Ichnology in oceanic islands; case studies from the Cape Verde Archipelago

Miocene and Pleistocene marine deposits in the Cape Verde Archipelago are represented by a series of thin, transgressive sandy-bioclastic limestones that typically occur between subaerial and/or submarine volcanic flows. This position within volcanic flows makes them an ideal paleobathymetric case study, because the submarine/subaerial passage zone above them provides a reliable and independent indicator of paleodepth. In terms of paleoichnology, the older Miocene sediments are characterized by the Thalassinoides ichnoassociation, whereas, Pleistocene sediments are characterized by the Macaronichnus–Dactyloidites ichnoassociation.

The fair-weather suite of the proximal Cruziana ichnosubfacies is represented by the ichnotaxa Dactyloidites, Bichordites, Cardioichnus, Phycodes, Teichichnus, aff. Taenidium, Palaeophycus and Thalassinoides in addition to the Thalassinoides ichnoassociation.

The Macaronichnus–Dactyloidites ichnoassociation is more complex, including representatives of both the proximal Cruziana and Skolithos ichnofacies. Only representatives of the Skolithos ichnofacies (Skolithos, Macaronichnus, Conichnus and Ophiomorpha) are represented in the post-storm suite.

So far, bioerosive structures were found on the Miocene/Pleistocene unconformity surface of Maio Island, and on basaltic paleocliffs at Ponta das Bicudas, on Santiago Island. They are represented by the ichnoassemblage Gastrochaenolites torpedo–Entobia isp., corresponding to the Entobia ichnofacies.

Bathymetric calibration deduced from the position of the following passage zone between submarine and sub-aerial lava flow allows for confirmation of the proximal Cruziana ichnosubfacies and Skolithos ichnofacies at a paleodepth between 12 and 18 m depth.