Stratigraphy, gravel provenance, and age of early Rio Grande deposits exposed 1-2 km northwest of downtown Truth or Consequences, New Mexico
— Daniel J. Koning, Andrew P. Jochems, Gary S. Morgan, Virgil Lueth, and Lisa Peters
Within 2 km northwest of downtown Truth or Consequences, the discovery of a fossil tooth identified as Neohipparion eurystyle and 40Ar/39Ar dating of cryptomelane in a fault zone indicate that a through-going, ancestral Rio Grande became established in the Engle and northern Palomas basins prior to 4.87 Ma (best estimate of 5.0 to 5.5? Ma). In the lower ~25 m of the ancestral Rio Grande deposits, referred to as the lower coarse unit (LCU), we differentiate three gravel-based petrofacies units. Gravel of the basal 3-5 m of the LCU in the southeastern part of the study area (petrofacies unit 1) consists almost entirely of Paleozoic and Mesozoic sedimentary rocks inferred to be derived from toe-cutting of the nearby Mescal-Ash Canyon paleofan during establishment of the ancestral Rio Grande. Gravel in the overlying petrofacies unit 2 is composed of felsic volcanic rocks plus minor Proterozoic clasts and Mesozoic-Paleozoic sedimentary clasts, reflecting mixing of gravel shed from highlands surrounding the Engle basin. Only petrofacies unit 3 contains notable exotic clasts transported by the ancestral Rio Grande, namely 10-40% quartzite and trace Pedernal chert, that are mixed with roughly subequal felsic and intermediate volcanic types shed from highlands located west-northwest of the study area. Stratigraphic relationships coupled with gravel transport paths indicate that early deposits of petrofacies 3, which contain a tooth of Neohipparion eurystyle and therefore predate 4.9 Ma, aggraded in a paleovalley inset into petrofacies 2. We interpret this paleovalley incision, as well as subsequent increased clast caliber and the first appearance of appreciable exotic clasts in petrofacies 3, to reflect an increase in Rio Grande stream power likely related to paleoclimate changes.
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- Koning, Daniel J.; Jochems, Andrew P.; Morgan, Gary S.; Lueth, Virgil; Peters, Lisa, 2016, Stratigraphy, gravel provenance, and age of early Rio Grande deposits exposed 1-2 km northwest of downtown Truth or Consequences, New Mexico, in: The Geology of the Belen Area, Frey, Bonnie A.; Karlstrom, Karl E.; Lucas, Spencer G.; Williams, Shannon; Zeigler, Kate; McLemore, Virginia; Ulmer-Scholle, Dana S., New Mexico Geological Society, Guidebook, 67th Field Conference, pp. 459-478. https://doi.org/10.56577/FFC-67.459