Biostratigraphic studies of Lower Ordovician successions in northwestern Argentina were traditionally based on trilobite faunas (e.g., Harrington and Leanza, 1957), whose endemic character has restricted the geological correlation. During the last decade, graptolite and conodont findings produced an increasing amount of information regarding biostratigraphy, paleoenvironments, and paleothermometry of the Tremadocian rock sequences (e.g., Moya et al., 1994; Ortega and Albanesi, 2002, and references therein). A recent integration of biostratigraphic data allowed the proposal of a preliminary biozonal scheme for the Tremadocian of the Eastern Cordillera, Famatina System, and the Precordillera of western Argentina (Albanesi et al., 2001; Ortega and Albanesi, 2002). This information has enabled the recognition of regional stratigraphic unconformities, faunal intervals, and the intercontinental correlation for the referred segment of the Gondwanan margin.

The current study deals, in particular, with the Lower Ordovician siliciclastic succession cropping out in the area between the Angosto de Lampazar and Parcha localities of the Eastern Cordillera, Salta Province (Figure 1), which provides relevant information for the establishment of a formal biostratigraphic scheme for northwestern Argentina.

The study area presents important Lower Ordovician exposures that correspond to the western borderland siliciclastic facies of the Eastern Cordillera (e.g., Keidel, 1943); i.e., the Lampazar, Cardonal, and Saladillo formations, which correlate with the Santa Rosita Formation of other areas. This succession culminates with the Parcha Formation ("early Arenig" sensu Harrington and Leanza, 1957) that has been considered equivalent to the Acoite Formation of other localities. The referred succession unconformably overlies the Meson Group (Upper Cambrian). Its top strata are in tectonic contact with Cretaceous units of the Salta Group, or covered by Quaternary deposits (Figures 1, 2).

Figure 1. Location map and geology of the study area.

Tremadocian rocks were deposited in relatively shallow-water marine environments interpreted as tide-dominated platforms that alternate successive transgressive systems tracts (Buatois et al., this volume). The effects of tectono-eustatic events on Lower Ordovician sequences of the Eastern Cordillera were discussed elsewhere (e.g., Ortega and Albanesi, 2002; Moya et al., this volume).

The Saladillo Formation tectonically overlies massive sandstones of the Cardonal Formation (Figure 2). A significant thickness of the lower part of the Saladillo Formation is suppressed by a fault in the studied section. This formation begins with a greenish-gray heterolithic succession of sandstones and interbedded shales, in a prograding sequence that includes isolated coquinas with shelly fauna. The Parcha Formation conformably overlies the latter, beginning with thin sandstones and siltstones, and massive strata of brownish calcarenites. Astini (2002) interpreted a seismically induced deposition for some particular layers within this sequence. The succession follows upwards with a heterolithic succession of greenish-gray shales and calcarenites that present some typical erosive structures in platform environments. A thick interval that mainly consists of gray shales, with conspicuous cone in cone structures and ellipsoidal flattened concretions, continues upwards to the coarsening succession that ends up the formation.

A monospecific graptolite fauna that consists of Bryograptus sp. was collected from fine sandstone strata of the basal Saladillo Formation at Angosto de Lampazar (Ortega et al., 1998). First mention of this species was given by Harrington and Leanza (1957, p. 28) and González-Barry and Alonso (1984) for the Parcha area and Mojotoro Range. Specimens are fairly abundant but their preservation is poor. Rhabdosomes showing three primary stipes and closely spaced branches are recorded from points S 5 to S 15 (Figure 2). This fauna was assigned to the Bryograptus Zone, indicating an early Late Tremadocian age for the bearer layers (Ortega and Albanesi, 2002). Trilobites of the Kainella meridionalis Zone were described from the same strata by Tortello and Rao (2000). An interval containing frequent phyllocarids but no graptolites is developed between points S15 and S18.

Figure 2. Composite stratigraphic column of the Parcha area showing graptolite ranges. G, graptolite zones. C, conodont zones.

Graptolites reappear with the entrance of the kiaerograptid fauna (S 18), which ranges through the interval referred to as the Kiaerograptus Zone in the studied section (Ortega and Albanesi, 2002). The fauna includes Kiaerograptus sp., Adelograptus sp. cf. A. altus Williams and Stevens, Adelograptus sp., Paradelograptus sp. cf. P. antiquus, and Paradelograptus sp. These graptolites show a typical isolated metasicula and sicular bitheca, which are characteristic of the mentioned fauna (Maletz, 1999). Kiaerograptus sp. is a common form ranging throughout the interval. Usually, it has a two-stiped rhabdosome with a right sicula and a sicular bitheca. Some specimens resemble Kiaerograptus kiaeri (Monsen) figured by Maletz and Egenhoff (2001, fig. 4 H). The absence of guide forms, such as Aorograptus victoriae, may be related to our restricted collections, considering that it is present in other sequences of the Eastern Cordillera (Monteros and Moya, 2002). Similar graptolite faunas are present in the Kiaerograptus kiaeri and K. stormeri zones from Scandinavia (Maletz, 1999), and the A. victoriae Zone of eastern North America (Williams and Stevens, 1991) and Bolivia (Maletz and Egenhoff, 2001).

Kiaerograptus supremus Lindholm appears at point P 25, 4.4 m above the base of the Parcha Formation. Rhabodosomes provisionally referred to as Didymograptus sp. 1 (Figure 3, O) are recorded few meters above the FAD of K. supremus. The interval that extends between the latter level and the first appearance of Araneograptus murrayi (J. Hall) is preliminary assigned to the K. supremus Zone. It can be correlated with the K. supremus Zone from Scandinavia (Lindholm, 1991). The key taxon was also found in southern Bolivia, in the A. murrayi Zone (Maletz and Egenhoff, 2001).

The entrance of A. murrayi is recorded at point P 29, ca. 35 m above the base of the Parcha Formation. It is a common form, particularly abundant in sandstones and calcarenites of the Abra de Sococha (P 32 – P 35). Layers bearing this fossil were assigned to the A. murrayi Zone (Ortega and Albanesi, 2002). The graptolite assemblage is composed of declined and deflexed two-stipes rhabdosomes, provisionally attributed to Didymograptus sp. 2 (Figure 3-R). Several sections bearing A. murrayi were mentioned in the Eastern Cordillera and Puna (e.g., Gutiérrez Marco and Aceñolaza, 1987; Zimmermann et al., 1999). It is to note that A. murrayi (= Dictyonema yaconense Turner) was considered a key graptolite for the lower Arenig in Argentina (Turner, 1960). The presence of this fossil below the Hunnegraptus copiosus Zone at Abra de Sococha indicates a late Tremadocian age for the bearer layers. However, it is still not possible to know the real age of other localities yielding A. murrayi in the Eastern Cordillera (e.g., Santa Victoria, Aguilar, and Mojotoro ranges). The A. murrayi Zone from Abra de Sococha can be correlated with equivalent intervals from other regions; e.g., Scandinavia (Lindholm, 1991), Australia (VandenBerg and Cooper, 1992), and Bolivia (Maletz and Egenhoff, 2001).

The FAD of Hunnegraptus copiosus Lindholm is registered at point P 36 in the Parcha Formation. A few specimens of Paradelograptus sp. cf. P. rallus Jackson and Lenz, and P. sp. cf. P. onubensis Erdtmann, Maletz and Gutiérrez Marco were collected from the same level. The entrance of H. copiosus allows us to identify the homonymous zone, which extends throughout the shaly part of the Parcha Formation. Specimens of H. copiosus were identified in the estern flank of the Incamayo Creek at La Pedrera and Barranca creeks sections, associated with H. novus (Berry), Hunnegraptus? sp., Paradelograptus sp. 3, and Paradelograptus sp. 4. Fossils from the Thysanopyge argentina trilobite fauna, and rhabdosomes of "Didymograptus v-deflexus" cited for these levels were referred to the early Arenig age (Harrington and Leanza, 1957). The graptolite assemblage described herein (H. copiosus Zone) supports the idea that the entire Parcha Formation is late Tremadocian in age. The absence of Tetragraptus phyllograptoides and T. approximatus reinforces this assertion. The H. copiosus Zone can be correlated with the same unit from Scandinavia (Lindholm, 1991) and Bolivia (Maletz and Egenhoff, 2001). The record of the key species in the Chiquero Formation suggests that an equivalent interval is present in the Puna of Jujuy (Benedetto et al., 2002).

Figure 3. Graptolites of the Parcha and Saladillo formations in the Angosto de Lampazar – Parcha area. A, K: Bryograptus sp., A, CORD-PZ 18939, K, CORD-PZ 20031; B, C: Kiaerograptus sp., B, CORD-PZ 19849, C, CORD-PZ 19207-B; D, G: Adelograptus sp. cf. A. altus, D, CORD-PZ 19192, G, CORD-PZ 19197; E: Paradelograptus sp. cf. P. antiquus, CORD-PZ 19646.; F: Adelograptus sp., CORD-PZ 19607; H: Paradelograptus sp. 3, CORD-PZ 19054; I: Paradelograptus sp. 1, CORD-PZ 19640; J, L: Paradelograptus sp. cf. P. rallus, J, CORD-PZ 19087, L, CORD-PZ 19084; M: Hunnegraptus novus, CORD-PZ 19028; N: Paradelograptus sp. 2, CORD-PZ 19781; O: Didymograptus sp. 1, CORD-PZ 19738; P: Hunnegraptus copiosus, CORD-PZ 19090; Q: Paradelograptus sp. 4; CORD-PZ 19139; R: Didymograptus sp. 2, CORD-PZ 19685; S: Hunnegraptus? sp., CORD-PZ 20116; T: Kiaerograptus supremus, CORD-PZ 18789. All figures in 4,5x, except for figure H, which is 10x.

Conodont faunas from the study area were previously reported by Albanesi et al. (1997) for the basal strata of the Parcha Formation at Abra de Sococha. Tortello and Rao (2000) registered some isolated key conodonts through the lower succession that correspond to the Cardonal and Saladillo formations. In the eastern flank of the neighboring Gólgota Hill, Rao and Tortello (1998) recovered conodonts from correlative levels. A synthesis of these conodont faunas, including other records from Eastern Cordillera was presented by Albanesi et al. (2001) and Ortega and Albanesi (2002), who recognized the presence of the Cordylodus angulatus, Paltodus deltifer, and Paroistodus originalis – Acodus deltatus zones. They are correlated with the Bryograptus, Kiaerograptus – K. supremus, and Araneograptus murrayi – Hunnegraptus copiosus zones, respectively (Figure 2). The precise position of zonal boundaries is still to be established due to the lack of favourable facies for conodont preservation in the study area. The conodont association of the Cordylodus angulatus Zone (upper part) includes some particular species, such as Rossodus beimadaoensis and Acanthodus lineatus, which range through the Rossodus manitouensis Zone in epicontinental environments of Laurentia (Ross et al., 1997). This unit corresponds to the upper part of the Cordylodus angulatus Zone for deep- or cold-water facies of other schemes. The conodont assemblage of the Paltodus deltifer Zone consists of characteristic cold-water species such as the eponymous one (Löfgren, 1997), and other endemic taxa (cf., Zeballo et al., this volume). A similar situation is observed for the P. proteus – A. deltatus Zone (lower part) in calcarenite basal beds of the Parcha Formation.

Outcrops of the Saladillo Formation in the study area are referred to the Upper Tremadocian according to the Bryograptus and Kiaerograptus graptolite zones, and the C. angulatus and P. deltifer conodont zones.

The K. supremus, A. murrayi and H. copiosus graptolite zones and the P. proteus – A. deltatus conodont Zone (lower part) are recognized in the Parcha Formation, indicating a Late Tremadocian age for this unit.

Particular fossils such as the graptolite A. murrayi and the trilobite T. argentina, previously referred to as the lower Arenig in Argentina, are associated with late Tremadocian graptolites and conodonts in the Parcha area.

Present results suggest that the Parcha Formation is positioned between the Saladillo and Acoite formations in a composite stratigraphic column of the Eastern Cordillera.

Accepted: June 15, 2003