Lower Ordovician calcareous microfossils from the San Juan Formation, Argentina: a new type of calcitarch and its paleoenvironmental implications

The calcareous microfossils present in the Ordovician and Silurian carbonate successions around world are limited to few studies their biological affinities environmental preferences remain indefinite. In San Juan Formation from Cerro La Chilca section, a group of was recognized and, based on size, they included calcitarch classification. Two types calcitarchs have been recognized, Type-III calcitarch: large spheres with thin medium wall (diameter ~250 μm), Type-0 small ~80 μm). microfacies analysis lower part allowed defining five successive microfacies: burrowed bioclastic wackestone (M1), peloidal intraclastic packstone-grainstone (M2), floatstone (M3), wackestone-packstone (M4) boundstone (M5). These interpreted range shallow subtidal facies below wave action shoal reef facies. It is possible infer that recovered show variations size relation contain them. wackestone-type those calcitarch. sizes variation probably related gradual increase energy within environment. Occurrences Floian Oepikodus evae-O. intermedius conodont zone extends fossil record into Early Ordovician.