MAGNETOSTRATIGRAPHIC CORRELATIONS OF THE LOWER CHINLE GROUP (UPPER TRIASSIC) IN NEW MEXICO: AGE ASSIGNMENTS AND DURATION OF THE TR-4 DISCONFORMITY

The lower Chinle Group (Upper Triassic) of northern New Mexico is defined as the Shinarump, Salitral and Poleo Formations. In the Zuni Mountains (west-central New Mexico), the upper Bluewater Creek Formation and Blue Mesa Member (Petrified Forest Formation) have been considered lithostratigraphically correlative to the Salitral Formation. Magnetostratigraphic and geochronologic data suggest different possible correlations of lower Chinle Group strata, as well as different age assignments (Carnian vs. Norian). Using vertebrate biostratigraphy and palynostratigraphy, the Salitral Formation has been interpreted as Carnian in age. Correlation using biostratigraphy alone is tenuous, given the poor quality of material from both the Zuni Mountains and Salitral Formation, as well as concerns raised by Irmis et al. (2010) regarding correlation of Triassic land vertebrate faunachrons to marine stages. If lithostratigraphic equivalence is used to correlate strata and one considers a maximum detrital zircon (MDZ) date of ~219 Ma from the Bluewater Creek Formation, strata in northern and western New Mexico are late early to middle Norian in age. This interpretation implies the Salitral Formation is equivalent to the Bluewater Creek-Blue Mesa interval, both are Norian in age and Carnian age strata are not preserved in northern New Mexico. Comparison of geomagnetic polarity chronologies from both localities to the GPTS of Hounslow and Muttoni (2010), with the MDZ age for the Bluewater Creek, shows the Bluewater Creek-Blue Mesa interval correlates well to the ~219 Ma segment of the GPTS, corresponding to a moderately long interval of normal polarity, and is Norian in age. The Salitral Formation has three normal and four reverse magnetozones and does not correlate well to the long normal interval at ~219 Ma in the GPTS. The polarity chronology for Salitral strata matches better with shorter polarity events at the Carnian/Norian boundary. It is possible that lower Chinle Group strata in northern New Mexico are substantially older than the Bluewater Creek-Blue Mesa interval. If a maximum depositional age of ~215 Ma (Dickinson and Gehrels, 2008) is assigned to the Poleo Formation and the Carnian/Norian boundary age of ~228 Ma is used, then the disconformity at the base of the Poleo Formation (Tr-4) may encompass up to 13 Ma of time.