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Aptian-Albian Stratigraphic Succession: An Example from the Adriatic-Dinaridic Carbonate Platform (Lakovići, West Istria, Croatia

Stratigraphic subdivision of Lower Cretaceous deposits of the Adriatic-Dinaridic carbonate platform is possible only on the basis of systematic research of microfossil assemblages. The microfossil assemblages are usually composed of two main microorganism groups: calcareous algae (Dasycladales) and benthic foraminifera (Orbitolinidae and others). The vicinity of Lakovići (Lakovići quarry) in West Istria is the part of the Adriatic-Dinaridic carbonate platform. Close to Lakovići quarry, in the vicinity of Heraki village, according to VELIĆ et al. (1989, 1995) Albian deposits transgressively overlie the Lower Barremian limestones. Rich foraminiferal, especially orbitolinid, assemblages, are very important for stratigraphic subdivision of Lower and Middle Cretaceous deposits, and have been determined within Aptian, Albian and Cenomanian limestones on the Adriatic-Dinaridic carbonate platform. The Lower Aptian is typically represented by Palorbitolina and Praeorbitolina limestone (“lower Orbitolina limestone” in VELIĆ & SOKAČ, 1978) and the Upper Aptian and basal Lower Albian by the Mesorbitolina limestone (“upper Orbitolina limestone” in VELIĆ & SOKAČ, 1978). The Mesorbitolina limestone is totally missing in West Istria (VELIĆ et al., 1989). The Lakovići Lower Cretaceous succession is one example in which both orbitolinid groups are totally absent. Stratigraphic subdivision of the Lakovići Lower Cretaceous succession is possible on the basis of the specific microfossil assemblages (rare calcareous algae-Dasycladales and abundant, but not very diversified foraminiferal, predominantlly miliolid assemblages), different sedimentary environments and emersion features. According to these microfossil assemblages, the Lakovići succession is subdivided into two stratigraphic levels: lower level (I) - Aptian; and upper level (II) - Albian. The boundary between this two levels is the emersion surface characterized by black breccia/conglomerates and yellow clay intercalations. In the lower stratigraphic level (I, Aptian), three different facies types are present: (1) algae mudstone (with only two species, Salpingoporella hasi and S. urladanasi); (2) foraminiferal wackestone-packstone (with Sabaudia minuta, Vercorsella scarsellai, Mayncina bulgarica, Glomospira urgoniana, Pseudonummoloculina aurigerica and miliolids); and (3) peloidal-foraminiferal (miliolid) grainstone (P. aurigerica, Rumanoloculina minima, R. cf. robusta, Istriloculina cf. granumtritici, Sigmoilina sp., Scandonea aff. phoenissa, G. urgoniana and fragments of gastropods). In the upper startigraphic level (II, Albian), two different facies types are present: (4) skeletal-peloidal grainstone with mollusks and microfossils ("Valdanchella" dercourti, Salpingoporella turgida); and (5) foraminiferal wackestone-packstone (Cuneolina pavonia, C. parva, Pseudonummoloculina heimi, V. scarsellai, Nezzazatinella picardi).

Adriatic-Dinaridic carbonate platform; West Istria; stratigraphic succession; Aptian-Albian; microfossil assemblages

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