New Mexico Geological Society Annual Spring Meeting — Abstracts

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 data on the age and provenance of the New Mexico Ogallala Formation are sparse. The current age of the southern Ogallala is ~13-5 Ma based on vertebrate biochronology in the northeastern part of the Llano Estacado in Texas. In an effort to improve the age constraints and correct this deficit, this study applied ⁴⁰Ar/³⁹Ar geochronology analysis to detrital sanidine (DS) from samples collected along the western escarpment of the Llano Estacado. By comparing the K/Ca ratios (determined from measuring ³⁹Ar/³⁷Ar) and ages of detrital sanidines to known K/Ca values and ages of regional volcanic units, the age and provenance of the Ogallala can be improved. This information is utilized to better understand the evolution of the Pecos River system.

The results for samples taken along the SW Llano Estacado (that part bounded on the west by Mescalero Ridge) suggest a Trans Pecos Volcanic Field source and perhaps a contribution from the Mogollon-Datil Volcanic Field as well. This supports paleocurrent data that implies that the paleo-Pecos river system flowed north (cf. Cather, 2011). While the SW Llano Estacado exhibits a significant population of Oligocene and Eocene DS, no crystals are younger than 26.6 Ma. These older ages do not improve our knowledge of the Ogallala depositional age. A study of outcrops and digital elevation data of the SW Llano Estacado shows both a steeper topographic gradient and different lithology compared to the typical northern Ogallala sections (i.e., north of Roswell). Northern Ogallala outcrops commonly contain fluvial deposits and reworked eolian sand and silt. SW Llano Estacado outcrops have very sparse fluvial deposits and are instead almost entirely eolian. These topographic and lithological differences may imply the existence of a here-to-fore unrecognized Oligocene formation in the SW Llano Estacado that is older than the Ogallala Formation and dominated by eolianites.

Samples from several outcrops currently mapped as Ogallala in east-central New Mexico have lower and upper Bandelier tuff (~1.6 and 1.2 Ma) represented in the DS age distributions. These samples are clearly too young to be Ogallala and will require changes to existing geological maps to coincide with these new DS data.