U-Pb Detrital Geochronology and Provenance Comparisons from the Nonmarine Strata of the Dakota Group, Lytle Sandstone, and Morrison Formation in Northeastern New Mexico
— Samatha R. Bartnik, Brian A. Hampton, and Greg H. Mack

U-Pb detrital zircon ages from sedimentary rocks of the Early–Late Cretaceous (Albian–Cenomanian) Dakota Group, Late Jurassic–Early Cretaceous(?) Lytle Sandstone, and Late Jurassic (Tithonian) Morrison Formation in northeastern New Mexico provide new geochronologic and provenance constraints on the age range and sources of detritus delivered to the Cordilleran foreland basin during Jurassic–Cretaceous time. Presented here are four U-Pb age spectra (n=978 analyses) from detrital zircons extracted from nonmarine stratigraphic horizons from these units where they crop out in the western Dry Cimarron Valley and near the Creston anticline just west of Las Vegas, New Mexico. All four stratigraphic units share strong similarities in the distribution of Paleo–Mesoproterozoic zircon ages with the majority falling between 1800–1600 (Yavapai-Mazatzal provinces), 1450–1350 (A-type granitoids), and 1300–1000 Ma (Grenville province). Neoproteozoic–Jurassic peak ages are also similar across all units with primary peaks occurring between 625–595, 430–415, 190–150 Ma. Neoproteozoic and early Paleozoic ages overlap ages of recycled Mesozoic eolionites of the Colorado plateau, whereas Jurassic ages overlap ages of magmatic sources of the Cordilleran arc. Although the Dakota Group contains elevated occurrences of Early–Late Cretaceous-age zircons with peak ages between 105–95 and 125–120 Ma, there are no occurrences of zircons younger than Late Jurassic in either the Morrison Formation or Lytle Sandstone. The nine youngest ages from the Morrison Formation fall between Early–Late Jurassic (between ~190–150 Ma), whereas the nine youngest ages from the Lytle are between Middle–Late Jurassic (~172–150 Ma). The nine youngest ages from overlying strata of the Mesa Rica Sandstone and Pajarito Formation are all Late Cretaceous (Cenomanian–earliest Turonian) and occur between ~100–92 Ma. The youngest detrital zircon ages from the Morrison Formation and Lytle Sandstone support a Late Jurassic (Tithonian) age for these units, whereas the youngest ages from both members of the Dakota Group indicate an age of earliest Late Cretaceous (Cenomanian). However, it is certainly possible that the youngest Cretaceous zircons in the Dakota Group originated from reworked, air-fall tuffs (rather than fluvial, water-laid deposits), thus the absence of these Cretaceous grains in the Lytle Sandstone could have resulted from a temporary hiatus in deposition of air-fall material to the Lytle Sandstone during the Early Cretaceous.