Relation between shell beds and trace fossils of the Albian-Cenomanian Aitamir Formation in Amirabad anticline- northeast Mashhad

Aitamir Formation (Albian-Cenomanian) in the Kopet-Dagh Basin has diverse trace fossils and several shell beds. These trace fossils contain diverse ethological patterns, including domichnia, fodinichnia, grazing, equilibrichnia, navichnia, chemichnia and fugichnia. The types of shell beds distinguished based on differences in sedimentological, palaeoecological and taphonomic features. Shell beds and sandstone layers containing characteristic trace fossils represent a storm-influenced succession. Bioturbation increases markedly towards the top of the shell beds as the energy of deposition declines to allow colonization by benthonic faunal assemblages. Types of shell beds generally including (1) initial firmground substrates in foreshore to offshore environments as indicated by the Glossifungites ichnofacies, including Gastrochaenolites, Rhizocorallium and Thalassinoides, (2) basal storm-erosive lag deposits, (3) re-worked shell beds of main storm deposits as indicated by the Glossifungites ichnofacies, including Ophiomorpha (4) fair-weather shell beds in a near-shore position and above fair-weather wave base. (5) Composite or multiple-event concentration from time averaged and multiple event deposit. Seven trace fossil assemblages are identified in Aitamir Formation deposits. They are (1) the Zoophycos Ichnofacies (2) the distal Cruziana ichnofacies, (3) the archetypical Cruziana ichnofacies, (4) the mixed Skolithos-Cruziana ichnofacies, (5) the distal Skolithos ichnofacies, (6) the archetypal Skolithos Ichnofacies, and (7) impoverished expression of the Skolithos ichnofacies. This succession of shell beds and trace fossil assemblages reflects a gradational decrease in hydraulic energy levels induced by storms.