Eocene Carbonates and Flysch Deposits of the Pazin Basin

Istrian flysch deposits (Istrian Eocene clastics) crop out in the Trieste-Pazin, Labin and Plomin basins, Mt. Ucka, and partly on Mt. Cicarija. They are generally characterized by an alternation of hemipelagic marls and gravity-flow deposits. The prevailing turbidite succesion of hybrid carbonate-siliciclastic sandstones and marls is randomly intercalated by several thick carbonate beds of debrite origin, i. e. megabeds. There are two distinctive types of contact between flysch and underlying deposits in the Istrian basin: a) in continuous and b) with erosional discordance between flysch and carbonate platform deposits. Continuous deposition in area of Boljun, Kotli, Buzet, Sterna etc. characterised gradual deepening from shallow-marine foraminiferal limestones to transitional beds with first influence of terigene contribution (Marls with crabs). Still further increase in clay content led to the deposition of homogeneous hemipelagic marls (Globigerina marls). Flysch deposits begin with occurences of alternation of sandstone and marl beds. On the southern border of flysch area (Motovun, Pican... ) flysch deposits discordantly overlies different stratigraphic horizons of platform carbonates i.e. Cenomanian limestones and Middle Eocene foraminiferal limestones. The flysch deposits correspond mainly to Middle Eocene planktonic foraminiferal zones Morozovella lehneri (P12) and Orbulinoides beckmanni (P13) of TOUMARKINE & LUTERBACHER (1985), and their uppermost portion to the Late Eocene (BENIC, 1991). Istrian flysch deposits are characterized by alternation of hemipelagical marls and gravity-flow deposits. Predominant deposits are 5-40 cm cm thick base-missing turbidites, developed mostly as laminated and cross- rippled sandstone beds (Tb-e, Tc-e and Td-e Bouma sequences). Rarely are up to 100 cm thick complete Ta-e Bouma sequences. Sandstones are of a mixed carbonate-siliciclastic composition. According to low ratio of arenites vs. marl thickness it could be concluded distal character of flysch deposits. Turbidites were deposited from low-density turbidity currents (sensu LOWE, 1982). The monotonous succesion of marls and mixed carbonate-siliciclastic sandstones is intercalated by several thick carbonate beds composed of breccia, conglomerate, bioclastic arenites/siltites and marl. They show significant thickness of 0.5 to 5 m sometimes even over 10 m. These carbonate beds (megabeds) are interpreted as complex sequences of debris and turbidite origin and characterise the lower part of basin fill. Some of them laterally thinned out and pinch out, but thicker are continuous at the distance more than 10 km. The base of large beds is often flat, but sometimes show shallow wide erosional depressions. Beside megabeds of Istrian (Pazin) Basin (e.g. 40 m thick megabed near Gracisce) occurences of megabeds of exceptional thickness are known from Julian (Friuli) Basin and Central Dalmatian Basin.