APPLICATION OF SEQUENCE STRATIGRAPHY TO THE UPPER CRETACEOUS TWO MEDICINE FORMATION, NORTHWESTERN MONTANA, AND ITS IMPLICATIONS FOR DINOSAUR NESTING SITES

The purpose of this research is to develop the sequence stratigraphic framework of the Upper Cretaceous Two Medicine Formation exposed along the western flank of the Sweetgrass Arch in Montana. This research is significant because: 1) it will provide a time-significant stratigraphic framework within which Egg Mountain dinosaur nesting sites can be correlated, providing an important contribution to paleontologists interpreting this site, and 2) within this framework changes in fluvial style can be related to relative sea level and a major change in tectonic style in western Montana (emplacement of the Boulder batholith and several volcanic events). The nonmarine Two Medicine Formation is ideally suited to test nonmarine sequence stratigraphic models because it grades laterally into the westernmost sections of the marginal marine Eagle, Virgelle, and Judith River Formations and marine Telegraph Creek and Claggett Formations. Correlation of the internal stratigraphy of the Two Medicine Formation to this stratigraphy allows a relationship to be developed relating relative sea level changes to alluvial architecture. This study complements earlier work that correlated two nonmarine discontinuities from the Two Medicine Formation type section to correlative unconformities in the Judith River Formation of central Montana, and expands this correlation southward to the Willow Creek anticline site west of Choteau, Montana. In this study, surface outcrops and subsurface wireline well-log data are used to describe and correlate the Two Medicine Formation. Preliminary work in the Willow Creek anticline area demonstrates similar changes in alluvial architecture through time as have been described at the upper discontinuity of the Two Medicine Formation type section. A bentonite located 2.8 m below this change in architectural style at the Willow Creek anticline has been dated (U-Pb) at approximately 76.9 Ma and gives a new bentonite date for Two Medicine Formation strata, as well as a means of estimating the time of this transition. Similarities in alluvial architecture at both areas suggest a shared control on alluvial architecture. The correlation between the two areas provides additional support for using nonmarine sequence stratigraphy to correlate strata within a Cretaceous nonmarine retroarc foreland basin.