Box Butte Formation

68 p., [3] folded leaves of plates : ill., maps (1 col.) ; 26 cm.Includes bibliographical references (p. 66-68)."Sedimentary deposits of northwestern Nebraska formerly assigned to the Box Butte Member of the Sheep Creek Formation are redefined as the Box Butte Formation, new rank. Two new members are proposed: the Red Valley Member, new name, the lower unit; and the Dawes Clay Member, new name, paraconformably overlying it. Type localities, type sections, and a series of reference sections are proposed for each new or redefined unit. Deposits assigned to the two members of the Box Butte Formation crop out in irregular exposures, some of which occur in well-defined paleovalleys in an area covering nearly 900 square miles situated high along the rims and ridges of tablelands forming the northern border of the High Plains. Sediments of the Red Valley Member are red siltstones or clayey siltstones predominantly, with occasional sandstone lenses or admixtures of sand. Coarser clasts are confined to the bottom and sides of paleovalleys and are mostly small cobble- or pebble-sized sandstone concretions reworked from the underlying Runningwater Formation = Upper Marsland Formation.¹ The Red Valley Member occurs only in paleovalleys, which are narrow and steep-walled on the more westerly outcrops, but widen to a few hundred yards along the easterly exposures and are incised as much as 140 feet into the Runningwater. White to gray, hard, nodular calcareous concretions are the most distinctive lithologic feature of the Dawes Clay Member; they are conspicuously absent in the Red Valley Member. The Dawes Clay Member commonly is 25 to 30 feet thick but may range from 20 to 40 feet. It crops out as a distinctive set of mottled reddish and greenish clayey siltstones on which the characteristic white calcareous nodules may be concentrated at certain horizons. Where the Red Valley Member is absent, the Dawes Clay lies unconformably on the Runningwater Formation, and on most outcrops it is in contact with the Platy Bench, a topographic feature formed at several levels in the upper part of the Runningwater. In western Dawes, western Box Butte, and eastern Sioux counties, the Dawes Clay Member is overlain by beds correlated with the Sand Canyon Member of the Sheep Creek Formation. Mammalian fossils collected in the Box Butte Formation show that the formation was deposited earlier than the earliest beds of the Sheep Creek Formation and later than any beds assigned to the Runningwater Formation; thus the Box Butte was deposited during a segment of time heretofore not recognized formally in the Nebraska geologic section. 1. The term Runningwater Formation = Marsland Formation is used here to direct attention to the existence of two formational names that currently are being applied to the same set of beds. Until clarification of the status of the beds is published, proponents of either of the terms are advised to read Runningwater Formation = Marsland Formation whenever Runningwater Formation appears in the present report"--P. 5