BIVALVE TRACE FOSSILS FROM AN EARLY MIOCENE DISCONTINUITY SURFACE: BURROWING BEHAVIOUR AND IMPLICATIONS FOR ICHNOTAXONOMY AT THE FIRMGROUND-HARDGROUND DIVIDE

Ichnofossils developed in a firmground at the contact between the middle Eocene-lower Miocene continental Sarmiento Formation and the lower Miocene marine Chenque Formation, in central-eastern Patagonia, Argentina, are assigned to Gastrochaenolites ornatus and interpreted as dwelling structures of suspension-feeding bivalves. These record the activities of pholadids, most likely belonging to the Pholadinae. Bivalves are usually preserved as casts and external moulds within the biogenic structures. The details of the internal ornamentation in G. ornatus allow identification of two sets of scratch marks, permitting comparison with the boring behaviour of modern pholadids. In terms of ichnotaxonomy, when the same organism is able to excavate and bore, and the excavation technique is identical to the mechanical perforation technique, the same name should be used. Accordingly, Gastrochaenolites, whilst more commonly assigned to bioerosion structures in hard substrates, is herein regarded as available also for bivalve burrows in firm, but unlithified substrates. The firmground studied represents a co-planar surface of lowstand erosion and transgressive erosion that produced exhumation of deposits belonging to the Sarmiento Formation, providing appropriate conditions for the development of the Glossifungites ichnofacies. Differential erosion of the apertural necks, the heterogeneity of the available casting material, and the wide size range of ichnofossils suggest a complex history of colonization and erosion for this surface.