The syntectonic Beaverhead Formation, a thick unit of conglomerate and sandstone, records major tectogenic and morphogenic events in the Lima region of southwestern Montana and adjacent Idaho. Pollen and spore assemblages selected from critical stratigraphic positions within the formation are used to establish its age limits and to date the prominent tectonic elements which generated the sediments.

Floral associations in the lowermost conglomerate beds, such as Tricolpites, Monocolpites, Eucommiidites, and monosulcate gymnosperm pollen, suggest a late Albian to late Cenomanian age for the basal Beaverhead. The middle part of the formation is assigned to the mid-Turonian to late Coniacian interval on the basis of relevant floral associations, such as Triatriopollenites, Proteacidites, Tricolpites, Eucommiidites, and Appendicisporites. The combination of Aquilapollenites, Momipites, and Proteacidites in the remaining third of the unit is indicative of a latest Cretaceous to mid-Paleocene age for the upper Beaverhead Formation. Although some reworking of Paleozoic spores is evident, most of the recorded palynomorphs suggest that Beaverhead sedimentation began as early as late Albia and persisted into the middle to late Paleocene.

The tectonic elements which generated the Beaverhead sediments and later deformed them are the Blacktail-Snowcrest arch, the Ancestral Beaverhead Range, uplift in the geosyncline of central Idaho, and northwest-trending upthrusts. The first three elements originated concurrently, approximately during the late Albian to late Cenomanian, and remained the dominant influence on Beaverhead deposition and deformation until the mid-Paleocene. The younger Tendoy, Cabin, and Fritz Creek northwest-trending upthrusts originated in mid-Paleocene time and continued into the late Paleocene or early Eocene.