Mineralogical and geochemical indicators of the polygenetic nature of terra rossa in Istria, Croatia

Abstract

Terra rossa in Istria is situated on the Jurassic–Cretaceous–Paleogene carbonate plain and is considered a polygenetic reddish soil which bears typical terra rossa Fe-oxide characteristics (e.g., Fe d and Fe d/Fe t). The difference in particle size, mineralogy and geochemistry observed between terra rossa and the insoluble residue of limestones and dolomites clearly indicates that the additions of external materials might have diminished the influence of insoluble residue of limestones and dolomites as the primary parent material of terra rossa in Istria. Terra rossa is clay and silty clay composed of quartz, plagioclase, K-feldspar, micaceous clay minerals (illitic material and mica), kaolinites (Kl D and Kl), chlorite, vermiculite, low-charge-vermiculite or high-charge smectite, mixed-layer clay minerals (other than illitic material), hematite, goethite and XRD-amorphous inorganic compound. Calcite, dolomite and boehmite are sporadically present and are of local importance. Kaolinites and illitic material are dominant clay mineral phases in the clay fraction of all terra rossa from Istria. Kaolinite which does not form intercalation compounds with dimethylsulfoxide (DMSO) is the dominant mineral phase in fine clay and is considered predominantly authigenic rather then inherited from parent materials. Neither the content and particle size distribution nor the bulk and clay mineralogy of the insoluble residue of limestone and dolomite support development of terra rossa entirely by dissolution of carbonate rock. If terra rossa has developed only from the insoluble residue of limestone and dolomite, its clay content, due to weathering should be higher than that in the insoluble residues which is not the case. Plagioclase was found only in one insoluble residue while all terra rossa samples contain this mineral. Moreover, insoluble residues do not contain vermiculite which was observed in small amounts in clay fraction of all terra rossa samples. (Na 2O/K 2O)×100, Zr/Nb and (Zr/Ti)×1000 ratios in the majority of terra rossa samples are much higher than in the insoluble residue of limestones and dolomites which also supports external material influence in the genesis of terra rossa. Materials other than the insoluble residue of limestones and dolomites which might have contributed to terra rossa are loess sediments, whose deposition was very important recurrent process in Istria probably since the early Middle Pleistocene, and flysch sediments which extended much more southwards from its present position. Analyses performed indicate that both loess older than that of the Upper Pleistocene age and flysch might have contributed in the genesis of terra rossa.