Paleoenvironments and Taphonomy of a Triassic Lacustrine System (Los Rastros Formation, Central-Western Argentina)

MANCUSO, ADRIANA CECILIA; MARSICANO, CLAUDIA

PALAIOS

SEPM

Año: 2008 vol. 23 p. 235 - 547

0883-1351

The Triassic Ischigualasto-Villa Unión Basin is an extensional basin located in central-western Argentina. It includes the Los Rastros Formation, a lacustrine-deltaic sequence comprising several coarsening-upward cycles of black shale, siltstone, and sandstone. We performed a taphonomic analysis of the floral and faunal fossils of the Los Rastros succession and have defined five plant taphofacies, four invertebrate taphofacies, and four vertebrate taphofacies. Our taphonomic model characterizes four sub-environments within the lacustrine-deltaic environment of Los Rastros Formation. These include (1) offshore lacustrine (2) prodelta (3) deltaic mouth-bar (4) deltaic plain sub-environment. Our analysis of fossil assemblages allows us to reconstruct the structure of the original ecosystem. The lake margins were vegetated with small ginkgoales, corystospermales, and sphenophyta. River margins were characterized by riparian forests of sphenophyta; the proximal floodplains supported closed woodlands of corystospermales, cycadales, and pteridophyta; whereas the more distal floodplains were covered with open conifer forests. The invertebrate fauna included insects (Blattoptera, Hemiptera, Coleoptera) associated with lakeshore vegetation and conchostracans that inhabited both the lake shoreline and smaller ponds in the floodplains. Fish and temnospondyl amphibians probably inhabited delta plain and incoming fluvial systems. The activity of non-mammalian therapsids, crurotarsal archosaurs, and putative dinosaurs is recorded by trackway surfaces in the lake shoreline sub-environment.