Preliminary Palynological Study of the Ordovician-Silurian Transition in the San Juan Precordillera, Argentina

GÓMEZ, JESSICA; DI PASQUO, MERCEDES; PERALTA, SILVIO; SILVESTRI, LEONARDO JOSÉ

Revista Serie de Correlación Geológica INSUGEO

INSUGEO Sede Miguel Lillo

Año: 2023

1514-4186

In the San Juan Precordillera, the Ordovician-Silurian Transition (OST) is represented 31 by sedimentological, biostratigraphic, and climatic change as evidence of glacial and 32 postglacial events, including significant transgressive processes. This interval was 33 identified in two stratigraphic units: in the La Chilca Formation (Middle Hirnantian34 Early Wenlockian) Central Precordillera, and in the Don Braulio Formation 35 (Hirnantian-Llandovery), Eastern Precordillera (Hirnantian-Llandovery). For a 36 palynological study, samples were collected from the basal strata of the La Chilca 37 Formation in the Talacasto area (Baños de Talacasto, Quebrada Ancha and Poblete 38 Norte sections), and from the Ocher Mudstone Member of the Don Braulio Formation, 39 Villicum Range. Several associations of palynomorphs composed of species of 40 acritarchs, chitinozoans and cryptospores were recorded, with variable diversity and 41 state of preservation between poor to good. In the Poblete Norte section, from the basal 42 strata of La Chilca a palynoassemblage composed of ten species of chitinozoans, seven 43 species of acritarchs and six species of cryptospores, represents the first record of 44 Hirnantian palynomorphs illustrated in the Argentine Precordillera. Among the 45 diagnostic taxa, the presence of Armoricochitina sp. cf. Armoricochitina nigerica 46 confirms the Hirnantian age in agreement with the record of the M. persculptus 47 graptolite Zone, documented in the Poblete Norte section, as well as in the Baños de 48 Talacasto and Quebrada Ancha sections. Characteristic Hirnantian palynomorphs have 49 been registered for the first time in the Ocher Mudstone Member of the Don Braulio 50 Formation in its type section. Paleoenvironmental and paleobiogeographic inferences 51 based on these preliminary data are also addressed.