Major successions as well as individual units of Neoproterozoic to Lower Jurassic strata and facies appear to be systematically offset left laterally from eastern California and western Nevada in the western United States to Sonora, Mexico. This pattern is most evident in units such as the “Johnnie oolite,” a 1- to 2-m-thick oolite of the Neoproterozoic Rainstorm Member of the Johnnie Formation in the western United States and of the Clemente Formation in Sonora. The pattern is also evident in the Lower Cambrian Zabriskie Quartzite of the western United States and the correlative Proveedora Quartzite in Sonora. Matching of isopach lines of the Zabriskie Quartzite and Proveedora Quartzite suggests ∼700–800 km of left-lateral offset. The offset pattern is also apparent in the distribution of distinctive lithologic types, unconformities, and fossil assemblages in other rocks ranging in age from Neoproterozoic to Early Jurassic. In the western United States, the distribution of facies in Neoproterozoic and Paleozoic strata indicates that the Cordilleran miogeocline trends north-south. A north-south trend is also suggested in Sonora, and if so is compatible with offset of the miogeocline but not with the ideas that the miogeocline wrapped around the continental margin and trends east-west in Sonora. An imperfect stratigraphic match of supposed offset segments along the megashear is apparent. Some units, such as the “Johnnie oolite” and Zabriskie-Proveedora, show almost perfect correspondence, but other units are significantly different. The differences seem to indicate that the indigenous succession of the western United States and offset segments in Mexico were not precisely side by side before offset but were separated by an area—now buried, eroded, or destroyed—that contained strata of intermediate facies.

Prebatholithic rocks are exposed near the town of San Felipe in northeastern Baja, California. Two spatially separated stratigraphic sections contain mature quartzite, marble, metaargillite, and interbeds of pelitic schist and micaceous quartzite. Eight mappable rock units of formational rank collectively measure at least 3,800 m. The depositional setting was in a shallow-marine environment, marginal to the craton. Based on lithology and stratigraphy, the sequence is provisionally correlated with miogeoclinal rocks of Late Proterozoic and Early Cambrian age in northwestern Sonora, Mexico. Unit G is correlated with the Proveedora Quartzite and units D, E, and F are correlated with the Puerto Blanco Formation of the Caborca area.

Abstract Cambrian and Ordovician shelf, platform, and basin rocks are present in Sonora, México, and southern Arizona and were deposited on the southwestern continental margin of North America (Laurentia). Cambrian and Ordovician rocks in Sonora, México, are mostly exposed in scattered outcrops in the northern half of the state. Their discontinuous nature results from extensive Quaternary and Tertiary surficial cover, from Tertiary and Mesozoic granitic batholiths in western Sonora, and from widespread Tertiary volcanic deposits in the Sierra Madre Occidental in eastern Sonora. Cambrian and Ordovician shelf rocks were deposited as part of the southern Laurentian miogeocline on the southwestern continental margin of North America. Lower Cambrian shelf units in Sonora consist mainly of quartzite, siltstone, and silty limestone; limestone increases upward in the sequence. Middle Cambrian shelf rocks consist mostly of limestone, dolostone, and siltstone. Upper Cambrian shelf rocks are sparse in Sonora; where present, they consist chiefly of siltstone and minor limestone. Cambrian shelf rocks display subtle facies changes from west to east across Sonora. In northwestern Sonora, these rocks attain their maximum thickness and may represent the Early Cambrian shelf margin. At the Sierra Agua Verde section, 110 km (68 mi) east of Hermosillo, these rocks thin, have greater proportions of clastic material, and were probably deposited in an inner-shelf setting. A major unconformity is present near the base of the Cambrian in Sonora and is similar to the Sauk I unconformity in the Wood Canyon Formation in Nevada and California. The top of the Cambrian is transitional with overlying Ordovician strata. Cambrian cratonic platform rocks are exposed in northern Sonora and southern Arizona and include the Middle Cambrian Bolsa Quartzite and Middle and Upper Cambrian Abrigo Limestone. The most complete sections of Ordovician shelf rocks in Sonora are 50 km (31 mi) northwest of Hermosillo. In these sections, the Lower Ordovician is characterized by intraclastic limestone, siltstone, shale, and chert. The Middle Ordovician is mostly silty limestone and quartzite, and the Upper Ordovician is cherty limestone and some argillaceous limestone. A major disconformity separates the Middle Ordovician quartzite from the overlying Upper Ordovician carbonate rocks and is similar to the disconformity between the Middle and Upper Ordovician Eureka Quartzite and Upper Ordovician Ely Springs Dolomite in Nevada and California. In parts of northwestern Sonora, Ordovician rocks are disconformably overlain by Upper Silurian rocks. Northeastward in Sonora and Arizona, toward the craton, Ordovician rocks are progressively truncated by a major onlap unconformity and are overlain by Devonian rocks. Except in local areas, Ordovician rocks are generally absent in cratonic platform sequences in northern Sonora and southern Arizona.

The Mojave-Sonora megashear model, which implies left-lateral strike-slip motion of northern México in Jurassic time, remains one of the most influential ideas concerning the geology of México. A comprehensive review of the literature related to this topic does not yet allow resolution of the controversy over the validity of this hypothesis. A clear conclusion is that the original hypothesis was based on a relatively simplistic model of the geology of Sonora, as the basement of the Caborca terrane is not simply a fragment of the Mojave Precambrian basement province of eastern California. Attempts to use quantitative techniques in testing the model have yielded results contrary to the hypothesis, such as clockwise rotations indicated by paleomagnetic data, and the diversity and complexity of the basement of Caborca indicated by geochemical and geochronological data. Other quantitative methods such as zircon provenance studies in quartzites of the sedimentary cover yield inconclusive results. The main conclusion of the studies of detrital zircons is that Grenvillean zircons are relatively abundant, but that their presence cannot be attributed solely to sources in the Grenville province in a fixist model. Stratigraphic correlations of upper Paleozoic and Mesozoic rocks in Caborca with similar sequences in California and Nevada do not provide convincing arguments of large displacement, but should be evaluated in more detail. Elements that have the potential to test the hypothesis with greater certainty include detailed studies of basement rocks, a refined stratigraphy of the Jurassic volcanic and volcaniclastic arc rocks south of the inferred fault trace, and an increased understanding of depositional trends in the miogeoclinal sequence. Structural studies are sparse in this region. It is particularly important to gain a better understanding of the effects in time and space of Late Cretaceous–Tertiary contractional deformation. A tectonic evolution model that does not conflict with the existing data is the proposal that displacement of a para-autochthonous Caborca terrane may have occurred in the late Paleozoic. Nonetheless, available data and geologic relations in the Caborca region do not require Late Jurassic slip of several hundred kilometers. El modelo de la megacizalla Mojave-Sonora, el cual implica desplazamiento lateral izquierdo en el norte de México durante el Jurásico, permanece como una de las ideas más influyentes en la geología del país. Una revisión general de la literatura relacionada con el tema no permite aún resolver la controversia sobre la validez de la hipótesis, pero una conclusión clara es que la hipótesis original estaba basada en un modelo relativamente simplista de la geología de Sonora, ya que el basamento del terreno Caborca no es un simple fragmento de la corteza Mojave del este de California. Intentos de utilizar métodos cuantitativos han dado resultados contrarios a la hipótesis, como el de las rotaciones horarias indicadas por el paleomagnetismo y la diversidad de basamentos en Caborca que sugieren la geocronología y geoquímica; otros métodos producen resultados indeterminados, como la proveniencia de circones en las cuarcitas de la cobertura del terreno Caborca. La conclusión más relevante de esos estudios es la abundancia de circones de edad Grenvilleana, pero su presencia no puede simplemente atribuirse a fuentes en la Provincia Grenville en un modelo fijista. Las correlaciones estratigráficas entre secuencias Paleozoico tardío y Mesozoico en Caborca y secuencias similares en California y Nevada no producen argumentos convincentes a favor de grandes desplazamientos, pero deben considerarse con datos más detallados. Elementos que podrían evaluar la hipótesis con mayor contundencia son estudios más detallados del basamento, una estratigrafía fina del arco volcánico Jurásico y de las rocas volcanoclásticas al sur de la traza inferida de la falla y un mejor conocimiento de la secuencia miogeosinclinal. Son pocos los estudios estructurales en la región y en particular un problema importante es resolver en tiempo y espacio los efectos de la deformación compresional Cretácico-Terciario. Un modelo que no entra en conflicto con la evidencia existente es la propuesta de que el desplazamiento del terreno parautóctono Caborca haya ocurrido en el Paleozoico tardío. Sin embargo, los datos existentes y las relaciones geológicas en la región de Caborca, no requieren de un desplazamiento de cientos de kilómetros en el Jurásico Tardío.