Geology of the late Cenozoic Alma Basin, New Mexico and Arizona
— Brenda B. Houser

The Alma Basin is an irregularly shaped late Cenozoic basin located just south of the Colorado Plateau at the intersection of the Morenci-Reserve fault zone and the Mangas trench. The sedimentary rocks of the Alma Basin record four stages in the history of the basin: (1) a pre-Basin-and-Range-faulting stage (23 to 18.7 Ma), when the basin had a different shape than the modern basin, (2) an early Basin-and-Range stage (18.7 to about 6.0 Ma), marked by only minor deposition, during which the basin began to take on its present shape, (3) a third stage (6.0 to about 1.5 Ma), marked by significant subsidence and deposition of a relatively thick section of basin-fill fluvial sedimentary rocks, and (4) waning subsidence during the Quaternary and erosion of basin-fill deposits by the San Francisco River and its tributaries. Structurally, the Alma Basin is an asymmetric graben composed of about six non-rotated fault blocks that step progressively downward toward the Mogollon Mountains on the east. Nearly all the faults are high-angle normal faults. A depositional hiatus between about 16 and 6 Ma may have been caused by shallow emplacement of an intrusive body beneath the west side of the Mogollon Mountains. The presence of such an intrusion at this time is inferred from the age of hydrothermal alteration in the Mogollon mining district.