Un paleosuelo mustersense con microgastrópodos terrestres (Charopidae) de la Formación Sarmiento, Eoceno de Patagonia central: significado paleoclimático

Abstract

The lower section of Puesto Almendra Member (Sarmiento Formation) at Gran Barranca (Chubut province, Argentina), is composed of tuffs, tuffaceous paleosols and fluvial sandstones and intraformational conglomerates of latest Middle Eocene age. In the lower pink paleosol (Rosado bed) terrestrial microgastropds, bee trace fossils and mammal remains referable to Mustersan SALMA have been recovered. This paleosol occurs within a pyroclastic loess-paleosol succession, that was originated as distal ash falls on loessic plains during a low humidity periodo Subsequently, the ash was calcretized through pedogenesis. The massive micritic fabric and frequent traces of Celliforma isp. indicate semiarid conditions. The snails are the second South American fossil record of Charopidae (Gyrocochlea? sp. d. S. mira bilis Hylton Scott, Stephadiscus sp. d. S. Iyratus (Couthouy in Gould), Zilchogyra sp. 1 and Z. sp. 2), living genera of wide geographic distribution in South America, from tropical to magellanic regions. In broad agreement with Paleogene global clima tic changes, a cooling and aridity trend in Patagonia through the late Eocene would ha ve likely occurred within a variable clima te scenario, perhaps involving high frequency fluctuations. The stratigraphic last occurrences of some groups of gastropods (i.e. Strophocheilus, Megalobulimus and Paleobu/imulus) may be related to these phenomena, whereas genera Gyrocochlea?, Stephadiscus and Zilchogyra may have survived in humid areas of southern Argentina and Chile.