Crassostrea patagonica (dÓrbigny, 1842) shell concentrations from the late Miocene of Río Negro province, NE Patagonia, Argentina.

DOMENECH R.; FARINATI E. A.; MARTINELL J.

Spanish Journal of Paleontology

Sociedad Española de Paleontología

Lugar: Madrid; Año: 2014 vol. 29 p. 165 - 182

0213-6937

The Río Negro Formation (late Miocene-early Pliocene) mainly consists of continental deposits, but it contains a middle member of marine origin. It represents a transgressive-regressive sequence that can be studied in several outcrops along the N Patagonian coast. The taphonomical approach to the El Espigón marine deposits permits the identification of four main layers containing different kinds of skeletal accumulations, mainly formed by oyster shells [Crassostrea patagonica (D?Orbigny, 1842)]. These concentrations display three different morphologies (pouches, pavements and bouquets) with a different taphonomic signature. These deposits were formed in shallow marine environments affected by waves that gave rise to valve concentrations of different entities. They bear several shell beds that represent simple event, composite and hiatal to lag concentrations. Traces of bioturbation in the sediment (Thalassinoides, Teichichnus) and bioerosion on the shells (Entobia, Gastrochaenolites), and encrusters (cirripeds, bryozoans), are also abundant in the outcrop and constitute common components of these Miocene materials.  Layers 1 and 2 of the sequence were deposited in shoreface/foreshore environments at the begining of a highstand systems tract, while layers 3 and 4 did so at the ending, or at the begining of a forced regression, in foreshore environments.  A final erosional episode cut the top of layer 4, which truncated the abundant bioturbation developed there.