PALYNOLOGY OF THE SILURIAN-EARLY DEVONIAN IN THE PRECORDILLERA, SAN JUAN PROVINCE: BIOSTRATIGRAPHY, PALAEOBIOGEOGRAPHY AND PALAEOENVIRONMENTS

VICTORIA JOSEFINA GARCÍA

Mendoza

Congreso; 4th International Palaeontological Congress. The history of life: A view from the Southern Hemisphere; 2014

CONICET

Palynological assemblages of the La Chilca and Los Espejos Formations, Precordillera Central, Argentina, were studied as part of the PhD thesis. These formations had been dated as Late Ordovician-Early Devonian based mainly on graptolite and brachiopod faunas. The aims were to accurately constrain the age of the stratigraphic units, proposing a new biostratigraphic scheme; besides to pinpoint the Silurian/Devonian limit, as well as to analyze the biodiversity trends of marine and terrestrial palynomorphs. The purpose was also to contribute to the understanding of the palaeogeographical and the palaeoenvironmental scenarios. Sixty-three samples from Río Jáchal, Río de Las Chacritas, Cerro La Chilca and Quebrada Ancha localities yielded acritarchs, clorophytes, trilete spores and cryptospores with variable abundance, diversity and preservation. The Devonian Talacasto Formation, showed a low number of poorly preserved palynomorphs, which led to enquire for the first time into a palinofacies analysis, including 15 levels of the upper section of the unit, at the Loma de Los Piojos locality. In the La Chilca Formation 105 species of marine and terrestrial palynomorphs were recorded, 48 of them constitute firsts records for Argentina. From palynological results, the base is dated as probably Hirnantian, subsequently upwards Llandovery, and Wenlock. The Los Espejos Formation yielded 271 species, 116 of them are recorded for the first time in Argentina. A Wenlock age is proposed locally, followed by the Ludlow, Pridoli, and Lochkovian. The palynological assemblages, mainly the miospores, allowed the recognition of the Silurian/Devonian boundary in Río Jáchal, the northernmost locality studied. This boundary has been poorly documented worldwide, especially by marine phytoplankton, since they are very rare or absent. The marine and terrestrial assemblages show strong similarities to those from other paleocontinents, indicating that provincialism was less significant than supposed before, even since the Lower Silurian. Strikingly, the La Chilca Formation, dated as Llandovery-Wenlock, presented rich and abundant phytoplankton diversity, but did not yield trilete spores in the whole unit. In the Los Espejos Formation the abundance and diversity of the terrestrial and marine palynomorphs throughout the studied sections, match fairly well with the sedimentary record. The marine phytoplankton is more abundant and diverse in the lower levels of the sections; considered as offshore facies. The miospores increase, in general, to the top of the sections, interpreted as a shoreface environment. The palynofacies analysis in the upper section of Talacasto Formation allowed the determination of a transition from suboxic marine to proximal marine environment.