Igor Vlahović

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.

Eocene Limestones Overlying Lower Cretaceous Deposits of Western lstria (Croatia): Did Some Parts of Present lstria Form Land During the Cretaceous?

In the region of the western Istrian anticline, erosional remains of the transgressive Eocene Foraminiferal limestones overly shallow water deposits of Early Cretaceous age (Valanginian, Hauterivian and Barremian). This evidence, together with the ocurrencc of bauxite deposits and Palaeogene beds overlying Albian, Cenomanian and Senonian limestones indicate the very high relief of the land transgressed by the Palaeogene sea. Contrary to the traditional connection beetween formation of the western Istrian anticline and the Laramian tectonic phase at the end of the Cretaceous (Maastrichtian), new investigations suggest sporadic tectonic movements with constant tectonic coordinate orientation from the Hauterivian to the end of the Cretaceous. With these movements, formation of the anticline structure began in the Early Cretaceous, with the emerged apical parts representing land areas throughout most of the Cretaceous.

Carbonate facies evolution from the late albian to Middle Cenomanian in Southern Istria (Croatia): influence of synsedimentary tectonics and extensive organic carbonate production

SummaryDuring the Late Albian, Early and Middle Cenomanian in the NW part of the Adriatic Carbonate Platform (presentday Istria) specific depositional systems characterised by frequent lateral and vertical facies variations were established within a formerly homogeneous area, ranging from peritidal and barrier bars to the offshore-transition zone. In southern Istria this period is represented by the following succession: thin-bedded peritidal peloidal and stromatolitic limestones (Upper Albian); well-bedded foreshore to shoreface packstones/grainstones with synsedimentary dliding and slumping (Vraconian-lowermost Cenomanian); shoreface to off-shore storm-generated limestones (Lower Cenomanian); massive off-shore to shoreface carbonate sand bodies (Lower Cenomanian); prograding rudist bioclastic subaqueous dunes (Lower to Middle Cenomanian); rudist biostromes (Lower to Middle Cenomanian), and high-energy rudist and ostreid coquina beds within skeletal wackestones/packstones (Middle Cenomanian).Rapid changes of depositional systems near the Albian/Cenomanian transition in Istria are mainly the result of synsedimentary tectonics and the establishment of extensive rudist colonies producing enormous quantities of bioclastic material rather than the influence of eustatic changes. Tectonism is evidenced by the occurrence of sliding scars, slumps, small-scale synsedimentary faults and conspicuous bathymetric changes in formerly corresponding environments. Consequently, during the Early Cenomanian in the region of southern Istria, a deepening of the sedimentary environments occurred towards the SE, resulting in the establishment of a carbonate ramp system. Deeper parts of the ramp were below fair-weather wave base (FWWB), while the shallower parts were characterised by high-energy environments with extensive rudist colonies, and high organic production leading to the progradation of bioclastic subaqueous dunes. This resulted in numerous shallowing- and coarsening-upwards clinostratified sequences completely infilling formerly deeper environments, and the final re-establishment of the shallow-water environments over the entire area during the Middle Cenomanian.

Cenomanian carbonate facies and rudists along shallow intraplatform basin margin-the island of Cres (Adriatic Sea, Croatia)

SummaryThe island of Cres is located in the northern part of the Adriatic Sea. The island is built up of predominantly Cretaceous carbonates deposited in north-western part of extensive and long-lasting Adriatic Carbonate Platform. Owing to the influence of synsedimentary tectonics supported by eustatic changes during the latest Albian/Early Cenomanian, different sedimentary environments were established: from shallow intraplatform basin and related slope, across basin margin to protected shallow-platform. During the Early to Middle Cenomanian rudist communities (ichthyosarcolitid/caprinid/radiolitid)flourished along a relatively high-energy intraplatform basin margin. Fair amounts of coarse-grained bioclasts, derived almost exclusively from broken rudist shells, were deposited over a marginal depocenter. Contemporaneously, pithonellid wackestone-packstones containing microbioclasts and planktonic foraminifera were deposited basinward while marginal bioclastic sediments and limestone blocks of the basin margin origin were sporadically deposited within the basin. The opening of the Cres intraplatform basin was aborted and the basin was finally filled up during the Late Cenomanian. Since the Cres intraplatform basin was established at the beginning of the Cenomanian it probably represented the initiation phase in the north-western extension of the later Adriatic Trough development.

Foraminiferal Assemblages in the Cenomanian of the Buzet-Savudrija Area (Northwestern Istria, Croatia)

In the Cenomanian shallow-water carbonates of Northwestern Istria five biostratigraphic zones have been established: CEN-1 Ovalveolina maccagnoae and Sellialveolina viallii biozone in the early Lower Cenomanian; CEN-2 O. maccagnoae to Chrysalidina gradata interval zone in the late Lower Cenomanian; CEN-3 C. gradata biozone in the early Middle Cenomanian, locally subdivided into two subzones: CEN-3a C. gradata and orbitolinids (lower part) CEN-3b C. gradata without orbitolinids (upper part); CEN-4 C. gradata and Broeckina (P.) balcanica biozone in the late Middle Cenomanian, and CEN-5 C. gradata, B. (P.) balcanica and Vidalina radoicicae biozone in the Upper Cenomanian.

Carbon-isotope record and palaeoenvironmental changes during the early Toarcian oceanic anoxic event in shallow-marine carbonates of the Adriatic Carbonate Platform in Croatia

Geochemical (δ 13 C, δ 18 O and Mn) compositions of Lower Jurassic shallow-water carbonates cropping out in Croatia were analyzed to elucidate the impact of the early Toarcian oceanic anoxic event (T-OAE) on the Adriatic Carbonate Platform (AdCP). The bulk-rock carbon-isotope records through the studied sections (Velebit-A, Velebit-B and Gornje Jelenje) are characterized by two significant excursions: (i) an initial positive trend interrupted by a pronounced negative shift ( c . 2.5‰) that is followed by (ii) an increasing trend of positive values (up to 4.5‰). A comparison with δ 13 C trends obtained from well-calibrated sections from other localities in Europe shows that the overall character of the early Toarcian negative excursion is clearly reproduced in the curves derived from Croatian shallow-water deposits, which helps to date the sequences and reinforces the global character of the carbon-cycle perturbation. Lower Jurassic sedimentary successions in the studied area show a gradual deepening trend corresponding to deposition of the Toarcian spotted limestones. Assuming that the distinctive negative excursion in the carbon-isotope curves is synchronous across the AdCP, the contact between the spotted limestones and the underlying beds rich in lithiotid bivalves appears to be diachronous within the study area. The Mn record through the Croatian Velebit-A section and, in particular, the rise in concentration (up to 100 ppm) coinciding with the beginning of the δ 13 C carb positive shift, reflects a change in the redox conditions in seawater that allowed diagenetic incorporation of reduced manganese into the calcite structure of the carbonate sediment during the onset of the T-OAE.

Permian deposits and the Permian–Triassic boundary in Croatia: palaeoclimatic implications based on palaeontological and geochemical data

Abstract The Late Palaeozoic deposits in Croatia were derived from the Palaeotethyan shelf areas along the northern Gondwana passive continental margin and consist of mixed carbonate–clastic facies. These sedimentary rocks accumulated in the equatorial area along northern Gondwana under a dominantly hot climate and laid the foundations for the future Adriatic Carbonate Platform (AdCP). Detailed classification and geochemical studies of the Permian and Permian–Triassic deposits in most parts of Croatia are still in progress, especially concerning palaeoclimate and regional palaeogeography. The best preserved Permian outcrops can be found in the Velebit mountain area, including continuous Permian–Triassic sections. Stable isotopes of carbon and oxygen from carbonates, with characteristic positive δ13Ccarb excursion during the Late Permian and negative δ13Ccarb excursion at the Permian–Triassic transition combined with palaeontological and sedimentological data, helped in palaeoclimate estimations and the positioning of the chemostratigraphical Permian–Triassic Boundary (PTB). Sedimentological, palaeontological and geochemical data indicate that deposition in the Permian took place mainly in shallow-water tropical areas under oligotrophic conditions. High carbon isotope ratios in Capitanian carbonate rocks (+4 to +6‰ VPDB) mark the ‘Kamura event’ characterized by temporary cooling, high productivity and burial rate. Evaporites in the central part of the Karst Dinarides and red beds of the ‘Gröden type’ indicate a warm and mostly arid climate. Calculations based on the oxygen isotopic compositions of whole-rock carbonate and fossil samples estimate latest Permian temperatures up to 26 °C.

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.

First rock magnetic and palaeomagnetic analyses of the Pre-Cenozoic rocks of the Velebit Mt. (Croatia): prospects for applications in palaeogeographic and geotectonic studies

Post-Carboniferous deposits of the Karst Dinarides were analysed in terms of their palaeomagnetic and rock magnetic properties in order to identify the best sections for comprehensive palaeomagnetic studies on geodynamic and palaeogeographic evolution of this mountain belt. A total of eighteen reconnaissance rock samples have been collected from the Permian red beds and limestones, and limestones of Triassic, Jurassic, Cretaceous and Eocene age, mainly from the Velebit Mt., part of the Dinaric orogenic belt finally uplifted in Oligocene–Miocene, and stretching NW–SE along the NE Adriatic coast. Experimental studies comprised identification of potential carriers of a natural remanent magnetization (NRM) by means of their properties of Curie temperature and coercivity accompanied by SEM and microprobe studies, demagnetization of standard palaeomagnetic specimens by means of alternating field and thermal cleaning, as well as measurement and analysis of magnetic susceptibility and its anisotropy. Haematite (in Permian red beds) and Ti-magnetite (in limestones) and sporadically goethite have been identified as remanence carriers. State of preservation, ascertained by SEM observations, suggests detrital origin of NRM in most of the samples. Characteristic NRM components (ChRM), both of high coercivity and unblocking temperatures, could be clearly identified in most of the specimens. These components show smeared distribution on the sphere and do not cluster around any known palaeomagnetic direction expected for the Velebit area from apparent polar wander paths, neither for Eurasia nor for Africa. This outcome suggests lack of overall remagnetization of the studied part of the Karst Dinarides after the last tectonic event and/or important contribution from tectonic rotations to the observed pattern of ChRM distribution.

Stratigraphy and Tectonic Relationships Along the Senj - Ogulin Profile (Velika Kapela Mt., Croatia)

Twelve lithostratigraphic units, representing a 5 km thick succession, have been determined from the Senj -Ogulin profile through the Velika Kapela Mt. This 45 km long sequence lies approximately normal to the Dinaric strike. Carbonate deposits ranging from the Middle Liassic to the Albian include laterally variable environments during the Kimmeridgian and Lower Tithonian. The deposits were deformed by compressive tectonics during the Tertiary tectonic cycle, and were consequently reshaped by weaker Neotectonic transpression. Therefore structures with the N-S strike, which are different to the common Dinaric strike (NW-SE), are interpreted as a consequence of syngenetic bending during the Tertiary cycle, rather than rotation during the Neotectonic cycle.