Geochemical (δ13C, δ18O 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 δ13C 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 δ13Ccarb 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.
The primordially structural-lithofacial relationships in the Adriatic Carbonate Platform (AdCP) of Croatia were formed by a Late Cretaceous synsedimentary tectonics. During Cenomanian, an extensional tectonic regime differentiated AdCP into several kilometres large paleoenvironmental segments which behaved as individual depocenteres. The latest Cenomanian and earliest Turonian were tectonically relatively quiet periods during which sediments only recorded a relative sea-level rise. Compression commenced during the middle Santonian and formed first (NW-SE) gentle folds in the frontal part of the Split-Dubrovnik thrust. These folds had amplitudes of tens to hundreds of metres and are up to ten kilometres in strike. The apical parts of the anticlines were dominated by shallow-marine deposition with short emergences simultaneously, slope deposition of pelagic sediments took place in the synclines. By the end of the Campanian, compression weakened and younger sediments infilled former depressions while the deposition ended in the Adriatic hinterland of Croatia. During the Maastrichtian the compression recommenced and the index of older folds increased while new folds and reverse faults were formed. Such deformations created a differentiated morphology at the surface subsequently overlaid by Palaeogene sediments. Clastic sediments accumulated indeed in this paleodepression during the Palaeogene and Miocene-Quaternary, forming favourable structural conditions for hydrocarbon generation.
13 The Jurassic-Cretaceous dolomites, Adriatic platform, Croatia resulted from climate-influenced 14 post-depositional reflux dolomitization (as opposed to synsedimentary peritidal-and deeper 15 burial dolomitization), and subsequent stabilization within a Mesozoic, "calcite sea" isolated 16 platform. The dolomites are stratiform (10 to 200 m thick), fabric destructive (20 to over 500 μm 17 crystals), nonluminescent, and zoned with respect to Ca. Bulk dolomites have low Mn (10 to 30 18 ppm), moderate Sr (60 to over 200 ppm), positive δ 13 C and δ 18 O values, only moderate ordering 19 (0.25 to 0.6) and single-phase fluid inclusions (temperatures <50 °C). High-Ca dolomite (HCD; 20 ~53 to ~59 mol% Ca; δ 18 O +1 to >+3‰ VPDB and Sr>100 ppm) composes most Jurassic and 21 40% of Cretaceous dolomites, making up turbid dolomite cores and initial clear dolomite rims. A 22 first generation of low-Ca dolomite (LCD-1; 50 to 53 mol% Ca; δ 18 O +1 to >+3‰ VPDB; 100-23 180 ppm Sr) forms cement and variably replaces (stabilizes) earlier HCD cores. HCD and LCD-24 1 formed in refluxing marine-dominated pore waters under semi-arid climate (<300m depth, 30 25 to ~40 °C). 87/86 Sr values of the HCD and LCD-1 dominantly were controlled by the refluxing 26 seawater 87/86 Sr with minor addition of 87/86 Sr from ascending cooling fluids. During maturation 27 of HCD, Sr loss was greatest from least stable, more calcic HCD (>55 mol%) phases. A second 28 generation of much younger, low-Ca dolomite (LCD-2; fracture-associated, more negative δ 18 O 29 from -1.4 to +1‰ VPDB, Sr <100 ppm, enriched in radiogenic 87 Sr) overgrows and variably 30 replaces earlier Cretaceous dolomites. It formed during Eocene-Oligocene deformation and uplift 31 to <1 km burial depths (<50 °C) from synorogenic, mixed marine-meteoric pore fluids, 32 circulating via faults, fractures and local permeable zones. Porosity loss by dolomite cementation 33 coupled with localized, rapid Cenozoic uplift, variably arrested stabilization to LCD. 34 35 3 Keywords: isolated platform, post-depositional dolomite, geochemistry, mesohaline reflux, 36 stabilization 37 38
Abstract:In this paper we present the results of the investigations on the Badenian (Middle Miocene) ostracods from the Pokupsko area. For the first time the presence of Badenian aged sediments in Croatia can be supported by the occurrence of ostracod biozonal markers. Four Badenian ostracod zones are established: Lower Badenian Biozone NO7 Acanthocythereis hystrix-Bythocypris lucida, Middle Badenian Biozone NO8 Eocytheropteron inflatum-Olimfalunia spinulosa, and the two Upper Badenian Biozones NO9 Neomonoceratina laskarevi-Miocyprideis sarmatica elongata and NO10 Carinocythereis carinata-Phlyctenophora farkasi. On the basis of the generally accepted paleoecology of selected genera, we identified the following ostracod faunas: shallow-water marine, shallow-water brackish-marine, shallow-water reef, and deep-water marine. The paleontological and trace element analyses suggest that the Pokupsko ostracod fauna lived in shallow (50 m deep), warm, and limpid waters, connected to a deeper sea and occasionally exposed to freshwater inflows.
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