SEQUENCE STRATIGRAPHY AND STABLE CARBON ISOTOPE RECORD FROM THE LATE JURASSIC PLATFORM INTERIOR, ADRIATIC PLATFORM, CROATIA

The Upper Jurassic upper Callovian-lower Kimmeridgian succession, southern Croatia contains 12 depositional sequences. The ~80 m thick upper Callovian-basal Oxfordian part contains three sequences (AdCal-x, AdCal-y, and AdCal-z) dominated by high-energy and grainy lithofacies stacked into shallowing-upward cycles (parasequences) consisting of intraclast-skeletal-peloid packstone-grainstone capped by fenestral-laminated carbonates. The ~270 m thick Oxfordian-basal Kimmeridgian part contains 9 sequences (AdOx-1 to AdOx-8, and AdKimm-1) consisting of low-energy and muddy lithofacies. The parasequences typically 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.

Stable carbon and oxygen isotopes were obtained from carbonate mud matrix of the upper Callovian-lower Kimmeridgian bulk carbonate matrix. The carbon-isotope curve shows two significant isotope excursions in the Oxfordian; these are associated with shallowing upward parts of sequences AdOx-3 and AdOx-6, respectively. The isotopic signature shows relatively constant values (0.8-1.8 ‰ VPDB) for the late Callovian, followed by a negative excursion to 0‰ at the beginning of Oxfordian, and a stepwise increase to 2.7‰ VPDB. The subsequent negative excursion (-2.3‰ VPDB) is followed by a stepwise increase to 3.8‰ VPDB in the middle of Oxfordian, slight decrease with values remaining between 1.5‰ VPDB and 3.3‰ VPDB, and finally by a stepwise increase with a positive excursion to 4.1‰ VPDB in the late Oxfordian. The remainder of the curve shows generally decreasing C-isotope values into the lower Kimmerdgian.

Given the duration of Oxfordian (~8 m.y.), the sequences likely represent third-order cycles with an average duration of ~1 m.y., and are suggestive of long-term obliquity forcing. Short-term eccentricity forcing is suggested by the total number of Oxfordian parasequences (57, ~100 k.y. duration). The parasequence stacking pattern supports overall late Callovian cooling with extensive tidal flats, followed by mid-Oxfordian warming and maximum flooding, as evidenced by widespread subtidal facies.