engleski

Main Events Recorded in the Sedimentary Succession of the Adriatic Carbonate Platform from Oxfordian to the Upper Santonian in Istria (Croatia)

During the Kimmeridgian-early Tithonian emersion area of the present Istria represented morphologically diversified terrain affected by significant karstification. On the Zlatni rt cape and Rovinj-1 bauxite deposit near Rovinj there are different types of the boundary between first two large-scale sequences of Istrian deposits: in the Rovinj-1 deposit, i.e. deepest part of the palaeorelief, approximately 20 m thick succession was a result of the "inner" transgression, comprising sequence from fresh-water to brackish and fully marine deposits. Succession from the Zlatni rt locality is a consequence of the fully marine transgression over a relatively higher point in the Late Tithonian palaeorelief, where only traces of reworked swamp deposits have been found in only 50 cm thick interval. Even more extreme example can be found in the area of Vrsar, where the entire hiatus can be seen in a thin-section, comprising Oxfordian/lowermost Kimmeridgian deposits with alveolar calcrete structures and Upper Tithonian intraclastic grainstones in a direct contact on a thin-section level. From this we can conclude that on a relatively small area a significant difference in the succession can be seen, as a consequence of differences in palaeogeomorphology of the area. Localities with bauxite occurrences represented palaeodepression into which the source material for bauxite was transported from neighbouring, hypsometrically higher areas. It is obvious that the relative sea-level fall during the forced regression was significant, since even in the lowest parts of the palaeorelief (palaeodepressions floors were more than 20 m below the surrounding area) vadose bauxites were formed, i.e. groundwater table was even deeper. Gradual sea-level rise caused slow rise in the salinity of swamps, but eventually caused obliteration of barriers and inundation of inner parts of the formerly emerged platform, first lower and subsequently hypsometrically higher areas. The quarry Fantazija is located within Berriasian dolomites, which continuously overlie either more or less late-diagenetically dolomitised limestones of the Kirmenjak or relatively thick succession of late-diagenetic dolomites (i.e. completely dolomitised upper part of the Kirmenjak unit). Boundary between Tithonian and Berriasian, i.e. between Jurassic and Cretaceous, is located within underlying late-diagenetic dolomites (it has been documented within incompletely dolomitised sequences). Thickness of alternation of early-diagenetic and late-diagenetic Fantazija dolomite or informal Rovinj unit is variable, approximately 35 m in average. In the Fantazija quarry on smoothly cut walls rhythmical alternation of 0.4 to 1.2 m thick layers of dark grey macrocrystalline dolomites and 0.3 to 0.6 m thick beds of light grey supratidal early-diagenetic dolomites can be seen. Rhythmical sedimentation of subtidal-intertidal lime muds (late-diagenetically dolomitised into the present dark grey beds) and light grey supratidal early-diagenetic dolomites compose typical shallowing-upwards cycles. These parasequences could be interpreted, especially in continuous succession with cycles of the Kirmenjak unit, as lowstand deposits - LST. Considering sequence-stratigraphic interpretation of dolomitisation it is possible that late-diagenetic dolomitisation was caused by significant oscillation of relative sea-level, when during the sea-level fall mixing zone of fresh and marine water migrated towards the sea. In this way, during the relatively early stage of late diagenesis subtidal-intertidal deposits were dolomitised within mixing zone. The boundary between Hauterivian and Barremian is characterised by significant emersion surface, filled with greenish-grey marls and clays, and very characteristic first appearance of typical LLH stromatolites. Barremian deposits of Istria are 70-80 m thick, composed of cyclical alternation of fine-grained packstones/grainstones and bindstones, as well as variable amount of peritidal and emersion breccia. Cross-bedded bioclastic rudstones and floatstones are especially common in the uppermost part of the unit. Sedimentary structures include desiccation cracks, small wave ripples, cross lamination and several levels with dinosaur tracks. LLH stromatolites represent the upper parts of peritidal shallowing upward cycles, being characterised by wavy lamination and fenestral fabric. Within the Uppermost Hauterivian peritidal limestones in the Kolone cove there are paleodepression composed of a swamp - lacustrine - deposits with dinosaur bones. Regional Late Aptian-Early Albian emersion is the boundary between the second and the third large-scale sequence of the Istrian carbonate succession. The Lower Aptian oncolites, plus only a 2-3 m thickness of Upper Aptian deposits in the quarry Kanfanar followed by a late Aptian-Early Albian emersion surface and Late Albian deposits. The Lower Aptian oncolite limestones are underlain by carbonate deposits of the informal Dvigrad Formation, deposited during the Barremian in peritidal environments of the inner part of the Adriatic Carbonate Platform. These are 68-78 m thick deposits, characterised by peritidal shallowing-upward cycles represented by alternations of mudstones, peloid wackestone/packstones to grainstones and LLH stromatolites. This unit is the culmination of a relative shallowing trend that is evident through the Lower Cretaceous succession, as indicated by common desiccation structures, dinosaur tracks, and large amounts of intertidal deposits. Shallowing-upward cycles in lower and middle part of the Barremian are usually composed of subtidal pelletal or algal mudstones or wackestones, followed by intertidal fenestral mudstones and LLH stromatolites or tidal breccia composed of reworked stromatolite fragments. At the beginning of the Aptian, spacious low-energy shallows and lagoons were formed, where large amounts of fine carbonate detritus were deposited. The first 2-5 metres of the succession are commonly characterised by variable amounts of requieniid shells, mostly of Toucasia sp., and different benthic foraminifera, as well as numerous relatively large (1-8 cm) oncoids of Bacinella irregularis RADOIČIĆ. The Aptian limestones, referred to informally as the Kanfanar Formation, can be divided into Lower Aptian massive oncolite limestones (Sv. Petar Member - known as the architectural-building stone "Istarski žuti", i.e. "Istrian Yellow", for its usually yellowish colour) and uppermost Lower Aptian and Upper Aptian deposits of locally very variable thickness (Begovac Member). Massive Lower Aptian limestones (facies of the architectural-building stone "Istarski žuti") are very recognisable by their morphology in the field. They are composed of cyclical alternation of two lithotypes: mudstones and oncolitic floatstones, which form thinner or thicker cycles. Bacinella oncoids, which are typical components of the Lower Aptian limestones throughout the Dinarides, are always irregular in shape, and relatively large (mostly 5-80 mm), and are therefore sometimes referred to as 2macroids". They have encrusting Bacinella skeletons in the central part, surrounded by a thinner or thicker oncoid envelope. Oncoids usually comprise up to 40 vol.% of the rocks, and in some parts, where they form oncoid crusts like, they are practically the only rock constituents. Hardgrounds shows irregular surphaces and burrowings. Mudstones were deposited during periods of higher accumulation rates, while during periods of low accumulation rates oncoids were formed, and deposits were intensely bioturbated. The entire unit was characterised by deposition in relatively deeper environments, resulting in relative accumulation rates approximately three times lower than the average for Lower Cretaceous deposits in Istria. The thickness of the Lower Aptian oncolite limestones is very variable, as a consequence of differences in the beginning and duration of the regional Aptian-Albian emersion on the Istrian part of the Adriatic Carbonate Platform. This was caused by variable amounts of synsedimentary tectonics, erosion and karstification during the Late Aptian and Early Albian. The Upper Aptian deposits are overlain by a 1-2 m thick bed of emersional breccio-conglomerates with clayey-marly matrix. The clasts are composed of fragments and pebbles of Aptian limestones and the matrix of emersional clays and/or marls and palaeosols. This bed represents the boundary between two large-scale sequences (one of the stratigraphic range Upper Tithonian-Upper Aptian, and the other one Upper Albian-Upper Santonian), and is therefore very important for the regional geological study of Istria and neighbouring areas (e.g. Ćićarija Mt., island of Cres, etc.). By the end of the Albian and during the Cenomanian, discrete depositional systems were established in different parts of the platform preserved in present-day Istria, which were characterised by lateral changes from peritidal and barrier bars to gently inclined carbonate ramp deposits. These changes also occurred in Upper Albian and Lower-Middle Cenomanian deposits at other localities on the Adriatic Carbonate Platform. The Upper Albian, Lower and Middle Cenomanian carbonate deposits of southern Istria (in the area of Veli Brijun Island, Banjole, Frašker Island, Vinkuran and Pomer) can be divided into seven facies units: Facies Unit 1: Thin-bedded peloidal and stromatolitic limestones (Upper Albian) ; Facies Unit 2: Well-bedded packstones/grainstones with sliding and slumping features (Vraconian-Lowermost Cenomanian) ; Facies Unit 3: Storm-generated and nodular limestones (Lower Cenomanian) ; Facies Unit 4: Massive bioturbated carbonate sand body (Lower Cenomanian) ; Facies Unit 5: Prograding rudist bioclastic clinostratified bodies (Lower to Middle Cenomanian) and Facies Unit 6: Rudist biostromes (Lower to Middle Cenomanian). The clinostratified bodies of Facies Unit 5 are interpreted as the foresets of subaqueous dunes, being a consequence of storm and wave reworking of highly productive rudist meadows, and the redistribution of the material into neighbouring areas. Such bioclastic bars are typical progradational features, resulting from downlapping into formerly deeper areas. Since the contemporaneous shallow-water carbonate system (rudist meadows) was very productive, depositional rates were very high, and the succession of numerous clinostratified bodies resulted in the shallowing-upward trend, followed by the lateral shift of the rudist congregations (Facies Unit 6) In AC Medulin (9 km SE from Pula) can be seen outcrops of the part of an approximately 120 m thick succession crossing the Cenomanian-Turonian transition, and recording a shift from a drowned platform environment into shallow-water deposition. The oldest part, approximately 35 m thick, is represented by limestones with calcispheres and very rare ammonites, which was described by POLŠAK (1965) as a "chronozone of limestones with ammonites". The microfossil assemblage contains typical autochthonous pelagic fauna and variable amounts of reworked shallow-water benthic organisms (miliolids, textularids, bioclasts of molluscs, gastropods and echinoderms). The second unit, approximately 20 m thick, is mostly composed of well-bedded (10-50 cm), rarely thick-bedded (60-80 cm), bioclastic, bioclastic-peloid and bioclastic-peloid-intraclastic grainstones. Besides variable amount of coarser rudist bioclasts (diameters up to 30 mm), these rocks contain rare benthic foraminifera (?Pseudocyclammina sphaeroidea GENDROT, Nezzazata sp., Cuneolina sp., Peneroplis sp., small verneulinids and textularids), rounded bioclasts of echinoderms and molluscs, coated bioclasts of Thaumatoporella and rounded intraclasts of shallow-water origin. This unit comprises exclusively shallow-water components, as a consequence of the re-establishment of shallow-water deposition. The third unit, which in this profile is approximately 35 m thick, consists of numerous alternations of thin-bedded mudstones, oncoid and/or skeletal floatstones, peloid-bioclastic and oncoid wackestone/packstones and foraminiferal grainstones. These deposits frequently contain small radiolitids and Sauvagesia (up to 6 cm in diameter), and in the middle and upper part of the unit, isolated thick-shelled radiolitids and radiolitid lithosomes with rare radiolitids in primary growth position (Sauvagesia/Durania, ?Lapeirousia, Radiolites/Praeradiolites).

platform carbonates; emersions; bauxite; early and late diagenetic dolomites; LLH-stromatolite; lacustrine sequence; dinosaur occurences; clinoforms; drowned platform; Adriatic carbonate platform; Upper Jurassic; Cretaceous; Istria; Croatia

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