Pozo Nuevo Formation: a new lithostratigraphic shelf sequence of early Ordovician age in central Sonora, Mexico

Abstract

The study of paleozoic sequences in northwestern Mexico allows to interpret the paleogeographic distribution and sedimentary environments from the southwestern margin of the North American craton. Near of the city of Hermosillo, in central Sonora, an Early Ordovician sedimentary succession crops out. This succession has been named Pozo Nuevo Formation and is characterized by fourteen members having a total thickness of 2,490 m. The main lithology includes cherty limestone, alternating with minor calcareous shale and quartz sandstone. The carbonate strata contain silicified trilobites (Ectenonotus sp., Trigonocerca sp., Perissopliomera sp., Kainella sp., and agnostids); brachiopods (Orthidiella sp., Aporthophyla sp., Hesperonomia sp., and Anomalorthis sp.); nautiloids (Coreanoceras sp., Phragmosiphon sp., and Protocycloceras sp.); briozoars; crinoids; and ichnofossils assignable to the genus Skolithos. The genera Trigonocerca, Perissopliomera, and Kainella are reported for the first time in Ordovician rocks of Mexico. The stratigraphic range of the fossils corresponds to the Early Ordovician (490–470 My) and the rocks that contain the fauna were deposited in a shelf setting at the southwestern margin of the North American craton. The primary sedimentary structures and the fauna diversity are indicative of inner and shallow shelf sedimentary environments.