Taphonomic and paleoenvironmental features of an Eocene oyster reef in the Río Turbio Formation, Patagonia, Argentina

RODRIGUEZ RAISING, MARTÍN; CASADIO, SILVIO; GRIFFIN, MIGUEL

Córdoba

Congreso; Congreso Argentino de Paleontología y Bioestratigrafía; 2006

Academia Nacional de Ciencias-Cipal

The Río Turbio Formation comprises Eocene rocks originated in marginal marine environments (Azcuy and Amigo, 1991; Malumián et al., 2000). It lies exposed in the southwestern corner of the province of Santa Cruz, Patagonia, within the Austral Basin. Herein we describe – from both taphonomic and paleoenvironmental points of view – an oyster reef exposed at an outcrop of this unit located to the North of the town of Río Turbio (51º29’47.7”; 72º14’1.8”). The lower 13 m of the studied section include beds of medium- to coarse-grained sandstone, sometimes conglomeratic, showing tangetial cross-stratification and – to a lesser degree – trough cross-stratification. Associated to these deposits are beds showing heterolithic lamination. This facies may have originated as a consequence of bedform migration (dunes) and the development of small associated channels in a shallow tide-dominated marine environment. An irregular (erosive) contact separates them from the overlying 40 cm of conglomerate, which includes clasts of up to 2 cm diameter and a coarse-grained sandy matrix. This conglomerate grades upwards into a conglomeratic sandstone containing plant remains, scarce fragments of shells of Ostrea sp., and burrows of up to 5 cm diameter referable to Thalassinoides isp. The bed is mainly massive, although tangential cross-stratification can be observed at the base. These deposits may have originated from landward derived turbulent fluxes later re-worked by tides. A subsequent lowering of sedimentation rates is suggested by the presence of Thalassinoides isp. Immediately overlying these beds and showing a sharp contact surface, lies the oyster reef. It shows lenticular geometry with an average thickness of 1.5 m and a lateral extension of about 300 m. The bed – the matrix of which is muddy – includes oysters in life position, both isolated and arranged in bunches, and articulate shells deposited concordant to the bedding plane. At 0.5 m from the base is the greatest concentration of oysters in life position. The taphonomic attributes of the shells (i.e., very little bioerosion and no encrustation) suggest a marginal shore environment subject to salinity changes. The low sedimentation rate, the firm substrate offered by the conglomerate, and possibly the proximity of fluvial environments, are factors that jointly would have contributed for the development of the reef. Presently, oyster reefs of this kind grow in the outermost depression of the main distributary channels of estuaries. Over the reef, there is a lenticular shell-bed 25 cm thick and constituted by valves of Ostrea sp. Most of the bivalves are disarticulated, concordant or in chaotic position, sometimes arranged in stacking pattern. They show poor size selection and there are juvenile and adult specimens. Packing is dense and disarticulation and fragmentation are high, while abrasion, bioerosion, and encrustation are very low. The matrix is fine sandstone. This facies suggests an increase in energy and the shifting of the conditions for reef growth. The section continues upwards with sandy facies with tangential cross-stratification suggesting a renewed marine influence.