OCR Text |
Show Lake Thatcher deposits, and therefore a maximum age on the cutting of Oneida Narrows, was determined to be 50 ± 10 ka. The initial capture of the Bear River by the Bonneville basin may have been considerably earlier than the - 50 ka establishment of Oneida Narrows, however. When the Bear River was diverted into Thatcher basin following the buildup of the northern basaltic rim, the basin filled and spilled south into Bonneville basin. To assess whether Lake Thatcher was fed by the Bear River en route to the Bonneville basin, David Bouchard, Jay Quade, and I used Sr isotopes in molluscan fossils as a tracer for Bear River water in the Thatcher basin. The results of this study are presented in detail elsewhere in this symposium. In brief, they indicate that, during the early Quaternary (> 620 ka), the Thatcher basin was occupied by locally fed shallow lakes that did not receive input from the Bear River. A single shell, together with sedimentological evidence, provides the first evidence of the Bear River in Thatcher basin - 140 ka. The river was apparently diverted back out of the basin ( and therefore the Bonneville basin) between - 140 and - 80 ka, then was finally diverted back into the basin - 50 ka. By - 20 ka, the incision of Oneida Narrows was complete and Lake Bonneville had backed up into the Thatcher basin. Beginning in 1993, and together with a parallel study in the Lahontan basin by Jeffery Bigelow ( MS Geology, Utah State University), I have been using amino acid geochronology to limit the ages of lacustrine deposits older than the range of radiocarbon dating (- 30 ka). Analyses to date reinforce previous results by William McCoy, who used the extent of isoleucine epimerization in snails to derive the original aminostratigraphic framework for the northern Great Basin. I am especially encouraged by recent results using new instrumentation and a recently developed analytical procedure ( reverse phase chromatography), to measure the extent of racemization for multiple amino acids. Some amino acids racemize at higher rates than isoleucine, which has been used in nearly all previous amino acid studies in the Great Basin, thereby providing enhanced temporal resolution. The new procedure also reduces sample sizes by nearly an order of magnitude, making analyses of sub- milligram quantities of ostracodes practical. Because ostracodes are more ubiquitous in lake deposits than are molluscs, the new technology enables amino acid analyses to be integrated into lake- core studies to derive more complete time series. Acknowledgements: The research highlighted here was accomplished through the collaborative effort of professional colleagues ( William Nash, Charles Oviatt, Andrei Sarna, and Cathy Whitlock), graduate students ( Jeffery Bigelow, David Bouchard, Amy Hochberg, and Adam Light), laboratory technicians ( Jordan Bright, Richard Hum, and Wendell Jolley), and previous workers in the Thatcher and Bonneville basins ( especially Robert Bright and William McCoy). |