Two Medicine Formation

Stebinger, Eugene, 1914, The Montana group of northwestern Montana, IN Shorter contributions to general geology, 1914: U.S. Geological Survey Professional Paper, 90-G, p. 62-68. Is mapped as formation in Montana Group in Lewis and Clark Co, MT in Montana folded belt province. Revised along western margin of Disturbed Belt in that the Big Skunk Formation (new), comprised of 2,100 ft of volcanic-rich, arkosic sedimentary rock, occupies the upper two-thirds of the Two Medicine stratigraphic interval. Two Medicine-Big Skunk contact drops stratigraphically from northeast to southwest across the Dearborn River zone of the Disturbed Belt as the Big Skunk thickens at the expense of the Two Medicine. Is divided into three units: a lower sandy facies, a middle shale-mudstone facies, and an upper tongue. Only the lower sandy facies is present in the Dearborn River zone. Gradational lower contact with Virgelle Sandstone. Upper contact with the Big Skunk is a widespread disconformity in the Dearborn River zone with breccia-filled channels along the contact. Fossils include brackish- and fresh-water pelecypods and gastropods of early Montana age in the lower sandy facies. Age is Late Cretaceous. [Schmidt, R.G., 1972, Geologic map of the Coburn Mountain quadrangle, Lewis and Clark and Cascade Counties, Montana: U.S. Geological Survey Geologic Quadrangle Map, GQ-975, 1 sheet, scale 1:24,000 [ Is formation differentiated into western and eastern facies in Lewis and Clark Co, MT (Montana folded belt province) and Cascade Co, MT (Sweetgrass arch). Overlies Virgelle Sandstone; underlies St. Mary River Formation. Western facies (divided into five map units) consists of a lower sedimentary member 340 ft thick and an upper volcanic member (equivalent to the Big Skunk Formation of Viele and Harris, 1965) about 1,525 ft thick. Volcanic member comprised of clastic volcanic rocks in lower part, 400 ft thick; latite flows, 550 ft thick; ash-flow tuffs, 475 ft thick; and clastic volcanic rocks in upper part, about 100 ft thick. Eastern facies consists of interbedded sandstone, siltstone, mudstone, and volcanic material and is mapped as single unit about 1,950 ft thick with tripartite division: lower part (equivalent to sedimentary member of western facies), about 400 ft thick; middle part, about 950 ft thick; and upper part, about 600 ft thick. Ash-fall tuff mapped locally in eastern facies 550 ft above base is probably equivalent to ash-flow tuff in western facies. Detailed lithologic descriptions given for all map units. Age is Late Cretaceous. Gill, J.R., and Cobban, W.A., 1973, Stratigraphy and geologic history of Montana Group and equivalent rocks, Montana, Wyoming, and North and South Dakota: U.S. Geological Survey Professional Paper, 776, 37 p. Rice, D.D., 1976, Stratigraphic sections from well logs and outcrops of Cretaceous and Paleocene rocks, northern Great Plains, Montana: U.S. Geological Survey Oil and Gas Investigations Chart, OC-71, 3 sheets, scale 1:500,000 For more information, please contact Nancy Stamm , Geologic Names Committee Secretary.