Geologic mapping and sedimentologic study of the upper Tesuque Formation refines interpretations regarding the influence of tectonics and climate in the aggradation of a Rio Grande rift basin in northern New Mexico. The upper Oligocene-upper Miocene Tesuque Formation is 1200-3000 m thick and is well exposed and well dated in the west-tilted Espaņola Basin half graben. The Tesuque Formation has been subdivided into Lithosome A, derived from the adjacent Sangre de Cristo Mountains to the east, and Lithosome B, derived from a more distant source across the Peņasco embayment to the northeast. Lithosome A represents deposition associated with westward drainages oriented transverse to the basin axis and is dominated by granitic gravel. Lithosome B southwest paleoflow is oriented obliquely to the basin axis and contains gravels dominated by Paleozoic carbonates, sandstone, and siltstone. New outcrop and well data indicate that Lithosome A has progressively prograded up to 9 km westward over Lithosome B since approximately 15-16 Ma. Also, stratal thickness variations (taken in part from past studies) between two middle Miocene ashes (White Ashes 2 and 4) indicate that relative subsidence of the basin floor increased southward away from the Peņasco embayment and westward away from the Sangre de Cristo Mts. The long-term basinward progradation of Lithosome A is interpreted to be a result of westward tilting of the basin floor and concomitant uplift of the Sangre de Cristo Mountains since the beginning of the middle Miocene. Significant upward-coarsening of sediment after 12-13 Ma may indicate an increase in the rate of this tectonic activity. An interruption of Lithosome A progradation occurred during faunal changes associated with the early/late Barstovian boundary (14-15 Ma), and may perhaps relate to geomorphic and hydrologic responses of the drainage basins to climatic changes.