Micromorphological characteristics of soils formed from Late-Tortonian to Late-Pleistocene in the Granada Basin (Southeastern Spain)

 

Ortiz I., Martín F., Simón M., García I. & Dorronsoro C.

Dpto Edafologia. Facultad Ciencias. Universidad de Granada. Granada. EspaĖa.

 

 

The Granada Basin is located in the central sector of the Betic Cordillera (southern Spain). The infilling of the basin began in the Serravalian-Tortonian and continentalization occurred towards the end of the Tortonian times (Fernández and Soria, 1986-1987). During Late Tortonian an important stage of uplift produced the thick deposits of the Block Formation (OTU-1 soil). A new stage of uplift in the Pliocene (Estévez and Sanz de Galdeano, 1983) gave rise to new coarse detrital inputs during the Early Pleistocene (DUR-1 soil). During the Middle-Late Pleistocene, low areas along the borders of the Sierra Nevada rose and were incorporated into the marginal zones of the range. These areas began to supply highly abundant coarse-grained sediment during the Quaternary, apparently reactivated by cold episodes (NIG soils). In NIG, we studied a series of 4 palaeosoils that presumably formed in warm episodes of the Middle-Late Pleistocene. The oldest of these, NIG-1, must have formed between 440,000 and 367,000 years B.P., NIG-2 between 347,000 and 297,000 years B.P., NIG-3 between 251,000 and 195,000 years B.P., and NIG-4 between 128,000 and 75,000 years B.P.

All the soils studied, being decarbonated and strongly weathered, with clay illuviation and rubefaction, and forming strongly developed Bt horizons, fit within the category Calcic Rhodic Cutanic Luvisols FAO (1998). The illuvial clay shows an intense reddish colour due to the presence of iron. Frequently, the Bt horizons present recarbonatization subsequent to the illuviation process, evident by the presence of carbonate coverings over the coatings.

The C horizons contain carbonate accumulations (Ck), which have intensely destroyed the illuvial-clay coverings that had formed previously. In the most developed soils (OTU-1, DUR-1 and NIG-3), the carbonates replaced the grains of the quartz and feldspar skeleton, these appearing with large aureoles in which the carbonates formed relatively large crystals (spatic limestone), which contrasts with the micritic limestone of the basal mass. Occasionally, epigenesis is total, and only a blurred trace remains of the silicate grain.

Finally, we evaluated the degree of evolution of the soils, using such characteristics as the degree of destruction of the mica grains, the abundance and thickness of the illuvial-clay coatings, and the quantity as well as orientation of the fine matter. The sequence from least to most evolved is: NIG-4 < NIG-2 < NIG-1< OTU-1 < NIG-3 < DUR-1.

Given that the duration of the warm periods of the Middle-Late Pleistocene en compassed some 55,000 years, the varying degrees of soil evolution must be attributed to differences in climatic conditions (presumably moisture). In this case, the period most conducive to soil development was between 251,000 and 195,000 years B.P., while the least propitious was 128,000 and 75,000 years B.P.