engleski

Facies stacking pattern in a Late Jurassic (Tithonian) Adriatic Platform: implications for greenhouse phase in Late Jurassic-Early Cretaceous cool mode

The Late Jurassic-Early Cretaceous has been considered to lie in the Mesozoic “cool” mode of Frakes et al. (1992), as evidenced by oxygen isotope signals (VEIZER et al. 2000) and by the pCO2 plots GEOCARB III of BERNER & KOTHAVALA (2001). At the long-term scale, these climate proxies are in agreement, but at shorter time scales, there is some conflict both in degree or warming or cooling and timing of the events (VEIZER et al. 2000 ; ROYER et al., 2004). This study uses data from the Adriatic platform of Croatia to evaluate whether the approximately 5 to 6 m.y. long Late Jurassic Tithonian, has facies compatible with a Mesozoic “cool” mode (FRAKES et al 1992). Well exposed Mesozoic sections of the Bahama-like Adriatic Platform reveal the detailed stacking patterns of cyclic facies within the rapidly subsiding Late Jurassic (Tithonian) shallow platform interior (over 770 m thick, approximately 5 to 6 m.y. duration). Accommodation plots show four 3rd-order relative sea-level cycles, the third being the largest both in duration and magnitude (HUSINEC & READ 2007). Facies within parasequences include dasyclad-oncoid mudstone-wackestone-floatstone and skeletal-peloid wackestone-packstone (shallow lagoon), intraclast-peloid packstone and grainstone (shoal), radial-ooid grainstone (shallow subtidal/intertidal shoals and ponds), lime mudstone (restricted lagoon), fenestral carbonates and microbial laminites (tidal flat). Maximum water depths (estimated from distance below tidal flat units capping parasequences) generally were less than four meters, and facies show strongly overlapping depth ranges suggesting facies mosaics. Facies in the overall transgressive, Lower Tithonian sections are arranged in 1 to 4.5 meter thick parasequences dominated by subtidal facies, some of which are capped by very shallow water grainstone-packstone or restricted lime mudstone ; laminated tidal caps only become common toward the interior of the platform. Parasequences in the regressive Upper Tithonian are dominated by peritidal facies with distinctive basal oolite units and well developed laminite caps. Parasequence durations appear to be within the precessional band and eccentricity bundling is weak. Modeling suggests that precessional sea level changes were small (several meters) as were eccentricity sea level changes, supporting a global, hot greenhouse climate for the Late Jurassic (Tithonian) within the overall “cool” mode of the Middle Jurassic to Early Cretaceous.

Late Jurassic; Adriatic Platform; Greenhouse; Sequence Stratigraphy

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