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Transgressive oversized radial ooid facies in the Late Jurassic Adriatic platform interior: Low energy precipitates from highly supersaturated hypersaline waters

Dark gray oolitic units characterized by oversized ooids with primary radial calcite fabrics occur in the interior of the Late Jurassic Upper Tithonian, Adriatic Platform, a large Mesozoic, Tethyan isolated platform in Croatia. They differ from open marine, platform margin ooid grainstones in their dark color, cerebroid outlines, broken and recoated grains, abundant inclusions, highly restricted biota, and lack of cross stratification. They were interpreted as being of vadose origin ("vadoids") at tops of upward-shallowing parasequences. However, detailed sections show that most oolitic units occur at bases of precessional parasequences, overlying erosional surfaces on fenestral carbonates. The oolitic units are similar to quiet water ooids forming today in low energy settings. They developed in an arid climate during initial transgression of supratidal flats, along low energy shores seaward of tidal flats, and along the margins of restricted lagoons and intertidal ponds. Superimposed fenestral fabrics, meniscus micrite cements and grain breakage occurred as they aggraded to high tide level and were subjected to wetting and drying, thermal expansion and contraction, and wind transport. They migrated landward with transgression, forming extensive sheets, and were overlain by subtidal lagoonal facies that shallow up into fenestral carbonates. These distinctive facies may have been overlooked in the geological record, or their geological distribution requires juxtaposition of calcite seas, high calcite supersaturation states, arid climate, and presence of flat topped carbonate platforms in a greenhouse world.

radial ooids; low energy; platform interior parasequences; Late Jurassic; Adriatic platform

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