Darriwilian (Middle Ordovician) conodont biofacies from the Central Precordillera of Argentina

Abstract

Conodont biofacies structure was recognized throughout detailed stratigraphic sampling at three localities fromthe Central Precordillera of Argentina; from north to south: Las Aguaditas Creek, Las Chacritas River and Cerro LaChilca sections. A total of 50 carbonate samples yielded over 9900 identified specimens, including a new conodontspecies, Fahraeusodus jachalensis, which is described due to its paleoecological connotations. TheYangtzeplacognathus crassus ? Eoplacognathus pseudoplanus and Periodon macrodentatus zones are dominatedby the ubiquitous species Periodon macrodentatus and Paroistodus horridus at the three study areas. This enablesthe recognition of the Periodon?Paroistodus biofacies in the Central Precordillera. Numerical cluster methodologyallowed the identification of conodont sub-biofacies: the Protopanderodus?Semiacontiodus sub-biofacies in theLas Aguaditas Creek, the Protopanderodus?Parapaltodus sub-biofacies in the Las Chacritas River, the Parapaltodussub-biofacies at the Cerro La Chilca and the Fahraeusodus?Ansella sub-biofacies in all of the study sections. These sub-biofacies present distinctive attributes, which are correlative with changes in the depositional environment.The diverse Protopanderodus?Parapaltodus sub-biofacies developed with a shallowing of the basin while theFahraeusodus?Ansella sub-biofacies is characteristic of deep environments. Shallower water settings resulted inhigher species richness, diversity, and evenness of conodont communities, the eustatic change that led to the drowning of the carbonate platform produced a shift to deeper, anoxic benthic conditions that in turn induceda change in the taxonomic composition of the fauna, and affected the diversity of the conodont communities.