The Pliocene Puye Formation of north-central New Mexico represents a rift-filling volcaniclastic alluvial fan sequence developed on the east flank of the Jemez Mountain volcanic pile. This coarse-grained sequence can be separated into two compositionally distinct members that are genetically unrelated. The basal Puye (here referred to as the Totavi Member) consists of a 25-m (82-ft) thick, clast-supported conglomerate with interbedded sandstone lenses. The Totavi is a coarse-grained braided stream deposit composed of reworked Precambrian terrane of the Sangre de Cristo Mountains and mafic lavas of the Taos basin. The sequence displays cross-bedded sandstones and conglomerates, and well-developed clast imbrication and long-axis orientation, which indicate a paleoflow direc ion to the southwest. The Totavi member is an axial-stream gravel deposit of the Rio Grande rift.

The Totavi member is conformably overlain by a 100+ m (300+ ft) thick volcanogenic alluvial fan deposit here referred to as the San Ildefonso Member. This member represents the eroded and reworked detritus of the growing silicic volcanic complex of the Jemez Mountains. The San Ildefonso Member consists of four major interstratified lithofacies associations. These include: (1) clast-supported, massive and horizontally stratified conglomerates, with interbedded sandstones; (2) matrix-supported conglomerates; (3) laminated claystones and mudstones; and (4) primary pyroclastic deposits of both silicic and mafic composition. The clast-supported conglomerates and interbedded sandstones are typical coarse-grained, braided-stream deposits representing longitudinal bar and minor transverse bar accumulations. Internal stratification and clast imbrication points to an easterly flow direction.

The matrix-supported units display a complete spectrum of types from clast-rich (debris flows) to clast-poor (mudflow) varieties. These deposits are sediment gravity flows developed in response to steep slopes, abundant unconsolidated volcanic detritus, and excessive loading by water uptake. A number of debris flows and mudflows are underlain by primary volcanic airfall material and were undoubtedly initiated by volcanism.

The laminated claystones and mudstones are lacustrine deposits that resulted from temporary damming of the alluvial fan and ancestral Rio Grande drainage systems. The San Ildefonso Member contains at least 7 primary silicic air fall ash beds and 3 basaltic ash beds. These not only provide a time framework for sedimentation but can be used to delineate lateral facies changes.

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