PALEOBIOGEOGRAPHIC IMPLICATIONS OF GUADALUPIAN FAUNAS FROM NORTHWESTERN SONORA

The Monos Formation is represented by a middle-upper Permian (Guadalupian) marine sequence exposed on the north side of the Sierra del Álamo, Caborca, Mexico. It is composed by a >600 m-thick succession of siltstone, sandstone and fossiliferous limestone deposited in a rimmed platform setting in an arc-related basin in the southwest margin of Laurentia. These deposits are in unconformable contact with the upper Permian-middle Jurassic rocks of the Antimonio Formation and are included in the Caborca terrane, which is considered as an allochthonous terrane displaced from northwestern North American to the southeastern during the Pennsylvanian-Permian time. The presence of certain fossils within the Monos Formation, such as Parafusulina antimoniensis, a species of fusulinid found in the exotic terrains of North America, support the existence of this tectonostratigraphic terrane. Nevertheless, recent studies on the Monos Formation document that Parafusulina antimoniensis as well as Waagenoceras dieneri and some species of crinoids show similarities with those described in Texas and New Mexico.

In the present work, we identify the species of conodonts Jinogondolella aserrata, Jinogondolella postserrata, Sweetognathus fengshanensis, and Hindeodus excavatus that show an indisputable affinity with the associations described for the Guadalupian Delaware basin (Texas and New Mexico) rather than with those from the Phosphoria basin (Wyoming, Utah, Idaho and Nevada). If the hypothesis of the Caborca terrane is correct, this correlation indicates that paleoclimatic and paleoecological factors control the distribution of the conodont associations more than purely paleogeographic factors. It could also suggest a non-displacement of the Permian deposits of the Monos Formation. In any case, a profound study on Permian conodont associations of northern Mexico is necessary to better explain the paleogeography of this region.