Variabilidad textural, mineralógica y geoquímica de la bentonita de Yuncos (Cuenca de Madrid)

Abstract

In this paper the compositional (mineralogy and geochemistry) and textural variability of the main bentonite bed (saponite) from Yuncos deposit, southwestern Madrid Basin, have been studied. Two stages from a mineralogical point of view have been differenciated: Lower stage (1), characterized by massive bentonites which laterally show microfacies from brecciated- bioturbated (section A) to intraclastic with glebules and organic matter (sections B and C), and the upper stage (11) constituted by fissile-Iaminated bentonite bearing calcite ooids, it shows a poor development in the sections studied. The interpretation is of palustrine stagnant water deposits on lacustrine mud-flat facies for the lower stage and as small ponds deposits on the lower bentonite -which is partially reworked- for the upper stage. Section A corresponds to the shallower zone whilst section e shows features of a longer stagnation period, section B being transitional between them. In the mineralogy a progressive decrease in inherited minerals content (quartz, feldspar, illite-mica) fram A to e together with a clay fraction increase, has been observed. The clay mineralogy is made up mainly of saponite with illite (higher content in section A) and sepiolite, the latter specially abundant in section B. Geochemically a similar behaviour (positive correlation) for A1₂O₃, K₂O, Fe₂O₃, Na₂O and TiO₂ decreasing from A to e has been observed. MgO is their antagonist (negative correlation) along the same direction. The trace elements specially Ba, Nb, Pb, V and Zn follow also, as major elements, an evolution related with the detrital contento The compositional and textural differences observed in the bentonite are related to: a) Location respecting detrital input. b) Degree of vertic paleoedaphic processes participation during shallowing-exposure stages. SEM-EDAX studies evidence textural and compositional differences in saponite. This suggests two genetic smectites, one related to transformation processes (Al/Mg > 0.3) and the other to neoformation (Al/Mg < 0.3).