CENOZOIC EXHUMATION IN THE GREAT PLAINS OF COLORADO FROM (U-TH)/HE THERMOCHRONOLOGY

Colorado’s Great Plains reside at anomalously high elevations (~1300-1800 m) for their continental interior setting, but why and when this region became elevated is poorly understood. Moreover, the history of the Great Plains is likely linked with that of the Rocky Mountains, where the cause(s) and timing of uplift are similarly debated. On the Great Plains, Cenozoic sedimentary rocks of the Ogallala Formation lie unconformably over older units to the south, suggesting that southeastern Colorado may have experienced a higher magnitude pre-Ogallala exhumation event than areas farther north. Apatite (U-Th)/He (AHe) thermochronology can be used to constrain the timing of this exhumation event, which in turn may be a proxy for the timing of surface uplift.

Here we present AHe data for 9 samples from Tertiary intrusive bodies along a ~200 km west to east transect across the Great Plains of southeastern Colorado. These data reveal kilometer-scale exhumation during the Cenozoic. AHe dates for the ~24 Ma East Spanish Peak batholith from the westernmost Great Plains are dominantly Mid-to-Late Miocene, recording a post-emplacement erosional event during this time. Two lamprophyric dikes, one of which was previously dated at 14 Ma, are located ~20-40 km farther east and yield AHe dates as young as 6 Ma, documenting that exhumation occurred until at least then. In far southeastern Colorado, the 36.8 Ma Two Buttes intrusive complex yields Late Oligocene dates. Here, the Ogallala Formation unconformably overlies the complex. Ages for the base of the Ogallala are estimated to be 12-9.5 Ma, marking the time when erosion would have ceased, and deposition dominated. The observation that AHe dates as young as 6 Ma are yielded by the intrusive rocks to the west suggests that the Ogallala may not have been deposited as far west as those sites. The AHe data presented here document a broad spatial footprint of kilometer-scale erosion across the southeastern Colorado High Plains in Cenozoic time. While exhumation can be triggered by a number of causes, a plausible driving force could be contemporaneous uplift, capable of elevating the region to modern heights.