Provenance and formation of the red palaeosol and lithified terra rossa-like infillings on the Island of Susak: A high-resolution and chronological approach

Abstract

Red palaeosol (RP) covering the carbonate basement exposed at the bottom of the loess section and lithified terra rossa-like materials situated in cavities (LTR1) and karstified fissures (LTR2) in Cretaceous limestones were investigated by means of high-resolution approach, to give a systematic and deeper palaeopedological insight into reddish materials and the exposed red palaeosols on the island of Susak. Both, selected geochemical indicators of weathering (Al/Si, (Na + K)/Al and Fed/Fet) and provenance (La/Ce), as well as detailed mineralogical analysis clearly showed differences among LTR1, LTR2 and RP, respectively. LTR1 and LTR2 represent erosional remains of different soils that existed on the surface of the limestone bedrock prior to the formation of the RP. LTR2 has the highest ZTR (zircon-tourmaline-rutile index) index and contains bauxite minerals (gibbsite and boehmite), clays (kaolinite and mixed-layer clay minerals), and haematite. LTR2 bears characteristics of reworked bauxitic materials mixed with other soils that existed on the surface of the carbonate basement. Those materials might have been mixed and transported until they were captured in the karstified fissure, and, subsequently interlocked with calcite veins and lithified. This might have happened from the Eocene age on. Our attempt to resolve the age of LTR2 formation more precisely, using palaeomagnetic investigations, did not give unambiguous results because the extremely viscous behaviour of the LTR2 samples hinders the acquisition of a stable remanent magnetisation. If we exclude the secondary calcite, LTR1 bears typical mineralogical and geochemical characteristics (e.g. Fed/Fet ratio) similar to those of Istrian terra rossa. Insoluble residues of Cretaceous limestone might have been the main parent materials for the LTR1 formation while the most likely additional fluxes influencing terra rossa formation on the island of Susak were aeolian dust and flysch. The erosion remains ended in limestone cavities and were mixed with cm to dm sized angular and subangular limestone clasts and, subsequently, interlocked with calcite veins and lithified. The investigated RP profile comprised the following mineral horizons: Bwb1-Bwb2-Bwb3-R. The RP formed in a lithologically uniform material that dominantly presents a mixture of loess and, partly, remains of older soils (e.g. rounded clay coatings not related to macrovoids) that existed on the surface of the Cretaceous limestone. Fed/Fet ratio in RP reflects a low degree of weathering of Fe-containing primary silicates. Therefore, we can conclude that the RP does not fit a rather limited variation of selected Fe-oxide characteristics typical for terra rossa. The investigated RP was classified independently of the overlying material. The full name of the palaeosol that we classified is as follows: Hypereutric Chromic Cambisol (Episiltic, Endoloamic). Since no reversal of the EMF was recorded in the RP, we concluded that the RP formed during the Brunhes epoch. Luminescence dating was applied to establish the age of the RP. Based on the acquired data, we assume that the RP very likely formed during the Penultimate Interglacial (OIS7).