LATE PALEOZOIC FUSULINIDS OF SOUTHWESTERN LAURENTIA: DEPOSITIONAL SETTINGS, BIOGEOGRAPHIC AFFINITIES, AND PALEOTECTONIC SIGNIFICANCE

Three sets of fusulinid faunas in Sonora, Mexico, record different depositional and paleotectonic settings along the southwestern margin of Laurentia (North America) during Pennsylvanian and Permian time. The settings include: offshelf continental rise and ocean basin (Rancho Nuevo Formation in the Sonora allochthon), continental shelf (La Cueva Limestone), and foredeep basin on the continental shelf (Mina México Formation). Our data represent 41 fusulinid collections from 23 localities with each locality providing one to eight collections.

Reworked fusulinids in the Middle and Upper Pennsylvanian part of the Rancho Nuevo Formation range in age from Desmoinesian into Virgilian. Indigenous Permian fusulinids in the La Cueva Limestone range in age from at least Wolfcampian to middle Leonardian, and reworked Permian fusulinids in the Mina México Formation range in age from early to middle Leonardian. Conodonts of Guadalupian age occur in some turbidites in the Mina México Formation, indicating the youngest foredeep deposit is at least Middle Permian in age. Our fusulinid collections indicate a hiatus of about 20 m.y. between the youngest Pennsylvanian (Virgilian) rocks in the Sonora allochthon and the oldest Permian (early Leonardian) rocks in the Mina México foredeep.

Most fusulinid faunas in Sonora show affinities to those of West Texas, New Mexico, and Arizona, and some genera and species are similar to those in southeastern California. As most species are similar to those east of the southwest-trending Transcontinental arch in New Mexico and Arizona, this arch may have formed a barrier preventing large-scale migration and mixing of faunas between the southern shelf of Laurentia in northwestern Mexico and the western shelf in the southwestern United States.

We conclude that the Sonora allochthon, consisting of pre-Permian (Lower Ordovician to Upper Pennsylvanian) deep-water continental-rise and ocean-basin rocks, was thrust northward 50–200 km over Permian and older shallow-water carbonate-shelf rocks and Permian deep-water foredeep rocks of southern Laurentia. The frontal part of the allochthon overrides Mina México foredeep strata, indicating terminal movement in Late Permian time.