Depositional History of the Chadron Formation in North Dakota by Clint A. Boyd1 and John R. Webster2 1North Dakota Geological Survey 2Geosciences, Minot State University REPORT OF INVESTIGATION NO. 120 NORTH DAKOTA GEOLOGICAL SURVEY Edward C. Murphy, State Geologist Lynn D. Helms, Director Dept. of Mineral Resources 2018 Table of Contents Abstract ........................................................................................................................................... v Acknowledgements ........................................................................................................................ vi Introduction ..................................................................................................................................... 1 Late Eocene Paleosols of North Dakota ......................................................................................... 3 Paleosols at White and Haystack Buttes (Stark County) ............................................................ 4 Description of Section 1 .......................................................................................................... 5 Description of Section 2 .......................................................................................................... 7 History of Paleosol Development ......................................................................................... 13 Interpretation ......................................................................................................................... 16 Identification ......................................................................................................................... 18 Time for Formation ............................................................................................................... 20 Geographic Distribution ........................................................................................................ 21 Paleosols at Fitterer Ranch ....................................................................................................... 23 Description of Section ........................................................................................................... 25 Interpretation ......................................................................................................................... 26 Identification ......................................................................................................................... 27 Time for Formation ............................................................................................................... 28 Geographic Distribution ........................................................................................................ 28 Regional Correlations ............................................................................................................... 28 Biostratigraphy of the Chadron Formation in North Dakota ........................................................ 32 New Chadron Formation Outcrops in Adams County .............................................................. 33 Discovery .............................................................................................................................. 33 The Stover Site Local Fauna ................................................................................................. 36 Geographic Distribution ........................................................................................................ 37 Stratigraphic Relationships ................................................................................................... 41 Medicine Pole Hills Local Fauna .............................................................................................. 43 Other Fossils from the Chalky Buttes Member ........................................................................ 44 Fossils from the South Heart Member ...................................................................................... 45 Heavy Mineral Analysis of the Chadron Formation in the Williston Basin ................................. 47 Sampling Locations .................................................................................................................. 48 Methodology ............................................................................................................................. 51 Disaggregation and Cleaning ................................................................................................ 51 Grain-size Separation ............................................................................................................ 51 i Heavy-liquid Separation ....................................................................................................... 51 Grain-mount Preparation ...................................................................................................... 52 Heavy Mineral Analysis ....................................................................................................... 52 Results ....................................................................................................................................... 53 Interpretation of Grain Size Differences in Heavy Minerals ................................................ 55 Cluster Analyses of Chalky Buttes Member Samples .......................................................... 55 Cluster Analysis Including South Heart Member Samples .................................................. 58 Cluster Analysis Without Biotite .......................................................................................... 58 Cluster Analysis Without Epidote ........................................................................................ 61 Cluster Analysis Without Epidote and Biotite ...................................................................... 62 Cluster Analysis Without Volcanic Minerals ....................................................................... 65 Discussion ................................................................................................................................. 65 Implications for the Stover Site Heavy Mineral Sources ...................................................... 67 Implications for Relative Ages of Chadronian Deposits ...................................................... 69 Reinterpretation of the Deposition of the Chadron Formation in North Dakota .......................... 72 The Chalky Buttes Member ...................................................................................................... 72 The South Heart Member ......................................................................................................... 76 Future Research Directions ........................................................................................................... 77 References ..................................................................................................................................... 79 Appendices .................................................................................................................................... 84 Tables Table 1. Faunal list for the Stover Site local fauna (NDGS L236) from the Chalky Buttes Member of the Chadron Formation in Adams County, North Dakota. .............. 39 Table 2. Measurement data for leptomerycid lower molars from the Medicine Pole Hills and Stover Site local faunas compared to reported values for the Chadronian taxa Leptomeryx mammifer, Leptomeryx speciosus, and Leptomeryx yoderi. ............... 40 Table 3. Heavy minerals identified (raw and normalized values) from the two grain size fractions from the Stover Site sample STOV-3. ............................................ 54 Table 4. Summary of dominant and minor heavy minerals by group and sub-group resulting from the analysis of all 36 Chadron Formation samples. ............................... 61 Table 5. Tracking the various positions of heavy mineral samples that move between groups in the cluster analyses depending on what minerals are included. ..................... 67 ii Table 6. Stratigraphic pattern of heavy minerals in the Chalky Buttes area (including Rattlesnake Butte) of North Dakota. ............................................................ 70 Table 7. Stratigraphic pattern of heavy minerals at South Cave Hills in South Dakota. .............. 71 Figures Figure 1. Updated North Dakota stratigraphic column of upper Paleocene through Miocene......... rocks................................................................................................................................ 2 Figure 2. Map showing geographic locations within the Williston Basin region in Montana, North Dakota, and South Dakota that are discussed in this study. ................................. 3 Figure 3. Diagram of White Butte paleosol at section one at White Butte (Stark County) ............ 5 Figure 4. Photograph of a sheer vertical outcrop of the White Butte paleosol near section one .... 6 Figure 5. Pedogenic features in the White Butte paleosols ............................................................ 8 Figure 6. A horizons within the White Butte paleosols .................................................................. 9 Figure 7. B horizons within the White Butte paleosols. ............................................................... 10 Figure 8. Diagram of White Butte paleosol at section two at White Butte (Stark County).. ........ 11 Figure