Josep Maria Salvany

Nonmarine Evaporitic Sedimentation and Associated Diagenetic Processes of the Southwestern Margin of the Ebro Basin (Lower Miocene), Spain

ABSTRACT In the southwestern margin of the Tertiary Ebro Basin (northeastern Spain) several evaporitic units are found among distal alluvial-fan deposits. They are made up of bioturbated microlenticular primary gypsum, secondary nodular and meganodular gypsum after anhydrite, chert nodules, and charophyte limestones. These evaporites formed in shallow saline lakes along the basin margin during the early Early Miocene (Autol gypsum unit) and the late Early Miocene (Ablitas, Gravalos, and Ribafrecha gypsum units). The lakes were fed principally by deeply circulated groundwaters loaded with dissolved salts from the old evaporite formations (mainly Triassic rocks) that make up the surrounding mountain chains. Shallow meteoric waters of low concentration also reached the lakes episodically. Concentration of these waters led to deposition of microlenticular gypsum and charophytic carbonates, which were then intensely bioturbated. Early-diagenetic chert nodules and nodular anhydrite also formed. In the early stages of burial large anhydrite nodules (meganodules) overprinted these sedimentary and early-di genetic deposits. With more advanced burial the primary gypsum was profoundly anhydritized in some areas. With subsequent exhumation the anhydritized zones were transformed to secondary gypsum. Marginal lakes acted as preconcentrators of drainage waters coming from the surrounding mountain chains, causing early precipitation of low-solubility salts as Ca-carbonates, gypsum, and silica. The remaining dissolved salts then drained toward the basin center, where gypsum, anhydrite, glauberite, polyhalite, and halite precipitated in contemporaneous high-salinity lakes.

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.

Pitfalls in the geological characterization of alluvial deposits: site investigation for reactive barrier installation at Aznalcóllar, Spain

The alluvial deposits of the Agrio River in SW Spain have been studied using terrace mapping, boreholes, trenching and vertical electrical sounding to select an adequate place for a Permeable Reactive Barrier. Geological and hydrogeological data available prior to the barrier construction suggested a simple geological model based on three terraces, the most modern being deeper than the oldest. The barrier was accordingly trenched through the youngest terrace. However, excavation of the barrier andsubsequent subsoil data demonstrated that the internal structure of the Agrio alluvial deposits does not follow such a simple model. A revised model, less favourable to the existing barrier design, revealed the oldest terrace to be deeper than the youngest, and to form a palaeochannel oblique to the surface terraces and river trends. The study methods used are standard, but proved to have insufficient resolution to detect key features of the alluvial geology. It is concluded that such characterization methods, though widely used, are not appropriate where alluvial terraces may display complex internal structures not reflected in the modern surface geomorphology. In hindsight, the study area should have been larger, so as to encompass at least the width of the alluvial plain and to extend for a hundred metres or so up and downstream from the proposed barrier location. Lithological logs of the boreholes should have been carefully described from both cuttings and downhole geophysical logs, which would have allowed more accurate delineation ofthe stratigraphy of the alluvial deposit. Subsequentgeophysical methods should have been calibrated to this stratigraphy to characterize the internal structure and basal contact of the alluvial deposit.