GEOLOGIC MAPPING IN THE FOOTSTEPS OF LEWIS AND CLARK: FLATHEAD/BELT RELATIONSHIPS CONSTRAIN THE NATURE OF SUB-CAMBRIAN PALEOTOPOGRAPHY, LEWIS AND CLARK PASS, MONTANA

Lewis and Clark Pass, Montana, is of both cultural significance and geologic importance. Culturally, Meriwether Lewis spotted Square Butte, some 200 km to the east, on July 7, 1806, and, for the first time since crossing the Continental Divide on his quest for the Pacific, recognized the landmark that would guide the expedition home. Geologically, stratigraphic relationships between the Mesoproterozoic Belt Supergroup and Middle Cambrian Flathead Sandstone constrain the nature of the sub-Cambrian paleosurface.

Geologic mapping of the Blowout Mountain 7.5’ Quadrangle under the auspices of the USGS EDMAP program focuses on the Proterozoic/Cambrian transition. The map area is within the leading edge of the Montana Fold and Thrust Belt, north of the Helena Salient. In this area, rocks of the Belt Supergroup and lower Paleozoic strata are imbricated in a series of east verging thrust faults in the hanging wall of the main frontal thrust fault.

Stratigraphic studies in this region demonstrate the unconformable relationship between the Flathead Sandstone and both the Ravalli Group (lower Belt) and the Missoula Group (upper Belt). The Flathead Sandstone is a very coarse sandstone to granule conglomerate with abundant trough cross stratification and ichnofauna, including Rusophycus, that suggests shallow water deposition in the upper shoreface. Deposition of the Flathead Sandstone on a slight angular unconformity on top of both the upper and lower Belt strata requires uplift (>5 km) and erosion of the Belt Supergroup prior to middle Cambrian time. The mechanism responsible for uplift of this magnitude in the absence of substantial deformation in the Belt Supergroup prior to Cambrian time is enigmatic. Evidence of uplift and erosion in the Belt Supergroup in this area contrasts strongly with the stratigraphic relationship within the Helena Salient, where the Missoula Group was apparently not deposited and the Flathead Sandstone sits disconformably on undeformed lower Belt strata.