Descriptions of volcanic rocks in the Sierra Madre Occidental of western Mexico have thus far emphasized the widespread and nearly continuous cover of ash flows tuffs and other units attributed to the Oligocene ignimbrite flare-up. However, much less attention has been given to the pre-Oligocene volcanic sequence beneath this ash flow blanket. At Nazas, Durango, on the eastern flank of the Sierra Madre Occidental, a well-exposed section includes voluminous felsic volcanic rocks of both Eocene and Oligocene age. The oldest igneous rocks at Nazas include a small exposure of intermediate volcanic breccias and lava flows that overlie Cretaceous limestones and apparently were deformed with them during Laramide tectonism. More commonly, the limestones are overlain by Tertiary volcanic rocks and continental clastic deposits. The Tertiary volcanic section is about 800 m thick, not including intercalated fanglomerates. The section records three distinct magmatic episodes: from 51 to 40 Ma, comprising felsic ash flow tuffs and intermediate lave flows and domes; a pulse of about 30 Ma, comprising voluminous felsic ash flow tuffs with an aggregate thickness of about 500 m; and from 24 to 20 Ma, comprising alkalic basalts. The Ahuichila Formation, a molasse-type conglomerate up to 200 m thick that underlies the Tertiary volcanic section, was deposited during or just after the Laramide deformation. The Santa In¿s Formation is a widespread fanglomerate, up to 110 m thick, that underlies the basalts and apparently accumulated during normal faulting. Eocene volcanism in the Nazas area is characterized by interfingering felsic ash flow tuffs and intermediate lave flows and domes.

A similar sequence of Eocene volcanic rocks has been mapped in central Chihuahua state, about 350 km to the northwest of Nazas. Felsic tuffs interbedded with intermediate volcanic rocks older than 40 Ma are also exposed at Tayoltita, Durango, about 200 km southwest of Nazas. These occurrences suggest that a wider spectrum of compositions and volcanic styles existed during the Eocene than during the Oligocene in the Sierra Madre Occidental. Eocene volcanism was more typical of orogenic magmatic belts developed at continental margins, whereas the Oligocene activity was dominated by voluminous felsic ash flow tuffs erupted during a transition of tectonic setting from subduction along a continental margin to intraplate extension. In addition to these three areas, there are several other localities in western Mexico having volcanic rocks with reported ages between 40 and 53 Ma. It is possible that the volcanic field during the Eocene in Mexico was comparable in extent to that of the Oligocene. The Eocene volcanism in western Mexico was in part contemporaneous with the Challis volcanic episode of northwestern United States and its extension into western Canada. However, Eocene magmatismin the Pacific Northwest apparently developed in an extensional tectonic setting, whereas in Mexico no evidence for Eocene extensional deformation is known. ¿1991 American Geophysical Union