DEPOSITIONAL TRENDS OF THE UPPER TESUQUE FORMATION, ESPAÑOLA BASIN, N.M., AND INFERRED TECTONIC AND CLIMATIC INFLUENCES ON AGGRADATION

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 Española 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 Peñasco 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 Peñasco 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.