Western Istrian Anticline as an ideal natural laboratory for the study of the regional unconformities in carbonate rocks

Bauxite deposit Rovinj-1. Lowermost Kimmeridgian–Upper Tithonian unconformity is accompanied with bauxites.

Lowermost Kimmeridgian–Upper Tithonian unconformity was of a relatively long duration (at least 6 Ma – Velić et al., 2003), resulting in a highly differentiated relief. This unconformity is accompanied with bauxites. The uppermost 20 to 30 cm of the bauxite is heavily altered: its colour is greenish-grey to yellowish-white with vertical to subvertical extensions penetrating the underlying deep red bauxite (Durn et al., 2003). The nature of the alteration of Fe-minerals is clearly redox-based and is closely related to environmental change associated with the deposition of the cover-beds. Three localities (1-3) are planned for the investigation. The rationale behind the selection of localities is to get the highest possible diversity of (a) unconformity surfaces, (b) unconformity-forming events, and (c) palaeosol profiles (in general, for all the materials related to regional unconformities as a group phrase, a common term palaeosol is used; once selected material is elaborated individually, the precise terminology will be applied). In order to reconstruct palaeoenvironmental changes that preceded and followed the periods of main subaerial exposure and to determine the beginning and the end of terrestrial phases, detailed bed-by-bed logs (several metres thick) in the rocks underlying and overlying the unconformities will be made. Depositional facies and early diagenetic alterations will be determined.

References:

Durn, G., Ottner, F., Tišljar, J., Mindszenty, A. and Barudžija, U., 2003. Regional subaerial unconformities in shallow-marine carbonate sequences of Istria: sedimentology, mineralogy, geochemistry and micromorphology of associated bauxites, palaeosols and pedo-sedimentary complexes. In: Vlahović, I., Tišljar, J. (Eds.), Evolution of Depositional Environments from the Paleozoic to the Quaternary in the Karst Dinarides and the Pannonian Basin. 22nd IAS Meeting of Sedimentology, Opatija, September 17–19, 2003, Field Trip Guidebook, 207–254.

Velić, I., Tišljar, J., Vlahović, I., Matičec, D., Bergant, S., 2003. Evolution of Istrian part of the Adriatic Carbonate Platform from the Middle Jurassic to the Santonian and formation of the flysch basin during the Eocene: main events and regional comparison. In: Vlahović, I., Tišljar, J. (Eds.), Evolution of Depositional Environments from the Paleozoic to the Quaternary in the Karst Dinarides and the Pannonian Basin. 22nd IAS Meeting of Sedimentology, Opatija, September 17–19, 2003, Field Trip Guidebook, 3–18.

Greenish-gray palaeosol in the Tri Jezerca quarry. Upper Aptian–Upper Albian unconformity is accompanied with greenish-grey clays, mainly in palaeokarst pits and coarse brecciated regolith.

Upper Aptian–Upper Albian unconformity in Istria had variable duration (11–19 Ma; Vlahović et al., 2005), and is accompanied with greenish-grey clays, mainly in palaeokarst pits and coarse brecciated regolith (Durn et al., 2003). Clays associated with this emersion range in thickness from several centimetres up to 1 metre. Transitional zones between the shallow-water carbonates and emerged parts of the platform are characterized by clay and marl deposition or by extensive coastal marshes with reductive conditions and deposition of black sediments (black pebbles). Preliminary field, mineralogical and chemical analyses gave only initial ideas about the formation and alteration of those clays (Durn et al., 2003). Weakly developed soil structure, presence of root remains, burrows and channels, now mainly filled with pyrite framboids, nests of the faecal products of soil dwelling fauna and nodular pedofeatures indicate that they were pedogenetically altered. Therefore, the colour of palaeosols, the presence of root remains only in the upper part of the profile and high abundance of pyrite framboids may imply that those were probably seasonally marshy soils or permanently waterlogged soils (Durn et al., 2003). Four localities (4-7) are planned for the investigation. The rationale behind the selection of localities is to get the highest possible diversity of (a) unconformity surfaces, (b) unconformity-forming events, and (c) palaeosol profiles (in general, for all the materials related to regional unconformities as a group phrase, a common term palaeosol is used; once selected material is elaborated individually, the precise terminology will be applied). In order to reconstruct palaeoenvironmental changes that preceded and followed the periods of main subaerial exposure and to determine the beginning and the end of terrestrial phases, detailed bed-by-bed logs (several metres thick) in the rocks underlying and overlying the unconformities will be made. Depositional facies and early diagenetic alterations will be determined.

References:

Durn, G., Ottner, F., Tišljar, J., Mindszenty, A. and Barudžija, U., 2003. Regional subaerial unconformities in shallow-marine carbonate sequences of Istria: sedimentology, mineralogy, geochemistry and micromorphology of associated bauxites, palaeosols and pedo-sedimentary complexes. In: Vlahović, I., Tišljar, J. (Eds.), Evolution of Depositional Environments from the Paleozoic to the Quaternary in the Karst Dinarides and the Pannonian Basin. 22nd IAS Meeting of Sedimentology, Opatija, September 17–19, 2003, Field Trip Guidebook, 207–254.

Vlahović, I., Tišljar, J., Velić, I., Matičec, D., 2005. Evolution of the Adriatic Carbonate Platform: palaeogeography, main events and depositional dynamics. Palaeogeography, Palaeoclimatology, Palaeoecology 220, 333–360.

In the area of Minjera the footwall of bauxites are Upper Cenomanian limestones (see geological hammer) and hangingwall are Palaeogene deposits. Upper Cenomanian/Upper Santonian–Lower Eocene unconformity is accompanied with bauxites.

Upper Cenomanian/Upper Santonian–Lower Eocene unconformity was of a very long duration (from c. 25 Ma in southern Istria and Mt. Učka to 40 Ma in northern Istria). However, in the western Istria at several localities erosional remnants of Middle Eocene foraminifera limestones covering the Lower Cretaceous limestones have been found, indicating a possible stratigraphic hiatus of up to c. 87 Ma (Matičec et al., 1996). During such a long hiatus part of the succession could have been chemically weathered, so aforementioned duration of hiatuses is probably somewhat overestimated. Although this stratigraphical hiatus was extremely long relatively thin deposits were formed, mostly bauxite in the lowest parts of the palaeorelief. As in the case of the Jurassic bauxites, the cover sequence also normally begins with transitional palustrine/lacustrine facies. When related to the initial transgression, the introduction of stagnant pore waters into the soil-derived sediment takes place at a stage when diffuse porosity is still unobstructed throughout the deposit, and iron oxide phases are still partly mineralized, so the bauxite may thoroughly react with reducing fluids resulting in a large-scale alteration. This is the case for the majority of bauxite deposits in the area of Minjera. The famous pyritic bauxites, in which whole bauxite bodies are thoroughly grey due to the finely disseminated eogenetic pyrite, clearly indicate that in the case of some deposits the above prerequisites were fulfilled, since marine pore waters came into contact with unconsolidated bauxite (Šinkovec et al., 1994). Three localities (10-12) are planned for the investigation. The rationale behind the selection of localities is to get the highest possible diversity of (a) unconformity surfaces, (b) unconformity-forming events, and (c) palaeosol profiles (in general, for all the materials related to regional unconformities as a group phrase, a common term palaeosol is used; once selected material is elaborated individually, the precise terminology will be applied). In order to reconstruct palaeoenvironmental changes that preceded and followed the periods of main subaerial exposure and to determine the beginning and the end of terrestrial phases, detailed bed-by-bed logs (several metres thick) in the rocks underlying and overlying the unconformities will be made. Depositional facies and early diagenetic alterations will be determined.

References:

Matičec, D., Vlahović, I., Velić, I. & Tišljar, J., 1996. Eocene limestones overlying Lower Cretaceous deposits of western Istria (Croatia): did some parts of present Istria form land during the Cretaceous? Geol. Croatica, 49(1), 117–127.

Šinkovec, B., Sakač, K., Durn, G., 1994. Pyritized bauxites from Minjera, Istria, Croatia. Natura Croatica 3, 41–65.

Savudrija loess-palaeosol sequence. Upper Eocene–Recent unconformity is accompanied with various sediments and soils/palaeosols among which terra rossa, loess–palaeosol sequences and pedo-sedimentary complexes predominate.

Upper Eocene–Recent unconformity was of a very long duration even in the areas characterized by the longest deposition – up to the flysch in the latest Eocene (resulting in stratigraphic hiatus of approximately 35 Ma). However, on the limbs of the Western Istrian Anticline due to the synsedimentary tectonics hiatus was longer since there was probably no younger deposits than Middle Eocene foraminifera limestones (40–45 Ma), while it is possible that in the apical part there was no carbonate deposition since the Lower Cretaceous times, so duration of stratigraphic hiatus could be more than 100 Ma. This unconformity is accompanied with various sediments and soils/palaeosols among which terra rossa, loess–palaeosol sequences and pedo-sedimentary complexes predominate. Zhang et al. (2018) investigated ∼8-m-thick loess-palaeosol sequence located near the town of Savudrija, on the northernmost cape of the Istrian Peninsula. Based on luminescence dating and palaeomagnetic age constraint they concluded that the Savudrija loess–palaeosol sequence was formed between ∼70 and ∼9 ka, correlated to the time span from the last glacial to early Holocene. Furthermore, the magnetic susceptibility record suggests that the age of ∼70 ka of the terra rossa-like soil (palaeosol) at the bottom of the section might be still ambiguous and needs further study. Three localities (13-15) are planned for the investigation. The rationale behind the selection of localities is to get the highest possible diversity of (a) unconformity surfaces, (b) unconformity-forming events, and (c) palaeosol profiles (in general, for all the materials related to regional unconformities as a group phrase, a common term palaeosol is used; once selected material is elaborated individually, the precise terminology will be applied).

References:

Zhang, J., Rolf, C., Wacha, L., Tsukamoto, S., Durn, G., Frechen, M., 2018. Luminescence dating and palaeomagnetic age constraint of a last glacial loess-palaeosol sequence from Istria, Croatia. Quat. Int. 494, 19–33.

Contact of the quartz diagenetic sediments layer in Gortan Cove with the overlying limestone, hammer for scale.

In addition to palaeosols/carbonate sediments related to the four regional unconformities the quartz-rich diagenetic sediments will also be studied. Namely, within the Upper Albian shallow-marine limestone sequences in Istria, there are also some dolomites, silicified deposits and clays. Through the analysis of these deposits, significant proxies for the interpretation of the Late Albian sea-level changes, palaeoclimate, duration of subaerial exposures, and the sources of the material incorporated could be obtained. A study of the silicified material will ensure that through the proposed project all non-carbonate materials will be analysed within the thick carbonate succession of the Western Istrian Anticline.