The Neoproterozoic and Cambrian strata of Sonora are divided into four major assemblages: (1) the Neoproterozoic (ca. 760-1000 Ma) Las Víboras and El Aguila Groups considered to be part of the Rodinian supercontinent, (2) the Cerro Las Bolas Group containing Neoproterozoic Sturtian glacial deposits (760-700 Ma) in the lowermost part, (3) uppermost Neoproterozoic and Cambrian strata of the Cordilleran miogeocline that formed along the newly formed continental margin of Laurentia after the fragmentation of Rodinia, and (4) cratonal cover strata of Middle and Upper Cambrian age inland of the Cordilleran miogeocline. The most widespread strata of Rodinian age comprise the Las Víboras Group that covers an area of ∼5000 sq. km in a region centered ∼70 km north-northeast of Hermosillo. It is ∼2400 m thick and consists of tidal, fluvial, and eolian quartzite of the El Tápiro Formation; fluvial feldspathic arenite of the El Alamo Formation; and the shallow water marine quartzite and stromatolitic dolomite of the Año Nuevo Formation. A second unit thought to represent Rodinian strata is the El Aguila Group that crops out ∼50 km north-northeast of Hermosillo, Sonora. It consists of fluvial, tidal, and shallow marine dolomite; fine-and coarse-grained quartzite; and siltstone. Its thickness is poorly known but may be >1000 m. The next youngest strata consist of the Cerro Las Bolas Group that is exposed at Sahuaripa ∼160 km east of Hermosillo. The Cerro Las Bolas Group is 2600 m thick and contains a thin Sturtian-age (760-700 Ma) glacial diamictite and associated cap carbonate in its lowermost part. The main part of the Cerro Las Bolas Group consists of subtidal to supratidal carbonates and lesser amounts of subtidal and tidal siltstone and arenite, fluvial arenites, and a few fluvial conglomerate layers. The next youngest assemblage of strata in Sonora comprise the Cordilleran miogeocline. These strata consist of Neoproterozoic and Cambrian shallow-water subtidal, tidal, and supratidal carbonate, and minor fluvial, tidal, and shallow marine arenite, conglomerate, dolomite, limestone, siltstone, and sandstone deposited on a broad continental shelf. These strata are exposed in the Caborca region ∼200 km north-northwest of Hermosillo, and in small outcrops of correlative rocks extending eastward nearly to Sahuaripa. Cratonal cover strata consist of the Middle Cambrian fluvial and shallow marine Bolsa Quartzite and Middle and Upper Cambrian marine carbonate rocks and siltstone of the Abrigo Formation in northernmost Sonora and of several formations at Cerro Mogallón and Cerro El Potrero south of Sahuaripa. These formations are relatively thin units inboard of the thick continental shelf deposits of the Cordilleran miogeocline. Sonora lies in a unique position in the reconstruction of the hypothetical supercontinent of Rodinia. Sonora has been proposed to lie near either East Antarctica, the Kalahari of South Africa, southern South America, Australia, or Siberia. Information from the Rodinian strata of Sonora does not reconcile these views, although a position near Australia or Antarctica seems the most likely.
|Number of pages
|Special Paper of the Geological Society of America
|Published - 2002