Ernesto Schwarz, Miquel Poyatos-Moré , Salvador Boya, Luz Elena Gomis-Cartesio, Ivar Midtkandal

Thick (>100 m-thick), highly bioturbated storm-influenced shallow-marine deposits are not frequent in the stratigraphic record, but they tend to be unusually common in aggradational to retrogradational successions. Individual storm-event beds have typically low preservation in these successions, yet depositional settings are characterized on the basis of storms processes. We present a sedimentological study of a thick, bioturbated exhumed succession deposited during the early post-rift stage of the Neuquén Basin (Argentina) and compare its stratigraphic record with examples developed worldwide, in order to discuss controlling factors on the total destruction of storm-event beds during several million years.
The Bardas Blancas Formation (170-220 m thick) is dominated by muddy sandstones and sandy mudstones, but it also includes subordinate proportions of clean sandstones and pure mudstones, collectively representing different environments of a storm-influenced shoreface-offshore system. The offshore transition and proximal offshore strata invariably comprises intensely bioturbated deposits, with only a few preserved HCS-sandstone beds. The unit shows a long-term aggradational pattern involving ca. 7-10 Myr and is associated with low riverine influence.
By combining the observations and interpretations of the Bardas Blancas Formation with other subsurface and exhumed intensely bioturbated, shallow-marine successions we dispute the general assumption that they can be associated to low frequency or low magnitude of storms. Alternatively, we propose that the long-lived efficiency of benthic fauna on destroying most if not all the storm-event beds that reached the offshore-transition sector, results from the combination of two or three factors: deposition in relatively confined marine depocentres, persistent low riverine influence, and long-term aggradational stacking pattern. As these conditions can be recreated in a variety of basin styles, such as rift, early post-rift, and foreland settings, the recognition of thick, bioturbated successions as the ones discussed here can be used to infer more realistic constrains for depositional models and better predict facies distribution in these storm-influenced systems.