Significado paleoambiental de las calcretas terciarias de la Fosa de Teruel. Microestructura e isótopos estables.

Abstract

Laminar and nodular calcretes in the Teruel Graben are from Palaeogene to Pleistocene in age. These calcretes developed under the influence of the same source rocks and groundwaters, so differences in their microstructure and isotope composition must be related to differences in host rock, climate, vegetation and time of development. The laminar calcretes formed on hard substrates or on coarse detrital deposits. Alveolar septal structures, root tubes, calcified organic filaments and root cells are common. Underside coatings and fibrous vadose cements occur around gravel clasts. Spherulites formed in association with bacterial mats in ponded waters that dried out, and are preserved only at the top of the youngest calcretes. Nodular calcretes developed on fine detrital substrates. The nodules consist of micrite and show desiccation cracks and mottling, micritic coatings, alveolar septal structures and root traces. Microcodium occurs only in calcretes of Palaeogene age. The stable isotope composition covers a wide range: -8.23 <δ180<-4.29 ‰ PDB and -7.29<δ13C<-3.42 ‰ PDB. These calcretes formed under semi-arid conditions with prevailing C3 vegetation. No clear shift in δ13C values, which might indicate an increase of C4 vegetation, was found during the Miocene or the transit to the Pliocene.