Ichnocoenoses and taphocoenoses of posidoniid-bearing marl-limestone rhythmites and event beds, Toarcian-Aalenian, Northern Apennines, Italy.

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, the calcium carbonate-poor bed and the calcium carbonate-rich bed, respectively (CacO3 between 10 to 85%). Event tempestite beds randomly occurs. The aim of this work is show ichnocoenoses and taphocoenoses and the faunistic variation of each semicouplet (and event beds). Each couplet is thick about 30 cm in Toarcian Rosso Ammonitico and up to 40 cm in Aalenian Calcari e Marne a Posidonia Formation, where the sedimentation rate increased. Here couplets were deposited in the dysaerobic/aerobic transition zone. (i) The clay-rich portion contains abundant thin-shelled poikilaerobic bivalves and benthic foraminifers (whithout radiolarians) and shows oxygen-poor ichnocoenosis with Chondrites (Ch), while the taphocoenosis exhibits convex-upward disposition (Cu) and disarticolation (Di) of bivalve shells. (ii) The limestone semicouplet, on the contrary, is rich in radiolarians and thin-shelled bivalves, usually more burrowed (ichnocoenosis Ch-Pl-Th, Chondrites and few Planolites and Thalassinoides) than the former, with grouping (Gr), dispersion (Dp) biogenic fragmentation (BFr) and orientation-reorientation (Or-ReOr). A result is that, in all marl-limestone semicouplets, foraminifers and radiolarians exhibit a periodic inverse correlation in abundance, while thin shelled bivalve 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 Sk-Pl-Tr-Ch-Th, Skolithos, Planolites, Trypanites, Chondrites and Thalassinoides). Taphocoenoses include fragmentation (Fr), grouping (Gr), surficial burrowing (Sb), biogenic reorientation (Br) and deep burrowing (Db). Ichnological and taphonomical features (10 ichnotaxa and 12 taphocharacheristics) indicate high physical and biogenical reworking affecting sediments. The Jurassic depocenters of Valdorbia and Fiuminata that received rhythmical dilution cycles (clay) or productivity cycles (carbonate) during Toarcian and Aalenian time can therefore strongly contribute to study the ichnological and taphonomical variation in rhythmic conditions.