engleski

Jurassic paleoenvironmental associations from the marginal depositional system of the Adriatic carbonate platform (Žumberak Mt., Croatia)

A Jurassic marginal depositional system of the Adriatic carbonate platform was analyzed in order to determine its depositional architecture and mayor depositional controls. Based on facies characteristics, four paleoenvironmental associations have been distinguished: (1) Top of the platform ; during the early and middle Early Jurassic the deposition took place mostly in the subtidal zone where successive coarsening-upward cycles were produced as the interplay of allocyclic and autocyclic depositional processes, (2) Upper foreslope ; during the late Early Jurassic the newly formed Adriatic carbonate platform experienced extensional faulting in its northeastern part in connection with rifting processes due to opening of the Dinaridic branch of the Neo-Tethys, which led to formation of the platform slope (ŠIKIĆ & BASCH, 1975 ; ŠIKIĆ et al., 1979). The occurrence of calcitized radiolarians, protoglobigerinids and skeletal fragments of pelagic bivalves or their prodissoconchs clearly points to deeper water conditions, (3) Toe-of slope ; from the late Early Jurassic to the early Late Jurassic faulting induced periodic seismic activity triggered multiple mechanical disintegration of the platform slope resulted in gravity mass movements along the slope and accumulation of calciturbidite interlayers with T-a Bouma features in the toe- of-slope environment. Also, sporadic more intense seismic shocks produced angular fragments of already consolidated deposits on the platform slope that were gravitationally moved down along the slope, forming a thick carbonate breccia wedge in the toe-of-slope environment. A multiple accumulation process can be assumed since pretty thick interval of breccia wedge was formed, and (4) Basin ; thin beds represent autochthonous pelagic carbonate mud deposition ("pelagic rain") within the deep-water environment distant from the platform margin, as indicated by the lack of any sedimentary structures and coarse-grained bioclastic intercalations. More argillaceous intervals correspond to increased influx of fine- grained siliciclastic detritus, probably derived from the north, i.e. from the Hercynian ranges (PAMIĆ et al., 1998). Therefore, the environmental changes are interpreted to be related to tectonic activity as a consequence of regional extensional movements, connected with the opening of the Dinaridic branch of the Neo-Tethys. These extensional movements resulted in multi-stage drowning on the northeastern part of the Adriatic carbonate platform, leading to its gradual back stepping and accordingly expanding of the Dinaridic branch of the Neo-Tethys basin (BABIĆ, 1976).

Jurassic ; Adriatic carbonate platform margin ; Žumberak Mt.

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