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 A1 2 O 3 , K 2 O, Fe 2 O 3 , Na 2 O and TiO 2 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).
Highlights
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
The trace elements specially Ba, Nb, Pb, V and Zn follow as major elements, an evolution related with the detrital contento
Summary
El muestreo de material bentonítico (capa inferior) se ha realizado en una cantera de dimensiones reducidas, sobre un frente de unos 25 m de longitud, localizada en las proximidades del arroyo del Carrizo, a unos 3 km al SW de la población toledana de Yuncos (Hoja 604, Villaluenga, serie L) (figs. la y b). En este frente de cantera se han recogido ocho muestras de bentonita y una de carbonato, distribuidas en tres puntos de la capa bentonítica, correlacionables lateralmente Lutitas marrones masivas a brechoides, con tintes rojizos locales de aspecto limoso que parecen rellenar superficies o grietas de desecación, observándose además evidencias de bioturbación y «slickensides» (A.l). A techo pasa transicionalmente a lutitas laminadas-Iajeadas que parecen ocupar depresiones, mostrando frecuentes inclusiones blancas milimétricas de carbonato esferulítico (A.2). Masivas glomerulares con frecuentes «slickensides» (B.l), que lateralmente pasan a materiales más céreos y compactos con tonalidad grisácea (B.3). Destaca la existencia de nódulos decimétricos de carbonato blanco, masivo y compacto, que se incluyen en la lutita, estando frecuentemente afectados por «slickensides». De forma sintética es posible establecer, por lo tanto, tres litofacies representativas de los puntos muestreados: lutitas masivas brechoides, lutitas laminadas-Iajeadas y carbonatos
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