QUATERNARY HISTORY OF THE THATCHER BASIN, IDAHO, RECONSTRUCTED FROM THE SR-87 SR-86 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 fro m 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 importan t to understanding the aquatic biogeography of the Bonneville Basin an d the climatological implications of its lake-level fluctuations. This study employs strontium (Sr) isotopes in lacustrine mollusc fossils a s a tracer of Bear River water that entered Lake Thatcher, a small lak e into which the redirected river flowed en route to the Bonneville Ba sin. The Sr-87/Sr-86 composition and Sr concentration of modern rivers were measured to construct a mixing model for Lake Thatcher water. Th e low Sr-87/Sr-86 ratio of the Bear River and its large discharge assu res that the presence or absence of its flow into Lake Thatcher is rea dily detectable. Temporal control is provided by amino acid geochronol ogy on lacustrine molluscs and calibrated using tephrochronology and C -14 dating. Sr-87/Sr-86 ratios of six fossil molluscs from the oldest exposed Quaternary deposits in Thatcher Basin (lower Main Canyon Forma tion, MCF) indicate that, during the early Quaternary (>620 ka), the b asin was occupied by shallow, locally fed lakes. A single shell from t he base of the upper MCF, together with sedimentological evidence, sug gests that the Bear River may have been diverted into Thatcher Basin b y similar to 140 ka. Sr-87/Sr-86 ratios in eight younger shells from t he upper MCF indicate that the Bear River was not present in the basin between similar to 140 and similar to 80 ka. By 50 +/- 10 ka, however , it was tributary to Lake Thatcher. A simple hydrologic model shows t hat, 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, b ut it was probably by similar to 100 ka. At that time, headward incisi on of Oneida Narrows was underway. By similar to 20 ka, the incision o f the narrows was complete and Lake Bonneville had backed up into That cher Basin. (C) 1998 Elsevier Science B.V. All rights reserved.