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Disambiguation of the Knoll Limestone, a Regional Late Devonian Reservoir-Prone Litho- and Allostratigraphic Carbonate Bench That Caps the Three Forks Formation Logan Gulch Member, Western Montana


Stratigraphy of the Three Forks Formation in outcrops and wells in western Montana and eastern cratonic and western shelf correlatives needs revision beginning with designation of a litho- and unconformity bound sequence stratigraphic unit called the Knoll Limestone. Early nomenclatures are confusing and new data describe carbonate facies bound by sharp regional sequence boundaries. The Three Forks Formation near Three Forks and throughout Montana was earlier (1950) subdivided into evaporative and post-evaporative units, and later into various named units (Rau, 1962, McMannis, 1962).

Sandberg (1962) constrained a type section for the mainly Famennian Three Forks and underlying Frasnian Jefferson formations at Logan Gulch along the Gallatin River in the Three Forks-Bozeman area. A poorly outcropping, lower evaporite, limestone, red-siltstone and rare barren green shale interval is called the Logan Gulch Member. These are restricted Lower Famennian rocks and are overlain by a sometimes complexly deformed, distinctive peloidal limestone that forms a knoll in the dry gulch at the Logan type section. What we call the Knoll Limestone is a widely mappable unit that is prone to outcrop well and was originally included in the top of the Logan Gulch Member, but occurs with disconformities with rocks below and with Middle Famennian rocks of the overlying Three Forks Trident Member. (These rocks are also subject to revisions in nomenclature). The Knoll Limestone has been described and measured at 40+ locations. Due to access questions we designate new prospective type section locations (e.g., public locations in the Bridger Range, along a road at Dry Hollow, near Morgan Creek in the Gravelly Range, or at easily accessed sections in the Little Belt Mountains).

In general the lower Logan Gulch Member is equivalent to the evaporative Potlatch and Stettler formations (northern Montana subsurface), the shallow water Morrow Member of the Palliser Formation (Alberta), the Torquay Formation (Saskatchewan), and the upper Jefferson Formation (Idaho). These units contain some of the same facies that produce in the subsurface Three Forks Formation of North Dakota. The differentiated but not easily dated Knoll Limestone (upper Logan Gulch Member) is likely a correlative of the False Birdbear of Idaho as defined by Grader et al. (2017).