Quaternary history of the Thatcher Basin, Idaho, reconstructed from the ⁸⁷Sr/⁸⁶Sr and amino acid composition of lacustrine fossils: Implications for the diversion of the Bear River into the Bonneville Basin

The Bear River, the largest river in the Great Basin, was diverted from its former course to the Pacific Ocean into the Bonneville Basin by Quaternary basalt flows that form the northern rim of Thatcher Basin, Idaho. Reconstructing the history of the river's diversion is important to understanding the aquatic biogeography of the Bonneville Basin and the climatological implications of its lake-level fluctuations. This study employs strontium (Sr) isotopes in lacustrine mollusc fossils as a tracer of Bear River water that entered Lake Thatcher, a small lake into which the redirected river flowed en route to the Bonneville Basin. The ⁸⁷Sr/⁸⁶Sr composition and Sr concentration of modem rivers were measured to construct a mixing model for Lake Thatcher water. The low ⁸⁷Sr/⁸⁶Sr ratio of the Bear River and its large discharge assures that the presence or absence of its flow into Lake Thatcher is readily detectable. Temporal control is provided by amino acid geochronology on lacustrine molluscs and calibrated using tephrochronology and ¹⁴C dating. ⁸⁷Sr/⁸⁶Sr ratios of six fossil molluscs from the oldest exposed Quaternary deposits in Thatcher Basin (lower Main Canyon Formation, MCF) indicate that, during the early Quaternary (>620 ka), the basin was occupied by shallow, locally fed lakes. A single shell from the base of the upper MCF, together with sedimentological evidence, suggests that the Bear River may have been diverted into Thatcher Basin by ~140 ka. ⁸⁷Sr/⁸⁶Sr ratios in eight younger shells from the upper MCF indicate that the Bear River was not present in the basin between ~140 and ~80 ka. By 50 ± 10 ka, however, it was tributary to Lake Thatcher. A simple hydrologic model shows that, if not for drainage through Oneida Narrows, Thatcher Basin would fill to its highest shoreline under present climate, even without the input of the Bear River. It is not clear when the lava flows that form the northern divide had been built high enough to allow Lake Thatcher to spill over its former southern divide into the Bonneville Basin, but it was probably by ~100 ka. At that time, headward incision of Oneida Narrows was underway. By ~20 ka, the incision of the narrows was complete and Lake Bonneville had backed up into Thatcher Basin.