Abstract
The B horizons of terra rossa soils developed on three different carbonate lithologies having variable insoluble residue contents were studied in Western Herzegovina. Comparison of their composition and properties illustrates to what extent mineral (especially clay mineral assemblage) and particle size composition of those horizons and the insoluble residue of the underlying carbonate rocks can be used as indicators of the polygenetic nature of terra rossa in this region. Terra rossa B horizons have characteristic red colours, neutral to slightly acid pH, high base saturation with calcium as the predominant cation and high CIA (Chemical Index of Alteration). The CIA values obtained are generally in accordance with mineral composition and particle size distribution of the analysed B horizons. The predominant clay mineral phases in B horizons and related insoluble residues match. Kaolinite is the predominant clay mineral phase in the B horizons overlying carbonate rocks containing low amounts of insoluble residue, while smectite predominates in calcarenites areas with a high insoluble residue content. However, the presence of plagioclase, gibbsite, chlorite-vermiculite mixed layer mineral and vermiculite in B horizons overlying carbonate rocks containing low amounts of insoluble residue support a polygenetic origin for the terra rossa. In contrast, terra rossa formed on calcarenites containing high amounts of insoluble residue might have formed almost exclusively from the parent carbonate rock although some influence of external materials (e.g. gibbsite) cannot be excluded. This investigation shows that in Western Herzegovina, an area with no important aeolian input, the content and mineral composition of carbonate rock insoluble residue plays a major role in terra rossa composition. We can tentatively conclude that the lower the insoluble residue content of the parent materials, the greater is the expectation of a more polygenetic origin for the terra rossa.
Highlights
Terra rossa is a reddish clayey to silty/clayey soil developed over limestones and dolomites and is especially widespread in the Mediterranean region
The terra rossa soils analysed from Western Herzegovina occur over carbonate rocks of different ages and lithologies (Figs. 2, 4, 5 and 6) having very variable insoluble residues (IR) contents (Table 3) ranging from extremely low (0.09 wt.% in Uzarići) to high (18.55 wt.% in Čitluk) values
According to SHELDON & TABOR (2009), parent materials that have already been cycled as sediments or which are clayrich, may start out with Chemical Index of Alteration (CIA) values of 60 to 70%
Summary
Terra rossa is a reddish clayey to silty/clayey soil developed over limestones and dolomites and is especially widespread in the Mediterranean region. A diagnostic feature of terra rossa is a bright red colour originating from rubification, a pedogenic process in which preferential formation of haematite over goethite takes place. Terra rossa is formed as a result of (1) decalcification, (2) rubification and (3) bisiallitisation and/or monosiallitisation (DURN et al, 1999). In other classification systems using the Mediterranean climate as the major soil differentiating criterion, the term terra rossa is Geologia Croatica 67/3 used to describe the soil subclass ‘‘Modal Fersiallitic Red soil’’ when situated on limestones (DUCHAUFOUR,1982). The Croatian classification puts terra rossa in the class of Cambic soils (ŠKORIĆ, 1985). Different authors have considered terra rossa to be a soil, vetusol, relict soil (non-buried-palaeosol), palaeosol or pedosedimentary complex, most scientists consider terra rossa a polygenetic relict soil, formed during the Tertiary and/or hot and humid periods of the Quaternary (e.g. ALTAY, 1997; BRONGER & BRUHN-LOBIN, 1997; DURN et al, 1999)
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