NEW CHRONOSTRATIGRAPHIC CONSTRAINT ON LATE JURRASIC (?) – EARLY CRETACEOUS SEDIMENTATION IN THE DISTAL CORDILLERAN FORELAND BASIN IN NORTHERN NEW MEXICO

Jurassic–Cretaceous nonmarine strata preserve the record of sedimentation across the distal-most Cordilleran foreland in northern New Mexico. Presented here are new U-Pb detrital zircon ages from the Jackpile and Lytle Sandstone that outcrop in the San Juan basin and westernmost Great Plains of northern New Mexico. These units stratigraphically overlie the Upper Jurassic Brushy Basin Member of the Morrison Formation and underlie the Albian–Cenomanian Dakota Sandstone (and equivalent strata) and share similarities regionally to Albian units across parts of Kansas (e.g., Cheyenne Sandstone), Colorado (e.g., Burrow Canyon Formation), Utah (e.g., Cedar Mountain Formation), and parts of Montana and Wyoming (e.g., Cloverly Formation). In northern New Mexico, these strata have been proposed to represent either the final Turonian stages of Morrison sedimentation, or a younger, Early Cretaceous depositional event that postdates the Morrison and predates deposition of the Dakota Sandstone. However, to date, there is very little bio- or chronostratigraphic constraint to confidently assign an age to this depositional event.

Detrital zircon ages from these intermediary strata overlap in age with the Yavapai and Mazatzal provinces (1800-1600 Ma), A-type Granitoids (1500-1350 Ma), and Grenville province (1300-1000 Ma). Latest Precambrian and Mesozoic peak ages overlap with Neoproterozoic-Jurassic zircons that span from 620-300 and 220-170 Ma and are interpreted to represent recycled Mesozoic eolianites of the Colorado Plateau. Jurassic ages are common and overlap with magmatic sources of the Cordilleran Arc, whereas Early Cretaceous zircons are extremely rare (limited to a single zircon). Maximum depositional ages for the youngest stratigraphic horizons support a latest Jurassic–Early Cretaceous age for the Jackpile Sandstone (143–147 Ma), and Lytle Sandstone (150–152 Ma).

Although these new ages support a Late Jurassic–Early Cretaceous age for these strata, there are scenarios where they could be interpreted to be younger in age. It is certainly possible that latest Jurassic zircons represent reworked, air-fall tuffs from the Cordilleran arc, thus absences of young Cretaceous grains could be interpreted as a temporary hiatus in air-fall material to the strata during the Early Cretaceous.