ABSTRACT A large portion of the Belt-Purcell Supergroup is well exposed in the vicinity of Glacier and Waterton Lakes national parks of northwestern Montana, USA, and southwestern Alberta, Canada. These strata were deposited in the northeastern part of the Mesoproterozoic Belt Basin. The dramatic rate of subsidence combined with dominantly fine-grained sediment influx produced thick units of broadly uniform lithology, which constitute the spectacular and unusually colorful mountain scenery of this region. Seemingly fairly simple at first glance, in detail these rocks exhibit a great deal of facies heterogeneity and a number of unusual attributes. This has resulted in contrasting and controversial interpretations of sedimentary features, depositional dynamics, sedimentary environments, and consequently the overall understanding of the entire basin. The Belt Basin reveals itself to be a unique setting in many respects, but ideas stemming from these rocks have implications for other strata, not just those of pre-Cambrian age, but for the entire Phanerozoic as well. The Belt Supergroup is therefore a particularly stimulating field-trip destination that challenges textbook interpretations.

The ca. 1.460 Ga Revett Formation is a gray and purple quartzite lithosome in northwestern Montana, and it interfingers eastward into red argillite of the Grinnell Formation in Glacier National Park. The Revett Formation was analyzed in northwestern Montana by identifying sedimentary structures in stratigraphic sections and by interpreting flow processes of the structures using the standard flow regime model (e.g., Simons et al., 1965). The sedimentary structures and thicknesses of the event beds were then organized into eight sediment types (lithofacies) that were grouped into three sediment complexes: the playa complex, the antidune complex, and the sheet sand complex. The arrangements of the sediment types and complexes within the stratigraphic framework of the lower informal Revett member indicated the configurations of the depositional environments in space, and the vertical configurations of the sediment types revealed the depositional history of the lower Revett member. The lower Revett member lithosome interfingers eastward into the red argillite of the Grinnell Formation lithosome, and has eight through-going descriptive, stacked, lithic units, called lithostromes. Lithostromes 2, 4, 6, and 8 (from the bottom up) are composed of the sheet sand complex and extend into playa complexes of the Grinnell Formation. They were deposited by sandy sheetfloods that flowed at grade and terminated as the water sank into the sand substrate. Between lithostromes 2, 4, 6, and 8 are lithostromes marked by playa lakes of the playa complex that spread from the east across western Montana during humid periods. They were overlain by sheetfloods of the antidune complex that built eastward over the playa complex as the playa lakes retreated with increasing aridity. The antidune complex was overlain by the sheet sand complex of a vast sand plain deposited by sheetfloods from the southwest that flowed at grade level across western Montana during arid periods. The sheetflood deposits of the Revett Formation were mostly deposited by the upper-flow regime element of the established fluvial facies model.

Abstract The evidence for interaction between Laurentide and Rocky Mountain glaciers east of Waterton-Glacier Park can best be studied by examining a number of sites in the zone of glacier overlap and possible confluence. The 10 stops described in this field guide are located in the Saint Mary, Belly, and Waterton drainages in Montana and Alberta (Fig. 1). With the exceptions of Stops 1 and 10, which occur on the Blackfeet Indin Reservation and in Waterton Park, respectively, all stops occur on public roads through private land. Landowner’s permission is not required for the excursion as described but should be obtaine before collecting samples or extensive off-route exploration. The trip can be completed in a long half day with two-wheel drive vehicle. Those with less time may limit their trip to Stops 1 through 5 on the U.S. side or the approximately equivalent Stops 6 through 10 in Canada. The U.S. portion of the trip is covered by the Cutbank 1:250,000 topographic sheet and the Saint Mary, Babb, Duck Lake, Wetzel, Hall Coulee, Emigrant Gap, and Pike Lake 1:24,000 sheets. The Canadian portion is covered by the Lethbridge 1:250,000 sheet and the Cardston and Waterton Lakes (82 H/3 and H/4) 1:50,000 sheets.