Upper Jurassic (Malm) Shallow-Water Carbonates in the Western Gorski Kotar Area: Facies and Depositional Environments (Western Croatia)

Shallow-water carbonates in the Upper Jurassic of the Gorski Kotar were deposited on a carbonate ramp, in beach-barrier island-lagoonal and peritidal environments. In the continuous sequence, more than 900 m thick, several facies have been found: (A) low-energy shallow-water wackestones/mudstones of the Lower Oxfordian, (B) high-energy shallow-water grainstones of the Middle Oxfordian, (C) low-energy, above the fair-weather wave-base packstones of the uppermost Oxfordian and transition to the Kimmeridgian, (D) shallowing-upward/coarsening-upward units formed through progradation of beach-tidal bars or barriers over the peritidal deposits of the Kimmeridgian and the beginning of the Tithonian and (E) peritidal shallowing-upward units capped by storm tidal deposits of the Tithonian and beginning of the Berriasian. Fossil assemblages adapted to the environmental changes: maximum of their abundance, in the number of the taxa, as well as in the number of individuals, corresponds to the high-energy facies B (Oxfordian), while their minimum corresponds to the peritidal shallowing-upward units of facies E (Tithonian)

Shoreline Cross-bedded Biocalcarenites (Middle Miocene) in the Podvrsko-Snjegavic Area, Mt. Psunj, and their Petroleum Significance (Pozega Subdepression - Eastern Croatia)

The extensively distributed bioclastic sedimentary bodies in the Podvrsko-Snjegavic Area, Mt. Psunj (Pozega Subdepression, Eastern Croatia) are mostly composed of fragments of bryozoans, echinoids, lamellibranchs and corallinaceans. Apart from this, a relatively compositionally uniform, but granulometrically variable bioclastic detritus occurs, which also contains a smaller proportion (5-30%) of siliciclastic grains of medium to coarse sand, as well as sporadic pebbles up to 60 mm in diameter. These sediments are characterised by remarkably large-scale cross-bedding with erosional surfaces clearly delimiting the sets. They are interpreted as shallow-marine shoreface subaqueous dunes, sand bars and barriers formed on the nearshore - mainly shoreface area during the Late Badenian in a high-energy depositional cycle with strong synsedimentary tectonics. With regard to the petroleum-geological reservoir characteristics, the described Middle Miocene cross-bedded biocalcarenites are compared with numerous large oil and gas pools globally, and in other localities in Croatia on the margins of inselberg massifs between the Drava and Sava rivers and south of the Sava river

The Karst Dinarides are Composed of Relics of a Single Mesozoic Platform: Facts and Consequences

Croatian geological literature during the mid nineteen-eighties and nineties was marked by the appearance of a new geotectonic concept of the Dinarides proposing that the recent, very complex structural setting is a direct consequence of a specific palaeogeographic relationship during the Mesozoic, i.e. the proposed existence of two carbonate platforms separated by a long-lasting deep marine area (interplatform trough). Although the idea was very interesting and provoked discussion, resulting in the questioning of formerly established concepts, detailed analysis of available data indicate that the Karst Dinarides (External Dinarides) were formed by the destruction of a single, although morphologically considerably variable shallow water carbonate platform. This platform was in some periods very dynamic because of its palaeogeographic position during the Mesozoic, resulting in many periods of large-scale facies differentiation, especially during the Late Cretaceous. The final disintegration of the platform area culminated in the formation of flysch trough(s) in the latest Cretaceous and Palaeogene and the subsequent uplift of the Dinarides. Recently there have been some misunderstandings resulting from the imprecise use of newly established terms, which are, by circular logic, used to connect recent geotectonic relationships with Mesozoic palaeogeography without adequate material proof. Therefore, the terms Dinaricum and Adriaticum should be redefined and used only for description of the recent tectonic pattern, without implying a palaeogeographic component, since during the Mesozoic they represented a single entity. Additional confusion is added by different names used for the same shallow water carbonate platform. Probably the best, although not the ideal name is the most frequently used one: the Adriatic Carbonate Platform. Its duration may be estimated from the Late Lias to the Late Cretaceous, representing the most important part of a thick carbonate succession in the Karst Dinarides (ranging from Carboniferous to Eocene)

Carbonate Platform Megafacies of the Jurassic and Cretaceous Deposits of the Karst Dinarides

Platform carbonate deposits of the Karst Dinarides area have a stratigraphic range from the Middle Triassic (or even Carboniferous in some places) to the Middle Eocene, forming a belt nearly 700 km long and, (after reduction by younger tectonics) 80–210 km wide. Besides their significant thickness (4500 to 8000 m) they are characterised by frequent lateral and vertical alternations of different facies, mostly associated with shallow marine environments. Environments ranging from peritidal through low-energy shallow subtidal–lagoons, restricted inner platform shallows, high-energy tidal bars, beach and shoreface to reefal–perireefal predominate, but there are also carbonate slope deposits and those representing temporarily drowned platform facies and intraplatform troughs. The Jurassic to Cretaceous part of this carbonate succession has been subdivided into 19 megafacies units (9 for the Jurassic and 10 for the Cretaceous), the majority of which represent an inner part of the ancient Adriatic Carbonate Platform. Marginal parts of the platform are mostly buried, either by the recent Adriatic Sea along the SW margin, or younger deposits along the NE margin; at some localities such Jurassic and Cretaceous deposits are represented by debrites and/or carbonate turbidites. An additional short review of the overlying Uppermost Cretaceous and Palaeogene deposits (4 megafacies units) enabled a better insight into the post-platform evolution. The very complex vertical and lateral alternation of different megafacies units, including emerged areas which were observed throughout the studied sequence in different parts of the Karst Dinarides, indicate the significant palaeogeographic dynamics of the region. This variability resulted from interaction of the global eustatic signal and local factors, including extensive organic production on the carbonate platform and synsedimentary tectonics controlled by the specific palaeogeographic position of the platform during its geological history

The Karst Dinarides are Composed of Relics of a Single Mesozoic Platform: Facts and Consequences

Croatian geological literature during the mid nineteen-eighties and nineties was marked by the appearance of a new geotectonic concept of the Dinarides proposing that the recent, very complex structural setting is a direct consequence of a specific palaeogeographic relationship during the Mesozoic, i.e. the proposed existence of two carbonate platforms separated by a long-lasting deep marine area (interplatform trough). Although the idea was very interesting and provoked discussion, resulting in the questioning of formerly established concepts, detailed analysis of available data indicate that the Karst Dinarides (External Dinarides) were formed by the destruction of a single, although morphologically considerably variable shallow water carbonate platform. This platform was in some periods very dynamic because of its palaeogeographic position during the Mesozoic, resulting in many periods of large-scale facies differentiation, especially during the Late Cretaceous. The final disintegration of the platform area culminated in the formation of flysch trough(s) in the latest Cretaceous and Palaeogene and the subsequent uplift of the Dinarides. Recently there have been some misunderstandings resulting from the imprecise use of newly established terms, which are, by circular logic, used to connect recent geotectonic relationships with Mesozoic palaeogeography without adequate material proof. Therefore, the terms Dinaricum and Adriaticum should be redefined and used only for description of the recent tectonic pattern, without implying a palaeogeographic component, since during the Mesozoic they represented a single entity. Additional confusion is added by different names used for the same shallow water carbonate platform. Probably the best, although not the ideal name is the most frequently used one: the Adriatic Carbonate Platform. Its duration may be estimated from the Late Lias to the Late Cretaceous, representing the most important part of a thick carbonate succession in the Karst Dinarides (ranging from Carboniferous to Eocene)

Palaeogeographic Variability and Depositional Environments of the Upper Jurassic Carbonate Rocks of Velika Kapela Mt. (Gorski Kotar Area, Adriatic Carbonate Platform, Croatia)

Synsedimentary tectonics caused significant differentiation of sedimentary environments of the Adriatic Carbonate Platform during the Kimmeridgian. The most important changes have been recorded in W and central Croatia: along the NW part in present day W Istria there was an emergence with bauxite deposits, while along the NE margin of the platform, in the Karlovac area, a former emerged area was submerged. Penecontemporaneously between these areas, in the wider area of Velika Kapela Mt., a shallow intraplatform trough was formed, characterised by deposition of dark mudstones with nodules and thin layers of cherts and thin interbeds of tuffs in the upper part of the succession. Occurrences of planktonic foraminifera, radiolarians, calcisphaeres and rare ammonites indicate the sporadic influence of the open sea. Along the margins of the trough, peri-reefal environments were established, with flourishing developments of different reef-building organisms – hydrozoans, stromatoporoids, corals and bryozoans. Reefs were continuously destroyed, and in this way derived material was reworked and transported towards the trough slopes. An enormous quantity of this material caused progradation towards the deeper central part of the area, which was gradually infilled and narrowed. In the final phase, the trough was completely infilled, and peri-reefal environments gradually disappeared, since they were covered by ooid bars, culminating in the establishment of shallow environments over the entire area. A similar situation was recorded in another contemporaneous, also tectonically formed environment – the Lemes trough, stretching from the vicinity of Bihac in NW Bosnia towards the south into Croatia, into E Lika and N Dalmatia. This trough had direct communication with the open Tethys realm, and thin-bedded and platy limestones with chert and pelagic organisms, including common ammonites, were deposited within it. The Lemes trough was also surrounded by coral–hydrozoan reefs, and it was infilled by the same depositional processes as the neighbouring trough in the area of Velika Kapela Mt., and finally covered by shallow-water deposits. Although both troughs were probably formed by the same tectonic act, and had approximately the same duration – during the Kimmeridgian and Early Tithonian – they differ according to their palaeogeographic and facies characteristics. The trough investigated in the Velika Kapela Mt. was isolated, surrounded by shallow-marine platform environments, and had only temporary, indirect contact with the open sea. The Lemes trough had a continuous connection with the open sea, as indicated by the relatively rich assemblages of pelagic organisms, especially ammonites, and is characterised by abundant cherts. However, both troughs are characterised by similar depositional sequences: both are underlain and overlain by shallow-water carbonate deposits, and they represent a consequence of a specific depositional event caused by tectonic deformation (formation of pull-apart basins) within the inner part of the Adriatic Carbonate Platform

The Origin and Importance of the Dolomite-Limestone Breccia Between the Lower and Upper Cretaceous Deposits of the Adriatic Carbonate Platform: An Example from Cicarija Mt. (Istria, Croatia)

On the NE slopes of Cicarija Mt. (N Istria) a 120-150 m thick complex composed of dolomite-limestone breccia crops out between the Lower and Upper Cretaceous deposits. This studied breccia sequence is of post-sedimentary, tectogenic-diagenetic origin. It was formed by polyphase tectonic fracture of the Upper Albian to Lower Cenomanian early- and late-diagenetic dolomite succession with relics of recrystallized limestone, which enabled very important subsequent diagenetic alteration. This included partial dissolution, dedolomitization, recrystallization and calcitization of the fine-grained, crushed dolomite matrix, and centripetal dissolution of dolomite fragments and their cementation by calcite and ferroan calcite cements, as well as the partial collapse of fragments from the roofs of dissolution cavities and limited late-diagenetic silicification (the silica surplus originating from layers of diagenetic quartz from underlying Upper Albian deposits). Such a complex pattern of different events resulted in the high variability of breccia characteristics over relatively small distances, especially near more intensively tectonized zones. The contemporaneous stratigraphic level (Lower to Upper Cretaceous transition) in other parts of the Adriatic Carbonate Platform is also characterised by predominantly late-diagenetic dolomites with relics of limestones (including local occurrences of early-diagenetic dolomites) which are, in more tectonized areas, late-diagenetically altered into tectogenic-diagenetic breccias

Glauconitic Materials from Lower Miocene Macelj-Sandstones of the Hrvatsko Zagorje, North-Western Croatia

The Lower Miocene Macelj-sandstones, from the western part of Hrvatsko Zagorje, are green in colour with variable amounts of glauconitic grains. This paper presents the results of mineralogical and some petrological analysis of three characteristic samples of these sandstones. The natural sandstone samples were analysed by polarising microscope and by X-ray powder diffraction (XRD). After separation, the pure or almost pure glauconitic materials were analysed by XRD, chemical analysis and thermal analysis (TG, DTA and DTG). The results show variation, not only in the glauconitic material of the sandstone samples, but also within individual samples. The amount of smectite layers varies from <5% to approximately 40% depending on the degree of order and the stage of glauconite evolution. This is indicated by the contents of K, Al, Fe, adsorbed water and cation exchange capacity as well as XRD powder patterns