Mass-gravity deposits and structures in the Lower Cretaceous of Sonora, Mexico

Rocks that form the south flank of Sierra Azul, northern Sonora, Mexico, ar e correlative with the Jurassic(?) and Lower Cretaceous Bisbee Group of sou thern 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 sur faces that cut down-section into footwalls. We believe that further mass mo vement 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 s hortening, but rather they reflect earlier (Jurassic and Cretaceous) high-a ngle crustal movement that produced the paleo-upland (the Cananea high), th e basin, and the slope that guided their development. We suggest their verg ence 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 sug gest that they may have been derived from the margin waters of the Cananea high. Confusion of structures such as these formed by mass-gravity processe s may be one reason that the pattern of Laramide deformation in northern So nora is still poorly defined.