Paleolimnology reconstruction of the Middle Triassic Los Rastros succession

MANCUSO, ADRIANA; MARSICANO, CLAUDIA

Mendoza

Congreso; Gondwana Conference; 2005

The Triassic Ischigualasto-Villa Unión Basin is an extensional basin located in the western margin of Gondwana; its origin is associated with the pre-breakup of Pangea during the earliest Triassic (Uliana and Biddle, 1988). Up to 4000 m of alluvial, fluvial and lacustrine deposits of Middle to Late Triassic age are the sedimentary infilling of the basin. The lacustrine-deltaic sequence is included in the Los Rastros Formation and consists of several coarseningupward cycles of black shales, siltstones and sandstones. The paleontological content of the succession includes abundant floral remains (related to the “Dicroidium-type Flora”) (e.g. Frenguelli, 1948; Stipanicic and Bonaparte, 1979; Ottone et al., 2005), invertebrates (clamp-shrimps, insects) (e.g. Gallego 1992, 1996; Martins-Neto et al. in press) and vertebrates (fishes, a temnospondyl amphibian) (e.g. Cabrera, 1944; Contreras et al., 1997; Mancuso, 2003). First integral taphonomic analysis that includes floral and faunal records of the Los Rastros levels has recently been performed (Mancuso, 2005). Thus, twelve plant taphofacies, four invertebrate taphofacies and four vertebrate taphofacies have been recognized for the whole succession. On this base, a taphonomic model that characterizes the lacustrine-deltaic environment of Los Rastros has been developed. The distal lake facies present mainly well preserved and abundant autochthonous elements, as conchostracans, in comparison with the allochthonous elements to water body, as plants, insects, and also conchostracans. The deltaic facies only content allochthonous elements to the lake body, as plant remains, conchostracans, fishes, and an amphibian, with different grades of preservation accordingly to the hydrological features of the material itself. The analyzed fossil assemblages allowed to reconstruct an original community from Gondwana during the Middle Triassic (Mancuso, 2005). Thus, the lake margins were characterized by shrubs and small trees of ginkgoales and corystospermales and reed of sphenophyta. The sphenophyta are also the dominant floral component in the river margins meanwhile the corystospermales, cycadales, pteridophyta and conifers formed the woodland upward stream probably related to the floodplains of affluent fluvial system. The invertebrate fauna includes both organisms of terrestrial habit, such as the insects (blattoptera, homoptera, coleoptera) which lived near to lake related to the shore-vegetation, and exclusively aquatic ones. The latter includes the conchostracans represented by taxa associated with shoreline of the lake and other taxa related with floodplains of the fluvial system. The fishes recorded were allochthonous to the lacustrine environment, and they, in contrast, inhabited the affluent fluvial system as the temnospondyls which probably preyed on them (Mancuso, 2003; 2005). Dicroidium-type Flora”) (e.g. Frenguelli, 1948; Stipanicic and Bonaparte, 1979; Ottone et al., 2005), invertebrates (clamp-shrimps, insects) (e.g. Gallego 1992, 1996; Martins-Neto et al. in press) and vertebrates (fishes, a temnospondyl amphibian) (e.g. Cabrera, 1944; Contreras et al., 1997; Mancuso, 2003). First integral taphonomic analysis that includes floral and faunal records of the Los Rastros levels has recently been performed (Mancuso, 2005). Thus, twelve plant taphofacies, four invertebrate taphofacies and four vertebrate taphofacies have been recognized for the whole succession. On this base, a taphonomic model that characterizes the lacustrine-deltaic environment of Los Rastros has been developed. The distal lake facies present mainly well preserved and abundant autochthonous elements, as conchostracans, in comparison with the allochthonous elements to water body, as plants, insects, and also conchostracans. The deltaic facies only content allochthonous elements to the lake body, as plant remains, conchostracans, fishes, and an amphibian, with different grades of preservation accordingly to the hydrological features of the material itself. The analyzed fossil assemblages allowed to reconstruct an original community from Gondwana during the Middle Triassic (Mancuso, 2005). Thus, the lake margins were characterized by shrubs and small trees of ginkgoales and corystospermales and reed of sphenophyta. The sphenophyta are also the dominant floral component in the river margins meanwhile the corystospermales, cycadales, pteridophyta and conifers formed the woodland upward stream probably related to the floodplains of affluent fluvial system. The invertebrate fauna includes both organisms of terrestrial habit, such as the insects (blattoptera, homoptera, coleoptera) which lived near to lake related to the shore-vegetation, and exclusively aquatic ones. The latter includes the conchostracans represented by taxa associated with shoreline of the lake and other taxa related with floodplains of the fluvial system. The fishes recorded were allochthonous to the lacustrine environment, and they, in contrast, inhabited the affluent fluvial system as the temnospondyls which probably preyed on them (Mancuso, 2003; 2005).