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Bioturbation structures from Ponta das Bicudas in Santiago Island, Cape Verde Archipelagoare located in coarse-grained biocalcirudite sandwiched between basaltic lavas of the Upper Pleistocene Assomada Formation. Four ichnospecies have been identified: Rhizocorallium commune irregulare, Rhizocorallium jenense versum , nov ichnosubsp., Alaichnus kabuverdiensis nov. ichnogen. nov. ichnosp. and cf. Dactyloidites . From a taphonomic point of view, most trace fossil specimens are undergoing throughout intense erosion. Nevertheless, the observations made on their taphonomy and state of preservation allowed i) to reconstruct the original, complete structures of the trace fossils, ii) to evaluate the existence and magnitude of erosional and depositional episodes, and iii) to quantify the reach of erosion on the seafloor sediment. In this context, it was possible to quantify the thickness of sediment being eroded at the final erosional episode, which happened right before the second extrusion episode of volcanic pillow lava, which buried all the traces previously formed: the total interval of sediment eroded was estimated in ca. 1 m. From a palaeoenvironmental point of view, in a synthesis, the traces were produced in a soft to stiffground substrate composed of very coarse-grained biocalcirudite with common rhodolith debris, in a shallow setting (4 to10 m of depth) with moderate wave energy, nearby rocky basalt knobs where corals and red coralline algae crusts were growing. Organisms producing Rhizocorallium commune irregular settled in the seafloor during a first phase of sedimentation, which was followed by an erosional episode. Once sedimentation resumed, specimens of R. jenense versum nov. ichnosubsp. were produced and overlapped with the former ones. Later, a new erosional event erased most of the previous traces, being preserved only the basal imprint of the specimens. This was followed by a rapid sedimentation episode where Alaichnus kabuverdiensis nov. ichnogen. nov. ichnosp. recorded intense bioturbation activity of the bivalve Panopea . Subsequently, under low-energy sedimentation, a new episode of intense bioturbation of the seafloor occurred, resulting in the development of abundant cf. Dactyloidites . Finally, a strong erosional event – probably related to the onset of the second volcanic episode – cut all previous structures before being buried by the overlying, well preserved pillow lava. [ABSTRACT FROM AUTHOR]

Abstract: 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 18m depth. [Copyright &y& Elsevier]