Cenozoic calcretes from the Teruel Graben, Spain: microstructure, stable isotope geochemistry and environmental significance

Abstract

In the Teruel Graben (northeast Spain), laminar and nodular calcretes formed in a variety of Cenozoic deposits, ranging in age from Palaeogene to Pleistocene. These calcretes developed in a relatively small area under the influence of the same highlands. Consequently, differences in their microstructure and isotopic composition (C and O) must be related to differences in host rock, climate, vegetation and duration of development. Nine profiles developed in different sedimentary settings, from proximal areas to lacustrine environments, were studied to determine whether the microstructure and stable isotope composition changed during the Cenozoic. These characteristics might be used as indicators of the prevailing climate and vegetation, and reveal any changes that occurred during this time. Most of the laminar calcretes studied are compound profiles that formed on hard substrates (e.g., Jurassic limestones) or coarse detrital deposits (e.g., Palaeogene, Pliocene and Plio-Pleistocene sandstones and conglomerates). These profiles formed in proximal areas of the basin and also in fluvial terraces. Microstructures include biogenic features such as alveolar septal structures, root tubes, calcified root cells and calcified organic filaments. Underside coatings of micrite and fibrous vadose cements are common around gravel clasts. On the other side, spherulites are only preserved at the very top of the youngest calcretes. Nodular calcretes developed on fine detrital substrates (e.g., on Palaeogene, Miocene and Pliocene red mudstones and sandy mudstones of distal alluvial and floodplain environments), and in some places grade vertically into palustrine limestones. The nodules consist of micrite and were elongated vertically, show desiccation cracks and mottling, micritic coatings, some alveolar septal structures, and root traces. Microcodium of type 1 occurs in both laminar and nodular calcretes, but only in those of Palaeogene age, while calcified root cells (Microcodium of type 3) are seen in the Pliocene calcretes. The stable isotope composition covers a wide range: −8.23<δ18O<−4.29‰ PDB and −7.29<δ13C<−3.42‰ PDB. Nodular, laminar and palustrine facies occupy overlapped isotopic fields, displaying an overall negative covariant trend. Location within the basin, prevailing climate and density of vegetation are responsible for differences in isotope values. The Palaeogene samples show a wide range of values and are difficult to separate from the others; however, there is as significant change to lighter carbon values from the Miocene to the Pliocene profiles. This change may be related to the cooler climatic conditions that prevailed during the Pliocene and is reverse to the expected shift to heavier δ13C values, which is related to the increase of C4 vegetation, that occurred in many areas of the world during the Miocene or to the transit to the Pliocene. The formation of these calcretes seems to have occurred in semi-arid conditions with prevailing C3 vegetation.