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Complex geological structure of western Biokovo Mt. on the route of Sv. Ilija tunnel

Tunnel “Sv. Ilija” has length of 4440 m and overlays of 1300 m that, together with inaccessible terrain at the tunnel route, makes an unique problem for geological interpretation. During geological mapping for the purpose of drawing a prognostic geological section, a numerous new data have been collected. Tunnel cut in its larger part through Jurassic-Cretaceous carbonate rocks, mostly limestones, that belong to different genetic and structural-tectonic units (Fig. 1). Structural complex of the western Biokovo Mt. generally conforms to the structures presented in MARINČIĆ et al. (1976), however an additional significant structural unit is made of deformed carbonate deposits of “Kotišina-Tilovica” (JELASKA et al., 2000, 2003) e.g. NE Adriatic trough (KORBAR, 2009), that is possibly a NW extension of „Budva-Cukali“ basin (CHOROWICZ, 1975). Four main structural units (from SW to NE) have been separated on the geological section. They belong to the Outer Dinarides, consisting of deformed layers deposited on different segments of the Adriatic Microplate: 1) Baška Voda – Vrulja: thick succession of intensely deformed mostly clastic flysch deposits with intercalations of grainy limestones (Eocene, Pg2) ; 2) Topići – Dubci: overturned hectometer sized anticline with SW vergence, made of rudist limestone from the Gornji Humac Formation (Coniacian-Santonian, K23-4) and overlying Paleogene Foraminiferal Limestone (Eocene, Pg2) ; 3) Bast – Gornja Brela: deformed massive deposits of Cretaceous-Paleogene slope-to-basin mostly crystalline limestone and breccia (K25-6, Pg) and overlying flysch clastics. 4) Sv. Ilija – Zadvarje: kilometer sized anticlinal structure (faulted to huge blocks) of the main West Biokovo massive, made of the Jurassic shallow-water carbonates in the southern limb (4a), popped-up fragmented core (4b) and generally monoclinal succession of the Jurassic to Upper Cretaceous shallow-water carbonates (the youngest is Gornji Humac Fm.) in the northern limb (4c). Anticline 2 is over the frontal fault thrusted upon the intensively deformed flysh of the Adriatic Segment, e.g. over the unit 1. Units 2, 3 and 4 are mutually delineated by steep (subvertical) faults. According to the Cretaceous and Paleogene stratigraphy, units 2 and 4 can be attributed to the same tectonostratigraphic unit (High Karst, Dinaridic Segment), but the unit 3 has specific development of deposits of the youngest Cretaceous and Paleogene, that suggest the complex paleogeography and polyphase tectonics. It can be assumed that deposits of the NE Adriatic trough have been squeezed through previously overtrusted parts of Dinaridic Segment (KORBAR, 2009). Structure of the West Biokovo is bordered from the NE by weekly expressed counter-directed reverse fault that reduced the thickness of the Gornji Humac Fm. (Coniacian-Santonian), overlain by the Eocene Foraminiferal Limestone. Geomechanical characteristics of limestone are closely correlated with the frequency of stratal discontinuities, but also with the structural-tectonic position of the particular unit. In such manner are massive limestones and breccias from the unit 3 highly fragmented in decimeter blocks, which is probably the consequence of their tectonic position. However, the Middle Jurassic massive limestones from the unit 4a are not so fragmented. During excavation of the tunnel occasionally thin (dm to m) occurrences of Paleogene clastics (silty claystone and clay) has been detected. These clastics have been squeezed into brecciated fault zones. In the middle part of the tunnel, there was a decameter cavern filled up with unconsolidated sediments (silt, silty clay, sand) that slow down the excavation process. Prognostic geological section has been largely confirmed during excavation of the tunnel.

Biokovo; stratigraphy; palaeogeography; tectonics; External Dinarides; tunnel; Croatia

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