Faunal shifts and climatic changes in the Upper ordovician of South America (Western Gondwana)

BENEDETTO, J.L., SANCHEZ, T.M., CASRRERA, M., HALPERN, K. Y BERTERO, V.

Ordovician of the World

Instituto Geológico y Minero de España

Lugar: Madrid; Año: 2011; p. 55 - 60

According to the microcontinental model the Precordillera terrane rifted off Laurentia in the late Early Cambrian and then drifted through a relatively narrow Southern Iapetus Ocean to finally collide with the Andean margin of Gondwana during the Late Ordovician (Benedetto, 1993; Astini et al., 1995). Such a trajectory from low to intermediate/high southern latitudes has been well documented by a progressive decrease of Laurentian faunal affinties and a correlative imput of Gondwanan taxa. By the end of the Ordovician the Precordillera basin was inhabited by the typical Hirnantia Fauna associated to glacigenic deposits (Benedetto, 1986), and during the Silurian its faunas had become indistinguishable from other Afro-South American assemblages. Such paleogeographic changes took place under highly variable climatic conditions at global scale documented by lithologic, biological and stable isotopic data (mainly ¦Ä13C and ¦Ä18O). Most evidence used hitherto to establish global paleoclimatic models for the Late Ordovician comes from the continuous, essentially carbonate successions of Laurentia, B¨¢ltica and China, as well as from high-latitude basins of North Africa and perigondwanan terranes such as Iberia, Sardinia, Armorica and Perunica (e.g. Boucot et al., 2003; Fortey and Cocks, 2005; Ainsaar et al., 2010). In this paper we analyze the paleoclimatic evidence from the well-known Precordillera terrane, which by the Late Ordovician had been accreted to the pre-Andean margin of Gondwana. Since carbon isotope data from the Precordillera are still limited (Marshall et al., 1997), we use lithofacial, stratigraphic and paleonto-logic evidence in order to support (1) a relatively warm paleoclimate during the Late Sandbian, (2) a Katian (Ka1-Ka3) warming interval ¨Cpartially equivalent to the Boda Event¨C, and (3) a cool-water posglacial trans-gression recording the first stages of development of the Hirnantia Fauna.