Named a formation in Livingston Group for town of Sedan, Gallatin Co, MT. Occurs on outcrop for 64 km on western edge of Crazy Mountains basin in Gallatin, Meagher, and Park Cos, MT in Montana folded belt province. Type section measured in secs 6, 7, 17, 18, T4N, R8E. Composed of interbedded sandstone, mudstone, siltstone, tuff, and conglomerate 1,075.4 m thick at type. Sandstones may be very fine-coarse grained, conglomeratic, friable, biotitic, feldspathic, zeolitic, carbonaceous, very poorly bedded, thin bedded, cross to contorted bedded, calcareous to noncalcareous. Volcanic sandstone beds may be fine-coarse grained, conglomeratic, calcareous, noncalcareous, or feldspathic. Mudstone is olive-gray to brown-black siliceous, zeolitic, calcareous to noncalcareous, massive or volcanic. Several gray mudflow conglomerates composed of pebbles and cobbles of volcanic rocks. Minor gray to olive-gray, fine-grained siltstone, crystal tuff, and lignite coals present. Divided sandstone, welded tuff, middle sandstone, and mudstone members, Bearpaw Shale, Lennep Sandstone Member (new rank). Conformable contact with underlying Eagle Sandstone. Gradational contact with overlying Billman Creek Formation. Equivalent to Elkhorn Mountains Volcanics, Cokedale and Miner Creek Formations and Maudlow Formation (new). Palynomorphs and ammonites indicate a Late Cretaceous, latest Santonian to Campanian age.

Source: GNU records (USGS DDS-6; Denver GNULEX).