Insights into the distribution of shallow marine/estuarine early Miocene oysters from southwestern Patagonia: sedimentologic and stable isotope constraints

CUITIÑO, JOSÉ IGNACIO; ROBERTO VENTURA SANTOS; SCASSO, ROBERTO ADRIÁN

PALAIOS

SEPM-SOC SEDIMENTARY GEOLOGY

Lugar: Lawrence; Año: 2013 vol. 28 p. 583 - 598

0883-1351

The early Miocene ?Patagoniense transgression? in southwestern Patagonia, Argentina (Estancia 25 de Mayo Formation), resulted in deposition of shallow marine to estuarine deposits that bear numerous oyster shell beds distributed throughout the sedimentary column. Sedimentary facies analysis reveals that oysters grew in the nearshore paleoenvironments of the stratigraphically lower Quién Sabe Member, as well as in the tide-dominated, shallow marine to estuarine paleoenvironments of the upper Bandurrias Member. Two species present in separate portions of the column were identified: Crassostrea(?) hatcheri, distributed in the lower two thirds of the column and Crassostrea orbignyi, distributed in the upper third of the column, within the transition from marine to fluvial deposits (present in the overlying Santa Cruz Formation). Petrographic, cathodoluminiscence, and carbon and oxygen stable isotope analyses of individual growth increments were performed on material from all the oyster beds, together with consecutive time-series analyses for one specimen of each species. The stable isotopic composition of the shell seems to be associated with the microstructure of the growth increments. Selective diagenetic alteration affected chalky growth increments whereas translucent (foliated and prismatic) growth increments are well preserved and can be used to infer paleoenvironmental conditions. Isotopic and sedimentologic data indicate that C.(?) hatcheri lived in shallow, normal marine waters with a range of paleotemperatures from 10.6º to 20.5ºC. Monospecific beds of this oyster are related to opportunistic and quick colonization of the sea bottom. Decreasing growth rates through the ontogeny of this oyster are registered. More negative δ13C and δ18O values in C. orbignyi shells likely reflects decreased paleosalinities, and the resultant environmental stress controlled the development of these low-diversity oyster accumulations. The separate stratigraphic distribution of each oyster species was therefore determined by the paleoenvironmental conditions.