Campanian to Eocene dinoflagellate cyst biostratigraphy from the Tahar and Sekada sections at Arba Ayacha, western External Rif, Morocco

Abstract

Our detailed palynological study of the Upper Cretaceous–Lower Eocene marly succession from the Sekada and Tahar sections in southern Arba Ayacha, westernmost External Rif Chain (northwestern Morocco), has provided precise age determinations based on dinoflagellate cyst biostratigraphy. Dinoflagellate cysts (dinocysts), which are the dominant palynomorphs, allowed us to recognize the following ages: Late Campanian, Early and Late Maastrichtian, Danian, Selandian, Thanetian and Early Ypresien. Published studies on the western External Rif based on lithostratigraphy, show conflicting ages of Late Cretaceous (Senonian) and Early Eocene. Our age determinations are based on dinocyst events, which are more reliable. We recognize the Late Campanian on the First Occurrence (FO) and Last Occurrence (LO) of Exochosphaeridium? masureae, and the FO of Cerodinium spp., and the LOs of Cribroperidinium wilsonii subsp. wilsonii, Odontochitina porifera and Trithyrodinium suspectum. The Early Maastrichtian is denoted by the FOs of Alterbidinium varium and Palaeocystodinium golzowense and the LO of Alterbidinium acutulum. We define the Late Maastrichtian on the FOs of Disphaerogena carposphaeropsis and Glaphyrocysta perforata, and the LOs of Alisogymnium euclaense, Dinogymnium spp., Isabelidinium cooksoniae, and Pterodinium cretaceum, plus the worldwide latest Maastrichtian acme of Manumiella seelandica. Marking the Danian is the overall range of Senoniasphaera inornata, the FOs of Carpatella cornuta, Damassadinium californicum and Membranilarnacia? tenella. Species having LOs in the Selandian incude Cerodinium diebelii, Manumiella seelandica, Senoniasphaera inornata, Palaeocystodinium australinum and Cerodinium speciosum. We recognize the Thanetian mainly on the FO of Homotryblium tenuispinosum and based the worldwide terminal Thanetian acme of Apectodinium spp. The Early Ypresian is characterized by the FO of Deflandrea phosphoritica and a high abundance of Apectodinium spp. and Kenleyia spp. Thus, we now know that the rocks outcropping in the Sekada and Tahar sections are Late Campanian to Early Ypresian in age.