| Abstrakt: |
This report re-evaluates the depositional history, biostratigraphic "age," and regional correlates of Chadron Formation rocks in North Dakota using a variety of data sources. A series of well-developed paleosols are herein recognized for the first time at the top of the Chalky Buttes Member across much of southwestern North Dakota. Those paleosols are contemporaneous with the Weta Paleosol Series of southern South Dakota developed on top of the Chamberlain Pass Formation, and together these paleosols indicate a period of nondeposition and geomorphic stability throughout the Great Plains region following a first phase of late Eocene deposition. Within the Williston Basin, that first phase of deposition begins with rocks dominated by group A heavy minerals (zircon > staurolite > aluminosilicates, tourmaline) that are overlain by rocks dominated by group B heavy minerals (epidote > garnet > zircon). Deposition of these group A and group B rocks would have occurred sometime in the early Chadronian (Ch1-Ch2: 36.9-35.8 Ma). The second phase of late Eocene deposition in North Dakota consisted of local downcutting of stream channels that were infilled with rocks dominated by group D heavy minerals (hornblende > diopside > epidote). Two faunas are now known from these group D rocks: the late early Chadronian (Ch2: 36.6-35.8 Ma) Medicine Pole Hills local fauna, and the newly reported middle Chadronian (Ch3: 35.8-34.8 Ma) Stover Site local fauna (Adams County, North Dakota). This second phase of deposition occurred after the onset of paleosol development in North Dakota as indicated by the common presence of ferruginous aggregate grains within the opaque heavy minerals from the Stover Site sample that appear to be derived from erosion of paleosols developed on older Chalky Buttes Member rocks that contained group B heavy minerals. Deposition of group D rocks in North Dakota was contemporaneous with portions of those rocks infilling the base of the Red River Paleovalley in southern South Dakota (Ahearn and Crazy Johnson Members). Deposition of the South Heart Member began gradually during the development of the paleosols within the Williston Basin, indicating those rocks are younger than the group A and group B rocks of the Chalky Buttes Member. Given the lack of biostratigraphic data from the South Heart Member and absence of those rocks within Bowman and Adams Counties where group D rocks of the Chalky Buttes Member are exposed, insufficient evidence is currently available to determine the relative timing of deposition of group D Chalky Buttes Member rocks and those of the South Heart Member. Overall, this study demonstrates that late Eocene deposition was more complicated than previously reported, though these patterns match those found elsewhere within the Great Plains region. [ABSTRACT FROM AUTHOR] |
