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Lacustrine strata of the Los Rastros Formation (Middle to Upper Triassic) at …


Biology Articles » Paleobiology » Micropaleobiology » Miospores and chlorococcalean algae from the Los Rastros Formation, Middle to Upper Triassic of central-western Argentina » Introduction

Introduction
- Miospores and chlorococcalean algae from the Los Rastros Formation, Middle to Upper Triassic of central-western Argentina

In the Triassic of southern South America, one of the best represented lacustrine successions is that of the Los Rastros Formation, which is part of the continental infilling of the Ischigulasto-Villa Unión Basin in central-westen Argentina (Stipanicic and Bonaparte, 1979; Stipanicic, 1983; López Gamundí et al ., 1989). The unit is characterized by thick deposits of black shales and fine grained sandstones that represent a cyclic lacustrine-deltaic environment. It crops out at different localities in La Rioja and San Juan provinces (figure 1) with slight lateral facies variation (de la Mota, 1946; Bossi, 1971; Milana and Alcober, 1994; Milana, 1998; Rogers et al ., 2001).

The fossil record of the Los Rastros Formation is mainly represented by floral remains. An abundant macrofloral assemblage, belonging to the ? Dicroidium - flora?, was collected at different levels and localities in the Los Rastros sequence although, the most important records are known from the thick lacustrine black shales, close to the base of the unit (Frenguelli, 1948; Kokogian et al ., 1999; Spalletti et al ., 1999; Stipanicic and Bonetti, 1969; Stipanicic and Bossi, 2002b; Zamuner et al ., 2001). Palynological analysis are known only from selected areas in the basin (Herbst, 1965, 1970, 1972; Yrigoyen and Stover, 1970).
Invertebrate remains exhumed from Los Rastros levels include different groups of conchostracans (clamp-shrimps), insects and bivalves, and they occurr throughout the sequence (Gallego, 1997, 1999). Also, a quite rich invertebrate ichnological data has been mentioned from the unit (Melchor et al ., 2001; Mancuso, 2003). The vertebrate fauna mainly comprises actinopterigian fishes (Cabrera, 1944; Forster et al ., 1995, Mancuso, 2000, 2003), whereas tetrapods are represented only by a temnospondyl amphibian specimen (Contreras et al ., 1997; Mancuso, 2002) and several tetrapod footprints related to different archosaur groups (von Huene, 1931; Arcucci et al ., 1995; Marsicano et al ., 2004).
In the present contribution an interesting association of miospores and Chlorococcalean algae recovered from the Los Rastros sequence outcropping at the Río Gualo area, about 10 km to the southeast of the steep-walled, picturesque canyons of the Puerta de Talampaya, La Rioja province (figures 1, 2), are described. The Río Gualo section is near the western margin of the Ischigulasto-Villa Unión Basin (Kokogian et al ., 2001). The palynoflora, together with sedimentological data, provide new interpretative insights into the basin evolution.


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