McGowan Creek Formation, new name for lower Mississippian flysch sequence in east-central Idaho

Abstract

The argillite sequence of Early Mississippian age in east-central Idaho that had been erroneously correlated with the Devonian Milligen Formation in central Idaho is here named the McGowan Creek Formation from exposures south of McGowan Creek on the west side of the Lost River Range. The McGowan Creek is dated and zoned on the basis of conodont faunas and its type section is described. It comprises a thick lower member, predominantly grayish-black argillite, as much as 1,000 metres (3,280 feet) thick, and a thin upper member, predominantly pinkish-gray-weathering calcareous siltstone, as much as 150 metres (492 feet) thick. Regionally, the McGowan Creek forms a westward-thickening wedge of flysch sediments that were deposited on the east side of the Antler foreland basin. Westward it correlates with the lower part of the Mississippian Copper Basin Formation near the axis of the foreland basin. Eastward it grades into the Lodgepole Limestone and lower part of the Mission Canyon Limestone on the cratonic platform in southwestern Montana.