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Exposed upper Proterozoic and Cambrian rocks in the Caborca region, Sonora, Mexico, consists of a shallow-water miogeoclinal sequence of quartzite, siltstone, dolomite, limestone, and minor amounts of conglomerate and greenstone. A revised stratigraphic sequence is divided into 14 formations, 11 of which have been previously named and 3 of which are named here. The sequence is as much as 3,300 m thick and rests unconformably on a basement terrane of 1,600-to l,750-m.y.-old metamorphic and igneous rocks, intruded by l,400-m.y.-old porphyritic granite and l,100-m.y.-old granite. The sequence contains a wide assortment of fossils that include algallike filaments, possible trace fossils, and conical stromatolites (Conophyton and related forms) in the upper Proterozoic rocks; a primitive shelly fauna in the lowermost Cambrian rocks; and archaeocyathids, trilobites, Salterella, Hyolithes, Gimanella, gastropods, and brachiopods in the overlying Cambrian rocks. Paleocurrent measurements in six different formations reveal no dominant trend, although individual studies characteristically show a single dominant direction or oppositely directed paleocurrents suggestive of the ebb and flow of tides. The revised stratigraphic sequence in the Caborca region is correlated, much of it unit for unit, with stratigraphic sequences in the southern Great Basin region of eastern California and southern Nevada, in the San Bernardino Mountains of southern California, and in the Sierra Agua Verde of central Sonora, Mexico. The upper Proterozoic and Cambrian rocks of the Caborca region have long been recognized as a southward extension of the Cordilleran miogeocline, but interpretations have varied as to whether or not many of these rocks have been tectonically displaced. The position of the Caborca rocks to the southeast of correlative rocks in the Southwestern United States may be due to an eastward curvature of the Cordilleran miogeocline into northern Mexico, to major left-lateral offset along the Mojave-Sonora megashear, or to a combination of both these factors. A complex pattern of tectonic disruption involving leftlateral and subsequent right-lateral offset is also possible.

Mickwitziids are a unique group of brachiopods, known exclusively from the Lower Cambrian. Herein are described the new speciesMickwitzia lochmanaandM. multipunctata; both species are from southwestern North America.Mickwitzia muralensisWalcott, 1913, the largest mickwitziid, is reported here for the first time from the United States. This paper also discusses the distribution of mickwitziids, their shell structure, and the possible functions of mickwitziid punctae.

Two new species of worm tubes referable to the genus Onuphionella occur in Lower Cambrian strata in eastern California and western Nevada. Onuphionella durhami n. sp. is found in the Campito Formation (in pre-trilobite strata, in the Fallotaspis and, possibly, the Nevadella Zones) and O. claytonensis n. sp. occurs in the Middle Member of the Poleta Formation (Nevadella Zone). The unusual tubes are armored with an imbricated coat of mica flakes, reminiscent of the modern genus Owenia. The lowest occurrence of Onuphionella in western North America corresponds closely with the lowest occurrence of the genus in the Baltic region, indicating that correlations between the two regions are not greatly in error.

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The Lower Cambrian sequence south of the town of Caborca in northwestern Sonora, Mexico, has yielded the trilobites Nevadia ovalis n. sp. and Judomia orbis n. sp., plus an incomplete fallotaspid cephalon. The Nevadella Zone of Fritz (1972) can now be recognized in the Puerto Blanco Formation, and fossils from the middle part of the formation strongly suggest the presence of the Fallotaspis Zone. The new fossils permit improved correlation of the Mexican sequence to sections in the United States and to Lower Cambrian stages of China and the Soviet Union.