CHARACTERIZATION OF BIOEROSION STRUCTURES IN MARINE QUATERNARY MOLLUSKS FROM BAHÍA BUSTAMANTE (PATAGONIA, ARGENTINA): PRELIMINARY RESULTS

SEBASTIÁN RICHIANO, MARINA L. AGUIRRE, KAREN DAVIES, IGNACIO CASTELLANOS & ESTER FARINATI

Simposio; II Simposio Latinoamericano de Icnología; 2013

Bioerosion structures represent several kinds of activities (boring, drilling, rasping and scraping) by different organisms on hard substrates and can be the result of mechanical, chemical or a combination of both processes. The most common substrates able to be eroded in littoral environments are bioclastic remains. Molluskan assemblages are abundant and exceptionally well preserved in the marine Quaternary deposits along Argentina. The richest and thickest skeletal accumulations (mostly bivalve and gastropod shells) occur in beach ridges which reflect littoral palaeoenvironmental parameters during the most recent high sea-level stands in the past. These deposits comprise parautochthonous assemblages accumulated during the last transgressive-regressive Mid-Late Pleistocene to Mid-Holocene marine cycles (Marine Isotope Stages, MIS 11 to 1). From a sedimentological point of view, the marine terraces from Patagonia have two different parts. Firstly, the central part is mainly composed of massive, clast-supported conglomerate with scarce sandy matrix interpreted as the core terrace. On the other hand, above the massive core there are well stratified sediments (fine conglomerates with abundant sandy matrix) are apparent, representing the foreshore and shoreface deposits. These sediments commonly show low angle planar cross stratification and trough stratification. All the shells analysed in this study came from the upper part of the terraces, where the shells are more abundant and better preserved. The bioerosive trace fossils presented in these deposits comprise three ethological categories Domichnia, Fixichnia and Praedichnia. Moreover, macroborings (12 ichnotaxa) and microborings have been recognized. In order of abundance the macroborings are: Oichnus, Iramena, Leptichnus, Maeandropolydora, Entobia, Caulostrepsis, Centrichnus, Renichnus, Pinaceocladichnus, Pennatichnus, Gastrochaenolites and Umbichnus. In Pleistocene deposits Domichnia traces are the more common bioerosion structures, with 5 ichnogenera (Caulostrepsis, Entobia, Iramena, Maeandropolydora and Pinaceocladichnus) distributed in 53 molluskan specimens. In Holocene deposits Praedichnia traces dominate, showing 75 shells with Oichnus. In the modern beach, Domichnia is again the more abundant with 82 shells altered by 8 ichnogenera (Caulostrepsis, Entobia, Gastrochaenolites, Iramena, Maeandropolydora, Pennatichnus and Pinaceocladichnus). The deposits of Bahía Bustamante exhibit the greatest ichnodiversity pattern recorded in marine Quaternary deposits of Patagonia up to present, in general terms including similar ichnotaxa as those identified along the Bonaerensian littoral. To conclude, this preliminary study will bring light into several topics, like interrelationships between different organisms, predation strategies and ethological behaviours, among others.