Lower Cambrian trilobites, zonation, and correlation of the Puerto Blanco Formation, Sonora, Mexico

Abstract

The Lower Cambrian sequence south of the town of Caborca in northwestern Sonora, Mexico, has yielded the trilobites Nevadia ovalis n. sp. and Judomia orbis n. sp., plus an incomplete fallotaspid cephalon. The Nevadella Zone of Fritz (1972) can now be recognized in the Puerto Blanco Formation, and fossils from the middle part of the formation strongly suggest the presence of the Fallotaspis Zone. The new fossils permit improved correlation of the Mexican sequence to sections in the United States and to Lower Cambrian stages of China and the Soviet Union.