NEW EVIDENCE FROM SONORA FOR CLOSE APPROACH OF GONDWANA TO LAURENTIA IN LATE DEVONIAN

Our reinterpretation of Late Devonian paleogeography is based on finding a globally important early Famennian deep-water conodont species Palmatolepis crepida in three samples from northwest Mexico. Two samples east of Hermosillo are from the leading edge of the Sonora allochthon, the western end of the Ouachita-Marathon-Sonora (OMS) orogen produced by the Gondwana-Laurentia continental collision. The third is from the southern Laurentia shelf northwest of Hermosillo. The allochthonous Los Pozos Formation at Arroyo Tesotitas and Cañón Máviro consists of deep-marine siliclastics deposited in a continental-rise setting. The “San Miguel Formation” at Rancho Placeritos consists of moderately deep marine bioclastic mudstones and grainstones.

We correlate the OMS orogen with the eastern Variscan belt in Western Europe and show that the Rheic Ocean was a narrow seaway in the Late Devonian. We show three southwest-trending land areas in the United States, which is part of the Old Red Continent; from east to west these are: the Appalachian Highland, resulting from the older Devonian Acadian orogeny; the Transcontinental Arch, extending from Wisconsin to Arizona; and the newly formed Antler Archipelago, formed by the Antler orogeny extending from California to Nevada and Idaho.

The abundant typical form of Palmatolepis crepida in the OMS orogen was originally described in Germany and has since been found in France, Belgium, and Poland as well as in Morocco. The form on the shelf autochthon differs in outline and curvature but resembles forms that have sparse occurrences in Nevada, Indiana, and Alberta. The species has been reported but not documented in sedimentary dikes in the Llano region of Texas. There, other species of Famennian conodonts closely resembling Western European forms have been well documented.