PALYNOLOGY AND PALYNOFACIES ANALYSIS OF THE LLANDOVERY-WENLOCK LA CHILCA FORMATION, SAN JUAN PRECORDILLERA, ARGENTINA

La Chilca Formation is exposed in the Central Precordillera of San Juan, western Argentina. It is sand-dominated with shaly intervals, showing gradual upward thickening and coarsening interpreted as offshore transition, inner shelf and shoreface facies associations.

Twenty-three samples from the Quebrada Ancha section were analyzed. Marine phytoplankton is predominant in the entire stratigraphic unit while terrestrial palynomorphs constitute less than 3% of the assemblage. Marine palynomorphs are abundant and fairly diversified, represented by approximately 58 species including statigraphically relevant taxa.

Correlations with marine palynomorph assemblages recognized worldwide allow constraining the age of the La Chilca Formation to the Late Ordovician?, Llandovery and Wenlock. Domasia trispinosa, Tylotopalla caelamenicutis, T. digitifera, Crassiangulina variacornuta, Stellinium rabians, Percultisphaera incompta and, P. cf. stiphrospinata are among the significant taxa. Terrestrial palynomorphs are exclusively represented by five cryptospore species; namely Gneudaspora divellomedia var. minor, Pseudodyadospora laevigata, Rugosphaera cerebra, Tetrahedraletes medinensis and Velatitetras retimembrana.

Comparisons to coetaneous assemblages from Gondwana and other paleoplates such as Laurentia, Baltica and Avalonia evidence strong similarities among them. This suggests the lack of provincialism of the Llandovery and Wenlock marine phytoplankton.

Two main associations have been identified based on the palynofacies analysis. Association B is mostly represented in the lower part of the section. Its major component is the amorphous organic matter characteristic of environments with low energy where it is well preserved as a result of anoxic/disoxic conditions and/or high productivity. These paleoenvironmental conditions are common in mid-distal platforms such as the lower part of the La Chilca Formation. Association A is characterized by a major amount of degraded non-biostructured translucent phytoclasts, suggesting an increase in the energy of the depositional environment. This association has been recognized in the upper part of the section where sandstone beds become more frequent, therefore indicating the shoreface facies.