U-Th Dating of Lacustrine Carbonates

Christine Chen, Ph.D., 2020
David McGee, Advisor
Carbonates are prevalent in many ancient lacustrine settings, but reconstructions of past lake levels or environments from such materials have been hindered by poor chronology. Poor understanding of the complex makeup of lacustrine carbonates has led to misguided conclusions on both the utility of certain geochronological tools as well as their age. This thesis showcases strategies for the successful application of uranium-thorium (U-Th) geochronology to two types of lacustrine carbonates: lake bottom sediments and tufa deposits. Chapter 2 presents a systematic approach to U-Th dating carbonate-rich lake sediments from Lake Junín, Peru. Chapters 3–5 demonstrate the descriptive power of combining precise U-Th dates on tufas and other carbonates with geologic observations of their depositional context from the outcrop to the microscale. With improved sample selection and leveraging of stratigraphic and coevality constraints, we test the validity of U-Th data. Combining quality-controlled geochronological constraints with characterization of different carbonate facies can yield new insight on the character of lake level changes. These case studies offer frameworks for interpreting scattered geochronologic data of any size or system. For the determination of the age of lacustrine carbonates, geologic context is of equal importance to the accuracy and precision of geochronological measurements.