hich, FIG. 1. Location map of Cordén de Lila in the southern part of Salar isthe de Atacama. discussion of the depositional environment and the paleogeographic significance of the Quebrada Grande Formation. The distribution of the Paleozoic lithostrati- graphic units at Cordén de Lila is shown in figure 2. The oldest stratified rocks exposed in the area belong to the ‘Complejo igneo-Sedimentario del Cordon de Lila’ (CISL; Niemeyer, 1989). The lower wo thirds of this approximately 3,000-m-thick succession consist of basalts interbedded with urbidites, whereas the upper third is formed mainly by rhyolitic and rhyodacitic volcanic flows with few ine grained sediment interbeds (Niemeyer, 1989; Pérez et al., 2006). The age of this complex is not well constrained and has been tentatively assigned o the Cambrian?-Lower Ordovician? on the basis of their stratigraphic relationships (Niemeyer, 1989; Damm et al., 1990). The CISL is unconformably overlain by a marine succession of about 1,600 m “IG. 4. Facies associations recognized in the Quebrada Grande Formation. a- Matrix- supported conglomerates; b- Calcareous matrix- supported conglomerates; c, d- Sandstones interbedded with siltstones; e- Sandstones with linguoid ripples; f- Siltstones containing graptolites; g- Imbricate clast-supported conglomerates; h- Non-imbricated clast-supported conglomerates. B and T indicate the base and top of the beds. THE ORDOVICIAN QUEBRADA GRANDE ForRMATION, CORDON DE LILA (ANTOFAGASTA REGION, NORTHERN CHILE)... FIG. 6. Directional data from sedimentary structures. a- measurements from linguoid ripples (in interbedded sandstones and siltstone facies association); b- measurements from imbricate clast (in the clast-supported facies association). The overall facies distribution throughout the Quebrada Grande Formation suggests a transgress- ive succession starting with coastal, shallow-water fan delta deposits and finishing with open-marine shelf deposits. Nevertheless, the repetitive interbed- Although the main rhyolitic and dacitic composi- tion of the conglomerates clasts derived from the CISL in detriment of the more easily altered basaltic clasts suggests that the climatic factor was also important in the erosion of the volcanic arc. The linguoid ripple marks in twelve beds of the inter- bedded sandstones and siltstones facies associa- tion indicate a paleocurrent direction towards the west (Fig. 6a). On the other hand, clast imbrication (seven beds) and flute marks (two beds), groove marks (one bed) and erosive direccional marks (five beds) in the coastal fan facies indicate paleocurrent direction toward the south-east (Fig. 6b). If the marine principal flow is assumed to be orthogonal to the coast, a nearly north-south coastal- line can be inferred. FIG. 7. Fossils from the Quebrada Grande Formation. a-b: Paralenorthis sp.; e-e: Monorthis transversa Benedetto; f-g: Mollesella? sp.: h, j: Cryptograptus? sp.; i: Tetragraptus sp. Argentina (Neuman, 1999; Benedetto, 1998, 2003). The Quebrada Grande Monorthis specimens do not differ in any significant way from Monorthis transversa found in the volcaniclastic Molles Formation of the Famatina basin (Benedetto, 2003) and coeval rocks exposed to the north in the Rio Chaschuil area (Benedetto, 1994). In both localities the horizons bearing M. transversa have yielded conodonts of well-constrained Arenigian age (Albanesi and Vaccari, 1994; Lehnert et al., 1997; Albanesi and Astini, 2000). Further south, in the Precordillera basin, the San Juan Formation con- tains the spe Benedetto of la cies Monorthis cumillangoensis e Arenigian age (Benedetto, 1998, 2002). Paralenorthis is a widespread Ordovician genus mostly strata (Benedet confined to Arenig-lower Llanvirn 0, 2002). Afew specimens question- ably referred to Mollesella Benedetto support the Arenigian age Grande Forma lying siltstones of the lower part of the Quebrada ion. The graptolites from the over- are Tetragraptus sp. (Fig 7i) and Cryptograptus? sp. (Fig. 7h, j), that also support a late Arenig to early Llanvirn (Darriwilian) age. FIG. 8. Paleogeographic distribution of the ‘Arco magmatico occidental’, AMO, and the ‘Faja eruptive dela Puna occidental’, FEPO in northern Chile and northwestern Argentina. There are several possible explanations, including some degree of diachronism of the faunas, and/or the existence of some kind of barriers (environmental, geographic). Coira et al. (1999) have critically evaluated the existing chemical and age data, along with new major and trace element data, from the ‘Faja eruptiva de la Puna occidental’ (FEPO; Fig. 8). They included the CISL into this paleogeographic unit. Nevertheless, some authors suggested that the CISL would belong to an independent and older volcanic arc called ‘Arco magmatico occidental’ (Omarini and Sureda, 1994) (AMO; Fig. 8). This magmatic arc has a different history, as is here demonstrated by the existence of the Quebrada Grande Formation, which has no equivalent inthe FEPO. The continuity to the south of this magmatic arc could be the Choschas (Cambrian) and Alto del Inca (Lower Ordovician) plutons at Sierra de Almeida (Davidson et al., 1981; Mpodozis et a/., 1983). At Sierra de Argomedo fine-grained dark clastic sedimentary rocks with volcanic intercalations containing Cruzianacf. furciferacrop out, which is characteristic of the Lower Ordovician of the Central Andes (Breitkreuz, 1985; Breitkreuz, 1986). At La En- cantada a basaltic succession unconformably
