CEFALOPODOS y BIVALVOS TITONIANOS y HAUTERIVIANOS DE LA FORMACION LAGO LA PLATA, CHUBUT

Abstract

Paleontological and stratigraphical features of the Lago La Plata Formation are described. Intercalated at various levels within this mainly volcanogenous Formation were found several marine fossiliferous beds. The recorded fossil assemblage comprises the following taxa (arranged in ascending stratigraphical order): a) Lucina sp., Anditrigonia sp. (Tithonian 1. s.); b) Trigonia aff. carinata Agassiz, Megatrigonia fontanaensis sp. nov., Anditrigonia aff. eximia (Philippi), Berriasella sp., Micracanthoceras ruedai sp. nov. (Upper Tithonian), and e) Paracrioceras sp., Belemnopsis sp. (Hauterivian). On the basis of detailed stratigraphical and lithological analysis carried out in several geological sections, the Lago La Plata Formation is considered to be made up mainly by pyroclastic rocks with minor volcanic and sedimentary intercalations which are recurrently disposed througout the unit. So it is concluded that these fossiliferous sedimentary intercalations, previously taken as independent stratigraphical units (e.g. Cotidiano and Tres Lagunas Formations), must be integrated to the Lago La Plata Formation. Moreover the fossil assemblage is indicative of an Upper Jurassic (Thithonian)-Lower Cretaceous (Hauterivian) age for this Formation instead of a Jurassic -pre Araucanian Movements it was previously considered.