Bivalve trace fossils from an early Miocene discontinuity surface: Burrowing behaviour and implications for ichnotaxonomy at the firmground-hardground divide

CARMONA, N.B.; MÁNGANO, M.G.; BUATOIS, L.A.; PONCE, J.J.

USA

Congreso; GSA Annual Meeting and Exposition; 2006

Ichnofossils developed in a firmground at the contact between the middle EocenelowerMiocene continental Sarmiento Formation and the lower Miocene marineChenque Formation, in central-eastern Patagonia, Argentina, are assigned toGastrochaenolites ornatus and interpreted as dwelling structures of suspensionfeedingbivalves. These record the activities of pholadids, most likely belonging to thePholadinae. Bivalves are usually preserved as casts and external moulds within thebiogenic structures. The details of the internal ornamentation in G. ornatus allowidentification of two sets of scratch marks, permitting comparison with the boringbehaviour of modern pholadids. In terms of ichnotaxonomy, when the same organismis able to excavate and bore, and the excavation technique is identical to themechanical perforation technique, the same name should be used. Accordingly,Gastrochaenolites, whilst more commonly assigned to bioerosion structures in hardsubstrates, is herein regarded as available also for bivalve burrows in firm, butunlithified substrates. The firmground studied represents a co-planar surface oflowstand erosion and transgressive erosion that produced exhumation of depositsbelonging to the Sarmiento Formation, providing appropriate conditions for thedevelopment of the Glossifungites ichnofacies. Differential erosion of the aperturalnecks, the heterogeneity of the available casting material, and the wide size range ofichnofossils suggest a complex history of colonization and erosion for this surface.