TERTIARY FAULTING AND ITS RELATION TO BASIN ARCHITECTURE IN THE UPPER ARKANSAS BASIN, NORTHERN RIO GRANDE RIFT

Highly incised exposures of the Tertiary Dry Union formation in the southern Upper Arkansas valley reveal faults that are parallel to basin bounding structures and therefore may have influenced the unusual basin morphology in this region. The Upper Arkansas valley is one of the northernmost basins of the Rio Grande Rift. This extensional basin is characterized by a northern rhomb-shaped sub-basin and a southern sub-basin that has a triangle shape. The western half of this southern sub-basin is elevated with respect to the surrounding basin regions to the east and north, and it is the location of the incised Dry Union formation. The Dry Union formation has its type section to the north near Leadville, CO and it correlates with the Sante Fe Group to the south. The unit consists of moderately to poorly indurated sand with lenses of gravel. The gravel consists of well-sorted, angular lithic clasts. Local ash layers are also present. Several minor faults in the area strike N60-70W and dip steeply to the NE. The strike of these faults is the same as the trend of the southern boundary of the southern sub-basin which has previously been mapped as a NE-dipping right-lateral, normal fault that juxtaposes Dry Union rocks with Precambrian crystalline basement at its eastern end and Dry Union against Dry Union at its western end. Several map-scale linear features with the N60-70W orientation cross the exposures of the Dry Union formation in the southern sub-basin and continue into the Precambrian crystalline basement of the Sawatch Range to the west. These features may also be fault-related. We interpret the current basin morphology to be due to the combined effect of original basin-bounding structures that localized the site of deposition of the Dry Union formation and of a number of post-depositional, WNW-striking normal faults that drop basin blocks down to the north and east. Tephrochronology of ash deposits in the southernmost Upper Arkansas basin indicates that some of the Dry Union deposition was occurring between 8.5 and 14.0 Ma.