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Shallow-water carbonate succession from Jazvina locality (Gorski kotar, Croatia): an example of typical Late Jurassic sedimentary events on the Adriatic carbonate platform

At the Jazvina locality, five facies units have been recognized ; the Jazvina 1 unit (Jz-1), with mudstones and pelletal wackestones ; the Jazvina 2 unit (Jz-2), with bioclastic floatstones and grainstone/rudstones ; the Jazvina 3 unit (Jz-3), with peloidal-skeletal wackestones and packstones ; the Jazvina 4 unit (Jz-4), composed of shallowing and coarsening-upward cycles with mudstones or pelletal wackestones as the lower cycle types, fenestral mudstones or pelletal wackestones as the middle cycle types, and ooid grainstones as the upper cycle types ; and the Jazvina 5 unit (Jz-5), composed of shallowing- and coarsening-upward cycles that differ from the underlying Jz-4 cycles by the presence of the pisoid-intraclastic grainstone/rudstones as the upper cycle types. The depositional environment for the Jz-1 unit is interpreted as a shallow, low-energy lagoon below the fair-weather wave-base, situated in the inner platform region. The Jz-2 unit is typical example of bioclastic carbonate sediments deposited on a carbonate platform in high energy shoals, in which large quantities of fossil debris, transported by waves and tidal currents, have been accumulated. High carbonate mud content within the Jz-3 unit suggests a subtidal depositional environment. However, contrary to the depositional environment of the Jz-1 unit, rich foraminiferal content (particularly in the packestones) indicates better water circulation above the fair-weather wave-base and thus more favorable ecological conditions than those in the Jz-1 unit. Within the Jz-4 unit, three sedimentary environments with different depositional styles can be recognized. By periodically changing conditions, ranging from shallow subtidal to oolite shoals, series of shallowing- and coarsening-upward cycles have been produced. The first two facies types of the Jz-5 unit originated under similar conditions as the first two cycle types of the underlying Jz-4 unit. However, after the final emergence of the middle cycle types, carbonate detritus (mainly intraclasts), thrown onto the emergent surface from the adjacent subtidal environments by action of storm waves and high tides, was exposed to vadose diagenesis. During these periods, some intraclasts developed pisoid envelopes, and internal sediment was produced.

Shallow-water carbonates; Late Jurassic; Gorski kotar; Croatia

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