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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 Tithonian carbonates of the southern part of the Adriatic Platform are more than 700 meters thick, and have an accumulation rate of up to 15 cm/k.y. during a time of relatively uniform subsidence. The platform interior consists of numerous shallowing upward parasequences 1 to 3 meters thick. Facies include dasyclad and oncoid wackestone, skeletal packstone-grainstone, lime mudstone, and microbial and fenestral laminites. Late Tithonian parasequences commonly contain radial-ooid grainstone that occurs at transgressive bases of parasequences, or less commonly, in upper regressive parts of parasequences. Fischer plots are a simple graphical method to define relative sea level changes on "cyclic" carbonate platforms, by graphing cumulative departure from mean cycle thickness (corrected for subsidence) as a function of time. The preliminary plots of the Late Jurassic data suggest that the Fischer plots are effective in extracting 3rd order accommodation cycles from the data, but there is only a weak 4th order (100 and 400 k.y. signal) evident. The accommodation plots measure the combined effects of sea-level plus subsidence. Accommodation (Fischer) plots of the Late Jurassic cyclic succession show four 3rd order accommodation events (0.5 to 5 m.y. long). Differences between the plots for the Adriatic Platform, the Arabian Platform and the Haq and Al-Qahtani (2005) curve may in part be due to the effects of local tectonics. A combination of spectral analysis of time series constructed using some measure of relative water depth plotted against stratigraphic thickness, plot of cycle thickness vs. cycle number, and Fischer plots are being used to examine the possible influence of Milankovitch forcing on the formation of the platform interior cycles.

Although the Mesozoic has long been considered greenhouse, the stacking patterns on Jurassic-Cretaceous platforms suggest that some intervals have formed under sea level changes that are too large for an ice free world. This is manifested in juxtaposition of facies or biotas in parasequences that represent 10 to 30 m shifts in water depths in excess of that expected from simple shallowing of several meters (the parasequence thickness). The presence of obliquity and short and long term eccentricity cycles also is suggestive of ice rather than the abundant precessional cycles that typify greenhouse platforms. We present data showing that the Late Tithonian of the Adriatic Platform represents a greenhouse phase. Well exposed Mesozoic sections along the southern Dalmatian islands (southern Croatia) can be used to evaluate the Jurassic-Cretaceous paleoclimate. The detailed stacking patterns of facies within the Late Jurassic (Tithonian) shallow platform interior (over 700 m thick) were examined. Facies include deeper lagoon dasyclad wackestone, oncoid wackestone, shoal-water skeletal and radial ooid packstone-grainstone, restricted lagoon lime mudstone, tidal flat microbial and fenestral laminites. They are arranged in 1 to 3 m thick parasequences. Parasequences likely are precessional (roughly 20 k.y. given the accumulation rates). They are arranged in sets 8 to 15 m thick composed of 3 to 6 parasequences, and possibly 100 k.y. duration. Lower Tithonian sets are dominated by subtidal parasequences passing up into thin, more shallow water parasequences. Upper Tithonian parsequences commonly have radial ooid grainstones at the base, and less commonly in upper regressive parts. Upper Tithonian parasequence sets have well developed tidal flat facies in upper parts. Estimation of water depths using the fenestral/microbial carbonate as a sea-level datum, shows that the subtidal facies formed in water depths of less than 5 m, and had overlapping depth ranges, implying mosaic-like facies patterns. These shallow water depths support very small precessional sea level changes forming the parasequences, compatible with very warm greenhouse climate. The high Tithonian subsidence rates favored relatively complete preservation of precessional cycles with few missing beats ; this also required that any eccentricity signal was very small, otherwise many precessional beats would not have been deposited on the platform top. Emergence near the Jurassic/Cretaceous boundary formed extensive sequence-bounding subaerial clayey and limestone breccia horizons interstratified with lowermost Cretaceous (Berriasian) cyclic carbonates. These Early Cretaceous emergence events were due to long term relative sea level fall, coupled with decreased subsidence rates. Thus the Late Tithonian appears to be a very warm greenhouse phase in the mid-Jurassic-Early Cretaceous cool mode. The Jurassic-Cretaceous sections of the Adriatic platform in Croatia may provide important paleoclimatic data for this time interval during which some of the world's major petroleum reservoirs were generated.

Introduction: Studying heavy mineral assemblages of beach sands and possible source rocks can often gain useful insight concerning supply patterns by which beaches are nourished with detritus. Beaches represent a delicate environment, one that is easily subject to destructive processes if imbalance is created between the supply of detritus and erosion. Such insights are thus of particular importance considering the rising pressure the Adriatic natural coast is experiencing with increased development of tourism and industry in recent years. Previous research on beach sands from the Island of Rab (CRNJAKOVIĆ et al., 1998) has shown that considerable variability exists in their heavy mineral compositions. The proposed sources of detritus were Eocene and Pleistocene siliciclastic rocks, which are present over a considerable area of the Island. In the current study new data is being obtained from both ancient rocks and recent beach sands with the intent of more precisely defining the source-beach relationship. Materials and Methods: Beach sands have been sampled at 8 beach locations (Figure 1). Several additional samples have been taken from Eocene and Pleistocene arenites to test similarities in heavy mineral associations. Obtaining heavy mineral concentrates was performed by commonly used procedures (MANGE & MAURER, 1992). Additionally, particle composition of arenites has been studied using thin sections made from artificially cemented sands. Results: Transparent heavy mineral associations from beach sands show specific differences. Most conspicious are the differences in the proportion of garnet and the ultrastable minerals zirkon, tourmaline and rutile. Garnet proportion varies between 12% and 72%, while ultrastable minerals vary from 11% to 67%. A higher proportion of garnet relates to a lower proportion of ultrastable minerals, and vice versa. Among the other most common heavy minerals are epidote/zoisite, Cr-spinel, staurolite and kyanite. Discussion and conclusion: The consistent variability in the garnet versus ultrastable mineral proportions in heavy mineral associations of the studied beach sands seems to be a consequence of the derivation of these sands. A comparison of Eocene and Pleistocene arenites shows similar difference. While the Eocene heavy mineral associations resemble that of the garnet-rich and ultrastable mineral-poor beach sands, the Pleistocene heavy mineral associations are similar to those beach sands rich in ultrastable minerals and poor in garnet. These relationships suggest that some beaches are dominantly supplied from source areas consisting of Eocene sandstones, some dominantly from source areas consisting of Pleistocene sands, while several beaches show HM associations suggesting a mixed source. Such relationships between beach sands and Eocene/Pleistocene arenites have previously been suggested based on the study of three beaches of the Island of Rab (CRNJAKOVIĆ et al., 1998), and the beaches in northern Dalmatia (PAVIČIĆ et. al., 2000). This study confirms previously proposed interpretations and shows that the relevant consistencies in heavy mineral variability in beach sands represent a rule for the northern half of the Croatian Adriatic coast. As the spatial relationship of beaches and their interpreted type of source areas show close vicinity, it follows that the supply of detritus to the beaches is greatly controlled by immediate hinterland erosion. Other possible supply paths such as longshore drift or a derivation from a distant source are much less significant. In conclusion, the source areas and the natural nourishment pathways of beach sands for the studied coasts can be successfully identified using a heavy mineral approach. The result of this is useful knowledge gained pertaining to the problem of sustaining Croatian beaches (PAVIČIĆ et. al., 2000). The need for evaluating how relevant natural processes will change with accelerated development of the coastal zone is growing. Otherwise, ignorance will inevitably bring damage to this natural and economic resource of the country.