Stratigraphy, gravel provenance, and age of early Rio Grande deposits exposed 1-2 km northwest of downtown Truth or Consequences, New Mexico

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. 459 INTRODUCTION The Rio Grande is an integral feature of the economy, culture, and landscape of New Mexico and the American Southwest. It also serves as the axial river of the Rio Grande rift and flows through the 2016 NMGS Fall Field Conference area in Belen. The geomorphic history of this river relative to the rift has received much study (Denny, 1940; Ruhe, 1962; Kottlowski, 1953, 1958; Kottlowski et al., 1965; Bachman and Mehnert, 1978; Manley, 1979; Baldridge et al., 1980; Smith et al., 2001; Smith, 2004; Connell et al., 2005). One particularly intriguing event in the history of the Rio Grande rift and its namesake axial river was a remarkable downstream elongation of the Rio Grande from a playa-lake system in the southern Albuquerque and Socorro basins (possibly including the Belen area), where the Rio Grande terminated in the late Miocene, to playa-lakes in the El Paso area (Mack et al., 1997, 2006; Connell, 2004; Connell et al., 2005). This southward expansion resulted in the fluvial integration of several previously closed (endorheic) basins in south-central New Mexico, including the Engle, Palomas, Rincon-Hatch, and Mesilla basins. Previous studies have only partly constrained the timing of this southward elongation. In the southwestern Socorro basin, stratigraphic relationships coupled with 40Ar/39Ar dating of volcanic rocks indicate that the transition between playa lake to through-going axial river occurred between 6.88±0.02 Ma and 3.73±0.1 Ma (respective ages of units Tbsh and Tbsc in Chamberlin and Osburn, 2006, and Chamberlin, 1999, respectively; R.M. Chamberlin, personal commun., 2016). Several independent datasets confirm that the Rio Grande was established in the Engle, Palomas, Hatch-Rincon, and Mesilla basins between 5.0 and 3.0 Ma. Age constraints from associated basin-fill strata include biostratigraphic data (Tedford, 1981; Repenning and May, 1986; Lucas and Oakes, 1986; Morgan and Lucas, 2003, 2011, 2012; Morgan et al., 2011), K/ Ar dating of basalts (Bachman and Mehnert, 1978; Seager et al., 1984), 40Ar/39Ar dating and intra-basin geochemical correlation of the Hatch Siphon pumice (3.12±0.03 Ma; Mack et al., 1996, 2009) and magnetostratigraphy (Mack et al., 1993, 1998, 2006). Magnetostratigraphic work indicates that the age of earliest Rio Grande deposits in the southern Palomas basin lies between 4.997 and 4.631 Ma (i.e., between the Thvera and Nunivak subchrons, ages per Ogg, 2012). However, the age of earliest Rio Grande deposits in the Engle and northern Palomas basins have not previously been constrained. This study explores the stratigraphy, provenance, and age of lowest exposed Rio Grande axial deposits within 2 km northwest of downtown Truth or Consequences (T or C) (Fig. 1). There, canyons and a west-east alignment of 5to 12-m-tall (15to 40-ft) bluffs display axial-fluvial deposits overlying a scoured contact developed on appreciably finer-grained, redder deposits. A preliminary geologic map of the study area is presented in Figure 2. A useful reference locality in the study area is a prominent quarry owned by BAR-2 Sand and Gravel Inc., located 1 km west of downtown T or C (Fig. 2). This study, combined with on-going investigations, helps elucidate the timing and manner of the arrival of the ancestral Rio Grande into the northern Palomas basin. Koning, Jochems, morgan, Lueth, and Peters 460 Ti Tfp Tsf LPz Tsf Tb QTs