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Posidoniids, a key for Early-Middle Jurassic

The Posidoniid-bearing rhythmic deposits, Toarcian to Aalenian (Early/Middle Jurassic) in age, of the Umbria-Marche basins (Fiuminata Colle Corno and Valdorbia type section) consist of hundred couplets, each composed by two semicouplets, a calcium carbonate-poor bed and a calcium carbonate-rich bed, respectively (CacO3 between 10% to 85%), where event tempestite beds randomly occurs. The aim of this work is to show ichnocoenoses and taphocoenoses as well as the faunistic variations of each semicouplet and event beds. Each couplet is 30 cm thick in the Toarcian Rosso Ammonitico and up to 40 cm in the Aalenian Calcari e Marne a Posidonia Formation, where the sedimentation rate increased. Here, couplets were deposited in the dysaerobic/aerobic transition zone. The clay-rich portion contains abundant thin- shelled poikilaerobic bivalves and benthic foraminifers (whithout radiolarians) and shows oxygen-poor ichnocoenosis with Chondrites, while the taphocoenosis exhibits convex-upward disposition and disarticulated bivalve shells. Conversely, the limestone semicouplet is rich in radiolarians and thin- shelled bivalves, usually more burrowed (ichnocoenosis including Chondrites and few Planolites and Thalassinoides) than the former, with grouping, dispersion, biogenic fragmentation and orientation- reorientation. In all marl-limestone semicouplets, foraminifers and radiolarians exhibit a periodic inverse correlation in abundance, while thin-shelled bivalves are always abundant. Rhythmically disposed within the rhythmites are also event beds indicating an occasional oxygen-rich substrate, fine- grained calcarenitic tempestites with hummocky cross-stratification, and winnowed bed (thin shelled bivalve concentrations). These deposits are intensely bioturbated (ichnocoenosis including Skolithos, Planolites, Trypanites, Chondrites, and Thalassinoides). Taphocoenoses include fragmentation, grouping, surficial burrowing, biogenic reorientation, and deep burrowing. Ichnological and taphonomical features indicate high physical and biogenical reworking affecting sediments. The Jurassic depocenters of Valdorbia and Fiuminata that received rhythmical dilution (clay) or productivity (carbonate) cycles during Toarcian and Aalenian time can therefore strongly contribute to the study of ichnological and taphonomical variations in rhythmic conditions.

Paolo Monaco (2016). Ichnocoenoses and taphocoenoses of posidoniid-bearing marl-limestone rhythmites and event beds, Toarcian-Aalenian, Northern Apennines, Italy. Geobios 49 (2016) 365–379

0016-6995/ß 2016 Elsevier Masson SAS.

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