Abstract

Rocks that form the south flank of Sierra Azul, northern Sonora, Mexico, are correlative with the Jurassic(?) and Lower Cretaceous Bisbee Group of southern Arizona. We interpret them as basinal marine deposits featuring both mass-gravity deposition and deformation. Thick (hundreds of meters) bodies of sediment showing some internal disaggregation are separated by slide surfaces that cut down-section into footwalls. We believe that further mass movement formed these stacked slide masses into northwest-trending, southwest-vergent folds, though they contain many features normally associated with direct tectonism. Thus, the folds are not the product of Laramide crustal shortening, but rather they reflect earlier (Jurassic and Cretaceous) high-angle crustal movement that produced the paleo-upland (the Cananea high), the basin, and the slope that guided their development. We suggest their vergence shows paleoslope, not tectonic transport.

This basinal marine sequence contains blocks of reefal limestone equivalent to the upper Mural Limestone (Bisbee Group) that have been widely used as evidence of an in situ carbonate bank; here, they are allochthonous. We suggest that they may have been derived from the margin waters of the Cananea high. Confusion of structures such as these formed by mass-gravity processes may be one reason that the pattern of Laramide deformation in northern Sonora is still poorly defined.

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