Big Snowy and Amsden groups and the Mississippian-Pennsylvanian boundary in Montana

Assessment of undiscovered continuous oil and gas resources in the Heath Formation, central Montana and western North Dakota, 2016

“…1). The Big Snowy Trough was an east-west depression during Meramec time, and the north and south boundaries of the trough were later enhanced by faulting and erosion (Maughan and Roberts, 1967). The Heath Formation of the Big Snowy Group is confined to the Big Snowy Trough.…”

Mississippian and Pennsylvanian stratigraphy in southwest Montana and adjacent Idaho

“…The Lombard Limestone consists of a geographically restricted lower unit of poorly fossiliferous, thin to thick indistinctly bedded lime-mudstone and packstone and an upper unit of fossiliferous, thin-to thick-bedded skeletal lime-mudstone, wackestone, and packstone with thin interbeds and partings of 1 The ages of Maughan and Roberts (1967) have been refined by abundant new data (Davis, 1983, Wardlaw;unpublished …”

“…Mississippian limestones were first described in southwest Montana by Hayden (1872). Though the rocks described here do not adequately fit into any previous nomenclature, the units of Upper Mississippian-Pennsylvanian rocks described by Scott (1935) and modified by Maughan and Roberts (1967) have generally been applied to them ( fig. 1).…”

“…The Big Snowy Group consisted of the Kibbey Sandstone and Otter Formation (previously members of the Quadrant of Weed, 1892Weed, , 1899, and the Heath Formation. Maughan and Roberts (1967) raised the Amsden Formation to group status in central Montana and divided the Amsden Group into, in ascending order: Tyler Formation (of Freeman, 1922), Alaska Bench Limestone (of Mundt, 1956), and Devils Pocket Formation (of Gardner, 1959). Maughan and Roberts ( 1967) also attempted to apply this terminology to southwest Montana by recognizing the Big Snowy Formation and Amsden Formation throughout the area, including Railroad Canyon in adjacent Idaho.…”

“…Maughan and Roberts (1967) raised the Amsden Formation to group status in central Montana and divided the Amsden Group into, in ascending order: Tyler Formation (of Freeman, 1922), Alaska Bench Limestone (of Mundt, 1956), and Devils Pocket Formation (of Gardner, 1959). Maughan and Roberts ( 1967) also attempted to apply this terminology to southwest Montana by recognizing the Big Snowy Formation and Amsden Formation throughout the area, including Railroad Canyon in adjacent Idaho. Though the rocks included in these two units may vary from author to author, these constitute the previously applied nomenclature.…”