Application of sequence stratigraphy to the nonmarine Upper Cretaceous Two Medicine Formation, Willow Creek anticline, northwestern, Montana

Abstract

The purpose of this research is to create a sequence stratigraphic framework for the Upper Cretaceous Two Medicine Formation exposed on the Willow Creek anticline along the western flank of the Sweetgrass Arch in Montana. The framework developed at this site was then correlated to the framework developed at the type section of the Two Medicine Formation located near Cut Bank, Montana. This research is significant because: 1) it provides a chronostratigraphically-significant 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 are related to relative sea level and a major change in tectonic style in western Montana (emplacement of the Boulder batholith and several volcanic events). In this study, surface outcrops were used to describe and correlate the Two Medicine Formation. The Willow Creek anticline area demonstrates similar changes in stratigraphic architecture through time as have been described at the upper discontinuity of the Two Medicine Formation type section.

A bentonite located 2.8 m (9.2 ft) below this change in architectural style at the Willow Creek anticline has been dated (U-Pb) in this study at approximately 76.5 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 stratigraphic architecture at both areas suggest a shared control on stratigraphic architecture interpreted to be primarily related to loading in the Lombard-Eldorado thrust slab due to the emplacement of the Boulder Batholith (~76-74 Ma). The correlation between the two areas provides additional support for using nonmarine sequence stratigraphy to correlate strata within a Cretaceous nonmarine retroarc foreland basin. Dinosaur nests located at the Willow Creek anticline are found in the deposits of both high and relatively lower accommodation strata, and can now be relatively dated based on their stratigraphic position above or below the newly dated bentonite. Finally, the new bentonite date suggests that the TM-003 bonebed is too old to be related to the Cretaceous Manson impact in Iowa.