MASS-GRAVITY DEFORMATIONAL STRUCTURES: A CHARACTERISTIC FEATURE OF UPPER CRETACEOUS ROCKS IN EASTERN SONORA

Geological mapping of the Upper Cretaceous rocks along the southwest shoulder of the Cananea High and Aldama Platform in eastern Sonora (Sierra Azul, San Antonio, Banamichi, Moctezuma, Lampazos, Sierra Los Chinos, and Arivechi) shows complex deformational structures assigned to Upper Cretaceous large –scale mass-gravity activity. The structures, which crop out in eastern Sonora where the Upper Cretaceous rocks constitute a sequence >6000 meters thick, can be traced for more than three hundred kilometers, from the Sonora-Arizona border to the Arivechi region and even further south. Structures associated with mass-gravity movements are well preserved within depositional bodies thereby avoiding the problem of discrimination between tectonic structures and syndepositional emplacement of megablocks (slumps or slides), sedimentary breccias and locally, turbidites. Some megablocks are over 5 km long and hundreds meters thick. These different characteristics suggest a new interpretation of the Late Cretaceous evolution in this region of northwestern Mexico, one in which uplift and exhumation played a primary role. Uplift and exhumation are possibly associated with Late Cretaceous extension. Deformation is recorded by a regional angular unconformity, separating tilted Late Cretaceous rocks sequences from underlying faulted and folded Proterozoic, Paleozoic, and Mesozoic megablocks in a Late Cretaceous matrix. The megablocks show a gradational transition from coherent beds in their upper and middle parts to fragmented beds along their edges and bases. The results provide new insights on the geological evolution of eastern Sonora and allow us to establish a correlation between units exposed in northern and northwestern Sonora and southern Arizona.