Ana Mestre

Darriwilian species of Histiodella (Conodonta) in the Argentine Precordillera

Mestre, A. & Heredia, S., iFirst article. Darriwilian species of Histiodella (Conodonta) in the Argentine Precordillera. Alcheringa, 141–150. ISSN 0311-5518. The Middle Ordovician successions in the Central Precordillera of Argentina are characterized by carbonate and fine mixed carbonate/siliciclastic deposits. Two middle Darriwilian sections spanning the Eoplacognathus pseudoplanus to Eoplacognathus suecicus Zones were sampled for conodonts, among which were found species of Histiodella. We revised the biostratigraphical distribution of these species in the Middle Ordovician of the Precordillera. This study confirms that the Darriwilian species of Histiodella present in the Argentine Precordillera have a similar distribution to those of equivalent age in Newfoundland, South China and the Baltic region. On the other hand, the overlap in the ranges of H. holodentata and H. kristinae seems to be restricted to a fairly short interval in the upper part of the E. pseudoplanus Zone. We also propose that the FAD of H. kristinae Stouge is accurately positioned in the upper part of the E. pseudoplanus Zone; thus this species should not be used to define the base of the E. suecicus Zone in the Ordovician biozonation of the Argentine Precordillera. Ana Mestre [amestre@unsj.edu.ar] and Susana Heredia [sheredia@unsj.edu.ar], Conicet—Instituto de Investigaciones Mineras, Facultad de Ingeniería, Universidad Nacional de San Juan. Av. Libertador y Urquiza (5400) San Juan, Argentina. Received 16.2.2011, revised 27.4.2011, accepted 13.5.2011.

La zona de Yangtzeplacognathus crassus (Conodonta), Darriwiliano de la precordillera central, San Juan, Argentina

Abstract. THE YANGTZEPLACOGNATHUS CRASSUS (CONODONTA) ZONE, DARRIWILIAN OF THE CENTRAL PRECORDILLERA, SAN JUAN, ARGENTINA. Elements of Yangtzeplacognathus crassus (Chen and Zhang) were recovered from samples collected in the San Juan Formation at three section in the Central Precordillera, San Juan Province, Argentina. The last meters of the San Juan Formation are exposed at the Quebrada del río de Las Chacritas, Cerro La Chilca and Quebrada Del Aluvión where these gray limestone strata are represented by grainstone, packstone-wackestone y dolopackstone. These limestone outcrops correspond to different carbonate ramp facies, from distal ramp, middle ramp, and lagoon. The key conodont Yangtzeplacognathus crassus documents the Yangtzeplacognathus crassus Zone in these sections, pointing towards a middle Darriwilian age for the conodont-bearing strata in the San Juan Formation and allowing correlation among these sections. The middle Darriwilian biostratigraphic frame of Baltica and central-south China is verified for the argentine Precordillera.

Biostratigraphic significance of Darriwilian conodonts from Sierra de La Trampa (Central Precordillera, San Juan, Argentina)

The Eoplacognathus pseudoplanus Zone is defined by a conodont fauna composed by E. pseudoplanus (Viira) and Dzikodus tablepointensis (Stouge) among others in the Central Precordillera, Sierra de La Trampra region. We revised the biostratigraphical distribution and control biofacies of the conodonts D. tablepointensis and E. pseudoplanus in the Middle Ordovician of the Precordillera. This study confirms that these species has a similar biostratigraphical distribution to those of equivalent age in south-central China, Baltica and Midcontinent region. We also corroborate that the presence of D. tablepointensis is accurately positioned in the E. pseudoplanus Zone. In this paper we discuss for the first time the morphological affinities of D. tablepointensis and E. pseudoplanus elements from the Precordillera with those same species from southcentral China and Baltica. On the other hand, we review the genus “Polonodus” galerus Albanesi and propose it as Kallidontus? galerus (Albanesi), extending the biostratigraphical record of the genus Kallidontus from the late Tremadocian to middle Darriwilian.

Ordovician Conodont Biostratigraphy of the Ponón Trehué Formation, San Rafael Block, Mendoza, Argentina

A review of the Middle Ordovician conodont fauna of the southern sector of the Ponon Trehue Creek, Mendoza Province, Argentina, is presented in this contribution. Different genera and species of conodonts were recovered from clastic–carbonate beds from the Ponon Trehue Formation. Two conodont zones and several subzones have been proposed for these outcrops, the Pygodus serra Zone with the E. robustus and E. lindstroemi Subzones, and the Pygodus anserinus Zone. An evaluation of conodont assemblages present in the last mentioned biozone allowed us recognizing two informal subzones.