Relationship of mineralogy to depositional environments in the non-marine Tertiary mudstones of the southwestern Ebro Basin (Spain)

Abstract During the Middle-Late Eocene, Oligocene and Miocene a non-marine succession, about 5000 m thick, was deposited in the western Ebro Basin. In the northern and southern margins of the basin, conglomerate deposits of alluvial fan origin were deposited. Towards the centre of the basin there is a lateral change to flood plain sandstones and mudstones and finally carbonate and evaporitic lacustrine deposits. Two types of saline lakes, where evaporitec precipitation occurred, can be distinguished: central saline lakes of high salinity, and marginal saline lakes, located near the southern margin, with lower salinity than the central ones. X-ray diffraction, TEM, SEM and EDX analysis were carried out on mudstones from central and southern deposits. The clay minerals identified were illite, chlorite, kaolinite, Mg-rich smectite, palygorskite and mixed-layers illite-smectite and chlorite-smectite. Apart from clays, carbonates (calcite, dolomite and magnesite) and other silicates (quartz, plagioclase, K-feldspar and analcime) were recognised. The palygorskite, Mg-rich smectite, analcime, dolomite and magnesite are of diagenetic origin. The other minerals are detrital, and derived from the surrounding mountain chains. The clay mineral assemblage of each environment is related to the diverse various source areas (Iberian Range and Pyrenees) and their compositional variation through time, as well as to diagenetic processes. On the basis of clay mineral distribution, the detrital clay mineral assemblages are preserved in the alluvial fans, the flood plain without saline influence, and the central saline lakes. In the saline flood plains and marginal saline lakes diagenetic palygorskite and Mg-rich smectites were formed. In the marginal saline lakes the formation of magnesium clays prevailed but in the central saline lakes, where no clay mineral diagenesis is observed, the magnesium precipitated as dolomite and magnesite.