A pedological and micromorphological study on Mediterranean loessial deposits near Gerona, NE-Spain

Abstract

During a detailed soil survey in the Ter valley (province of Gerona, NE-Spain), loamy sediments were observed, which because of their geomorphological position and sediment characteristics were interpreted as loessial deposits. Previous studies suggested that loess deposition was of very minor importance in this part of the Mediterranean, which seemed to be in conflict with the results of the survey. To establish the nature and stratigraphy of these deposits considerable attention was paid to their distribution, pedology and stratigraphy, as well as to the fluvial deposits in the Ter valley. Results from the field survey strongly indicate that the loamy sediments are of aeolian provenance, but have been more or less reworked after their deposition. A fluvial origin can be excluded, as such would be in conflict with their distribution and composition, as well as with their soil development relative to that in the fluvial terraces. They can be divided into two units. The Younger Unit, marked by an at most moderate soil formation and weathering, has two intercalated palaeosols. The Older Unit is characterized by the presence of a deep rubified soil and a strongly developed calcic/petrocalcic horizon. Micromorphological studies on a number of representative sections confirmed that the deposits largely consist of more or less reworked material of aeolian origin. The texture of this material ranges from silt to fine sand, indicating a provenance from a nearby source area. Sedimentary structures allowing for a more precise indentification of the colluvial processes were not observed. Within the Younger Unit three members could be distinguished, separated by palaeosols. The lower palaeosol is characterized by brunification, bioturbation, and yellow argillaceous cutans, and regularly shows hydromorphic features. The upper palaeosol is also characterized by brunification and bioturbation, but has yellowish brown to orange argillaceous cutans and some matric cutans, and only locally shows hydromorphic features. In both palaeosols secondary carbonate redistribution is observed, dating from after the truncation and subsequent burial of the soil. The soil developed in the top of the deposits has a crystic matrix and is marked by a strong redistribution of carbonates (calcitans, neocalcitans, and nodules). Bioturbation in this soil is prominent. The truncated soil in the Older Unit shows iron segregation, giving rise to a red-brown argillicol plasma and a weak rubification of the matrix. In addition, common biological features and red-brown illuviation ferri-argillans are observed. Correlation of the various units and palaeosols with those described in the literature leads to the following tentative stratigraphy: deposition of the Older Unit during the Rissian, followed by soil formation during the Eemian. Deposition of the lower member of the Younger Unit during the Early Würm, followed by soil formation in Early to Middle Würm. Subsequently, deposition of the middle member before and soil formation during the Denekamp interstadial. The upper member probably dates from the Late-Würm.