Trypanites ichnofacies: Palaeoenvironmental and tectonic implications. A case study from the Miocene disconformity at Foz da Fonte (Lower Tagus Basin, Portugal)

A well preserved bioeroded surface occurs at the unconformity separating Cretaceous limestones and Lower Miocene sediments, outcropping on the western coast of the Peninsula of Setْbal (Central West Portugal). The ichnoassemblage present in this bioeroded surface is herein assigned to the Trypanites ichnofacies. The preservation characteristics of the borings reflect several episodes of encrustation/boring and physical erosion. The erosional truncation of bioerosive structures, and the predominant preservation of the largest borings (Gastrochaenolites isp.) in the ichnocoenoses are herein related with repeated phases of bioerosion and physical abrasion occurred during an Early Miocene transgressive pulse. The recognition of this bioeroded transgressive surface also allowed confirming the presence, at that time, of an emergent topographic relief related to salt domes formed earlier, probably already during Palaeogene times.