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Late Jurassic Callovian-Kimmeridgian Sequence Development and Carbon-Isotope Signature of Adriatic Platform, Croatia


A continuous, ∼350 m thick succession of cyclic, predominantly muddy upper Callovian-lower Kimmeridgian shallow-marine carbonates was logged in southern Croatia to define the sequence stratigraphy. The basal, late Callovian part contains three sequences (AdCal-x to AdCal-z) dominated by parasequences consisting of intraclast-skeletal-peloid packstone-grainstone capped by fenestral-laminated carbonates. The bulk of the Oxfordian (eight sequences: AdOx-1 to AdOx-8) and lower Kimmeridgian (AdKimm-1) is represented by low energy and muddy lithofacies. The parasequences consist of basal skeletal-microbial lump wackemudstone capped by barren lime mudstone. Lack of tidal flat facies caps to parasequences indicates incomplete shallowing to intertidal depths, and suggests that some parasequences are amalgamated. Given the duration of Oxfordian (∼8 m.y.), these likely represent third-order cycles with an average duration of ∼1 m.y. (suggestive of long-term obliquity forcing). The total number of Oxfordian parasequences (57, ∼100 k.y. duration) is suggestive of short-term eccentricity forcing. Samples for δ13C analysis were collected every meter from bulk carbonate matrix. The carbon-isotope values vary from −2.3 to +4.0‰ PDB, and show two significant isotope excursions in the Oxfordian The isotopic signature shows relatively constant values (0.8–1.8 ‰PDB) for the late Callovian, followed by a negative excursion to 0‰ at the beginning of Oxfordian, and a stepwise increase to 2.7‰. The subsequent negative excursion (-2.3‰) is followed by a stepwise increase to 3.8‰ in the middle of Oxfordian, slight decrease with values remaining between 1.5‰ and 3.3‰, and finally by a stepwise increase with a positive excursion to 4.1‰ in the late Oxfordian. Stepwise positive shifts in carbon-isotope values in the middle and late Oxfordian are both associated with shallowing upward parts of sequences AdOx-3 and AdOx-6, respectively. The remainder of the curve shows generally decreasing C-isotope values into the lower Kimmerdgian. The studied succession supports overall late Callovian cooling (marked by tidal flats) followed by mid-Oxfordian maximum flooding (widespread subtidal facies), with parasequence development suggestive of short-term eccentricity forcing and superimposed long-term obliquity forcing. It is interesting that the dominantly subtidal Oxfordian of the Adriatic platform corresponds to time of intrabasin source rock formation in the Middle East.