Tertiary stratigraphy and tectonic development of the Alamosa basin (northern San Luis Basin), Rio Grande rift, south-central Colorado

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 late Laramide basin. Initiation of Oligocene volcanism is marked by andesitic lava flows and volcaniclastic rocks of the Conejos Formation (0 to 2,300 m thick), also limited in extent to the western half of the Alamosa basin. Ash-flow tuffs (380 to 580 m thick) correlative to 29 to 27 Ma tuffs of the San Juan volcanic field cap the Conejos Formation in the western half of the basin and rest directly on denuded Precambrian basement in the eastern half of the basin. These tuffs exist in deep wells across the Alamosa basin and together represent a basinwide time marker. Extension related to the Rio Grande rift resulted in eastward-tilting of the entire basin area following emplacement of the ash-flow tuffs. Filling the resulting half graben is the upper Oligocene-middle Pleistocene Santa Fe Group (as much as 5.6 km thick) composed of variegated mudstones and coarse lithic sandstones and conglomerates. Lithic fragments in the Santa Fe Group represent two sources: variable-composition volcanic rocks from the San Juan volcanic field to the west (majority) and plutonic-metamorphic-sedimentary, basementderived rocks from the Sangre de Cristo Range to the east (minority). An angular unconformity within the Santa Fe Group documents strong early tilting due to movement on the Sangre de Cristo fault zone during an early phase of rifting (late Oligocene-early Miocene). The rift-related geometry of the crust beneath the Alamosa basin is that of two east-tilted crustal blocks creating two second-order half grabens within the basin.