Abstract
Cerro Seco is one of six post-caldera rhyolite domes in the northern moat of the Valles Caldera, New Mexico. This study presents detailed mapping as well as petrographic and geochemical analysis of Cerro Seco’s eruptive units and surrounding lacustrine deposits. Cerro Seco’s eruptive members were previously mapped as three separate units: two flow units (lavas Qvse1 and Qvse2), and a pyroclastic unit Qvset. This study has revealed that the pyroclastic unit should be classified as two units: one ignimbrite and one hydromagmatic tuff. Outcrop morphology and pumice clast morphology support a hydromagmatic eruption for the newly classified unit; geochemical analysis illustrates that significant post-emplacement alteration involving water also occurred. This new study offers an in-depth characterization of Cerro Seco and a model for it its eruptive behavior, with primary focus on the hydromagmatic tuff; a new designation of Qvshy is proposed for the hydromagmatic unit. These findings are important because they identify Cerro Seco as the only eruptive center in the Valles caldera that produced a v hydromagmatic eruption. Findings and conclusions put forth by this study have not been identified until now, and are unique within the Valles caldera system