THE BONNEVILLE SHORELINE

This presentation is dedicated to Blair Jones, who was an intelligent, knowledgeable, and meticulous scientist as well as a compassionate cynic with a sense of humor, and who inspired many people to learn more about geochemistry, hydrology, and geology. The Bonneville shoreline of Pleistocene Lake Bonneville in the eastern Great Basin marks the time when the lake reached its highest level in the basin. Many previous authors have assumed the lake overflowed for an extended period (hundreds of years, or longer) after reaching its highest altitude and prior to the Bonneville flood. In some places the Bonneville shoreline appears to consist of a broad abrasion platform, and this appearance likely led to this assumption. However, in the places examined so far, the geomorphic features that appear to be abrasion platforms actually consist of coarse debris deposited offshore in sloping depositional platforms. The depositional platforms were created at higher and higher altitudes as Lake Bonneville transgressed to its highest level. The Bonneville flood probably occurred as soon as the transgressing lake reached the low point on its basin rim, the surface of Marsh Creek alluvial fan, just north of Red Rock Pass, ID, and began to spill northward into the Snake River drainage basin. The fan had been weakened by groundwater sapping before the lake reached it highest level and was catastrophically washed out during the Bonneville flood. The traditional interpretation that a prolonged period of overflow and stable lake level is required to explain the landforms at the Bonneville shoreline, for which there is no known evidence, should be reassessed and probably abandoned.