CAMBRIAN-ORDOVICIAN SHALLOW-WATER FAUNA FROM EAST-CENTRAL SONORA

Cambrian to Ordovician shallow-water strata have been described from several localities in the state of Sonora, northwestern Mexico, such as Caborca, Rancho Sobechi, Sierra Agua Verde, San José de Gracia, Rancho Las Norias, Sierra Martínez, Rancho Pozo Nuevo, and Rancho Bízani. In most of these localities, however, only informal lithostratigraphic units have been defined, and therefore a regional stratigraphic framework lacks. In particular, in the Arivechi area, east-central Sonora, the El Mogallón Formation includes the middle Cambrian, the La Huerta Formation comprises the upper Cambrian and the El Santísimo Formation the Lower Ordovician. Several fossil groups have been described from these strata, being the most important brachiopods, sponges, trilobites, and conodonts.

We studied several stratigraphic sections in the Arivechi area including the type sections of the Cambrian-Ordovician formations. In the El Mogallón Formation, sponges species such as Choia carteri and Diagonella cyathiformis are described for the first time in the area, which allow the identification of the Ptychagnostus atavus biozone. The La Huerta Formation is precisely dated as the Aphelaspis biozone because of the presence of the brachiopod Angulotreta triangularis. In the El Santísimo Formation, Early Ordovician conodonts are reported for the first time, and the association includes Scolopodus subrex, Drepanodus arcuatus, Drepanodus sp., Acodus sp., Paltodus deltifer, and Rossodus manituensis. These faunas are similar to those from equivalent strata in Nevada and Utah, which indicate paleogeographic connections during early Paleozoic times.