacustre probablemente evolucionó desde un cuerpo de agua dulce y condiciones eutróficas, a un ambiente de carácter oligotrófi- co hacia la parte media y superior de la sección de Río Gualo. Así mismo, se enmienda el género Variapollenites a fin de ampliar su diagnosis original. Key words. Argentina. Triassic. Miospores and chlorococcalean algae. Systematics. Lacustrine environment. Palabras clave. Argentina. Triásico. Miosporas y algas Chlorococcales. Sistemática. Ambiente lacustre. Introduction The fossil record of the Los Rastros Formation is mainly represented by floral remains. An abundant In the Triassic of southern South America, one of macrofloral assemblage, belonging to the “Dicroi- the best represented lacustrine successions is that of dium- flora”, was collected at different levels and lo- the Los Rastros Formation, which is part of the con- calities in the Los Rastros sequence although, the tinental infilling of the Ischigulasto-Villa Unión most important records are known from the thick la- Basin in central-westen Argentina (Stipanicic and custrine black shales, close to the base of the unit Bonaparte, 1979; Stipanicic, 1983; López Gamundí et (Frenguelli, 1948; Kokogian et al., 1999; Spalletti et al., al., 1989). The unit is characterized by thick deposits 1999; Stipanicic and Bonetti, 1969; Stipanicic and of black shales and fine grained sandstones that rep- Bossi, 2002b; Zamuner et al., 2001). Palynological resent a cyclic lacustrine-deltaic environment. It analysis are known only from selected areas in the crops out at different localities in La Rioja and San basin (Herbst, 1965, 1970, 1972; Yrigoyen and Stover, Juan provinces (figure 1) with slight lateral facies 1970). variation (de la Mota, 1946; Bossi, 1971; Milana and Invertebrate remains exhumed from Los Rastros Alcober, 1994; Milana, 1998; Rogers et al., 2001). levels include different groups of conchostracans (clamp-shrimps), insects and bivalves, and they oc- curr throughout the sequence (Gallego, 1997, 1999). Departamento de Ciencias Geológicas, Facultad de Ciencias Also, a quite rich invertebrate ichnological data has Exactas y Naturales, Universidad de Buenos Aires, Pabellón n° 2, been mentioned from the unit (Melchor et al., 2001; Ciudad Universitaria, C1428EHA Buenos Aires, Argentina. [email protected], Mancuso, 2003). The vertebrate fauna mainly com- [email protected], [email protected] prises actinopterigian fishes (Cabrera, 1944; Forster ©Asociación Paleontológica Argentina AMGHB2-0002-7014/05$00.00+.50 348 E.G. Ottone, A.C. Mancuso and M. Resano Figure 1. Location map of the Río Gualo section, Ischigualasto-Villa Unión Basin, La Rioja province, Argentina / mapa de ubicación de la sección de Río Gualo, Cuenca de Ischigualasto- Villa Unión, provincia de La Rioja, Argentina. Modified from Caselli (1998)/ modificado de Caselli (1998). et al., 1995, Mancuso, 2000, 2003), whereas tetrapods several tetrapod footprints related to different ar- are represented only by a temnospondyl amphibian chosaur groups (von Huene, 1931; Arcucci et al., specimen (Contreras et al., 1997; Mancuso, 2002) and 1995; Marsicano et al., 2004). AMEGHINIANA 42 (2), 2005 Triassic miospores and chlorococcalean algae 349 In the present contribution an interesting associa- (Romer and Jensen, 1966), and the Agua de la Peña tion of miospores and Chlorococcalean algae reco- Group, which comprises five units (de la Mota, 1946; vered from the Los Rastros sequence outcropping at Bossi, 1971; Stipanicic, 1983). The lower Chañares the Río Gualo area, about 10 km to the southeast of Formation (Bossi and Stipanicic, 2002a) is dominated the steep-walled, picturesque canyons of the Puerta by tuffaceous sandstones and siltstones deposited in de Talampaya, La Rioja province (figures 1, 2), are a fluvial-lacustrine environment and grades upward described. The Río Gualo section is near the western into the lacustrine black shales and deltaic sand- margin of the Ischigulasto-Villa Unión Basin stones of the Ischichuca and Los Rastros formations (Kokogian et al., 2001). The palynoflora, together (Bossi et al., 2002; Stipanicic and Bossi, 2002a). This with sedimentological data, provide new interpreta- sequence passes upward into the sandstones, mud- tive insights into the basin evolution. stones and tuffs of the Ischigualasto Formation (Stipanicic and Bossi, 2002b), deposited by a moder- ate- to high-sinuosity river system. The succession Material and methods culminates in the red beds of the Los Colorados Formation (Bossi and Stipanicic, 2002b). A regional A stratigraphic section of the Los Rastos For- unconformity separates the Triassic succession from mation have been recognized at Rio Gualo. A de- the overlying ?Cretaceous Cerro Rajado Formation tailed description of the sedimentology of the section (Caselli et al., 2001). was used to interpret the facies assemblage and to re- The lacustrine-deltaic Los Rastros Formation is construct the depositional environment. Systematic unconformably overlain by the fluvial Ischigualasto palynological sampling was conducted; and this pa- Formation and conformably succeeds the fluvial-la- per is based on the study of 15 samples. Laboratory custrine deposits of the Chañares and Ischichuca for- procedures followed conventional practices. Carbo- mations. The Los Rastros Formation deposits are nates and silicates were removed by treatment with generally interpreted as lacustrine-deltaic cycles with hydrochloric and hydrofluoric (70%) acids. Residues a predominance of lacustrine sedimentation (Stipa- were sieved on a 25 µm mesh and mounted in un- nicic and Bonaparte, 1979; López Gamundí et stained glycerine jelly as well as in Cellozise on glass al.,1989; Milana and Alcober, 1994; Milana, 1998; slides. Specimens were examined under a Leitz Rogers et al., 2001). Orthoplan binocular microscope. Photomicrographs The age of the Los Rastros Formation has been re- were taken with a Leitz Orthomat camera using AG- ferred to the early Late Triassic by some authors, FAPAN APX25 and ILFORDPAN NF50 films. based on its palynological content (Zavattieri and Palynological slides are prefixed BAFC-Pl. The illus- Batten, 1996; Morel et al., 2001). Nevertheless, an old- trated specimens are identified with a slide number er age (Middle Triassic) has been proposed by other and England Finder coordinates. The slides are de- authors due to its stratigraphic relationships (Koko- posited at the Geological Sciences Department, gian et al., 2001). Thus, a radioisotopid age data from Buenos Aires University. E.G. Ottone prepared the the base of the overlying Ischigualasto Formation palynological section of the paper. A.C. Mancuso col- (227.8 ± 0.3 Ma) suggests a minimun late Middle lected the samples and provided geological informa- Triassic age for the Los Rastros Formation (Rogers et tion. M. Resano collaborated in the study of the al., 1993, 2001). Chlorococcales. Sedimentology Geological setting and age Fieldwork at the Río Gualo section indicated five During the pre-breakup stage of Pangea, during lacustrine-deltaic cycles each with two facies assem- the Late Permian and Early Triassic, several exten- blages. sional basins were formed along the western margin of Gondwana (Uliana and Biddle, 1988). The Ischigualasto-Villa Unión Basin is situated in central- Lake facies assemblage western Argentina, with a NW-SE orientation, be- tween San Juan and La Rioja provinces (figure 1).The This assemblage is dominated by dark gray to sedimentary infilling of the basin consists of up to black carbonaceous claystones. Iron mudstones