Taphonomic analysis in lacustrine environment: two very different Triassic lake paleoflora contexts from Western Gondwana

MANCUSO, ADRIANA CECILIA

Barcelona, España

Congreso; 4º International Limnogeology Congress; 2007

During the earliest Triassic associated with the pre-breakup of Pangea several extensional basins were developed in the western margin of Gondwana. They were filled by exclusive non-marine sediments including alluvial, fluvial and lacustrine deposits of Triassic age. In Ischigualasto-Villa Unión Basin, the lacustrine-deltaic succession is included in the Los Rastros Formation and consists of several coarsening-upward cycles of black shales, siltstones and sandstones. The paleontological content of the succession includes abundant floral remains (related to the “Dicroidium-type Flora”), invertebrates (clamp-shrimps, insects) and vertebrates (fishes, a temnospondyl amphibian, ichnites). In Cuyana Basin at Cerro Puntudo depocenter, the lacustrine succession forms the upper section of the Cerro Puntudo Formation and consists of alternation of limestones, stromatolitic limestones, mudstones, sandstones and tuff. The paleontological content includes scarce floral remains and rhizoliths, invertebrate represented exclusively by traces (associated to ichnofacies of Skolithos and Scoyenia) and vertebrates represented by a fragment of pelvic girdle of basal arcosaurs. The taphonomic analysis performed in these Triassic lacustrine successions allows recognising two different taphonomic histories for plant remains. On one hand, the Los Rastros lake preserved both autochthonous and allochthonous elements. The distal lake is dominated by autochthonous well-preserved elements and allochthonous debris material and woods, which formed time-average accumulations. The delta is characterized by allochthonous elements with varied preservation conditions, usually showing evidence of mechanic degradation and short time accumulations. On the other hand, the Cerro Puntudo lake preserved only autochthonous elements including rhizoliths and folia material, forming autochthonous and parautochthonous accumulations. The Los Rastros lake allowed preservation of autochthonous and allochthonous material by means of its anoxic conditions in the distal lake and the high sediment rate in the delta. In contrast, the Cerro Puntudo lake allowed the preservation of only autochthonous material in spite of oxidant conditions. This condition was produced by the tuff material which enhanced the preservation. The analyzed fossil assemblages allowed reconstructing the original communities from this section of Gondwana during the Triassic. Thus, the Los Rastros lake margins were characterized by shrubs and small trees of ginkgoales and corystospermales and reed of sphenophyta. The sphenophyta is 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 Cerro Puntudo lake submerge margins were dominated by mainly reed of sphenophyta and herbaceous lycophytas.