Bioerosion on Crassostrea orbignyi (Ihering, 1897) as an indicator of paleoenvironmental conditions during the early Miocene in southern Patagonia, Argentina

PARRAS, A.; GRIFFIN, M.

Eger

Workshop; 8° International Bioerosion Workshop; 2014

Eszterházy Károly College, Department of Geography - Mátra Museum of Hungarian Natural History Museum

Bioerosion is an important feature to be considered in paleoenvironmental reconstructions because it is a process closely tied to local environmental conditions. Oyster shells are one of the substrates with the highest chances of preservation and provide an excellent record of bioerosion structures. In addition, the morphology of oyster shells is strongly influenced by ecological factors. The shells also preserve other taphonomical features, providing important clues to the prevailing environmental conditions and the processes acting on them. The bottom of the Santa Cruz Formation (Burdigalian, early Miocene), exposed along the coastal cliffs of southern Patagonia (Austral Basin, Santa Cruz Province, Argentina), contains monospecific oyster reefs of Crassostrea orbignyi (Ihering, 1897). These oysters lived in southern Patagonia during the final stages of the Atlantic transgression known, with the informal name of Patagoniano, occurring through the Paleogene/Neogene boundary. Although these oysters have been mentioned since the late 19th century and early 20th century (Ihering, 1897, 1907; Ortmann, 1902), studies on the bioerosion traces they carry are lacking. In this contribution we studied bioerosion structures on 80 specimens (left and right valves in the same proportion) of C. orbignyi. The material comes from four localities (Cañadón Feruglio, Cerro Monte León, Cerro Observación and Cerro Redondo), lying along ~80 km of the coast, from Monte León National Park in the north to the mouth of Coyle river in the south. The bioerosion structures were identified and Chi-squared test was used to test bioerosion trace preferences for the outer or inner surface and for different sectors of the valves. In addition, taphonomic processes and the main paleoenvironmental conditions were analyzed. Crassostrea orbignyi reefs show a bed- or lens-geometry from 0.20 to 1.5 m thick and 10 m to over 100 m long. They are formed by mostly thin shelled specimens of between 7 and 14 cm (exceptionally 20 cm) high, embedded in fine silty sandstone to siltstone matrix. Specimens are both juveniles and adults, in life position, forming nests of including two to ten specimens, or building bioherms formed by clusters of successive generations of specimens. Taphonomic observations indicate that the shells are very little altered by disarticulation, bioerosion, encrustation and abrasion; there is no evidence of high fragmentation of the shells. Morphological analysis of different bioerosion structures preserved on these oysters reveals the presence of structures attributed to clionaid sponges (Entobia isp.), polychaete annelids (Maeandropolydora isp., Caulostrepsis isp.), endolithic bivalves (Gastrochaenolites isp.), and predatory gastropods (Oichnus isp.). Of these, only ploychaetes are abundant and were recorded in 62% of the valves. Sponges are only present in less than 15% of the specimens. Bivalve and gastropod bioerosion structures are very scarce and appear in only 2% of the valves. No encrusters were observed other than oysters of the same species. Results revealed that traces of bioerosion are more common on the outer surface of the valves, and especially on that of the thinner right valves. The activity of polychaetes is more significant at the ventral margin and in the central sector of the valves. Sponge traces are very sparse, and occur indistinctly in different parts of the valves. The only two valves with Oichnus are left and the trace is on the central area of the external surface. Gastrochaenolites was recorded also in the central sector of the exterior of a right and a left valve. Bioerosion is positively correlated with primary productivity and negatively with sedimentation rate and energy. Bioerosion abundance on Crassostrea orbignyi is lower than that recorded in other Miocene species of Crassostrea (e.g. Farinati and Zavala, 2002; El-Hedeny, 2005; Hosgör and Okan, 2010), that were interpreted as living in nearshore, shallow marine environments. The low richness and abundance of bioerosion traces, together with sedimentological and taphonomic features, indicate that the monospecific C. orbignyi reefs were established in shallow low salinity water, probably subject to temporary subaereal conditions. The absence of encrusters and the low abundance of clionaid sponge bioerosion traces suggest moderate to high energy environments and high sedimentation rates. The results suggest that conditions prevailing during the early Miocene in the studied area were marginal marine environments, such as coastal plains or estuaries, that favoured the establishment of the oyster reefs. These conditions were short-lived and the oyster reef environment was replaced by mostly fluvial environments that reveal the onset of the frankly continental conditions occurring in southern Patagonia during the deposition of the rest of the Santa Cruz Formation.