Most of brachiopods described below come from the uppermost part of the San Juan Formation cropping out at Quebrada Potrerillos, a tributary of the Los Piojos River. A few specimens were recovered from equivalent beds exposed at the Gualcamayo river (figure 1). In these sections, the upper 30 m of the San Juan Formation is characterized by thin-bedded limestones with nodular stratification interbedded with shale partitions. Limestones consist essentially of bioturbated bioclastic wackestones and nodular, often pyritic dark-gray mudstones. According to Cañas (1995, 1999) these nodular limestones were deposited in quiet-waters, below the storm wave base, in a distal ramp setting. The brachiopod assemblage described in this paper was recovered from these levels. They are overlain by a package ca. 15 m thick of parted limestones formed by couplets of dark mudstones and graptolitiferous black shales deposited in a deep ramp-to-basin transition (Cañas, 1995). This interval grades upward to the basinal black shales of the Gualcamayo Formation.
The age of brachiopods is well constrained by conodonts (Albanesi et al ., 1999) and graptolites (Ortega and Albanesi, 1999). The uppermost nodular limestones of the San Juan Formation and the overlying parted limestones have yielded abundant conodonts belonging to the T. laevis Zone, which correlates with the Baltoniodus triangularis Zone of Baltoscandia (Bergström, 1995). The FAD of Tripodus laevis has been proposed as the marker of the base of the Middle Ordovician Series (Webby, 1998), but currently the FAD of Protoprioniodus aranda was selected a the key horizon for the Lower/Middle Ordovician boundary owing to its wider geographic and environmental range (Albanesi and Carrera, 2001; Albanesi et al ., 2003). In the Potrerillos river section, P . aranda has been recorded from the top of the San Juan Formation and the base of the overlying Gualcamayo Formation (Albanesi et al ., 1999). The basal levels of the latter contain a diverse graptolite assemblage referred to the Isograptus victoriae maximus Zone of Ca3 age (Ortega and Albanesi, 1999). Ecologically, it can be ascribed to the deep-water isograptid biofacies ( cf . Cooper, 1998).
Outside the Guandacol depocenter, in the central Precordillera, coeval strata are represented by middle-ramp highly fossiliferous wackestones punctuated by thin intraclast lime conglomerates. Cañas (1995) interpreted this facies as deposited in low energy settings within the photic zone, with low sedimentation rates, affected sporadically by storm events. Megafossils include sponges, trilobites, bryozoans, brachiopods, echinoderms, gastropods, rostroconchs, cephalopods, receptaculitids and algae. The brachiopod assemblage encompassing the T. Laevis Zone was referred to the Niquivilia Zone (Herrera and Benedetto, 1991), which was recently redefined and renamed as Niquivilia extensa Zone (Benedetto, 2002). The nominal species of this zone is present everywhere the mid-ramp facies are exposed. In the Gualcamayo area (Gualcamayo River section) it has been recorded about 40 m below the contact with the Gualcamayo Formation (Benedetto and Herrera, 1993), from shallower facies than those analyzed herein.
Skeletal remains in the nodular limestones are mostly silicified. Acid etching of a sample of ca. 7 kg yielded well preserved but relatively scarce brachiopod shells and abundant trilobite sclerites. Fragments of other megafossils are rare. Although less well preserved than the etched specimens, some calcified brachiopods have also been recovered directly from the outcrops. High-quality internal molds were obtained by dissolution of calcareous shells.