OXFORDIAN U/PB AGES FROM SHRIMP ANALYSIS FOR THE UPPER JURASSIC MORRISON FORMATION OF SOUTHEASTERN WYOMING WITH IMPLICATIONS FOR BIOSTRATIGRAPHIC CORRELATIONS

The Morrison Formation of the western interior of North America previously has been radiometrically dated using many different methods, including: 1) Rb-Sr on chlorite grains; 2) single-crystal Ar-Ar laser-fusion methods on biotite, plagioclase, and sanidine crystals; 3) K-Ar on biotite; and 4) zircon and apatite fission-track. These various methods have yielded a range of ages from 154.87 ± 0.52 Ma (Ar-Ar on sanidine from the Tidwell Mbr.) to 134 Ma (K-Ar on biotite from the Brushy Basin Mbr.). All but one of these dates comes from western parts of the depositional area.

In preliminary work to understand the zircon population of the Morrison Fm. in southeastern Wyoming and to determine the utility of U/Pb dating for the Morrison Fm., we performed U/Pb SHRIMP analyses of single zircon crystals separated from a smectitic mudstone collected from the upper third of the formation exposed at Ninemile Hill near Medicine Bow. The sample locality is at the same stratigraphic level as three microvertebrate quarries, most notably Quarry Nine at Como Bluff. Analyses of 8 individual, euhedral zircons, including shapes commonly associated with ash-falls, yielded a weighted mean 206Pb/238U date of 156.3 +/- 2 Ma. We interpret this as the age of the ashfall component of the smectite. Additional SHRIMP dates document detrital components with ages of 297, 433, 1087 and 1150 Ma.

This result is at least 7 million years older than previous estimates based on biostratigraphic correlations. We envision two endmember possibilities: either the zircons have been reworked from sources farther west, and so do not give the age of deposition of the Wyoming strata, or deposition began earlier than previously assumed in eastern parts of the formation. As the euhedral, 156 Ma zircons are the dominant zircon population, however, the first scenario would imply little or no primary volcanic input to the Morrison Fm. in Wyoming, which seems unlikely. Our favored interpretation is that 156 Ma represents the depositional age. This interpretation challenges previous stratigraphic correlations and has great implications for the ages of the ecosystems of the Morrison Fm. in southeastern Wyoming as well as faunal and floral associations worldwide that are thought to be contemporaneous with those of the Morrison Fm.