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First Direct Evidence of Jurassic Evolution of the Central Adriatic Basin Based on Radiolarian Biostratigraphy

Palagruža is an isolated group of islets (small archipelago) situated in the central Adriatic Sea (Croatia, northern Mediterranean) and represents a central part of the common Adriatic foreland of the Dinaridic and Apenninic orogenic belts (SCISCIANI & CALAMITA, 2009). On palaeogeographic maps this area appears within the Adriatic Basin, situated between Apulia and Adriatic carbonate platforms (GELLETI et al., 2008). Mesozoic pelagic successions of the central Adriatic Basin are buried today beneath Cenozoic deposits and the Adriatic Sea, and are known only from boreholes and seismic profiles. Because of active diapirism in the region (GELLETI et al., 2008), Palagruža is the only place where some of these units are exposed above sea level (KORBAR et al., 2009, 2012). However, because of their distance from the mainland, steep slopes and rocky coast, access to these islets is difficult and information about their geology is still sparse. Here we present the first preliminary results on stratigraphy based on radiolarians from Mala Palagruža islet. The islet of Mala Palagruža is built predominantly of unique succession of thin-bedded siliceous dolomites alternating with chert (Figure 1) that is recognized as a specific lithostratigraphical unit - Mala Palagruža formation, tentatively assigned to a late Triassic to early Jurassic age (KORBAR et al., 2012). Based on correlation with southwestern Apennines proximal or transitional carbonate platform- to-basin environment is suggested for these deposits. During this research poorly preserved radiolarian fauna was extracted from a chert sample after treatment with diluted hydrofluoric acid (HF 5%). Identified radiolarians indicate Middle Jurassic age for this sample according to zonation by BAUMGARTNER et al. (1995) based on Unitary Associations (UA). Maximum range of the sample is late Bajocian (UA Zone 4) to late Bathonian-early Callovian (UA Zone 7) defined by the presence of Protunuma turbo MATSUOKA. Stichomitra(?) takanoensis AITA and Dictyomitrella(?) kamoensis MIZUTANI & KIDO, associated in the sample, both also have their last appearance in UA Zone 7, giving further proof that the sample can not be younger than late Bathonian-early Callovian. A more precise age constraint is not possible because of poor preservation of radiolarians. These new data provide a rare insight into poorly known stratigraphy of the deeply buried early Adriatic Basin succession and its Jurassic evolution.

Radiolarians, Jurassic, Palagruža, Adriatic Basin

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