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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.

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