Stratigraphy and lithology of the Logan Gulch Member of the Three Forks Formation in the type area and its regional stratigraphic framework

摘要

The revised Logan Gulch Member of the Three Forks Formation in Montana and Wyoming contains three primary lithological facies including muddy to dolomitic conglomerates, dolomitic interbedded siltstones, and evaporitic dolostone. The revised Logan Gulch Member can be subdivided into two distinct stratigraphic units. The lower Logan Gulch Member comprises abundant vuggy evaporitic dolostones and was likely deposited in a hypersaline restricted tidal flat. The upper Logan Gulch Member comprises muddy to dolomitic, red, gray, and green conglomerates that were likely deposited in a coastal plain setting, and highly interbedded dolomitic rippled siltstones and mudstones likely deposited in a tidal flat setting. The upper Logan Gulch can be further subdivided into two stratigraphic sequences, the Logan Gulch 2 at the base and the Logan Gulch 1 at the top. Both sequences have muddy conglomerates at the base that are overlain by the interbedded siltstones. The revised Logan Gulch Member from Montana and Wyoming is related to the Three Forks Formation of North Dakota and both are located in the same relative stratigraphic position, comprise similar lithological facies and demonstrate the same large scale stratigraphic stacking patterns. The newly described Sunlight Basin roadcut in Wyoming is proposed as the neostratotype section for the revised Logan Gulch Member because it contains the most complete and easily accessible exposure of the member.