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Toston-Townsend Valleys WGI has completed a geologic map of the Toston-Townsend valleys, southwestern Montana. The map is a 1:100,000 scale topographic metric. We have mainly concentrated on the Cenozoic strata/structure, although bedrock mapping is included. This area contains the Sixmile Creek type section. Consequently, we have paid particular attention to facies changes in the type section area, and thus we demonstrate how the type section is simply a coarse-grained, basin-margin facies - not a common lithostratigraphic unit to use as a basis for a late Tertiary Sixmile Creek Formation. We use the sequence stratigraphic approach in our areal mapping! Project AbstractTERTIARY STRATIGRAPHY AND CENOZOIC STRUCTURE OF THE SIXMILE CREEK FORMATION IN THE SIXMILE CREEK AREA, SOUTHWESTERN MONTANA.The Sixmile Creek Formation was originally described for strata of the Sixmile Creek area that contain predominantly conglomerate and sandstone and range in age from Arikareean to Hemphillian. Subsequently, the Sixmile Creek Formation has been redefined as a predominantly coarse-grained unit that lies immediately above a regional Hemingfordian unconformity. However, recent mapping in the Sixmile Creek area near Toston, Montana shows that the Sixmile Creek type section and much of the unit mapped as the Sixmile Creek Formation contain a variety of lithologies, and that these lithologies are mainly Arikareean in age.Lithologies of the Sixmile Creek Formation in this area range from clast-supported conglomerate to mudstone. Clast-supported conglomerate is present primarily near the eastern part of the map area where it occurs within alluvial fan deposits. Mudstone, sandstone, and minor conglomerate comprise the bulk of strata in the western part of the map area. These lithologies are interpreted as flood-plain and channel-fill deposits. Volcanic ash is also present in the western part of the map area both as reworked and air-fall deposits. Paleosols occur at the top of underlying Chadronian-Orellan strata and provide excellent stratigraphic markers for delineating the base of Arikareean strata. There are at least two small erosional remnants of Hemphillian strata within the Sixmile Creek area. The remnants contain lithologies similar to those of Arikareean strata in the western map area and lie disconformably on the Arikareean strata.The Cenozoic strata of the Sixmile Creek area overlie a relay ramp developed within a zone of overlapping normal faults that separate the Sixmile Creek area from the Toston Valley and delineate the eastern margin of the Sixmile Creek basin. The entire Sixmile Creek basin has been recently tilted to the northwest over an intrabasin high associated with the relay ramp.(Amer. Association of Petroleum Geologists Rocky Mountain sectional meeting, abstract, 1996, by Hanneman,D.L.,Wideman,C.J.,and Halvorson,J.W.)
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An ultisol (location shown by arrow) is developed atop
Sequence 2 in the Toston area.
Fine- coarse-grained couplets of
fan complex in Sequence 3 (Sixmile Creek type section!!).