Correlation chart of Frontier Formation from Greenhorn Range, southwestern Montana, to Mount Everts in Yellowstone National Park, Wyoming

Abstract

The Upper Cretaceous Frontier Formation in southwestern Montana was deposited in the foreland basin of the Cordillera in the west and shelf facies of the foreland in the east. Lateral and vertical changes in the lithology of the formation reflect both depositional environments and regional tectonism. The measured sections presented here form a 120-km (75-mi) transect that is about normal to the axial trend of the mid-Cretaceous seaway in which the marine rocks of the formation were deposited, and about normal to the orogenic trend in the hinterland to the west (fig. 1). During the Late Cretaceous to early Tertiary Laramide orogeny, Cretaceous strata in southwestern Montana were deformed and foreshortened (telescoped) by eastward transport of strata above major thrust faults. Frontier rocks of the Madison Range composite section and the Mount Everts section are not known to be separated by any major Laramide-age thrust faults that would have moved the sections closer together than they were when deposited, and hence, the facies present are broadly similar. Frontier strata of the Ruby River section, in the Greenhorn Range, however, lie within a thrust plate that is bordered on the east by the Hilgard thrust fault (fig. 1). The plate moved eastward during the Laramide orogeny and the facies of the Frontier present within it differ significantly from those exposed farther east, both in depositional environments and thicknesses. The purpose of this report is to review stratigraphic relationships and nomenclature for the Frontier Formation in the region of the measured sections, and to discuss contact relationships with overlying and underlying formations. These goals are accomplished through the presentation of new data, including the faunal and palynofloral identifications and the four measured sections displayed graphically in this report, and the reinterpretation of existing data. Regional stratigraphic relationships are shown in figure 2, and thickness relationships-for the measured sections are illustrated in figure 3. In discussing stratigraphic relationships, we proceed from eas* to west, from shelf to foreland basin. All megafossils reported here, including those in cited references, were identified by co-author W.A. Cobban and are shown in tables 1 and 2. All palynomorphs, including those in cited references, were identified by co-author D.J. Nichols and are shown in tables 3 and 4. In the citation of previous studies, we have attempted to include all relevant M.S. and Ph. D. theses, but others, of which we are unaware, may exist.