Nature and origin of spectacular marine Miocene shell beds of northeasternPatagonia (Argentina): Paleoecological and bathymetric significance

Biofacies and taphonomic analysis has allowed for the reconstruction of the paleoenvironmental history of accumulation of a series of spectacular moll usk-dominated shell-concentrations from the Puerto Madryn Formation and the benthic assemblages that inhabited the Miocene sea in northern Patagonia, Argentina. An upward-shallowing from open. mid-shelf to more restricted sho reface-foreshore environment's has been recognized. Transgressive, Maximum Highstand and Regressive phases are recognized based on the integrated appr oach of assemblages and lithofacies arrangement. Eleven mollusk-dominated f ossil assemblages were defined and grouped into Associations A, B, and C. T ransgressive and Maximum Highstand phases preserve three main shell beds th at record an upwards change from dynamic to complete bypassing conditions. These were deposited in, tidal current-dominated mid- and inner-shelf envir onments and belong within Association A. The top bedding surface records th e maximum depth att attained by the sea. The Regressive Phase is characteri zed by three upward-deepening cycles comprised of foreshore-shoreface sandb ar deposits containing Association C. Sandbars are capped by environmentall y condensed shell-beds of Association B and record deposition from the shor eface (wave-breaking zone) to mid-shelf-environments, all above storm-wave base. Based on, these fossil assemblages, seven. benthic life associations can be identified. The deepest ones inhabited the mid-to-inner shelf and were rep resented by suspension-feeders from gravel-substrata swept by strong tidal currents and by suspension-feeders from lower energy firm bottoms. Lower sh oreface sandy bottoms, close to fairweather wave base and affected by weak tidal currents, were inhabited by epifaunal suspension-feeders, whereas san dy bottoms close to the fairweather wave-breaking zone were characterized b y semi-infaunal deposit feeders and suspension feeders. The shallowest livi ng assemblages inhabited intertidal and foreshore settings and were represe nted by soft-bottom infaunal suspension feeders, as well as by firm bottom, fragile carnivorous and suspension-feeding epifauna.