1 Department of Geology, R.A.U. University, Auckland Park 2006, Johannesburg, South Africa. E–mail: uz@na.rau.ac.za

2 Geologisches-Paläontologisches Institut, Univiversität. Münster, Corrensstr. 24, 48149 Münster, Germany. E–mail: bahlbur@uni-muenster.de

Key words: Stratigraphy. Clastic rocks. Tectonic processes. Puna Basin. Northwest Argentina.

For many years the association of Ordovician sedimentary and pre-Silurian magmatic rocks of the Puna has been object of numerous investigations. In the last years, the petrological, geochemical and isotope-geochemical composition was determined, as well as the provenance of the sedimentary rocks (Bahlburg, 1998; Bock et al., 2000; Zimmermann and Bahlburg, in press). The basin shows an evolution from a rifted margin to a retro-arc basin during the Arenig, and the deposition of siliciclastic turbidites in the Llanvirn to Llandeilo in a foreland basin (Bahlburg, 1990, 1998; Zimmermann et al., 2002; Zimmermann and Bahlburg, in press). During the Tremadoc, the first occurrence of volcanism and volcanic debris is observable (Las Vicuñas and Tolar Chico formations). The geochemical data of the Las Vicuñas Formation (Lower Tremadoc; Moya et al., 1993) document volcanism related to a subduction zone. In contrast, the shallow marine facies of the Tolar Chico Formation is dominated by quartz, which indicates a rifted margin provenance. During the Upper Tremadoc, a volcanism related to a continental volcanic arc could be recognized both in the southern as well as in the northern Puna. The basin was structured into several sub-basins by structural highs such as the "Cobres high" in the central basin region (Moya, 1997). The Tolillar, Diablo, volcanic successions, and Aguada de la Perdíz formations reflect this event in their petrological, geochemical, isotope-geochemical, and composition of lava flows (andesites to rhyolites). The sedimentary rocks were deposited mainly in volcanic aprons by turbidity currents and debris flows. In the Middle Ordovician the volcanic influence disappeared (Falda Ciénaga and Lina, Turbidite System formations). The predominance of quartz and the abundance of metamorphic and sedimentary lithoclasts confirm this tendency together with geochemical and isotope geochemical data. Extensional movements, which started during the uppermost Arenig and Lower Llanvirn (Lork and Bahlburg, 1993; Coira et al., 1999) produced a magmatism in the northern Puna with important plutonic bodies.

The deposition of recycled basement material and volcanic arc debris in turbiditic systems was dominant; only locally shallow marine facies could be observed. During the entire Ordovician, the deposition of detrital material was transported from south to north (n> 200).

The objective of this contribution is the reinterpretation of the stratigraphic succession of Ordovician deposits in the Puna (NW Argentina). An extended system of formation names, containing in many cases similar lithologies and same facies, complicates the understanding of the history of this basin. We propose to group the existing formations of the Ordovician in the Puna Region (NW Argentina) into facies complexes. This proposal results from basin analysis, provenance and facies studies, and helps to understand the Puna basin evolution during the Ordovician.

We propose a division into three groups based on outcrop evidence, facies analysis (borne out by sedimentology, tectonic studies, and petrology of light and heavy minerals), and geochemical, as well as isotope-geochemical methods:

1. Puna Platform Complex (+/– Tremadoc): Tolar Chico, Taique, Potrerillo and Chiquero formations

2. Puna Volcanic Complex (+/– Arenig): Las Vicuñas, Tolillar, Coquena, Diablo, Volcanic Successions and Aguada de la Perdíz formations

3. Puna Turbidite complex (+/– Llanvirn): Falda Ciénaga and Lina formations, Lower and Upper Turbidite System

The magmatism related to the continental arc occurs together with the volcanic debris in the sediments, but it is diachronic. Synsedimentary silicic lava flows of the Las Vicuñas Formation erupted in the Lower Tremadoc. Intermediate to silicic lavas in the Diablo Formation are Arenig in age. Lavas in the Aguada de la Perdíz Formation, the volcanic successions, and the igneous complex of Pocitos are 482 +/– 2 Ma (Lower Arenig).

The analysis of the Ordovician Puna basin, including a provenance study, shows the potential of clastic rocks as tracers of tectonic processes.

This is a contribution to the IGCP 436 "Pacific Gondwana Margin".