Lower Cretaceous Mount Pablo formation, northwestern Montana

Abstract

The newly established Mount Pablo Formation includes strata that crop out in the Sawtooth Range that are equivalent to the lower part of the Kootenai Formation in and west of the Sweetgrass Arch. Palynomorphs indicate an age of early Albian to possibly late Aptian. The Mount Pablo Formation unconformably overlies Jurassic rocks and is overlain unconformably by the Lower Cretaceous Kootenai Formation. Lateral equivalents of the Mount Pablo belonging to the Kootenai are widespread in the subsurface west of the crest of the Kevin-Sunburst dome. The Mount Pablo consists of nonmarine sandstone and conglomerate in the lower part, variegated mudstone interbedded with some sandstone in the middle part, and limestone interbedded with and overlain by mudstone in the upper part. The lower sandstone and conglomerate is the Cut Bank Sandstone Member, a previously established unit assigned as the basal member of the Kootenai Formation in the subsurface in the Cut Bank area that is extended to the Sawtooth Range and assigned to the Mount Pablo. This member ranges in thickness from 9 to 30 m (meters), whereas the formation ranges from 34 to 90 m. The Mount Pablo Formation is correlative with the Cut Bank Sandstone Member, Lander Member as described in 1966 by M. H. Oakes, and the basal brown lime unit of the Moulton Member, all of which are included in the lower part of the Kootenai Formation in the Cut Bank area. Lateral equivalents of the Mount Pablo pinch out eastward onto the Kevin-Sunburst dome, where the Sunburst Sandstone Member forms the basal unit of the Kootenai Formation and is correlative with the upper part of the Moulton Member. In the Canadian Foothills, the Cadomin Formation, which is a prominent conglomerate, correlates with the Cut Bank Sandstone Member of the Mount Pablo Formation or of the Kootenai Formation. The upper part of the Mount Pablo is equivalent to the Gladstone Formation in the southern foothills, to the Gething Formation in the northern foothills, and to the McMurray Formation in the central plains of Alberta. The palynological assemblage in the Mount Pablo is similar to that found in other Lower Cretaceous formations in the Western Interior and is somewhat similar to assemblages in the early to early middle Albian McMurray Formation of Alberta.