Non-depositional and erosional “offshore” bioeroded hardgrounds from the Lower Cretaceous of the Neuquén Basin, Argentina: Insights into their sequence-stratigraphic implications and controls

Hardgrounds are important surfaces for basin analysis and oil exploration, representing in some cases significant markers of the sedimentary basin fill. In the Neuquén Basin, bioerosion structures are common on the shell surfaces of benthic organisms, but bioeroded hardgrounds are comparatively uncommon. In the Pilmatué Member of the Lower Cretaceous Agrio Formation (Valanginian-Hauterivian) two examples of bioeroded hardgrounds are described in detail, allowing to recognize contrasting palaeoenvironmental and sequencestratigraphic contexts, and in turn, discussing the potential major controlling factors in each example. In both cases Trypanites isp. and Gastrochaenolites isp. are described, interpreted as belonging to a Trypanites ichnofacies. The two cases are developed in different environmental conditions: one in non-depositional offshore conditions at the top of a parasequence (i.e., transgressive surface); and the other in offshore erosional settings with the omission surface located in an intra-parasequence position, with no obvious sequence-stratigraphic connotation. These marked differences in the palaeonvironmental conditions (non-depositional vs erosive) and sequence-stratigraphic context (inter vs intra-parasequence occurrence) interpreted for these example allowed us to propose different controlling factors related to the generation of the hardgrounds. Despite these differences, namely long periods of non deposition and low-energy conditions vs shorter time and high-energy (erosive) conditions, in both cases a Trypanites ichnofacies is developed, representing an important aspect to be considered in the characterization of the basin evolution based on omission surfaces.