MESOZOIC-CENOZOIC TECTONIC EVOLUTION OF THE NORTHERN SIERRA LOS AJOS, SONORA, MEXICO

Geologic mapping in the northern Sierra Los Ajos reveals stratigraphic and structural data important in determining the tectonic evolution of the range. The range is cored by Proterozoic, Cambrian, Devonian, Mississippian, and Pennsylvanian strata equivalent respectively to the Pinal Schist, Bolsa Quartzite and Abrigo Limestone, Martin Formation, Escabrosa Limestone, and Horquilla Limestone. The Proterozoic-Paleozoic sequence is mantled by rocks equivalent to the Upper Cretaceous Cabullona Group.

Absence of the Upper Jurassic-Lower Cretaceous Bisbee Group below the Cabullona Group and above the Proterozoic-Paleozoic rocks indicates that the Sierra Los Ajos area was part of the Cananea high, a paleotopographic highland and site of non-deposition within the Bisbee basin. Deposition of Cabullona Group equivalent rocks directly on Paleozoic rocks indicates that the Sierra Los Ajos area had subsided as part of the Cabullona basin during Late Cretaceous time. Basal beds of the Cabullona Group are clast-supported conglomerate composed locally of Paleozoic clasts. The conglomerate represents erosion of Paleozoic basement in the Sierra Los Ajos area coincident with development of the Laramide Cabullona basin.

The present-day Sierra Los Ajos is the highest mountain range in Sonora and was uplifted during Cenozoic basin and range extension. Cabullona Group equivalent rocks are exposed at high elevation in the northern Sierra Los Ajos and are an uplifted part of the Cabullona basin. Cenozoic uplift of the Sierra Los Ajos was largely accommodated by vertical movement along the north to northwest-striking Sierra Los Ajos fault zone flanking the west side of the range. This fault zone structurally controls the configuration of the upper San Pedro river basin, an important water resource in the U.S.-Mexico border region.