Deltaic Deposits of the Upper Cretaceous Dad Sandstone Member of the Lewis Shale, South-Central Wyoming

Abstract

The Dad Sandstone Member is a progmding unit of sandstone and minor mudstone that sepamtes upper and lower shale members of the Lewis Shale. Outcrops of this Maestrichtian-age unit have been examined in southcentml W)vming along the eastern margin of the Great Divide basin north of Rawlins, and in the Hanna basin to the east. These outcmps form two groups with very different chamcteristics, including thickness of sendstone and interbedded mudstone units, sedimen- tary structures, and fossils. In the western part of the area studied, the Dad Sandstone is interpreted to represent progmda- tion in the lower delta-fmnt reglon of the Sheridan delta that existed to the northwest. In the eastern portion of the study area the Dad Sandstone probably represents upper shorefece iacies of interdistributary barrier bars. These deposits mey be related to either the Sheridan delta to the north, or a north-trending uplift to the south. The Dad Sendstone in the eastern part of the study area is probably slightly younger than In the western part of the study area.