THE AGE OF THE MAROON LOESSITE (COLORADO), AND IMPLICATIONS FOR RESOLVING HIGH-FREQUENCY CLIMATE CHANGE IN THE LATE PALEOZOIC ICEHOUSE

The Maroon Formation is an unfossiliferous redbed unit in the Eagle basin (Colorado), and has a published age of Middle Pennsylvanian (Desmoinesian) to Early Permian (Wolfcampian) on the basis of enclosing strata. A 700 m exposure of loessite within the Maroon Formation near Basalt, Colorado, however, contains detrital zircons that indicate a substantially younger age. Several acicular zircons were recovered in random, 60-80 grain samples intended for provenance analysis. These magmatic grains occur in samples from both the base of the loessite, and the top, 700 m apart. Multiple SHRIMP spot analyses on one grain from the base yielded a weighted mean ²⁰⁶Pb/²³⁸U age of 293.0±4.1 Ma, while similar analyses on two grains from upper parts of the section yielded ages of 293.3±5.3 Ma (weighted mean) and 295 Ma. Although these ages were determined in reconnaissance analytical mode for provenance study and have relatively large uncertainties, they nevertheless show that the published age of this unit is off by several million years or more-- enough to shift it entirely out of the Pennsylvanian, using the latest Pennsylvanian-Permian boundary age. Because the grains are detrital, they furnish only a maximum age of deposition rather than a strict age of deposition; however, the acicular, sharply terminated morphology of these magmatic grains suggests that they have not undergone significant reworking. They are thus likely wind-transported from broadly coeval tuffs, and effectively constrain the duration of the 700 m loessite and intervening >200 paleosols to <10 My. This is substantially shorter than the >40 My duration implied by previous age constraints, and indicates that the loess (glacial) – paleosol (interglacial) couplets of the Maroon system archive a high-resolution record of climate cycling that operated on orbital (eccentricity to possibly precession) frequencies, akin to those documented in the Quaternary icehouse. High-precision dating (ID-TIMS, in progress) of targeted zircon single grains or sub-grains may define a far shorter interval, and correspondingly higher-frequency climate cycling. This study illustrates the potential for using detrital zircons to help constrain maximum absolute ages of non ash-bearing units, and thereby resolution and rates of climate shifts in Earth’s deep-time record.