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Geologic models describing Cenozoic uplift and erosion of the southern Rocky Mountains predict a topographic inversion in the Four Corners region (i.e., present day basins were previously topographic highs and vice versa) during the transition from the Laramide orogeny to Rio Grande rifting. This predicts a change in the flow directions of rivers, as well as a distinct change in the sources from which basin sediments were derived. This process is investigated in the Abiquiu Embayment that lies at the junction of the modern Chama and Espanola Basins. The embayment is bound to the northeast by the Tusas-Brazos Uplift and to southwest by the Nacimiento Uplift. The Cañones Fault zone and Embudo Transfer zone bound the embayment along its width. The two studied sedimentary units exposed in the embayment include the Eocene El Rito formation, which unconformably overlies Precambrian to Cretaceous units and was deposited during the waning stages of shortening, and the syn-rift Oligocene Ritito Conglomerate that unconformably overlies the El Rito and was deposited between 25.1 Ma to 27.0 Ma. These two units record the transition from Laramide shortening to Rio Grande extension both directly, through their sedimentology, and indirectly, through their underlying unconformities. Outcrops of these units are located on the Colorado Plateau to the west at an elevation of 2600 m, in the Tusas-Brazos Mountains to the east at 2730 m, and within the rift valley near the Chama River at 1900 m. This study investigates the unconformity at the base of the Eocene El Rito Formation. Detailed field work addressing the sedimentology, paleoflow, and provenance of the El Rito is used in conjunction with a digital reconstruction of the unconformity to build a new paleogeographic map. The results show that Eocene canyons trend in the same direction as the modern ones, but are slightly wider and are inset into similar basement rock sequences. This indicates that the modern landscape is mimicking the Eocene geography. I interpret this to result from structural inversion whereby rifting drives reactivation of reverse faults, associated with Laramide basement-cored uplifts, resulting in inversion of the hanging wall to the Tusas-Brazos Laramide thrust fault into a footwall to a normal fault. This implies that structural/topographic high and lows switch places resulting in a minor modification to the landscape.



Abiquiu Embayment, El Rito