Lingulid shell beds from the Ordovician of Argentina, with notes on other peri-Gondwanan occurrences

Ordovician shell beds have been classified from different points of view, considering their genetic processes (Kidwell et al., 1986), their stratigraphic features and concentrating processes (Kidwell, 1991), their sedimentation patterns (Davies and Miller, 1992), the shell concentrating processes (Fürsich and Oschmann, 1993), and on the basis of the taxonomic composition of their shell concentrations (Li and Droser, 1999). Along the Ordovician peri–Gondwanan shelves from high to intermediate paleolatitudes there are numerous occurrences of various types of shell beds, which are mainly intercalated in siliciclastic sequences. They are frequently monotaxic, e.g., concentrations of articulated brachiopod shells, bivalve shells, bryozoan remains, echinoderm debris, etc. Polytaxic shell beds are mainly composed of trilobite sclerites combined with gastropod, bivalve and rostroconch shells. A typical monotaxic shell bed, widely recorded in the Darriwilian of SW Europe, is formed by lingulid brachiopods, which occur in tens of localities in the Armorican and Hesperian massifs (Figure 1), around the transition between the Armorican quartzite and the overlying fossiliferous shales (Tristani Beds) (Emig and Gutiérrez–Marco, 1997). The same type of lingulid concentration is recorded in various stratigraphic positions in the Ordovician of northwestern Argentina, where it has been considered for its economic potential as sedimentary phosphorite ore (Leanza, 1972; Mastandrea and Leanza, 1975; Fernández, 1984, 1987) as well as in a few localities from the Upper Ordovician of Bolivia (Suárez Soruco, 1976 with references; Gagnier, 1987). The purpose of the present study is to consider the actual distribution of these lingulid shell beds, with detailed description of some typical occurrences in the Ordovician of Argentina. Apart from the peri–Gondwanan records, somewhat comparable lingulid shell beds have been previously described from the uppermost Cambrian sandstones of the Baltic area (Heinsalu, 1992; Puura, 1996) and along the present coast of Namibia (Hiller, 1993).