SHALLOW-WATER CONODONTS OF THE MIDDLE EXPANSA ZONE (LATEST DEVONIAN) IN SONORA, MEXICO

We report the results of conodont studies from two localities in Sonora, Mexico, which provide the first detailed and well-constrained dating of Devonian rocks in these areas. Among the identified conodont species, the most significant in terms of biostratigraphy are “Icriodus” darbyensis Klapper, Polygnathus extralobatus Schäfer and Polygnathus semicostatus Branson and Mehl morphotype 1, whose presence indicates an age in the Middle expansa Zone (late Famennian). Other important species identified within these faunas are Bispathodus aculeatus aculeatus (Branson and Mehl), Bispathodus stabilis (Branson and Mehl), Polygnathus cf. inornatus E.R. Branson, Polygnathus obliquicostatus Ziegler, and Polygnathus aff. purus Voges.

The northernmost studied section is located in northeast Sonora at the Cerros Las Pintas, just south of the Cerros Las Mesteñas, and is related to the southwesternmost extension of the North American platform. The second section is located in central Sonora at the Cerro La Cueva, and lies within the continental shelf developed on the southern margin of the North American craton. Both localities are included in the shallow-water facies rocks described in Sonora, which overlay the southern margin of the North American craton and the epicontinental sea that extended through most of the Midcontinent region of the United States during the Paleozoic. In spite of the different paleogeographic scenarios, preliminary conclusions deduced from our conodont studies point to the biofacies III of polygnathid-“icriodid”, indicating similar paleoenvironmental features for both localities. The most notable difference, however, is the occurrence of the offshore conodont genus Palmatolepis in the lower part of the Las Pintas section, which is absent in the La Cueva section. Farther studies on the distribution of these conodont faunas may help to improve the paleogeographic configuration of Sonora and northwestern Mexico during the Late Devonian.