EARLY DEVONIAN BRACHIOPODS IN WEST-CENTRAL SONORA INDICATE DEPOSITIONAL CONTINUITY ALONG SOUTHERN MARGIN OF LAURENTIA

An Early Devonian (Pragian) silicified brachiopod fauna occurs in a thin limestone bed 105 m above the base of the Devonian San Miguel Formation (>240 m thick) near Rancho Placeritos, 40 km northwest of Hermosillo, Sonora, Mexico, and ~100 km southwest of the trace of a hypothetical Jurassic megashear. The fauna is dominated by Patriaspirifer aff. P. murchisoni and Meristella sp., but also includes Rensselaeria aff. R. ovoides, Leptocoelia sp., Plicoplasia aff. P. cooperi, Discomyorthis sp., Platyorthis sp., Anoplia? sp., and a single specimen of Costellirostra cf. peculiaris. The small Rensselaeria and conodonts of the E.s. sulcatus or E.s. kindlei Zone date the brachiopod fauna as early or middle Pragian. This fauna belongs to the Appohimchi Subprovince of the Eastern Americas Realm, and is the westernmost known Subprovince occurrence in North America. Similar brachiopod faunas occur in correlative strata in north-central Mexico and the southern and eastern United States, but have not been recognized in the western United States. Platyorthis and Leptocoelia are abundant in the Appohimchi Subprovince, but do not occur in the Nevadan Subprovince of the western United States.

Biogeographic distribution of the Appohimchi brachiopod fauna at Rancho Placeritos indicates continuous Early Devonian shelf deposition along the southern margin of Laurentia. However, the largely emergent southwest-trending Transcontinental arch formed a barrier preventing migration and mixing of many genera and species of brachiopods from the southern shelf of Laurentia in northern Mexico to the western shelf (Cordilleran miogeocline) in the western United States. These data negate the hypothesis of large magnitude (600-l,l00 km) left-lateral offset of Cordilleran carbonate-shelf rocks from the Death Valley region (California-Nevada) to Sonora along a northwest-striking Jurassic Mojave-Sonora megashear.