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

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)

Qualitative–Quantitative Analyses of the Influence of Depth and Lithological Composition on Lower Pontian Sandstone Porosity in the Central Part of Bjelovar Sag (Croatia)

Results of several tests (porosity–depth graphic relation, t-test, F-test, Pearson’s R), were used to analyse and interpret the regularity in porosity values of Lower Pontian oil-bearing sandstones from the central part of Bjelovar sag. Data came from 7 cored intervals within 5 wells in the Pepelana member and from 6 cored intervals (in 4 wells) in the Poljana member. The expected porosity decrease with increasing depth was checked against lithological variations. Sandstones are mostly fine-grained lithoarenites. The detritus is composed of quartz, various micas, carbonate fragments and feldspars. Across the study area, the depth range of the sandstones varies from 430 m (top of the Pepelana member near Sandrovac) to 2046 m (base of the Poljana member near Velika Ciglena). Cores included in the analyses vary between 2.75 to 15.5 m in length. Mean porosity and relative depth data were collected for two groups: Group 1 comprised wells Pav–1, Pav–2, Rov–1, Sa–5, Sa–35, and Group 2 included well VC–1. These groups were subdivided for analysis into two (litho)stratigraphic units (Pepelana and Poljana ss.). Porosity variation within each group is explained with reference to the silt or clay fraction. Differences between the porosities of the two groups (~10% lower absolute porosity near Velika Ciglena) is the result of compaction and other processes. Interval of 400 m thickness in particular sandstone member is set as minimum value for observing influence of compaction. Such statement is based on sandstone’s tops and bottoms comparison as well as graphical presentation of relation core porosity–relative depth interval. The analysis was improved by statistical calculation of Pearson’s R, t-test and F-test, which more precisely described the relationship between porosity and depth. Using these statistical tests and regression equitation, the depth difference is calculated as 621 m in the Pepelana and 667 m in the Poljana sandstones, as the limits when the influence of compaction in the porosity–depth relationship could be noticed. Compaction was observed, in the study area, when data from Velika Ciglena are compared to data from the other wells

Quaternary Deposits as the Hydrogeological System of Eastern Slavonia

The area of eastern Slavonia, situated between the Drava and Sava rivers, comprises three geotectonic units: the eastern part of the Drava depression in the north, part of the Slavonia-Srijem depression in the south and the central Djakovo-Vinkovci plateau together with the Vukovar plateau. These units are separated by deep faults that reach the base of the Tertiary sediments. The first 200 m of Quaternary deposits are saturated with fresh water. The aim of this study was to find out whether the faults form impermeable boundaries separating the waterbearing deposits into independent hydraulic systems, or if a singular hydraulic entity exists. Results of the analysis indicate that lithological continuity of the aquifers exists along the fault zones on the margins of the Djakovo-Vinkovci and the Vukovar plateaux, which means that there is no impermeable hydraulic boundary on the watershed between the Sava and Drava river valley. The part of eastern Slavonia between the Sava and Drava rivers is one hydraulic system consisting of zones with different transmissivity values. In the zones of reduced transmissivity, the hydraulic connections are weakened, but not broken. Such zones exist not only along the fault zones of the Djakovo-Vinkovci plateau and the Vukovar plateau, but also within the Sava and Drava depressions. The terrain morphology influenced formation of both the surface and the underground watershed, parallel to the extension of the Djakovo-Vinkovci and Vukovar plateau. Therefore, within this single hydraulic entity, when the draw-down reaches the watershed due to excessive pumping, the watershed will be displaced from its natural position

Lithologic Composition and Stratigraphy of Quaternary Sediments in the Area of the “Jakusevec” Waste Depository (Zagreb, Northern Croatia)

In the area covered by the “Jakusevec” waste depository, to a depth of 101 m, six lithological units were determined based on fieldwork and laboratory geologic-geophysical investigations. It was discovered that the silty-clayey units (units 1, 3 and 5) are covered by sandy-gravely (units 2 and 4) and gravely ones (unit 6), respectively. These units constitute the sediments of the Middle and Upper Pleistocene and Holocene and are separated by erosional unconformities. The Pleistocene gravels are predominantly of quartz-quartzite composition, while the Holocene ones are composed of carbonate cobbles and pebbles. In contrast, the sands exhibit a fairly uniform mineral composition throughout the column. The Pleistocene silt and clay are mostly composed of muscovite-illite and quartz with lesser amounts of chlorite, kaolinite and smectite. There is a difference in composition of this fraction in unit 6, where the quartz, calcite and dolomite particles prevail and smectite and illite/smectite are absent. Unit 3 is characterised by the goethite content. The Pleistocene layers were formed in a lacustrine-marshy environment while the Holocene sediments are fluviatile. This sedimentary sequence is interrupted by occasional terrestrial phases, or drying-up periods, dependent on the palaeoclimate conditions, particularly the interchange of cold and dry glacials with the warmer and more humid interglacial stage

Origin of Hydrocarbons in the Eastern Part of the Drava Depression (Eastern Croatia)

Oil and gas in the eastern part of the Drava depression originated from source rocks of very good generating capabilities - dark gray to black marlstones of Ottnangian, Karpathian, Badenian, Sarmatian and partly Pannonian age. During different tectonic regimes, algal-lipid rich kerogen formed in anoxic and suboxic marine environments. The typical terrestrial maceral vitrinite only occurs as traces

Quaternary Deposits as the Hydrogeological System of Eastern Slavonia

The area of eastern Slavonia, situated between the Drava and Sava rivers, comprises three geotectonic units: the eastern part of the Drava depression in the north, part of the Slavonia-Srijem depression in the south and the central Djakovo-Vinkovci plateau together with the Vukovar plateau. These units are separated by deep faults that reach the base of the Tertiary sediments. The first 200 m of Quaternary deposits are saturated with fresh water. The aim of this study was to find out whether the faults form impermeable boundaries separating the waterbearing deposits into independent hydraulic systems, or if a singular hydraulic entity exists. Results of the analysis indicate that lithological continuity of the aquifers exists along the fault zones on the margins of the Djakovo-Vinkovci and the Vukovar plateaux, which means that there is no impermeable hydraulic boundary on the watershed between the Sava and Drava river valley. The part of eastern Slavonia between the Sava and Drava rivers is one hydraulic system consisting of zones with different transmissivity values. In the zones of reduced transmissivity, the hydraulic connections are weakened, but not broken. Such zones exist not only along the fault zones of the Djakovo-Vinkovci plateau and the Vukovar plateau, but also within the Sava and Drava depressions. The terrain morphology influenced formation of both the surface and the underground watershed, parallel to the extension of the Djakovo-Vinkovci and Vukovar plateau. Therefore, within this single hydraulic entity, when the draw-down reaches the watershed due to excessive pumping, the watershed will be displaced from its natural position

Interdependence of Petrophysical Properties and Depth: Some Implications of Multivariate Solution on Distinction Between the Lower Pontian Hydrocarbon-bearing Sandstone Units in the Western Part of the Sava Depression

Statistical analysis of reservoir data from the Lower Pontian clastics (the most important hydrocarbon reservoir rocks in the Sava depression), supports established knowledge of the interdependence of petrophysical properties and depth. Irrespective of the focus that the reservoir data may be studied and presented, depth always emerges as a fundamental reservoir descriptor. This is particularly evident when studying the differences between widely spaced oil and gas fields, when the numerical model completely separates the two sets of descriptor variables, indicating two different sources of their internal variability. Porosity and permeability belong to “intrinsic rock properties” while depth must be ascribed to other sources, e.g. tectonic subsidence. Discriminant function weighted with depth (DF1) has such group centroid values, that zones can be drawn within a particular field that coincide with structural relationships. On the function marked with reservoir properties (DF2), group centroid values are higher close to the axes of palaeotransport channels, where sandstone layers are the thickest and particles are best sorted. Group centroid values on the third function (DF3) depict the areas of relatively higher permeability in the apical parts of structures, possibly caused by fracturing due to folding, or by cementation of other parts of reservoirs, where the circulation of pore waters was more pronounced. In the case of the most thoroughly investigated Zutica field, the inverse relationship between depth and porosity becomes evident when compared with the direction of palaeotransport and thickness of reservoir rocks on the respective structure and thickness maps

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)