LATE ORDOVICIAN PALYNOMORPHS FROM THE BASAL STRATA OF THE LA CHILCA FORMATION, CENTRAL PRECORDILLERA, ARGENTINA

JESSICA GOMEZ; DI PASQUO MERCEDES; SILVIO PERALTA

Lille

Encuentro; 2021 Virtual Annual Meeting of IGCP 653; 2021

Universidad de Lille

The first record of Late Ordovician palynomorphs is documented in the basal strata of the La Chilca Formation, Hirnantian-early Wenlockian in age, in the Poblete Norte section, Talacasto area, Central Precordillera of San Juan Province, Argentina. This unit unconformably overlies Early Ordovician shelfal limestone of the San Juan Formation, and in turn is paraconformably overlain by shallow-water deposits of the Los Espejos Formation of middle Wenlockian-Pridolian age. In Talacasto area, palynologic works carried out in the La Chilca and Los Espejos formations at Quebrada Ancha and Baños de Talacasto sections were published by several authors. In the Poblete Norte section, the basal cherty pebble conglomerate of the La Chilca Formation is not recognized. Instead, a ferruginous reddish matrix-supported sandstone level, with a low carbonate cement content, occurs. Overlying in sharp contact there is a ferruginous reddish matrix-supported sandstone level bearing phosphate nodules and pebble- sized clasts of chert. This bed followed by a shaly-siltstone layer with scattered ooids passing to a Fe-phosphate oolitic bed in sharp contact with the base of the Cuarcitas Azules Member. It is noteworthy that the Hirnantian-Rhuddanian rich-graptolite siltstones and shales of the Salto Macho Member has not been recognized in this section. From this oolitic bed, a diverse assemblage of well-preserved palynomorphs composed of 10 species of chitinozoans, 7 acritarchs and 6 cryptospores documented. Several age- diagnostic of the late Ordovician are recorded such as an acritarch Cheleutochroa diaphorosa Turner and chitinozoans Ancyrochitina sp. cf. Ancyrochitina merga (Jenkins), Armoricochitina sp. cf Armoricochitina nigerica (Bouché), Calpichitina lenticularis (Bouché), Conochitina minnesotensis (Stauffer), Euconochitina lepta (Jenkins), Lagenochitina deunffi Paris, Spinachitina bulmani (Jansonius). This association of chitinozoans and especially the presence of the Hirnantian Armoricochitina sp. cf. Armoricochitina nigerica allows confirming a late Ordovician age (Hirnantian), mostly related with Gondwana?Perigondwana regions, in accordance with the dating of the studied interval based on graptolites from the M. persculptus Zone in the Talacasto area (Baños de Talacasto section). At Talacasto area, Silurian species Armoricochitina nigerica and Crassiangulina tesselita Jardiné et al. emend. Wauthoz et al. are forms of the Atavograptus atavus Graptolite Zone (early-mid Llandoverian) at Baños de Talacasto section, and of the Llandoverian-Wenlockian of the top of the La Chilca Formation at Quebrada Ancha section, respectively. The Quadraditum deunffi Pöthe de Baldis has been mentioned in the lower Member of the Los Espejos Formation of the Precordillera region (Los Azulejitos section). The temporal distribution of these species has been modified recently, therefore, a review is recommended to rule out reworking on the younger sediments.Keywords: Palynomorphs, Hirnantian, La Chilca Formation, Precordillera, Argentina