The Ordovician black shales of the Cerro Viejo of Huaco were originally mentioned by Borrello & Gareca (1951), who reported Nemagraptus gracilis in its upper part. The succession was known by different names in the geological literature (Harrington & Leanza, 1957; Turner, 1960) until Cuerda & Furque (1975), who referred it to the Los Azules Formation, and considered a Llanvirn to Caradoc age. The unit crops out on the western flank of the Cerro Viejo, between 30º 11’ 40" and 30º 15’ 30" S Latitude, and 68º 34’30" and 68º 35’20" W Longitude, in the Central Precordillera, San Juan Province (Figure 1).

The Los Azules Shale is composed of three members informally named as lower, middle and upper members, cropping out in many sections on the western flank of the Cerro Viejo anticline (Ortega, 1987) (Figures 2, 3). The lower member (ca. 6 m thick) is made up of silicified black shales and K–bentonite levels, which conformably overlie the San Juan Limestone. A rich graptolite fauna occurs. It was originally assigned to the Paraglossograptus tentaculatus Zone (Alfaro & Cuerda, 1986; Ortega, 1995) and subsequently to the Undulograptus austrodentatus (Darriwilian Da1) and Undulograptus dentatus (Da2) zones (Mitchell et al., 1998). Recently, Undulograptus dentatus and Holmograptus lentus zones were identified in the lower member (Brussa et al., 2003; Ortega & Brussa, 2003).

A study of the K–bentonites of the Cerro Viejo succession infers an isotopic age of approximately 464 Ma for the lower levels of the Los Azules Formation (Huff et al., 1997).

These strata are covered by brownish micaceous sandstones of variable thickness, and succeeded by grey siltstones and thin K–bentonites layers of the middle member. This member is ca. 220 m thick at the El Nido creek and ca. 95 m thick at the Los Azules creek. In these deposits the Pterograptus elegans, probably, Pseudamplexograptus distichus, and Hustedograptus teretiusculus zones (Darriwilian Da4) (Ortega, 1987; Ortega & Rickards, 2003; Brussa et al., 2003) were recognized. The basal micaceous sandstone yielded conodonts of the Pygodus anitae Subzone (Eoplacognathus suecicus Zone) (Ottone et al., 1999). A lower palynological assemblage (LAL) composed main by acritarchs and chitinozoans were also described for this member (Ottone et al., 1999, 2001).

The upper member (ca. 20 m thick) comprises calcareous siltstones and subordinate mudstone lenses. It is only recognized in the Los Azules, Amarilla, and Árbol Seco creeks. The graptolite fauna correspond to the Climacograptus bicornis Zone (Gisbornian, Gi2) and it is associated with conodonts of the Amorphognathus tvaerensis Zone (Ottone et al., 1999). An upper palynological assemblage (LAU) integrated by acritarchs and profusely ornamented chitinozoans, was identified in these levels (Ottone et al., 1999; 2001).

A stratigraphic gap is present between the middle and upper members embracing the N. gracilis Zone interval, and possibly the uppermost part of the H. teretiusculus Zone. The Los Azules Formation is unconformably overlain by Carboniferous strata of the Guandacol Formation.

The succession of the Los Azules Formation at Los Gatos creek is incomplete. The lower member and part of the middle member of the formation, which is unconformably covered by the Carboniferous deposits (Guandacol Formation), are exposed.

A rich graptolite fauna including Loganograptus logani (Hall), Pseudobryograptus parallelus Mu, Pseudotrigonograptus ensiformis (Hall), Tetragraptus bigsbyi (Hall), T. serra (Brongniart), T. reclinatus Elles & Wood, T. headi (Hall), T. quadribrachiatus (Hall), Xiphograptus lofuensis (Lee), Holmograptus bovis Williams & Stevens, Holmograptus sp. nov., Acrograptus sp., Glossograptus acanthus Elles & Wood, Glossograptus sp., Paraglossograptus tentaculatus (Hall), P. tricornis Mu, Geh & Yin, Cryptograptus antennarius (Hall), Arienigraptus zhejiangensis Yu & Fang, Isograptus victoriae divergens Harris, I. caduceus caduceus Salter, Undulograptus austrodentatus (Harris & Keble), U. sinicus (Mu & Lee), U. dentatus (Brongniart), U. primus (Legg), and U. cumbrensis (Bulman) occurs in the basal part of the lower member of the Los Azules Formation (Figure 4, 5). Most of these graptolites were recorded in the lower part of the Gualcamayo Formation, northern Precordillera (Ortega & Albanesi, 1999).

Approximately 0.60 m above the base of the member, Arienigraptus angulatus (Mu) and Arienigraptus sp. make their appearance. Some forms, such as I. v. divergens are restricted to the base of the unit, while others, e.g. T. headi, are just localized in the middle part of the lower member. A. zhejiangensis, A. angulatus and U. dentatus range upwards throughout the middle part of the member but they were not recorded in our collections from the uppermost strata.

This graptolite assemblage was referred to the U. dentatus Zone in accordance with the presence of the nominal species, and A. angulatus and Arienigraptus sp. It correlates with the U. dentatus Zone of the North America (Maletz, 1997b) and the Undulograptus intersitus Zone of Australasia (VandenBerg & Cooper, 1992).

In the upper part of the lower member, a thick K–bentonite bed (ca. 0.50 m) contains Pseudobryograptus sp. (proximal ends and young colonies), P. ensiformis, Tetragraptus sp. (a reclinated form), X. lofuensis, X. cf. disermus, Brachiograptus etaformis, I.. c. caduceus, A. angulatus, Arienigraptus sp., Glossograptus sp., P. tentaculatus, Cryptograptus sp. (siculae and early stages), and U. primus. The appearance of Archiclimacograptus, Haddingograptus and Hustedograptus allow reference to the Holmograptus lentus Zone (upper Da2), following the scheme of Maletz (1995) (see Brussa et al., 2003; Ortega & Rickards, 2003).

In the Precordillera, a similar assemblage was studied in the Corridita creek, to the north of the Cerro Viejo area (Máspero Castro et al., 2003), where scarce rhabdosomes of H. lentus were found in the middle member of the Gualcamayo Formation.

We do not have the definitive fauna of Da3 age, equivalent to the Nicholsonograptus fasciculatus Zone of Scandinavia and North America (Maletz, 1995; 1997a, b) or Diplograptus? decoratus of Australasia (VandenBerg & Cooper, 1992). It is possible that rocks of this age are not represented in the Cerro Viejo area. If that is the case, a hiatus might be present between the lower and middle members of the Los Azules Formation.

The basal coarse sandstone of the middle member bears conodonts, inarticulate brachiopods uniserial stipes and biserial graptolite remains. Immediately above this sandstone we record the entrance of Pterograptus elegans Holm, Acrograptus euodus (Lapworth), Kalpinograptus parallelus (Ni) and Wuninograptus sp., associated to Reteograptus geinitzianus (Hall), C. schaeferi Lapworth, and biserial graptolites (e.g. Archiclimacograptus, Haddingograptus, Hustedograptus, and Eoglyptograptus) which become more abundant constituents of the fauna (Figure 6). This graptolite assemblage was assigned to the P. elegans Zone, late Darriwilian (Da4a) in age (Ortega, 1995). Equivalent faunas were described in the Potrerillo range and the Corridita creek, northern Precordillera (Ortega & Albanesi, 2000; Máspero Castro et al., 2003).

The occurrence of Drepanodus robustus Hadding, Paroistodus horridus (Barnes & Poplawski), Periodon aculeatus Hadding, Polonodus tablepointensis (Stouge), Polonodus sp. A of Albanesi, Protopanderodus robustus (Hadding), Pygodus anitae Bergström, and Spinodus spinatus (Hadding) as casts on bedding plane surfaces, in the lower 20 m of the middle member, indicates the presence of the Pygodus anitae Subzone (Eoplacognathus suecicus Zone) for the bearer strata (Ottone et al., 1999) (Plate 1). The conodont assemblage represents the Periodon–Pygodus biofacies that characterizes deep–cold waters slope environments (Albanesi & Ortega, 1998).

The graptolite fauna of the lower member of the Los Azules Formation corresponds to the isograptid biofacies (Cooper et al., 1991) of noteworthy Pacific affinity. The graptolite assemblage of the middle member is dominated by biserial rhabdosomes, gradually increases pandemic forms, and incorporates some Baltic elements.

This is the classic locality of the Los Azules Formation. A complete sequence of this formation crops out therein; it includes the lower, middle, and upper members (early Darriwilian Da2 to early Gisbornian Gi2) (Figure 3).

The lower member and the lower part of the middle member contain the graptolite assemblages mentioned for the Los Gatos creek (U. dentatus, H. lentus, P. elegans, and probably P. distichus zones).

In the upper part of the middle member graptolites dominated by biserial rhabdosomes were referred to the Hustedograptus teretiusculus Zone (Ortega, 1987). The assemblage is composed by Thamnograptus sp., Acrograptus euodus (Lapworth), Reteograptus geinitzianus (Hall),, Cryptograptus schaeferi Lapworth, Glossograptus sp., Hustedograptus teretiusculus (Hisinger), H. vikarbyensis (Jaanusson), Hustedograptus sp., Gymnograptus sp., and Haddingograptus sp. cf. H. olivieri (Boucek) (Figure 6). Same levels yielded conodonts of the Pygodus serra Zone. The mentioned fauna is assigned to a late Llanvirn age (late Darriwilian Da4) (Ortega, 1987, 1995).

In the El Nido creek, located few kilometers to the south of the Los Azules creek, the higher strata of the middle member are exposed. The sequence contains Nemagraptus sp. cf. N. subtilis Hadding, Dicellograptus sp., H. teretiusculus, Orthograptus sp., conodonts of the Pygodus serra Zone, and the lower palynological assemblage (LAL) (Ottone et al., 1999, 2001).

The upper member of the Los Azules Formation paraconformably overlies the middle member. In the Los Azules creek the upper member is ca. 20 m thick. Graptolites of the Climacograptus bicornis Zone are associated to trilobites (Porterfieldia jachalensis Harrington & Leanza, and Guandacolithus sp. aff. G. furquei Harrington & Leanza) and conodonts of the Amorphognathus tvaerensis Zone, indicating an early Caradoc age for the bearer strata. A palynological assemblage (LAU) dominated by chitinozoans is present in these layers (Ottone et al., 1999, 2001).

The graptolite fauna is integrated by Aspidograptus sp., Callograptus sp., Dictyonema sp., Ptilograptus sp. cf. P. delicatulus Ruedemann, Thamnograptus capillaris (Emmons), Acrograptus euodus, Reteograptus geinitzianus, Glossograptus ciliatus Emmons, Cryptograptus tricornis (Carruthers), Nemagraptus gracilis (Hall), Dicellograptus salopiensis Elles & Wood, Dicellograptus divaricatus (Hall), Dicellograptus sp., Dicranograptus nicholsoni Hopkinson, Psudoclimacograptus scharenbergi Lapworth, Climacograptus bicornis (Hall), Orthograptus ex gr. calcaratus (Lapworth), and Glyptograptus? sp.

In calcirudite, the presence of Amorphognathus tvaerensis Bergström, Baltoniodus variabilis (Bergström), Drepanoistodus suberectus (Branson & Mehl), Panderodus gracilis (Branson & Mehl), Periodon aculeatus Hadding, Pragmodus undatus Branson & Mehl, Protopanderodus varicostatus (Sweet & Bergström), and Scabbardella altipes (Henningsmoen), among others, was recorded (Ortega & Albanesi, 1998).