engleski

Kimmeridgian-Tithonian cyclic carbonates of Adriatic platform: implications for climate and eustatic fluctuations in western Tethys during a global greenhouse

Late Jurassic Kimmeridgian and Tithonian carbonates, Adriatic platform, Croatia record detailed sea level fluctuations. In the Kimmeridgian section measured, 3 sequences occur composed of lower units dominated by thick wackestone-mudstones which shallow up into cyclic peritidal facies, capped by breccia. The Tithonian is dominated by several sequences dominated by shallow subtidal to peritidal parasequences. Parasequences are 1 to 5 m thick units of basal subdital skeletal-oncoidal wackestone-mudstone, up into intraclast-peloid packstone grainstone. In more interior areas bases of parasequences commonly have transgressive radial-ooid grainstone ; oolitic units are less common in upper parts of parasequences. Parasequence caps vary regionally, with restricted, unfossiliferous lime mudstone capping most parasequences closer to the platform margin, while elsewhere fenestral, planar, and microbial carbonates cap cycles. Clayey breccias cap sequences in the interior of the platform. The cyclic facies formed by high-frequency sea level fluctuations of low amplitude (<10 m) but significant sea level falls were associated with sequence boundaries. Duration of the Tithonian coupled with the number of cycles suggest a precessional origin for the basic cycle. Carbon and oxygen isotopes were obtained from lime mud matrix of dasyclad-oncoid mudstone-wackestone (shallow lagoon) at 5 m intervals from a 200 m thick section of the Lower Tithonian. Isotope values PDB of all analyzed samples fall into a narrow range of &#948; 18O values (-0.2 to -3.0 &#8240; ) and &#948; 13C values (0.0 to 3.0 &#8240; ). Comparison of sequence framework and the isotopes suggests that there appear to be two positive excursions in carbon associated with major flooding events, and that the negative excursions are related to lowered sea level phases.

Kimmeridgian; Tithonian; Adriatic Platform; Greenhouse

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