1 CONICET – Museo de Paleontología, Universidad Nacional de Córdoba, Casilla de Correo 1598, 5000 Córdoba, Argentina. E–mail: galbanesi@arnet.com.ar

2 School of Geology, Mongolian Technical University, Ulaanbataar, Mongolia. E–mail: minjin@mtu.edu.mn

Key words: Conodonts. Biostratigraphy. Late Ordovician. Tsagaan Del Formation. Mongolia.

During a field trip to southern Mongolia organized by IGCP project 410 (IUGS–UNESCO), "The Great Ordovician Biodiversity Event", in August–September 2001, the authors were led to examine the best available stratigraphic sections and their accompanying Lower Paleozoic biotas. One of the most interesting visited sites was the Tsagaan Del area (Figure 1), where macrofossils and carbonate samples for conodonts were collected in order to get a more accurate biostratigraphy for the Tsagaan Del Formation.

One of the most complete fossiliferous sections of the Upper Ordovician in Mongolia is exposed in the Tsagaan Del area. This significant section is located ca. 40 km west of Bayankhongor City in Central Mongolia (Figure 1). The Tsagaan Del Section was discovered by German geologist D. Andres and colleagues during a geological survey in 1960. On the basis of the registered fauna, which includes trilobites, brachiopods, gastropods, corals, and conodonts, a Silurian age was originally assigned to the section (Blagonravov, 1973). In 1974, Kh.S. Rozman and Ch. Minjin began to study some of the invertebrate fossil groups; in particular, the brachiopods and corals, and proposed a preliminary biostratigraphy for the Tsagaan Del Section (Bondarenko and Minjin, 1977; Rozman and Minjin, 1981; Bondarenko et al., 1981; Bondarenko, 2002). The Rugosa and Stromatoporoidea, were also studied by Bolshakova and Ulitina (1985), and the bryozoans by Kopaevich (1984). The first record of a conodont form–species from the Tsagaan Del Formation, Icriodella irregularis, was in the book "Geology of Mongolian People Republic" (Blagonravov, 1973). A conodont assemblage including Icriodella superba, Panderodus compressus, Oistodus cf. inclinatus, Belodina sp., and Paltodus sp., recovered from bed 7 of the formation, was determined by Moskalenko (1981), while Panderodus gracilis and Erismodus sp. from bed 6 of same unit were determined by S.V. Dubinina (unpublished). Although these faunas are Late Ordovician in age, the absence of key species precluded a precise age determination. Subsequently, an Ashgill age was given from the associated macrofauna (Minjin and Undarya, 2001) (Figure 3).

Figure 1. Location and schematic geological map of the Tsagaan Del area.

Figure 2. Conodonts from bed 6 of the Tsagaan Del Formation.

As result of the first geological mapping of Mongolia, seven isolated outcrops of the Tsagaan Del Formation were recognized in tectonic blocks of the Bayankhongor suture zone (Blagonravov, 1973). The type section of the Tsagaan Del Formation is located within that suture zone, where outcops show west–east strike, have an almost vertical dip, and are ca. 5 km in length. These outcrops are mostly represented by red sandstones, continental conglomerate facies, and basalts. To date, only two localities, Tsagaan Del and Melden Teeg (= Tsul Tskhir), have yielded fossil remains. At Melden Teeg, fossils are rare and have been less thoroughly studied than those from Tsagaan Del, although a list of registered forms includes a similar fauna to the present one from white–grayish limestones.

Seven beds can be recognized in the Tsagaan Del Formation (Figures 1, 3) following descriptions by Minjin and Undarya (2001). Bed 6, 20–25 m in thickness, which yielded the conodont fauna described below, is characterized by gray–brownish argillaceous limestones, with rich and well–preserved fossils, including rugosans, tabulates, heliolitoids, stromatoporoids, bryozans, brachiopods, trilobites, crinoids, and conodonts. Regional correlation of the Tsagaan Del Formation allows two regional stages to be distinguished in Mongolia; i.e., Bayankhongor (Caradoc – Lower Ashgill) and Khangai (Middle Ashgill) (Bondarenko et al., 1981).

One carbonate sample (ca. 2.5 kg) from Bed 6 of the Tsagaan Del Formation, was completely digested by conventional acid etching techniques (10% acetic acid) in search of conodonts. Insoluble residues yielded 59 conodont specimens that belong to four species: Amorphognathus tvaerensis Bergström, Icriodella superba Rhodes, Panderodus gracilis Branson and Mehl, and Panderodus serratus Rexroad (Figure 2). The multielement apparatuses of the four taxa have been fully described by previous authors, and only comments on the taxonomy of the species are herein included. Recovered conodonts show Conodont Alteration Index 1.5, which indicates overburden temperatures lower than 90 ºC (Epstein et al., 1977). Specimens are housed at the Museum of Paleontology of the National University of Córdoba, Argentina, under code CORD–MP. Following review on taxonomy, biostratigraphy, and paleoenvironmental meaning of represented species considers original and most recent studies.

Figure 2: 5a,b

Material: 3 elements (1M, 1Sa, 1Sb).

Remarks: The apparatus of Amorphognathus tvaerensis was originally reconstructed by Bergström (1971) to include Pa, Pb, M, Sa, Sb, Sc, Sd elements of typical balognathid morphology. Recently, Dzik (1994) described ontogenetic variations in the species, phylogenetic relationships with other closely related taxa, and migratory patterns for the cold–water Mediterranean Province (cf., Bergström, 1983). A. tvaerensis most commonly occurs in deep–cold water biofacies like those described for the Las Plantas Formation in the Argentine Precordillera (Albanesi, 1998).

The A. tvaerensis Zone, reference for the early Upper Ordovician of the Atlantic Realm, was originally defined by Bergström (1971) from the Dalby Limestone, Fjäcka section of Dalarna, in Central Sweden. In the type area, the key species occurs in rocks characterized by the presence of graptolites from the upper part of the Nemagraptus gracilis Zone and the lower part of the Diplograptus multidens Zone. These units correspond to the lower portion of the Caradoc British Series; i.e., the lower (yet not nominated) Stage of the global Upper Ordovician Series (Bergström et al., 2000).

Icriodella superba Rhodes, 1953

Figure 3. Vertical distribution of taxa in the Tsagaan Del Formation. Bayankhongor suture zone, Central Mongolia.

Panderodus serratus Rexroad, 1967

The presence of Amorphognathus tvaerensis Bergström in Bed 6 of the Tsagaan Del Formation at Bayankhongor suture zone, Central Mongolia, indicates an Early Caradoc age (early Late Ordovician). This new record allows us to revise the preliminary age assignment based on the macrofauna. The conodont assemblage suggests a relatively shallow–water shelfal environment of deposition in low latitude domains.

The authors thank to Drs. E. Serpagli, R. Aldridge and G. Sarmiento for reviewing present contribution, and acknowledge support given by IGCP project 410, IUGS–UNESCO.

Accepted: June 15, 2003