New Mexico Geological Society Annual Spring Meeting — Abstracts

A late Miocene- early Pliocene unconformity has been found along almost the entire length of the Rio Grande rift, from the San Luis Basin in Colorado to the Mesilla Basin in southern New Mexico. The unconformity generally spans from about 8-5 Ma, and has often been described as an angular unconformity. It is most pronounced on the hanging wall sides of the asymmetric grabens. The unconformity was preceded by rapid basin subsidence and sediment infill, and is followed by continued basin subsidence. It is broadly synchronous with uplift of the Ogallala Formation on the western Great Plains. What may have caused this almost rift-wide unconformity? We have calculated geoid-to-elevation ratios that give information on the mechanism of topographic compensation. North of 35°N, these ratios suggest that mantle-driven dynamic uplift has occurred during the late rifting phase. Such dynamic uplift has also been predicted by geodynamic modeling studies (Moucha et al. 2008). We propose therefore that dynamic uplift, focused in the northern rift-region, promoted wide development of the unconformity in the Rio Grande rift, promoting erosion along basin margins and restricting aggradation to basin depocenters. Such dynamic uplift could also explain tilting of the Ogallala formation.