Dragutin Slovenec

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 2 O/K 2 O)×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.

Distribution of Iron and Manganese in Terra Rossa from Istria and its Genetic Implications

Haematite and goethite are the predominant pedogenic mineral phases in terra rossa from Istria. Limited variation of selected Fe oxide characteristics in analysed samples indicates the specific pedogenic environment in which terra rossa is formed. The mean value of the Fed/Fet ratio, taken as an index of weathering is 0.7 and reflects quite a high degree of weathering of Fe-containing primary silicates. Relatively uniform Fed/clay ratios clearly indicate a predominance of co-illuviation of clay and Fe oxides. Low values of Feo point to the low content of poorly crystalline Fe oxides in analysed terra rossa. Feo/Fed ratios in the analysed profiles vary. In the upper part of the Pomer, Porec and Novigrad profiles Feo/Fed ratios are higher than in the lower part of those profiles which may be explained as a result of different pedogenic environments and/or additions of external materials in the upper part of those profiles. The lack of positive correlation between Mnd and clay and Mnd and Fed is a consequence of remobilization of manganese due to hydromorphic processes which post-dated co-illuviation of clay and Fe oxides. During the Late Tertiary and Quaternary the pedogenic environment on hard carbonate rocks of the Jurassic-Cretaceous-Palaeogene carbonate plain of southern and western Istria generally remained suitable for rubification. However, (Neo)tectonic activity and the input of external material, as well as various climatic fluctuations might have significantly effected terra rossa through the processes of erosion, colluviation, yellowing and secondary hydromorphy.

The Mineralogical Characteristics of the Lamboglia 2 Roman-Age Amphorae from the Central Adriatic (Croatia)*

The mineralogical characteristics of four Lamboglia 2 Roman-age amphorae excavated from ancient underwater shipwreck localities in the central Adriatic were investigated. X-ray diffraction and optical microscopy analyses showed that the amphorae have similar primary mineral compositions, mainly consisting of quartz, plagioclase, potassium feldspar, diopside, illite and calcite, most probably originating from clayey Eocene flysch deposits used as raw materials in manufacture. The difference in mineral composition between these samples is recognized from the presence or absence of melilite, high-magnesium calcite, aragonite, gypsum and analcime. It was shown that melilite is a secondary mineral formed during the firing process, while the others, which build up through incrustation of marine benthonic communities on the amphora walls and fill the cracks in the amphora bodies, were formed by crystallization in seawater. The firing of the original pieces of amphorae showed that melilite could be formed at temperatures higher than 800°C, most probably through interaction between calcite and silicate minerals present in the raw materials. The presence of melilite, and the absence of illite and calcite, were used to estimate the firing temperature of the amphorae during their production.

The origin and composition of flysch deposits as an attribute to the excessive erosion of the Slani Potok valley („Salty Creek“), Croatia

Flysch deposits that crop out in the elongated coastal area of the Vinodol Valley (External Dinarides), and particularly in the Slani Potok (‘Salty Creek’) catchment, are characterized by excessive erosion. Formation of badlands, creeping and sliding processes have been related to crystallization of thenardite. Unusually intense erosion is caused by the dispersive effect of sodium on clay particles (derived from pelitic flysch intervals) in solution, as well as by expansion of thenardite during transformation into a deckahydrate (with a four fold increase in volume). The search for a sodium source required for thenardite crystallization has focused on a particular composition of Slani Potok Eocene fl ysch rock types. Slani Potok flysch is predominantly composed of pelitic intervals, with subordinate sandstones and biocalcirudites. Sandstones and biocalcirudites contain appreciable amounts of siliciclastic detritus. Lithic fragments are represented by volcanic rocks revealing porphyry structures - possibly andesite or altered volcanic glass fragments, quartzite, schists, shales, and carbonate grains. High amounts of sodium and barium (0.93–1.09 % and 267–276 mg/kg respectively), in marls and sandstones, is essentially ascribed to a particular composition of flysch, refl ecting its area of provenance. A Palaeozoic clastic complex with barite mineralization, (black shales, sandstones and conglomerates), as well as a Mesozoic hornblende andesite located near Fužinski Benkovac in the Gorski Kotar region, are interpreted as flysch source rocks. Sodium at least could be partly leached from the lithic and feldspar grains in clastic rocks, due to circulation of pore fluids. An increased amount of sulphur, barium and strontium in flysch can be related to that of a barite ore complex in the source area of the Gorski Kotar region. Transportation and deposition of material eroded from the hinterland (North of Slani Potok), suggests turbidity current flow was transverse to the main axis of the Eocene foreland basin striking NW-SE.

Verification of Engineering-Geological/Geotechnical Correlation Column and Reference Level of Correlation (RNK) Method by Observations in the Slip-Plane Zone

The engineering-geological/geotechnical correlation column can be established in zones of limited extent using one or several characteristic layers, one of which is selected as the reference layer. The Reference Level of Correlation method, i.e. the RNK (Referentni nivo korelacije in Croatian) method, is a confirmed procedure enabling the establishment of such columns. In the engineering geological/ geotechnical correlation column, the plasticity index is the most significant indicator of the peak friction angle, full-softening friction angle and residual friction angle for coherent soils and soft rock formations. As a rule, maximum plasticity index values correspond to the minimum values of such friction angles. This opens up the possibility of an exact engineering-geological/geotechnical model, with accurate differentiation of minimum shear strength zones, zones of different permeability, and zones of various degrees of natural compaction. This procedure was applied inter alia on the successfully improved Granice landslide located in the Zagreb area, where it was proven that elements for verification based on the RNK-method exist for all three areas of investigation: sliding body – drain trench section – detail in the central portion of the sliding zone. The procedure is recommended as a means for finding solutions to similar problems.