TIMING OF THE BELLE FOURCHE-LITTLE MISSOURI RIVER STREAM PIRACY EVENT AND RELATED BLACK HILLS TERRACES SYSTEMS FROM OPTICALLY STIMULATED LUMINANCE DATING TECHNIQUES, WYOMING AND SOUTH DAKOTA

Current interest in climate change has warranted further study of the effects that climatalogical changes have on dynamic river systems. Numerous studies have been conducted to explore the Quaternary history of the rivers that drain the Black Hills Uplift in an effort to relate their development to possible regional uplift, climate, and glacial events. The conclusions of many of these studies concerning steam terrace development in the Black Hills are hampered by a common weakness, the timing of terrace formation. This study was able to use optically stimulated luminance (OSL) dating techniques to provide accurate terrace formation dates. This allows for past research to be revisited and reevaluated creating a more accurate history of the fluvial development of the Black Hills.

The goal of this study is to constrain the timing of the capture of the Little Missouri River by the Belle Fourche River using OSL dating techniques on steam terrace sediments associated with the capture in an attempt to accurately date the event. This research will consider two study areas, one located along and between the Belle Fourche and Little Missouri Rivers in northeastern Crook County, Wyoming, and southern Carter County, Montana, (the site of the capture event also known as the Stoneville Flats). The other area is located along the Belle Fourche River in western Meade County, South Dakota, near the town of Volunteer and will be used to help correlate upstream and downstream terraces as well as provide information on terraces not found in the capture area. The timing of the capture event, documented by the formation of the Rapid terrace, and the formation of the other Black Hills terraces found in the study areas is compared to the occurrence of other geologically significant events, including tectonic events, glaciations, and climate changes, in order to determine driving forces that could have provided the conditions necessary to cause the piracy of the Little Missouri River and the formation of the Black Hills terrace system. Preliminary OSL ages on the youngest terrace level (known as the Farmingdale terrace) indicate that these terraces formed between 8 ka to 10 ka. OSL data from the older Sturgis and Rapid terraces also are included in this analysis.