Los minerales de la arcilla en los ambientes deposicionales de la fm Escucha (Albiense medio, Sistema Ibérico)

Abstract

The Escucha Fm. (Middle Albian, Iberian Range, NE Spain) consists of alternating coal bearing and fine siliciclastic deposits up to 550 m thick. During Albian times fluvio-deltaic, swamp and marine environments prevailed in this area. Paleoenvironmental modelling was realized from sedimentology data in an earlier study (Querol, 1990) and allow us to distinguish five depositional zones (a/fluvio- deltaic plain, b/peat bog flat, c/brackish water swamp belt, d/delta front, e/prodelta) in order to recognize different mineral assemblages. Mineralogical analysis in the whole rock and the clay fraction from 49 samples of sediments interbedded in coal sequences were performed by X-ray diffraction means. A semiquantitative estimation of mineral contents in the whole rock and of the clay minerals in fine fraction was obtained following the criterion of Chung (1974) and Inglés & Anadón (1991). The mineral assemblages in the rocks studied are formed mainly by quartz and kaolinite. Substantial amounts of illite and mixed layer illite/smectite are present, and it is also possible occasionally to identify feldspars, calcite, gypsum, jarosite, hematite and pyrite. There is no correlation between the mineral features of assemblages and the location of the samples in the vertical profiles. The obtained results show a relationship between mineral distribution and the different depositional zones. In this sense, it can be regarded that the amount of the mixed layer illite/smectite increases to the prodelta sediments, the ranges of quartz distribution and illite crystallinity spreading to the fluvio-deltaic deposits, and the kaolinite/illite ratio is lower in prodeltaic environment, reflecting the changes in the physico-chemical conditions of deposition.