40Ar/ 39Ar Detrital Sanidine Dating of the Ogallala Formation in Southeastern New Mexico and West Texas

Abstract

Despite the potential for the use of the Ogallala Formation as a constraint on the sedimentary response to uplift of the Southern Rocky Mountains during the Tertiary, primary age and provenance data for New Mexico Ogallala units are sparse. The current estimated depositional age of the southern Ogallala is between ~13 and 5 Ma based on vertebrate biochronology in the northeastern part of the Llano Estacado in west Texas. In an effort to improve the age constraints, detrital sanidine (DS) 40 Ar/ 39 Ar geochronology is utilized on samples from the western escarpment of the Llano Estacado and elsewhere in NM. Coupling DS age and associated K/Ca data (determined from measuring 39 Ar/ 37 Ar) and comparing this to age and K/Ca data of regional volcanic units allows estimates of maximum depositional age (MDA) and provenance. This information is utilized to better understand the evolution of the Pecos River system. DS data were determined from the Bridwell and Couch formations of the Ogallala Group near Lubbock, TX (samples courtesy of Dr. Tom Lehman). Based on biostratigraphy, the Bridwell formation is Hemphillian (10.3-4.9 Ma) and the Couch is Clarendonian (13.6-10.3 Ma). DS data yield an MDA of 6.77 Ma thus restricting the sampled interval of the Bridwell to no older than 6.77 Ma In contrast, the MDA of the Couch formation sample is 27.1 Ma with no Miocene DS grains detected. Four Miocene DS grains are found in samples from Mescalero Ridge in SE New Mexico and they provide an 11.44 MDA for Ogallala Formation in this area. The combined DS data and lithologic (eolian) similarities suggest that the Ogallala of SE New Mexico is correlative to the upper Couch Formation of west Texas. The 6-8 Ma youngest grains in the Bridwell formation indicate a New Mexico Peralta tuff source, whereas significant late Cretaceous DS grains are likely derived from reworked Cretaceous or younger sedimentary rocks. The 11.44 Ma DS grains from the Llano Estacado may be derived from Socorro area volcanics or perhaps much more distal Yellowstone Hotspot Track eruptions in Idaho, although long transport of sanidine is tephra is problematic. As a whole there are multiple DS ages between the Oligocene and Eocene that could reflect derivation from several regional volcanic fields. In detail, age and K/Ca data of Trans-Pecos volcanic field sanidines provide the best matches to some of the DS data, thereby implying an overall southern source for the Ogallala sediments. This coupled with paleocurrent data supports a north flowing paleo-Pecos river system that has been proposed by Cather (2011). Several samples from north-central New Mexico that are mapped as Ogallala yield Pleistocene DS ages with grains likely sourced from Valles Caldera eruptions. The young ages demonstrate the difficulty of correctly mapping the Ogallala and suggest that these units are likely either the Blackwater Draw or Blanco formation.