Ar-40/Ar-39 age and oxygen isotope temperature of the Centinela Formation,southwestern Argentina: An Eocene age for crustacean-rich "Patagonian" beds

The presence of a prominent volcanic ash enclosed within sediments of the C entinela Formation, southwestern Argentina, permits establishment of an Ar- 40/Ar-39 age of approximately 46 Ma for these rocks. Oxygen isotopic analys is of shell material from the oyster Crassostrea? hatcheri Ortmann, 1897, s uggests seasonal fluctuation of temperature from about 15 degrees C to abou t 21 degrees C. In concordance with this, a diverse crustacean fauna, inclu ding nine families within the order Decapoda and one within Isopoda, bears strong affinities with temperate and subtropical faunas of the Atlantic Oce an basin and documents the southernmost extension of low latitude oceanogra phic influence on this region during the Paleogene. Establishment of an Eoc ene age for the rocks in the Centinela Formation provides the first definit ive documentation of rocks of that age in the region of Calafate, permits c orrelation of these rocks with Eocene strata southward to the vicinity of R io Turbio, and suggests that these rocks, assigned to the Patagonian sequen ce, may be substantially older than those in eastern Argentina, (C) 2000 El sevier Science Ltd. All rights reserved.