Analysis of borehole and reflection seismic data from the Alamosa basin (northern San Luis Basin, Rio Grande rift) reveals tectonic development in response to three Tertiary events, each with an associated package of rocks distinguished by mineralogy and petrology. Eocene redbeds of the Blanco Basin Formation (0 to 696 m thick) are micaceous, sandy mudstone and coarse arkosic sandstone units containing lithic pebbles derived from granitic basement rock. They were deposited in a late Laramide basin formed during wrench-fault-related segmentation of the early Laramide San Luis-Brazos uplift. The western half of the younger, rift-related Alamosa basin is superposed over this...