Naslov
Regional Subaerial Unconformities in Shallow-Marine Carbonate Sequences of Istria: Sedimentology, Mineralogy, Geochemistry and Micromorphology of Associated Bauxites, Palaeosols and Pedo-sedimentary Complexes

Autori
Durn, Goran ; Ottner, Franz ; Tišljar, Josip ; Mindszenty, Andrea ; Barudžija, Uroš

Vrsta, podvrsta i kategorija rada
Radovi u zbornicima skupova, cjeloviti rad (in extenso), znanstveni

Izvornik
Field trip guidebook : evolution of depositional environments from the palaeozoic to the quaternary in the Karst Dinarides and the Pannonian Basin / 22nd IAS Meeting of Sedimentology / Vlahović, Igor ; Tišljar, Josip - Zagreb : Hrvatski geološki institut, 2003, 209-255

Skup
22nd IAS Meeting of Sedimentology

Mjesto i datum
Opatija, Hrvatska, 17.09.2003. - 19.09.2003

Vrsta sudjelovanja
Predavanje

Vrsta recenzije
Međunarodna recenzija

Ključne riječi
Subaerial unconformities; emersions; bauxite; palaeosols; Pedo-sedimentary complexes; Eocene; Cretaceous; Upper Jurassic; Quaternary; Istria; Croatia

Sažetak
The following are examinated regional subaerial unconformities: clayey bauxites associated with Kimmeridgian to Early Tithonian emergence, greenish-gray clays associated with Late Aptian - Late Albian regional emergence, regional scale silicification of Upper Albian peritidal limestones, bauxites assiciated with the emergence between the Upper Cenomanian and Eocene which are almost completely pyritized, and Quaternary pedo-sedimentary colluvial complex, Upper Pleistocene loess in Savudrija situated on red interglacial soil and polygenetic relict terra rossa soil. The Rovinj bauxite occurs at the contact of Upper Oxfordian-Lower Kimmeridgian and Upper Tithonian strata in an apparent stratigraphic gap of relatively short duration (about 6 My). There is no noticeable angular unconformity associated with the erosional surface. According to TIŠLJAR et al (1995) the subaerial phase is supposed to have lasted from the latest Oxfordian/earliest Kimmeridgian to the Middle Tithonian. Paleokastified thick-bedded ooid grainstones and bioclastic rudstones (Muča unit) interupted by subaerial unconformity associated with bauxites. They are overlain by Kirmenjak unit consists of clay, breccia and mudstone followed by black-pebble breccia, stylolitised mudstone and fenestral mudstone. Uneven karstic surface of the bedrock contais medium-size (2, 0 to 2, 5 m high) sub-soil pinnacles exposed by mining activity. A closer look at the bedrock reveals the evidence of several superimposed phases of dissolution, brecciation and cementation. The "blue hole" sequence Above the altered bauxite the lowermost part of the cover records the establishment of a palustrine-lacustrine environment with greyish, organic-rich marl and brecci-ated limestone, characterized by a very restricted fauna (mainly ostracods according to VLAHOVIĆ et al., 2000), and probably representing a freshwater pond, formed as the groundwater table was pushed upwards by seawater rising through karstic channels from below. The dominant mineral phases in both red and grey bauxite are kaolinite and boehmite. The main iron bear- ing phase in red bauxite is haematite and in grey bauxite pyrite. Both bauxites also contain chlorite and mixed layer chlorite/vermiculite/illite. Compared to the red bauxite, grey bauxite is depleted in iron and arsenic, and, at the same time, enriched in sulphur, and slightly enriched in cobalt, copper, zinc and lead, i.e, in elements which have a general tendency to accumu- late under reducing conditions. The mineralogy of the iron bearing phases and chemical composition clearly indicates that red bauxites were deposited under oxidizing condi-tions (vadose bauxites), while grey bauxite at the top of red bauxite, formed in a reducing environment as a result of hydrological change due to deposition of the coverbeds. Consequently, grey bauxite at the top of a red one can be considered phreatic bauxite. This is also supported by the distribution of REE. The total REE content is 376.35 ppm in red and 261.76 ppm in grey bauxite. The (La/Yb)ch ratio in grey bauxite (6.36) is lower than that in red bauxite (8.50), and significantly lower than that of ES indicating HREE enrichment relative to LREE. Like their Jurassic counterparts, Palaeogene bauxites of Istria are also indicators of long lasting exposure and hot, humid climatic conditions. Their occurrence coincides with one of the most widespread bauxite events of the peri-Mediterranean area. Bauxites between Mesozoic and Palaeogene strata are abundant from Hungary through Slovenia, Croatia and Herzegovina, as far south as Albania and Greece, all belonging to the Apulian Promontory palaeogeographic domain. Palaeogene bauxites of Istria occur in an apparent strati- graphic gap of over 30 My, between the Lower to Upper Cretaceous (Albian to Turonian-Senonian) and Middle Eocene (Cuisian) strata (ŠINKOVEC, 1973). However, the actual hiatus may have been different. For example in northwestern Istria near Karojba (approximately 12 km southwest from Minjera) bauxites are situated on Upper Albian limestones and are covered by Foraminiferal limestones of Eocene age, i.e the stratigraphic gap is about 54 My. In the area of Minjera the footwall of bauxites are Upper Cenomanian limestones and hangingwall are Liburnian deposits (Early Eocene), i.e. the stratigraphic gap lasted about 38 My. This stratigraphic gap is shorter in southern Istria (about 28 My), where Eocene limestones overlie Upper Santonian beds. Bauxite deposits, which were exploited in the area of Minjera, are situated along the steep southern and northern slope of the Mirna river canyon in the vicinity of Sovinjak and Mlun. Seventeen locations have been found in the vicinity of the former processing plant. Bauxites were excavated by underground mining, because the configuration of the ground favoured adit exploitation. The footwall of the bauxites in the area of Minjera are Upper Cenomanian limestones. The bauxite-filled karst relief is rather dissected: vertically sided sinkholes and karstic canyons developed at joint-intersections are of decametres depth. This indicates that the Cenomanian limestones with karstic canyons must have been uplifted at least several tens of metres above sea level. In perfect accordance with the deep-sinkhole-filling nature of the deposits, the Palaeogene bauxites in Istria are mainly of an overall red colour, with prevailing oolitic intraclastic texture and with less abundant pelito-morphic lithotypes (typical of the "vadose" lithofacies of D'ARGENIO & MINDSZENTY, 1995). Associated with the hydrological change during the deposition of the cover beds, the transition from the subaerial to the marine environment is represented by earlydiagenetic processes under stagnant, reducing conditions and the resulting mobilization and removal of Fe3+. This process generally affects only the uppermost few tens of centimetres of bauxite deposits (i.e. the least consolidated, most permeable part of the accumulated bauxite). Black bauxite either forms a thin layer on the top of grey bauxite or is incorporated in grey bauxites as small detrital fragments. In places almost the entire deposit was affected by pyritization. The yellowish-red mottled bauxite is usually found on the contact with the bedrock, but can also be found on the top of the deposit, and is the result of recent (subrecent) subaerial exposure. The textures of pyritic bauxites are identical to those of red coloured Palaeogene Istrian bauxites. Two phases of pyritization of bauxite can be recognized. Sulphur from the first (earlier) phase of pyritization originated from organic matter from the hangingwall sediments. The poorly crystallised pyrite which was formed in this phase gives a black colour to the bauxite. During the second (later) phase, sulphur mainly originated from sea-water (pore waters of ma- rine composition). It is interesting to note, that exploitation of grey bauxites in Istria dates back as far as the XVIth century when it was mined in order to make "alum" and sulphuric acid for industrial purposes, i.e. in the history of bauxite extraction, Istria was the forerunner. K2O-content in the upper part is around 5 wt.% and decreases to 4 wt.% in the lower part of the profile (Fig. 27). Lower Cretaceous deposits of Istria, characterized by shallow-marine deposition, sporadically interrupted by periods of emersion are sedimentologically and palaeo-geographically very similar to some Jurassic-Cretaceous marine carbonate sequences of NW Europe, called Purbeckian sediments. Important members of these sedi-ments are greenish-grey marls, which form thin films between the limestone beds (DECONINCK et al., 1988). According to DECONINCK & STRASSER (1987) they are characterized by a mineralogical composition of smec-tite-illite-kaolinite, and usually occur at the top of small In the Tri jezerca quarry only the upper part, about 10 m thick, of these limestones crops out. In the up- permost part there are indications of a relative sea-level fall, and the beginning of a regression phase culminating in the regional Late Aptian emersion. Besides intense vertical bioturbation, indicating lowered sedimentation rates, weakly expressed palaeokarstification effects oc- cur. A further relative sea-level fall resulted in a clearly visible emersion with palaeokarstified relics of Istarski žuti, greenish-grey clays in palaeokarst pits and/or coarse brecciated regolith. This unit represents a stratigraphic gap, which started during the Middle Aptian and lasted until the beginning of the Late Albian (VELIĆ & TIŠLJAR, 1987 ; VELlĆ et al., 1989). Greenish-grey clays, i. e, deposits within the Late Aptian - Early Albia emersion phase in the Tri jezerca quarry are up to 93 cm thick and are situated in palaeokarst pits. Morphologically, pits are conical, and compound and resembl those recognized in the Late Dinantian palaeokarst of England and Wales, where VANSTONE (1998) recognized four types of palaeokarst depression morphology. We can tentatively conclude that the morphology of palaeokarst in the Tri jezerca quarry resembles that of hummocky palaeokarstic depressions. We consider that the clays are erosional remains of surficial soils and sediments, which were accumulated in palaeokarst pits following an oscillating marine transgression that terminated emergence. Samples of greenish-grey clay (5BG 4/1 to 10BG 6/1 Munsell colour chart) were taken along one profile for detailed analyses (Fig. 25). A sample of limestone (Istarski žuti) situated immediately bellow the clay was also taken. Clays are composed of phyllosilicates and pyrite. Gypsum was detected only in the uppermost sample. The clay mineral distribution along the profile shows a clear trend. The main clay mineral in the upper part is illitic material (up to 78 wt.%). In the lower part of the profile illite values are below 50 wt.%. The second main clay mineral group are regularly ordered, and randomly ordered illite/smectite mixed layer minerals. Chlorite is subordinate and present only in the lower part of the profile. The insoluble residue of limestone is dominated by smectite and illitic material. In contrast to the clays, mixed-layer minerals were not detected in the insoluble residue of the limestone. The higher content of illitic material in the upper part of the profile corresponds well with chemical data. The clay mineral composition of clays in Tri Jezerca quarry clearly indicates the influence of both pedogenic and diagenetic processes. Mineralogical as well as chemical data indicate that transformation of smectite (from the mixed-layer minerals) to illite must have occurred. Wetting and drying experiments and layer charge measurements support this theory (DURN et al., 2000). K-fixation necessary for illitization of smectites could have been achieved on the palaeolandscape by repeated wetting and drying. Potassium may have been provided by plants, marine wa-ters, volcanic dust and other sources. ä34S in pyrites (ä34S is -36&#8240; ) from Tri jezerca quarry may also result from repeated cycles of oxidation and reduction, i.e. fluc-tuation of the water-table in wetland marshy soils. Based on micromorphological research, one can also conclude that greenish-grey clays from the Tri jezerca quarry were pedogenetically altered, i.e. they are palae-osols. The following facts favour of this statement: (i) weakly developed soil structure, (ii) presence of root remains, burrows and channels, now mainly filled with pyrite framboids, (iii) nests of the faecal products of soil dwelling fauna, (iv) nodular pedofeatures and (v) microfabric. The colour of palaeosols, the presence of root remains only in the up-per part of profile and high abundance of pyrite framboids may imply that they were probably seasonally marshy soils (partly or wholly within the zone of water-table fluctua-tion), to permanently waterlogged soil (entirely bellow the water-table). This is also supported by the distribution of U, V and Mo along the profile and REE distribution. The total REE content of the clay profile ranges from 87.80 to 123.56 ppm. (La/Yb)ch ratios in grey waterlogged palaeosols vary from 4.51 to 6.82 and are significantly lower than that of ES indicating HREE enrichment relative to LREE. The total REE content in the insoluble residue of limestone is 148.04 ppm and the (La/Yb)ch ratio is 15.67. Enrichment of HREE in clays from the Tri jezerca quarry is probably the result of the higher mobility of LREE in an acidic pedogenic environment. Very low values of ä34S in pyrites (ä34S is -36&#8240; ) from Tri jezerca quarry (sample SB2 ; 8-16 cm, Fig. 29) may also result from repeated cycles of oxidation and reduction, i.e. fluc-tuation of the water-table in wetland marshy soils. Within the inner platform lagoonal and shallow subtidal oncoidal and peloidal mud-bearing limestones of the Upper Albian also occur "quartz sediments" (ŠUŠNJARA et al., 1971 ; GALOVIĆ, 1991) or "quartz diagenetic sediments" (TIŠLJAR, 1985, 1994, 2001). They are observed in Istria, Ćićarija Mt. (SE Istria), and on the Dinara Mt. and Svilaja Mt. in central Dalmatia, as well as on the island of Vis (southern Dalmatia). They were previously known as "mechano-chemogene quartz sediments" (CRNKOVIĆ & MARIĆ, 1969) or "quartz sands" and "quartz sandstones" (POLŠAK, 1970). "Quartz diagenetic sediments" (QDS) occur in Istria within the upper part of the Upper Albian "Thin-bedded peloidal and stromatolitic limestones" unit (Facies Unit 1 - TIŠLJAR et al., 1998) in three separate layers trending in two directions: NE-SW (Tinjan-Pula region) and NW-SE (Savudrija-Buje-Oprtalj region).The quartz layers are occasionally broken by minor faults, which sometimes result in a lenticular appearance of the "quartz diagenetic sediments". Thicknesses of the first, second and the third bed are 0.5-0.8 m, 1-6 m and 1 m. They vary from well-lithified rocks to, "sandy" loose sediments. The well-lithified examples often exhibit well-preserved primary sedimentary features The sharp contact of the QDS with the overlying layer (Fig. 48) indicates silicification with limited influence on the overlying bed. The lower contact is also sharp (Fig. 49), but the transition downwards can be sometimes gradual, with stronger silicification restricted to the joints of the underlying bed. Solitary quartz grains in the underlying limestone are the result of later silicification. "Quartz diagenetic sediments" are composed of at least three structural types of quartz and chalcedony: idiomorphic authigenic quartz crystals (Type 1), microcrystalline quartz aggregates (Type 2) and cryptocrystalline to microcrystalline chert-like clusters. Dense clusters of unidentified small grains can be observed along the boundaries between the last two structural types. These grains are probably the residue of the primary material, which was the source of the silica. Concentrations of REE in insoluble residues from QDS are much lower than the concentrations of REE in insoluble residues from the overlying and underlying limestones. Insoluble residues from overlying and underlying limestones have similar REE-distributions and are enriched in LREE. (La/Yb)es values range from 2.64 (sample 2/1) to 2.67 (sample 1/2). REE-distribution in insoluble residues from QDS show quite the opposite trend, namely they are enriched in HREE. These relationships may imply either a possible volcanic source of silica, probably aeolian in origin. An alternative explanation for such a distribution can be fractionation of REE (increased HREE/LREE ratio) during the silicification process. Origin of QDS is interpreted as a result of the early-dia- genetic silicification of the unlithified carbonate deposits, in general deposited in shallow-marine environments characterized by a low sedimentation rate and low intensity of resedimentation (TIŠLJAR, 1985, 1994, 2001). A possible source of silica can be aeolian dust of probable volcanic origin, which fell into the seawater, was redeposited and mixed with shallow-marine carbonate deposits. Primarly volcanic minerals (feldspars, biotite, sanidine, etc.), which can indicate a possible volcanic source of silica, have not been found within the QDS. However, the mineral assemblage from the underlying montmorillonitic clay in the Loborika mine, near Pula (CRNKOVIĆ & MARIĆ, 1969) supported the possibility of the aeolian volcanic dust as a source of silica. A model explaining the origin of these deposits, in- cluding the aeolian volcanic dust as a source of silica in shallow-marine carbonate deposits largely explains the regional distribution of "quartz diagenetic sediments" and allows further comparison with similar occurrences on the Adriatic Carbonate Platform and in the surrounding area. Since the end of the Eocene (in central parts of Istria probably even much earlier - MATIČEC et al., 1996) the surface of the Istrian Peninsula has been affected by karst processes and weathering, which has led to the development of both surficial and underground features. Different types of sediments, polygenetic palaeosols and soils have been formed. Among them the most important are loess and terra rossa. The oldest Quaternary sediments were discovered in the Šandalja cave near Pula and are represented by red breccia with faunal remains of Early Pleistocene age (MALEZ, 1981). Upper Pleistocene loess situated on red interglacial soil on Savudrija cape is situated in the northwestern part of Istria. It represents a loess complex about 400 cm thick situated on red palaeosol. The palaeosol is about 20 cm thick and is situated on more or less late-diagenetically dolomitized wackestone of Upper Cretaceous age. Although the loess complex can generally be considered silty loam, the insoluble residues of loess are clayey silts. According to DURN et al. (1999) loess is composed of calcite, dolomite, quartz, plagioclase, K-feldspar, goethite, micaceous clay minerals (illitic material and mica), chlorite, vermiculite, low-charge vermiculite or high-charge smectite, chlorite/vermiculite and both kaolinites that do not form intercalation compounds with DMSO (Kl), and kaolinite which intercalates with DMSO (KlD). The fine clay (<0.2 &micro ; ; ; m fraction) contains only kaolinite which does not form intercalation compounds with DMSO (Kl). Coarse and medium clay (2-0.2 &micro ; ; ; m fraction) contains both kaolinites in similar amounts. Low-charge vermiculite or high-charge smectite is quite abundant in the fine clay. A heavy mineral assemblage of loess from Savudrija strongly resembles that of loess derived from deflation of the Po plain sediments during the Middle and Late Pleistocene (DURN, 1996 ; DURN & ALJINOVIĆ, 1995). The pedo-sedimentary colluvial complex in Sjenokoša (Stop 2b) overlies the Berriasian dolomites, which were in the neighbouring abandoned Fantazija quarry exploited as a building stone. The middle and lower part of the Sjenokoša profile is a colluvial complex showing superimposed features of colluviation and soil formation. It seems as if it were an aggradational system, intermittently receiving moderate amounts of fine, probably preweathered clayey material mixed with detrital silt. The increments of material supply were probably not too great and thus vegetation could always survive (a type of "keep-up" system). Pedological features suggest a strongly sesonal, warm humid climate (clay illuviation requires water to transport clays downwards into the section, but also seasonal aridity to make sure that clays dry up and become attached to the porewalls). It is not unlikely that part of the aggrading material was blown to the site of deposition by winds during the dry season and was trapped by the vegetation which could easily "digest" the excess dust with the help of moisture provided in the rainy season. The amount of clay, the argillic horizons, and the voluminous clay coatings all suggest that the ecosystem which this kind of colluvial soil complex has "nurtured" must have been that of a woodland. True, that there are no large root-traces which one would expect from trees, but it very often happens that trees in a subtropical or even in a temperate forest do not let their roots penetrate the soil deeply, because the soil is not very rich in nutrients, and because in such ecosystems, the actual organic soil is restricted to a thin layer at the very top of the profile, roots spread laterally rather than vertically and may not survive because of efficient decay. Root traces are rather those of the shrubs and grasses of the undergrowth. Had it been atrue open grassland, there must have been many more pedofeatures suggesting aridity (e.g. calcic horizons, as in most intersta- dial loess-related soils). Polygenetic relict terra rossa soil in the Novigrad town area (western Istria) is situated on the coast. It represents a polygenetic terra rossa soil about 150 cm thick situated on fine-grained pelletal wackestone of Lower Cretaceous age (Upper Albian). The contents of SiO2, TiO2, MnO and Na2O are significantly higher, and Al2O3 and Fe2O3 lower in the upper part of the profile. Although K2O does not show a clear trend along the profile, decreas-ing content of Na2O with depth is also manifested as a decreasing of the Na2O/K2O index with depth. The same trend is observed for other weathering indices. The trends of increasing contents of Al2O3 and Fe2O3, and decreas-ing contents of SiO2 and Na2O with depth are compatible with the increasing amount of the clay fraction with depth. The heavy mineral fraction of the Novigrad profile is enriched in epidote-zoisite group minerals, amphiboles and garnets, which may indicate that at least part of the parent material from which terra rossa was formed belongs to the Po plain- Adriatic loess region provenance. However, the contents of epidote-zoisite group minerals, amphibole and garnet decrease, while contents of zircon, tourmaline and rutile increase with depth along the profile. This may indicate, as in the case of the red interglacial soil below the Savudrija loess complex, that the parent material is of mixed provenance.

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