Sedimentación lacustre neógena en el sector oriental de la cuenca de Almazán

Resumen

The Neogene lacustrine sedimentation in the eastern part of the Almazán basin occupies an elongated area next to the southern border of the basin (Castilian Branch, Iberian Ranges). The lacustrine system is encircled and nourished by distributary petromictic fluvial systems which were more than fifty kilometres long and by came from northern and eastern borders (Cameros massif, Aragonian Branch, Iberian Ranges), and by carbonate alluvial fan sourced in the nearby southern border, This location indicates an asymmetrical disposition of the lacustrine environments in the basin that was generated by the active rise of the eastern border and, because of this reason, they were installed next to the less active south border. The lacustrine system is represented by shallow and well mixed carbonated lakes, rich in gastropods and with ostracods and charophytes. The terrigenous supply to the lakes was minor. Two main types of lacustrine sedimentation are showed in the section studied. The first one, mainly developed in the lower part of the section, is characterised by the presence cycles of dark fossiliferous soft limestones from 0.2 to 1.5 m thich alternating with dark fossiliferous marls/marly limestone in the same thickness range. The correspond to the alternation of open calcareous mineralized lakes and lakes with eutrophic trend. The second type of lacustrine sedimentation consists of shallowing carbonate sequences from 0.4 to 1.3 m thick, which are adjusted to a palustrine model, although the degree of exposure and pedogenesis is commonly scare and restricted to the top of the sequences.