Distinguishing core and flank facies based on shell fabrics in Lower Jurassic lithiotid shell beds

Lower Jurassic larger bivalves, mostly represented by the monospecific lithiotid genera Lithiotis, Cochlearites and Lithioperna, formed large shell accumulations in the shallow-water carbonate Tethyan and Panthalassa margins. A quantitative analysis of lithiotid accumulations from the Trento Platform (northern Italy) was carried out in order to distinguish the distribution of autochthonous, parautochthonous and allochthonous lithiotids in the accumulation core and flanks. Various representative accumulations are characterized with respect to systematic content, shell cover, shell density, disarticulation, shell fragmentation and orientation of individual shells. Lithioperna shows a high shell cover within the accumulation core while that of Cochlearites is variable and Lithiotis is rare. All three lithiotids occur in high densities in the accumulation flanks. As expected, autochthonous individuals of all three genera are frequent in the accumulation cores where they are preserved in life position. In the flanks, most bivalves are parautochthonous except for Lithioperna which can grow in life position. All bivalves in allochthonous shell beds are highly fragmented and disarticulated. The studied traits are potentially useful as proxies for distinguishing core or flank facies when the accumulation does not crop out as a whole.