Ordovician Fossils of Argentina

BENEDETTO, J.L.

Secretaria de Ciencia y Tecnología, Universidad Nacional de Córdoba

Lugar: Cordoba; Año: 2003 p. 560

987-43-5910-2

The paleobiogeography of the Ordovician fossils of Argentina is particularly interesting because they come from sedimentary basins with different depositional, paleoclimatic and geodynamic histories (Precordillera, Famatina, Central Andean, Puna; see Chapter 1). This complex scenario includes autochthonous basins, volcanic islands, marginal seas and displaced terranes, as well as a varied array of carbonate, siliciclastic and volcaniclastic facies. In this context, benthic assemblages have proved to be a valuable tool for paleogeographic reconstruction of the proto-Andean margin of Gondwana (Benedetto, 1993; Astini et al., 1995; Benedetto and Sánchez, 1996a; Benedetto, 1998; Benedetto et al., 1999). The biogeographic affinities of the high-diversity Precordilleran faunas have been the focus of great attention over the past decade, mostly due to the key role they have played in establishing the identity and allochthony of the Precordillera terrane and  also in defining its provenance, trajectory and timing of accretion to Gondwana. At a broad scale, the Precordillera terrane is among the few clear conjugate margins of Laurentia, and its history have led to a clearer understanding of the Iapetus Ocean development.  Taxonomic studies of Ordovician faunas have increased substantially over the past 15 years, and the current level of information is adequate to make accurate biogeographic comparisons with other paleocontinents and microplates, including the use of statistical analyses. The aim of this chapter is to present a comprehensive synthesis of the main biogeographic evidence from the whole described biota of each basin. Biogeographic affinities have been assessed at stage-level time intervals. The Arenig is particularly important because rocks of that age are represented in all the Argentine basins, making possible a direct faunal comparison between them (Waisfeld et al., in press). Although several proto-Andean configurations have been proposed (Mannheim, 1993; Dalziel et al. 1994, Dalziel, 1997; Dalla Salda et al., 1992, 1998; Keller et al., 1998; Buggisch et al., 2000; Aceñolaza et al., 2002, among others), the so-called ‘microcontinental model’ or ‘far-travelled microplate model’ proposed by the author in 1993, depicted in text-figure 3.4, provides a better explanation for the changing biogeographic affinities deduced from different groups of benthic organisms (Benedetto, 1993). Moreover, this configuration is also consistent with the main non-paleontologic evidence, including climatically sensitive rocks and stratigraphic features (Astini, 1995, 1998, 1999), paleomagnetism (Conti et al., 1996; Rapalini and Astini, 1998), K-benthonite ashes (Huff et al., 1998) and geodynamic constraints (Benedetto and Astini, 1993; Thomas and Astini, 1996, 1999; Benedetto, 1998; Ramos et al., 1996; Benedetto et al., 1999, among others). The text-figure 3.1 is global paleogeographic reconstruction for the Early Ordovician time showing the hypothetical pattern of paleo-aceanic circulation. A summary of the faunal constraints on the Ordovician paleogeography of the proto-Andean margin is presented at the end of this chapter.