THE STRATIGRAPHY OF THE BRULE FORMATION IN THE NORTH UNIT, BADLANDS NATIONAL PARK

The lower Oligocene Brule Formation is exposed along the Badlands Wall that extends for 80 km in the North Unit of Badlands National Park. The Brule averages 136 m thick in its most complete sections in the North Unit. The Brule Formation includes two members, the Scenic Member, characterized by alternating widespread thick sheets of mudstone beds and muddy sandstone beds, and the Poleslide Member, characterized by massive siltstone beds. The Scenic Member is exposed extensively in the lower Badland Wall between the northern upper prairies and the southern low prairies of the White River drainage. The Scenic rocks were deposited on the Chadron claystone beds on a contact that was eroded into a relief of as much as 25 m. The Scenic Member has two widespread but thin (1 to 3 m thick) mudstone marker beds that extend across the entire Badlands Wall. The lower of these two marker units contains a thin but persistent tuff bed. The lower Poleslide is characterized by thick tan siltstone beds interspersed with thick muddy sandstone sheets. The detailed stratigraphy of the lower Poleslide is remarkably consistent, with 12 thick siltstone, sandstone and mudstone units (averaging 4 m thick) being traceable for at least 50 km along the Wall. The upper Poleslide is characterized by very light gray massive siltstone beds, structureless at the base and well, but thickly bedded at the top. The upper and lower Poleslide intervals are separated by a thick persistant white layer in the eastern half of the North Unit. This white layer superficially resembles a tuff, but does not have the internal features characteristic of tuffs and pinches out to the west. The Poleslide is capped by the Sharps Formation that is represented by thick but laterally restricted paleovalley sequences (the “Sharps Channels”) in the east, and by a prominent tuff on Sheep Mountain Table in the west. The Poleslide Member has been eroded away from most of the North Unit except where it is protected from erosion by the Sharps Channels at the top of scattered high buttes. The transition from thick mudstone intervals in the Scenic to the thick siltstone intervals of the Poleslide reflect a change from fluvial and moist pedogenic processes during Scenic deposition to dry aeolian dust accumulation in the Poleslide.