The Nazas Formation: A Detailed Look at the Early Mesozoic Convergent Margin along the Western Rim of the Gulf of Mexico Basin
Jon F. Blickwede, 2001. "The Nazas Formation: A Detailed Look at the Early Mesozoic Convergent Margin along the Western Rim of the Gulf of Mexico Basin", The Western Gulf of Mexico Basin: Tectonics,Sedimentary Basins, and Petroleum Systems, Claudio Bartolini, Richard T. Buffler, Abelardo Cantú-Chapa
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The lower Mesozoic Nazas Formation, where it crops out in the Sierra de San Julián in northern Zacatecas state, Mexico, consists of slightly metamorphosed volcanic rocks and unfossiliferous continental red beds measuring a combined total thickness of approximately 1 km. The age of the Nazas remains equivocal, although limited radiometric dates from the Nazas and adjacent units indicate that the formation is probably Triassic and/or Early to Middle Jurassic in age.
The Nazas Formation, as defined in the study area, consists of two members: (1) a thick (average 946-m) lower volcanic member and (2) a thin (average 146-m) upper red-bed member. The lower volcanic member consists of interstratified ash-fall tuffs, ash-flow tuffs, lava flows, and lahars. More than two-thirds of the volcanic rocks are andesitic and dacitic pyroclastic rocks. Rhyolites and latites also occur. The lower volcanic member is thought to have been deposited as a composite cone complex. The upper red-bed member contains channel-fill, sheet-flood, and debris-flow deposits, representing prograding medial to proximal alluvial-fan facies. These sediments are compositionally and texturally immature volcarenites, volcanic lithic arkoses, and arkoses; all detritus was derived from the lower volcanic member. The alluvial fan(s) probably developed in an active graben system that formed immediately after the deposition of the lower volcanic member. Graben formation was likely a result of crustal collapse above a depleted magma chamber.