MIDDLE CENOZOIC LANDSCAPE EVOLUTION OF THE SOUTHERN ROCKIES RECORDED IN DETRITAL ZIRCON PROVENANCE

The high-elevation, high-relief landscape of the southern Rocky Mountains (Rockies) reflects interactions among tectonics, paleoclimate, and surface processes. The southern Rockies experienced several episodes of uplift, extension, and major climate changes during the Cenozoic, and the landscape and drainage evolution remain poorly constrained. Here we apply detrital zircon U-Pb geochronology to Eocene-Miocene strata in south-central Colorado to constrain the depositional ages and sediment provenance. A total of 1284 concordant U–Pb ages are predominantly in 75–11 Ma, 1500–1300 Ma, and 1800–1500 Ma populations. Intense late Eocene-Oligocene regional volcanism provided abundant airfall zircons into the sedimentary systems, and the chronostratigraphy of the upper Eocene-Oligocene is improved by maximum depositional ages. The new data are integrated with interpretation of sedimentary environments, and the detrital zircon signatures of potential source terranes and age-equivalent strata in nearby basins to interpret the landscape and paleodrainage evolution. Specifically, the provenance data record that 1) after the main phase of the Laramide deformation, the Wet Mountains, but not the Sangre de Cristo Range was the dominant local topographic feature, and a southward river connected the Wet Mountain Valley with the Huerfano and Raton Basin to the south; 2) during the Eocene-early Oligocene, aggradation of the Wet Mountain Valley and the Huerfano Basin formed a low-relief surface, and subsequent river erosion changed the drainage pattern eastward and likely formed the modern lower Arkansas River Valley; 3) during the Miocene, dissection of the low-relief surface by the opening of the Rio Grande Rift formed the upper Arkansas River Valley, and the upper Valley was connected with the lower Valley to establish the modern drainage pattern of the Arkansas River in the southern Rockies. Constraints on the timing and changes of landscape and paleodrainage provide a better understanding of the topographic evolution of the southern Rockies.