Road Log Geology of Mexico: Mexican State of Sonora

Northern and central Mexico underwent east to northeast extension during the mid- to late- Cenozoic. North to northwest orientations of numerous epithermal vein systems indicate extension occurred as early as 30 mya north of the Trans-Mexican volcanic belt. Major episodes of faulting began 24 to 23 and 13 to 122 mya in Mexico and the Southwestern U.S. Faulting was accompanied by alkali basalts typical of intraplate rifting.

The Bisbee Basin makes up most of central and eastern parts of the State of Sonora up to the Chihuahua state line. The area is now occupied by the Sierra Madre Occidental volcanic plateau. During Jurassic time, volcanic rocks covered the area. Rhyolitic and andesitic rocks are also present aged late middle Jurassic. The Bisbee Basin developed during trans-tension of a rift as the Chihuahua trough formed.

Western Sonora was predominantly composed of volcanic and plutonic rocks and associated tectonic structures. Paleozoic and Triassic clastics deformed into folds appearing as the oldest rocks. These rocks were capped by erosional unconformities below the Tarahumara Formation of Cretaceous age. Ashfall tuff, volcaniclastics, ignimbrite regolith belong to the formation. The upper part is composed of rhyolitic ignimbrite, rhyolitic and siliceous rock types. Granodiorite intrusives, basalt porphyritic dikes, brecciated deposits, hydrothermally altered volcanics, and ash deposits were commonly observed along Federal Highway 16.

Eastern Sonora continued with similar observations of Western Sonora. Volcanic and volcaniclastics included basaltic lava flows, calc-alkaline granitoids, calc-alkaline andesites, silty sandstones and conglomerates, pyroclastic flows, porphyritic basalts and andesites, tuffaceous and scoraceous ash deposits, welded ignimbrites, meta-volcanics, welded breccias, granodiorite intrusives all were subjected to deformational folding and faulting. Tertiary continental clastics forming canyonland topography occurred between Yecora and Maycoba.

Chapter 1 introduces a general summary and overview. Chapter 2 discusses the tectonic evolution of Mexico. Chapter 3 presents the paleogeography of Mexico. Chapter 4 details the Basin and Range geology of Sonora. Chapter 5 presents the Western Sonora road log. Chapter 6 presents the Eastern Sonora road log while Chapter 7 continues the Eastern Sonora road log between Yecora and the Chihuahua state line. Chapter 8 summarizes road log observations.