ADDING INFORMATION ABOUT SOILS AND PALEOSOLS TO GEOLOGICAL MAPPING. A GIS APPLICATION IN CENTRAL ITALY

Soil survey and study of paleosols can add a great deal of information useful to the comprehension of Quaternary geological and geomorphological processes, but they are rarely reported in geological maps, perhaps due to the difficulty in individuating and mapping pedostratigraphic units. This study was conducted to qualify Quaternary formations in terms of pedostratigraphic units and to test the possibilities offered by the use of GIS and 3D tools in implementing existing geological maps with soil survey and information about paleosols. The territory studied, measuring around 35 km2 and situated in Central Italy, is part of a broad ridge emerging from the sea during the Miocene age and undergoing intense geomorphological evolution during the Pliocene and Quaternary Periods. Many rock types and sediments outcrop in the area. They are metamorphic rocks, mainly free of carbonates, calcareous rocks, and slope and alluvial deposits derived from the local bedrock. We used as geological reference the recent map published by the Department of Earth Science of the University of Siena, at 1:25,000 scale, where the Quaternary sediments are described as alluvial or eluvial-colluvial deposits. The GIS was built on ArcGIS8® and ArcScene® was used to provide a tri-dimensional view of spatial distribution of pedostratigraphic units as well as to obtain cross sections of the area. Three pedotratigraphic units were proposed, according to the presence of different diagnostic horizon assemblages and selected chemical and physical characteristics of horizons and parts of them. Following former paleopedological studies, they were attributed to Early, Middle and Late Pleistocene, and Holocene. Three soil maps at 1:10,000 and 1:25,000 scales were combined and harmonized, 27 benchmark profiles were analyzed, and each soil map unit was classified in terms of occurrence and thickness of the three pedostratigraphic units. The pedostratigraphy map obtained was then combined with the geological map, resulting in a more detailed subdivision of Quaternary surface deposits. The tri-dimensional view of the geology-pedostratigraphy map, as well as the cross sections, permitted appraisal of the geographical and chronological relationships between rocks and overlaying soils.