Named as the basal member of the Tullock Formation for the Nelson Ranch, Sand Creek Arroyo quad, McCone Co, MT in the Williston basin. Overlies upper member of Hell Creek Formation; lower contact placed at base of upper Z-coal. Underlies the newly named Collins Ranch Member of Tullock; contact placed at top of W-coal. Type section measured west side of hwy where MT 24 crosses line between secs 24 and 25, T23N, R43E and continues northwest 600 m to SW corner sec 24, Bug Creek quad. Includes gray sandstone, shale, mudstone, and the upper Z-, Y-, X-, and W-coal zones. Is 42.7 m thick at type where the boundary Z-coal is overlain by (generalized; in sequence): 1) brown gray to black carbonaceous shale, gray claystone with ironstone concretions and laminated iron-rich bands, minor gray siltstone and sandstone; 2) Y-coal; 3) clayey mudstone with siderite bands "zebra striped," gray, salt-and-pepper sandstone, silty carbonaceous shale; 4) X-coal; 5) brown to gray sandstone, unnamed coal, brown claystone, gray sandstone, brown claystone, brown carbonaceous to coaly shale; 6) W-coal. Columnar section. Geologic map. Early Paleocene age.

Source: GNU records (USGS DDS-6; Denver GNULEX).