New Mexico Geological Society Annual Spring Meeting — Abstracts

The Puye Formation is an infilling of the Velarde graben on the western side of the Espanola Basin. It overlies the Chamita Formation, in places with a gradational contact. This sedimentological transition represents a pronounced change in fluvial systems associated with the linking of contiguous anddtainage-independent rift basins by a common axial drainage. The lower member of the Puye Formation (Totavi member) is a coarse-grained conglomeratic sequence of braided stream origin and represents the newly acquired axial drainage deposits. The upper unit of the Puye (San Ildefonso member) is composed of the marginally-derived epiclastic and pyroclastic deposits shed from a growing dacitic Tschicoma dome complex in the northeastern Jemez Mountains. The volcaniclastics prograded eastward as a large ailuvial fan
system that displaced the rift axial drainage. Concomitant basaltic volcanIsm in the southern Velarde graben is represented in the Puye Formatibn by intercalations of lavas and phreatomagmatic ashes.

Recent fossil evidence has established a late Hemphillian age (4.0 + 0.5 m.y.) for the basal San Ildefonso member in the type section. The youngest San Ildefonso beds are considered to be of early Pleistocene age and thus the member represents continuous fan sedimentation close to the time of the Lower BandelIer Tuff eruption (1.4 m.y.).