engleski

Late Jurassic Tithonian greenhouse climate inferred from facies stacking: Adriatic cyclic platform interior, Croatia

Well exposed Late Jurassic sections (over 750 m thick, approximately 5 to 6 m.y. duration) of the Bahama-sized Adriatic platform along the Dalmatian coast, southern Croatia, have well developed parasequences dominated by shallow subtidal to peritidal facies. These include dasyclad-oncoid mudstone-wackestone-floatstone (shallow lagoon), skeletal-peloid wackestone-packstone (very shallow lagoon), intraclast-peloid packstone and grainstone (shoal), radial-ooid grainstone (hypersaline shallow subtidal/intertidal shoals and ponds), lime mudstone (restricted lagoon), fenestral carbonates and microbial laminites (tidal flat). Oversized, whole, broken and recoated radial ooids that form distinctive units resemble ooids of modern, low energy lake and marginal marine ponds. Their common position at bases of parasequences indicates formation during initial inundation of supratidal flats, along the shores of shallow hypersaline ponds and restricted lagoons of the platform interior. The abundance of these distinctive ooid facies in the Tithonian of Tethys may be due to arid climate, high calcite saturation state of platform waters, and the flat-topped, platform-interior morphology. The Tithonian parasequences appear to be less than 20 k.y. average duration and likely were formed by small sea-level changes driven by precessional forcing. During long-term (2.5 m.y.) sea-level rise, predominantly subtidal cycles were deposited, whereas peritidal cycles developed on the long-term (2.5 m.y.) sea-level fall. Evidence of an eccentricity signal in the parasequence bundling is relatively weak and produced parasequence sets whose component facies become successively more restricted upward. Paleosols are conspicuously absent from the Tithonian, suggesting that relative sea-level falls driven by precession were suppressed by high accommodation rates (12 to 15 cm/k.y.). The parasequence stacking patterns strongly suggest that the Tithonian likely was a time of hot global greenhouse during the postulated Middle Jurassic to Early Cretaceous "cool" mode.

Jurassic; Greenhouse; Paleoclimate; Carbonate Platform; Croatia

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