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Unraveling stratigraphic complexities in Lower Cretaceous (Barremian to Albian) strata of Istria, Croatia.

The Istrian peninsula in western Croatia is the northwestern part of the Adriatic-Dinaridic carbonate platform (ADCP) – one of the largest Mesozoic platforms in the Perimediterranean (Tethys) region. The ADCP deposits comprise the carbonate succession of the Karst or External Dinarides, which is in places >8000 m thick [1, 2], and spans Late Triassic to Eocene in age. This study focuses on a Lower Cretaceous (Barremian to Albian) succession of strata at five localities in western Istria. Stratigraphic determinations are based on identification of microfossil assemblages (benthic foraminifera and calcareous algae-Dasycladales) and on using their taxa as index fossils. Rich foraminiferal, especially orbitolinid, assemblages are very important for stratigraphically subdividing Lower and Middle Cretaceous deposits, and have been determined within the Aptian, Albian and Cenomanian limestones of the ADCP. At one of the investigated localities (Lakovići North, Late Aptian?-Albian) the orbitolinid groups are completely absent. Stratigraphic subdivision of this succession is based on recognition of a specific microfossil assemblage (rare Dasycladaceans and abundant, but not very diversified foraminiferal, predominantly miliolid assemblages), various depositional environments, and characteristic emersion features. The age of strata with this microfossil assemblage, however, is questionable. Most of the age uncertainties in the Lower Cretaceous succession from the study area are associated with a regional, locally (e.g. Istria) long-lasting, emersion, which occurred on the ADCP during the Upper Aptian or close to the Aptian-Albian transition. It is unclear what portions of the Upper Aptian and/or Lower Albian are missing along this unconformity. A stable isotope study was conducted on homogenous micritic matrix samples in an attempt to resolve some of these uncertainties. Variations in carbon isotope compositions proved useful for stratigraphic correlation between the examined successions of strata and for improving their age determination. The δ 13C values suggested that the best marker for stratigraphic correlation is the regional unconformity on top of the biostratigraphic Zone III of the Early Aptian age, and that the Lakovići North succession is stratigraphically younger (with only a small possible overlap in the lowermost part) than other investigated successions in Istria. A tightly constrained δ 13C variation curve for the strata below the unconformity suggests that their syndepositional composition has not been significantly modified by meteoric diagenesis. The δ 13C variation trends, in general, correspond to those observed for the Upper Barremian to Lower Aptian marine carbonate strata world-wide, thus supporting the proposed biostratigraphic interpretations. Near the biostratigraphic boundary between Zone II (Late Barremian) and Zone III (Early Aptian) there is a small shift towards more negative δ 13C values. Such a shift is known from the Lower Aptian strata globally (before a large positive shift in the Middle Aptian), and may suggest that the Barremian-Aptian boundary is somewhat lower in the successions examined than indicated by biostratigraphy. The shift, however, is associated with a lithofacies change, which likely reflects a sea-level fall that led to the restriction and isolation of parts of the carbonate platform, and ultimately resulted in the subaerial exposure and regional unconformity on top of Zone III. The platform isolation and the resulting local to regional variations in the isotopic composition of seawater are reflected in the δ 13C values of the strata immediately below the unconformity, which are by several permils more negative than those of the Lower Aptian normal marine carbonates elsewhere, and vary between localities from about -2 to 0‰ VPDB. The δ 13C values of the strata above the unconformity are highly variable and depart mainly towards more negative values relative to the underlying strata. This likely reflects a variable organic carbon input during deposition and early diagenesis of the carbonate deposits in very shallow and periodically exposed restricted platform environments as indicated by common brecciated intervals. It is also possible that some of the documented variations represent expressions of the highly variable Late Aptian normal marine global δ 13C signature. Thus, even though questionable, the proposed Late Aptian age for the strata above the unconformity and for the lowermost part of the Lakovići North section is the best choice given the poor exposure of parts of this interval, its possible regional diachroneity, and the present limits of bio- and chemostratigraphy.

microfossil assemblages; carbon isotopes; Lower Cretaceous; Istria; Croatia

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